ELECTROSTATIC MODEL FOR THE VISCOELASTICITY OF IONIC SURFACTANT MONOLAYERS

In a previous paper (Langmuir 1993, 9, 8), we have reported the measurement of low-frequency dilational viscoelasticity for monolayers of an ionic surfactant (sodium dodecyl sulfate (SDS)) at the interface between water and dodecane. We have compared the results with a model in which it is assumed that the exchanges between the surface and bulk are diffusion controlled, the surfactant undergoing simple Fickian diffusion. In the presence of added salt, very good agreement was found with the model, whereas large discrepancies arised in the absence of salt. In this paper, we present similar experimental data with another ionic surfactant (sodium bis(ethylhexyl) sulfosuccinate (AOT)) and we propose a new model in which we take into account the role of the electric field created by the charged monolayer on the diffusion process. This new model leads to very good agreement with the experiments on SDS and AOT monolayers, when assuming that the surfactant molecules at the oil-water interface are fully ionized. These results demonstrate that no charge condensation occurs in these monolayers in the absence of salt and that the surface charge plays a very important role in surface theology.