Structural study of low concentration LiCl aqueous solutions in the liquid, supercooled, and hyperquenched glassy states (original) (raw)
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Neutron and X-ray diffraction measurements on highly concentrated aqueous LiCl solutions
Journal of Molecular Liquids, 2012
New neutron and X-ray diffraction experiments have been performed on concentrated aqueous lithium chloride (LiCl) solutions under ambient conditions, as a function of the salt concentration. The concentration range covers a wide region, between ion-pair:water ratios of about 1:3 (which is about the solubility limit) and 1:15 (which is still rather high a concentration value). Data have been interpreted by means of the Reverse Monte Carlo structural modelling method. It has been established that meaningful ion-water and water-water partial radial distribution functions may be derived even from such limited amount of diffraction data, provided that the diffraction measurements and the method of interpreting them are chosen sensibly. The new data and their analyses suggest the presence of Li + cations with only 3 (or less) coordinated water molecules around them. At the highest concentration, one counter-ion has been found to penetrate into the first hydration sphere of the ions.
Journal of Molecular Structure, 2008
A detailed analysis of aqueous LiCl solution structures is performed by molecular dynamics (MD) simulations and X-ray scattering at two salt concentrations (6 and 7 m) at 298 K. Another run is performed for C = 14 m. MD simulations are carried out with rigid SPC water model, while the ions are treated as charged Lennard-Jones particles. Both MD and X-ray scattering investigations show a significant reorganization of the solvent. The degree of hydrogen bonding of which clearly decreases when compared to pure fluid. MD results also show that the first hydration shell of the cation is greatly distorted at high concentrations whereas the corresponding one of the anion appears nearly invariant. Moreover, the ion-pairing process is accented at highest salt concentration. X-ray data are analyzed to deduce the structure factor and the pair correlation functions of each system. A good concordance is highlighted between experimental and theoretical results. A model of intermolecular arrangement, via MD results, is then proposed to describe the local order in each system, as deduced from X-ray scattering data.
The Journal of Physical Chemistry B, 2014
Aqueous solutions of LiCl have recently received much attention in connection with the study of the anomalies of supercooled water and its polyamorphism. From the point of view of computer simulation, there is need for a force field that can reproduce the structural and dynamical properties of this solution, and more importantly it is also simple enough to use in large scale simulations of supercooled states. We study by molecular dynamics the structure of the LiCl−water solutions with the force field proposed by Joung and Cheatham (J. Phys. Chem. B 2008, 112, 9020) appropriate for the water TIP4P-Ew model potential. We found that this force field does not reproduce the experimental ion pairing when the Lorentz−Berthelot (LB) rules are used. By incorporating deviations to the LB rules to obtain the crossed interactions between the ions, it is possible to get agreement with experiment. We have studied how the modification of the LB rule affects the structural and thermodynamic properties of the solution at increasing concentration of the solution from the low (around 2%) to medium (around 14%) concentration regimes. We also tested the transferability of the Joung and Cheatham force field to the water TIP4P/2005 model that works very well for supercooled water.
1 H NMR study of water clusters in supercooled LiCl / water solution
2015
We have performed a proton Nuclear Magnetic Resonance (NMR) study on a water solution of lithium chloride at eutectic concentration (6.76 M) as a function of the temperature. We have acquired the NMR spectra from 320K to 205K each 5K and executed the line-shape analysis by means of a Gaussian deconvolution. Our data reveal the existence of different local water structures within the solution and suggest the occurrence of a liquidliquid transition at about 225K below which water is able to develop its characteristic tetrahedral network. Keywords—lithium chloride, dynamical crossover, water solution, NMR.
Journal of Molecular Structure, 2007
X-ray scattering and molecular dynamics (MD) simulations are carried out on pure water and aqueous lithium chloride solutions at four salt concentrations 0.5, 1.5, 3 and 4 molal. Both experimental and theoretical results indicate a decrease of hydrogen bonding in liquid water as compared with pure fluid. MD results also show that the hydration number around the cation decreases with increasing salt concentration; simultaneously the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. The local order in the systems, as deduced from MD simulation by considering the rigid SPC water model and the Lennard-Jones potential, is directly confronted with that extracted from X-ray data. A good agreement is shown between the theoretical and experimental pair correlation functions of each system.
