Acoustic Study of Solvent Coordination in the Hydration Shells of Potassium Iodide

The isentropic compressibilities of aqueous solutions of potassium iodide, from dilute to almost saturated, were determined at 288 to 308 K based on precise measurements of the speed of ultrasound. Using proper correlations, the hydration numbers (h) were calculated as well as the molar volume and compressibility parameters of the hydrated complexes (V _h , β _h V _h ) of water in the hydration shell (V _1h , β_1h V _1h), and of the cavity containing stochiometric mixtures of K⁺ and I⁻ ions (V _2h, β_2h V _2h). It is revealed that under the studied conditions, the obtained values of h and β _h V _h are independent of temperature whereas the molar compressibility of the hydration shell β _h V _h) is independent of concentration. The electrostatic field of the ions is shown to influence the temperature dependence of the molar volume of water in the hydration shell more substantially than a change of pressure alone influences the temperature dependence of the molar volume of pure water.