Effect of a cosolvent on the hydrophobic interactions. A calorimetric study of alkane-m,n-diols in concentrated aqueous solutions of ethanol

Calorimetric measurements were carried out at 298.15 K on concentrated aqueous solutions of ethanol, ranging from 0 to 9 m, containing alkane-m,n-diols from C-2 to C-9. The purpose of this study is to obtain more information about the influence of ethanol on hydrophobic hydration and interactions. The pairwise interaction coefficients of the virial expansion of the excess enthalpies were evaluated, and the results rationalized according to the preferential configuration model. The effect of ethanol depends on its concentration, on the alkyl chain length of the diol and on the OH positions. The hydrophilic interactions between the hydroxy groups determine the strength of hydrophobic interactions. The presence of the cosolvent ethanol, which lowers the relative permittivity of the medium, at first enhances the forcing action of hydrophilic interactions, making the hydrophobic interactions more cooperative. The enthalpic interaction coefficients are larger than those in water and reach a maximum at 2-3 m ethanol. Then, they decrease with the increasing structuring effect of the cosolvent on water. Evidences are given that, at low concentration of ethanol, the alkyl chains of the diols are preferentially hydrated. At higher concentration, instead, they are to some extent ethanolated. The results obtained are compared with those found for diols in concentrated solutions of urea: some comments are made on the possible mode of action of ethanol and urea as protein denaturants.