Physical chemistry of foams and aerosols: educational manual 9786010421004

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Physical chemistry of foams and aerosols: educational manual
 9786010421004

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AL-FARABI KAZAKH NATIONAL UNIVERSITY

Zh. B. Ospanova K. B. Musabekov

PHYSICAL CHEMISTRY OF FOAMS AND AEROSOLS Educational manual

Almaty «Qazaq university» 2016

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UDC 544.77.051.13 O-83 Recommended for the Academic Council of the Faculty of Chemistry and Chemical Technology and Editorial and Publishing Council al-Farabi KazNU (protocol №1 dated 02.11.2016) Reviewers: Candidate chemical sciences, assistant professor A.O. Adilbekova

O-83

Ospanova Zh.B. Physical chemistry of foams and aerosols: educational manual / Zh.B. Ospanova, K.B. Musabekov. – Almaty: Qazaq university, 2016. – 72 p. ISBN 978-601-04-2100-4 The educational manual consists of a theoretical part and an experimental part. The manual presents the laboratory works with a brief theoretical introduction and control questions for self work. The purpose of manual – to teach students to experimental methods of obtaining of foam and aerosol systems and study of their properties. Designed for university students studying in the field «Chemistry», «Chemical technology of organic substances» and «Chemical technology of inorganic substances». Учебное пособие состоит из теоретической и экспериментальной частей. В пособии представлены лабораторные работы с кратким теоретическим введением и контрольные вопросы для самопроверки. Цель данного пособия – научить студентов экспериментальным методам получения пенных и аэрозольных систем и исследования их свойств. Разработано для студентов высших учебных заведений, обучающихся по специальностям «Химия», «Химическая технология органических веществ» и «Химическая технология неорганических веществ».

UDC 544.77.051.13 ISBN 978-601-04-2100-4

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© Ospanova Zh.B., Musabekov K.B., 2016 © Al-Farabi KazNU, 2016

INTRODUCTION Foams are dispersion of gas in a liquid or solid body; aerosols are disperse systems consisting of liquid or solid particles of a substance suspended in a gaseous atmosphere (usually in air). Such disperse systems are quite abundant in nature and engineering. Foamed and aerosol detergents are widely used for cleaning bath, carpets and furniture. Of great importance are foams in firefighting, especially, against ignition of tanks with easily inflammable liquids, when extinguishing fire in closed spaces – basements, vessels, airplanes. Foams are used for heat insulation, for example, to prevent freezing of fields for open-cast mining of minerals. Foams with solid thin walls (aerogels) are widely used for production of heat insulation and sound-proofing materials, foam plastics, rescue facilities, etc. Confectionaries, cakes refer to solid foams, too. Particles of aerosols get into the atmosphere from the Earth in a ready form, but a significant part of them is formed due chemical reactions between gaseous, liquid and solid substances, including steam. A great amount of aerosols is formed as a result of natural processes, and some of them have an anthropogenic origin. The amount of particles released to the atmosphere due to the activity of man is approximately equal to 1 billion tons a year. The chemical composition of the particles is different; it is silicon dioxide – sand, toxic metals, pesticides, hydrocarbons and others. The main source of anthropogenic aerosols is the process of combustion. The power industry and transport release 2/3 of the total amount of anthropogenic aerosols. Among other sources of aerosols are metallurgical plants, production of construction materials, chemical industry. The properties of foam and aerosol systems are significantly influenced by processes taking place at the interface disperse phase – disperse medium. Therefore, the knowledge of the properties of systems characterized by the presence of phase boundary, and the ability to control their behavior are of both practical and scientific importance. 3

The manual «Physical chemistry of foams and aerosols» intended for students of chemical and chemical-technological specialties of higher schools: «Chemistry», «Chemical technology of organic substances», «Chemical technology of inorganic substances». The aim of this manual is to teach students master experimental methods of obtaining foam and aerosol systems and studying their properties. The manual presents laboratory work with a brief theoretical introduction and questions for self-control. The practical work is designed taking into account the prerequisites stipulated by the program of students (higher mathematics, physics, inorganic chemistry, organic chemistry, colloid chemistry). Therefore, it was not necessary to consider in detail determination of measurement errors, processing of experimental data and constructing plots and tables. Methodical developments of the experiments are given taking into consideration the experience of teachers and workers of the department in carrying out laboratory works. The authors hope that the manual «Physical chemistry of foams and aerosols» will contribute to a more profound understanding of colloid chemistry by the students. Any remarks of the readers will be admitted with gratitude.

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1 THE BASIC NOTIONS ABOUT FOAMS AND METHODS OF OBTAINING THEM 1.1. The structure of foam Foam is a disperse system consisting of bubbles of gas separated by interlayers of liquid. Usually, gas is considered to be a disperse phase and liquid is considered as a continuous disperse medium. The structure of foams is determined by the ratio of volumes of gaseous and liquid phases and depending on that ratios, foam cells can have a spherical or polyhedral form (Figure 1.1).

a)

b)

Figure 1.1. An element of spherical foam consisting of three bubbles (a) and an elementary cell of polyhedral foam (b)

A transition form of cells from a spherical to a polyhedral one was called a cellular form by Manegold due to its similarity with the structure of the honeycomb. Foam cells take a spherical form if the 5

volume of a gaseous phase exceeds the volume of liquid not more than 10-20 times. In such foams, films of bubbles have a relatively large thickness. The less the ratio of volumes of gaseous and liquid phases, the larger is the thickness of the film. Foam cells which have this ratio being equal to several tens and even hundreds are separated by very thin liquid films; their cells are polyhedrons. In the process of construction, a spherical form of foam bubbles transforms into a polyhedral one due to the films getting thin. The change of the form of foam bubbles from a spherical to a polyhedral one is easily observed in a burette containing foamed liquid. The state of foam with polyhedral cells is close to the state of equilibrium. Therefore, such foams are more stable than foams with spherical cells. According to the data of Plato, a polyhedral structure of foams is described by two geometric rules. 1. In each edge of a polyhedron three films converge, the angles between which are equal and make up 120°. The places of film joints (edges of polyhedrons) are characterized by increased thickness formed in the cross-section of triangles (Figure 1.2). These thicknesses are called canals of Plato or Gibbs. They are a bound system and they permeate the whole structure of the foam. A system of canals formed by plane (or distorted) films, the section area of which is less than that of Plato’s canals, is similar. These canals consist of two adsorption layers of SAS molecules and a layer of solution between them. 2. Four canals of Plato converge in one point forming identical angles of 109´28´.

Figure 1.2. A cross-section of a Plato-Gibbs canal

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A qualitative study on the rearrangement of polyhedral bubbles in foam has shown that in the process of foam destruction owing to gaseous diffusion the bubbles subsequently take the form of a parallelepiped, triangular prism and tetrahedron independently on the original structure. In the last stage (tetrahedron) there takes place transformation of a volumetric figure into a node (the place of joints of Plato’s canals). The surface of faces of foam cells can be plane only in case of pentagonal polyhedrons. For other forms of polyhedrons, faces have no curvature only under the condition of pressures in separate bubbles.

1.2. Methods of producing foams The main stages of foam formation can be traced back on the example of the behavior of bubbles coming to the surface. When bubbles of gas appear on the surface of their boundary with the solution of SAS, there takes place adsorption of SAS. Coming to the surface of solution every bubble forms a semi-spherical dome which is a liquid film consisting of two adsorption layers of SAS and an internal layer of solution (Figure 1.3).

