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

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 (ΔW_elec) and surface (ΔW_surt)—in the process J^± ·NSG (g) → J^± (aq). ΔW_elec is the major contributor. ΔW_surf 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, ΔW_surt < 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).