Sonochemistry: Fundamentals and Evolution 9783110566178, 9783110566123

Table of contents :
Preface for Volume 1
Contents
Chapter 1 Historical introduction
Chapter 2 Fundamental aspects of sonochemistry
Chapter 3 Ultrasonically assisted extraction (UAE)
Chapter 4 Environmental protection
Index

Citation preview

Timothy J. Mason, Mircea Vinatoru Sonochemistry

Also of Interest Sonochemistry. Volume : Applications and Developments Timothy J. Mason and Mircea Vinatoru,  ISBN ----, e-ISBN ---- Also available as a set - Set ISBN: ---- Flow Chemistry. Volume : Fundamentals nd Edition Ferenc Darvas, György Dormán, Volker Hessel and Steven V. Ley (Eds.),  ISBN ----, e-ISBN ---- Flow Chemistry. Volume : Applications nd Edition Ferenc Darvas, György Dormán, Volker Hessel and Steven V. Ley (Eds.),  ISBN ----, e-ISBN ---- Green Chemisty. Water and its Treatment Green Chemical Processing, Volume  Mark Anthony Benvenuto and Heinz Plaumann (Eds.),  ISBN ----, e-ISBN ----

Green Chemistry. Principles and Designing of Green Synthesis Syed Kazim Moosvi, Waseem Gulzar Naqash and Mohd. Hanief Najar,  ISBN ----, e-ISBN ---- Process Technology. An Introduction nd Edition André B. de Haan and Johan T. Padding,  ISBN ----, e-ISBN ----

Timothy J. Mason, Mircea Vinatoru

Sonochemistry

Volume 1: Fundamentals and Evolution

Authors Prof. Dr. em. Timothy J. Mason Faculty of Health and Life Sciences Coventry University Priory Street Coventry CV1 5FB United Kingdom [email protected] Dr. Mircea Vinatoru Faculty of Chemical Engineering and Biotechnology University POLITEHNICA of Bucharest Spl. Independentei nr. 313 060042 Bucharest Romania [email protected]

ISBN 978-3-11-056612-3 e-ISBN (PDF) 978-3-11-056617-8 e-ISBN (EPUB) 978-3-11-056623-9 Library of Congress Control Number: 2022941772 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the internet at http://dnb.dnb.de. © 2023 Walter de Gruyter GmbH, Berlin/Boston Cover image: NiPlot/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck. www.degruyter.com

Preface for Volume 1 This two-volume book “Sonochemistry” is not written in the style that might be expected of such a comprehensive history of the subject. This is because the authors, Tim Mason and Mircea Vinatoru, were active participants in its development from the 1990s to the present day and the text reflects their experiences. In the early days it was used mainly in the field of chemistry but within a few years the subject had begun to extend into other disciplines including environmental protection, the extraction of natural materials, food technology and medicine. In his opening address at the 17th meeting of the European Society of Sonochemistry in Jena in August 2022 Tim Mason used a term to describe his entry into the subject as a case of “Serendipity” which is defined as the occurrence and development of events “by chance” and “in a happy way”. It certainly applied when he gained his first permanent teaching post at Coventry Polytechnic because it corresponded exactly in time to the appointment of another chemist, Phil Lorimer, in the same department. It was these two who were to go on and establish the Sonochemistry Centre in Coventry. There are many other examples of serendipity recounted in the book, one of which was the arrival of an unsolicited letter from Mircea Vinatoru to Tim Mason in July 1990 which asked for some guidance on sonochemistry. This led to the two scientists meeting in Bucharest and resulted in a long-lasting friendship. Many years later and after continued research collaboration it led to the writing of this book. An important source of information for the authors was the paperwork that Tim Mason had collected from the very start of his time in Coventry. He had remained in the same building for the whole of his 40 years there and amassed a wealth of material, the earliest parts of which were not stored electronically amongst which were some significant but faded faxes that have now become very difficult to read. Coventry University closed the Sonochemistry Centre in 2018 and some years later the whole building within which it had been housed was demolished. The collected historical material was saved and transferred in several filing cabinets to Tim’s garage at home. Mircea Vinatoru, with a background in chemical engineering, has worked in several countries applying his knowledge to the design of ultrasonic reactors. This experience has not only added significantly to industrial interest in sonochemistry but has also established some best practice guidelines for laboratory experiments. These contributions are well represented in this book. Volume 1 traces the evolution of sonochemistry from the 1920’s, when the effects of acoustic cavitation were first reported, almost as a scientific curiosity, to the present day. A chapter is devoted to ultrasonically assisted extraction (UAE), a field in which both authors are pioneers. In this field the different ways in which sonochemical technology can be applied from laboratory to large-scale processing are illustrated. It is an application of particular importance for the nutraceutical and food industries. A

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Preface for Volume 1

chapter on environmental protection shows the remarkable and wide-ranging uses of sonochemistry for both biological and chemical decontamination. It also highlights the particular advantages of sonochemistry when used in conjunction with Advanced Oxidation Processes.

Contents Preface for Volume 1

V

Chapter 1 Historical introduction 1 1.1 Introduction 1 1.2 Sound waves and some basic elements of acoustics 2 1.3 A note on the physiology of hearing 5 1.4 Cavitation – some background information 7 1.5 Sonochemistry – the first uses of the term 10 1.6 Sonochemistry: some personal reflections from Tim Mason 13 1.6.1 The beginnings of sonochemistry in Coventry 16 1.6.2 The first international meeting on sonochemistry 21 1.6.3 Textbooks produced by the Coventry Sonochemistry Group 22 1.7 The development of sonochemistry 24 1.7.1 The European Society of Sonochemistry 24 1.7.2 Meetings and conferences of importance to the development of sonochemistry 28 1.7.3 International conference series involving sonochemistry 36 1.8 European Union research programmes involving sonochemistry 40 1.8.1 European research programmes 40 1.8.2 European research projects to support Eastern Europe and the Soviet Union (1997–2001) 46 1.8.3 European Framework Programmes (2005–2013) 47 1.9 Origins of the journal Ultrasonics Sonochemistry 48 1.10 The Sonochemistry Centre at Coventry University 49 1.10.1 Sonochemistry in Coventry University after closure of the Sonochemistry Centre in 2015 51 1.11 Sonochemistry some personal reflections from Mircea Vinatoru 52 1.11.1 EU COPERNICUS and COST D10 programmes involving ultrasonic extraction from renewable natural resources 55 1.11.2 The “ordering effects” mechanism in sonochemistry 58 1.11.3 Work in Japan 61 1.11.4 Work in Canada (2001 and 2005) 62 1.11.5 Work in Texas (2007–2009) 64 1.11.6 Work in the Coventry Sonochemistry Group (2009–2015) 65 1.11.7 Return to Romania and ULTRAMINT 67 1.12 Some comments on the links between sonochemistry and nuclear fusion 68

VIII

1.12.1 1.12.2 1.12.3 1.12.4 1.13

Contents

Energy derived from nuclear fusion Cold fusion 69 The film Chain Reaction 69 Nuclear fusion in a cavitation bubble Concluding remarks 71 References 72

68

70

Chapter 2 Fundamental aspects of sonochemistry 79 2.1 Introduction to acoustic waves 79 2.2 Acoustic waves and their propagation 79 2.3 Ultrasound parameters: velocity, wavelength and frequency 85 2.4 Sound attenuation 86 2.5 Ultrasound power measurements and dosimetry 87 2.6 The importance of power distribution and power density 89 2.7 Sonochemistry reaction conditions and scale-up 91 2.7.1 Scaling up; a note of caution about computer optimization 92 2.7.2 Continuous and loop reactors 93 2.8 Bubble collapse 96 2.8.1 Symmetric collapse of cavitation bubbles 98 2.8.2 Asymmetric collapse of cavitation bubbles 99 2.8.3 Some comments about sonoluminescence 100 2.9 Concluding remarks 101 References 101 Chapter 3 Ultrasonically assisted extraction (UAE) 105 3.1 Introduction to the extraction of natural medicinal compounds 105 3.2 A brief history of medicinal plants 109 3.2.1 Notes on the Romanian Pharmacopoeia 111 3.2.2 Some information about allelopathy 112 3.3 Classical extraction procedures for vegetal materials 113 3.3.1 Distillation 113 3.3.2 Solvent extraction 114 3.4 Non-conventional extraction procedures (excluding UAE) 116 3.4.1 Supercritical fluid extraction (SFE) 116 3.4.2 Microwave-assisted extraction (MAE) 117 3.4.3 Pulsed-electric field extraction (PEF) 117 3.4.4 Enzyme-assisted extraction (EAE) 118 3.4.5 Pressurized liquid extraction (PLE) 118 3.5 The origins of ultrasonically assisted extraction (UAE) 118

Contents

3.6 3.6.1 3.6.2 3.6.3 3.6.4 3.7 3.7.1 3.7.2 3.8 3.9 3 9.1 3.9.2 3.9.3 3.10

The development of UAE from the 1990s in Coventry and Bucharest 120 UAE research projects supported by the European Union 121 Programmes supported by the UK 138 UAE applied to the valorization (increasing the value) of edible oils 147 UAE and links with non-classical extraction procedures 147 Some important parameters to consider in UAE 152 Swelling index (SI) 152 Extractive value – EV 153 The way in which UAE works with associated mechanisms 155 Practical aspects of UAE 159 Some guidance for good practice in UAE 160 Laboratory equipment for UAE 161 Larger scale systems for UAE 164 Concluding remarks 168 References 169

Chapter 4 Environmental protection 177 4.1 Historical introduction 177 4.1.1 Biological effects of ultrasound 177 4.1.2 Chemical effects of ultrasound 181 4.2 Studies in Coventry 185 4.2.1 Water disinfection with chlorine 188 4.2.2 Cryptosporidium 189 4.2.3 Report from the Foundation for Water Research 190 4.3 Environmental protection – microbiology 191 4.3.1 INCO-COPERNICUS project 1999–2001 191 4.3.2 Flow systems for the treatment of suspensions of bacteria 194 4.3.3 Ultrasound treatment of bacteria in conjunction with other methods 198 4.3.4 Ultrasound for the control of algae 204 4.4 Environmental protection – chemical 217 4.4.1 Sonoelectrochemical methods 217 4.4.2 Removal of volatile and non-volatile pollutants 223 4.4.3 Removal of endocrine disruptor chemicals (EDCS) 225 4.4.4 Removal of dyes 226 4.4.5 Decontamination of soil 231 4.5 Large-scale low-frequency sound 236 4.5.1 Low-frequency sound plus ozone for dye destruction 238 4.5.2 Low-frequency sound for soil remediation 240

IX

X

4.5.3 4.6 4.6.1 4.6.2 4.6.3 4.7

Index

Contents

Hydration (slaking) of ash waste from fluidized bed combustors (FBC) 241 Environmental involvement of Mircea (in Japan and Romania) 243 Dioxin degradation 243 Chlorobenzene decomposition using Fenton-type aqueous systems 244 Removal of heavy metals with alginate beads 245 Concluding remarks 247 References 247 255

Chapter 1 Historical introduction 1.1 Introduction All chemists are aware of the general methods of enhancing chemical reactions such as the use of heat or the application of pressure. Some more specific types of reaction can be excited by light (photochemistry) or by the passage of an electric current (electrochemistry) or through the use of a catalyst. Over the last few decades, another method known as sonochemistry has been developed. Sonochemistry involves the use of sound (almost always it is ultrasound) as a source of energy to drive chemical reactions. The study of sound is not normally one that is familiar to chemists and so, before we embark upon a discussion of what is involved in the science of sonochemistry, we should perhaps first consider how it is possible for sound to influence chemical reactions. Photochemistry can be explained as the activation of molecules through excitation of electrons in them by light. However, sound energy cannot directly excite electrons because it is a very different form of energy. To illustrate this difference, consider the fact that both light and sound can be transmitted through air but light can also pass through a vacuum, whereas sound cannot. An everyday example is that we can see light from the Sun through the vacuum of outer space but we cannot hear any sound transmitted through outer space [1]. A classical physics experiment is to place a ringing bell inside a sealed glass jar and evacuate the air from the jar. As the pressure is reduced, the volume of sound audible outside the jar is reduced and it disappears completely when a vacuum is achieved. The difference between sound and light transmission is that the former requires a solid, liquid or gaseous medium (i.e. molecules) to transmit vibrational energy but light does not because it is part of the spectrum of electromagnetic radiation. Chemists are familiar with several types of electromagnetic radiation used in spectroscopic analysis (Table 1.1). The corresponding frequencies and wavelengths of audible sound in air are included here for comparison. An electromagnetic wave such as light passes through a medium in the form of a transverse wave which involves photons [2]. In contrast, sound passes through a medium by longitudinal waves and is transmitted through the vibrational movement of molecules. In other words, the transmission of sound requires the presence of molecules and therefore cannot travel through a vacuum.

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Chapter 1 Historical introduction

Table 1.1: Relationship between wavelength and frequency for different types of radiation in air. Name

Wavelength

Frequency (Hz)

Electromagnetic radiation X-ray Ultraviolet Visible Infrared Microwave Radio

. to  nm  to  nm  to  nm  nm to  mm  mm to  m  m to , km

 EHz to  PHz  PHz to  THz  THz to  THz  THz to  GHz  GHz to  MHz  MHz to  Hz

Vibrational radiation Audible sound Ultrasound

 m to . cm . cm to . mm

 Hz to  kHz  Hz to  MHz

1.2 Sound waves and some basic elements of acoustics The way in which sound is propagated via molecular motion was demonstrated many years ago with the observation of the “sensitive flame”. A simple flame burning symmetrically in a draft-free environment will react to a nearby sound source in a similar way that it would react to a slight movement of the air. This can be used to detect sound even though it is inaudible, that is to say beyond the range of human hearing (ultrasound). John Tyndall developed the concept of the sensitive flame as a standard laboratory tool that was used well into the early 1900s for the study of audible as well as inaudible acoustic waves. He used it in one of his demonstration lectures delivered at the Royal Institution in London [3, 4]. His book Sound, published in 1869, is a compilation of eight of his demonstration lectures and provides an excellent insight into the ways in which sound was understood in those days [5]. Tyndall is also the scientist who gave his name to the explanation for why the sky is blue (the Tyndall effect). An extremely good review of “A history of ultrasonics” was published in 1981 by Karl Graff, in which he describes the developments in understanding and applications of this field of acoustics up to the mid-1950s [6]. Ultrasound is defined as a sound which is at a frequency higher than that to which the human ear can respond. Thus, one of the first questions that we should consider is: if ultrasound is not audible, then how did the early scientists know that it existed? The upper limit of hearing was determined by the work of Francis Galton who was interested in establishing the threshold levels of human hearing [7]. His whistle was constructed from a brass tube with an internal diameter of about 2 mm and is operated by passing a jet of gas through an orifice into a resonating cavity. By moving a piston, the volume of the cavity can be changed to alter the “pitch” or frequency of the sound emitted (Figure 1.1).

1.2 Sound waves and some basic elements of acoustics

3

annular slit gas flow resonant cavity

piston

Figure 1.1: The Galton whistle.

Galton used his whistle to investigate the hearing of animals. The whistle was attached to the end of a long stick and activated from a rubber squeeze ball connected to the whistle by a tube. Galton would go the enclosures of different animals in the London Zoo and use the long tube to reach towards the ear of the animal and adjust the plunger to change the pitch. He then would watch for the animal’s ears to prick up as an indication of hearing the whistle. A present-day version of this can be found in some dog whistles which have adjustable pitch. In 1902, a modification of this device known as the Edelman Galton-Pfeife (pfeife is the German for whistle) was used to test human hearing and was described in an article by H. McNaughton Jones following his visit to a clinic in Munich [8]. The device was used to test problems with infections of the inner ear. Like Tyndall and many other scientists who lived at that time, Sir Francis Galton was a well-to-do Victorian scientist with a wide range of interests in science and discovery. He was a half-cousin of Charles Darwin and was influenced by the publication in 1859 of Darwin’s celebrated treatise On the Origin of Species. This led eventually to his development of the concept of eugenics: the “science” of improving the human race through selective breeding [9, 10]. When a sound is inaudible to a listener, its presence can be detected using the “sensitive flame” but how do you measure the frequency of that sound? This can be accomplished using the disturbances produced in lycopodium powder (a very light and fine powder composed of moss or fern spores) distributed along a horizontal glass tube sealed at one end. When sound is transmitted through the air in the tube from the open end the powder is disturbed at regular intervals and the nodes of the sound wave become visible. The distance between the nodes gives the wavelength of the sound wave from which the frequency can be calculated. The device was described by a German scientist August Kundt in 1866 [11] and followed 2 years later with an English description of its experimental use [12]. The device is now known as the Kundt’s tube, and this method of sound frequency measurement was referred to in the paper by McNaughton Jones [8]. The work of Galton and others led to an estimate of the upper limit of human hearing as 20 kHz which is still generally accepted. However, from experience it is clear that younger researchers in laboratories engaged in sonochemistry can definitely hear 20 kHz sound. The reason is that the original upper limit was determined by scientists of mature years and it is now recognized that the ability to hear higher pitched sounds diminishes with age. Indeed, this is reflected in the frequencies used

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Chapter 1 Historical introduction

for ultrasonic cleaning which are generally around 40 kHz and thus well beyond the audible range for workers of any age. The difference in the frequency limit of hearing between teenagers and older people has been used as a method of deterring younger people from congregating in certain areas of towns and cities. A speaker emitting frequencies around 20 kHz causes irritation to teenagers and they tend to move away, whereas older residents are unaffected. This same effect of using irritation caused by sound has been used in small loudspeaker devices to repel mosquitoes and rodents. At the lower end of the frequency range, the physical effects of sound vibrations transmitted through air can be experienced by standing near to a loudspeaker-emitting bass notes, for example at a music festival. The actual sound vibrations are transmitted through the air and are not only audible but can also be sensed by the body through the skin. Lower frequency sound of this type can be weaponized and used to make anyone exposed to it nauseous and thus unfit for duty. At frequencies below 20 Hz the sound becomes inaudible, and this lower range is known as infrasound [13]. A sound wave travels through a medium through the sequential transfer of vibrational motion between adjacent molecules. The oscillation of the molecules is around their equilibrium position because the molecules themselves do not change in overall position regardless of the media through which the sound is transmitted. The molecules in solids are packed very tightly but in liquids they are not packed as tightly and in gases the molecules are very loosely packed. This is the reason why sound is transmitted more quickly through solids than liquids and sound speeds are slowest in gases. The acoustic range of sound from very low frequency infrasound through audible and on to ultrasound is shown (Figure 1.2). Within the overall acoustic range, there are some typical examples of the frequencies of naturally occurring audible sound from thunder to a mosquito. The range of human hearing is somewhat restricted compared to that of the animal kingdom, but all Infrasound 0

Sound 102

10

Thunder 100 Hz

Ultrasound 103

104

Bumble bee Middle C Mosquito 270 Hz 261.6 Hz 1.5 kHz

Human hearing 16 Hz – 20 kHz

Power ultrasound 20 kHz – 100 kHz

Figure 1.2: Spectrum of sound.

105

106

107

Dog hearing up to 50 kHz

Dolphin and Bat ranging 30 – 100 kHz

Extended range for sonochemistry 100 kHz – 1 MHz

High frequency, mainly for medical use 1 MHz – 10 MHz

1.3 A note on the physiology of hearing

5

animals will respond to sounds within the human range. In the higher ranges lie the echolocation frequencies of dolphins and bats [14]. These are usually in the form of clicking. Higher pitched sounds have shorter wavelengths, and this means that the echo distance can be more accurately located. It was echo location that became the first commercial use of ultrasonics when, in 1917, Paul Langevin designed a submarine detector using piezoelectric quartz crystal transceivers, that is transducers that could both send and receive ultrasonic signals. The device was based on the idea of echo sounding, which simply sent a pulse of ultrasound from the keel of a boat to the bottom of the sea from which it was reflected back the distance to the bottom could be gauged from the time taken for the signal to return to the boat. If some foreign object (e.g. a submarine) were to come between the boat and the bottom of the seabed, an echo would be produced from this in advance of the bottom echo. This system was very important to the Allied Submarine Detection Investigation Committee during the second war and became popularly known by the acronym ASDIC. Later developments resulted in the system known as SONAR (SOund Navigation And Ranging), which allowed the surrounding sea to be scanned rather than just the water below the ship [15]. Such devices are now commonly available in the form of fish-finders which can locate shoals of fish in the ocean or individual fish for the angler. Essentially all imaging from medical ultrasound to non-destructive testing relies upon the same pulse echo type of approach but with considerably refined electronic hardware. These enable the equipment not only to detect reflections of the sound wave from the hard, metallic surface such as a submarine in water but also much more subtle changes in the media through which sound passes (e.g. those between different tissue structures in the body). It is high-frequency ultrasound (in the range 2–10 MHz) which is used primarily in this type of application because by using these much shorter wavelengths it is possible to detect much smaller volumes of phase change, thus giving better “definition”.

1.3 A note on the physiology of hearing In the outline given above of the early experiments of Galton and his contemporaries, the question of why there is an upper limit to human hearing was not addressed, and nor was the question of why other mammalian species should have different upper frequency limits. We are grateful to Matthew Mason of the Department of Physiology, Development and Neuroscience at Cambridge University for information on this and also some informal discussions [16]. In any description of sound transmission, the term “impedance mismatch” will appear. This concerns the problem of sound passing from one medium into another, for example the transfer of sound energy from the air into water. If the acoustic impedances of the two substances are different, as is the case for air and water,

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some of the sound energy will reflect back at the interface and transmission is thereby reduced. A simple example is when you are listening to the radio while taking a bath: if you dip your head below the surface the sound becomes somewhat muffled and indistinct. This is because much of the sound energy is reflected at the water surface and only a fraction penetrates the water. This is the reason why a special coupling fluid is used between the transducer and skin when ultrasound is specifically intended to be introduced into the body, for example for physiotherapy or scanning. A question then arises about how sound vibrations in air can be converted into a hearing response within the body: what has happened to the impedance mismatch between air and tissue? Sound vibrations through the air are first collected by the external ear and channelled to the tympanic membrane or eardrum. Because the eardrum is very light its impedance is close to that of air, so less sound energy is reflected. The vibrations induced in the eardrum are passed across the middle ear cavity and into the inner ear by three small bones – the auditory ossicles which are the smallest bones in the body (malleus, incus and stapes). The cochlea is the part of the inner ear responsible for converting sound vibrations, which are now travelling in the inner ear fluids, into electrical impulses that are transmitted to the brain. The cochlea is named from the Greek for snail as it is in the form of a spiral. Within the cochlea is a long membrane called the basilar membrane, and different parts of which vibrate best at different frequencies. Sitting on the basilar membrane are thousands of special cells called hair cells, each of which has a tuft of hair-like structures projecting from the top. When the basilar membrane vibrates, the hairs on the hair cells are also vibrated. This opens up ion channels, which leads to electrical changes to the cells. The hair cells, therefore, act as organic transducers, converting vibrational energy into electrical signals, just as piezoelectric materials are inorganic transducers which perform a similar task. As the frequency of the sound becomes higher, the tympanic membrane and auditory ossicles become less able to adequately transmit the vibrations to the cochlea, the basilar membrane ceases responding and, in the limit, no electrical sound impulses are transmitted to the brain. As a person gets older, degeneration of cells within the cochlea often results in a progressive reduction in response to higher frequencies. The high-frequency limit of hearing is different in different mammals, and this is related to factors including the size and vibrational properties of the tympanic membrane, auditory ossicles and basilar membrane within the cochlea. The hair cells in the cochlea can also be excited through a different mechanism which does not involve the tympanic membrane. When Beethoven began to lose his hearing, he is thought to have heard music by gripping a wooden rod between his teeth and placing the other end of it on his piano. Vibrations through the wood caused his jaw to vibrate, and this was conducted through his skull to the cochlea. A similar effect is produced if a tuning fork is pressed against the skull. Here again the vibrations pass through the bone and can excite the hair cells in the cochlea.

1.4 Cavitation – some background information

7

This mechanism is known as bone conduction, and it can be used to diagnose where a hearing defect may be (if the problem lies in the cochlea, bone conduction fails to work too, but if only the middle ear is compromised, bone-conducted hearing remains). It is also the basis of hearing aids which are used for people who cannot hear by the normal pathway of tympanic membrane and auditory ossicles. A cautionary paper was published by Angeluscheff in 1956 suggesting that there may be some danger to hearing if scientists were exposed to powerful ultrasound during sonochemistry experiments [17]. Entitled “Sonochemistry and the organ of hearing”, it suggested that ultrasonic waves might produce intense agitation, heating and possibly chemical reactions in the liquid contained in the inner ear but there is no real evidence for these claims. The transmission of sound from air into the inner ear is attenuated by an impedance mismatch, which increases as the frequency is increased beyond the normal range. At very low intensities, acoustic cavitation cannot occur in a liquid and so sonochemistry will not take place in the inner ear.

1.4 Cavitation – some background information In the general classification of ultrasound given within the sound spectrum shown in Figure 1.2, ultrasound is defined as sound with a frequency beyond that to which the human ear can respond. The normal range of hearing is between 16 Hz and about 20 kHz and ultrasonic frequencies lie between 20 kHz and beyond 100 MHz. For many years it has been accepted that sonochemistry is driven by acoustic cavitation and most studies from the 1980s used frequencies between 20 and 40 kHz because this is the range employed in common laboratory equipment. However, since acoustic cavitation can be generated well above these frequencies, a much wider range can now be considered. To produce cavitation, a period of time is required during the rarefaction phase of a sound wave to pull molecules apart. At 20 kHz, for example, the rarefaction cycle lasts 25 μs, whereas at 20 MHz the rarefaction cycle lasts only 0.025 μs. As the frequency increases, the production of cavitation bubbles becomes more difficult to achieve in the available time and so it could be assumed that the application of greater sound intensities (i.e. greater amplitudes) over these shorter periods would ensure that the cohesive forces of the liquid are overcome. This however is not correct and a limit is reached at around 5 MHz, where cavitation can no longer be achieved even at high intensities, and frequencies beyond this are used only in diagnostic studies [18]. Essentially ultrasound can be divided into the two ranges of power and diagnostic but the division between the two is not clear-cut because frequencies between 1 and 2 MHz can be used for both purposes. In recent years, there has been some discussion about the possibilities of obtaining sonochemical effects at subcavitation levels through an “ordering effect” (see 1.11.2) [19]. Nevertheless, discussions about sonochemistry

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Chapter 1 Historical introduction

generally focus on liquid systems into which sufficient acoustic energy is delivered to induce cavitation and that it is cavitation which drives both sonochemistry and ultrasonic processing. Lord Rayleigh published a theoretical investigation of the behaviour of an incompressible fluid within which he imagined the formation of a void. His work suggested that cavitation generated enormous turbulence, heat and pressure [20]. Perhaps before moving on to a more detailed discussion of cavitation in liquids it is worth mentioning another important application of ultrasound – welding – which uses lower frequency power ultrasound but does not involve cavitation and so is not considered to be part of sonochemistry. The equipment employed in plastic welding is similar to the probe system used in sonochemistry, and so, just as ultrasonic cleaning provided the baths used in early work on sonochemistry, welding gave rise to developments of the probe system. Plastic welding generally operates at around 20 kHz and uses a shaped die which compresses two pieces of material together against a rigid surface [21]. The mechanical vibration of the die contact face (the vibrational amplitude employed is typically 50–100 µm) is passed through the upper component which vibrates against the lower part. Heat is developed at the joint, and the rise in temperature causes localized melting of the material in a very short time (only a matter of seconds). The two parts are continuously held together under pneumatic pressure during this time and then for a short time after ultrasound is turned off. When the vibrations are stopped, the bulk material becomes a heat sink, giving rapid cooling which will set the molten plastic at the interface to produce the welded joint. Thermoplastics are ideal for ultrasonic welding because they have a low thermal conductivity and have melting or softening temperatures between 100 and 200 °C. An advantage of the use of ultrasound is the high joint strength of the weld, reaching 90–98% of the material strength. Indeed, test samples usually break in the body of the material and not at the weld itself. Ultrasonic welding is not only restricted to plastics but can also be used for metals [22]. Cavitation is the production of microbubbles, generally containing some vapour and/or gas in a liquid when subjected to a large negative pressure. It was characterized at the end of the nineteenth century in connection with problems associated with the underperformance of a torpedo boat destroyer HMS Daring. A torpedo boat destroyer, as the name suggests, was a ship designed to defend shipping against enemy torpedo boats. To do this, the destroyer had to be very fast and manoeuvrable. HMS Daring was built at the Thornycroft shipyard at Chiswick, UK, in 1893 and equipped with guns plus three torpedo tubes and was bigger than a torpedo boat. In 1894, she reached a maximum speed of 24 instead of the projected 27 knots. Sydney Barnaby suggested, together with John Thornycroft (the owner of the shipyard that built the Daring), that this underperformance was due to a limitation in the efficiency with which water could follow the rapidly moving propeller blade [23]. This resulted in a loss of thrust due to loss of contact between the blade and the water because of the formation of vapour-filled voids caused by surface (hydrodynamic) cavitation. With

1.4 Cavitation – some background information

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suitable corrections to the shape of the propeller blades, HMS Daring eventually achieved a speed of 28.21 knots over the measured mile and as a result was referred to at that time as the “Fastest Boat Ever”. The cavitation caused by propeller motion was of particular interest at the beginning of the Second World War. The Allies were developing SONAR (see above) which was an “active” detection system, that is to say a short burst of sound (a ping) was sent out from the detecting ship and the operator waited to pick up any echo from a submarine. From the time between emission and reception of the echo, a distance for the target could be estimated. The ping was also detectable by the target which then knew it was under surveillance. Meanwhile, the German forces had developed a “passive” sonar system, which simply listened for any underwater noises generated by submarines. That this noise was the result of cavitation produced by the propellers was proven by the fact that the cavitation noise was easily detectable when the submarine was running at periscope depth (some 50 feet under the surface) but became less and less as the ship dived. This can be explained by the increase in water pressure with depth which makes cavitation more difficult to produce. Shortly after the war, Knapp began investigating the mechanics of cavitation and cavitation damage induced by a pressure drop over a blade. He used high-speed photography and found cavitation bubbles were strongly deformed by the flow [24]. Many of the early investigations into cavitation were linked with the type of cavitation produced by propellers driving through water (hydrodynamic cavitation). Hydrodynamic cavitation can also be generated simply by the passage of the liquid through a constriction such as an orifice, valve or Venturi. When the liquid emerges from the constriction, there is a sudden pressure reduction and if that falls below the threshold pressure for cavitation, then cavities are generated. Further downstream the liquid jet expands, the pressure recovers and the cavities collapse. These are sometimes referred to as “liquid whistles” and when used in processing are very efficient for emulsification [25]. The technology for producing such systems has developed dramatically with the original single Venturi-type jets replaced with liquid pumped through plates containing multiple holes of various geometries [26]. Although hydrodynamic cavitation continues to be of interest in the processing field, it is cavitation generated through sound waves, acoustic cavitation, that has become predominant in cavitation-driven chemical and processing applications due to its wide versatility and ease of implementation. Studies of acoustic cavitation began many years ago in the late 1920s. A pioneer in this field was Alfred Loomis, who started studies with large quartz plate transducer systems which he experimented with in his garage in New York. Later, as his studies expanded, he moved to a large house which was converted into what became known as the Tuxedo Park Laboratory. Together with Robert Wood, who had worked earlier with Paul Langevin on underwater detection systems, they collaborated on some groundbreaking experiments. In 1927, they published a paper about the physical and biological effects of

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high-frequency sound waves of great intensity [27]. The authors were clearly aware of the importance of this field of study as evidenced by the first sentence of the paper: In the present paper we shall give an account of a preliminary survey of what appears to be a wide field for investigation, opened up by the study of the very surprising and remarkable effects obtained with sound-waves of high frequency and great intensity generated in and oil-bath by a piezo-electric oscillator of quartz operated at 50,000 volts and vibrating 300,000 times per second.

Loomis also collaborated with E. Newton Harvey, a professor from Princeton University, to extend this work and directly observe through a microscope the effects of much lower power ultrasound on living cells [28]. Loomis also reported some preliminary results on the chemical effects of high-frequency sound waves with William T. Richards from the Chemical Department at Princeton [29]. In the early days of the investigation of the effects of sound, another phenomenon was identified – sonoluminescence. It is generally agreed that the studies of two teams (Marinesco and Trillat in 1933 and also Frenzel and Shultes in 1934) provided the first evidence that light was emitted from ultrasonic irradiation of a liquid sample through the darkening of photographic plates in the complete absence of ambient light [30, 31]. By 1939, the effects of ultrasound were becoming more familiar and the subject of many reports. These were reviewed by Richards (who was now based at the Rockefeller Institute for Medical Research, New York) in an excellent paper entitled “Supersonic phenomena” that contained 348 references [32]. From those early days, the study of the use of ultrasound in chemistry and related fields, later to become known as sonochemistry, was developed.

1.5 Sonochemistry – the first uses of the term There is a general feeling that sonochemistry (as a recognized and discreet topic) had its origins in the 1980s. Certainly, it was around this time that small off-the-shelf cleaning baths and also ultrasonic horn systems aimed at cell destruction became more available and familiar in chemistry laboratories. It is important to recognize the historical significance of the development of ultrasonic cleaning bath technology on the growth of sonochemistry because the use of ultrasonic cleaners was probably the first method used when chemical laboratories attempted research into sonochemistry [33]. It is undeniable that the 1980s saw the real entrance of sonochemistry into mainstream chemistry in terms of the number of papers which were produced around that time. However, we have seen above that there were papers from the early and middle twentieth century which reported the use of ultrasound to cause chemical and physical changes [34, 35]. But what about the specific term “sonochemistry” when was this first used in a scientific paper? Karl Graff in his excellent review article “A history of ultrasonics”

1.5 Sonochemistry – the first uses of the term

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[6] considered that Weissler was the first scientist to use the term in the title of his paper “Sonochemistry: the production of chemical changes with sound waves” published in 1953 [36]. In fact, there is a reference to sonochemistry in a paper published before this in 1951 by Weyl entitled “Surface structure of water and some of its physical and chemical manifestations” [37]. Section VII of this paper is entitled “Chemistry of the Nascent Surface of Water (Sono-Chemistry)” and he uses the term sonochemistry, again but this time not hyphenated, in the last paragraph of this same paper. In those last few sentences, it is clear that he is referring to what we now recognize as sonochemistry: The cavitation of water in an ultrasonic field produces nascent surfaces which must contain protons. These nascent surfaces tear apart those oxygen and nitrogen molecules which dissolved in the water in order to produce anions which can screen the potential fields of the exposed protons. Protons are present only in the nascent surface of water, but not in equilibrium surfaces. This concept is used for explaining the sonoluminescence of water and the oxidation reactions which accompany its cavitation in ultrasonic fields (sonochemistry).

To the best of our knowledge, these papers by Weyl and Weissler are the first publications in chemistry which use a new term “sonochemistry” to describe the effects of sound on chemistry. Weissler was a true pioneer in the study of uses of ultrasound in chemistry with many papers produced in the 1940s through to the 1960s. At the beginning of his review of ultrasound in chemistry in the Journal of Chemical Education [38], he takes care to differentiate between: a) the use of velocity measurements of low-power ultrasound to investigate the molecular properties of liquids and b) the use of high-power ultrasound to cause or accelerate chemical reactions. This difference between the terms power and diagnostic ultrasound is a key to the understanding of sonochemistry which owes its effects to cavitation generated using power ultrasound and not to the measurement of acoustic parameters. Notable also in this paper is a picture of the large ultrasonic generator used in his laboratory at the Naval Research Laboratory, Washington, DC. The name of Weissler is closely associated with sonochemistry for another reason. In one part of his work, he studied the oxidation of potassium iodide to iodine in water induced by sonication which is now known as the Weissler reaction [39]. This is used as a chemical dosimeter for assessing the efficiency of a sonochemical reactor by monitoring the quantity of iodine liberated. There were some earlier reports of the oxidation of iodide to iodine using ultrasound from Liu and Wu who were working in Peiping Union Medical Centre (Peiping later became Peking and then Beijing) [40]. Based on this and some earlier work on the oxidative destruction of a number of indicators [41], they suggested that ultrasonic oxidations arose from cavitation in water and did not require the presence of dissolved oxygen as had

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been suggested by Schmitt et al. [42]. However, it is the work of Weissler, which is most widely known. In Weissler’s paper on the oxidation of iodide, he made some important observations [39]: – The amount of iodine liberated by ultrasonic waves from potassium iodidecarbon tetrachloride aqueous solutions depends on the dimensions and material of the reaction vessel. – This observation was neglected by sonochemists for many years. However it was re-emphasized in 1962 when it was reported that the amount of cavitation produced in an ultrasonic generator depends on factors such as tank geometry, water height, and continuous operation time [43]. Some typical results were presented in that paper. These may help to explain the quantitative variability found in, for example, industrial cleaning and biochemical applications. – Although dissolved oxygen has formerly been considered essential, nitrogen or helium are found to serve almost as well. – This is a confirmation that the source of HO radicals is not the oxygen present in the reaction mixture. Weissler had previously determined that it was HO radicals that were the precursor of hydrogen peroxide formation during sonication [44]. – As the power input is increased, no iodine is produced until cavitation occurs. Then the yield increases almost linearly if the volume of solution is large enough, but for smaller volumes the yield first increases and then decreases sharply. – This is also the first observation that the well-known cavitation threshold must be exceeded in order to produce the chemical effect (oxidation in this case). The rise followed by a fall in ultrasonic effects is also significant and has since been ascribed to a “cushion effect” whereby at sufficiently high powers an excess of cavitation bubbles are formed near the transducer which reduces the efficiency of acoustic energy transmission into the bulk liquid [45]. – The main reaction is that between water and dissolved carbon tetrachloride, the potassium iodide being primarily an indicator of the oxidizing chlorine set free. – Rate studies indicate that the reaction takes place in two steps, each of which liberates two chlorine atoms per carbon tetrachloride molecule. The first step is approximately ten times faster than the second. – In the absence of carbon tetrachloride, the oxidation of potassium iodide solution proceeds only about one-fifteenth as rapidly as in the presence of carbon tetrachloride. Despite the groundbreaking publications of Weissler, the real acceptance of sonochemistry did not materialize until the 1980s. From 1953, when Weissler used the word “sonochemistry”, to 1980, an electronic search (using Scopus) for the term “sonochemistry” returned only eight hits. This reflects the limited number of studies

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performed in the field which can probably be attributed to the lack of ultrasonic resources for laboratory experiments. Weissler used equipment developed from naval research into SONAR, these types of generators were not commonly available. In those days and throughout the early history of investigations on ultrasound effects in chemistry, each scientist would need to improvise a setup for experimental work. Hence, ultrasonic devices were constructed on an individual basis normally using quartz as the piezoelectric material or magnetostrictive transducers to produce ultrasonic vibrations. Ultrasonic apparatus only became more widely commercially available in the 1980s. During this time, ultrasonic cleaning baths and horn systems driven by magnetostrictive or piezoceramic transducers began to appear in laboratories. Horn systems had been found to be useful in microbiology laboratories for cell disruption as an alternative to vortex stirring and were developed from plastic welding equipment. For most would-be sonochemists, however, it was the availability of the ultrasonic cleaning bath that played the major role. As a result, papers containing the term “sonochemistry” between 1980 and 1990 increased to around 100, indicating a growing interest in this new branch of chemistry.

