Luís M. N. B. F. Santos

High-Accuracy Vapor Pressure Data of the Extended [CnC1im][Ntf2] Ionic Liquid Series: Trend Changes and Structural Shifts

For the first time, two distinct trends are clearly evidenced for the enthalpies and entropies of vaporization along the [Cnmim][Ntf2] ILs series. The trend shifts observed for Δ(l)(g)H(m)(o) and Δ(l)(g)S(m)(o), which occur at [C6mim][Ntf2], are related to structural modifications. The thermodynamic results reported in the present article constitute the first quantitative experimental evidence of the structural percolation phenomenon and make a significant contribution to better understanding of the relationship among cohesive energies, volatilities, and liquid structures of ionic liquids. A new Knudsen effusion apparatus, combined with a quartz crystal microbalance, was used for the high-accuracy volatility study of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series ([Cnmim][Ntf2], where n = 2, 3, 4, 5, 6, 7, 8, 10, 12). Vapor pressures in the (450–500) K temperature range were measured, and the molar standard enthalpies, entropies, and Gibbs energies of vaporization were derived. The thermodynamic parameters of vaporization were reported, along with molecular dynamic simulations of the liquid phase structure, allowing the establishment of a link between the thermodynamic properties and the percolation phenomenon in ILs.

Specific Solvation Interactions of CO2 on Acetate and Trifluoroacetate Imidazolium Based Ionic Liquids at High Pressures

New classes of acidic or basic ionic liquids (ILs) are gaining special attention, since the efficiency of many processes can be enhanced by the judicious manipulation of these properties. The absorption of sour gases can be enhanced by the basic character of the IL. The fluorination of the cation or the anion can also contribute to enhance the gas solubility. In this work these two characteristics are evaluated through the study of the gas-liquid equilibrium of two ionic liquids based on similar anions, 1-butyl-3-methylimidazolium acetate ([C4mim][Ac]) and 1-butyl-3-methylimidazolium trifluoroacetate ([C4mim][TFA]), with carbon dioxide (CO2) at temperatures up to 363 K and pressures up to 76 MPa. The data reported are shown to be thermodynamically consistent. Henrys constants estimated from the experimental data show the solubility of CO2 on the [C4mim][Ac] to be spontaneous unlike in [C4mim][TFA] due to the differences in solvation enthalpies in these systems. Ab initio calculations were performed on simple intermolecular complexes of CO2 with acetate and trifluoroacetate using MP2/6-31G(d) and the G3 and G3MP2 theoretical procedures to understand the interactions between CO2 and the anions. The theoretical study indicates that although both anions exhibit a simultaneous interaction of the two oxygen of the carboxylate group with the CO2, the acetate acts as a stronger Lewis base than the trifluoroacetate. 13C high-resolution and magic angle spinning (HRMAS) NMR spectra provide further evidence for the acid/base solvation mechanism and the stability of the acetate ion on these systems. Further similarities and differences observed between the two anions in what concerns the solvation of CO2 are discussed.

Evaluation of Cation−Anion Interaction Strength in Ionic Liquids

Electrospray ionization mass spectrometry with variable collision induced dissociation of the isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) ions of imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ionic liquids (ILs) combined with a large set of anions, such as chloride, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, and bis[(trifluoromethyl)sulfonyl]imide, was used to carry out a systematic and comprehensive study on the ionic liquids relative interaction energies. The results are interpreted in terms of main influences derived from the structural characteristics of both anion and cation. On the basis of quantum chemical calculations, the effect of the anion upon the dissociation energies of the ionic liquid pair, and isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) aggregates, were estimated and are in good agreement with the experimental data. Both experimental and computational results indicate an energetic differentiation between the cation and the anion to the ionic pair. Moreover, it was found that the quantum chemical calculations can describe the trend obtained for the electrostatic cation-anion attraction potential. The impact of the cation-anion interaction strengths in the surface tension of ionic liquids is further discussed. The surface tensions dependence on the cation alkyl chain length, and on the anion nature, follows an analogous pattern to that of the relative cation-anion interaction energies determined by mass spectrometry.

