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–OW(H2O) distance of 2.82 Å. To further describe the hydration properties of K+, a hydration factor (fh) was defined based on the orientational angle between the water O–H vector and the ion-oxygen vector. The fh 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–ON(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 rN–OW = 3.63 Å. K+–NO3– ion association is characterized by K–ON 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 gK–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.