The influence of the sublayer on the surface dilatational modulus

The frequency dependence of the surface dilatational modulus is valuable for an assessment of surface models of aqueous surfactant solutions. Precise data of the complex surface dilatational modulus can be obtained with an oscillating bubble device. The technique covers a broad frequency range (3-500 Hz) and has been used to study several surfactant systems. So far, the surface rheology has been mainly described by the well-established Lucassen-van den Tempel-Hansen model (LvdTH model). This model can describe the dynamic surface behavior of many surfactant solutions, but a few puzzles remain. First, the measured data do not match the corresponding model fit, in particular near the CMC or the limit of solubility. Moreover, the frequency dependence of the modulus of some surfactant systems provides clear evidence for the existence of an intrinsic surface dilatational viscosity. All contradictions and observational facts can be bridged by applying the Guggenheim convention with an extended surface phase, which explicitly includes a molecular exchange between the monolayer and an adjacent sublayer as intrinsic properties. Neglecting these substructures one obtains the classical Gibbs model of the surface. Furthermore, the model takes into consideration the dissipative losses in the extended effective surface layer. The significance of this finding requires an independent verification; therefore, independent measurements with surface second harmonic generation, SHG, have been carried out. SHG is a non-linear optical technique with an inherent surface specificity. It can be used for the measurement of the monolayer coverage under dynamic conditions. SHG measurements carried out on an oscillating bubble provide evidences for the extended surface model. The surface dilatational viscosity is a system quantity which has consequences for variety of technological processes such as high speed coating processes. Furthermore, it could be demonstrated that surface dilatational viscosity is a prerequisite for stable foams. The paper discusses also the surface properties of high concentrated solutions and solutions of surfactant/polyelectrolyte mixtures, which require a different treatment.