Abstract
A New Model is proposed, to replace the existing 'charged sphere' model for electrolyte solutions. Charge transport in such systems is examined, and equations are derived, based on this model, predicting the conductivity of solutions containing symmetrical, unsymmetrical or mixed electrolytes, both associated and unassociated. Techniques are developed for the analysis of experimental conductance data for solutions of associated unsymmetrical salts. The new model, developed and extended from the original 'cosphere' concept of Gurney(16),represents each ion as being surrounded by a cosphere of radius Ri. The experimentally determinable distance parameter R, now introduced, is the distance between the ions at which cation and anion cospheres overlap. R is defined as the distance at which short-range repulsive forces (arising from the effects of dielectric saturation about each ion, hard-core interactions, and the possible formation of solvent-separated ion-pairs) become sufficiently significant to impede further approach of the ions, allowing the formation of contact or solvent-separated ion-pairs. All cation-anion pairs closer than R are thus defined as paired. Outside the cosphere of each ion, all interactions between the central ion and the solvent molecules are assumed to be negligible; hence in this region the solvent can be rigorously represented as a dielectric continuum and the Bogoliubov, Born, Green, Kirkwood and Yvon (BBGKY) hierarchy of equations can be applied without approximation to a system of such cospheres. Inside R the discrete nature and finite size of the solvent molecules becomes significant. The new model represents an advance upon the 'charged sphere' model, making allowance for the discontinuous nature of the solvent and the existence of ion-solvent interactions; when such interactions are strong (and in such systems ion association and the formation of solvent separated ion-pairs are normally important) the new model is particularly suitable. When ion-dipole interactions are weak R → a and the charged sphere model becomes rigorous. The existing conductance equations of Pitts, Fuoss & Onsager, Fuoss, Onsager and Skinner, Kraeft and Ebeling, Murphy and Cohen, Carman and Quint and Viallard (all based on the Primitive Model) are examined and compared. Conductance measurements have been made on solutions of some 2:1 salts in methanol at 25°C. This data, and a large amount of other data available from the literature, has been analysed using the New Model conductance equation; excellent 'fits' of the theoretical to the experimental data are found, and values of the parameters Lambdao, R and KA have been determined. The R values obtained show clearly the formation of solvent separated ion-pairs in many systems.