Optical Kerr effect spectroscopy of CS2 in monocationic and dicationic ionic liquids: insights into the intermolecular interactions in ionic liquids.

A comparative study of the intermolecular dynamics of CS2 in monocationic and dicationic ionic liquids (ILs) was performed using optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The reduced spectral densities (RSDs) of mixtures of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 3-5) and 1,2n-bis(3-methylimidazolium-1-yl) alkane bis[(trifluoromethane)sulfonyl]amide ([(C1im)2C2n][NTf2]2 for n = 3-5) were investigated as a function of concentration at 295 K. An additivity model was used to obtain the CS2 contribution to the RSD of a mixture in the 0-200 cm-1 region. One of the aims of this study is to show how CS2 can be used as a probe of intermolecular/interionic interactions in ILs. The concentrations were chosen such that the CS2-to-imidazolium ring mole fraction of a mixture with [(C1im)2C2n][NTf2]2 (DIL(2n)) is the same as that of a mixture with [CnC1im][NTf2] (MIL(n)). As found previously for CS2 in monocationic ILs, the intermolecular spectrum of CS2 in dicationic ILs is lower in frequency and narrower than that of neat CS2. The new result is that the intermolecular spectrum of CS2 is higher in frequency in DIL(2n) than in the corresponding MIL(n), indicating that CS2 molecules experience a stiffer potential in dicationic ILs than in monocationic ILs. The intermolecular dynamics of CS2 being higher in frequency in DIL(2n) than in MIL(n) is consistent with recent molecular dynamics simulations (Lynden-Bell and Quitevis, J. Chem. Phys., 2018, 148, 193844) that show the stiffer potential is the result of greater confinement of CS2 in DIL(2n) than in MIL(n). We also show in this study how effects due to dilution and the intermolecular potential seen by a solute molecule in solution are unraveled.

Ionic liquids at the surface of graphite: Wettability and structure.

The aim of this work is to provide a better understanding of the interface between graphite and different molecular and ionic liquids. Experimental measurements of the liquid surface tension and of the graphite-liquid contact angle for sixteen ionic liquids and three molecular liquids are reported. These experimental values allowed the calculation of the solid/liquid interfacial energy that varies, for the ionic liquids studied, between 14.5 mN m-1 for 1-ethyl-3-methylimidazolium dicyanamide and 37.8 mN m-1 for 3-dodecyl-1-(naphthalen-1-yl)-1H-imidazol-3-ium tetrafluoroborate. Imidazolium-based ionic liquids with large alkyl side-chains or functionalized with benzyl groups seem to interact more favourably with freshly peeled graphite surfaces. Even if the interfacial energy seems a good descriptor to assess the affinity of a liquid for a carbon-based solid material, we conclude that both the surface tension of the liquid and the contact angle between the liquid and the solid can be significant. Molecular dynamics simulations were used to investigate the ordering of the ions near the graphite surface. We conclude that the presence of large alkyl side-chains in the cations increases the ordering of ions at the graphite surface. Benzyl functional groups in the cations lead to a large affinity towards the graphite surface.

Comparative study of the intermolecular dynamics of imidazolium-based ionic liquids with linear and branched alkyl chains: OHD-RIKES measurements.

This article describes a comparative study of the low-frequency (0-450 cm-1) Kerr spectra of the branched 1-(iso-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([(N - 2)mCN-1C1im][NTf2] with N = 3-7) ionic liquids (ILs) and that of the linear 1-(n-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([CNC1im][NTf2] with N = 2-7) ILs. The spectra were obtained by use of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular spectrum of a branched IL is similar to that of a linear IL that is of the same alkyl chain length rather than of the same number of carbon atoms in the alkyl chain. This similarity and the lack of a correlation of the first spectral moments and widths of the intermolecular spectra with chain length is mainly attributed to the increase in the dispersion contribution to the total molar cohesive energy being compensated by stretching of the ionic network due to the increasing size of the nonpolar domains, which is dependent only on the length of the alkyl chain.

The importance of polarizability: comparison of models of carbon disulphide in the ionic liquids [C1C1im][NTf2] and [C4C1im][NTf2].

