UNIQUAC — Universal Quasi-Chemical method

The Universal Quasi-Chemical (UNIQUAC) equation was developed by Abrams and Prausnitz based on statistical-mechanical considerations and the lattice-based quasichemical model of Guggenheim. As in the Wilson and non-random two-liquid (NRTL) equations, it uses local compositions. However, it uses the local surface area fractions as the primary composition variable instead of volume fractions. It characterizes each molecule i by a volume parameter, ri, and a surface area parameter, qi. See Wilson method and NRTL — Non-random two-liquid method for more information.

The excess Gibbs energy (and therefore the logarithm of the activity coefficient) is divided into a combinatorial and a residual part. The combinatorial part depends only on the sizes and shapes of the individual molecules; it contains no binary parameters. The residual part, which accounts for the energetic interactions, has two adjustable binary parameters. Like the NRTL equation, the UNIQUAC equation is quite successful in correlating a wide variety of systems.