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PubMed2005

Why are viscosities lower for ionic liquids with -CH2Si(CH3)3 vs -CH2C(CH3)3 substitutions on the imidazolium cations?

We have prepared novel room temperature ionic liquids (RTILs) with trimethylsilylmethyl (TMSiM)-substituted imidazolium cations and compared the properties of these liquids with those for which the TMSiM group is replaced by the analogous neopentyl group. The ionic liquids are prepared with both tetrafluoroborate (BF(4)(-)) and bis(trifluoromethylsulfonyl)imide (NTf(2)(-)) anions paired with the imidazolium cations. At 22 degrees C, the TMSiM-substituted imidazolium ILs have shear viscosities that are reduced by a factor of 1.6 and 7.4 relative to the alkylimidazolium ILs for the NTf(2)(-) and BF(4)(-) anions, respectively. To understand the effect of silicon substitution on the viscosity, the charge densities have been calculated by using density functional theory electronic structure calculations. The ultrafast intermolecular, vibrational, and orientational dynamics of these RTILs have been measured by using femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular dynamical spectrum provides an estimate of the strength of interactions between the ions in the RTILs, and provides a qualitative explanation for the observed reduction in viscosity for the silicon-substituted RTILs.

Cations↗
PubMed2006

Solvation of yttrium with ammonia revisited. Di-amide formation in the reaction of yttrium with ammonia.

The reactivity of yttrium atoms toward ammonia is revisited using expanded density functional theory calculations. The new results reveal that absorption of NH3 on YNH is dissociative to form Y(NH2)2. The di-amide species can adsorb further NH3 molecules molecularly to form Y(NH2)2NH3 and Y(NH2)2(NH3)2. The calculations aimed to reveal the detail of the potential energy curves between the imide and the di-amide forms. The Y(NH2)2(NH3)x species are more stable than those of YNH(NH3)x by more than 20 kcal/mol.

Journal Article↗
PubMed2006

Surface characterization of 1-butyl-3-methylimidazolium Br-, I-, PF6-, BF4-, (CF3SO2)2N-, SCN-, CH3SO3-, CH3SO4-, and (CN)2N- ionic liquids by sum frequency generation.

Sum frequency generation spectroscopy, SFG, was used for the surface characterization at the gas-liquid interface of the 1-butyl-3-methylimidazolium cation combined with the following anions: Br-, I-, PF6-, BF4-, (CF3SO2)2N- (imide), SCN-, CH3SO3- (MeSO3), CH3SO4- (MS), and (CN)2N- (DCN). The SFG spectra obtained for the different ionic liquids were similar independent of the anion selected; therefore, a comprehensive analysis for the surface characterization of the ionic liquids' cation was focused only on the PF6- and Br- anion combinations. For an accurate identification of the vibrational modes observed, FT-IR and Raman spectroscopy in combination with isotopic labeling with deuterium and polarized Raman spectroscopy was used. The cation orientation was determined by analysis of polarization-dependent SFG spectra. For a compound dried in a vacuum to < or = 2 x 10(-5) Torr, the cation appears to be oriented with the ring laying flat along the surface plane and the butyl chain projecting into the gas phase independent of the anion identity.

Journal Article↗
PubMed2006

Ultrafast solvation dynamics of coumarin 153 in imidazolium-based ionic liquids.

The dynamic Stokes shift of coumarin 153 has been measured in two room-temperature ionic liquids, 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-3-methylimidazolium tetrafluoroborate, using the fluorescence up-conversion technique with a 230 fs instrumental response function. A component of about 10-15% of the total solvation shift is found to take place on an ultrafast time scale < 10 ps. The amplitude of this component is substantially less than assumed previously by other authors. The origin of the difference in findings could be partly due to chromophore-internal conformational changes on the ultrafast time scale, superimposed to solvation-relaxation, or due to conformational changes of the chromophore ground state in polar and apolar environments. First three-pulse photon-echo peak-shift experiments on indocyanine green in room-temperature ionic liquids and in ethanol indicate a difference in the inertial component of the early solvent relaxation of <100 fs.

Journal Article↗
PubMed2006

Bimolecular networks and supramolecular traps on Au(111).

