Molecular dynamics simulations of equilibrium and transport properties of amino acid-based room temperature ionic liquids.

Molecular dynamics simulations are used to study liquid-state equilibrium and transport properties of the 1-ethyl-3-methylimidazolium salts of the 20 naturally occurring amino acids [emim][AA] that all form room temperature ionic liquids. These ionic liquids have been recently synthesized by Ohno and co-workers [J. Am. Chem. Soc. 2005, 127, 2398], but other than measured ionic conductivity at 25 degrees C, there is a dearth of quantitative measurements on the physiochemical properties of these liquids. The goal is to computationally study the density, polarity, transference number, and ionic conductivity of this family of solvents. We also study the spatial correlations among the imidazolium cation and amino acid anions in these liquids by computing atomic and charge radial distribution functions and preparing polarity maps. The microscopic dynamics behavior of these materials is determined by studying the mean square displacements (MSD) and velocity autocorrelation functions (VACF). The diffusion coefficients of the liquids are determined using the MSD and VACF, and the contributions of the anions and cations to the transport of charge in the ionic liquids are studied. Ionic liquids of this family that show strong anion-anion and anion-cation associations in the simulations are experimentally observed to show anomalously low electrical conductivities. Knowledge of the microscopic structures and dynamics of these liquids can allow for an intelligent choice of a solvent from this class that has required polarity and ionic conductivity.