Heat capacities of water + organic-solvent mixtures

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Abstract
Apparent molar heat capacities ϕC have been measured in water + organic-solvent mixtures at both ends of the concentration range at 10, 25 and 40 °C. The ϕC of water in the following organic compounds shows a maximum against concentration with dϕC/dT < 0: acetone, tetrahydrofuran, methylacetate, ethylacetate and methylethylketone. For water in diethylamine, ϕC decreases monotonically with increasing water concentration and dϕC/dT < 0. In all systems at this concentration end the excess heat capacity CE p is large and positive with dCE p/dT > 0. The data are explained using an extended version of the Treszczanowicz–Kehiaian (TK) model for associated mixtures in terms of water self-association and complex formation through hydrogen bonds with the proton-acceptor organic solvent. The water-diethylamine behaviour reflects especially strong complexing. Water is suggested to have similar thermodynamic behaviour to a lower alcohol when at low concentration in an organic solvent. This is supported by positive CE p data for methanol in methylacetate. Interpretation with the TK model predicts that CE p should be negative for a higher alcohol, e.g. decan-1-ol, in methylacetate and this is confirmed. At the water–rich end, ϕC for the organic substances decreases linearly with the concentration of the organic solvent, and CE p is again large and positive but dCE p/dT < 0. The data are inconsistent with traditional concepts of aqueous solutions of hydrophobic solutes, where water structuring around the hydrophobe lowers solution stability. They support the views of Shinoda that water structuring has the opposite effect. The Shinoda treatment provides an interpretation for the LCST in aqueous solution and is consistent with the thermodynamic effect of structure in alkanes.

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