Publications

Abstract The conformational preferences of several potential anticancer dihydroxycinnamic esters with a variable length alkyl chain were studied by quantum-mechanical (DFT) calculations (both for the isolated molecule and for aqueous solutions). The orientation of the hydroxyl ring substituents and of the alkyl ester moiety relative to the carbonyl group showed these to be the most determinant factors for the overall stability of this type of phenolic systems, strongly dependent on an effective pi-electron delocalization. Compared to the parent caffeic acid (dihydroxycinnamic acid), esterification was found to lead to a higher conformational freedom, and to affect mainly the energy barrier corresponding to the (O=)C-OR internal rotation. No particular differences were verified to occur upon lengthening of the ester alkyl chain, except when this is branched instead of linear. The vibrational spectra of the whole series of compounds were simulated, based on their calculated harmonic vibrational frequencies, and a preliminary assignment was performed.

Abstract Vesicles composed of an anionic and a cationic surfactant, with a net negative charge, associate strongly with a hydrophobically modified polycation (LM200) and with an unmodified polycation with higher charge density (JR400), forming viscoelastic gel-like structures. Calorimetric results show that in these gels, LM200 induces a rise of the chain melting temperature (T-m) of the vesicles, whereas JR400 has the opposite effect. For both polymer-vesicle systems, the shear viscosity exhibits an inflection point at T-m, and for the LM200 system the measured relaxation times are significantly higher below T-m. The neat vesicles and the polycation-bound vesicles have a polygonal-like faceted shape when the surfactant chains in the bilayer are crystallized, as probed by cryo-transmission electron microscopy. Above T-m, the neat and the LM200-bound vesicles regain a spheroidal shape, whereas those in the JR400 system remain with a deformed faceted shape even above T-m. These shape changes are interpreted in terms of different mechanisms for the polymer-vesicle interaction, which seem to be highly dependent on polymer architecture, namely charge density and hydrophobic modification. A crystallization-segregation mechanism is proposed for the LM200-vesicle system, while, for the JR400-vesicle one, charge polarization-lateral segregation effects induced by the polycation in the catanionic bilayer are envisaged.

Abstract A static bomb calorimeter has been used to measure the standard molar energy of combustion, in oxygen, at T = 298.15 K, of a commercial sample of cytosine. From this energy, the standard (p degrees = 0.1 MPa) molar enthalpy of formation in the crystalline state was derived as -(221.9 +/- 1.7) kJ center dot mol(-1). This value confirms one experimental value already published in the literature but differs from another literature value by 13.5 kJ center dot mol(-1). Using the present standard molar enthalpy of formation in the condensed phase and the enthalpy of sublimation due to Burkinshaw and Mortimer [J. Chem. Soc., Dalton Trans. 1984, 75], (155.0 +/- 3.0) kJ center dot mol(-1), results in a value for the gas-phase standard molar enthalpy of formation for cytosine of -66.9 kJ center dot mol(-1). A similar value, -65.1 kJ center dot mol(-1), has been estimated after G3MP2B3 calculations combined with the reaction of atomization on three different tautomers of cytosine. In agreement with experimental evidence, the hydroxy-amino tautomer is the most stable form of cytosine in the gas phase. The enthalpies of formation of the other two tautomers were also estimated as -60.7 kJ center dot mol(-1) and -57.2 kJ center dot mol(-1) for the oxo-amino and oxo-imino tautomers, respectively. The same composite approach was also used to compute other thermochemical data, which is difficult to be measured experimentally, such as C-H, N-H, and O-H bond dissociation enthalpies, gas-phase acidities, and ionization enthalpies.

Abstract Batch experiments were carried out for the sorption of methylene blue onto Paspalum notatum. The operating variables studied were initial dye concentration, initial solution pH, adsorbent dosage and contact time. Experimental equilibrium data were fitted to Freundlich, Langmuir and Redlich-Peterson isotherms by non-linear regression method. Six error functions was used to determine the optimum isotherm by non-linear regression method. The present study shows r(2) as the best error function to determine the parameters involved in both two- and three-parameter isotherms. Langmuir isotherm was found to be the optimum isotherm for methylene blue onto Paspalum notatum. The monolayer methylene blue sorption capacity of P. notatum was found to be 31 mg/g. The kinetics of methylene blue onto P. notatum was found to follow a pseudo second order kinetics. A Boyd plot confirms the external mass transfer as the rate-limiting step in the dye sorption process. The influence of initial dye concentration on the dye sorption process was represented in the form of dimensionless mass transfer numbers (Sh/Sc-0.33) and was found to vary as C-0(-5x10-6)

Abstract This letter reports the way to solve the Redlich-Peterson isotherm. ln addition the importance of constraints for the Redlich-Peterson isotherm exponent was discussed in detail.