Ion Specific Effects on the Mutual Solubilities of Water and Hydrophobic Ionic Liquids

Ion specific effects on the mutual solubilities between hydrophobic ionic liquids (ILs) and water are complex and not fully understood. The aim of this work is to obtain further evidence about the molecular mechanism behind this phenomenon by evaluating the effect of a large series of inorganic and organic salts on the mutual solubilities of water and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(4)mim][Tf(2)N]. The magnitudes of the salting-in and salting-out effects were assessed by changing either the cation or the anion, in a series of salts, as well as the salt concentration. It was observed that the influence of the ions on the solubility followed the Hofmeister series. Both salting-in and salting-out effects were observed and they showed to be dependent on both the nature of the salt and its concentration, while the pH had only a marginal effect on the studied solubilities. On the basis of the solubility changes of the ionic liquid in water in the presence of salts and on NMR spectroscopic data, it will be shown that salting-out inducing ions (high charge density) and salting-in inducing ions (low charge density) act through different mechanisms. While the former act mainly through an entropic effect resulting from the formation of water-ion hydration complexes which cause the dehydration of the solute and the increase of the surface tension of the cavity, the salting-in results from a direct ion binding of the low charge density ions to the hydrophobic moieties of the solute.

Alkylimidazolium Based Ionic Liquids: Impact of Cation Symmetry on Their Nanoscale Structural Organization

Aiming at evaluating the impact of the cation symmetry on the nanostructuration of ionic liquids (ILs), in this work, densities and viscosities as a function of temperature and small-wide angle X-ray scattering (SWAXS) patterns at ambient conditions were determined and analyzed for 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (asymmetric) and 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide (symmetric) series of ionic liquids. The symmetric IL series, [CN/2CN/2im][NTf2], presents lower viscosities than the asymmetric [CN-1C1im][NTf2] counterparts. For ionic liquids from [C1C1im][NTf2] to [C6C6im][NTf2], an odd-even effect in the viscosity along the cation alkyl side chain length was observed, in contrast with a linear increase found for the ones ranging between [C6C6im][NTf2] and [C10C10im][NTf2]. The analysis of the viscosity data along the alkyl side chain length reveals a trend shift that occurs at [C6C1im][NTf2] for the asymmetric series and at [C6C6im][NTf2] for the symmetric series. These results are further supported by SWAXS measurements at ambient conditions. The gathered data indicate that both asymmetric and symmetric members are characterized by the occurrence of a distinct degree of mesoscopic structural organization above a given threshold in the side alkyl chain length, regardless the cation symmetry. The data also highlight a difference in the alkyl chain dependence of the mesoscopic cluster sizes for symmetric and asymmetric cations, reflecting a different degree of interdigitation of the aliphatic tails in the two families. The trend shift found in this work is related to the structural segregation in the liquid after a critical alkyl length size (CALS) is attained and has particular relevance in the cation structural isomerism with higher symmetry.

Energetics of 6-methoxyquinoline and 6-methoxyquinoline N-oxide: the dissociation enthalpy of the (N-O) bond

Abstract The standard (p o =0.1 MPa ) molar enthalpies of formation, at T=298.15 K , were determined using static-bomb calorimetry for crystalline 6-methoxyquinoline N -oxide (6MeOQUINO), as −(85.6±3.7) kJ · mol −1 and liquid 6-methoxyquinoline (6MeOQUI), −(25.8±2.2) kJ · mol −1 . The standard molar enthalpies of sublimation/vaporization, at T=298.15 K , were measured by microcalorimetry and from the enthalpies of formation of the gaseous compounds, the dissociation enthalpy D m 0 of the (N + –O − ) dative covalent bond was derived, as (269.2±5.0) kJ · mol −1 .

Cation Symmetry effect on the Volatility of Ionic Liquids

This work reports the first data for the vapor pressures at several temperatures of the ionic liquids, [C(N/2)C(N/2)im][NTf(2)] (N = 4, 6, 8, 10, 12) measured using a Knudsen effusion apparatus combined with a quartz crystal microbalance. The morphology and the thermodynamic parameters of vaporization derived from the vapor pressures, are compared with those for the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide series, [C(N-1)C(1)im][NTf(2)] (N = 3 - 9, 11, and 13). It was found that the volatility of [C(N/2)C(N/2)im][NTf(2)] series is significantly higher than the asymmetric cation ILs with the same total number of carbons in the alkyl side chains, [C(N-1)C(1)im][NTf(2)]. The observed higher volatility is related with the lower enthalpy of vaporization. The symmetric cation, [C(N/2)C(N/2)im][NTf(2)], presents lower entropies of vaporization compared with the asymmetric [C(N-1)C(1)im][NTf(2)], indicating an increase of the absolute liquid entropy in the symmetric cation ILs, being a reflection of a change of the ion dynamics in the IL liquid phase. Moreover both the enthalpy and entropy of vaporization of the [C(N/2)C(N/2)im][NTf(2)] ILs, present a clear odd-even effect with higher enthalpies/entropies of vaporization for the odd number of carbons in each alkyl chain ([C(3)C(3)im][NTf(2)] and [C(5)C(5)im][NTf(2)]).

