Wettability and Surface Tension of Imidazolium, Ammonium, and Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids: Comparison between Pentyl, Ethoxyethyl, and Ethylthioethyl Groups.

This study particularly compares the surface tensions and contact angles for molten bis(fluorosulfonyl)amide salts of imidazolium, ammonium, and phosphonium cations with the pentyl, ethoxyethyl, or ethylthioethyl group. The examined substrate plates for contact angle measurements include silicate glass, platinum, copper, graphene, and polytetrafluoroethylene (PTFE). In addition, quantum chemistry calculations were performed to obtain the optimized structures of the cations and anions of the ionic liquids (ILs) that were studied here along with some typical anions and their dipole moments, mean polarizabilities, and charge distributions. All ILs showed the same order of contact angles with respect to the substrates: PTFE > graphene ≈ copper ≈ platinum > silicate glass. By comparing the three functional groups, i.e., pentyl, ethoxyethyl, and ethylthioethyl, the ILs with the ethylthioethyl group featured a higher work of adhesion than the respective ILs with the pentyl or ethoxyethyl group. The values of the surface tensions of the ILs followed the same trend for the three functional groups. Based on the Fowkes theory, it was found that the larger surface tensions of the ILs with the ethylthioethyl group compared with pentyl and ethoxyethyl groups were because of the increase in both dispersive and nondispersive components. The quantum chemistry calculations of the ions showed a larger dipole moment and mean polarizability for the cations with the ethylthioethyl group as compared with the pentyl and ethoxyethyl groups. This is consistent with the analysis results of the surface tensions based on the Fowkes theory. By comparing other anions, the dispersive component of the surface tension of the ILs with bis(fluorosulfonyl)amide was large, which is attributed to the small dipole moment of the anion.

Effects of aromaticity in cations and their functional groups on the temperature dependence of low-frequency spectrum.

In this study, we investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3-200 cm-1 for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf2]- salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr]+, 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm]+, N-cyclohexylmethylpyridinium [CHxmPy]+, 1-benzyl-1-methylpyrrolidinium [BzMPyrr]+, 1-benzyl-3-methylimidazolium [BzMIm]+, and N-benzylpyridinium [BzPy]+ cations. The aim of this study is to better understand the effects of aromaticity in the cations' constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but the temperature-dependent spectrum of [CHxmMPyrr][NTf2] is different from that of other ILs. While [CHxmMPyrr][NTf2] shows spectral changes with temperature in the low-frequency region below 50 cm-1, the other ILs also show spectral changes in the high-frequency region above 80 cm-1 (above 50 cm-1 in the case of [BzMPyrr][NTf2]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.

Glass transition dynamics and conductivity scaling in ionic deep eutectic solvents: The case of (acetamide + lithium nitrate/sodium thiocyanate) melts.

A detailed investigation on the molecular dynamics of ionic deep eutectic solvents (acetamide + lithium nitrate/sodium thiocyanate) is reported. The study was carried out employing dielectric relaxation spectroscopy covering seven decades in frequency (10(-1)-10(6) Hz) and in a wide temperature range from 373 K down to 173 K, accessing the dynamic observables both in liquid and glassy state. The dielectric response of the ionic system has been presented in the dynamic window of modulus formalism to understand the conductivity relaxation and its possible connection to the origin of localized motion. Two secondary relaxation processes appear below glass transition temperature. Our findings provide suitable interpretation on the nature of secondary Johari-Goldstein process describing the ion translation and orientation of dipoles in a combined approach using Ngai's coupling model. A nearly constant loss feature is witnessed at shorter times/lower temperatures. We also discuss the ac conductivity scaling behavior using Summerfield approach and random free energy barrier model which establish the time-temperature superposition principle. These experimental observations have fundamental importance on theoretical elucidation of the conductivity relaxation and glass transition phenomena in molten ionic conductors.

Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: a femtosecond Raman-induced Kerr effect spectroscopic study.

