COMPARISON OF THE SOLUBILITY OF POLYCYCLIC AROMATIC-HYDROCARBONS IN NONASSOCIATED AND ASSOCIATED SOLVENTS - THE HYDROPHOBIC EFFECT

A remarkable improvement of the predictions of the solubility, PHI(B), of a solute B in a solvent S is achieved by the theory of the mobile order of Huyskens. In this theory, the hydrophobic effect of the associated solvents like alcohols against inert substances is no longer considered as a result from a change in the energy of the molecular interactions, but as a decrease of the entropy due to temporary correlated displacements of two or more hydroxylic groups of solvent molecules. Such correlated displacements create a kind of mobile order. Quantitatively, the hydrophobic effect reduces In PHI(B) by an amount of r(S)PHI(S)V(B)/V(S). A direct consequence of this effect is that an increase in the ratio V(B)/V(s) of the molar volumes, which in non-H-bonded solvents is favourable for the solubility, becomes unfavourable in alcohols. For polycyclic aromatic hydrocarbons, the prediction of the solubilities in apolar, polar and associated solvents by the mobile order theory necessitates the knowledge of a single parameter only which can be deduced from one experimental solubility.