Gas–liquid partition coefficients of n-alkane solutes at infinite dilution in binary n-alkane solvents

Abstract

Theoretical expressions due to Janini and Martire and to Orwoll and Flory are extended to allow calculation of the value of the infinite dilution activity coefficient of an n-alkane dissolved in a mixture of two other n-alkanes of any composition. Activity and partition coefficients are calculated and compared with experimental (g.l.c.) values for C₅-C₈ n-alkanes dissolved in C₁₈H₃₈+ C₃₆H₇₄ mixtures of mole fraction of 0 to 1. The theoretical treatments provide essentially identical numerical values and predict a very small negative curvature of plots of relative partition coefficients against volume fraction of C₁₈H₃₈. The predicted deviation from linearity is, at most, 1 %, a value within which the experimental data are linear. To this extent the theory provides a quantitative description of the experimental findings although curvature cannot be unequivocally identified. However, since it has been shown elsewhere that the mixtures of a remarkable range of solute and mixed solvent types also provide data showing apparent linearity of such plots, the results obtained here have interesting implications in that the underlying theories are specific to random mixtures of chain molecules. It is concluded that comprehensive experimental studies, with non-alkane systems, designed to provide more accurate data than have been common are now essential in order more precisely to define the problem.