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Molecular Simulations of Structure and Dynamics of Neat and Hydrated Imidazolium Ionic Liquids


Copyright to ACS, J. Phys. Chem. B 2014, 118, 8839−8847

The understanding of complex interactions of Ionic Liquids (ILs) with water or other solvents and its effect on structure and dynamics can determine its suitability in applications to gas absorption, metal ion extraction, liquid extraction, electrochemistry, catalysis etc. The objectives of this research program is to use MD simulations (with all atom force-fields) to elucidate the structure and dynamics in imidazolium ILs in neat and hydrated environments. For example, a molecular understanding on the effect of anion on structural and dynamical properties in neat imidazolium ILs shows the following trends: ILs with smaller anions like Cl−, Br− , BF4− have relatively higher cation-anion interactions, unlike larger anions like PF6-, OTf−, NTf2−. The cationic and anionic diffusion (ionic conductivity) are lowest in ILs containing the Cl− and Br− anions, and highest with BF4−, OTf− and NTf2− anions. While the magnitude of diffusion coefficients is primarily dependent on anionic size and shape, we conclude that ion-pair lifetimes will decisively provide a direct qualitative trend with diffusion coefficients (conductivity) of ILs. The addition of water to a hydrophobic IL ([Hmim][NTf2]) IL shows the following structural features: (a) At low water concentration, small regions of water molecules are surrounded by several cation-anion pairs. (b) At medium water concentration, cation tail aggregation starts, and phase separation between the IL and water is observed. At high water concentration, increasing cationic tail aggregation leads to micelle formation. Further aggregates of cations and anions are solvated by large water channels. The Radial Distribution Functions show that cation-anion, cation-cation, and anion-anion interactions decrease and water-water interaction increases with water concentration. The simulations demonstrate the need for long simulation time scales to observe converged rotational correlation functions in neat ILs, compared to hydrated conditions. The dynamical properties can lead to opportunities for experimental measurements of transport properties in a family of several imidazolium ILs in hydrated environments.