In this study, we have investigated the ion concentration dependent collective dynamics in two series of deep eutectic solvent (DES) systems by femtosecond Raman-induced Kerr effect spectroscopy, as well as some physical properties, e.g., shear viscosity (η), density (ρ), and surface tension (γ). The DES systems studied here are [0.75CH3CONH2 + 0.25{f KSCN + (1 - f )NaSCN}] and [0.78CH3CONH2 + 0.22{f LiBr + (1 - f )LiNO3}] with f = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. γ of these DES systems shows near insensitivity to f, while ρ shows a moderate dependence on f. Interestingly, η exhibits a strong dependence on f. In the low-frequency Kerr spectra, obtained via the Fourier transform of the collected Kerr transients, a characteristic band at ∼70 cm(-1) is clear in [0.78CH3CONH2 + 0.22{f LiBr + (1 - f )LiNO3}] DES especially at the larger f. The band is attributed to the intermolecular hydrogen bond of acetamide. Because of less depolarized Raman activities of intermolecular/interionic vibrational motions, which are mostly translational (collision-induced or interaction-induced) motions, of spherical ions, the intermolecular hydrogen-bonding band is clearly observed. In contrast, the intermolecular hydrogen-bonding band is buried in the other intermolecular/interionic vibrational motions, which includes translational and reorientational (librational) motions and their cross-terms, in [0.75CH3CONH2 + 0.25{f KSCN + (1 - f )NaSCN}] system. The first moment (M1) of the intermolecular/interionic vibrational band in these DES systems is much higher than that in typical neutral molecular liquids and shows a weak but contrasting dependence on the bulk parameter √γ/ρ. The time constants for picosecond overdamped Kerr transients in both the DES systems, which are obtained on the basis of the analysis fitted by a triexponential function, are rather insensitive to f for both the DES systems, but all the three time constants (fast: ∼1-3 ps; intermediate: ∼7-20 ps; and slow: ∼100 ps) are different between the [0.78CH3CONH2 + 0.22{f LiBr + (1 - f )LiNO3}] and [0.75CH3CONH2 + 0.25{f KSCN + (1 - f )NaSCN}] systems. These results indicate that the intermolecular/interionic interactions in DES systems is strongly influenced by the ionic species present in these DES systems.

Low-Frequency Spectra of Hydrated Ionic Liquids with Kosmotropic and Chaotropic Anions.

In this study, we investigated the water concentration dependence of the intermolecular vibrations of two hydrated ionic liquids (ILs), cholinium dihydrogen phosphate ([ch][dhp]) and cholinium bromide ([ch]Br), using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES). The anions of the former and latter hydrated ILs are kosmotropic and chaotropic, respectively. We found that the spectral peak of ∼50 cm-1 shifted to the low-frequency side in hydrated [ch][dhp], indicating the weakening of its intermolecular interactions. In contrast, no change in the peak frequency of the low-frequency band at ∼50 cm-1 was observed with increasing water concentration in hydrated [ch]Br. The vibrational density of states (VDOS) spectra generated from molecular dynamics (MD) simulations were in qualitative agreement with the experimental results. Decomposition analysis of the VDOS spectra for each component revealed that the red shift of the low-frequency band in the hydrated [ch][dhp] upon water addition was essentially due to the contributions of anions and water rather than that of the cholinium cation. We also found from the low-frequency spectra of the two hydrated ILs that they differed in the concentration dependence of the 180 cm-1 band, which is assigned as a hindered translational motion of water molecules combined to form O···O stretching motions. From the relationship between the peak frequency of the low-frequency band and the bulk parameter, which is the square root of the surface tension divided by the density, we found that the peak frequency in the hydrated IL with kosmotropic [dhp]- depends on the bulk parameter, similar to the case for an aqueous solution of the typical deep eutectic solvent reline. However, the peak frequency of the hydrated IL with chaotropic Br- is constant with the bulk parameter.

