Large Scale Molecular Dynamics Simulation of Aqueous NaCl Solutions

The understanding of aqueous solution properties is recognized as being important in many areas of physical chemistry and molecular biophysics. Computer simulation has become an important tool in understanding the structure and dynamics of aqueous electrolyte solutions at the atomic or molecular level. Many works of molecular dynamics (MD) simulations have been carried out to investigate hydration structure and dynamic properties of aqueous solutions [1–4]. We carried out large scale molecular dynamics simulations to investigate the concentration dependence of solute ion’s behaviors in aqueous NaCl solution. In the dilute limit of solute, there must be no Na-Cl pair formation. However, it is very difficult to detect the existence of Na-Cl pairs experimentally. This prompts us to study the microscopical structure of aqueous NaCl by using MD simulation at three concentration conditions; 0.05 mol% (0.028M), 0.10 mol% (0.055M) and 0.50 mol% (0.276M). In this work, we carried out two series of MD simulations. In the first series, Amber potential set [5] was adopted for NaCl-water and NaCl-NaCl interactions. This potential is constructed by a Lennard-Jones term and a Coulomb term. Parameters of this potential are taken from Amber force field [6,7]. These parameters were determined based on the energy of hydration of Na-water and Cl -water. In the second series, Tosi-Fumi potential [8] was adopted as inter-ionic ones. This potential was made for solid alkali halide systems, such as crystalline NaCl and KCl. And, it is possible to reproduce their liquid structures, diffusion constants, electrical conductivities and viscosities very well [9]. Amber potential set was also employed for solution-water interactions in the second series. MD simulation of low concentration condition requires a large scale system. We have developed a special-purpose computer, MDM (Molecular Dynamics Machine) [10–13], to carry out a large scale MD simulation. The MDM can perform very high-speed MD simulation of a Coulomb system using the Ewald method. The MDM consists of a MDGRAPE-2 part, a WINE-2 part and a host computer. The calculation speed of the MDM is 10100 times faster than that of a general purpose computer. We used the MDM in the present work.