The local environment of CS2 and in solution in two ionic liquids ([C1C1im][NTf2] and [C4C1im][NTf2]) are investigated by atomistic simulation and compared with that in neat CS2. The intermolecular vibrational densities of states of CS2 are calculated and compared with experimental OHD-RIKES spectra. The fair agreement of the results from solutions but poor agreement of the results from neat CS2 suggest that while collective effects are unimportant in solutions, they have a major effect on the OHD-RIKES spectrum of neat CS2. Comparing polarizable and unpolarizable models for CS2 emphasizes the importance of polarizability in determining local structure.

Solubility of n-butane and 2-methylpropane (isobutane) in 1-alkyl-3-methylimidazolium-based ionic liquids with linear and branched alkyl side-chains.

The solubility of n-butane and 2-methylpropane (isobutane) in three ionic liquids - 1-(2-methylpropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(2mC3)C1im][Ntf2], 1-(3-methylbutyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(3mC4)C1im][Ntf2] and 1-methyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide [C5C1im][Ntf2] - has been measured at atmospheric pressure from 303 to 343 K. Isobutane is less soluble than n-butane in all the ionic liquids. Henry's constant values range from 13.8 × 10(5) Pa for n-butane in [C5C1im][Ntf2] at 303 K to 64.5 × 10(5) Pa for isobutane in [(2mC3)C1im][Ntf2] at 343 K. The difference in solubility between the two gases can be explained by a more negative enthalpy of solvation for n-butane. A structural analysis of the pure solvents and of the solutions of the gases, probed by molecular dynamics simulations, could explain the differences found in the systems: (i) the nonpolar domains of the ionic liquids accommodate better the long and more flexible n-butane solute; (ii) the small differences in solubility of each gas in the ionic liquids with the same number of carbon atoms in the alkyl side-chains are explained by the absence of large structural differences in the pure solvents. In all cases, the structural analysis of the four ionic liquids confirms that the studied gases can act as probes of the molecular structure of the ionic liquids, the simulations being always compatible with the experimental solubility data.

An OHD-RIKES and simulation study comparing a benzylmethylimidazolium ionic liquid with an equimolar mixture of dimethylimidazolium and benzene.

The principal difference between 1-benzyl-3-methyl-imidazolium triflimide [BzC1im][NTf2] and an equimolar mixture of benzene and dimethylimidazolium triflimide [C1C1im][NTf2] is that in the former the benzene moieties are tied to the imidazolium ring, while in the latter they move independently. We use femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) and molecular simulations to explore some properties of these two systems. The Kerr spectra show small differences in the spectral densities; the simulations also show very similar environments for both the imidazolium rings and the phenyl or benzene parts of the molecules. The low frequency vibrational densities of states are also similar in the model systems. In order to perform the simulations we developed a model for the [BzC1im](+) cation and found that the barriers to rotation of the two parts of the molecule are low.

Probing the interplay between electrostatic and dispersion interactions in the solvation of nonpolar nonaromatic solute molecules in ionic liquids: an OKE spectroscopic study of CS2/[C(n)C(1)im][NTf(2)] mixtures (n = 1-4).

The intermolecular dynamics of dilute solutions of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 1-4) were studied at 295 K using femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectra of the CS2/ionic liquid (IL) mixtures were analyzed using an additivity model to obtain the CS2 contribution to the OKE spectrum from which information about the intermolecular modes of CS2 in these mixtures was gleaned. The intermolecular spectrum of CS2 in these mixtures is lower in frequency and narrower than that of neat CS2, as found previously for CS2 in [C5C1im][NTf2]. Moreover, a dependence of the spectra on alkyl chain length is observed that is attributed to the interplay between electrostatic and dispersion interactions. The surprising result in this study is the solubility of CS2 in [C1C1im][NTf2], which involves the interaction of a nonpolar nonaromatic molecular solute and only the charged groups of the IL. We propose that the solubility of CS2 in [C1C1im][NTf2] is determined by three favorable factors - (1) large polarizability of the solute molecule; (2) small size of the solute molecule; and (3) low cohesive energy in the high-charge density regions of the IL.

Local structure and intermolecular dynamics of an equimolar benzene and 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide mixture: Molecular dynamics simulations and OKE spectroscopic measurements.

