Alfonso S. Pensado

Solvation and Stabilization of Metallic Nanoparticles in Ionic Liquids

Some room-temperature ionic liquids can hold stable suspensions of nanoparticles without additional surface-active agents through mechanisms of solvation and stabilization that are not understood at present, particularly for metallic nanoparticles. These systems are relevant for applications in catalysis, lubrication, electrochemical devices, and chemical processes. We address this issue by studying the interactions and ordering of ionic liquids around metallic nanoparticles using molecular dynamics simulations, which is a suitable tool because the arrangement of the ions around a 2 nm particle is difficult to observe experimentally. The fundamental obstacle to modeling resides in the description of the interactions between metals and ionic fluids, a problem not only for nanometer-scale objects but for extended surfaces as well. In this work we devised an original strategy to represent accurately the molecular interactions and gain insight into the solvation and stabilization mechanisms of nanoparticles in ionic liquids. Experimental studies of metallic nanoparticles in ionic liquids provide different clues about the stabilization of the colloid. Some postulate an electric double layer (the Deryagin–Landau–Verwey–Overbeek model) in which a first solvation shell of anions surrounds the metal cluster, followed by a less ordered layer of cations, and so on. Other studies present evidence of close interactions of the nanoparticles with the cations, through deuterium exchange on positively charged imidazolium rings and through surface-enhanced Raman spectroscopy on gold nanoparticles in imidazolium liquids. Correlations have been established between the size of metallic nanoparticles synthesized in situ with the anion volume. Still other studies suggest that nanoparticles are solvated in nonpolar regions formed by aggregation of the hydrophobic alkyl side chains of the ions, as there is a relationship between the length scale of the structural heterogeneities of the ionic liquid and the size of nanoparticles synthesized therein. Measurements of the thickness of the electrostatic double layer of ionic liquids at metal surfaces have been performed by different techniques. Atomic force microscopy of two ionic liquids [C2C1im][Ntf2] and [C4C1pyrr][NTf2] at the Au(111) surface yielded a surface layer with a thickness of 6 . Capacitance and Stark effect measurements on [C4C1im][BF4] at a Pt surface yielded an interfacial layer with one-ion thickness of 3.3 to 5 . This is consistent with the Debye length of the order of 1 estimated for an electrolyte with a concentration around 4 or 5 m, such as a pure ionic liquid, and constitutes an argument against DLVO-type stabilization. However, measurements on macroscopic flat surfaces may not be immediately transposed to nanoparticle suspensions. Suspensions of metallic nanoparticles in an ionic liquid are governed by three kinds of molecular interaction: ion–ion, metal–metal, and metal–ion, which are all nontrivial and each offers its own difficulties to a description. We adopted an atomistic description for both the nanoparticle and for the ionic liquid, providing a high level of detail regarding the interactions and conformations. We considered a ruthenium nanoparticle in [C4C1im][Ntf2], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (Figure 1). This system was chosen in the context of hydrogenation catalysis using metallic nanoparticles.

Interaction between the π-System of Toluene and the Imidazolium Ring of Ionic Liquids: A Combined NMR and Molecular Simulation Study

The solute-solvent interactions and the site-site distances between toluene and ionic liquids (ILs) 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [BMMIm][NTf2] and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIm][NTf2] at various molar ratios were determined by NMR experiments (1D NMR, rotating-frame Overhauser effect spectroscopy (ROESY)) and by molecular simulation using an atomistic force field. The difference in behavior of toluene in these ILs has been related to the presence of H-bonding between the C2-H and the anion in [BMIm][NTf2] generating a stronger association (>20 kJ.mol-1) than in the case of [BMMIm][NTf2]. Consequently, toluene cannot cleave this H-bond in [BMIm][NTf2] which remains in large aggregates of ionic pairs. However, toluene penetrates the less strongly bonded network of [BMMIm][NTf2] and interacts with [BMMIm] cations.

Relationship between viscosity coefficients and volumetric properties using a scaling concept for molecular and ionic liquids.

