The role of solvent cohesion in nonpolar solvation

Abstract

Understanding hydrophobic interactions requires a molecular-level picture of how water molecules adjust to the introduction of a nonpolar solute. New insights into the latter process are derived from the observation that the Gibbs energies of solvation of the noble gases and linear alkanes by a wide range of solvents, including water, correlate well with linear combinations of internal pressure (P_i) and cohesive energy density (ced) of the solvent. P_i and ced are empirical solvent parameters that quantify two different aspects of solvent cohesion: the former reflects the cost of creating a cavity by a subtle rearrangement of solvent molecules, whereas the latter captures the cost of creating a cavity with complete disruption of solvent–solvent interactions. For the solvation of smaller solutes the internal pressure is the dominant parameter, while for larger solutes the ced becomes more important. The intriguing observation that the solubility of alkanes in water decreases with increasing chain length, whereas the solubility of noble gases increases with increasing size, can be understood by considering the different relative influences of the ced and P_i on the solvation processes of both classes of compounds. Also the solvation enthalpy, but not the entropy, correlates with linear combinations of solvent ced and P_i, albeit poorly, suggesting that the good correlations observed for the Gibbs energy are largely due to enthalpy, most likely that related to cavity formation.