Properties of aqueous mixed micelles formed by hexadecyltrimethylammonium bromide and polyoxyethylene 23-lauryl ether obtained by ion selective electrodes

Properties of Aqueous Mixed Micelles Formed by Hexadecyltrimethylammonium Bromide and Polyoxyethylene 23-Lauryl Ether Obtained by Ion Selective Electrodes L. SEPI3LVEDAt AND W. J. CABRERA Department of Chemistry, Faculty of Sciences, University of Chile, Las Palmeras 3425, Casilla 653, Santiago, Chile Received February 23, 1988; accepted September 16, 1988 Surface properties of aqueous mixed micelles formed by hexadecyltrimethylammonium bromide (CTABr) and the nonionic surfactant polyoxyethylene 23 lauryl ether (Brij 35 ) have been studied. The Br- and CTA + intermicellar activities have been measured by using specific electrodes for both ionic species. When log(acrA+ ) are plotted vs log(aar-) linear relations are obtained and the slopes of the plots are used to calculate the degree of dissociation (/~) for the different molar fractions of the CTABr-Brij 35 mixed micelles. Surface miceUar potentials (~0) were also calculated, compared with reported values, and related with the experimental/3 values./3 was found to be an almost linear function of xIt0. Mean activities for the intermicellar CTA + and Br- ions were also calculated. The obtained mean activities suggest that the CTA + monomers are preferentially sorbed by micelles richer in the nonionic surfactant. CMCs of the mixed miceUes, calculated from the electromotive force of the CTA ÷ electrode are in very good agreement with CMCs values reported for the same systems. © 1989AcademicPress,Inc. INTRODUCTION

Specific electrodes for determining activities of ions have been used extensively in recent years and many types of electrodes have been reported (1). Of particular interest are the electrodes sensitive to surfactant ions and their corresponding counterions. Several reports in this field have been published (2-6). The use of specific electrodes to determine the actual activities of the ions present in surfactant aqueous solutions not only provides an easy and direct method for surfactant analysis in research and technological problems but also is an important method to be applied in relation to the models proposed for micellar formation and other organized systems. Micelles composed of mixtures of ionic and nonionic surfactants (mixed micelles) are of great interest from both a theoretical and technological point of view. Properties of mixed ionic nonionic micelles depend on the t Deceased in February 1989.

0021-9797/89 $3.00 Copyright© 1989by AcademicPress,Inc. All rightsof reproductionin any formreserved.

relative composition of the surfactant components. This relative composition also determines the micellar surface charge density which, in turn, may affect the micellar behavior with respect to properties such as solubilization, ion exchange, miceUar catalysis and other properties closely related to the micellar charge or to the micellar surface potential. Thus, the association degree of counterions, t, of mixed ionic nonionic micelles is a key parameter to be obtained and interpreted in these systems. Some attempts have been made in this sense by Rathman and Scamehorn (7), by Sasaki et al. (6), by Corkill et al. (8) and by Sepfilveda and Meyer ~(9), among others. However, in no one case have the ionic activities of both the surfactant ion and the corresponding counterion been measured simultaneously in mixed ionic nonionic micelles in order to obtain fl values. Instead, the ionic surfactant activities have been estimated by semiempirical methods (7). In this work we have used selective electrodes for measuring the activities of Br- ions

Journalof ColloMandInterfaceScience,Vol. 131,No. 1, August1989

9

AQUEOUS MIXED MICELLES

and cetyltrimethylammoniumions (CTA +) in mixtures of different composition formed by the cationic surfactant CTABr and the nonionic surfactant polyoxyethylene 23 laury ether (Brij 35). Single ion activities are required for calculating/3 from the charged phase separation model proposed by Sasaki et al. ( 6 ) . The so obtained/3 values were compared with reported values found by a different method (9). The results have been analyzed in terms of thermodynamic parameters and with an electrostatic approach in order to correlate/3 with the micellar surface potential and with the composition of the mixed micelles. EXPERIMENTAL

R e a g e n t s . Brij 35, Atlas C.I., was used as received. It was characterized by NMR spectroscopy according to the following procedure: a NMR spectrum of a pure monodisperse sample ofoctaethyleneglycolmono n-dodecyl ether (OEDE), (Nikko Chem. Co.) in D20 was taken using a Varian T-60 NMR spectrophotometer. The number of protons of the alkyl chain (na) and those carried by the polyoxyethylene residue ( n p ) w e r e related to the magnitude of the integrated signals (Ia) and (Ip) of the corresponding protons according to the relation np/na = l o / l a .

