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.