High-Pressure Densities and Derived Thermodynamic Properties of Imidazolium-Based Ionic Liquids

This work addresses the experimental measurements of the pressure (0.10 <p/MPa <10.0) and temperature (293.15 <T/K <393.15) dependence of the density and derived thermodynamic properties, such as the isothermal compressibility, the isobaric expansivity, the thermal pressure coefficient, and the pressure dependence of the heat capacity of several imidazolium-based ionic liquids (ILs), namely, 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4]; 3-methyl-1-octylimidazolium tetrafluoroborate, [omim][BF4]; 1-hexyl-3-methylimidazolium hexafluorophosphate, [hmim][PF6]; 3-methyl-1-octylimidazolium hexafluorophosphate, [omim][PF6]; 1-butyl-2,3-dimethylimidazolium hexafluorophosphate, [bmmim][PF6]; and 1-butyl-3-methylimidazolium trifluoromethansulfonate, [bmim][CF3SO3]. These ILs were chosen to provide an understanding of the influence of the cation alkyl chain length, the number of cation substitutions, and the anion influence on the properties under study. The influence of water content in the density was also studied for the most hydrophobic IL used, [omim][PF6]. A simple ideal-volume model was employed for the prediction of the imidazolium molar volumes at ambient conditions, which proved to agree well with the experimental results.

Mutual Solubilities of Water and Hydrophobic Ionic Liquids

The ionic nature of ionic liquids (ILs) results in a unique combination of intrinsic properties that produces increasing interest in the research of these fluids as environmentally friendly "neoteric" solvents. One of the main research fields is their exploitation as solvents for liquid-liquid extractions, but although ILs cannot vaporize leading to air pollution, they present non-negligible miscibility with water that may be the cause of some environmental aquatic risks. It is thus important to know the mutual solubilities between ILs and water before their industrial applications. In this work, the mutual solubilities of hydrophobic yet hygroscopic imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ILs in combination with the anions bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, and tricyanomethane with water were measured between 288.15 and 318.15 K. The effect of the ILs structural combinations, as well as the influence of several factors, namely cation side alkyl chain length, the number of cation substitutions, the cation family, and the anion identity in these mutual solubilities are analyzed and discussed. The hydrophobicity of the anions increases in the order [C(CN)3] <[PF6] <[Tf2N] while the hydrophobicity of the cations increases from [Cnmim] <[Cnmpy] [Cnmpyr] <[Cnmpip] and with the alkyl chain length increase. From experimental measurements of the temperature dependence of ionic liquid solubilities in water, the thermodynamic molar functions of solution, such as Gibbs energy, enthalpy, and entropy at infinite dilution were determined, showing that the solubility of these ILs in water is entropically driven and that the anion solvation at the IL-rich phase controls their solubilities in water. The COSMO-RS, a predictive method based on unimolecular quantum chemistry calculations, was also evaluated for the description of the water-IL binary systems studied, where it showed to be capable of providing an acceptable qualitative agreement with the experimental data.

Mutual Solubilities of Water and the [Cnmim][Tf2N] Hydrophobic Ionic Liquids

Ionic liquids (ILs) have recently garnered increased attention because of their potential environmental benefits as "green" replacements over conventional volatile organic solvents. While ILs cannot significantly volatilize and contribute to air pollution, even the most hydrophobic ones present some miscibility with water posing environmental risks to the aquatic ecosystems. Thus, the knowledge of ILs toxicity and their water solubility must be assessed before an accurate judgment of their environmental benefits and prior to their industrial applications. In this work, the mutual solubilities for [C2-C8mim][Tf2N] (n-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and water between 288.15 and 318.15 K at atmospheric pressure were measured. Although these are among the most hydrophobic ionic liquids known, the solubility of water in these compounds is surprisingly large, ranging from 0.17 to 0.36 in mole fraction, while the solubility of these ILs in water is much lower ranging from 3.2 × 10-5 to 1.1 × 10-3 in mole fraction, in the temperature and pressure conditions studied. From the experimental data, the molar thermodynamic functions of solution and solvation such as Gibbs energy, enthalpy, and entropy at infinite dilution were estimated, showing that the solubility of these ILs in water is entropically driven. The predictive capability of COSMO-RS, a model based on unimolecular quantum chemistry calculations, was evaluated for the description of the binary systems investigated providing an acceptable agreement between the model predictions and the experimental data both with the temperature dependence and with the ILs structural variations.

