Determination of Interfacial Co-ion Concentration in Ionic Micelles by Chemical Trapping: Halide Concentration at the Interface of Sodium Dodecyl Sulfate Micelles

Abstract

Products from the spontaneous reaction of a long-chain arenediazonium salt, 2,6-dimethyl-4-hexadecylbenzenediazonium tetrafluoroborate (16-ArN2BF4), 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 of N2 to give an aryl cation that traps available nucleophiles, i.e., H2O, Cl-, and Br-, to give stable phenol, 16-ArOH, and halobenzene products, 16-ArCl and 16-ArBr, respectively. Product yields, determined by HPLC, are related to local concentrations using calibration curves obtained from independent standards. The local concentrations determined 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 interfacial concentrations of Cl- and Br- are identical, indicating no specific interactions in the interfacial co-ion compartment. PBE calculations predict that, in micellar SDS, increasing the concentration of a particular halide salt (NaX) at constant concentration of another halide (NaY) should result in an increase in the local concentrations of both co-ions. Using this chemical-trapping method, this prediction was demonstrated experimentally.