Publications

Abstract The Knudsen mass-loss effusion technique was used to measure the vapor pressures at different; temperatures of the following crystalline dicarboxylic acids: methylmalonic acid, between 341.13 K and 354.74 K, dimethylmalonic acid, between 347.18 K and 363.26 K, methylmalonic acid, between 347.52 K and 362.34 K and butylmalonic acid, between 348.20 K and 362.18 K. From the temperature dependence of the vapor pressure, the standard molar enthalpies of sublimation were derived by the Clausius-Clapeyron equation and the molar entropies of sublimation at equilibrium pressures were calculated. Using estimated values for the heat capacity differences between the gas and the crystal phases of the studied compounds the standard (pressure p(0) = 10(5) Pa) molar enthalpies, entropies and Gibbs functions of sublimation at T = 298.15 K, were derived.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapor pressures at different temperatures of the following substituted quinoxalines: 2-hydroxyquinoxaline, between 383.17 K and 399.15 K; 2-hydroxy-3-methylquinoxaline, between 375.16 K and 391.15 K; 2,3-dichloroquinoxaline, between 313.15 K and 329.15 K; 2,3,6,7-tetrachloroquinoxaline, between 347.16 K and 361.17 K; 2,3-dimethylquinoxaline between 294.14 K and 308.14 K; 2,3-bis(bromomethyl)quinoxaline, between 351.14 K and 365.14 K. From the temperature dependence of the vapor pressure, the standard molar enthalpies of sublimation at the mean temperature of the experimental range were derived by the Clausius-Clapeyron equation. From these results the standard molar enthalpies, entropies, and Gibbs functions of sublimation at T = 298.15 K were calculated. An empirical equation for estimating vapor pressure-temperature data from enthalpies of sublimation values is presented.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation at the temperature T = 298.15 K of crystalline nickel(II) acetate, Ni(CH3COO)(2), nickel(II) acetate tetrahydrate Ni(CH3COO)(2) . 4.00H(2)O, cadmium(II) acetate Cd(CH3COO)(2), and cadmium(II) acetate dihydrate Cd(CH3COO)(2) . 2.00H(2)O were determined by solution-reaction calorimetry. The enthalpies of dehydration were derived for both complexes. [GRAPHICS] (C) 2000 Academic Press.

Abstract The phase behavior and phase structure for the catanionic pair sodium taurodeoxycholate - didodecyl-dimethylammonium bromide (DDAB) are investigated, at 25 degreesC. A combination of techniques is used including light and electron microscopy, small-angle X-ray scattering, and pulsed field gradient NMR self-diffusion. The bile salt micellar solution incorporates large amounts of the double-chained amphiphile, with the solution region extending to equimolarity. On the contrary, the hexagonal liquid-crystalline phase is destabilized by the addition of small amounts of DDAB. At equimolarity, coacervation instead of precipitation is observed, with formation of a viscous isotropic solution and a very dilute one. In the water-rich part of the phase diagram, a peculiar type of phase separation occurs, involving the formation of very fine bluish dispersions and a region of coexistence of two dispersions (double dispersion region). Microscopy and self-diffusion data for the solution region indicate limited growth of the mixed micelles. Large domains in which the micellar structure appears to be maintained are imaged in the bluish dispersions by electron microscopy. No other type of aggregate such as vesicles or precipitates is observed in the dilute bile salt-rich area of this mixture.

