United atom model for ionic liquids: UAM- IL

The knowledge of molecular interactions of room temperature ionic liquids is highly relevant to understand and to predict their behavior in many applications. A non-polarizable and transferable force field to be used in molecular simulations is developed where the hydrocarbon groups of positive ions are defined as united atoms. The model is applied to positive ions (1-alkyl-3-methyl-imidazolium [CnMIM], alkyl-trimethyl-ammonium [N-111n], N-alkyl-N-methyl-pyrrolidinium [Pyr(1n)] and N-alkyl-pyridinium [Pyrid(n)]) with a negative ion (bis(trifluoromethanesulfonyl)imide [NTf2]). The total charge on every ion is +/- 1e. The short ranged parameters are fitted to reproduce the experimental liquid density and self-diffusion coefficient at room conditions. The transferability of the parameters is evaluated through the calculation of the shear-viscosity and ionic conductivity of several ionic liquids as a function of temperature. The simulation results are in excellent agreement with experimental data, except for the ionic conductivity that has an average error of 21%, and with those from a polarizable force field. The motion of the center of mass of ions and their sites is analyzed. The clustering process in systems with imidazolium ring is discussed. The united atom model is a good option to simulate ionic liquids with a low computational cost. (C) 2021 Published by Elsevier B.V.