No abstract

It is shown that the excess thermodynamic properties calculated from the mean spherical approximation (via the energy integrals) for the primitive model of ionic mixtures, that is, electrolytes atid molten salts, can be expressed as functions of a single parameter . The expressions are quite simple, and for what we call low concentrations (up to 1 N of a simple salt) reduce to the same relations that Debye found for the finite size ions, only that 2 takes the place of the Debye length. The interpretation that 2 is in fact the correct screening parameter for finite size ions is borne by the asymptotic form of the pair correlation function at small densities.

Departures from ideality in electrolytes are described in the framework of the primitive model of ionic solutions in which the solvent is a dielectric continuum, using the mean spherical approximation (MSA). To include solvation and solvent concentration effects, we consider that the permittivity of the solvent and the sizes of the ions are concentration-dependent parameters. New expressions are derived for the activity coefficients and the osmotic coefficient. They are applied to pure ionic aqueous solutions of 18 salts, taking simple functions for the adjusted parameters. Good fittings are obtained in the concentration range 0−6 mol/kg.

An invariant expansion of the two-body statistical correlation function of a fluid is proposed. This expansion does not depend on any particular reference frame used to define the orientation of the molecules, and therefore can be reduced to the expansions of the literature in a simple way. The new expansion permits a rather convenient way of including the effects of molecular symmetry into it. The expressions for a few thermodynamic properties in terms of this expansion are obtained. The equations for x-ray, neutron, and light scattering are somewhat simpler using this expansion. The Ornstein—Zernike equation has a very convenient form, and is given in Fourier transformed form in terms of 6j angular recoupling coefficients.