Solvation shell

The first solvation shell of a sodium ion dissolved in water

A solvation shell, in physical chemistry, is a structure used to describe solvation. A solvation shell is a shell of any chemical species that acts as a solvent and surrounds a solute species. When the solvent is water it is often referred to as a hydration shell or hydration sphere.

A classic example is when water molecules form a sphere around a metal ion. The electronegative oxygen atom contained in the water molecule is attracted electrostatically to the positive charge on the metal ion. The result is a solvation shell of water molecules that surround the ion. This shell can be several molecules thick, dependent upon the charge of the ion.

Solvation number

The solvation number could be determined from the compressibility of the solvent and solution.

$n_s = (1 - \frac {\beta}{\beta_0}) \frac{n_i}{n_0}$

where $\beta$ and $\beta_0$ denote compressibilities of solution and the solvent and $n_i$ and $n_0$ the amounts of solvate and solvent.

Hydration shells of proteins

The hydration shell (also sometimes called hydration layer) that forms around proteins is of particular importance in biochemistry. This interaction of the protein surface with the surrounding water is often referred to as protein hydration and is fundamental to the activity of the protein.[1] The hydration layer around a protein has been found to have dynamics distinct from the bulk water to a distance of 1 nm with effects on the surrounding water network extending beyond 2 nm.[2] The duration of contact of a specific water molecule with the protein surface may be in the subnanosecond range while molecular dynamics simulations suggest the time water spends in the hydration shell before mixing with the outside bulk water could be in the femtosecond to picosecond range.[1]

With other solvents and solutes, varying steric and kinetic factors can also affect the solvation shell.