Structural similarity of an ionic liquid and the mixture of the neutral molecules

Insight to the Local Structure of Mixtures of Imidazolium-Based Ionic Liquids and Molecular Solvents from Molecular Dynamics Simulations and Voronoi Analysis

While the physicochemical properties as well as the NMR and vibration spectroscopic data of the mixtures of ionic liquids (ILs) with molecular solvents undergo a drastic change around the IL mole fraction of 0.2, the local structure of the mixtures pertaining to this behavior remains unclear. In this work, the local structure of 12 mixtures of 1-butyl-3-methylimidazolium cation (C₄mim⁺) combined with perfluorinated anions, such as tetrafluoroborate (BF₄^-), hexafluorophosphate (PF₆^-), trifluoromethylsulfonate (TFO^-), and bis(trifluoromethanesulfonyl)imide, (TFSI^-), and aprotic dipolar solvents, such as acetonitrile (AN), propylene carbonate (PC), and gamma butyrolactone (γ-BL) is studied by molecular dynamics simulations in the entire composition range, with an emphasis on the IL mole fractions around 0.2. Distributions of metric properties corresponding to the Voronoi polyhedra of the particles (volume assigned to the particles, local density, radius of spherical voids) are determined, using representative sites of the cations, anions, and the solvent molecules, to characterize the changes in the local structure of these mixtures. By analyzing the mole fraction dependence of the average value, fluctuation, and skewness parameter of these distributions, the present study reveals that, around the IL mole fraction of 0.2, the local structure of the mixture undergoes a transition between that determined by the interionic interactions and that determined by the interactions between the ions and solvent molecules. It should be noted that the strength of the interactions between the ions and the solvent molecules, modulated by the change in the composition of the mixture, plays an important role in the occurrence of this transition. The signature of the change in the local structure is traced back to the nonlinear change of the mean values, fluctuations, and skewness values of the metric Voronoi polyhedra distributions.

Pressing matter: why are ionic liquids so viscous?

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

Hydrogen Bond Redistribution Effects in Mixtures of Protic Ionic Liquids Sharing the Same Cation: Non-ideal Mixing with Large Negative Mixing Enthalpies

We report a joint experimental and theoretical study characterising the hydrogen bond (HB) redistribution in mixtures of two different protic ionic liquids (PILs) sharing the same cation: triethylammonium-methanesulfonate ([TEA][OMs]) and...

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids – a molecular dynamics study

Both viscosity and the shear-thinning of ionic liquids are determined mainly by ionic interaction, with density having a secondary effect.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.