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

Chemical trapping of bromide ions in reverse micelles prepared with hexadecyltrimethylammonium bromide, CTAB, in n -dodecane/CHCl 3 and isooctane/ n -hexanol has been obtained for 2,4,6-trimethylbenzenediazonium (1-ArN 2 + ) and 2,4-dimethyl-4-hexadecylbenzenediazonium (16-ArN 2 + ) tetrafluoroborates. Quantitative analysis of the reaction products of 1-ArN 2 + and 16-ArN 2 + 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/CHCl 3 and isooctane/ n -hexanol. The determination of [Br] b in reverse micelles by chemical trapping with 16-ArN 2 + can be obtained after correction for probe distribution between the reverse micelle and the organic solvent, especially in the case of n -dodecane/CHCl 3 . This correction was possible after demonstrating that 16-ArN 2 +, upon dediazoniation in wet n -dodecane/CHCl 3, 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-ArN 2 + 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/CHCl 3 reaches a value of ca. 0.2 above W/S 15.