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.

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.

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.

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.

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.

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.

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.

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- ).

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.

Properties of the liquid-vapor interface of acetone-methanol mixtures, as seen from computer simulation and ITIM surface analysis.

Molecular dynamics simulations of the liquid-vapor interface of acetone-methanol mixtures of different compositions, including the two neat systems, have been performed on the canonical (N,V,T) ensemble at 293 K. The intrinsic liquid surface has been determined in terms of the Identification of the Truly Interfacial Molecules (ITIM) method. The results have revealed that the proximity of the interface influences the properties of only the first molecular layer of the liquid phase, while the second layer already turns out to be bulk-like in every respect. The two molecules are distributed uniformly along the macroscopic surface normal axis, as no strong preference for surface adsorption is shown by any of them. However, similarly to the bulk liquid phase, both molecules exhibit a marked tendency for self-association within the surface layer. Surface orientations are found to be composition independent; all the preferred orientations of both molecules correspond to the same alignment of the molecular dipole vector, which is nearly parallel to the macroscopic surface plane, declining only 10-20° from it towards the vapor phase. The surface properties are thus primarily governed by dipolar interactions, whereas hydrogen bonding within the surface layer, which decreases steadily with an increase in the acetone mole fraction, plays only a minor role in this respect.

Probing structural patterns of ion association and solvation in mixtures of imidazolium ionic liquids with acetonitrile by means of relative (1)H and (13)C NMR chemical shifts.

Mixtures of ionic liquids (ILs) with polar aprotic solvents in different combinations and under different conditions (concentration, temperature etc.) are used widely in electrochemistry. However, little is known about the key intermolecular interactions in such mixtures depending on the nature of the constituents and mixture composition. In order to systematically address the intermolecular interactions, the chemical shift variation of (1)H and (13)C nuclei has been followed in mixtures of imidazolium ILs 1-n-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4), 1-n-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6), 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate (BmimTfO) and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimTFSI) with molecular solvent acetonitrile (AN) over the entire composition range at 300 K. The concept of relative chemical shift variation is proposed to assess the observed effects on a unified and unbiased scale. We have found that hydrogen bonds between the imidazolium ring hydrogen atoms and electronegative atoms of anions are stronger in BmimBF4 and BmimTfO ILs than those in BmimTFSI and BmimPF6. Hydrogen atom at position 2 of the imidazolium ring is substantially more sensitive to interionic hydrogen bonding than those at positions 4-5 in the case of BmimTfO and BmimTFSI ILs. These hydrogen bonds are disrupted upon dilution in AN due to ion dissociation which is more pronounced at high dilutions. Specific solvation interactions between AN molecules and IL cations are poorly manifested.

Non-covalent interactions in ionic liquid ion pairs and ion pair dimers: a quantum chemical calculation analysis.

Ionic liquids (ILs) being composed of bulky multiatomic ions reveal a plethora of non-covalent interactions which determine their microscopic structure. In order to establish the main peculiarities of these interactions in an IL-environment, we have performed quantum chemical calculations for a set of representative model molecular clusters. These calculations were coupled with advanced methods of analysis of the electron density distribution, namely, the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction (NCI; J. Am. Chem. Soc., 2010, 132, 6499) approaches. The former allows for profound quantitative characterization of non-covalent interactions between atoms while the latter gives an overview of spatial extent, delocalization, and relative strength of such interactions. The studied systems consist of 1-butyl-3-methylimidazolium (Bmim(+)) cations and different perfluorinated anions: tetrafluoroborate (BF4(-)), hexafluorophosphate (PF6(-)), trifluoromethanesulfonate (TfO(-)), and bis(trifluoromethanesulfonyl)imide (TFSI(-)). IL ion pairs and ion pair dimers were considered as model structures for the neat ILs and large aggregates. Weak electrostatic hydrogen bonding was found between the anions and the imidazolium ring hydrogen atoms of cations. Weaker but still appreciable hydrogen bonding was also noted for hydrogen atoms adjacent to the imidazolium ring alkyl groups of Bmim(+). The relative strength of the hydrogen bonding is higher in BmimTfO and BmimBF4 ILs than in BmimPF6 and BmimTFSI, whereas BmimTfO and BmimTFSI reveal higher sensitivity of hydrogen bonding at the different hydrogen atoms of the imidazolium ring.

The local environment of the molecules in water-DMSO mixtures, as seen from computer simulations and Voronoi polyhedra analysis.

