Molecular dynamics simulation study of sodium ion structure & dynamics in water in ionic liquids electrolytes using 1-butyl-3-methyl imidazolium tetrafluoroborate and 1-butyl-3-methyl imidazolium hexafluorophosphate

Abstract

In this paper, we have performed an all-atom molecular dynamics simulation to understand the structure and dynamics of Na⁺ ions in water mixed Ionic liquids (Water in Ionic liquid). Two ionic liquid (IL) systems consist of (1) 1-butyl-3-methylimidazolium [BMIM] tetrafluoroborate [BF₄] and (2) 1-butyl-3-methylimidazolium [BMIM] hexafluorophosphate [PF₆] were considered in this work. We understand various inter^-molecular structures and dynamic and thermodynamic behaviours of Na⁺ ions in the water^-mixed IL systems. The water (H₂O) mole fractions (x) varied from 0.33 to 0.71. The neat ILs [BMIM][BF₄] and [BMIM][PF₆] pairwise radial distribution functions show a decrease with an increase in x. The [BMIM][PF₆] exhibits a strong coordination structure with Na⁺ ions across the entire range of x values. The rdf between the pairs of Na⁺-[PF₆] presents a significant interaction compared to Na⁺ and [BF₄]. The Na ⁺ ions manifested greater coordination with H₂O In H₂O-[BMIM][PF₆] compared to H₂O-[BMIM][BF₄]. The self-diffusion coefficient (D) values of Na ⁺ ions increase with the rise in x in both ILs. The D values of Na ⁺ ions are 10-fold higher in [BMIM][BF₄] than [BMIM][PF₆]. The ionic conductivity values are higher for [BMIM][BF₄]. Overall, this paper unveils molecular-level insights for understanding the behavior of Na⁺ ions in the water in ionic liquid systems.

Keywords: MD simulation; Na(+) ion; Water in ionic liquids; [BF(4)] and [PF(6)]; [BMIM].