On the hydration of simple ions and polyions

Abstract

Hydration of polymeric ions is investigated by examination of salting-out behaviour in relation to dielectric saturation effects and to the significance of partial molal volumes of polysalts. The validity of previous distribution theories of salting-out is examined and limitations are shown to arise from an unjustified expansion of an exponential distribution factor, and neglect of solvent dielectric saturation effects in the primary solvation shell of the ion where, relatively, the maximum salting-out effect is developed. A new theory is proposed, as a basis for extension to the case of polyions, in which the calculation of the salting-out behaviour is carried out over two regions: ( a ) in the primary hydration shell where a high degree of solvent dielectric saturation prevails and the exponential distribution function must be retained, and ( b ) beyond the primary region where weaker polarization occurs and Kirkwood’s theory of dielectrics can be applied. Comparison of the predictions of various forms of the salting-out equation with experimental data is made. Comparative calculations of the variation of solvent dielectric constant with distance from ions of various geometries have been carried out. The cases examined are those for simple spherical ions, linear polyions and planar colloidal ions having various charge densities. The dependence of solvent dielectric constant on distance from the ions enables a semi-empirical primary hydration radius r d to be evaluated. Values of r d for various simple and polymeric ions are examined in relation to salting-out by polyions and their corresponding monomers and in relation to hydration radii deduced by other methods. In the case of the polyions and planar ions, the extent to which increasing charge density causes increased dielectric saturation can be evaluated. Experimental studies on hydration of polyions and corresponding monomer salts have been carried out by examination of the salting-out of selected non-electrolytes by the polysalts, and by partial molal volume measurements. The effect of variation of the degree of neutralization or charge density has been examined; in general, it is found that the salting-out and partial molal volume behaviour varies with degree of ionization in the expected manner but differs from the corresponding behaviour of the monomer salts owing to stronger electrostriction effects due to condensation of the ionic charge into a chain configuration. The results are examined in relation to the dielectric saturation calculations for polyions, and theoretical salting-out equations are developed for the polyion case, and applied to the present experimental results. The partial molal volume data for the same materials lead to complementary information on the effective hydration radius and solvation numbers of the polysalts.