Thermodynamic properties of tetraalkylammonium halides: volumes, heat capacities, and expansibilities in H2O, D2O and urea–water mixtures from 278 to 328 K

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Abstract
The densities and heat capacities per unit volume of the symmetrical tetraalkylammonium bromides (R4NBr) were measured in H2O, D2O and 3 m(molal) aqueous urea from 0.01-1 mol kg-1 and from 5-55.degree. C with a flow digital densimeter and a flow microcalorimeter. Expansibilities were measured at 25.degree. C for the same electrolytes in H2O and urea-H2O mixtures with a dilatometer. Apparent molal volumes (.vphi.V), heat capacities (.vphi.C) and expansibilities (.vphi.E) were derived. The .vphi.C of R4NCl and R4NI were measured in H2O at 25.degree. C. The effect of urea concentration was investigated at 25.degree. C in the case of Bu4NBr. Once allowance is made for the anion, the properties of the larger R4N+ behave essentially as hydrophobic nonelectrolytes in H2O. The transfer functions from H2O to D2O have the same sign as the hydration functions, and the transfer functions from H2O to urea-H2O mixtures the opposite sign. Whatever is the origin of the interactions giving rise to the peculiar behavior of hydrophobic R4N+ in H2O, these interactions are larger in D2O and smaller in the presence of urea. The excess volumes, heat capacities and expansibilities of Bu4NBr and Pen4NBr, once corrected for the long-range Debye-Hueckel interactions, all have the same sign as the hydration functions at infinite dilution, in contrast with excess free energies and enthalpies. This suggests some kind of cooperative effect as 2 hydrophobic solutes interact with each other without the formation of a hydrophobic bond. No conclusions can be drawn from the difference in excess functions in the various aqueous solvents.

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