Stabilization of some emulsions with nonionic surfactants

Stabilization of some emulsions with nonionic surfactants

Stabilization of Some Emulsions with Nonionic Surfactants PIMOLPAN PITHAYANUKUL AND NEITON PILPEL Chelsea College o f Science, University o f London, ...

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Stabilization of Some Emulsions with Nonionic Surfactants PIMOLPAN PITHAYANUKUL AND NEITON PILPEL Chelsea College o f Science, University o f London, Manresa Road, London S W 3 6LX, United Kingdom Received October 26, 1981; accepted February 15, 1982 Measurements were made o f the density, droplet size, ~"potential, viscosity, surface viscosity, and elasticity of O / W emulsions o f dodecyl benzene with mixtures of tetradecyl polyoxyethylene alcohol and nonyl phenol polyoxyethylene alcohol as the emulsifying agents. Stabilization o f the emulsions against coalescence o f droplets appears to be due to the formation around the oil droplets o f bimolecular films o f the surfactants, approximately 10 n m thick with viscosities of about 10 -2 surface poise and shear elasticities of the order of 1 N m -l.

(8, 9), others have demonstrated the presence of lyotropic liquid crystals, which have a high viscosity and constitute a third phase in the emulsion ( 1O, 11). In the present work measurements have been made of the density, droplet size, ~"potential, viscosity, surface viscosity, and elasticity of some oil-in-water emulsions in an attempt to obtain more information about the nature of the steric barrier.

INTRODUCTION

Instability in emulsions is normally detected by changes in droplet size, the onset of creaming, or phase separation. Measurements of density at a fixed point in the emulsion can provide an accurate method for following flocculation and coalescence of droplets, which cause the density at the sampling point to vary. Considerable work has been done in recent years to develop a theory of stabilization for emulsions containing nonionic surfactants (1-4). The total interaction energy between the droplets is written as (5)

EXPERIMENTAL

Materials

Commercial dodecyl benzene (Dobane JN, Shell Chemicals) was purified by heating twice with 5% w/v of Fullers earth under nitrogen and then filtered (bp 283°C; d 2° 0.875; n 2° 1.486; interfacial tension at 20°C 46.2 mN m-l). Purified samples of tetradecyl polyoxyethylene alcohol CH3(CH2)I3(OCH2CH2)zOH (Et 252) and of nonyl phenol polyoxyethylene alcohol

Aatotal = Aaelectrostatic + AGvan der Waals "31-AGsteric,

where AGexectrostatic is due to electrostatic forces of repulsion; A G w aer w~s is an attraction due to van der Waals forces; and AG~t~nc is a repulsion due to steric effects. It is generally accepted that in emulsions stabilized against flocculation and coalescence of droplets with nonionics the term AG~tenc CH3(CHp)~B( (OCH2CH2)B.BOH can considerably outweigh the other two. The detailed structure of the steric barrier is uncertain. Some authors (6, 7) refer to (Et 77) were supplied by Lankro Chemicals. molecular complexes, others postulate Triple-distilled water (surface tension at 20°C monolayers or multilayers of the surfactants 72.0 mN m-l; specific conductivity at 20°C 494 0021-9797/82/100494-10502.00/0 Copyright© 1982 by AcademicPress,Inc. All rightsof reproductionin any formreserved.

Journalof Colloidand InterfaceScience, Vol. 89, No. 2, October 1982

495

EMULSION STABILIZATION

1.5 X 10 -6 ~2-1 cm 1; p H 5.8) was used throughout. Methods

Oil/water emulsions in 50-ml quantities were prepared containing either a fixed a m o u n t of 20% w/w of water with different amounts and ratios o f the two surfactants, or containing different amounts o f water with the ratios of the surfactants fixed to each other and to the oil. The compositions are given in Table I and the type o f emulsion was confirmed as O / W by dilution, staining, and electrical conductivity. They were stored for 20 days at 20°C in stoppered measuring cylinders and measurements were made of the changes that occurred in their densities at a fixed depth of 5 cm due to creaming (the overall conclusions about stability are not affected by the depth of sampling), using 1 ml of sample and a Paar digital density meter (model DMA 45) which works on the principle of resonance (12). The rheological properties of the emulsions were measured with a Ferranti-Shirley cone and plate viscometer, using a 7-cmdiam. cone, a 1200-g-cm torque spring, and an automatic gap-setting device with a clear-