Structural Properties of High and Low Density Water in a Supercooled Aqueous Solution of Salt
The Journal of Physical Chemistry B, 2011
We consider and compare the structural properties of bulk TIP4P water and of a sodium chloride aqueous solution in TIP4P water with concentration c = 0.67 mol/kg, in the metastable supercooled region. In a previous paper [D. Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)] we found in both systems the presence of a liquid-liquid critical point (LLCP). The LLCP is believed to be the end point of the coexistence line between a high density liquid (HDL) and a low density liquid (LDL) phase of water. In the present paper we study the different features of water-water structure in HDL and LDL both in bulk water and in the solution. We find that the ions are able to modify the bulk LDL structure, rendering water-water structure more similar to the bulk HDL case. By the study of the hydration structure in HDL and LDL, a possible mechanism for the modification of the bulk LDL structure in the solution is identified in the substitution of the oxygen by the chloride ion in oxygen coordination shells.
Physical Chemistry Chemical Physics, 2011
We study with the method of molecular dynamics simulation the structural properties of aqueous solutions of NaCl, KCl and KF salts at ambient conditions and upon supercooling at constant pressure. The calculations are performed at increasing concentration of the salt starting from c = 0.67 mol kg À1 up to 3.96 mol kg À1. We investigate the modifications of the hydration shells and the changes in the water structure induced by the presence of the ions. The oxygen-oxygen structure is strongly dependent on the ionic concentration while it is almost independent from the cation. The hydrogen bonding is preserved at all concentrations and temperatures. The main effect of increasing the ionic concentration is the tendency of the water structure to assume the high density liquid form predicted for pure water upon supercooling. An important consequence of our analysis is that the concept of an ion as a structure maker or a structure breaker must be revisited to take into account the other ionic species, the ionic concentration and more generally the thermodynamic conditions of the solutions.
Temperature and Concentration Effects on Li + -Ion Hydration. A Molecular Dynamics Simulation Study
The Journal of Physical Chemistry B, 2003
Molecular dynamics simulations of aqueous LiCl solution have been carried out over wide concentration (from 0.1 to 11.4 mol/kg) and temperature (from -30 to 110°C) ranges. Three different interaction potentials are investigated: the recent Li + -water effective pair potential, derived from ab initio molecular dynamics simulations Laaksonen, A. J. Chem. Phys. 2001, 114, 3120], as well as earlier potentials of Lennard-Jones type with two widely different sets of parameters [Dang, L. X.; J. Chem. Phys. 1992, 96, 6970 and Heinzinger, K.; Physica B 1985, 131, 196]. Hydration structure and residence times around Li + are studied with focus on the still somewhat controversial issue of hydration structure: both tetrahedral and octahedral water coordination have been predicted from the experiments. Besides classical MD simulations, even complementary Car-Parrinello simulations were employed to investigate the stability of a possible six-coordinated hydration shell around lithium. Self-diffusion coefficients for lithium were calculated for Li + from the simulations and compared to NMR spin-echo measurements. The new ab initiobased exponential Li + -H 2 O interaction potential appears to be robust giving the overall characteristic hydration properties in agreement with experiments. However, while it reproduces the radial distribution function (RDF) features from a recent neutron diffraction with isotopic substitution (NDIS) experiments with a well-pronounced tetrahedral water structure, the same experiment is interpreted to give octahedral water structure around lithium at the same concentrations and temperatures as were used in our simulations.
Journal of Molecular Structure, 2008
A neutron diffraction study has been carried out on pure water and sodium chloride aqueous solutions ranging from 0.5 to 3 m under ambient conditions. The present contribution extends our earlier study by means of X-ray scattering and molecular dynamics simulations. The recorded data are analyzed to yield the total structure factor and the pair correlation function of each system. It was shown that the D-D and O-D intramolecular distances remain independent on concentration. Whereas, the degree of hydrogen bonding in solutions decreases in comparison with that of pure water. To describe the local order of solutions, simulated results using a rigid SPC water model are used to propose a model of intermolecular arrangement. Good agreement is found between computed structure factors and experimental ones.
The Journal of Chemical Physics, 2008
We present the results of a molecular dynamics simulation study of thermodynamic and structural properties upon supercooling of a low concentration sodium chloride solution in TIP4P water and the comparison with the corresponding bulk quantities. We study the isotherms and the isochores for both the aqueous solution and bulk water. The comparison of the phase diagrams shows that thermodynamic properties of the solution are not merely shifted with respect to the bulk. Moreover, from the analysis of the thermodynamic curves, both the spinodal line and the temperatures of maximum density curve can be calculated. The spinodal line appears not to be influenced by the presence of ions at the chosen concentration, while the temperatures of maximum density curve displays both a mild shift in temperature and a shape modification with respect to bulk. Signatures of the presence of a liquid-liquid critical point are found in the aqueous solution. By analysing the water-ion radial distribution functions of the aqueous solution we observe that upon changing density, structural modifications appear close to the spinodal. For low temperatures additional modifications appear also for densities close to that corresponding to a low density configurational energy minimum.