The behaviour of surfactants in concentrated acids part II. Formation of micelles from cationic surfactants in concentrated sulphuric acid

Abstract The process of the alkylation of olefins with isoalkanes catalysed by sulphuric acid to form high-grade alkylate benzines can be positively influenced in several ways by cationic surfactants. It is still unclear whether the surfactants intervene in the course of the process at the sulphuric acid phase by means of micelle catalysis or by adsorption at the phase boundary. For this reason studies of the aggregation and interface behaviour of cationic surfactants in concentrated sulphuric acid are very important. The surface tension/concentration dependency of n-alkylammonium, n-alkyltrimethylammonium and n-alkylpyridinium salts and of 4-(n-alkylammonium) butane-1-sulphonates was determined by du Nouy's ring method. The evaluation of the measurement data took place by means of approximation to the Von Szyskowski equation. The maximum deviation amounted to ⩽ 1.5%. The c.m.c. of the homologous series in relation to the chain length were determined. They showed a good correlation with the Klevens equation. The change in free energy on the formation of micelles Δ M G was calculated. Compared with the situation in aqueous systems, the chemical surfactant properties of cationics are less pronounced in concentrated sulphuric acid as a solvent. The surface tension is less strongly decreased, the c.m.c. occur at much higher concentrations and the energy obtained from micelle formation is considerably less.