Water is extremely unique in that it has a high dielectric constant (=80). A high dielectric constant suggests that the solvent (in this case, water) has the ability to screen charges. This means that water molecules will surround both anions and cations in a solution, and ultimately, diminish the attraction of the two charges. For example, if you place a cation (such as sodium) in water, the positive charge (+) of sodium is able to interact with anions and other molecules. However, because water has such a high dielectric constant, there is a decreasing effect on the interaction between sodium and its counter-ion. This is primarily because the charges interact with the electric dipole of water more than with each other, decreasing the overall interaction. In comparison to water, the interaction between anions and cations is much greater when placed in solvents with lower dielectric constants.
- Berg, Jeremy M., ed. (2002), Biochemistry (6th ed.) New York City, NY: W.H. Freeman and Company,