We demonstrate the formation of intermixed phases and self assembled molecular templates on the Au(111) surface. The templates are stabilized by hydrogen bonding between melamine molecules with trigonal symmetry and linear PTCDI (perylene tetra-carboxylic di-imide) molecules. When annealed, these molecules spontaneously form either a chiral intermixed phase or a honeycomb arrangement in which vertexes and edges correspond respectively to melamine and PTCDI molecules. We also observe minority phases with more complex intermolecular junctions. The use of these networks as templates is demonstrated by the controlled capture of fullerenes within the pores of the network to form dimers, hexamers, and heptamers. Our results confirm that bimolecular templates can be realized on a range of substrates.

Journal Article↗
PubMed2006

Dielectric response of imidazolium-based room-temperature ionic liquids.

We have used microwave dielectric relaxation spectroscopy to study the picosecond dynamics of five low-viscosity, highly conductive room temperature ionic liquids based on 1-alkyl-3-methylimidazolium cations paired with the bis((trifluoromethyl)sulfonyl)imide anion. Up to 20 GHz the dielectric response is bimodal. The longest relaxation component at the time scale of several 100 ps reveals strongly nonexponential dynamics and correlates with the viscosity in a manner consistent with hydrodynamic predictions for the diffusive reorientation of dipolar ions. Methyl substitution at the C2 position destroys this correlation. The time constants of the weak second process at the 20 ps time scale are practically the same for each salt. This intermediate process seems to correlate with similar modes in optical Kerr effect spectra, but its physical origin is unclear. The missing high-frequency portion of the spectra indicates relaxation beyond the upper cutoff frequency of 20 GHz, presumably due to subpicosecond translational and librational displacements of ions in the cage of their counterions. There is no evidence for orientational relaxation of long-lived ion pairs.

Journal Article↗
PubMed2006

Solvent effect on redox properties of hexanethiolate monolayer-protected gold nanoclusters.

The capacitance of monolayer-protected gold nanoclusters (MPCs), C(MPC), in solution has been theoretically reconsidered from an electrostatic viewpoint, in which an MPC is considered as an isolated charged sphere within two dielectric layers, the intrinsic coating monolayer, and the bulk solvent. The model predicts that the bulk solvent provides an important contribution to C(MPC) and influences the redox properties of MPCs. This theoretical prediction is then examined experimentally by comparing the redox properties of MPCs in four organic solvents: 1,2-dichloroethane (DCE), dichloromethane (DCM), chlorobenzene (CB), and toluene (TOL), in all of which MPCs have excellent solubility. Furthermore, this set of organic solvents features a dielectric constant in a range from 10.37 (DCE) to 2.38 (TOL), which is wide enough to probe the solvent effect. In these organic solvents, tetrahexylammonium bis(trifluoromethylsulfonyl)imide (THATf2N) is used as the supporting electrolyte. Cyclic and differential pulse voltammetric results provide concrete evidence that, despite the monolayer protection, the solvent plays a significant effect on the properties of MPCs in solution.

Journal Article↗
PubMed2006

Hydrogen storage properties of Li-Mg-N-H systems with different ratios of LiH/Mg(NH2)2.

In this work, the hydrogen desorption and structural properties of the Li-Mg-N-H systems with different LiH/Mg(NH2)2 ratios are systemically investigated. The results indicate that the system with the LiH/Mg(NH2)2 ratio of 6/3 transforms into Li2NH and MgNH, and then, the mixture forms an unknown phase by a solid-solid reaction, which presumably is the ternary imide Li2Mg(NH)2; the system with the LiH/Mg(NH2)2 ratio of 8/3 transforms into 4Li2NH and Mg3N2 after releasing H2 at T < 400 degrees C; the system with the LiH/Mg(NH2)2 ratio of 12/3 transforms into 4Li3N and Mg3N2 after releasing H2 at T > 400 degrees C, where the LiMgN phase is formed by the reaction between Li3N and Mg3N2. The characteristics of the phase transformations and the thermal gas desorption behaviors in these Li-Mg-N-H systems could be reasonably explained by the ammonia mediated reaction model, irrespective of the difference in the LiH/Mg(NH2)2 ratios.

