Estimation of the “hydrophobic reactivity” of SDS micelles by the use of BPh4− anions

Abstract

Above the cmc, aqueous solution of sodium dodecyl sulfate SDS is an ionic mixture constituted of micelles of apparent charge number Zap, free counterions Na+ and free monomers SD-. The degree of micellization and the degree of condensation of Na+ on the micelles are calculated according to the mass-action model and by the generalization of the Fuoss ionic association theory.In the first part of this work, conductance data of this mixture are analyzed within the framework of the Onsager–Kim–MSA conductivity theory of mixed electrolytes and that of the Zwanzig dielectric friction theory. It appears that the micellization process is a progressive continuous “micro-phase transition” highly correlated to the degree of condensation of counterions. In the second part, we show how (in contrast with spectroscopic techniques) the high precision conductivity technique, combined to the theories mentioned above, can detect the absorption of the hydrophobic BPh4- anions by SDS micelles in dilute aqueous solutions: Because of the affinity of the BPh4- ions to the nonpolar side chains inside SDS, each micelle incorporates nBPh4–anions (hydrophobic reactivity). This number is determined by adjusting theoretical specific conductance to its corresponding experimental value. Results show that Na+ condensation facilitates this absorption and that “nBPh4–” increases with dilution.