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

Abstract

The densities and heat capacities per unit volume of the symmetrical tetraalkylammonium bromides (R 4 NBr) were measured in H 2 O, D 2 O, and 3 m aqueous urea from 0.01 to 1 mol kg −1 and from 5 to 55 °C with a flow digital densimeter and a flow microcalorimeter. Expansibilities were also measured at 25 °C for the same electrolytes in H 2 O and urea–water mixtures with a dilatometer. Apparent molal volumes [Formula: see text] heat capacities [Formula: see text] and expansibilities [Formula: see text] were derived. The [Formula: see text] of R 4 NCl and R 4 NI were also measured in H 2 O at 25 °C. The effect of urea concentration was investigated at 25 °C in the case of Bu 4 NBr.Once allowance is made for the anion, the properties of the larger R 4 N + behave essentially as hydrophobic nonelectrolytes in water. The transfer functions from H 2 O to D 2 O have the same sign as the hydration functions and the transfer functions from H 2 O to urea–water mixtures the opposite sign. Whatever is the origin of the interactions giving rise to the peculiar behavior of hydrophobic R 4 N + in water, these interactions are larger in D 2 O and smaller in the presence of urea.The excess volumes, heat capacities, and expansibilities of Bu 4 NBr and Pen 4 NBr, once corrected for the long-range Debye–Hückel 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 two 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.