Application of the Centrifuge Method to the Evaluation of the Stability of Pharmaceutical Emulsions

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PURDUE UNIVERSITY

TH IS IS TO CERTIFY THAT THE T H E SIS PR EPA R ED U N D E R MY SU PE R V ISIO N

BY__________ Alfred

e n title d

Nicholas Martin, J r ._________„ __________

APPLICATION QF THE CENTRITniftF: METHOD TO THft

EVALUATION OF THE STABILITY OF PHARMACEUTICAL EMULSIONS

COMPLIES WITH THE UNIVERSITY REGU LA TIO N S ON GRADUATION T H E SE S

AND IS APPROVED BY ME A S FU LFILLIN G THIS PART OF THE REQUIREM ENTS

FO R THE D EG R EE O F

Doctor of Philosophy

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TO THE LIBRARIAN:----THIS T H E SIS IS NOT TO B E REGARDED A S CONFIDENTIAL.

PROFESSO R

GRAD.

SCHO O L FORM

9 —3 - 4 9 —1M

CT C H A R G E

APPLICATION OF THE CENTRIFUGE METHOD TO THE EVALUATION OF THE STABILITY OF PHARMACEUTICAL EMULSIONS

A Thesis

Submitted to the Faculty

of

Purdue u n iv e rsity

by

Alfred Nicholas Martin, J r .

In p a r tia l Fulfillm ent of the

Requirements fo r the Degree

of

Doctor of Philosophy

August, 1950

ProQuest N um ber: 27714175

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TABLE OF CONTENTS Page INTRODUCTION........................................................................................................................

1

THEORY OF EMULSIONS.....................................................................................................

5 ®

S t a b i l i t y ......................................................................................................................

8

Creaming..........................................................................................................................

9

Breaking..........................................................................................................................

H

Other Physico-chemical Changes.............................................................. 12 A

SURVEY OF THE PROBLEM......................................................................... 14 Surface Tension and I n te r f a c ia l Tension S t u d i e s . . . . . .

14

Viscosity S tu d ie s................................................................................................ 15 Determination of S ta b ility by th e Microscopic Method.....................................................................................................................17 Determination of s t a b i l i t y by the Shelf Ageing Method.....................................................................................

23

Determination of S ta b ility by the Centrifuge Method..................................................................................................................... Summary of the L ite ra tu re ............................................................

25 30

EXPERIMENTAL........................................................................................................................ 31 Procedure for Making the Emulsions..............................

31

Preparation of the Em ulsifier................................................................

32

Part I .

Comparison of the S ta b ility of Emulsions Prepared with VariousO ils...............................................

32

Shelf T ests....................................................

33

Microscopic T e s ts ...

34

........................

Centrifuge T ests................................................................... 39

Page

The jsffect of Increase in Emulsifier Concentration as Determined by the Centrifuge Method.............................. Part I I .

41

Comparison of the S ta b ility of Emulsions S tab ilized with Various Emulsifying Agents.

..........................................................................

45

Extended Centrifuge T ests...........................................47 The Centrifuge Method as a Measure of Film Strength. Part I I I .

..................................................... 49

Use of the Centrifuge Method in the Evaluation of the S ta b ility of Other Type Pharmaceuticals........................... Calamine Lotion.

54

..............................

Hydrophilic Ointment

............................... 58

Lime Liniment. . ...................... Lotion B ases.

54

59

................................................................ 60

DISCUSSION............................................................................................................................. 63 SUMMARY..................................................................................................................................... 64 BIBLIOGRAPHY. . . ABSTRACT VITA

............................................................................................................

65

lis t

o f ta b le s

Table I.

PaSe The S ta b ility of

©mil si on 3 Containing

Various Oils as Determined by the Microscopic Method# II#

........................

37

Representative Results of a Centrifuge T est........................................................................................................................ 40

III#

The S ta b ility of

©nuisions of Various

Oils as Determined by the Centrifuge Method.................................................................................................................. 41 IV.

The S ta b ility of

©nuisions Containing

Various Concentrations of Emulsifier as Determined by the centrifuge V.

The S ta b ility of

Method...................................43

Emulsions Containing

Various Emulsifiers as Determined by the Centrifuge Method# ................ VI.

46

Order of S ta b ility of Mineral Oil Emulsions S tabilized with Various Emulsify­ ing Agent s ................................................................................................... 47

VII.

The S ta b ility of Ageing Emulsions as Determined by the Centrifuge Method.

........................... 50

LIST OF FIGURES Figure 1.

Page The S ta b ility of ©nuisions Containing Various Oils as Determined by the Shelf Method.....................................................................................................................

2m

55

The S ta b ility of Emulsions Containing Various Oils as Determined by the Micro scopic Method#

5,

• • 38

The S ta b ility of Emulsions Containing Various Oils as Determined by the Centrifuge

4*

Method#................................................................................... 42

The S ta b ility of a Cottonseed Oil Emulsion at Various Emulsifier Concentrations as Determined by the Centrifuge Method...........................

5.

44

The S ta b ility of Emulsions S tab ilized With Various Emulsifiers as Determined by I n i t i a l Centrifuge Tests#

6#

...........................................

48

The S ta b ility of Ageing Emulsions S tabilized with Various Emulsifiers as Determined by Extended Centrifuge T ests................................................................

7.

51

The S ta b ility of a Cottonseed Oil Emulsion S tabilized with Acacia as Determined by Extended Centrifuge T ests.......................................................

8#

55

The S ta b ility of Various Types of Pharma­ ceu tic als as Determined by the Centrifuge Method.....................................................................................................................

57

L is t of Figures

(Continued)

Figure

9.

Page

The S ta b ility of Lotion Bases as Determined by the Centrifuge Method.

............ 62

ACKNOWLEDGEMENTS

The author wishes to express h is sincere appreciation for the d irectio n and assistance of Dr. C. 0. Lee and Dr. Glenn L. Jenkins under whose supervision th i s work was done; and to h is wife, Mary, for her sympathetic under­ standing and constant encouragement which helped him through many discouraging moments• The author i s g ra te fu l to the American Foundation for Pharmaceutical Education whose generous grant made th is i n ­ vestigation possible.

international Centrifuge, Size 1, Type SB.

1*

APPLICATION OF THE CENTRIFUGE METHOD TO THE EVALUATION OF THE STABILITY OF PHARMACEUTICAL EMULSIONS

INTRODUCTION

The United S tates Pharmacopoeia XIV* contains monographs fo r only one pharmaceutical preparation under the t i t l e

of

emulsion, and the National Formulary VIII* only fou r, but c e rta in other o f f i c i a l preparations such as linim ents, o in t­ ments, and even hydrous wool f a t , per se. may be considered

under

the general c la s s if ic a tio n of "emulsion.w The problems of emulsion chemistry are becoming in crea s­

ingly more important to pharmacy.

The introduction of the

newer synthetic emulsifying agents has raise d the cu rtain on a host of new products and processes in many fields»

The

theory and p rin cip les of emulsion chemistry have been found useful not only in the formulation of pharmaceuticals but also in the development of in d u s tria l processes fo r the preparation of adhesives; bituminous products used in road construction, paper fab ric a tio n , and e le c tr ic a l in su la tio n ; cosmetics such as lo tio n s and creams; detergents and wetting agents; foods such as mayonnaise, milk products, in s e c tic id a l sprays;

and flavo rs;

and emulsion paints»

Henceforth the United s ta te s pharmacopoeia w ill be r e ­ ferred to in th is work as the U.S.?. and the National Formulary as the N.F»

2.

I f the pharmacist Is to remain abreast of th is rap id ly advancing science, he must not only become fa m ilia r with the newer surface— active agents, but must also attempt to under­ stand the physico-chemical p rin c ip le s of surface phenomena and co llo id a l behavior and be able to apply them to h is problems of emulsion formulation. In 1933, Longevin (1) c la s s if ie d th e experimental in ­ v estigations of pharmaceutical emulsions in to the following periods:

between 1830 to 1865 the publication of formulas

of emulsions was undertaken; in 1860 to 1900 studies were made o f the methods of ém ulsification with the development of the Continental, English and Forbes B o ttle methods; dur­ ing the period, 1900 to 1933 l i t t l e

consideration was given

to research on pharmaceutical emulsions. also pointed out th at the g re a te st

In 1916, Roon (2)

s trid e s in the f ie ld of

emulsion chemistry had been made between 1870 and 1900.

He

made a plea to th e pharmacists of h is day to accept the chal­ lenge o f th is fascin atin g study and extend the th e o re tic a l knowledge of ém ulsification to the profession of pharmacy. Much progress indeed has been made In recent years in the development of pharmaceutical emulsions.

The Introduction

of the newer surface-active agents in about 1930 and th e ir increasingly wider application to th e preparation of lo tio n s , creams, ointments, end other pharmaceutical and cosmetic emulsions a t t e s t s to the in te r e s t of the pharmacist in th is important aspect of pharmaceutical technology.

5.

The Stabl 11 ty of “ Emulsions» - Coutlnho (5)

stated th a t

probably the most important and d esirab le property of cos­ metic and pharmaceutical emulsions was the s t a b i l i t y of the f i n i died product.

The evaluation of the s t a b i l i t y of emul­

sions, however, remains today as one of the most d i f f i c u l t problems in emulsion chemistry.

Sut helm (4) re fe rre d to the

problem of s t a b i l i t y as "the nightmare of the emulsion chemist."

King and Mukherjee (5)

stated th a t "the most

fundamental q u a lity of emulsion s t a b i l i t y i s undefined, mis­ understood and, a t the b e s t, regarded as a vague, empirical concept, while at the same time being of the utmost impor­ tance."

In 1947, Hollehberg (6 ) noted th a t "a wholly ade­

quate and r e lia b le method of accelerated to replace

s ta b ility te s tin g

shelf te s tin g has not yet been devised."

Sut helm

(4) believed th a t "there i s no such thing as a s t a b i l i t y which i s ty p ic al of a l l emulsions, but only conditional s t a b i li t y with regard to the purpose of the emulsion. " The present co stly and time-consuming method employed in industry fo r s t a b i l i t y te s tin g includes storage of the emulsions at various co ntrolled temperatures fo r long periods of time, ing, o il

periodic inspections are made with regard to cream­ separation, and general appearance.

