A simple empirical model describing the thermodynamics of hydration of ions of widely varying charges, sizes, and shapes

Cations and anions and small and large ions are hydrated in different manners, due to the orientation of water molecules around them. Still, a very simple model accounts reasonably well for the thermodynamics of hydration of ions with chargesz varying from ± 1 to ± 4, depending only onz and their radiir, when taken as spheres. The model specifies the width of the hydration shell, Δr, the numbern of water molecules immobilized and electrostricted in it, proportional to |z|/r, and the bulk and high-field dielectric properties of the water. It accounts for the Gibbs energy, enthalpy, entropy, heat capacity of hydration and the partial molar volumes of the ions.