The local structure and intermolecular dynamics of an equimolar mixture of benzene and 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([dmim][NTf2]) were studied using molecular dynamics (MD) simulations and femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectrum of the benzene/[dmim][NTf2] mixture at 295 K was analyzed by comparing it to an ideal mixture spectrum obtained by taking the volume-fraction weighted sum of the OKE spectra of the pure liquids. The experimental mixture spectrum is higher in frequency and broader than that of the ideal mixture spectrum. These spectral differences are rationalized in terms of the local structure around benzene molecules in the mixture and the intermolecular dynamics as reflected in the density of states from the MD simulations. Specifically, we attribute the deviation of the OKE spectrum of the mixture from ideal behavior to benzene molecules seeing a stiffer intermolecular potential due to their being trapped in cages comprised of ions in the first solvation shell.

Nanostructural organization in acetonitrile/ionic liquid mixtures: molecular dynamics simulations and optical Kerr effect spectroscopy.

The nanostructural organization and subpicosecond intermolecular dynamics in mixtures of acetonitrile and the ionic liquid (IL) 1-pentyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(5)mim][NTf(2)]) are studied as a function of concentration using molecular dynamics (MD) simulations and optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The MD simulations show the IL to be nanostructurally organized into an ionic network and nonpolar domains, with CH(3)CN molecules localized in the interfacial region between the ionic network and nonpolar domains, as found previously by other researchers. The MD simulations indicate strong interactions between CH(3)CN and the hydrogen atoms on the imidazolium ring of the cation. The low-frequency (0-200 cm(-1)) intermolecular part of the reduced spectral densities (RSDs) of the mixtures narrows and shifts to lower frequency as the concentration of CH(3)CN increases. These spectral changes can be partly attributed to the increasing contribution of the low-frequency intermolecular modes of CH(3)CN to the RSD. At a given composition, the RSD of a mixture is found to be broader and higher in frequency than the corresponding ideal RSD given by the volume-fraction-weighted sum of the RSDs of the neat liquids. This difference is rationalized in terms of the competition between CH(3)CN-cation interactions and solute-induced disruption of the ionic networks.

Nanostructural organization in carbon disulfide∕ionic liquid mixtures: molecular dynamics simulations and optical Kerr effect spectroscopy.

In this paper, the nanostructural organization and subpicosecond intermolecular dynamics in the mixtures of CS(2) and the room temperature ionic liquid (IL) 1-pentyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(5)mim][NTf(2)]) were studied as a function of concentration using molecular dynamics (MD) simulations and optical heterodyne-detected Raman-induced Kerr effect spectroscopy. At low CS(2) concentrations (<10 mol.% CS(2)/IL), the MD simulations indicate that the CS(2) molecules are localized in the nonpolar domains. In contrast, at higher concentrations (≥10 mol.% CS(2)/IL), the MD simulations show aggregation of the CS(2) molecules. The optical Kerr effect (OKE) spectra of the mixtures are interpreted in terms of an additivity model with the components arising from the subpicosecond dynamics of CS(2) and the IL. Comparison of the CS(2)-component with the OKE spectra of CS(2) in alkane solvents is consistent with CS(2) mainly being localized in the nonpolar domains, even at high CS(2) concentrations, and the local CS(2) concentration being higher than the bulk CS(2) concentration.

Intermolecular vibrational motions of solute molecules confined in nonpolar domains of ionic liquids.

In this study, we address the following question about the dynamics of solute molecules in ionic liquids (ILs). Are the intermolecular vibrational motions of nonpolar molecules confined in the nonpolar domains formed by tail aggregation in ILs the same as those in an alkane solvent? To address this question, the optical Kerr effect (OKE) spectrum of CS(2) in the IL 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C(5)mim][NTf(2)]) was studied as a function of concentration at 295 K by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The OKE spectrum broadens and shifts to higher frequency as the CS(2) concentration is decreased from 20 to 10 mol %; at lower concentrations, no further change in the width of the OKE spectrum is observed. Multicomponent line shape analysis of the OKE spectrum of 5 mol % CS(2) in [C(5)mim][NTf(2)] reveals that the CS(2) and [C(5)mim][NTf(2)] contributions to the spectrum are separable and that the CS(2) contribution is similar to the OKE spectrum of 5 mol % CS(2) in n-pentane with the spectrum being lower in frequency and narrower than that of neat CS(2). These results suggest that, at this concentration, CS(2) molecules are isolated from each other and mainly localized in the nonpolar domains of the IL.