As an example,

Christenson and Shelton (7) reported an in d u s tr ia l method wherein samples of the products were stored a t temperature, at 37° and at 45° C*

a t room

They were examined weekly

for the f i r s t month and th e re a fte r at In te rv a ls .

4° ,

six-month and one-year

These workers stated th a t from the standpoint of

4.

sales and d is trib u tio n ,

a liq u id emulsion should not show

any noticeable change in physical p ro p erties fo r at le a s t one year at room temperature and at 4° C#

Even though the

product showed slig h t creaming at 37° at the six-month period, the emulsions were accepted i f no fu rth e r change in the roomtemperature sample was noted within one year*

Samples were

required to show no change at 45° fo r at le a s t one month to be considered stable*

in the absence of rapid q u a n titativ e

methods of s t a b i li t y measurement, the manufacturer must r e ­ ly upon empirical methods such as these* Because of the aforementioned d i f f i c u l t i e s encountered in the determination of the s t a b i l i t y of emulsions, the present study was undertaken with the object of evaluating and c o rre la tin g ex istin g emulsion s t a b i li t y t e s t methods and of attempting to develop a method which might be both rapid and accurate*

5.

THEORY OF EMULSIONS

The th e o re tic a l discussion of emulsions resolves i t s e l f into a consideration of two phenomena, ém ulsification and s ta b ility ; principles*

i t must be re a liz e d th a t these two are separate The problems of ém ulsification find importance

in the preparation of the emulsion*

S ta b ility , on the other

hand, r e f e r s to a permanent or enduring s ta te of the emul­ sion, re fle c te d in the absence of a lte ra tio n or m aterial change fo r a su ita b le length of time following emuLsifica­ tio n .

Before considering the p rin c ip le s of emulsion s ta ­

b ility

and the t e s t methods used for i t s

evaluation. I t i s

necessary to discuss the fundamental p rin cip le s of émulsi­ fic a tio n . Em ulsification. - When two Immiscible liq u id s such as o il and water are mixed by a g ita tio n , then allowed to stand, they separate rapidly Into two c le a rly defined la y e rs.

This

i s due to the fa c t th a t each liq u id independently manifests an in te rn a l force or tension by which the molecules at i t s surface are drawn toward the center of the liq u id , re su ltin g in a minimum surface area with the molecules packed closely together*

Stated in another way, the cohesive forces of

the surface molecule s of each liq u id are greater than the adhesive forces which would permit the molecules of the two liq u id phases to mix#

The composite of these unbalanced

forces' exerted upon the surface of the two liq u id s at the

6

in te rfa c e between these immiscible phases is referred to as the in te r f a c ia l tension»

Antonow stated the rule th a t the

i n te r f a c ia l tension of the two liq u id s i s approximately equal to the difference between th e ir surface tensions measured at the vapor- 1 1 quid boundary (8 )»

A c e rta in "sur­

face energy" resid es a t the in te rfa c e as a r e s u lt of the in ­ ward forces of the liquids»

As the liq u id s are ag itated to ­

gether and are broken in to small p a r tic le s , of the globules increases enormously. (9), i f we take a t e s t tube of 1 cm. troduce

1

p

the surface area

According to Sutheim cross-sectio n and in ­

cc. of water upon which i s layered

1

cc. of o i l ,

a two-phase system of water and o il with an in te rfa c e of 1 cm.2 and a volume of 2 cc. i s obtained.

I f an emulsify­

ing agent Is now added and the mixture shaken, we s t i l l have

2

cc. of a two-phase system but i t

i s now an emulsion

with one liq u id dispersed in the other as small globules# The area of the in te rfa c e has increased enormously.

I f we

assume th at the size of the p a r tic le s of the dispersed phase i s 5 micron (p.) in diameter, we may make some calcu latio n s. 1.

The volume of one droplet of 5 (1

M=

10*4

diameter

cm.) i s :

Ve .àS l f = iâ® x tfx 10*'L£cœ3. 6 6 2.

m

From one cc. of o i l ,

64.6 x 10*1 2 cm3.

the number of droplets formed

Is;

NS I = 2 ^ . 6*1018 » 1.54 x 1010 s 16,400,000,000 V

^

126X

t

7

5#

The surface area of one droplet I s : S z

4.

àfirt = Z b T t'x

10 " 8 cm2.

= 77.5 x 10"8 cin2.

The in te r f a c ia l area of the emulsion is : A= Nx S s

= É x 10^cm2, s 12,000 cm2.

aV

5

The increased area i s acconç>anled by a tremendous increase in surface energy as re fle c te d In the work expended during the agitation*

This energy, by the laws of thermodynamics,

seeks to become a minimum; th is decrease in energy may be accomplished only i f the enormous surface area i s reduced. Thus, the divided p a r tic le s rapidly coalesce to form larg er and larg er p a r tic le s , re su ltin g in a continued reduction In surface area, u n til two separate liq u id phases again e x is t. The problem of forming an emulsion then l i e s

firs t,

in e f­

fecting the division of the globules of the in te rn a l phase with the minimal expenditure of energy (work) and secondly, in providing a s ta b iliz in g facto r to keep the p a r tic le s in th is divided sta te .

The l a t t e r consideration w ill be d is­

cussed under the heading of s t a b i l i t y . I t i s impossible to disperse more than about 2% of o il in water for any length of time because of the phenomena re ­ ferred to above.

The problem has been solved by the addi­

tion of a th ird phase in to the system.

This substance,

c alled the emulsifying agent or em ulsifier, by v irtu e of an o il-so lu b le and a water-soluble portion, i s oriented at the liq u id /liq u id in te rfa c e in such a way as to reduce the free surface energy without the accompanying coalescence of the

8.

globules.

I t i s generally agreed th a t the emulsifying agent

acts by lowering the in te r f a c ia l tension thus increasing the ease of ém ulsification, and then surrounds the dispersed globules with a tough, e la s tic film to prevent th e ir coa­ lescence.

I f the emulsifying agent i s soluble in ,

or r e ­

duces the surface tension of the water to a g reater extent than the o i l ,

the em ulsifier w ill surround the o i l and form

an o i l in water ( 0/W) emulsion. in a W/0 emulsion.

The opposite e ffe c t r e s u lts

Admittedly, t h i s i s an over-sim plifica­

tio n of the mechanism of ém ulsification; many fa c to rs remain unexplained despite the numerous theo ries proposed and the preponderance of experimental work conducted in an e ffo rt to describe the phenomenon of émulsification* S t a b ility . - The s t a b i l i t y of a pharmaceutical emulsion i s recognized by the absence of a separated o i l layer (break­ in g ), absence of creaming, and the maintenance of elegance with regard to appearance, odor, color, and other general physical c h a ra c te ris tic s ,

during a su ffic ie n t period of time

commensurate with the purpose for which the emulsion Is In ­ tended to be used.

The physical chemist often re fe rs to the

in s ta b i l i t y of an emulsion as the separation of the in te rn a l phase as a separate layer from the body of the preparation. Creaming, the ris in g and concentration of the emulsified in ­ tern al phase a t the top of the preparation without the actual separation of the o i l as a separate la y e r, considered by him as a mark of i n s t a b i l i t y .

is generally not (In the present

9

discussion, 0/W emulsions are referred to exclusively*) Nevertheless, en emulsion i s a dynamic system and the con­ tin u a l increase in the concentration of the emulsified o il at the surface of the preparation, p a r t i a l l y as a r e s u lt of the increase in the size of the o il p a r tic le s , p o te n tia l step toward separation of the o i l .

represents a Furthermore,

in the case of pharmaceutical emulsions which find in te rn a l use, creaming r e s u lts in a decreased uniformity of dosage unless the preparation i s thoroughly shaken before adminis­ tr a tio n .

Certainly the eye-appeal of the emulsion i s a f­

fected by creaming and th is i s ju s t as r e a l a problem to the pharmacist as i s the separation of o i l .

The classes of in ­

s ta b ility of a pharmaceutical emulsion are therefore l i s t e d as: (a)

creaming

(b)

breaking or "cracking"

(c)

other physico-chemical changes, v i z ., change of color, change of odor, phase re v e rsa l, and o th ers.

Creaming. - This type of i n s t a b i l i t y i s evidenced in an accumulation of the emulsified o i l globules at the top of the emulsion, the film of the emulsifying agent remaining in ta c t.

I t i s a rev ersib le process,

since merely shaking

the preparation w ill redisperse the cream layer in the body of the emulsion to re e sta b lis h a uniform emulsion. Those facto rs which are involved in the creaming of an

10

emulsion are re la te d by Stokes 1 Law;

v = 2 r2 (d1 - d") g where v i s the ra te of sedimentation, r i s the radius of the dispersed globules, df i s the density of the dispersed phase, d* i s the density of the continuous phase, n i s the v isc o sity of the continuous phase, and g i s the force of gravity* analysis of the equation, i t

i s seen th a t i f

By

the dispersed

phase i s le s s dense than the continuous phase, the v elo city of sedimentation, v, would become negative; th a t i s , ing** e ffect would result*

a "cream­

The g reater the difference be­

tween the density of the two phases, the la rg er the o il globules,

and the less the v isc o sity , the g re ate r would be

the r a te of creaming as predicted from the above equation* Also by increasing the force of gravity by cen trifu gatio n, the ra te of creaming may be increased*

The radius of the

globules i s seen to be a major fa c to r in th is equation fo r, since the v elocity of creaming increases as the second power of the rad iu s,

doubling the radius of the o il globules would

increase the creaming r a te by four times. Much confusion e x ists in the l i t e r a t u r e from a f a ilu r e to d if f e r e n tia te between the phenomena of creaming and of breaking as contributing to the i n s ta b i l i t y of an emulsion* They must be considered as two separate and d is tin c t pro­ cesses*

Creaming i s re la te d to breaking only in so fa r as

a concentration of the o i l phase leads to agglomeration of the dispersed phase re su ltin g in a g re ate r p ro b a b ility of

o i l separation* Some workers doubt the v a lid ity of Stokes* Law as ap­ p lie d to concentrated emulsions*

Coutlnho (3) argued th a t

the r i s e o f the o il globules,

as a r e s u lt of the difference

of the density of the phases,

i s o ffs e t by Brownian movement

which keeps the globules in incessant movement*

Increasing

the v isco sity of the emulsion reduces the Brownian movement and thus reduces the diffusion of the p a r tic le s and allows them to agglomerate*

An optimum vl sc o sity » then,

and not a

continued increase in v isc o sity i s necessary fo r re ta rd a tio n of creaming*

Coutlnho believed that reduction in p a r tic le

size was the predominant fa c to r in sta b ility *

Reduction of

p a r tic le size below a c r i t i c a l value, however, r e s u lts in an increased frequency o f c o llis io n due to Brownian movement with the formation of large aggregates*

When the aggregates

became la rg e r than 3 to 5u the Brownian movement ceases to keep the p a r tic le s in constant motion and the emulsion creams rapidly*

Capricious e ffe c ts of th is nature lead to the com­

p lex ity of emulsion investigation* Breaking* - The phenomenon of creaming may be considered as a rev ersib le process,

a homogeneous system again ex istin g

upon a g itatio n of the emulsion*

Breaking, on the other hand,

represents an irr e v e r s ib le process*

Simple a g ita tio n f a i l s

to resuspend the globules throughout the external phase in a stab le, emulsified form since the film i s destroyed* A considerable amount of in v estig a tio n has been

12.

undertaken with reference to th is type of i n s t a b i l i t y . ( 1 0 ) showed th a t reduction in p a r tic le s a r ily lead to increased s t a b i l i t y .

