Solvation Structures and Dynamics of the Magnesium Chloride (Mg²⁺–Cl^–) Ion Pair in Water–Ethanol Mixtures

Abstract

We have performed constrained molecular dynamics simulations of magnesium chloride in water-ethanol mixtures. From the potentials of mean force (PMFs) of the Mg(2+)-Cl(-) ion pair, we notice that, as the mole fraction of ethanol increases, the depths of the minima of the contact ion pair (CIP) and solvent assisted ion pair (SAIP) increase, but the depth of the CIP minimum increases more in comparison to the SAIP minimum. This shows that ion pairing becomes more favorable with an increase in the mole fraction of ethanol. Significant differences in the PMFs between the Mg(2+) and the Cl(-) ion (depending upon whether the second Cl(-) ion is present in the first coordination shell of the Mg(2+) ion or not) seem to have been reported for the first time in this work. The local mole fraction of water molecules in the first solvation shell of ions is generally greater than in the bulk. The diffusional behavior of solvent molecules in solvation shells of the ion-pair indicates that the ions as well as the first solvation shells of the ions diffuse at much slower rates. Also, the diffusion constant of bulk water in the mixtures is greatly reduced compared to the pure solvent value.

Hydrogen‐Bond Structure and Low‐Frequency Dynamics of Electrolyte Solutions: Hydration Numbers from ab Initio Water Reorientation Dynamics and Dielectric Relaxation Spectroscopy

“…The last configuration was then used to generate MgCl 2 (aq) with concentrations ranging from 0.1 to 2.8 mol kg −1 by randomly replacing N water molecules with N /3 Mg 2+ and 2 N /3 Cl − ions. As the formation of Mg 2+ /Cl − contact ion pairs (CIPs) has been subject to some debate, [42,43] for the 0.63 and 1.30 mol kg −1 solutions we generated two set of initial configurations, with and without CIPs. Each system was subject to 6 ns of classical MD (NPT) simulations to equilibrate the cell volume using the Mg−O and Mg−Cl Lennard Jones potential parameterized by Aqvist [44] together with the SPC/E water model [45] .…”

Ion association characteristics in MgCl2and CaCl2aqueous solutions: a density functional theory and molecular dynamics investigation

“…Theoretical calculations have been proven to be a valuable tool in understanding the complex behaviour of electrolyte solutions [9]. Many authors have studied the ion-pair formation between alkali metal and halide ion, theoretically by classical molecular dynamics (MD) or Monte Carlo simulations [10,11] and experimentally by X-ray and neutron diffraction methods [12][13][14][15]. Phutela and Pitzer suggested the formation of contact ion pair (CIP) in CaCl 2 aqueous solution, compared to the MgCl 2 aqueous solution [16].…”

Thermodynamics of Na+ Cl− ion – pair association in acetonitrile–dimethyl sulfoxide mixtures

“…However, the PMFs obtained from RDFs (system B1 to D5) go to the asymptotic value of zero in the range of 10-12Å for concentrations beyond 0.1 M. Since simulations with less than 5 ion pairs do not give reliable values of RDFs, we need to use constrained MD to get PMFs when a single ion pair is present. Thus, there will be differences in the asymptotic behaviour of the PMFs obtained from the constrained MD method [25,34,35] and RDF method [15,36]. Although there are small systematic differences of the order of 1-3 kJ/mol between the PMFs obtained from different reference values of r 0 , there is hardly any difference in the ion association structures.…”