Agílio A. H. Pádua

Density and viscosity of several pure and water-saturated ionic liquids

Densities and viscosities were measured as a function of temperature for six ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium ethylsulfate and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide . The density and the viscosity were obtained using a vibrating tube densimeter from Anton Paar and a rheometer from Rheometrics Scientific at temperatures up to 393 K and 388 K with an accuracy of 10−3 g cm−3 and 1%, respectively. The effect of the presence of water on the measured values was also examined by studying both dried and water-saturated samples. A qualitative analysis of the evolution of density and viscosity with cation and anion chemical structures was performed.

Three commentaries on the nano-segregated structure of ionic liquids

Abstract The concept that ionic liquids are nano-segregated fluids has allowed the rationalization at a molecular level of many of their complex and unusual properties, either as pure substances or as solvents. In this work we will use molecular dynamics simulation results to discuss in a semi-quantitative manner different aspects of such segregation: how it varies within a homologous ionic liquid family; the influence of the nature of the ions in the morphology of the segregated domains; and the interactions of those domains with molecular solutes or solvents.

CL&P: A generic and systematic force field for ionic liquids modeling

In this account, we review the process that led to the development of one of the most widely used force fields in the area of ionic liquids modeling, analyze its subsequent expansions and alternative models, and consider future routes of improvement to overcome present limitations. This includes the description and discussion of (1) the rationale behind the generic and systematic character of the Canongia Lopes & Padua (CL&P) force field, namely its built-in specifications of internal consistency, transferability, and compatibility; (2) the families of ionic liquids that have been (and continue to be) parameterized over the years and those that are the most challenging both in theoretical and applied terms; (3) the steps that lead to a correct parameterization of each type of ion and its homologous family, with special emphasis on the correct modeling of their flexibility and charge distribution; (4) the validation processes of the CL&P and other force fields; and finally (5) the compromises that have to be attained when choosing between generic or specific force fields, coarse-grain or atomistic models, and polarizable or non-polarizable methods. The application of the CL&P and other force fields to the study of ionic liquids using quantum- and statistical-mechanics methods has led to the discovery and analysis of the unique nature of their liquid phases, that is, the notion that ionic liquids are nano-segregated fluids with structural and dynamic heterogeneities at the nanoscopic scale. This successful contribution of theoretical chemistry to the field of ionic liquids will also serve as a guide throughout the ensuing discussion.

Polarity, Viscosity, and Ionic Conductivity of Liquid Mixtures Containing [C4C1im][Ntf2] and a Molecular Component

In this study, we have focused on binary mixtures composed of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide, [C(4)C(1)im][Ntf(2)], and a selection of six molecular components (acetonitrile, dichloromethane, methanol, 1-butanol, t-butanol, and water) varying in polarity, size, and isomerism. Two Kamlet-Taft parameters, the polarizability π* and the hydrogen bond acceptor β coefficient were determined by spectroscopic measurements. In most cases, the solvent power (dipolarity/polarizability) of the ionic liquid is only slightly modified by the presence of the molecular component unless large quantities of this component are present. The viscosity and electrical conductivity of these mixtures were measured as a function of composition and the relationship between these two properties were studied through Walden plot curves. The viscosity of the ionic liquid dramatically decreases with the addition of the molecular component. This decrease is not directly related to the volumetric properties of each mixture or its interactions. The conductivity presents a maximum as a function of the composition and, except for the case of water, the conductivity maxima decrease for more viscous systems. The Walden plots indicate enhanced ionic association as the ionic liquid gets more diluted, a situation that is the inverse of that usually found for conventional electrolyte solutions.

Effect of fluorination and size of the alkyl side-chain on the solubility of carbon dioxide in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids.

It is proven in this work that it is possible to significantly increase the carbon dioxide uptake by an ionic liquid relying on physical interactions only. The solubility and thermodynamics of solvation of carbon dioxide in the ionic liquids 1-octyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide [C(8)mim][Ntf(2)], 1-decyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide [C(10)mim][Ntf(2)], and 1-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium bis[trifluoromethylsulfonyl]amide [C(8)H(4)F(13)mim][Ntf(2)] were determined experimentally between 298 and 343 K at pressures close to atmospheric. The solubility of carbon dioxide is significantly higher in the fluorine-substituted ionic liquid with Henrys law constants at 303 K of 33.3 and 30.7 bar for [C(8)mim][Ntf(2)] and [C(10)mim][Ntf(2)], respectively, and of 28.0 bar for [C(8)H(4)F(13)mim][Ntf(2)]. Molecular simulation was used for interpreting the molecular mechanisms of solvation of carbon dioxide in the studied ionic liquids and coherent molecular mechanisms of solvation are proposed in light of the solute-solvent radial distribution functions. It is shown that the increase of the size of the hydrogenated or fluorinated alkyl chain in the imidazolium cation does not lead to a steady augmentation of the gaseous uptake by the liquid probably due to an increase of the nonpolar domains of the ionic liquid, carbon dioxide being solvated preferentially in the charged regions of the solvent.

Organized 3D-alkyl imidazolium ionic liquids could be used to control the size of in situ generated ruthenium nanoparticles?

