Surface relaxations as a tool for studying dynamic interfacial behaviour

Abstract

Dynamic properties of surfactant adsorption layers at fluid interfaces are of significance for many technological processes. Relaxation studies at interfaces are useful means of determining dynamic interfacial parameters since they provide information about the exchange of matter and the dilational elasticity of the respective adsorption layers. In the last few years, the accuracy of interfacial tension measurements has decreased to a point that not only periodic disturbances but also transient interfacial tension responses of interfaces can be used for relaxation experiments. Different relative area changes are possible when performing relaxation experiments . For the most common ones relations are derived which predict the interfacial tension response after a disturbance of the equilibrium state. It is necessary to derive interfacial response function models to describe the exchange of matter. System theory provides a general relation to combine different relative area changes with models of matter exchange : diffusion-controlled exchange of matter for solutions containing one or more surfactants and the exchange of matter at liquid/liquid interfaces taking into account the solubility of surfactants in both phases adjacent to the interface. The application of non-periodic relaxation studies is demonstrated by experimental data obtained at the solution air interface with n-dodecyl dimethyl phosphine oxide solutions. Some computer simulations show the effect of surface active impurities on the system response. The effect of the distribution equilibrium coefficient K of a surfactant in a liquid/liquid system on the interfacial tension response after area changes is also discussed.