Quantitative Investigation of Ion Clusters in a Double Salt Ionic Liquid by Both Vibrational Spectroscopy and Molecular Dynamics Simulation.

Abstract

Vibrational spectroscopy and molecular dynamics simulations are powerful tools frequently used to elucidate interactions among ions in ionic liquid electrolyte solutions. We apply these techniques to characterize ionic interactions in mixtures of 1-butyl-1-methylpyrrolidinium trifluoromethansulfonate, [C1C4pyr][CF3SO3], and lithium trifluoromethanesulfonate, LiCF3SO3, namely, [Li]0.091[C1C4pyr]0.909[CF3SO3] and [Li]0.167[C1C4pyr]0.833[CF3SO3]. The computational and experimental data indicate that extensive, LiCF3SO3-rich regions exist within the solutions, and most of the anionic species that are composed of these domains are either [Li2CF3SO3]+ or LiCF3SO3 moieties. The [Li]0.167[C1C4pyr]0.833[CF3SO3] system contains a larger number of [Li2CF3SO3]+ and [Li3CF3SO3]2+ species than [Li]0.091[C1C4pyr]0.909[CF3SO3], which may explain, in part, the reduction in ionic conductivity when LiCF3SO3 is added to [C1C4pyr][CF3SO3]. The charge-organized liquid structure inherent to [C1C4pyr][CF3SO3] supports the dynamic coupling of vibrationally induced dipole moments to form optical phonons. Consequently, intense, IR-active vibrational modes are split into transverse optical and longitudinal optical components. Band splitting is reduced when LiCF3SO3 is added to the ionic liquid, suggesting that ionically associated anions impede the ability of the ionic liquid to support optical phonons.