Analysis of the Bromide Ion Distribution in the Water Pool of Reverse Micelles of Hexadecyltrimethylammonium Bromide in Chloroform/n-Dodecane and Isooctane/ n-Hexanol by Chemical Trapping Iolanda M. Cuccovia,* Luı ´s G. Dias, Fla ´ vio A. Maximiano, and Hernan Chaimovich Departamento de Bioquı ´mica, Instituto de Quı ´mica, Universidade de Sa ˜ o Paulo, CEP 05508-900, Sa ˜ o Paulo, SP, Brasil Received September 8, 2000. In Final Form: November 21, 2000 Chemical trapping of bromide ions in reverse micelles prepared with hexadecyltrimethylammonium bromide, CTAB, in n-dodecane/CHCl3 and isooctane/n-hexanol has been obtained for 2,4,6-trimethylben- zenediazonium (1-ArN2 + ) and 2,4-dimethyl-4-hexadecylbenzenediazonium (16-ArN2 + ) tetrafluoroborates. Quantitative analysis of the reaction products of 1-ArN2 + and 16-ArN2 + with water and bromide ion, the corresponding phenol and bromo derivatives, and comparison with appropriate standard curves yielded the local concentrations of Br - in the water pool, [Br]f, and micellar interface, [Br]b, in reverse CTAB micelles prepared in n-dodecane/CHCl3 and isooctane/n-hexanol. The determination of [Br]b in reverse micelles by chemical trapping with 16-ArN2 + can be obtained after correction for probe distribution between the reverse micelle and the organic solvent, especially in the case of n-dodecane/CHCl3. This correction was possible after demonstrating that 16-ArN2 + , upon dediazoniation in wet n-dodecane/CHCl3, yields exclusively the corresponding bromo derivative. A Poisson-Boltzmann (PB) equation above a water/ detergent molar ratio, W/S, of 14 appropriately describes the values of [Br]f. Comparison of the experimental values of [Br]b with those predicted by PB with changing W/S suggest that 16-ArN2 + extends from the interface 0.6-1.2 nm with increasing W/S. Both PB calculations and experimental data indicate that the degree of counterion dissociation from CTAB reverse micelles in n-dodecane/CHCl3 reaches a value of ca. 0.2 above W/S 15. Introduction The local concentrations of ions and reactive species in interfaces control a variety of chemical transformations in interfaces. In particular, essential biological properties, such as energy biotransformation and excitability, are determined by the local ion concentration at relevant proteolipid sites. 1 Experimental determinations of local ion concentrations in biological membranes are trouble- some and complicated by their compositional complexity. 1 Micelles and vesicles are convenient and simpler models used to understand selected properties of biological membranes. 2 Even in these models the determination of local interfacial ion concentrations is particularly difficult due, in part, to the lack of positional definition of the ions in the surface and the selection of the volume elements used in the calculations. Interfacial ion concentrations in charged micelles rang- ing from 1.5 to 4.0 M can be calculated by assuming a spherical form and using independently determined parameters such as the total micellar volume, radius, aggregation number, and ion dissociation degrees. 3 A major problem in this approach is the definition of the volume elements used for concentration calculations. 4 Poisson-Boltzmann equations, PBE, have also been used to calculate the local counterions and co-ions at several distances from the surface, taking into account the same physical parameters used in the geometrical calcula- tions. 4,5 Local ion concentrations have also been deduced from kinetic experiments. 4 When a bimolecular reaction is studied in micelles, it is possible to estimate the local concentration of reactive ions by using the rate constants in the micellar pseudophase as the only adjustable parameter. Again in this case the reactive volume has to be chosen. 5,6 Estimation of reaction volume elements has been attempted by incorporation of probes at the micellar surface, using molecules bearing a charge opposite to that of the micelle and changing the distance between the fixed center at the surface of micelle and the reaction center in the probe, thus mapping local ion concentrations. 7 A similar approach was used in the study of ion adsorption in zwitterionic surfactants with a variable distance between the charged groups. 8 Recently Romsted and co-workers 9-16 introduced a method that directly determines local composition of interfaces without recourse to a definition of the reactive * Address correspondence to this author at Instituto de Quı ´mica, Universidade de Sa ˜ o Paulo, Av. Prof. Lineu Prestes 748, Sa ˜ o Paulo SP, Brasil, CEP 05508-900. E-mail imcuccov@quim.iq.usp.br; phone 55 11 3818-3810, ext 228; fax 55-11-3818-2186. (1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (2) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Inter- science: New York, 1982. (3) (a) Romsted, L. S. Thesis, Indiana University, 1975. (b) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 509. (4) Bunton, C. A.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (5) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 7851. (6) Blasko, A.; Bunton, C. A.; Armstrong, C.; Gotham, W.; He, Z.-M.; Nickes, J.; Romsted, L. S. J. Phys. Chem. 1991, 95, 6747. (7) Zanette, D.; Chaimovich, H. J. Phys. Org. Chem. 1992, 5, 341. (8) Kamenka, N.; Chorro, M.; Chevalier, Y.; Levy, H.; Zana, R. Langmuir 1995, 11, 4234. (9) Loughlin, J. A.; Romsted, L. S. Colloids Surf. 1990, 48, 123. 1060 Langmuir 2001, 17, 1060-1068 10.1021/la001291s CCC: $20.00 © 2001 American Chemical Society Published on Web 01/27/2001