Molecular Dynamics Simulation of Na⁺–Cl^– Ion-Pair in Water–Methanol Mixtures under Supercritical and Ambient Conditions

Abstract

Constrained molecular dynamics simulations have been performed to investigate the structure and thermodynamics of Na(+)-Cl(-) ion-pair association in water-methanol mixtures under supercritical and ambient conditions in dilute solutions. From the computed potentials of mean force (PMFs) we find that contact ion pairs (CIPs) are more stable than all other associated states of the ion pairs in both ambient and supercritical conditions. Stabilities of CIPs increase with increase in the mole fraction of methanol. In supercritical conditions, major changes in PMFs occur as we go from x(methanol) = 0.00 to x(methanol) = 0.50. The stable solvent shared ion pair (SShIP) which occurs in x(methanol) = 0.00 and 0.25, vanishes when x(methanol) is 0.50 or greater. The stabilities of these ion pairs increase with increasing temperature. Local structures around the ions are studied using the radial distribution functions, density profiles, angular distribution functions, running coordination numbers and excess coordination numbers. Preferential solvation analysis shows that both Na(+) and Cl(-) ions are preferentially solvated by water. From the calculation of enthalpies and entropies, we find that Na(+)-Cl(-) ion-pair association in water-methanol binary mixtures is endothermic and driven by entropy both in ambient as well as under supercritical conditions.

Structural and Dynamical Properties of Alkaline Earth Metal Halides in Supercritical Water: Effect of Ion Size and Concentration

“…As a result, CIPs, SAIPs, and SSIPs are more stable in supercritical conditions compared to those in ambient conditions. This has been previously observed in ion pair association studies by several authors. ,, Because the decrease in the dielectric constant facilitates ion association, one may expect the contact ion pair to be more stable in the 5.55 m solution in supercritical conditions, but in reality, we observe the opposite. There is hardly any decrease in the dielectric constants as we go from the 0.27 m (∼3.47) to 5.55 m solution (∼3.30) in the supercritical conditions.…”

“…For the Cl − ion, the anion−oxygen peak is centered at 0.33 nm, which is in good agreement with that deduced from MD simulations 5,6,28,29,78,81 and from X-ray diffraction studies. The first peaks of the Cl − −O (H 2 O) radial distribution functions are centered at around 0.33 nm in both the thermodynamic states under study.…”

“…In recent years, there have been several experimental and theoretical studies on the solvation structure and dynamics of alkali metal halides in SCW, but detailed studies of alkaline earth metal halides in high-temperature, high-pressure systems are rare. Several studies have been reported on the effect of solvent composition on the ion–ion (both alkali and alkaline earth metal halides) potentials of mean force (PMFs) in supercritical conditions via a constrained method (infinite dilution). − It has been established that the contact ion pair (CIP) is the most stable structure in supercritical conditions as compared to that in ambient conditions. However, solvation in a concentrated salt solution may proceed in a different manner than in dilute solutions because the solutes may be present as ion clusters rather than as single ions.…”

Influence of Cosolutes on Chemical Equilibrium: a Kirkwood–Buff Theory for Ion Pair Association–Dissociation Processes in Ternary Electrolyte Solutions

“…For simple salts in water–dimethyl sulfoxide (DMSO) mixtures, it was observed that sodium ions are preferentially solvated by water molecules, whereas chloride ions favor a coordination by DMSO, which stands in sharp contrast to the corresponding donor numbers of the solvents in pure solution . Furthermore, it was observed that sodium chloride reveals a homoselective preferential solvation behavior by water molecules in water–methanol and DMSO–acetonitrile (AN) solutions, , as well as in water–AN mixtures . For more complex organic ions, recent simulations of DMSO–water mixtures also revealed a homoselective solvation mechanism, where water molecules form the first solvation shell around both ion species.…”