Publications

Abstract Thermotropic liquid crystal formation by salt-free catanionic surfactants (alkyltrimethylammonium alkylsulfonates, herein designated as TA(m)So(n)) has been investigated as a function of chain length mismatch (asymmetry). Previous studies on these compounds have revealed an unusual and rich asymmetry-dependent lyotropic phase behavior. Herein, phase transition temperatures and transition enthalpies/entropies were determined by differential scanning calorimetry, while mesophases were assigned by polarized light microscopy. Three series of compounds were investigated, namely: the TA(16)So(n) series, where n = 6-10; the TA(m)So(8) series, where m = 12-16; and a constant m + n series, TA(m)So(n) where m + n = 22. Typically, several solid phases and two smectic mesophases are found prior to isotropization to the liquid phase. As asymmetry decreases, two somewhat counterintuitive tendencies emerge: a general decrease in enthalpy/entropy for solid-solid and solid-first mesophase transitions, and an increase in solid-first mesophase transition temperatures. Yet, solid phases are seen to be more stable for the most asymmetric compounds, while the second mesophase is more stable for the least asymmetric ones, in what appears to be a more complex behavior than expected. The results are globally interpreted in terms of subtle differences in chain interdigitation and packing, and odd-even chain effects.

Abstract Herbicides are important for the control of weed growth but increasing use causes environmental problems and undesirable side effects in crops. Phenoxy acid herbicides with auxin-like activity have been used against grass and broad leaf weeds in many crops, such as rice, winter wheat and soybean. Carcinogenicity in humans and embryotoxicity in animals have been described as the main hazards of phenoxy acid herbicides. Thermal denaturation of double stranded DNA has been used as a measure of the effect of harmful chemical substances on the structure and stability of the DNA molecule. Hence, the interaction between several phenoxy acid herbicides (phenoxyacetic acid, 4- chlorophenoxyacetic acid, MCPA, mecoprop, 2,4-D and dichlorprop) and Calf thymus DNA was assessed by UV-Vis absorption spectroscopy. UV spectra and melting curves have been recorded for solutions at constant DNA concentration and using different concentrations of each of the herbicides under study. The transition temperature values and the thermodynamic parameters of DNA thermal denaturation have been determined. The studies performed so far help to understand herbicide-DNA interactions gathering suitable information for the (re)design of new molecules with lower potential risk to humans.

Abstract Thermodynamic properties of ortho, meta and para methoxybenzamides were determined using the Knudsen effusion method and calorimetric experiments as well as computational approaches. The vapour pressure of the crystalline phase of the three isomers was measured and values of the standard (po = 0.1 MPa) molar enthalpy, Gibbs energy and entropy of sublimation, at T = 298.15 K, were derived. Static bomb combustion calorimetry was used to measure the standard molar enthalpies of combustion from which the standard molar enthalpies of formation in the crystalline state, at T = 298.15 K, were derived. Together with the standard molar enthalpies of sublimation, these results yielded the standard molar enthalpies of formation in gaseous phase of the three isomers. The standard Gibbs energies of formation in crystalline and gaseous phases were also derived and used to differentiate the thermodynamic stability of the three isomers. Moreover, differential scanning calorimetry analysis enabled determination of the temperature and molar enthalpies of fusion of the studied compounds. Gas-phase enthalpies of formation of the three compounds were estimated computationally at the G3 and G4 levels of theory and compared with the experimental results.

Abstract The topographic structural analysis of thin films of triphenylamine (TPA) derivatives, obtained by vacuum deposition, is presented. The topology of the films is highly dependent on the number of phenyl rings as well on the presence of methyl groups. The glass transition temperatures can be predicted from the melting temperatures, according to the molecular symmetry, flexibility, and molecular size of the amorphous nonplanar materials suitable for long-time use in OLED based devices. A phase transition thermodynamics study, including vapor pressure and heat capacity determinations of some hole transport materials based triphenylamine derivatives (DDP, p-TTP, TPB, TPD, TDAB, and m-MTDAB) is presented. Analysis of the enthalpic and entropic contributions provides an understanding of the fusion temperature differentiation, which results in higher melting points for TDB and TDAB and lower values for TPD and m-MTDAB, due to the presence of meta-CH3 groups. The thermodynamic parameters of sublimation were used to explain the slightly higher volatility of solid DDP relative to p-TTP and the similarity between the volatilities of solids TPB and TDAB.