Densities and Derived Thermodynamic Properties of Imidazolium-, Pyridinium-, Pyrrolidinium-, and Piperidinium-Based Ionic Liquids

In the present work, experimental density measurements are reported along with the derived thermodynamic properties, such as the isothermal compressibility (?T), the isobaric expansivity (ap), and the thermal pressure coefficient (?v) for imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ionic liquids (ILs), namely, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2mim][CF3SO3], 3-methyl-1-propylpyridinium bis(trifluoromethylsulfonyl)imide [C3mpy][NTf2], 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C3mpyr][NTf2], 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C4mpyr][NTf2], and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide [C3mpip][NTf2] in the pressure (0.10 <P/MPa <35.00) and temperature (293.15 <T/K <393.15) domains. These ILs were chosen to provide an understanding of the influence of the cation and anion on the properties under study. Experimental densities are correlated with the Tait equation with an average absolute relative deviation (AARD) better than 0.02 %. It is shown that experimental densities are in good agreement with the densities obtained by the predictive method previously proposed by us.

Thermodynamic Studies of Ionic Interactions in Aqueous Solutions of Imidazolium-Based Ionic Liquids [Emim][Br] and [Bmim][Cl]

Experimental measurements of density at different temperatures ranging from 293.15 to 313.15 K, the speed of sound and osmotic coefficients at 298.15 K for aqueous solution of 1-ethyl-3-methylimidazolium bromide ([Emim][Br]), and osmotic coefficients at 298.15 K for aqueous solutions of 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) in the dilute concentration region are taken. The data are used to obtain compressibilities, expansivity, apparent and limiting molar properties, internal pressure, activity, and activity coefficients for [Emim][Br] in aqueous solutions. Experimental activity coefficient data are compared with that obtained from Debye-Hückel and Pitzer models. The activity data are further used to obtain the hydration number and the osmotic second virial coefficients of ionic liquids. Partial molar entropies of [Bmim][Cl] are also obtained using the free-energy and enthalpy data. The distance of the closest approach of ions is estimated using the activity data for ILs in aqueous solutions and is compared with that of X-ray data analysis in the solid phase. The measured data show that the concentration dependence for aqueous solutions of [Emim][Br] can be accounted for in terms of the hydrophobic hydration of ions and that this IL exhibits Coulombic interactions as well as hydrophobic hydration for both the cations and anions. The small hydration numbers for the studied ILs indicate that the low charge density of cations and their hydrophobic nature is responsible for the formation of the water-structure-enforced ion pairs.

Extension of the Ye and Shreeve group contribution method for density estimation of ionic liquids in a wide range of temperatures and pressures

An extension of the Ye and Shreeve group contribution method [C. Ye, J.M. Shreeve, J. Phys. Chem. A 111 (2007) 1456–1461] for the estimation of densities of ionic liquids (ILs) is here proposed. The new version here presented allows the estimation of densities of ionic liquids in wide ranges of temperature and pressure using the previously proposed parameter table. Coefficients of new density correlation proposed were estimated using experimental densities of nine imidazolium-based ionic liquids. The new density correlation was tested against experimental densities available in literature for ionic liquids based on imidazolium, pyridinium, pyrrolidinium and phosphonium cations. Predicted densities are in good agreement with experimental literature data in a wide range of temperatures (273.15–393.15 K) and pressures (0.10–100 MPa). For imidazolium-based ILs, the mean percent deviation (MPD) is 0.45% and 1.49% for phosphonium-based ILs. A low MPD ranging from 0.41% to 1.57% was also observed for pyridinium and pyrrolidinium-based ILs.

Acoustic and volumetric properties of aqueous solutions of imidazolium based ionic liquids at 298.15 K

The experimental measurements of the speed of sound and density of aqueous solutions of imidazolium based ionic liquids (IL) in the concentration range of 0.05 mol · kg-1 to 0.5 mol · kg-1 at T = 298.15 K are reported. The data are used to obtain the isentropic compressibility (ßS) of solutions. The apparent molar volume (phiV) and compressibility (phiKS) of ILs are evaluated at different concentrations. The data of limiting partial molar volume and compressibility of IL and their concentration variation are examined to evaluate the effect due to IL–water and IL–IL interactions. The results have been discussed in terms of hydrophobic hydration, hydrophobic interactions, and water structural changes in aqueous medium.

