Bogdan A. Marekha

Translational Diffusion in Mixtures of Imidazolium ILs with Polar Aprotic Molecular Solvents

Self-diffusion coefficients of cations and solvent molecules were determined with (1)H NMR in mixtures of 1-n-butyl-3-methylimidazolium (Bmim(+)) tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethylsulfonyl)imide (TFSI(-)) with acetonitrile (AN), γ-butyrolactone (γ-BL), and propylene carbonate (PC) over the entire composition range at 300 K. The relative diffusivities of solvent molecules to cations as a function of concentration were found to depend on the solvent but not on the anion (i.e., IL). In all cases the values exhibit a plateau at low IL content (x(IL) < 0.2) and then increase steeply (AN), moderately (γ-BL), or negligibly (PC) at higher IL concentrations. This behavior was related to the different solvation patterns in the employed solvents. In BmimPF6-based systems, anionic diffusivities were followed via (31)P nuclei and found to be higher than the corresponding cation values in IL-poor systems and lower in the IL-rich region. The inversion point of relative ionic diffusivities was found around equimolar composition and does not depend on the solvent. At this point, a distinct change in the ion-diffusion mechanism appears to take place.

Free Energy of Mixing of Acetone and Methanol: A Computer Simulation Investigation

The change of the Helmholtz free energy, internal energy, and entropy accompanying the mixing of acetone and methanol is calculated in the entire composition range by the method of thermodynamic integration using three different potential model combinations of the two compounds. In the first system, both molecules are described by the OPLS, and in the second system, both molecules are described by the original TraPPE force field, whereas in the third system a modified version of the TraPPE potential is used for acetone in combination with the original TraPPE model of methanol. The results reveal that, in contrast with the acetone-water system, all of these three model combinations are able to reproduce the full miscibility of acetone and methanol, although the thermodynamic driving force of this mixing is very small. It is also seen, in accordance with the finding of former structural analyses, that the mixing of the two components is driven by the entropy term corresponding to the ideal mixing, which is large enough to overcompensate the effect of the energy increase and entropy loss due to the interaction of the unlike components in the mixtures. Among the three model combinations, the use of the original TraPPE model of methanol and modified TraPPE model of acetone turns out to be clearly the best in this respect, as it is able to reproduce the experimental free energy, internal energy, and entropy of mixing values within 0.15 kJ/mol, 0.2 kJ/mol, and 1 J/(mol K), respectively, in the entire composition range. The success of this model combination originates from the fact that the use of the modified TraPPE model of acetone instead of the original one in these mixtures improves the reproduction of the entropy of mixing, while it retains the ability of the original model of excellently reproducing the internal energy of mixing.

Thermodynamics of Mixing Water with Dimethyl Sulfoxide, as Seen from Computer Simulations

The Helmholtz free energy, energy, and entropy of mixing of eight different models of dimethyl sulfoxide (DMSO) with four widely used water models are calculated at 298 K over the entire composition range by means of thermodynamic integration along a suitably chosen thermodynamic path, and compared with experimental data. All 32 model combinations considered are able to reproduce the experimental values rather well, within RT (free energy and energy) and R (entropy) at any composition, and quite often the deviation from the experimental data is even smaller, being in the order of the uncertainty of the calculated free energy or energy, and entropy values of 0.1 kJ/mol and 0.1 J/(mol K), respectively. On the other hand, none of the model combinations considered can accurately reproduce all three experimental functions simultaneously. Furthermore, the fact that the entropy of mixing changes sign with increasing DMSO mole fraction is only reproduced by a handful of model pairs. Model combinations that (i) give the best reproduction of the experimental free energy, while still reasonably well reproducing the experimental energy and entropy of mixing, and (ii) that give the best reproduction of the experimental energy and entropy, while still reasonably well reproducing the experimental free energy of mixing, are identified.

Local Structure in Terms of Nearest-Neighbor Approach in 1-Butyl-3-methylimidazolium-Based Ionic Liquids: MD Simulations.

