Dynamical heterogeneities of rotational motion in room temperature ionic liquids evidenced by molecular dynamics simulations

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged N_head–H group and the hydrophobic C_tail–H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the N_head–H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the C_tail–H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the N_head–H group slows down with increasing cationic chain length, while the rotation of the C_tail–H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the N_head–H group with increasing cationic chain length is associated with a lower number of nitrate anions near the N_head–H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the N_head–H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of N_head–H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents.