Adsorption of polyvinyl alcohol on the paraffin—water interface. I. Interfacial tension as a function of time and concentration

Abstract

A systematic experimental study has been made of the interfacial tension γ of paraffin-water interfaces on which polyvinyl alcohol (PVA) was adsorbed. Special attention has been paid to time effects and to the influence of the acetate content and molecular weight of the polymer. The measurements have been performed by the static Wilhelmy plate technique. Prolonged time dependencies up to several tens of hours, are generally observed. At very low polymer concentration ( c p ) the process is diffusion controlled, at high c p it is reconformation controlled. Plots of steady-state values of γ against c p over the 0.4–4000 ppm range are convex with respect to the concentration axis for PVA samples in which the content of acetate groups exceeds 10%. In this respect the trend is opposite to that of low molecular weight surfactants below the CMC. For samples of PVA with 2% acetate groups, however, the γ-log c p plots consist of two linear stretches separated by a discontinuity. At polymer concentrations below that corresponding to the discontinuity, γ is almost independent of c p . In this region the equilibration time is also very short. The effect of molecular weight is not univocal and depends on the source of the PVA. Explanations of these findings are sought in terms of the conformation of the adsorbed polymer.