Studies in ion solvation in non-aqueous solvents and their aqueous mixtures. Part 18.—Enthalpies of transfer of alkali-metal halides from water to dioxan + water mixtures; structural effects and comparison with large ions.

Abstract

Molar enthalpies of transfer at infinite dilution, ΔH^°_t, have been measured for H⁺Cl^–, Li⁺Cl^–, Na⁺Cl^–, Cs⁺I^– and Na⁺BPh^–₄ for transfer from water to dioxan + water mixtures up to 30%(w/w) dioxan, at 25°C, and for transfer of Li⁺Cl^– and Na⁺Cl^– to 10%(w/w) dioxan at 10°C.

At low mole fraction (x₂) of dioxan, ΔH^°_t is uniformly negative for the alkali-metal halides and H⁺Cl^–, in contrast to their behaviour in alcohol + water mixtures in which ΔH^°_t is positive and passes through a maximum. It is argued that such a maximum, for electrolytes consisting of small ions, indicates a region of maximum solvent strucuture, but that its absence does not necessarily imply the absence of such a region.

Previous work using electrolytes with large organic ions is also examined. It is suggested that the ions BPh^–₄ and NMe⁺₄ could be classified with the smaller ions in their effects on solvent structure rather than with the larger tetra-alkylmmonium ions, whose behaviour is dominated by hydrophobic interactions. If so, the maxima in ΔH^°_t found for Na⁺BPH^–₄ and NMe⁺₄Br⁺ at low x₂ indicate a maximum in solvent structure in the dioxan + water system.