ance of 2.5 ~zm. The instrument was set at a sweep time of 60 sec and a m a x i m u m shear rate of 1663 sec -1. Rheograms on four replicates were plotted automatically and the areas of the hysteresis loops were measured with a planimeter. Changes in droplet size distribution, due to coalescence, were followed with a Coulter counter (model ZB). Measurements were also made o f the ~- potentials of the droplets using a Zetameter. The interfacial tensions o f dodecyl benz e n e / w a t e r systems c o n t a i n i n g various amounts and ratios of the two surfactants were measured using a ring detachment method (13), equilibrating the samples in the dark and employing the equation (14) y = P{0.725 + [0.0145P/C 2 X (DI

-

D2)11/2},

[11

where y is the interfacial tension, P the rupture reading, C the circumference of the platinum ring, and DI and D2 are the densities of water and dodecyl benzene, respectively. Results were corrected to 20°C by a factor o f - 0 . 0 4 m N m -1 °C-L The interfacial viscosities 0s of selected solutions o f the surfactants were measured with a thermostatted torsional surface viscometer (15) calculating the values from

TABLE I Composition of O/W Emulsions (% w/w)

Water

Oil

Total surfactant

1 2 3 4 5 6 7 8 9

2O 20 20 20 20 20 20 20 20

79.99 79.95 79.90 79.80 79.5 79.2 77.6 76.0 73.6

0.01 0.05 0.10 0.20 0.50 0.80 2.40 4.00 6.40

1:7999 1:1599 1:799 1:399 1:159 1:99 1:32.3 1:19 1:11.5 J

10 11 12 13 14

30 40 50 60 70

66.5 57.0 47.5 38.0 28.5

3.5 3.0 2.5 2.0 1.5

1:19

Sample

Ratio suffact/oil

Ratio Et 77/Et 252

Each sample

1:9 3:7 5:5 9:1

5:5

Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982

496

P I T H A Y A N U K U L AND PILPEL

40'

~s- 2~ x

4~_2+x2),/2

(4~2+4)1/2

,

[2] 3O

where r (= 178.5 mN m -1) is the torsion constant of the suspending tungsten wire, I (=2604 g cm 2) is the moment of inertia of the apparatus, rl (=2.5 cm) is the radius of the stainless-steel disk, h (=9 cm) is the radius of the containing dish, ~r = 3.142, and X is the logarithmic decrement when there is an adsorbed film at the interface and Xo when there is not. The sensitivity and range of the instrument could be altered by using different lengths/diameters of the torsion wire or by attaching weights to the disk, but it was found to be insufficiently sensitive for measuring the interfacial shear elasticity, Es, of the adsorbed films quantitatively. (The presence of shear elasticity was, however, indicated by the curvature of the graphs of amplitude of oscillation versus number of oscillations when the surfactant concentration was greater than 0.1% (16).) Instead their dilational elasticities, Eo, were derived from the interfacial tension results by (17)

Dn3p~ aarn~er lam FIG. 2. Coulter size analyses. Number percent versus droplet diameter. Sample 4, n, initially, A, after 15 days. Samples 6 to 8, O, between 0 and 15 days. Sample 9, n, between 0 and 15 days.

where A0 is the limiting area per molecule of the mixed film, II is the surface pressure, k is Boltzmann's constant, and T is the temperature K. RESULTS

Eo = II + (~T)II2 ,

[3]

Densitygcm-a 0.9020~ 0.8960 A ,t ~

~

0.89000.8840 Time

(days)

FIG. 1. Density of emulsion versus storage time (days). II, Sample 9 (Et 77/Et 252 5:5); A, sample 10. Journal of Colloid and Interface Science. Vol. 89, No. 2, October 1982

Figure 1 shows typical plots of density versus storage time. The stability of each emulsion to flocculation (primarily) and also to coalescence was conveniently taken as the time in days over which no change in density could be detected. On the basis of Fig. 1, sample 9 with a ratio o f E t 77/Et 252 of 5:5 was more stable than sample 10. The decrease in density of the latter between 3 and 6 days and its subsequent increase was due to initial flocculation and/or coalescence of droplets, as shown by the results of the droplet size analysis, followed by upward creaming to above the sampling point. Typical Coulter size analysis results for sample 4 and for samples 6 to 9 are shown in Fig. 2, from which it is seen that sample 4 was less stable than the others. In general,

497

EMULSION STABILIZATION ~ 10

1.0

2.0

3.0

10

20

30

7, w/w

4.0

5.0

6.0

40

50

60

Water content

~ w/w

Stal~ay

Time Days

8.