Journal Article↗
PubMed2006

Mechanistic investigations on the heterogeneous solid-state reaction of magnesium amides and lithium hydrides.

Isothermal and non-isothermal kinetic measurements on the chemical reaction between Mg(NH2)(2) and LiH, as well as the thermal decomposition of Mg(NH(2))(2), give apparent activation energies of 88.1 and 130 kJ/mol, respectively, which reveal that the thermal decomposition of Mg(NH2)(2) is unlikely to be an elementary step in the chemical reaction of Mg(NH2)(2) and 2LiH. The H-D exchange between H(delta+) in Mg(NH2)(2) and D(delta-) in LiD gives evidence for the coordinated interaction between amide and hydride. The observed linear and nonlinear kinetic growth in the reaction of Mg(NH2)(2)-2LiH indicates that the reaction rate is controlled by the interface reaction in the early stage of the reaction and by mass transport through the imide layer in the later stage. Both particle size and degree of mixing of the reacting species affect the overall kinetics of the reactions.

Journal Article↗
PubMed2006

The [BMI][Tf2N] ionic liquid/water binary system: a molecular dynamics study of phase separation and of the liquid-liquid interface.

We report molecular dynamics (MD) simulations of the aqueous interface of the hydrophobic [BMI][Tf2N] ionic liquid (IL), composed of 1-butyl-3-methylimidazolium cations (BMI+) and bis(trifluoromethylsulfonyl)imide anions (Tf2N-). The questions of water/IL phase separation and properties of the neat interface are addressed, comparing different liquid models (TIP3P vs TIP5P water and +1.0/-1.0 vs +0.9/-0.9 charged IL ions), the Ewald vs the reaction field treatments of the long range electrostatics, and different starting conditions. With the different models, the "randomly" mixed liquids separate much more slowly (in 20 to 40 ns) than classical water-oil mixtures do (typically, in less than 1 ns), finally leading to distinct nanoscopic phases separated by an interface, as in simulations which started with a preformed interface, but the IL phase is more humid. The final state of water in the IL thus depends on the protocol and relates to IL heterogeneities and viscosity. Water mainly fluctuates in hydrophilic basins (rich in O(Tf2N) and aromatic CH(BMI) groups), separated by more hydrophobic domains (rich in CF3(Tf2N) and alkyl(BMI) groups), in the form of monomers and dimers in the weakly humid IL phase, and as higher aggregates when the IL phase is more humid. There is more water in the IL than IL in water, to different extents, depending on the model. The interface is sharper and narrower (approximately 10 A) than with the less hydrophobic [BMI][PF6] IL and is overall neutral, with isotropically oriented molecules, as in the bulk phases. The results allow us to better understand the analogies and differences of aqueous interfaces with hydrophobic (but hygroscopic) ILs, compared to classical organic liquids.

Journal Article↗
PubMed2006

Comparing an ionic liquid to a molecular solvent in the cesium cation extraction by a calixarene: a molecular dynamics study of the aqueous interfaces.

We report a molecular dynamics (MD) study of the interfacial behavior of key partners involved in the Cs(+) cation extraction by a calix[4]arene-crown-6 host (L), comparing an ionic liquid (IL) to a classical molecular solvent (chloroform) as receiving "oil" phase. The IL is composed of hydrophobic 1-butyl-3-methylimidazolium cations (BMI(+)) and bis(trifluoromethylsulfonyl)imide anions (Tf(2)N(-)) and forms a biphasic system with water. The simulations reveal similarities but also interesting differences between the two types of interfaces. Much longer times are needed to "equilibrate" IL systems, compared to classical liquid mixtures, and there is more intersolvent mixing with the IL than with chloroform, especially concerning the water-in-oil content. There is also some excess of the BMI(+) cations over the Tf(2)N(-) anions in the aqueous phase. Simulations on the Na(+)NO(3)(-) and Cs(+)NO(3)(-) ions show that they sometimes interact at the interface with the IL ions, forming hydrated intimate ion pairs, whereas they are "repelled" by the classical interface. The LCs(+) complex and L ligand also behave differently, depending on the "oil phase". They are better solvated by the IL than by chloroform and thus poorly attracted at the IL interface, whereas they adsorb at the chloroform interface, adopting well-defined amphiphilic orientations. The results are discussed in the context of assisted ion transfer and provide a number of arguments explaining the specificity and efficiency of IL based, compared to classical extraction systems.

