Phase Transition in the Adsorbed Layer of Catanionic Surfactants at the Air/Solution Interface

Abstract

Dynamic and equilibrium surface tension measurements were performed on aqueous solutions of three symmetric chain catanionic surfactants (dodecyltrimethylammomium dodecyl sulfate, DTA·DS, decyltrimethylammonium decyl sulfate, DeTA·DeS, and octyltrimethylammonium octyl sulfate, OTA·OS). The surface tension of DTA·DS and DeTA·DeS solutions shows a peculiar time dependence. Depending on the surfactant concentration, there is a latent period while the surface tension is almost constant followed by a pronounced decrease toward the equilibrium value. The equilibrium surface tension against the logarithm of the surfactant concentration can be characterized by two linear regions. The surface tension of OTA·OS solutions changes regularly with both the time and concentration as in the case of ordinary surfactant solutions. The dynamic and equilibrium surface tension data were interpreted in terms of a two-dimensional gas/condensed phase transition. A molecular interaction model was developed to describe the adsorption of the catanionic surfactants. In agreement with the experimental observations, the model predicts phase transition in the adsorbed layer of the DTA·DS and DeTA·DeS.