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.