A consideration concerning the surface potential of mixed micelles of sodium dodecyldecaoxyethylene sulfate and dodecyldimethylamine oxide

A Consideration Concerning the Surface Potential of Mixed Micelles

of Sodium Dodecyldecaoxyethylene Sulfate and Dodecyldimethylamine Oxide the critical micelle concentration. As is evident in Fig. 1, the surface potential is decreased by addition of SDPS. This decrease results from a decrease in the surface charge density of the micelle through the formation of the mixed micelle of DDAO and SDPS. This effect by the long chain ion should be distinguished in the mechanism from that by small ions as already explained by these authors (3). pK = pKi(eo¢o/2.3OkT), [i] The net surface charge density and the surface potential of the mixed micelle, which is composed of equal where e0 is the elementary charge, k is the Boltzmann moles of the anion of SDPS and the cation of DDAO constant, and Ki is the K value at ~b0 = 0. The pK regardless of the moles of the nonion of DDAO, should value in micellar systems is determined by the potenbe zero, unless one of the sodium and chloride ions tiometric and spectroscopic methods (2-8). preferentially binds to this micelle. This therefore indiTokiwa and Ohki have already obtained the surface cates that the surface potential of the mixed micelle is potential of mixed micelles of sodium dodecyldecaoxyethylene sulfate (SDPS) and dodecyldimethylamine zero at fl--* 1 in the case of the molar ratio 1/1 of SDPS to DDAO, and at fl = 0.333 in the case of the oxide (DDAO) in the presence of 0.1 iV sodium chloride at 25°C (3). The calculation of the surface potential ratio 1/3. The solid lines shown in Fig. 1, however, contradict this expectation. The above consideration by these authors, however, is somewhat unreasonable. leads to the more reasonable evaluation of pKi and the This paper deals with the more reasonable evaluation surface potential ~02 in Eq. Ell, which are shown in of surface potential of the micelles. The surfactant studied, DDAO, exists in nonionic Table I and by the dotted lines in Fig. 1. As a result, the isoelectric point pHi.e, of the mixed and cationic forms depending upon the pH of the solumicelles also changes with the two procedures as shown tion. These two forms are related by the equilibrium in Table I. I t is likely that the isoelectric point of the Ci2i'i25(Ci'i3) 2N+OH ~ C~2H2~(CH3) 2NO + H + mixed micelle of eqnimolar SDPS and DDAO is present at the strongly acidic region rather than at the neutral in the micelle as well as in the bulk solution (3). These region. authors have determined the degree of ionization/3 and This table also shows that the pK¢ value depends the pK of DDAO in mixed micelles of SDPS and DDAO upon the micellar composition. The pK value of monoin the presence of 0.1 M NaC1. Here the pK value is meric DDAO in 0.1 M NaC1 solution is 4.95, which calculated from differs from the pKi value of micellJzed DDAO, viz., pK = pH + log ~ / ( 1 -- fl)~. [2~ 5.95 (2, 9). However, any explanation for the difference between these pK's has not been given. A similar Furthermore, they obtained the surface potential ~b01 difference between pKi of a dissociable group of proteins of the mixed micelles from and pK of the group of amino acids or their derivatives is found in a number of proteins (10). Some of these ~o~ = 2 . 3 0 k r ( p K ~ ° -- p K ) / e o , E3] abnormal pKi's are explained in terms of the location where the pKi ° is 5.95, viz., the pK value at fl = 0 of of the group, viz., the hydrophobic interior of the the micelle of DDAO alone in 0.1 M NaC1 solution. protein (10). Such explanation is also given for changes These surface potentials ¢01 are shown by the solid in p K values of phenols solubilized at the miceUar lines in Fig. 1 as a function of ~. The numerical values surface of a nonionic surfactant containing a polyoxyin this figure indicate the overall molar ratios of SDPS ethylene chain; the more hydrophobic phenols are to DDAO, which are expected to equal the values in located more deeply below the surface (6). A widely the mixed micelles, since the experiment has been applicable and rigorous theory that predicts the variacarried out at considerably higher concentration than tion in pK,- of a particular group is proposed by Tanford The surface potential of surfactant micelles produces significant effects upon the kinetics and equilibrium of chemical reactions on or in the micelle and also upon other properties of surfactants (1). The experimental value of the surface potential ~b0 of micelles has been evaluated from a change in the acidic dissociation constant K with the ~bo (2-8) ; that is,

189 Copyright © 1977 by Academic Press. Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 62, No. i, October 15, 1977' ISS:b[

QQ~.I,-9797'

190

NOTES TABLE I

/1

The pKi Values and the Isoelectric Points of Mixed Micelles of SDPS and DDAO in the Presence of 0.1 M NaC1 at 25°C

100

1/3

~.113 50

E v

SDPS/DDAO

iii

0

0/1 1/3 1/1

-100

/

FIGURE 1

Journal of Colloidand InterfaceScience,VoL 62, No. i,

Tokiwa, Ohki

This work

Tokiwa, Ohki

This work

5.95 5.95 5.95

5.95 5.51 4.63

>8 6.72 6.24

>8 5.81 <4

1. FENDLXR,J. H., AND FENDLER, R. J., "Catalysis in Micellar and Macromolecular Systems," Academic Press, New York, 1975. 2. ToI~IWA, F., Advan. Colloid Interface Sci. 3, 389 (1972). 3. TOKIWA,F., AND OHI~I, K., J. Colloid Interface Sci. 27, 247 (1968). 4. HAI~TLXY,G. S., AND ROE, J. W., Trans. Faraday Soc. 36, 101 (1940).

.J

(11). He has shown that pK~, viz., pK at the zero net charge, strongly depends upon the configuration of cationic and anionic charges fixed at the protein surface (i1). The pK~ changes shown in Table I may be explained on the basis of this discrete-charge effect since the configuration of SDPS and DDAO is expected to change with the micellar composition. The micellar pK's of weak acids, the minute amount of which is solubilized at the micellar surface, have been determined spectroscopically (4-8). In this case the pKi value may be affected principally by the solvent property of the micellar surface rather than by the configuration of micellar charges, since the way of distributing the charges is extremely limited. The pKi, 4.63, of protonated DDAO in the mixed micelle of equimolar SDPS and DDAO, which contains the relatively small fraction of DDAO, is smaller than the value of monomeric DDAO, viz., 4.95. This is in line with the micellar pKi data on cation acids which have been obtained spectroscopically (6, 8).

pHi.e.

REFERENCES

-50

jr"

pKi

5. MUKERJEE, P., AND BANERJEE, K., Y.

Phys. Chem.

68, 3567 (1964). 6. FUNASAKI,N., Nippon Kagaku Kaishi, 722 (1976). 7. FUNASAKI,N., J.

ColloidlnterfaceSci. 60, 54 (1977).

8. FUNASAKI,N., J. Colloid Interface Sci., in press. 9. ToKIWA, F., AI~BOHI% K., J. Phys. Chem. 70, 3437 (1966). 10. STEINHARDT, J., AZ~D REYNOLD, ~. A., "Multiple Equilibria in Proteins," p. 185. Academic Press, New York, 1969. 11. TANFORD,C., f . A mer. Chem. Soc. 79, 5340 (1957). NORIAKI F U N A S A K I

Department of Physical Chemistry, Kyoto College of Pharmacy, Kyoto 607, Japan

October 15,

Received December30,1976; acceptedFebruary 28, 1977

1977