Effect of intermolecular interactions on the formation of micelles. Proton spin–lattice relaxation study in water + dimethyl sulphoxide solutions of hexadecyltrimethylammonium bromide

Abstract

Effects of intermolecular interactions on the formation of micelles and molecular motions of surfactant molecules were investigated by measuring proton spin–lattice relaxation rates for the methyl, N-methyl, and methylene groups of hexadecyltrimethylammonium bromide (HTAB) in water and water + dimethyl sulphoxide (DMSO) mixtures at concentrations below and above the critical micellar concentration. The increased “structuring” in the water + DMSO liquid system disturbs the formation of micelles by overcoming “the hydrophobic effect” of the long alkyl chain of HTAB which is the main driving force to form micelles. The generalized picture that emerges for the HTAB micelles consists of three regions: (1) the head groups consisting of N-methyl groups and the corresponding counter ions where the mobility of head groups is significantly restricted, (2) a quite mobile region consisting of the methylene groups and (3) the centre that contains the terminal methyl groups in a rather small region.