The acoustical and structural changes of aqueous solutions due to transition to molten supercooled salts

Abstract

The analysis of concentration dependencies of acoustical properties of aqueous electrolyte solutions has been carried out in a wide range of concentration. Among the large number of systems, including binary and multicomponent aqueous solutions of salts, some systems (∼20) are singled out where linear concentration behaviour of the square of sound velocity (u 2) is observed down to saturation. The linear extrapolation of u 2 up to 100% of saline component for solutions of halogens and nitrates of alkali metals yields a value of u 2 for the molten, supercooled salt. The interpretation of such acoustical dependencies is provided on the basis of a structural scheme including bulk water (1), water in hydration shells (2) and ionic clusters (3) in two concentration bands. The main principle of connection of these subsystems is the complementary organization of configuration (1), (2), (3). In the framework of this model the analytical expressions were deduced for a sound velocity of water in hydration shells in the first and second concentration bands. The condition of linearity of u 2 from 100% of water to 100% of salt was obtained. The deviations from linear concentration dependence are explained by the concentration variations of hydrophobic and hydrophilic hydration, or formation of ionic complexes. The calculated magnitudes of partial molar adiabatic compressibility of salts at infinite dilution ( k 2x 0 ) are in a good agreement with results obtained from concentration extrapolation of experimental data of compressibility coefficients and densities in dilute solution. The expression for the calculation of nonlinear acoustic parameters of solution and molten, supercooled salt (B/A) L was deduced using experimental data of dilute solution and density of molten supercooled salt. The packing factor of pure water and salt in molten, supercooled state was estimated.