Effect of cation symmetry and alkyl chain length on the structure and intermolecular dynamics of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids.

In this article, the structure and intermolecular dynamics of 1,3-alkylmethylimidazolium bis(trifluoromethanesulfonyl)amides [C(n)mim][NTf(2)] with n = 2-5 are compared to those of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amides [(C(n))(2)im][NTf(2)] with n = 2-5. The structures of these room-temperature ionic liquids (RTILs) were studied by small-wide-angle X-ray scattering (SWAXS), and their intermolecular dynamics were studied by optical Kerr effect (OKE) spectroscopy. The SWAXS measurements indicate that, on a microscopic scale, the liquid structure of RTILs with symmetric cations is similar to that of RTILs with asymmetric cations. The OKE measurements indicate that the intermolecular dynamics of RTILs with symmetric cations are higher in frequency than those of RTILs with asymmetric cations. These results suggest that the local structure of RTILs with symmetric cations is more solid-like than that of RTILs with asymmetric cations. Further evidence for this difference in local structure on a mesoscopic spatial scale is that the width of the low-Q peak in the SWAXS data is narrower for [(C(5))(2)im][NTf(2)] than for [C(5)mim][NTf(2)]. Moreover, the structure and intermolecular dynamics of the RTILs with ethyl-substituted cations appear to be quite different from those of other RTILs within a given series. This difference is evidenced by a clear change in the dependence of the spectral parameters of the intermolecular part of the OKE spectrum on the alkyl chain length in going from n = 2 to n = 3. The dependence of the SWAXS and OKE data on alkyl chain length is discussed within the context of the nanoscale heterogeneities of RTILs.

Dissolution of cotton cellulose in 1:1 mixtures of 1-butyl-3-methylimidazolium methylphosphonate and 1-alkylimidazole co-solvents.

During the past decade, ionic liquids (ILs) have attracted increasing attention as efficient, novel solvents for dissolving cellulose. In this study, 1-butyl-3-methylimdazolium methylphosphonate ([C4C1im][(OMe)(H)PO2]) was used in the dissolution of cotton cellulose and the role of 1-methylimidazole, 1-ethylimidazole, 1-propylimidazole, and 1-butylimidazole as co-solvents was investigated. The progress of the dissolution was monitored using polarized light microscopy (PLM) and the regenerated cellulose was characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The effect of 1-alkylimidazoles as co-solvents in cellulose dissolution was examined in terms of the basicity (hydrogen-bond acceptor capability), conductivity, viscosity, and ionicity of the IL and IL/co-solvent mixtures. These studies showed that the addition of 1-alkylimidazole co-solvents enhances cellulose dissolution by the IL and that the role of these co-solvents is mainly to increase mass transport by reducing the viscosity of the mixtures.

Nanostructural organization and anion effects in the optical Kerr effect spectra of binary ionic liquid mixtures.

This article reports a study of the effect of anions on the optical Kerr effect (OKE) spectra of binary ionic liquid mixtures with one mixture comprising the 3-methyl-1-pentylimidazolium ([C 5mim] (+)) cation and the anions PF 6 (-) and CF 3CO 2 (-) (TFA (-)), and another mixture comprising the [C 5mim] (+) cation and the anions Br (-) and bis(trifluomethanesulfonyl)imide (NTf 2 (-)). The spectra were obtained by the use of optical heterodyne-detected Raman-induced Kerr Effect Spectroscopy at 295 K. The OKE spectra of the mixtures are compared with the calculated mole-fraction weighted sum of the normalized OKE spectra of the neat liquids. The OKE spectra are nearly additive for [C 5mim]Br/[C 5mim][NTf 2] mixtures, but nonadditive for [C 5mim][PF 6]/[C 5mim][TFA] mixtures. In the case of the equimolar [C 5mim][PF 6]/[C 5mim][TFA] mixture, the nonadditivity is such that the experimental OKE spectrum is narrower than the calculated OKE spectrum. The additivity or nonadditivity of OKE spectra for IL mixtures can be explained by assuming ionic liquids are nanostructurally organized into nonpolar regions and ionic networks. The ionic networks in mixtures will be characterized by "random co-networks" for anions that are nearly the same in size (PF 6 (-) and TFA (-)) and by "block co-networks" for anions that differ greatly in size (Br (-) and NTf 2 (-)).