Figure 1.3. A scheme elucidating formation of a foam film when a bubble comes to the surface of SAS solution

Adsorption layers of SAS provide a long-term existence of the emerging films. The increase in the number of bubbles results in 7

their drawing together. The process of further drawing together and deformation of surfaces of bubbles is also contributed by capillary attraction of bubbles, owing to that between the neighbor bubbles there emerge thin liquid films. As a result, on the surface of solution, there at first forms a monolayer of gaseous bubbles, then the subsequent layers are formed, that leading to formation of a volumetric foam. Like any disperse system, foam can be obtained by two methods: amalgamation of very small (microscopic) gaseous bubbles into larger ones (the method of condensation) or breaking of large air bubbles to small ones (the method of dispersion). When using the dispersion method, foam is formed as a result of intensive joint dispersion of a foam forming solution and air. Technologically, dispersion is realized by the following methods:  When jets of gas pass through a layer of liquid ( in bubbling or aeration installations, in aerators with «a foam layer») used for purification of waste gases, in foam generators of some types having a grid irrigated with a foam forming solution. A simple and widely spread method of foam production is dispersion of gas with the help of porous partitions (filters) fixed in the bottom part of a foam device. This method is used in a laboratory practice, in flotation, adsorption of gases and dust collection. Dispersity of foam depends on the diameter of pores of the filter, the wetting ability of the material of the filter, physic-chemical properties of foam generator and conditions of dispersion. Foam being obtained by bubbling methods is initially a gaseous emulsion. The rate of its transformation into a polyhedral foam depends on the rate of bubbles coming to the surface and the following out flowing of the «excess» liquid. The rate of bubbles coming to the surface is determined by their sizes, fraction of gas dispersed in liquid (concentration of gas in the liquid) and concentration of SAS. In laboratory practice, porous plates obtained by sintering of glass powders are most widely used. A characteristic peculiarity of these dispersing systems is a considerable inhomogeneity of pores by sizes as well as inconstancy of the cross-section of pores by the height of the plate. Because of these inhomogeneities the number of acting pores depends on the pressure of gas and surface tension of solution. With the increase in pressure all smaller pores start «working». Foam 8

generators of a jet type and foam generators with a grid as a dispersion system are widely spread, especially for production of anti-fire foams and foams designed for dust suppression.  When acting of flowing units on the liquid in the atmosphere of gas or in the course of action of flowing liquid on the barrier (in technological devices during mixing by agitators – floatation machines, during shaking up, beating, mixing solutions).  When ejecting air with a flowing jet of solution (in some types of foam generators designed for extinguishing fires). Ejection – the condensation method of almost instantaneous foaming – serves as a visual illustration of the law of a gaseous state: the increase in pressure or the decrease in temperature results in the increase of solubility of gas in liquid (Henry’s law). When decreasing pressure or increasing temperature, gas will momentarily start releasing and foam the liquid. In the liquid subjected to purification from solid particles, gas is dissolved under pressure and then SAS is added. The liquid is transferred into the other reservoir with decreased pressure, the dissolved gas releases and, foaming water, carries over pollution with bubbles.  Electrolysis of water is used in electric floatation, foaming occurs on account of gas bubbles and SAS introduced into the solution. The condensation method of foam forming can be realized according to three different schemes:  Changing the parameters of the physical state of the system, for example, decreasing pressure under the solution, increasing the temperature of solutions or introducing substances which reduce solubility of gases;  As a result of chemical reactions (the main of them are: interaction of soda with acid, of hydrogen peroxide with potassium permanganate, decomposition of ammonium carbonate) with the release of CO2 or O2: NaHCO3+HClNaCl+H2O+CO2 2KMnO4+H2O2+3H2SO42MnSO4+K2SO4+4H2O+3O2 (NH4)2CO32NH3+CO2+H2O  Using microbiological processes accompanied by the release of gases, most of them CO2. The condensation methods are widely 9

used in baking bread and confectionaries, in production of foam plastics, fire extinguishers, in the technology of aerated concrete and foam metal. It should be noted that the process of foam formation is extremely complex due to the joint effect of numerous factors. The regularities conditioning formation of foam significantly change depending on the conditions of the specific technological process or experiment.

1.3. Characteristics of foam To evaluate foam forming solutions and foams obtained from them, investigators use various criteria: the volume or height of the foam column obtained under definite conditions of the experiment, the ratio of the foam height to the time of its complete destruction, the change of the volume (the column height) in time presented in the form of plots, etc. Up to the present time, there is no and, obviously, can not be a universal criterion of foam formation which will objectively evaluate all foaming systems under any conditions. However, if is possible to point out the following properties which thoroughly characterize a foam system: 1. The properties of a single foam. It was stated by Plato that the life-time of the film is inversely proportional to its surface area, therefore, it is necessary to compare the foam forming ability of different SAS at the same area of the film surface. 2. The foam formation ability of solution is the amount of foam expressed by the volume of foam (ml) or the height of its column (mm) which is formed from the constant volume of solution under definite conditions during the given time. The both indexes are semiquantitative as they depend on the conditions of foam production, vessel, rate of gas supply, etc. The volume or the height of the foam column sharply decreases depending on the size of bubbles. These indexes can be quantitative if the dispersion state of foam is taken into account. The total surface of foam bubbles is a more objective characteristic not depending on the methods of production. 3.The criterion which is more objective than the volume or height of a foam column is the foam ratio K-the ratio of the foam volume to the volume of liquid contained in foam: 10

K

Vfoam Vliquid in

foam

=

(Vgas  Vliquid ) =1+ Vgas , Vliquid Vliquid

(1)

where Vfoam, Vliq. in foam – are the volume of foam and the volume of liquid in foam, Vliq., Vgas – are volumes of a liquid and gaseous phases. In civil engineering and industry of building materials, foams with the ratio of 5-10 are used, in laundries – with the ratio of 10-20, in fire extinguishing – with the ratio of 70-90. In foams with a solid dispersion medium, the ratio is determined by the following expression:



V foam V (V  V ) = gas s. =1+ gas , Vs. inf oam Vs . Vs.

(2)

where Vfoam, Vs in foam – are volumes of foam and a solid substance in foam, Vs, Vgas – are volumes of solid and gaseous phases. To compare the technological properties of foams, criterion q is used which is the function of foam rise Нf in time τe and life-time of foam τf: q=

Hf  e .

f

(3)

4. Foam dispersity is characterized by an average size of air bubbles: the less are bubbles, the more is the dispersity of foam. Foam with a large size of cells is called rough disperse. Foam is always polydispersive. Polydispersity has not only structural ratios in real foams but also effects transfer processes – electric conductivity and thermal conductivity of foams, their filtration properties and syneresis, significantly changes mechanical and thermal physical properties of rejected foam materials (foam plastics) and determines the rheological peculiarities of foam flow. In the process of ageing, the degree of foam polydispersity increases. The kinetics of dispersity change indicates the rate of internal destruction of the foam structure due to coalescence and diffusion transfer. The rate of many technological processes in microbiological and chemical industries as well as the efficiency of fire extinguishing depend on the dispersity of foam. 11

5. An inalienable property of polyhedral foam is a reduced (compared to atmospheric pressure) equilibrium capillary pressure in canals of Plato-Gibbs. At the moment of foam production, the pressure in its canals depends mainly on the ratio, dispersity and surface tension. 6. Resistance and stability of foam are characterized by the time of its existence up to complete or partial destruction. There are a number of devices for determining the life-time or half-decay time. The destruction of a foam column is observed or the life-time of separate bubbles is measured (the less is the bubble, the more stable it is). As a rule, the destruction time of half of the foam volume is determined. 7. The ultimate shearing stress of foam is often expressed by hardness. It characterizes the ability of foam to react definite mechanical loads, for example, pressure of higher positioned column of foam, without deformation, i.e. change of the volume and form. Foams possess some hardness even if their films are liquid. This is explained by the fact that the equilibrium state corresponds to minimum surface energy, and any deformation enhances this energy, i.e. requires external work. 8. Viscosity of foam is a flow characteristic (rheology is the science of flow) the knowledge of which allows to determine the conditions of foam transfer by pipes, spreading of foam mass over a surface (when extinguishing fire), the ability to free outflowing from openings. All the main properties of foam depend, in the first place, on the following: with the help of what substances it is produced, i.e. on the kind and amount of the foaming agent. When analyzing different processes in foams and interpreting experimental results, the polyhedral model of the foam structure is most frequently used. According to this model, foam canals have a constant section along the whole length. Analyzing this model, Krotov V.V. showed that the volume of liquid contained in the nodes of the capillary structure (at the joints of Plato-Gibbs canals) is negligibly small. Many calculations of foam parameters are based on this supposition. So, using the notions of the polyhedral structure of foam, an equation relating foam dispersity, its ratio and capillary pressure in Plato-Gibbs canals was derived. 12

Capillary pressure (Pσ) is equal to the difference between pressure inside foam bubbles (PB) and pressure in Plato-Gibbs canal (PG): P  PB  PG .

