Heat capacities of hydration of saturated uncharged organic compounds at 25° C
- 1 January 1978
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases
- Vol. 74, 2408-2417
- https://doi.org/10.1039/f19787402408
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 ΔCh 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 ΔCh 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 ΔCh 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.Keywords
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