Prediction of the mutual solubilities of hydrocarbons and water with COSMO-RS

A critical collection of data for the mutual solubilities of water and hydrocarbons (HC) recently presented by Tsonopoulos is used for careful analysis of the ability of a priori prediction method conductor-like screening model–real solvents (COSMO-RS) to predict the infinite dilution activity coefficients in such dissimilar systems. Although only few data of hydrocarbon–water mixtures had been used in the parameterization of COSMO-RS, the mutual solubilities of hydrocarbons (n-alkanes, 1-alkenes, alkylbenzenes, and alkylcyclohexanes) and water are quite well predicted by COSMO-RS. All trends of the solubilities with respect to carbon number (CN) of the hydrocarbons are very well reproduced. Some moderate deviations are observed for the temperature dependence (solvation enthalpy) of hydrocarbon solubility in water, while the temperature dependence of the water solubility in various hydrocarbons is reproduced almost within the experimental error up to temperatures of about 473 K. od qualitative and even satisfactory quantitative prediction of the solubilities of hydrocarbons in water proves that COSMO-RS correctly reflects the physics of the hydrophobic solvation, i.e. the fact that the largest part of the free energy of solution of hydrocarbons in water is of entropic nature due to the re-organization of the water molecules around non-polar compounds. This is especially unexpected, because the COSMO-RS method does not takes into account the 3D-geometry of solute–solvent arrangements. An explanation is given, how the physics of water re-organization around non-polar surface is nevertheless described by COSMO-RS. Some ideas for further improvement of COSMO-RS regarding the mutual solubilities of hydrocarbons and water are discussed.