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

Abstract

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-trimethylbenzenediazonium (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.