A proposal for the estimation of binary mixture activity coefficients from surface tension measurements throughout the entire concentration range

A method to estimate the concentration dependence of the bulk activity coefficients of both binary mixture components from experimental surface tension data covering the whole concentration range is presented. To this end, high-quality experimental surface tension data as a function of the concentration, paying special attention to the diluted regions, are needed. The method is based on the application of equilibrium conditions to the surface chemical potential arisen from the Volmer equation (the simplest non-ideal surface EOS) and to the bulk chemical potential coming from the three-suffix Margules equation for activity coefficients. It can be applied provided that positive aneotropy is not present and the second derivative of the surface pressure as a function of the mole fraction is not positive in any composition region, which means rather unrestrictive conditions. In order to test the method, a compilation of 25 systems for which surface tension and activity coefficient data are simultaneously available in the literature was performed, turning out that most of them involved water. It was found that surface tension data with enough concentration coverage and good quality are really scarce, which makes the development of this kind of methods difficult.