ment with the expected value of 8.00. For Brij 35,/~ = 114 mm, Ip = 30 mm, and na = 23 giving N = 21.4 and a mol wt of 1111. This value also agrees well with reported values (10). This mol wt was used in the present work. CTABr, Matheson C., was recrystallized twice from ethanol-ether mixtures. All other reagents were p.a. grade unless specified. Specific electrodes. The CTA ÷ electrode was made according to the procedure proposed by Satake et al. ( 11 ). A water insoluble cetyltrimethylammonium dodecylsulphate compound was prepared by mixing quantitatively a concentrated solution of CTABr with an aqueous solution of sodium dodecylsulphate. After centrifuging and washing, the compound was recrystallized several times from acetone. A certain amount of the compound was dissolved in a solution of THF containing polyvinylchloride (12) and dioctyl phthalate (bis2-ethylhexyl phthalate) as a plasticizer. The solvent was slowly evaporated in a fiat container leaving a transparent membrane about 0.5 mm thick. A disc of about 0.5 cm in radius was cut and fixed to the end of a PVC tube. The Br- specific electrode was a Radiometer F 1022. The complete electrochemical cells were as follows:

[1 ] X 10 -n M sample

Considering that each oxyethylene group contains 4 protons and that the first neighboring protons of the alkyl chain behave as those in the oxyethylene groups, the number of oxyethylene groups (N) is given by N=

( n o - 2)/4.

[2]

CTA + selective membrane

for CTA + and BrCTABr or NaBr electrode sample Calomel

Combining Eqs. [1 ] and [2], we obtain N = (/~ X h a - 2 ) / 4 .

t

[3]

For OEDE, la = 80 ram, I o = 119 mm, na = 23, and Nbecomes 8.05, in excellent agree-

for Br-. The calibration of the electrodes was performed using solutions of CTABr below its CMC and also solutions of NaBr for the BrJournal of Colloid and Interface Science, Vol. 131,No. 1, August1989

10

SEPI3LVEDA AND CABRERA

electrode. In both cases the response of the electrodes followed the equation

response for the Br- electrode in standard solutions of NaBr was the same as that found in CTABr solutions below the CMC (Fig. 1).

[4]

Ei = Eo +- K l o g ( a i ) ,

where ai stands for the activities of the CTA ÷ RESULTS AND DISCUSSION and Br- ions. The response of the CTA ÷ electrode was Nernstian (K = 59 mV) from 5 Figures 2-5 show the E values for the difX 10 -7 t o 8 X 10 -4 M while the response of ferent CTABr-Brij 35 mixtures as a function the Br- electrode was linear (K = 55 mV) of the actual concentration of CTABr (total from 7 X 10-s to 1 X 10-2 M. In case of using surfactant concentration X XCrABr). The gena combination cell the ai term of Eq. [4] might eral pattern is that at low concentrations of be replaced by a -+(4). CTABr, E, for both electrodes, change linearly The E0 values in Eq. [4] were obtained by with the CTABr concentration. At XcrABr replacing ai by ~i X C~and using the equation = 1.0, the linear response of both electrodes changes abruptly when the CTABr concentra3'i = -0.592 CTABr~/2/1 + CTABr 1/2, [5] tion equals 8.5 X I0-5 M which closely corwhere the CTABr concentrations are below responds to reported values of the CMC of the CMC. The E0 values found for CTA ÷ were CTABr. On the other hand, for systems con0.232, 0.235, 0.230, 0.236, and 0.238 V at taining Brij 35, the breaks in the linear beXCTABr of 1.0, 0.8, 0.6, 0.4, and 0.2, respec- havior become less defined as the Brij 35 tively, and -0.172, -0.163, -0.172, -0.163, composition increases. If one assumes that this and -0.172 for the Br- electrode at the same is due to the formation of mixed micelles, the XcrABr molar fraction composition. The linear concentration at which E deviates from lin-

XCTABr=I.O

-20 20-

10-"20 0 -"~0 -10

~ --5O

/i

i

i

i i i

I

I

I

I

I

I

I II

I

I

I

I

I

I

I I I

'0"2

I

~.l

J

I

I

I

I

*~0

[cTABr],M

FIG. 1. Electromotive potentials (E) for pure CTABr as a function of actual total concentration of CTABr for the CTA + and Br- cells. The slopes rn at low CTABr are also shown. ( e ) CTA ÷ electrode, (O), (A), response of the Br- electrode in NaBr solutions.