6. 4,

7'0

FIG. 3. Stability versus concentration of water and concentration of surfactant. O, Samples 8, 10 to 12; A, samples 6 to 9.

for samples 5 to 11, the mean droplet diameter, dvs, was about the same in the region of 3 gm and remained sensibly constant for 15 days, but it was larger for the remaining samples and increased with time to >6 #m. The stability times of the emulsions to flocculation and coalescence depended on their water content, on their total surfactant content, and on the ratios in which the surfactants had been mixed. With fixed ratios of surfactant to oil o f 1:19 and of Et 77 to Et 252 of 5:5 (samples 8 and 10 to 12) the stability decreased as the water was increased from 20 to 50%. When the water was kept Stability time [days)

10-

~

9-

/



constant at 20%, then increasing the total surfactant content (maintaining the ratio of Et 77 to Et 252 at 5:5) increased the stability. The stability was greatest when the ratio of Et 77 to Et 252 was between 5:5 and 9:1. These effects are shown in Figs. 3 and 4. The most stable emulsions to both flocculation and coalescence, i.e., samples 6 to 9 containing ratios of Et 77 to Et 252 between 5:5 and 9:1, exhibited non-Newtonian pseudoplastic types of flow with various degrees of clockwise (rheopectic) hysteresis. The areas of the hysteresis loops and the sizes of their spurs increased with surfactant concentration and with the ratio of Et 77 to Et 252 as shown in Table II. But there was no evidence of any yield values. Defining the apparent viscosity of the emulsion 'Oappas the shear stress/shear rate at maximum shear rate, relations were established between the apparent viscosity and the water content, the total surfactant content and the ratios of surfactants in the different systems. These are plotted in Figs. 5 and 6. The general pattern of viscosity behavior is seen to be similar to that exhibited by the stability times in Figs. 3 and 4. The ~'potentials of sample 5 to 9 decreased from about -5.0 to -2.5 millivolts as the total amount of surfactant was increased from 0.5 to 6.4%. The potentials tended to be smaller when the ratio of Et 77 to Et 252 was increased in the range 1:9 to 9:1. How-

87-

TABLE II /0"

-0

Effect of Concentration and Ratio of Surfactants on Hysteresis 5-

-rn

4" 3"

1" 00)10 1'/9 . . . . 5)5 ' ' ' 9)1 10/0 Ratio of ET 77/ET 252

FIG. 4. Stability versus ratio of Et 77/Et 252. Sample 6, A; sample 7, n; sample 8, e; sample 9, 0.

Sample

Ratio Et 77 to Et 252 Total surfactant % w/w

1:9

3:7

5:5

9:1

6 7 8 9

0.8 2.4 4.0 6.4

0 0 0 7

0 0 2.0 10

0 2 5.5 16

0 4 10 39

Area hysteresis loop (cm2)

Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982

498

P I T H A Y A N U K U L A N D PILPEL

ever, for all the emulsions examined the ~" potentials were very low and this was as expected since nonionic surfactants had been used. Turning next to the m e a s u r e m e n t s of interfacial tension, graphs (not shown here) were plotted between interfacial tension % and log concentration, c, of surfactant. The Gibbs' equation I~ =

1

dr

. ~ 100

2p

3p

60-

50-

40-

[4]

2.303RT d log c

was used in the usual way (18) to calculate the surface pressure, II, vs area, A, characteristics of the adsorbed films. Typical plots of IIA versus II and of II versus (A - Ao) (where Ao is the limiting area per molecule of the mixed surfactants) are given in Figs. 7 and 8. The limiting value o f Ao of 0.47 n m 2 obtained from Fig. 8 is in good agreement with that expected from the known cross-sectional areas of the surfactants concerned, i.e., Et 77 = 0.55 n m z, Et 252 = 0.40 n m 2 (19). The dilational elasticities of the films, E0, were calculated from Eq. [3]. For all ratios

~.o

Apparent viscosity (~mPa sec / 70-

30.

20.