In this work, a scaling concept based on relaxation theories of the liquid state was combined with a relation previously proposed by the authors to provide a general framework describing the dependency of viscosity on pressure and temperature. Namely, the viscosity-pressure coefficient (partial differentialeta/partial differentialp)T was expressed in terms of a state-independent scaling exponent, gamma. This scaling factor was determined empirically from viscosity versus Tvgamma curves. New equations for the pressure- and temperature-viscosity coefficients were derived, which are of considerable technological interest when searching for appropriate lubricants for elastohydrodynamic lubrication. These relations can be applied over a broad range of thermodynamic conditions. The fluids considered in the present study are linear alkanes, pentaerythritol ester lubricants, polar liquids, associated fluids, and several ionic liquids, compounds selected to represent molecules of different sizes and with diverse intermolecular interactions. The values of the gamma exponent determined for the fluids analyzed in this work range from 1.45 for ethanol to 13 for n-hexane. In general, the pressure-viscosity derivative is well-reproduced with the values obtained for the scaling coefficient. Furthermore, the effects of volume and temperature on viscosity can be quantified from the ratio of the isochoric activation energy to the isobaric activation energy, Ev/Ep. The values of gamma and of the ratio Ev/Ep allow a classification of the compounds according to the effects of density and temperature on the behavior of the viscosity.

Density scaling of the transport properties of molecular and ionic liquids

Casalini and Roland [Phys. Rev. E 69, 062501 (2004); J. Non-Cryst. Solids 353, 3936 (2007)] and other authors have found that both the dielectric relaxation times and the viscosity, η, of liquids can be expressed solely as functions of the group (TV (γ)), where T is the temperature, V is the molar volume, and γ a state-independent scaling exponent. Here we report scaling exponents γ, for the viscosities of 46 compounds, including 11 ionic liquids. A generalization of this thermodynamic scaling to other transport properties, namely, the self-diffusion coefficients for ionic and molecular liquids and the electrical conductivity for ionic liquids is examined. Scaling exponents, γ, for the electrical conductivities of six ionic liquids for which viscosity data are available, are found to be quite close to those obtained from viscosities. Using the scaling exponents obtained from viscosities it was possible to correlate molar conductivity over broad ranges of temperature and pressure. However, application of the same procedures to the self-diffusion coefficients, D, of six ionic and 13 molecular liquids leads to superpositioning of poorer quality, as the scaling yields different exponents from those obtained with viscosities and, in the case of the ionic liquids, slightly different values for the anion and the cation. This situation can be improved by using the ratio (D∕T), consistent with the Stokes-Einstein relation, yielding γ values closer to those of viscosity.

Effect of Dispersion on the Structure and Dynamics of the Ionic Liquid 1‐Ethyl‐3‐methylimidazolium Thiocyanate

We present a comprehensive density functional study, using the Perdew-Burke-Ernzerhof (PBE) functional, to elucidate the effect of including or neglecting the dispersion correction on the structure and dynamics of the ionic liquid 1-ethyl-3-methylimidazolium thiocyanate. We have investigated the structure of the liquid phase and observed that specific interactions between the anions and cations of the ionic liquid were not accurately represented if the dispersion was neglected. The dynamics of the system is more accurately described if the dispersion correction is taken into account and its omission also leads to an incorrect representation of the hydrogen-bonding dynamics. Finally, the power spectrum is predicted and in good agreement with experimental results. Thus, we conclude that it is possible to represent the structure and dynamics of systems containing ionic liquids accurately using ab initio molecular dynamics and a correction for dispersion.

Absorption of Carbon Dioxide, Nitrous Oxide, Ethane and Nitrogen by 1-Alkyl-3-methylimidazolium (Cnmim, n = 2,4,6) Tris(pentafluoroethyl)trifluorophosphate Ionic Liquids (eFAP)

