Determination of halide concentrations at the interface of zwitterionic micelles by chemical trapping: Influence of the orientation of the dipole andthe nature of the cation

The interfacial concentrations of Cl- and Br- in aqueous zwitterionic micel les were determined by chemical trapping by analyzing product yields from s pontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzened iazonium ion. Interfacial concentrations of Cl- and Br- in 3-(N-hexadecyl-N , N-dimethylammonio) propane sulfonate, HPS, micelles were higher than in b ulk solutions prepared with Li+, Na+, Rb+, Cs+, tetramethylammonium (TMA(+) ), Mg+2, and Ca+2 salts. In contrast, the interfacial concentrations of Cl- and Br- were generally lower than in bulk solution in hexadecylphosphoryl choline, HDPC, micelles for all salts except Mg+2 and Ca+2. In both HPS and HDPC micelles the interfacial concentration of Br- was higher than that of Cl-, showing that binding is anion selective. The cation had a large effec t on the interfacial concentration of halide ions with HDPC micelles decrea sing in the order Ca2+ > Mg2+ much greater than Li+ > Na+ > K+ > Cs+ > Rbmuch greater than TMA(+), These results are the first direct and extensive determination of local halide ion concentration at the surface of zwitterio nic micelles, and they demonstrate that chemical trapping methodology will work in membranes at physiologically relevant salt concentrations. (C) 1999 Academic Press.