A thermodynamic analysis of the first solvation shells of alkali and halide ions in liquid water and in the gas phase

Abstract

Abstract Let n denote the number of water molecules in the nearest-neighbour shell (NS) of an ion J± in liquid water, and denote J± ·nH2O in the gas phase by J± · NSG (g). The standard free energy of hydration ΔG°hyd (J± ·NSG) can then be deduced by a thermodynamic cycle involving ΔG°n for the formation of J± · NSG (g) and ΔG°hyd for the transfer of J± (g) to water. Values of ΔG°hyd (J± ·NSG) for alkali and halide ions are substantial, ranging from 48 % to 86 % of ΔG0 hyd. The values of ΔG0 hyd(J± ·NSG) can be accounted for largely by the calculated work—electrostatic (ΔWelec) and surface (ΔWsurt)—in the process J± ·NSG (g) → J± (aq). ΔWelec is the major contributor. ΔWsurf depends on whether (i) J± ·NSG (g) can be represented by a cluster consisting of the ion and n separate water molecules, or (ii) there is some molecular complex formation within that cluster. In fact, ΔWsurt < 20 kJ mol−1 for the alkali ions and of the order of 100 kJ mol−1 for the halide ions. A reasonable case can be built that the alkali ions and some of the n water molecules form molecular complexes while the anions are better represented by case (i).