Further terms primary and secondary solvation

There are further terms which are used to describe the strength of the interaction between the ion and the solvent molecules and the region in which strong interaction takes place. 

If this occurs it takes place in the first solvation shell. These are water molecules with which the ion interacts strongly and are coordinated to the ion. They thus move with the ion and could be termed bound water. Often all of the first solvation shell water molecules can be considered to be primary solvation, but some workers feel that this need not necessarily be the case. Some water molecules may be in the first solvation shell but are not bound, and do not move with the ion. It becomes necessary to define a criterion for distinguishing bound from unbound molecules, i.e. primary solvation or not. The criterion used will be determined by the experimental technique which is being used to measure it. Unfortunately, different experiments may actually measure different properties. 

This is made up of molecules which are affected by the ion, but are not coordinated to them and often do not move with the ion. They generally are found in the secrmd solvation shell or subsequent shells if the water molecules in them have different properties fiom bulk wata-. It may also include molecules from the first solvation shell which are not included in the primary solvation. 

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