King

size does not neces­

Rather,

an optimum de­

gree of dispersion for each p a r tic u la r emulsion system ex­ i s t s fo r maximal s t a b i l i t y .

S ta b ility ,

furthermore, i s not

a d ire c t function of the reduction of the in te r f a c ia l tension, as some emulsifying agents which f a i l to a ffe c t the tension or lower i t only s lig h tly , form stable emulsions ( e . g . , gela­ tin ).

The in te r f a c ia l adsorption of the emulsifying agent

which accompanies the reduced surface tension i s the im­ portant facto r here.

V iscosity, likew ise, alone does not

estab lish a stable system; however, very viscous emulsions may be more stable than mobile ones by v irtu e of the r e ­ tard atio n of coalescence of the globules.

Tragacanth forms

viscous emulsions which are le s s stab le than the f lu id acacia preparations.

King concluded th at the compactness and the

strength of the I n te r f a c ia l film i s facto r favoring s ta b ility *

the most important single

Other fa c to rs , as the lowering

of in te r f a c ia l tension and the increase of v isc o sity ,

are

effe c tiv e only in so fa r as they modify the p ro p erties of the film . Other Physico-chemical Changes. - Other changes which are of importance to the pharmacist, as color and odor, must also be considered in a discussion of emulsion s t a b i l i t y . Very l i t t l e to r s .

work has been conducted in regard to these fac­

I t has been recognized th a t the addition of perfumes

13

may cause a color change in the preparation by chemical re ­ actions upon ageing (11)•

The addition of coloring agents

to the pharmaceutical or cosmetic emulsion may re a c t chemical­ ly with one of the phases causing a change in color and pos­ sibly an inversion or breakage of the emulsion.

The perma­

nence of medicinals incorporated in the emulsion i s an important consideration fo r the pharmacist.

The reduction

of the vitamin content of a cod li v e r o i l emulsion exempli­ f ie s t h i s type of i n s t a b i l i t y .

C ry sta ll!z a tio n of waxes

contained in emulsified ointments has received l i t t l e ten tio n .

a t­

These problems are complex and varied and demand

considerable research in v e stig a tio n .

These many complex

facto rs probably cannot be evaluated by any rapid s t a b i l i t y te s t.

Factors such as color change, odor change, and c ry s ta l­

liz a tio n develop slowly over a period of time.

There seems

to be no other method for the evaluation of these e ffe c ts than the time-honored, a lb e it time-consuming,

sh elf t e s t s .

The present study, however, as set fo rth in the objective resolves i t s e l f into an evaluation of s t a b i li t y te stin g methods with emphasis upon a study of the breaking of emul­ sions by rapid methods#

14*

A SURVEY OF THE PROBLEM

A survey of the l i t e r a t u r e

shows th a t much work has been

conducted in an e f f o r t to find a q u a n tita tiv e method for the evaluation of emulsion s t a b i l i t y .

This l i t e r a t u r e

search has

been undertaken in an e ffo rt to find one or several phenomena whose rapid evaluation might predict the s t a b i li t y of an emul­ sion system over a period of time under normal conditions of shelf storage.

The approach which i s found to have yielded

the b est r e s u lts in the hands of other workers might con­ ceivably be applied in the present study to the evaluation of the s t a b i li t y of pharmaceutical emulsions. Surface Tension and I n te r f a c ia l Tension Studies. - I t i s generally agreed th a t a low in te r f a c la l tension i s the chief condition favoring the ease of em ulsification.

An

emulsifying agent which lowers the in te r f a c ia l tension be­ tween the two phases promotes rapid dispersion of one liq u id in the other.

As an example, the I n te r f a c la l tension in

dynes/cm. between Nujol* and water i s 64.8.

I f Aerosol OT**,

a powerful wetting agent. I s added to the water to produce a solution of 0*01$ concentration, I s reduced to 21.7 dynes/cm.

the I n te r f a c ia l tension

A 1$ solution of Aerosol OT

w ill produce an I n te r f a c ia l tension of

1 *97

dynes/cm. be­

tween Nujol and water (1 2 ). *

A refined mineral o i l , product of Stanco, Inc.

**

Dioctyl sodium sulfosuccinate, product of American Cyanamid Co.

15

I t Is very doubtful# however, whether I n te r f a c la l ten ­ sion plays any p art In the s t a b i li t y of an emulsion over a period of time (10) (13)

(14).

V Iscosity Studies. - I t has been explained previously th at the v isc o sity of the system cannot be used as a c r i t e r ­ ion of s t a b i li t y .

An Increase in v isc o sity lowers the r a te

of creaming to some extent, but an optimum and not neces­ sa rily an increased v isco sity i s the more effective» (1 0 )

King

stated th a t v isco sity i s a symptom ra th e r than a cause

of s t a b i l i t y .

Although the v isco sity of an emulsion may in ­

crease as the p a r tic le size of the emulsion i s reduced, th is phenomenon was a ttrib u te d by Sibree (15) to the e n tra inment of a ir during émulsification»

When the emulsions were pre­

pared in a vacuum apparatus, the v isco sity was found not to be dependent upon the p a r tic le

size.

The v isco sity of the

emulsions studied by Slbree, furthermore, was found to r e ­ main constant for at le a s t 28 days*

A formula was developed

by Hatschek (16) to r e la te the v isco sity of an emulsion to the v isco sity of the external phase and the volume of the in te rn a l phase*

The formula was te ste d experimentally and

modified by Sibree (15) into the following form; n °- ‘ T

in which n i s the v isco sity of the emulsion re fe rre d to by some as

11s tru c tu ra l

v is c o s ity ;w n^ the v isco sity of the ex­

te rn a l phase, and V the volume of the in te rn a l phase* symbol, h, i s a constant with a value of about 1*3.

The I t was

16.

added to the equation by Slbree to account for the d is ­ crepancy between the volume* V, calculated from the formula of Hat schek and the measured volume.

Prom th is equation,

i t may be seen th a t the v isco sity of an emulsion depends on the v isco sity of the external phase but not on th a t of the in te rn a l phase.

This i s understandable when i t

is

realiz e d th a t the in te rn a l phase i s enveloped within a film of emulsifying agent and i s unable to exert i t s viscous ef­ f e c t.

The v isc o sity of the emulsion i s also dependent upon

the phase-volume r a tio o r,

as seen in the equation, upon the

volume of the dispersed phase.

Furthermore,

i t may be ob­

served th at an increase in th is volume w ill r e s u lt in an in ­ crease in the stru c tu ra l v isc o sity of the emulsion.

This

phenomenon has frequently been encountered, p a rtic u la rly in the formation of W/0 emulsions, by those who have done any emulsion work. In 1933, Richardson (17) developed the equation; log

— no

= kV

-Sr Where k i s rithm.

a constant and e i s the base of the n atu ral loga­

This equation Indicates th at the r e la tiv e v isco sity

of an emulsion i s an exponential function of the volume of the dispersed phase.

Broughton and Squires (18) prepared

emulsions of Nujol, benzene, and oliv e o il sta b iliz e d with 1#

sodium o le a te , 2% saponin, and

t r i ethanol ami ne o leate.

17.

By determining the v isco sity of these emulsions (entrained a ir removed), they were able to develop the equation 3 log nr r a where nr i s

bV

the lim itin g r e la tiv e v isc o sity ,

the r a tio of

the lim iting v isco sity at i n f i n i t e ra te of shear of the vi scosimeter to the v isco sity of the continuous phases n„

lim itin g v isco sity _________ v isco sity of continuous phase

The symbols, a and b, are constants which may be determined experimentally.

This equation was found to describe the ex­

perimental r e s u lts of v isco sity studies on concentrated e~ mulsions with g reater accuracy than any of the equations referred to previously.

Broughton and squires found th a t

the emulsifying agent which most read ily formed an emulsion gave the lowest lim itin g r e la tiv e v isc o sity .

Next to the

concentration of the in te rn a l phase, the emulsifying agent used was considered to be the moat important facto r in the determination of the v isco sity of any phase p a ir .

It

seems,

from these r e s u l ts , th a t the emulsifying agent possessing the g re a te st efficiency and producing the emulsion with the g reatest

s t a b i li t y might be found from a determination of

the lim iting r e la tiv e v isco sity of the emulsion. Determ inat1on of S ta b ility by the Microscopic Method. Pinkie, Draper, and Hildebrand (19) were probably the f i r s t workers to determine the size of dispersed p a r tic le s under the microscope.

The diameters of a hundred p a r tic le s were

18.

measured and a size«*frequency d is trib u tio n curve was made* In 1929, Harkins and Beeman (20) recognized the need fo r an accurate method of determining the d is trib u tio n of the droplet sizes in emulsions*

The d ilu ted emulsion was con­

tained in a ruled chamber and the image was projected through the micro scope onto a screen where the p a r tic le s could be counted and measured.

