Simulations of liquids with ionic interactions

Molecular Dynamics Simulations of Liquids with Ionic Interactions... 

Molecular dynamics (MD) simulations of liquids with ionic interaction have so far been performed for molten salts and aqueous electrolyte solutions. The characteristic problem for this kind of simulation are the far ranging Coulombic forces. 

Some problems of numerical stabilities in molecular dynamic calculations of liquids with ionic interactions have been discussed. Mainly a sufficient high accuracy seems to be necessary for the intergration and force routine in order to get the kinetic properties of the system from long simulation runs. 

Deschan5)s, J. and Padua, A.A.H., Interactions of gases with ionic liquids Molecular simulation, in Ionic Liquids Ilia Fundamentals, Progress, Challenges, and Opportunities, Properties and Structure, ACS Symposium Series, bl. 901, eds. R.D. Rogers and K.R. Seddon, American Chemical Society, Washington, DC (2005), pp. 150-158. 

In addition to AIMD simulation of bulk-phase ionic liquids, scientists have extended the simulation to mixed systems. Kirchner et al. studied a one-to-one mixture of EmimSCN and EmimCl. The results showed that the coordination of the anion to the most acidic hydrogen atoms in the system correlates with its basicity. The addition of the chloride anion has an important non-ideal influence on the complete system and changes the types of interactions present. Bhargava and Balasubramanian implemented an AIMD simulation of BmimPF6-C02 system. The results implied that the solvation of CO2 in Bmim PFg is primarily facilitated by the anion, as observed from the radial and spatial distribution functions. CO2 molecules were aligned tangent to the PFg spheres and were most probably located inside the octahedral voids of the anion. 

The studies discussed above have been concerned with ionic systems in the gas phase or in small clusters. This work has provided information about the structural and energetic properties of these systems as well as their reactions. However, the practical interest in these materials is in condensed phases. The inclusion of intermolecular interactions is essential for realistic descriptions of these materials. It is important to consider the electrostatic, long-range interactions. A practical way to consider these interactions is to perform simulations of solids or liquids with periodic boundary conditions. 

The first ah initio simulation of a room temperature molten salt, dimethylimi-dazolium chloride (]MMIM]Q), appeared at the beginning of 2005 [21]. The work aimed at providing information on the liquid structure, in order to compare with results from classical force-field simulations and neutron diffraction experiments. Urdike non-associating fluids, in ionic liquids the distribution of ions around certain chemical bonds may depend strongly on the instantaneous electronic structure. Therefore, site-site distribution functions and three-dimensional densities may change when passing from a classical to a quantum mechanical description of the interactions. 

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