Widom Line in Supercritical Water in Terms of Changes in Local Structure: Theoretical Perspective.

Performing molecular dynamics simulations with the TIP4P/2005 water model along 9 isobars (from 175 to 375 bar) in the temperature range between 300 and 1100 K, we have found that the loci of the extrema in the rate of change of specific structural properties can be used to define purely structure-based Widom lines. We have examined several parameters that describe the local structure of water, such as the tetrahedral arrangement, nearest neighbor distance, local density around the molecules, and the size of the largest dense domain. The last two parameters were determined using the Voronoi polyhedral and density-based spatial clustering of applications with noise methods, respectively. By analyzing the moments of the associated distributions, we show that along a given isobar, the temperature at which we observe a maximum in the fluctuation, the rate of change of the average values, or in the skewness values unambiguously determines the Widom line that is in agreement with the experimentally detected, thermodynamic response function-based ones.

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

We conducted a study on the photophysics of three indoline dyes, D102, D149, and D205, in binary mixtures of ionic liquids (IL) and polar aprotic molecular solvents (MS). Specifically, we examined the behavior of these dyes in IL-MS mixtures containing four different imidazolium-based ILs and three different polar aprotic MSs. Our investigation involved several techniques, including stationary absorption and emission measurements, as well as femtosecond transient absorption (TA) spectroscopy. Through our analysis, we discovered a peculiar behavior of several photophysical properties at low IL mole fractions (0 < XIL < 0.2). Indeed, in this range of mixture composition, the absorption maximum wavelength decreases noticeably, while the emission maximum wavelength and the Stokes shift, expressed in wavenumbers, reach a maximum. while a minimum occurs in the relative quantum yield and the excited state lifetime. These results indicate that the solvation of dye undergoes a large change in this range of mixture composition. We found that, at high ionic liquid content, the excited relaxation times are correlated with the high viscosity, while at low content, it is the polarity of the solvent that influences the behavior of the excited relaxation times. At a mixture composition of around 0.10, the behavior of the photophysical properties of the studied IL-MS mixtures indicates a crossover between situations where the solvation is dominated by that of ions and that dominated by the solvent.

Solvation structure and dynamics of coumarin 153 in an imidazolium-based ionic liquid with chloroform, benzene, and propylene carbonate.

In order to determine the self-diffusion coefficients D of all the species in the solutions at 298.2 K, 1H and 19F NMR diffusion ordered spectroscopy (DOSY) has been conducted on coumarin 153 (C153) in binary mixed solvents of an imidazolium-based ionic liquid (IL), 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C12mimTFSA), with three molecular liquids (MLs) of chloroform (CL), benzene (BZ), and propylene carbonate (PC) as a function of ML mole fraction xML. Below xML ≈ 0.8, the D values of each species do not significantly depend on the MLs. However, above this mole fraction, the diffusion of C153 becomes smoother in the order of BZ ≈ CL > PC systems. The interactions among C153, C12mim+, TFSA-, and ML molecules have been investigated using infrared (IR) and 1H and 13C NMR spectroscopic techniques. The relations of the diffusion of the species with the interactions among them have been discussed on the molecular scale. In the IL solution, the C153 carbonyl oxygen atom is hydrogen-bonded with the imidazolium ring C2-H atom of C12mim+. C12mim+ also forms an ion pair with TFSA-. Thus, C153, C12mim+, and TFSA- cooperatively move in the CL and BZ solutions at a lower ML content, xML < ∼0.8. On the other hand, at a higher ML content, xML > ∼0.8, the C153 molecule diffuses with CL and BZ molecules because of the hydrogen bonding between the C153 carbonyl O atom and the CL H atom and the π-π interaction between the C153 and BZ ring planes, respectively. For the PC system, the change in the relative self-diffusion coefficients of each species with increasing xML differs from those for the CL and BZ systems because of both hydrogen bonding donor H and acceptor O atoms of PC for C153, the IL cation and anion, and PC themselves.

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.

Surface Properties of N,N-Dimethylformamide-Water Mixtures, As Seen from Computer Simulations.

