On the structure and thermodynamics of solvated monoatomic ions using a hybrid solvation model

The hydration free energies relative to that of the proton are calculated for a representative set of monatomic ions Z±. These include cationic forms of the alkali earth elements Li, Na, and K, and anionic forms of the halogens F, Cl, and Br. In the current model the relative ion hydration free energy is defined as Δ[ΔGhyd(Z±)]=G(Z±[H2O]n(aq))−G(H+[H2O]n(aq))−G(Z±(gas))−G(H+(gas)), where the solvated ions are represented by ion–water clusters coupled to a dielectric continuum using a self-consistent reaction field cycle. An investigation of the behavior of Δ[ΔGhyd(Z±)] as the number of explicit waters of hydration is increased reveals convergence by n=4. This convergence indicates that the free energy change for the addition of water to a solvated proton–water complex is the same as the free energy change associated with the addition of water to a solvated Z±–water complex. This is true as long as there are four explicitly solvating waters associated with the ion. This convergence is independent of the type of monatomic ion studied and it occurs before the first hydration shell of the ions (typically ⩾6) is satisfied. Structural analysis of the ion–water clusters reveals that the waters within the cluster are more likely to form hydrogen bonds with themselves when clustering around anions than when clustering around cations. This suggests that for small ion–water clusters, anions are more likely to be externally solvated than cations.