Estimation of speed of sound of ionic liquids using surface tensions and densities: A volume based approach

The limited availability of experimental data and their quality have been preventing the development of predictive methods and Computer Aided Molecular Design (CAMD) of ionic liquids (ILs). Based on experimental speed of sound data collected from the literature, the inter-relationship of surface tension (s), density (?), and speed of sound (u) has been examined for imidazolium based ILs containing hexafluorophosphate (PF6), tetrafluoroborate (BF4), bis(trifluoromethanesulphonyl) amide (NTf2), methyl sulphate (MeSO4), ethyl sulphate (EtSO4), and trifluoromethanesulphonate (CF3SO3) anions, covering wide ranges of temperature, 278.15–343.15 K and speed of sound, 1129.0–1851.0 m s-1. The speed of sound was correlated with a modified Auerbach's relation, by using surface tension and density data obtained from volume based predictive methods previously proposed by the authors. It is shown that a good agreement with literature data is obtained. For 133 data points of 14 ILs studied a mean percent deviation (MPD) of 1.96% with a maximum deviation inferior to 5% was observed. The correlations developed here can thus be used to evaluate the speeds of sound of new ionic liquids.

Electrochemical reduction of benzoic acid and substituted benzoic acids in some room temperature ionic liquids

Using cyclic voltammetry, the electrochemical reduction of benzoic acid (BZA) has been studied at Pt and Au microelectrodes (10 and 2 mu m diameter) in six room temperature ionic liquids (RTILs), namely [C(2)mim][NTf2], [C(4)mim][NTf2], [C(4)mpyrr][NTf2], [C(4)mim][BF4], [C(4)mim][NO3], and [C(4)mim][PF6] (where [C(n)mim](+) = 1-alkyl-3-methylimidazolium, [NTf2](-) = bis(trifluoromethylsulfonyl)imide, [C(4)mpyrr](+) = N-butyl-N-methylpyrrolidinium, [BF4](-) = tetrafluoroborate, [NO3](-) = nitrate, and [PF6](-) = hexafluorophosphate). In all cases, a main reduction peak was observed, assigned to the reduction of BZA in a CE mechanism, where dissociation of the acid takes place before electron transfer to the dissociated proton. One anodic peak was observed on the reverse sweep, assigned to the oxidation of adsorbed hydrogen, and a reductive

Extraction of electrode kinetic parameters from microdisc voltammetric data measured under transport conditions intermediate between steady-state convergent and transient linear diffusion as typically applies to room temperature ionic liquids

The extraction of electrode kinetic parameters for electrochemical couples in room-temperature ionic liquids (RTILs) is currently an area of considerable interest. Electrochemists typically measure electrode kinetics in the limits of either transient planar or steady-state convergent diffusion for which the voltammetic response is well understood. In this paper we develop a general method allowing the extraction of this kinetic data in the region where the diffusion is intermediate between the planar and convergent limits, such as is often encountered in RTILs using microelectrode voltammetry. A general working surface is derived, allowing the inference of Butler-Volmer standard electrochemical rate constants for the peak-to-peak potential separation in a cyclic voltammogram as a function of voltage scan rate. The method is applied to the case of the ferrocene/ferrocenium couple in [C(2)mim][N(Tf)(2)] and [C(4)mim][N(Tf)(2)].

Oxidation of several p-phenylenediamines in room temperature ionic liquids: Estimation of transport and electrode kinetic parameters

The electrochemical oxidation of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) has been studied by cyclic voltammetry and potential step chronoamperometry at 303 K in five ionic liquids, namely [C(2)mim] [NTf2], [C(4)mim] [NTf2] [C(4)mpyrr] [NTf2] [C(4)mim] [BF4], and [C(4)mim] [PF6] (where [C(n)mim](+) = 1-alkyl-3-methylimidazolium, [C(4)mpyrr](+) = N-butyl-N-methylpyrrolidinium, [NTf2](-) = bis(trifluoromethylsulfonyl)imide, [BF4](-) = tetrafluoroborate, and [PF6](-) = hexafluorophosphate). Diffusion coefficients, D, of 4.87, 3.32, 2.05, 1.74, and 1.34 x 10(-11) m(2) s(-1) and heterogeneous electron-transfer rate constants, k(0), of 0.0109, 0.0103, 0.0079, 0.0066, and 0.0059 cm s(-1) were calculated for TMPD in [C(2)mim] [NTf2], [C(4)mim] [NTf2], [C(4)mpyrr] [NTf2], [C(4)mim] [BF4], and [C(4)mim] [PF6], respectively, at 303 K. The oxidation of TMPD in [C4mim][PF6] was also carried out at increasing temperatures from 303 to 343 K, with an activation energy for diffusion of 32.3 kJ mol(-1). k(0) was found to increase systematically with increasing temperature, and an activation energy of 31.4 kJ mol(-1) was calculated. The study was extended to six other p-phenylenediamines with alkyl/phenyl group substitutions. D and k(0) values were calculated for these compounds in [C(2)mim] [NTf2], and it was found that k(0) showed no obvious relationship with the hydrodynamic radius, r.