The liquid-vapor interface of N,N-dimethylformamide (DMF)-water mixtures, spanning the entire composition range, is investigated in detail at 298 K by molecular dynamics simulation and intrinsic surface analysis. DMF molecules are found to adsorb strongly at the liquid surface, but this adsorption extends only to the first molecular layer. Water and DMF molecules mix with each other on the molecular scale even in the surface layer; thus, no marked self-association of any of the components is seen at the liquid surface. The major surface component prefers such orientation in which the molecular dipole vector lays parallel with the macroscopic plane of the surface. On the other hand, the preferred orientation of the minor component is determined, at both ends of the composition range, by the possibility of H-bond formation with the major component. The lack of H-donating ability of DMF leads to a rapid breakup of the percolating H-bond network at the surface; due to the strong adsorption of DMF, this breakup occurs below the bulk phase DMF mole fraction of 0.03. The disruption of the surface H-bond network also accelerates the exchange of both species between the liquid surface and bulk liquid phase, although, for water, this effect becomes apparent only above a bulk phase DMF mole fraction of 0.4. H-bonds formed by a DMF and a water molecule live, on average, 25-60% longer than those formed by two water molecules at the liquid surface. A similar, but smaller (i.e., about 10-20%) difference is seen in the bulk liquid phase. The enhanced surface mobility of the molecules results in 2-6 times larger diffusion coefficient and 2-5 times shorter H-bond lifetime values at the liquid surface than in the bulk liquid phase. The diffusion of both molecules is slowed down in the presence of the other species; in the case of DMF, this effect is caused by the formation of water-DMF H-bonds, whereas for water, steric hindrances imposed by the bulky DMF neighbors are responsible for this slowing down.

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.

Effects of self-hydrogen bonding among formamide molecules on the UCST-type liquid-liquid phase separation of binary solutions with imidazolium-based ionic liquid, [Cnmim][TFSI], studied by NMR, IR, MD simulations, and SANS.

The upper critical solution temperature (UCST)-type liquid-liquid phase separation of imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][TFSI], where n represents the alkyl chain length of the cation, n = 6, 8, 10, and 12) binary solutions with formamide (FA) was examined as a function of temperature and the FA mole fraction xFA. The two-phase region (immiscible region) of the solutions is much larger and expands more with the increase in n, in comparison with the previous [Cnmim][TFSI]-1,4-dioxane (1,4-DIO) systems. An array of spectroscopic techniques, including 1H and 13C NMR and IR combined with molecular dynamics (MD) simulations, was conducted on the present binary systems to clarify the microscopic interactions that contribute to the phase-separation mechanism. The hydrogen-bonding interactions of the imidazolium ring H atoms are more favorable with the O atoms of the FA molecules than with 1,4-DIO molecules, whereas the latter interact more favorably with the alkyl chain of the cation. Upon lowering the temperature, the FA molecules gradually self-aggregate through self-hydrogen bonding to form FA clusters. Concomitantly, clusters of ILs are formed via the electrostatic interaction between the counter ions and the dispersion force among the IL alkyl chains. Small-angle neutron scattering (SANS) experiments on the [C6mim][TFSI]-FA-d2 and [C8mim][TFSI]-FA-d2 systems revealed, similarly to [Cnmim][TFSI]-1,4-DIO systems, the crossover of the mechanism from the 3D-Ising mechanism around the UCST xFA to the mean-field mechanism at both sides of the mole fraction. Interestingly, the xFA range of the 3D-Ising mechanism for the FA systems is wider compared with the range of the 1,4-DIO systems. In this way, the self-hydrogen bonding among FA molecules most significantly governs the phase equilibria of the [Cnmim][TFSI]-FA systems.

Computer Simulation of the Surface of Aqueous Ionic and Surfactant Solutions.

The surface of aqueous solutions of simple salts was not the main focus of scientific attention for a long while. Considerable interest in studying such systems has only emerged in the past two decades, following the pioneering finding that large halide ions, such as I-, exhibit considerable surface affinity. Since then, a number of issues have been clarified; however, there are still several unresolved points (e.g., the effect of various salts on lateral water diffusion at the surface) in this respect. Computer simulation studies of the field have largely benefited from the appearance of intrinsic surface analysis methods, by which the particles staying right at the boundary of the two phases can be unambiguously identified. Considering complex ions instead of simple ones opens a number of interesting questions, both from the theoretical point of view and from that of the applications. Besides reviewing the state-of-the-art of intrinsic surface analysis methods as well as the most important advances and open questions concerning the surface of simple ionic solutions, we focus on two such systems in this Perspective, namely, the surface of aqueous mixtures of room temperature ionic liquids and that of ionic surfactants. In the case of the former systems, for which computer simulation studies have still scarcely been reported, we summarize the theoretical advances that could trigger such investigations, which might well be of importance also from the point of view of industrial applications. Computer simulation methods are, on the other hand, widely used in studies of the surface of surfactant solutions. Here we review the most important theoretical advances and issues to be addressed and discuss two areas of applications, namely, the inclusion of information gathered from such simulations in large scale atmospheric models and the better understanding of the airborne transmission of viruses, such as SARS-CoV-2.

