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

The structure of aqueous KNO₃ 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 KNO₃ 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(H₂O) 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 ion-oxygen 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 NO₃^- with the average CN from 0.3 ± 0.7 to 2.6 ± 1.7, respectively, and the K-O_N(NO₃^-) distance of 2.82 Å. The average number of water molecules around NO₃^- slightly reduces from 10.5 ± 1.5 to 9.6 ± 1.7 with r_N-O = 3.63 Å. K⁺-NO₃^- 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 corner-shared monodentate (MCIP) contact ion pairs. The structure and stability of the KNO₃(H₂O)₁₀ cluster were investigated by a DFT method and cross-checked with the results from WAXS.