Carlos E. S. Bernardes

Structure and Aggregation in the 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series

A new comprehensive Molecular Dynamics study using large simulation boxes has been performed in order to complete and extend the structural analysis on the mesoscopic segregation observed in the ionic liquids of the 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide homologous series, [CnC1im][Ntf2] (2 ≤ n ≤ 10). The analysis includes the discussion along the whole family of the corresponding 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 intermediate low-q peak equivalence; and the introduction of five statistical functions that probe the existence and characterize the polar network and the nonpolar aggregates that are formed along the [CnC1im][Ntf2] series. The later functions comprise aggregate size distributions, average number of contact neighbors within an aggregate, neighbor distributions, distributions of aggregate maximum length, and distributions of aggregate volume.

The Structure of Aqueous Solutions of a Hydrophilic Ionic Liquid: The Full Concentration Range of 1-Ethyl-3-methylimidazolium Ethylsulfate and Water

Several structural features of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate were analyzed in the entire concentration range using molecular dynamics simulation results. Different analysis tools developed in-house were applied to describe the size and connectivity of different water and ion aggregates as a function of the solution concentration. Four concentration ranges-x(H(2)O)<0.5, 0.50.95-with four distinct structural regimes--isolated water molecules, chain-like water aggregates, bicontinuous system, and isolated ions or small ion clusters--were identified and discussed, including two different percolation limits: that of water in the ionic liquid network (around x(H(2)O)=0.8) and that of the ionic liquid in water (around x(H(2)O)=0.95).

Inorganic salts in purely ionic liquid media: the development of High Ionicity Ionic Liquids (HIILs).

This work explores the possibility of increasing the ionicity of ionic liquids via the solubilization of inorganic salts in their midst. The resulting purely ionic media-distinct ionic liquid plus inorganic salt mixtures-are liquid in an extensive concentration range and can be aptly denominated High Ionicity Ionic Liquids (HIILs).

Mutual Solubility of Water and Structural/Positional Isomers of N-Alkylpyridinium-Based Ionic Liquids

Despite many previous important contributions to the characterization of the liquid-liquid phase behavior of ionic liquids (ILs) plus water systems, a gap still exists as far as the effect of isomers (of ILs) is concerned. Therefore, in this work, a comprehensive study of the liquid-liquid equilibria between water and isomeric pyridinium-based ionic liquids has been performed. Atmospheric pressure mutual solubilities between water and pyridinium-based ionic liquids combined with the common anion bis[(trifluoromethyl)sulfonyl]imide were experimentally determined between (288.15 and 318.15) K. The main goal of this work is to study the isomeric effects on the pyridinium-based cation, namely, the structural and positional isomerism, as well as the alkyl side chain length. To the best of our knowledge, the influence of both structural and positional isomerism on the liquid-liquid behavior in ionic-liquid-water-containing systems is an unexplored field and is here assessed for the first time. Moreover, from the experimental solubility data, several infinite dilution molar thermodynamic functions of solution, namely, the Gibbs energy, the enthalpy, and the entropy, were estimated and discussed. In addition, aiming at gathering a broader picture of the underlying thermodynamic solvation phenomenon, molecular dynamics simulations were also carried out for the same experimental systems.

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.

Energetics and structure of nicotinic acid (niacin).

The standard molar enthalpies of formation and sublimation of crystalline (monoclinic, space group P2(1)/c) nicotinic acid (NA), at 298.15 K, were determined as Delta(f)H(m)(o)(NA, cr) = -344.7 +/- 1.2 kJ x mol(-1) and Delta(sub)H(m)(o)(NA) = 112.1 +/- 0.5 kJ x mol(-1) by using combustion calorimetry, drop-sublimation Calvet microcalorimetry, and the Knudsen effusion method. The experimental determinations were all based on a sample of NIST Standard Reference Material 2151, which was characterized in terms of chemical purity, phase purity, and morphology. From the above results, Delta(f)H(m)(o)(NA, g) = -232.6 +/- 1.3 kJ x mol(-1) could be derived. On the basis of this value and on published experimental data, the enthalpy of the isodesmic reaction nicotinic acid(g) + benzene(g) --> benzoic acid(g) + pyridine(g) was calculated as -3.6 +/- 2.7 kJ x mol(-1) and compared with the corresponding predictions by the B3LYP/cc-pVTZ (-3.6 kJ x mol(-1)), B3LYP/aug-cc-pVTZ (-3.7 kJ x mol(-1)), B3LYP/6-311++G(d,p) (-4.2 kJ x mol(-1)), G3MP2 (-4.3 kJ x mol(-1)), and CBS-QB3 (-4.0 kJ x mol(-1)) quantum chemistry models. The excellent agreement between the experimental and theoretical results supports the reliability of the Delta(f)H(m)(o) (NA, cr), Delta(sub)H(m)(o)(NA), and Delta(f)H(m)(o)(NA, g) recommended in this work. These data can therefore be used as benchmarks for discussing the energetics of nicotinic acid in the gaseous and crystalline states and, in particular, to evaluate differences imparted to solid forms by the production and processing methods. Such differences are perhaps at the root of the significant inconsistencies found between the published enthalpies of sublimation of this important active pharmaceutical ingredient and thermochemical standard. The molecular packing in the crystalline phase studied in this work was also discussed and its influence on the molecular structure of nicotinic acid was analyzed by comparing bond distances and angles published for the solid state with those predicted by the B3LYP/cc-pVTZ method. No advantage in terms of accuracy of the structural predictions was found by the use of the larger aug-cc-pVTZ or 6-311++G(d,p) basis sets.

