Connecting Correlated and Uncorrelated Transport to Dynamics of Ionic Interactions in Cyclic Ammonium-Based Ionic Liquids.

The correlations present in the ionic liquids (ILs) are essential to interpret the ion association and dynamics processes. The correlated and uncorrelated ionic conductivities reported from two different types of experiments were calculated employing molecular dynamics methods by exercising appropriate care to obtain the diffusive regions. The ionic conductivity is found to be correlated with lifetimes of the ion-pair and ion-cage formation. In this study, the structure and dynamic properties of five cyclic ammonium-based ILs were investigated by comparing the experimental results with the calculated transport properties: (1) 1-allyl-1-methylpyrrolidinium, (2) 1-propyl-1-methylpyrrolidinum, (3) 1-methyl-1-allylpiperidinium, (4) 4-allyl-4-methylmorpholin-4-ium, and (5) 4-allyl-4-ethylmorpholin-4-ium with bis(trifluoromethanesulfonyl)imide anion as the common anion. We observed the linear relationship between the inverse of ion-pair lifetimes and ionic conductivities. The diffusion coefficients obtained from velocity autocorrelation functions follow a trend similar to that of the experiment as compared to mean square displacements (MSDs). The ionic conductivities from correlated MSD and current autocorrelation functions are compared to the ionic conductivities from the conductometer experiment. The time correlation functions of the ion-pair and ion-cage dynamics were calculated. The correlation functions were used to obtain the lifetimes. Pyrrolidinium-based ILs show lower lifetimes than other ILs, which correlates with the conductivity. Morpholinium-based ILs show higher interaction between ions than other ILs. This result supports the slower dynamics present in morpholinium-based ILs than in other ILs. In this work, our objective is to give atomic insight into the dynamics of IL, which could not be extracted from the experiment, and relate microscopic properties with macroscopic properties.