Abstract
The alkyl chains attached to the cation in ionic liquids play a central role in modulating the stability of the cation-anion ion pair. To elucidate how the alkyl chain length influences the local structure, we performed classical molecular dynamics (MD) simulations on imidazolium chloride ionic liquids. Vibrational frequency analysis was used to probe cation-anion interactions, and the resulting structural, hydrogen-bonding (HB) dynamics, and ion-transport consequences were systematically examined. Local mode analysis reveals a progressive change in the peak shape of the ring CH vibrational frequencies as the alkyl chain length increases. This change reflects enhanced hydrogen bonding interaction between the imidazolium cation and the chloride anion. To support this observation, the radial distribution functions show that the first peak corresponding to the interaction between the CH hydrogen of the ring and the Cl− anion shifts towards shorter distances with increasing chain length, indicating a closer approach and stronger HB character. Additionally, the presence of bifurcated hydrogen bonds formed through both the methyl CH group and the ring CH moiety significantly influences the HB dynamics. Finally, the mean squared displacement of both cations and anions increases with the alkyl chain length, demonstrating enhanced ion mobility. This trend suggests that longer alkyl chains weaken cation-anion interactions despite strengthening specific ring CH⋯Cl hydrogen bonds, ultimately facilitating greater translational motion.
•We analyzed the cation-anion interactions with an increase in the alkyl chain length attached to the imidazolium ring.•The alkyl chain length effect is studied by analyzing the ring CH vibrational frequencies.•The hydrogen bond dynamics and the mean squared displacements are studied as a consequence of changes in cation-anion interactions.