An initial discussion of a quasi-chemical approach appeared in Section 4.6. This chapter gives a fuller development of those ideas. The idea of our initial discussion was to introduce a statistical model capable of a natural description of strong association phenomena in solutions, and the example of ion clustering in electrolyte solutions was considered. But the quasi-chemical approach may be founded on broader concepts, and given a more extensive development. The most primitive idea is to identify an inner-shell region from the rest of the neighborhood of a distinguished solute, and to rely on a painstaking treatment of the inner shell, with full molecular resolution. The remainder of the neighborhood of that distinguished solute – the outer-shell region – can be given an alternative statistical description, and then a proper matching of results for inner and outer shells must be accomplished. The pragmatic approach of using alternative methods for physically distinct spatial regions is important.

Many problems of solution theory cry out for chemical treatment of an obvious inner shell. For example, complexes such as Fe(H2O)63+ naturally present themselves as important solution structures when Fe3+(aq) is considered. But discussion of the thermodynamics of Fe3+(aq) on that basis requires a satisfactory parsing of the thermochemistry associated with the ligand species.