Assessment of the UCST-type liquid-liquid phase separation mechanism of imidazolium-based ionic liquid, [C8mim][TFSI], and 1,4-dioxane by SANS, NMR, IR, and MD simulations.

Liquid-liquid phase separation of binary systems for imidazolium-based ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][TFSI], where n represents the alkyl chain length of the cation), with 1,4-dioxane (1,4-DIO) was observed as a function of temperature and 1,4-DIO mole fraction, x1,4-DIO. The phase diagrams obtained for [Cnmim][TFSI]-1,4-DIO systems showed that the miscible region becomes wider with an increase in the alkyl chain length, n. For n = 6 and 8, an upper critical solution temperature (UCST) was found. To clarify the mechanism of the UCST-type phase separation, small-angle neutron scattering (SANS) experiments were conducted on the [C8mim][TFSI]-1,4-DIO-d8 system at several x1,4-DIO. The critical exponents of γ and ν determined from the SANS experiments showed that phase separation of the system at the UCST mole fraction occurs via the 3D-Ising mechanism, while that on both sides of UCST occurs via the mean field mechanism. Thus, the crossover of mechanism was observed for this system. The microscopic interactions among the cation, anion, and 1,4-DIO were elucidated using 1H and 13C NMR and IR spectroscopic techniques, together with the theoretical method of molecular dynamics (MD) simulations. The results on the microscopic interactions suggest that 1,4-DIO molecules cannot strongly interact with H atoms on the imidazolium ring, while they interact with the octyl chain of the cation through dispersion force. With a decrease in temperature, 1,4-DIO molecules gradually aggregate to form 1,4-DIO clusters in the binary solutions. The strengthening of the C-H⋯O interaction between 1,4-DIO molecules by cooling is the key to the phase separation. Of course, the electrostatic interaction between the cations and anions results in the formation of IL clusters. When IL clusters are excluded from 1,4-DIO clusters, liquid-liquid phase separation occurs. Accordingly, the balance between the electrostatic force between the cations and anions and the C-H⋯O interaction between the 1,4-DIO determines the 3D-Ising or the mean field mechanism of phase separation.

Calculation of the Free Energy of Mixing as a Tool for Assessing and Improving Potential Models: The Case of the N,N-Dimethylformamide-Water System.

The Helmholtz free energy, energy, and entropy of mixing of N,N-dimethylformamide (DMF) and water are calculated in the entire composition range by means of Monte Carlo computer simulations and thermodynamic integration using all possible combinations of five DMF and three widely used water models. Our results reveal that the mixing of DMF and water is highly non-ideal. Thus, in their dilute solutions, both molecules induce structural ordering of the major component, as evidenced by the concomitant decrease in the entropy. Among the 15 model combinations considered, only 4 reproduce the well-known full miscibility of DMF and water, 3 of which strongly exaggerate the thermodynamic driving force of the miscibility. Thus, the combination of the CS2 model of DMF and the TIP4P/2005 water model reproduces the properties of the DMF-water mixtures far better than the other combinations tested. Our results also reveal that moving a fractional negative charge from the N atom to the O atom of the DMF molecule, leading to the increase in its dipole moment, improves the miscibility of the model with water. Starting from the CS2 model and optimizing the charge to be moved, we propose a new model of DMF that reproduces very accurately both the Helmholtz free energy of mixing of aqueous DMF solutions in the entire composition range (when used in combination with the TIP4P/2005 water model) and also the internal energy of neat DMF.

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.

Dynamics in the BMIM PF6/acetonitrile mixtures observed by femtosecond optical Kerr effect and molecular dynamics simulations.

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for a mixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF6) ionic liquid and acetonitrile in the whole mole fractions range. The long delay line in our experimental setup allowed us to capture the complete reorientational dynamics of the ionic liquid. We have analysed the optical Kerr effect signal in the time and frequency domains with help of molecular dynamics simulations. In our approximation of the slow picosecond dynamics with a multi-exponential decay, we distinguish three relaxation times. The highest two are assigned to the reorientation of the cation and acetonitrile molecules that are in the vicinity of the imidazolium ring. The third one is recognized as originating from cation rotations and reorientation of acetonitrile molecules in the bulk or in the vicinity of the aliphatic chains of the cation. With help of the simulation we interpret the intermolecular band in the reduced spectral density, obtained from Kerr signal, as follows: its low-frequency side results from oscillations of one of the components in the cage formed by its neighbors, while the high-frequency side is attributed to the librations of the cation and acetonitrile molecule as well as the intermolecular oscillations of system components involved in specific interactions. We use this assignment and concentration dependence of the spectra obtained from velocity and angular velocity correlations to explain the mole fraction dependence of Kerr reduced spectral density.