Journal of Colloid and Interface Science, V ol. 131, No. 1, August 1989

AQUEOUS MIXED MICELLES

11 80

60.

XCTAB r : 0.8

E(mv)

E(mv)

-60

50.z0 z, 0-20 30 -0 20 :20

0. ,60 -10i

i

80

i

1C)4

10-3

10-2

[ CTABr],M

10-I

FIG. 2. E potentials at 0.8 molar fraction of CTABr as a function of actual concentration of CTABr. The initial slopes m are indicated. (e), CTA + electrode, (©) Br- electrode.

earity would correspond to the CMC of the mixed micelles. The CMCs values so calculated are shown in Fig. 6 together with the values found by Sepfilveda and Meyer (9). There is a good agreement between both sets of data.

L o g a C T A + a n d log aBr- increase e q u a l l y a n d linearly at low total C T A B r c o n c e n t r a t i o n s and, after a certain C T A B r c o n c e n t r a t i o n , log aca-A+ levels o f f w h i l e log aBr- increases alm o s t linearly a n d w i t h o u t a n a p p r e c i a b l e change in slope. This finding w o u l d indicate

t.O

30-

eo

EtmV)

- ~

X

:0.6

E(mV)

60

20,

40

10-

20

0-

o

-I0-

-20

-20-

--t,o

-30-

~6o

-40

'

I()Z' F I G . 3.

10-3

10-2

,

-80

[CTABr], M

As in Fig. 2 but at 0.6 molar fraction of CTABr. Journal of Colloid and Interface Science, Vol. 131, No. 1, August 1989

12

SEP[TLVEDA AND CABRERA

30,

B0

-(rnV) 20 J :

~

'"O .. "~-~bz,

XCTABr= Q4

E(mV)

-60

10-4

-40

O.

-20

-10.

-0

-20.

-'20

-30.

;40

-40-

"60

-5C

I

I

,

,

L ,

t

"80

[CTABr],M ~ G . 4. As in Figs. 2-3 but at 0.4 molar fraction of CTABr.

that Br- behaves in a similar manner below and above the CMC which suggests that at first the mixed micelles are rich in Brij 35. For increasing CTABr concentration, the CTA ÷ ions are incorporated preferentially in the micelles while the intermicellar Br- ion concentrations slightly decrease. Corkill et al. (8) as-

sume this type of behavior for treating their results in mixed micelles of anionic and nonionic surfactants. In spite of the small changes in log aBr-, the association degree of Br- ions to the mixed micelles can still be evaluated applying the mass action law to the formation of mixed ,100

20 10-

E( m ~

XCTABr= 0.2

E(mv)

80

0-

-60

-10-

-40

-20.

-20

-30.

-0

-/,0.

--20

-50-

--40

-60

'

16~"

10-3

~0-2

FIG. 5. As in Figs. 2-4 but at 0.2 molar fraction of CTABr.

Journal of Colloid and Interface Science, Vol. 131, No, 1, August 1989

'

'

'

' ' ' "60

[CTABr ]jM lff1

13

AQUEOUS MIXED MICELLES

where q/p is the association degree 13 = (1 a), (a = dissociation degree). This treatment considers the micelles to be a charged, separated pseudophase in equilibrium with the free surfactant monomers and the corresponding free counterions. Unfortunately, this model and the experimental values presented here cannot yield the concentrations of free and micellized nonionic surfactant. In a further study we shall try to use the method proposed by Funasaki and Hada (13) to obtain the concentration of the free surfactant species and the mixed micellar composition. Equation [8] predicts that a plot of 1og(acTA+) VS 1og(aBr-) would result in a straight line with slope ft. The straight lines are shown in Fig. 12 for different values of XCTAB~and the/3 values are presented in Fig. 13 which also shows the/3 values reported by Sepfilveda and Meyer (9) for the same systems. The values of Ref. 9 were calculated using the formal definition of/3, i.e., -

8

7

6

o

5

x o o

4

3

2

I

!

!

!

!

0.2

0,4

0.6

0.8

i.O-

XCTABr FIG. 6. CMCs values of the mixed surfactant solutions as a function of the molar fraction composition of CTABr. (©), values calculated at which E is not anymore constant (see text). (A) Values from Ref. 9.

micelles. According to this model, the following equilibrium is established: r (Brij) + p (CTA +) + q (Br-) (Brij)r(CTA +)p (Br-)q.

