Calculations on ionic solvation. Part 6.—Structure-making and structure-breaking effects of alkali halide ions from electrostatic entropies of solvation. Correlation with viscosity B-coefficients, nuclear magnetic resonance B′-coefficients and partial molal volumes

Abstract

The electrostatic entropy of solvation of an ion, ΔS^°_e, or the contribution to ΔS^°_e from the co-ordination sphere of the ion, ΔS_{I, II}, have been shown to be quantitative measures of the structure-making and structure-breaking effects of ions of the alkali halide series in water and in non-aqueous solvents. Both entropy criteria indicate that in water the ions Li⁺, Na⁺, Ag⁺ and F^– are net structure-makers, the ions Rb⁺, Cs⁺, Cl^–, Br^–, I^– and ClO^–₄ are structure-breakers, and K⁺ is a borderline case. In the non-aqueous solvents formamide, methanol, N-methylformamide, dimethylformamide, dimethylsulphoxide and acetonitrile, all the above ions are structure-makers with the exceptions of the weak structure-breaking ion ClO^–₄ in formamide and the borderline cases of ClO^–₄ in methanol and I^– in formamide.

It is shown that the ΔS^°_e or ΔS^°_{I, II} values may be used to assign single-ion B- or B′-coefficients and that for water and several non-aqueous solvents there are good linear correlations between the entropy values and the single-ion coefficients. There are also good linear correlations between the entropy values and single-ion V° values when the latter are based on V°(H⁺, aq, 1 mol dm^–3)=–5.4 cm³ mol^–1 and when values of V° in non-aqueous solvents are assigned by the correspondence method. It is further shown that the general conclusions reached do not depend on any particular choice of ionic radii, although the Goldschmidt–Pauling set is preferred, and it is suggested that the derived ΔS^°_e and ΔS^°_{I, II} values are close to ‘absolute’ values and hence provide an ‘absolute’ measure of ion–solvent interactions.