Thermodynamics of mixing methanol with supercritical CO2 as seen from computer simulations and thermodynamic integration.

The changes in extensive thermodynamic quantities, such as volume, energy, Helmholtz free energy and entropy, occurring upon mixing liquid methanol with supercritical CO2, are calculated using Monte Carlo simulations and thermodynamic integration for all eight combinations of four methanol and two CO2 potential models in the entire composition range at 313 K. The obtained results are also compared with experimental data whenever possible. The transition of the system from liquid to a supercritical state is found to occur at this temperature around a CO2 mole fraction value of 0.95 with all model combinations considered. This liquid to supercritical transition is always accompanied by positive Helmholtz free energy of mixing values and, consequently, by the non-miscibility of the two components. Furthermore, both this non-miscibility around the liquid to supercritical transition and also the miscibility of the two components below this transition, in the liquid regime, are found to be primarily of the energetic rather than entropic origin; the entropy of mixing turns out to be very close to zero, and around the liquid to supercritical transition even its qualitative behaviour is strongly model dependent. Finally, it is found that the methanol expansion coefficient is not sensitive to the details of the potential models, and it is always in excellent agreement with the experimental data. On the other hand, both the volume and the energy of mixing depend strongly on the molar volume of neat CO2 in the model being used, and in this respect the TraPPE model of CO2 [J. J. Potoff and J. I. Siepmann, AIChE J., 2001, 47, 1676] performs considerably better than that of Zhang and Duan [Z. Zhang and Z. Duan, J. Chem. Phys., 2005, 122, 214507].

Mixing states of imidazolium-based ionic liquid, [C4mim][TFSI], with cycloethers studied by SANS, IR, and NMR experiments and MD simulations.

The mixing states of an imidazolium-based ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][TFSI]), with cycloethers, tetrahydrofuran (THF), 1,4-dioxane (1,4-DIO), and 1,3-dioxane (1,3-DIO), have been clarified on the meso- and microscopic scales using small-angle neutron scattering (SANS), IR, and NMR experiments and molecular dynamics (MD) simulations. SANS profiles of [C4mim][TFSI]-THF-d8 and -1,4-DIO-d8 solutions at various mole fractions xML of molecular liquid (ML) have shown that [C4mim][TFSI] is heterogeneously mixed with THF and 1,4-DIO on the mesoscopic scale, to a high extent in the case of the latter solution. In fact, [C4mim][TFSI] and 1,4-DIO are not miscible with each other above the 1,4-DIO mole fraction x1,4-DIO of 0.903, whereas the IL can be mixed with THF over the entire range of THF mole fraction xTHF. The results of IR and 1H and 13C NMR measurements and MD simulations showed that cycloether molecules are more strongly hydrogen-bonded with the imidazolium ring H atoms in the order of THF > 1,3-DIO > 1,4-DIO. Although 1,4-DIO and 1,3-DIO molecules are structural isomers, our results point out that 1,4-DIO cannot be strongly hydrogen-bonded with the ring H atoms. The solvation of [TFSI]- by cycloethers through the dipole-dipole interaction promotes hydrogen bonding between the ring H atoms and cycloethers. Thus, 1,4-DIO with the lowest dipole moment cannot easily eliminate [TFSI]- from the imidazolium ring. This results in the weakest hydrogen bonds of 1,4-DIO with the ring H atoms. 2D-NMR of 1H{1H} rotating-frame nuclear Overhauser effect spectroscopy (ROESY) showed the interaction of the three cycloethers with the butyl group of [C4mim]+. 1,4-DIO mainly interacts with the butyl group by the dispersion force, whereas THF interacts with the IL by both hydrogen bonding and dispersion force. This leads to the higher heterogeneity of the 1,4-DIO solutions compared to the THF solutions.

Correlation between the conformational crossover of carbamazepine and its polymorphic transition in supercritical CO2: On the way to polymorph control.

