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 (φ ^o_v ) and heat capacities (φ ^o_c ) 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 φ ^o_v and φ ^o_c of Bu₄PBr and Bu₄NBr differ only in magnitude by a few percent, the properties of Ph₄PBr are sharply distinct, especially φ ^o_c . NaBPh₄ also exhibits a distinct behavior in the temperature dependence of its φ ^o_c ; the effect, however, is much less pronounced than that previously reported by other investigators. Within the data presently available, φ ^o_c of hydrophobic ions appears rather invariant with temperature; for the compounds studied here, the relative change in φ ^o_c with temperature is significantly smaller than the relative change in φ ^o_v . 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 φ ^o_c of the hydrophobic ions.