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation-anion H-bond-like motifs, cation-cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C4mim(+)) cation is coupled with the acetate (OAc(-)), chloride (Cl(-)), tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), or bis(trifluoromethanesulfonyl)amide (TFSA(-)) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc(-) and Cl(-), prefer to be located in the imidazolium ring plane next to the C-H(2/4-5) sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H(2) site are found to be particularly enhanced in comparison with the ones at H(4-5) in the case of asymmetric and/or more basic anions (C4mimOAc, C4mimCl, C4mimTfO, and C4mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation-cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion.

Inhomogeneous Distribution in Methanol/Acetone Mixture: Vibrational and NMR Spectroscopy Analysis

The main aim of this paper is to quantify the inhomogeneous distribution of the components of acetone/methanol mixture and to give detailed insight into the interplay between the dipole-dipole and hydrogen bonding interactions inducing this inhomogeneity. To this end, we used the concept of infrared excess molar absorption of a given vibrational mode as an observable which contains all the information on the collective interactions in the mixture. Indeed, the changes in the infrared excess molar absorption may be associated with the inhomogeneous distribution (clustering, self-association, or high-density domains) of the components and consequently with the interaction between the two components of the mixture. The results show that acetone molecules are not homogeneously distributed in the mixture, particularly in the mole fraction range of acetone between 0.05 and 0.55. The spectral signature of this inhomogeneity is associated with the appearance of a shoulder in the C═O and C-C stretching vibrational profiles of acetone. This inhomogeneity is driven by the prevalence of the dipole-dipole interactions over those of hydrogen bonding between acetone and methanol molecules. The inhomogeneous distribution of methanol molecules is found to occur in the mole fraction range of acetone between 0.55 and 1. In this case, the hydrogen bond interactions between methanol molecules prevail over those between methanol and acetone. However, the extent of this inhomogeneity is small compared with that of acetone in the low mole fraction range. The spectral signature of this inhomogeneity is not visible in the O-H stretching vibrational mode; however, a second peak appears as a shoulder of the C-O stretching vibrational mode in this range of acetone mole fraction.

Competition between Cation-Solvent and Cation-Anion Interactions in Imidazolium Ionic Liquids with Polar Aprotic Solvents.

The subtle interplay between ion solvation and association was analyzed in mixtures of imidazolium-based ionic liquids (ILs) with polar aprotic solvents. A site-specific pattern of cation-solvent and cation-anion interactions was disclosed by a careful analysis of the 1 H and 13 C NMR chemical shift dependence of the mixture composition. It was established that the less polar but more donating γ-butyrolactone is more prone to establish H-bonds with the imidazolium-ring hydrogen atoms of the IL cations than propylene carbonate, particularly at the H2 site and at high dilutions xIL <0.1. The H2 site was found to be more sensitive to intermolecular interactions compared to H4, 5 in the case of ILs with asymmetric anions like trifluoromethanesulfonate (TfO- ) or bis(trifluoromethylsulfonyl)amide (TFSA- ).

Transport Properties and Ion Aggregation in Mixtures of Room Temperature Ionic Liquids with Aprotic Dipolar Solvents

