Vapor pressure group contribution models

It is very satisfying and useful that the COSMO-RS model—in contrast to empirical group contribution models—is able to access the gas phase in addition to the liquid state. This allows for the prediction of vapor pressures and solvation free energies. Also, the large amount of accurate, temperature-dependent vapor pressure data can be used for the parameterization of COSMO-RS. On the other hand, the fundamental difference between the liquid state and gas phase limits the accuracy of vapor pressure prediction, while accurate, pure compound vapor pressure data are available for most chemical compounds. Therefore, it is preferable to use experimental vapor pressures in combination with calculated activity coefficients for vapor-liquid equilibria predictions in most practical applications. 

UNIFAC Approach Jensen et al. [16] have employed the UNIFAC group contribution approach to develop an estimation method for pure-component vapor pressures. The model developed applies to hydrocarbons, alcohols, ketones, acids, and chloroalkanes of less than 500 molecular mass and in the vapor pressure region between 10 and 2000 mmHg. Burkhard et al. [8] extended this model to chlorinated aromatic compounds such as chlorobenzenes and PCBs. 

Alternatives to Soave s approach and group contribution adaptations would be better focused on capabilities not offered by such approaches. For example, pure component properties like vapor pressure are assumed to be available when applying Soave s methodology. In the coming world of chemical product design, this assumption may not be satisfactory. Molecular simulation offers the prospect of being able to make these predictions for transport properties as well as equilibrium properties. Proximity effects would also be naturally included within molecular modeling. While the National Research Council has estimated that such predictive capability may not be available for a decade or two, viable preliminary versions may come much sooner than that. 

The group contribution technique employs a special type of mathematical function for each property analyzed [14-20], whose parameters are calculated with the contributions of defined fragments in the molecule. The choice of the functions is made on the basis of physical meaning and/or previous practices. For example, Iwai et al. [ 14] estabHshed a model for the vapor pressure of alkane isomers by resorting to the following formula... 

---