Effects of constant pressure and constant volume conditions on the composition dependence of chemical potentials and activity coefficients

Abstract

Experimental data are generally obtained at constant pressure whereas many theoretical models are derived at constant volume. It is shown that the substitution of a constant pressure condition to that of a constant volume one affects very markedly the composition dependence of the chemical potential and the fugacity, but only slightly (and often negligibly) that of the activity and activity coefficient. Under conditions of constant volume, the choice of the reference state also affects the composition dependence of the activity and activity coefficient as well as the definitions of interaction coefficients. Finally, it is important that constant volume conditions be specific as to what volume is kept constant: molar volume, volume per mole of solvent or per mole of substitutional elements, etc.