Chemical Association via Exact Thermodynamic Formulations

It can be fruitful to view two-component physical systems of attractive monomers, A and B, ``chemically'' in terms of a reaction A + B <-> C, where C = AB is an associated pair or complex. We show how to construct free energies in the three-component or chemical picture which, under mass-action equilibration, exactly reproduce any given two-component or ``physical'' thermodynamics. Order-by-order matching conditions and closed-form chemical representations reveal the freedom available to modify the A-C, B-C, and C-C interactions and to adjust the association constant. The theory (in the simpler one-component, i.e., A = B, case) is illustrated by treating a van der Waals fluid.