1.6 Sonochemistry: some personal reflections from Tim Mason My own entry into the field of sonochemistry was one of those serendipitous events which occur from time to time in science. The paths which led me there can be traced to two phases of my early career. The first was my research background in physical organic chemistry and the second was my move to an academic post teaching organic chemistry at Lanchester Polytechnic at exactly the same time as Phil Lorimer who was a physical chemist. Lanchester Polytechnic, many years later, became Coventry University. My background had been in physical organic chemistry with specific interest in factors influencing the rates of reaction in solution. This had been my first research interest dating from my PhD studies at Southampton University in the UK. The title of my thesis was “The relationship between structure and reactivity within rigid systems” which I had studied under the guidance of Professor Ray Baker. Ray had been at UCLA with Saul Winstein and had only recently arrived at Southampton (1964, the same year that I had started my undergraduate course in chemistry there); in fact, I was only his second PhD student. Later in his career he went on to be executive director of medicinal chemistry at Merck Sharp & Dohme and Chief Executive of the Biotechnology and Biological Sciences Research Council. He was awarded a CBE (Commander of the Order of the British Empire, a medal awarded for the national importance of the work of an individual) and elected as a Fellow of the Royal Society of London (FRS). My first paper with Ray Baker was published in 1969 and concerned the solvolyses of p-toluenesulphonates in rigid polycyclic systems [46]. I published eight

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papers from my PhD studies all of which concerned the way in which chemical structure and choice of solvent affected the rate of a reaction. I got married in 1969, the last year of my PhD at Southampton, to Christine Bull, who had been in the same year as me at Southampton University, also studying chemistry. I was heavily engaged in writing up my thesis at the time. This is so much easier nowadays with the advent of word processors and internet searches for references but in those days involved hours in the library reading journals and Chemical Abstracts. For me it was also a case of using stencils to draw diagrams, handwriting the text of the thesis and taking it weekly to a specialist local typist who would give me top and carbon copies to proofread. After corrections were made with whiting-out fluid and retyping a final version could be photocopied ready for compiling into the bound thesis. It was a lengthy process. In those days, it was quite a normal career progression for researchers in the UK to look for a post-doctoral position in the USA; fortunately, my wife was also keen on this idea. So, in the last few months of my studies at Southampton in 1970, I applied for a NATO Fellowship which required a written application followed by an interview in London. I was successful and it enabled me to spend 2 years as a research fellow in the USA. As with others like me, this meant that I was not able to attend the formal award of my doctorate as I had already travelled to the USA and so the PhD degree was awarded “in absentia”. I had decided to study with G. Dann Sargent at Amherst College in Western Massachusetts. I knew of him from his work in physical organic chemistry and in particular from his review article “Bridged, non-classical carbonium ions” which I had referenced a lot in my thesis [47]. Amherst itself was a delightful town with several universities close by including the University of Massachusetts, Mount Holyoke, Smith and Amherst itself. Amherst College is a liberal arts college set in beautiful countryside and a great place to adapt to living in the different culture of the USA compared with the UK. In the 2 years I spent there, I generated two papers in JACS, the first of which was “The influence of bond angle distortion and sigma-pi sigma delocalisation on the stability and chemistry of allylic cations” [48]. I remember well that part of this work involved going across the leafy and green Amherst campus to the computing building to use Fortran programs to analyse kinetic data. I spent many hours punching out computer cards that were then fed into a card reader and the next day returning to get the results on long continuous perforated sheets of printed output. Despite the amount of work required to pursue my research, my wife and I found time to travel and managed two long journeys in our beloved used car – a Dodge Dart. One took us down to Mexico City crossing the border through Brownsville and returning to the USA via Nogales, and our other adventure was to travel the whole way across the USA to San Francisco, up the coast to Portland where some of my mother’s family had moved after the second world war and then back to Amherst through Canada. At the end of my 2 years in the USA, I was awarded a British Research Council Resettlement Fellowship which, as the name implies, was aimed at bringing British

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scientists back to the UK. Christine and I decided to come home in style and travelled back by sea on the SS France an ocean liner billed as “le plus long paquebot du monde”. I used the fellowship to study at York University where I worked with Professor R.O.C. Norman who I knew of from his book Principles of Organic Synthesis. He went on to become president of the Royal Society of Chemistry (RSC), was elected an FRS and was knighted in 1987. I was there for nearly a year and started a new line of research on phenyl group participation. I published one paper with him entitled “Kinetics and mechanism of addition and cyclialkylation reactions of ωarylakenes with trifluoroacetic acid” [49]. Permanent academic posts were very hard to find in those days, so I moved to a temporary lectureship in organic chemistry which had been advertised at nearby Bradford University. This was when I began to realize that there was a difference between lecturing and teaching. It was brought home to me when I was assigned a course of basic organic chemistry for chemical engineering students. They were not chemists and had little interest in the “nuts and bolts” of organic chemistry being much more enthused with physical chemistry and physics. Teaching was required because a recitation off the facts did not work for them. Despite a full timetable, I found time to continue working on the research project I had started at York. Professor Norman was happy to let me write up and publish this stage of the work entirely on my own because he considered that he had no involvement in the Bradford studies. As a result of this, I reached what I considered to be an important milestone in the research career of any scientist when in 1975 I published my first full research paper as a single author. It concerned phenyl group participation in the cyclialkylation reactions of ω-arylalkenes in trifluoroacetic acid [50]. My temporary lectureship at Bradford was for 2 years, so when it was coming to an end, I was looking around for any possibilities of a full-time job. It had been the case for some years that university teaching posts were in very short supply. However, in late 1974 a lectureship in organic chemistry was advertised at Lanchester Polytechnic in Coventry and so I applied for it. In those days in the UK, polytechnics differed from universities in that the courses were more applied rather than theoretical, which made them more relevant for industry. During their undergraduate studies, all students would spend their third year out of college working in industry and during that period they were monitored by a member of staff. This was in my opinion an excellent training period and students would return to their final year of studies after this with an awareness of what chemistry in the world outside of academia was all about. The interviews for the post were on 21 November 1974, during which I was told that there was a very heavy teaching load and that I should not expect to do any research because Polytechnics were known for teaching and not for research. I was offered the post and accepted it on the spot but something else happened that day to make it even more memorable. I took a train back to Bradford in the evening which stopped as per schedule at Birmingham New Street Station in the centre of

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the city. When I got home, my wife had been extremely worried about me because she had heard some major news about bombings in Birmingham. This was in the days before mobile phones and so I was out of contact throughout the trip home and was unaware of the explosion of bombs in two public houses in the city centre of Birmingham just after 8 pm. This was only a few minutes after my train had left New Street Station. The bombings killed 21 people and injured a further 182 others. They were thought to have been part of the Provisional Irish Republican Army’s campaign for the unification of Ireland. I had been lucky to avoid these horrific events and maybe this good fortune was also an indication that taking the post at Coventry would be a good career move.

1.6.1 The beginnings of sonochemistry in Coventry The department that I joined at Lanchester Polytechnic in 1975 was chemistry and metallurgy, and it was here that my interest in what was to become generally known as sonochemistry began. The city of Coventry in those days was famous for engineering, and there were numerous factories within and around it. The name “Lanchester” was derived from George Lanchester who was claimed to be the manufacturer of the first British motor car in 1895. Therefore, with such strong engineering links, metallurgy seemed to be an appropriate subject for the college.The engineering industry was a big employer and the individual factories had sports and social clubs attached. The reason I mention this is that several of them had their own cricket grounds and in my younger days I played for the Polytechnic cricket club and we had fixtures against many of these engineering companies. It was my friend and colleague Phil Lorimer who had encouraged me to join the cricket club. Phil had joined Lanchester Polytechnic as a physical chemist at exactly the same time as me in January 1975. Today, sadly, those factories have mostly disappeared together with their sports facilities and have been replaced by housing or by shopping centres. I had never been involved in metallurgy before arriving in Coventry and the office I was allocated was shared by two metallurgists. It was through general conversation that I started to learn about the subject. There were some laboratories used in common between chemistry and metallurgy, and in one of these labs, metallurgy samples for testing were routinely cleaned by immersion in an ultrasonic bath. Ultrasonic cleaning was new to me and I was intrigued by the way the water in the bath fizzed and moved under the stimulus of ultrasound. It was clear that some kind of energy was being generated, and so with my background in physical organic chemistry, I wondered if ultrasonics could provide (what I thought of at the time) a new form of energy that might be able to enhance chemical reactions. After a few exploratory experiments, with the help of a technician in the physical chemistry laboratory some results were obtained that looked promising.

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I discussed the idea of using ultrasound to accelerate reactions with my chemistry colleague, John Philip (Phil) Lorimer. His background was also in kinetic studies but he was an electrochemist having worked with David Feakins in Dublin on Washburn numbers. These concerned the solvation and transport of ions [51]. Phil was also intrigued with the idea, so we began a study of the influence of ultrasound on the solvolysis reactions of 2-chloro-2-methylpropane (t-butyl chloride) in aqueous ethanol by dipping our reaction vessels in an ultrasonic bath. We had both been told at interview that we were not likely to find time to do research due to our heavy teaching loads. The reactions we were interested in sometimes took several hours to complete and individually we never had enough free time between classes to follow lengthy reactions. However, we came up with a way in which we could monitor them using a sort of “relay” system so that whichever of us was not teaching in a particular slot could be in the lab and we would swap over as our timetables allowed. We thought that the results obtained were exciting, and we truly believed we were opening up a new area of research. One of the big problems that we both had at that time was a lack of knowledge about ultrasound. We were very fortunate, however, to make contact with an experienced ultrasonic engineer John Perkins who had worked for many years in the 1950s for a company called Mullard in the general field of power ultrasound. Later he had formed his own company “Sonic Systems” working from his home in Marlborough Witshire. I remember well the visits that Phil and I made to him at his house, which was called “the Old Bakery” in Silverless Street. He had set up an acoustics lab in the basement and we would sit for hours chatting about ultrasound. He was involved with the series of conferences “Ultrasonics International” (UI) and was also on the editorial board of the journal Ultrasonics, both of these were connected through the publisher Butterworths. John was a great help to us, and I invited him as a speaker at the first sonochemistry conference in 1986 (see Figure 1.5). He did not want to write a paper based on his lecture to be included in a special issue of Ultrasonics but I did persuade him to contribute a chapter to a book I was putting together for the RSC in 1990 in which he included much of the valuable information on power ultrasound presented in 1986 [52]. Our association with John continued for many years. He passed away in 2010, although the company which he had founded continued to operate at new premises near the town of Ilminster in North Somerset. By 1979 Phil and I had accumulated enough data from the solvolysis studies to prepare a paper for Chemical Communications and it came as a great disappointment to us that the referees did not like it and it was rejected. We had difficulty in persuading them that this was a real effect induced by ultrasound and not simply the result of heating. However, we persevered and eventually achieved our first publication in sonochemistry in 1980 entitled “Effect of ultrasonic irradiation on the solvolysis of 2-chloro-2-methylpropane in aqueous ethanol mixtures” [53]. Publication meant recognition of our research, and things became a little easier but in

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those early days there was still quite a lot of scepticism in the UK about the effects of ultrasound on the rates of chemical reactions. In 1982, Phil and I presented our work at the second EUCHEM Conference on Correlation Analysis in Organic Chemistry at the University of Hull and the Third International Conference on the Mechanisms of Reactions in Solution at the University of Canterbury in Kent. In that same year, we took on our first research student Bharat Mistry and he completed his PhD in 1985 with the thesis “The effects of ultrasound on the solvolysis of 2-chloro-2methyl-propane in aqueous alcohol mixtures”. This greatly helped our research effort, particularly in terms of the experimental work. With Bharat we published four further papers. In 1985, Phil Lorimer and I went to our first Ultrasonics International meeting (UI85) which was held that year at Kings College London. There we presented a poster “The effect of ultrasound on a homogeneous chemical reaction” [54]. The UI conferences had started in 1973 at Imperial College, London, and were to become significant in the development of sonochemistry (see later). At this particular conference, however, we were the only participants presenting work on the chemical effects of ultrasound. In 1984, I wrote a review entitled “Sonochemistry” for a magazine called Laboratory Practice [55]. I introduced the article with the statement that “Ultrasonics is emerging from the world of cleaning baths and disintegration cabinets into the realm of the chemist. This article looks at sonochemistry, the use of ultrasonics in enhancing and modifying chemical reactions.” In it I referred to some other names involved in sonochemistry at that time – Phil Boudjouk, Christian Petrier, Jean-Louis Luche, Ken Suslick and Takashi Ando all with publications in the subject dating from 1980. It was clear that there were researchers in other parts of the world who had also developed an interest in the influence of ultrasound on chemical reactions although at that time Phil and I seemed to be the only ones in the UK. As Phil Lorimer and I became more involved in sonochemistry, we also started to receive invitations to give presentations at conferences, universities and industrial research groups. Although these did not result in monetary reward, our attendances at such meetings (many of which are described elsewhere in this book) were very enjoyable and we often travelled to them with younger members of our research group. In the 1980s, I had begun to develop a series of sonochemistry experiments which could be set up using a few simple chemicals and a small ultrasonic cleaning bath and I used these demonstrations in talks I gave to schools and university students. In the beginning, I had to take one of our own small ultrasonic baths to the presentations because they were not commonly available around the country. There were many simple demonstrations of sonochemistry that I used, and eight of these are described in chapter 2 of the Oxford Primer book on Sonochemistry [56]. As time went by, Phil and I started to receive a different type of invitation in which we were asked to visit several groups within a country. I suppose that these could be best described as “lecture tours”. He and I would depart the UK and spend a couple of weeks exploring different parts of the world. If an ultrasonic bath could

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be provided at our venue, Phil and I formed a sort of “double act” with my demonstrations first which were then followed by his interpretation of the chemistry and physics underlying sonochemistry. The first of these extended visits took us to Hong Kong, Taiwan and Japan in November 1988. After that we visited the far east several times and included stays in P.R.China and Japan. One regular “booking” was from Astrid Rehorek, who we had visited in Leipzig in 1989 (see Section 1.6.3). After the unification of Germany, she moved to Aachen where she held a post at the Fachhochschule in Cologne. For several years from 1997, we went over to Cologne and nearby Aachen to deliver Christmas lectures to chemistry and chemical engineering students, including my demos. Phil was very willing to drive his car all the way on such trips because I did not like driving in Europe. We would go across by ferry, drive to Cologne, deliver our talks there and in nearby Aachen and then enjoy the Christmas markets that were so famous in both cities. In 1990, the opportunity arose for me to visit Ukraine; this was made much easier because one of our post-doctoral researchers – Larysa Paniwnyk – had family ties with Ukraine but had never visited. She could also speak the language and was very happy to help with the organization and accompany me on the adventure which started in Moscow with a visit to Milia Margulis. Moscow was not really a tourist city in those days and we had a great time visiting the sites. The next stage was to fly to the Ukraine to meet up with our host Professor E. M. Mokry, head of General Chemistry at the Polytechnic Institute of Lviv, and Milia drove us to the airport used for internal USSR flights which was some distance from the centre of Moscow. It was a difficult journey due to bad visibility as a result of what was referred to as fog but seemed to have a degree of pollution involved. When we got to the airport, all flights were grounded due to poor visibility and so we had to re-plan our journey. Milia bought tickets for us on a sleeper train for the 24 h journey to Lviv. The sonochemistry group in Lviv were friends of ours having regularly attended European Sonochemistry Society meetings. They came to the very first European meeting earlier in 1990 at Autrans, France (ESS1), where they had presented material on sonochemistry in catalytic oxidation processes. Professor Mokry is on the front row, extreme right of Figure 1.7, next to me with his colleague Volodymyr Starchevsky behind him. At all of the subsequent meetings that they attended, the Lviv group would arrive by car having travelled long distances over several days to get there. One of the items of food that they brought with them was called “Hunters Sausage”, which involved the cooking of small pieces of sausage by lighting some purified ethanol poured over them. It became a ritual at ESS conferences for the Ukranian group to have a small party involving vodka and this tasty sausage. Professor Mokry was quite a character (Figure 1.3) and had been a weightlifter for the USSR in his younger days. With Volodymyr he wrote a review in 1993 entitled “Initiation and catalysis of oxidation processes of organic compounds in an acoustic field” [57].

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Figure 1.3: Larysa Paniwnyk, Tim Mason and Eugeny Mokry in Lviv, 1990.

Professor Mokry died in 2002 but Volodymyr would go on to host ESS13 in Lviv which was the first of the series ever to be held in Eastern Europe 22 years after ESS1. This meeting was special for me in that during a formal ceremony I was awarded “Doctor Honoris Causa” by the Lviv Polytechnic National University. When I looked at a board listing these awards I saw that the first such degree had been awarded in 1912 which was exactly 100 years previously, my name was at the top of a new column. I was amazed to see that the first-ever recipient whose name was atop column one, and thus next to mine was Marie Curie (in Cyrillic script) (Figure 1.4).

Figure 1.4: Tim Mason with list of Honoris Causa recipients in Lviv, 2012.

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1.6.2 The first international meeting on sonochemistry Over the first few years of the 1980s, international interest in the effects of ultrasound on chemical reactions expanded, although very little was published on kinetic studies which was the aspect which had first attracted the Coventry group. There were several scientists around the world involved in the study of ultrasound in chemistry and so we thought it would be a good idea to organize an international sonochemistry symposium. In April 1986, it was made part of an RSC Annual meeting at Warwick University. Warwick was a convenient place for our group as it was on the outskirts of Coventry. It brought together scientists who were at that time active in sonochemistry and some of these would go on to become pioneers in this “new” field (Figure 1.5).

Figure 1.5: Speakers at the First International Symposium on Sonochemistry in 1986. Back row: R. S. Davidson (City University, London, UK), J-L. Luche (Universite de Grenoble, France), B. Pugin (CIBA-GEIGY, Basle, Switzerland) and J. P. Lorimer (Coventry Polytechnic, UK). Middle row: J. Lindley (Coventry Polytechnic, UK), P. Riesz (NIH, Bethesda, USA), T. J. Lewis (University College North Wales, Bangor, UK), J. Perkins (Sonic Systems, Marlborough, UK), E. J. Einhorn (Universite de Grenoble, France) and P. Kruus (Carleton University, Ottawa, Canada). Front row: R. Verrall (University of Saskatchewan, Saskatoon, Canada), A. Henglein (Hahn-MeitnerInstitut, Berlin, Germany), T. J. Mason (Coventry Polytechnic, UK), P. Boudjouk (North Dakota State University, Fargo, USA), K. S. Suslick (University of Illinois, Urbana-Champaign, USA) and C. Dupuy (Universite de Grenoble, France).

In the lead-up to the conference, I had approached the RSC about publishing some of the papers from the sonochemistry symposium, but they were not interested in doing so at that time. However, I had established good links with Butterworths through our attendance at UI85 and they had already accepted a review paper from me on

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ultrasound in chemical synthesis for the journal Ultrasonics [58]. In January 1986, Marija Vukovojak who was Executive Editor of Ultrasonics wrote to J. F. Gibson at the RSC asking for permission to publish some of the papers from the meeting. She received an interesting reply from P. G. Gardam, who was the RSC Manager of Books. He wrote: Dear Ms Vukovojac, Dr Gibson has passed your letter of 27th January 1986 on to me. We have no plans to publish the proceedings of the sonochemistry symposium at Warwick so I am happy to agree that you may publish individual papers based on the symposium lectures in a special issue of your journal Ultrasonics. However, we will have an editorial representative present at the symposium so we reserve the right to compete with you for individual papers should he feel that any of the lectures would be suitable as contributions to our review journal Chemical Society Reviews.

Marija was surprised about the comment regarding reviews but we proceeded to collect the papers from the Warwick meeting and they appeared in the very first Ultrasonics special issue devoted to sonochemistry which was published in January 1987 [59]. The RSC followed up on their interest in reviews on sonochemistry and asked me if I could provide two that would address separately the physical and synthetic aspects. The first was written with Phil Lorimer and the second with another colleague Jim Lindley who was an inorganic chemist at Coventry. It was a large undertaking and together the reviews cited over 350 references [34, 35].

1.6.3 Textbooks produced by the Coventry Sonochemistry Group The two major reviews were to form the foundation from which Phil and I compiled our first textbook. It was commissioned by Terry Kemp a professor of Chemistry at Warwick University. Terry had telephoned in September 1986 and then followed up with an invitation letter for Phil and I to ask us if we would write a monograph on sonochemistry for the Ellis Horwood Series in Physical Chemistry for which he was editor. This text was to become the first-ever published that included the term “sonochemistry” in its title “Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry” and it was released in 1988 [60]. Terry Kemp was interested in spin trapping experiments, and at Warwick university a guest researcher in his laboratories was Detlef Rehorek from the Karl-Marx University in Leipzig. Detlef also had an interest in sonochemistry from his work in Canada in the 1980s on the ultrasonic decomposition of organotin compounds [61], which he had continued with Terry [62]. As a result of this contact, Phil and I visited Detlef and his wife Astrid in Leipzig in October 1989. That was an interesting period of history because it was before the reunification of Germany and Leipzig was in East Germany. Detlef took us to East Berlin; we saw the Berlin wall and the famous “Check-Point Charlie”. Leipzig was a centre for protest against the communist regime and we were there just before the resignation of the President of East Germany (Erich

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Honecker) and the fall of the Berlin Wall on 9 November 1989 less than 1 month after we left Leipzig. The visit also introduced us to a type of ultrasonic device of which we were previously unaware. It was a small ultrasonic bath made by a local company Meinhardt. The bath was water cooled with a small concave transducer at the base operating at 800 kHz much higher than the normal baths in the UK which were normally 40 kHz. I gave a demonstration lecture on sonochemistry in the Karl-Marx University, and Astrid allowed me to use this high-frequency bath. It was particularly good for a demonstration of the Weissler reaction due to its high frequency producing many radicals. Some years later, the Meinhardt company would produce the first ultrasonic bath system for sonochemistry which used a single transducer and bath to deliver three different discrete frequencies. The years around 1990 were very busy in terms of book publications. In December 1986, Celia Gould from the Society for Chemical Industry in the UK had contacted me by telephone to ask if I would act as senior editor for a book on sonochemistry. She was the managing editor of the series entitled Critical Reports in Applied Chemistry (CRAC). This I agreed to do and “Chemistry with Ultrasound” was published as my second book in 1990 with three chapters from our group [63]. The RSC rekindled its interest in sonochemistry and commissioned a short book for which I was editor entitled “Sonochemistry: Uses of Ultrasound in Chemistry” also published in 1990 [64]. This had a broader remit and included a chapter from John Perkins on “Power ultrasound” from his presentation at the 1986 Warwick conference (see earlier), a chapter on diagnostic ultrasound and one from Jean-Louis Luche on his ideas about ultrasound and radical chemistry. A contributor to both of these books was T. J. (Terry) Goodwin, who wrote about sonochemistry equipment and the scale-up of sonochemistry, respectively. In 1986, Terry was one of the organizers of the Harwell Sonochemistry Development Club, more details of which are given later in this chapter. The year 1990 also saw the birth of what was to become a total of six volumes in the series Advances in Sonochemistry [65]. This came about because in March 1987 I had been contacted by Piers Allen of JAI Press in London who wanted to explore the possibility of developing a new serial publication in the area of sonochemistry. I replied that I was already heavily involved in writing and editing texts, but I suggested that if I was to take on a series to be tentatively called Advances in Sonochemistry, it must differ from the way in which other volumes in the existing JAI series were organized. I suggested that the chapters should be articles involving in-depth reports on the contribution by the authors on their individual research in the area of sonochemistry. This was accepted and he was happy to wait for me to discuss possible contributions from attendees that I would meet at upcoming conferences. Volume 1 appeared in 1990, with contributions from David Bremner (Historical), Milia Margulis (Sonoluminescence), Ben Pugin (Metallic Reactions), Jean-Louis Luche (Carbonyl Additions), Wilf Tomlinson (Corrosion), Ken Suslick (Surface Chemistry) and Gareth Price (Polymer Degradation). The breadth was indeed very wide, which fitted with my intention for such a series.

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Ellis Horwood, who had published the first text book by T. J. Mason and J. P. Lorimer which was entitled Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry [60], also published a series of books on organic chemistry for which the series editor was John Mellor. I knew John because he had taught me during my undergraduate years at Southampton University, and so I was interested in a project that he approached me with about a book on practical sonochemistry. The book was to be aimed predominantly at organic chemistry and was originally co-authored with JeanLouis Luche. This was accepted in a letter from Ellis Horwood himself in October 1989. I had agreed to discuss the book in detail with Jean-Louis at the Pacifichem conference in Hawaii in December 1989 which we were both due to attend. Unfortunately, he was taken ill in Paris just before departure for Hawaii and we did not have that opportunity to meet. Jean-Louis contacted me to say that his illness was associated with stress and he felt that he could not take on the book project, but he promised that he would contribute some examples of practical experiments. So it was that I became sole author of “Practical Sonochemistry: A User’s Guide to Applications in Chemistry and Chemical Engineering” which was published in 1991 [66]. The last chapter was devoted to laboratory experiments and I wrote to many of my colleagues to ask for contributions. In the end, the chapter contained 38 different experiments and 9 of these came from JeanLouis. This proved to be a popular book and, some years later, the publisher asked me to update it with a second edition (Practical Sonochemistry: Power Ultrasound Uses and Applications (2nd edition), 2002). I asked Dietmar Peters, a colleague from Rostock University if he would be willing to join me in this project, and in 2002, we produced the second edition [67]. In that same year (2002), Phil Lorimer and I co-authored a comprehensive text for Wiley entitled Applied Sonochemistry: The Uses of Power Ultrasound in Chemistry and Processing [68].

1.7 The development of sonochemistry From the late 1980s, sonochemistry began to develop rapidly along two different paths. The first led to the establishment of the European Society of Sonochemistry (ESS) which led to wider cooperation in joint research projects and the second was the launch of a new journal to be known as Ultrasonics Sonochemistry.

1.7.1 The European Society of Sonochemistry Jean-Louis Luche was one of the pioneers of this “new” science of sonochemistry and was also instrumental in the establishment of the ESS [69]. He was one of the speakers at the first Sonochemistry Symposium at Warwick University in 1986 while he was based in Grenoble at Joseph Fourier University. In the following year, he was involved in the organization of a EUCHEM Conference “Unusual Methodologies

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in Synthetic Chemistry” held in 10–15 May 1987 at the Centre Paul Langevin at Aussois, France. The conference centre belonged to CNRS, the French National Centre for Scientific Research, and was beautifully located in the Alps close to the Italian Border. The meeting was very significant in the development of sonochemistry. It brought together scientists interested in a range of high-energy processes including high-pressure (piezochemistry) and high-temperature chemistry as well as sonochemistry. It was a forerunner of the EU COST programmes involving sonochemistry and it was here that the idea for the formation of ESS was first discussed. However it would be another 3 years before this could be realized. The EUCHEM meeting was memorable for another reason. It was to be the first of many meetings which were attended by a large group of researchers from the Coventry Sonochemistry Centre (see later). For this one, we travelled in a Coventry University minibus. It was a long journey taking several days but the driving was shared between Phil Lorimer, Jim Lindley and a research student Andrew Turner. The return journey was a lot faster because while we were in Aussois, Phil had received news that his wife had obtained several tickets to the FA cup final at Wembley Stadium in London on 16 May. The match was important because it involved our home team Coventry City who were to play Tottenham Hotspur. We got there with a few hours to spare and watched a tense match in which Coventry won the cup 3–2 but only after extra time. In September 1988, in conjunction with the RSC a Residential Course was organized at Coventry Polytechnic entitled Sonochemistry: The Uses of Ultrasound in Chemistry and it attracted 56 delegates. I invited Jean-Louis to present not only some of his research but also to contribute to the laboratory sessions which were an important part of the course. He brought with him some of the laboratory glassware that had been specially made for his work on organometallic sonochemistry. This was a further opportunity for discussions about the way in which we might organize an ESS and I recall enthusiastic discussions involving Phil Lorimer and I with leading sonochemists including Jean-Louis, Jacques Berlan and Steven Davison. The next major international gathering was a sonochemistry session at UI (UI89) 3–7 July 1989 held in Madrid. This was a significant meeting in the UI series because it was the first to include a full session on sonochemistry. After this groundbreaking session, all subsequent UI meetings included at least one. The speakers at that session are shown in Figure 1.6 and in the centre is M. A. (Milia) Margulis from Moscow. I had organized the invitation and it was the first time that he presented his work in the West. His studies on ultrasound were well known to us from reading the Russian literature, and his theories on cavitation were to become contentious in later years. This was the first time also that an audience would experience the difficulty that Milia had in keeping his talks within the time limit of a conference presentation. It was always a problem as chairman of a session to stop him, I think the only time I remember it working was at a conference where a low noise would start near the end of a presentation that would slowly grow louder and eventually drown out the

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speaker’s voice. He was thrilled to be there in Western Europe for the first time and presented me with one of his paintings. Milia was an artist and as I was to find out later when I visited him in Moscow, he was quite an expert on expressionist painters. When he visited the UK in 1992, he tutored my middle son Daniel in water colour painting, and Daniel, while still at school, went on his own to stay with him and his family in Moscow. The Coventry group presented eight papers at that meeting and we all thoroughly enjoyed our time in Madrid although it was extremely hot that July.

Figure 1.6: Sonochemistry session speakers at Ultrasonics International 89 in Madrid. From the left: D. J. Walton (Coventry Polytechnic, UK), T. J. Mason (Coventry Polytechnic, UK), J. V. Sinisterra (U Complutens, Madrid, Spain), J-L. Luche (University Grenoble, France), M. A. Margulis (Institute of Organic Synthesis, Moscow, Russia), J. P. Lorimer (Coventry Polytechnic, UK), A. Chyla (T U Wroclaw, Poland), S. Leeman (Dulwich Hospital, UK).

In October of the following year, there was the first meeting of what was later to become formally established as the ESS. It was organized by Jean-Louis and others in Autrans, France (Figure 1.7). The Coventry group made seven presentations and were again able to use a Polytechnic minibus for the trip with a very similar route but this time there was no cup final to come back too. There were many discussions amongst delegates at that meeting about the best way to take things forward. We needed formal arrangements to set up a new scientific society and Jean-Louis decided to undertake this. It was proposed that the society would be legally established at the next European Sonochemistry meeting arranged by Vittorio Ragaini in September 1991 at a conference venue owned by the

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Figure 1.7: Attendees at the first meeting of the European Society of Sonochemistry Autrans, France, 1990.

Figure 1.8: Speakers and attendees at the second meeting of the European Society of Sonochemistry, Gargnano, Italy, 1991.

University of Milan at Gargnano on Lake Garda (Figure 1.8). Vittorio is in the middle of the front row wearing a grey suit. The first officers elected were Tim Mason as president, Jacques Reisse as vicepresident (Universite Libre de Bruxelles) and Jean-Louis as organizing secretary. It

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was decided that the treasurer of ESS should be the organizer of the latest ESS conference and so the first was Vittorio Ragaini (Università di Milano), and for a few years, the treasurer changed with each conference. Jean-Louis was much more interested in organizing the society and its statutes than becoming president and he had sent by fax his ideas of the possible statutes of our new society based on those of the European Society for Rare Earths. After the meeting, he proceeded to establish ESS based on a French model for scientific organizations. We were to find out much later that the model he used had some restrictions which included the requirement for the bank to be in France resulting in some ongoing problems with membership fees. Although the foundations for the ESS had been formalized, at this meeting in Gargnano, it was thought best, in retrospect, to refer to Gargnano as the second meeting since the previous meeting in Autrans had been named unofficially as the First Meeting of the European Society of Sonochemistry and was the first major conference entirely dedicated to sonochemistry. The birth of ESS proved to be a tremendous driving force for the development of sonochemistry – which up until then had been regarded as a new and unexplored branch of chemical science. The topics embraced by sonochemistry began to expand beyond its original scope which was “chemistry under the influence of ultrasound” to include many other fields. This was the result of the policy of the ESS scientific board who were open minded about such things. Topics such as Ultrasonically Assisted Extraction (UAE), therapeutic ultrasound, food processing, environmental protection together with cavitation theory and sonoluminescence made more regular appearances in ESS meetings. This expansion of interests will be reflected in other chapters of this book, where some of these topics will be addressed from their historical roots and followed on to current interests.

1.7.2 Meetings and conferences of importance to the development of sonochemistry It is interesting to reflect on the geography of the meetings and conferences which were influential in the development of sonochemistry after that first meeting at Warwick University in 1986. Almost all of these were held in Europe and a few that were originally organised as unique events turned into series which added to the growing strength of the subject. 1.7.2.1 Meetings in the UK Immediately following the Warwick University conference in 1986, the RSC sanctioned the formation of a new interest group devoted to sonochemistry and I agreed to act as the organizer and secretary for it. The first meeting was arranged in November 1986 at City University in London, where Stephen Davidson who had been one of the speakers

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at the Warwick conference was professor of chemistry. He was duly elected chair and together we went on to consolidate the group. Then, and because we were now part of the RSC, we were able to receive their support and join larger conferences, and many of these are referred to in other chapters of this book. In parallel with the formation of the RSC group that was mainly but not exclusively aimed at academia, there was interest from the industrial sector. In December 1985, before the Warwick conference I received a letter from the Chemical Technology Centre at the Harwell Laboratory in Oxfordshire. The letter was from Terry Goodwin and he referred to an article I had written on sonochemistry for the magazine Laboratory Practice in 1984 [55]. He was interested in introducing sonochemistry to Harwell and we met in early 1986 and later in April when he attended the Warwick conference. In a letter written later that year, he told me that Harwell had decided to launch a “Sonochemistry Development Club” from April 1987 for 3 years. The club would be funded partly by the Department of Trade and Industry, and partly by subscriptions from member companies. The research would address two themes: 1. Problems with operating sonochemical reactions on a large (multi-litre) scale, that is, the fundamental principles of “scale-up”. 2. Development of equipment which would allow the commercial operation of these reactions. Harwell had some new ideas in this area. These areas of research were “applied” sonochemistry and would complement the more fundamental research performed by others in the academic world. The Club, if successful, would represent a major sonochemical research effort within the UK and, together with the RSC Subject Group, help the promotion and development of sonochemistry within the British Chemical Industry. By the end of 1987, the club had attracted 19 companies. Our group in Coventry contributed to their meetings and we shared a PhD research student (Darren Bates) who completed his doctorate in 1992 “The effect of ultrasound and other physical parameters on the reactivity of powders and catalysts” [70]. Three years after the first residential course in sonochemistry in Coventry, we organized a second, again in co-operation with the RSC in September 1991. In addition to our own team and Jean-Louis Luche, we had three other speakers from the UK: Peter Martin from the Harwell Laboratory talking about the scale-up of sonochemistry, John Perkins of Sonic Systems on power ultrasound and R. T. (Tudor) Roberts, Leatherhead Food Research Association (LFRA) on food processing. I had also invited Ken Suslick from the University of Illinois, USA. The presence of Tudor Roberts from LFRA was an indication of the interest being generated in the food industry and this will be further expanded in Volume 2, Chapter 8 of this book. Mircea Vinatoru also attended as a delegate. Between these two residential courses came the first of two 1-day meetings organized by the LFRA together with The RSC Sonochemistry group. In the 1990s, we worked with a group at LFRA on applications of ultrasound in food technology. At

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Leatherhead they hosted conferences (mostly on food-related subjects) but two were significant in the promotion of sonochemistry in the UK. The first was in May 1990 with the title “Applications of High-Power Ultrasound”. The speakers are shown in Figure 1.9. Martin Wiltshire (far right) who was based at LFRA and worked with us on a number of projects presented a review of applications of power ultrasound in the food industry but the remainder of the speakers were chemists, all with notable contributions to sonochemistry. One interesting aspect of this meeting was that it brought together Arnim Henglein and Milia Margulis and they spent a good deal of time discussing their shared interest in the mechanisms underlying sonochemistry.

Figure 1.9: Speakers at the LFRA meeting “Applications of High-Power Ultrasound” in 1990. From the left: T. J. Mason (Coventry Polytechnic, UK), J. P. Lorimer (Coventry Polytechnic, UK), Arnim Henglein (Hahn-Meitner Institut, Berlin, Germany), M.A. Margulis (All Russia Institute of Organic Synthesis, Moscow), J. Berlan (ENSIGC, Toulouse, France), R. S. Davidson (City University, London, UK) and M. P. Wiltshire (LFRA, Leatherhead, UK).

The second meeting in September 1992 concentrated on High-Powered Ultrasound in the Processing Industries. In this meeting, there was a wider range of topics and the speakers include several whose researches will appear in later chapters of this book; the topics and speakers were: Crystallization (Tudor Roberts, LFRA), Airborne ultrasound (Juan Gallego-Juarez, Institute of Acoustics, Madrid), Particle manipulation in an ultrasonic field (John Schram, Sonic Research), Filtration processes (Frank Rawson, FFR Ultrasonics), Atomization (Andy Morgan, Courtaulds Coatings), Ultrasonic modification of polymers (Gareth Price, University of Bath), Ultrasonic cleaning (Bill Lambert, Kerry Ultrasonics) and Scale-up (Jacques Berlan, ENSIGC, Toulouse).

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It was at this meeting that I first met Oleg Abramov from Moscow. I had organized a Royal Society grant for his visit to the UK, which included this conference but also covered his visits to discuss applications of ultrasound in metallurgy which was to lead to research carried out with Alcan Aluminium and Kaye Presteigne which will be discussed in Volume 2, Chapter 6. Although these two meetings in Leatherhead were at the headquarters of a food research association, they contained very little material on food technology. Nevertheless interest was gathering in this area of processing, and in September 1996, Malcolm Povey and I organized a 2-day meeting at Tetley Hall, Leeds University, entirely devoted to ultrasound in food processing. We produced a book from the proceedings of that meeting [71]. In October 2005, a 2-day workshop on the effects of non-ionizing radiations of ultrasound and infrasound relevant to human health was held in Oxford. It was an unusual meeting in that it brought together experts in acoustics, therapeutic ultrasound and sonochemistry to discuss the effects of sound waves on the human body from high frequencies used in diagnosis and therapy through to sub-audible frequencies (infrasound). The proceedings were published in Progress in Biophysics and Molecular Biology [72]. A conference organized in 2008, which proved to be popular with industrialists, was “Molecules to Particles: The Use of Ultrasound in Synthesis, Particle Engineering and Measurement”. It was a joint venture by the Sonochemistry and Formulation Science groups of the RSC. It was held at Weetwood Hall, Leeds, and I was the scheduled first speaker followed by my colleague Giancarlo Cravotto from the University of Turin, who was to talk on “Recent advances in ultrasound-promoted synthesis”. A combination of heavy traffic and unclear directions to the venue meant that Larysa Paniwnyk and I, as we drove to the meeting from Coventry were severely delayed. For me it was somewhat distressing to have to telephone the organisers while en route and minutes before the start to ask them to reschedule the programme and ask Giancarlo to open the meeting. It was the first, and only, time that I had failed to make a conference slot time – but we did arrive just in time for me to deliver the second talk. Prosonix, who were sponsors for this meeting, and Graham Ruecroft, from that company, presented an overview of sonocrystallization. Previously, he had been a part of Accentus which had itself derived from the Harwell Sonochemistry Development Club (see Section 1.7.2.1). At Accentus, he had developed an interest in the use of ultrasound to improve industrial crystallization [73]. Subsequently, he and others from Accentus had formed Prosonix at the Oxford Science Park. Our Coventry Group had strong ties with all three companies particularly with Graham himself. The second part of the meeting dealt with analytical ultrasound used for particle size analysis and to observe the dynamic processes involved in crystallization. One of the speakers in this section was Malcolm Povey who I had worked with in a previous conference on food technology in Leeds (see earlier) and who later wrote a review on the uses of ultrasound in particle sizing [74].

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1.7.2.2 Meetings in France In the early days of sonochemistry, a focal point for its development was the Ecole Nationale Supérieure d’Ingénieurs de Génie Chimique (ENSIGC) in Toulouse. Henri Delmas (a chemical engineer) and Jacques Berlan (an organic chemist) were involved in the subject in the 1980s and it was on the recommendation of Henri Delmas that a young researcher, Anne-Marie Wilhelm, obtained a post there in 1989. She had received an introduction to sonochemistry when she accompanied Jacques to the residential course in Coventry in September 1988 and she was employed specially to work on sonochemistry with him. He was very keen to progress his pioneering studies on sonochemistry and they worked together for some years with her first paper published in 1992 [75]. All three of them attended the very first ESS meeting in Autrans in 1990 where Jacques gave a plenary talk “Scale-up in sonochemistry”. One of my first collaborations with Anne-Marie was a Course on Sonochemistry for French Industrialists “Sonochimie Industrielle” which began at ENSIGG in March 1991. The majority of the programme was in French, but I have never been a linguist and so my contributions were always in English as were those of my colleague Phil Lorimer who also taught on the course. It ran every year for 5 years and sadly Jacques Berlan passed away during its last year 1995. In 1997, Anne-Marie and Henri decided to adapt this course, which was originally aimed at industrialists to produce an international conference in English “Applications of Power Ultrasound in Physical and Chemical Processing”. It was also held in Toulouse and was administered by INP ENSIGC Antenne Formation Continue. The titles for the first meetings in 1997 and 1999 were abbreviated to USOUND1 and USOUND2. There was a change in venue for USOUND3 due to a major disaster in Toulouse on 21 September 2001. In the morning of that day, an explosion occurred in the AZF fertilizer factory in a warehouse containing about 300 metric tons of production rejects of granulated ammonium nitrate. The explosion was heard up to 80 km away and caused 31 deaths, hundreds of casualties and enormous damage. The disaster was close to the campus of ENSIGC and put a stop to all research including sonochemistry. As a result, the venue of the following conferences had to be changed and USOUND3 was held in Paris in 2001. USOUND4, the fourth and last of the series, was held in Besancon in 2003 and organized by JeanYves Hihn. It was unfortunate that this last conference coincided with a swine fever outbreak in Southern China and the SARS virus had begun to spread. For this reason, all travel from China was cancelled and I delivered a talk on behalf of one of my former students, Zhao Yiyun, who sent her presentation to me. The paper concerned the application of ultrasound on electroless coating which was an extension of her PhD program and would become an important research field for the sonochemistry centre in later years [76]. Although the USOUND series finished in 2003, Anne-Marie continued to attend sonochemistry meetings and was active in research. Tragically, she died in 2009 when flight AF 447 from Rio de Janeiro to Paris crashed into the Atlantic in June of that year.