Abstract This work reports an experimental and computational thermodynamic study performed on two 9-fluorene derivatives: fluorene-9-methanol and fluorene-9-carboxylic acid. The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the crystalline phase of these compounds were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. Astatic method, based on a capacitance diaphragm gauge, and a Knudsen effusion method were used to perform the vapour pressure study of the referred compounds, yielding accurate determination of the standard molar enthalpies and entropies of sublimation and vaporisation. For fluorene-9-carboxylic acid, the enthalpy of sublimation was also determined using Calvet microcalorimetry. The enthalpy of fusion of both compounds was derived indirectly from vapour pressure results and directly from DSC experiments. Combining the thermodynamic parameters of the compounds studied, the standard Gibbs energy of formation in crystalline and gaseous phases were derived as well as the standard molar enthalpy of formation in the gaseous phase. A theoretical study at the G3 and G4 levels has been carried out, and the calculated enthalpies of formation have been compared with the experimental values.

Abstract 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, [ChinCNnim][NTf2], presents lower viscosities than the asymmetric [C-N/2 C-N/2 im][NTf2] counterparts. For ionic liquids from [C(1)C(1)im][NTf2] to [C(6)C(6)im][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 [C(6)C(6)im][NTf2] and [Ci(10)C(10)im][NTf2]. The analysis of the viscosity data along the alkyl side chain length reveals a trend shift that occurs at [C(6)Cim][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.

Abstract A computational study has been carried out for xanthene and thioxanthene homologous derivatives with keto, hydroxyl, carboxyl, and carboxamide functional groups on position 9, contributing to the understanding of their energetics and reactivity. For that it is presented and compared with the molecular structures, the electrostatic potential energy maps, and the electronic properties of all these heteropolycyclic compounds. The estimation of the standard molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, was made only for the thioxanthydrol, thioxanthene-9-carboxylic acid, and thioxanthene-9-carboxamide using the experimental values available in the literature for the homologous compounds containing oxygen.

Abstract Thermochemical data of dibenzofuran, a compound of considerable industrial and environmental significance, obtained from experimental calorimetric and computational techniques are reported in this work. The enthalpy of fusion, (19.4 +/- 1.0) kJ mol(-1), at the temperature of fusion, (355.52 +/- 0.02) K, was determined by differential scanning calorimetry measurements of dibenzofuran. From the standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline dibenzofuran, (-29.2 +/- 3.8) kJ mol(-1), and of sublimation, (84.5 +/- 1.0) kJ mol(-1), determined at T = 298.15 K by static bomb combustion calorimetry and by vacuum drop microcalorimetry, respectively, it was possible to calculate the enthalpy of formation of the gaseous compound, (55.0 +/- 3.9) kJ mol(-1), at the same temperature. The enthalpy of formation in the gaseous phase was also determined from G3(MP2)//B3LYP calculations. The same computational strategy was employed in the calculation of the standard molar enthalpies of formation, at T = 298.15 K, in the gas-phase, of single methylated derivatives of benzofuran and dibenzofuran.

Abstract A new and efficient process for organic synthesis in aqueous media based on a direct ohmic heating reactor is described. Four representative organic transformations, a Diels-Alder cycloaddition, a nucleophilic substitution, an N-alkylation and a Suzuki cross-coupling reaction, were performed using this process. The results, when compared with those obtained under conventional external heating (oil bath) and microwave heating, showed that ohmic reactor allows faster and more uniform heating and an induced increase of dynamics/mobility of charged species leading in several cases to higher reaction yields and shorter reaction times.