The excess surface tensions of simple binary mixtures

Abstract

A simple hypothesis is proposed which relates the local mole fraction at each height in the surface layer of a binary mixture to the local density at that height. The hypothesis is consistent with computer-simulation studies of mixtures of similar Lennard-Jones molecules but overestimates the adsorption of the more volatile component when the vapour-pressure ratio is 100 or more. It is used to calculate the excess surface tension of ten binary mixtures and (without any adjustable constants) fits well the four systems with vapour-pressure ratios below 4. It accounts for the size and shape of the excess tension, but the magnitude is too great by factors of up to 1.5 for a further four systems with pressure ratios of between 12 and 80. For one system, with a pressure ratio of 274, the error is a factor of ca. 2.5, which is consistent with an overestimate of the adsorption of the more volatile component.