Prediction of surface tension of highly nonideal aqueous-organic mixtures as a function of composition by a partitioning model between surface and bulk phases and use of partial molar surface areas

Abstract

The composition dependence of the surface tension of highly nonideal aqueous-organic nonelectrolyte solutions is described in detail. The Wilson, NRTL, UNIFAC and ideal solution models are used for activity coefficients of surface and bulk phases and compared with one another and with experimental data. Paquette and Rasmussen molar surface areas are employed for prediction of the surface tension of the mixtures and results compared. A new procedure consisting of a partitioning model between surface and bulk phases combined with determination of partial molar surface areas is evolved and tested with experimental data taken on the surface tension of aqueous-organic systems. Comparisons of the calculated surface tensions with experimental data yield mean absolute errors of only 2.8% for the systems studied, all of which exhibit highly nonideal behavior, with excess surface tensions as high as 35 mN m−1. The surface tension predictions are found to be extremely sensitive to the values of the molar surface areas used in the computation. Use of the partitioning model with partial molar surface areas improves surface tension predictions of highly nonideal aqueous-organic mixtures by 30% over use of pure component molar surface areas. Moreover, this improvement in predictions is found to occur for every value of composition without exception for each aqueous-organic system studied. Analysis of the data suggests the attractive hypothesis that the extent of nonideality of aqueous-organic mixtures is characterized by and is directly proportional not only to the difference in pure component surface tension of the binary constituents but also to the ratio of pure component molar (or partial molar) surface areas of the organic to that of water. It is proposed that this physical area parameter is correlated to the value of the partition constant between the surface and the bulk. It therefore constitutes an important determinant of the surface chemical properties of the organic species in the aqueous-organic mixture, in addition to the intrinsic chemical nature of the organic compound, although further research is needed to find out the exact relationship between them and to arrive at a quantitative theory.