A frequency d is trib u tio n analysis was

made by determining the number of globules in various size ranges, 0.0 to 0*5 microns, 1 to 2 microns, 2 to 3 microns, etc*

The diameter of the globules in microns was p lo tted

against the number of globules in each size range*

These

workers found th a t the size d is trib u tio n increased toward the larg e r classes as the emulsion aged. In 1930, Smith and Grinling (21) developed a method for the analysis of pharmaceutical emulsions.

They stated th a t

the best c r ite r io n fo r the q u ality of an emulsion was the degree of dispersion.

Smith and Grinling recognized the

need fo r a more rapid method than th a t e n ta ilin g the tedious task of measuring the size of several thousand d ro p lets, required by the size-frequency analysis*

These workers

reasoned th a t a large percentage of o il in an emulsion i s contained in a re la tiv e ly small number of large globules, whereas the g re a te r number of globules may be of the smaller size, causing frequency d is trib u tio n to give maximal values for the smaller c lass Intervals*

A large percentage of the

o il may be poorly emulsified and yet y ie ld r e la tiv e ly few very large globules, while a very small amount of fin e ly

divided o il gives an enormous number of small globules*

The

elegancy of the emulsion i s then not a function of the sige d is trib u tio n of the globule diameters but ra th e r of the mean of the volume of the globules*

These workers concluded th a t

the mean volume of a globule in m i l l i l i t e r s or i t s re c ip ro c a l, namely the number of globules in to which 1 ml* of o il i s

sub­

divided, i s the most sa tisfa c to ry single c r ite r io n for the efficiency of ém ulsification and i s also a measure of the ex­ te n t to which creaming occurs on standing. tity ,

A weighed quan­

1 Qm., of the emulsion was d ilu ted with 25 cc. of a

25$ acacia mucilage, preserved with formalin,

and enough

water was added to give a sa tisfa c to ry number of globules when observed under the microscope, 10 to 20 drops per square, using a Thoma hemocytome te r as a counting chamber* The degree of ém ulsification, H, was defined as the number of m illions of globules contained in one mm.^ of the oil* Van der Muelen and Rieman (22) had previously used a sim ilar method employing the hemocytome t e r for counting the emulsion p a r tic le s .

The emulsion was d ilu ted 500 times and

placed on the counting chamber*

The globules contained in

8 to 10 squares were counted and averaged, Munzel (25) used the method of Smith and Grinling (21) with slig h t modification in the study of pharmaceutical emulsions*

The value, K, defined as the number of globules

(in b illio n s ) produced from one gram of o i l , measure of ém ulsification.

was used as a

The microscopic f i e l d was pro­

jected upon a ground g lass p la te to allow easier measurement

of the p a r tic le s .

Munzel pointed out th a t the number of

globules in a given amount of the emulsion i s not the c r i ­ te rio n of a superior emulsion as there are many coarse emul­ sions on the market which do not cream, while some of a fine it

p a r tic le size may show a measure of in s ta b ility *

Munzel

compared the efficiency of emulsifying agents by the use of the equation: Emulsification number = —£— x K where 0 i s the grams of o il in the emulsion,

i s the grams

of emulsifying agent, and K i s the globule number obtained from the microscopic study* Many other workers have contributed to the development of the p a r tic le size determination as a measure of emulsion sta b ility *

Hixson and Pain (24) measured the s ta b iliz a tio n

of aqueous dispersions of asphalt by a determination of the average p a r tic le s iz e , based on the volume of the globules rath er than the diameter*

Fischer and Harkins (25) deter­

mined the s t a b i l i t y of emulsions prepared with soap as an emulsifier*

The emulsions were permitted to cream and

samples of the cream were removed for study* sion objective was used to view the field*

An oil-immerIn 1933, Longe-

vln (1) made a comparative study of the s t a b i li t y of pharma­ ceu tical emulsions by use of a microscope with a ca lib rate d eye-piece*

Berkman (26) determined the s t a b i l i t y of emul­

sions by a size-frequency d is trib u tio n method involving the use of a L eltz microprojection apparatus*

A mathematical

21.

analysis was made to study the k in e tic s of the emulsion.

In

1937, Cooper (27) reviewed the size-frequency analysis methods fo r the determination of emulsion s t a b i l i t y and conducted experimental work on sixty emulsions.

The microscopic f i e l d

was photographed and magnified and the size of the globules on the p rin ts was determined by use of a glass scale marked with rings of various sizes.

The 1 1sp ecific

surface area*

d is trib u tio n was found to be more acceptable than the siz e frequency.

King and Mukherjee (5) studied the s t a b ility of

emulsions s ta b iliz e d with soaps, hydrophilic c o llo id s, solid em ulsifiers.

and

The s t a b i l i t y of the emulsions was de­

termined by a frequency d is trib u tio n analysis at various in te rv a ls of time.

The to t a l area of the in te rfa c e per Gm.

of emulsified o i l was calculated and found to decrease l i n e ­ arly with time.

The reciprocal of th is decrease was defined

as the s t a b i li t y of the emulsion.

These in v e stig a to rs

emphasized the difference between creaming and o il tio n ,

separa­

sta tin g th a t the former i s re la te d to a rath er small

specific area of in te rfa c e whereas a breaking emulsion ex­ h ib its a decreasing In te r f a c ia l area.

Aherne and R eilly (28)

proposed th at the King-Mukherjee s t a b i li t y fa c to r, when de­ termined in the absence of undue mechanical disturbances, be renamed "factor of permanence” because in many technical processes emulsions are exposed to shaking, v ib ratio n , temperature changes, modify the l i f e Maclay (29)

and other e ffe c ts ,

any of which may

and s t a b i li t y of the emulsion.

Lotzar and

studied the emulsifying p ro p e rties of pectin as

22 compared with acacia, tragacanth, and karaya in the prepara­ tio n of emulsions of o liv e , cottonseed, and mineral oils* The change of the sp ecific in te r f a c ia l

surface with time was

measured by the microscopic method of King and Mukher jee (5) * They p lo tte d the specific in te r f a c ia l area against the time on semi-logarithm paper.

A mathematical in te rp re ta tio n of

the experimental r e s u lts could not be made.

They reported

th a t the dispersion of the o il and the i n i t i a l v isco sity of the emulsions were unrelated to the s t a b i l i t y . Other methods of measuring the p a r tic le size of emul­ sions were reported by Kraemer and Stamm (30), Stamm and Svedberg (31), Bailey, Nichols, and Kraemer (32), Schweyer (33), Cohan and Hackerman (34), and Persoz (35).

These

workers a l l used various forms of o p tical measurements to determine the size of the p a r tic le s of the in te rn a l phase of the emulsions. From the r e s u lts of these in v estig atio n s, i t

i s con­

cluded th a t a single size-frequency d is trib u tio n analysis does not serve to determine the s t a b i li t y of an emulsion. King (10)

stated th a t the s t a b i l i t y of an emulsion i s not

n ecessarily re la te d to the degree of dispersion and a l l com­ parisons based on th is conception are therefore fundamental­ ly unsound.

He observed that the only precise method in ­

volved a size-frequency analysis of the emulsion from time to time as the emulsion aged or, in the case of rapidly breaking systems, the mere macroscopic measurement of the separated o i l lay e r.

King fu rth er reported however, th a t

23

according to an unpublished work by Gelpin, the method of size-frequency analysis c a rried out over a period of time may give misleading r e s u lts in c e rta in cases, especially where the concentration of the emulsifying agent i s low* Determination of S ta b ility by the Shelf Ageing Method. Only a lim ited number of experimental observations have been carried out on "ageing" emulsion samples. r e la tiv e ly

If

an emulsion i s

sta b le , months or even years may be required to

obtain s u ffic ie n t data upon which to base conclusions* In 1935, Lederer (36) developed a formula for the break­ ing of an emulsion upon ageing: dV

dt ~

k (1-V)

-Jv“

which becomes upon in teg ratio n : kt a log — 1— 1— 1 -

-nr

where V i s the volume of the separated phase, t i s the time, k is a constant, dv/dt i s the change In the volume of the separated phase with time. the equation.

Experimental r e s u lts agreed with

Chessman and King (37) prepared samples of a

50% emulsion of amyl alcohol s ta b iliz e d by e le c tro ly te s . They applied the formula of Lederer to the breaking of the emulsions and found i t

to apply•

The dém ulsification of

stab le, n eu tral and acid emulsions, however, did not apply. Ross (38) reviewed the work of Lederer and of Chessman and King.

He found th a t the equation of Lederer Is the same as

th a t for a chemical reaction of one-and-one-half order.

The

24*

xnechanîsm of demulslficsitîlon of an unstable emulsion must therefore depend on the In terac tio n of the droplets of the emulsion*

This in te ra c tio n of p a r tic le s would not be pos­

sible in a stab le emulsion where each drop i s

surrounded by

a tough, e la s tic film of em ulsifier which prevents coales­ cence upon mere contact of the particles*

Ross derived a

new equation which was thought to be suitable for a mathe­ matical description of emulsion s ta b i li t y :

Vq ~ v a = V0 in which VQ is

i (bt + 1)&

the i n i t i a l volume of the o il in the emulsion,

i s the volume of the o il demul si fled a f te r time t , a constant calculated from the experimental r e s u lts ,

b is and VG -

V& I s the amount of emulsified o il remaining in the emulsion at time t* Chessman and King (39) determined the p ro p erties of dual emulsions using s h e lf-to s t measurements*

The c rite rio n

of s t a b i l i t y was taken as the time required fo r 5 cc* of the dispersed phase to separate when 50 cc* of the emulsion was placed in an eudiometer, the walls of which were thoroughly wetted with the dispersion medium*

On several occasions i t

was found necessary to measure the time taken for the sepa­ ratio n of the same quantity of the external phase because of the creaming in the dispersed phase which obscured the meniscus*

These two values were not s t r i c t l y

comparable but

the procedure was considered permissible since the r e s u lts

25

depended upon the order of s t a b i l i t y and not on the exact­ ness of measurement e Schulman and Cockbain (40) determined the s t a b i l i t y of Nujol emulsions by a measure of the time required for the f i r s t v is ib le sign of separation of the two phases. The ageing t e s ts although f a ilin g to give c le a r-c u t r e s u lts ,

are the most r e lia b le measure of emulsion s t a b i li t y .

