Ionic volumes, compressibilities and electrostriction of electrolyte solutions.

A theory of electrostriction of solvent water near ions is developed. The treatment is based on a calculation of the effective pressure which would, in the absence of the field, cause the she change in volume as does the field. It is shown that when the dissolution process is from the crystalline phase, the electrostriction at small ions arises mainly on account of changes of effective volume of the solvent rather than from compression of the ion cavity in the dielectric. The change in solvent volume near ions can be related, using an appropriate molecular model, to the apparent change in the volume of the salt upon dissolution. This treatment avoids the difficulties of previous calculations where use of the Born equation involves an integration over a distance from the ion. Explicit approximate solutions for high-and low-field conditions, and a complete numerical solution for the effective pressure-field and field-volume relations are given. A method for calculation of the compressional entropy associated with hydration of ions is proposed on the basis of the above electrostriction calculations, taking into account dielectric saturation and pressure dependence of dielectric constant. Relations of the electrostatic contributions to the entropy of hydration of ions to corresponding volume changes are investigated and it is shown that the statistical mechanical method of Eley and Evans and the method of Laidler and Pegis, allowing for dielectric saturation, give entropies of hydration that are in better agreement with experimental values. The partial molal volumes V¯ of a series of homologous tetraalkylammonium chlorides, bromides and iodides have been precisely measured by means of a differential buoyancy technique. The extrapolated values of V¯ to infinite dilution give V¯° data which are almost additive for successive homologues and enable the absolute individual partial g. ionic volumes of the halide ions to be estimated by a new principle to an accuracy of about 0.5 ml. (g. ion)-1. The anomalously large negative concentration dependence of V¯ is discussed in terms of mutual salting-in by the large cations and also salting-out due to the anion. Estimates of a and d ln a/dP are also made. Non-electrostatic effects arising from the large differences in size between the cations and the solvent molecules are suggested as one of the reasons for the anomalous activity coefficient and partial molal volume behaviour. It is concluded that the hydrophobic character of these ions exerts a strong structural influence on the solvent water, as indicated in other ways in previous publications. The partial molal volumes of some alkylamine hydrogen halides series have been studied with special reference to the role of solvation in the process of ionisation of methylammonium bases. It is shown that the change in volume associated with the ionisation is almost invariant as the degree of hydrophobic character of the cation is increased. The unexpected behaviour of DeltaS°, DeltaC p and DeltaV¯° is discussed in terms of structural influences upon the solvent by the different species. Partial molal adiabatic compressibility studies have been carried out on the homologous series of tetraalkylammonium bromides and iodides, methylammonium chlorides and methylamines. The extrapolated values of fKs to infinite dilution give f&j0;Ks data which confirm the suspected structure promoting effect of these salts upon solvent water structure; these results complement those obtained from the partial molal volume studies