Correlation between film properties and the purity of surfactants

Abstract

The disjoining pressure Π as a function of the film thickness h of aqueous tetraethyleneglycol-monodecylether (C₁₀E₄)-solutions was measured by means of a thin film pressure balance (TFPB). The Π(h)-curves were obtained for three surfactant concentrations for each of three samples, which differed in purity. Apart from a pure and an aged sample of C₁₀E₄, we investigated C₁₀E₄ solutions, which had been contaminated with decanol. In addition, the surface tension and the miscibility gap of the binary system water–C₁₀E₄ were measured. The aim of these investigations was to obtain a correlation between the above-mentioned properties, especially the disjoining pressure, of a non-ionic surfactant and its purity. We found that the impurities of the investigated sample strongly influence the position of the miscibility gap, whereas the effect on the surface tensions was unexpectedly low. With respect to the properties of thin films the impurities were found to decrease the electrostatic repulsion between the two monolayers of the film. As a consequence, common black films (CBF) are destabilized, whereas at concentrations around the c.m.c. an increasing stability of Newton black films (NBF) is observed. At first sight, these observations are in contrast to the results found by other groups, according to which the CBF is usually stabilized by the presence of impurities or the addition of alcohol. An explanation of this discrepancy is given.