Structure of Aqueous KNO₃ Solutions by Wide-Angle X-ray Scattering and Density Functional Theory

Abstract

The structure of aqueous KNO 3 solutions was studied by wide-angle X-ray scattering (WAXS) and density functional theory (DFT). The interference functions were subjected to empirical potential structure refinement (EPSR) modeling to extract the detailed hydration structure information. In aqueous KNO 3 solutions with a concentration from 0.50 to 2.72 mol•dm −3 , water molecules coordinate K + with a mean coordination number (CN) from 6.6 ± 0.9 to 5.8 ± 1.2, respectively, and a K−O W (H 2 O) distance of 2.82 Å. To further describe the hydration properties of K + , a hydration factor (f h ) was defined based on the orientational angle between the water O−H vector and the ionoxygen vector. The f h value obtained for K + is 0.792, 0.787, 0.766, and 0.765, and the corresponding average hydration numbers (HN) are 5.2, 5.1, 4.8, and 4.5. The reduced HN is compensated by the direct binding of oxygen atoms of NO 3 − with the average CN from 0.3 ± 0.7 to 2.6 ± 1.7, respectively, and the K−O N (NO 3 − ) distance of 2.82 Å. The average number of water molecules around NO 3 − slightly reduces from 10.5 ± 1.5 to 9.6 ± 1.7 with r N−Od W = 3.63 Å. K + −NO 3 − ion association is characterized by K−O N and K−N pair correlation functions (PCFs). A K−N peak is observed at 3.81 Å as the main peak with a shoulder at 3.42 Å in g K−N (r). This finding indicates that two occupancy sites are available for K + , i.e., the edge-shared bidentate (BCIP) and the cornershared monodentate (MCIP) contact ion pairs. The structure and stability of the KNO 3 (H 2 O) 10 cluster were investigated by a DFT method and cross-checked with the results from WAXS.