10

~1/10 1;9

' 3;7 ' 5;5 ' ' ' 9;1 1()/0 Ratio of ET 77/ET 252

FIG. 6. Apparent viscosity versus ratio of Et 77/EL 252. Sample 6, &; sample 7, 121; sample 8, 0; sample 9,0.

of Et 77 to Et 252 they were found to increase rapidly as the limiting area A0 was approached and their maximum values were

content ~o w/w

4p

sp

6p 35-

Appaeent v~co~ty m Pa sec

RA 5040-

30"

302520. 201015-

10-

o

:6

,~

~

=

water content ~ow/w

FIG. 5. Apparent viscosity versus water content and surfactant content. I , Samples 9 to 14; samples 6 to 9, Ratio Et 77/Et 252, A, 1:9; x, 3:7; 0, 5:5; D, 9:1. Journal of Colloid and Interface Science,

Vol. 89, No. 2, October 1982

i

0

10

i

20 n

30

40

50

mN .rn :"~

FIG, 7. 12A versus 12. Ratio of Et 7 7 / E t 252, 0, 1:9; 121, 3:7; &, 5:5; O, 9:1.

499

EMULSION STABILIZATION 50 n

m

mN. m -~

po- V+ ~S'

[5]

4O ps

~ -

30

20

100 0

0.10

0.20

0.30

( A - Ao~

0.40

0.50

nm 2

FIG. 8. I/A versus (A - A0). Symbols same as in Fig. 7.

0.43 N m -1 for an Et 77 to Et 252 ratio of 1:9 3:7 5:5 9:1

0.37 0.33 0.37

The results for the interfacial viscosity measurements are summarized in Figs. 9 and 10. Figure 9 shows that the interfacial viscosity of the adsorbed film increased as the ratio of Et 77 to Et 252 was increased from 1:9 to 9:1. Figure 10 shows that at all ratios there was an initial increase in interfacial viscosity with surfactant concentration. This was followed by a decrease and then by another increase, suggesting that structural changes were occurring in the adsorbed film.

m V

.

Here m is the mass o f the emulsion, po its measured density, ps its theoretical density, V is the total volume o f oil + water + surfactants, a is the mean radius o f the oil droplets, q~ is the volume of the oil phase/total volume of the emulsion, and S is the surface area o f the droplets. The values o f 6 for samples 10 to 13 were between 9.2 and 9.8 nm. Figure 11 shows in more detail how 6 varied with the surfactant ratio and with surfactant content for samples 6 to 9. The general finding is that for all the above samples 6 was in the neighborhood of 10 nm. O f the emulsions examined in the present work, three types could be distinguished based on their stability to flocculation and coalescence as measured by the absence of change in their densities and droplet sizes with time. Samples 1 to 4 were unstable. They exhibited rapid changes in density and in droplet size which lead to creaming and tn*,Nrf~ vi~oei*y(sp) 4 ~,lff2

1,10":

DISCUSSION

F r o m the densities of the emulsions and their mean droplet sizes it was possible from their narrow size distributions to calculate a hypothetical effective thickness, 6, o f the surfactant layer adsorbed on each oil droplet. The relevant expressions are (20) 6-

aAp 34'

where

Ap=IP°psP~ ,

5.10 a

1 ~10-3

Ratio of ET 77/ET 252

FIG. 9. Interfacial viscosity versus ratio of Et 7 7 / E t 252. Total surfactant % w/w, e , 5.0 × 10-6; 0, 5.0 X 10 -4, &, 1.0 × 10-5; II, 5.0 X 10 2 Journal of Colloid and Interface Science, Vol. 89, No. 2, O c t o b e r 1982

500

P I T H A Y A N U K U L A N D PILPEL ~t~

CsP)

4~,K)-2

1 =10-2

s,lo ~

1=1ff a

'

I

-6

'

I

-5

'

I

-4

Log. ~ t r a t i o n

'

-3

I

I

-2

-1

of ET 77/ET 252 ( 7,w/w)

FIG. 10. Interfacial viscosity versus log concentration o f surfactants Ratio o f Et 77/Et 252, &, 1:9; e , 5:5; D, 9:1.

separation of phases within a period of a few hours or days. Sample 9, containing ratios ofEt 77/Et 252 of 5:5 or 9:1, was completely stable to both flocculation and coalescence. There was no detectable change in its properties over a period of 10 days. The remaining samples were, to varying extents, moderately stable. During the period of 10 days they exhibited slow flocculation of droplets and creaming. However, the droplets did not coalesce, as shown by the size analyses, and the phases did not separate. All the systems that have been examined had very low ~"potentials and it was therefore inappropriate to attempt to explain their different stabilities in terms of the DLVO theory. In the present systems additional mechanisms to van der Waals' attraction and electrostatic repulsion between droplets are involved (21). These are due to the physical nature of the continuous, aqueous phase and of the film of surfactants adsorbed on each oil droplet. The increasing viscosities and hysteresis areas exhibited by samples 6 to 9 (Table II and Fig. 5) indicate the development of a gel network in the continuous phase which could act as a primary barrier to flocculation and Journal of Colloid and Interface Science,

