Rodrigo M. A. Silva

Abstract The thermodynamic properties of ionic liquids (ILs) bearing alkylsilane and alkylsiloxane chains, as well as their carbon-based analogs, were investigated. Effects such as the replacement of carbon atoms by silicon atoms, the introduction of a siloxane linkage, and the length of the alkylsilane chain were explored. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the thermal and phase behavior (glass transition temperature, melting point, enthalpy and entropy of fusion, and thermal stability). Heat capacity was obtained by high-precision drop calorimetry and differential scanning microcalorimetry. The volatility and cohesive energy of these ILs were investigated via the Knudsen effusion method coupled with a quartz crystal microbalance (KEQCM). Gas phase energetics and structure were also studied to obtain the gas phase heat capacity as well as the energy profile associated with the rotation of the IL side chain. The computational study suggested the existence of an intramolecular interaction in the alkylsiloxane-based IL. The obtained glass transition temperatures seem to follow the trend of chain flexibility. An increase of the alkylsilane chain leads to a seemingly linear increase in molar heat capacity. A regular increment of 30 JK-1mol(-1) in the molar heat capacity was found for the replacement of carbon by silicon in the IL alkyl chain. The alkylsilane series was revealed to be slightly more volatile than its carbon-based analogs. A further increase in volatility was found for the alkylsiloxane-based IL, which is likely related to the decrease of the cohesive energy due to the existence of an intramolecular interaction between the siloxane linkage and the imidazolium headgroup. The use of Si in the IL structure is a suitable way to significantly reduce the IL's viscosity while preserving its large liquid range (low melting point and high thermal stability) and low volatilities.

Abstract This work reports the thermodynamic and morphological study and characterization of four salts consisting of a divalent/trivalent cobalt complex with pyrazole-pyridine ligands (FK 102 and FK 209 samples) and bis(trifluoromethylsulfonyl)imide (TFSI) moieties as counter anions. The oxidation state of the central metal (Co(II) or Co(III)) and the presence of tert-butyl (t-Bu) groups in the ligand structure were found to have a strong impact on the thermal behavior, phase stability, heat capacities, and thin-film morphology of each salt. The Co(II) complexes exhibited good thermal stability up to 600 K. Lower thermal stability was observed for the Co(III) congeners. The FK 209 Co(III) displayed a higher melting temperature but a partial decomposition during or above melting was detected. The higher melting temperatures observed for the Co(III) complexes were found to be entropically driven. However, the addition of t-Bu in the ligand (FK 209) leads to an increase in the melting temperature, which is driven by the enthalpy of fusion. The four compounds studied evidenced a large glass-forming ability. Moreover, the thermal stability of the glassy state was clearly increased when the ligands comprised t-Bu groups. The contribution of the t-Bu group for the molar heat capacity in the solid phase, at T = 298.15 K, was found to be (110 ± 3) J·K−1·mol−1 and (98 ± 4) J·K−1·mol−1 for the Co(II) and Co(III) complexes, respectively. These results are in good agreement with the contribution of the t-Bu group observed for both solid and liquid phases in other materials, indicating that the t-Bu groups are relatively unhindered in the crystalline phase of the salts. The morphological behavior of the thin films of FK 102 samples was found to be quite similar to the observed for typical ionic liquids, with the formation of micro- and nanodroplets onto different substrates. The introduction of t-Bu substituents in the ligand structure was found to have a strong impact on the formation of homogeneous and compact nanofilms for the FK 209 salts. © 2022 Elsevier Ltd

Abstract The importance of choline chloride (ChCl) is recognized due to its widespread use in the formulation of deep eutectic solvents. The controlled addition of water in deep eutectic solvents has been proposed to overcome some of the major drawbacks of these solvents, namely their high hygroscopicities and viscosities. Recently, aqueous solutions of ChCl at specific mole ratios have been presented as a novel, low viscous deep eutectic solvent. Nevertheless, these proposals are suggested without any information about the solid-liquid phase diagram of this system or the deviations from the thermodynamic ideality of its precursors. This work contributes significantly to this matter as the phase behavior of pure ChCl and (ChCl + H2O) binary mixtures was investigated by calorimetric and analytical techniques. The thermal behavior and stability of ChCl were studied by polarized light optical microscopy and differential scanning calorimetry, confirming the existence of a solid-solid transition at 352.2 +/- 0.6 K. Additionally, heat capacity measurements of pure ChCl (covering both ChCl solid phases) and aqueous solutions of ChCl (x(ChCl) < 0.4) were performed using a heat-flow differential scanning microcalorimeter or a high-precision heat capacity drop calorimeter, allowing the estimation of a heat capacity change of (ChCl) approximate to 39.3 +/- 10 J K-1 mol(-1), between the hypothetical liquid and the observed crystalline phase at 298.15 K. The solid-liquid phase diagram of the ChCl + water mixture was investigated in the whole concentration range by differential scanning calorimetry and the analytical shake-flask method. The phase diagram obtained for the mixture shows an eutectic temperature of 204 K, at a mole fraction of choline chloride close to x(ChCl) = 0.2, and a shift of the solid-solid transition of ChCl-water mixtures of 10 K below the value observed for pure choline chloride, suggesting the appearance of a new crystalline structure of ChCl in the presence of water, as confirmed by X-ray diffraction. The liquid phase presents significant negative deviations to ideality for water while COSMO-RS predicts a near ideal behaviour for ChCl.