The behaviour of surfactants in concentrated acids 6. Micellization and interfacial behaviour of n-hexadecyltrimethylammonium bromide in mixtures of methane sulphonic acid and concentrated sulphuric acid

The surface and interfacial activity of cationic surfactants depends on the polarity of the bulk phase. If concentrated sulphuric acid is mixed with water, physical properties such as density, and surface and interfacial tension go through a maximum as the molar composition is changed. Also the properties of surfactants, i.e. surface activity and CMC, do not vary linearly in such aqueous mixtures. However, if concentrated sulphuric acid is mixed with methane sulphonic acid we find a linear relationship.

The adsorption isotherms of n-hexadecyltrimethylammonium bromide at the surface (interface) of mixtures of H₂SO₄ and CH₃SO₃H/air (n-hexane) and the CMC were determined for various compositions of the acid phase and approximated by the Von Szyskowski equation. The change in the energy of micelle formation, — Δ_MG, and the energy of adsorption, — Δ_AG°, were calculated as a function of solvent composition. The surface concentration as a function of bulk concentration, the maximum surface coverage and the percentage surface concentration were also calculated.

We found a linear dependence of the CMC, and of the lowering of σ and γ at the CMC, on the mole fraction of the acid phase. Also the constant A of the Von Szyskowski equation decreases and the logarithm of B increases linearly with the mole fraction of methane sulphonic acid. Therefore it is possible to calculate all the data needed for the characterization of systems of different acid composition. A special pattern was found for acid mixtures with X_CH3SO3H < 0.1. This can be explained by the transformation of micelle structure and the possibility that micelles can solubilize hydrocarbons in this region.