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

Description of the local microscopic structure in ionic liquids (ILs) is a prerequisite to obtain a comprehensive understanding of the influence of the nature of ions on the properties of ILs. The local structure is mainly determined by the spatial arrangement of the nearest neighboring ions. Therefore, the main interaction patterns in ILs, such as cation–anion H-bond-like motifs, cation–cation alkyl tail aggregation, and ring stacking, were considered within the framework of the nearest-neighbor approach with respect to each particular interaction site. We employed classical molecular dynamics (MD) simulations to study in detail the spatial, radial, and orientational relative distribution of ions in a set of imidazolium-based ILs, in which the 1-butyl-3-methylimidazolium (C₄mim⁺) cation is coupled with the acetate (OAc^–), chloride (Cl^–), tetrafluoroborate (BF₄^–), hexafluorophosphate (PF₆^–), trifluoromethanesulfonate (TfO^–), or bis­(trifluoromethanesulfonyl)­amide (TFSA^–) anion. It was established that several structural properties are strongly anion-specific, while some can be treated as universally applicable to ILs, regardless of the nature of the anion. Namely, strongly basic anions, such as OAc^– and Cl^–, prefer to be located in the imidazolium ring plane next to the C–H^2/4–5 sites. By contrast, the other four bulky and weakly coordinating anions tend to occupy positions above/below the plane. Similarly, the H-bond-like interactions involving the H² site are found to be particularly enhanced in comparison with the ones at H^4–5 in the case of asymmetric and/or more basic anions (C₄mimOAc, C₄mimCl, C₄mimTfO, and C₄mimTFSA), in accordance with recent spectroscopic and theoretical findings. Other IL-specific details related to the multiple H-bond-like binding and cation stacking issues are also discussed in this paper. The secondary H-bonding of anions with the alkyl hydrogen atoms of cations as well as the cation–cation alkyl chain aggregation turned out to be poorly sensitive to the nature of the anion

SANS, Infrared, and ⁷Li and ²³Na NMR Studies on Phase Separation of Alkali Halide–Acetonitrile–Water Mixtures by Cooling

Phase separation of alkali halide (MX) (M = Li⁺, Na⁺, and K⁺ and X = Cl^– and Br^–)–acetonitrile (AN)–water mixtures by cooling has been investigated at the molecular level. The phase diagram obtained for the MX–AN–H₂O ternary systems showed that the temperatures of phase separation for the mixtures with MCl are higher than those with MBr. The phase-separation temperatures of the mixtures with MCl and MBr are higher in the sequence of NaX > KX > LiX, although the magnitude of the hydration enthalpies for the alkali metal ions is larger in the sequence of Li⁺ > Na⁺ > K⁺. To elucidate the reasons for the sequence of phase separation on the meso- and microscopic scales, small-angle neutron scattering (SANS), infrared (IR), and ⁷Li and ²³Na NMR measurements have been conducted on MX–AN–water mixtures with lowering temperature. The results of SANS and IR experiments showed that the mechanism of phase separation of the mixtures by cooling is the same among all of the mixtures but did not clearly reveal the reasons for the phase separation sequence. In contrast, the spin–lattice relaxation rates and the chemical shifts of ⁷Li and ²³Na NMR for the mixtures suggested the different solvation structure of Li⁺ and Na⁺ in the mixtures. In conclusion, the solvation of acetonitrile molecules for Li⁺ and the formation of Li⁺–X^– contact ion pairs in the mixtures cause the weakest effect of LiX on phase separation of the mixtures by cooling among the alkali metal ions

Effects of Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide Anions on the Microscopic Structures of 1‑Methyl-3-octylimidazolium-Based Ionic Liquids and Benzene Mixtures: A Multiple Approach by ATR-IR, NMR, and Femtosecond Raman-Induced Kerr Effect Spectroscopy

The microscopic aspects of the two series of mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm]­[BF₄])–benzene and 1-methyl-3-octylimidazolium bis­(trifluoromethylsulfonyl)­amide ([MOIm]­[NTf₂])–benzene were investigated by several spectroscopic techniques such as attenuated total reflectance IR (ATR-IR), NMR, and fs-Raman-induced Kerr effect spectroscopy (fs-RIKES). All three different spectroscopic results indicate that the anions more strongly interact with the cations in the [MOIm]­[BF₄]–benzene mixtures than in the [MOIm]­[NTf₂]–benzene mixtures. This also explains the different miscibility features between the two mixture systems well. The <i>x</i>_C₆H₆ dependences of the chemical shifts and the C–H out-of-plane bending mode of benzene are similar: the changes are large in the high benzene concentration (<i>x</i>_C₆H₆ > ∼0.6) compared to the low benzene concentration. In contrast, the linear <i>x</i>_C₆H₆ dependences of the first moments of the low-frequency spectra less than 200 cm^–1 were observed in both the [MOIm]­[BF₄]–benzene and [MOIm]­[NTf₂]–benzene systems. The difference in the <i>x</i>_C₆H₆ dependent features between the chemical shifts and intramolecular vibrational mode and the intermolecular/interionic vibrational bands might come from the different probing space scales. The traces of the parallel aromatic ring structure and the T-shape structure were found in the ATR-IR and NMR experiments, but fs-RIKES did not observe a clear trace of the local structure. This might imply that the interactions between the imidazolium and benzene rings are not strong enough to librate the imidazolium and benzene rings together. The bulk properties, such as miscibility, density, viscosity, and surface tension, of the two ionic liquid-benzene mixture series were also compared to the microscopic aspects

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 <i>x</i>_OS 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 (<i>x</i>_OS = 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 <i>x</i>_OS = ∼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 <i>T</i>₁ 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 <i>x</i>_AN > 0.9. Especially, the pair correlation functions <i>g</i>(<i>r</i>) 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 <i>x</i>_AN rises to the unity

Solvent-Dependent Properties and Higher-Order Structures of Aryl Alcohol + Surfactant Molecular Gels

Molecular organogels, comprising small organic gelators in solvents, can be applied for dispersal of optical devices, such as emitters. Phenolic compounds and the surfactant bis­(2-ethylhexyl) sulfosuccinate (AOT) are known examples of self-assembly organogels. However, conventional phenol + AOT gels in aromatic and acyclic alkane solvents are optically turbid, which is an obstacle for use as host materials in optical devices. In this study, a variety of aryl alcohol–AOT–solvent sets have been investigated systematically, and the correlation between the molecular architecture and optical transparency of the gels was considered. Accordingly, <i>p</i>-chlorophenol + AOT gels in cyclic alkane solvents were shown to form optically transparent gels. In contrast, aromatic and acyclic alkane solvents gave rise to turbid or opaque gels, even when utilizing the same gelators. AFM, NMR, SAXS, and FTIR were employed to determine the organogel structures. Consequently, we found that the gel transparency strongly depends on the size of the fibrous network of the gel, the structure of which is attributed to higher-order aggregates of the gelators. The average contour length and diameter of the fibrous network, <i>l</i>_av and <i>d</i>_av, respectively, were determined from AFM images. The transparent gels were shown to have <i>l</i>_av = 4–9 μm and <i>d</i>_av ≤ 0.3 μm, whereas the turbid gels had <i>l</i>_av = 15 μm and <i>d</i>_av = 0.4–0.6 μm. Such differences in the size of the fibrous network significantly affected the mechanical response of the gels, as shown by stress–strain measurements