The synthesis of ruthenium nanoparticles, RuNPs from the organometallic complex (η4-1,5-cyclooctadiene)(η6-1,3,5-cyclooctatriene)ruthenium(0), Ru(COD)(COT) in various imidazolium derived ionic liquids, ILs: [RMIm][NTf2] (R = CnH2n + 1 with n = 2; 4; 6; 8; 10), and [R2Im][NTf2] (RBu) and [BMMIm][NTf2] has been performed, under 0.4 MPa of H2, at 25 °C or at 0 °C with or without stirring. A relationship between the size of IL non-polar domains calculated by molecular dynamics simulation and the RuNP size measured by TEM has been found, suggesting that the phenomenon of crystal growth is probably controlled by the local concentration of Ru(COD)(COT) and consequently is limited to the size of the non-polar domains. Moreover, the rigid 3D organization based on C2–H⋯anion bonding and chosen experimental conditions, could explain the non-aggregation of RuNPs.

Understanding the role of co-solvents in the dissolution of cellulose in ionic liquids

The dissolution of microcrystalline cellulose in 1-butyl-3-methylimidazolium acetate [C4C1Im][OAc] was studied using a solid–liquid equilibrium method based on polarized-light optical microscopy from 30 to 100 °C. We found that [C4C1Im][OAc] could dissolve as much as 25 wt% of cellulose at temperatures below 100 °C. The structure of the composite phase obtained after cooling a solution of 16 wt% of cellulose in [C4C1Im][OAc] was analyzed by low angle X-ray diffraction showing the absence of microcrystalline cellulose, but depicting an extensive long range isotropic ordering. With the aim of improving the dissolution of cellulose in the ionic liquid, dimethyl sulfoxide, DMSO, was added as a co-solvent. It was observed that it enhances the solvent power of the ionic liquid by decreasing the time needed for dissolution, even at low temperatures. In order to understand what makes DMSO a good co-solvent, two approaches were followed. Firstly, we studied experimentally the mass transport properties (viscosity and ionic conductivity) of [C4C1Im][OAc] + DMSO mixtures at different compositions and, secondly, we assessed the molecular structure and interactions around glucose, the structural unit of cellulose, by means of molecular dynamics simulations. As expected, DMSO dramatically decreases the viscosity and increases the conductivity of the mixtures, but without inducing cation–anion dissociation in the ionic liquid. These results were confirmed by molecular simulation as it was found that the presence of a 0.5 mole fraction concentration of DMSO does not significantly affect the hydrogen-bond network in the ionic liquid. Furthermore, molecular dynamics shows that in the [C4C1Im][OAc] + DMSO equimolar mixture, DMSO does not interact specifically with glucose. We conclude that DMSO improves the solvation capabilities of the ionic liquid because it facilitates mass transport by decreasing the solvent viscosity without significantly affecting the specific interactions between cations and anions or between the ionic liquid and the polymer. The behavior of DMSO as a co-solvent was compared with that of water and it was found that water molecules are more probably found near glucose than those of DMSO, thus interfering with ionic liquid–glucose interactions, which might explain the unsuitability of water as a co-solvent for cellulose in ionic liquids.

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.

Molecular Force Field for Ionic Liquids V: Hydroxyethylimidazolium, Dimethoxy-2- Methylimidazolium, and Fluoroalkylimidazolium Cations and Bis(Fluorosulfonyl)Amide, Perfluoroalkanesulfonylamide, and Fluoroalkylfluorophosphate Anions

In this article, the fifth of a series that describes the parametrization of a force field for the molecular simulation of ionic liquids within the framework of statistical mechanics, we have modeled cations belonging to the hydroxyethylimidazolium, dimethoxy-2-methylimidazolium, and fluoroalkylimidazolium families and anions of the bis(fluorosulfonyl)amide, perfluoroalkanesulfonylamide, and fluoroalkylfluorophosphate families. The development of the force field, created in the spirit of the OPLS-AA model in a stepwise manner and oriented toward the calculation of equilibrium thermodynamic and structural properties in the liquid and crystalline phases, is discussed in detail. Because of the transferability of the present force field, the ions studied here can be combined with those reported in our four previous publications to create a large variety of ionic liquids that can be studied by molecular simulation. The present extension of the force field was validated by comparison of simulation results with experimental crystal structure and liquid density data.

Validation of an accurate vibrating-wire densimeter: Density and viscosity of liquids over wide ranges of temperature and pressure

A new vibrating-wire instrument for the meaasurement of the density of fluids at high pressures was described in a previous paper. The technique makes use of the buoyancy force on a solid sinker and detect, this force with a vibrating wire placed inside the measuring cell. Owing to the simple geometry of the oscillating element there exists a complete theoretical description of its resonance characteristics. enabling the calculation of the density of the fluid from their measurement. In the present paper a new method for the determination of the cell constants is outlined which permits the operation of the densimeter essentially as an absolute instrument. Furthermore. it is shown that the viscosity ol the fluid can be measured Simultaneously with the density. New results for three fluids are presented: for cyclohexane at temperatures from 298 to 348 K and pressures up to 40 MPa. for 2,2,4-trimethylpentane between 197 and 348 K at 0.1 MPa, and for 1,1,1,2-tetrafluoroethane from 197 to 298 K close to saturation. The sets of measurements where chosen with the intention of testing the performance of the apparatus. complementing previous work at higher pressures. The densities and viscosities measured exhibit the same accuracy for all of the three fluids over the entire temperature and pressure ranges and were obtained using the same set of cell parameters The precision of the densities is ±0.03% and their estimated accuracy is ±0.05%. File viscosities have a precision of ±0.6%, and an estimated accuracy of ±2%.