A solvation shell is the solvent interface of any chemical compound or biomolecule that constitutes the solute. When the solvent is water it is often referred to as a hydration shell or hydration sphere. The number of solvent molecules surrounding each unit of solute is called the hydration number of the solute.
A classic example is when water molecules arrange around a metal ion. For example, if the latter were a cation, the electronegative oxygen atom of the water molecule would be 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, its distribution and spatial dimensions.
A number of molecules of solvent are involved in the solvation shell around anions and cations from a dissolved salt in a solvent. Metal ions in aqueous solutions form metal aquo complexes. This number can be determined by various methods like compressibility and NMR measurements among others.
Relation to activity coefficient of an electrolyte and its solvation shell number
The solvation shell number of an dissolved electrolyte can be linked to the statistical component of the activity coefficient of the electrolyte and to the ratio between the apparent molar volume of a dissolved electrolyte in a concentrated solution and the molar volume of the solvent (water):
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. The hydration layer around a protein has been found to have dynamics distinct from the bulk water to a distance of 1 nm. 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.
With other solvents and solutes, varying steric and kinetic factors can also affect the solvation shell.
A dehydron is a backbone hydrogen bond in a protein that is incompletely shielded from water attack and has a propensity to promote its own dehydration, a process both energetically and thermodynamically favored. They result from an incomplete clustering of side-chain nonpolar groups that "wrap" the polar pair within the protein structure. Dehydrons promote the removal of surrounding water through protein associations or ligand binding. Dehydrons can be identified by calculating the reversible work per unit area required to span the aqueous interface of a soluble protein, or the "epistructural tension" at the interface.:217–33 Once identified, dehydrons can be used in drug discovery, both to identify new compounds and to optimize existing compounds; chemicals can be designed to "wrap" or shield dehydrons from water attack upon association with the target.:1–15
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