782 Langmuir 1992,8, 702-707 Articles Electrochemical Studies Associated with Micellization of Cationic Surfactants in Ethylene Glycol H. Gharibi, R. Palepu, D. M. Bloor, D. G. Hall, and E. Wyn-Jones* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4 WT, U.K. Received August 14, 1991. I n Final Form: November 12, 1991 Membrane electrodes selective to a series of alkyltrimethylammonium bromides and also alkylpyri- dinium bromides have been constructed and used to investigate the aggregation properties of the sur- factants in ethyleneglycol. During these electrochemical measurementsthe emf of the surfactant selective electrode was measured relative to a sodium electrode and also a bromide ion selective electrode. As a result of these measurements it is possible in principle to evaluate the monomer surfactant and counterion concentrations. The data for all the surfactants reveal nonideality which is interpreted as a critical mi- cellar concentration whose value increasesas the chain length of the surfactant decreases. At concentrations immediately below this critical micellar concentration nonideality is interpreted as the existence of pre- micellar aggregation. The data are analyzed to yield the effective degree of dissociation of the micelles and also equilibrium and thermodynamic information concerning aggregation in the surfactants. Introduction Surfactants are widely used in both industry and everyday life and the properties of aqueous solutions of surfactants have received considerable attention.' As a result of extensive experimental studies over a long period of time and covering a wide variety of different experi- mental techniques, the relationship between surfactant chemical structure and both micellar and lyotropic liquid crystalline structures is fairly well understood for aqueous systems, and their practical applications, which require a knowledgeand control of their aqueous solution properties, have enhanced cor~siderably.'-~ In many applications of surfactant the fluid medium is a polar nonaqueous solvent rather than water and at present these systems are not as well understood as those in aqueous solutions. Studies on nonaqueous polar solvents are extremely important in the sense that they underpin current and anticipated devel- opment in the technological use of these system^.^^^ In addition a t the molecular level a range of nonaqueous polar solvents can be used which gives the experimentalist the opportunity to gain information on the molecular mech- anisms of the so-called solvophobic effect. Although sur- factant association structures in nonaqueous polar solvents have been known for some time, there are still many controversial issues which remain unsolved. There is currently much interest in the solution properties of sur- factants in these nonaqueous polar solvents. In particular there are certain controversies associated with the ex- perimental data on these systems especially related to estimating critical micellar concentrations.'-20 One of the (1) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and (2) Wyn-Jones, E.; Gormally, J. Aggregation Processes in Solution; (3) Tiddy, G. J. T. Modern Trends of Colloid Science in Chemistry (4) Bloor, D. M.; Wyn-Jones, E. The Structure, Dynamics and Equi- (5) Friberg, S. E.; Liang, Y. C. Microemulsions; CRC Press: Cleveland, (6) Evans, D. E. Langmuir 1988, 4, 3. Hall: London, 1983. Elsevier: Amsterdam; 1983; Chapters 2, 3, and 7. and Biology, Berkhauer Verlag, 1985; Chapter 7. librium Properties of Colloidal Systems; Kluwer: Dordrecht, 1990. OH, 1988 Chapter 3. key issues in this area is the design of experimental methods which can lead to unambiguous conclusions. In connection with the understanding of aggregation behavior, one of the fundamental measurements which can lead to a better understanding of these systems is the measurement of surfactant monomer concentration as a function of total concentration. In previous studies of the aggregation behavior of aqueous solutions of a series of trimethylammonium bromides21 and also alkylpyridin- ium bromides,21p22 we constructed surfactant-selective electrodes in order to measure monomer concentration as a function of total surfactant concentration. In all cases these electrodes displayed almost ideal Nernstian behavior and could be used in a routine fashion to determine the concentration of monomer surfactants in these systems. As a result of the interest shown in the aggregation behavior of these surfactants in nonaqueous polar solvents and in particular ethylene glycol, we decided to investigate whether or not these electrodes could operate in this (7) Backlund, S.; Bergenstah, B.; Molander, 0.; Warnheim, T. Colloid (8) Biname-Linbe, W.; Zana, R. Colloid Polym. Sci. 1989, 267, 440. (9) Ray, A. Nature 1971, 231, 313. (10) Ray, A. J. J. Am. Chem. SOC. 1969, 91, 6511. (11) Singh, H. N.; Salem, S. M.; Singh, R. P.; Birdi, K. S. J. Phys. 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Soc., Faraday Trans. 1990, 86, 1535. 0743-7463/92/240~-0782$03.00/0 0 1992 American Chemical Society