[6]

If micelles are now considered as a charged pseudophase (6), their chemical potentials are considered constant and, assuming that the nonionic surfactant does not affect the activity of the ions in the bulk solution, we can write (a+_) (q+q) = (aCTA+) p X ( a B r - ) q q-

constant

[7] or

1og(acTA+) = --q/P 1og(aBr-) + (constant)',

[8]

/3=(1

-o~)=

IS,] - [Cr] [ s d - [Sw] '

[91

where [St] is the total ionic surfactant concentration, [Cr] the free counterion concentration, and [Sw] the monomer ionic surfactant concentration that remains unmicellized. [Sw] has been usually taken as equal to the CMC when pure ionic surfactants are under consideration ( 14, 15). However, it has been experimentally established, and confirmed in this work, that [Sw] (or the activity of unmicellized monomers) decreases with [ St ] above the CMC (15) and therefore Eq. [9] is not consistent with the charged pseudophase model represented by Eqs. [ 6 ]- [ 8 ]. The constancy in [Sw] above the CMC is even more unlikely for mixed ionic nonionic micelles. The use of Eqs. [6 ]-[8] implies that all intermicellar CTA + and Br- ion concentrations correspond to the equilibrium represented by Eq. [6] while the [Sw] values occurring in Eq. [9] represent the CTA + and Br- concentrations species which are considered constant Journal of Colloid and Interface Science, Vol. 131,No. 1, August1989

14

SEPOLVEDA AND CABRERA

and equal to the CMC. The values obtained here increase with CTABr concentration and then tend to the value of pure CTABr (XCTAB~ = 1.0). This last value of fl agrees relatively well with reported values found by other methods ( 17, 18). However, the shape of the curve of/3 vs XCTAB~(Fig. 13) is opposite to what was reported by Seprlveda and Meyer (9) but is in accordance with the results presented by Scamehorn (7) for similar systems. The discrepancy with the data of Ref. 9 could arise because specific electrodes detect ionic activities while the fluorescence quenching of a probe by counterions depends on the counterions that actually interact with the probe at the micellar interface. In this context, the quenching capability ofcounterions would be less at lower XCTABrsince the surface density of the counterions would be low. An extensive analysis for explaining the discrepancies in/3 values obtained by different methods have been presented by Gunnarsson (16). For all reasons given above we believe that fl values obtained from single ion activities are more reliable than those obtained by other methods. The mean activity (a_+)a above the CMC can be chosen as that given by Eq. [ 7 ]. Taking logarithms, using the definition of fl = q/p, and rearranging, we obtain log(a-+)a = (1/1 +/~)log aCTA+

culation of ( 3"-+)a, (Eq- [ 12 ] ) requires the actual intermicellar concentrations of both CTA ÷ and Br- ions which is not possible unless assumptions about the activity coefficients of the intermiceUar CTA ÷ and Br- ions are made. The mean ionic activities (a +-)b below the CMC are given by

(a+-)b = (acta+ X aBr-) 1/2

[13]

and above the CMC (a_+)a corresponds to the chosen definition established in Eqs. [7] and [ 10 ]. Some authors (6), have calculated a +_ values below and above the CMC, not considering the conceptual difference between (a-+)a and (a_+)b. Figs. 7-11 include the log ( a-+)b and the log ( a +-)avalues as a function of total concentration of CTABr. Log(a_+)b increases with log CTABr and the slope of the curve increases as the micellar composition becomes richer in the nonionic surfactant component (Brij 35). On the other hand, log( a +-)aremains constant at XCrAB,= 1.0 and drastically decreases as XCTAartends to zero. According to Eqs. [ 7 ] and [ 10 ], the pseudophase separation model requires that (a+)a remain constant, i.e., that (p + q) must also be a constant. However, (p + q) is only constant at XCTAB,= 1.0 (Fig. 7) but at XCTAB, less than 1.0, log(a+-)a decreases suggesting that (p + q) increases. In addition, the decrease

+ (fl/1 + fl)log aBr=

log K/(p + q).

[10] -2.2

In a similar way, if the mean activity coefficients (3" +) are chosen as (3"++-)(P+q) ---= ( 3 " C T A + ) p X ( 3 " B r - ) q

[11]

-2.8

-3.4

log(3,+)~ = ( 1/1 +/3)log 3'CTA+

-3.8

The (3, +)b values calculated below the CMC of pure CTABr were found close to unity ( 1.00 _+ 0.05) in accordance with the values predicted by the Debye-Hiickel theory. The calJournal of Colloid and Interface Science, Vol. 131, No. 1, August 1989

~

-

one obtains above the CMC

+ (/3/1 + fl)log3"B~-. [12]

XcrAB~ 1 . ~ . .

