VISCOELASTICITY OF MONOLAYERS AT OIL-WATER INTERFACES

We have measured the low frequency dilational viscoelasticity for monolayers of a nonionic (Triton X100) and an ionic surfactant (SDS) at the interface between water and dodecane. We have compared our results with a model in which it is assumed that exchanges between surface and bulk are diffusion controlled, the surfactant undergoing simple Fickian diffusion. In the case of the nonionic surfactant, or of the ionic surfactant in the presence of salt, a very good agreement was found with the model. On the other hand, large discrepancies are observed for the ionic surfactant in the absence of salt. These results are also inconsistent with the presence of adsorption-desorption barriers. Similar results are reported for the dynamic surface tension of these systems: agreement with diffusion controlled absorption, except for the ionic surfactant in the absence of salt. The surface charge plays therefore a very important role in the dynamic surface processes, possibly modifying the diffusion processes or the intrinsic properties of the monolayers. This role remains to be elucidated.