Dynamics Simulations Of NaCl Solutions Research Articles

On the calculation of solubilities via direct coexistence simulations: Investigation of NaCl aqueous solutions and Lennard-Jones binary mixtures.

Direct coexistence molecular dynamics simulations of NaCl solutions and Lennard-Jones binary mixtures were performed to explore the origin of reported discrepancies between solubilities obtained by direct interfacial simulations and values obtained from the chemical potentials of the crystal and solution phases. We find that the key cause of these discrepancies is the use of crystal slabs of insufficient width to eliminate finite-size effects. We observe that for NaCl crystal slabs thicker than 4 nm (in the direction perpendicular to the interface), the same solubility values are obtained from the direct coexistence and chemical potential routes, namely, 3.7 ± 0.2 molal at T = 298.15 K and p = 1 bar for the JC-SPC/E model. Such finite-size effects are absent in the Lennard-Jones system and are likely caused by surface dipoles present in the salt crystals. We confirmed that μs-long molecular dynamics runs are required to obtain reliable solubility values from direct coexistence calculations, provided that the initial solution conditions are near the equilibrium solubility values; even longer runs are needed for equilibration of significantly different concentrations. We do not observe any effects of the exposed crystal face on the solubility values or equilibration times. For both the NaCl and Lennard-Jones systems, the use of a spherical crystallite embedded in the solution leads to significantly higher apparent solubility values relative to the flat-interface direct coexistence calculations and the chemical potential values. Our results have broad implications for the determination of solubilities of molecular models of ionic systems.

Molecular dynamics simulations of ionic concentration gradients across model bilayers

To model a concentration gradient across a biomembrane, we have performed all-atom molecular dynamics simulations of NaCl solutions separated by two oppositely charged plates. We have employed the recently formulated three-dimensional Ewald summation with correction (EW3DC) technique for calculations of long-range electrostatics in two-dimensionally periodic systems, allowing for different salt concentrations on the two sides of the plates. Six simulations were run, varying the salt concentrations and plate surface charge density in a biologically relevant range. The simulations reveal well-defined, atomic-level asymmetries between the two sides: distinct translational and rotational orderings of water molecules; differing ion residency times; a clear wetting layer adjacent only to the negative plate; and marked differences in charge density/potential profiles which reflect the microscopic behavior. These phenomena, which may play important roles in membrane and ion channel physiology, result primarily from the electrostatics and asymmetry of water molecules, and not from the salt ions. In order to establish that EW3DC can accurately capture fundamental electrostatic interactions important to asymmetric biomembrane systems, the CHARMM force-field (with the corrected Ewald sum) has been used. Comparison of the results with previously published simulations of electrolyte near charged surfaces, which employed different force-fields, shows the robustness of the CHARMM potential and gives confidence in future all-atom bilayer simulations using EW3DC and CHARMM.

Computer simulation studies of aqueous sodium chloride solutions at 298 K and 683 K

We have carried out molecular dynamics simulations of NaCl solutions at room temperature (298 K) and at a supercritical temperature of 683 K using discrete simple point charge (SPC or SPC/E) molecular models for the water solvent. The solvent densities were 0.997 g cm−3 at 298 K and 0.35 g cm−3 and 0.175 g cm−3 at 683 K. The ion–ion and ion–solvent distribution functions were calculated and compared with corresponding functions for a continuum model of the solvent also determined by computer simulation. Our studies confirm the presence of significant amounts of ion pairing and clustering at supercritical conditions as seen in visualizations of the equilibrium configurations of the solution. However, the degree of pairing and clustering of ions in supercritical solutions is significantly different for discrete and continuum representations of the solvent. Simulations of a 1 molal solution of NaCl at 683 K, using a discrete molecular model for the solvent at a density of 0.35 g cm−3, show the presence of a single megacluster of 10 sodium and chloride ions in a system of 555 water molecules. Three smaller clusters containing positive and negative charges are observed at 683 K when the electrolyte concentration is reduced to 0.5 molal at a solvent density of 0.35 g cm−3 and also at a lower solvent density of 0.175 g cm−3. Molecular dynamics simulations of the velocity auto correlation functions of Na+ and Cl− ions have distinct forms related to the cluster to which the ion belongs. The diffusion coefficients of Na+ and Cl− ions, at infinite dilution, are larger at 683 K than at 298 K, and decrease with increasing electrolyte concentration. They are nearly equal to each other in the one molal solution at 683 K, which may correspond to a supersaturated solution in which the large cluster of sodium and chloride ions moves as an entity over an observed lifetime greater than 200 ps.

Molecular dynamics studies on the structure of methanol-water solutions of NaCl

Molecular dynamics simulations of NaCl solutions in water and in two methanol-water mixtures with methanol concentrations of 10 and 90 mol% at room temperature have been performed. Three-site flexible models of water and of methanol have been applied. The structural properties of the systems have been discussed on the basis of radial and angular distribution functions, the orientation of the solvent molecules and their geometrical arrangement in the solvation shells of ions. A preferential solvation of ions has been found. In the methanol-deficit mixture cations and anions are preferentially solvated by methanol molecules. In the methanol-rich system no preferential solvation of cations has been found, but the anions are solvated only by the methanol molecules.