Water structure enhancement in water-rich binary solvent mixtures. Part II. the excess partial molar heat capacity of the water

The excess partial molar isobaric heat capacity of water, C _PW ^E, in water-rich mixtures with co-solvents has been calculated. When the excitation of internal modes of the water molecules is deducted in part, the remainder indicates, if positive, that the relative extent of fully hydrogen bonded domains of the water is enhanced. This was demonstrated in the cases of aqueous methanol, ethanol, 1- and 2-propanol, t-butanol, 2-methoxyethanol, 1,2-dimethoxyethane, tetrahydrofuran, acetone, N,N-dimethylformamide, and N,N-dimethylacetamide. Some solutes such as ethylene glycol, 1,4-dioxane, acetonitrile, N-methylformamide, formamide (and urea), ethanolamine, and dimethylsulfoxide, many of which hydrogen-bond very strongly with water, do not enhance the water structure according to this criterion. The results are compared qualitatively with conclusions from the partial molar excess volumes of the solutions and these agree with the present ones.