Trapping of counterions and water on the surface of cationic micelles

The concentration ratios of three nucleophiles, bromide and chloride ions and water, at the micellar surface were determined from the yields of trans-1,2-dibromocyclohexane, trans-1-bromo-2-chlorocyclohexane, and trans-2-bromocyclohexanol, respectively, products of the bromination of cyclohexene in aqueous (i) cetyltrimethylammonium bromide (CTAB), (ii) cetylpyridinium bromide (CPyB), and (iii) cetylpyridinium chloride (CPyC), with added NaX (X = Br-, Cl-). The generally small values of [X(-)]/[H2O] obtained are attributed to a highly ordered interfacial structure in which the halide ions occupy well defined positions. A sphere to rod transition caused by the addition of NaX for all the surfactants was followed by the variation of the ratio [X(-)]/[H2O] and by H-1 NMR.