![Na+H2O](https://upload.wikimedia.org/wikipedia/commons/6/67/Na%2BH2O.svg)
A solvation shell or solvation sheath is the solvent interface of any
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
or biomolecule that constitutes the
solute
In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...
. When the solvent is
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
it is called a hydration shell or hydration sphere. The number of solvent molecules surrounding each unit of solute is called the
hydration number
The hydration number, or solvation number of a compound is defined as the average number of molecules bound to the compound more strongly (by 13.3 kcal/mol or more) than they are bound to other water molecules. The hydration number is dependent o ...
of the solute.
A classic example is when water molecules arrange around a metal ion. If the metal ion is a cation, the
electronegative
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
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 solution
A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula (H2O)nsup>z+. The solvation number, ''n'', determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 ...
s form
metal aquo complex
In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...
es. 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 a dissolved electrolyte can be linked to the statistical component of the
activity coefficient
In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. In an ideal mixture, the microscopic interactions between each pair of chemical species are the same ( ...
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
Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of t ...
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,
and that near features conventionally regarded as attractive to water, such as hydrogen bond donors, the water molecules are actually relatively weakly bound and are easily displaced.
With other solvents and solutes, varying steric and kinetic factors can also affect the solvation shell.
See also
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Activity coefficient
In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. In an ideal mixture, the microscopic interactions between each pair of chemical species are the same ( ...
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Metal ions in aqueous solution
A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula (H2O)nsup>z+. The solvation number, ''n'', determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 ...
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Ion transport number
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Ionic radius
Ionic radius, ''r''ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the catio ...
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Water model
In computational chemistry, a water model is used to simulate and thermodynamically calculate water clusters, liquid water, and aqueous solutions with explicit solvent. The models are determined from quantum mechanics, molecular mechanics, experim ...
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Poisson-Boltzmann equation
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Hydration energy
In chemistry, hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergoes hydration. Hydration energy is one component in the quantitative analysis of solvation. It is a particular special case of ...
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Solvation
Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
References
{{DEFAULTSORT:Solvation Shell
Solutions
Chemical properties
Chemical bonding