In the fin a l an alysis,

the other methods are v alid only in

so fa r as they compare with the s t a b i l i t y of the emulsion determined by the sh elf method.

As stated before, the major

disadvantage of the shelf-ageing t e s t method l i e s in the ex­ tended period of time required for evaluation of a r e la tiv e ly stable preparation.

I t was mentioned In the introduction

th a t, in the absence of rap id ,

accurate methods fo r the de­

termination of the s t a b i l i t y of emulsions, the manufacturer necessarily resorted to th is method. Determination of S t a b i l i t y by the Centrifuge Method» Centrifuge methods have been used to demuls i f y as well as to te s t the s t a b i l i t y of emulsions.

Many workers believe, how­

ever, th a t the centrifuge method fo r s t a b i l i t y te s tin g ,

a l­

though rap id , does not give p a r a lle l r e s u lts in a l l cases with the sh elf te s tin g or with the microscopic method.

It

is

argued th a t centrifugation causes a rapid creaming and break­ ing of the emulsion and thus does not permit an evaluation of ageing phenomena which develop over a period of time. I t has been mentioned previously that the creaming of an

26

emulsion appears to follow the equation of Stokes: VIt

2r2 (d«_- d") g 9 n

i s In te re stin g to note th at under the influence of a

cen trifu g al force,

the creaming of an emulsion w ill follow

the same general equation i f

the g ra v ita tio n a l force i s re ­

placed by cen trifu g al force to yield: v where w i s

2r 2 (d* - d") w2 jc 9n

the angular v elo city of the centrifuge and x i s

the distance between any given p a r tic le of o il and the axis of ro ta tio n .

It

i s seen from these equations th a t the

creaming of an emulsion may be represented in a q u an tita­ tiv e manner,

Svedberg (41) used modifications of these

equations with the u ltra c e n trifu g e to determine the size and the molecular weight of substances in the co llo id al sta te . The only formulas available for the description of the breaking of an emulsion are those of Lederer (36) and Ross (38) previously mentioned.

The equations apply to the

separation of the in te rn a l phase of an emulsion over a period of time and under the influence of g ravity.

No

mathematical formula has been proposed, to the w r ite r 1s knowledge, to describe the breaking of an emulsion subject­ ed to cen trifu g al treatm ent,

M errill

(42) reported that

Lederer*s equation did not apply to the data obtained by the centrifuge method.

27

Ungerer (43) measured the p a r tic le dimensions of a clay suspension by cen trifu g atio n , and removal of a sample from each layer in which the number of p a r tic le s counted*

The

size of the p a r tic le s was calculated from Stokes’ Law and the calculated and ejqperimental r e s u lts were compared*

The

author suggested th is method fo r the measurement of emulsions. Ayers (44) discussed the common c h a ra c te ris tic s of crude petroleum emulsions*

He referred to the cen trifu g al and

gravity s e ttlin g of water from crude petroleum emulsions and a ttrib u te d i t to the difference of the sp ecific gravity of the two phases*

Ayers stated th a t the highest cen trifu g al

force would cause no g reater degree of coalescence of the o il p a r tic le s than would g ravity.

Furthermore, cen trifu ga­

tion cannot pack the suspended globules any closer than can gravity*

In re la tio n to the time element, i f

f if te e n to

eighteen thousand times the force of gravity i s exerted by cen trifu g atio n ,

the gravity separation would take f if te e n to

eighteen thousand times as long as cen trifu g al separation* Puchkovskii and Strukova (45) determined the s t a b i l i t y and type of margarine emulsions by centrifuging a 10 cc* sample for 60 seconds at 1000 r.p.m* M errill (42) determined the mechanical s t a b i l i t y of emulsions by use of the centrifuge method.

He measured the

rate of separation of the in te rn a l phase of le c ith in ,

soap,

and gum-stabilized emulsions under a constant cen trifu g al force,

applying the method to both 0/W and W/0 emulsions*

He pointed out th a t a single size-frequency measurement is

28

not a su itab le measure of emulsion s t a b i l i t y fo r i t

i s not

a function of the degree of dispersion but ra th e r a change in the p a r tic le period of time.

size

(change in I n te r f a c la l area ) over a

Furthermore, one must e sta b lish whether the

change in droplet size actu ally represents i n s ta b i l i t y or merely a step toward a more thermodynamically stable s ta te of la rg e r, more permanent p a r tic le s . measure of s t a b i li t y ,

The only r e lia b le

th erefo re, is a determination of the

ra te of breaking of the emulsion.

The application of c e n trifu

gal force accelerates th is dém ulsification and allows a rapid determination of emulsion s t a b i li t y ,

according to M errill.

The recipro cal of the ra te of o il separation was taken as a q u an titativ e measure of the mechanical s t a b i l i t y of the e~ mulsion and was designated by the symbol, K.

The volume of

dispersed phase which had broken from each emulsion was noted and recorded at in te rv a ls of five to t h i r t y minutes.

These

values were p lo tte d as a function of time at a constant cen trifu gal speed.

Since the data yielded sim ilar curves,

the i n i t i a l ra te of separation of the dispersed phase was taken as an index of mechanical I n s t a b i l i t y .

M errill con­

cluded th at the cen trifu g al method must await comparison with other methods such as microscopic and s h e lf-te s tin g before i t could be accepted as v alid . Hollenberg (6) discussed the evaluation of em ulsifiers in cosmetic manufacture.

He l i s t e d the commonly used methods

of s t a b i li t y determination as temperature t e s t s , te s tin g ,

and p a r tic le

cen trifu g al

size measurement using an emulsion of

29 e

time-*tested s t a b i l i t y as a control#

To evaluate the cen­

trifu g e method, Hollenberg undertook t e s t s which indicated the differences between the r e s u l ts obtained by s h e l f - te s t ­ ing methods and cen trifu g atio n .

Five minutes centrifuging

at 3000 r.p.m . was taken as the equivalent of one week shelf life *

The tab les presented showed l i t t l e

c o rre la tio n

between the two methods. Avis (46) used, among other t e s t s ,

the centrifuge

method for the evaluation of em ulsifiers In the develop­ ment of an acid lo tio n . 1700 r.p.m .

The centrifuge was operated a t

and readings of creaming and o il

separation

were made at 15 minute in te rv a ls over a period of two-andone-half hours; or the centrifuge was run at 2000 r.p.m . for one-and-one-half hours. Christenson and Shelton (7), in the development of an acidic germicidal lo tio n ,

subjected the preparations to

various s t a b i li t y t e s t s .

The centrifuge te s t method was

considered by these workers to be inaccurate for the quanti­ ta tiv e determination of s t a b i l i t y .

I t was applied, however,

as an ind ication of the le s s stable lo tio n s which were then reje cted from fu rth er te s tin g . De Navarre (47) suggested the use of a small laboratory centrifuge to check the s t a b i l i t y of an emulsion against a sample of known s t a b i li t y . The evaluation of the cen trifu g al method from ex istin g lite ra tu re is d if f ic u lt.

Apparently no systematic or ex­

tended experimentation has been conducted to determine

conclusively the efficacy of th is method in the evaluation of the s t a b i l i t y of pharmaceutical emulsions. Summary of the L ite ra tu re # —I t i s concluded from the c r i t i c a l survey of the l i t e r a t u r e

th a t several methods of

s t a b i l i t y determination may be applicable to the study of pharmaceutical emulsions;

the microscopic measurement of

the decrease of specific in te r f a c ia l area as a function of time;

the microscopic method of Smith and Grinling;

and the

centrifuge method. lo

The microscopic method requires the tedious and

time-consuming procedure e n ta ilin g the measurement of a large number of globules under the microscope.

Repeated

measurements must be made from time to time as the emulsion ages. 2«

The method of Smith and Grinling, while le s s tedious,

has not been applied to the measurement of the s t a b i l i t y of an emulsion over a period of time.

I t i s generally agreed

by workers in the f i e l d today th a t an i n i t i a l microscopic measurement i s in v alid as fa r as predicting the future s t a ­ b i l i t y of the emulsion.

Any microscopic or shelf method

must be extended over a considerable period of time for the evaluation of stable emulsions which break slowly. 3.

The centrifuge method remains as the most promising

rapid q u a n titativ e method.

This study was thus devoted to an

evaluation of the cen trifu g al method with emphasis upon i t s possible pharmaceutical applies tio n .

31.

EXPERIMENTAL

The experimental In v estig atio n l a divided Into three p a rts as follows* F irs t,

a comparison of the s t a b i lit y of four emulsions

composed of four d iffe re n t o ils was made by means of the centrifuge method.

All emulsions were s ta b iliz e d with the

same emulsifying agent. Second, the comparative s t a b ility measurement of emulslons s ta b iliz e d with five d ifferen t emulsifying agents was made by the centrifuge method. Third, the centrifuge method was applied also to the evaluation of the s t a b i l i t y of several other types of pharmaceuticals such as suspensions, hydrophilic ointments, linim ents, end lo tio n bases. Procedure for Making the Etaulsions. - I t was necessary to standardize the preparation of a l l emulsions f i r s t in order to obtain consistent re s u lts .

A d e fin ite quantity of

the emulsifying agent was p ip etted from a stock solution in ­ to a 250 cc. beaker and enough water was added to make 16 cc. of solution. to t h i s

A fine

stream o f 50 cc. of the o il was added

solution with mechanical s tir r in g

five minutes.

over a period of

During the following five minutes, 100 cc.

of water was gradually added with s t i r r i n g .

The mechanical

a g ita tio n was accomplished with an Eberbach nPower-StirM f i t t e d with a four-bladed s t i r r e r .

