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.

Aggregation of the Dipeptide Leu-Gly in Alcohol-Water Binary Solvents Elucidated from the Solvation Structure for Each Moiety.

The aggregation of a dipeptide, l-leucine-glycine (Leu-Gly), at 100 mmol dm-3 has been observed in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)-water and 2-propanol (2-PrOH)-water solvents at various alcohol mole fractions, xA, using the dynamic light scattering technique and molecular dynamics (MD) simulations. Leu-Gly was dissolved into the HFIP solvents at the concentration over the entire xA range, while the dipeptide was not dissolved in the 2-PrOH solvents above xA = 0.6. Interestingly, the MD snapshots showed different shapes of Leu-Gly aggregates in the HFIP and 2-PrOH solvents. A linear-shaped aggregate forms in the former; in contrast, a spherical-shaped aggregate is generated in the latter. The solvation structure of each moiety of Leu-Gly in the HFIP and 2-PrOH solvents was observed using experimental and theoretical techniques,1H and 13C NMR, IR, and 19F-1H HOESY measurements and MD simulations. These results gave us the reasons for the different shapes of Leu-Gly aggregates in both solvents. In the HFIP solvents, most of the moieties of the dipeptide are easily solvated by HFIP. This induces the elongated structure of Leu-Gly, leading to the electrostatic interaction between the N- (NH3+ group) and C- (COO- group) terminals of dipeptide molecules. On the other hand, in the 2-PrOH solvents, water molecules that initially solvate the moieties of Leu-Gly, such as the N- and C-terminals and the peptide linkage, are not easily eliminated even as the xA is close to 0.6. The water molecules can bridge such moieties of Leu-Gly to form spherical-shaped aggregates. The diffusion coefficients of Leu-Gly in both alcohol-water binary solvents were experimentally determined by NMR DOSY to estimate the geometries of the aggregates in the solvents. The sizes of Leu-Gly aggregates obtained by DOSY for both solvent systems were consistent with those estimated from the MD snapshots.

Solvation Structures of Tetraethylammonium Bromide and Tetrafluoroborate in Aqueous Binary Solvents with Ethanol, Trifluoroethanol, and Acetonitrile.

The solvation structures of tetraethylammonium bromide and tetrafluoroborate (TEABr and TEABF4) in aqueous binary solvents with ethanol (EtOH), 2,2,2-trifluoroethanol (TFE), and acetonitrile (AN) have been clarified by molecular dynamics (MD) simulations. In addition, 1H and 13C NMR chemical shifts of the H and C atoms within TEA+ in the binary solvents have been measured as a function of the mole fraction of the organic solvent, xOS. The variations of the chemical shifts with an increase in xOS were interpreted according to the solvation structures of TEA+, Br-, and BF4- obtained from the MD simulations. It has been found that TEABF4 at 130 mmol dm-3 cannot be dissolved into the EtOH and TFE solvents above xOS ≈ 0.7 and 0.6, respectively, while TEABr can be done in both solvents. Interestingly, TEABr and TEABF4 at the concentration can be dissolved in the AN solvents over the entire xOS range. The solvation of TEA+, Br-, and BF4- in each solvent has been discussed in terms of the electrostatic force, the weak hydrogen bond of C-H···F-C, and the dipole-dipole interaction.

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.

Solvation Structure of 1,3-Butanediol in Aqueous Binary Solvents with Acetonitrile, 1,4-Dioxane, and Dimethyl Sulfoxide Studied by IR, NMR, and Molecular Dynamics Simulation.

The solvation structure of 1,3-butanediol (1,3-BD) in aqueous binary solvents of acetonitrile (AN), 1,4-dioxane (DIO), and dimethyl sulfoxide (DMSO) at various mole fractions of organic solvent xOS has been clarified by means of infrared (IR) and 1H and 13C NMR. The change in the wavenumber of O-H stretching vibration of 1,3-BD in the three systems suggested that water molecules which are initially hydrogen-bonded with the 1,3-BD hydroxyl groups in the water solvent (xOS = 0) are more significantly replaced by organic solvent molecules in the order of DMSO ≫ DIO > AN. This agrees with the order of the electron donicities of the organic solvents. The 1H and 13C chemical shifts of 1,3-BD also revealed the most remarkable replacement of water molecules on the hydroxyl groups by DMSO. In contrast to the DMSO system, the O-H vibration band of 1,3-BD in the AN and DIO systems suggested the formation of the intramolecular hydrogen bond between the two hydroxyl groups of 1,3-BD above xOS = ∼0.9. To further evaluate the intramolecular hydrogen bonding of 1,3-BD in AN-water binary solvents, molecular dynamics (MD) simulations and NMR experiments for spin-lattice relaxation times T1 and 1H-1H nuclear Overhauser effect (NOE) were conducted on 1,3-BD in the AN system. These results showed the intramolecular hydrogen bond within 1,3-BD in the AN-water binary solvents in the high AN mole fraction range of xAN > 0.9. Especially, the pair correlation functions g(r) of the OH-O interactions of 1,3-BD obtained from the MD simulations indicated that the intramolecular hydrogen bond remarkably increases in the AN solvent as the xAN rises to the unity.

