Origin of the Difference in Ion-Water Distances Determined by X-ray and Neutron Diffraction Measurements for Aqueous NaCl and KCl Solutions

<jats:title>Abstract</jats:title> <jats:p>Experimental evidence has been presented on the difference in intermolecular ion-water distances obtained from X-ray and neutron diffraction methods. Simultaneous least squares fitting procedures were performed for X-ray and neutron interference terms observed for (NaCl)x(*H2O)1−x, (x = 0, 0.02, 0.05, and 0.098) and (KCl)x(*H2O)1−x, (x = 0, 0.02, 0.05, and 0.075) solutions at 25 °C, respectively. The null-water mixture was employed for neutron diffraction measurements for these solutions to eliminate structural contribution from hydrogen atoms. It has been revealed that the hydration numbers of Na+ and K+ are concentration dependent and the values for lower-concentration limit are 5 and 6, respectively. The nearest neighbor Na+⋯H2O and K+⋯H2O distances are obtained to be 2.36–2.37 and 2.75–2.82 Å, respectively. In order to examine the effect of the separate treatment of interactions between ion-oxygen and ion-hydrogen atoms in the X-ray model function, simultaneous fitting procedures were carried out for X-ray and neutron diffraction data observed for 9.8 mol% NaCl and 7.5 mol% KCl solutions by employing the individual atom model for the X-ray interference term. Obtained Na+⋯O and K+⋯O distances are ca. 0.02 Å shorter than those determined by the simultaneous fit employing the usual united model for water molecules. The nearest neighbor Cl−⋯O distance derived from the simultaneous fit by means of the individual atom model exhibits ca. 0.1 Å shorter than that obtained from the fit using the united atom model. The present Cl−⋯O distance agrees with that obtained from neutron diffraction measurements on 35Cl/37Cl isotopically substituted aqueous 5 mol% Na*Cl solutions in D2O. The simultaneous fitting analyses employing X-ray model function with the united and individual atom models of water molecule have revealed that the ion-oxygen (water) internuclear distance is significantly shorter than the average separation of electron clouds between ion and neighboring water molecule. The present results indicate that the difference in ion-water distance observed from X-ray and neutron diffraction studies mainly arises from the united atom model of X-ray diffraction data analysis assuming a spherical electron density around oxygen atom within the water molecules.</jats:p>