Surface Rheology of Surfactant Solutions Close to Equilibrium

Abstract

In this paper we present surface rheological measurements of various surfactant solutions close to equilibrium in a Langmuir trough. We find that the storage modulus is, in the systems investigated, higher than the loss modulus. The rheological behavior depends strongly on the surfactant concentration, even at concentrations exceeding the CMC. Films with quite different surface rheological properties are found to show similar drainage rates. This supports earlier work (Baets and Stein, Langmuir 8, 3099 (1992)), in which the velocity of film drainage was found to be determined by bulk viscosity effects. A number of possible explanations for the cause of the surface rheological effects found in our solutions are examined. The rheological effects at concentrations exceeding the CMC can best be ascribed to two-dimensional ordering of surfactant molecules at the surface combined with interaction of these molecules with micelles in the nearby solution.