Effect of temperature on the hydration of hydrophobic lons. Apparent molal volumes and heat capacities of Bu4PBr, Ph4PBr, and NaBPh4 in aqueous solutions at various temperatures

Abstract

The density and specific heat of aqueous solutions of Bu4PBr, Ph4PBr, and NaBPh4 have been measured at various temperatures between 10 and 45°C. The derived limiting apparent molal volumes (φvo) and heat capacities (φco) have been examined for specific effects in the hydration of alkyl- and aryl-substituted quaternary ions, using also similar results on the tetraalkylammonium series. While φvo and φco of Bu4PBr and Bu4NBr differ only in magnitude by a few percent, the properties of Ph4PBr are sharply distinct, especially φco. NaBPh4 also exhibits a distinct behavior in the temperature dependence of its φco; the effect, however, is much less pronounced than that previously reported by other investigators. Within the data presently available, φco of hydrophobic ions appears rather invariant with temperature; for the compounds studied here, the relative change in φco with temperature is significantly smaller than the relative change in φvo. These results are correlated with spectroscopic data reported earlier, using a phenomenological description of the two-state behavior of liquid water. The latter is used to calculate the solvent relaxational contribution to the heat capacity of water and to φco of the hydrophobic ions.