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1.7.2.3 Meetings in Belgium Another centre for European Sonochemistry was Brussels, and in the early 1990s, two significant workshops were held there. Jacques Reisse organized the first of these on 21 October 1991 a “One Day Workshop Devoted to Sonochemistry” which involved the Coventry group and Jean-Louis Luche. Also speaking at the meeting was Eric Cordemans who was based in Belgium and a very good friend who we had first met at the EUCHEM conference in 1987. In Coventry we used specialist equipment provided by his company Undatim. His interchangeable ultrasonic probe system allowed us to investigate the effects of different frequencies, a study that was not possible for many groups at that time due to the lack of such specialist equipment (Volume 2, Chapter 5 Figure 5.3) [45]. The second 1-day workshop in Brussels was held 2 years later to celebrate the Centenary of the Institute Meurice. Working at that institute was a man and wife team who contributed greatly to the more physical chemistry aspects of sonochemistry with their research on cavitation; they were Thierry LePoint and Francoise Mullie. Also presenting were myself, Jean-Louis Luche, Jacques Reisse, Jacques Berlan and Gareth Price. 1.7.2.4 Meetings in Germany Up to the 1990s, there were two major international conferences that attracted regular contributions from sonochemists; these were UI and the International Congress on Acoustics (ICA). I had been involved in these from 1985 and 1992, respectively. Of course, there were other acoustics conferences with perhaps the main ones being the Acoustics Society of America and Forum Acusticum but both covered a wide range of acoustics and neither had much content relating to sonochemistry. In 1993, an idea was suggested for the launch of a new conference series to be entitled World Congress on Ultrasonics (WCU). This was the brainchild of Joachim Herbertz from the University of Duisburg whose field was medical ultrasound. I joined an organizing committee together with a group of acoustics experts from many disciplines which met in 1993 at Humboldt University in Berlin (Figure 1.10). The rationale for this new conference was that the ultrasonics community required: – A conference under academic management so the resulting fees would be affordable for scientists from all over the world. UI conferences were generally regarded as quite expensive, especially for young researchers. – It would be supported by major acoustical societies. – This would be the first of a new series of WCU acoustics meetings to be held worldwide. This was a conference that would clearly be a rival to the existing UI series, which was seen as a more commercial operation run by the publisher Butterworths.

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Figure 1.10: Organizing committee members for first WCU meeting at the Humboldt University, Berlin, in 1993. From the left: T. J. Mason (UK), L. Crum (USA), S. Ueha (Japan), N. Chubachi (Japan), A. Zarembowich (France), L. Bjørnø (Denmark), N. G. Pace (UK), J. Herbertz (Germany), A. Alippi (Italy), J. A. Gallego-Juarez (Spain), E. Benes (Austria), A. Sliwinski (Poland), O. Leroy (Belgium) and R. Reibold (Germany).

The first WCU meeting was held in Berlin in September 1995 at Humboldt University, and I had contacted both Jean-Louis Luche and Christian Petrier to alert them about the possibility for the inclusion of European COST groups in the conference (see Section 1.8.1.1). Joachim Herbertz was in favour of this and so we were able to set aside several sessions devoted to sonochemistry. As a result, we recruited a good number of ESS members to sign up and deliver talks and posters. We even managed to bring Milia Margulis and Oleg Abramov from Russia although it was also necessary to subsidize their conference fees. Our group from Coventry provided several talks including a plenary entitled “Cavitation in the service of science and technology” together with five others. The conference was a success attracting 562 delegates from 37 countries and the proceedings were published in 2 volumes [77]. Leif Bjorno of the Technical University Denmark, Lyngby, asked me to join the WCU steering group and the terms of reference for future conferences were ratified in 1997 at WCU2 in Yokohama. Eventually, the WCU conferences were to be merged with UI series to form ICU, the International Congress on Ultrasonics (see Section 1.7.3.2).

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In Harburg, Germany, there was a group of scientists with interests in environmental protection led by Uwe Neiss. Three residential meetings were held at the Technical University of Hamburg-Harburg starting in 1999. Each had the title “Ultrasound in Environmental Engineering”, and the proceedings were edited by Uwe with his colleague Andreas Tiehm and published as part of a series of books Reports on Sanitary Engineering. For my contribution to the first meeting in 1999, I chose the broad ranging topic “Ultrasound in environmental protection – an overview” [78]. I then updated the topic for subsequent meetings in 2002 and 2005. An interesting aspect of these meetings was that the morning talks were in English, and the shorter afternoon presentations were in German. The great majority of the audience was from Germany. 1.7.2.5 Meetings in the USA Another series of conferences which helped with the establishment of sonochemistry was the International Chemical Congress of the Pacific Basins Societies also known as the Pacifichem Conferences. In 1989, because of the rapid growth of sonochemistry, it included for the first time a symposium on the chemical effects of ultrasound. Ken Suslick, Peeter Kruus and Takashi Ando were the organizers representing the American Chemical Society, the Canadian Society for Chemistry and the Chemical Society of Japan, respectively. It ran between 17 and 22 December in Honolulu, and this was a wonderful chance to visit the famous islands of Hawaii. So good was this opportunity that my wife decided that maybe we should go as a family and extend the trip to a 3-week holiday over the Christmas period and to visit other islands. Thus, after many weeks of planning, my wife and sons – and my 75 year old mother-in-law – embarked on the adventure. My presentation was sonoelectrochemistry and selected papers from the session were published in a special issue of Ultrasonics in 1990 for which Ken Suslick wrote the introduction [79]. Another reason why this meeting was memorable for me was that I had left the FAX number of our hotel in Honolulu with Coventry University and it was by this means that they contacted me to let me know that I had been elected to a full professorship. I met Ray Hunicke at this conference, he had founded the company Lewis Corporation in 1965 and I would later be grateful to that company for their advice and the use of some of their specialized equipment. In the sonochemistry session, Ray presented a paper reviewing various industrial applications of high-power ultrasound for chemical reactions [80]. Ray and his wife Barbara invited my whole family to dinner one evening and from there on our families developed a close relationship. The Pacifichem conferences were every 5 years and my wife was very keen to accompany me and take similar extended breaks in 1995 and 2000 when my topics were “Food technology” and “Ultrasonically assisted extraction”, respectively. In 1994, Karl Graff invited me to deliver a talk at the 25th Anniversary Congress of the Ultrasonics Industries Association (UIA) in Columbus, Ohio. This was a different

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sort of meeting from those which I normally attended because it was aimed at industrialists. As one of the only chemists there, I gave a talk on “Sonochemistry in Europe – Current Trends and Future Prospects”. What made that meeting even more special for me was that David Hunicke, son of Ray, who I had met in Hawaii the year before suggested that after this meeting he would take me on a road trip in his Lexus car back to his home town in Oxford, Connecticut. It was over 600 miles directly by road, but I accepted readily because we took the “scenic route” and he showed me some interesting parts of Eastern USA, including the Amish Country and the Gettysburg battlefield in Pennsylvania. When we got back, I said that I was planning to visit Amherst College, where I had been a research fellow 24 years earlier. Ray offered to let me use an old and treasured family car belonging to the Hunicke family – a Cadillac. I then drove the 100 miles and visited my old college of Amherst. The car had a number plate SONIC 3. Over a period of 2 weeks in August 1997, a NATO Advanced Study Institute (ASI) was held at the Sleeping Lady Conference Centre, Leavenworth, Washington State. It was co-organized by Jacques Reisse, Larry Crum, Ken Suslick and myself and entitled Sonochemistry and Sonoluminescence [81]. The Centre was a complex of comfortable log cabins in the Cascade mountains with no televisions allowed but we were encouraged to enjoy the food and indulge in discussions either in the bar or even around a campfire at night. The aim of this ASI meeting was to “bridge the knowledge gap between the disciplines by bringing together physicists with their expertise in the physics of acoustic cavitation and bubble dynamics, and chemists with their expertise in electron transfer, chemical reactions and spectroscopy”. It attracted 75 participants from 19 countries and the proceedings of that conference were put together as a book [82].

1.7.3 International conference series involving sonochemistry 1.7.3.1 International Congress on Acoustics (ICA) One of the long-established conferences involving ultrasound was the ICA; it had held meetings in various cities with the first conference in Delft in June 1953. An opportunity arose in 1992 for me to present a talk at the 14th meeting of the (ICA14) in Beijing in September, and although I had travelled to Japan and Taiwan previously with Phil Lorimer, I had never visited China. I had a Chinese student at that time Zhao Yiyun, a teacher from Yunnan University in Kunming. She encouraged me to go and said that she would organize a tour of some universities in China before the ICA meeting and travel with me to make sure all went smoothly. It was a wonderful opportunity and so I took her up on her offer. Sonochemistry as a topic was not well known in those days and so I presented an introduction to the subject at each of the five locations arranged by Yiyun which involved a lot of travelling and included the universities of Nankai, Shandong and Peking. At ICA14, my talk

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was placed in a section on high-power ultrasonics within which the other presentations were mostly related to welding. The conference itself did not contain much about the chemical effects of ultrasound at all although bubble dynamics and cavitation were well represented. This first visit to China was particularly memorable because I arranged for my wife and three sons to join me in Beijing while the ICA conference was taking place. Yiyun was invaluable as an organizer and managed to find a taxi for us to use all day. Including the driver there were seven of us in that taxi which was licensed for six and so periodically, when we spotted police around, one of my sons would have to duck down out of sight. It gave us the opportunity as a family to visit the sights including the Forbidden City and the Summer Palace. I particularly remember that the first McDonald’s had just opened in Beijing that year and we appreciated a taste of Western food albeit hamburgers. There was a post-conference tour by coach and air to sights such as the Great Wall, Terracotta Warriors in Xian and a trip on the river Li departing from Guilin where we saw fishermen using cormorants to catch fish and finally ending up in Hong Kong before returning to the UK. It was an odd coincidence that one of the participants on that after conference trip was Peter Narins from UCLA who none of us knew at that stage. Some years later my oldest son Matthew would work in Peter’s lab when he did post-doctoral research in the USA. The next in the ICA series that I attended was ICA16 in 1998 which was a joint meeting with the Acoustical Society of America (ACS) and held in Seattle. I gave a lecture with demonstrations in the section “Sonochemistry and Sonoluminescence” using an ultrasonic bath kindly supplied by David Hunicke of the Lewis Corporation who was a good friend to us. This was a very special meeting for me because it was the only conference I have ever attended, where one of my sons (Matthew) also gave a presentation. This was because the conference was joint with the ACS and had a very wide range of topics that included a session on “Insect, Insectivore and Avian Acoustics”. Matthew was studying the hearing of moles for his PhD and his talk was entitled “Functional anatomy of the middle ear of insectivores”. Sonochemistry became more prominent in later ICA conferences. It occupied three sessions at ICA18 in Kyoto, Japan, in April 2004. We were privileged to be there during the short but beautiful spring cherry blossom festival. I addressed the possibility of using audible sound frequencies for large-scale processing from the work which I had done with ARC Sonics in Canada (see Chapter 4) [83]. Scientists in Japan had been involved with sonochemistry from the early days with major contributions from Takashi Ando at Shiga University of Medical Science. It was Takashi who had first identified the phenomenon of sonochemical switching in 1984 [84]. Phil Lorimer and I had visited Takashi some years earlier in November 1988 after we had attended the International Kyoto Conference on New Aspects of Organic Chemistry (IKCOC-4). A Japan Society of Sonochemistry was established in 1992 and it went on to become very active in the promotion of sonochemistry.

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ICA19 was held in Madrid, 2–7 September 2007, and there were enough ICU board members amongst the delegates present that an ICU board meeting was held during the meeting. There were three sessions on sonochemistry, involving Water Treatment (Astrid Rehorek), Sonoelectrochemistry (Jose Gonzales and David Walton) and Large-Scale Reactors which was my responsibility. Five of the Coventry group attended this conference and I remember that we all got together on the eve of the conference to cheer up Tony Collings and his wife Berwin at a local bar. They had travelled from Australia, but the airline had lost their baggage (it was recovered after 2 days). Tony was involved in environmental sonochemistry and more details of his work and how we became acquainted can be found in Chapter 4. Sadly this was the last conference that we had together with Oleg Abramov who passed away in 2008 [85]. Mircea and I shared a table with Oleg at the conference banquet (Figure 1.11).

Figure 1.11: Mircea Vinatoru, Tim Mason and Oleg Abramov at ICA17 in Madrid (2007).

The next ICA meeting was in Sydney, Australia, in August 2010, which included three sonochemistry sessions. My topic was “Sonochemistry – a proven tool for Process Intensification” [86], and Larysa Paniwnyk spoke about environmental remediation [87]. The trip to Australia was made particularly pleasant by the hospitality extended to us by our host Muthupandian Ashokkumar (Ashok) who was at the conference and then came with us to his home university in Melbourne where we stayed in its guest house. 1.7.3.2 International Congress on Ultrasonics (ICU) From 1995, there were two acoustics conferences operating with similar subject matter. One was the new World Congress on Ultrasonics (see Section 1.7.2.4) and

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the other the much longer established UI (see Section 1.6.1). This was clearly not a comfortable situation with participants finding interest in both conferences within the same year. A meeting was arranged in the summer of 1999 in Denmark between Leif Bjorno from the Technical University of Denmark representing WCU and Wolfgang Sachse from Cornell University representing UI about holding a joint conference. However, the next WCU (number 4) was again separate from UI and held in Seattle where Larry Crum was in charge. Having missed conferences 2, 3 and 4, I attended the next in September 2003, held in Paris where Pascal Laugier was president. Eventually the merger was agreed and the last ever meetings of WCU and UI were jointly held in Beijing between 29 August and 1 September 2005. It was at this meeting that a formal merger of the two conferences was decided, and a new title was given to future combined meetings – the ICU. The WCU committee had previously decided to initiate a new but small Young Member Advisory Board to assist in the general organization of WCU and in helping to set up conferences. The qualification criteria were under 35 years of age and researching in a relevant discipline. I was deputed to oversee this venture and the idea of a younger advisory board continued into the new ICU organization. The Technical University of Vienna (Austria) hosted the first of the new ICU meetings between 9–12 April 2007 with Ewald Benes as President. I was on the board, but I had still not organized any young scientists to join an advisory board. Later that year, in September, a further ICU board meeting was arranged during the ICA meeting in Madrid. The next meeting was in South America in January 2009 organized by the University of Santiago de Chile under the Presidency of Luis Gaete Garretón. I travelled to Santiago with Larysa Paniwnyk. In Santiago, we met up with Jose Gonzalez Garcia and Veronica Saez from the University of Alicante, Veronica had worked with our group in Coventry for several years. We had all arranged to stay in the same hotel within reasonable walking distance of the conference. Mircea Vinatoru also joined the conference during the time that he was based in Texas (see Section 1.11.5). Luis had invited me to put together a session on sonochemistry, which consisted of six presentations and they were dedicated to the memory of Oleg Abramov who had passed away the previous year. Jose had prepared a special session on sonoelectrochemistry consisting of nine papers dedicated to Joachim Herbertz who had passed away the previous year and had organized the very first WCU meeting (see Section 1.7.2.4). In total, the Coventry group presented five talks and six posters at ICU2009. I had by this time had some success recruiting young scientists and the Board had three: Nico Declercq (Belgium), Robin Cleveland (USA) and Stefan Radel (Austria). An agreement was reached with Elsevier that the proceedings of this ICU meeting and subsequent would be published in the journal Physics Procedia. An innovation introduced with ICU was the idea of two awards to be presented to scientists for outstanding contributions to the promotion of the ultrasound in the world community. The recipients were to be the decision of the board and was in the form of a certificate and a gold or silver coloured tri-tone Samba whistle made

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by the Acme whistle company in the UK. These had a strong overtone spectrum in the ultrasonic frequency range. The whistle symbolized leadership in ultrasound and the first award winners were Sadayuki Ueha, Japan (Silver Whistle) and Leif Bjørnø, Denmark (Gold Whistle). The next ICU was held in Gdansk in 2011 with Bogumil Linde as president and I was invited to present a keynote lecture entitled “Recent Trends in Sonochemistry and Ultrasonic Processing” [88]. Four of us travelled from Coventry to be joined by Mircea and also Ken Yamamoto from Kansai University in Japan who had visited us in Coventry and would later return to work on the physical effects of cavitation on the destruction of algal cells [89] (Chapter 4).

1.8 European Union research programmes involving sonochemistry 1.8.1 European research programmes The COST organization was founded in 1971 by the European Community to promote inter-European cooperation in any field of scientific and technological research. COST is the acronym for “Cooperation in Science and Technology”, and for many years, chemistry was not a part of COST. When the Management Committee of the COST Programme met in Bruxelles on 9 December 1992, they made the important decision to include chemistry and divided the whole of chemistry into seven divisions: DI Coordination chemistry in the context of biological and environmental studies D2 Selective synthesis D3 Theory and modelling of chemical systems and processes D4 Design and preparation of new molecular systems with unconventional electrical, optical and magnetic properties D5 Chemistry at surfaces and interfaces D6 Chemical processes and reactions under extreme or non-classic conditions D7 Molecular recognition chemistry 1.8.1.1 COST D6 (1992–1998) When Jean-Louis Luche heard about this decision, he sent a FAX to me on 12 January 1993. He was quite excited about the division D6 and wrote: For the first time, a COST programme is specifically devoted to Chemistry, particularly Chemistry under unusual and extreme conditions. Under this heading, we find high pressure and high temperature chemistries, microwaves, plasma chemistry, reactions in the absence of solvent, and, of course, our common point of interest: Sonochemistry. This is the reason of this letter. During the meeting in Brussels, it appeared clearly that sonochemistry is rather advanced

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in comparison to other new chemistries. We can envisage the creation of networks involving a multilateral cooperation in various aspects of sonochemistry.

In fact, the extreme conditions referred to were included in the subject areas which had been highlighted at the EUCHEM meeting in Aussois in 1987. I supported JeanLouis, and in November 1993, the newly formed D6 held a meeting in Lausanne Switzerland at the Royal Savoy Hotel. Within this meeting, there was a “Working Group Session” on sonochemistry in the Crystal Lecture room at 9.00 am on Sunday 21 November for which I agreed to be secretary. The case was made for two separate sonochemistry groups to be included in D6 one to be devoted to “Synthesis” with designated leader Jean-Louis Luche, and the other to “Fundamentals” with designated leader Christian Petrier. In the minutes, it was suggested that the “Synthesis” group might have as themes chemistry on solid supports, phase transfer catalysis and cyclization reactions while “Fundamentals” might involve the investigation of cavitation phenomena and the measurement of sonochemical power. It was decided that there would be no members in common between the two groups and I joined the fundamentals team. There were definite overlaps of interest but joint meetings would be held to cover these. An official “Activity Report” was published on all COST Chemistry Actions in 1998, and this summarized the two Sonochemistry COST projects [90]. It is worth reproducing part of this in Table 1.2 as it gives the participants and abstracts for both sonochemistry projects that started on 1 September 1993. Some words written at the end of this report were very significant: The COST D6 programme on sonochemistry has been of great benefit. The links with other groups in Europe allowed rapid progress in fundamental research. Other European programmes have grown out of the initial COST collaboration (COPERNICUS programmes involving UK, Romania and Slovakia; INTAS programmes between UK and Russia; Human Capital and Mobility programme involving Belgium, UK and Portugal).

In 1994, a COST workshop was held, entitled “Fundamentals of Cavitation” as an extra day added to ESS4 in Blankenberge, Belgium. Jacques Reisse had been Chairman of ESS4 and was instrumental in the organization of this and several subsequent COST meetings. The principle of “Open seminars” was introduced for this meeting, meaning that enough time would be made available to discuss topics and allow unrestricted exchanges. The point was made that any idea could be challenged but not the person who formulated it, and courtesy would always be present. This was the case with the presentation of Mircea Vinatoru which he comments on later. Nevertheless, it is my strong impression from that meeting in Blankenberge that Milia Margulis suffered from a lot of criticism of his views on electrical theories of cavitation expressed in his book which had just appeared as preprints prior to its publication in 1995 [91]. I was unhappy with the treatment that he received because it is my view that a scientist should be allowed more freedom of expression in a book than in a peer

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Table 1.2: Summary of activities of the first two COST Sonochemistry Groups. Project number: D//

“Organic Sonochemistry, Mechanisms and Synthetic Applications”

Keywords

synthetic sonochemistry, formation of carbon-carbon and carbon-heteroelement bond, cycloadditions, isomerization, oxidation/reduction

Participants

J.L. Luche (Univ. Toulouse), G. Descotes (Univ. Villeurbanne), S. Toma (Univ. Bratislava), R. Miethchen (Univ. Rostock), H. Fillion (Univ. Lyon), A. Campos Neves (Univ. Coimbra), J. Jurczak (Academy of Sciences Warsaw)

Abstract

Applications of sonochemistry in organic synthesis have developed along two main lines: direct synthetic methods and mechanistic studies. Among the latter, a proposal was made to explain the effects of sonication on Diels–Alder cycloadditions, namely the existence of an ultrasound-induced redox step. The formation of radical species generated sonochemically, although not yet optimized, leads to a number of efficient reactions used for synthesis (halogenations, organofluorine and organometallic chemistries). Applications in carbohydrate and steroid chemistry have represented an important piece of work, for oxidative processes, and the transformation of sugars to fluorine containing molecules. Innovative results were obtained in a preliminary exploration, the combination of sonochemistry with photochemistry. Other applications of ultrasounds such as the effects on the extraction of drugs from medicinal plants are developed

Project number: D//

“Fundamental in sonochemistry”

Keywords

fundamental in sonochemistry, energy measurements, sonoluminescence, frequency effects, industrial scale-up

Participants

C. Petrier (Univ. de Savoie), J. Reisse (Univ. Libre de Bruxelles), T. Lepoint (Inst. Meurice Chimie Bruxelles), T. Mason (Univ. Coventry), J. Berlan (ENSIGN Toulouse), H. Delmas (ENSIGN Toulouse), G. Portenlanger (TU München).

Abstract

The objectives are concerned with identification and quantification of the phenomena leading to chemical reactions induced by ultrasonic cavitation. Main subjects – Study on the connection between physical and chemical primary processes leading to sonoluminescence and sonochemistry. – Quantification and location of the acoustical and cavitational energy in a sonochemical reactor. – Study of model chemical reactions which are affected by ultrasound. A standard procedure to calibrate sonochemical reactors was proposed. Development and design of the sonochemical equipment results of short-term scientific missions as well as materials exchange. The control of model chemical reactions in different experimental conditions in different laboratories permits to evaluate the contributions of specific sonochemical parameters (power and frequency). Design of industrial reactors and studies on potential uses of ultrasound in wastewater treatment is under development.

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reviewed paper. I had helped Milia with some parts of his book and had even supplied the design for the front cover. One of the advantages of the COST organization was that within it there were grants for so-called short-term missions. These provided travel and living expenses for scientists to visit other COST groups to discuss research. There was a problem for me because the UK was not a signatory to the D6 programme and so I did not qualify for funding. However, in July 1994, I wrote to Andre Merbach at the University of Lausanne asking if there was any way that I might be able to obtain funding for a COST visit to Jean-Louis Luche and Christian Petrier to discuss matters relating to sonochemistry. Andre was the Chairman of the Management Committee of the COST Action D6 at that time. He wrote back to say that there would be no problem if one or both of the coordinators invited me as an expert for a tour among the partners of the two working groups. This was organized and I visited Jean-Louis in Toulouse (Universite Paul Sabatier) and Christian in Chambery (Universite de Savoie). I made the journey in January 1995 to discuss COST matters and two other key issues. One was the request from Vittorio Ragaini for the inclusion of sonochemically assisted catalysis in D6 and the other about the locations for future COST meetings. Vittorio wanted catalysis to be a separate action to join fundamentals and synthesis, but this proved difficult. He had assembled a group of laboratories and the case itself was quite strong, but the COST organization did not like the idea of a separate group. In the end it was decided that catalysis could be introduced as a subgroup of “Fundamentals”. As for future meetings, the next was already in place as a full COST D6 Workshop that was to be held in April 1995 at the Dorint Hotel RheinLahn in Lahnstein near Koblenz. This involved all of the chemistry disciplines and attracted 88 participants. Within sonochemistry we were very keen to attach D6 meetings focused on ultrasound to other major conferences just as we had done at ESS4 in Blankenberge. Our next target was the inaugural World Congress in Ultrasound (WCU) meeting later that year (September) to be held in Berlin. Over the following years, there were two full COST workshops covering all of the D6 disciplines and therefore involving sonochemistry. The next was in Chambery in 1996 at the at the University de Savoie. In that year, both Jean-Louis Luche and Christian Petrier were based there. It was timed to run over a weekend between 29 November and 1 December. On the last day, Sunday, the progress of the two networks “Fundamentals” and “Synthesis” were reviewed, and I was asked to chair a discussion on what future networks might be possible when the COST D6 action finished. At that stage, I suggested that three areas should be targeted, and my suggestions were: sonoelectrochemistry, environmental sonochemistry and the conversion of natural resources. Interconnecting these three themes would be the key subject of dosimetry. The final workshop of D6 was on the Greek Island of Santorini which was organized by Jacques Reisse who was President of that meeting. He praised the efforts made in D6 and introduced the new D10 action which was a follow-on and would also contain sonochemistry.

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In 1997, an important book was produced from the D6 programme entitled “Chemistry Under Extreme or Non-Classical Conditions” [92]. The contributions included chapters written by participants in D6 involved in studies of chemistry at high temperature, pressure, in supercritical fluids, and using plasma and microwaves. Originally, there were to be three chapters related to sonochemistry: Organic Synthesis by JeanLouis and myself, Polymer Chemistry by Gareth Price and Heterogeneous Reactions by Phil Boudjouk but Phil had to withdraw and so in 1995 following a request from Rudi van Eldik, one of the editors, I volunteered to incorporate the heterogeneous chemistry section into organic synthesis. Thus, there were only two chapters related to sonochemistry and the one that I co-authored with Jean-Louis Luche was expanded and retitled as “Ultrasound as a new Tool for Synthetic Chemists”. 1.8.1.2 COST D10 (1998–2004) When COST D6 finished, the new action D10 started in 1998 entitled “Innovative Methods and Techniques for Chemical Transformations”. It was D10 that would provide support for the project, Sonochemical Application to Food Additives, Flavours, Fragrances and Pharmaceuticals Extraction from Renewable Natural Resources, (SAFE) with Mircea Vinatoru as coordinating manager (see Section 1.11.1). Within this overall programme, three working groups involving sonochemistry were submitted to the D10 Committee. The main difference between D10 and the old D6 was that international experts not based in Europe could now become involved in the teams. Since I was not named in any of the groups I was asked to act as an evaluator. The decision was that all three would be accepted: – Jacques Reisse proposed “Acoustic Cavitation and Its Chemical Effects” in February 1998. This had similarities to a very successful conference which he, Larry Crum, Ken Suslick and I had put together as a NATO Advanced Study Institute in August 1997 at the Sleeping Lady Conference Centre, Washington State, USA. It comprised seven members: J. Reisse (Belgium), J. Gallego-Juarez (Spain), J. L. Migeot (Belgium), G. Price (United Kingdom), C. von Sonntag (Germany), F. Grieser (Australia) and K. Suslick (USA). The first workshop for this group was at the Club de la Fondation Universitaire, Brussels, 11–13 March 1999. – Jean-Louis Luche proposed “Towards Environmentally Benign Chemical Processes using Sonochemistry”. His network had eight members Takashi Ando (Japan), Werner Bonrath (Switzerland), Pedro Cintas (Spain), J. L. Luche (France), Dietmar Peters (Germany), Yves Queneau (France), Luisa Sa e Melo (Portugal) and Stefan Toma (Slovakia). The first meeting of this network was organized in Chambéry on 28–29 May 1999. Three recognized research experts in green chemistry were invited to speak D. Brunel (Univ. Montpellier, France), J. Clark (Univ. York, UK) and R. Sheldon (T.U. Delft, Netherlands). During this short conference, a basic question “is there a common future in sonochemistry and green chemistry?” was examined together with discussion of the results obtained in the laboratories involved.

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David Walton, my colleague from Coventry University, proposed “Sonoelectrochemical Processes”. There were five participants: D. J. Walton (UK), A. Aldaz (Spain), R. G. Compton (UK), J.-L. Delplancke (Belgium) and A. Oliveira Brett (Portugal), and their first workshop was held in Alicante between 17 and 22 February 1999.

1.8.1.3 COST D32 (2004–2009) The last of the COST chemistry programmes involving sonochemistry was D32 “Chemistry in High-Energy Microenvironments” which began in 2004. As its name suggests, it had very similar objectives to those that preceded it but with a more restricted range of disciplines. Here sonochemistry and microwave-enhanced chemistry were involved either singly or in combination with each other or with electrochemistry and/or photochemistry. The chair was David Walton and the vice-chair was Bernd Ondruschka (Friedrich Schiller University Jena). The total number of working groups increased to 10 and by 2006 involved 83 Laboratories from 21 European Countries plus 6 Laboratories outside of the EU from the USA, Australia and Japan. The groups were: – High-Energy Micro-Environments in Biotechnology Coordinator: Andreas Tiehm, Water Technology Center, Karlsruhe, Germany – Ultrasonic and Microwave-Assisted Synthesis of Nanometric Particles Coordinator: Cristina Leonelli, University of Modena, Italy – Electrochemistry with Ultrasound Coordinator: Jose Gonzalez-Garcia, University of Alicante, Spain – Microwaves and Ultrasound Activation in Chemical Analysis Coordinator: Antonio Canals, University of Alicante, Spain – Microwave and High-Intensity Ultrasound in the Synthesis of Fine Chemicals Coordinator: Giancarlo Cravotto, University of Turin, Italy – Cavitation and Environmental Remediation (CAVEMEN) Coordinator: David Bremner, University of Abertay, Dundee, UK – Development and Design of Reactors for Microwave-Assisted Chemistry in Lab and Pilot Scale Coordinator: Matthias Nüchter, Friedrich Schiller University, Jena, Germany – Fundamentals in Cavitation, Sonochemistry and Sonoluminescence Coordinator: Thierry Lepoint, Institute Meurice, Brussels, Belgium – Diversity-Oriented Synthesis Under (Highly Efficient) Microwave Conditions Coordinator: Dariusz Bogdal, Politechnika Krakow, Poland – High-Energy Microenvironments in Textiles Coordinator: Artur Cavaco-Paulo, University of Minho, Portugal

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A number of workshops and meetings were held involving these groups in Alicante, Spain, in July 2004; Minho, Portugal, in November 2004; Oxford, UK, in June 2005; and Dundee, Scotland, in May 2007.

1.8.2 European research projects to support Eastern Europe and the Soviet Union (1997–2001) Several collaborative research grants were obtained in the late 1990s involving Coventry were linked with scientists in the Eastern Europe. At the outset we did not know many of the scientists except from their publications, but they served to initiate and/or consolidate what were to become long-lasting relationships. Grants from the EU that were designed specifically to foster such research collaborations were unusual in that they required a coordinator from Western Europe who would receive the funding from Brussels and disperse it to the Eastern European participants. However, the Scientific Leader would always be from the Eastern Europe research groups. Competition for such funds was intense, and the first that was received in Coventry was via the INTAS programme (International Association for the Promotion of Cooperation with Scientists from the former Soviet Union) and ran for 30 months between 1998 and 2000. It was entitled “Cavitation-induced sonoelectrochemical and sonoluminescent processes in aqueous solutions” (INTAS-96-1051). The scientific coordination and leadership was from Belarus and involved Nikolai Dezhkunov, who was particularly interested in theoretical aspect of cavitation. The project dealt with the investigation of the mechanisms of the impact of ultrasound on the fundamental parameters of electrochemical processes (charge-transfer kinetics, parameters of double layer, etc.). Sonoluminescence measurements provided the basis for the development of novel electrochemical sensors for measuring cavitation activity. Two other grants were won in that same time period and both involved Oleg Abramov as scientific coordinator. We did not know Oleg at that time, only by reputation as a distinguished scientist from Moscow. Both grants involved industrial applications of power ultrasound. The first ran from 1997 to 1999 “Study of ultrasonic mass transferring processes and development of design principles of scale up equipment and technology” (INTAS-96-0986). It was aimed at the design of ultrasonic reactors for the enhancement of mass transfer processes such as disintegration, homogenization, extraction, emulsification, sorption, filtration and distillation. The second came through a new scheme INCO-COPERNICUS, a programme of scientific cooperation with the countries of Central Europe (CCE) and with the New Independent States of the former Soviet Union. With the acronym ULTRAWAT, it ran from 1999 to 2001 “Development of new generation of ultrasonic equipment and processes of physical and chemical action on water treatment” (IC 15-CT98-0110). The project covered a range of topics, including ozone treatment for the degradation of

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different organic substances with an emphasis on equipment design. It was the COPERNICUS programme which also funded the ultrasonic extraction of natural products from herbs research programme coordinated by Mircea Vinatoru in 1994 which gave rise to the COST collaboration on extraction in 1999 (see 1.11.1).

1.8.3 European Framework Programmes (2005–2013) Amongst subsequent European grants in sonochemistry were two which centred upon the use of ultrasound associated with nanoparticles. This resulted in a considerable broadening in the interests of the Coventry group and both involved collaboration with the Aharon Gedanken group at Bar Ilan University in Tel Aviv, Israel. The first was awarded in 2005 under the FP6-NMP framework aimed at nanotechnologies and new production processes. The project was entitled “The development of multifunctional nanometallic particles using a new process – sonoelectrochemistry” (SELECTNANO) and ran for 3 years until 2008. The original idea for the synthesis of nanometallic powders using a sequence of electrochemistry to deposit and ultrasound to remove the deposited metal had been suggested by Jacques Reisse in 1995 [93]. This had been developed further and the aim of the project was to scale up the process for industrial preparation of metal nanoparticles. With a healthy overall budget of €3.3 million, it fitted neatly into our interests in sonoelectrochemisry and extended our developing work on nanoparticles (Volume 2, Chapter 5). It also brought Bruno Pollet back into the Sonochemistry Centre as a European research fellow attached to the programme. Bruno had completed his PhD within the group in 1998 and his return in 2005 as research fellow was soon upgraded to project development manager. The year after the SELECTNANO programme finished, another successful bid was made under the nanoparticle call in the next EU framework FP7. Under the title “A pilot line of antibacterial and antifungal medical textiles based on a sonochemical process” (SONO) it ran from 2009 until 2013. The project was based on some earlier work using ultrasonic equipment from Oleg Abramov in Russia which was used in Gedanken’s laboratory in Tel Aviv [94]. It was aimed at preparing fabrics containing embedded nanoparticles of metal oxides, for example copper or zinc, which would be antimicrobial and suitable for the manufacture of bandages, uniforms, sheets and curtains that would combat hospital acquired infections. With an overall budget of €12 million it involved 14 different laboratories (Volume 2, Chapter 9). We were fortunate to find Jamie Beddow, a microbiologist, within the Faculty of Health and Life Sciences in Coventry who was ideally qualified to become research fellow. This was also the project which brought Mircea Vinatoru to Coventry to work with us.

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1.9 Origins of the journal Ultrasonics Sonochemistry After the RSC conference in 1986 from which the first special issue of Ultrasonics devoted to sonochemistry had been published [59], I continued the links with Butterworths with three contributions from the group for the next UI conference in 1987, held at Kings College London. This was followed by our major participation in UI89 in Madrid with eight contributions and the organisation of the first special session on sonochemistry (Figure 1.6). In September 1990, I was invited by Ann Horscroft from Butterworths to join the International Advisory Board of Ultrasonics which I accepted and began in January 1991. In May 1991, I received an agreement to proceed with a special section in the journal that would be allocated to sonochemistry. Ultrasonics was still owned by Butterworths based in Guildford, Surrey, at that time but changed ownership to Butterworth Heinemann who opened new offices based in Linacre House, Oxford, in June 1991. This was the trigger to begin serious discussions with the publishers about the launch of a sister journal to Ultrasonics which would be dedicated to sonochemistry with a provisional title of Ultrasonics Sonochemistry. Strong justification for this came from the publication of more individual papers on sonochemistry in Ultrasonics and two further special editions derived from conferences in 1990 (covering the sonochemistry section of Pacifichem 89, held in Hawaii [79]) and in 1992 (covering the second meeting of the ESS in Gargano, Italy [95]). In January 1993, Lynn Clayton who was Group Editor of Ultrasonics wrote to me about producing a sister journal dedicated to sonochemistry, and questionnaires were sent canvassing opinions as to the chances of such a dedicated journal succeeding. Very soon after this in 1994, Ultrasonics Sonochemistry was launched with an introduction from the European, American and Asian editors who were myself, Ken Suslick and Takashi Ando, respectively [96]. It concluded with the words: We now know that the chemical effects of ultrasound are diverse and can include substantial improvements in both stoichiometric and catalytic reactions. In some cases, ultrasonic irradiation has increased reactivities by nearly a million-fold. Sonochemistry is finding widespread applications in diverse areas, for both laboratory and industrial scale applications. It is appropriate that sonochemistry have its own central publication, and the Editors and Editorial Board of Ultrasonics Sonochemistry wish to acknowledge the farsightedness of Butterworth-Heinemann for making this project possible.

The content of that first issue was devoted to papers from the Third Meeting of the European Society of Sonochemistry, Figueira da Foz, Portugal [97]. In the history of the development of sonochemistry, the launch of Ultrasonics Sonochemistry was a significant milestone and both of the authors of this book contributed, independently, papers included in the very first issue [70, 98, 99]. In 1996,

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Butterworth Heinemann were taken over by Elsevier who became the publishers of both Ultrasonics and Ultrasonics Sonochemistry. I met most of the Ultrasonics subject editors and advisory board members at a dinner held at UI’97 in Delft, and everything seemed to be proceeding smoothly with both journals. However, in February 1998, I received a letter from Dolors Alsina one of the editors responsible for Ultrasonics who had some bad news. She said that while in 1997, four issues of Ultrasonics Sonochemistry were published unfortunately the current manuscript situation was less promising and not enough material had been received for Volume 5, Issue 1. She was also worried that it was proving difficult to find an enthusiastic American editor willing to work for the journal. There was also a problem with the flow of papers to the original journal Ultrasonics. She made a proposal that there should be a re-amalgamation of Ultrasonics Sonochemistry with Ultrasonics. Naturally I and others put up some arguments against this and fought for the preservation of the journal and because of our resistance to the merger the title Ultrasonics Sonochemistry continued, submissions began to increase and the Impact Factor rose and exceeded that of Ultrasonics. For me it was a real pleasure to serve as an editor of the journal from its foundation in 1994 for 30 more years. I recall how in the early days there was no online submission and no pro forma letters. I had to build up a list of colleagues willing to referee papers and it was all done from my home with the help of my wife who was very good at organizing the letters, chasers and so on. It was very time-consuming work, but I continued through its ups and downs and finally finished in the post of editor-in-chief in 2015 [100]. Now, as I finish writing this book, the journal is extremely successful and one of the world’s leading journals on acoustics.

1.10 The Sonochemistry Centre at Coventry University In its original form, sonochemistry research at Coventry Polytechnic had started with Phil Lorimer and I. We slowly gathered more researchers into our team which was based within the Department of Chemistry and Metallurgy. In 1992, the UK government decided to convert all polytechnics to universities, and as a consequence, the internal structures were changed and our group became part of the newly formed “school” (or faculty) of Natural and Environmental Sciences. Our interests expanded into other fields besides chemistry, by 1993 the sonochemistry group had 20 staff members (excluding research students and assistants). Within this, in addition to chemists were three engineers, three biologists and two physicists. In order to improve its research profile, Coventry University decided to set up a number of “Centres of Excellence”. These were to become the foci of research within the institution and receive special treatment in terms of funding. So it was that Phil Lorimer and I began putting together a case for a Centre of Excellence in

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Sonochemistry. It was a complex task and it was necessary to obtain support from external organizations. We approached 14 potential supporters and were rewarded with positive responses from each. Two of these that carried particular weight were from the National Physical Laboratory (NPL) and the government Department of Trade and Industry (DTI). Statements from these two emphasized the need for a centre of excellence in sonochemistry at Coventry University. NPL wrote “Coventry University could act as a unique focus for joint projects involving industry, higher education and government agencies”. The DTI made several points “there is a need for more information and advice on the potential uses of power ultrasound . . . . A centre of excellence in the UK would act as a focus . . . pleased if Coventry were to set up such a centre.” Thus, it was that in September 1993 Centre of Excellence status was granted, and within it were five main sections: materials including polymers and metallurgy, chemistry including synthesis and electrochemistry, environment protection, food technology and process development. Shortly afterwards, therapeutic ultrasound was added. When the Centre was first set up within the School of Natural and Environmental Sciences, the school subjects included chemistry, physics, metallurgy, biology and geography. As the years went by, the university began to look at student recruitment and the cost of running science degrees. What had been a very sciencebased faculty in the Polytechnic days changed as the university removed in turn the departments of metallurgy (temporarily replaced by materials), then physics and materials, and finally in 2005 it axed its course in chemistry. I had worked with Phil for many years until he passed away in June 2009 but we were a great team and I firmly believe that together we made more progress in the field of sonochemistry than we could ever have made working separately (Figure 1.12).