(4)

For a polydisperse foam, the value of PB is equal to: PB  Pe 

2 ,  3

(5)

where PB – is atmospheric pressure; ε – is the ratio of the total area of foam surface to the total volume of gas in foam; σ – is surface tension of solution. For polyhedral structure from pentagonal dodecahedrons, ε can be shown to be equal to: 2,7 , (6)  a where a is the length of a dodecahedron edge ( a cell parameter). In the gravitational field, pressure in the canal makes up: PG  PG0  P ,

(7)

where PG° – is pressures in the lower layer of foam; ΔP – is the difference of pressures in the lower layer and at the height H. If foam contacts with liquid, PG° = Pe, and ΔP is capillary rarefaction in relation to the atmosphere, at hydrostatic equilibrium:

P  gH ,

(8)

where ρ is density of liquid; g is acceleration of free fall; H is the height of a foam column. In general case: P 

 2 1,8  Pe    (Pe  P)   P , R 3 a

(9)

where R is the radius of Plato-Gibbs canal curvature. If the foam ratio K is expressed by cell parameters, we have the expression: 13

K 1

7,66  a 2 10,5  a  h  1,6  R 2

,

(10)

2

where h is thickness of a foam film. If the volume of liquid in films is small compared to the volume of liquid in films is small compared to the volume of liquid in canals, it can be neglected and the expression gets simplified: K  1  4,8 

a2 . R2

(11)

Solving the equations in combination, we can finally have the expression:  . (12) a  (0,46 K  1  1,8)  P The derived expression is well confirmed experimentally, it can be used for calculation of foam dispersity according to the data on measuring the ratio of foam and capillary pressure in foam canals. 1.4. Surface-active substances All the main properties of foam depend, in the first place, on the following: with the help of what substances it is produced, i.e. on the kind and concentration of the foaming agent. As the most important foaming agents refer to the class of surface-active substances (SAS), let us consider the structure and main properties of these substances in more detail. The foam forming ability of SAS solutions is conditioned by the diphilic structure of their molecules. According to the ability to dissociation in aqueous solutions, surface-active substances are divided into ionogenic and nonionogenic ones. In their turn, ionogenic SAS are subdivided into anionic, cationic and ampholyte (amphoteric) ones. Anionic SAS – are organic compounds which, dissociating in water, form an anion with a large hydrocarbon radical – a carrier of surface activity; and a cation is not surface active. SAS of this type which made up the greater part of the world production are: 14

 carbonic acids and their salts (soaps) with the general formula RCOOMe (where Me is metal: Na+ in solid soaps, K+ or NH4+ in liquid soaps, R is organic radical C8 – C20), for example sodium palmitate C15H31COONa, sodium stearate C17H33COONa, sodium oleate C17H31COONa. Such SAS are distinguished by simplicity of their production, low cost, complete biodecomposition. Synthetic fatty acids of a normal structure containing 10-20 atoms of carbon in the molecule are widely used in production of SAS. Soaps of carbonic acids have a good detergent power only in an alkaline medium, but in an acid medium (due to formation of hardly soluble fatty acids) and in hard water (due to formation of insoluble calcium and magnesium salts) the detergency of these SAS is low.  alkylarylsulphonates (most often alkylbenzensulphonates) salts of aromatic sulphonic acids with general formula R

SO3-Me +

are the cheapest and the most available of synthetic SAS. Owing to fast biodecomposition in the biosphere, alkylbenzenesulphonates with the linear structure of alkyl radical are produced. Sodium propyl – and butylnaphthalenesulphonates refer to this group too. The foam forming ability of solutions of alkylbenzenesulphonates with an alkyl chain of a normal structure increases from homolog C4 to a compound containing 14 atoms of carbon and then decreases.  alkylsulphonates – RSO3Me ( R is usually C10-C20) form a promising group of SAS possessing a good detergent power under the conditions of different pH and in hard water and a good biodecomposition. Solutions of alkylsulphonates beginning with C11 have a good foam forming ability at concentrations of about 0.5 g/l, maximum foam forming ability is achieved for homolog C15, for more high molecular alkylsulphonates it is lower. It is noted that the foam forming ability of SAS solutions depends on the location of a hydrophilic group in a molecule: the closer it is to the middle of the molecule, the higher is the foam forming ability of solutions.  alkylsulphates are salts of sulphonates with the general formula ROSO3Me (R is usually C10-C18). SAS of this group are 15

quite promising from the ecological point of view but more expensive than alkylarylsulphonates. The foam forming ability of aqueous solutions of primary alkylsulphates increases with the increase in the length of hydrocarbon radical reaching the maximum value for dodecylsulphate. Homologs with a greater molecular mass have a low foam forming ability at room temperature because of their less solubility in water.  alkylsulphinates, alkylthiosulphates, alkylphosphates, alkylphosphonates refer to anionic SAS, too. Anionic SAS are used as wetting agents, the main components of detergents are foaming agents. Anionic SAS develop their properties most actively in alkaline media, though they may be used also in acid solutions and solutions of salts, for example alkylsulphates and alkylsulphonates. Cationic SAS dissociate in water with formation of organic cation – a carrier of surface activity. Fatty amines dissociate and develop surface activity mainly in acid media. There are primary, secondary, tertiary and quarternary amines.  Compounds with the general formula RNX, where R is usually C10 – C16, X is anion ( Cl-, Br-), refer to primary amines.  Secondary amines have the formula R’R’’NX, for example, dodecylmethylammoniumchloride.  Tertiary amines have the formula RR’R’’NX or RN(R’)2X, for example, dodecyldimethylammoniumchloride.  Quarternary amines with the general formula RN(R’)3X (where r is C12-C18, R’ is CH3, C2H5, X is Cl-, Br-) have found the widest application.  Alkylpyridinium salts where X is Cl-, Br-, SO3-, acetate and other anions.  Salts of tetrasubstituted ammonium and pyridine bases are soluble in both acid and alkaline media. At the length of hydrocarbon radical C12-C18 they can have a bactericide action. Cationic SAS are used as corrosion inhibitors, floating reagents, bactericide, disinfection and fungicide preparations, their use in CMC is limited because of their high cost. Salts of pyridine bases are used in textile industry as colour fixing agents. SAS of this group are the most toxic and the less biodecomposable of all SAS. 16

Ampholyte (amphoteric) SAS are compounds containing in the composition of molecules both types groups – acidic (most often carboxylic) and basic (usually an aminogroup of different degrees of substitution). Depending on pH, of the medium they developthe properties of both the cationic (at рН4) and anionic SAS (at pH 9-12): alkaline environment