Journal Article↗
PubMed2006

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.

Letter↗
PubMed2006

Dynamics in supercooled ionic organic liquids and mode coupling theory analysis.

Optically heterodyne-detected optical Kerr effect experiments are applied to study the orientational dynamics of the supercooled ionic organic liquids N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS). The orientational dynamics are complex with relaxation involving several power law decays followed by a final exponential decay. A mode coupling theory (MCT) schematic model, the Sjögren model, was able to reproduce the PMPIm data very successfully over a wide range of times from 1 ps to hundreds of ns for all temperatures studied. Over the temperature range from room temperature down to the critical temperature Tc of 231 K, the OHD-OKE signal of PMPIm is characterized by the intermediate power law t(-1.00+/-0.04) at short times, a von Schweidler power law t(-0.51+/-0.03) at intermediate times, and a highly temperature-dependent exponential (alpha relaxation) at long times. This form of the decay is identical to the form observed previously for a large number of organic van der Waals liquids. MCT analysis indicates that the theory can explain the experimental data very well for a range of temperatures above Tc, but as might be expected, there are some deviations from the theoretical modeling at temperatures close to Tc. For EMImTOS, the orientational dynamics were studied on the ps time scale in the deeply supercooled region near its glass transition temperature. The orientational relaxation of EMImTOS clearly displays the feature associated with the boson peak at approximately 2 ps, which is the first time domain evidence of the boson peak in ionic organic liquids. Overall, all the dynamical features observed earlier for organic van der Waals liquids using the same experimental technique are also observed for organic ionic liquids.

Journal Article↗
PubMed2006

An electrochemical study of PCl3 and POCl3 in the room temperature ionic liquid [C4mpyrr][N(Tf)2].

Voltammetric studies of PCl3 and POCl3 have not been reported in the literature to date, probably due to the instability of these molecules in conventional aprotic solvents giving unstable and irreproducible results. From a previous study [Amigues et al. Chem. Commun. 2005, 1-4], it was found that ionic liquids have the ability to offer a uniquely stable solution phase environment for the study of these phosphorus compounds. Consequently, the electrochemistry of PCl3 and POCl3 has been studied by cyclic voltammetry on a gold microelectrode in the ionic liquid [C4mpyrr][N(Tf)2] (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). For both compounds, reduction and oxidation waves were observed and a tentative assignment of the waves is given. For PCl3, the reduction was thought to proceed via the following mechanism: PCl3 + e- <=> PCl3-, PCl3- <=> Cl- + P*Cl2, and Cl- + PCl3 <=> PCl4-. For POCl3, the suggested reduction mechanism was analogous to that of PCl3: POCl3 + e- <=> POCl3-, POCl3- <=> Cl- + P*OCl2, and Cl- + POCl3 <=> POCl4-. In both cases P*Cl2 and P*OCl2 are likely to engage in further reactions. Potential step microdisk chronoamperometry was carried out on the reductive waves of PCl3 and POCl3 to measure diffusion coefficients and number of electrons transferred. It was found that the diffusion of PCl3 was unusually slow (3.1 x 10(-12) m2 s(-1)): approximately 1 order of magnitude less than that for POCl3 (2.2 x 10(-11) m2 s(-1)). For both PCl3 and POCl3, a "split wave" was observed, with an overall electron count of 1. This observation is shown to be consistent with and to "fingerprint" the mechanisms proposed above.

Journal Article↗
PubMed2006

Li+ transport in lithium sulfonylimide-oligo(ethylene oxide) ionic liquids and oligo(ethylene oxide) doped with LiTFSI.

