The presence of local anisotropy in the bulk, isotropic, and ionic liquid phases-leading to local mesoscopic inhomogeneity-with nanoscale segregation and expanding nonpolar domains on increasing the length of the cation alkyl-substituents has been proposed on the basis of molecular dynamics (MD) simulations. However, there has been little conclusive experimental evidence for the existence of intermediate mesoscopic structure between the first/second shell correlations shown by neutron scattering on short chain length based materials and the mesophase structure of the long chain length ionic liquid crystals. Herein, small angle neutron scattering measurements have been performed on selectively H/D-isotopically substituted 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids with butyl, hexyl, and octyl substituents. The data show the unambiguous existence of a diffraction peak in the low-Q region for all three liquids which moves to longer distances (lower Q), sharpens, and increases in intensity with increasing length of the alkyl substituent. It is notable, however, that this peak occurs at lower values of Q (longer length scale) than predicted in any of the previously published MD simulations of ionic liquids, and that the magnitude of the scattering from this peak is comparable with that from the remainder of the amorphous ionic liquid. This strongly suggests that the peak arises from the second coordination shells of the ions along the vector of alkyl-chain substituents as a consequence of increasing the anisotropy of the cation, and that there is little or no long-range correlated nanostructure in these ionic liquids.

The liquid state structure of the ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C(2)mim][OAc]), an excellent nonderivitizing solvent for cellulosic biomass, has been investigated at 323 K by molecular dynamics (MD) simulation and by neutron diffraction using the SANDALS diffractometer at ISIS to provide experimental differential neutron scattering cross sections from H/D isotopically substituted materials. Ion-ion radial distribution functions both calculated from MD and derived from the empirical potential structure refinement (EPSR) model to the experimental data show the alternating shell structure of anions around the cation, as anticipated. Spatial probability distributions reveal the main anion-to-cation features as in-plane interactions of anions with the three imidazolium ring hydrogens and cation-cation planar stacking above/below the imidazolium rings. Interestingly, the presence of the polarized hydrogen-bond acceptor (HBA) anion (acetate) leads to an increase in anion-anion tail-tail structuring within each anion shell, an indicator of the onset of hydrophobic regions within the anion regions of the liquid. MD simulations show the importance of scaling of the effective ionic charges in the basic simulation approach to accurately reproduce both the observed experimental neutron scattering cross sections and ion self-diffusion coefficients.

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We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.

Abstract:Mixtures of room temperature ionic liquids (IL) with neutral organic molecules provide a valuable testing ground to investigate the interplay of the ionic and molecular-dipolar state in dense Coulomb systems at near ambient conditions. In the present study, the viscosity η and the ionic conductivity σ of 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6])/naphthalene mixtures at T ) 80°C have been measured at 10 stoichiometries spanning the composition range from pure naphthalene to pure [bmim] [PF 6]. The viscosity grows nearly monotonically with increasing IL mole fraction (x), whereas the conductivity per ion displays a clear peak at x ≈ 15%. The origin of this maximum has been investigated using molecular dynamics simulations based on a classical force field. Snapshots of the simulated samples show that the conductivity maximum is due to the gradual transition in the IL component from an ionic state at high x to a dipolar fluid made of neutral ion pairs at low x. At concentrations x < 0.20 the ion pairs condense into molecular-thin filaments bound by dipolar forces and extending in between nanometric droplets of IL. These results are confirmed and complemented by the computation of dynamic and transport properties in [bmim][PF 6]/naphthalene mixtures at low IL concentration.

In this work, using H and F PFG NMR, we probe the effect of temperature, ion size/type and glucose dissolution on the rate of transport in 1-ethyl-3-methylimidazolium ([EMIM] )-based ionic liquids by measuring self-diffusion coefficients. Using such data, we are able to establish the degree of ion pairing and quantify the extent of ionic aggregation during diffusion. For the neat 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) a strong degree of ion pairing is observed. The substitution of the [OAc] anion with the bis{(trifluoromethyl)sulfonyl}imide ([TFSI] ) anion reduces the pairing between the ions, which is attributed to a lower electric charge density on the [TFSI] anion, hence a weaker electric interaction with the [EMIM] cation. The effect of glucose, important for applications of ionic liquids as extracting media, on the strongly paired [EMIM][OAc] sample was also investigated and it is observed that the carbohydrate decreases the degree of ion pairing, which is attributed to the ability of glucose to disrupt inter-ionic interactions by forming hydrogen bonding, particularly with the [OAc] anion. Calculations of aggregation number from diffusion data show that the [OAc] anion diffuses as a part of larger aggregates compared to the [EMIM] cation. The results and analysis presented here show the usefulness of PFG NMR in studies of ionic liquids, giving new insights into ion pairing and aggregation and the factors affecting these parameters.

The liquid structure of 1-methyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, a prototypical ionic liquid containing an electron-withdrawing group on the cation, has been investigated at 368 K. Experimental neutron scattering combined with empirical potential structure refinement analysis of the data and classical molecular dynamics simulations have been used to probe the liquid structure in detail. Both techniques generated highly consistent results that provide valuable validation of the force fields and refinement approaches. A significant degree of apparent charge ordering is found in the liquid structure, although the nonspherical shape of the ions results in interpenetration of cations into the first shell of adjacent cations, with much shorter closest contact distances than the averaged center-of-mass cation-cation and cation-anion separations.

Alkyl-n-cyanopyridinium and 1-alkyl-n-(trifluoromethyl)pyridinium salts have been synthesised and characterised in order to compare the effects of different electron-withdrawing functional groups on their ability to form ionic liquids. The presence of the electron-withdrawing nitrile or trifluoromethyl substituent on the pyridinium ring leads to salts with higher melting points than with the corresponding 1-alkylpyridinium or 1-alkylpicolinium cations. Solid-state structures were determined by single crystal X-ray crystallography for seven salts; 1-methyl-4-cyanopyridinium methylsulfate, and 1-methyl-3-cyanopyridinium, 1-methyl-4-cyanopyridinium, 1-ethyl-2-cyanopyridinium, 1-ethyl-3-cyanopyridinium, 1-ethyl-4-cyanopyridinium and 1-ethyl-4-(trifluormethyl)pyridinium bis{(trifluoromethyl)sulfonyl}imide, and show the effects of ring-substitution position on hydrogen-bonding in the solid-state and on melting points.