The results of experimental (conductometry, NMR-diffusometry) and computational (MD simulations) studies on the binary mixtures of room-temperature imidazolium- and pyridinium-based ionic liquids (RTILs) with acetonitrile (AN), γ-butyrolactone (γ-BL) and propylene carbonate (PC) over the wide composition range are presented. The conductometric analysis was carried out in the RTILS mole fraction (χ(RTIL),) range between 0.0 and 0.5 in the temperature ranges from 278.15 to 328.15 K. Notably, all binary systems exhibit conductivity maximum at, χ(RTIL), between 0.1 and 0.2. This maximum slightly shifts towards smaller χ(RTIL), as counter-ion gets larger. Self-diffusion coefficients of solvent molecules and cations were obtained by means of 1H-NMR-diffusometry in mixtures of 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide tetrafluoroborate, trifluoromethanesulfonate and hexafluorophosphate with PC, γ-BL and AN over the whole concentration range at 300 K. The relative diffusion coefficients of solvent molecules to cations as a function of composition were established to be depended on a solvent but not on the anion of RTIL. In all cases the relative diffusion coefficients demonstrate a plateau at χ(RTIL) < 0.2 and then increase significantly for AN, moderately for γ-BL or negligibly for PC at higher RTIL content. Such behavior was attributed to the different solvation ability of the investigated solvents. In the mixtures with [BMIM][PF6] anion diffusion coefficients derived from 31P NMR were found to be higher than the corresponding values for cation in RTIL-depleted systems and lower in the RTIL-enriched systems. The inversion of relative ion diffusion is observed near the equimolar composition and being insensitive to the solvent. At this point a remarkable change in the diffusion mechanism of ion of RTIL is expected. Additionally, molecular dynamics simulations on the binary mixtures of 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium tetrafluoroborates with AN were performed. The conductivity correlates with a composition of ion aggregates simplifying its predictability. Large amounts of AN stabilize ion pairs, although destroy greater ion aggregates. Based on the simulation results, we show that conductivity of the studied mixtures significantly depend on the ion aggregation.

Local lateral environment of the molecules at the surface of DMSO-water mixtures

Molecular dynamics simulations of the liquid-vapour interface of dimethyl sulphoxide (DMSO)-water mixtures of 11 different compositions, including two neat systems are performed on the canonical (N, V, T) ensemble at 298 K. The molecules constituting the surface layer of these systems are selected by means of the identification of the truly interfacial molecules (ITIM) method, and their local lateral environment at the liquid surface is investigated by performing Voronoi analysis. The obtained results reveal that both molecules prefer to be in a mixed local environment, consisting of both kinds of molecules, at the liquid surface, and this preference is even stronger here than in the bulk liquid phase. Neat-like patches, in which a molecule is surrounded by like neighbours, are not found. However, vacancies that are surrounded solely by water molecules are observed at the liquid surface. Our results show that strongly hydrogen bonded DMSO·H2O complexes, known to exist in the bulk phase of these mixtures, are absent from the liquid surface.

Local structure of dilute aqueous DMSO solutions, as seen from molecular dynamics simulations

The information about the structure of dimethyl sulfoxide (DMSO)-water mixtures at relatively low DMSO mole fractions is an important step in order to understand their cryoprotective properties as well as the solvation process of proteins and amino acids. Classical MD simulations, using the potential model combination that best reproduces the free energy of mixing of these compounds, are used to analyze the local structure of DMSO-water mixtures at DMSO mole fractions below 0.2. Significant changes in the local structure of DMSO are observed around the DMSO mole fraction of 0.1. The array of evidence, based on the cluster and the metric and topological parameters of the Voronoi polyhedra distributions, indicates that these changes are associated with the simultaneous increase of the number of DMSO-water and decrease of water-water hydrogen bonds with increasing DMSO concentration. The inversion between the dominance of these two types of H-bonds occurs around XDMSO = 0.1, above which the DMSO-DMSO interactions also start playing an important role. In other words, below the DMSO mole fraction of 0.1, DMSO molecules are mainly solvated by water molecules, while above it, their solvation shell consists of a mixture of water and DMSO. The trigonal, tetrahedral, and trigonal bipyramidal distributions of water shift to lower corresponding order parameter values indicating the loosening of these orientations. Adding DMSO does not affect the hydrogen bonding between a reference water molecule and its first neighbor hydrogen bonded water molecules, while it increases the bent hydrogen bond geometry involving the second ones. The close-packed local structure of the third, fourth, and fifth water neighbors also is reinforced. In accordance with previous theoretical and experimental data, the hydrogen bonding between water and the first, the second, and the third DMSO neighbors is stronger than that with its corresponding water neighbors. At a given DMSO mole fraction, the behavior of the intensity of the high orientational order parameter values indicates that water molecules are more ordered in the vicinity of the hydrophilic group while their structure is close-packed near the hydrophobic group of DMSO.