Vol. 89, No. 2, October 1982

coalescence of oil droplets. The higher the ratio of Et 77 to Et 252 the more effective the barrier. This could be because the larger ethylene oxide residues in Et 77, i.e., 6.5 units, more readily promote hydrogen bonding with water molecules through their ether oxygen than the smaller residues of 2 units in Et 252 (22). Addition of more water-samples 9 to 14--causes dilution of this gel Interracial film thickness ii (nm.) 11.0-

10.5" 10.0" 9.5" 9,0"

8.5

/

i

0

1.0

i

i

i

i

i

2.0 3.0 4.0 5.0 6.0 7, w/w total concentrationof s~cfaetants

FIG. 11. Interfacial film thickness versus concentration o f surfactants Ratio o f E t 77/Et 252, A, 1:9; 0, 3:7;

e , 5:5, 121,9:1.

EMULSION

STABILIZATION

network and reduces the effectiveness of the primary barrier to flocculation and/or coalescence of droplets; Figs. 3 and 5. The resuiting creaming that occurs is then due to the differences in density between the oil and water phases. It seems probable that, provided they are not coalescing, flocculated droplets are being held together by hydrophobic bonding (8) between bimolecular films of the two surfacrants as illustrated in Fig. 12. The formation of these films constitutes a secondary barrier to coalescence of droplets because of their mechanical properties. It is assumed that their viscosity is likely to be comparable to that of the monomolecular films at the plane oil/water interface (23, 24) even though it cannot be measured directly. Similarly their shear elasticity, Es, is likely to be at least twice the dilational elasticity of the monolayers, of the order of 1 N m-L Their thickness deduced from Eq. [5], i.e., about 10 nm, is in good agreement with the figure of 9 nm predicted from the molecular models in Figs. 12 and 12a. Appendix 1 shows that except in the most unstable emulsion systems, samples 1 to 3, there was more than sufficient surfactant present to allow bimolecular films to form. Some of the elements in the bimolecular films may be similar in function, though not

H OIL

ii

necessarily in physical form, to the liquidcrystalline phases which Friberg et al. (10, 1 l) have shown to occur in other emulsion systems. Alternatively they might be like the bilayers which are now thought to constitute the vesicles in certain phospholipid systems (25). Their presence appears to reduce the magnitude of the van der Waals' forces of attraction between oil droplets which, in the virtual absence of repulsive, electrostatic forces, would otherwise cause them to coalesce. The present findings therefore provide confirmation for the theory of steric stabilization of (practically uncharged) emulsions (5), according to which there is an overall steric repulsion between the droplets of AGR

=

AG ~ter + AG ~t~a + AG~Om,

[6]

where AG~ m is the energy change due to compression of the adsorbed film which, following the original theory of elastic collisions given by Hertz (26), has been expressed (5) as

AGp. °m-

Es 1.32

t~ \5/2

(~ - - ~ 1

(a + ~)1/2. [7]

The value Es is the elastic modulus of the layer and Ho is the droplet surface separation; AG~ ter is the energy change due to inter-

WATER I

"

i

H

H

Ida2

50 1

I

I

1

I

OIL

-===

t

l~-t,,,~,, isurfac~nt

r'l I

H

, / |

-

i

"n~r,~.

I

9. O,~.

~" I

HYDFtOI:~(2IliC BONDING

FIG. 12. H y d r o p h o b i c b o n d i n g b e t w e e n b i m o l e c u l a r films a t t h e o i l / w a t e r i n t e r f a c e . Journal of Colloid and Interface Science, Vol. 89, No. 2, October 1982

502

PITHAYANUKUL AND PILPEL Et 7 7

.% I

I

I

I

,o.as

0.24

I nm

nm

I_ I

o .c..c.

I

o .%.%

o

.%

.

I

O.ad" nm

1.5nm





2.4nm



I

IiNonylphenol Hydrocarbon chain

polyoxyethylene chain

I

Et 2 5 2

.%.% .% .% .%.% ft..%

o .% .

