Response of Ionic Hydration Structure and Selective Transport Behavior to Aqueous Solution Chemistry during Nanofiltration

The effect of aqueous solution chemistry on the ionic hydration structure and its corresponding nanofiltration (NF) selectivity is a research gap concerning ion-selective transport. In this study, the hydration distribution of two typical monovalent anions (Cl- and NO3-) under different aqueous solution chemical conditions and the corresponding transmembrane selectivity during NF were investigated by using in situ liquid time-of-flight secondary ion mass spectrometry in combination with molecular dynamics simulations. We demonstrate the inextricable link between the ion hydration structure and the pore steric effect and further find that ionic transmembrane transport can be regulated by breaking the balance between the hydrogen bond network (i.e., water-water) and ion hydration (i.e., ion-water) interactions of hydrated ion. For strongly hydrated (H2O)nCl- with more intense ion-water interactions, a higher salt concentration and coexisting ion competition led to a larger hydrated size and, thus, a higher ion rejection by the NF membrane, whereas weakly hydrated (H2O)nNO3- takes the reverse under the same conditions. Stronger OH--anion hydration competition resulted in a smaller hydrated size of (H2O)nCl- and (H2O)nNO3-, showing a lower observed average hydration number at pH 10.5. This study deepens the long-overlooked understanding of NF separation mechanisms, concerning the hydration structure.

Molecular modeling and simulation of aqueous solutions of alkali nitrates

A set of molecular models for the alkali nitrates (LiNO3, NaNO3, KNO3, RbNO3, and CsNO3) in aqueous solutions is presented and used for predicting the thermophysical properties of these solutions with molecular dynamics simulations. The set of models is obtained from a combination of a model for the nitrate anion from the literature with a set of models for the alkali cations developed in previous works of our group. The water model is SPC/E and the Lorentz–Berthelot combining rules are used for describing the unlike interactions. This combination is shown to yield fair predictions of thermophysical and structural properties of the studied aqueous solutions, namely the density, the water activity and the mean ionic activity coefficient, the self-diffusion coefficients of the ions, and radial distribution functions, which were studied at 298 K and 1 bar; except for the density of the solutions of all five nitrates and the activity properties of solutions of NaNO3, which were also studied at 333 K. For calculating the water the activity and the mean ionic activity coefficient, the OPAS ( osmotic pressure for the activity of selvents) method was applied. The new models extend an ion model family for the alkali halides developed in previous works of our group in a consistent way.

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-OW = 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.

Solubilities in aqueous nitrate solutions that appear to reverse the law of mass action

Non-ideal aqueous electrolyte solutions have been studied since the start of the application of thermodynamics to chemistry in the late 19th century. The present study examines some of the most extreme non-ideal behavior ever observed: solubilities of alkali and NH₄⁺ nitrate salts in water that appear to behave the opposite of how the Law of Mass Action would predict. A literature review discovered that the solubilities of NH₄NO₃ and many alkali nitrate salts increases when another nitrate-bearing electrolyte is added to solution. These occurrences were in concentrated solutions with insufficient water to provide all ions their preferred hydration number without sharing waters between ions. This water deficit results in the formation of contact ion-pairs as well as larger ion-clusters. These ion-clusters may be favored when there is more than one type of monovalent cation present.