Structure and aggregation in the 1,3-dialkyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liquid family: 2. From single to double long alkyl side chains.

A systematic molecular dynamics study using large simulation boxes has been performed in order to extend the analysis of the mesoscopic segregation behavior observed in ionic liquids of the 1,3-dialkyl-imidazolium bis(trifluoromethylsulfonyl)imide homologous series, [C(n)C(mim)][Ntf2] (2 ≤ n ≤ 10, 2 ≤ m ≤ n). The analyses include the discussion of the structure factors, S(q), in the low-q range (1.6 ≤ q/nm(-1) ≤ 20); the confirmation of the periodicity of the polar network of the ionic liquid and its relation to the so-called intermediate peaks; and the characterization of the polar network and the nonpolar regions that are formed along the series using aggregate analyses by means of five different statistical tools. The analyses confirmed that the percolation of the nonpolar regions into a continuous domain occurs when the total number of carbon atoms in the alkyl chains exceeds six but that this is not a sufficient condition for the emergence of a distinct and intense prepeak. The existence of such a peak also requires that the longer alkyl chain contains more than a critical alkyl length (CAL) of five carbon atoms.

Host−Guest Complexation: A Convenient Route for the Electroreduction of Diazonium Salts in Aqueous Media and the Formation of Composite Materials

Electrochemical grafting of a water-insoluble diazonium salt in aqueous media onto an electrode surface was achieved by host-guest complexation. 1-(2-Bisthienyl)-4-aminobenzene (BTAB) was solubilized in a water/beta-cyclodextrin solution (beta-CD). The corresponding diazonium salt was generated in situ then electroreduced. This process leads to the attachment of bithiophene or short oligothiophene groups to the electrode surface. The modified surfaces were analyzed by cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and atomic force microscopy (AFM). The electrochemical investigations show that the water-based modified surface is similar to one generated in acetonitrile without beta-CD. Thus, the attached organic layer behaves like an electrochemical switch (above some threshold potential, a soluble external probe is oxidized, but the oxidized form cannot be reduced). The modified surfaces consist of grafted bisthienylbenzene (BTB) and cyclodextrins that can be removed from the surface. This procedure may be considered as a new means of creating a surface made of submicrometric holes in an organic semiconducting layer.

Vapor pressures of 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids with long alkyl chains

This work presents the vapor pressure at several temperatures for the 1,3-dialkylimidazolium bis(trifluoromethylsulfonyl)imide series, [CN/2CN/2im][NTf2] (N = 14, 16, 18, and 20), measured by a Knudsen effusion method combined with a quartz crystal microbalance. The thermodynamic properties of vaporization of the ionic liquids under study are analysed together with the results obtained previously for the shorter alkyl chain length [CN/2CN/2im][NTf2] (N = 2, 4, 6, 8, 10, and 12), in order to evaluate the effect of the alkyl side chains of the cation and to get additional insights concerning the nanostructuration of ionic liquids. The symmetry effect is explored, based on the comparison with the asymmetric imidazolium based ionic liquids, [CN-1C1im][NTf2]. A trend shift on the thermodynamic properties of vaporization along the alkyl side chains of the extended symmetric ionic liquids, around [C6C6im][NTf2], was detected. An intensification of the odd-even effect was observed starting from [C6C6im][NTf2], with higher enthalpies and entropies of vaporization for the odd numbered ionic liquids, [C7C7im][NTf2] and [C9C9im][NTf2]. Similar, but less pronounced, odd-even effect was found for the symmetric ionic liquids with lower alkyl side chains length, [CN/2CN/2im][NTf2] (with N = 4, 6, 8, 10, and 12). This effect is related with the predominant orientation of the terminal methyl group of the alkyl chain to the imidazolium ring and their influence in the cation-anion interaction. The same Critical Alkyl length at the hexyl, (C6C1and C6C6) was found for both asymmetric and symmetric series indicating that the nanostructuration of the ionic liquids is related with alkyl chain length.