The speed of the s t i r r e r

was regulated a t 800 r.p.m#, having been standardized

52

previously with a Waltham tachometer»

The coarse emulsion

obtained was then d ilu ted to 200 cc, to y ie ld a 25% 0/W emulsion, a fte r which i t was passed three times through a one pint hand homogenizer» Preparation of the Em ulsifier. «• In the preparation of the stock solution of em ulsifier, the emulsifying agent was weighed out accurately on an a n a ly tic a l balance,

tran sferre d

q u an titativ e ly from the watch glass to a beaker containing enough d i s t i l l e d water to dissolve i t ,

and the mixture was

s t i r r e d u n t i l complete solution was effected#

The solution

was then tran sferre d to a volumetric flask and d ilu ted to the volume, the foam being di spelled with a few drops of alcohol#

An aliquot of t h i s stock solution was p ip etted out

for use in the preparation o f the emulsion»

The stock solu­

tio n s were prepared fresh ly each week as ageing effe c ts were found to a lte r the resu lts*

Part I . COMPARISON OP THE STABILITY OP EMULSIONS PREPARED WITH VARIOUS OILS

Emulsions of U.S.p. heavy mineral o i l , liv e r o i l ,

castor o i l ,

cod

and cottonseed o i l , were prepared using as the

em ulsifier a mixture of equal q u a n titie s of sorbitan monolau rate *

(Span 20)* and polyoxyethylene sorbitan monolaurate

A v a ila b le D e la w a r e .

fr o m t h e A t l a s

P o w d er C om pany, W ilm in g t o n ,

35

(Tween 20)*.

To permit r e la tiv e ly rapid evaluation of the

emulsions th at quantity of em ulsifier which ju s t emulsified a l l the o i l present was added.

This concentration i s r e f e r ­

red to as the c r i t i c a l concentration of the emulsifier*

The

value was determined by preparing several emulsions at v a ri­ ous concentrations and selectin g that emulsion which con­ tained no o il droplets on i t s

surface a f te r homogenization.

The c r i t i c a l concentration of the emulsifying agent fo r the o i l s was as follows ;

about 0.08^ fo r mineral o i l , 0*6#

for castor o i l , 0*3# for cod liv e r o i l , and 0.1# for cotton­ seed o i l . Shelf T ests. - A 200 cc.

sample o f each of the emulsions

was prepared and 40 cc » samples were placed in graduated cylinders, tu re .

stoppered and allowed to stand a t room tempera­

At assigned periods of time, the volume of o il which

had separated from the samples was noted and recorded.

It

was often d i f f i c u l t to read the meniscus of the o i l layer in these shelf samples because of the obstruction of view caused by the cream layer which remained in uneven deposits on the sides of the containers.

When necessary, the sides of the

graduates were scraped with a rubber-tipped rod to clear away the obstruction. the same d if f ic u lty .

Cheesman and King (37) experienced When they found i t

impossible to read

the meniscus of the o il la y e r, they measured the time taken

*

A v a ila b le D e la w a r e .

fr o m

th e A t l a s

P ow der C om pany, W ilm in g t o n ,

34

for the separation of the same quantity of dispersion medium. The two methods did not give comparable r e s u lts but the cor­ r e la tio n was found su ffic ie n t for t h e i r purpose*

In the

present in v estig a tio n the r e s u lts of the sh elf t e s t s have been p lo tted and may be seen in Figure 1. of measurements make the r e s u l ts ,

The d if f i c u lt i e s

at b est, approximate but

the r e la tiv e order of s ta b ility of these emulsions may be observed* Micro scopic T ests. - A modification of the Smith and Grinling method (13) was used to determine the s t a b i l i t y of the emulsions by the microscopic method*

Smith and Grinling

weighed 1 Gm. of the emulsion and dilu ted i t with 25 cc* of a 25% solution of acacia and enough d i s t i l l e d water to give a sa tisfa c to ry number of globules,

10 to 20 per square, when

observed under the microscope using a Thoma hemocytometer as a counting chamber*

The ém ulsification number, H, was ob­

tained by use of an equation.

In the present in v estig atio n ,

th i s procedure was modified slig h tly by p ip e ttin g 1 cc* of the emulsion in to a volumetric flask and adding a mixture of equal p a rts of glycerin and water as the d iluting solution. This method obviated the time-consuming procedure of weigh­ ing a l l the emulsion and made the determination of the sp ecific g ravity of the o il unnecessary.

Glycerin was found

to be superior to a solution of acacia for the purpose of immobilizing the globules in the counting changer.

The con­

sistency of glycerin is p a rtic u la rly adaptable to the

F ig u r e

1.

The S ta b ility of Emulaions Containing Various Oils as Determined by the Shelf Method

35

OI L S E P A R A T I O N

2

50

100

150

20 0

DAYS -e

Ind.

Eng. Chem.,

67,

39.

Cheesman, D* F ., and King, A., Trans. Faraday Soc., 34, 594 (1938).

40.

Schulman, J. H., and Cockbain, E. 0 ., Trans. Faraday Soc., 36, 651 (1940).

41.

Svedberg, T., (In; Alexander, J . , "Colloid Chemistry,” Vol I , 1926, p. 838. The Chemical Catalog Co., In c., N. Y. ).

42.

M errill, R. C., J. Ind. Eng. Chem., Anal. Ed., 15, 743 (1943).

43.

Ungerer, E ., Kolloid Chem. Beihefte, 14, 63 (1921); C. A., 16, 1169 (1922).

44.

Ayers, J r . ,

45.

Fuchkovskli, B., and Strukova, E ., Masloboino Zhirovoe Delo, 9, N 6 , 28 (1933); C. A., 28, 7106 (1934).

46.

Avis, K. E ., Am. J. Pharm., 119, 271 (1947).

47.

De Navarre, M. G. , "The Chemistry and Manufacture of Cosmetics," 1941, p. 206. D. Van Nostrand Co., In c•, N. Y.

48.

Mellor, J. W., "Higher Mathematics for Students of Chemistry and Physics," 1946, p. 94. Dover Publica­ tio n s, N. Y.

49.

Sharp, P. F . , J.

50.

Serrallaeh, J. A., Jones, G., and Owen, R. J . , Eng. Chem. , 25, 816 (1933).

51.

Nadkarni, M. V., and Zopf, L. C., J. Am. Pharm. Assoc., Pract. Pharm. Ed., 9, 212 (1948).

52.

Fantus, B ., and Dyniewicz, H. A., J . Am. Pharm. Assoc., 27, 878 (1938).

53.

Jannaway, S. P., Perfumery Essen t. Oil Record, 27, 351 (1936).

54.

Osborne, G. E., and DeKay, H. G., J. Am. Pharm. Assoc., Pract. Pharm. Ed., 2, 420 (1941).

55.

Dean, S* J . , Brodie, D. C. and Chin, D. P., J. Am. Pharm. Assoc., Pract. Pharm. Ed., 10, 430 (1949).

56.

Marcus, A. D., and DeKay, H. G., J. Am. Pharm. Assoc., Pract. Pharm. Ed., 11, 227 (1950).

E. E ., J. Ind. Eng. Chem*, 13, 1011 (1921).

Dairy S ci., 11, 259 (1928). Ind.

68

57.

The Pharmacopoeia of the United States o f America, 13th r e v . , 1947, p. 92. Mack Publishing Co., Easton, Pa.

58.

The pharmacopoeia of the United States o f America, 14th r e v . , 1950, p. 97. Mack publishing Co., Easton, Pa.

59.

Beeler, E. C., Bulletin of the National Formulary Committee, 11, 27 (1943).

60.

Munzel, K., pharm. Acta Helv., 21, 145 (1946); ibid, 22, 86 (1947).

61.

The Pharmacopoeia of the United States of America, 14th r e v . , 1950, p. 391. Mack Publishing Co., Easton, Pa.

62.

The National Formulary, 8 th Ed., 1946, (1947) p. 299, American Pharmaceutical Association, Washington, D.C.

63

.

McConnell, W . E ., Unpublished Investigation, Purdue University, 1950.

APPLICATION

of the centrifuge method to the evaluation of

THE STABILITY OF PHARMACEUTICAL EMULSIONS

-to Abstract of

A Thesis

Submitted to the Faculty

of

Purdue University

by Alfred Nicholas Martin, J r .

In P a r tia l Fulfillment of the

Requirements for the Degree

of

Doctor of Philosophy

August, 1950

APPLICATION OF THE CENTRIFUGE METHOD TO THE EVALUATION OF THE STABILITY OF PHARMACEUTICAL EMULSIONS* je -H

By Alfred Nicholas Martin, J r .

and C. 0. Lee

Coutinho (1 ) stated that probably the most important and desirable property of cosmetic and pharmaceutical emulsions was the s t a b i l i t y of the finished product.

Sutheim

( 2 ) re fe rre d to the problem of s t a b i li t y as ltthe nightmare of the emulsion chemist.”

In 1947, Hollenberg (3) noted

th a t ”a wholly adequate and r e lia b le method of accelerated s t a b i l i t y testin g to replace shelf testing has not yet been devised.”

The present costly and time-consuming Ind ustrial

method of s t a b i l i t y te s tin g includes storage of the product at various controlled temperatures for long periods of time - weeks, months,

and even years - in an effort to determine

the permanency of the emulsion before i t i s marketed. Because of the disadvantages In the microscopic method and shelf ageing t e s t s used today for the determination of the s t a b i l i t y of emulsions, t h i s study was undertaken with the object of developing a centrifuge method which might be both rapid and accurate* The microscopic method involving a size-frequency d is trib u tio n analysis of the emulsion has been used by a *

A contribution from the laboratories of the Purdue University School of Pharmacy, Lafayette, Indiana. Fellow of the American Foundation for Pharmaceutical Education.

2 number of workers (4)

(5) ( 6 ) (7)

(8 ).

Only a limited number of Investigations has been car­ rie d out In which sh e lf te stin g has been used to determine the s t a b i l i t y of the ageing emulsion (9) (10) (11).

The

major disadvantage of th is method l i e s In the extended period of time required for the evaluation of a r e la tiv e stable preparation#

In the fin al analysis, however, the other

methods are valid only in so fa r as they compare with the shelf method for

s t a b i l i t y evaluation.

The centrifuge has been used to demulsify as well as t e s t the s t a b i l i t y of emulsions. ever, that

Many workers believe, how­

the centrifuge method of s t a b i l i t y te stin g ,

though rapid,

a l­

does not give r e s u lts p arallel in a l l cases

to those obtained by the shelf or microscopic method.

Sever­

al workers have applied t h i s method to the evaluation of s ta b ility

(3)

(12)

(13) (14).

The centrifuge method ap­

pears to o ffer the greatest promise as a rapid quantitative method; the other methods discussed require a considerable time when applied to re la tiv e ly

stable pharmaceutical e-

mulsions.

