Atomistic Molecular Dynamics Simulation of Pyridinium Bromide Ionic Liquids: Quantitative Insights into the Effects of Alkyl Chain Length on Ion Transport and Microstructure
Volume 23, Issue 1, Spring 2026, Pages 87-107
https://doi.org/10.22034/ijche.2026.572366.1584
Zahra Fakhri, Azim Soltanabadi
Abstract This study employs classical molecular dynamics simulations using the OPLS-AA force field to systematically investigate the influence of the length of alkyl chain on the structural, thermodynamic, and dynamical properties of a homologous series of pyridinium-based ionic liquids (methyl- to pentyl-pyridinium bromide). The main objective is to elucidate how the gradual elongation of the alkyl chain affects intermolecular interactions and ion transport behavior at the molecular level. The model demonstrates good agreement with available experimental density data, confirming its reliability for predicting physicochemical trends in these systems. The results indicate that increasing the length of the alkyl chain weakens electrostatic interactions and enhances free volume, leading to a systematic reduction in density and cohesive energy density. The structural analysis reveals well-defined cation–anion coordination shells, reflecting strong local ionic organization across all systems. The dynamical analysis shows a consistent decrease in the ionic mobility with the elongationof chains, due to stronger van der Waals interactions and steric effects, which in turn reduce diffusion and ionic conductivity. Importantly, the ionic transference numbers calculated from ion mobilities clearly demonstrate that cations contribute more to charge transport than anions in all investigated systems. This cation-dominated transport behavior provides a direct molecular-level explanation for the observed decrease in ionic conductivity by increasing the length of chains.