On the Interaction of Insoluble Alkyl Alcohol Monolayers with Aqueous Solutions of Ionic Surfactants

The interaction between so-called alkyl alcohol (Cs-Cs) monolayers and aqueous solutions of ionic surfactants with different hydrophilic properties, i.e. calcium di-2-ethylhexyl sulfosuccinate and sodium dodecyl sulfate, has been studied with the help of a Langmuir-Adam film balance. A decrease or increase in surface pressure of the alkyl alcohol monolayers with the addition of surfactants is observed. The shift of the critical concentration (cmc) to increasing surfactant concentration occurs at intermediate surface pressures. This is interpreted as due to the formation of mixed alcohol-surfactant aggregates. The maximum of the cmc as a function of the pressure can be explained by a thermodynamic treatment assuming competition between a pressure dependent solubility of alcohol molecules and the formation of mixed alcohol surfactant complexes. The amount of alkyl alcohol required to react with the surfactant has been evaluated at different surface pressures, which shows values similar to those known from cosurfactant studies in the bulk phase. Critical micelle concentrations of the surfactants were verified by conductivity measurements.