In this paper we have established a correlation between the conformation crossover of carbamazepine and associated polymorph transformation. This was achieved by using a combination of quantum chemical calculations and in situ IR spectroscopy for performing a conformational analysis of carbamazepine molecules in its saturated solution in scCO2 being in permanent contact with the carbamazepine solid form. Using quantum calculations, we determined two carbamazepine conformers, whose spectral signatures were then found in experimental IR spectra. Further analysis of the IR spectra allowed us to quantify the distribution of these conformations in supercritical CO2. We found that this distribution can be changed by heating from 40°C to 110°C along two isochores at 1.1 and 1.3 of the critical CO2 density. Using in situ Raman spectroscopy we proved that the appearing conformational crossover correlates with the polymorphic transformation of the carbamazepine solid form. Moreover, this transformation was proved by the results of IR diffuse reflection spectroscopy.

Distance Angle Descriptors of the Interionic and Ion-Solvent Interactions in Imidazolium-Based Ionic Liquid Mixtures with Aprotic Solvents: A Molecular Dynamics Simulation Study.

The aim of this paper is to quantify the changes of the interionic and ion-solvent interactions in mixtures of imidazolium-based ionic liquids, having tetrafluoroborate (BmimBF4), hexafluorophosphate (BmimPF6), trifluoromethylsulfonate (BmimTFO), or bis(trifluoromethanesulfonyl)imide (BmimTFSI), anions, and polar aprotic molecular solvents, such as acetonitrile (AN), γ-butyrolactone (GBL), and propylene carbonate (PC). For this purpose, we calculate, using the nearest-neighbor approach, the average distance between the imidazolium ring H atom in positions 2, 4, and 5 (H2,4,5) and the nearest high-electronegativity atom of the solvent or anion (X) as distance descriptors, and the mean angle formed by the C2,4,5-H2,4,5 bond and the H2,4,5···X axis around the H2,4,5 atom as angular descriptors of the cation-anion and cation-solvent interactions around the ring C-H groups. The behavior of these descriptors as a function of the ionic liquid mole fraction is analyzed in detail. The obtained results show that the extent of the change of these descriptors with respect to their values in the neat ionic liquid depends both on the nature of the anion and on the mixture composition. Thus, in the case of the mixtures of the molecular solvents with BmimBF4 and BmimTFO, a small change of the distance and a drastic increase of the angular descriptor corresponding to the cation-anion interactions are observed with decreasing mole fraction of the ionic liquid, indicating that the anion moves from the above/below position (with respect to the imidazolium ring plane) to a position that is nearly linearly aligned with the C2-H2 bond and hinders the possible interaction between the C2-H2 group and the solvent molecules. On the other hand, in the case of mixtures of BmimTFSI and BmimPF6 with the molecular solvents, both the observed increase of the distance descriptor and the slight change of the angular descriptor with decreasing ionic liquid mole fraction are compatible with the direct interactions of the solvent with the C2-H2 group. The behavior of these descriptors is correlated with the experimentally observed 1H chemical shift of the C2-H2 group and the red shift of the C2-H2 vibrational mode, particularly at low ionic liquid mole fractions. The present results are thus of great help in interpreting these experimental observations.

The local structure in the BmimPF6/acetonitrile mixture: the charge distribution effect.

The changes of the local structure in the binary mixture of 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) ionic liquid and acetonitrile are investigated over the entire composition range. Two charge distribution models of the ions are considered: in the first one, the atomic fractional charges of the cations and anions are kept equal with those in the neat ionic liquid, and hence they are independent from the mole fraction of the ionic liquid, while in the second one the charge distribution is scaled up by a mole fraction dependent factor. The sum of these charges converge to +1e and -1e on the cation and anion, respectively, at infinite dilution. All the other interactions and geometry parameters of the ions (i.e., Lennard-Jones, bond stretching, angle bending and dihedral parameters) are identical in the two cases. The effect of the fractional charge distribution on the hydrogen bonding between the counterions themselves and between the ions and solvent molecules, as well as on the stacking interactions between the cations, is analyzed. To this end, two distances, characteristic of the hydrogen bond formed by the donor moiety and its nearest neighbor acceptor, as well as a coordinate system that defines unambiguously the orientation between a reference cation and its nearest neighbor, are introduced. It is shown that, with the variable charge model, the neighboring cation-anion pairs maintain their relative arrangement similar to the neat ionic liquid down to an ionic liquid mole fraction of xIL = 0.10, whereas in the case of the constant charge model such changes occur already at xIL = 0.20. Furthermore, the analysis of the first and the second nearest neighbor distance distributions of an anion around a reference cation indicates that, at this mole fraction range, there are two different preferred arrangements of the anions around the cations. In the first one, similarly to the local structure around a reference cation in the neat ionic liquid, the anion forms a distorted hydrogen bond with the cation, while in the second one the anion is located farther from the cation, forming no hydrogen bond with it. The relative population of these two types of preferred nearest neighbor cation-anion arrangements is found to be sensitive to further decrease of the ionic liquid mole fraction. These findings correlate with experimental results concerning the behavior of many physical chemical properties (e.g., excess volume, excess viscosity, chemical shift, infrared and Raman vibrational mode shifts, diffusion, etc.) that were found to undergo a drastic change in this mole fraction range. Our results show that in this composition range a transition occurs from the situation where the macroscopic physical chemical properties of the system are determined primarily by the cation-anion hydrogen bonding interactions to that where they are determined by the solvation of the cation and the anion by the molecular solvent.