Li(+) Local Structure in Li-Tetraglyme Solvate Ionic Liquid Revealed by Neutron Total Scattering Experiments with the (6/7)Li Isotopic Substitution Technique.

Equimolar mixtures of lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and tetraglyme (G4: CH3O-(CH2CH2O)4-CH3) yield the solvate (or chelate) ionic liquid [Li(G4)][TFSA], which is a homogeneous transparent solution at room temperature. Solvate ionic liquids (SILs) are currently attracting increasing research interest, especially as new electrolytes for Li-sulfur batteries. Here, we performed neutron total scattering experiments with (6/7)Li isotopic substitution to reveal the Li(+) solvation/local structure in [Li(G4)][TFSA] SILs. The experimental interference function and radial distribution function around Li(+) agree well with predictions from ab initio calculations and MD simulations. The model solvation/local structure was optimized with nonlinear least-squares analysis to yield structural parameters. The refined Li(+) solvation/local structure in the [Li(G4)][TFSA] SIL shows that lithium cations are not coordinated to all five oxygen atoms of the G4 molecule (deficient five-coordination) but only to four of them (actual four-coordination). The solvate cation is thus considerably distorted, which can be ascribed to the limited phase space of the ethylene oxide chain and competition for coordination sites from the TFSA anion.

Li(+) solvation in glyme-Li salt solvate ionic liquids.

Certain molten complexes of Li salts and solvents can be regarded as ionic liquids. In this study, the local structure of Li(+) ions in equimolar mixtures ([Li(glyme)]X) of glymes (G3: triglyme and G4: tetraglyme) and Li salts (LiX: lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]), lithium bis(pentafluoroethanesulfonyl)amide (Li[BETI]), lithium trifluoromethanesulfonate (Li[OTf]), LiBF4, LiClO4, LiNO3, and lithium trifluoroacetate (Li[TFA])) was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectra and ab initio molecular orbital calculations have shown that the glyme molecules adopt a crown-ether like conformation to form a monomeric [Li(glyme)](+) in the molten state. Further, Raman spectroscopic analysis allowed us to estimate the fraction of the free glyme in [Li(glyme)]X. The amount of free glyme was estimated to be a few percent in [Li(glyme)]X with perfluorosulfonylamide type anions, and thereby could be regarded as solvate ionic liquids. Other equimolar mixtures of [Li(glyme)]X were found to contain a considerable amount of free glyme, and they were categorized as traditional concentrated solutions. The activity of Li(+) in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li(+) as a function of concentration, by using concentration cells against a reference electrode. At a higher concentration of Li salt, the amount of free glyme diminishes and affects the electrode reaction, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the significantly high electrode potential found in the solvate ILs indicates that the solvation of Li(+) by the glyme forms stable and discrete solvate ions ([Li(glyme)](+)) in the molten state. This anomalous Li(+) solvation may have a great impact on the electrode reactions in Li batteries.

Structures of [Li(glyme)](+) complexes and their interactions with anions in equimolar mixtures of glymes and Li[TFSA]: analysis by molecular dynamics simulations.

Molecular dynamics simulations of equimolar mixtures of glymes (triglyme and tetraglyme) and Li[TFSA] (lithium bis(trifluoromethylsulfonyl)amide) show that the glyme chain length affects the coordination geometries of Li(+), which induces the changes in interactions between the [Li(glyme)](+) complex and [TFSA](-) anions and diffusion of ions in the equimolar mixtures.

Theoretical characterization of the "ridge" in the supercritical region in the fluid phase diagram of water.