RNH(CH2)nCOOanionic properties

acidic environment

RNH(CH2)nCOOH

RN+H2(CH2)nCOOH cationic properties

At рН=4-9 they can act behave as nonionic compounds. This type of SUS includes many natural substances including all aminoacids and proteins. Alkylaminoacids can serve as examples of their synthetic analogs. Production of such substances is quite complex and expensive. Nonionogenic SAS are soluble in both acid and alkaline media. They are compound not dissociating in water. They are:  oxiethoxylated primary and secondary fatty alcohols RO (CH2CH2O)nH, RR’CHO(CH2CH2O)nH.  polyethyleneglycol esters of fatty acids R COO(CH2CH2O)nH.  oxyethylated alkylphenols R C6H4O (CH2CH2O)nH. Here, R is usually С8-С9, n – is an average number of oxyethyl groups.  pluronics are block copolymers of ethylene oxide and propylene oxide HO(CH3CHCH2O)n(C2H5)mH with the molecular mass of 2000-20000, the solubility, surface activity of which are determined by the ratio of the length of a polyoxypropylene (a carrier of hydrophobicity) and polyoxyethylene (a carrier of hydrophobicity) chains. They are promising noniogenic SAS.  a large group of natural SAS refer to noniogenic SAS. These glycerides, glucosides, saccharides. Mono- and diesters of long-chain acids and multiatomic alcohols are oil soluble SAS. Sulfoeterifikation and the subsequent neutralization of these substances allow producing water-soluble SAS. Many SAS from this group, for exemple sucrose ester, are not toxic at all, they have no odour and taste and are successfully used in food industry, medicine and cosmetics. 17

Ethers of phosphoric acid can also be subjected to oxyethylation. Oxiethoxylated fatty alcohols are easily decomposed in the biosphere. Unlike them, In contrast, oxiethoxylated alkylphenols poorly decompose in the biosphere even if straight-chain alkyl radicals are used. The foam foaming ability of oxiethoxylated SAS depends on both the length and the number of CH2 – CH2O – groups. In the series of oxiethoxylated fatty alcohols, the foaming ability increases from C4 to C10-12 and then decreases. Alcohols with a normal structure are characterized by a great foaming ability, the higher alcohols with a branched chain having its maximum. As components of detergents, nonionogenic SAS are not inferior to soaps of high quality and are successfully used in soft and hard water, in neutral, acidic and alkaline media. They have a low foaming ability and can be used as foam suppressors. Nonionogenic SAS are widely used in textile industry. Oxyethoxylated SAS are widely used in oil production: they are introduced into solutions which are pumped into wells at edge water flood, this contributing to pressing oil back from the pool to the production well. Various biologically active substances refer to natural SAS. Lipids are esters of glycerine or sphingosin (long-chain aminoalcohol) and fatty acids (saturated and unsaturated acids) containing C12 – C18 in the main a hydrocarbon radical. Most lipids have two such chains in the molecule. Polar parts can include different chemical groups: ether groups (mono-, di- and triglycerides), phosphoric acid residues (phospholipids) and carbohydrate residuals (in a large group of glycolipids). The surface activity of proteins dependens on the tertiary structure of protein molecules, which is conditioned by a space arrangement of their polypeptide chains. The surface of a protein globule has a mosaic character, i.e. contains polar and non-polar sites; and their fractions are approximately equal. Protein is usually adsorbed in a globular form at the interface and in a number of cases there may take place changes in the conformation of molecules in the adsorption layer. Adsorption of the protein molecules are to a great extent irreversible. Another large and promising group includes synthetic high molecular SAS. 18

Foam generators are divided into two types. Foam generators of the first kind are lower alcohols, acids which are present in the volume of solution and adsorption layer in the molecular state. Foams containing these SAS quickly disintegrate (as a rule, their life timedoes not exceed 20 seconds). Foam stability maxim responds to a definite concentration of the foaming agent. The concentration at which foam stability maximum is observed decreases with increase number of carbon atoms in a homolog series. For example, in a series of alcohols the optimum concentration of the foaming agent decreases from 0.3 to 3 • 10-4 M when going from ethyl alcohol to oxiethoxylated alcohol. The higher members of the homolog series passess insufficient solubility and, therefore, they are not good foam generators. Soaps give much more stable foams than acids and alcohols, evidently, due to the presence of ionic groups in their molecules, as in case of alcohols and acids, maximum stability of foam responds to soaps with a mean length of hydrocarbon radical and their solutions of average concentration. Foam generators of the 2nd kind. These are: different high-molecular compounds – proteins, saponins – glycosides extracted from plants and others. Stability of foams stabilized HMC is considerably higher (they can exist hundreds and thousands of seconds), as on the surface of bubbles there form strong gel like films. Stability of foams stabilized by foam generators of the 2nd kind, continuously increases with the increase in concentration of HMC. The frame of foam is very stable and canhold back outflowing of liquid from films. The foaming ability ionogenic surfactants are much higher that that of nonionogenicones, which is related to a higher rate of formation of adsorption layers. To increace foam stability, special additives called stabilizers are used. The action of stabilizers is based on the increase of solution viscosity and thereby slowing down outflowing of liquid from foams. Sometimes molecules of stabilizer get introduced into «the palisade» of molecules of the foam generator in foam films and binding blowing them into strong stable compounds. The reinforcing agents of foam can be soluble and insoluble, organic and mineral. According 19

to the principle of reinforcing action all stabilizers are divided into five groups. 1. Substances increasing viscosity of the foam forming solution (glycerin, ethylene glycol, methylcellulose). 2. Substances increasing viscosity of liquid in foam films (gelatine, starch, agar-agar). 3. Substances increasing strength of foam films on account of their polymerization in the volume of foam (synthetic resins, latexes). 4. Substances forming with a foam generator insoluble in water highly disperse precipitates which armour films and prevent their destruction (heavy metal salts). 5. Substances decreasing surface tension at the interface «liquid – gas» (higher fatty alcohols). In practice, depending on the requirements to foam stability and technological conditions of production, this or that group is chosen. For example, at confectionaries, stabilizers of the 2nd group are used in production of thermal insulation and acoustic materials, stabilizers of the 3rd group are effective. For stabilization of foams, sometimes the method of armouring bubbles is used: finely milled solid substances (talcum, asbestos and quartz), are introduced into foam. They are uniformly distributed in films at the surface of bubbles increasing their strength. Such foams are called mineralized. Concentration of foam generators plays an important role. For foam generators – colloid surfactant maximum foam foaming ability is reached in a definite range of concentrations, with the further increase of concentrations it remains constant or even decreases. In case of high molecular foam generators, foam forming ability increases with the increase of concentration. Besides, the value of pH of the medium is of importance for foam generators – proteins. They develop maximum foaming ability at the isoelectric point. In hard water (i.e., in the presence of a great number of salts) the ratio and stability of foams are low. In a number of cases, foam formation is not a desirable process. Destruction foams can be caused by introduction of foam suppressors. Their action is specific and determined by the type of a foam 20

generator in the foam subjected to destruction. All foam suppressors can be divided into 2 groups. Substances reacting with the foam generator and changing its nature refer to the first group. So, if HCl is added to the foam stabilized by sodium oleate we will have the reaction: C17 H33COONa  НС1  С17 Н33СООН  NaCl and the foam generator will transform from a strong electrolyte (C17H33COONa) to a weak one (C17H33COOH), that considerably decreasing the electrostatic factor of foam stability and leading to destruction of the foam. The second group includes substances having a high surface activity but not able to form strong adsorption films. There are average homologs of alcohols, for example, octyl alcohol, ketones, polyamides of fatty acids, propileneglycols, esters and others. They displace foam generators from the surface layer, make them less strong and contribute to breakage of walls of foam bubbles. This is the case, for example, when acetone is added to foam stabilized by gelatine.