Figure 1.12: Phil Lorimer and Tim Mason in 1998.

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A restructuring of Coventry University occurred in 2015 when a new Faculty of Health and Life Sciences was formed by incorporating the science that was left in the old Faculty of Natural and Environmental Sciences into the larger Faculty of Health. Naturally, the management of the health faculty was unfamiliar with chemical sciences but there was a more obvious link with biological sciences. As a direct result, sonochemistry became less well appreciated within the newly formed faculty and the centre was closed shortly afterwards.

1.10.1 Sonochemistry in Coventry University after closure of the Sonochemistry Centre in 2015 In 2016, Mircea left the employment of Coventry University but remained in the UK continuing to live in his house in Kenilworth for a further 18 months during which time we extended our research interests (see later). In that same year, I became an emeritus professor at the university. I then joined the Centre for Research in Built and Natural Environment. This was because of my previous association some 20 years earlier with what was then known as the Faculty of Civil Engineering. In the early days, it had been Phil Lorimer who worked closely with them through a group led by Peter Claisse on various aspects of cement and concrete technology [101]. I had been involved with some collaborative work on chloride transport in concrete with Peter Claisse and a student Tom Beresford (who received his MPhil in 2000). Later, I worked with Eshmaiel Ganjian on the uses of finely powdered (pozzolanic) materials (e.g. waste gypsum and silica fume) in cement mortars to improve durability. The last of the civil engineering research students for whom I was on the supervisory team was Ahmad Ehsani from Iran. His thesis concerned the effects of power ultrasound on Portland cement pastes and mortars. From this work, we had produced the first review paper on the effects of direct application of power ultrasound on Portland cement pastes [102]. The paper outlined the possible mechanisms involved in the uses of sonication to promote the hydration kinetics of Portland cement. Unfortunately, Coventry University decided to close that centre at the end of 2020 but we managed to complete a further publication on the influence of ultrasound on the setting of cement [103]. The paper reported observations on the effects of several frequencies and power densities on two Portland cement pastes at water-to-cement ratios of 0.50 and 0.80. These were the first reported investigations of the ways in which ultrasound could affect the setting and subsequent ageing of cement by disturbing the early formation of calcium sulphoaluminate (ettringite) which can weaken concretes but it promotes the formation of the beneficial amorphous aluminium hydroxide hydrate by releasing aluminium ions into the pore solution. Overall, it was concluded that ultrasound could provide a promising technique for improving the characteristics of cements.

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In 2018, I was invited to join as honorary member, an EU-funded project “Competitiveness Operational Programme (POC 2014–2020)”, with a group in Romania, from the Faculty of Applied Chemistry and Material Science, Politehnica University of Bucharest, where Mircea Vinatoru was the project director. The project was entitled “Ultrasonic/Microwave Nonconventional Techniques as new tools for nonchemical and chemical processes” (ULTRA – MINT Technologies). The group was involved in microwave and sonochemistry and more details of this are given later in this chapter (1.11.7) and elsewhere in this book.

1.11 Sonochemistry some personal reflections from Mircea Vinatoru In late 1989, I was working as a synthetic organic chemist in the “C. D. Nenitzescu” Institute of Organic Chemistry, Bucharest, Romania. I was involved in a project dealing with industrial research aimed at producing glycerol from propene. At that time, glycerol was an important product for Romania (it had pharmaceutical uses and was also employed as a testing fluid for pipes in nuclear plants and other nuclear devices working under pressure). My research was at the stage of implementation on an industrial scale (the design of a production factory had been started). It was at that time when my interest was drawn to a new topic in chemistry involving ultrasound and I wondered if it might help in some of the chemical processes for glycerol production. I was looking for ways to enhance syntheses in addition to using photochemistry, catalysis or other methods to achieve more selective reactions and process intensification. Sonochemistry was at that time in its infancy, with most of the papers linked to ways of using ultrasound in organic synthesis or to attempted explanations of mechanisms [58, 84, 104, 105]. In my institute in Romania, I did not have any ultrasonic equipment and I did not know anyone else in the country studying this subject. This was the situation that I found myself in the end of 1989, a year which found me responsible, as principal researcher, for such a big industrial project (over 1 billion Lei, Romanian currency at that time). In some ways, it was fortunate for my own future that this project stopped after the political regime changed. I had to think about my future research plans. So, it was in early 1990, when I decided that it would be a good time to turn my full attention to sonochemistry and I began a deeper study this topic. It took me almost 1 year of reading and trying to understand papers dealing with the use of ultrasound to influence chemical synthesis. It was a tough time for me; I was not able to link cavitation with chemical transformations and I asked myself why a mechanical energy like this should interfere with chemistry. Trying to get more knowledge in the field, I found several names of scientists who at that time were prolific in publishing papers related to the use of ultrasound in chemistry. I did not know

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any of them personally but nevertheless I started to write letters in the hope that they would reply and maybe send me some reprints of their work. Among the names I was attempting to contact for help was Tim Mason, to whom I wrote on 4 July 1990 opening with a simple statement “Quite by chance I came across your two part review on sonochemistry which appeared Chem. Soc. Rev. Nr 16 in 1987 and in this way your sonochemistry group is the first one I became acquainted with”. I was very pleased to receive a reply dated 16 July beginning with the very positive response “I would like to help you in any way that I can to promote a research programme in sonochemistry at your university” and enclosing copies of his publications. He also expressed an interest in visiting my institute. It was in May of the following year (1991) that Tim decided to pay a visit to Bucharest (this was a brave decision by him because Romania had still not fully settled after its 1989 revolution). He arrived with a priceless gift for me and my group, an ultrasonic cleaning bath. The bath was a donation from a company called Langford Ultrasonics that he knew well and was located near Coventry. To us, this equipment was a sort of Holy Grail of sonochemistry; finally, I could start doing some experimental work using this bath as a source of ultrasonic energy. Tim gave a lecture to our scientists from the C. D. Nenitzecu Institute of Organic Chemistry with others from the Faculty of Chemical Engineering of Polytechnic University of Bucharest. The topic raised a lot of interest and many discussions followed. I showed him around Bucharest, but in those days, it was a city in recovery from former times. Nevertheless, we had time for some relaxation and the weather was good for the visit (Figure 1.13).

Figure 1.13: Mircea Vinatoru and Tim Mason at a restaurant in Snagov city, near Bucharest (May 1991).

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After the visit, I started to work on real practical sonochemistry but only as a novice using my new (and only) ultrasonic device. In common with many other chemists at the beginning of my research, I believed that using an ultrasonic device would be very similar to using a heater or a stirrer. At that time, I was interested in performing research on the reaction of triphenylmethyl methane and triphenylmethyl chloride with nitrobenzene as an example of an electron transfer reaction. Jean-Louis Luche who was a pioneer in synthetic sonochemistry had identified electron transfer reactions as types that would be greatly influenced by ultrasound. I switched from conventional heating to sonication (Volume 2, Chapter 5). Soon after, we got results that allowed our group to have the first-ever paper published from my institute involving sonochemistry, which happened to be in the very first issue of the new journal Ultrasonics Sonochemistry [98]. In September 1991, a few months after the visit of Tim to Bucharest, I had the chance to attend a Residential School in Coventry related to sonochemistry, strengthening the connection with Tim. I coupled this trip with a visit to Jean-Louis Luche who was teaching at that meeting. He was working in Grenoble at that time and we travelled from there in his car to attend the second ESS meeting in Gargnano, Italy. It was a long trip of around 300 miles from Grenoble to the conference venue so there was plenty of time for conversation. In the conference photo (Figure 1.8), I am sitting (six from the left) next to Jacques Berlan (seven from left) and next to him is Vittorio Ragaini the conference organizer. Tim is sitting on the extreme right. I think that perhaps this was the launch pad for me to fully enter into the science of sonochemistry. After the ESS meeting, Vittorio Ragaini, along with Jacques Berlan and Jean-Louis Luche became my new sonochemical friends. However, it was Vittorio who pushed me to write my first paper on ultrasonically assisted extraction of plant materials which was published in an Italian journal (Tecnologie Chimiche), in the Italian language. This was an important milestone for my group and for our COPERNICUS project (see 1.11.1), because it was the very first that we published on UAE, which was to become one of my major research interests [106]. In my institute, only my group was able to do any work using the already “famous” ultrasonic cleaning bath. No one else had such a tool. I received many requests to use it to check if their reaction or process under study could be influenced by ultrasound, but I gently declined these requests. Everybody wanted to take advantage of this new technique which had now become available in a Romanian laboratory. We were able to expand our cooperation with other groups after we had been funded by the COPERNICUS project on extraction and acquired new ultrasonic equipment (see Section 1.11.1). When my group became more experienced in sonochemistry, one of my older colleagues (Ilie Dinulescu, no longer among us) was very sceptical about using ultrasound in organic synthesis. He asked me if I could give him at least one example of a chemical reaction which would benefit from using ultrasound. By chance I

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needed to prepare a Grignard reagent and so I suggested that this was one example. Grignard reagents are normally prepared from metallic magnesium turnings and a halogenated carbon compound in purified dry diethyl ether and I told him that he could watch me prepare a Grignard reagent using simply laboratory-grade diethyl ether (from the bottle and not purified). He was very doubtful that this would work but we did the experiment together and he was amazed that the reaction went so well. So, he decided to make an experiment by himself, using one of the ultrasonic cleaning baths that my group then had, but he forgot to use ordinary unpurified diethyl ether straight from the bottle and instead used the purified type, prepared for conventional Grignard synthesis. Under ultrasound, the reaction was very violent and the whole mixture hit the laboratory ceiling. Since then, he became one of the greatest advocates in my institute for this new science of sonochemistry. The sonochemistry topic took root in my institute, and chemical engineering students preparing their graduation theses began to study it. In Romania, these students have a two-part graduation thesis, one involves testing their chemical engineering skills but the other concerns research capabilities on a specific topic (usually organic synthesis). Several of my students graduated with research projects related to sonochemistry which was a rather exotic topic in those days. The general activity in the field of sonochemistry involving both our own experiments and making some tests for colleagues in other labs raised questions about how ultrasound could possibly interact with chemical reagents. It was clear that the wavelengths used for sonochemistry were far away from conditions that might directly influence molecular bonds. It was a subject that I would return to again later in my career.

1.11.1 EU COPERNICUS and COST D10 programmes involving ultrasonic extraction from renewable natural resources A group from a neighbouring research institute (ICECHIM), run by Sanda Velea, asked me to try to do some experiments using ultrasound to enhance the extraction of active principles from vegetable materials. This was how I began my investigations into the Ultrasonically Assisted Extraction of bioactive principles from plants – which was a very new topic for me. It was a logical progression from this point to an application for a European project within the COPERNICUS framework. A consortium, including Tim Mason’s group, my group and two groups from Slovakia, was assembled for this project, the bid was submitted at the end of the summer in 1994. The bid was successful, and so, COPERNICUS ERB-CIPA-CT94-0227-1995 entitled “The development of methodologies for the ultrasonically assisted extraction of biologically active components from plants and seeds” was selected to be funded. When I received the news that the COPERNICUS project was approved for funds, I happened to be spending 1 month (October 1994) in the laboratories of Jacques Berlan from ENSIGC in Toulouse. I was there under the terms of a French CNRS grant for the interchange of scientists with the

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Romanian Academy of Science. It was Jacques who brought me the news. I was so happy that I jumped up and started to dance, that surprised him because he was not expecting such an explosion of joy. It was a typical reaction for Romanians receiving great news but not something that would be expected from the French. In the same year, Jacques Berlan asked me if I would be willing to join his group to build up a new research centre in which ultrasound and microwaves were to be the main sources of activation of reactions and processes. To consolidate this idea, and also to help me become accustomed to the French style of teaching, he asked me to give a lecture to his first-year students on the so-called bizutage or initiation course. He proposed the name of the lecture as “The ordering effect in chaos”; this was an amusing reference to my COST D6 lecture given in Blankerberge: “Ordering effects in sonochemistry”. I presented it as a humorous lecture and it caught the attention and interest of the students. I had given the lecture in English, my French being not so good at that time, but they asked me if I was really a Romanian and if I was staying there until they needed to pass their exams. I showed them my passport to prove my nationality and this satisfied them for a few hours until Jacques told them that “The ordering effect in chaos” was not a serious lecture and then they became rather unhappy. Unfortunately, Jacques died in the following year, and so his plan to establish the ultrasound and microwave centre was never followed through. However, many years later (in 2014), this dream, the use of combined ultrasound and microwaves as energy sources to promote chemical processes, came true through the ULTRA-MINT project (see Section 1.11.7). In that year spent in the labs of Jacques Berlan (1994), I had the opportunity to link up with a French company who were working with him on the extraction of chemicals from herbs. They had already built a reactor incorporating ultrasound but unfortunately this collaboration did not work out. It was probably because the COPERNICUS project raised a concern for the company that there might be a conflict of interest. I never really discovered how their reactor was designed and built but I retained a great interest in UAE reactors as a way to attract the interest of industrialists in this new emerging technology. Thus, we (my group) started the design of new type of ultrasonic reactor devoted to large-scale herbal extracts. We were successful and built it in Romania at the Plafar factory in Brasov as a stand-alone device. We went on to apply for a Romanian patent for this ultrasonic reactor dedicated to herbal extraction in 1998 (Figure 1.14) [107]. The topic of UAE will be dealt with in more detail in Chapter 3. Several of Jacques’ PhD students were involved in experiments related to the influence of ultrasound on chemical reactions, among them was Farid Chemat, now professor at the university of Avignon where he has established a group who have embraced the field of natural products extraction via ultrasound and microwaves. Farid was one of the students who appreciated my jokes; they were unusual types of jokes for a Frenchman to appreciate because they were based on Romanian humour. He told me that I was a “blagueur” which when translated into English means joker.

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Figure 1.14: Ultrasonic reactor for herbal extracts which received a Romanian patent in 1998 [107].

The COPERNICUS project, which had been awarded in 1994 and started in 1995, turned out to be one of the most successful programmes in my group as it involved not only research publications but also visits to European Countries and generated many new international research contacts [108]. This project also attracted the attention to some investors who were interested in this new way to obtain active principles from medicinal herbs. Soon after it concluded, I successfully applied for and coordinated a new European project under the COST programme “Sonochemical Assistance to Food additives, flavors, fragrances and pharmaceuticals Extraction from renewable natural resources” acronym SAFE (COST D10/0016/1999). More details about both of these programmes are to be found in Chapter 3. In February 1999, Tim secured a 4-week Royal Society grant under the programme “Short term visit to the UK” which was worth £1680 to allow me to visit him in the UK. At that time, the rules stipulated that the money must be sent to the host and could then be used (by Tim) to support my stay (daily allowance, accommodation, trips within UK, etc.). This allowed us to develop our ideas on future cooperation in sonochemistry. The accommodation provided was in the academic visitor’s part of a

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student hall of residence at Coventry University. The room was on the 14th floor of that building and I remember very well the wind that whistled around outside which was loud enough to wake me up during the night.

1.11.2 The “ordering effects” mechanism in sonochemistry In October 1994 when the COPERNICUS project was approved, both Jacques and Jean-Louis were more involved in organic chemistry than extraction. I remember many discussions with them about the mechanisms underlying sonochemistry because I had the opinion that it could not be explained only by the results of cavitation bubble collapse. Why for example were Diels–Alder reactions when they are thermally allowed not influenced by ultrasound since it was known that a collapsing cavitation bubble develops high temperatures which should promote Diels–Alder reactions? I had an idea that ultrasound itself (without cavitation) could produce chemical effects through an ordering of molecules in the fluid to produce solid-like structures (Figure 1.15). This idea continued to be the subject of long discussions with Jacques Berlan and Jean-Louis Luche. – – – – – – + + + + + + – – – – – – – ∆U

+ + + + + + + – – – – – – + + + + + +

Sonic wave direction

Figure 1.15: Charged solid-like structure induced by ultrasound.

It was not long after the COPERNICUS project had been approved that I was invited to present these ideas to a COST D6 meeting which was held in 1994 at the end of ESS4, in Blankenberge, Belgium (Figure 1.16). In my lecture “Ordering effects in sonochemistry”, I proposed that the compression phase could induce order in liquids and this might allow charges to be developed on the solid-like structure (Figure 1.15). This is similar to the structures developed in piezoelectric materials and would have a limited lifetime τ, as a function F related to ultrasonic frequency (f), reaction temperature (T ), external pressure (P), molecule polarizability (α) and a solvent rearrangement factor (the availability of the liquid to return to initial state) (λ):

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τ = F ð f , T, P, α, λÞ These new ideas that the audience had not anticipated generated very vivid discussions at the meeting and in my estimation some 50% of auditorium were with me and 50% being against what I presented. This idea disappeared from view for a while but was not lost. Tim Mason referred back to my ideas at a talk he gave at ESS11 in 2008. The meeting was held at “La Grande Motte” (near Montpellier) in Southern France with Jean-Yves Hihn as the Chairman. Tim wanted to bring my ideas back for discussion and he asked me what he should include. I suggested two things: the induced rotation of polarized light by very symmetric molecules (such as benzene) when subjected to sonication and also the idea of a solid-like structure that might become electrically charged. The actual slide used in his presentation is shown in Figure 1.17.

Figure 1.16: ESS4 at Blankenberge in Belgium.

In 2019, Ultrasonics Sonochemistry celebrated its 25th year of existence with the publication of Volume 52. Tim and I decided to submit our paper on ordering effects to this special anniversary edition. It became the first “opinion paper” - a new class of submissions for the journal. In his editorial, Muthupandian Ashokkumar, wrote that Tim Mason had suggested the introduction of this new category of papers [109]:

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Mircea Vinatoru – some unusual observations polariser

polariser

Ultrasound will cause benzene to rotate the plane of polarised light

benzene

transducer

∆U

Ultrasound could induce order in liquids

Sonic wave direction The Sonochemistry Centre, Coventry University

“The Home of Sound Science”

Figure 1.17: Slide presented by Tim Mason at ESS11 France, 2008, regarding the “ordering effect”. Recently, I received a proposal from Professor Timothy J. Mason, one of the Founding Editors of Ultrasonics Sonochemistry, who played a key role in the establishment and development of the journal. He suggested the introduction of a new category of articles, “Opinion Papers”, and volunteered to contribute the first paper in this category. In his correspondence, Professor Mason mentioned, Quote: “it is perhaps time to reflect a little on some of the issues within sonochemistry which are as yet not completely resolved. I have always been of the opinion that ideas on such issues even if they are unorthodox should not be strangled at birth but rather put out into the scientific community for considered comment. With this in mind it seems like an excellent time to publish occasional papers which are aimed at engendering considered thought and discussion amongst our readers”. Following some discussion with Journal Management and Executive Editors, we have decided to introduce this category of papers as part of our 25th Anniversary celebration. Such Opinion Papers would undergo the normal screening process, in particular they will be reviewed by Executive Editors and other leaders who have strong expertise in areas relevant to the submitted articles. The current issue of Ultrasonics Sonochemistry features the first paper published in this category: Can Sonochemistry take place in the absence of cavitation? – A complementary view of how ultrasound can interact with materials, by Dr. Mircea Vinatoru and Prof. Timothy J. Mason. It is perhaps fitting that both authors published papers in the very first issue of our journal.

This paper was written during the years when I was working with Tim in his group in Coventry [19]. In 2019, I had a further opportunity to reopen the topic of ordering effect and sonochemistry in the absence of cavitation as a topic during the 4th Meeting of the Asia-Oceania Society of Sonochemistry (AOSS4) in Nanjing, China, 19–21 September. It was entitled: “Exploring Alternative Mechanisms and Pathways in Sonochemistry”, and in this presentation, the ordering effect along with other developments in sonochemistry was emphasized as a complementary and more relevant explanation of some of the interactions of ultrasound with materials [110].

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1.11.3 Work in Japan Sonochemistry allowed me to travel to many countries, among them was Japan. In 1998, through a Japan Society for Promotion of Science (JSPS) grant I visited Takashi Ando’s group at Shiga University of Medical Science. He was a pioneer of sonochemistry in Japan and had hosted Tim Mason and Phil Lorimer 10 years previously and gone on to help to establish the Japanese Sonochemistry Society in 1992. My first visit to Japan produced several new ideas for the use of ultrasound in areas other than synthesis including the activation of manganese dioxide catalysts [111] and environmental remediation where ultrasound was used to degrade dioxin contained within contaminated soil [112]. During this visit, Takashi asked me to give a lecture to the research group of Professor Yasuaki Maeda at Osaka Prefecture University about the development of sonochemistry in Romania. This happened on 1 December 1998 (which, coincidentally, is the National Day of Romania). One of the Japanese students decided to draw a cartoon sketch of me on a piece of paper (Figure 1.18). I really loved this drawing and I then used it almost all the time as the end of my presentations to conferences or workshops.

Figure 1.18: Mircea Vinatoru as seen by a Japanese student on 1 December 1998.

Subsequently, I was asked whether I would be willing to join the Maeda group in Osaka Prefecture University. I was happy to do this and it happened 3 years later through another JSPS grant. One of the topics that was of interest to them at that time was related to the potential use of ultrasound in the production of biodiesel via

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a transesterification reaction – using commercially available vegetable oils. This provided me with the chance to expand the list of topics involving ultrasound. My aim, as a chemical engineer, was to develop a continuous process for the production of biodiesel using sonication, and after some fruitful research, a series of papers were published on this theme. The first of these was also the first-ever published involving the use of ultrasound to assist biodiesel production making me one of the pioneers of this field [113]. This groundbreaking paper was followed by a number of others on ultrasonically assisted biodiesel synthesis and also some patents. In 2007, it was an honour for me and my co-authors to receive from the JSS the award of “best paper of the year”. It was published with the Japanese group I was working at that time and was entitled “Fatty acids methyl esters from vegetable oil by means of ultrasonic energy” [114].1 The topic of biodiesel synthesis will be expanded upon in Volume 2, Chapter 10. When I returned to Romania, we continued to work on some of the environmental projects which had started in the Maeda group, in particular, the sonochemical degradation of chlorobenzene in water containing various salts [115]. In this paper, we were able to show that there is an interesting mechanism for water decomposition under sonication and it will be further discussed in chapter 4 dealing with “Environmental Sonochemistry”. This became the subject of the PhD dissertation of one of my co-workers Carmen Stavarache, from the Institute of Organic Chemistry “C. D. Nenitzescu”, who was preparing her PhD in Japan, under the supervision of professor Yasuaki Maeda.

1.11.4 Work in Canada (2001 and 2005) In 2001, I was asked to make a short visit to the Food Technology Centre (FTC) in Prince Edward Island, a province in Canada. The purpose was to draft a proposal to help FTC extend their activities into a new field for them – the extraction of natural products using ultrasound and supercritical fluids. I enjoyed the challenge, and after we finished the proposal it was submitted to the funding agency with a request for around 50 million Canadian dollars. To celebrate the completion and submission of the proposal, the manager of the centre offered me a fishing trip in a boat out on the Atlantic Ocean which we did on 11 September 2001. It was quite a successful trip and we caught a lot of mackerel and even a small shark. At the end of the fishing trip, the skipper made the catch into fillets to take home and cook. Unfortunately, that particular day was significant in the USA and the world – it is

 Granted by JSS: Best Paper Award From fiscal 2007, the award was renamed Academic Paper Award. “Fatty acids methyl esters from vegetable oil by means of ultrasonic energy” (http://chem istch.shiga-med.ac.jp/JSS/data/ronbunichiran.pdf).

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often referred to as 9–11 or the Attack on the Twin Towers. The Canadian project did not receive funding, possibly due to repercussions relating to 9–11 and worries about the financial future and so it was put on hold to await better economic times. I returned to Europe and after a couple of years, in 2004, I received an email from FTC, asking for my phone number in order to have a discussion related to the extraction project. It seemed that it had now been approved although with a much smaller budget. The local government rules stated that I must be part of the FTC staff (because I was one of the two key scientists who proposed the project – the other was Ron Skinner still working at FTC) – and without both of us the project could not be funded. Over a phone call from Ron Skinner lasting almost 30 min, I learned that there were now only two programmes aimed at extraction: 1) To develop a pilot plant dedicated to UAE of natural compounds from medicinal herbs 2) Supercritical-assisted extraction (with refrigerating fluids and supercritical CO2) The phone call convinced me to accept a job position in FTC. I went through a lot of bureaucracy that was required to obtain a visa and a work permit for Canada. In the end, I got it and FTC purchased my flight ticket for 5 January 2005. I was a bit worried about Canadian winters, but there was no snow at that time in Charlottetown, the capital city of Prince Edward Island. On arrival at Montreal airport, late in the evening, I had to go through Canadian immigration and the immigration officer was looking suspiciously at my papers, where it was written that I was employed at the FTC as “ultrasonic chemist”. The immigration officer opened an impressive big book, containing all possible job qualifications of Canada. The entries in that book under the letter “U” recorded no such occupation. He asked me if perhaps what I was to be employed could be listed under a different name, since ultrasonic chemist did not appear in the book. I told him to look to sonochemist, and then at supersonic chemist but still there was no record of such an occupation in the book. In the end he said to me that it was too late to ask someone in the main immigration office of Canada about this type of job. He asked if I minded if he wrote on the work permit that the job was just a chemist. I agreed and so it was that I was able to take the last plane to Charlottetown that day. Perhaps this was an opportunity for me to add the new occupation of ultrasonic chemist to the Canadian job qualifications book – but I had no chance to confirm its inclusion. I arrived late in the night, around 1 am, and I took a taxi to the hotel, where I had accommodation until I could find a house to rent. Also staying in the same hotel was Roberto Armenta, another member of FTC staff of Mexican origins. He helped me to find the FTC location on 6 January 2005. Unfortunately, during the night after my visit to FTC, a very powerful snowstorm started and all the roads were closed. Both Roberto and I were forced to stay in the hotel next day, but the hotel did not have a restaurant. However, there was a pub just across the street (the TransCanada Highway) and we were able to eat and have

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a beer there. Overnight another snowstorm, more powerful than that before, hit PEI, so, my start at FTC was postponed again. It was only after a third snowstorm that we eventually could go to the office and that was because one of our future colleagues Edward Charter arrived with his car a big SUV (which was able to deal with the snow-covered road). My first day at FTC was on 10th of January 2005 and after this I became more used to the local rules and geography and began to settle into my job. First, I had to work out a way to furnish the laboratories with ultrasonic apparatus. I ordered an ultrasonic probe system (1 kW power) together with a large cleaning bath (~ 50 L) and several smaller routine laboratory ultrasonic baths. I shared an office with Roberto Armenta and Stephen Porter and in a separate office with a large window facing us was Ron Skinner, director of our group. He was intrigued by our behaviour because we often burst into laughter, but this was only because we three had a similar sense of humour and it was a way for us to alleviate the daily stress. The experience that I had accumulated in Osaka Prefecture University on biodiesel impressed my new employers when I started my work in FTC in January 2005. My background in biofuels persuaded another company, Ocean Nutrition of Canada (ONC), to propose a small project on transesterification of fish oil with ethanol using ultrasound as the source of energy (to replace the normal method involving heating). The research group from FTC and ONC finalized this work with a paper which was published after I left the food research facility [116]. For the population of Prince Edward Island, the news that an “ultrasonic chemist” was working at FTC spread rapidly through the media. Many people wanted to meet me and have a chat. This was the reason why one day a journalist from the magazine Hatchery International a Canadian publication came for a chat in my office. He asked me if there are any effects of ultrasound upon microbes in water. After I had replied yes, he invited me to contribute a one-page article about the sterilization of drinking water obtained from wells (a common source of domestic water in Canada) which I was happy to write [117]. At the end of the same year 2005, I decided to resign from my position at FTC mostly due to a misunderstanding of my role in the scientific research at FTC. As an academic researcher, I was used to publishing or patenting my results, but the management was not very supportive of this.

1.11.5 Work in Texas (2007–2009) Upon returning from Canada I resumed my work at the “C. D. Nenitzescu” Institute of Organic Chemistry in Bucharest and continued there until a new challenge arrived in 2007. I received an offer to join a private company located in Texas, USA. The post required the setting up of a research laboratory from scratch to study and commercialize biodiesel technologies involving ultrasound. After a long and complicated

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procedure, I got the work permit visa – O1 – which is only awarded to individuals with extraordinary ability or achievement. The task there was to set up a fully functional research laboratory. The first one was in Dallas not far from Love Field airport, but after 1 year we needed to move to a different lab in a brand new building in Frisco, a city located at the north of Dallas. Thus during this time in Texas, together with a small group of researchers including my colleague from Romania Carmen Stavarache, we put together two sonochemistry laboratories in which we were able to perform experiments for biodiesel production. We finished up with four new and potentially patentable technologies that could be used on an industrial scale. Unfortunately, this coincided with a strange decision of the investor to stop his financial backing of this business because of the economic crises which hit the USA. As a result, the projects could not be followed through to commercialization and came to an end, despite very optimistic feedback from the patent attorneys, see Volume 2 Chapter 10.

1.11.6 Work in the Coventry Sonochemistry Group (2009–2015) It was late in 2009 (I was still in Texas), when I got an email from Tim Mason asking me how long I will be involved in this private company. He asked if I was interested in a position in Coventry University, Sonochemistry Centre but I could not believe that this could be true. It had been my long-time dream to join the Sonochemistry Centre in Coventry University, the most famous and world-renowned sonochemical hub. For me, Tim Mason and Coventry University were the centre of the sonochemistry world. In those days, if someone acquainted with the sonochemistry field mentioned Tim Mason, this would automatically point to Coventry University and sonochemistry. The reverse was also true, and for many years the association of “Coventry University = sonochemistry = Tim Mason” was well known. I replied that I would be free of duties in the private company in Texas from about mid-February and then I would fly back to Romania. And so it was that on the 1st of April 2010, I arrived in Coventry to start my new position as researcher in a European project entitled “A pilot line of antibacterial and antifungal medical textiles based on a sonochemical process” with the acronym SONO. This work will be discussed in more detail in Volume 2 Chapter 9. After the SONO project finished and before leaving Coventry University, we developed an alternative that we thought would be a more efficient method for the impregnation of fabrics with metal oxide nanoparticles, to make them antimicrobial/antifungal. Through Coventry University Enterprises (the commercial wing of the university), we applied for and were granted a UK patent on “Method for producing antimicrobial yarns and fabrics by nanoparticle impregnation” [118]. We dreamed that this technology would soon find applications in the textile industry to produce fabrics able to protect people from many potential infections although

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the fabrics were mainly aimed at combating the spread of superbugs associated with hospital-acquired (nosocomial) infections. In addition to my main work on the SONO project, I had the chance to attend several conferences in the UK and Europe and beyond. I also worked on other projects, involving both foreign exchange students and the co-supervision of research fellows. This allowed me to maintain and expand my wider interests in sonochemistry. A post-doctoral researcher, Meral Dükkanci from Ege Üniversitesi, Bornova/ Izmir, Turkey, came on a 1-year grant in October 2010 to study “Degradation of Textile Dyes by Individual and Combined Processes of Ultrasound, UV Irradiation and Fenton-like Reagents” [119]. Marina Shestakova from Lappeenranta University of Technology, Finland, was also a research fellow involved in environmental remediation. She worked with us for 4 months during 2014, investigating sonoelectrocatalytic experiments with metal oxide electrodes for the oxidation of organic compounds [120]. Both of these topics will be dealt with in more detail in Chapter 4. A researcher from Egypt was to become an important link for future studies of UAE. He was Ibrahim Ahmed Saleh from the National Research Centre, Cairo. He arrived in January 2013 with a project entitled “A comparative study of innovative and traditional techniques for extraction of bioactive phytochemicals from certain economically important medicinal plants used in the treatment of liver diseases”. This work will be described in more detail in Chapter 3 and an important outcome was the elucidation of a possible general mechanism for ultrasound-assisted extraction which was developed from the results of UAE of chlorogenic acid from Cynara scolymus L. (artichoke) leaves [121]. After I left Coventry, I stayed in contact with Ibrahim, and through him, our group from the Politehnica University of Bucharest was able to set up a Memorandum of Understanding (MoU) with Cairo. Through this MoU, both of us (Tim and I) were able to visit Cairo, and researchers from there came to us (in Bucharest) and some common research topics were initiated. Around this period, we found some support for a project with the aim of developing a methodology for delivering cancer drugs by encapsulating them for later release locally within the body using high-intensity focused ultrasound (HIFU). For this project, the Sonochemistry Centre recruited Carmen Stavarache, a researcher from Romania who had worked with me on many occasions in the past. The project also involved the Churchill Hospital in Oxford, UK. In 2002, this hospital was the first in the western world to obtain such an instrument from a company in China. The story of this development in HIFU through the links of Tim Mason with Nanjing and Chongqing together with more details of the encapsulation project are to be found in Volume 2, Chapter 7 “Therapeutic Ultrasound”. While working with Tim we found that we had a common interest in fishing, and Tim introduced me to his favourite Meadowland Fishery just 15 min from our homes in Kenilworth. This made it easy to share lifts to go carp fishing, and at the

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beginning, Tim lent me some of his gear to fish with because mine was back in Romania. Tim kept records of all of his fishing trips and so I know that we first went together on 12 October 2012. I was unprepared for the fierce bites and runs of the carp especially with borrowed tackle, but it was a great day out. The fishery was socalled catch and release which meant that no fish could be taken away; on that day I bagged 3 carp against 13 for him. Our catches evened out when I got used to fishing in the UK and bought my own tackle. It was a great way for us to relax and chat and develop new research ideas (one example was the new technology for textile coating with nanoparticles of metal oxides for which we obtained a patent).

1.11.7 Return to Romania and ULTRAMINT Before leaving Coventry University (at the end of 2015), I had been approached by Professor Ioan Calinescu from the Faculty of Applied Chemistry and Material Science of Politehnica University of Bucharest. He wanted to know if I was willing to join him on a new European programme “Competitiveness Operational Programme (POC 2014–2020)”, under the name of “Ultrasonic/Microwave Nonconventional Techniques as new tools for nonchemical and chemical processes”, ULTRA-MINT. I said yes, and so, over the next 18 months, there was a sort of “transition period” when I remained in the UK living in my house in Kenilworh, but began spending periods of time in my home flat in Bucharest. It was fortunate that not long after I accepted the offer, a new and inexpensive direct flight was introduced between Birmingham and Bucharest operated by the budget airline Blue Air. Tim was my taxi driver for my 20 min trips between Kenilworth and the airport. When I finished living in the UK (in April 2018), I drove back to Bucharest with as many of my belongings that I could pack into my old car that I had bought in England – a Vauxhall Astra estate. My son Bogdan was co-driver for the long 3-day trip and I was then able to take up the half-time employment position as director of the project at the Politehnica University of Bucharest. It was part of a Romanian Government scheme aimed at attracting high-level personnel from abroad back to Romania in order to enhance the national RD capacity (Figure 1.19). I was working on this project when Tim and I began writing this book. The grant finished in 2021, but fortunately, our group won a new national grant – an Exploratory Research Project and several other domestic and European projects. We recommended that the project management should appoint Tim Mason as an honorary member of our team. It was the wise decision of the Politehnica University to accept him. Since then several important topics have been investigated and published from the ULTRAMINT group [19, 122–129] and will be explored in more detail in other chapters of this book.

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Figure 1.19: The ULTRA-MINT official poster.

1.12 Some comments on the links between sonochemistry and nuclear fusion 1.12.1 Energy derived from nuclear fusion Historically, research into nuclear fusion as an alternative energy source to fossil fuels involves the generation and control of a high-energy plasma composed of

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light element nuclei. At a sufficiently high temperature, the nuclei will acquire enough kinetic energy to undergo nuclear fusion. During the writing of this book, a plasma reactor in the Culham Laboratories in Oxfordshire, UK, has produced such a nuclear fusion reaction over a time of 5 s [130]. The process involves heating hydrogen to over 100 million degrees Celsius confined by a magnetic field to produce a plasma – hydrogen nuclei in the plasma combine to form a helium nuclei. For each fusion reaction a neutron is liberated and the associated heat produced is used to generate energy.

1.12.2 Cold fusion As the name suggests, this is an attempt to get nuclear fusion to occur under less extreme conditions than in a plasma. In 1989, a claim was made by Pons and Fleischmann that this could be done in an experiment using ultrasound during the electrolysis of heavy water (D2O) using a palladium electrode [131]. They calculated that more energy was being released from the process, as heat, than was being put into it as electricity [132]. It is known that palladium is very good at absorbing hydrogen within its lattice structure. In cold fusion, it was proposed that deuterium liberated at the cathode during the electrolysis of heavy water would become enmeshed within the metal lattice as nuclei and in this situation might somehow be induced to fuse, produce helium and liberate energy together with neutrons. These cold fusion experiments were greeted with enormous enthusiasm when they were first reported but subsequently became the subject of criticism and were debunked.

1.12.3 The film Chain Reaction The discovery by Seth Putterman in the early 1990’s that sonoluminescence could be observed from a single cavitating bubble was a major breakthrough in sonochemistry [133]. This phenomenon was later to be referred to as “a star in a jar” [134]. In 1996, sonoluminescence came to the notice of the general public with the release of the film Chain Reaction. Unfortunately, the film was not well received and the scientific content proved to be confusing but to sonochemists it was of interest. The film involves a project aimed at producing cheap energy from hydrogen generated from water. At one point in the film the project leader – Dr Shannon played by Morgan Freeman – does mention sonoluminescence. A possible interpretation of the science involved in the film by the authors of this book is as follows. The project to produce hydrogen from water could have involved electrolysis and ultrasound can enhance this through sonoelectrochemistry. The effect of changing the sound frequency to one that is optimum for the generator could improve its efficiency

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and result in the production of light (sonoluminescence). The fact remains, however, that it did not excite any great interest in sonochemistry at the time but nowadays there is a genuine scientific interest in the production of hydrogen as a clean fuel.

1.12.4 Nuclear fusion in a cavitation bubble In 2002, Rusi Taleyarkhan of the Oak Ridge National Laboratory and colleagues claimed that they had achieved nuclear fusion of deuterium within a collapsing acoustic cavitation bubble produced in degassed deuteroacetone (CD3COCD3) [135]. The emission of weak sonoluminescence from the system confirmed that cavitation collapse was occurring. One of the outcomes from the fusion of deuterium is the production of helium (He) or Tritium (3H) and 2.5 MeV neutrons, and the team claimed to have detected the generation of such neutrons. They revisited the experiments and confirmed their findings in 2004 [136]. This work created great interest – and also doubt about the findings – so much so that several groups attempted to reproduce the results but without success. It became a cause celebre, and in 2004, the BBC popular science programme Horizon decided to feature the subject in one of its broadcasts, and they invited me to contribute as an expert on sonochemistry. The episode was entitled An Experiment to Save the World; it was broadcast in 2005 and contained not only interviews with Taleyarkhan but also some comments from Fleishmann about his work some years previously on “cold fusion”. An important part of the programme was that the BBC commissioned some experiments from Seth Putterman at UCLA [134]. Seth set up experimental apparatus that paralleled those described by Taleyarkhan but with a more accurate neutron detection system. The results were presented by Seth and also discussed by a panel of three scientists who were invited to London on Thursday, 17 February. With me on that panel were two physicists Nigel Hawkes from the NPL and Mike Loughlin from the UK Atomic Energy Authority. The programme is available on YouTube [137] and as a transcript [138]. The programme concluded that: Our experiment failed to find any evidence of fusion. We put this conclusion to Rusi Taleyarkhan. He said that it had taken him several years to perfect the exact conditions necessary for fusion. And that because our experiment was not an identical copy of his any one of several differences might have affected the outcome. Never the less we followed his fusion recipe as closely as possible, on the principle that if the key scientific conditions are reproduced the results would be too. But we found nothing. It is possible that other scientists may succeed in reproducing Rusi Taleyarkhan’s results, but for now, all we can say is that the dream of a shortcut to unlimited clean energy forever must remain just that, a dream.