-3.8

-3.2

-2,8

uog

-2.4

-2

-1.8

-1.2

[ct^sr]

FIG. 7. Logs of: acrA+(O), abe- (O), (a2 from Eq. [13] (A), and (a_+) from Eq. 10 (×) at 1.0 molar fraction of CTABr.

AQUEOUS MIXED MICELLES - I°8

15

-1.8

XCTABr =0-4

-2 -2.4 -3./, -3.0

-3.8

-3.2

-3, 6

~

~+

-3,8 -4,2

-4 I

I

-3,2

-3.4

i -3

I

l

l

I

-2.8

-2.2

-1.~

-1./,

| -4

-3 Loo

Loo[CTABr]

-2

-1

[CTABr]

FIG. 8. As in Fig. 7 but at 0.8 molar fraction of CTABr.

FIG. 10. As in Fig. 7 but at 0.4 molar fractionof CTABr.

in log(a_+)a values is more pronounced as the mixed micelle composition becomes richer in the nonionic surfactant component. These results can be explained in terms of an increment in the p values since q, related to the B r - activity, changes only slightly with CTABr composition (Figs. 7-11 ). This would m e a n that more CTA + than that expected from an ideal mixing model is incorporated into the mixed micelles as the molar fraction of the nonionic surfactant c o m p o n e n t tends to 1.0. This explanation is consistent with the electrostatic model of ionic micelles in the sense that when more ionic surfactant molecules are taken up into the mixed micelles, the addition of another ionic surfactant molecule into the miceUe will be m o r e difficult due to electrostatic

repulsions. The CMCs values shown in Fig. 6 support this explanation.

Surface Micellar Potentials (~o) In principle, the surface micellar potential depends on the ratio of the ionic to nonionic surfactants in the mixed micelles. R a t h m a n and Scamehorn (7) have developed two electrostatic models relating ~0, the surface potential (Stern layer potential) to the mixed micellar composition. We have chosen the localized counterion adsorption model for treating our/3 values in terms of xI'0 since the other model (mobile adsorption) gives almost the same results (7). The final equation for the localized adsorption model is (7)

-2.0

-1.8

= .

= ,

-2.8

-2.8

-3.6 -3.4

-4.4 -4.2 ,,, -5

| -4

I -3

L,~

I -2

-1

[CTA.,]

FIG. 9. As in Fig. 7 but at 0.6 molar fraction of CTABr.

-4.8

-8.8

-3,0

-2.2

Log [CTABr]

FIG. 11. As in Fig. 7 but at 0.2 molar fraction of CTABr. Journal of Colloid and Interface Science, Vol. 131, No. I, August 1989

16

SEPI3LVEDA AND CABRERA -3°q

O -3.,

-3.8

-1

-4,2

-,6

I. -2 8

I -2.6

1 -2.4

I -2.0

-2~2

_1=8

- 1 .16

Log a B r

FIG, 12. Log of CTA+ activitiesvs log of Br- activities for the differentfractions of sttrfactant compositions. (O), XcrgBr= 1.0; (O), XcrgB~= 0.8; (×), XcrABr= 0.6; (V1),XcrgBr= 0.4; (A), XC~AB~= 0.2,

KB × Cc e x p ( - Z c e q J o / k T ) = 1 + KB × C c e x p ( - Z ~ e a J o / k T ) '

20

60

80

100

[14]

where C~ is the concentration of counterions at zero potential, KB a localized adsoption constant, e, the charge per electron and Zc the valence of the counterions. In order to calculate ~0 from Eq. [ 14] KB must be known. We calculated KB for pure CTABr solutions above the CMC using a value of 103 m V for X C T A B r = 1.0. The XIto value for this system was estimated from Eq. [ 15 ] proposed by Davies and Rideal (20): ~0 = 50.4 s i n h - l ( 1 3 4 / A × cl/2),

40

0.41-

'

L-~

k,"

/ v

/."-/

[15]

where A is the area available in the surface to each ionogenic long chain group and c refers to ionic molar concentrations. In this work, c has been chosen as the average mean activity remaining constant above the C M C of pure CTABr solutions (Fig. 7). A was calculated using a micellar radius of 23 A and an aggregation number of 85 (21 ). In our calculations, Journal of Colloid and Interface Science, Vol. 131, No. 1, August 1989

pit

~

FIG. 13. Countcrion association degrees (/~) obtained in this work ( • ) and from Rcf. 9 ( • ) and surface potentials (~o), (11) as a function of the molar fraction of CTABr.