Thermodynamics of Mixing Primary Alkanolamines with Water.

The volume, energy, entropy, and Helmholtz free energy of mixing of the seven simplest primary alkanolamine molecules, i.e., monoethanolamine, monoisopropanolamine, 2-amino-propan-1-ol, 2-amino-butan-1-ol, 2-amino-2-methyl-propan-1-ol, 1-amino-2-methyl-propan-2-ol, and 1-amino-butan-2-ol, with water is investigated by extensive computer simulations and thermodynamic integration. To check the force field dependence of the results, all calculations are repeated with two commonly used water models, namely, SPC/E and TIP4P. The obtained results show that the thermodynamics of mixing of alkanolamines and water is largely independent from the type of the alkanolamine molecule. The Helmholtz free energy of mixing is found to be negative for all alkanolamines at every composition, in accordance with the experimentally known full miscibility of these molecules and water. This free energy decrease occurring upon mixing is found to be clearly of energetic origin, as the energy of mixing always turns out to be negative in the entire composition range, while the entropy of mixing is also negative up to high alkanolamine mole fractions. The obtained results suggest that alkanolamines form, on average, stronger hydrogen bonds with water than what is formed by two water molecules, and they induce some ordering of the hydrating water molecules both through the hydrophobic hydration of their side chains and through the strong hydrogen bonding.

Hydrogen bonds of the imidazolium rings of ionic liquids with DMSO studied by NMR, soft X-ray spectroscopy, and SANS.

The hydrogen bonds of the imidazolium-ring H atoms of ionic liquids (ILs), 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amides ([Cnmim][TFSA], n = 2 to 12 where n represents the alkyl chain length), with the O atom of dimethyl sulfoxide (DMSO) have been elucidated using 1H, 13C, and 15N NMR spectroscopy and soft X-ray absorption and emission spectroscopy (XAS and XES). Density functional theory (DFT) calculations have been performed on an isolated DMSO molecule and two cluster models of [Cnmim]+-DMSO by hydrogen bonding to interpret the XES spectra for the [Cnmim][TFSA]-DMSO solutions. The 1H and 13C NMR chemical shifts of the imidazolium ring showed that deshielding of the ring H and C atoms is moderate as the DMSO mole fraction xDMSO increases to ∼0.8; however, it becomes more significant with further increase of xDMSO. This finding suggests that the hydrogen bonds of the three ring H atoms with the DMSO O atoms are saturated in solutions with xDMSO increased to ∼0.8. The 1H and 13C chemical shifts of the alkyl chains revealed that the electron densities of the chain H and C atoms gradually decrease with increasing xDMSO, except for the N1-bound carbon atom C7 of the chain. The 15N NMR chemical shifts showed that the imidazolium-ring N1 atom which is bound to the alkyl chain is shielded with increasing xDMSO in the range from 0 to 0.8 and is then deshielded with further increase of xDMSO. In contrast, the imidazolium ring N3 atom is simply deshielded with increasing xDMSO. Thus, the electron densities of the alkyl chain may be condensed at the C7 and N1 atoms of [Cnmim]+ by the hydrogen bonding of the ring H atoms with DMSO. The hydrogen bonding of DMSO with the ring results in low-energy shifts of the XES peaks of the O K-edge of DMSO. Small-angle neutron scattering experiments showed that [Cnmim][TFSA] and DMSO are homogeneously mixed with each other on the mesoscopic scale. This results from the strong hydrogen bonds of DMSO with the imidazolium-ring H atoms.