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1.13 Concluding remarks In this chapter, we have attempted to outline some of the origins and history of sonochemistry. We do not claim that this is an exhaustive historical review; it is much more of a personal account of the way the subject developed as seen through our own experiences. At the first Sonochemistry Symposium in 1986, Arnim Henglein presented a paper entitled “Sonochemistry: historical developments and modern aspects” [96] and this was expanded some years later as a chapter in Advances in Sonochemistry [139]. His first published paper on sonochemistry was in 1952 relating to the initiation of the polymerization of acrylamide by ultrasound [140]. Arnim had been a little reticent about using the term “sonochemistry” preferring cavitation chemistry which he used as his chapter title “Aspects of Cavitation Chemistry” but within it he referred to sonochemistry quite freely. In that chapter, he talked about cavitation bubble collapse and the early electrical discharge theory of Frenkel that was accepted for many years before the advent of the ideas about “hot-spot” chemistry (see earlier). It is intriguing to note that he was interested in the discharge theory which Margulis had advocated and they had discussed it when they first met at the meeting in Leatherhead in 1990 (Figure 1.9). He did not dismiss these ideas as had some other participants at ESS4 (Figure 1.16), but in his final remarks in the chapter, he noted that charge separation should be facilitated if a sonicated liquid had a high dielectric. As far as we are aware, his suggestion that one might study the sonolysis of liquids that have equal vapour pressures but possess different dielectric constants has not been followed up. Sonochemistry was only one area within the wide contributions of Arnim Henglein to chemistry. Arnem passed away in January 2012 in Freiburg at the age of 85. Another pioneer of the effects of ultrasound in chemistry was David Gillings. David had studied the influences of ultrasonic waves on the viscosity of colloidal solutions with Herbert Freundlich at University College London in the 1930s and had published a paper with him in 1938 “The influence of ultrasonic waves on the viscosity of colloidal solutions” [141]. Freundlich was a pioneer of ultrasound and colloidal science, and so David was in an excellent position to recall and write about these early developments in sonochemistry. He was invited to the ESS5 meeting in Cambridge and wrote a short chapter on the history of the use of ultrasound in colloid science for Advances in Sonochemistry [142]. The articles by Gillings and Henglein were the first two chapters in Volume 3 of Advances in Sonochemistry. Perhaps, the field of research involving mechanochemistry should have been more closely linked with the history of sonochemistry. This research field became recognized in the 1960s [143]. A simple definition of mechanochemistry is the influence of mechanical actions on the reactions of inorganic solids usually in the form of powders, and much of the early work was pursued in Russia. In 1995, Boldyrev from Novosibirsk State University published a review comparing mechanochemistry and sonochemistry [144]. He claimed that mechanochemistry and sonochemistry

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are related and have similar mechanistic problems in common: (a) determination of basic principles allowing a distinction to be drawn between the direct and indirect effects of stress fields on chemical processes; (b) study of metastable states formed as a result of mechanical and sonochemical activation of solids. This is indeed an intriguing comparison but it has not been followed up to any great extent although in 2014 a Faraday Discussions conference on mechanochemistry contained some comments on the links [145]. The first investigations of the influence of ultrasound on chemical reactions were in 1927 when Richards and Loomis reported “The chemical effects of high frequency sound waves” in the Journal of the American Chemical Society [29]. Thereafter, the subject did not “take off” because of the lack of availability of simple laboratory sonication apparatus. It was much later in 1951 that the term “sonochemistry” was used for the first time [37] and even later than that when apparatus in the form of ultrasonic cleaning baths became widely available in the 1980s and sonochemistry came of age.

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[37] W.A. Weyl, Surface structure of water and some of its physical and chemical manifestations, Journal of Colloid Science, 6 (1951) 389–405. [38] A. Weissler, Ultrasonics in chemistry, Journal of Chemical Education, 25 (1948) 28–30. [39] A. Weissler, H.W. Cooper, S. Snyder, Chemical effect of ultrasonic waves: Oxidation of potassium iodide solution by carbon tetrachloride, Journal of the American Chemical Society, 72(1950), 1769–1775. [40] S.C. Liu, H. Wu, Mechanism of oxidation promoted by ultrasonic radiation, Journal of the American Chemical Society, 56 (1934) 1005–1007. [41] S.C. Liu, H. Wu, Effect of ultrasonic radiation on indicators, Journal of the American Chemical Society, 54 (1932) 791–793. [42] F.O. Schmitt, C.H. Johnson, A.R. Olson, Oxidations promoted by ultrasonic radiation, Journal of the American Chemical Society, 51 (1929) 370–375. [43] A. Weissler, E.J. Hine, Variations of cavitation intensity in an ultrasonic generator, Journal of the Acoustical Society of America, 34 (1962) 130–131. [44] A. Weissler, Formation of hydrogen peroxide by ultrasonic waves: Free radicals, Journal of the American Chemical Society, 81 (1959) 1078–1081. [45] T.J. Mason, J.P. Lorimer, D.M. Bates, Quantifying sonochemistry: Casting some light on a ‘black art’, Ultrasonics, 30 (1992) 40–42. [46] R. Baker, T.J. Mason, Solvent effects on the exo: Endorate ratios of solvolysis of tetracyclo [8,2,1,02,903,8]trideca-3,5,7-trien-11-yl toluene-p-sulphonates, Journal of the Chemical Society D: Chemical Communications (1969) 120–121. [47] G.D. Sargent, Bridged, non-classical carbonium ions, Quarterly Reviews, Chemical Society, 20 (1966) 301–371. [48] T.J. Mason, M.J. Harrison, J.A. Hall, G.D. Sargent, Influence of bond angle distortion and .sigma.-.pi. .sigma.-delocalization on the stability and chemistry of allylic cations, Journal of the American Chemical Society, 95 (1973) 1849–1859. [49] T.J. Mason, R.O.C. Norman, Kinetics and mechanism of addition and cyclialkylation reactions of ω-arylakenes with trifluoroacetic acid, Journal of the Chemical Society, Perkin Transactions, 2 (1973) 1840–1844. [50] T.J. Mason, Phenyl participation in the generation of carbocations from the reactions of some 4-methyl-w-phenylalkyl toluene-p-sulphonates and o-phenylalk-7-enes in trifluoroacetic acid, Journal of the Chemical Society, Perkin Transactions, 2 (1975) 1664–1669. [51] D. Feakins, J.P. Lorimer, Washburn numbers. Part 1 – The electromotive force method for alkali-metal chlorides in the methanol + water and dioxan + water systems, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 70 (1974) 1888–1901. [52] J.P. Perkins, Power Ultrasound, in: T.J. Mason (Ed.) Sonochemistry – The Uses of Ultrasound in Chemistry, Royal Society of Chemistry, London, (1990) 47–59. [53] J.P. Lorimer, T.J. Mason, Effect of ultrasonic irradiation on the solvolysis of 2-chloro-2methylpropane in aqueous ethanol mixtures, Journal of the Chemical Society. Chemical Communications (1980) 1135–1136. [54] T.J. Mason, J.P. Lorimer, B.P. Mistry, The Effect of Ultrasound on a Homogeneous Chemical Reaction, Ultrasonics International 85, Butterworths, King’s College, London, (1985) 839–844. [55] T.J. Mason, Sonochemistry, Laboratory Practice, 33 (1984) 13–16. [56] T.J. Mason, Sonochemistry, Oxford University Primers #70, Oxford University Press, Oxford, 1999.

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E.M. Mokry, V.L. Starchevsky, Initiation and Catalysis of Oxidation Processes of Organic Compounds in an Acoustic Field, in: T.J. Mason (Ed.) Advances in Sonochemistry, JAI Press, Greenwich, CT, 3 (1993) 257–292. T.J. Mason, Use of ultrasound in chemical synthesis, Ultrasonics, 24 (1986) 245–253. T.J. Mason, Sonochemistry symposium, annual chemical congress, Ultrasonics, 25 (1987) 5. T.J. Mason, J.P. Lorimer, Sonochemistry – The Theory, Applications and Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1988. D. Rehorek, E.G. Janzen, Spin trapping of radicals generated by ultrasonic decomposition of organotin compounds, Journal of Organometallic Chemistry, 268 (1984) 135–139. D. Rehorek, S. Di Martino, S. Sostero, O. Traverso, T.J. Kemp, Spin trapping of radicals formed by sonolysis of some organometallic compounds, Inorganica Chimica Acta, 178 (1990) 1–3. T.J. Mason, Chemistry with Ultrasound, Society for Chemical Industry, London, 1990. T.J. Mason, Sonochemistry the Uses of Ultrasound in Chemistry, Royal Society of Chemistry, Cambridge, UK, 1990. T.J. Mason (Ed.), Advances in Sonochemistry, a series of 6 volumes (vol 1 published in 1990, JAI press, Greenwich, CT). T.J. Mason, Practical Sonochemistry, A Users Guide to Applications in Chemistry and Chemical Engineering, Ellis Horwood Publishers, Chichester UK, 1991. T.J. Mason, D. Peters, Practical Sonochemistry, Power Ultrasound Uses and Applications, 2nd ed., Horwood Publishing, Chichester UK, 2002. T.J. Mason, J.P. Lorimer, Applied Sonochemistry: The Uses of Power Ultrasound in Chemistry and Processing, Wiley, VCH, Weinheim 2002. T.J. Mason, In memory of Jean-Louis Luche, Ultrasonics Sonochemistry, 25 (2015) 4–7. P.W. Cains, L.J. McCausland, D.M. Bates, T.J. Mason, Sonochemical hydrogenation over metal catalysts, Ultrasonics Sonochemistry, 1 (1994) S45–S46. M. Povey, T.J. Mason, Ultrasound in Food Processing, Blackie Academic and Professional, London 1998. A. McKinlay, The ultrasonic boom – Focus on health and safety, Progress in Biophysics and Molecular Biology, 93 (2007) 1–2. G. Ruecroft, D. Hipkiss, T. Ly, N. Maxted, P.W. Cains, Sonocrystallization: The use of ultrasound for improved industrial crystallization, Organic Process Research & Development, 9 (2005) 923–932. M.J.W. Povey, Ultrasound particle sizing: A review, Particuology, 11 (2013) 135–147. N. Ratoarinoro, A.M. Wilhelm, J. Berlan, H. Delmas, Effects of ultrasound emitter type and power on a heterogeneous reaction, The Chemical Engineering Journal, 50 (1992) 27–31. Y. Zhao, C. Bao, R. Feng, T.J. Mason, New etching method of PVC plastic for plating by ultrasound, Journal of Applied Polymer Science, 68 (1998) 1411–1416. J. Herbertz, First World Congress in Ultrasonics, in: J. Herbertz (Ed.) First World Congress in Ultrasonics, GEFAU Duisberg, Berlin, (1999), 1127. T.J. Mason, Ultrasound in Environmental Protection – An Overview, in: A. Tiehm, U. Neis (Eds.) Reports on Sanitary Engineering, TU Hamburg, Harburg, 1999 1–9. K.S. Suslick, Editorial for Special Issue on Pacifichem 89, Ultrasonics, 28 (1990) 279. R.L. Hunicke, Industrial applications of high power ultrasound for chemical reactions, Ultrasonics, 28 (1990) 291–294. G. Price, Sonochemistry and sonoluminescence: NATO ASI, Leavenworth, Washington, USA, August 1997, Ultrasonics Sonochemistry, 4 (1997) 325–326. L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick, Sonochemistry and Sonoluminescence, NATO ASI Series, Kluwer Academic Publishers, the Netherlands, 1999.

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[83] T.J. Mason, Sonochemistry using audible sound frequencies: A route to large-scale processing, in: Proceedings of 18th International Congress on Acoustics, ICA, Kyoto, Japan, 2004. [84] T. Ando, S. Sumi, T. Kawate, J. Ichihara, T. Hanafusa, Sonochemical switching of reaction pathways in slid–liquid two-phase reactions, Journal of the Chemical Society. Chemical Communications (1984) 439–440. [85] T.J. Mason, Oleg Abramov 1936–2008, Ultrasonics Sonochemistry, 16 (2009) 439. [86] T.J. Mason, Sonochemistry – A proven tool for process intensification, in: Proceedings of 20th International Congress on Acoustics, ICA, Sydney, Australia, 2010. [87] L. Paniwnyk, O. Larpparisudthi, T.J. Mason, Degradation of water pollutants using ultrasound, in: Proceedings of 20th International Congress on Acoustics, ICA, Sydney, Australia, 2010. [88] T.J. Mason, Trends in sonochemistry and ultrasonic processing, AIP Conference Proceedings, 1433 (2012) 21–26. [89] K. Yamamoto, P.M. King, X. Wu, T.J. Mason, E.M. Joyce, Effect of ultrasonic frequency and power on the disruption of algal cells, Ultrasonics Sonochemistry, 24 (2015) 165–171. [90] S. Benefice-Malouet, B. Reichert, A. Abou-Hamdan, Activity Report 1995–1996 Cost Chemistry Actions D1-D7, Office for Official Publications of the European Communities, 1997. [91] M.A. Margulis, Sonochemistry and Cavitation, Gordon and Breach Science Publishers, Luxembourg, 1995. [92] R.v. Eldik, C.D. Hubbard, Chemistry under Extreme and Non-Classical Conditions, John Wiley & Sons, New York, 1997. [93] A. Durant, J.-L. Delplancke, R. Winand, J. Reisse, A new procedure for the production of highly reactive metal powders by pulsed sonoelectrochemical reduction, Tetrahedron Letters, 36 (1995) 4257–4260. [94] O.V. Abramov, A. Gedanken, Y. Koltypin, N. Perkas, I. Perelshtein, E. Joyce, T.J. Mason, Pilot scale sonochemical coating of nanoparticles onto textiles to produce biocidal fabrics, Surface & Coatings Technology, 204 (2009) 718–722. [95] T.J. Mason, Editorial for Special Issue on ESS2, Ultrasonics, 30 (1992) 144. [96] T. Ando, T.J. Mason, K.S. Suslick, Editorial, Ultrasonics Sonochemistry, 1 (1994) S3. [97] T.J. Mason, Third meeting of the European Society of Sonochemistry Figuera da Foz, Portugal, 28 March–1 April 1993, Ultrasonics Sonochemistry, 1 (1994) S4. [98] M. Vinatoru, A. Iancu, E. Bartha, A. Petride, V. Badescu, D. Niculescu-Duvaz, F. Badea, Ultrasonically stimulated electron transfer in organic chemistry. Reaction of nitrobenzene with triphenylmethane and its derivatives, Ultrasonics Sonochemistry, 1 (1994) S27–S31. [99] D.J. Walton, S.S. Phull, D. Colton, P. Richards, A. Chyla, T. Javed, L. Clarke, J.P. Lorimer, T.J. Mason, Ultrasonic enhancement of electrochemiluminescence from arylacetate electrooxidation, Ultrasonics Sonochemistry, 1 (1994) S23–S26. [100] M. Ashokkumar, Editorial: In Honour of Professor Timothy J. Mason, Ultrasonics Sonochemistry, 37 (2017) A1–A3. [101] P.A. Claisse, J.P. Lorimer, M. Al Omari, Workability of cement pastes, ACI Materials Journal, 98 (2001) 476–482. [102] E. Ganjian, A. Ehsani, T.J. Mason, M. Tyrer, Application of power ultrasound to cementitious materials: Advances, issues and perspectives, Materials & Design, 160 (2018) 503–513. [103] A. Ehsani, E. Ganjian, T.J. Mason, M. Tyrer, M. Bateman, Insights into the positive effects of power ultrasound on the pore solution of Portland cement pastes, Cement and Concrete Composites, 125 (2022) 104302. [104] P. Boudjouk, B.H. Han, Organic sonochemistry. Ultrasound promoted coupling of chlorosilanes in the presence of lithium wire, Tetrahedron Letters, 22 (1981) 3813–3814. [105] K.S. Suslick, Organometallic Sonochemistry, Advances in Organometallic Chemistry, 25 (1986) 73–119.

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[106] M. Vinatoru, D. Stilpeanu, S. Velea, M. Petcu, M. Vija, G. Bacneanu, C. Brinzan, L’Estrazione Ultrasonica Dei Principi Bioattivi Dalle Piante, Tecnologie Chimiche, 4 (1996) 76–79. [107] M. Vinatoru, M. Toma, P. Filip, T. Achim, N. Stan, T.J. Mason, P. Mocanu, G. Livezeanu, D. Lazurca, RO 115789, Ultrasonic Reactor Dedicated to the Extraction of Active Principles from Plants in, Plafar SA, Romania, 2000. [108] M. Vinatoru, M. Toma, T.J. Mason, Ultrasonically Assisted Extraction of Bioactive Principles from Plants and Their Constituents, in: T.J. Mason (Ed.) Advances in Sonochemistry, JAI Press, Stamford, CT, 5 (1999), 209–248. [109] M. Ashokkumar, Editorial, Ultrasonics Sonochemistry, 52 (2019) 1. [110] M. Vinatoru, T.J. Mason, Exploring Alternative Mechanisms and Pathways in Sonochemistry, in: Asia-Oceania Society of Sonochemistry (AOSS4), Nanjing, China, 2019. [111] R. Stavrescu, T. Kimura, M. Fujita, M. Vinatoru, T. Ando, Active manganese dioxide supported on alumina, Synthetic Communications, 29(1999), 1719–1726. [112] R. Stavrescu, T. Kimura, M. Fujita, T. Ando, M. Vinatoru, Ultrasonic Degradation of a Dioxin Type Molecule, Science Direct Working Paper No S1574-0331(04) 70525-1, 2001 (2001) 101–107. [113] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Conversion of vegetable oil to biodiesel using ultrasonic irradiation, Chemistry Letters, 32 (2003) 716–717. [114] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Fatty acids methyl esters from vegetable oil by means of ultrasonic energy, Ultrasonics Sonochemistry, 12 (2005) 367–372. [115] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Short-time sonolysis of chlorobenzene in the presence of Pd(II) salts and Pd(0), Ultrasonics Sonochemistry, 11 (2004) 429–434. [116] R.E. Armenta, M. Vinatoru, A.M. Burja, J.A. Kralovec, C.J. Barrow, Transesterification of fish oil to produce fatty acid ethyl esters using ultrasonic energy, Journal of the American Oil Chemists’ Society, 84 (2007) 1045–1052. [117] M. Vinatoru, Will ultrasound find a place for disinfections in your hatchery?, Hatchery International, 6, (2005) 11. [118] M. Vinatoru, T.J. Mason, J.A. Beddow, Method for producing antimicrobial yarns and fabrics by nanoparticle impregnation, US 10,415,179 B2, 2014. [119] M. Dukkanci, M. Vinatoru, T.J. Mason, The sonochemical decolourisation of textile azo dye Orange II: Effects of Fenton type reagents and UV light, Ultrasonics Sonochemistry, 21 (2014) 846–853. [120] M. Shestakova, M. Vinatoru, T.J. Mason, M. Sillanpaa, Sonoelectrocatalytic decomposition of methylene blue using Ti/Ta(2)O(5)-SnO(2) electrodes, Ultrasonics Sonochemistry, 23 (2015) 135–141. [121] I.A. Saleh, M. Vinatoru, T.J. Mason, N.S. Abdel-Azim, E.A. Aboutabl, F.M. Hammouda, A possible general mechanism for ultrasound-assisted extraction (UAE) suggested from the results of UAE of chlorogenic acid from Cynara scolymus L. (artichoke) leaves, Ultrasonics Sonochemistry, 31 (2016) 330–336. [122] I. Calinescu, A. Vartolomei, I.A. Gavrila, M. Vinatoru, T.J. Mason, A reactor designed for the ultrasonic stimulation of enzymatic esterification, Ultrasonics Sonochemistry, 54 (2019) 32–38. [123] I. Călinescu, M. Vinatoru, D. Ghimpețeanu, V. Lavric, T.J. Mason, A new reactor for process intensification involving the simultaneous application of adjustable ultrasound and microwave radiation, Ultrasonics Sonochemistry, 77 (2021) 105701. [124] P. Chipurici, A. Vlaicu, I. Calinescu, M. Vinatoru, M. Vasilescu, N.D. Ignat, T.J. Mason, Ultrasonic, hydrodynamic and microwave biodiesel synthesis – A comparative study for continuous process, Ultrasonics Sonochemistry, 57 (2019) 38–47. [125] A. Diacon, I. Călinescu, M. Vinatoru, P. Chipurici, A. Vlaicu, A.C. Boscornea, T.J. Mason, Fatty acid ethyl esters (FAEE): A new, green and renewable solvent for the extraction of carotenoids from tomato waste products, Molecules, 26 (2021) 4388.

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[126] C.M. Simonescu, T.J. Mason, I. Călinescu, V. Lavric, M. Vînătoru, A. Melinescu, D.C. Culiţă, Ultrasound assisted preparation of calcium alginate beads to improve absorption of Pb+2 from water, Ultrasonics Sonochemistry, 68 (2020) 105191. [127] M. Vinatoru, T. Mason, Comments on the use of loop reactors in sonochemical processes, Ultrasonics Sonochemistry, 39 (2017) 240–242. [128] M. Vinatoru, T.J. Mason, Jean-Louis luche and the interpretation of sonochemical reaction mechanisms, Molecules, 26 (2021) 755. [129] M. Vinatoru, T.J. Mason, I. Calinescu, Ultrasonically assisted extraction (UAE) and microwave assisted extraction (MAE) of functional compounds from plant materials, TrAC – Trends in Analytical Chemistry, 97 (2017) 159–178. [130] E. Gibney, Nuclear-fusion reactor smashes energy record, Nature, 602 (2022) 371. [131] M. Fleischmann, S. Pons, Electrochemically induced nuclear fusion of deuterium, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 261 (1989) 301–308. [132] M. Fleischmann, S. Pons, M.W. Anderson, L.J. Li, M. Hawkins, Calorimetry of the palladiumdeuterium-heavy water system, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 287 (1990) 293–348. [133] B.P. Barber, S.J. Putterman, Observation of synchronous picosecond sonoluminescence, Nature, 352 (1991) 318–320. [134] S. Putterman, Sonoluminescence: The star in a jar, Physics World, 11 (1998) 38–42. [135] R.P. Taleyarkhan, C.D. West, J.S. Cho, R.T. Lahey Jr., R.I. Nigmatulin, R.C. Block, Evidence for nuclear emissions during acoustic cavitation, Science, 295 (2002), 1868–1873. [136] R.I. Nigmatulin, R.P. Taleyarkhan, R.T. Lahey, Evidence for nuclear emissions during acoustic cavitation revisited, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 218 (2004) 345–364. [137] BBC-Horizon, An Experiment to Save the World Youtube, in: https://www.youtube.com/ watch?v=0CN1gAu1Hys, 2005. [138] BBC-Horizon, An Experiment to Save the World Transcript, in: http://www.bbc.co.uk/sn/tvra dio/programmes/horizon/experiment_trans.shtml, 2005. [139] A. Henglein, Aspects of Cavitation Chemistry, in: T.J. Mason (Ed.) Advances in Sonochemistry, JAI Press, Greenwich, CT, 3 (1993) 17–84. [140] A. Henglein, R. Schulz, Notizen: Die Auslösung der Polymerisation des Acrylamids durch Ultraschall, Zeitschrift Für Naturforschung B, 7 (1952) 484–485. [141] H. Freundlich, D.W. Gillings, The influence of ultrasonic waves on the viscosity of colloidal solutions, Transactions of the Faraday Society, 34 (1938) 649–660. [142] D.W. Gillings, Ultrasound and Colloid Science – The Early Years, in: T.J. Mason (Ed.) Advances in Sonochemistry, JAI Press, Greenwich, CT, 3 (1993) 1–16. [143] L. Takacs, The historical development of mechanochemistry, Chemical Society Reviews, 42 (2013) 7649–7659. [144] V.V. Boldyrev, Mechanochemistry and sonochemistry, Ultrasonics Sonochemistry, 2 (1995) S143–S145. [145] Challenges and opportunities of modern mechanochemistry, in: Faraday Discussions 170, Royal Society of Chemistry, Montreal, Canada, (2014) 1–432.

Chapter 2 Fundamental aspects of sonochemistry 2.1 Introduction to acoustic waves Some years before sonochemistry became recognized as a specific discipline, there were mathematicians and physicists fascinated by the nature of bubbles in water. The interest can be traced back to the early work of Lord Rayleigh [1] and it has been discussed extensively in more modern books in the context of acoustic cavitation [2, 3]. For the chemist, much of the mathematical material contained in such studies can be daunting particularly if, like the authors of this book, your basic interests are in organic chemistry. Early scientific meetings that included sonochemistry often contained papers exploring the theories involved in cavitation which did not really grab the attention of the majority of the chemists present. But in the mid1990s, this changed markedly when we were presented not just with the physics and mathematics of cavitation but also with photographic evidence of the behaviour of cavitation bubbles, and at around the same time, sonoluminescence became more accessible as a practical laboratory study [4, 5]. Sonochemistry is part of the broad spectrum of acoustic science and some appreciation of the fundamentals of acoustics is needed by those who are involved. With this knowledge, it becomes easier to understand not only how sonochemistry works but also it enables the practitioner to modify conditions leading to an optimization of sonochemical reactions. In this chapter, a brief presentation of the principles that govern sonochemistry is given for readers that are not familiar with the theory of this branch of science. This chapter is thus effectively a tutorial and is based on some of the information to be found in Chapter 2 of a book co-written in 1988 by Tim Mason and Phil Lorimer entitled Sonochemistry – The Theory, Applications and Uses of Ultrasound in Chemistry [6], revised as Applied Sonochemistry in 2002 [7].

2.2 Acoustic waves and their propagation Acoustic waves are propagated via the movement/oscillation of molecules in the medium through which sound is passed (Figure 2.1). As the acoustic power is increased, the displacement of the molecules intensifies and the compression and rarefaction cycles become larger. For a liquid medium, when the acoustic power reaches a sufficiently high value, the rarefaction cycle (sometimes referred to as negative pressure) is large enough to exceed the normal attractive forces of the molecules and when this happens voids will form in the overall structure each enclosing a vacuum. Molecules from the surrounding liquid will enter these voids so that in the following compression cycle they do not fully collapse as they would have done if they had enclosed complete https://doi.org/10.1515/9783110566178-002

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vacuums. As illustrated in Figure 2.1, subsequent cycles cause the bubbles to grow as more molecules from the surrounding liquid are drawn in after each rarefaction cycle. This process of bubble growth is known as rectified diffusion [8] and it continues until the bubble reaches a resonant size which depends upon the sound frequency applied. The higher the applied frequency, the smaller the resonant bubble size, for example in water at 20 kHz, the bubble is approximately 200 µm in diameter, but at 1 MHz, this is reduced to only 2 µm. Such resonating bubbles may exist for many cycles in a homogeneous sound field, but most common transducers do not generate perfect acoustic fields and the presence of other bubbles can produce some disturbances within the acoustic field. This causes the bubble to become unstable and collapse. It is an extremely rapid collapse which produces a high temperature and large pressure on the molecules within the bubble. This results in the decomposition of these molecules and the generation of active radical species and sonoluminescence. Cavitation can only occur in a liquid and because sonochemists are almost exclusively concerned with reactions in liquid media cavitation bubble collapse will naturally be a subject of interest to them. Hence, in this chapter, we will concentrate on the way in which ultrasound interacts with homogenous liquids and heterogeneous liquid mixtures since these are the conditions present in the majority of chemical reactions.

compression

rarefaction

compression

rarefaction

compression

rarefaction

compression

rarefaction

rarefaction

5000°C 2000 ats

bubble forms

bubble grows in successive cycles

reaches unstable size

undergoes violent collpse

Figure 2.1: Bubble initiation, expansion and collapse.

Sound can also travel through gases and solids, but this is of less interest to sonochemists although the former is important for the destruction of foams, mists and smokes while a knowledge of the latter finds use in the design of acoustic devices particularly acoustic horns and waveguides. In Chapter 1, the transmission of sound

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81

through air was discussed in terms of hearing and sound transmission in a gas follows the same mechanism as in a liquid (compression and rarefaction of longitudinal waves). The speed of sound through air is 343 m/s, much lower than in liquids (1,482 m/s in water) because liquid molecules are much closer together than in gases and so the vibrations are transmitted more efficiently. The way in which sound travels through solids is somewhat different in that solids e.g. metals are not easily compressible and the molecules or atoms are closely packed so sound transmission is even more rapid (5,960 m/s in steel). There is another important difference in the way in which sound is transmitted in solids compared with liquids and gases. In the latter the oscillations of particles or molecules take place in the direction of the sound wave and produce longitudinal waves (Figure 2.2a). Solids, however, since they also possess shear elasticity, can also support tangential stresses giving rise to transverse waves as well as longitudinal waves and for transverse waves the particle movement is perpendicular to the direction of the wave (Figure 2.2b). vibration of particle

vibration of particle

direction of wave direction of wave (a)

(b)

Figure 2.2: Waves and particle (or molecule) movement (a) longitudinal and (b) transverse.

How the sound waves are transmitted through a gas (air in the case of hearing) is illustrated in Figure 2.3. The waves are essentially longitudinal (there is a very weak transverse wave component) (see Chapter 1, Section 1.3 for more information about the physiology of hearing). Rarefaction

Compression Figure 2.3: Illustration of a longitudinal wave.

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If ultrasound is introduced at the end of a solid metal bar, vibrational energy will be transmitted longitudinally producing vibrations at the other end. To obtain maximum transmission of vibrational energy, the length of the bar should be equivalent to a finite number of wavelengths of sound. In solids there is a significant contribution from transverse waves which are at right angles to the direction of vibration and induce movement perpendicular to the direction of the wave. In a metal bar, the transverse waves (also known as radial vibrations) are much weaker than longitudinal waves. Despite this some types of ultrasonic processing employ radial vibrations [9]. One example is the Telsonic system which consists of a hollow, gas filled, metal tube sealed at one end and driven at the other by a standard piezotransducer. The device looks like a conventional probe system but is significantly different in that the sealed end is at a null point and is not the major source of vibrational energy. Instead of this, the ultrasound is emitted radially at half wavelength distances along its length. Another system from Martin Walter involves a cylindrical bar of titanium (cut to a precise number of half wavelengths at the frequency used). Opposing piezoelectric transducers are attached at each end connected through a central wire. With these transducers operating together in a push–pull mode, the “concertina” effect makes the bar of metal expand and contract at half-wavelength distances along the entire length (Figure 2.4). Any erosion in this system should not affect the performance since the bar is essentially composed of solid metal material and any loss by erosion on the surface does not change the overall resonant length. This is in contrast to a conventional horn where any erosion of the tip shortens the horn length and thus affects its resonance. An understanding of the core essentials of wave propagation theory in liquids must involve the use of some mathematical concepts to support more qualitative arguments. There are a number of texts available for readers who would like to delve more deeply into the physics and mathematics of acoustic cavitation and bubble dynamics [2, 3, 10] or about the theories of the chemical and physical effects related to cavitation [11]. For those more interested in practical applications rather than the theoretical considerations of power ultrasound more details about the major physical factors influencing sonochemical events are available in the literature [7, 9, 12]. As with any sound wave, ultrasound is transmitted through a material by molecular or atomic motion having been generated initially by a vibrating source at a frequency above 20 kHz. In most cases, the source is a piezoceramic or magnetostrictive transducer (details of which can be found in any of the basic textbooks [7, 9]). Each of the molecules (or in the case of metals it will be the atoms) transmits its motion to an adjoining molecule or atom before returning to approximately its original position. For liquids and gases, particle oscillation takes place in the direction of the wave and produces longitudinal waves. An example of longitudinal waves is when a loudspeaker emits sound waves, the air molecules transmit their vibrational energies in the same linear direction as the sound waves.

2.2 Acoustic waves and their propagation

83

Figure 2.4: Schematic of action of push–pull system.

Accompanying the linear progression of the wave, there is a transverse sinusoidal component at right angles to the longitudinal wave where the fluid particles are moving perpendicular to the wave direction (Figure 2.5).

Figure 2.5: Illustration of a transverse wave.

This transverse motion can be visualized as rather like the ripples produced when a pebble is dropped into a pool of still water. The waves move away from the initial splash of the pebble but the water molecules which constitute the radial wave do not move horizontally and revert to their normal positions after the wave has passed. Thus, if an object such as a cork is floating in the pool, it will certainly move up and down when the wave reaches it but there will be no movement in the direction of the wave. When sound is propagated through air, it is also transmitted via compression and rarefaction sequences, the air molecules moving forward and backward to their rest position. The pitch (or note) of the sound produced by this series of waves depends upon their frequency, that is the number of waves which pass a fixed point in unit time. For example, middle C is 261.6 times per second (261.6 Hz). It is important to understand the relationship between particle motion and pressure during the passage of an acoustic wave. Figure 2.1 shows the simple concept that sound is transmitted as a series of compression and rarefaction phases of the molecules. This shows that at the point of maximum compression the particles are closest together and at maximum rarefaction they are furthest apart. Exactly

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midway between these extremes of pressure, the particles are essentially in their normal equilibrium positions, that is the distances apart which they would maintain in the absence of a sound wave. The behaviour of these molecules during the passage of a sound wave can be described in terms of displacement (Figure 2.6). Displacement has a specific meaning in physics and is used to describe a particle in motion in terms of how far it has moved from a given point. In the case of a sound wave, the given point is the normal equilibrium position. Since the pressure wave is sinusoidal, there will be a corresponding sinusoidal response in terms of particle separations. Separation is shown on the displacement graph in terms of slope, that is tangent, and the largest tangents correspond to the maximum changes in separation from equilibrium which occur at the pressure maxima and minima. Thus, the particle displacement curve (Figure 2.6) is out of phase with the pressure curve (Figure 2.7).

Figure 2.6: Displacement (x) graph.

Figure 2.7: Pressure (P) graph.

When the tangent is zero (horizontal) on the displacement curve the molecules are at equilibrium separation and this occurs (as described above) exactly midway between a pressure maximum and a pressure minimum. The reason there are two positions for this “equilibrium” distributions is because the molecules can attain equilibrium distribution as a result of being moved to the left or to the right during the passage of the sound wave. Now we can engage in a brief mathematical treatment of sound propagation which is required in order to appreciate more fully the terms used in acoustics [7]. From Figure 2.6, at any time (t), the displacement (x) of an individual air molecule from its mean rest position is given as follows:

2.3 Ultrasound parameters: velocity, wavelength and frequency

x = xo sin 2πft

85

(2:1)

where xo is the displacement amplitude and f is the frequency of the sound wave. Differentiation of the above with respect to time leads to an expression for the particle velocity V: V=

dx = vo cos 2πft dt

(2:2)

where vo is the maximum velocity of the particle (vo = 2πfxo) The pressure (P) at any instant in time (t) is dependent on sound frequency (f): Pa = PA sin 2πft

(2:3)

where PA is the pressure amplitude at that instant. Figures 2.6 and 2.7 show that the maximum displacement of particles appears at the point of minimum pressure (P = 0), in other words the displacement and pressure are out of phase. In the case of a liquid (as is also the case for air), the molecules are under the action of the applied acoustic field Pa but also and in addition to this the ever-present ambient hydrostatic pressure Ph. Thus, the total pressure P, in a liquid at any time, t, is given as follows: P = Ph + Pa

(2:4)

2.3 Ultrasound parameters: velocity, wavelength and frequency Ultrasound has the same characteristics as audible sound with the equivalent relationship between sound velocity c, wavelength λ and frequency f as any sound wave: c = λf – – –

(2:5)

Frequency (f) will remain the same whatever medium that the sound wave passes through, it is determined only by the source of acoustic vibration. Velocity (c) has a characteristic value for a given medium at whatever frequency is used. Wavelength (λ) will change with the velocity.

From eq. (2.5), we can calculate the wavelength range corresponding to the ultrasonic frequencies of 20–1,000 kHz the range normally employed in sonochemistry. Using an approximate sound velocity c in water of 1,500 m/s, this produces a wavelength range of between 7.5 and 0.15 cm. These wavelengths are far longer than any molecular bond length and thus we can deduce that sonochemical effects cannot be the

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result of direct interaction between the chemical reagent and the sound wave. This differentiates sonochemistry from photochemistry where direct interaction can occur. In the case of diagnostic ultrasound, which is the type used for examining the foetus during pregnancy, higher frequencies (1–50 MHz) are employed. This corresponds to a much shorter wavelength range of 0.15 to 0.003 cm; but these are still much longer than molecular bond lengths and so they are also unable to directly produce any chemical changes. The shorter wavelength is important because an ultrasound image is generated through a series of sound echoes that are reflected from changes in tissue structure [13]. Ultrasound cannot detect any changes that are smaller than its wavelength and so higher frequencies of ultrasound are used produce better image resolution. While transducers with higher frequencies produce a higher resolution image, there is a problem that very high frequencies do not penetrate very far into the body. They are used for imaging small structures at shallow depths. More powerful slightly lower frequency probes are required for imaging at greater depths, but as a result the image will not have the fine detail of one produced at higher frequencies. For this reason, ultrasonic probes used for sonography are normally designed to operate at more than one frequency to accommodate different image depths.

2.4 Sound attenuation Sound attenuation (sound absorption) is a very important parameter in the transmission of sound but it is often neglected by researchers when carrying out sonochemical reactions. Attenuation means a loss of strength of the sound energy over distance. During the propagation of a sound wave through a medium, the intensity of the wave and its power density decreases as the distance from the radiation source increases. Mathematically, this effect is best expressed in terms of sound intensity rather than sound power density. It is also important to understand that the calculation is made assuming that there is no cavitation produced in the medium. The intensity, I, at some distance, d, from the source is given as follows: I = I0 expð − 2αdÞ

(2:6)

where α is the absorption (attenuation) coefficient. According to Stokes [14], the absorption coefficient in a liquid due to frictional losses (αs), is given as follows:
8ηs π2 f 2 αs = (2:7) ð3ρc3 Þ where ηs is the shear viscosity of the liquid. The value of αs/f2 is a constant for a given liquid at a given temperature and so any increase in sound frequency, f, must result in a compensatory increase in αs

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and thus a more rapid attenuation of the sound intensity with distance (eq. (2.7)). The value of αs/f2 for a wide variety of frequencies in water is 21.5 × 10–17 cm−1 [15]. Using this value, we can calculate that the distances required for sound intensities to be reduced to half of their original values at 21.5 and 127.0 kHz will be 35 and 1 km, respectively. However, in sonochemistry, where reaction vessels are not likely to exceed dimensions of much more than 1 m, any differences in sound attenuation between them at the normal frequencies used will be small and so can be ignored. Attenuation can also result in heating from friction developed between molecules as they are set in motion by the acoustic wave. In sonochemistry, there will also be additional heat generated by cavitation bubbles which themselves present barriers to sound transmission through reflection, refraction, diffraction or scattering of the ultrasonic wave. Such is the case of a reaction mixture in a vessel dipped into an ultrasonic cleaning bath. Experimental temperatures often rise very quickly, generally by around 5 °C, over the surrounding liquid during the first few minutes of applying ultrasound. For an ultrasonic probe system, this rise can be even higher but if the reaction vessel is equipped with an efficient cooling system the temperature can be maintained effectively constant, but even then, it is still normally some 2–5 °C higher than the liquid used for cooling. It is heat generation that provides the basis for calorimetric assessment of acoustic power (see Volume 2, Chapter 5).

2.5 Ultrasound power measurements and dosimetry The quantification of the effects of power ultrasound on chemical systems is necessary in order to obtain optimum and reproducible effects from ultrasonic irradiation. Inextricably tied in with this approach is the need for accurate acoustic power measurement, also referred to as acoustic dosimetry. When a sound wave travels through a liquid, it creates an additional pressure called the sound or acoustic pressure; this is the pressure normally measured with a hydrophone in liquid media. It is a measure common in diagnostic ultrasound. The pressure of a sound wave at any point in the medium is given by eq. (2.3). For a planar progressive wave, the sound pressure is related to the vibrational velocity of the particles as follows: Pa = ρc ν

(2:8)

where ρ is the density of the medium and c is the sound speed in the medium [16, 17]. The product of the density of the medium and the sound velocity in this medium (ρc) is known as the specific acoustic impedance of that medium. The greater the acoustic impedance the greater is the sound pressure amplitude:

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PA = ν0 ρc

(2:9)

where ν0 is the maximum particle velocity in the medium. It is obvious that a sound wave propagated through a medium carries a certain amount of energy with it. From a chemist point of view, it is important to characterize this energy. If we consider the amount of energy carried by sound waves in 1 s passing a cross-sectional area of 1 cm2 with a velocity c this will give the intensity of sound I, usually measured in watts/square centimetre. The intensity of the sound wave may be expressed as follows: I=

P2 A 2ρc

(2:10)

Thus, the sound intensity is proportional to the square of acoustic amplitude. Clearly, to measure the sound intensity at a particular point in a medium, either the maximum particle velocity, v0 (eq. (2.2)) or the maximum pressure amplitude, PA must be determined. In practice, this is extremely difficult, and for most sonochemical applications where the power is high and cavitation is present, a form of calorimetric determination of the total ultrasonic energy delivered to the medium is considered to be sufficient. We shall see later that this is a very approximate measure of energy input, but it is both simple and easy to apply. In the laboratory, a chemist generally deals with a reaction or process taking place in a fixed volume rather than in a flow system and this can be used to define one of the most useful acoustic parameters: ultrasonic power density – in W/cm3. The acoustic field generated in a chemical process is not generally uniform. Moreover, the acoustic pattern depends enormously on the reaction medium and the configuration of the reactor e.g., the presence of a stirrer and its shape. If the medium is homogenous (a clear liquid reaction mixture), the acoustic pattern is dependent on the reaction vessel shape and volume and the way the ultrasound is introduced into the reaction mixture. This is the reason why a straightforward scale up of a sonochemical process is not possible. The acoustic pattern for systems under ultrasonic irradiation is far more complicated than in the case of homogenous media. If the medium is heterogeneous there are a number of additional factors which can influence the acoustic field: – Suspended particles in the medium (e.g. catalysts or herbal materials during an extraction processes) – Immiscible liquid mixtures producing multiple interfaces when they undergo emulsification – The generation of products which will have different acoustic properties compared with the starting materials – The presence of acoustic cavitation bubbles

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There are three general methods of measurement that can be applied to dosimetry and these are thermal, mechanical and electrical [18]. Within these, there are many different approaches which were reviewed some years ago [19]. The most popular laboratory method adopted by sonochemists has proved to be a form of calorimetry which is easy to perform even though it may not be the most accurate [20]. More details of dosimetry are to be found in the chapter on synthesis (Volume 2, Chapter 5).

