Solvation of hydrocarbons in aqueous-organic mixtures

• Thermodynamic functions of solvation in mixtures of water with acetone and acetonitrile are measured at T = 298.15 K.
• Solvation of n-octane and toluene in aqueous-organic mixtures is studied.
• When increasing water content, Gibbs free energies grow up steadily, while enthalpies have a maximum.
• Hydrocarbons are preferentially solvated with organic cosolvent even in mixtures with rather high water content.
• Acetonitrile suppresses the hydrophobic effect less than acetone.

We study the solvation of two hydrocarbons, n-octane and toluene, in binary mixtures of water with organic cosolvents. Two polar aprotic cosolvents that are miscible with water in any proportions, acetonitrile and acetone, were considered. We determine the magnitudes of thermodynamic functions of dissolution and solvation at T = 298.15 K in the mixtures with various compositions. Solution calorimetry was used to measure the enthalpies of solution, and GC headspace analysis was applied to obtain limiting activity coefficients of solutes in the studied systems. For the first time, the enthalpies of solution of alkane in the mixtures with high water content were measured directly. We observed well-pronounced maxima of the dependencies of enthalpies of solvation from the composition of solvent and no maxima for the Gibbs free energies of solvation. Two factors are concluded to be important to explain the observed tendencies: high energy cost of reorganization of binary solvent upon insertion of solute molecules and preferential surrounding of hydrocarbons with the molecules of organic cosolvent. Enthalpy-entropy compensation leads to a steady growth of the Gibbs free energies with increasing water content. On the other hand, consideration of the plots of the Gibbs free energy against enthalpy of solvation clearly shows that the solvation properties are changed dramatically after addition of a rather small amount of organic cosolvents. It is shown that they suppress the hydrophobic effect very effectively even at low concentration, and acetonitrile suppresses the hydrophobic effect less than acetone.