SOME NEW EXPRESSIONS CONCERNING INDIVIDUAL IONIC ACTIVITIES AND THEIR PHYSICAL SIGNIFICANCE

According to local thermodynamic arguments, at constant temperature (T) and zero field strength (E), individual ionic chemical potentials-mu-i are functions of the local number densities and vice versa. In this paper it is shown that the quantities L(ki) defined by [GRAPHICS] where delta-ki = 0; i not-equal k delta-ki = 1; i = k are given by L(ki) = n(i) integral-o/infinity [g(ki)(r) - g"ki(r)]4-pi-r2 dr in homogeneous solutions, where n(i) is the number density of i, g(ki)(r) is the pair distribution function and g"ki(r) is the limiting form of g(ki)(r) as r --> infinity. Expressions are derived in terms of physically significant quantities for the liquid-junction potential and Donnan potential differences between two solutions. It is also shown that as n(k) --> 0 [GRAPHICS] where n'i refers to a solute-free solution with the same mu-i at the mid-plane between identical overlapping double layers. These results show that the L(ki) have a similar significance to the Kirkwood-Buff integrals in solutions of non-electrolytes. By considering a hypothetical solution in which the valency of solute --> 0 a simple method is developed for treating solution non-ideality in double-layer theory at the Debye-Huckel level. The work is compared with the statistical mechanical treatment of Stell and Lebowitz (J. Chem. Phys., 1968, 48, 3706).