~0 vs/3 is also shown (O).

AQUEOUS MIXED MICELLES A = 76 (ilk) 2 a n d KB = 1.9 × 105 m o l e -1. KB a n d the e x p e r i m e n t a l values of/3 were i n t r o d u c e d in Eq. [14] a n d the surface p o t e n t i a l ( ~ o ) c a l c u l a t e d for the different c o m p o s i t i o n s o f the m i x e d micelles. T h e results o f such calc u l a t i o n s are s h o w n in Fig. 13. As expected, 9 0 decreases as the m i x e d micelles c o n t a i n less C T A + a n d / 3 is a l m o s t a l i n e a r f u n c t i o n o f 9 0 (Fig. 13). I n d o i n g the a b o v e calculations we h a v e n o t c o n s i d e r e d the p r o b l e m in r e l a t i o n to the p o l y o x y e t h y l e n e chains o f Brij 35 which w o u l d b e l o c a t e d in the w a t e r phase a n d m a y affect the c o u n t e r i o n a s s o c i a t i o n d u e to the h y d r a t i o n w a t e r b e t w e e n the p o l y o x y e t h y l e n e chains.

4. 5. 6. 7. 8. 9. 10.

11. 12.

ACKNOWLEDGMENTS Support of this work by the fondo Nacional de Investigaci6n Cienffficay Tecnolrgica (FONDECYT), Proyecto #502 and by the Departarnento de Investigaci6n de la Universidad de Chile is gratefully acknowledged. We also thank Miss M. Luz Pefia for providing the drawings and for technical assistance. REFERENCES 1. Freiser, H., "Ion Selective Electrodes in Analytical Chemistry." Plenum Press, New York and London, 1978. 2. Koshinuma, M., Bull. Chem. Soc. Jpn. 54, 3128 (1981). 3. Yamamuchi, A., Kunisaki, T., Minematsu, T., To-

13. 14. 15. 16. 17. 18. 19. 20. 21.

17

mokiyo, T., and KJmizuka, H., Bull. Chem. Soc. Jpn. 51, 2791 (1978). Vikingstad, E., J. Colloid Interface Sci. 72, 68 (1979). Botrr, C., Hall, D. G., and Scowen, R. W., Kolloid Z.Z. Polym. 250, 900 (1972). Sasaki, T., Hattori, M., Sasaki, G., and Nukina, K., Bull. Chem. Soc. Jpn. 48, 1397 (1975). Rathman, J. F., and Scamehorn, J. F., J. Phys. Chem. 88, 5807 (1984). Corkill, J. M., Goodman, J. F., and Tare, J; R., Trans. Faraday Soc. 60, 986 (1964). Sepfilveda, L., and Meyer, M., J. Colloid Interface Sci. 99, 536 (1984). Shinoda, K., Nakagawa, T., Tamamushi, B., and Isemura, T. "Colloidal Surfactants." Academic Press, New York and London, 1962. Maeda, T., Ikeda, M., Shibahara, M., Haruta, T., and Satake, I., Bull. Chem. Soc. Jpn. 54, 94 ( 1981 ). Hayakawa, K., and Kwak, J. C. T., J. Phys. Chem. 86, 3866 (1982). Funasaki, N., and Hada, S., J. Phys. Chem. 83, 2471 (1979). Shirahama, K., Bull. Chem. Soc. Jpn. 47, 3165 (1974). Nishikido, N., J. Colloid Interface Sci. 92, 588 (1983). Gunnarsson, G., J6nsson, B., and Wennerstrrm, H., J. Phys. Chem. 84, 3114 (1980). Sepfilveda,L., and Cortrs, J., J. Phys. Chem. 89, 5322 (1985). Romsted, L. S., Ph.D. thesis, Indiana University, 1975. Rusanov, A. I., and Krotov, V., Proc. Int. Congr. Surf Act. Subst. 6th 2, Part 2, 795 (1973). Davies, J. T., and Rideal, E. K., "Interfacial Phenomena." Academic Press, New York and London, 1963. Leibner, J. E., and Jacobus, J., 3". Phys. Chem. 81, 130 (1977).

Journal of Colloid and Interface Science, Vol. 131, No, I, August 1989