The Li+ environment and transport in an ionic liquid (IL) comprised of Li+ and an anion of bis(trifluoromethanesulfonyl)imide anion (TFSI-) tethered to oligoethylene oxide (EO) (EO(12)TFSI-/Li+) were determined and compared to those in a binary solution of the oligoethylene oxide with LiTFSI salt (EO(12)/LiTFSI) by using molecular dynamics (MD) simulations and AC conductivity measurements. The latter revealed that the AC conductivity is 1 to 2 orders of magnitude less in the IL compared to the oligoether/salt binary electrolyte with greater differences being observed at lower temperatures. The conductivity of these electrolytes was accurately predicted by MD simulations, which were used in conjunction with a microscopic model to determine mechanisms of Li+ transport. It was discerned that structure-diffusion of the Li+ cation in the binary electrolyte (EO(12)/LiTFSI-) was similar to that in EO(12)TFSI-/Li+ IL at high temperature (>363 K), thus, one can estimate conductivity of IL at this temperature range if one knows the structure-diffusion of Li+ in the binary electrolyte. However, the rate of structure-diffusion of Li+ in IL was found to slow more dramatically with decreasing temperature than in the binary electrolyte. Lithium motion together with EO(12) solvent accounted for 90% of Li+ transport in EO(12)/LiTFSI-, while the Li+ motion together with the EO(12)TFSI- anion contributed approximately half to the total Li+ transport but did not contribute to the charge transport in IL.

Journal Article↗
PubMed2006

Self-assembly of heterogeneous supramolecular structures with uniaxial anisotropy.

Uniaxial anisotropy in two-dimensional self-assembled supramolecular structures is achieved by the coadsorption of two different linear molecules with complementary amine and imide functionalization. The two-dimensional monolayer is defined by a one-dimensional stack of binary chains, which can be forced to line up along steps in vicinal surfaces. The competing driving forces in the self-organization process are discussed in light of the structures observed during single molecule adsorption and coadsorption on flat and vicinal surfaces and the corresponding theoretical calculations.

Molecular Structure↗
PubMed2004

Formation of Langmuir layers and surface modification using new upper-rim fully tethered bipyridinyl or bithiazolyl cyclodextrins and their fluorescent metal complexes.

Seven new amphiphilic cyclodextrins bearing bipyridyl or bithiazolyl moieties at the narrow rim and free hydroxyl or methoxyl groups at the wide rim of the cyclooctaamylose crown were synthesized using a one step "phosphine imide" approach. These ligands form metal complexes that have fluorescence properties with potentials for optical applications. Here, the cyclodextrin derivatives were used as probes for evaluating the role of different moieties in the self-assembly process, providing crucial information in creating functional devices. The behavior of these molecules and of complexes with EuIII in some cases was studied in Langmuir films using surface pressure (pi) and surface potential (deltaV) measurements performed as a function of film compression (compression isotherms). For chosen cyclodextrins, Brewster angle microscopy (BAM) in monolayers was performed. Films formed with derivatives 1, 3, 7, and 2compl were transferred on mica using the Langmuir-Blodgett technique. The properties of the films deposited on mica were analyzed with fluorimetry and, in the case of derivative 7, using fringe of equal chromatic order technique (FECO). The monolayer structure and the fluorescence properties of the Langmuir-Blodgett films indicate that the derivatives studied can be used for preparing cyclodextrin-based optical devices.

Cyclodextrins↗
PubMed2005

Nanostructured polyamic acid membranes as novel electrode materials.

This paper describes a new approach for the preparation of polyamic acid (PAA) composites containing Ag and Au nanoparticles. The composite film of PAA and metal particles were obtained upon electrodeposition of a PAA solution containing gold or silver salts with subsequent thermal treatment, while imidization to polyimide is prevented. The structural characterization of the films is provided by 1H NMR and Fourier transform infrared spectroscopy (FTIR), while the presence of metallic nanoparticles within the polymeric matrix was confirmed by scanning electron microscopy (SEM), cyclic voltammetry (CV), energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). This approach utilizes the unique reactivity of PAA by preventing the cyclization of the reactive soluble intermediate into polyimides at low temperature to design polymer-assisted nanostructured materials. The ability to prevent the cyclization process should enable the design of a new class of electrode materials by use of thermal reduction and/or electrodeposition.

Benzene Derivatives↗