Hydrogen Bonding and Local Electrostatic Interactions in a Non-Empiric Analytical Model of Ion Hydration Shell

As well-known, a confusion related to the volume of the ion hydration shell is more imaginary and artificial than real because the experimental hydration number determined for any ion generally depends upon the method of measurement. The currently existing theoretical models do not possess the required universality and do not allow one to do without empirical parameters. In the proposed study, being an attempt to prevent improper data, a new model of the hydration shell of the ion, based on its topological, electrostatic, and hydrophilic properties, is developed. A serious difficulty in developing a nonempirical modeling approach for describing the solvation phenomenon is to know how the dielectric permittivity depends on the distance from the central ion at the site of the hydration shell formation. In the author’s recent papers, a useful non-empiric expression for the dielectric permittivity dependence on the distance was analytically derived. The hydration energy is described by taking into account not only the local dielectric permittivity, but also the type of interaction between ions and water molecules and the shape of the multilayer hydration shell. Geometric representation of the first hydration layer of a spherical ion in the form of Platonic solids is proposed. So, an icosahedron relates to “structure-making” ions, and a dodecahedron – to “structure-breaking” ones. It is shown, how the ion hydration number depends on the ionic radius, charge, and ability of the ion to hydrogen bonding. The calculations related to series of cations and anions are made.