Comparison between Phosphonium Docusate Ionic Liquids and Their Equimolar Mixtures with Alkanes: Temperature-Dependent Viscosity, Glass Transition, and Fragility.

In this study, we determined the temperature-dependent viscosities, glass transition temperatures, and fragilities of tetraalkylphosphonium docusate ionic liquids (ILs) and their equimolar mixtures with alkanes to elucidate the effects of the alkyl groups on the phosphonium cation. The target ILs were the docusate salts with tributylheptylphosphonium ([P4447][doc]), tributyltetradecylphosphonium ([P444,14][doc]), butyltrihexylphosphonium ([P4666][doc]), trihexylheptylphosphonium ([P6667][doc]), and trihexyltetradecylphosphonium cations ([P666,14][doc]). The comparable IL/alkane mixtures were equimolar mixtures of IL and alkane with the same carbon numbers of the target ILs: [P4447][doc]/hexane to [P6667][doc]; [P4447][doc]/heptane to [P444,14][doc]; [P444,14][doc]/hexane to [P666,14][doc]; [P4666][doc]/decane to [P666,14][doc]; and [P6667][doc]/heptane to [P666,14][doc]. The viscosities and glass transition temperatures of the neat ILs were higher than those of their respective IL/alkane mixtures. Based on the analysis of temperature-dependent viscosities, including a viscosity value of 1013 mPa·s at the glass transition temperature using the Vogel-Fulcher-Tammann equation, the neat ILs were stronger liquids than the corresponding IL/alkane mixtures. By comparing several combinations of the neat ILs and IL/alkane mixtures, we found that the larger the alkane, the more fragile the mixture.

Physical Properties and Low-Frequency Polarizability Anisotropy and Dipole Responses of Phosphonium Bis(fluorosulfonyl)amide Ionic Liquids with Pentyl, Ethoxyethyl, or 2-(Ethylthio)ethyl Group.

This study compared the physical properties, e.g., glass transition temperature, melting point, viscosity, density, surface tension, and electrical conductivity, and the low-frequency spectra under 200 cm-1 of three synthesized ionic liquids (ILs), triethylpentylphosphonium bis(fluorosulfonyl)amide ([P2225][NF2]), ethoxyethyltriethylphosphonium bis(fluorosulfonyl)amide ([P222(2O2)][NF2]), and triethyl[2-(ethylthio)ethyl]phosphonium bis(fluorosulfonyl)amide ([P222(2S2)][NF2]), at various temperatures using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and terahertz time-domain spectroscopy (THz-TDS). The [P222(2S2)][NF2] had the highest viscosity and glass transition temperature, whereas the [P222(2O2)][NF2] had the lowest. Among the three ILs, the [P222(2S2)][NF2] had the highest density and surface tension, and the [P222(2O2)][NF2] had the highest electrical conductivity. The RIKES and THz-TDS spectral line shapes for the three ILs varied significantly. For the [P2225][NF2], molecular dynamics simulations successfully reproduced the line shapes of the experimental spectra and indicated that the RIKES spectrum was mainly due to the cation and cross-term and their rotational motions, whereas the THz-TDS spectrum was mainly due to the anion and its translational motion. This shows that it is desirable to utilize both fs-RIKES and THz-TDS methods to reveal molecular motions at the low-frequency domain. The [P222(2S2)][NF2] had higher frequency peaks and broader bands in the low-frequency spectra via fs-RIKES and THz-TDS than those for the [P2225][NF2] and [P222(2O2)][NF2].

Exploring the Microscopic Aspects of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Formamide, N-Methylformamide, and N,N-Dimethylformamide by Multiple Spectroscopic Techniques and Computations.

