Ionic Pairing and Selective Solvation of Butylmethylimidazolium Chloride Ion Pair in DMSO-Water Mixtures: A Comprehensive Examination via Molecular Dynamics Simulations and Potentials of Mean Force Analysis
Authors: Meti Manjunath D. , Srivastava Srishti , Srivastava Amar , Hajari Timir , Dixit Mayank 
Published: September 2023

Chloride-based ionic liquids, exemplified by 1-butyl-3-methylimidazolium chloride (BMIM+ − Cl−), possess the capacity to dissolve cellulose, a predominant constituent of biomass. However, their inherently high viscosity poses a hindrance to efficient biomass dissolution for biofuel production. An intriguing solution emerges through the utilization of chloride-based ionic liquids in combination with solvents like water and DMSO, offering a promising avenue to reduce ionic liquid viscosity and enhance the efficacy of biomass dissolution. Utilizing constrained molecular dynamics simulations, we have conducted an extensive exploration of the potentials of mean force (PMFs) governing the behavior of the 1-butyl-3-methylimidazolium chloride (BMIM+ − Cl−) ion pair within dimethyl sulfoxide (DMSO)-water mixtures. Analysis of the BMIM+ − Cl− ion pair PMFs has revealed a noteworthy trend: with increasing DMSO mole fraction, there is a conspicuous augmentation in the depths of the minima associated with both the contact ion pair (CIP) and the solvent-assisted ion pair (SAIP). Notably, the CIP minimum exhibits a more pronounced increment relative to the SAIP minimum. This compelling observation underscores the heightened thermodynamic favorability of ion pairing as the DMSO mole fraction elevates. The credibility of the PMFs is corroborated through the meticulous computation of ion pair residence times for various inter-ionic separations. Thermodynamic assessments discern an intriguing trend: within the range of DMSO mole fractions (xDMSO) spanning 0.10, 0.21, 0.35, and 0.48, the stabilization of both CIPs and SAIPs is driven by entropy. In contrast, for xDMSO values of 0.0, 0.91, and 1.00, enthalpy plays a pivotal role in stabilizing the CIP and SAIP states. Further insights emerge from the meticulous analysis of radial distribution functions (RDFs) characterizing the arrangement of water and DMSO molecules surrounding the BMIM+ − Cl− ion pair. This scrutiny reveals the propensity of water molecules to form hydrogen bonds with the chloride ion, while DMSO molecules preferentially engage in hydrogen bonding with the BMIM+ ion, both within the CIP and SAIP states. Re- markably, water emerges as the preferred solvent for the solvation of the BMIM+− Cl− ion pair, superseding the affinity of DMSO. A notable transition surfaces as the DMSO mole fraction transitions from 1.0 to 0.91, resulting in a pronounced diminishment in the stability of both CIP and SAIP states, attributed to a substantial amplification in the local water density. Significantly, the preferential binding coefficients (γ) values for DMSO consistently show negativity, indicating its preferential exclusion from the BMIM+ − Cl− ion pair in DMSO-water mixtures. The calculated decay times for the survival probabilities of water and dimethyl sulfoxide (DMSO) molecules in the vicinity of the BMIM+ − Cl− ion pair suggest that the water cluster surrounding the Cl− ion exhibits greater stability compared to the DMSO cluster around the Cl− ion, while conversely, the trend is reversed for the BMIM+ ion. These findings advance our understanding of ion pairing kinetics in DMSO-water mixtures, providing valuable insights applicable to a wide spectrum of endeavors, notably including the incorporation of BMIM+ − Cl− ionic liquids in the conversion of biomass into biofuels.