An apparatus for investigating dynamic surface properties of adsorption layers and spread monolayers

A new set-up for studying the dynamic behaviour of soluble adsorption layers and insoluble monolayers is presented. The apparatus consists of a hydrophobic funnel and a counterbody. By moving the funnel through the liquid surface the surface area inside the funnel is expanded or compressed. The counterbody compensates for the liquid displaced by the funnel by moving in the direction opposite to the funnel. This allows to keep the interface fixed and to measure accurate surface tensions. Dynamic surface tensions are measured by the Wilhelmy plate technique in order to follow the relaxation process with time. The system can be used as a standard method for dynamic interfacial investigations and works in a time window from is up to hours. A disturbance of the thermodynamic equilibrium of the adsorption layer performed by the movement of the funnel induces a relaxation process of the interface without any significant lateral motion of adsorbed molecules. For such a stress relaxation experiments response functions which contain characteristic parameters for the surfactant studied exist. The results obtained for the nonionic alkyl dimethyl phosphine oxide C12DMPO are discussed in terms of a diffusional theory for the exchange of matter. The theory describes the experimental relaxation data.