- The UNIFAC method for predicting activity coefficients in mixtures of nonelectrolyte solutions has been widely used in recent years (7). I t is based on the group contribuHoh concept which assumes that the liquid mixture consists of functional groups such as CHP, CH, O rather than molecules of various components. The activity coefficients are divided into a combinatorial part and a residual part. The combinatorial part is a function of the mole fractions and the reduced van der Waals parameters. The residual part depends on adjustable parameters called group interaction parameters. The equations of r; and r? and a bt of parameters are given elsewhere (7, 2).