EXPERIMMTAL

Procedure fo r Making the Emul si one. - Emulsions of U.S.P. liq u id petrolatum and cottonseed o i l were prepared using as emulsifying agents polyoxyethylene sorbltan monolaurate

3.

(Tween

2 0 )*,

a mixture of equal q uan tities of polyoxyethyleœ

sorbltan monolaurate and sorbltan monolaurate (Span 20)*, sodium lau ry l

su lfate, dloctyl sodium sulfosuccinate (Aero­

sol OT)**, and acacia.

The emulsions were prepared by an

English-type method, adding the o i l

slowly to a concentrated

stock solution of the emulsifier with mechanical agitation* An Eberbach "power-Stir* f i t t e d with a four-bladed paddle which was rotated at 800 r.p.m. was used. sion was d ilu ted with d i s t i l l e d water to

The coarse emul­ 200

cc. to yield a

25$ emulsion, a f te r which i t was passed three times through a one pin t hand homogenlzer. To permit re la tiv e ly rapid evaluation of the s t a b i l i t y of the emulsions, emulsified a l l

that quantity of emulsifier which just

the o il was added*

This concentration i s

referred to as the c r i t i c a l concentration of the emulsifier* The value was determined by preparing several emulsions at various concentrations and selecting th at emulsion which contained no o il droplets on i t s surface a fte r homogeniza­ tion. The concentration of the emulsifiers in the various emulsions was as follows: 1.

Tween 20

0.04$

2*

Tween 20 - Span 20

0.08$

3.

sodium laury l su lfate

0.05$

*

Available from Atlas Powder Co., Wilmington, Delaware

**

Available from American Cyanamid Co., N. Y.

4

4.

Aerosol OT

0.025#

5.

Acacia

2.0

Shelf T ests. - A 200 cc. sions was prepared and 40 cc. ated cylinders, temperature.

#

sample of each of the emul­ s a m p l e s were placed in gradu­

stoppered, and allowed to stand at room

At assigned periods of time,

the volume of

o i l which had separated from the samples was noted. often d if f i c u lt

I t was

to read the meniscus of the o i l layer in

these shelf samples because of the obstruction of view caused by the cream layer which remained in uneven deposits on sides of the containers.

The d i f f i c u l t i e s of measure­

ments make the r e s u l ts , at b e st,

approximate, but the r e l a ­

tiv e order of s t a b i l i t y of these emulsions could, be observed. The r e s u l ts are tabulated in Table I . Table I . Order of S t a b ility of Mineral Oil Bmlsions

Test Method Shelf Test

(90 days)

Order of S ta b ility TS >

T = Ac > Sis > ACT*

I n i t i a l Centrifuge Test

TS s

T > Sis >A0T >Ac

Extended Centrifuge Test (90 days)

TS >

T r Ac > Sis > AOT

TS Bnulsion stabilized with Tween 20 - Span 20. T Bnulsion stabilized with Tween 20. Ac Emulsion stab ilized with acacia. Sis Emulsion stab ilized with sodium 1eurylsulfate. AOT Emulsion stab ilized with Aerosol OT. The emulsions are arranged in the order of decreasing r e l a t iv e s t a b i l i t y from l e f t to right*

Centrifuge Test Method. - A size 1, type SB, I n t e r ­ national centrifuge with an eight place head, a s lid e wire r e o s t a t , and a Waltham indicating tachometer reading 0 to 6000 r.p.m. was used.

Calibrated centrifuge tubes of 60

cc. capacity contained the emulsion samples under test*

In

order to determine the effect of temperature on the r e s u lts , the temperature change was noted during centrifugation in a sample of water contained in one of the tubes.

The re s u lts

showed th at the temperature increased about 7° C. during a 90 minute period.

Centrifuge t e s ts carried out at d ifferen t

temperatures within th is range, however. Indicated the ab­ sence of effect of moderate temperature variations on the rate of démulsification.

The 25# emulsions of liq u id pe­

trolatum and cottonseed o il stabilized with the various em ulsifiers were freshly prepared and 40 cc. placed in the calibrated centrifuge tubes.

samples were Water was placed

in the cups to surround the tubes and prevent t h e i r break­ age from the Internal pressure developed during centrifuga­ tio n .

The centrifuge was operated at 5000 Î 20 r.p.m.

for

a period of 90 minutes, being stopped every 15 minutes to record the volume of o i l separation.

A gradual slowing of

the revolving head was made possible by a braking device pro vided with the centrifuge.

The periods of centrifugation

were timed with a mechanical clock provided with a warning b e ll. The volume in cubic centimeters of o i l which separated from the emulsion was read and recorded at d e fin ite in terv al

6

.

Several centrifuge runs of four samples each were made with each emulsion.

The values obtained were averaged and the o i l

separation in cubic centimeters was plo tted on the ordinate with time in minutes on the abscissa as seen in Figure 1. The slope of the curves was determined by the two-point form­ u la (15) and the reciprocal of the slope, K, was taken as a measure of the s t a b i l i t y of the emulsion according to the procedure of Merrill

(12)•

The data has been tabulated in

Table I I . Table II* The S ta b il it y of Efrmlsions Containing Various Emulsifiers as Determined by the Centrifuge Method Emuls. agent

15 So 45 60 9b 75 min. min. min. min. min. min. cc . cc. cc. cc. cc. cc.

Slope cc./min.

S ta b ility K

Tw. -Sp. 20

0.1

0.3

0.3

0.6

0.9

0.9

0.0112

89.4

Tw. 20

0.1

0 .2

0.4

0.7

0.8

1.0

0.0129

77.5

3 .1 .s.

0.3

0.5

0.8

1.1

1.3

1.5

0.0183

54.5

AOT

0.3

0.7

1 .2

1.4

1.6

1.9

0.0231

43.3

Acacia

1.5

3.4

6.8

9.3

—— - -

0.1081

9.2

The r e s u l ts of the I n i t i a l centrifuge t e s t does not compare with those of the shelf t e s t in the liq u id petrolatum emul­ sions as seen In Table I .

The emulsion stab ilize d with

acacia appears to be r e la tiv e ly stable when evaluated by the shelf t e s t method but exhibits a considerable i n s t a b i l i t y according to the i n i t i a l centrifuge method.

F ig u r e 1 T h e S t a b i l i t y o f E m u ls io n s S t a b i l i z e d w i t h V a r io u s E m u l s i f i e r s a s D e te rm in e d by I n i t i a l C e n t r i f u g e T e s t

7

OI L S E P A R A T I O N IN C C .

4

2



30

45

60

75

i

90

MINUTES *— -e — -A—



A-

-0— —O

Figure

A cacia A - e r o s o 1 OT Sodium Laury 1 Tween 20 T w e e n - Span 2 0

The Stability of Emulsions Stabilised with Various Emulsifiers as Determined by Initial Centrifuge Tests. (Mineral Oil Emulsions)

Extended Qentrifuse Tests# - Since the i n i t i a l c e n t r i ­ fuge t e s t s did not agree s a t i s f a c t o r i l y with the shelf t e s t s , repeated t e s t s were made over a period of several months up­ on preserved samples of emulsions which were prepared in

1 00 0

cc# q u a n titie s. The e ffe c t of ageing on the s t a b i l i t y of some emulsions is clearly demonstrated in the case of a cottonseed o il emul­ sion s ta b iliz e d with acacia as seen in Figure 2*

During a

period of 27 days th is emulsion increased from the le a s t to the most stable of the ten emulsions under test#

These re­

s u lts explain the lack of correlation between the shelf t e s t r e s u l ts and those obtained from the i n i t i a l centrifuge t e s t s . At two-week periods, four 40 cc# samples were removed from each emulsion sample and centrifuged.

In some cases,

i t was d i f f i c u l t to read the volume of o i l separation at each time in te rv a l since f i s suring and contraction of the cream layer allowed the separated o il to run back into the emul­ sion, re su ltin g in abnormally low values.

An accurate

volume determination was obtained at the 90 minute period, however, by pouring the separated o i l from the tube into a graduated cylinder.

From these extended t e s t s ,

the change

in s t a b i l i t y of the emulsions over a period of time could be noted.

The s t a b i l i t y , K, was calculated and t h i s value

for each emulsion was p lo tted against the time of ageing in weeks as seen in Figure 3* The r e la tiv e order of emulsion s t a b i l i t y ,

as determined

by the extended centrifuge te s t method, compared very

F ig u r e

2

T h e S t a b i l i t y o f a C o tto n s e e d O i l E m u ls io n S t a b i l i z e d w i t h A c a c ia as D e te r m in e d b y E x te n d e d c e n t r i f u g e T e s t s

9

OI L S E P A R A T I O N IN C C ,

4

2

15

30

45

60

75

90

Mi nute s -e------ #-o------ o-6 a•o— o -

Figure

0 Days 2 Days 13 D a y s 2 7 Days

The Stability of a Cottonseed Oil Emulsion Stabilized with Acacia as Determined by Extended Centrifuge Tests,

F ig u r e

3o

T h e S t a b i l i t y o f id n u ls io n s S t a b i l i z e d w i t h V a r io u s E m u l s i f i e r s as D e te r m in e d b y E x te n d e d C e n t r i f u g e T e s t

10

2 0 Q SIAB1L.TY'K

100

7

9

II

13

WEEKS •x— —x—

-0—— o—ûr

Tween- Span 20 Tween 2 0 Sodium Lauryl S u lfa te A cacia A e r o s o l OT

Figure $ e The Stability of Emulsions Stabilized with Various Emulsifiers as Determined by Extended Centrifuge Tests. (Mineral Oil Emulsions )

11

favorably with the r e s u l t s of the shelf t e s t s as observed in Table I .

Although the sodium lauryl su lfate emulsion had a

higher order of s t a b i l i t y than the acacia emulsion during the t e s t period,

the emulsion sta b iliz e d with acacia showed

an increase of s t a b i l i t y over the f i r s t two months about equal to th a t of the emulsion stab ilize d with Tween 20*

The

acacia emulsion i s therefore given a higher s t a b i l i t y r a t ­ ing.