We measured the densities of 1-alkyl-3-methylimidazolium (C(n)mim, n = 2,4,6) tris(pentafluoroethyl)trifluorophosphate ionic liquids (eFAP) as a function of temperature and pressure and their viscosities as a function of temperature. These ionic liquids are less viscous than those based in the same cations but with other anions such as bis(trifluoromethylsulfonyl)imide. The ionic liquids studied are only partially miscible with water, their solubility increasing with the size of the alkyl side-chain of the cation and with temperature (from x(H(2)O) = 0.20 ± 0.03 for [C(4)mim][eFAP] at 303.10 K to x(H(2)O) = 0.49 ± 0.07 for [C(6)mim][eFAP] at 315.10 K). The solubility of carbon dioxide, nitrous oxide, ethane, and nitrogen in the three ionic liquids was measured as a function of temperature and at pressures close to atmospheric. Carbon dioxide and nitrous oxide are the more soluble gases with mole fraction solubilities of the order of 3 × 10(-2) at 303 K. The solubility of these gases does not increase linearly with the size of the alkyl-side chain of the cation. The solubilities of ethane and nitrogen are much lower than those of carbon dioxide and nitrous oxide (mole fractions 60% and 90% lower, respectively). The higher solubility of CO(2) and N(2)O can be explained by more favorable interactions between the solutes and the polar region of the ionic liquids as shown by the enthalpies of solvation determined experimentally and by the calculation of the site-site solute-solvent radial distribution functions using molecular simulation.

Molecular Dynamics Simulations of the Liquid Surface of the Ionic Liquid 1-Hexyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide: Structure and Surface Tension

Molecular dynamics simulations of the liquid-vacuum interface of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide were performed with an all-atom force field. Structural properties of the interface, such as orientational ordering and density profiles, were calculated. The hexyl side chain of the cation is likely to protrude outward from the surface layer. There is a region with enhanced density from that of the bulk where the cation preferably slants with the imidazolium ring tending to be perpendicular to the interface. The surface tensions are calculated using mechanical and thermodynamic definitions via profiles along the direction normal to the interface. We also discuss the different contributions to the surface tension due to the repulsion-dispersion and electrostatic interactions. The use of local pressure profiles provides an explanation to the systematic problems encountered by several researchers to obtain accurate values of the surface tension at low temperature. Even when macroscopically the system looks in equilibrium, locally this is not accomplished.

Side chain fluorination and anion effect on the structure of 1-butyl-3-methylimidazolium ionic liquids

We present a comprehensive molecular dynamics simulation study on 1-butyl-3-methylimidazolium ionic liquids and their fluorinated analogs. The work focused on the effect of fluorination at varying anions. The main findings are that the fluorination of the cations side chain increases overall structuring, especially the aggregation of cation side chain. Furthermore, large and weakly coordinating anions tend to occupy on-top positions of the cation and decrease the aggregation of cation side chains, most likely due to enhanced alkyl-anion interaction.

2D or not 2D: Structural and charge ordering at the solid-liquid interface of the 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ionic liquid

Molecular dynamics simulations of a 5 nm-thick layer of the ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate, [(OH)C2C1im][BF4], over silica, alumina and boro-silicate glass substrates have been performed. The structure of the ionic liquid at the solid-liquid interface has been interpreted taking into account the corresponding normal density profiles, lateral interfacial structure, orientational ordering and planar density contours. Comparisons with experimental data suggest that the adsorption and stratification process of ionic liquids over solid substrates can be correctly modeled using a realistic rendition of a non-uniform amorphous substrate such as a glass material.

Volumetric behaviour of the environmentally compatible lubricants pentaerythritol tetraheptanoate and pentaerythritol tetranonanoate at high pressures

Knowledge of proper lubricant selection and its handling can substantially influence the reliability of a refrigeration system. In this sense the awareness of several thermophysical properties of refrigerants, lubricants, and their mixtures under different conditions of pressure and temperature is highly important for designing refrigeration systems. Polyol ester oils have been proposed as lubricant candidates for refrigeration systems. In this work, we have studied the density of two polyol esters, pentaerythritol tetraheptanoate and pentaerythritol tetranonanoate, in the range 278.15 ≤ T/K ≤ 353.15 and 0.1 ≤ p/MPa ≤ 45. In addition, the behaviour of two other essential volumetric properties, namely the thermal expansion coefficient and the isothermal compressibility coefficient, as well as the internal pressure have been analysed.