1.5. The effect of different factors on the foaming ability It is known that «pure» liquids are not able to form a stable foam. This statement is confirmed thermodynamically. The change in Gibbs energy for a one-component system with a quite large surface is determined by the equation: dG =VdP – SdT – dA,

(13)

where V is volume of the system; P is pressure; T is temperature; S is entropy;  is surface tension; A is specific surface of the system. At constant pressure and temperature: = – (dG/A),T or G= – A.

(14)

The decrease in Reducing Gibbs energy G can be accompanied only by the decrease of the value ΔA, this contributes to destruction 21

of foam bubbles. Conseguently, foam from pure liquid is thermodynamically instable. For development of foaming, solution must contain at least one component possessing surface active properties. It is difficult to compare the foaming ability of surfactants of different classes. This is explained by different conditions of tests and especially the use of different methods for determination of this characteristic. However, some regularity can be noted. Anion active surfactants possess grate foaming ability in comparison with the nonionogenic ones. This may be related to the rate of formation of the adsorption layer, which is larger in case of anion active surfactants, i.e. less time is required to reach adsorption equilibrium. The foaming ability of SAS depends on the mutual effect of many factors. The foaming ability of surfactants usually increases with the increase in concentration. The increase of the foaming ability with growth concentration is related to micelle formation, as, when reaching critical concentration of micelle formation (CCM), maximum volume of foam is observed. Besides, in the area of CCM, a formation of the adsorption layer with maximum strength gets complete. With the further increase in concentration of surfactants in solution (higher than CCM) the rate of molecules diffusion into the surface layer decreases, resulting in the decrease of the foaming ability. In the area of positive temperatures, foaming of anion active surfactants usually increases with the growth of temperature and then, after reaching maximum starts decreasing. Investigations on the foaming ability of aqueous solutions of sodium n-alkyl sulphonates in a wide range of temperature have shown that in order to evaluate the ability of surfactants to foaming, it is necessary to take into account the character of CCM value dependence on temperature. Fatty acids and alkaline salts in acid medium practically do not form foam. Maximum foam formation of fatty acids is usually observed at pH = 8÷9, foaming of sodium oleate starts in fact only at pH = 9 but even at pH =12 does not reach maximum value yet. At pH=8, decane acid does not form foam, maximum of foaming ability of this acid solutions is observed at pH = 9. Under these conditions, molecule in the monolayer with increase in the sodium 22

salts of of fatty acid occupies the largest area per molecule in a monolayer with increasing length of the hydrophobic chain in the series of the sodium salts saturated fatty acids, maximum of the foaming ability is shifts to the alkaline area: if for sodium laureate the optimum value is pH = 7, for sodium palmitate it is pH = 10. As a rule protein solutions develop maximum foaming ability, at the isoelectric point. Solutions of gelatin and lactalbumin have maximum foaming at pH = 4.5. Addition of electrolytes results in the shift of the isoelectric point with a simultaneous shift of foaming maximum. The foaming ability of solution increases with the decrease in its surface tension. The decreasein the surface tension reguires less work for production of the equal volume of foam and the store of free surface energy decreases. As a rule, salts of hardness suppress foam formation. So, solutions of alkylbenzenesulfonates in hard water have much less foaming ability than in desalted water. The effect of hardness salts is especially noticeable for compounds containing 14 and more carbon atoms in the alkyl chain. Solutions of substances with 10 carbon atoms in the molecule are less subjected to the action of hardness salts. To improve the foaming ability of detergents under different conditions of application (including in hard water), inorganic electrolytes – potassium and sodium chlorides as well as phosphates which cause the increase of the volume of foam and its stability are introduced into the composition. However, the amount of introduced phosphates must not be extremely large as at their concentration, maximum of foaming ability is reached, and the further introduction of phosphates decreases foaming of solution. Soda acts similarly, besides, it decreases stability of the formed foam. Carboxymethylcellulose increases the foaming ability of synthetic detergents, too, especially at high temperatures. Introduction of excess amounts of carboxymethylcellulose inhibits formation of foam and increases its stability due to the increases of solution viscosity. The optimum action of this substance is reached at concentrations 0,2-0,75% depending on the kind of the synthetic detergent, the presence and concentration of other additives and the conditions of application. 23

With the aim to increase the deterging and bleaching action of detergents it is proposed to introduce polyacrylamide as an antisorbtion substance.

1.6. Stability of foams The practice of scientific investigations on the properties of foam films and foams, and development of effective methods of production and usage of foams for the preparation and application of foam in industry, agriculture, for fire extinguishing regure the knowledge of physico-chemical parameters of surfactants and their relation with foam-stabilizing ability of surfactant solutions. Usually, as a criterion for evaluation of the efficiency of surfactants as a foaming agent researchers use the value of adsorption of these substances at the interface «solution air» and the related properties such as the decrease of surface tension, the work of adsorption, the limit adsorption. Often, CCM is taken as a foaming characteristic, if micelle formation is possible in SAS solution. Also, parameters to stability of foam, for example, the life-time, the height of a foam column are used. The effect of temperature on stability of foams is complex and related to the procedure of a number of competing processes. Evaporation of the solvent and foaming agent increase with the increase of temperature, and stability of foam can increase or decrease depending on concentration of the foaming agent and its structure. With the increase in temperature adsorption decreases of surfactants decreases, this can lead to the decrease in stability of foam. At the same time solubility of the foaming agent improves contributing to the increase in stability of foam. At high temperatures, thermal oscillations of adsorbed molecules strengthen resulting in reduction of the mechanical strength of the surface layer formed by the foaming agent molecules. Besides, viscosity of the foaming solution decreases, correspondingly the rate of liquid flowing from the foam increases and the conditions for hydration of polar groups of the foaming agent change. 24

Stability of hydrate layers decrease increases in temperature, this causing the decrease in stability of foam. The increase in concentration of surfactants in solution leads to the increases in stability of foams reaching the maximum value at CCM, then stability decreases. The increase in stability of foams with the increase in concentration of SAS up to a definite limit corresponds to saturation of the adsorption layer. For alkylsulfonates, alkylphenol and sodium soaps, maximum of stability is shifts to the side of less concentration with the increase in the length of radical and becomes more blurring. Stability and resistance of foam are greatly affected by pH of the medium. So, stability of foams from solutions of anionic surfactants in acid medium noticeably increases and in alkaline medium it decreases. Compounds with a short hydrocarbon chain are characterized by the decrease in foam stability in acid medium and some increase in alkaline medium. This is due to the effect of hydrogen ions and hydroxyl on the interaction of the hydrophilic and hydrophobic parts of molecules shifting equilibrium adsorption of surfacetants and micelle formation to this or that side. Stabilization of foams is achieved by introduction of substancesstabilizers: carboxymethylcellulose, polyacrylamide, polyvinyl alcohol and others into solution. These substances increasing viscosity of solution and films contribute to slowing down the process of liquid flowing from foams. Stabilizers cause a considerable decrease CCM of surfactants solution. The most effective are those stabilizers the molecule of which has a branched chain and polar groups are able to form hydrogen bonds with molecules of water (-OH, -NH2, = NH, and others). If the solution contains surfactants of different types, the effect of stabilization can be conditioned by formation of mixed micelles consisting of molecules of nonionogenic and anionic surfacetants. When introducing small amounts of fatty alcohol into solution of anionic surfactants, CCM increases on account of the increase in SAS solubility. Figure 6 presents the structure of foams stabilized with different compositions of surfactants with polymers. The nature of a gaseous phase effect stability of foam. The time of foam column destruction can differ tens and more of times, if hydrogen and argon are used for the process of foam formation. Hydrogen gives the less stable foam and argon gives the most stable25