The microscopic aspects of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with formamide (FA), N-methylformamide (NMF), and N,N-dimethylformamide (DMF) were investigated using spectroscopic techniques of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), FT-IR, and NMR. Molecular dynamics simulations and quantum chemistry calculations were also performed. According to fs-RIKES, the first moment of the low-frequency spectrum bands mainly originating from the intermolecular vibrations in the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems changed gradually with the molecular liquid mole fraction XML but that in the [MOIm][BF4]/NMF system was constant up to XNMF = 0.7 and then gradually increased in the range of XNMF ≥ 0.7. Excluding the contribution of the 2D hydrogen-bonding network due to the presence of FA in the low-frequency spectrum band, the XML dependence of the normalized first moment of the low-frequency band in the [MOIm][BF4]/FA and [MOIm][BF4]/NMF systems revealed that the normalized first moment did not remarkably change in the range of XML < 0.7 but drastically increased in XML ≥ 0.7. FT-IR results indicated that the amide C═O band shifted to the low-frequency side with increasing XML for the three mixtures due to the hydrogen bonds. The imidazolium ring C-H band also showed a similar tendency to the amide C═O band. 19F NMR probed the microenvironment of [BF4]- in the mixtures. The [MOIm][BF4]/NMF and [MOIm][BF4]/DMF systems showed an up-field shift of the F atoms of the anion with increasing XML, and the [MOIm][BF4]/FA system exhibited a down-field shift. Steep changes in the chemical shifts were confirmed in the region of XML > 0.8. On the basis of the quantum chemistry calculations, the observed chemical shifts with increasing XML were mainly attributed to the many-body interactions of ions and amides for the [MOIm][BF4]/FA and [MOIm][BF4]/DMF systems. Meanwhile, the long distance between the cation and the anion was due to the high dielectric medium for the [MOIm][BF4]/NMF system, which led to an up-field shift.

Structural Origins of Viscosity in Imidazolium and Pyrrolidinium Ionic Liquids Coupled with the NTf2- Anion.

Ionic liquid viscosity is one of the most important properties to consider for practical applications. Yet, the connection between local structure and viscosity remains an open question. This article explores the structural origin of differences in the viscosity and viscoelastic relaxation across several ionic liquids, including cations with alkyl, ether, and thioether tails, of the imidazolium and pyrrolidinium families coupled with the NTf2- anion. In all cases, for the systems studied here, we find that pyrrolidinium-based ions are "harder" than their imidazolium-based counterparts. We make a connection between the chemical concept of hardness vs softness and specific structural and structural dynamic quantities that can be derived from scattering experiments and simulations.

Comparison of low-frequency spectra between aromatic and nonaromatic cation based ionic liquids using femtosecond Raman-induced Kerr effect spectroscopy.

In this minireview, the features of low-frequency spectra within the frequency range 0-200 cm(-1) for aromatic and nonaromatic cation based ionic liquids obtained by femtosecond Raman-induced Kerr effect spectroscopy are reviewed. For aromatic cation based ionic liquids, the aromatic group of the cations is largely responsible for the spectral line shape due to their larger polarizability anisotropy volume and more polarizable nature than nonaromatic cations. The low-frequency Kerr spectra are also compared with the bulk properties of the ionic liquids. The differences in the relationship of the first moment of the low-frequency spectrum band and the bulk properties between the aromatic and nonaromatic cation based ionic liquids with respect to the natures of constituent ions and microscopic aspects are discussed. For aromatic cation based ionic liquids, the ionic region is more pronounced than the nonpolar region in their low-frequency Kerr spectra. In contrast, the ionic and nonpolar regions of nonaromatic cation based ionic liquids are likely to be captured due to the weaker signal of the cationic region. Also, the low-frequency Kerr spectra of the nonaromatic analogues are influenced by their weaker segregation as compared to aromatic cation based ionic liquids.

Intermolecular Dynamics of Positively and Negatively Charged Aromatics and Their Isoelectronic Neutral Analogs in Aqueous Solutions.

