(262s) Modification of Staverman-Guggenheim Combinatorial Entropy Expression

Staverman-Guggenheim¹ (S-G) expression has been widely used to describe the non-ideal entropy of mixing in state of art activity coefficient models, e.g. UNIQUAC²/UNIFAC³, COSMO-SAC⁴ etc. It accounts for both size and shape differences of molecules, while Flory-Huggins⁵ (F-H) expression only considers size difference and results in an upper limit for entropy of mixing. However, for molecules with large size and shape differences, S-G may return unrealistic results. In this study, a modification is proposed by introducing an exponent on both volume and surface area parameters of S-G. The modified S-G is employed to predict infinite dilute activity coefficients for mixtures containing linear, branched, and cyclic alkanes. It shows significant improvement than original S-G model by comparing to experimental data. The modification is further explained from statistical mechanics point of view. Finally, the modified S-G expression is applied to activity coefficient models like UNIQUAC and NRTL-SAC⁶ to show the improvement in correlation and prediction.

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