INTERPRETATION OF PREFERENTIAL INTERACTION COEFFICIENTS OF NONELECTROLYTES AND OF ELECTROLYTE IONS IN TERMS OF A 2-DOMAIN MODEL

For a three-component system consisting of solvent (1), polymer or pol yelectrolyte (2j), and a nonelectrolyte or electrolyte solute (3), a t wo-domain description is developed to describe thermodynamic effects o f interactions between solute components (2j) and (3). Equilibrium dia lysis, which for an electrolyte solute produces the Donnan distributio n of ions across a semipermeable membrane, provides a fundamental basi s for this two-domain description whose applicability is not restricte d, however, to systems where dialysis equilibrium is established. Expl icit expressions are obtained for the solute-polymer preferential inte raction coefficient Gamma(3,2j) (nonelectrolyte case) and for Gamma(+, 2j) and Gamma(-,2j), which are corresponding coefficients defined for single (univalent) cations and anions, respectively: Gamma(+,2j) = \Z( j)\ + Gamma(-,2j) = 0.5(\Z(j)\ + B--,B-2j + B-+,B-2j) - B(1,2j)m(3)/m( 1) Here B-+,B-2j, B--,B-2j are defined per mole of species j, respecti vely, as the number of moles of cation, anion, and water and B include d within the local domains that surround isolated molecules of j; Z(j) is the charge on j; m(3) is the molal concentration of uniunivalent e lectrolyte, and m(1) = 55.5 mol/kg for water. Incorporating this resul t into a general thermodynamic description (derived by us elsewhere) o f the effects of the activity a(+/-) of excess uniunivalent salt on an equilibrium involving two or more charged species j (each of which is dilute in comparison with the salt) yields: SaKobs = d In K-obs/d In a(+/-) = Delta(Gamma(+,2j) + Gamma(-,2j)) = Delta(B-+,B-2j + B--,B-2j - 2B(1,2j)m(3)/m(1)) where K-obs is an equilibrium quotient defined in terms of the molar concentrations of the participants, J, and Delta d enotes a stoichiometrically weighted combination of terms pertaining t o the reactant(s) and product(s). The derivation presented here does n ot depend on any particular molecular model for salt-polyelectrolyte ( or solute-polymer) interactions; it therefore generalizes our earlier (1978) derivation.