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

Abstract

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 (C 4 mim + ) combined with perfluorinated anions, such as tetrafluoroborate (BF 4 − ), hexafluorophosphate (PF 6 − ), 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.

New Local Composition Model for Correlating of the Molar Conductivity of Ionic Liquid–Solvent Systems over the Whole Concentration Range

“…It should be noted that the anion–cation, cation–solvent, and anion–solvent interactions mentioned here refer to the average of a large number of molecules or ions in the IL–solvent systems over the whole concentration range and do not refer to interactions between individual molecules or ions at a specific defined composition. Comparing the values of ( g sc – g ac ) and ( g sa – g ca ) listed in Table , it is interesting to note that they are all positive, i.e., | g sc | < | g ac | and | g sa | < | g ca |, indicating that the anion–cation interactions are larger than the cation–solvent or anion–solvent interactions, which is consistent with the previous results from MD simulations, DFT calculations, and FT-IR spectra. − According to the magnitude of the ( g sc – g ac ) and ( g sa – g ca ) values, the studied IL–solvent systems can be classified into two types. The first type of systems is those where the solvents are H 2 O and MeOH, and ( g sc – g ac ) is smaller than ( g sa – g ca ), i.e., | g sc | > | g sa |, indicating that the cation–solvent interaction is greater than the anion–solvent interaction.…”

“…As the amount of polar solvent increases, hydrogen-bonding interactions can be formed between the aromatic hydrogen atoms in [Rmim] + and the polar solvent . Moreover, it should be noted that the strength of the anion–solvent and cation–solvent interactions is modulated by changing the composition of the mixture and the polarity of the solvent. , Hu et al compared bonding energy of the cation–solvent and anion–solvent of [Bmim]­[BF 4 ] in different polar solvents by theoretical calculations, which showed that the bonding energy of the cation–solvent was greater than that of the anion–solvent when the solvents are H 2 O and MeOH, whereas the opposite was the case when the solvents are ACN, AC, and DMSO . Therefore, it is not difficult to understand why the anion–solvent and cation–solvent interactions of the studied IL–solvent systems obtained from Λ m by the new model show a different order with the type of the solvent.…”

“…Because of the presence of the anion−solvent, cation−solvent, anion−cation, and solvent−solvent interactions in the mutually soluble IL−solvent systems, 39 there is a discrepancy between the microscopic local molar fraction and the macroscopic molar fraction of the systems. As shown in Figure 1, it is assumed that three types of cells centered on the cation, anion, and solvent molecule are formed in the microscopic state, and then the IL−solvent binary system is considered as an anion− cation−solvent quasi-ternary system.…”

“…67 Moreover, it should be noted that the strength of the anion−solvent and cation−solvent interactions is modulated by changing the composition of the mixture and the polarity of the solvent. 60,68 71 all of which contribute to the enhancement of cation−solvent interactions. As for the anion−H 2 O/MeOH interactions, their strength is related to the degree of compactness and dispersion of negative charges of the anion.…”

Effect of Mixture Composition on the Photophysics of Indoline Dyes in Imidazolium Ionic Liquid-Molecular Solvent Mixtures: A Femtosecond Transient Absorption Study

“…It was observed that a crossover occurs between cation−anion and cation-solvent/ anion-solvent distances, with the mole fraction at which this crossover happens being influenced by the solvent's competition for cation−anion interactions (higher for PC and γ-BL compared to ACN). Furthermore, the analysis of local structure using Voronoi polyhedral as a statistical observable 132 revealed that density fluctuation reaches a maximum at low mole fraction, indicating a looser radial position of mixture components compared to their fluctuation in neat states. Moreover, preliminary results from molecular dynamics simulations of D205 in a BmimBF 4 -ACN mixture also show that the average distance between the dye and ions remains relatively unchanged from the neat IL, but increases significantly for X IL values below 0.2.…”