A new calorimetric system to measure heat capacities of solids by the drop method

A new electrically calibrated calorimetric system to measure heat capacities of solids has been designed and tested by determining the heat capacity of sapphire and benzoic acid in the temperature range 298–393 K. Comparison of the obtained results with recommended adiabatic calorimetry data showed that the maximum and average absolute deviations were 3.3% and 1.6%, respectively. The technique provides a convenient alternative to routine heat capacity determinations by DSC, since the experiments are faster and, in general, the results have better accuracy. In addition, the sample cells can be kept under argon or nitrogen atmosphere thus enabling the study of air sensitive compounds.

A general strategy for the experimental study of the thermochemistry of protic ionic liquids: enthalpy of formation and vaporisation of 1-methylimidazolium ethanoate.

A general strategy to determine enthalpies of formation of protic ionic liquids, based solely on enthalpy of solution measurements, was conceived and tested for 1-methylimidazolium ethanoate, leading to Δ(f)H°(m){[Hmim][O(2)CCH(3)], 1} = -(425.7 ± 1.2) kJ mol(-1). This result in conjunction with the enthalpy of formation of gaseous 1-methylimidazole (mim) proposed in this work, Δ(f)H°(m)(mim, g) = 126.5 ± 1.1 kJ mol(-1), and Δ(f)H°(m)(CH(3)COOH, g) taken from the literature, allowed the calculation of the enthalpy of the vaporisation process [Hmim][O(2)CCH(3)](l) → mim(g) + CH(3)COOH(g) as Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 119.4 ± 3.0 kJ mol(-1). The agreement between this value and Δ(vap)H°(m){[Hmim][O(2)CCH(3)]} = 117.3 ± 0.5 kJ mol(-1), obtained for the direct vaporisation of [Hmim][O(2)CCH(3)], by Calvet-drop microcalorimetry, gives a good indication that, as previously suggested by Fourier transform ion cyclotron resonance mass spectrometry, Raman spectroscopy, and GC-MS experiments, the vaporisation of [Hmim][O(2)CCH(3)] essentially involves a proton transfer mechanism with formation of the two volatile neutral precursor molecules (mim and CH(3)COOH). Although being a low ionicity protic ionic liquid, [Hmim][O(2)CCH(3)] was chosen to validate the methodology proposed here, since its vaporisation mechanism has been unequivocally demonstrated by different methods and for different pressure ranges.

Energetics of aqueous solutions of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate.

The energetics of aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate in the concentration range m < 165 mol·kg(-1) was analyzed on the basis of the enthalpies of solution of the IL in water (Δ(sol)H(m)) and the enthalpies of dilution (Δ(dil)H(m)) of solutions with different IL concentrations. The data were both obtained experimentally, by calorimetry, and theoretically, by using Molecular Dynamics (MD) simulations. Particular attention was given to the low-concentration range (m < 5 mol·kg(-1)), which had not been covered in previous experimental studies of this system. The dependence of Δ(sol)H(m) from the molality of the IL observed within this m < 5 mol·kg(-1) range could be fitted to a fourth-order polynomial with an average relative deviation of ∼0.13%. This polynomial function shows a minimum of Δ(sol)H(m) at m ≈ 0.6 mol·kg(-1) (or a molar fraction x(IL) ≈ 0.01) that could be approximately captured by the MD simulations performed in this work but not through extrapolations based on previously reported experimental or simulation data. The decomposition of our MD simulation Δ(sol)H(m) results in contributions from different types of interaction (IL-IL, H(2)O-H(2)O, and IL-H(2)O), indicated that the minimum essentially results from two opposite effects: the differences between the IL-IL and H(2)O-H(2)O interactions in the solution and in the pure liquids are both positive and increase with the dilution of the IL, while the contribution of the IL-H(2)O interactions (that is only present in the solution) is negative and decreases with the IL dilution. It was also found that the observed trends in Δ(sol)H(m) are dominated by electrostatic rather than dispersion interactions.

Evaluation of the OPLS-AA Force Field for the Study of Structural and Energetic Aspects of Molecular Organic Crystals

Motivated by the need for reliable experimental data for the assessment of theoretical predictions, this work proposes a data set of enthalpies of sublimation determined for specific crystalline structures, for the validation of molecular force fields (FF). The selected data were used to explore the ability of the OPLS-AA parametrization to investigate the properties of solid materials in molecular dynamics simulations. Furthermore, several approaches to improve this parametrization were also considered. These modifications consisted in replacing the original FF atomic point charges (APC), by values calculated using quantum chemical methods, and by the implementation of a polarizable FF. The obtained results indicated that, in general, the best agreement between theoretical and experimental data is found when the OPLS-AA force field is used with the original APC or when these are replaced by ChelpG charges, computed at the MP2/aug-cc-pVDZ level of theory, for isolated molecules in the gaseous phase. If a good description of the energetic relations between the polymorphs of a compound is required then either the use of polarizable FF or the use of charges determined taking into account the vicinity of the molecules in the crystal (combining the ChelpG and MP2/cc-pVDZ methods) is recommended. Finally, it was concluded that density functional theory methods, like B3LYP or B3PW91, are not advisable for the evaluation of APC of organic compounds for molecular dynamic simulations. Instead, the MP2 method should be considered.