Abstract The phase behavior of several binary sodium bile salt-water systems is investigated over the entire concentration range, with emphasis on concentrated regions beyond the isotropic solution phase. The studied bile acid salts comprise the free salt sodium deoxycholate (SDC), the taurine conjugates sodium taurocholate (STC), sodium taurodeoxycholate (STDC), and sodium taurochenodeoxycholate (STCDC) and the glycine conjugate sodium glycodeoxycholate (SGDC). A combination of classical techniques is used, including phase diagram determination, polarizing microscopy, H-2 NMR, and small-angle X-ray scattering (SAXS). The aggregation behavior in the isotropic micellar solutions of STC and STDC is also investigated by pulsed-field gradient NMR self-diffusion. The optical textures and the data from SAXS and H-2 NMR clearly point to the formation of hexagonal liquid crystals, possibly of the reverse type, beyond the micellar solution for all the bile salts. Several unusual kinetic effects, such as very slow equilibration times and the formation of transient spherulitic crystals in biphasic regions, are observed. The phase diagrams and structural data are qualitatively discussed in terms of the molecular structure and solubility of the different salts. The formation of lyotropic liquid crystals by bile salts, which has remained unknown for decades, is clearly demonstrated in this work.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen of crystalline N-benzoyl-N',N'-diethylurea, PhCONHCON(Et)(2) (HBETU) and N-benzoyl-N',N'- diisobutylurea, PhCONHCON(iBu)(2) (HBIBU), were measured at T = 298.15 K by static bomb combustion calorimetry. The standard molar enthalpies of sublimation of both compounds were measured using microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in their crystalline and gaseous phases, respectively. -Delta (c)u(o)(cr)/(J . g(-1)) Delta H-g(cr)m(o)(kJ . mol(-1)) N-benzoyl-N',N'-diethylurea 29756.9 +/- 9.6 132.2 +/- 1.4 N-benzoyl-N',N'-diisobutylurea 33047.0 +/- 9.3 137.5 +/- 2.9 (C) 2000 Academic Press.

Abstract The association between alpha -cyclodextrin and three monoalcohols (1-pentanol, 1-hexanol, and 1-heptanol) in (glycerol + water) was studied by titration microcalorimetry at T = (288.15, 298.15, and 308.15) K. The apparent thermodynamic parameters for the binding reactions, the equilibrium constant K(obs) and enthalpy of reaction Delta H-r(m)o(obs), were calculated from the calorimetric results. Knowing from a prior study that glycerol associates with alpha -cyclodextrin in aqueous solution, the Observed values were corrected for this equilibrium, assuming as a model that there is identical linkage. The intrinsic values of the thermodynamic parameters for the binding reaction, K(int) and Delta H-r(m)o(int). were then calculated for the three studied temperatures. From these values, the change in Gibbs free energy and entropy upon binding, Delta (r)G(m)(o)(int) and Delta S-r(m)o(int), respectively, were also calculated. From the temperature dependence of the enthalpy changes, the change in heat capacity for the binding reactions Delta C-r(p,m)o was obtained. The values of the intrinsic thermodynamic parameters are compared with the corresponding ones in water. For all compounds and temperatures, there is an increase in binding strength, as reflected by higher K values. The fact that the intrinsic parameters have different values from the ones observed in water shows that the glycerol linkage to the monoalcohols binding to alpha -cyclodextrin explains just part of the change in the observed parameters, indicating that glycerol is also acting as a co-solvent, thus affecting the solution interactions. (C) 2000 Academic Press.

Abstract The antiradical activity of caffeic acid (1), dihydrocaffeic acid (5), and their corresponding n-alkyl esters was evaluated by using the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH.) method. Dihydrocaffeic acid (5) was the most potent compound, having an antiradical effect higher than that of (+/-)-alpha-tocopherol, whereas caffeic acid (1) was less efficient. Esterification of the carboxyl group of dihydrocaffeic acid (5) had a dramatic effect on its antiradical potency, but similar effects were not observed for caffeic acid (1) derivatives. The n-alkyl esters of both phenolic series had similar potencies, and their antiradical activities were independent of the alkyl chain length, Dose-dependent scavenger effects were found in both series. Acid-base properties of the compounds, evaluated by using potentiometry and spectrophotometry, showed that the catechol moiety had pK(a2) and pK(a3) values of 9.24-9.02 and 11.38-10.99 in the dihydrocaffeic series and 8.48-8.24 and 11.38-11.07 in the caffeic series, respectively. Antiradical activity and pK(a) values of the compounds were not related.

Abstract The standard (p(O) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline D-, L- and DL-valine were measured by static-bomb calorimetry, in the presence of an auxiliary of combustion to improve the complete oxidation reaction. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds, at T = 298.15 K, as -(612.9 +/- 1.5) kJ . mol(-1), -(614.7 +/- 1.3) kJ . mol(-1) and -(617.1 +/- 1.0) kJ . mol(-1), respectively, for the D-, L- and DL-valine. The reliability of the results obtained is commented upon and compared with literature results for other alpha-amino acid isomers. (C) 2000 Academic Press.