Solution Theory in the Twenty-First Century

IONIC SOLUTION THEORY IN THE TWENTY-FIRST CENTURY 

Looking first back to the publication date of the famous theory of Debye and Hiickel (1923), there is no doubt that their ionic-atmosphere calculation (Section 3.3) of the activity coefficient at very low concentrations is still the dominating peak of 

By the 1980s, it had to be admitted (Rasaiah, 1987) that several versions of the post-1950 theories (mostly developments based on Mayor s theory) could not be distinguished as to virtue when ranked by their ability to replicate experiments, nor were they markedly better than those obtainable from later developments of Debye and Htickel s ionic atmosphere theory. 

However, there is no doubt that from the 1980s on, a very hopeful type of development has been taking place in ionic solution theories. It is the correlation function approach, not a theory or a model, but an open-ended way to obtain a realistic idea of how an ionic solution works (Fig. 3.58). In this approach, pair correlation functions that are experimentally determined from neutron diffraction measurements represent the truth, without the obstructions sometimes introduced by a model. From a knowledge of the pair correlation function, it is possible to calculate properties (osmotic pressure, activities). The pair correlation function acts as an ever-ready test for new models, for the models no longer have to be asked to re-replicate specific properties of solutions, but can be asked to what degree they can replicate the known pair correlation functions. 

It is rather easy to make a list of milestones in ionic solution theory  

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