In this study, we investigated the temperature dependence of intermolecular vibrations and orientational dynamics in the aqueous solutions of imidazole hydrochloride, imidazole, sodium triazolide, and triazole using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and steady-state Raman spectroscopy. The difference low-frequency Raman spectra under 250 cm-1 of the aqueous solutions relative to the neat water showed that the spectral shoulder in the high-frequency region at 60-100 cm-1, assigned to the libration of an aromatic ring, was higher in frequency for the imidazolium cation but lower for the triazolide anion than those of the respective neutral aromatics. The results of the ab initio quantum chemistry calculations of the clusters of the aromatics and water molecule(s) were consistent with the experimental spectra of the aqueous solutions. Further, the results of the temperature-dependent experiments showed that the signal intensity in the low-frequency region below 50 cm-1 increased for all solutions with an increase in temperature. In contrast, the spectral density in the high-frequency region above 80 cm-1 exhibited almost no shift for the 1.0 M solutions, while a significant red shift was observed for the 5.0 M solutions. In addition, the temperature-dependent densities, viscosities, and surface tensions were characterized for the aqueous aromatic solutions from 293 to 353 K.

Intermolecular Dynamics and Structure in Aqueous Lidocaine Hydrochloride Solutions.

We investigated the intermolecular dynamics and static structure in the aqueous solutions of lidocaine hydrochloride (LDHCl) in the concentration range of [LDHCl] = 0-2.00 M using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), small- and wide-angle X-ray scattering (SWAXS), and dynamic light scattering (DLS). For the fs-RIKES experiments, the concentration dependence of the difference low-frequency spectra of the aqueous LDHCl solutions relative to the neat water, which was mainly due to the intermolecular vibrations, was characterized using an exponential function with a characteristic concentration of ∼1 M. For the SWAXS experiments, we observed a manifestation of an excess scattering component centered within a range of 8-10 nm-1 in the aqueous LDHCl solutions. The results of Fourier inversion and further deconvolution analyses unambiguously demonstrated that lidocaines assemble into a nanometer-sized micelle-like structure with the innermost core (∼0.3 nm) and outer shell (∼0.5 nm), respectively. The DLS experiments also found nanometer-sized aggregates and further indicated evidence of the clusters of the aggregates. The results of viscosities, densities, and surface tensions of the solutions and the quantum chemistry calculations supported the unique features of the microscopic intermolecular interaction and the micelle-like aggregation.

Intermolecular vibrations and diffusive orientational dynamics of Cs condensed ring aromatic molecular liquids.

The ultrafast dynamics, including the intermolecular vibrations and the diffusive orientational dynamics, of the neat C(s) symmetry condensed ring aromatic molecular liquids benzofuran, 1-fluoronaphtalene, and quinoline were investigated for the first time by means of femtosecond Raman-induced Kerr effect spectroscopy. To understand the features of these C(s) condensed ring aromatic molecular liquids, reference singular aromatic molecular liquids, furan, fluorobenzene, pyridine, and benzene, were also studied. High quality low-frequency Kerr spectra of the aromatic molecular liquids were obtained by Fourier-transform deconvolution analysis of the measured Kerr transients. The Kerr spectra of the C(s) condensed ring aromatic molecular liquids are bimodal, as are those of the reference singular aromatic molecular liquids. The first moment of the intermolecular vibrational spectrum and the peak frequencies of the high- and low-frequency components in the broad spectrum band were compared with their molecular properties such as the rotational constants, molecular weight, and intermolecular (bimolecular) force. The comparisons show that the molecular volume (related to molecular weight and rotational constants) is a dominant property for the characteristic frequency of the entire intermolecular vibrational spectrum. The observed intramolecular vibrational modes in the Kerr spectra of the aromatic molecular liquids were also assigned on the basis of the ab initio quantum chemical calculation results. In their picosecond diffusive orientational dynamics, the slowest relaxation time constant for both the condensed ring and singular aromatic molecular liquids can be accounted for by the simple Stokes-Einstein-Debye hydrodynamic model.

Intermolecular vibrational spectra of C3v CXY3 Molecular Liquids, CHCl3, CHBr3, CFBr3, and CBrCl3.

