- Of dissolved ions. This results in a hydration shell (also hydrate sphere ).
- To polar neutral molecules, especially when hydrogen bonds can be formed.
- In solids (minerals ) as water of crystallization in the formation of hydrates.
By " hydration " thus refers to the special case of solvation of the solvent water.
Water of dissolved ions
The hydration takes place ( ion-dipole interaction) due to the electrostatic forces between the charged ions and the water dipoles. With the formation of hydrogen bonds to the first hydration shell for additional water molecules can accumulate and form a further hydrate sphere.
As a measure of the tendency of the ions to combine with water molecules, the hydration energy is used. This is the energy that is required to bring the ions out of the aqueous solution in the vacuum.
The number of bound water molecules, and the strength of the bond will depend on the size and charge of the ions. With the same charge, the smaller ionic build a larger hydration shell than the larger ones. An aq ion marked on it that the ion is present hydrated:
For small and / or multiply charged cations, the bound water molecules can release protons, one speaks of cation acids (see Lewis acids). For solutions containing cation acids, the solvent water can not be removed under certain circumstances:
In other solvents, such as ammonia, similar effects occur, which are commonly referred to solvation.
Water to non-polar particles ( hydrophobic hydration)
When non-polar and uncharged ( ie water- hostile ) particles are in aqueous solution, there is the so-called hydrophobic effects. This includes the hydrophobic ( wassermeidende ) hydration. The water in the immediate vicinity of the nonpolar solute (eg a xenon atom ) or a non-polar molecule group (eg, an alkyl group ) then has a little more structure, and the translational and rotational mobility of the adjacent water molecules is reduced. Since the hydrophobic effect also takes place in the vicinity of large biomolecules, where water is disposed adjacent to both polar and non-polar molecular groups in addition, this effect plays an extremely important role in many biochemical processes.