State of Hydration Shells of Sodium Chloride in Aqueous Solutions in a Wide Concentration Range at 273.15-373.15 K

On the basis of the measurements of the speed of ultrasound, density, and isobaric heat capacity, isentropic compressibilities of aqueous solutions of sodium chloride (from dilute up to highly concentrated) have been determined at 273.15-373.15 K. With the use of correct correlations, at electrolyte concentrations below complete hydration limit (21.53% wt), the following values have been calculated: solvation numbers (h), molar parameters of volume and compressibility of hydration complexes (V-h, beta Y-h(h)), water in a hydration shell (V-1h, beta V-1h(1h)), and a cavity containing a stoichiometric mixture of ions without a hydration shell (V-2h, beta V-2h(2h)). The values of h and beta V-h(h) have been found to be independent of temperature in the indicated temperature interval, while the molar compressibilities of the hydration shell (beta(1h),V-1h) and of the stoichiometric mixture of ions (beta V-2h(2h), of concentration. The electrostatic field of ions has been shown to influence the temperature dependence of the molar volume of water in the hydration shell much stronger than the mere change of pressure affects the temperature dependence of molar volume of water in the hydration shell of pure water. It is suggested that the reason for this effect is connected with the change in the dielectric permeability of water in direct proximity to ions. Below the complete solvation limit (CSL), the functions Y-K,Y-S = f(beta V-1(1)*), beta V-h(h) = f(h), V-1h = f(T), and beta(1h) = f(T) are linear with a high correlation coefficient. Thus, at all studied concentrations of sodium chloride solutions in the temperature interval 273.15-323.15 K, the dependence Y-K,Y-S = f(beta V-1(1)*) enables h and beta V-h(h) to be calculated using the obtained correlations; at higher temperatures (323.15-373.15 K), the dependence is reverse. Beyond the CSL, the dependence of the apparent molar volume phi(V)' upon the hydration number h' remains linear and allows V-2h', and V-1h' to be estimated at various temperatures.