foam due to the difference in the relative density of these gases and diffusion coefficient of molecules in liquid films. The changes in dispersity of foams in time depending on the nature of gas – filler and the content of additives of higher fatty the alcohol in solution. The structural stability of foams decreases, when using gases with high solubility in water. Introduction of stabilizers – higher fatty alcohol – selectively 4-5 times increases the structural stability of foams obtained on hydrogen, propane and ethylene. In practice, the foams often interact with different solid fine disperse substances. This takes place in flotation, purification of waste gases, dust suppression, etc. The influence of a solid phase on stability of foams depends on concentration of the foaming agent. With a small content of the foaming agent (of about 0.1%) introduction of a solid phase causes a sharp increase in stability of foams with simultaneous increase in rate of the liquid outflowing. At high concentrations of the foaming agent (1%) the effect of a solid phase is less noticeable: the increase in stability and some slowing down of the rate of liquid outfiowing are insignificant. This difference is explained by the fact that at small concentrations of surfactants its greater part is adsorbed on the surface of solid phase particles owing to which concentration of SAS in solution decreases and correspondingly, surface tension increases, causing acceleration of the process of liquid outflowing from foams. The increase in stability of three-phase foams and the decrease in the rate of solution outflowing are conditioned by narrowing and clogging of Plato-Gibbs canals with solid particles.

1.7. Syneresis of foams The problem of stability of foams, like other disperse systems, is central in colloidal chemistry; many aspects of this problem have not been cleared up yet. At present, there is no theory explaining quantitatively the behavior of foams in time. Such theory is hardly possible as for different foams; these thermodynamic and kinetic factors of stability can be previling. Stability of foam should be studied taking into consideration three aspects: stability outflowing of liquid (syneresis), the change of the disperse composition and the decrease in the total volume of foam. 26

Study of foam syneresis pen is of great practical importance as the technological properties of foams of different purposes (for example, the efficiency of surface concentration and separation, fire extinguishing ability) are determined by the content of liquid in foam and the rate of the ratio change. The methods of investigation of syneresis come to determination of the amount of liquid flowing out of foam in the unit of time. A widely distributed characteristic of syneresis is duration of half of the initial volume of liquid flowing out from foam. Determination of these characteristics is specific in GOST on antifire foams. Hydrostatic stability of foams is conditioned by their ability to prevent liquid from outflowing under the effect of gravitational forces. Motion of the liquid against gravity is explained by capillary effects due to pressure gradient of liquid in Plato canals. As the liquid from flow out, the pressure gradient of liquid in Plato canal increases in height, when maximum value of the gradient is reached syneresis ceases. Further, outflowing is only possible due to the appearance of excess liquid as a result of destruction of bubbles. Hydrostatic stability is conserved only during several minutes after the formation of foam, then hydrostatic stability is disturbed and outflowing of liquid begins. The process of foam destruction is graphically described by a straight line in coordinate’s lg K – τ, where K is the ratio of foam, τ is time. The slope of the curve can serve as a measure of foam stability at the given moment of time. The aggregative stability of foams is related to their ability, to preserve a constant disperse composition. Destruction of the foam structure (the change of its dispersed composition) takes place due to diffuzion transfer of gas between foam bubbles and destruction of bubble films (coalescence). These processes lead to the decrease of the interface surface in foam.

1.8. The kinetic factor of foam stability The kinetic factor slows down the process of film thinning and thereby contributes to the increase in the viability of foam. Thin films possess the ability to respond to local changes in thickness 27

owing to which the weakened areas get «repaired». This «repaired» takes place on account of surface flow of solution from the area of low surface tensions to the area of large values of σ (Marangoni effect), i.e. thinning of films results in the increase of σ, i.e. SAS molecules of the surface layer are in the state of rarefaction. Marangoni effect depends on the volume of liquid bearing against the surface layer: this effect must increase with the decrease in the volume of liquid. When films get thinning the role of Marangoni effect increases, however, for films of a very small thichness consisting practically of two solvated layers this, effect is negligible, as in the adsorption layer vacuum rarefaction of SAS molecules «palisade» is difficult. Thus, the influence of Marangoni effect on stability of films is maximum at its optimum thickness. For a long time, Marangoni effect was considered, in combination with Gibbs effect. Only since the 60-ies of the XXth century the mechanism of their action has been considered separately. Stabilization of films according to Gibbs is explained by emergence of local differences of surface tension under the influence of mechanical or thermal disturbances causing the film spread. The surface tension of the spread part of film increases as the increase in the film surface leads to the decrease in average concentration of SAS in the surface layer. The local difference in surface tensions in conditions the emergence of SAS molecules flow in the adsorption layer to the side of low concentration to the spread part of film. Gibbs theory is based on the supposition of film elasticity. When the film spreads due to surface tension, its elasticity develops, the forces of which lead to its tightening back. Elasticity of film E according to Gibbs is determined by the ratio of the increase in surface tension to the relative increment in the area d / S due to local extension: Е = 2S . d/S.

(15)

At E>0, the film is resistant to any local disturbances as elastic forces are able to prevent the destructing action of disturbances and bring in back to the initial state. The kinetic factor is determining only for low-resistant (seconds and minutes) foam structures. 28

1.9. Thermodynamic factor of foam stability The thermodynamic factor or splitting pressure of double layers of ions. If SAS containing ionogenic groups are used for stabilization of films and concentration of electrolytes in water is small, stability of film can be related to the emergence, of electrostatic repulsion forces during overlapping of diffusive parts of double electric layers being formed by ionized molecules of SAS on the film surface. According to the electrostatic theory of stability of lyophobic colloidal systems (Deryagin-Landau-Ferwei-Overbek theory – DLVO), the electrostatic component of film energy of the is approximately equal to: Fsel 

64n0 kT 2  h ,  e 

(16)

where n0 is concentration of electrolyte in the volumetric phase; h is thickness of film;



8 e2 z 2  k T

is the value inverse to the thickness

of ionic atmosphere in the theory Dubai-Gukkel ( is dielectric constant of film substances, e is electron charge, z is valency of electrolyte counter ion); and value  = thzl0/4kТ is a function of 0 – potential of the layer of potential determining ions and at small 0:  = zl0/4kT, and at large 0: 1. Electrostatic interaction leads to emergence of forces of "splitting pressure" equal to: П

dFsel  64n0 kT  h . dh

(17)

The splitting action of double diffusive layers of ions can prevent the tendency of a film to thinning under the effect of capillary forces in «Gibbs triangle» (Figure 1.4) and the forces of intermolecular interaction Fsel = – const/h2. As a result, the dependency of film energy on its thickness, at ho, can have a sufficiently deep minimum (Figure 1.5), to which a stable state corresponds. Destruction of a film in this case is related to local overcoming of energy barrier due to thermal oscillations of the surface. The existence time of such films can reach minutes and tens of minutes. 29

Figure 1.4. Gibbs triangle

Figure. 1.5. Dependency of film energy on its thickness

1.10. The structural – mechanical factor of stability The structural-mechanical theory of film stability was developed by P.A. Rebinder, B.V. Deryagin and other researchers. When studying separate bubbles, it was noticed that maximum stability of bubbles continuously increases with SAS concentration reaching the limiting value at saturation of the adsorption layer. On the basis of these and more recent investigations it was supposed that stability of adsorption layers (including foams) is determined by their mechanical properties alongside with surface activity. Production of highly stable foams is only possible when using which are able to form gel-like structures in surface layers and the volume of film. This is related to specific strengthening of thin films on account of hydration of polar groups of SAS molecules preventing the motion of liquid under the action of capillary forces and gravity and at the same time decreasing the forces stretching the film. 30