Temperature Dependence of Volumetric and Dynamic Properties of Imidazolium-Based Ionic Liquids.

Atomistically detailed molecular dynamics simulations were used to investigate the temperature dependence of the specific volume, dynamic properties, and viscosity of linear alkyl chain ([CnC1Im][NTf2], n = 3-7) and branched alkyl chain ([(n - 2)mCn-1C1Im][NTf2]) ionic liquids (ILs). The trend of the glass transition temperature (Tg) values obtained in the simulations as a function of the alkyl chain length of cations was similar to the trend seen in experiments. In addition, the system relaxation behavior as determined from the temperature dependence of the diffusion coefficient, rotational relaxation time, and viscosity close to Tg was observed to follow the Vogel-Fulcher-Tammann expression. Furthermore, the reciprocal of the diffusion coefficient of the anion and cation in both linear and branched IL systems showed a linear correlation with viscosity, thus confirming the validity of the Stokes-Einstein relationship for these systems. Similarly, the average rotational relaxation time of the ions was also found to correlate linearly with the viscosity of the ILs over a wide range of temperatures, thereby validating the Debye-Stokes-Einstein relationship for the ILs. These simulation findings suggest that the temperature dependence of the relaxation time of ILs is very similar to that of other glass-forming liquids.

Nanostructural organization and anion effects on the temperature dependence of the optical Kerr effect spectra of ionic liquids.

The intermolecular spectra of three imidazolium ionic liquids were studied as a function of temperature by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The ionic liquids comprise the 1,3-pentylmethylimidazolium cation ([C(5)mim]+), and the anions, bromide (Br-), hexafluorophosphate (PF(6)-), and bis(trifluoromethanesulfonyl)imide (NTf(2)-). Whereas the optical Kerr effect (OKE) spectrum of [C(5)mim][NTf(2)] is temperature-dependent, the OKE spectra of [C(5)mim]Br and [C(5)mim][PF6] are temperature-independent. These results are surprising in light of the fact that the bulk densities of these room temperature ionic liquids (RTILs) are temperature-dependent. The temperature independence of the OKE spectra and the temperature dependence of the bulk density in [C(5)mim]Br and [C(5)mim][PF(6)] suggest that there are inhomogeneities in the densities of these liquids. The existence of density inhomogeneities is consistent with recent molecular dynamics simulations that show RTILs to be nanostructurally organized with nonpolar regions arising from clustering of the alkyl chains and ionic networks arising from charge ordering of the anions and imidazolium rings of the cations. Differences in the temperature dependences of the OKE spectra are rationalized on the basis of the degree of charge ordering in the polar regions of the RTILs.

Additivity in the optical kerr effect spectra of binary ionic liquid mixtures: implications for nanostructural organization.

Low-frequency spectra of binary room-temperature ionic liquid (RTIL) mixtures of 1-pentyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-pentyl-3-methylimidazolium bromide in the 0-250 cm(-1) region were studied as a function of mole fraction at 295 K. The spectra were obtained by use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The spectra of these binary mixtures are well described by the weighted sums of the spectra for the neat RTILs. This surprising result implies that the intermolecular modes giving rise to the spectra of the neat liquids must also produce the spectra of the mixtures. Additivity of the OKE spectra can be explained by a model in which locally ordered domains are assumed to exist in the neat liquid with the structures of these locally ordered domains preserved upon mixing. Recently published molecular dynamics simulations show that RTILs are nanostructurally organized with ionic networks and nonpolar regions. If ionic networks also exist in the mixture, the additivity of the OKE spectra implies that there are "blocks" along the network of the mixture that are ordered in the same way as in the neat liquids. These "block co-networks" would have a nanostructural organization resembling that of a block copolymer.

Molecular Topology and Local Dynamics Govern the Viscosity of Imidazolium-Based Ionic Liquids.

