DETERMINATION OF INTERFACIAL CO-ION CONCENTRATION IN IONIC MICELLES BY CHEMICAL TRAPPING - HALIDE CONCENTRATION AT THE INTERFACE OF SODIUM DODECYL-SULFATE MICELLES

Products from the spontaneous reaction of a long-chain arenediazonium salt, 2,6-dimethyl-4-hexadecylbenzenediazonium tetrafluoroborate(16-Ar N2BF4), in aqueous micellar solutions of sodium dodecyl sulfate (SDS)? are used to estimate the local concentration of chloride and bromide ions at the micellar surface. The arenediazonium ion, 16-ArN2+, which is totally bound to the SDS micelle, reacts by rate-determining loss o f N-2 to give an aryl cation that traps available nucleophiles, i,e., H2O, Cl-, and Br-, to give stable phenol, 16-ArOH, and halobenzene pro ducts, 16-ArCl and 16-ArBr, respectively. Product yields, determined b y HPLC, are related to local concentrations using calibration curves o btained from independent standards. The local concentrations determine d by this method are consistent with co-ion concentrations calculated, using a cell model, by numerical integration of the Poisson-Boltzmann equation (PBE) taking into account salt-induced micellar growth. The salt dependence of the intel facial concentrations of Cl- and Br- are identical. indicating no specific interactions in the interfacial co-i on compartment. PBE calculations predict that, in micellar SDS, increa sing the concentration of a particular halide salt (NaX) at constant c oncentration of another halide (NaY) should result in an increase in t he local concentrations of both co-ions. Using this chemical-trapping method, this prediction was demonstrated experimentally.