Abstract In isoperibol temperature-rise calorimetry, different approaches can lead to the corrected temperature rise. The description of the methodology, as well as an improvement in the approach to the accurate numerical calculation of the corrected temperature rise in isoperibol calorimetry, based on the comparison between the exponential functional description of the initial and final periods and an independent third order polynomial fitting, is presented. The latter is focused on the strategy of minimizing the result dependency on the definition of the main period of the reaction. Furthermore, the dedicated software application Labtermo is introduced.

Abstract Batch experiments were carried out for the sorption of eosin yellow, malachite green and crystal violet onto jute fiber carbon (JFC). The operating variables studied are the initial dye concentration, initial solution pH, adsorbent dosage and contact time. Experimental equilibrium data were fitted to Freundlich, Langmuir and Redlich-Peterson isotherm by non-linear regression method. Langmuir isotherm was found to be the optimum isotherm for eosin yellow/JFC system and Freundlich isotherm was found to be the optimum isotherm for malachite green/JFC and crystal violet/JFC system at equilibrium conditions. The sorption capacities of eosin yellow, malachite green and crystal violet onto JFC according to Langmuir isotherm were found to 31.49 mg/g, 136.58 mg/g, 27.99 mg/g, respectively. A single stage batch adsorber was designed for the adsorption of eosin yellow, malachite green and crystal violet onto JFC based on the optimum isotherm. A pseudo second order kinetic model well represented the kinetic uptake of dyes studied onto JFC. The pseudo second order kinetic model successfully simulated the kinetics of dye uptake process. The dye sorption process involves both surface and pore diffusion with predominance of surface diffusion at earlier stages. A Boyd plot confirms the external mass transfer as the rate limiting step in the dye sorption process. The influence of initial dye concentration on the dye sorption process was represented in the form of dimensionless mass transfer numbers (Sh/Sc-0.33) and was found to be agreeing with the expression: Sh/Sc-0.33 = xC(0)(y). (C) 2006 Published by Elsevier B.V.

Abstract This letter reports the importance and advantages of the constraints in the Redlich Peterson isotherm exponent.

Abstract The standard molar enthalpy of formation at T = 298.15 K of 6-chloro-2,3-dimethylquinoxaline 1,4-dioxide in the gas-phase was derived using the values of the enthalpies of combustion for the crystalline solid, measured by using totating-bomb combustion calorimetry, and of the enthalpies of sublimation, measured by using Calvet microcalorimetry. This value was used for calibrating the computational procedure employed to estimate the gas-phase enthalpies of formation for the deoxygenated quinoxaline derivatives and also to compute the first, second, and mean (N-O) bond dissociation enthalpies in the gas-phase. The B3LYP/6-311+G(2d,2p) calculations were also extended to the fluorine, bromine, and iodine derivatives and to the isomers with a halogen substituted at position 5. When the halogen atom enters the same position in the quinoxaline ring, it was found that the substituent had almost no influence on the computed (N-O) bond dissociation enthalpies, except for 5-fluoro-2,3-dimethylquinoxaline 1,4-dioxide.

Abstract The huge number of possible combinations of binary mixtures of alcohols and ionic liquids (ILs) make the exhaustive measurement of all these systems impracticable requiring the use of a predictive model for their study. In this work, the predictive capability of COSMO-RS, a model based on unimolecular quantum chemistry calculations, was evaluated for the description of the liquid-liquid equilibria and the vapour-liquid equilibria of binary mixtures of alcohols and several imidazolium and pyridinium-based ILs. The effect of the ions and alcohols conformers on the quality of the predictions was assessed and the quantum chemical COSMO calculation at the BP/TZVP level derived from the lower energy conformations was adopted. Although a degradation in the liquid-liquid equilibria predictions with increasing size length of the alkyl chain of the alcohol or of the ionic liquid and with the short chain alcohol methanol in the vapour-liquid equilibria predictions were observed, in general a reasonable qualitative agreement between the model predictions and experimental data for a large combination of structural variations of both alcohols and ILs was obtained. COSMO-RS can thus be very useful in the scanning of the growing number of known ILs to find suitable candidates, or help designing new ILs, for specific applications before extensive experimental measurements.