The UNIQUAC (universal quasi-chemical) equation was developed by Abrams and Prausnitz based on statistical-mechanical considerations and the lattice-based quasichemical model of Guggenheim. Similar to the Wilson and NRTL models, local compositions are used. However, local surface-area fractions are used as the primary composition variable instead of the volume fractions. Each molecule is characterized by a volume parameter and a surface-area parameter.
The excess Gibbs energy is divided into a combinatorial part and a residual part. The combinatorial part depends only on the sizes and shapes of the individual molecules. The residual part, which accounts for the energy interactions, has two adjustable binary parameters.
The expression for the activity coefficient is:
ln gi = ln giC + ln giR
where: C, R = the combinatorial and residual contributions to the liquid activity coefficient.
Two binary parameters are used in the computation of the residual portion of the activity coefficient. The parameters may be expanded to include temperature dependence if desired. Thus, either two or four constants may be supplied for the equation.
Refer "UNIQUAC" in the PRO/II Reference Manual, Volume I, Chapter 2 for more information on this method.