The structure of aqueous lithium chloride solutions at high concentrations as revealed by a comparison of classical interatomic potential models

Abstract

Highly concentrated aqueous lithium chloride solutions were investigated by classical molecular dynamics (MD) and reverse Monte Carlo (RMC) simulations. At first MD calculations were carried out applying twenty-nine combinations of ion-water interaction models at four salt concentrations. The structural predictions of the different models were compared, the contributions of different structural motifs to the partial pair correlation functions (PPCF) were determined. Particle configurations obtained from MD simulations were further refined using the RMC method to get better agreement with experimental X-ray and neutron diffraction data. The PPCFs calculated from MD simulations were fitted together with the experimental structure factors to construct structural models that are as consistent as possible with both the experimental results and the results of the MD simulations. The MD models were validated according to the quality of the fits. Although none of the tested MD models can describe the structure perfectly at the highest investigated concentration, their comparison made it possible to determine the main structural properties of that solution as well. It was found that four nearest neighbors (oxygen atoms and chloride ions together) are around a lithium ion at each concentration, while in the surroundings of the chloride ion hydrogen atom pairs are replaced by one lithium ion as the concentration increases. While in pure liquid water four water molecules can be found around a central water molecule, near the solubility limit nearly all water molecules are connected to two chloride ions (via their hydrogen atoms) and one lithium ion (by their oxygen atoms).Ion-water potential model; Structure nowadays. Unfortunately, for highly concentrated LiCl solutions they are almost absent, and the comparison with experimental data sets (e.g. XRD or ND data) is completely missing.Several interaction models (force fields, FF) have been developed and used for MD simulations in the context of electrolyte solutions. However, for concentrated aqueous LiCl solutions their applicability is very limited, as was shown recently [65]. In that work 29 different models were investigated; it turns out that their predictions about density, dielectric constant and self-diffusion constants scatter widely. Neutron and X-ray weighted structure factors were also calculated and compared with experimental data: it was found that the agreement between the experimental and simulation curves was poor, especially for the highest investigated concentration (19.55 mol/kg, near the solubility limit).The reverse Monte Carlo method [66] is a useful tool to find three dimensional configurations consistent with experimental data sets. However the number of experimental data sets (mostly one or two structure factors from neutron and/or X-ray diffraction measurements) is low compared to the number of the independent partial pair correlation functions (10). This lack of information results in a highly underdetermined system (see the 'pure' RMC results in R...

Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs

“…We wished to facilitate comparability with literature by taking exactly the same salt concentrations as in previous, related works from our laboratory 15,16,22,23 , namely 3.74 m (mol/kg; corresponding to 6.3 molar %), 8.3 m (13 molar %), 11.37 m (17 molar %) and 19.55 m (26 molar %). In fact, the calculations providing the atomic assemblies ('particle configurations') were identical to those reported recently by one of us 15 .…”

“…Most importantly, H-bonded Cl -…H maximum distances are somewhat longer than O…H ones, as it is defined by the first minimum of the Cl --H and O-H partial radial distribution functions (see, e.g., Refs. 15,16,23 ). The potential model called 'JC-S' from Ref.…”

“…The potential model called 'JC-S' from Ref. 16 is used just below for demonstrating features of the 'mixed' H-bond network concept.…”

“…As 'good' force field, the JC-S combination from Ref. 16 was taken, that consists of the SPC/E water model 28 which was found to be one of the worst when comparing simulated and experimental total structure factors, cluster sizes with Clions (part (c)) appear to be smaller than the percolation limit, whereas pure water clusters (part (d)) are large even at high salt concentrations. This indicates non-perfect mixing (i.e., 'microphase-segregation' between water and salt), which is against the observation that all solution considered are homogeneous.…”

“…hydrogen bonds 15,16 (without specific orientations). In other words, the number of H-bonds formed by chloride ions is limited by steric effects only.…”