Ionic Interactions

Michael Faraday was the first to recognize the important part played by ions in the behavior of an electrolytic solution. Fifty years later, Arrhenius proposed and demonstrated that the molecules of an electrolyte are dissociated to a certain extent into free ions even when no external field is acting on them. He was led to this conclusion by van't HofPs theory of solutions and his own extensive researches on the conductivity of electrolytic solutions. The theory of Arrhenius is, however, only valid for weak electrolytes in which only a small proportion of the molecules are dissociated into ions. The behavior of strong electrolytes shows considerable departures from Arrhenius's theory. The ideas of Milner, Debye, Hückel, Onsager, and Falkenhagen made it possible to explain the properties of strong electrolytes, at least in sufficiently dilute solutions. Even today it is only with difficulty that the theory can be extended from the field of dilute solutions to embrace more concentrated electrolytes. Such an extended theory must take into account the short-range forces between the ions. The theoretical treatment of these complicated factors has, so far, been only partially successful. Various attempts to treat theoretically the properties of more concentrated solutions were given since the pioneering work of Debye and his co-workers (1923). Although these theories have led to valuable insight into the nature of ionic solutions, there remain many unsolved questions, e.g., the nature of the short-range forces between the ions.