A series of branched ionic liquids (ILs) based on the 1-(iso-alkyl)-3-methylimidazolium cation from 1-(1-methylethyl)-3-methylimidazolium bistriflimide to 1-(5-methylhexyl)-3-methylimidazolium bistriflimide and linear ILs based on the 1-(n-alkyl)-3-methylimidazolium cation from 1-propyl-3-methylimidazolium bistriflimide to 1-heptyl-3-methylimidazolum bistriflimide were recently synthesized and their physicochemical properties characterized. For the ILs with the same number of carbons in the alkyl chain, the branched IL was found to have the same density but higher viscosity than the linear one. In addition, the branched IL 1-(2-methylpropyl)-3-methylimidazolium bistriflimide ([2mC3C1Im][NTf2]) was found to have an abnormally high viscosity. Motivated by these experimental observations, the same ILs were studied using molecular dynamics (MD) simulations in the current work. The viscosities of each IL were calculated using the equilibrium MD method at 400 K and the nonequilibrium MD method at 298 K. The results agree with the experimental trend. The ion pair (IP) lifetime, spatial distribution function, and associated potential of mean force, cation size and shape, and interaction energy components were calculated from MD simulations. A quantitative correlation between the liquid structure and the viscosity was observed. Analysis shows that the higher viscosities in the branched ILs are due to the relatively more stable packing between the cations and anions indicated by the lower minima in the potential of mean force (PMF) surface. The abnormal viscosity of [2mC3C1Im][NTf2] was found to be the result of the specific side chain length and molecular structure.

Effect of cation symmetry on the morphology and physicochemical properties of imidazolium ionic liquids.

In this paper, the morphology and bulk physical properties of 1,3-dialkylimidazolium bis{(trifluoromethane)sulfonyl}amide ([(C(N/2))(2)im][NTf(2)]) are compared to that of 1-alkyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(N-1)C(1)im][NTf(2)]) for N = 4, 6, 8, and 10. For a given pair of ionic liquids (ILs) with the same N, the ILs differ only in the symmetry of the alkyl substitution on the imidazolium ring of the cation. Small-wide-angle X-ray scattering measurements indicate that, for a given symmetric/asymmetric IL pair, the structural heterogeneities are larger in the asymmetric IL than in the symmetric IL. The correlation length of structural heterogeneities for the symmetric and asymmetric salts, however, is described by the same linear equation when plotted versus the single alkyl chain length. Symmetric ILs with N = 4 and 6 easily crystallize, whereas longer alkyl chains and asymmetry hinder crystallization. Interestingly, the glass transition temperature is found to vary inversely with the correlation length of structural heterogeneities and with the length of the longest alkyl chain. Whereas the densities for a symmetric/asymmetric IL pair with a given N are nearly the same, the viscosity of the asymmetric IL is greater than that of the symmetric IL. Also, an even-odd effect previously observed in molecular dynamics simulations is confirmed by viscosity measurements. We discuss in this paper how the structural heterogeneities and physical properties of these ILs are consistent with alkyl tail segregation.

Translational diffusion in sucrose benzoate near the glass transition: probe size dependence in the breakdown of the Stokes-Einstein equation.

The translational diffusion coefficient D(trans) for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (Tg=337 K) was studied as a function of temperature from Tg+3 K to Tg+71 K by use of the holographic fluorescence recovery after photobleaching technique. The values of D(trans) vary by five to six orders of magnitude in this temperature range. Contrary to the predictions of the Stokes-Einstein equation, the temperature dependence of probe diffusion in SB over the temperature range of the measurements is weaker than that of T/eta, where eta is the shear viscosity. In going from the crossover temperature Tx approximately 1.2Tg to Tg, D(trans)eta/T increases by factors of 2.4+/-0.2 decades for rubrene, 3.4+/-0.2 decades for BPEA, and 3.8+/-0.4 decades for tetracene. The decoupling between probe diffusion in SB and viscosity is characterized by the scaling law D(trans) approximately T/eta(xi), with xi=0.621 for tetracene, 0.654 for BPEA, and 0.722 for rubrene. Data for probe diffusion in SB are combined with data from the literature for probe diffusion in ortho-terphenyl and alphaalphabeta-tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter rho(m)=(m(p)m(h))(1/3), where m(p) and m(h) are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at rho(m) approximately 1. By applying temperature shifts, D(trans) for probe diffusion in SB and the dielectric relaxation time tau(D) can be superimposed on a single master curve based on the Williams-Landel-Ferry equation. This suggests that the dynamics of probe diffusion in SB is described by the scaling relationship D(trans) approximately 1/tau(D)(T+DeltaT), where tau(D)(T+DeltaT) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids.