On theory of apparent molar volumes in concentrated aqueous solutions of strong electrolytes

Abstract

On the basis of the method reported in the previous work, according to which close-packed hydration sheaths of ions are surrounded by excess voids which in the above model were replaced by contact gap of width d0 independent of temperature and concentration, it is inferred in the present study that on extrapolating the apparent volumes of strong electrolytes to zero volume fraction of water in solution, temperature-independent water-free ion volumes Φ’i are obtained. These volumes correspond to the state in which the contact gap is shifted up to the very surface of ions, without changing its width d0 = 39 ± 2 pm. The volumes Φ’i show additivity and acquire thus the properties of parameters which are applicable to calculation of the density of aqueous solutions of strong electrolytes as well as of their mixtures up to the highest possible concentrations. With Li+ and Na+ ions, the condition of temperature invariance of Φ’i results in that the hydrate nucleus is not naked cation but a species MH2O+ analogous to H3O+ ion. This explains likely the different function of Na+ and K+ ions in biological systems (especially concerning their permeation through membranes). In the case of Mg2+, Ca2+, Sr2+, and Ba2+ ions, the hard-body centre of their hydrates is formed by hexaaqua-complexes.