Alkane vapor and surfactants co-adsorption on aqueous solution interfaces

The co-adsorption of hexane from the vapor phase and surfactants from an aqueous solution at the surface of drops are studied in dependence of surfactant concentration. For the anionic surfactant SDS as well as the cationic surfactant DoTAB a significant surfactant–hexane co-adsorption and a remarkable decrease in surface tension is observed. This hexane co-adsorption effect increases with surfactant concentration and reaches a maximum value at intermediate surfactant concentrations below the CMC values. Dynamic surface tension measurements by drop profile analysis tensiometry and calculation of the standard deviation from Laplacian shape fitting, demonstrate three different adsorption regions. The kinetics and equilibrium state of adsorption depend significantly on the surfactant concentration. For the initial effects (regular adsorption region) which appear quickly, a thermodynamic model is applied which is based on a mixed adsorption layer approach.