Standard thermodynamics of transfer. Uses and misuses

The standard free energy of transfer of a solute A between two solvents a and b is discussed at both a thermodynamic and a statistical mechanical level. It is shown that whereas thermodynamics alone cannot be used to choose the "best" standard quantity, statistical mechanics can help to make such a choice. It is shown that Δμ_A°^ρ, the standard free energy of transferring A, computed by the use of the number density (or molarity) scale, has the following advantages: (1) it is the simplest and least ambiguous quantity; (2) it is the quantity that directly probes the difference in the solvation properties of the two solvents with respect to the solute A; (3) it can be used, without any change of notation, in any solution, not necessarily a dilute one, and including even pure A; (4) by straightforward thermodynamic manipulations one obtains the entropy, enthalpy, volume changes, etc. for the same process. All of these quantities have advantages similar to the ones indicated for the free energy change. Because of the advantages of this particular choice of standard quantities, we propose to "standardize" the use of the standard thermodynamic quantities of transfer and refer to them as the local-standard quantities. Some common misconceptions and misinterpretations of other standard quantities are indicated.