Effects of Ions on Water Structure and Vice Versa

Effects of Ions on Water Structure and Vice Versa 

The structure of liquid water was dealt with in detail in Sect. 1.1.2. Once a solute, whether an ion or a neutral solute and whether hydrophilic or hydrophobic, is placed in the water, it is reasonable to expect it to affect the structure around it. The effects may be limited to a hydration shell surrounding the solute that has a structure differing from that in pure water. For instance, around monatomic ions the water molecules in the hydration shell are oriented towards the ion in a more or less spherical symmetry. Around hydrophobic solutes cages of water molecules are formed, that may be icelike but also resemble the structure of clathrates or crystal hydrates. In many cases the effects of the presence of an ion are manifested also beyond the hydration shell or shells. 

Water structural effects of ions may be assessed by a variety of experimental methods as well as by computer simulations. These methods lead to the recognition that some ions enhance the native structure of the water whereas other ions destroy it, up to some distance in the water away from the ion. The former ions are called structure-makers, or cosmotropic, a term favoured in the biophysical literature, and the latter kind of ions are structure-breakers or chaotropic. Naturally, there are also ions that are borderline between these groups, neither structure-makers nor -breakers to a significant extent. 

Following are the results of several of the methods that have been used to categorize ions as belonging to the structure-making or -breaking groups, the subject having been also recently reviewed (Marcus 2009,2010). 

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