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

Abstract

Ionic Liquids (ILs) are highly ionic compounds where its physical and chemical proper- ties can be appropriately tuned by varying the choice of cations and anions. The complex interactions of ILs with water or other solvents can also influence the structure and dynam- ics of these mixtures and can determine its suitability in applications to gas absorption, metal ion extraction, liquid extraction, electrochemistry, catalysis etc. Molecular Dynam- ics simulations (using all atom forcefields) can provide enormous physical insights into the structure and dynamics of 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−, BF−4 haverelativelyhighercation-anioninteractions,unlikelargeranionslikePF−6 ,OTf− , NTf−2 . The cationic and anionic diffusion (ionic conductivity) are lowest in ILs with Cl− and Br− anions, and highest with BF−4 , OTf− and NTf−2 anions. While the magnitude of diffusion coefficients is primarily dependent on anionic size and shape, 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]) shows the follow- ing 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. (c) At high water concentration, increasing cationic tail aggregation leads to micelle forma- tion. 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 in- teractions decrease and water-water interaction increases with water concentration. The translational and rotational dynamics in hydrated [Hmim][Cl] ILs and [Hmim][NTf2] ILs is also investigated. At low, intermediate, and high water concentration, dynamical prop- erties (diffusion coefficient) of hydrophobic/hydrophilic ILs shows the following trends: Cl− > Hmim+ ; Cl− > NTf−2 ; Hmim+ ( [Hmim][Cl]) > Hmim+ ([Hmim] [NTf2]). At ultra-low water concentration, hydrophilic ([Hmim][Cl]) IL shows several bridges likeconfigurations between water molecules and Cl− anions which is supported by a complex distribution of water clusters and positive value of water-water interaction energy. The ro- tational relaxation time of Hmim+ cations is larger in neat [Hmim][Cl] compared to neat [Hmim][NTf2] IL with an opposite trend seen with hydration.