Molecular dynamics simulations of water-DMSO mixtures, containing 10, 20, 30, 40, 50, 60, 70, 80, and 90 mol% DMSO, respectively, have been performed on the isothermal-isobaric (N,p,T) ensemble at T = 298 K and at the pressure equal to the experimental vapor pressure at each mixture composition. In addition, simulations of the two neat systems have also been performed for reference. The potential models used in the simulations are known to excellently reproduce the mixing properties of these compounds. The simulation results have been analyzed in detail by means of the Voronoi polyhedra (VP) of the molecules. Distributions of the VP volume and the asphericity parameter as well as that of the radius of the spherical intermolecular voids have been calculated. Detailed analyses of these distributions have revealed that both molecules prefer to be in an environment consisting of both types of molecules, but the affinity of DMSO for mixing with water is clearly stronger than that of water for mixing with DMSO. As a consequence, the dilution of the two neat liquids by the other component has been found to follow different mechanisms: when DMSO is added to neat water small domains of neat-like water persist up to the equimolar composition, whereas no such domains are found when neat DMSO is diluted by water. The observed behaviour is also in line with the fact that the main thermodynamic driving force behind the full miscibility of water and DMSO is the energy change accompanying their mixing, and that the entropy change accompanying this mixing is negative in systems of low and positive in systems of high DMSO mole fractions. Finally, we have found a direct evidence for the existence of strong hydrogen bonded complexes formed by one DMSO and two water molecules, but it has also been shown that these complexes are in equilibrium with single (monomeric) water and DMSO molecules in the mixed systems.

Microscopic interactions of the imidazolium-based ionic liquid with molecular liquids depending on their electron-donicity.

Microscopic interactions of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C2mimTFSI), with dimethyl sulfoxide (DMSO), methanol (MeOH), and acetonitrile (AN) have been analyzed by means of Raman, attenuated total reflectance infrared (ATR-IR), (1)H and (13)C NMR spectroscopy techniques. The magnitude of the red-shift of the C(2)-H vibration mode of the imidazolium ring and the deshielding of the C(2)-H hydrogen and carbon atoms, compared with that of the other atoms of the ring or the anion, indicated a strong interaction between the C(2)-H hydrogen atom and the molecular liquids in the following order; DMSO ≫ MeOH > AN. This correlates with the order of the electron donicities of these molecular liquids which allows us to suggest a hydrogen bonding character of these interactions. The behavior of S= O vibration of DMSO as a function of the DMSO molar fraction xDMSO also suggested that DMSO molecules are stoichiometrically hydrogen-bonded with the three hydrogen atoms, C(2,4,5)-H, of the ring. In contrast, the hydrogen bonding between MeOH and the C(4,5)-H atoms is much weaker than that in DMSO. AN hardly forms hydrogen bonds with the C(4,5)-H atoms. Instead, AN molecules may interact with the imidazolium ring through the π-π interaction. The interactions between the imidazolium ring and the molecular liquids lead to the loosening of the TFSI anion from the cation; this correlates with both the blue-shift of the S=O stretching vibration of TFSI and the deshielding of the trifluoromethyl carbon atoms with an increase in the molar fraction of the molecular liquid xML. The latter is weak in the MeOH solutions, and may be explained by the possible hydrogen bonding of the MeOH hydroxyl group as an electron-acceptor with the TFSI anion. Furthermore, the organization of MeOH molecules around the ethyl and methyl groups of the cation is discussed in terms of the chemical shift of the hydrogen and carbon atoms in these groups as a function of xML.

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.

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.

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.

Detailed insight into the hydrogen bonding interactions in acetone-methanol mixtures. A molecular dynamics simulation and Voronoi polyhedra analysis study.

Voronoi polyhedra (VP) analysis of mixtures of acetone and methanol is reported on the basis of molecular dynamics computer simulations, performed at 300 K and 1 bar. The composition of the systems investigated covers the entire range from neat acetone to neat methanol. Distribution of the volume, reciprocal volume and asphericity parameter of the VP as well as that of the area of the individual VP faces and of the radius of the empty voids located between the molecules are calculated. To investigate the tendency of the like molecules to self-associate the analyses are repeated by disregarding one of the two components. The self-aggregates of the disregarded component thus turn into large empty voids, which are easily detectable in VP analysis. The obtained results reveal that both molecules show self-association, but this behavior is considerably stronger among the acetone than among the methanol molecules. The strongest self-association of the acetone and methanol molecules is found in their mole fraction ranges of 02-0.5 and 0.5-0.6, respectively. The caging effect around the methanol molecules is found to be stronger than around acetones. Finally, the local environment of the acetone molecules turns out to be more spherical than that of the methanols, not only in the respective neat liquids, but also in their mixtures.