Revealing Hydrogen Bond Dynamics between Ion Pairs in Binary and Reciprocal Ionic Liquid Mixtures

Molecular dynamics simulations are performed to probe the molecular-level interactions between various ionic pairs in the reciprocal mixture consisting of equimolar amounts of the cations 1-ethyl-3-methylimidazolium [C2mim]+ and 1-n-hexyl-3-methylimidazolium [C6mim]+ and dicyanamide [DCA]− and bis(trifluoromethanesulfonyl)imide [NTf2]− anions. Any enhancement or depletion in these interactions is compared with those existing in equimolar binary mixtures [C2mim][C6mim][DCA], [C2mim][C6mim][NTf2], [C2mim][DCA][NTf2], and [C6mim][DCA][NTf2] and pure ionic liquids [C2mim][DCA], [C6mim][DCA], [C2mim][NTf2], and [C6mim][NTf2]. The simulation results indicate that the [C2mim]+ cation prefers to interact favorably with the strongly coordinating [DCA]− anion through enhanced hydrogen-bonding interactions, while showing no preference towards the other anion in the reciprocal mixture. The average hydrogen bond lifetimes between [C2mim]+ cation and [DCA]− anion increases by a factor of two in the reciprocal mixture compared to that in the pure system. We find that the hydrogen bond lifetimes in the various systems are directly correlated to the first peak intensity in the center-of-mass radial distribution functions for corresponding ion pairs and have a direct bearing on the self-diffusion coefficients of the ions. Our results point to possibilities of tuning interactions between various species in reciprocal ionic liquid mixture by appropriately changing the cation and anion combinations.