Hydrated molecules contribute to formation of surface layers increasing stability of bubbles. Also, hydration of part of the hydrocarbon chain, especially molecules occupying a large area in the surface layer is possible. Stabilization is conditioned by the presence of cohesive forces between separated molecules of the adsorption layer as well as the mobility of these molecules which provides quick restoration of deformations emerging due to outflowing of liquid from the film of bubble. A definite role in stability of foams can be played by viscosity of inter film liquid the increase of which decreases the rate of the process of liquid outflowing from the film of bubbles. This is possible because in the middle part of the film between the adsorption layers there can form a spatial structure which considerably increases the viscosity of this part of film. The presence of a volumetric structure in films significantly increases stability increases stability of films in a whole. The process of stabilization is evidently in a sharp decrease of the rate of liquid outflowing and consequently, the rate of film thinning. Deceleration of outflowing can be due to both a slower outflowing of liquid in thin capillaries and the result of drawing of the liquid back into capillaries. Control questions 1. What systems are called foams? What structure do foams have? 2. Name the methods of foam production. 3. Are foams thermodynamically stable systems? 4. What is aggregate stability of foams? 5. What is the sedimentation stability of foams? How is it possible to increase it? What substances are used as foam generators? 6. Where are foams used? 7. Name the methods of foam destruction.

31

2 AEROSOLS 2.1. Classification of aerosols Aerosols are free called disperse systems with a gaseous dispersive medium and disperse phase consisting of solid and liquid particles. Aerosols are classified according to the aggregate state of the disperse phase, dispersity and methods of production. The most known aerosols are: 1) fog – condensation aerosol with a liquid disperse phase; 2) smoke – condensation aerosol with a solid disperse phase; 3) smog – mixture of fog and smoke usually containing products of photochemical reaction and vapors of water; 4) dust – disperse aerosol with solid particles formed in processes of grinding explosion, drilling and pouring of powders; Spray – disperse aerosol with a liquid disperse phase formed due to disintegration of jets or liquid films, for example, when spraying liquid under the action of the source of acoustic vibrations or destruction of jets under the action of electric field. It should be noted that often in practice «smoke» is considered as a system formed due to combustion of fuel and containing both solid particles of soot and ash and liquid particles of fuel distillation and drops of water formed owing to condensation of steam. Smokes in which particles of a disperse phase adsorbed a considerable amount of moisture from the atmosphere are evidently at the same time smokes and fogs. Such systems which are especially often formed at great content of moisture in smoked atmosphere over large industrial cities are called by English term «smog» (smoke+ fog). Often there are formed mixed aerosols consisting of particles of different origin. 32

According to dispersity, aerosols with a solid disperse phase are divided into the smokes with particles from 10-9 to 10-5 m and dusts with the size of particles usually more than 10-5 m. Fogs, as a rule, consist of rather large droplets with the size from 10-7 to 10-5 m. According to the origin, systems with a gaseous disperse medium are divided, like all disperse systems, into dispersion and condensation systems with both liquid and solid condensed phase. Dispersion aerosols are formed due to dispersion (grinding, spraying) of solid or liquid bodies or due to transition of powder like bodies to suspension. Condensation systems are formed due to volumetric condensation of supersaturated vapour as a result of gaseous chemical reactions evaporation of bodies including under the action of plasma and laser radiation followed by condensation of vapour. Dispersion aerosols formed due to grinding of solid bodies or spraying of liquids, like lyosols obtained by dispersion, have rather large particles and are, as a rule, polydisperse. Aerosols obtained by the method of condensation from supersaturated vapours or as results of chemical reactions are on the contrary, are highly disperse systems with more size homogeneous particles. Table 2.1 presents classification of aerosols according to the mentioned indications. Table 2.1 Classification of aerosols Name of the system Smoke Dust Fog Smog

Type of the system S/G S/G L/G L,S/G

Sizes of particles, m 10-9 – 10-5 >10-5 10-7 – 10-5 -

Methods of obtaining condensation dispersion condensation condensation

There are aerosols formed in the terrestrial atmosphere due to procedure of different natural processes aerosols of technogenic origin. The first ones are called natural; the second ones are called technogenic. Technogenic aerosols are formed in the process of mining and processing of ores, coal, grinding of materials, production of cement, combustion of fuel and other technological processes. Aerosols are systems with the sizes of particles in the range of dispersity from 10-3 to 103 mm of which particles from 10-2 – 102 mm, 33

are of interest for the science of aerosols as the lower boundary of sizes is in the area of transition from a molecule to a particle, and large particles cannot be suspended in air for a long time. The size of particles is the most important parameter for prediction of the behavior of aerosol. The particle size is usually characterized by the radius or diameter. Particles of aerosol with a liquid disperse phase have a regular spherical shape and, coagulating they coalesce forming again a spherical individual particle. Solid particles are of various shapes and, when coagulating, form loose aggregates also of diverse shapes. Monodisperse aerosol contains particles of only one size (seldom occurs), while polydisperse aerosol contains particles of different sizes. For particles of an irregular shape, the particle diameter greatly differs for the characteristic size. It is expedient to divide diverse forms of aerosol particles into three classes according to the relative extent of in three dimensions: 1. Isometric particles for which all three dimensions have approximately the same size (sphere, regular polyhedral, and particles close to them by shape); 2. Particles having a greater extent in two dimensions than in the third one (plates, leaves, flakes, discs); 3. Particles with a greater extent in one dimension (prisms, needles, fibers or filaments). Aerosols cover a wide range of dispersivity, however, high- and coarse-disperse aerosols are unstable. The first ones are unstable due to frequent collisions of particles between each other and in a closed system with walls; the second ones are unstable because of a high rate of sedimentation. Therefore, aerosols are practically of the size 10-4 – 10-7 m, as is seen in Table 2.2. Table 2.2 Size of particles of some typical aerosols Name of the system 1 Fog (Н2О) Natural dust Spores and pollen of plants Strata clouds Rain clouds

34

The size of particles, m 2 5·10-7 1·10-6 – 1·10-4 1·10-6 -1·10-5 1·10-6 -1·10-5 1·10-5 – 1·10-4

1

2 1·10-6 – 1·10-5 1·10-7 – 1·10-6 5·10-6 – 1·10-6

Fog (Н2SО4) Tobacco smoke Smoke (Р2О5)

The curve of particle distribution in aerosol, i.e. the content of particles of different radii, depends on the origin of aerosols processes taking place in aerosol after it is obtaining (aggregation, coalescence, and isothermal distillation). The shape of aerosol particles depends on the aggregate state of the substance of the disperse phase. In fogs, droplets of liquid are spherical. In smokes, particles can be of various shapes, for example: needle, plate, star like. In smokes, particles can be complex aggregates, while in fogs; collision of droplets usually results in coalescence formation of larger droplets. Owing to the looseness (porosity) of aerosol particles, the apparent densities of these particles determined by common methods are often less than the density of the substance of which they consist. This can be seen in the values of density of some smoke particles obtained by different methods (Table 2.3.). Table 2.3 Density of particles in smokes Substance Gold Mercury Magnesium oxide Mercuric chloride

Density ρ·10-3, kg/m3 True apparent 19.3 0.2-8.0 13.6 0.07-10.8 3.6 0.24-3.48 5.4

0.62-4.3

The method of production of smoke evaporation in electric arc boat heating combustion of metallic magnesium boat heating

The size and shape of particles are determined using usual microscopy, ultra- or electron microscopy. 2.2. Methods of obtaining aerosols Like for lyosols, the methods of obtaining aerosols are divided into dispersion and condensation ones. Let us consider the most frequently used dispersion methods. 35

