Structure Of Room Temperature Ionic Liquid Research Articles

Effect of room temperature ionic liquid structure on the enzymatic acylation of flavonoids

Enzymatic acylation reactions of flavonoids (rutin, esculin) with long chain fatty acids (palmitic, oleic acids) were carried out in 14 different ionic liquid media containing a range of cation and anion structures. Classification of RTILs according to flavonoid solubility (using COSMO-RS) was the basis for structural selection. Overall, anion selection had a far greater influence on lipase activity than choice of cationic moiety. RTILs containing TF 2N −, PF 6 − and BF 4 − anions were most successful as reaction media while RTILs containing anions with stronger solvating properties (i.e. H-bonding ability) resulted in decreased yields, likely due to increased interactions with the protein structure of the lipase. Biosynthesis of rutin proceeded much slower than of esculin. All-in-all, judicious selection of RTILs was central to achieving high yields (>98% after 6 days for TOMA·TF 2N) since a balance must be struck that maximized flavonoid solubility with minimum negative impact on lipase activity. The process also benefitted from an increased reaction temperature which may have helped to reduced mass transfer limitations.

In situ Raman study of dissolved carbon-dioxide induced changes of imidazolium-based ionic liquids

In situ Raman spectroscopic analysis has been performed for room temperature ionic liquid (RTIL) + CO2 systems under two-phase equilibrium conditions at room temperature. The Raman spectra indicate the followings: the vibration state and configuration of 1-ethyl-3-methylimidazolium tetrafluoroborate change by CO2 dissolution; the liquid structure of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide barely changes by addition of CO2 pressure up to 30 MPa. In addition, the information of the RTIL structure suggests that the large change of the vibration state and configuration of the RTIL does not necessarily relate to the high CO2 solubility in the RTIL.

Influence of the aliphatic chain length of imidazolium based ionic liquids on the surface structure

The molecular surface structure of four imidazolium based ionic liquids was studied with two surface sensitive techniques. Angle resolved neutral impact collision ion scattering spectroscopy (ARNICISS) allows us to determine elemental concentration depth profiles and to obtain information about the topography of the surface. Angle resolved X-ray photoelectron spectroscopy (ARXPS) can be used to study the chemical composition of the surface. The room temperature ionic liquids (RTILs) 1-ethyl-3-methylimidazolium [EMIM], 1-butyl-3-methylimidazolium [BMIM], 1-hexyl-3-methylimidazolium [HMIM], and 3-methyl-1-octylimidazolium [OMIM] bis(trifluoromethylsulfonyl)imide [Tf(2)N] were investigated at 293 K. No evidence of surface active impurities was observed. The majority of previous studies about these RTILs with ARXPS or other surface sensitive techniques dealt only with single examples of these substances or with different combination of the anion and cation. In this present study a homologous series of the four RTILs mentioned above was investigated. This means that only the number of carbon atoms in the aliphatic chain of the cation is varied. Due to this procedure it is possible to study the influence of the chain length, which is a part of the imidazolic ring, on the composition of the surface and the surface near region. In this paper we demonstrate the potential of ARNICISS as a surface sensitive technique to study the surface structure of the RTILs. Furthermore, we combine our NICISS data with ARXPS data, to get a better comprehension of the influence of the aliphatic chain length. After the presentation of the results we develop a model of the surface structure of different RTILs. We have discovered two different surface structures that depend on the number of carbon atoms inside the aliphatic chain.

Room-Temperature Ionic Liquids: Excluded Volume and Ion Polarizability Effects in the Electrical Double-Layer Structure and Capacitance

We study structures of room-temperature ionic liquids at electrified interfaces and the corresponding electrical double-layer capacities using a self-consistent mean-field theory. Ionic liquids are modeled as segmented dendrimers and the effective dielectric constant is calculated from the local distribution of ions to accommodate the excluded volume and the local dielectric screening effects. The resulting camel-shaped capacitance curve is further analyzed in terms of the thickness of alternating layers and the polarization of ions at electrified interfaces.

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.

Studies on Thermal Properties of Selected Aprotic and Protic Ionic Liquids

We describe herein the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) investigations of the thermal properties of selected room-temperature ionic liquids (RTILs). The dependence of the thermal properties on both cation and anion structures of RTILs was systematically studied. The ionic liquids (ILs) investigated here include 28 different imidazolium-based ILs, 22 ammonium-based ILs, and 16 amide-based ILs. In general, these three cation classes exhibit different thermal behaviors but follow a quite systematic trend as expected from the corresponding structural variation. The ILs with bromide as the conjugate anion have lower thermal stabilities than those with bis(trifluoromethane sulfonyl) imide or bis(perfluoroethyl sulfonyl) imide as the conjugate anion. The mass of TGA samples and scan rate were found to have a systematic effect on the decomposition temperature of ILs, highlighting the caution needed in reporting TGA results.

Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid

Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.

Hydrogen Bond Stabilization in 1,3-Dimethylimidazolium Methyl Sulfate and 1-Butyl-3-Methylimidazolium Hexafluorophosphate Probed by High Pressure: The Role of Charge-Enhanced C−H···O Interactions in the Room-Temperature Ionic Liquid

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C-H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C-H...O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C-H stretching mode at ca. 2960 cm(-1). Our results indicated that the imidazolium C-H groups are more favorable sites for hydrogen bonding than the methyl C-H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C-H and imidazolium C-H groups are favorable sites for C-H...O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/D(2)O mixture. Hydrogen bond-like C-H...F interactions were observed between PF(6)(-) and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.

Electronic structure of ionic liquids at the surface studied by UV photoemission

Electronic structures of room-temperature ionic liquids (RILs) were studied by ultraviolet photoemission spectroscopy (UPS) with synchrotron radiation. The samples contain 1-buthyl-3-methylimidazolium ion [bmim] + as the common organic cation, combined with fluorine-containing inorganic anions. We also performed molecular orbital (MO) calculations for the isolated ions. Comparing the calculated density of the states with the observed spectra, we found that the top of the valence states in the liquid salts is derived from the organic cation, although the highest occupied molecular orbitals (HOMOs) of the isolated anions are higher than that of the isolated cation. The Madelung energies at the surface region, which are probably responsible for this reversal, are estimated by comparison between UPS and MO calculation. We also simulated the observed spectra based on the independent-atomic-center (IAC) approximation.

Comparison of Polarities of Room-Temperature Ionic Liquids Using FT-IR Spectroscopic Probes

The solvent properties, especially the polarity, of room-temperature ionic liquids (RTILs) of the general type [Cnmim]X, where [Cnmim] is the 1-CnH2n+1-3-methylimidazolium cation and X is [BF4]−, [PF6]−, [NO3]−, [SCN]−, or [NTf2]− (Tf = CF3SO2), have been investigated using Fourier-transform infrared (FTIR) spectroscopy. Comparing the experimental shifts in the νC=O stretching frequencies of acetone and N,N-dimethylformamide (DMF) as probe molecules suggests that the polarities of most common RTILs are between that of DMF and methanol. A more sensitive probe, Fe(CO)5, can distinguish small polarity differences between RTILs with similar structures. The effects of varying the structure of RTILs by introducing functional groups and different anions on their polarity are also compared.