A relationship between the zeta potential and surface free energy changes of the sulfur/n-heptane—water system

Measurements of the zeta potential of sulfur in doubly distilled water were carried out by the streaming potential technique. The zeta potential of sulfur changes from −25.6 to −120 mV if the sulfur surface is wetted with n-heptane. The relationship between the amount of heptane wetting the surface and the zeta potential has been determined. On this basis, the film pressure for heptane and then the dispersion part of surface free energy of sulfur have been determined. For comparison, similar measurements have been made with a Teflon (PTFE)/n-hexane-water system. If the determined values of film pressure are treated as those resulting from immersional wetting, good values of dispersion energy are obtained, e.g., 97.6 and 19.7 ergs/cm2 for sulfur and Teflon, respectively. It was assumed that n-heptane molecules are vertically oriented against the sulfur surface, at least in the first layer. This conclusion was verified theoretically by Fowkes' relation for calculation of the relative pressure, pP0 for first monolayer. It has been assumed here that relative zeta potential correlates to pP0. The calculated relative zeta value is 0.28, and the experimental one is 0.26.