Prediction of Vapor Pressures and Enthalpies of Vaporization Using a COSMO Solvation Model

Abstract

We have developed a general predictive method for vapor pressures and enthalpies of vaporization based on the calculation of the solvation free energy that consists of three components; the electrostatic, dispersion, and cavity formation contributions. The electrostatic contribution is determined using the quantum mechanical COSMO solvation model. Thermodynamic perturbation theory for hard-core molecules is used for the cavity term, and the dispersion term is modeled using a mean field term proportional to the density and molecular surface area. The proposed model uses one set of van der Waals atomic radii to describe molecular shape, two universal interaction parameters for the electrostatic interaction, one set of atom-specific dispersion coefficients, one universal parameter to scale the atomic exposed surface area, and a single universal parameter for the ratio of the hard-core to atomic radii. The model parameters have been determined using 371 pure substances of varying molecular structure, functionality, and size. The average accuracy of the model for vapor pressures and enthalpies of vaporization at the normal boiling temperature is found to be 76% and 4.81 kJ/mol, respectively, with temperature-independent parameters. The average error in the normal boiling temperature is found to be 16 K for species whose boiling points range from 191 to 610 K.