Surface rheology as a tool for the investigation of processes internal to surfactant adsorption layers

The description of adsorption at liquid interfaces has been largely improved after incorporation into models of dynamic processes internal to the adsorption layers, such as surfactant re-orientation, aggregation and chemical reactions. Evidence for most of these processes has been given by qualitative studies utilising direct imaging techniques or by tensiometric investigations. These processes strongly influence the dilational rheological features of the adsorption layer, i.e. the response of interfacial tension to perturbations of the interfacial area. The investigation of the dilational rheology is then very effective to assess the existence of these processes and to characterise their kinetic parameters and equilibrium properties. To this aim specific models and experimental techniques have been recently developed. Here the surface visco-elasticity of adsorbed surfactant layers, epsilon, at liquid-air and liquid-liquid interfaces is measured by means of different oscillating drop/bubble methods, which are based on the interfacial tension response to harmonic perturbations of the interfacial area and have been implemented in capillary pressure and in drop shape tensiometers. The comparison of this data with theoretical predictions allows at the same time the validation of the models and the quantification of the kinetic features of the internal processes.