The density fluctuation of water in the supercritical region was investigated theoretically using the reference interaction site model theory combined with the Kovalenko-Hirata closure relation, the so-called RISM-KH theory. The density fluctuation was evaluated by the numerical differentiation of density with respect to pressure at constant temperature. The density fluctuations plotted against density show finite maxima along a line slightly off from the critical isochore, in accordance with experimental results. The microscopic structures of water on both regions that were separated by the line were investigated by analyzing the site-site radial distribution functions. The analysis clearly indicates that the structure is determined by the two effects featuring liquid states: the packing or volume exclusion effect and the screening of the Coulomb interaction or the hydrogen bond, both becoming more important at higher densities. An interplay of the two effects creates maxima of the density fluctuation in the supercritical region of water.

Specific solvation of benzyl methacrylate in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquid.

It has been established that poly(benzyl methacrylate) in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm(+)][TFSA(-)]), exhibits lower critical solution temperature (LCST)-type phase separation. In this work we investigated the solvation structure of benzyl methacrylate monomer in the ionic liquid by using high-energy X-ray diffraction with the aid of a molecular dynamics simulation. It was found that the C2mIm cation orderly distributes above and below a benzyl group within benzil methacrylate (BnMA), while the TFSA anion distributes around the equatorial position of the benzyl group where no cation distribution is found, with a weak interaction between TFSA and BnMA. The relationship between specific solvation and the LCST phase separation mechanism was considered at the molecular level.

Thermal properties and mixing state of diol-water mixtures studied by calorimetry, large-angle X-ray scattering, and NMR relaxation.

Differential scanning calorimetry (DSC) has been performed on aqueous mixtures of three diols, which involve a linear carbon chain, HO-(CH 2) n -OH ( n = 3, 4, and 5), over the whole mole fraction range of diols. The DSC results have shown the alkyl chain parity for the freezing process of the aqueous mixtures: aqueous mixtures of 1,3-propanediol (PrD) and 1,5-pentanediol (PeD) are kept in the supercooled state or vitrified over a wide mole fraction range, while those of 1,4-butanediol (BuD) are easily crystallized. The structure of PrD-water mixtures has been elucidated by using the large-angle X-ray scattering (LAXS) technique. It has been suggested that the structural change of PrD-water mixtures occurs at PrD mole fractions of x PrD = 0.4 and 0.8: in the range of x PrD < or = 0.4 where the tetrahedral-like structure of water predominates, in the range of 0.4 < x PrD < 0.8 where both PrD and water structures coexist, and in the range of x PrD > or = 0.8 where the inherent structure of PrD is mainly formed. (17)O and (1)H NMR relaxation measurements have been made on aqueous mixtures of ethylene glycol (EG, n = 2), PrD, and BuD to clarify the dynamics of H 2 (17)O and diol molecules. The (17)O NMR relaxation rates have suggested that the rotational motion of water molecules is gradually retarded in the diol-water mixtures with increasing diol content and that the restriction of the motion is more remarkable in the order of EG < PrD < BuD. On the basis of all the results, together with comparison with those of methanol-water, ethanol-water, and 1-propanol-water mixtures previously reported, the mixing state of diol-water mixtures has been discussed at the molecular level.

Thermal properties and mixing state of ethylene glycol-water binary solutions by calorimetry, large-angle X-ray scattering, and small-angle neutron scattering.

Thermal properties and mixing states of ethylene glycol (EG)-water binary solutions in the entire mole fraction range of EG, 0 < or = x(EG) < or = 1, have been clarified by using differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), and small-angle neutron scattering (SANS) techniques. The DSC curves obtained have shown that the EG-water solutions over the range of EG mole fraction 0.3 < or = x(EG) < or = 0.5 are kept in the supercooling state until approximately 100 K, and those in the range of 0.6 < or = x(EG) < or = 0.8 are vitrified, and those in the ranges of 0 < x(EG) < or = 0.2 and 0.9 < or = x(EG) < 1 are crystallized. The radial distribution function (RDF) for pure EG obtained from the LAXS measurements has suggested that a gauche conformation of an EG molecule is favorable in the liquid. The RDFs for the EG-water solutions have shown that the structure of the binary solutions moderately changes from the inherent structure of EG to the tetrahedral-like structure of water when the water content increases. The SANS intensities for deuterated ethylene glycol (HOCD2CD2OH) (EGd4)-water solutions at x(EG) = 0.4 and 0.6 have not been significantly observed in the temperature range from 298 to 173 K, showing that EG and water molecules are homogeneously mixed. On the other hand, the SANS intensities at x(EG) = 0.2 and 0.9 have been strengthened when the temperature decreases due to crystallization of the solutions. On the basis of all the present results, a relation between thermal properties of EG-water binary solutions and their mixing states clarified by the LAXS and SANS measurements has been discussed at the molecular level.