We report the quality anisotropic intermolecular vibrational spectra within the frequency range 0.5-800 cm(-1) of four C(3v) CXY(3) molecular liquids, CHCl(3), CHBr(3), CFBr(3), and CBrCl(3), by means of femtosecond optical-heterodyne-detected Raman-induced Kerr effect spectroscopy. The results show that the first moment of the intermolecular vibrational spectrum is proportional to the square root of the value of the surface tension divided by the liquid density. This implies that the intermolecular vibrational spectrum reflects the bulk properties of the liquids. To understand the molecular-level aspects of the intermolecular vibrational spectra of the liquids, the spectra are compared with the molecular properties such as molecular weight, rotational constants, and bimolecular interaction energy. Overall, the first moment of the spectrum moderately correlates to the inverse square roots of both the molecular weight and the fast rotational constant. Therefore, the molecular properties are responsible for the intermolecular vibrational spectrum. Plots of the first moment of the intermolecular vibrational spectrum vs the square root of the value of the simple bimolecular interaction energy divided by the molecular surface area and the molecular weight show a linear correlation in the case of the oblate symmetric top molecular liquids, CHCl(3), CHBr(3), and CFBr(3). However, CBrCl(3), which is a prolate symmetric top molecular liquid, does not show the same correlation for the oblate molecular liquids.

Intermolecular vibrational modes and orientational dynamics of cooperative hydrogen-bonding dimer of 7-azaindole in solution.

We observed the low-frequency Raman-active intermolecular vibrational modes of 7-azaindole in CCl(4) by femtosecond Raman-induced Kerr effect spectroscopy. To understand the dynamical aspects and vibrational modes of 7-azaindole in the solution, the ultrafast dynamics of 1-benzofuran in CCl(4) was also examined as a reference and ab initio quantum chemistry calculations were performed for 7-azaindole and 1-benzofuran. The cooperative hydrogen-bonding vibrational bands of 7-azaindole dimer in CCl(4) appeared at 89 cm(-1) and 105 cm(-1) represent the overlap of stagger and wheeling modes and the intermolecular stretching mode, respectively. They are almost independent of the concentration in the solution. We further found from the low-frequency differential Kerr spectra of the solutions with neat CCl(4) that the intermolecular motion in the low frequency region below 20 cm(-1) was less active in the case of 7-azaindole/CCl(4) than in the case of 1-benzofuran/CCl(4). The slow orientational relaxation time in 7-azaindole/CCl(4) is ~3.5 times that in 1-benzofuran/CCl(4) because of the nature of the dimerization of 7-azaindole.

Low-frequency modes of the benzoic acid dimer in chloroform observed by the optical Kerr effect.

The low frequency Raman spectral density associated with the intermolecular hydrogen-bonding interaction of benzoic acid in chloroform was investigated through the ultrafast optically-heterodyne-detected optical Kerr effect. The low-frequency solute Raman spectrum was obtained by Fourier transform analysis and subtraction of the solvent spectrum from the solution spectrum. The resulting difference spectrum has a broad band below 150 cm(-1) with a peak at around 80 cm(-1). Previous studies of aromatic liquids suggest that the origin of such a low-frequency band is librational motion, although intermolecular hydrogen-bonding modes in benzoic acid may also contribute. To clarify these contributions to the low-frequency band, methyl benzoate was used to estimate the librational component; its structure is similar to benzoic acid, but it forms no intermolecular hydrogen bonds. Both librational and intermolecular modes were found to contribute to the low-frequency Raman spectrum of the dimer and thus can be separated. These experimental results were compared with the results of density functional theory calculations. In addition, the effect of deuteration on the Raman spectrum was also investigated.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Poly(ethylene glycol) by Femtosecond Raman-Induced Kerr Effect Spectroscopy.

