Heat capacities of hydration of saturated uncharged organic compounds at 25° C

Abstract

The procedures so far adopted for interpreting the partial molal heat capacities at infinite dilution of organic compounds in water at 25°C, Φ°_Cp, are critically reviewed and a new method of correlating Φ°_Cp data to the molecular structure is proposed. In this method, the ΔC^h_p values, obtained by substracting the heat capacities in the gas phase from the Φ°_Cp data, are interpreted as the sum of the following terms: (i) formation of a cavity inside the solvent; (ii) interaction between the hydrocarbon part of the molecule and water; (iii) interaction between hydrophilic centres, Y, and water; (iv) a term to account for interactions among centres, Y, when more than one hydrophilic group is present in the molecule.

A general equation which accounts for the formation of the cavity and for the interaction water–hydrocarbon surface is obtained on the basis of the ΔC^h_p values of the first six n-alkanes. The values of the (iii)rd and (iv)th terms are then calculated for several types of groups Y and of couples Y₁, Y₂, by using the above mentioned equation in conjunction with the ΔC^h_p values.

This analysis has revealed that both the formation of the cavity and the hydrocarbon group combine to produce large positive Φ°_Cp values, while all hydrophilic groups always bring about a decrease of the partial molal heat capacities. It is also shown that the interaction between hydrophilic centres in the same molecule is important in determining the Φ°_Cp values, and that this may cause either ancrease or a decrease of the Φ°_Cp values predicted on the basis of the Φ°_Cp contribution for non-interacting groups.