Density of salt solutions: Effect of ions on the apparent density of water

The apparent density of water in aqueous solutions is analyzed by assuming that the volume occupied by the ions, in the absence of contact ion pairs, can be approximated reliably by spheres with appropriate ionic radii X-ray data for ionic solutions are used to identify the ionic radii with crystal radii. Small corrections for the sizes of chloride and fluoride are made. Addition of ions to water changes the packing density of the water as well as the average hydrogen bond length. The slope of the apparent density of water versus concentration (taken at infinite dilution) varies from negative to positive for univalent electrolytes at 298 K, 1 arm. in the case of sucrose solutions, the apparent density of water does not change with sucrose concentration. For salts that have a positive slope of apparent water density with concentration, a maximum in apparent density as a function of concentration is generally observed depending on the solubility range. Apparent density maxima at room temperature are more frequently observed with polyvalent electrolytes. These phenomena are discussed in terms of the equilibrium structure of water and the influence of added salts on the structure mediated by changes in hydrogen bond strength. This analysis anticipated that at low temperatures (higher hydrogen bond strength) sodium chloride solutions should show a maximum in the apparent density of water as a function of concentration. Data confirming the existence of such a maximum has been available since 1937. Changes in hydrogen band strength for water are interpreted in terms of electron delocalization involving cations, anions, and water. A. perturbation molecular orbital (PMO) model gives a good account of the Limiting slope of apparent water density. Model values far solution densities at low concentrations compare favorably with experimental results. Changes in the structural equilibrium for liquid water caused by the presence of ions will be reflected in changes in the free energy of solution for even the lowest concentration of ions.