2.6 The importance of power distribution and power density A conventional sonochemical reaction in a laboratory involves the delivery of ultrasonic power, from whatever source is employed, into a known reaction volume. In such situations, the overall ultrasonic power density Pd, in W/cm3 is a logical measure of the total energy delivered to the ultrasonic system rather than the ultrasonic power intensity which is the power delivered from the surface of the vibrating source in watts/cm2. In the case of a probe device, the ultrasonic power intensity is only valid at the surface of the probe. The acoustic energy from the surface is distributed into the reaction through a cone-like shape which carries the cavitation field and is not propagated as a cylindrical planar wave of the same diameter as the vibrating surface.

Figure 2.8: Acoustic zone generated by different probe shapes.

The transmission of energy into the system through the cone of energy (or whatever shape emerges from the emitting surface, see Figure 2.8) is best represented as ultrasonic power density. The resulting energy entering the reaction is quickly attenuated because the reaction mixture is no longer homogeneous (see Section 2.9) but containing millions of (cavitation) bubbles which will interfere with sound transmission. The reaction volume outside of this zone (often the bulk of the reaction mixture) is not directly subjected to sonication unless there is good mixing. This can be achieved through acoustic streaming or mechanical stirring both of which are critical for the correct application of sonication to the whole reaction mixture. Some experimental set-ups to achieve this will be discussed later in the book. A simple example of the use of acoustic streaming alone for the efficient total sonication of a reaction is the use of a Rosette cell (Figure 2.9). In its basic form (Figure 2.9a) a probe is

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(a)

(b)

Figure 2.9: Rosette cell used in sonochemistry (a) basic form (b) adapted for laboratory use.

dipped into the cell and rapid streaming from the tip of a probe drives the liquid downwards and forces it through the three external tubes back into the cell towards the top of the vessel. This system also allows for efficient temperature control or cooling when the Rosette cell is dipped into a cooling bath. Figure 2b shows the Rosette cell as part of a reactor for use in laboratory synthesis in which the open top of the cell has been modified with the addition of a glass flange. A corresponding glass flanged seal has been placed on top of it equipped with entry ports for an ultrasonic probe and and glassware such as addition funnels and condensers. When an ultrasonic cleaning bath is used for sonochemistry, the intensity distribution is different, and the acoustic field is much broader, radiating directly from transducers usually attached to the outside of the tank base or walls. Then, it is the tank wall itself which vibrates and transmits the energy through what is effectively a large emitting surface. The amplitude of vibration is however smaller than that of a probe. It is because the metal of the whole tank is vibrating that a rubber insert is fitted between the tank and its outer casing to cushion the inner tank vibration from the casing and so reduce frictional wear. Sonication using a bath or a probe both require stirring to ensure that the reaction mixture is brought repeatedly into the region in front of the surface that is emitting ultrasonic vibrations. From this, it is obvious that when a reaction mixture is treated with ultrasound, whatever the source, it is not wise to assume that the acoustic field entering the mixture is homogeneous. Some parts of the reaction mixture will not receive the same amount of ultrasonic energy and the situation becomes even more complicated when the reaction mixture contains suspended solids. The solids will not only

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further disturb the passage of the acoustic waves but also interfere with cavitation bubble generation and collapse. Indeed, it has been argued that if cavitation is involved then there is no such thing as a truly homogeneous sonochemical process. This is because the bubbles themselves change the acoustic characteristics of the solution. The problem of changes in the acoustic wave propagation is particularly important when considering the scale-up of a sonochemical process. It is very difficult to apply calorimetric measurements of approximate power density that are made on a laboratory scale in a batch reactor to a scaled-up system. This is because the acoustic pattern can never be the same in a larger volume when the reactor shape is different or the configuration for stirring is changed and particularly when the source of ultrasound is different.

2.7 Sonochemistry reaction conditions and scale-up Scale-up in sonochemistry is not a matter of simply enlarging a laboratory system. There is an important dependence of product formation on the intensity of ultrasonic irradiation as outlined above. To achieve scale-up, the sonochemical installations are likely to be very different from a bath or probe system in laboratory experiments. Early attempts to do this were reported some 30 years ago [21] when sonochemistry became a recognised route to follow for process intensification [22]. What is clear is that the results obtained in laboratory experiments must be carefully recorded in order to be able to use them scaling up. Unfortunately, a simple response surface method (RSM) or other computer software programme that would normally be employed for the optimization of chemical processes cannot be applied to sonochemical processes. The reason is that sonochemical processes are non-linear; the acoustic power distribution is totally different when the reaction volume is changed, and there will be a difference in acoustic behaviour when the reaction mixture changes composition due to the generation of products. There are a number of guidelines that should be followed and some of these have been identified in the use of ultrasonically assisted extraction (UAE) of herbs [23]. A summary of this guidance is provided here with regard to “best practices” and this information is required when reporting sonochemical reactions so that it can be understood and replicated by other workers. The points that should be included are: – the ultrasonic source (in terms of: bath or probe, working frequency, nominal power and manufacturer); – if an ultrasonic cleaning bath is used then the location of the reaction vessel within it; – when an ultrasonic probe is used its shape and also its position in the reaction vessel (including depth of immersion); – reaction vessel shape and reaction volume; – the type of stirring used, if any;

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effective ultrasonic power measured via one of the methods described in the literature [19, 24–26]; the ultrasonic power should be reported as power density (W/cm3); accurate measurement of temperature inside reaction (or extraction) vessel, because this could be 2–5 °C higher than the surrounding cooling/heating bath.

The use of calorimetric measurements can give an approximate value for the ultrasonic power entering into a given system. This is a very good guide for other scientists attempting to reproduce the reaction at a similar scale. However, in terms of scaling up the process: – These values are a good guide only if the scale-up method employed for either a batch or continuous reaction is transferred to a group of many reactors of the same dimensions, working in parallel. The so called numbering up approach [27, 28]. – These values cannot be used to scale up to a single larger reactor of a similar or different configuration.

2.7.1 Scaling up; a note of caution about computer optimization In the preceding section, we mentioned that it is not possible to apply computer optimization to sonochemical reactions. We can emphasize this by considering the simple scale up of a sonochemical reaction using the calculated ultrasonic power (or better the ultrasonic power density) required to drive a laboratory reaction. Direct scale-up would suggest a much bigger probe of the same shape inserted into a greatly increased volume of solution in a reactor of the same geometry as that used in the laboratory. However, this cannot provide the same acoustic pattern in the larger reaction mixture as that obtained on a smaller scale. The normal option for a batch scale-up would be for either multiple probes to be used or, as an alternative, to use a flow-through system. But, just as in the case of direct scale-up, the acoustic pattern produced using these two options will never be the same as that produced on a laboratory scale. It is the opinion of both of the authors that it is perfectly possible to optimize a batch experiment to obtain better yields – this is normal sonochemistry laboratory practice. However it is very difficult, if not impossible, to mathematically model the transition from laboratory to large batch scale using existing software. We want to make clear to any scientist who is looking to transfer a sonochemical process from a small to a larger scale, that all ultrasonic processes used in this way are nonlinear. In other words, what is applicable to a small volume experiment cannot directly applied to a larger scale system even if it is only increased fivefold (e.g. from 70 to 350 mL). This observation is made because of the way in which many scientists brought up using standard chemical engineering methodology seem convinced that the scale up of sonochemical reactions follow normal rules. It is not appropriate to use software such as Box-Behnken Design (BBD), or Central Composite Design (CCD) to

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predict the paths of sonochemical reactions. This type of approach does not work for the reasons given above because sonochemical processes are nonlinear, which means that the outcomes of the process are not proportional to the changes in the input.

2.7.2 Continuous and loop reactors The simplest configuration for a flow system is one in which the reaction mixture is pumped from a reservoir through a small powerful ultrasonic reactor and from there on to post-reaction processing. Such a system has the advantage of subjecting the reaction to high-powered ultrasound in a small volume rather than lower powered ultrasound applied to the whole reaction mixture (Figure 2.10). Post reaction processes Ultrasonic processor

Reaction mixture

Pump

Figure 2.10: Schematic of a flow through sonochemical process.

This is often referred to as a continuous flow reactor [29]. It involves only a single pass of the reaction mixture through an active ultrasonic zone. This kind of reactor can be found in sonochemical processes where the reaction is completed or almost completed after one pass. But many sonochemical reactions require more time in the sonication zone than can be achieved in this simple single pass configuration. To achieve this extended time in the reaction zone, the system can be adapted either by (1) lowering the flow rate (2) adding an extra ultrasonic processor or (3) passing the reaction mixture through the zone more than once. It is the last adaptation (3) which is often chosen and the system becomes a multi pass reactor or “loop reactor” by recirculating the reaction mixture (Figure 2.11). In this configuration, the reaction mixture from a reservoir is passed through an external ultrasonic reactor and back to the reservoir. Continuous pumping will allow high-powered ultrasound to be applied to a small volume of the total reaction mixture at each pass around the loop. An important characteristic of this system, that should be carefully considered, is the fact that it is only the first pass of the reaction mixture through the ultrasonic processor that has a definite and known chemical composition – it is the mixture that was made up at the start of the reaction. After this and as a result of each pass through the ultrasonic processor the mixture will have been sonochemically altered. We should perhaps mention here that there are

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Ultrasonic processor Discharge Stock solution tank

Pump

Stock solution Figure 2.11: Schematic of a loop reactor [29].

many papers in which authors (probably chemists rather than chemical engineers) have used the term ultrasonic “continuous reactor” when the system used was in fact a “loop reactor”. One of the problems for chemists is the use of the term “sonication time” when referring to loop reactors which is certainly not the same as the total running time that the loop reactor is in operation. This is because, unlike a batch reactor where the whole mixture is exposed to sonication during its “on” time, only a part of the reaction mixture is exposed to ultrasound in a flow loop system. The running time of such a system is very definitely not the same as the sonication time. For a chemical engineer the calculation of the total exposure time in such a system is relatively simple but for those who are not engineers it is not so easy to appreciate how short the actual exposure times can be. As an example, whatever the volume of the stock solution in the reservoir if the ultrasonic reactor in the loop has a volume of 0.1 L and the pump flow rate is 0.5 L/min the actual residence time of the solution within the reactor is only 0.2 min [29]. If this loop reactor is run for 60 min with a stock solution of 10 L, the actual exposure time of the reaction mixture i.e. the whole of the reaction mixture has been exposed to sonochemical treatment is only 0.6 min! Care must therefore be taken when reading reports of a sonochemical reaction taking place in a loop reactor where the processing time is reported as “one hour”, the real sonochemical treatment (exposure) time will be very much shorter. We believe that it is essential to emphasis this aspect of loop processing to highlight the differences between this and a continuous sonochemical process, very often considered the same. In Section 2.7.2.1, a model calculation is shown which estimates the real sonication time for a sonochemical reaction using a looptype arrangement. Another important characteristic of the loop reactor mentioned above but reemphasized here (because it is often ignored) is the fact that it is only the first pass of the reaction mixture through the ultrasonic processor when the chemical composition is precisely defined. This composition during the first pass is “starting materials” but after a single pass through the sonochemical zone of the whole reaction mixture, some

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reagents are converted to products therefore at the next pass some product will be present. Each pass will change the quantity of products formed and this will change the consistency of the solution and as a consequence the way in which the acoustic energy is transferred into it. This may, or may not, result in a significant difference from the products obtained from the same sonochemical reaction performed in a batch configuration but sometimes it is necessary to consider all potential changes during sonochemical reactions. 2.7.2.1 How to calculate the true ultrasonic exposure time for a loop system Consider a loop system containing a sonochemical reactor such as that shown in Figure 2.11 in which the following parameters are known: – reactor volume, Rv; – flow rate, Fr; – stock solution volume, Sv; and – total sonication (operation) time, Op. The spread sheet for calculation of the real sonication time, which is always different from the operation time can be seen in the example shown in Table 2.1 [29]. Table 2.1: Example of calculation of the real-time sonication for a loop reactor. Reactor volume, Rv (L) Flow rate, Fr (L/min) Rt = Rv/Fr (= L/L/min = min)

Residence time, Rt (min)

  .

Stock solution volume, Sv (L)



Overall operation time, Op (min)



Np = Fr/Sv × Op (=L/min/L × min = dimensionless)

Number of passes, through reactor Np

.

St = Rt × Np (= min)

Actual sonotreatment time, St (min)



What is significant about this type of calculation is that it reveals an actual exposure to sonication which is significantly lower than the overall operation time of the reactor. For the example given in Table 2.1, a loop reactor with an overall operation time of 20 min produces 10 passes through reactor at the flow rate specified which converts to only 5 min actual sonication time.

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2.8 Bubble collapse At the beginning of this chapter, we showed (Figure 2.1) that when acoustic power exceeds the cohesive forces of liquid during rarefaction, a small “hole” is formed in that liquid and the fate of that hole or cavitation bubble will dictate the course of any subsequent sonochemical event. Potentially, the rarefaction phase can produce four different types of cavities in a liquid before its subsequent collapse [7]: 1. simple expansion without rectified diffusion produces a void; 2. vapours of liquid enter the first-formed void by rectified diffusion; 3. dissolved or entrained gas enters void; 4. a combination of gas and vapour is contained in the void. In sonochemistry, however, the predominant type of cavitation bubble contains vapour (or vapour with a gas) as the result of rectified diffusion. Subsequent collapse of the bubble when it becomes unstable releases a tremendous amount of energy at the point of implosion often referred to as a hotspot. This term was used for the first time in a paper published in 1956 by Fitzgerald et al. “Chemical Effects of Ultrasonics – ‘Hot Spot’ Chemistry” [30]. Thirty years later, in 1987, Henglein wrote in a review paper that “Four types of sonochemical reactions are known” [31]: – The acceleration of conventional reactions – Redox processes in aqueous solution – The degradation of polymers – The decomposition of and reactions in organic solvents This was based on his presentation at the first sonochemistry symposium in Warwick, UK, in which he attempted to link different types of chemical reaction that were influenced by ultrasound and were recognized at that time as being under the general umbrella grouping of sonochemistry. Today it is accepted that the number of types of sonochemical reaction is much larger and the range of applications has extended to many more fields than chemistry but most can still be explained by acoustic cavitation. It was therefore a very wise move one might say even a brave decision of the European Society of Sonochemistry to include a range of disciplines which are not pure chemistry under the name of sonochemistry. This expansion of the range of applications of the subject was confirmed with the successful introduction of the journal Ultrasonics Sonochemistry (see Chapter 1). The topics included: – sonoelectrochemistry in processing and analysis [32, 33]; – ultrasonically assisted extraction (UAE) of biocomponents from herbs [34]; – environment remediation using ultrasound [35]; – therapeutic application of ultrasound in medicine [36]; – material science [37]; – renewable fuels [38];

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– – – –

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textile finishing [39, 40]; surface engineering [41]; ultrasound in food technologies [42]; green chemistry [43].

Sonochemistry is based on cavitation – the collapse of acoustically generated bubbles that release high energy in a very small point within a sonicated liquid. This has always raised doubts in the minds of some scientists about what is meant by a “homogeneous” liquid because it can be argued that as soon as bubbles are formed a homogeneous liquid becomes heterogeneous. In May 2000, during the seventh meeting of European Society of Sonochemistry held in Biarritz, France, this was part of a round-table discussion in which scientists tried to define these terms. In the end, it was agreed that: – a “homogenous” sonochemical reaction should be considered to be a reaction in which there is always just a liquid phase; – a “heterogeneous” sonochemical reaction was one in which a solid material is present or suspended in the reaction media. This is of course the classical definition but there remained some disagreement about a sonochemical reaction which was homogeneous until cavitation was induced because in this classical definition cavitation made the reaction a heterogeneous gas/ vapour/liquid mixture. The situation with regard to sonochemistry remained unresolved. However, the authors of this text – retaining the view which they held at that time – would like to suggest a refinement of the definition of a homogeneous sonochemical reaction as “a reaction in which the starting materials are in the form of a single liquid phase”. In other words, the description of the state of a sonochemical reaction is defined by the conditions before sonication. Thus, heterogeneous sonochemical reactions are those that use from the beginning a suspension of solids or are composed of two or more immiscible liquids as reagents. A supporting argument for this can be found in the well-known technique used by a chemist who wishes to perform a conventional homogeneous reaction in inert atmosphere – this is achieved by bubbling an inert gas through it – the bubbled gas is not considered to render the reaction heterogeneous – so why should cavitation bubbles be considered to be any different from bubbled gas? In a way, this also parallels the situation when a homogeneous mixture is subjected to reflux, it is still referred to as a homogeneous reaction, but bubbles are present. The type of reaction (either homo- or heterogeneous before sonication) will however affect the type of cavitation collapse (symmetric or asymmetric) during sonication [44].

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2.8.1 Symmetric collapse of cavitation bubbles The cavitation bubbles created in a homogenous liquid are symmetric and their collapse in the bulk liquid is also symmetric leading to localized hotspots (~5,000 K and ~2,000 atm). The interaction of cavitation bubble collapse in the vicinity of a solid surface is somewhat different, however, and was observed and investigated many years ago. An illustration of a symmetric cavitation bubble collapse is shown in Figure 2.12.

Figure 2.12: Symmetric collapsing bubble, instantaneously from large to a tiny “hotspot” [30].

A bubble with a radius of several microns will collapse in a fraction of second and in the last instant before finally disappearing the temperature and pressure inside will be very high which is the so-called hotspot. To create a bubble in a liquid it is necessary to pull the molecules of the liquid apart. In the case of pure water the estimated acoustic pressure necessary to cause cavitation (the cavitation threshold) has a value of around 1,500 atm. Cavitation threshold values can be considered to be a characteristic of a pure liquid in a similar way, for example, that refractive index has a specific and unique value for a liquid. These values for thresholds can be found in the literature for liquids and are temperature dependent. It should be emphasized that the values quoted are for absolutely pure liquids but the situation in chemical reactions is different, the liquids are seldom pure, they often contain dissolved gases or contain microscopic particles both of which can act as weak spots in the liquid. Such weak spots substantially lower the cavitation threshold values. In the case of water the cavitation threshold of non-purified water is less than 20 atm (much lower than that the 1500 atm. recorded for purified water) [45]. Nevertheless, some general points can be made regarding factors which influence the initiation of a cavitation: – Ultrasonic power: The power must be sufficient to overcome the cavitation threshold and after that point is reached, any further rise in the power will generally increase the amount of cavitation until a point is reached when excess power causes too much cavitation close to the transducer surface and the

2.8 Bubble collapse

–

–

–

–

–

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formation of a “cushion” of bubbles. This results in a situation known as “decoupling” which stops any further increase in power transmission into the fluid. Applied frequency: As the frequency is increased, the rarefaction phase of the wave shortens, and more US power is needed to initiate cavitation. The bubble size and consequent collapse energy reduces and lowers the hotspot energy. Temperature: A rise in bulk temperature increases the vapour pressure of a liquid which makes cavitation easier. It also allows more gas/vapour to enter the bubble which then reduces the energy of the last stages of bubble collapse and dampens the hotspot energy release. The presence of a gas: A gas can be present in a reaction either from being initially dissolved or deliberately entrained. Degassing is a natural consequence of cavitation and is a visible result in the case of water used in a cleaning bath. It is important to degas the water in a cleaning bath in order for it to become more powerful in terms of hotspot energy generation. Almost all liquid chromatographs use this feature of ultrasound to de-gas eluents before use. Any gas that is entrained into the reaction will enter the cavitation bubbles and thus reduce hotspot collapse temperature. When an inert gas is used, e.g. argon, it serves a dual purpose of providing an inert atmosphere and forcing residual air from the reaction which would otherwise lead to the production of oxidizing radicals. Note also that argon, which is a monatomic gas, has a significant positive effect on the hotspot temperature [7]. Viscosity and vapour pressure: Viscous liquids are difficult to cavitate because they have a low vapour pressure and raising the temperature will improve the chances of cavitation. The “hotspot” energy produced on collapse then depends upon the amount of vapour or gas accumulated inside of the cavitation bubble during rectified diffusion. Highly volatile liquids such as ether cavitate very easily and generate cavitation bubbles so full of vapour that they do not collapse. In this situation cooling the liquid will reduce vapour pressure and so with less vapour in the bubbles they will collapse with the release of energy. Surface tension: Any reduction in surface tension will facilitate cavitation. It is for this reason that detergent is added to a cleaning bath used as a source of energy for sonochemistry. This is not in order to improve its cleaning efficiency but rather to reduce surface tension and allow more even cavitation throughout the tank to facilitate more efficient acoustic energy transfer into any reaction flask dipped into it.

2.8.2 Asymmetric collapse of cavitation bubbles The discussion above relating to symmetric cavitation bubble collapse only applies when cavitation occurs in a bulk homogeneous liquid. In any real sonochemical experiment – even in a homogeneous mixture – some cavitation will occur near to the reactor surface or any surface within the reaction medium, for example a stirrer. A

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surface will disturb the symmetry of the surrounding liquid medium and cause asymmetric collapse such as would happen in the presence of suspended particles or a surface boundary between immiscible liquids. In such situations, the spherical shape deforms and adopts a doughnut like structure (Figure 2.13).

Figure 2.13: Asymmetric bubble collapse.

The jet arising from an asymmetric collapsing bubble is directed towards the solid surface or the boundary of immiscible liquids. The velocity of the jet can attain speeds of 100 m/s or more [45]. This type of jetting was identified many years ago in 1915 when hydrodynamic cavitation was found to be responsible for the pitting of a ship’s propeller (see Chapter 1). For many years after that, it was assumed that acoustic cavitation bubbles, unlike hydrodynamic cavitation bubbles, collapsed symmetrically. It was not until some 50 years later, in 1966, that Benjamin and Ellis recorded the asymmetric collapse of an initially spherical vapour cavity near a rigid boundary and clearly showed the formation of a liquid jet [46].

2.8.3 Some comments about sonoluminescence Sonochemistry is driven by acoustic cavitation as is the production of any accompanying sonoluminescence. The generation of light from cavitation bubble collapse provides evidence of the intense energy involved in the process. However, the mechanism by which this conversion of sound energy (vibrational) to light energy (electromagnetic) remains a subject for discussion. It was first reported in the 1930s as an observation that light was emitted from a liquid subjected to ultrasonic irradiation based on the darkening of photographic plates in the complete absence of ambient light [47, 48]. For many years, this phenomenon remained mainly in the domain of physicists and mathematicians. In the early days some members of the Coventry sonochemistry group visited Alan Walton in Cambridge to witness at first

References

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hand the experimental work involving a glass-sided ultrasonic tank to visualize sonoluminescence [49]. We did see a faint glow in the tank when the lights were turned off. The study of sonoluminescence was of increasing interest to chemical science as it provided a means of measuring the temperature within collapsing cavitation bubbles resulting in estimates up to 5,000 K [50]. The discovery by Putterman that sonoluminescence from a single bubble could be observed with the naked eye was a major breakthrough [51, 52] which was later to be referred to as “a star in a jar” [53, 54]. It was Thierry LePoint and his wife Francoise Mullie who began to study the sonochemistry which might occur in the liquid immediately around these single bubbles [55].

2.9 Concluding remarks Acoustic cavitation bubbles play a key role in many uses of power ultrasound including topics such as surface cleaning [56], catalytic reactions [57, 58], nanoparticle preparations [59, 60], emulsification and biodiesel synthesis [61, 62]. Regardless of whether the collapse is symmetric or asymmetric there are many useful applications of acoustic cavitation beyond sonochemistry in a wide range of processing, materials science and medicine. The majority are described in a recent book Power Ultrasonics: Applications of High-Intensity Ultrasound containing five different sections and a total of 36 chapters [63]. Well-known applications of non-cavitating ultrasound at high frequency also exist in the very broad field of non-destructive testing and medical scanning. There are also some effects of ultrasonic waves in chemistry which do not involve cavitation which we have described in Chapter 1 as the “ordering effect”.

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[2] [3] [4] [5] [6] [7]

L. Rayleigh, VIII. On the pressure developed in a liquid during the collapse of a spherical cavity, The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 34 (1917) 94–98. F.R. Young, Cavitation, McGraw Hill, Maidenhead, 1989. T. Leighton, The Acoustic Bubble, Academic Press, London, 2012. L.A. Crum, Sonoluminescence, Physics Today, 47 (1994) 22–29. W. Lauterborn, C.-D. Ohl, Cavitation bubble dynamics, Ultrasonics Sonochemistry, 4 (1997) 65–75. T.J. Mason, J.P. Lorimer, Sonochemistry – The Theory, Applications and Uses of Ultrasound in Chemistry, Ellis Horwood,, Chichester, 1988. T.J. Mason, P. Cintas, Sonochemistry, Chapter 16 in: J.H. Clark, D.J. Macquarrie (Eds.) Handbook of Green Chemistry and Technology, Blackwell, Oxford, UK, (2002) 372–396.

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Chapter 2 Fundamental aspects of sonochemistry

L.A. Crum, Acoustic cavitation series: Part five rectified diffusion, Ultrasonics, 22 (1984) 215–223. T.J. Mason, Sonochemistry, Oxford University Primer Series #70, Oxford Science Publications, Oxford, UK 1999. J.R. Blake, J.M. Boulton-Stone, N.H. Thomas, Bubble dynamics and interface phenomena, in: Proceedings of an IUTAM Symposium Springer Science & Business Media, Birmingham, UK., (1994) 456. L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick, Sonochemistry and Sonoluminescence, Kluwer, Dordrecht, Netherlands, 2013, 1999. T.J. Mason, D. Peters, Practical Sonochemistry, 2nd ed., Horwood Publishing, Chichester, UK, 2002. M.M.G.H. Harisinghani, J.W. Chen, R. Weissleder, Primer of Diagnostic Imaging, 6th ed., Elsevier, 2018. G.G. Stokes, On the theories of the internal friction of fluids in motion, and of the equilibrium and motion of elastic solids, Transactions of the Cambridge Philosophical Society, 8 (1849) 287–305. F.E. Fox, G.D. Rock, Ultrasonic absorption in water, Journal of the Acoustical Society of America, 12 (1941) 505–510. I.E. Ėlʹpiner, Ultrasound: Physical, Chemical, and Biological Effects, Consultants Bureau, New York, 1964. A.B. Wood, A Textbook of Sound: Being an Account of the Physics of Vibrations with Special Reference to Recent Theoretical and Technical Developments, 3rd (revised) ed., G. Bell, London, 1960. T.G. Leighton, P.R. Birkin, M. Hodnett, B. Zeqiri, J.F. Power, G.J. Price, T.J. Mason, M. Plattes, N.V. Dezhkunov, A.J. Coleman, Characterisation Of Measures Of Reference Acoustic Cavitation (COMORAC): An Experimental Feasibility Trial, in: A.A.Doinikov (Ed.) Bubble and particle dynamics in acoustic fields: modern trends and applications, Research Signpost, Kerala, India, 2005, 37–94. T.J. Mason, J. Berlan, Dosimetry in Sonochemistry, in: T.J. Mason (Ed.) Advances in Sonochemistry, JAI Press, Greenwich, CT, 4 (1996) 1–74. S. Koda, T. Kimura, T. Kondo, H. Mitome, A standard method to calibrate sonochemical efficiency of an individual reaction system, Ultrasonics Sonochemistry, 10 (2003) 149–156. J. Berlan, T.J. Mason, Sonochemistry: From research laboratories to industrial plants, Ultrasonics, 30 (1992) 203–212. T.J. Mason, E.D. Cordemans, Ultrasonic intensification of chemical processing and related operations: A review, Transactions of the Institute of Chemical Engineers, 74 (1996) 511–516. M. Vinatoru, Ultrasonically assisted extraction (UAE) of natural products some guidelines for good practice and reporting, Ultrasonics Sonochemistry, 25 (2015) 94–95. T. Kimura, T. Sakamoto, J.M. Leveque, H. Sohmiya, M. Fujita, S. Ikeda, T. Ando, Standardization of ultrasonic power for sonochemical reaction, Ultrasonics Sonochemistry, 3 (1996) S157–S161. R.F. Contamine, A.M. Wilhelm, J. Berlan, H. Delmas, Power measurement in sonochemistry, Ultrasonics Sonochemistry, 2 (1995) S43–S47. T.J. Mason, D. Peters, Practical Sonochemistry, 2nd ed., Horwood Publishing, Chichester, UK, 2002, 33–46. M. Freemantle, ‘Numbering up’small reactors, Chemical and Engineering News, 81 (2003) 36–36. Y. Su, K. Kuijpers, V. Hessel, T. Noël, A convenient numbering-up strategy for the scale-up of gas–liquid photoredox catalysis in flow, Reaction Chemistry & Engineering, 1 (2016) 73–81.

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[29] M. Vinatoru, T. Mason, Comments on the use of loop reactors in sonochemical processes, Ultrasonics Sonochemistry, 39 (2017) 240–242. [30] M.E. Fitzgerald, V. Griffing, J. Sullivan, Chemical effects of ultrasonics – “hot spot” chemistry, The Journal of Chemical Physics, 25 (1956) 926–933. [31] A. Henglein, Sonochemistry: Historical developments and modern aspects, Ultrasonics, 25 (1987) 6–16. [32] T.J. Mason, J.P. Lorimer, D.J. Walton, Sonoelectrochemistry, Ultrasonics, 28 (1990) 333–337. [33] R.G. Compton, J.C. Eklund, F. Marken, Sonoelectrochemical processes: A review, Electroanalysis, 9 (1997) 509–522. [34] M. Vinatoru, An overview of the ultrasonically assisted extraction of bioactive principles from herbs, Ultrasonics Sonochemistry, 8 (2001) 303–313. [35] T.J. Mason, A. Tiehm, Ultrasound in Environmental Protection, Elsevier, Amsterdam, the Netherlands 2001. [36] G. ter Haar, Therapeutic ultrasound, European Journal of Ultrasound, 9 (1999) 3–9. [37] A.J. Cobley, L. Paniwnyk, T.J. Mason, V. Saez, Aspects of Ultrasound and Materials Science, in: D. Chen, S.K. Sharma, A. Mudhoo (Eds.) Handbook on Applications of Ultrasound: Sonochemistry for Sustainability, CRC Press, Boca Raton, FL (2011) 41–74. [38] Y. Maeda, M. Vinatoru, C. Stavarache, K. Iwai, H. Oshige, Method for Producing Fatty Acid Alcohol Ester, Cosmo Engineering Co., Ltd, Tokyo (JP), United States of America, 2005. [39] M.R. Mathur, Ultrasound and textiles, Man-Made Textiles in India, 41 (1998) 523–526. [40] S. Vajnhandl, A. Majcen Le Marechal, Ultrasound in textile dyeing and the decolouration/ mineralization of textile dyes, Dyes and Pigments, 65 (2005) 89–101. [41] L.H. Thompson, L.K. Doraiswamy, Sonochemistry: Science and engineering, Industrial & Engineering Chemistry Research, 38 (1999) 1215–1249. [42] T.J. Mason, F. Chemat, M. Ashokkumar, Power Ultrasonics for Food Processing, in: J.A. Gallego-Juárez, K.F. Graff (Eds.) Power Ultrasonics, Woodhead Publishing, Oxford, (2015) 815–843. [43] P. Cintas, J.-L. Luche, Green chemistry. The sonochemical approach, Green Chemistry, 1 (1999) 115–125. [44] C.E. Brennen, Cavitation and Bubble Dynamics, Cambridge University Press, Cambridge, 2013. [45] W. Lauterborn, R. Mettin, Acoustic Cavitation: Bubble Dynamics in High-Power Ultrasonic Fields, in: J.A. Gallego-Juárez, K.F. Graff (Eds.) Power Ultrasonics: Applications of HighIntensity Ultrasound, Woodhead Publishing, Oxford UK (2015) 37–78. [46] T.B. Benjamin, A.T. Ellis, The collapse of cavitation bubbles and the pressures thereby produced against solid boundaries, Philosophical Transactions for the Royal Society of London. Series A, Mathematical and Physical Sciences 260 (1966) 221–240. [47] N. Marinesco, J. Trillat, Action of supersonic waves upon the photographic plate, Proceedings of the National Academy of Sciences, 196 (1933) 858–860. [48] H. Frenzel, H. Schultes, Luminescenz im ultraschallbeschickten Wasser, Zeitschrift Für Physikalische Chemie, 27B (1934) 421–424. [49] A.J. Walton, G.T. Reynolds, Sonoluminescence, Advances in Physics, 33 (1984) 595–660. [50] M. Ashokkumar, F. Grieser, A comparison between multibubble sonoluminescence intensity and the temperature within cavitation bubbles, Journal of the American Chemical Society, 127 (2005) 5326–5327. [51] B.P. Barber, S.J. Putterman, Observation of synchronous picosecond sonoluminescence, Nature, 352 (1991) 318–320.

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[52] G. D. F., L.A. Crum, C.C. Church, R.A. Roy, Sonoluminescence and bubble dynamics for a single, stable, cavitation bubble, The Journal of the Acoustical Society of America, 91 (1992) 3166–3183. [53] S. Putterman, Sonoluminescence: The star in a jar, Physics World, 11 (1998) 38. [54] W.C. Moss, D.B. Clarke, D.A. Young, Star in a Jar, in: L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick (Eds.) Sonochemistry and Sonoluminescence, Springer, Dordrecht, Netherlands, (1999) 159–164. [55] T. Lepoint, F. Lepoint-Mullie, A. Henglein, Single Bubble Sonochemistry, in: L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick (Eds.) Sonochemistry and Sonoluminescence, Springer, Netherlands, Dordrecht, (1999) 285–290. [56] T.J. Mason, Ultrasonic cleaning: An historical perspective, Ultrasonics Sonochemistry, 29 (2016) 519–523. [57] Y.L. Pang, A.Z. Abdullah, S. Bhatia, Review on sonochemical methods in the presence of catalysts and chemical additives for treatment of organic pollutants in wastewater, Desalination, 277 (2011) 1–14. [58] G. Cravotto, E. Borretto, M. Oliverio, A. Procopio, A. Penoni, Organic reactions in water or biphasic aqueous systems under sonochemical conditions. A review on catalytic effects, Catalysis Communications, 63 (2015) 2–9. [59] A. Gedanken, Using sonochemistry for the fabrication of nanomaterials, Ultrasonics Sonochemistry, 11 (2004) 47–55. [60] V. Sáez, T.J. Mason, Sonoelectrochemical synthesis of nanoparticles, Molecules, 14 (2009) 4284–4299. [61] J.P. Canselier, H. Delmas, A.M. Wilhelm, B. Abismaïl, Ultrasound emulsification – An overview, Journal of Dispersion Science and Technology, 23 (2002) 333–349. [62] C. Stavarache, M. Vinatoru, R. Nishimura, Y. Maeda, Conversion of vegetable oil to biodiesel using ultrasonic irradiation, Chemistry Letters, 32 (2003) 716–717. [63] J.A. Gallego-Juárez, K.F. Graff, Power Ultrasonics: Applications of High-intensity Ultrasound, Woodhead Publishing, Oxford, UK, 2015.

Chapter 3 Ultrasonically assisted extraction (UAE) 3.1 Introduction to the extraction of natural medicinal compounds Plants are well known as resources in daily life and are used not only for food but also for flavourings and fragrances. Their uses in colours and pharmaceuticals are generally less known but these also date from antiquity. Plants as sources for dyeing will not be covered in this chapter but it continues to be an active area of research that dates back several thousand years. Traditional colours include blue from the woad plant (Isatis tinctoria) or red from madder (Rubia tinctorum) [1, 2]. Many useful herbs were selected in the past for use in medical treatments, but it can only be assumed that the choices were made by luck or by some kind of trial and error. Nowadays, with the help of modern science, plant materials can be targeted by genus or assessed by rapid analysis for the chemicals contained in them. There is an increasing interest in natural remedies, in part due to some disillusionment with modern medicine and drugs that either do not perform entirely as expected or are accompanied by unwanted side effects. Natural remedies have the advantage that they have passed the test of time. Synthetic medicines have a much shorter history compared to natural remedies, and some will need more time to have absolute proof that they are totally harmless. There is little doubt that natural remedies will always have a place. This is not simply because of any “fashion” for those who are forever seeking an alternative greener lifestyle instead of adopting modern drug consumerism, because there is no doubt that synthetic drugs are generally highly efficient in the treatment of many diseases. Rather, the use of traditional remedies often involves a combination of materials, since traditional medicines are mixtures and not single chemicals, and these combinations can afford, by chance, a broader treatment than a single compound. Also, there is no doubt that traditional medicine can be the source for modern drugs and the beneficial effects of most of the world’s flora have yet to be explored. Currently, in Western Europe and in the so-called developed countries, people have become reliant on modern synthetic medicines. In the UK, for example, this can be traced back to the beginning of the National Health Service, which provided doctors who did not charge patients for their treatment and who would prescribe the latest cures. Prior to this, however, ailing people would not have had this type of access to free medicine, and so remedies used as traditional cures by past generations were often derived from plants. A herbal extract can be obtained from fresh or dried plants, or parts of plants: leaves, flowers, seeds, stems, roots and barks by different solvents and extraction procedures. Generally, the active constituents are obtained together with other materials present in the vegetal mass and, in some cases, further isolation https://doi.org/10.1515/9783110566178-003

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of the main components may be required. A good example of this is the crude extract using water and alcohol from the opium poppy, Papaver somniferum. This simple extract was well known as a sleeping draught in the form of laudanum and was widely used in Europe and America up until the nineteenth century. The concentrated extract, known simply as opium, also found its way into numerous patent medicines. Several important drugs have been isolated from this extract of poppy, including morphine and codeine. Some well-known examples of traditional herbal medicines are shown in Table 3.1. There has been a resurgence of interest in “natural”, that is non-synthetic materials and many companies offer medicinal compounds that are actual plant materials or are derived from plants [3]. Examples are readily found in the field of aromatherapy (aromatic oils), pain relief (marijuana) and sweeteners (stevia). Often, however, the reason for a particular usage of a plant has been lost in the history of folklore and, without doubt, the beneficial uses of other plants are still to be fully explored. Examples of traditional remedies in the west are less common for the reasons given above but in Eastern Europe, the practice remains strong. In the Far East, there are whole research institutes and hospitals dedicated to herbal medicine and these have done much to preserve the knowledge. Table 3.1: Extraction of natural medicines from vegetal materials. O

Discovered in willow bark and leaves. Hippocrates ( BC) prescribed extracts of willow leaves to relieve pain during childbirth. The extract contains acetyl salicylic acid (aspirin), which is now one of the most commonly used of all synthetic drugs

OH O O ASPIRIN

HO

O H

H3C H N CH 3

O

O H

Together with other alkaloids used in pain control are compounds that originated as plant extracts from the opium poppy

H N CH 3

HO

HO MORPHINE

HO

N H

O N

QUININE

CODEINE

Extracted from cinchona bark and used to alleviate fever caused by malaria, often referred to as Jesuits’ bark since it was the Jesuits who popularized this medicine

3.1 Introduction to the extraction of natural medicinal compounds

107

Table 3.1 (continued) O

The ancient Incas chewed coca leaves to overcome pain, cure high altitude sickness and boost energy levels.

CH3 O O

H 3C N

O COCAINE

Produced by yew tree bark, it shows significant anticancer activity O

O NH

O O

HO

O

OH

O OH H O O O

O

O

TAXOL

One example of a herbal medicine comes from an experience by Tim Mason as a child in his home in Torquay, a seaside town in England where his mother and her family had lived for generations. When anyone in the family suffered from toothache, a bottle of whole cloves was found and one clove was applied to the gum near the problematic tooth. Cloves contain eugenol, which is a natural anaesthetic that helps to numb and reduce pain. A more modern and more expensive substitute is clove oil, obtained by distilling the dried cloves. Mircea Vinatoru, having been brought up in Romania, has much more experience with herbal remedies – both positive and negative. In his hometown (Bailesti, Romania) there was a veterinarian living near his parents’ house who became sick (diagnosed as flu) and the folk tradition was to make tea from a single leaf of birthwort weed, Aristolochia clematitis L. (Figure 3.1), for the sick person to drink. The wife of the vet was hoping to heal him faster and so put two leaves in the tea, instead of one, and he drank it. The unfortunate consequence was that immediately after drinking the tea he started to sweat profusely and became dizzy. The wife asked for help and when Mircea went with his mother to see him, his face was red, indicating a high level of blood in the facial skin. The plant is commonly used as a traditional tonic for several illnesses, but especially in childbirth (hence the name). It was an ingredient in theriac (a medical concoction originally formulated by the Greeks in the first century AD), which was used as a panacea against ulcers, fevers and snake bites. but one of its constituents, aristolochic acid, may be toxic in larger doses [4]. Fortunately, the vet recovered. This incident, together with background knowledge, influenced his choices of herbs to investigate in a COPERNICUS programme that he coordinated (see later in this chapter).

