Approximate theory for effect of ionic polarizability on equilibrium constant for contact ion pairs

Abstract

By considering an ion to be a sphere of low dielectric constant in a liquid of higher dielectric constant, an expression is derived to modify the Denison-Ramsey theory or other “hard sphere” theory of contact ion pairs for the effect of ionic polarizability. The modified theory decreases a, the distance between the centres of the ions at contact which is computed from the measured equilibrium constant for contact ion pairs; the effect is greater the less symmetrical the electrolyte. Contact ion pairing may be completely prevented with some highly unsymmetrical electrolytes, although the Debye ionic atmosphere, Bjerrum long-range ion pairs, and other longer-range effects of the coulombic forces will still occur. The theory may also be applied to pairwise interactions between ions and neutral molecules.