Predictions of Infinite Dilution Activity Coefficients from Ab Initio Solvation Calculation

Shiang-Tai Lin and Stanley I. Sandler
Center for Molecular and Engineering Thermodynamics
Department of Chemical Engineering
 University of Delaware
Newark, DE 19716

We have developed a Group Contribution Solvation model (GCS) to calculate infinite dilution activity coefficients (g¥) based on modern computational chemistry. The GCS model results in an average error of 7% in g¥ for the limited number of data points among water, n-hexane, acetonitrile and n-octanol that we could test with this new model, whereas the errors are 47% and 52% with the UNIFAC model and the modified UNIFAC model, respectively.
We also use GCS to calculate infinite dilution partition coefficients, which can be used to determine how a dilute solute partitions between two solvents. We have examined solutes in 3 different liquid-liquid systems: water/n-hexane, water/acetonitrile and water/n-octanol. With GCS, the average errors are 22% (for 18 solutes), 18% (for 14 solutes) and 14% (for 15 solutes) for these solvent systems; comparable errors are 237%, 286% and 226% with UNIFAC; and 342%, 414% and 306% with modified UNIFAC. Finally, we show that the GCS model is a powerful new tool to predict the octanol-water partition coefficients.

Last updated on June 16, 1999
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