Langmuir 1996,11, 2393-2398
2393
Kinetic Evidence for Temperature-Induced Demixing of a Long Chain Dioxolane in Aqueous Micellar Solutions of Sodium Dodecyl Sulfate: A New Application of the Pseudophase Ion Exchange Model k g e l o Adolfo Ruzza, Faruk Nome, and Din0 Zanette" Departamento de Quimica, Universidade Federal de Santa Catarina, 88040-900, Florianbpolis, Santa Catarina, Brasil
Laurence S. Romsted Department of Chemistry, Wright and Rieman Laboratories, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received October 17, 1994@ The effect of anionic micelles of sodium dodecyl sulfate (SDS)on the rates of acid-catalyzed hydrolyses of 2-@-alkoxyphenyl)-l,3-dioxolaneswith alkoxy groups of different chain lengths, methoxy @-MPD), nonoxy (p-NPD),and tetradecoxy (p-TPD),were determined as a function of SDS and NaCl concentrations and temperature. First-order rate constants, kobs, were obtained from plots ofln(A, -A) vs time that were linear for at least three half-lives. The k,b,-[SDS] profiles were fitted by using the pseudophase ionexchange (PPIE) model and substrate binding constants, K,, and second-orderrate constants for reaction in the micellar pseudophase, k2m, were estimated from the simulations. Values of& and kzm forp-MPD andp-NPD are completely consistent with micellar effects on other acid-catalyzed hydrolyses in SDS and the assumptions of the PPIE model; i.e. hydrolysis of monomeric substrate occurs in either the aqueous or the micellar pseudophases. However, kinetic profiles for p-TPD in SDS are more consistent with this substrate acting as a nonionic surfactant that does not mix ideally with SDS. The K8forp-TPD decreases about 800-fold as the temperature is increased from 25 to 50 "C.In 0.012 M SDS, kobs forp-TPD passes through a sharp maximum at 32 "C with increasing temperature whereas the plot for p-NPD is linear. At constant SDS concentration, added NaCl initially speeds the hydrolysis ofp-TPD instead of inhibiting the reaction as observed forp-NPD and salt effects on other micellar-catalyzedreactions. All these results can be interpreted by assuming that increasing the temperature induces demixing ofp-TPD and SDS t o give two populations of mixed micelles each enriched in one of the surfactants and in dynamic equilibrium. Thus the PPIE model can be used to identifynonrandom distributionsof reactive and nonreactivesurfactants in aggregated systems.
Introduction Surfactants or detergents spontaneously aggregate above a certain concentration called the critical micelle concentration or cmc to form mice1les.l Micelles act as microreactors which both speed and inhibit the rates of a wide variety of uni- and bimolecular reactions and shift the equilibrium constants of many indicators.2-6 Aggregate effects on chemical reactivity are generally interpreted by using pseudophase models which treat micelles and water as separate reaction media, i.e. as separate phases or pseudophases. This approach has been successfully applied to a wide range of chemical reactions in micellar solution^,^-^ including mixed micelle^,^,^ and in other types association colloids, such as microemul@Abstractpublished in Advance ACS Abstracts, May 1, 1995. (1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (2) Romsted,L. S. InSurfactants inSolution;Mittal, K. L.,Lindman, B., Ed.; Plenum Press: New York, 1984; Vol. 2, p 1015. (3) Chaimovich, H.; Aleixo, F. M. V.; Cuccovia, I. M.; Zanette, D.; Quina,F. H. InSolutionBehuuiorofSurfactants: Theoretical andApplied Aspects; Mittal, K. L., Fendler, E. J., Ed.; Plenum Press: New York, 1982: Vol. D 949. - 2. c - -(4j Bunton, C. A.; Nome, F.; Quina, F. H.;Romsted, L. S. Acc. Chem. Res. 1991,24, 357. ( 5 ) Bunton, C. A. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M.,Kalyanasundaram,K., Ed.;Marcel Dekker: New York, 1991. (6) Bunton, C. A.; Savelli, G. Adu. Phys. Org. Chem. 1986,22, 213. (7) Wright, S.;Bunton, C. A.; Holland, P. M. In Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Ed.; American Chemical Society: Washington, DC, 1992; Vol. 501, p 227. (8)Bunton, C. A.; Wright, S.; Holland, P. M.; Nome, F. Langmuir 1993, 9, 117. ~
~~
-7
sionsgJOand v e s i c l e ~ . ~ J Surfactant ~J~ aggregates affect chemical reactivity primarily by binding or excluding reactants and only secondarily by changing the free energy of activation. For example, anionic micelles speed bimolecular reactions of neutral organic substrates with cations such as H+ by compartmentalizing them within the micellar pseudophase, but inhibit bimolecular reactions with anions such as OH- by binding the organic substrate and excluding the anion. Estimated values of secondorder rate constants within surfactant aggregates for many types of bimolecular reactions are often similar in value to the second-order rate constants for the same reaction in water, consistent with the generally held assumption that reactions in micelles occur a t the water rich surface and not in the hydrocarbon core. The major problem in modeling chemical reactivity in many surfactant systems has been to describe the effect of surfactant and counterion concentration and counterion type on the distribution ofionic reactants. Several models have been developed. In the Poisson-Boltzmann equation (PBE) model, electrostatic interactions are assumed to attract counterions to the micellar surface but repel coions, and ion distributions are estimated by solving the (9)Bertoncini, C. R. A.; Nome, F.; Cerichelli, G.; Bunton, C. A. J. Phys. Chem. 1990,94, 5875. (10) Da Rocha Pereira, R.; Zanette, D.; Nome, F. J. Phys. Chem. 1990, 94, 356. (11) Kawamuro, M. K.; Chaimovich, H.; Abuin, E. B.; Lissi, E. A.; Cuccovia, I. M. J. Phys. Chem. 1991, 95, 1458. (12) Chaimovich, H.; Bonilha, J. B. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Ed.: Plenum Press: New York, 1984: Vol. 2,p 1121.
0743-746319512411-2393$09.00/0 0 1995 American Chemical Society
Ruzza et al.
2394 Langmuir, Vol. 11, No. 7, 1995 Scheme 1
IO
be Poisson-Boltzmann equation in the appropriate symmetry.13 In the pseudophase ion exchange (PPIE)model, the surfaces of ionic micelles are viewed as ion exchangers and competition between reactive and inert counterions is characterized by a n empirical ion exchange constant.2 This approach works well a t low to moderate concentrations of surfactant and added salt (