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Chapter 3 Ultrasonically assisted extraction (UAE)

O O

OH NO2

O

OCH3

Aristolochia clematitis

(a)

(b)

Figure 3.1: (a) Aristolochia clematitis L. (birthwort) and (b) aristolochic acid.

Preparations that include herbal extracts are part of the pharmacopoeias of many countries [5]. One of the most famous English herbalists, Culpeper, described in a wonderful book, published over 350 years ago, several rules for collecting plant parts for herbal preparations [6]. These rules are still applicable nowadays. Readers will find them valuable as a guide to select the part of the plant to collect. Leaves, flowers, seeds, roots, barks, rhizomes or other parts of plants can be selected in order to achieve the best results in formulations. These herbal extracts are also useful in the food industry as additives, flavours, fragrances, colours and so on. In pharmaceuticals, there are some that are antimicrobial [7] or antioxidants [8]. Parts of complete plants are used in cooking, but extracts are preferred in the food industry because these are easier to handle and provide more concentrated flavours, smells and there is also less risk of insect or microbial contamination. When one begins any research aimed at developing new extraction techniques from medicinal and/or aromatic herbs, the first requirement is to gather the relevant herbs. The harvesting time as well as the methods of preservation are of crucial importance for industrial scale processing of medicinal and/or aromatic herbs, requiring the setup of a precise technique for this purpose [9]. For a chemist not familiar with botany, this is an unfamiliar problem, but as will be seen later in this chapter, it was an important consideration of some research done in Coventry on the extraction of the herb rosemary. In Culpeper’s book, the chemical content and, therefore, the efficacy of natural treatment is found to depend on the days used in the harvesting season and even the time of day during which the collection took place.

3.2 A brief history of medicinal plants

109

3.2 A brief history of medicinal plants The use of medicinal and/or aromatic herbs is as old as humanity. Humans used plants initially as food from natural sources (food was later obtained from cultivated flora). The medicinal benefits of some of these plants were probably discovered by accident and then passed down through generations by so-called “witch doctors”. Such wild plants are normally referred to as herbs. Those that were aromatic, such as lavender, hyssop and thyme, were cultivated for their essential oils or medicinal components. Thus, Egyptian papyruses describe thousands of recipes showing that coriander and castor oil were used for medicinal application, cosmetics and as preservatives [10]. Hebrew and Chinese manuscripts related to traditional natural remedies describe over 2,000 herbs having medicinal properties [11, 12]. Hippocrates is often referred to as the father of medicine [13]. He lived during the Greek empire and prescribed treatment to an ill person in the following way: at first, talk to the patient and use counselling (psychotherapy). In the second stage, use plant-derived medications in the treatment (phytotherapy) and only when these therapies fail, turn to surgery. He gave advice on the use of 236 medicinal plants with full details for hygienic and prophylactic purposes [14]. He is credited with composing the legendary Hippocratic Oath, which can be translated as follows: I swear by Apollo Physician, by Asclepius, by Hygieia, by Panacea, and by all the gods and goddesses, making them my witnesses, that I will carry out, according to my ability and judgment, this oath and this indenture. To hold my teacher in this art equal to my own parents; to make him partner in my livelihood; when he is in need of money to share mine with him; to consider his family as my own brothers, and to teach them this art, if they want to learn it, without fee or indenture; to impart precept, oral instruction, and all other instruction to my own sons, the sons of my teacher, and to indentured pupils who have taken the physician’s oath, but to nobody else. I will use treatment to help the sick according to my ability and judgment, but never with a view to injury and wrongdoing. Neither will I administer a poison to anybody when asked to do so, nor will I suggest such a course. Similarly, I will not give to a woman a pessary to cause abortion. But I will keep pure and holy both my life and my art. I will not use the knife, not even, verily, on sufferers from stone, but I will give place to such as are craftsmen therein. Into whatsoever houses I enter, I will enter to help the sick, and I will abstain from all intentional wrong-doing and harm, especially from abusing the bodies of man or woman, bond or free. And whatsoever I shall see or hear in the course of my profession, as well as outside my profession in my intercourse with men, if it be what should not be published abroad, I will never divulge, holding such things to be holy secrets. Now if I carry out this oath, and break it not, may I gain for ever reputation among all men for my life and for my art; but if I break it and forswear myself, may the opposite befall me.

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The modern history of plant extracts began with the work of Aelius Galenus, who was a Greek physician, surgeon and philosopher in the Roman Empire in the second century. He wrote around 30 papers and recipes that are still useful today [15, 16]. In 1493, Theophrastus Philippus Aureolus Bombastus von Hohenheim was born in Switzerland. He was to become a noted physician and philosopher, who is better known by the name he adopted later in life – Paracelsus. He was one of the first to use opiates as anaesthetics in surgery and also to recommend the healing and therapeutic properties of mineral waters used at many spas and hot spring resorts situated in the Alps [17]. The medicinal and aromatic “hot bath” is still in use nowadays in Romania, at the Ana Aslan Geriatric Institute in Otopeni. The first extract to be produced commercially as a perfume (sometimes referred to as “Hungarian Water) appeared in 1380. It was an alcohol-based extract from rosemary, which was used extensively for five centuries throughout Europe [18]. In the nineteenth century, French researchers broadened investigations in the field of herbal products by developing methods of obtaining desirable commercial extracts, mainly for perfumes, but the extracts were known as “palace secrets” until the emergence of the first apothecaries. In Romania, the use of herbs for the treatment of diseases has been known since antiquity. For example, Leonurus cardiaca, L., known also as motherwort (Figure 3.2), was mentioned in an old manuscript written by Herodotus (fifth century BC). This document is preserved in the Romanian History Museum, which contains a drawing of this herb. He refers to Tracs, the people living to the north of the river Danube, who discovered its properties and were using it to treat heart conditions. This herb contains the chemical “leonurine” together with alkaloids, flavonoids, iridoids, tannins (5–9%), terpenoids, citric acid, malic acid, oleic acid, some bitter substances, carbohydrates, choline and a phenol glycoside.

O O

O

N

NH2 NH2

HO O

(a)

(b)

Figure 3.2: (a) Leonurus cardiaca (motherwort) and (b) leonurine.

Wild and cultivated herbs were used to make extracts such as herbal teas, infusions, aromatic vinegar and “tonic” wine. Many of these plant products passed into scientific medicine and “Coltea” hospital, the first hospital in Bucharest, Romania, was built

3.2 A brief history of medicinal plants

111

in 1704 as a hospital for the poor. It included a pharmacy selling medicinal herbs and herbal preparations. In the nineteenth century, vegetable products were introduced into the Romanian Pharmacopoeia [19, 20] and the first worldwide Institute of Medicinal Plants was established in 1904 in Cluj-Romania. In Britain, herbal tradition can be traced back over a thousand years. The British Library in London owns an ancient ‘leech book’ (a medical reference book, also known as Medicinale Anglicum), which contains herbal recipes to prevent diseases. It is thought to have belonged to a physician called Bald, from Worcester, who probably lived in the eighth century.

3.2.1 Notes on the Romanian Pharmacopoeia The first Romanian Pharmacopoeia was approved by the Medical Council on 5 December 1862 and was officially used from January 1863. It comprised 790 pages and was divided into three parts. The edition was written in two columns – one column was in the Romanian language and the other column was in the Latin language [19]. The first part was entitled Materia Pharmaceutica, and comprised 301 monographs, out of which 207 were herbal drugs, 23 were of animal origin, 46 were inorganic substances and 15 were organic compounds. More than half of the vegetal products came from the country and many of them are still used today for therapeutic purposes, examples of which are shown in Figure 3.3.

Matricaria chamomilla

Hypericum perforatum

Chelidonium majus

Artemisia absinthium

Arnica Montana

Leonurus cardiaca

Figure 3.3: Some examples of traditional medicinal plants.

The second part of the volume, Preparata Pharmaceutica, contained 547 monographs of galenic products and pharmaceutical formulations as well as organic and inorganic

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substances, which were developed in pharmacy (boric acid from borax, barium chloride from barium sulphate and so on). The final part of the Romanian Pharmacopoeia, Reagentia et Tabulae Variae, comprised 52 tables and a list of both the foreign names and their more common synonyms of some of the most important drugs, as well as instructions for the preparation of popular drugs. In 1874, with the approval of the first national health law in Romania, the second edition of the Romanian Pharmacopoeia was printed by the State Printing Office in Bucharest. Two decades later, in 1893, the third edition was published by a special commission appointed by the Ministry of Interior. A further 32 years passed until 1926 when the fourth edition appeared. The fifth and the sixth editions were published before and after the Second World War. The seventh edition of the Romanian Pharmacopoeia involved a much larger collaboration, including the Research Institute of Bucharest together with the Faculties of Pharmacy in Bucharest, Cluj and Targu Mures, as well as the Faculty of Veterinary Medicine at the Dr. I. Cantacuzino Institute and the branch of the Pharmaceutical Research Institute in Cluj. The commission included a total of 23 specialists under the guidance of Professor C. N. Ionescu. Review work began in 1952 and ended in 1956. The eighth edition appeared in 1965 and the ninth edition in 1976. The current edition, the 10th, has been available since 1993 and, subsequently, supplements have been published, with the latest in 2006 (the 3rd). This contained a chapter concerning “Compliance Pharmacopoeias”, and “Homeopathic Preparations” as well as updated standard Romanian terminologies. The evolution of the Romanian Pharmacopoeia together with similar publications in other countries provides a bank of knowledge about the origins of various drugs and can trigger renewed interest in a range of drugs. On a recent visit to Egypt, the authors of this book were shown an impressive inventory of native plants in several volumes of The Egyptian Encyclopedia of the Wild Medicinal Plants published by the National Research Centre in Cairo [21].

3.2.2 Some information about allelopathy This chapter on UAE predominantly deals with the uses of vegetal extracts in medical applications. However, there are other uses of plant extracts that include their use as insecticides and for the control of plant growth. A well-known example of the former is Pyrethrum, made from the dried flower heads of Chrysanthemum flowers and used for the control of aphids and whiteflies, but there are many others [22, 23]. Some plants also have the ability to deter or even stop other plants from growing around them. The first record of this can be traced to Theophrastus, around 300 BC, relating to the way chickpea exhausted the soil in which it grew and destroyed the surrounding weeds. Observations such as this have led to the development of the science of allelopathy,

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defined as the chemical inhibition of the germination or growth of one plant by another through the release of chemicals into the environment. In the mid-1990s, an active group of researchers comprising chemists, plant physiologists, ecologists and horticulturalists developed this new field of research and its history has been recorded in a book by Willis [24]. Allelochemicals are mostly “secondary metabolites”, which means that they are not needed for the basic (primary) metabolism of the plant. An example is juglone [25], produced by the leaves and green husks of walnut, which effectively stops other plants from growing around a tree. Allelochemicals affect other organisms, either by altering their physiology, growth, behaviour or life history [26].

3.3 Classical extraction procedures for vegetal materials Ultrasonically assisted extraction (UAE) is one of the newer methods of extraction that are generally referred to as non-classical which also includes the use of microwaves or supercritical fluids. However, before looking into these in more detail, it is important to understand those methods that have been used for centuries and for which, the newer techniques offer alternative and sometimes significantly improved methodologies [27]. The methods that can be considered as classical are shown in Table 3.2. Table 3.2: Classical extraction technologies. Distillation

– –

Direct distillation of essential oils Water and steam distillation

Solvent extraction

– – – –

Percolation (solvent extraction) Maceration with a solvent Infusion (boiling with water) Extraction using fat (enfleurage)

Cold expression (pressing)

–

Using pressure to squeeze liquid components from vegetal material, for example apples or olives

3.3.1 Distillation In the extraction of natural products, the term distillation almost always means steam distillation, that is, distillation in the presence of water because this achieves distillation at temperatures lower than the normal boiling point of the target components. It is applicable when the materials to be distilled are both immiscible and chemically unreactive with water. Distillation involves either the direct introduction of live steam (sometimes, superheated) into a reaction vessel containing the plant materials or mixing the herbs with water and heating the mixture to boiling point.

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In either case, the targeted volatile plant components co-distil with steam. This method is especially suitable for the isolation of essential (volatile) oils from fresh or dried plant materials [14]. The resulting vapours are cooled and collected in a separator where essential oils separate from the condensed water. A typical scheme for the implementation of such a procedure is shown in Figure 3.4.

Herbs

Condenser Essential oil Steam generator Water Separator Figure 3.4: Typical scheme used for the distillation of essential oils.

When water is mixed with plant material and heated without the use of external steam, the volatile materials distil with the steam produced from the boiling water. This procedure has some disadvantages compared with those in which the steam is injected into plant material, such as non-uniform heating and overheating near the vessel walls. Both of these lead to some degradation of the product apart from difficulty in temperature control. It is a procedure that is mainly used in smaller scale extraction units.

3.3.2 Solvent extraction 3.3.2.1 Percolation This is the most common methodology and involves passing the solvent through the herbal material supported on a filter; a simple example is the domestic coffee filtering machine. The solvent can be hot or cold according to the extract required and can be used in the laboratory as well as in industry (Figure 3.5). Cold or hot water (or, in some cases, water/steam) is introduced into the percolation unit via a distribution head and passes through the vegetal mass. The liquid phase accumulates in the bottom of the vessel from where it is collected and further processed. This type of extraction unit can be used with different solvents, such as supercritical CO2, subcritical water [28] or organic solvents. Another liquid that has been used is pressurized polyfluoroalkanes (refrigeration liquids). These can be readily

3.3 Classical extraction procedures for vegetal materials

High pressure, high temperature water or steam inlet

115

Pressure - temperature gauge

Water or steam distribution head High pressure resistant cyclinder

Vegetal biomass (dry or fresh)

Coarse filter

Liquid extract outlet

Figure 3.5: Percolation reactor for laboratory or larger scale use.

recovered and recycled by evaporation and were introduced by Peter Wilde (see Section 3.6.4.2). It is a method that has limited uses but has been employed with added cold ethanol for the extraction of Cannabidiol (CBD) from hemp (cannabis) with good results. A recent example of this is an extraction unit that was built in Bailesti in Dolj county, Romania, by the StillCanna company from Canada, together with Dragonfly BioSciences from UK, under the name ORIGIN. 3.3.2.2 Maceration It is a process that involves immersing the plant materials in a liquid solvent for a period of time, from hours to days or even months (depending on the plant material), the immersion is to allow softening of the material as well as to optimise the yield. The plant material can be fresh, dry or ground, but small pieces provide a better penetration of the solvent. For industrial maceration processes, the herbal material is placed in bags (made of vegetable fibre) attached to a slow stirring device. By this means, the bags holding the herbs are moved slowly through the solvent for the necessary time period. It is widely used with water or ethanol/water mixtures to obtain tinctures. Perhaps, the best-known example of maceration is in the making of wine. 3.3.2.3 Infusion This involves suspending a plant material in a solvent over a short time period to extract chemical compounds or flavours from it. The concentration of chemical components in the extract is rather low because the time for infusion is short. Often, the infusion itself is the final product; a common example of which is the brewing of tea.

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3.3.2.4 Enfleurage This process is most commonly used in perfumery and involves the extraction of essential oils and perfumes from flowers using odourless animal or vegetable fats. It is based on the ability of fatty oils (grease) to absorb, dissolve and bind ethereal oils, principally from flowers. It can involve the use of either cold solid fat (cold enfleurage) or at higher temperatures ranging from 50 to 70 °C when the fat or oil (usually vegetable oil is used) becomes a liquid. In both cases, the herbal material is repeatedly extracted with the same fat. This type of extract is used to make pomades for styling hair. The essential oils can be extracted from these with ethanol or other solvents, followed by further purification to obtain the fragrances used in perfumery [9]. 3.3.2.5 Cold expression (pressing) This method is used to extract juices from the pulp of fruits or other juicy parts of a plant. It is a common technology for use at an industrial scale and the vegetable product obtained by this method is not heated or contaminated with a solvent and is thus considered to be of good quality. It is the method of production for olive oil and fruit juices.

3.4 Non-conventional extraction procedures (excluding UAE) In addition to UAE, there are a number of so-called non-conventional extraction technologies that have been considered for adoption by the industry. Five of these are listed and summarised below but will not be discussed in detail. – Supercritical fluid extraction (SFE) [29] – Microwave-assisted extraction (MAE) [30] – Pulsed-electric field extraction (PEF) [31] – Enzyme-assisted extraction [32] – Pressurized liquid extraction [33]

3.4.1 Supercritical fluid extraction (SFE) The first patent for SFE was awarded to Zosel in 1964 and concerned the decaffeination of coffee [34]. The process involved supercritical carbon dioxide at a temperature in the range of 40–80 °C, a pressure within the range of 120–180 atm and a contact time between 5 and 30 h. The rapid development of this process together with several others were reviewed some 20 years later [35], but since then the field has expanded to include the extraction and purification of various natural compounds [36]. Supercritical extraction processes can be accelerated with ultrasound at pilot plant scale [37]. When used in the extraction of oil and coixenolide from adlay seed

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(Coix lacryma-jobi L. var. Adlay), ultrasound increases the yield of supercritical CO2 extraction by 14% [38]. The overall yield of malagueta pepper oleoresin from malagueta pepper (Capsicum frutescens L.) is increased by up to 30%, when compared to SFE alone [39].

3.4.2 Microwave-assisted extraction (MAE) Microwaves are electromagnetic waves and they have been employed for the heating of polar materials for many years. Microwave heating (2.45 GHz) generates heat throughout the entire volume inside the irradiated medium, while in conventional extraction, heat is transferred from the heating medium through the outer part of the sample to the interior. This results in an important difference between conventional and microwave heating. In the former, heat transfer depends on thermal conductivity and on the temperature difference across the sample and the exterior and in the case of fluids, on convection currents. As a result, the temperature increase is often rather slow. In contrast, in microwave heating, due to the heating effect on the entire volume, much faster temperature increases can be obtained. MAE has a relatively short history having been first applied to the extraction of materials for chemical analysis in 1994 [40]. Since then many applications have been found in the extraction of bioactive compounds from plants [41]. Any heating of liquids within vegetal cells will cause some breakdown of cell walls and the release of contents. The acceleration of extraction rates under microwaves compared with conventional heating may be due to a synergetic combination of mass and heat transfer acting in the same direction. Comparisons have been made between the efficiencies of extraction using microwaves and ultrasound [42, 43].

3.4.3 Pulsed-electric field extraction (PEF) Pulsed electrical discharges within an extraction mixture were also claimed to increase the extraction yield. This was first reported by Issaev and Mitev, and used in the extraction of alkaloids from the seed of Cytisus laburnum (broom) [44]. PEF treatment was recognized as useful for improving the pressing, drying, extraction, and diffusion processes during food processing [45, 46]. The principle of PEF is to destroy the cell membrane structure in order to facilitate the extraction of contents. During the suspension of a living cell in electric field, an electric potential passes through the membrane of that cell. Based on the dipole nature of the membrane molecules, electric potential separates molecules according to their charge in the cell membrane. After crossing a critical value of approximately 1 V of transmembrane potential, repulsion occurs between the charge-carrying molecules and this creates pores in weak areas of the membrane resulting in a drastic

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increase in its permeability. This method has been used to improve the extractability of a number of bioactive species from plants, including polyphenols from grape pomace (Vitis vinifera L.) [47], carotenoids from tomato peel [48] and anti-inflammatory compounds from brown rice [49].

3.4.4 Enzyme-assisted extraction (EAE) This method involves the addition of specific enzymes, like cellulase, α-amylase or pectinase, during extraction. The action of these enzymes is the hydrolysis of the structural polysaccharides that form the cell wall of oilseeds or the proteins that form the cell and lipid body membrane. The content, for example, edible oils, [50] or phenolic compounds from grape pomace [51], then become more accessible for solvent extraction. The process is recognized as an eco-friendly extraction technology for bioactive compounds and oil, because it uses water as solvent instead of organic chemicals.

3.4.5 Pressurized liquid extraction (PLE) The process was described by Richter in 1996 as a method for accelerated solvent extraction for sample preparation [52]. Essentially, the process allows solvents to be used in the liquid form beyond their normal boiling point but below the critical points. As a result, mass transfer rates are enhanced, solvent surface tension and viscosity are decreased and the solubility of extracted materials is increased [33]. Currently this methodology is mainly applied in analytical rather than in preparative processes.

3.5 The origins of ultrasonically assisted extraction (UAE) The use of ultrasound to enhance extraction has a rather longer history than the other non-conventional methods described above and was originally reported around the middle of the twentieth century, but until the 1990s, it remained somewhat unexplored. Later, when it was recognized as a viable method to increase extraction yields, it became known as Ultrasonically Assisted Extraction (UAE) – a terminology that was introduced and first used by the authors of this book in 1997 [53, 54]. In 1955, one of the first papers to be published in this field related to the extraction of peanut oil with hexane as solvent [55]. The process which used 400 kHz ultrasound showed the main characteristics of UAE: the extraction efficiency was 2.76 times greater than the control with a 90.2% yield produced in the reduced extraction time of 6 min. Another application concerned improvements to the aqueous extraction of hops in the brewing of beer [56, 57]. The production of beer generally consists in mashing

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ground malt with hot water, after which the extraction liquor is separated from the malt residues to produce wort. This solution is boiled with the addition of the required quantity of hops, passed over a filter to separate the hops and, subsequently, cooled, to allow insoluble materials to settle. The wort is then fermented with yeast and after the fermentation is completed, the beer is transferred to storage for aging. The ultrasonically assisted extraction of hops, using brewing water or wort, is much gentler and more productive at 50 °C than that can be achieved by boiling in the absence of ultrasound [56]. The process was patented because the inventor claimed that the production of beer was more economical than the traditional methods and the resulting beers were of superior quality [57]. Different sonication times revealed that the beer taste remained unchanged up to 1 h sonication and became sweeter after 2 h sonication. Some years later, in 1968, a paper was published in the British journal Ultrasonics by Truman’s Brewery in Burton-on-Trent, extolling the virtue of the use of ultrasound in hop extraction [58]. The article highlighted the combined advantages that it offered in the extraction of the bittering element of the hops and also the increase in the yield of aroma-giving oils. In the conventional brewing process this involves boiling with malt extract, and as a result some of the aromatic oils are lost through evaporation. A pilot plant was constructed to treat 420 gal of liquid and it yielded a product of improved quality with significant materials savings. The author concluded very positively that “We are convinced that the possibilities of ultrasonic hop extraction are greater than we have shown above. Further lines of investigation should contribute towards greater economies and better quality”. However, there is no evidence that UAE has been adopted in modern breweries. In the 1960s, a number of researchers used ultrasound to improve the extraction of alkaloids. Demaggio and Lott investigated its use on the maceration process as well as during the actual extraction process of Thorn apple (Datura stramonium) [59]. This plant is a member of the nightshade family and is the source of a range of alkaloidbased medicines, including atropine. Its extracts have also been used as a hallucinogen. The use of ultrasound during short periods of maceration was effective in producing a greater yield of alkaloids than that obtained by conventional procedures. When used during the process of continuous solvent extraction, ultrasound proved to be slightly more efficient in liberating the desired alkaloids. The efficiency of an ultrasonic probe system, rather than an ultrasonic bath, on the extraction of alkaloids has been investigated by Ovadia and Skauen using a 20 kHz ultrasonic generator under controlled temperature conditions [60]. The plant materials used were cinchona bark (Cinchona succirubra), ipecac root (Cephaelis ipecacunha) and Jaborandi leaf (Pilocarpus micro-phyllus). Forty-five minutes of ultrasonic treatment proved to be nearly equivalent to 7 h of Soxhlet extraction and in the case of ipecac root, the extraction solvent was chloroform. Four years later, Skauen applied the ultrasonic probe technique for the extraction of senna (Cassia acutifolia) [61]. This plant is traditionally prepared as a water infusion, a tea, which is

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a laxative. Under conditions similar to the infusion method, ultrasound achieved more rapid extraction and greater yields of aglycones (steroidal alkaloids).

3.6 The development of UAE from the 1990s in Coventry and Bucharest It was in 1990 that the Coventry group became involved in the use of ultrasound to enhance the extraction of medicinal compounds from plants. In that year, an application arrived out of the blue from Zhao Yiyun, a researcher in China, to work with Tim Mason for 12 months. Zhao Yiyun was a chemistry teacher from Yunnan University in Kunming and that university had given her a grant to study abroad. She had come to Coventry because of a recommendation by Professor Feng Ruo, her mentor in China. Professor Feng worked in the Acoustics Institute in Nanjing and had interests in many different applications of ultrasound that were mostly in the medical field (see Volume 2, Chapter 7 Therapeutic Ultrasound). He knew about the work in Coventry and exchanged letters with Tim about the student. He strongly recommended her as a researcher. While she was busily preparing the necessary paperwork for her travel to the UK, Yiyun sent the draft of a paper that she and a colleague, Bao Ciguang, had prepared. It concerned the extraction of helicid, an antidepressant drug, from the seeds of a plant of the Proteaceae family. Zhao Yiyun travelled to the UK on 9 May 1991. Despite the fact that it was her first visit to the West, she travelled alone and simply turned up at the Coventry University. She had her heart set on presenting the extraction work at a conference and the next appropriate one was Ultrasonics International 91, which was to be held in le Touquet, France. I helped her to prepare a presentation on the work and then agreed to deliver it for her, since she was not very confident of her proficiency in English [62]. This provided an introduction to a topic that would later become very important to the Coventry group – ultrasonically assisted extraction. Helicid (Figure 3.6) is conventionally extracted from dried seeds with ethanol for 2 h at 80 °C. The sonochemical route produced a 50% better yield at 40 °C in only half the time. CHO

HOCH2

O

HO

O OH

OH

Figure 3.6: Helicid, CAS number: 80154-34-3.

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UAE was not the main theme of the work which Yiyun had come to study at Coventry. Her topic was “Sonochemistry on the surface and at the interface of materials” (see volume 2 chapter 6). However, we set aside some time for further studies on extraction and in 1994, a second paper was published, this time involving the extraction of tea solids with water [63]. Tea solids are the main ingredients of instant tea and this was part of a preliminary investigation aimed at improving the process. Temperature is one of the most important factors that influences flavour in tea extraction, because raising the temperature or extending the extraction time can increase the leaching effect of water for bitter components such as tannins, making the tea less palatable. Sonication improved the yield of tea solids by nearly 20% at 60 °C, to a level that approached the efficiency of conventional thermal extraction with water at 100 °C.

3.6.1 UAE research projects supported by the European Union The real breakthrough that led to the involvement of both authors of this book with UAE came mainly from the support provided by Europe in this emerging field of research. 3.6.1.1 COPERNICUS The first grant that the authors of this book obtained to work together on ultrasonic extraction came in 1995 through the COPERNICUS scheme. This funding was driven mainly through the efforts of Mircea Vinatoru and it also established the Institute of Organic Chemistry “C. D. Nenitzescu” Bucharest, as a main player in this field of research. The purpose of the COPERNICUS scheme was to allow the European Union to support scientific and technological cooperation between groups based in the European Union and in the countries of Central and Eastern Europe. The application was assembled in Coventry since the rules required that the submission and coordination of these grants should come from a Western European partner, although scientific leadership would be from an Eastern European group. The consortium assembled included the UK (Tim Mason as coordinator at Coventry University), together with partners from Romania (Mircea Vinatoru as Scientific Lead from Costin D. Nenitzescu Institute for Organic Chemistry) and Slovakia (Stefan Toma at Comenius University, Bratislava together with Anna Ebringerova and Josef Sandula at the Institute of Chemistry – Slovak Academy of Sciences). The group also included an industrial partner from Slovakia (Pavol Cupka of Mediplant Ltd, Modra). We applied for funding for a project entitled “The development of methodologies for the ultrasonically assisted extraction of biologically active components from plants and seeds”. As far as we were aware, this was the first detailed investigation across several laboratories of what has become a well-accepted technology. The summary attached to the proposal outlined our thinking at that time:

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The classical techniques for the solvent extraction of materials from vegetable sources are based mainly upon the correct choice of solvent coupled with the use of heat and/or agitation. Most of these techniques use relatively high temperatures and long agitation or exposure times and thus are costly in terms of energy consumption. An additional disadvantage is that high temperatures may induce some decomposition of the target extracts and cause some processes to be less discriminatory. The extraction of organic compounds contained within the body of plants and seeds by a solvent is significantly improved by the use of power ultrasound. The mechanical effects of ultrasound transmitted through a solvent assists cell disruption. It also provides greater penetration of solvent into cellular materials and improved mass transfer. The use of ultrasound has been shown to generally reduce the temperatures required for extraction and in some cases to result in an enhanced specificity for component extraction not only at lower temperatures but also in a faster time. The project will be targeted at improved methodologies for a number of commercially important extractive processes. One such is the isolation of xylan from hardwoods and annual cereal crops. These materials are novel biopolymers which could enter a broad field of application in paper technology, food and pharmaceutical industries, textile printing and cosmetics. A second concerns the extraction of active components from vegetable materials for use in the pharmaceutical, food and cosmetic industries. Coupled with these basic research efforts in ultrasonically enhanced extraction it is intended that a pilot plant scale operation may be developed. This will allow the exploitation of the technology for industrial applications.

As usual, in Coventry, there was a last-minute rush to complete the paperwork of an EU grant in order to meet the Brussels deadline, but in this case, it was even tighter. Four copies of the application were taken and delivered by hand to Brussels by a colleague, who happened to be going there for other reasons on 27 April 1994 – only a day before the deadline. The effort was however worthwhile and the programme was approved under the reference number ERB–CIPA–CT94–0227–1995, and ran from 1 February 1995 until 31 January 1998. As with all such grants, the funding was sent periodically to the Western European university (in this case, Coventry) and from there, distributed to the other partners. The grant allowed colleagues in Romania and Slovakia to purchase ultrasonic equipment and consumables and, in some cases, fund researchers so that a serious study of UAE could be carried out. The first requirement was to demonstrate the advantages of using ultrasound, compared with conventional extraction technology. Normal extraction procedures were routinely used in Romania and Slovakia, and the addition of ultrasound led to sequential publications in the journal Ultrasonics Sonochemistry in 1997 [53, 54]. The first paper concentrated on pharmaceutically active compounds from the herb sage (Salvia officinalis). In particular, the chemicals cineole, thujone and borneol could be extracted more efficiently when sonicated [53]. This was closely connected with the research program of another member of COPERNICUS, Mediplant, in Modra. For them, the extraction of pharmacologically active compounds from Salvia officinalis was of great interest, since it had been shown that the tincture

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obtained could be used effectively for the treatment of a range of oral conditions. These include inflammation of the mouth and pharynx after tonsillectomy and other surgical operations in the oral cavity, in the reduction of halitosis and in the treatment of chronic atrophic laryngitis. Summaries of the possible uses of the different herbal extracts in medicine can be found in several publications that were available around that time, and one of these that was particularly favoured by the Slovakian group was entitled “The Medicinal Plant Industry” [64]. This text was a compilation of information that included the impact of traditional medicines on health treatment, together with their sources, extraction and formulation. The laboratory method used for the extraction of Salvia officinalis involved the use of 15 g dried material cut to about 1 cm pieces in a 250 cm3 Erlenmeyer flask and 125 cm3 of 65% ethanol. The flask was partially immersed in an ultrasonic bath in which the temperature was controlled using a submerged cooling coil. Ultrasound was applied for 12 h under mechanical stirring which kept the sample in suspension and aliquots (5 cm3) were withdrawn by pipette periodically for gas chromatography (GC) analysis. The mixture was then allowed to stand undisturbed at room temperature for a further 12 h (24 h from the start of experiment), after which time the mixture was filtered and the last sample for GC was withdrawn from the filtrate. Table 3.3 shows these results compared with a control (stirred) experiment. Table 3.3: Extraction of Salvia officinalis with ethanol (65%) at 20 °C. Time (h)

Cineole (mg/kg)

Thujone (mg/kg)

Borneol (mg/kg)

UAE

Control

UAE

Control

UAE

Control



.

.

.

.

.

.



.

.

.

.

.

.



.

.

.

.

.

.



.

.

.

.

.

.

Stand for  h

.

.

.

.

.

.

The results in Table 3.3 show that after 5 h, there was approximately 45% more biologically active compounds in the extracts produced under sonication. After 12 h, the improvement induced by sonication was 26%. The maximum quantity of bioactive compounds that could be extracted from Salvia officinalis (after a series of repeated extractions) was 41 mg/kg of cineole, 292 mg/kg of thujone and 11 mg/kg of borneol. UAE can be improved further using an ultrasonic horn where nearly 60% of the active compounds can be extracted after 2 h sonication. However, there is a problem in applying this type of extraction because by using this technique, it is difficult to control the extraction temperature.

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In the second paper, tinctures of various plants of interest to the Plafar company in Brasov were extracted with aqueous ethanol at the “Costin D. Nenitzescu” Institute of Organic Chemistry in Bucharest. The plants chosen were Mentha piperita (mint), Matricaria chamomilla (chamomile), Salvia officinalis (sage), Gentiana lutea (gentian), Arnica montana (arnica) and Calendula officinalis (marigold) [54]. The dry vegetal material used was mixed with aqueous alcohol in a ratio selected according to the Romanian Pharmacopoeia and allowed to stand for 15 min. The mass was subjected to ultrasonic treatment by placing it directly in an ultrasonic bath, and the whole extraction mass was mechanically stirred slowly (60 rpm). The extraction was monitored by withdrawing samples at intervals over a period of 2 h. After this time, the mixture was allowed to stand and settle at room temperature, without agitation, for 18 h. The mixture was filtered and the crude ethanolic extract was separated, giving a total extraction time of 20 h. In the control experiments based on conventional extraction techniques, the samples were macerated without ultrasound for 7 days and allowed to stand and settle for a further 14 days. In all cases, the crude extracts after separation were allowed to stand for sedimentation at 4 °C for a period of 18 h, after which, the supernatant liquid was removed as the final “matured” extract (Table 3.4). The results suggest that the diffusion process continues to a small extent after sonication has been stopped. However, the time required to obtain a tincture using UAE is effectively 2 h, compared to 7 days for the classical maceration procedure. Table 3.4: Dry residue of tinctures obtained from several plant materials. Sonication time (min)

Mint

Chamomile

Marigold

Sage

Arnica

Gentian

    

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

– . . . .

After  h standing in solvent and settling without ultrasound

.

.

.

.

.

.

After  days of maceration and  days standing and settling (control)

.

.

.

.

.

.

During these investigations, photographic evidence was obtained for the damage caused by ultrasound. In Figure 3.7, the fibrils on a marigold “petal” are shown, before and after a brief exposure to ultrasonic irradiation during extraction. The images clearly show how ultrasound can fragment the fibril cells that are on the outside of the plant material to release their contents [65]. Another objective of the COPERNICUS project was to improve the methodologies for a number of commercially important extractions of xylan, from hardwoods

3.6 The development of UAE from the 1990s in Coventry and Bucharest

(a)

125

(b)

Figure 3.7: Marigold fibrils (a) before and (b) after sonication.

and annual cereal crops. This was an area of particular interest from the participants of the Slovak Academy of Sciences [66, 67]. Xylans (polysaccharides found in plant cell walls that hydrolyse to xyloses) are novel biopolymers that have the potential to be used in a broad field of paper technology, food and pharmaceutical industries, textile printing and cosmetics. The efficiency of the UAE procedures exceeded that of the classical methods. Using a short application of ultrasound, similar yields of the total extracted xylan could be achieved at lower extraction temperatures and substantially shorter extraction time. In addition, about 50% higher yield of the immunologically active xylan can be obtained using 1% NaOH at a temperature of about 60 °C in the first extraction step. The biological activity of the sonically extracted xylans was rather higher than that of the classically extracted materials. In 2001, a paper from the project was published, in which we tried to identify the mechanism of the ultrasonic enhancement of solvent extraction through the effect of ultrasound on the vegetal material involved [65]. At that time, it was assumed that the most probable mechanism for ultrasonic enhancement of extraction was an intensification of mass transfer and easier access of the solvent to the vegetal cell. The collapse of cavitation bubbles near the cell walls is expected to produce cell disruption, together with a good penetration of the solvent into the cells, through the ultrasonic jet. It was noted, however, that the mechanism of UAE depends upon the state of the biomass used, that is, whether it was dried or fresh. In the case of dry biomass, the first step involves soaking the dried material in a solvent, which results in a swelling of the dry material by solvent uptake into the plant cells. Solvent uptake is one of the most important steps in dry herb extraction. Such is its importance that it is defined in the British Pharmacopoeia [68] as Swelling Index (SI) and it is the volume estimated as millilitres of liquid taken up by the swelling of 1 g of plant material under specified conditions. Ultrasound speeds up this process by the collapse of asymmetrical bubbles near to the plant surface, which forces the solvent into the dry material (see 3.7.1). A comparison of the classic and ultrasonic SI for a range of dry plant materials is given in Figure 3.8 [42, 65].

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30

Swelling index (ml/gram)

25 Classic 20

Ultrasonic

15 10 5 0

Figure 3.8: The effect of ultrasound on swelling index for a range of dry plant material.

In the case of fresh biomass, the extraction was mostly considered to be the result of solvent diffusion into and out of the material. This is due to the concentration gradient between the plant material and the surrounding solvent. In order to quantify any improvement caused by the application of ultrasound, we used a parameter known as the extractive value (EV) (see 3.7.2). This is normally defined in terms of the mass of the target material (g) extracted from 100 g of a starting material by a standard method. In Table 3.5, the ultrasonic EV was chosen as being the dry residue of the extracted material obtained after a half hour steeping, followed by one hour of sonication. This was compared with a control, which involved classical 23 h steeping and 1 h stirring [42, 65]. Ultrasound was found to be beneficial for improving both the swelling and extractive indices. In addition, the data shows that EV is dependent upon the solvent used. It is because of this that it is always useful to test the EV for several solvents or solvent mixtures before performing any experimental work regarding herbal extraction. It is important to recognize that these effects occur only when the plant material is in the ultrasonically agitated zone of the reactor and many reactors have “dead” zones. If an ultrasonic bath is used as the reactor, a stirrer should also be used not only to keep the biomass moving through the active zones but also to keep all particles in suspension. This avoids any accumulation of biomass at the bottom of the reactor, which would cushion the transfer of vibrations into the biomass from the transducers on the base. One of the participants in the COPERNICUS programme (Mediplant based in Modra, Slovakia) applied ultrasound to some larger extraction vessels, aiming an industrial scale [69]. They chose to investigate the production of tinctures from sage

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Table 3.5: Comparison of extractive values of sonicated and silent methods for some vegetal species. Vegetal species

Method

Eth

Eth/W

Fennel seeds Foeniculum vulgare ” ”

Classic

.

.

. h US  h US

. .

Hop strobiles Humulus lupulus ” ”

Classic

W

Gly/W

Ether

.

.

.

. .

. .

. .

. .

.

.

.

.

.

. h US  h US

. .

. .

. .

. .

. .

Marigold flowers Calendula officinalis ” ”

Classic

.

.

.

.

.

. h US  h US

. .

. .

. .

. .

. .

Peganum seeds Peganum harmala ” ”

Classic

.

.

.

–

.

. h US  h US

. .

. .

. .

– –

. .

Mint leaves Mentha piperita ” ”

Classic

.

.

.

.

.

. h US  h US

. .

. .

. .

. .

. .

Lime flowers Tilia cordata ” ”

Classic

.

.

.

.

–

. h US  h US

. .

. .

. .

. .

– –

Elecampane root Inula helenium ” ”

Classic

.

.

.

.

–

. h US  h US

. .

. .

. .

. .

– –

Eth, 94 v/v ethanol/water; Eth/W, 70 v/v ethanol/water; W, water; Gly/W, 3.5 v/v glycerol/water; ether, diethyl ether.

and valerian, both of which were grown in local fields and were widely used in the Slovak pharmaceutical industry. A variety of formulations containing sage tincture (tinctura salviae) are still to be found in the pharmaceutical and food markets of European countries, in the form of drops, tablets, pastilles, ointments and gels. Products containing the valerian tincture (tinctura valerianae), a sedative, are in competition with synthetic drugs, but as a natural tranquilizer, they have the advantage that there are negligible side effects. The factory at that time was equipped with typical extraction equipment in rows, known as batteries (Figure 3.9).

