Ions in Solutions: The Liquid Plasma of Chemistry

Much of chemistry occurs in ionic solutions and all of biology. Ionic solutions are plasmas in both the physical and biological meanings of the word. The composition of these ionic mixtures has profound effects on almost all chemical reactions that occur in them.

Ionic solutions have been studied mostly in the tradition of perfect gases, an understandable tradition. But perfect gases are defined by their lack of interactions while ions are defined by their interactions, chiefly electrical. So it is not surprising that traditional approaches have not worked very well.

Interactions in ionic solutions of spherical ions are (almost) the interactions of charged hard spheres. Effects of water are surprisingly unimportant and can be well approximated by the screening produced by a dielectric. At higher concentrations, ions are crowded and steric interactions are important: ions cannot overlap. When water is absent, ionic solutions become ionic liquids, the ultimate liquid plasma. We will discuss how well theories using just hard spheres can account for the properties of ionic solutions and in some devices important in biology and nanotechnology.