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 ΔChp 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 ΔChp 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 ΔChp 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.