Publications

Abstract The Knudsen mass-loss effusion technique was used to measure the vapour pressure as a function of temperature for trans-cinnamic acid and nine substituted methoxy and dimethoxy cinnamic acids. From the temperature dependence of the vapour pressure the standard molar enthalpies and entropies of sublimation, at 298.15 K, were calculated for all the studied compounds using the estimated value Delta(cr)(g)c(p,m)(o) = -50 J.K-1.mol(-1). The results obtained for trans-cinnamic acid and for the trans-methoxy and trans-dimethoxy cinnamic acids together with literature results for benzoic acid and substituted methoxy and dimethoxy benzoic acids allowed us to derive the correlation: Delta(cr)(g)H(m)(o){T(p = 0.5 Pa)}/(kJ.mol(-1)) = (0.623 +/- 0.026).{T(p = 0.5 Pa)/K} + (-106.3 +/- 9.4) where T(p = 0.5 Pa) is the temperature at which the equilibrium vapour pressure of each crystalline compound is 0.5 Pa. (C) 1999 Academic Press.

Abstract The phase behavior in the cationic-rich side of the phase diagram of the mixed system sodium dodecyl sulfate (SDS)-didodecyldimethylammonium bromide (DDAB)-water at 25 degrees C is presented. DDAB is a double-chained surfactant and thus it tends to self-assemble in water into bilayer structures-vesicles and lamellar phases. The phase diagram of the binary system DDAB-water has been studied, and some features of the diluted region as revealed by surfactant NMR self-diffusion and light microscopy are shown. The structural and phase behavior effects resulting from the addition of SDS are then investigated by complementary microscopy and NMR methods. Upon adding SDS to DDAB dispersions, the area for which a single phase of vesicles occurs is largely extended and a lobe is defined in the phase diagram. The DDAB-rich vesicles are essentially unilamellar and characterized by large sizes (range 0.1-5 mu m) and high polydispersity, as probed by combined cryo-TEM and light microscopy. Self-diffusion measurements show a nonmonotonic variation of water self-diffusion coefficients with the molar fraction of SDS in the mixture, which is correlated to a nonmonotonic variation of mean vesicle size. Microscopy results support this picture. The trends are qualitatively reproduced if initially sonicated (nonequilibrium) DDAB vesicles are used to prepare the catanionic mixtures. The observations are rationalized in terms of an interplay between two opposing effects associated with the presence of SDS in the bilayer-electrostatic effects and packing effects.

Abstract DNA conformational behavior in the presence of nonstoichiometric mixtures of two oppositely charged surfactants, cetyltrimethylammonium bromide and sodium octyl sulfate, was directly visualized in an aqueous solution with the use of a fluorescence microscopy technique, It was found that in the presence of cationic-rich catanionic mixtures, DNA molecules exhibit a conformational transition from elongated coil to compact globule states, Moreover, if the catanionic mixtures form positively charged vesicles, DNA is adsorbed onto the surface of the, vesicles in a collapsed globular form. When anionic-rich catanionic mixtures are present in the solution, no change in the DNA conformational behavior nas detected. Cryogenic transmission electron microscopy, as well as measurements of translational diffusion coefficients of individual DNA chains, supported our optical microscopy observations. (C) 1999 Federation of European Biochemical Societies.

Abstract Some recent illustrations of the phase behavior of polymer-amphiphile systems in solution are presented. Surfactant-polymer association is demonstrated for various amphiphilic synthetic and biological polymers both on a macroscopic and on a single molecular level.

Abstract The Knudsen mass-loss effusion technique was used to measure the vapour pressures of five crystalline dicarboxylic acids as a function of temperature. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation were derived by using the Clausius-Clapeyron equation and the standard molar entropies of sublimation were calculated. The standard molar enthalpies and entropies of sublimation at T = 298.15 K were also calculated by using the estimated value of the standard molar heat capacity of sublimation Delta(cr)(g)C(p,m)(O), = -50 J K-1 mol(-1): [GRAPHICS] The sublimation enthalpy increments for a CH2 group in straight-chain dicarboxylic acids with less than nine methylene groups were calculated as (7.9 +/- 0.4) kJ.mol(-1). The available values for the enthalpies of sublimation of straight-chain dicarboxylic acids with more than eight methylene groups are discussed in terms of partial cyclization. The standard molar enthalpies of formation of dicarboxylic acids in the gaseous phase have been derived. (C) 1999 Academic Press.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for crystalline 3-methoxy-2-nitro-, 4-methoxy 3-nitro-, and 3-methoxy-4-nitrobenzoic acid were derived from the standard molar enthalpies of combustion, in oxygen, at the temperature 298.15 K, measured by static bomb combustion calorimetry, the Knudsen mass-loss effusion technique was used to measure the vapour pressures of these crystals as functions of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by the Clausius-Clapeyron equation. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three nitrobenzoic acids were also determined using the Knudsen mass-loss effusion technique. [GRAPHICS] The standard molar enthalpies of formation in the gaseous state are shown to fit a group additivity scheme. (C) 1999 Academic Press.

Abstract Cyclic, differential pulse and square wave voltammetry were applied to develop an electroanalytical procedure for the determination of a group of herbicides used to treat rice crops: molinate, bensulfuron-methyl, mefenacet and thiobencarb. The oxidation mechanism causes adsorption problems and consequent poisoning of the electrode surface by the products of the electrochemical reaction. Parameters such as pH, frequency and electrochemical electrode surface treatment were optimized. The analytical methodology developed using square wave voltammetry was applied to the determination of molinate and bensulfuron-methyl in simple or mixed commercial products.

Abstract In this work several methods for evaluation of the degree of lipid oxidation and antioxidant activity are reviewed. Some aspects related to the recent advances mentioned in the literature are also reported.