Local structure in sub- and supercritical CO2: A Voronoi polyhedra analysis study

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.

Local Structure of DMF-Water Mixtures, as Seen from Computer Simulations and Voronoi Analysis

Molecular dynamics simulations of mixtures of N,N-dimethylformamide (DMF) with water of various compositions, covering the entire composition range, are performed on the canonical (N,V,T) ensemble. The local structure of the mixtures is analyzed in terms of radial distribution functions and the contributions of the first five neighbors to them, various order parameters of the water molecules around each other, and properties of the Voronoi polyhedra of the molecules. The analyses lead to the following main conclusions. The two molecules are mixing with each other even on the molecular scale; however, small self-aggregates of both components persist even at their small mole fraction values. In particular, water-water H-bonds exist in the entire composition range, while water clusters larger than 3 and 2 molecules disappear above the DMF mole fraction values of about 0.7 and 0.9, respectively. The O atoms of the DMF molecules can well replace water O atoms in the hydrogen-bonding network. Further, the H-bonding structure is enhanced by the presence of the hydrophobic CH₃ groups of the DMF molecules. On the other hand, the H-bonding network of the molecules gradually breaks down upon the addition of DMF to the system due to the lack of H-donating groups of the DMF molecules. Finally, in neat DMF, the molecules form weak, CH-donated H-bonds with each other; however, these H-bonds disappear upon the addition of water due to the increasing competition with the considerably stronger OH-donated H-bonds DMF can form with the water molecules.

Voronoi Polyhedra as a Tool for the Characterization of Inhomogeneous Distribution in 1-Butyl-3-methylimidazolium Cation-Based Ionic Liquids

The inhomogeneity distribution in four imidazolium-based ionic liquids (ILs) containing the 1-butyl-3-methylimidazolium (C₄mim) cation, coupled with tetrafluoroborate (BF₄), hexafluorophosphate (PF₆), bis(trifluoromethanesulfonyl)amide (TFSA), and trifluoromethanesulfonate (TfO) anions, was characterized using Voronoi polyhedra. For this purpose, molecular dynamic simulations have been performed on the isothermal-isobaric (NpT) ensemble. We checked the ability of the potential models to reproduce the experimental density, heat of vaporization, and transport properties (diffusion and viscosity) of these ionic liquids. The inhomogeneity distribution of ions around the ring, methyl, and butyl chain terminal hydrogen atoms of the C₄mim cation was investigated by means of Voronoi polyhedra analysis. For this purpose, the position of the C₄mim cation was described successively by the ring, methyl, and butyl chain terminal hydrogen atoms, while that of the anions was described by their F or O atom. We calculated the Voronoi polyhedra distributions of the volume, the density, and the asphericity parameters as well as that of the radius of the spherical intermolecular voids. We carried out the analysis in two steps. In the first step, both ions were taken into account. The calculated distributions gave information on the neighboring ions around a reference one. In the second step, to distinguish between like and oppositely charged ions and then to get information on the inhomogeneity distribution of the like ions, we repeated the same calculations on the same sample configurations and removed one of the ions and considered only the other one. Detailed analysis of these distributions has revealed that the ring hydrogen atoms are mainly solvated by the anions, while the methyl and butyl terminal H atoms are surrounded by like atoms. The extent of this inhomogeneity was assessed by calculating the cluster size distribution that shows that the dimers are the most abundant ones.

On the characterization of inhomogeneity of the density distribution in supercritical fluids via molecular dynamics simulation and data mining analysis

We combined molecular dynamics simulation and DBSCAN algorithm (Density Based Spatial Clustering of Application with Noise) in order to characterize the local density inhomogeneity distribution in supercritical fluids. The DBSCAN is an algorithm that is capable of finding arbitrarily shaped density domains, where domains are defined as dense regions separated by low-density regions. The inhomogeneity of density domain distributions of Ar system in sub- and supercritical conditions along the 50 bar isobar is associated with the occurrence of a maximum in the fluctuation of number of particles of the density domains. This maximum coincides with the temperature, Tα, at which the thermal expansion occurs. Furthermore, using Voronoi polyhedral analysis, we characterized the structure of the density domains. The results show that with increasing temperature below Tα, the increase of the inhomogeneity is mainly associated with the density fluctuation of the border particles of the density domains, while with increasing temperature above Tα, the decrease of the inhomogeneity is associated with the core particles.