I I

"

1.8nm Tetradecyl Hydrocarbon chain

I I

"a' ~ 0 . 7 2 nm I I polyoxyethylene I chain I

"1 II I I

FIG. 12a. Structures of surfactant molecules.

molecular interpenetration; AG ~tra that due to intramolecular self-penetration (5) AGil~tra

= 2IIe{~ [ , -

~ ] 2 1 3 a + 2' + ~ ] } ,

[8]

where l-le is the excess osmotic pressure generated. Approximate values can be inserted in these expressions for 6, a, E~, and Ho (calculated from the mean droplet diameter and the oil contents of the emulsions) but more precise knowledge about AG ~ter and IIe will be needed to evaluate the total steric repulsion AGR explicitly. APPENDIX 1

The total surface area of the oil drops of density 1)2 and diameter dvs is Journal of Colloid and Interface Science,

Vol. 89, No. 2, October 1982

S =

6 X 1018 X 070 (W/W) oil D2 X d~

The amount of mixed surfactants of cross section A0 (=0.47 nm 2) needed to produce a bimolecular film on each droplet is 2 X S X mean MW of surfactants %. A0 X 6.02 X 1023 For samples 6 to 9 this works out at between 0.2 and 0.4% (w/w) depending on the ratio of Et 77 to Et 252. REFERENCES 1. Mackor, E. L., and van der Waals, J. H., J. Colloid Sci. 7, 535 (1952). 2. Ottewill, R. H., in "Colloid Science," Vol. 1, pp. 173 iT. The Chemical Society, London, 1973. 3. Napper, D. H., .1. Colloid Interface Sci. 58, 390 (1977). 4. Overbeek, J.Th, G., jr. Colloidlnterface Sci. 58, 408 (1977).

EMULSION STABILIZATION 5. OttewiU, R. H., in "Non-Ionic Surfactants" (M. J. Schick, Ed.), p. 645 ft. Edward Arnold, London, 1967. 6. Schulman, J. H., and Cockbain, E. K., Trans. Faraday Soc. 36, 661 (1940). 7. Cockbain, E. K., and McRoberts, T. S., £ Colloid Interface Sci. 8, 440 (1953). 8. Cockbain, E. K., Trans. Faraday Soc. 48, 185 (1952). 9. Shotton, E., Wibbedey, K., Warburton, B., and Davis, S. S., RheoL Acta 10, 142 (1971). 10. Friberg, S., Mandell, L., and Larsson, M., J. Colloid Interface Sci. 29, 155 (1969). 11. Friberg, S., in "Advances in Liquid Crystals," Vol. 2, p. 1. Academic Press, New York, 1967. 12. Picker, P., Tremblay, E., and Jolicoeur, C., J. Solution Chem. 3, 377 (1977). 13. Harkins, W. D., in "Physical Methods of Organic Chemistry" (A. Weissberger, Ed.), Vol. 1, Chap. 6. Interscience, New York, 1945. 14. Zuidema, H. H., and Waters, G. W., Ind. Eng. Chem. Anal Ed. 13, 312 (1941).

503

15. Joly, M., in "Surface and Colloid Science" (E. Matijecvic, Ed.), Vol. 5, p. 1. Wiley-Interscience, New York, 1972. 16. Alexander, A. E., and Johnson, P., "Colloid Science," Chap. 12. Oxford Univ. Press (Clarendon), Oxford, 1949. 17. Buscall, R., Progr. ColloidPolym. Sci. 63, 15 (1978). 18. Pilpel, N., J. ColloidSci. 11, 51 (1956). 19. Florence, A. T., and Rogers, J. A., J. Pharm. Pharmac. 23, 153, 233 (1971). 20. Martynov, V. M., Kolloid Zh. 9, 255 (1949). 21. Pilpel, N., Chem. Rev. 66, 29 (1966). 22. Rosch, M. Z., Ges. Textile lnd. 59, 1033 (1957). 23. Carless, J. E., and Hallworth, G. W., J. Colloid Interface Sci. 26, 75 (1968). 24. Davies, J. T., Proc. 2nd lnt. Congr. Surf. Act. 1,220 (1957). 25. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., J. Chem. Soc. Faraday Trans 2 72, 1525 (1976). 26. Hertz, H., Mathematick 92, 155 (1888).

Journal of Colloid and Interface Science, VoL 89, No. 2, October 1982