A parameter predict ing the foam stability of mixtures of aqueous ionic amphiphile solutions with indifferent electrolyte

Abstract

Sodium dodecyl sulfate (SDS) is a well-defined model substance for anionic molecular surfactants. Aqueous mixtures of SDS in a surface chemically purified state and high purity NaCl were studied with respect to their equilibrium and dynamic surface tensions at a constant mean ionic activity Imean of 1mmolL−1. All studied solutions possessed the same equilibrium surface tension, however they showed striking differences in foam column and foam lamella stability experiments. This observation suggested the interpretation of the results in terms of dynamic interfacial properties. Oscillating bubble experiments were conducted to determine the frequency dependence of the surface dilatational modulus in a range from 5 to 400Hz. The obtained surface dilatational modulus could be described by the extended Lucassen-van den Tempel (LvdT) model, which allows capturing dissipative processes by the intrinsic surface viscosity κ within the interfacial region. A new parameter derived from the fitting parameters of the LvdT model correlates reasonably with direct measurements of the solutions’ foam stability. The data suggest the important role of surface rheology for the stability of foams.