Molecular dynamics simulation studies of 1,3-dimethyl imidazolium nitrate ionic liquid with water

The fundamental understanding of intermolecular interactions of ionic liquids with water represents a vital extent in predicting ionic liquid properties. Intermolecular or noncovalent interactions were studied for 1,3-dimethyl imidazolium [DMIM]+ cation and nitrate [NO3]- anion with water, employing quantum mechanics (QM) and molecular dynamics (MD) simulations. Extensive electronic structure calculations were performed first for molecular dimers, using various levels of theory and basis sets to pinpoint dimer optimized geometries and estimate binding energies in the gas phase. Many calculations resulted in planar dimer geometries for the cation-anion and anion-water pairs using 6-311++G(d,p) basis set. Dispersion corrected exchange correlation functionals resulted in more favorable binding energies for all tested pairs in comparison with energies obtained using Moller-Plesset second order perturbation theory (MP2). Molecular dynamics simulations were performed next using a revised multipolar polarizable force field (PFF). The effect of water on ionic liquids was evaluated in terms of thermodynamic properties. Thermodynamic properties included liquid densities, excess molar volumes, and liquid structures g(r) as a function of water concentration. Densities of ionic liquid-water mixtures monotonically decrease while increasing the concentration of water. A negative excess volume is obtained for low water concentrations demonstrating strong intermolecular interactions of water with ionic liquid components. Liquid structures of ionic liquid - water mixtures revealed a tendency for anions to interact with cations at shorter intermolecular distances when water concentration is increased.