Thus the centrifuge t e s t , when conducted over a period

of several months, appears to give a r e la tiv e measure of the s t a b i l i t y of emulsions stab ilize d with various emulsifiers. Use of the Centrifuge Method In the S ta b ility Evalua­ tion of Other Type Pharmaceuticals. - The centrifUge method was also applied to the evaluation of the s t a b i l i t y of the o f f i c i a l calamine lotion, hydrophilic ointment, and lime liniment. Calamine Lotion,. . One-thousand cubic centimeter samples of the U.S.P* XIV (16) formula was prepared and 40 cc. samples were centrifuged at 3000 r.p.m* for a period of 30 minutes, reading and recording the volume of the clear aque­ ous layer which separated a t the top of the centrifuge tubes. A curve representing the type of r e s u lts obtained may be seen in Figure 4. Hydrophilic Ointment.. .No investigation to date has been undertaken to determine the s t a b i l i t y of

0/W

emulsified o in t­

ments although much l i t e r a t u r e has been compiled re la tiv e to the formulation and incom patibilities of such pro ducts.

F ig u r e

4

The s t a b i l i t y of Various Types of Pharmaceutic a l s as Determined by the Centrifuge Method*

12

SEPARATION IN C C .

4

2

15

10

20

25

MINUTES Lime L inim ent Ol i ve Oil L i n i m e n t C alam in e Lotion Hydrophilic

Ointment

Figure 4 » The Stability of Various Types of Pharmaceuticals as Determined by the Centrifuge Method•

30

13.

I t I s believed, that new formulas might be b e tt e r evaluated by application of the centrifuge t e s t to the determination of t h e i r stability*

The ü.S.P. XIV Hydrophilic Ointment

(17) was diluted by the Incorporation of twice i t s weight of water with an e l e c tr ic mixer.

The liquid which was j u s t

pourable was transferred to calibrated centrifuge tubes and centrifuged a t 3000 r.p.m. for 30 minutes.

The volume of

aqueous phase which separated a t the bottom of the tubes was read at 5 minute in te rv a ls.

The re s u lts may be seen in

Figure 4. Lime Liniment. . .The o f f i c i a l product of the N.F. VIII (18) was prepared and i t s

s ta b i l i t y was compared by the

centrifuge method with a liniment prepared by agitating to ­ gether by hand equal q uan tities of olive o il and calcium hydroxide solution. 1000 r.p.m.

These products were centrifuged at

The re su lts are found in Figure 4.

DISCUSSION

From the r e s u lts of th is in vestigation,

i t i s concluded

that the centrifuge method has merit in the determination of the r e l a ti v e s t a b i l i t y of emulsions and suspensions.

I t has

been pointed out that while an i n i t i a l centrifuge s t a b i l i t y evaluation of an emulsion may be of value In the q u a lita tiv e selection of the most stable preparation, i t serve to pre dict the re la tiv e

does not always

s t a b i li t y of the various prep­

arations over a period of shelf ageing.

It

i s therefore

advised th a t those who wish to use the method, run consecu­ tive centrifuge te s t s a t two-week or monthly in te rv a ls for a reasonable period of time i f a f a i r l y accurate measure of emulsion s t a b i l i t y I s desired. I t i s proposed that th is method be used by the manufac­ tu re r for the determination of the re la tiv e merit of various emulsifying agents in the development of new emulsified pro­ ducts* 1*

A suggested method of procedure follows: Determine the c r i t i c a l concentration of each emul­

sifying agent for the p a rtic u la r emulsion under study by preparing samples at various emulsifier concentrations* That concentration which just emulsifies a l l of the o il Is taken as the c r i t i c a l concentration of the emulsifying agent© 2*

Prepare small batches of preserved emulsions ( about

5 l i t e r s ) stabilized with the various emulsifying agents, each agent at i t s c r i t i c a l concentration. 3*

Allow the emulsions to stand undisturbed for 24

hours then gently mix and remove a 40 cc.

sample from each.

Centrifuge these emulsion samples at 3000 r.p.m.

for 90

minutes, making readings of the volume of separated o i l at 15 minute I n te r v a ls . 4.

Make about 5 such centrifuge runs.

From the overall resu lts of the several centrifuge

runs, determine the slope of each curve and calculate I t s reciprocal which is taken as a measure of the s t a b i l i t y , K, of the emulsion on that p articu lar date. 5.

Repeat these te s ts every two weeks for about 3

months and from the r e s u lts , plot the s t a b i l i t y ,

K, against

15.

the time in weeks.

The curves obtained ( see Figure 5) w ill

give a relative measure of the efficiency of each emulsifying agent and of the s ta b i l i t y of the p a rtic u la r emulsions studied. 6.

I f fiesuring of the cream layer causes inaccuracies

in the r e s u lts ,

the volume of separated in tern al phase may

be measured

at one time in terv al

( say at the 60 minute

period) and

the reciprocal of t h i s value used instead of the

reciprocal of the slope as a measure of s t a b i l i t y . 7.

Increasing the concentration of the emulsifier, which

i s found by

th is study to be the most acceptable, by about 25

to 50 times

commensurate with economicalconsiderations,

should produce an emulsion of the desired s t a b i l i t y for mar­ keting.

I f i t is anticipated that the emulsions will be sub­

jected to adverse conditions a f t e r marketing, the samples may be stored under these conditions during the several months te s tin g period.

SUM M ARY

!•

Experiments have been undertaken to determine the

value of the centrifuge method. certain d e fin ite advantages. ever,

I t has been found to have

I n i t i a l centrifuge t e s t s ,

how­

are lim ited to a q ualitative d iffe re n tia tio n between

emulsifying agents and o il used in various emulsion systems. 2.

I t was shown that an emulsion which appeared in ­

i t i a l l y to be very unstable by the centrifuge method could

16

continually increase in s t a b i l i t y for a period of time by ageing e ffects, presumably at the in te rfa c e , u n t i l i t might become the most stable emulsion of the group under test* This was seen in the case of a cottonseed o il emulsion stab ilize d with acacia*

Extended centrifuge t e s t s were found

to give a re la tiv e measure of the s t a b i l i t y of ageing emul­ sions and a method based on these findings was proposed for the manufacturer to follow in the evaluation of new emul­ sifying agents* 3*

The application of the centrifuge t e s t method to

the evaluation of the s t a b i l i t y of calamine lo tio n , hydro­ p h ilic ointment, and lime liniment has been demonstrated* 4*

I t i s recommended that the centrifuge method be

used by the manufacturer and research investigato r as an aid to the formulation of new and more stable pharmaceuticals*

17.

BIBLIOGRAPHY

1*

Coutinho, H., Am. Perfumer Essent. Oil Rev., 5Z» (July 1938).

2*

Sutheim, G. M., "Introduction to Bmilsiona,n 1946, p. 117. Chemical Publishing Co., I n c . , N. Y.

3*

Ho11enberg. I* R., Proc. Sci. NO. 8 , 1 (1947).

4.

Smith, E. L ., and Grinling, G. N., Quart. J • Pharm. Pharmacol., 3, 554 (1930).

5.

Cooper, F. A., J. Soc. Chem. Ind .,

6*

King, A., and Mukherjee, L. N., J. Soc. Chem. I n d ., 58, 243T (1939).

7.

Lotzar, H . , end Maclay, W. D., Ind. 1294 (1943).

8.

King, A., Trans. Faraday Soc., 257, 168 (1941).

9.

Lederer, E. L. , Kolloid Z., 71, 61 (1935).

Sect. Toilet Goods Assoc.,

56, 477T (1937).

Eng. Chem., 35,

10.

Chessman, D. F . , and King, A., Kolloid Z., 83, 33 (1938).

11.

Ross, S ., J . Phys. Chem., 47, 266 (194 3).

12*

M errill, R. C., J. 743 (1943).

13.

Avis, K. E ., Am. J. Pharm., 119, 271 (1947).

14.

Christenson, G. L ., and Shelton, R. S., J. Am. Pharm, Assoc. , Sci. e d ., J57, 354 (1948).

15.

Mel lo r , J. W., ”Higher Mathematics for Students of Chemistry and Physics, ” 1946, p. 94. Dover Publications.

16.

The Pharmacopoeia of the United States of America, 14th r e v ., 1950, p. 97. Mack Publishing Co., Easton, Pa.

17.

The pharmacopoeia of the United States of America, 14th r e v ., 1950, p. 391. Mack Publishing Co., Easton, Pa.

Ind. Eng. Chem., Anal.

e d ., 15,

18.

Bibliography

18.

(Continued)

The National Formulary, 8 th e d ., 1946 (1947), p. 299 American Pharmaceutical Association, Washington, D. C i•

VITA

Alfred

n.

Martin, J r . was bom May 1» 1919, at P i t t s ­

burgh, Pennsylvania,

He received his elementary and second­

ary school education in Hampton Township near Pittsburgh, He was graduated from the Philadelphia College of Pharmacy and Science in 1942 with the degree of Bachelor of Science in Pharmacy. In July of 1942, he was called into active duty involv­ ing f l i g h t train ing with the united States Naval Reserves, In May of 1943, he was graduated from the Pensacola Air Training Station and was commissioned a Second Lieutenant in the United States Marine Corps Reserves.

In January,

1946, he was honorably discharged from the United States Marine Corps Reserves as a F i r s t Lieutenant. He attended Duquesne University as a graduate student in the pharmacy Department during 1946 and 1947 while work­ ing in the pharmacy of his father in Pittsburgh.

In June,

1947, he entered Purdue University Graduate School and r e ­ ceived h is Master of Science degree in August, 1948.

He

completed the requirements for the degree of Doctor of Philosophy in August, 1960. Mr. Martin married Mary W . Ziegler on July 27, 1946, in the Second Presbyterian Church of WiIkinsburg, Pennsyl­ vania. He has published with G. E. Osborne and C. 0. Lee W A Suggested L ist of Books Governing Pharmacy and I t s Related

F ield s," Am. J . Pharm. Ed., 13, 669 (1949). Mr. Martin i s a registered pharmacist in the State of Pennsylvania, and is a member of the American Pharmaceuti­ cal Association, Sigma XI, Rho Chi, Kappa P s i, and Phi Lambda Upsilon.

He has been a Fellow of the American Founda­

tion for Pharmaceutical Education during 1949 and 1960*