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Figure 3.9: Batteries of extraction equipment at the company Mediplant in Modra, Slovakia.

The control experiments were performed by static maceration with solvent circulation through a column loaded with the plant material. The extractions were carried out in a cylindrically shaped glass extractor with volume 56 L, height 85 cm and diameter 29 cm. The quantity of the ethanol/water extraction solvent mixture pumped into the extractor was dependent on the amount of plant material used. Ultrasonic extractions were made with an identical device under the same conditions as the control. An ultrasonic probe operating at 20 kHz was placed at the top centre of the extractor. The acoustic horn was made of stainless steel and was 79 cm high and 5 cm in diameter. Sonication was introduced in three different ways during the working day of 8 h: 1. Alternate periods of 0.5 h of sonication and 0.5 h of silence, 8 h daily over a period of 3 days (termed broken mode). 2. Sonication run for 2 h at the beginning of each of the 3 days of the extraction (termed short time mode) 3. Sonication run continuously for 8 h each day for 2 days (termed continuous mode). Solvent circulation was carried out at the same time as ultrasonic treatment and the broken mode method was found to be most effective. A small amount of plant material was milled as a result of ultrasonic action and fine particles entered the flowing stream. These particles were captured in a gauze filter at the bottom of the extractor vessel and built up a filter cake, which effectively trapped the fine particulate matter. The efficiency of the ultrasonic process was evaluated by GC analysis and/or by determination of the dry residue content and compared with the control. In the case of sage, the greatest efficiency for ultrasonic irradiation of the mixture was observed when the ratio of dry sage to solvent was 1:6. Under these

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conditions, the content of dry residue in the final tincture was 26.56 % higher than in the control experiment. Similarly, the content of essential oils was higher than in the control (increase of cineole 15.26%, borneol 2.13%, α- and β-thujone 23.76%). A slightly smaller influence was observed in the effect of ultrasound on the extraction of valerian root, compared with sage. The best effects of ultrasound were observed using a solid/liquid ratio of 1:3. The content of dry residue from the ultrasound experiment was 9.8% higher than in the control experiment. There were three meetings of the entire group during the period of the programme; the first at the Comenius University, Bratislava, Slovakia, in August 1995; the second at Sinaia, Romania, in May 1997; and the third in Slovakia, at a conference centre in the Tatra Mountains, in October 1997. The COPERNICUS project was a success because it provided the basic ideas underpinning UAE. Through the efforts of all the participants during the project meetings, it was possible to design and build the first reactor dedicated to industrial scale UAE of plant material [70]. The reactor (Figure 3.10) had a working volume of about 650 L, operating at a controlled temperature via a cooling/heating jacket using three push–pull-type ultrasonic transducers, providing a total of 4.5 kW ultrasonic power.

Piezoceramics

Figure 3.10: Industrial scale ultrasonic reactor for natural products extraction.

This reactor was used for extraction experiments in the Romanian factory of Plafar in Brasov, which was well known for its production of vegetal extracts using conventional technologies. In one of the industrial experiments carried out using this equipment, a comparison was made between the classical and UAE extraction processes in aqueous ethanol (40% ethanol in water). The classical unit (2,000 L working volume) used herbs contained in bags made of vegetable yarn suspended on slow stirrer arms. The UAE was performed under similar experimental conditions in

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terms of alcohol concentration and ratio of herb to solvent, but the herbs and solvent were mixed throughout the reactor, that is without using vegetable yarn bags. The extracts, for both units were monitored by dry residue value over time and ultimately by FT-IR spectroscopy, which confirmed the improvement in extraction (Figure 3.11). The major difference between the two was the time involved, with the classical extraction taking 28 days and the UAE process taking only 10 h. The resulting extracts were also tested in the traditional manner, by tasting in a sensory room. The ultrasonic extract was found to be much better since it was not only higher in dry residue content but it was also preferred in terms of its taste.

Absorbance

with ultrasound

normal

Wavenumbers (cm-1 )

Figure 3.11: FT-IR spectra for the ethanolic extraction of the same herb mixture using conventional and UAE industrial scale extraction.

The COPERNICUS project laid the foundation for more extensive work on UAE, both in Romania and in the UK. Part of the funding was used by the Coventry sonochemistry group to support a student to investigate a related UAE project. Her name was Elaine Beaufoy, who had graduated with a BSc in pharmaceutical chemistry in 1996. Elaine was enrolled for an MSc “The application of ultrasound in the extraction of essential oils from Chinese plants”. This concentrated on the extraction of rutin, a flavonoid (Figure 3.12), from the dried flower buds of the Chinese Scholar Tree (Sophora japonica). Flavonoids are products of secondary metabolism in plants and are of interest to the pharmaceutical and food industries because of their reported antioxidant

3.6 The development of UAE from the 1990s in Coventry and Bucharest

HO

HO

131

OH

O CH2 O

OH

O Me

O O OH

O

HO OH OH OH

Figure 3.12: Structure of rutin.

activity. Antioxidants can interact with free radicals and so prevent the damage that radicals might otherwise cause to cell membranes and biological molecules. They find use in the treatment of arthritis. In water, UAE reduced the yields of rutin when compared with conventional methods. This reduction was believed to be the result of degradation of the rutin by its interaction with the highly reactive hydroxyl radicals formed during sonication of the aqueous solvent. The extractions were, therefore, carried out in methanol, which is not only a better solvent for conventional extractions but it also does not produce oxidizing species, when sonicated. Experiments were performed at room temperature without allowing the temperature to rise since (a) cavitational effects are reduced as temperature is increased and (b) although the solubility of rutin in methanol is much higher than in water over the entire temperature range, the difference is particularly significant at lower temperatures. UAE in methanol enhanced the yields as shown in Figure 3.13 [71]. The higher yields obtained from conventional methanol extractions compared with water is attributed to the much greater solubility of rutin in this solvent, together with its apparently increased stability to air oxidation in this solvent. The further increase in yield when ultrasound is applied can be attributed to the normal effects of cavitation-induced breakdown of cells and the minimal formation of hydrogen peroxide during the sonication of methanol so that the extracted rutin is not degraded. A conclusion from this study was that the extraction of antioxidants with aqueous solvents appeared to be unsuitable for UAE because of degradation of the sample through the ultrasonic generation of free radicals. Subsequently, this problem of extraction of antioxidants was addressed in more detail in the SAFE programme (see later) during extensive studies of the extraction of the herb rosemary. The COPERNICUS project had generated considerable interest, to the extent that we decided, together with Maricela Toma, to produce an in-depth review of the subject for Advances in Sonochemistry [14]. Maricela was working in Bucharest at the same Institute as Mircea and she had expertise in plant biology. In that chapter we wrote:

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yield (g) 1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

20

40

60

80

100

120 140 time (minutes)

conventional (reflux) ultrasound 20 kHz, 27W, Room Temp., Figure 3.13: Effect of ultrasound on the extraction of rutin from Sophora japonica using methanol. The use of plants in daily life not only as food but also as flavoring, coloring or as medicine, has a long history all over the world. It is obvious that the use of plants as food, from wild and cultivated flora is as old as human history. Some of the plants are cultivated at a very large scale, like cereals, vegetables, while others used as food additives and folk medicine are cultivated on a small scale or harvested from wild flora.

3.6.1.2 COST D10 (SAFE) At a dinner held during the successful COPERNICUS programme, an idea was discussed to look in more detail at the application of ultrasonic extraction for the food industry. We decided to apply for a concerted action in the framework of what was then the new EU COST Programme (D1O), “Innovative Methods and Techniques for Chemical Transformations” (see Chapter 1, Section 1.8.1.2). In order to determine whether other groups might be interested in such a project, one of the proposers, Petru Filip, who was Director of the C. D. Nenitzescu Institute, sent out a letter to those that we considered might be interested in joining such a group. He wrote that the proposed title would be “Sonochemical Assistance to Food and Pharmaceutical Material Extraction from Renewable Natural Resources” acronym – S A F E. Also that there would be an initial organized meeting for the new COST programme to be held in Brussels during 14–15 February and at this meeting, Professor Mason would outline the SAFE proposed action. Those interested in joining this group were thus advised to contact him there. Although such programs did not cover salaries, they were designed to enable travel and living expenses for personnel between laboratories and to help finance group meetings. The proposal was successful (SAFE COST D10/0016/99) and ran between 2000 and 2002. The COST management team decided that as a condition of the award, Mircea must act as coordinator because of his considerable experience

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in UAE. Several members of the previous COPERNICUS project also joined: Tim Mason and Larysa Paniwnyk (Coventry University, UK) and Anna Ebringerová, (Institute of Chemistry, Slovakia). Others who were added to this team included Rosemary Cole (National Herb Centre, UK), who had been working on projects involving the herb rosemary with the Coventry group (see later). Moschos Polissiou (Agriculture University of Athens) had been introduced because of his work on saffron. Also included were Francois Pellissier (Universite de Savoie, France) and Marja Marjoniemi (Tampere University of Technology, Finland). An outline of the project was given in the project proposal as follows: An international team: will collaborate on research topics connected with ultrasound assisted extraction of active principles from renewable resources, isolation and identification of major components from some extracts, testing the bio-activity of the extracts targeting from pharmaceutical uses to allelopathy studies. The first task in all contributions will be comparing ultrasound assisted processes with classical extraction procedures, aiming for higher extraction yields or targeting for components located in cell walls. Industrial tests will be included, the Romanian team having an industrial ultrasound assisted extraction unit already build, based on their previous results. The extracts will be processed in order to isolate and establish the structure of the main components. Bioactivity will also be tested as well as the other potential uses of the extracts (tanning agents, bio-surfactants, additives for food preservation, etc.). Each group will focus on common effort having as tasks: process optimization, specific equipment development, isolation and structure determination techniques, bio-activity testing.

Participants took advantage of the travel funding from COST to attend a range of conferences, but there was also a main meeting arranged by Mircea for members of the SAFE network. This was held in Athens, Greece, from 12 to 14 April 2002. Tim Mason and Rosemary Cole travelled to Athens together but their arrival in Greece was not smooth. They boarded the bus from Athens airport to the city centre without buying tickets in advance thinking that they would buy them on board the bus. However, they were made to feel like real villains when a security guard on the bus whose job it was to find “fare dodgers” discovered they had no tickets. It was apparently not legal in Athens to buy tickets on the bus and a spot fine was incurred. This was not a great introduction to Athens and neither of them would make that mistake again. The hotels were in Athens but the conference was held outside the city at the Agricultural University of Athens, which involved a long bus journey. The meeting itself consisted of reports from the different groups involved and the attendees are shown in Figure 3.14. The Slovak group had continued their interest in UAE of polysaccharides from vegetal materials (particularly waste corn hulls) and from yeast and fungi wastes. They had developed an interest in extracting polysaccharides from valerian roots from the material remaining after tincture preparation [72]. The authors noted that

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Figure 3.14: Members of the SAFE group at the meeting in Athens. Back row: Timothy J. Mason, Takahide Kimura, Prof. M. Polissiou, Mircea Vinatoru, Charalampos Kanakis, Rosemary Cole, Anna Ebringerova, Jozef Sandula, Christos Pappas. Front row: Maricela Toma, P.A. Tarantilis, Dimitra Daferera.

sonication causes depolymerization of the solubilized polysaccharides, yielding aqueous ethanol soluble fragments. The water-soluble polysaccharide fractions from both the conventional and ultrasonic experiments exhibited significant immunostimulatory activity. In a separate study, conventional and ultrasonic extraction methods were applied to yeast biomass to extract polysaccharides and glycoproteins. These showed a wide range of biological activity, including antioxidative effects, based on their ability to scavenge reactive oxygen radicals. The highest activity was achieved with the protein-glucomannan complex that was ultrasonically isolated from the industrially important Candida utilis yeast. The Greek group had also been studying the extraction of polysaccharide (cellulose) but in their case from kenaf (Hibiscus cannabinus L.) and eucalyptus (Eucalyptus rodustrus Sm.) [73]. Their yields are shown in Table 3.6. All values are calculated relative to the mass of untreated starting material. These results showed that the yield of cellulose increased using the ultrasound-assisted technique and the total time of treatment decreased from 170 to 21.5 h. The percentage of cellulose in eucalyptus is higher than in kenaf but kenaf is an annual plant and so it can be reproduced every year.

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Table 3.6: Yields of cellulose extracted from kenaf and eucalyptus using classical and ultrasound-assisted extraction methods. Cellulose (%) Classical ( h)

Ultrasound assisted (. h)

Kenaf

Eucalyptus

Kenaf

Eucalyptus

.

.

.

.

In another study, they reported on their work with UAE of the petals of the flower Crocus sativus L. (Iridaceae) [74]. This plant is much prized as a source of the spice saffron, which is the dried crimson stigma of the flower, but the purple petals are a by-product that are normally discarded. They are however rich in flavonols and anthocyanins, which can be used as colourants in many applications. The results of a study of the extraction of flavonols and anthocyanins with ethyl acetate and n-butanol respectively are given in Table 3.7. The two advantages of the ultrasound-based method over classical extraction are that (a) a smaller amount of solvent was used and (b) the ultrasound-assisted method produced larger amounts of the pigments. Table 3.7: Extraction of flavonols and anthocyanins from crocus petals. Method

Compounds Flavonols (ethyl acetate)

Anthocyanins (n-butanol)

Conventional

 mg/ g

 mg/ g

UAE

 mg/ g

 mg/ g

The Greek group also compared hydro distillation (simultaneous distillation solvent extraction using the Lickens–Nickerson apparatus) with UAE for essential oil extraction from a range of aromatic plants, including oregano, thyme, marjoram, lavender, rosemary, sage, pennyroyal and anise seeds. The methodologies employed were later published in an article related to extraction methods for the isolation of sensitive aroma compounds from garlic [75]. In this paper, it was concluded that the UAE procedure (which they referred to as USE – UltraSound-assisted Extraction) reduced the danger of thermal degradation of the sensitive aroma compounds. Moreover, the method was easy to carry out and could be applicable for large-scale industrial use. The UK group reported the results obtained from their extraction of antioxidants from the herb rosemary. UAE using conventional solvents was compared with the results from liquid and supercritical carbon dioxide extraction, and also a fluorocarbon

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solvent 1,1,1,2-tetrafluoroethane (134a). This was part of a research programme that is described in more detail in Section 3.6.2.2 relating to the project RAPFI. The Romanian group presented work on one of the lesser known applications of ultrasound in botanical studies – the use of ultrasound for improved seed germination and plant growth. Ultrasonically stimulated seed germination offers the possibility of increased productivity for large-scale farm crops and in more general horticulture. There had been a number of reports on this topic that had been published in the middle of the twentieth century, but for some reason, they had not been followed up. One such report appeared in 1954 relating to a faster germination of barley seeds and a faster growth of seedlings when treated with 30, 80, and 960 kHz ultrasound at 1–2 W/cm2 [76]. This was attributed to increased water uptake of the seeds during treatment, possibly due to cavitation-induced cracks in the outer layers of the seeds. A few years later, one of the authors published a review that reported the beneficial effects of ultrasound on seeds and advocated the use of ultrasound in agriculture [77]. In Russia, it had been reported that ultrasound produced a faster germination in corn seeds when they were treated for 5 min with 380 kHz ultrasound [78]. It is interesting to note that in these studies, the seeds were not immersed in water but were held in the “ultrasonic fountain” produced at the surface of water when ultrasound was applied from below. In 1971, Andrew Gordon reviewed the more general beneficial effects of ultrasound on plants, including enhanced seed germination, and although he proposed that there was a significantly beneficial effect of ultrasound, the causes of, and inconsistencies in the effects were barely understood at that time [79]. The Romanian group reported on the effect of ultrasound on the germination of a range of seed species. Ultrasonic treatment was applied using a Langford Sonomatic cleaning bath (33 kHz) for different periods of time: 5, 15, 30, 45 and 60 min. The ultrasonic intensity of treatment was 50 and 100 watt (electrical input power applied to the ultrasonic transducer). The germination of the sonicated seeds was recorded only after all the control batch seeds had germinated. Germination was considered complete when the radicle had reached 2 mm, which depended on the species e.g. 96 h in the case of tomato and carrot and 72 h for wheat. The data obtained for every species tested in this experiment showed that sonication could significantly promote the germination process. Two of these relating to wheat and tomato are shown in Figure 3.15(a) and (b). The error bars are expressed as percent from the control value ± 0.5SD. The results obtained in the case of wheat show an important enhancement of germination rate for seeds sonicated for a short time period, between 5 and 15 min. In this example, sonication produces a beneficial effect, up to a maximum period of 30 min, but after this time, a substantial decrease in germination occurs, especially at 100 W ultrasonic power. The most beneficial effect for tomato was when ultrasound was applied for 45 min. Here, the enhancement was up to 10% greater than the silent control. The data obtained suggested that sonication at the lower 50 W level was better than that at 100 W.

Wheat 10

50 100

0

–10

–20 5

(a)

15

137

Tomato

30

Time, min

45

Germination, percent of control

Germination, percent of control

3.6 The development of UAE from the 1990s in Coventry and Bucharest

50 100

20

10

0

60

5

15

30

45

60

Time, min (b)

Figure 3.15: Effect of ultrasonic irradiation on the germination of seeds (a) wheat and (b) tomato.

The Romanian team had also investigated UAE in conjunction with the extraction of chemicals related to allelopathy (see Section 3.2.2). Here, the goal was to obtain extracts that could be used to prevent germination, and the target herb was the common weed aristolochia (in the UK, it is called birthwort because of its original usage to induce childbirth). Extracts from it are also known to contain a number of secondary metabolites that have the potential to be used directly as herbicide substitutes, one of these is aristolochic acid. Because ultrasound raises the swelling index (SI) it seemed reasonable to suppose that an allelopathic material could be introduced into a seed during this swelling process. Acoustic cavitation should enhance material transfer into the seed after it has been “softened” during swelling. The plant seed chosen for the trial was pearl millet (Pennisetum glaucum L.). The seeds were immersed in an aqueous solution of aristolochic acid solution (10–4 M) under the same conditions as were used in the study of enhanced germination. The result showed a significant decrease in germination rate for seeds sonicated at 100 W. The shoots developed from seeds treated in this way showed a dramatic decrease in growth after germination. The programme also contained two invited lectures. One was “Prospects for scale-up in the ultrasonic extraction of natural materials”. In this lecture, Tim Mason outlined the current and future technologies for scale-up and for possible industrial application of the research work emanating from SAFE. The other lecture was from Takahide Kimura, an invited speaker from Shiga University of Medical Science in Japan, who presented “Antioxidants from natural resources”. The lecture showed the role of antioxidants in the human body and the natural sources from which they can be obtained. An important addendum to his lecture was an approach to the standardization of ultrasonic power input in an extraction process, a topic of great importance in the move towards industrial applications.

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3.6.2 Programmes supported by the UK In the 1990s, interest had been growing in the UK concerning the uses of plant-derived materials as sources of chemicals, food additives and medicines. The Ministry of Agriculture, Fisheries and Food (MAFF) decided to make a more positive entry into this field. To this end, a national group, “Alternative Crops Technology Interaction Network” to be known by the acronym ACTIN, was formed in August 1995. 3.6.2.1 Maximizing the extraction of oils and alkaloids from herb species using ultrasonics (HERBSONICS) In March 1997, Tim Mason received a letter from Sally Runham of the Agricultural Development and Advisory Service (ADAS) expressing an interest in the use of ultrasound in the extraction of essential oils. She had heard about the sonochemistry research work in Coventry from an interview broadcast on BBC radio for the series Science Now. ADAS was a privatized agency of MAFF and they were involved with a group of farmers in East Anglia, who were growing herb crops as a source of oil for the first time in the UK. Sally was interested in the possibility of using ultrasound in the extraction process. I contacted Rosemary Cole at the National Herb Centre near Coventry about this because I had visited Ryton Gardens, which was a national centre for organic gardening and it was where the Herb Centre was located. Rosemary had great expertise in plant materials and she knew of a new government programme entitled “Competitive Industrial Materials for Non-Food Crops” (CIMNFC). Together we submitted a proposal entitled “Maximising the extractable oils and alkaloids from herb species using ultrasonics” with the acronym HERBSONIC with Sally Runham as the project coordinator. The aim was to determine the scope for using ultrasonic techniques in association with standard methods of extraction to improve the efficiency of extraction per hectare (ha) of herb biomass grown in the UK. It would examine the extraction of volatile oils from herbs that are normally in low yields (1–2%). These yields were obtained from traditional methods of extraction such as steam distillation and solvent extraction, including the use of carbon dioxide. The competitiveness of the essential oil industry in the UK (based on established and new small- and medium-sized enterprises in rural locations) was dependent on the productivity of crops. The climate is good in the UK for the production of quality oils from some specific species (e.g. lavender, chamomile and sage) but the yields were lower than those obtained in warmer climates. It was hoped that ultrasound would improve standard extraction techniques in terms of reduced heat and/or time required. We had high hopes of success but unfortunately it was rejected perhaps because of its novelty and we had to rethink our approach.

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3.6.2.2 Potential for extracting high concentrations of antioxidants from rosemary (RAPFI) Despite the fact that our HERBSONIC application had failed, we had at least put together a wealth of information on the UK herbal extraction industry. Two years later, interest in the field of plant-derived chemicals had greatly increased and we were ready to try again. The increased interest was encapsulated in an excellent brief article for the magazine, Chemistry and Industry, written by Ian Bartle in 1999 and entitled “Realizing the potential of plants” [80]. Ian was then an executive of ACTIN (see earlier) and in the article, he pointed out that while the prevailing commercial interests in plants was for the supply of food and feed, other possibilities were ready for exploitation. He emphasized factors that were the same as those that we had identified in our work on COPERNICUS, that plant crops have tremendous potential as renewable sources of high-quality raw materials for industry. Some of these were already under active investigation in the UK at that time, including: – rapeseed for oil-based lubricants; – wheat and maize as sources of starch used in the manufacture of biodegradable plastics; – hemp or flax fibres as components in composite materials for the building industry. In the years leading up to 2000, stimulated by a requirement of the EU Common Agricultural Policy (CAP) that land should be set aside from food production, industry had begun to make greater use of vegetable oils, carbohydrates and fibres. The conclusion to the article by Ian Bartle was: Under the banner of the European Renewable Raw Materials Association (ERMA), representations to the European Commission are being made to underline the seriousness of the situation. The Commission is being urged to allow non-food crops to compete with mainstream crops through uniform area payments for all arable crops and to encourage a focused and coordinated EU-wide approach. National governments are being asked to provide a level of support to industry and agriculture which reflects the environmental and social benefits of non-food crops.

OH

OH

HO

COOH

HO O

COOH

C

O O

H3C

H CH3

Carnosic acid

H3C

OH O

H CH3

Carnosol

Rosmarinic acid

Figure 3.16: Antioxidant compounds extracted from Rosmarinus officinalis.

OH

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This was the background to an application in 1999 put together by Rosemary Cole for another funding opportunity through the same call as we had used before (CIMNFC). In this case, however, the lead partner was the National Herb Centre (NHC). The title of the proposal was “Potential for extracting high concentrations of antioxidants from Rosemary for the food and pharmaceutical industry” (RAPFI). The main rosemary antioxidant targets are shown in Figure 3.16. The project aimed to: – identify the economics of production for UK farmers, including production, drying and logistics; – set up trials to determine the best agronomic practice to produce high levels of candidate antioxidants; – develop a commercial extraction process using ultrasonic or CO2 extraction methods. To deliver this, a team was assembled that included the NHC, Coventry University Sonochemistry Centre, Ray Marriott who was the Chief Executive of Botanix (a company specializing in extraction, including the use of liquid and supercritical CO2), Reading University, who would investigate growth patterns and John K Kings of Coggeshall, a seed company from Colchester (as potential commercial growers). Our application was one of many different proposals and a short list had to be drawn up. We were delighted to be selected and the short-listed teams were invited to a meeting of the Programme Management Committee on 19 May 1999 to make a presentation and answer any questions that they might have. The meeting was held in the prestigious headquarters of the Royal Society, 6 Carlton Terrace, London and our 20–min presentation was scheduled for 11.00 am. The interview went well and in July we heard that it would be co-funded by the Scottish Executive Rural Affairs Department and CIMNFC. It started in late 1999 and continued until 2003. By autumn 2000, the project was well established, with Ian Bartle acting as the Programme Coordinator and Ray Elliot of Zeneca as the Programme Monitor. We recruited two ex-students from Coventry University for the project. One was Elaine Beaufoy, who had been working on the COPERNICUS project, and Tony Arkell, who was employed at the National Herb Centre as a laboratory manager and his contribution was on the identification and activity of the antioxidant compounds isolated from selected Rosmarinus officinalis accessions. The term accession is used quite generally in botany as a way to identify plants by the order in which they entered the collection. Accession is used simply to categorize. Working with Tony at the NHC was Carol Wellwood. A project report published in the LINK programme magazine of autumn 2000 gave some details of the work as follows [81]: Many compounds have been identified in rosemary (Rosmarinus officinalis) extracts at the National Herb Centre, where Carol Wellwood and Anthony Arkell have carried out various assays for antioxidant activity, each corresponding to a phase of autoxidation. The first assessed the antioxidant's ability to 'mop up' free radicals, the second, how far it slowed the production of hydroperoxides,

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and the third how much it inhibited their breakdown. A correlation between the concentration of one or more of the compounds and the antioxidant activity of the extract allowed a comparison of the potential yield of antioxidants from each accession (clone of a known plant). Different solvents were investigated to determine their ability to extract a wide range of antioxidants. The concentrations of known antioxidant compounds in 80 rosemary accessions have been assessed with the aim of selecting accessions that will produce the best yield of antioxidant extracts when grown on a field scale in the UK. Field scale trials of six high yielding accessions will be planted at three trial sites in Spring 2001 to determine how yield can be enhanced by cultivation and growing conditions. At the School of Plant Sciences, University of Reading, Cristo Luis-Jorge is growing different accessions of R. officinalis under controlled conditions under a range of supplementary UV-B lights (280–320 nm). Wavelength, intensity and time of irradiation are varied to establish the relationship between UV light, secondary product formation and antioxidant activity on different plants of different ages. In the near future aspects such as effects of UV light on tissue differentiation, variation in leaf surface wax, essential oil composition and the modification of plant enzyme content will be studied. Elaine Beaufoy at the Sonochemistry Centre, Coventry University, is trying to obtain higher yields of antioxidants using ultrasonic extraction techniques. Extractions have been carried out using several different solvents of varying polarity. Ethanol and methanol were found to give the highest yields, with ultrasonic extraction giving similar yields to conventional extraction but at an increased extraction rate. Both the conventional and ultrasonic extracts were found to have similar antioxidant activity. The effect of temperature, novel solvents, acoustic power and frequency on extraction yields will be studied in the future.

During the three years of the project, several rosemary accessions with high antioxidant activity were successfully selected. Spain and North Africa were the normal sources of rosemary with high antioxidant content, because these countries have a dry climate and the antioxidants produced within the rosemary plant have the function to prevent damage from the free radicals produced during drought stress. The climate in UK does not generally result in drought stress and so the new accessions were important for UK growth. The three selected rosemary accessions were planted in field-scale trials using 5 ha of land in Oxfordshire, Norfolk and Cornwall. Due to the problems encountered in producing large number of plants (30,000/ha) using clonal propagation, some of them were planted very late in the season and they did not survive the winter. In the past, rosemary crops had not been grown in the UK in large acreages and so experience in planting, growing and, particularly, harvesting and drying the crop was not available to growers. A potential customer for the product was PINUS in Slovenia, who were already extracting antioxidants from rosemary and so were interested in purchasing the rosemary crop. The logistics of transporting the dried crop to Slovenia was organized by J. K. Kings of Coggeshall. Although this showed that there was a market for rosemary antioxidants, it was not an ideal solution because transportation of light, bulky products such as dried rosemary is expensive and the growers did not benefit from the value added that could have been obtained from

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selling the extracts from the crop. This indicated the need for an efficient and largescale extraction route to be made available in the UK. Encouraging results showed that high concentrations of antioxidants could be extracted efficiently from rosemary using conventional solvents such as ethanol using ultrasound. In late 2001, a young Romanian lady knocked on the office door of Tim Mason to ask whether there was any possibility of working in the sonochemistry group. There had not been any post advertised but she knew about the extraction work we were doing. She discovered that we already had strong links with Romania and she saw on the bookshelves in my office two chemistry books written in Romanian by Costin D. Nenitzescu. These had been a present many years before from Petru Filip on the occasion of my first visit to the Costin D. Nenitzescu Institute of Organic Chemistry in 1991 (see Chapter 1). Her name was Silvia Albu, a chemist with an M.Sc. from Romania in pharmaceuticals and cosmetics. Silvia did not know Mircea Vinatoru but she did know of Petru Filip. This was an interesting coincidence and so it was decided that Silvia should be accepted on an M.Sc. in sonochemistry in January 2002, entitled “The effect of various parameters and techniques on the efficiency of extraction of antioxidant materials from the herb Rosmarinus Officinalis”, with funding from the RAPFI project. Romania was not part of the EU at that time and so it was necessary to write a letter in late 2002 to the consulate to extend her visa to allow her to extend her stay in the UK to complete her M.Sc. She used UAE to increase the extraction efficiency of carnosic acid from the leaves of rosemary under different extraction conditions [82]. Using dried rosemary leaves, the extraction was compared at different temperatures (25, 35 and 50 °C) and in different solvents (butanone, ethanol or ethyl acetate). At a solid/liquid ratio of 1/10, the leaves were extracted using a conical flask in a shaking water bath for 30, 60, 120 and 180 min (Figure 3.17). The results showed that the extent of carnosic acid extraction from dried rosemary leaves (mg/g) was dependent upon the temperature. Butanone appeared to be the most effective extraction solvent under silent conditions, followed by ethyl acetate and then ethanol. The different efficiencies of these solvents were attributed to their polarities. UAE was performed using a conical flask, dipped into a 40 kHz bath (Langford Sonomatic) and sonicated for 15, 30 and 45 min. The temperature was maintained between 47 and 53 °C and an overhead stirrer was used in the flask to obtain a good solvent/plant material contact (Figure 3.18). Good extraction was achieved in only 15 min and the difference in efficiency between the solvents was markedly reduced. A significant result from these experiments was that at 50 °C, slightly more carnosic acid (17 mg/g) was extracted into ethanol at 15 min than could be obtained in 3 h with a shaking water bath (15 mg/g). This indicated that ethanol was the best of the three solvents. From the point of view of the herb grower, it would be more convenient to use the fresh herb rather than wait until after the drying process, especially in the UK where outdoor drying under ambient conditions is not always possible. The same

20 18 16 14 12 10 8 6 4 2 0

mg/g

20 18 16 14 12 10 8 6 4 2 0

mg/g

mg/g

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60 120 Time (min)

180

30

25ºC

60 120 Time (min)

180

20 18 16 14 12 10 8 6 4 2 0

143

butanone ethanol ethyl acetate

30

60 120 Time (min)

35ºC

180

50ºC

Figure 3.17: Shaking water bath extraction of carnosic acid from dried rosemary.

20 18 16

mg/g

14 12

butanone

10

ethanol

8

ethyl acetate

6 4 2 0 15

30

45

60

Time (min) Figure 3.18: Extraction of carnosic acid from dried rosemary using an ultrasonic bath.

UAE methodology used for the extraction of dried rosemary was therefore applied to the fresh herb, using ethanol as the solvent (Figure 3.19). The extraction of carnosic acid from the dried herb proved to be more efficient than that using fresh material. An explanation for this may lie in the co-extraction of the water present in the fresh rosemary leaves (40% by weight). This would result in an increased level of water content in the ethanol extraction solvent and this in turn could reduce its extraction efficiency. Also, the amount of carnosic acid obtained from the extraction of fresh rosemary reduced over time. This reduction in yield is probably connected with the formation of more radicals during the sonication of aqueous ethanol, compared to ethanol itself. The additional radicals formed on sonication of the aqueous

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20

20

18

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14

14

12

mg/g

mg/g

solvent would react with the antioxidant carnosic acid, thus reducing the overall yield.

10

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8

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2 0

0 15

30 Time (min) Fresh Rosemary

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Figure 3.19: Extraction of carnosic acid from fresh and dried rosemary using an ultrasonic bath.

3.6.2.3 Rosemary and the supply chain to end users (RADSC) The RAPFI programme ended in 2003, which co-incidentally corresponded to the end of the ACTIN organization in November of that year. Many of its activities were taken on by a new body the National Non-Food Crops Centre (NNFC) based at the Innovation Centre at York Science Park near the University of York. Its aims were to cover all uses of plant-derived materials, derivatives and by-products for commercial non-food purposes. This classification excluded plants grown for ornamental reasons and forestry (grown solely for timber). In addition, it would function as a national information centre. In 2004, NNFCC began setting up Thematic Working Groups, with the aim of bringing together all elements of the supply chain to look at reducing any barriers to the introduction of bio-based materials into the marketplace. There were four potential thematic working groups: – Biolubricants – Biopolymers – Plant-derived pharmaceuticals – Products for buildings Clearly, Coventry had an interest in plant-derived pharmaceuticals and so Rosemary Cole brought together a group of us to look at submitting a project proposal to a Department for Environment, Food, and Rural Affairs (DEFRA) call entitled “Non-Food uses of crops supply chain assessment and development”, to extend the RAPFI

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project. The idea was that following the completion of RAPFI, the programme management board had agreed that there was potential to take it further as a demonstration project. This recent call for proposals from DEFRA seemed to be an ideal fit. The potential team consisted of the National Herb Centre (NHC), the Sonochemistry Centre at Coventry University and Botanix, who were producers and extractors of specialty chemicals. We held a meeting to discuss the plan in Coventry University in October 2004, involving the Sonochemistry Centre, Rosemary Cole, Ray Marriot who was CEO of Botanix and Kosmas Tsuvaras, an agronomist from the National Herb Centre. A plan was put together and submitted in the first week of November, entitled “Rosemary Assessment and Development of the Supply Chain” (RSDSC). The main objective of this project was to provide convincing evidence that high value specialized antioxidants from rosemary could be produced and extracted economically in the UK, overcoming any barriers to market uptake by the food and pharmaceutical industries in the UK, Europe and worldwide. The official start-up meeting of the DEFRA RADSC project was held at the National Herb Centre at 11.00 on 23 February 2005. Groundwork on the identification and selection of Rosmarinus officinalis (L.) to produce an optimum antioxidant activity had been pursued by Rosemary Cole and Carol Wellwood [83]. This paper reported the variation of carnosic acid concentrations in extracts of 29 accessions grown in field trials at the three trial sites in England during the RAPFI programme. Extracts from different rosemary accessions using three solvents of varying polarity were assayed for their antioxidant activity and their major antioxidant compounds were identified and quantified by high-performance liquid chromatography (HPLC). Carol had been part of the RAPFI team while at the National Herb Centre, but had recently moved to Plymouth University. The Sonochemistry group would apply itself to two issues of the project. The first was to determine whether the residues of rosemary plant material (after CO2 extraction at Botanix) could be further extracted using UAE to provide additional antioxidant material. The second was to scale up UAE extraction. 3.6.2.3.1 UAE of the residue after CO2 extraction at Botanix Botanix had a pilot scale CO2 extraction unit that could handle small quantities of dried rosemary, down to 1 kg of charge. It was intended that a two-stage process for rosemary extraction could be developed to obtain three major target compounds: carnosic acid, carnosol and rosmarinic acid (Figure 3.16). – Stage 1: lipophilic antioxidants e.g. carnosic acid and carnosol were removed from dried rosemary leaves using non-polar liquid CO2 at Botanix – Stage 2: more polar antioxidants, for example, rosmarinic acid were to be extracted from the residue using UAE in Coventry.

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This part of the project was not entirely successful due to a very limited supply of “spent” rosemary, that is material remaining after CO2 extraction. We did obtain six separate samples from Botanix, each from different accessions of the herb. We subjected these samples to both conventional and ultrasonic extraction and in every case, reasonable yields of both carnosic and rosmarinic acids were obtained. Supercritical CO2 extraction causes the destruction of cell walls and produces a material that was reduced in water content and constituents, with little remaining structure. It was dry and crumbly. The lack of a structure meant that although UAE still tended to give slightly more extract than when using the conventional process, the differences between them were not as pronounced as was the case for fresh or dried leaves. The results are shown in Figure 3.20, but these quantities are related to the amount of antioxidant remaining in the “spent” rosemary and not to the amount of antioxidants in the original leaves because that information was not made available to us. Carnosic Acid

CA (mg/g)

40

30 US Post-CO2 20

Thermal Post-CO2

10 0 1

2

3

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5

6

Extract Sample (a)

Rosmarinic Acid

RA (mg/g)

4 3 US Post-CO2 2

Thermal Post-CO2

1 0

1

2

3

4

5

6

Extract Sample (b) Figure 3.20: Results of extracting post CO2 extracted rosemary: (a) carnosic acid and (b) rosmarinic acid.

Details of the scale-up achieved within RADSC are to be found in Section 3.9.3.

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3.6.3 UAE applied to the valorization (increasing the value) of edible oils Valorization is a term used in finance, referring to increasing or maximizing the value of an investment. It can now be found in papers on extraction technologies, particularly when applied to increasing the worth of low-cost materials such as vegetable oils by the incorporation of more expensive additives to increase their health benefits and, of course, improve the financial returns to the manufacturers. This topic is of importance primarily to the food industry but also of great interest in extraction technology. One example is some work undertaken with Farid Chemat. Both authors had known Farid since his early days of research in Toulouse with Jacques Berlan. He moved to the University of Réunion and whilst there, collaborated with the group in Coventry to produce a book chapter on ultrasound as a preservation technology [84]. We continued working with him after he moved to the University of Avignon, mainly in terms of the uses of ultrasound in extraction, and in 2012, a paper was published directly relating to UAE and valorization. It described the enrichment of edible oil with sea buckthorn [85]. Improving the value of vegetable oils is important for the food, cosmetic and nutraceutical industry. Normally, it is accomplished by adding ingredients such as carotenoids that have been previously extracted from a vegetable source using hexane or other solvents. This process involves several steps, which include (a) extracting the vegetal material with a solvent (e.g. hexane) (b) evaporation of the solvent and (c) mixing the extract with the low-grade oil. In the UAE process, the residual pomace remaining after the juice has been extracted from sea buckthorn (all such residues from vegetable extractions are known as pomace) was extracted directly with an oil, which could be sunflower, rape seed, olive or soya. This approach was effectively UAE by maceration of the pomace in the oil without the use of solvent, in a single step. The results of this study identified a new use for agro-industrial waste, where a low-quality edible oil is used as an efficient means of extraction of the valuable residual carotenoids from waste thus providing a means to the commercial and nutritional value of that oil.

3.6.4 UAE and links with non-classical extraction procedures Towards the end of the RATFI project, two important conferences related to extraction technologies were held in the UK. Together, they were to prove useful in making connections for our future research programmes and funding. The first was organized by ACTIN and entitled “A Workshop on Extraction Technologies”. It was held in March 2000 at Chilford Halls, near Cambridge. The second was organized by the Society for Chemical Industry in London in March 2001, entitled “Developments in Extraction Technology”. Both were aimed at reviewing various novel extraction technologies seen as important in developing vegetable sources for the

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sustainable production of high value materials. At each conference, RATFI was the only group that presented reviews of UAE. There were several other extraction methods presented at both meetings and amongst them, there were two with which we were to become involved during our investigation of rosemary. These were liquid and supercritical CO2 (Botanix and English Hop Products, both based in Tonbridge, Kent) and a low-boiling fluorocarbon solvent (Advanced Phytonics in Northalerton, Yorkshire). 3.6.4.1 Extraction with CO2 At the time when these two conferences were held, the use of carbon dioxide on an industrial scale to extract natural products had been in existence for some 20 years. There were more than forty carbon dioxide extraction plants in operation throughout the world, but of these, fifteen plants accounted for in excess of 86% of the available capacity. The largest industrial applications were in the decaffeination of tea and coffee. Caffeine could then be purified for food and drug use. Other applications included the extraction of hops, defatting of cocoa powder and the extraction of valuable constituents of oil seeds, spices and aromatic plants. Carbon dioxide, when compressed and liquefied, is able to dissolve volatile and non-volatile components at relative low temperatures. Unlike organic solvents, carbon dioxide, because of its far greater volatility, can be recovered at much lower temperatures (often, sub ambient). This gives it a great advantage over organic solvents (and steam distillation) because it causes little or no deterioration of thermally labile components. Two physical phases of the cooled gas are used: liquid and supercritical liquid. In both cases, non-polar and slightly polar molecules are more soluble than polar materials (Table 3.8). Table 3.8: Characteristic properties of compressed and liquefied CO2. Property

Sub-critical (liquid)

Super-critical

Pressure

– bar

– bar

Temperature

– °C

– °C

mol wt range