Publications

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for liquid 1-phenylimidazole (1PhIMI) and 1-phenylpyrazole (1IPhPYR) were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, Delta(c)H(m)(o), measured by static bomb combustion calorimetry. The molar heat capacities of both liquids were measured, in the temperature range (268 to 381) K, by differential scanning calorimetry. The standard molar enthalpies of vaporisation, at the temperature 298.15 K, Delta(l)(g)H(m)(o), were measured by Calvet microcalorimetry. The derived standard molar enthalpies of formation in the gaseous state are discussed comparatively. [GRAPHICS] (C) 2000 Academic Press.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapour pressures at different temperatures of the following substituted benzoic acids: 2-iodobenzoic acid, between T = 345.20 K and T = 359.17 K, 3-iodobenzoic acid, between T = 347.15 K and T = 363.17 K, 4-iodobenzoic acid, between T = 363.15 K and T = 379.11 K, 2-fluorobenzoic acid, between T = 309.15 K and T = 323.15 K and 3-fluorobenzoic acid between T = 303.17 K and T = 317.16 K. From the temperature dependence of the vapour 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, p(0) = 10(5) Pa, molar enthalpies, entropies and Gibbs energies of sublimation at T = 298.15 K, were derived: [GRAPHICS] (C) 2000 Academic Press.

Abstract This paper reports on an automated method based on a flow injection analysis single stream system with amperometric detection for the determination of acetylsalicylic acid in pharmaceutical preparations. The amperometric detection is based on the measurement at 0.8 V vs Ag/AgCl of the current generated when the acetylsalicylic acid hydrolysis product, salicylic acid, is oxidised at a glassy carbon electrode. The current produced is directly related to the concentration of the electroactive specie and the salicylic acid content is proportional to the level of acetylsalicylic acid present in the sample, thus it is possible to determine the concentration of acetylsalicylic acid electrochemically. This system allows a sampling rate of 150 samples per hour without requiring any pre-treatment, apart from dissolving them in a NaOH/KCl buffer, pH 13.0. The results obtained were compared with those obtained using reference methods and demonstrated excellent agreement with relative deviation always lower than 2%.

Abstract The majority of vesicles prepared from dispersions of phospholipids in water are nonequilibrium structures. Thermodynamically stable vesicles must display distinctive physicochemical properties. To investigate the size and stability properties of the vesicles formed in the catanionic mixture of single-tailed (SDS) and double-tailed (DDAD) surfactants, we evaluate the influence of the formation path on size, polydispersity, and equilibration time, by means of microscopy methods and water NMR self-diffusion. The different paths used involve mixing of solutions, mixing of solids, or dilution of preequilibrated concentrated samples. Water self-diffusion and microscopy show that all of the methods are able to yield vesicle solutions, but their size and polydispersity present some differences. Under the effect of a short sonication time, all solutions gain identical macroscopic appearance and with time they apparently equilibrate to a common state (as probed by water self-diffusion data). This stable state coincides with one of the previous formation paths - mixing of solutions.

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.