Single-ion contributions to activity coefficient derivatives, second moment coefficients, and the liquid junction potential

We discuss several interrelated single-ion thermodynamic properties require d to calculate the liquid junction potential Psi between two solutions of t he same binary electrolyte. According to a previously reported molecular th eory of nonuniform electrolyte solutions in nonequilibrium, Psi is determin ed by the transport numbers of the ions, and by the second moment coefficie nts H-alpha((2)) of the charge densities around the ions. The latter may be viewed as the single-ion contributors to the second moment condition of St illinger and Lovett. For a solution of a single binary electrolyte, we rela te the H-alpha((2)) (R) to the derivatives of the single-ion activity coeff icients gamma(alpha) with respect to the ionic strength. In the light of th ese results, we examine, in some detail, the role played by the specific sh ort-range interionic interactions in determining Psi. We investigate this m atter by means of integral equation calculations for realistic models of Li Cl and NaCl aqueous solutions in the 0-1 mol-dm(-3) range. In addition to t he hypernetted-chain (HNC) relation, we perform calculations under a new in tegral equation closure that is a hybrid between the HNC and Percus-Yevick closures. Like the HNC approximation, the new closure satisfies the Stillin ger and Lovett condition. However, for the models considered in this study, the two closures predict different dependence of the H-alpha((2)) and of P si on the specific part of the interionic interactions.