Formation of Micelles of Cetyltrimethylammonium Bromide in Water-Dimethyl Sulfoxide Solutions

Abstract

The critical micellar concentration of aqueous solutions of cetyltrimethylammonium bromide (CTAB) containing different amounts of dimethyl sulfoxide (DMSO) has been determined at 25° and 40° C by means of surface tensiometry. In general, the experimental results indicate that micelle formation is hindered by a raise in temperature. In addition, DMSO was found to have an inhibitory effect on the formation of micelles of CTAB. This effect is relatively small at low cosolvent concentrations, but it increases dramatically as the mole fraction of DMS0 ap-proaches 0.33. This mole fraction corresponds to the formation of the stoichiometric complex DMSO.2H20. At mole fractions of DMSO higher than 0.33 the formation of CTAB micelles does not take place. Although the free energy values determined for the process of micellization in the mixed solvent system are comparable to those determined for pure water, they increase linearly with the mole fraction of DMSO. The values determined for ∆S m o indicate that an increase in the ordering of the surfactantwater-cosolvent system takes place as the mole fraction of DMSO is increased. This is consistent with a strong interaction, such as hydrogen bonding, between water and DMSO. The inhibitory effect on micelle formation can be explained in terms of a decrease of hydrophobic forces in the ternary system due to interactions between water and DMSO.