1. Spraying of solution by compressed air. This is one of the oldest methods. For its realization, sprayers of different constructions are used. 2. Spraying in electric field. According to this method, aerosols are obtained by spraying of a substance from a spray connected with one of the poles of electric voltage source. The obtained aerosols are quite stable. At present, there are industrial apparatus for production of medicine aerosols by this method. 3. Spraying with the help of ultrasound. This method allows producing aerosols with a high concentration of the disperse phase. It is used to obtain aerosols of aqueous solutions of antibiotics. 4. Spraying of liquids by ultracentrifuge. Using this method it is possible to obtain aerosols of different aqueous solution in considerable volumes. Production and use of many important materials and preparations are based on dispersion methods. For example, thus is production of powders by grinding solid materials, spraying by jets liquid fuel injectors (for intensification of combustion processes), pesticides for protection of plants from pests, lacquers and paints for protective coatings, etc. In nature, formation of dust is related to production of aerosols by dispersion. The most important physical method of obtaining aerosols is condensation of vapours – for example, formation of fog. When the change in the parameters of the system, in particular, with the decrease in temperature, pressure of vapour can become higher than the of equilibrium vapour over liquid pressure (or solid) and a new liquid (solid) phase is formed in the gaseous phase. As a result, fog (smoke) is formed. So, for example, masking aerosols are obtained which are formed due to cooling of vapors of P2O5, ZnO, and other substances. Condensation formation of aerosols causes the emergence of cumulus clouds containing drops of water or cirrus clouds consisting of ice crystals due to their heterogeneous nucleation on specks of dust and microcrystal’s of salt. Such microcrystal’s are formed due to drying out of the finest drops of seawater and raised to great height by convection flows of air. The chemical reactions in which formation of aerosols is possible can be of different character. So, oxidation in the course of fuel 36

combustion results in formation of flue gases containing products with quite low pressure of vapour. Being mixing with cooler air, these products condense and form smoke. Smokes are product during combustion phosphorus (P2O5), interaction of gaseous ammonia and hydrogen chloride (NH4Cl), as a result of photochemical reactions, for example, when lighting wet chlorine (fog of hydrochloric acid). Oxidations of metals in air proceeding in different metallurgical and chemical processes are often accompanied by formation of smokes consisting of metal oxides (ZnO, MgO, etc.). Stable fogs mixed with air can give substances such as HCl and SO3. Finally, the smoke is formed in contact with most air AlCl3 – finely formed Al(OH)3. 2.3. Optical properties Optical properties of aerosols obey the same laws as those optical properties of lyosols. So, diffusion of light of aerosols is described by Rayleigh's equation: 2

 n 2  n02  v 2  4 I0 , Ip  24  21 2   n1  2n0   3

(4)

where I0 and Ip are intensities of the incident and diffused light, respectively; n0 and n1 are indexes of refraction of the dispersion medium and disperse phase; ν is partial concentration of the disperse system; v is volume of particle, λ is wavelength of incident light. It should be noted, however, that due to a great difference in density and consequently, in refraction indexes of the dispersion medium and disperse phase the optical properties and first of all diffusion of light, are developed quite noticeably. Owing to a great ability to diffuse light, aerosols are widely used for creation of smokescreens. Of all smokes, smoke of P2O5 has the greatest ability to diffuse and reflect light – its masking ability is usually taken as a unit. Concentration of aerosols hardly available for investigations, for example, concentration of water in a cloud, can be determined with the help of radars. A directed radio beam sounding the space is passed by the source in the form of impulses in definite periods of time 37

and is registered on the screen of oscillography. Radiation coming back as a result of diffusion by the object (cloud) is also registered by oscillography. It is possible to determine the distance to the object by the interval of time passed from application of a signal till direction of diffused beam. During opalescence under the action of white light, colorless disperse systems acquire blueish coloration. When under the influence of opalescent white light, colorless dispersions exhibit a bluish tint. As the value Ip is inversely proportional to the fourth degree of the incident light wavelength, short waves are mainly diffused. On the contrary, in transmitted light, these systems are red colored, because beams of blue color disappears due to diffusion, when passing through the disperse system from the spectrum. The primary diffusion of light with a short wavelength explains the colour of the sky at different times of the day. The blue colour of the sky is explained day by diffusion of short waves of sunlight by the atmosphere of the Earth. The absolute value of the intensity of light diffused by 1m3 of air is negligible, but it becomes noticeable due to a great thickness of the terrestrial atmosphere and fluctuations of gaseous molecules. The orange or red color of the sky at sunrise or sunset is explained by the fact in the morning or the evening we observe, mainly light passing through the atmosphere. Also, the use of blue light for black-out and red light for signaling is based on the dependence of light diffusion on the length of a light wave. Lamps of blue colour are used, when they are not desired to be noticed from airplanes as blue beams completely diffuse, when passing through a sufficient thick layer of air, especially if it contains particles of dust or fog. On the contrary, when light is desired not to be diffused and must be noticed in fog, lanterns shining with red colour are used.

2.4. Molecular-kinetic properties The principal difference of aerosol from systems with liquid dispersion medium is in the fact that the length of free path of molecules in gas can be more than the sizes of particles of the disperse phase. According to the molecular-kinetic theory of gases, the length of 38

free path of the molecule equal to the mean path between its collisions with other molecules is calculated by the formula



1 , 2Vnd 2

(5)

where d is diameter of molecules, V is volume of the system, n = p/kT is the number of molecules in the volume unit. Consequently,



kT . 2Vpd 2

(6)

According to the order of magnitude at atmospheric pressure, the length of free path of a gaseous molecule is about 10-7 m. The length of free path of a liquid molecule is approximately equal to its radius, i.e. by the order of magnitude it is close to 10-10 m. When studying molecular-kinetic properties of aerosols, it is expedient to divide them into two classes: Aerosols with quite large particles (r>>λ), for which the regularities are of a hydrodynamic character (to be more exact, of aerodynamic character). Motion of particles in a continuous viscous medium is described by Stokes law: f  6 rU ,

(7)

where η is viscosity of the medium, U is velocity of the particle motion. Highly disperse aerosols (r >r>10-8 m), there are only empiric equations, in particular Kenningham's equation 1 f  6 rU (8) 1  A / r , 39

transforming at r >> λ to Stokes' law and giving a square-law characteristic dependence at r 20-30 mkm). Hydrodynamic resistance of the medium during sedimentation of large particles is described by Osseen equation:

f  6 rU (1  3r U / 8 ) ,

(9)

where ρ is density of the medium. Equations 8 and 9 are only applicable for description of motion of molecules of solid spherical particles. For aerosols with a liquid disperse medium, equations taking into account viscosity of the disperse phase are proposed. The viscosity of a gaseous dispersion medium is lower the viscosity of liquid by several orders, therefore Brownian motion of aerosol particles is more intensive. Experimental investigations verify the applicability of Einstein-Smolukhovski theory of Brownian motion. However, in this case also, it is necessary to take into account the ratio of the length free path of the molecule to the size of the disperse phase particles. At r >> λ, the formula of Einstein –Smolukhovsky has the form: kTt . (10) x 3 r For smaller particles Kenningham's a correction is applied and the formula has the form: x

kT (1  A / r ) . t 3 r

(11)

Now, let us consider the phenomena of thermophoresis, photophoresis and termoprecipitation related to the molecula-kinetic pro40

perties and characteristic for disperse systems with a gaseous dispersion medium. Thermophoresis is motion of aerosol particles in the direction of the temperature decrease. Under the conditions of λ / r >> 1 (i.e., when particles are small) thermophoresis occurs because particles of gas molecules come flying to the heated side with a higher rate than to the less heated side and consequently, impart the particles with an impulse in the direction of the temperature decrease. If λ / r