This is the first report on low-frequency spectra of ionic liquid (IL)/polymer mixtures using femtosecond Raman-induced Kerr effect spectroscopy. We studied mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) and poly(ethylene glycol) (PEG) with Mn = 400 (PEG400) at various concentrations. To elucidate the unique features of the IL/polymer mixture system, mixtures of PEG400 with a molecular liquid, 1-octhylimidazole (OIm), which is a neutral analog of the cation, were also studied. In addition, mixtures of [MOIm][BF4] with ethylene glycol (EG) and poly(ethylene glycol) with Mn = 4000 (PEG4000) were also investigated. The first moments of broad low-frequency spectra, mainly due to intermolecular vibrations for the [MOIm][BF4]/PEG400 and OIm/PEG400, increased slightly with increasing concentration of PEG400, indicating that microscopic intermolecular interactions, in general, are slightly enhanced. We also compared the [MOIm][BF4] mixtures with EG, PEG400, and PEG4000 at concentrations of 5 and 10 wt % PEG or EG. The low-frequency spectra of samples with the same concentrations were quite similar, but a comparison of the normalized spectra showed that the spectral intensity in the low-frequency region below ∼50 cm-1 of the [MOIm][BF4] mixtures with PEG400 and PEG4000 is somewhat lower than that of the [MOIm][BF4] mixtures with EG. Although the effect of the polymer is small compared to other polymer solution systems, this feature is attributed to a suppression of translational motion in the mixtures of [MOIm][BF4] with PEG compared to the mixtures of [MOIm][BF4] with EG due to the greater mass of PEG than EG. Density, surface tension, viscosity, and electrical conductivity were also estimated. From Walden plots, it was found that the [MOIm][BF4]/PEG4000 system showed more ideal electrical conductive behavior than the [MOIm][BF4]/PEG400 and [MOIm][BF4]/EG systems.

Anion Effects on the Mixing States of 1-Methyl-3-octylimidazolium Tetrafluoroborate and Bis(trifluoromethylsulfonyl)amide with Methanol, Acetonitrile, and Dimethyl Sulfoxide on the Meso- and Microscopic Scales.

The mixing states of two imidazolium-based ionic liquids (ILs) with different anions, 1-methyl-3-octylimidazolium tetrafluoroborate (C8mimBF4) and bis(trifluoromethylsulfonyl)amide (C8mimTFSA), with three molecular liquids (MLs), methanol (MeOH), acetonitrile (AN), and dimethyl sulfoxide (DMSO), have been investigated on both mesoscopic and microscopic scales using small-angle neutron scattering (SANS), infrared (IR), and 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. Additionally, molecular dynamics (MD) simulations have been conducted on the six combinations of ILs and MLs to observe the states of their mixtures on the atomic level. The SANS profiles of the IL-ML mixtures suggested that MeOH molecules only form clusters in both C8mimBF4 and C8mimTFSA, whereas AN and DMSO were homogeneously mixed with ILs on the SANS scale. MeOH clusters are more enhanced in BF4--IL than TFSA--IL. The microscopic interactions among IL cations, anions, and MLs should contribute to the mesoscopic mixing states of the IL-ML mixtures. In fact, the IL cation-anion, cation-ML, anion-ML, and ML-ML interactions observed by IR, NMR, and MD simulations clarified the reasons for the mixing states of the IL-ML binary solutions observed by the SANS experiments. In neat ILs, the imidazolium ring of the IL cation more strongly interacts with BF4- than TFSA- due to the higher charge density of the former. The interaction of anions with the imidazolium ring is more easily loosened on adding MLs to ILs in the order of DMSO > MeOH > AN. It does not significantly depend on the anions. However, the replacement of the anion on the imidazolium ring by an ML depends on the anions; the replacement is more proceeded in the order of MeOH > DMSO > AN in BF4--IL, while DMSO > MeOH > AN in TFSA--IL. On the other hand, the solvation of both anions by MLs is stronger in the order of MeOH > DMSO ≈ AN. Despite the stronger interactions of MeOH with both cations and anions, MeOH molecules are heterogeneously mixed with both ILs to form clusters in the mixtures. Therefore, the self-hydrogen bonding among MeOH molecules most markedly governs the mixing state of the binary solutions among the abovementioned interactions.

