Structural Origins of Viscosity in Imidazolium and Pyrrolidinium Ionic Liquids Coupled with the NTf₂^– Anion

Abstract

Ionic liquid viscosity is one of the most important properties to consider for practical applications. Yet, the connection between local structure and viscosity remains an open question. This article explores the structural origin of differences in the viscosity and viscoelastic relaxation across several ionic liquids, including cations with alkyl, ether, and thioether tails, of the imidazolium and pyrrolidinium families coupled with the NTf 2 − anion. In all cases, for the systems studied here, we find that pyrrolidinium-based ions are "harder" than their imidazoliumbased counterparts. We make a connection between the chemical concept of hardness vs softness and specific structural and structural dynamic quantities that can be derived from scattering experiments and simulations.

Do Ionic Liquids Slow Down in Stages?

“…REMD produces smooth S(q) changes as a function of temperature as opposed to regular MD starting from well equilibrated initial conditions where S(q) features, particularly those in the region 0.5–0.6 Å –1 , appear to be very sensitive to the actual glass or low temperature liquid being trapped as can be gleaned from Figure S10 ( vide infra ). Figure shows between 1 and 2 Å –1 what we have commonly termed an “adjacency” peak which is associated with all sorts of intramolecular and intermolecular interactions between atoms that are close by (this peak is present for all liquids, but ILs often have two other peaks at lower q value). − Around 0.7–0.8 Å –1 there is what we have called a “charge alternation” peak linked with the spacing between positive moieties separated by an anion or with anions separated by positive charge. − The lowest q-peak below ≈0.5 Å –1 is what is commonly described as a prepeak or a first sharp diffraction peak and is associated with the distance between charge networks spaced by apolar domains. − Notice that at low temperature there is an additional small peak at around 0.5–0.6 Å –1 (blue bar). This peak has been previously observed experimentall...…”

Structure-Dependence and Mechanistic Insights into the Piezoelectric Effect in Ionic Liquids

“…1-Alkyl-3-methylimidazolium RTILs with TFSI – are systematically less viscous than their 1-alkyl-1-methylpyrrolidinium counterparts. BMPyrr + TFSI – has also been characterized as having less charge localization and organization than those of BMIM + TFSI – , which was characterized by a greater extent of directional order . If this finding is true, it could imply a much more efficient dipole arrangement for piezoelectric effects in the imidazolium RTILs than that in the pyrrolidinium RTILs.…”

“…BMPyrr + TFSI – has also been characterized as having less charge localization and organization than those of BMIM + TFSI – , which was characterized by a greater extent of directional order. 80 If this finding is true, it could imply a much more efficient dipole arrangement for piezoelectric effects in the imidazolium RTILs than that in the pyrrolidinium RTILs. A lower order in the pyrrolidinium RTILs would also be consistent with the many degrees of conformational freedom of pyrrolidinium aliphatic cations, resulting in many conformers and in systematically higher pressure for glass transition than that for the more rigid, planar aromatic imidazolium cations, as has been reported in a comparative study.…”

The Coiling Effect in Ether Ionic Liquids: Exploiting Acetate as a Probe for Transport Properties and Microenvironment Analysis

“…Understanding the interactions in these ILs is crucial for unraveling the origins of the modification in their macroscopic properties. Several authors have employed different structural characterization techniques, such as X-ray diffraction, , X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy to study these systems. , Allied to these techniques, classical and ab initio molecular dynamics (AIMD) simulations, as well as density functional theory (DFT) calculations, have been used to elucidate the contributions of the ether chain to the properties of ILs, particularly in comparison to their alkyl analogs. − …”