Paper title:
Molecular dynamics simulation of solution structure and dynamics of aqueous sodium chloride solutions from dilute to supersaturated concentration
Journal:
JMS 139 (1998) 1-16
Author/s:
Hirohisa Uchida and Masakuni Matsuoka
Department of Chemical Engineering, Tokyo University of Agriculture and Technology, (TUAT), 24-16 Nakacho-2, Koganei, Tokyo 184-8588, Japan
Summary:
This article studies the effects of salt concentration on the solution structure and dynamics of aqueous NaCl solutions by using molecular dynamics (MD) simulations. Concentrations were varied from dilute to supersaturated NaCl solutions.
To construct the simulation system, water molecules were represented by rigid single-point charge/Ewald (SPC/E) model while the sodium and chloride ions were represented as charged Lennard-Jones particles. A total of 256 particles were present in the simulation cell. Through the Berendsen thermostat and barostat, simulations were done at constant temperature and pressure of 298 K and 0.1 MPa, respectively. The concentrations varied from 0.22 to 6.12 mol/dm3. The saturated point of the solution was at 5.42 mol/dm3.
Results are divided into two areas: (1) thermodynamical and structural properties, and, (2) dynamical properties.
1. First, simulated molar volumes of the different aqueous NaCl solutions were compared with experimental values to ensure the reliability of potential models and parameters. When it was confirmed, radial distribution functions (RDFs) were obtained for Na+-Na+, Cl?-Cl? and Na+-Cl? pairs. An RDF can suggest the existence of local ion-ion structural correlation. Results show that both contact ion pairs and solvent-separated ion pairs exist. The change in coordination numbers was also observed. It was found out that the coordination numbers of Na+-Cl? ion pairs increased with salt concentration from dilute to saturated condition. But for supersaturated solution, the number of contact ion pairs had negligible change (or was almost constant).
2. Using the velocity autocorrelation functions (VACFs) and/or the mean square displacement (MSD), the tracer diffusion coefficients may be calculated. The tracer diffusion coefficients were found to decrease with salt concentration until the saturated concentration. For Na+, there was no significant change at the supersaturated region. Also, the tracer diffusion coefficients of both ions were relatively close to each other.
Because the diffusion coefficients were close to each other and the number of contact ion pairs was constant, it was concluded that clusters would form in supersaturated solutions.
Contribution and application:
This paper gave example of models that can be used to model water and NaCl solution for simulating seawater at different concentrations. This will be helpful when ESEL simulates desalination processes and for investigating the transport of water and ions. Although the paper is aimed at elucidating nucleation and crystallization mechanisms in supersaturated solutions, the paper also shows a good description of how to analyze MD raw data to evaluate both thermodynamic and structural properties, and, dynamic properties of a system.
By: Hannah Ebro
hannah@gist.ac.kr