Synergism in Cationic Surfactant Mixtures and Counterion Mixing in Adsorbed Films and Micelles Studied by Surface Tension Measurement

Abstract

The surface tension measurements revealed that an appropriate coincidence in the difference in the hydrocarbon chain lengths between two surfactants with the hydrated hydrophilic group diameter caused the synergetic adsorption and micelle formation in the aqueous solutions of the mixture of hexadecyltrimethylamonium bromide (HTAB) - dodecyltrimethylammonium bromide (DTAB). This is explained by the theory of the staggered structure formation driven by the strong dispersion interaction between the alkyl chains of different surfactants. Furthermore, it was found that the synergism was considerably strengthened in the mixed system without common ions; hexadecyltrimethylammonium chloride (HTAC) - dodecyltrimethyammonium bromide (DTAB) system by the preferential adsorption of bromide ions over chloride ions onto the adsorbed film and micelle. A useful analytical procedure was developed to evaluate the composition of individual ions in the adsorbed film and micelle and applied to the HTAC-DTAB system.