Effect of Cosolvent on Bulk and Interfacial Characteristics of Imidazolium Based Room Temperature Ionic Liquids

Ionic Liquid Solutions Show Anomalous Crowding Behavior at an Electrode Surface

X-ray reflectivity was used to study the several-nanometer-thick "crowded" layers that form at the interfaces between a planar electrode and concentrated solutions of ionic liquids. The ionic liquid [P_14,6,6,6]⁺[NTf₂]^- was dissolved in either strongly polar propylene carbonate or weakly polar dimethyl carbonate. In the range of 19-100 vol % ionic liquid, between working electrode potentials +2 and +2.75 V, uniform 2-7 nm thick interfacial layers were observed. These layers are not pure anions but contain three to five times as many anions as cations and about the same percentage of solvent as the bulk solution. On the other side of the layer, the density is that of the bulk solution. These features are inconsistent with a picture of the crowded layer as a region of pure, close-packed counterions. Not only the layer thickness but also the charge density decrease with increasing dilution at any given applied voltage. This appears to indicate, counterintuitively, that a thinner layer with lower net charge density will screen an electric field as effectively as a thicker layer with higher charge density.

Solvent Polarity Governs Ion Interactions and Transport in a Solvated Room-Temperature Ionic Liquid

We explore the influence of the solvent dipole moment on cation-anion interactions and transport in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl), [BMIM⁺][Tf2N^-]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation-anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid-solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes.