Ionic solutions under high pressures V : pressure effects on the walden products and hydration of Et4N+ and ClO4- ions in water

Abstract

The electrical conductivities of aqueous solutions at high pressures up to 5, 000 atm have been measured in the concentration range from 10^-4 to 10^-3N for tetraethylammonium chloride, Et_4NCl at 25 and 40℃ and for tetraethyl-ammonium perchlorate. Et_4NClO_4 at 25℃. The equivalent conductances of the electrolytes at infinite dilution have been determined by means of the Onsager equation which was verified to be valid in the dilute solutions at high pressures in the previous papers. The limiting equivalent conductances determined at high pressures were separated into the single-ion ones on the basis of the same assumption as used in the previous paper. Although the limiting equivalent conductance of Et_4N^+ has a maximum against pressure at 25℃ like other tetraalkylammonium ions, that of ClO_4^- , surprisingly and exceptionally, has no maximum even at 25℃ where the viscosity of solvent water has a minimum at about 650 atm. The Walden product of Et_4N^+ decreases slightly with increasing pressure at 25℃ and, probably, so at 40℃ like that of Me_4N^+. On the other hand, the Walden product of ClO_4^- at 25℃ dramatically decreases with increasing pressure. Thus, it is considered that the pressure dependence of the limiting equivalent conductance of the ion in water can not be explained merely in terms of such bulk properties of water as the viscosity and dielectric constant. These differences in the pressure coefficients of the Walden products were ascribed to the differences between the density of the hydration shell and that of the bulk water.