Phase transition in the adsorbed layer of catanionic surfactants at the air/solution interface

Dynamic and equilibrium surface tension measurements were performed on aque ous solutions of three symmetric chain catanionic surfactants (dodecyltrime thylammomium dodecyl sulfate, DTA.DS, decyltrimethylammonium decyl sulfate, DeTA.DeS, and octyltrimethylammonium octyl sulfate, OTA.OS). The surface t ension of DTA.DS and DeTA.DeS solutions shows a peculiar time dependence. D epending on the surfactant concentration, there is a latent period while th e surface tension is almost constant followed by a pronounced decrease towa rd the equilibrium value. The equilibrium surface tension against the logar ithm of the surfactant concentration can be characterized by two linear reg ions. The surface tension of OTA.OS solutions changes regularly with both t he 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 interactio n model was developed to describe the adsorption of the catanionic surfacta nts. In agreement with the experimental observations, the model predicts ph ase transition in the adsorbed layer of the DTA.DS and DeTA.DeS.