Low-Frequency Vibrational Motions of Polystyrene in Carbon Tetrachloride: Comparison with Model Monomer and Dependence on Concentration and Molecular Weight.

In this study, the low-frequency vibrational dynamics of polystyrene (PS) in CCl4 was investigated by femtosecond Raman-induced Kerr effect spectroscopy. Ethylbenzene (EBz) was also investigated as a model monomer of the polymer to elucidate the unique dynamical features of PS in solution. The broadened low-frequency spectrum of the PS/CCl4 in the frequency region below 150 cm-1 is significantly different from that of the EBz/CCl4. Difference spectra between the PS or EBz solutions and neat CCl4, normalized to an internal vibrational mode of CCl4, clearly show a much lower spectral intensity for the PS/CCl4 than the EBz/CCl4 in the low-frequency region below ca. 20 cm-1. This indicates that translational motions are suppressed in the PS/CCl4 compared to the EBz/CCl4. Moreover, the high-frequency motion at ca. 70 cm-1, mainly due to phenyl ring librations, occurs at higher frequency in PS (78 cm-1) than EBz (65 cm-1). In addition, the results of concentration-dependent experiments show that the first moment (M1) of the low-frequency difference spectra of both PS/CCl4 and EBz/CCl4 is almost independent of the concentration. The molecular weight dependence of the low-frequency spectrum in the PS/CCl4 shows that the M1 value of the low-frequency spectral band of PS shifts to higher frequencies when the molecular weight of PS increases up to Mw = ∼1000, which corresponds approximately to the decamer, and then remains constant upon further increasing the molecular weight.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Methanol, Acetonitrile, and Dimethyl Sulfoxide: A Combined Study of Femtosecond Raman-Induced Kerr Effect Spectroscopy and Molecular Dynamics Simulations.

In this study, we examined the low-frequency spectra of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF4]) mixtures with methanol (MeOH), acetonitrile (MeCN), and dimethyl sulfoxide (DMSO), which were obtained by femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and molecular dynamics (MD) simulations. In addition, we estimated the liquid properties of the mixtures, such as density ρ, surface tension γ, viscosity η, and electrical conductivity σ. The line shapes of the low-frequency Kerr spectra of the three [MOIm][BF4] mixture systems strongly depend on the mole fraction of the molecular liquid, XML. The spectral intensity increases with increasing XML of the [MOIm][BF4]/MeCN system but decreases for the [MOIm][BF4]/MeOH and [MOIm][BF4]/DMSO systems. These behaviors of the spectral intensities reasonably agree with the vibrational density-of-states spectra when the polarizability anisotropies of MeOH, MeCN, DMSO, and ion species are considered. The characteristic frequencies (first moments, M1) of the low-frequency spectra of the three mixture systems are almost insensitive at XML = 0-0.6. However, the frequencies vary mildly at XML = 0.6-0.9 and dramatically at XML = 0.9-1. The XML-dependent M1 in the Kerr spectra are well reproduced by the MD simulations. Plots of M1 versus bulk parameter, (γ/ρ)1/2, for the three mixture systems show that the mixtures at XML = 0-0.6 behave like aromatic cation-based ionic liquids (ILs), those at XML = 0.9-1 are molecular liquids (MLs), and those at XML = 0.6-0.9 are transitioning between aromatic cation-based ILs and MLs. MD simulations show that the solvent molecules localized at the interface between the ionic and the alkyl group regions without forming large solvent networks at XML = 0-0.6. However, solvent networks or regions develop largely at XML = 0.6-0.9 and the constituent ions of the IL disperse in the MLs at XML = 0.9-1. The MD simulations corroborate the results obtained by fs-RIKES.