The behaviour of surfactants in concentrated acids part IV. Investigation of the synergism of binary cationic/cationic mixtures in concentrated sulphuric acid

Abstract

The dependence of the c.m.c. and of the surface tension depression on the mole fraction of surfactants was determined for all combinations of even-numbered n-alkylammonium hydrogen-sulphates in concentrated sulphuric acid. With the help of the Rubingh formula we calculated the molecular interaction parameter β, the mole fraction of one component in the micelle x M, the mole fraction of one component at the point of maximum synergism x M max, and the c.m.c. of the binary mixtures of surfactants at maximum synergism c.m.c. 12,min. A relative enrichment of the surfactant with the longer chain length takes place in the micelle compared with the bulk phase when the mole fraction of the surfactant with the longer chain length decreases in the bulk phase or with an identical molecular composition, when the chain length difference increases. The conditions for the occurrence of synergism are only met when the C number of two homologues differs by a maximum of two. Otherwise the concentration of the homologue with the shorter alkyl group in the mixed micelle is too low. Attractive interactions can only be explained if the partial deprotonation of the ammonium head group is assumed. Accordingly, no synergistic effect was found, either with n-alkyltrimethylammonium salts or with n-alkylpyridinium salts.