Derivation of an Accurate Forcefield for Ionic Liquid Simulation

T. YOUNGS, C. PINILLA, M. G DEL PÓPOLO , J. KOHANOFF AND R. M. LYNDEN-BELL

Salzburg, Austria

Congreso; 1st International Congress on Ionic Liquids; 2005

Ionic liquids (ILs) are currently receiving a great deal of attention owing to their prospective use as replacements for common solvents used in industrial organic synthesis. However, like many molecular liquids they are not easily investigated by experimental techniques. For this reason simulation of ILs has become a rapidly expanding area, allowing previously unobserved structural and electronic properties to be quantified.In this work we present results for  the first ab-initio Molecular Dynamics simulation of an IL [1].  Dimethylimidazolium chloride([DMIM][Cl]) is our model liquid since its structure is know experimentally [2].  We discuss results on the basis of site-site and three-dimensional distribution functions, electronic structure features and the nature of the interactions between the hydrogens of the cations and the anions. Agreements and discrepancies with experiment are highlighted.The main drawback of ab-initio simulation is its limitation to small system sizes and short integration times. Classical models offer a complementary level of description. Three forcefields for  [DMIM][Cl] are constructed from OPLS-AA dispersive parameters, with the cation geometry and three alternative sets of atomic charges taken from ab initio calculations. Molecular dynamics (MD) simulations of a cubic system of 200 ion pairs are performed for each model. Simulation results are compared with previous classical MD studies of the same liquid [3,4], neutron diffraction data [2], and the former ab initio results [1].  In general terms the structural features of the liquid agree across the different data sets; however major discrepancies are evident when considering atomic site-site RDFs between the classical and ab initio MD results. That suggests that an improved forcefield could be obtained using a force matching method over the ab initio trajectories. Preliminary results to this end are presented.            Ionic liquids have been recently used as solvent in the development of dye-sensitized solar cells (Gratzel cell [5]). These devices are based on a thin film of dye-sensitized nanocrystalline TiO2, interpenetrated by the solvent containing a redox couple. Experimental evidence suggests that the charge transport mechanism in the liquid confined within the TiO2  pores, affects the efficiency of the cell. We study the behaviour of [DMIM][Cl] under confinement. Structural properties together with relevant dynamical properties such as the diffusion coefficient for each atomic species are presented.[1] C. Hardacre, J. D. Holbrey, S. E. J. McMath, D. T. Bowron and A. K. Soper, J. Chem. Phys., 118, 273 (2003).[2] M. Del Popolo, J. Kohanoff and R. M. Lynden-Bell, Submitted to J.Chem. Phys. B, in press (2004).[3] C. G. Hanke, S. L. Price and R. M. Lynden-Bell, Mol. Phys., 99, 801 (2001).[4] S. M. Urahata and M. C. C. Ribeiro, J. Chem. Phys., 120, 1855 (2004).[5] M. Gratzel, Nature, 414, 338 (2001)