Publications

Abstract The electrochemical behaviour of propanil and related N-substituted amides (acetanilide and N,N-diphenylacetamide) was studied by cyclic and square wave voltammetry using a glassy carbon electrode. Propanil has been found to have chemical stability under the established analytical conditions and showed an oxidation peak at +1.27 V versus Ag/AgCl at pH 7.5. N,N-diphenylacetamide has a higher oxidation potential than the other compounds of +1.49 V versus Ag/AgCl. Acetanilide oxidation occurred at a potential similar to that of propanil, +1.24 V versus Ag/AgCl. These results are in agreement with the substitution pattern of the nitrogen atom of the amide. A degradation product of propanil, 3,4-dichloroaniline (DCA), was also studied, and showed an oxidation peak at +0.66 V versus Ag/AgCl. A simple and specific quantitative electroanalytical method is described for the analysis of propanil in commercial products that contain propanil as the active ingredient, used in the treatment of rice crops in Portugal.

Abstract The oxidative stability of refined sunflower oil in the presence and in the absence of propyl caffeate (PC), propyl hydrocaffeate (PHC), propyl ferulate (PF), and propyl isoferulate (PI) has been evaluated according to the Rancimat method. The antioxidant activity of the phenolic derivatives was compared with that obtained with native [alpha -tocopherol (alpha -TOH)] and synthetic [propyl gallate (PG)] antioxidants. The results allow the establishment of a decreasing order of antioxidant power: PG > PHC > PC much greater than alpha -TOH > PI > PF. The oxidative stability was improved neither by the addition of PF nor by a supplement of alpha -TOH. Moreover, a positive antioxidant effect was obtained for PC that was placed between those of alpha -TOH and PG. The antioxidant activity of PHC was higher than that of its analogue (PC). A dose-dependent effect was observed for PG, PHC, and PC. A chain-breaking mechanism was proposed for the antioxidant activity of propyl phenolic esters because the same ranking order of efficacy was obtained for their antiradical activities evaluated by using the 2,2-diphenyl-1-picrylhydrazyl radical method.

Abstract Square-wave voltammetry (SWV) has been used to study the transfer of zinc(II) ion under static conditions, assisted by 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (DPT), through gel-supported microinterfaces. Microhole arrays created by laser photoablation of thin polyester films were used to support an organic gel phase prepared by addition of 1,3 : 2,4-dibenzylidenesorbitol (DBS) to a solution of o-nitrophenyloctyl ether (o-NPOE) with the appropriate supporting electrolyte. The results show that SWV can be used with the gel-supported microinterfaces if a gelified aqueous reference is used for the organic phase. Under such conditions a preliminary estimate of the detection limit for the determination of Zn2+ is 5 x 10(-8) mol L-1.

Abstract Determination of total sulfur in environmental samples can be accomplished using automated dry combustion techniques. The analytical performance of an elemental analyzer (CE Instruments) was compared with a method involving dry ashing followed by ion chromatographic detection of sulfate (IC). Samples included certified reference materials, confirmation materials with a known content of sulfur, and several soils and plants. There was a close agreement between both methods. Superior accuracy, precision and detection limits were obtained using the dry combustion method. Current results suggest that the automated dry combustion technique has developed into an adequate method for the determination of the relatively low total sulfur contents commonly encountered in soils and plants.

Abstract The capacitance of the polarisable aqueous \ organic interface was measured with the impedance method, varying both the solvent and the electrolyte of the organic phase. The results were analysed using the idea of interfacial ion-pairing presented earlier (J. Chem. Soc. Faraday Trans. 87 (1991) 107). It appeared that this model was capable of explaining the measured capacitances quite well for 2-heptanone, 2-octanone and 1,3-dibromopropane as the organic solvent, while for 1,2-dichloroethane - which is perhaps the most common solvent used in liquid \ liquid electrochemistry - the model needed to be modified, assuming a mixed layer between the bulk solvent phases as suggested earlier in the literature.

Abstract The inhibition by an adsorbed nonionic surfactant (sorbitan monostearate) of electron transfer across an aqueous/organic interface, between ferro-/ferricyanide and dimethyl (DiMFc) or decamethyl (DcMFc) ferrocene, is related to the fraction of the surface covered. However, the apparent surface geometry depends on the direction of electron transfer: the blocked interface behaves as an array of isolated microelectrodes for the oxidation of DiMFc or DcMFc and as a uniformly accessible surface for the reduction of DiMFc(-). Interfacial capacitance and interfacial tension measurements gave the surface coverage of the surfactant, and scanning electrochemical microscopy approach curves yielded the overall reaction rate constant fur ferrocene oxidation. Four-electrode voltammetry revealed the asymmetry of the interface geometry induced by surfactant adsorption. It is concluded that sorbitan monostearate forms patches on the interface, which the uncharged ferrocene derivative cannot penetrate. The packing density of surfactant molecules in the surface patches increased with increasing surface coverage. From the limiting current values fur DiMFc oxidation obtained from cyclic voltammograms for near-complete coverage of surfactant, the diameter of the holes in the surfactant layer is calculated to be approximately 200 nm. In contrast, it semis that the oxidized, charged ferricenium can induce a local rearrangement of the adsorbed layer and hence diffuse freely through the layer.

Abstract Mixed solutions of catanionic and anionic surfactants are known to form thermodynamically stable vesicles over wide concentration ranges. This phase behaviour is reviewed and the conditions of vesicle formation, and their microstructure is discussed. Vesicles are either rich in cationic or in anionic surfactant. The present work reports novel studies of the interaction between thermodynamically stable vesicles and water-soluble polymers. In particular, we have investigated the interaction between catanionic vesicles and oppositely charged polymers, either anionic (DNA) or cationic (cellulose derivatives). The experimental work included phase diagram determination, rheology, cryo transmission electron microscopy and direct visualization of single DNA molecules by fluorescence microscopy. Observations include an associative phase separation, a compaction of DNA with cationic but not with anionic vesicles, and the formation of novel polymer-vesicle gels.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation for crystalline 2-hydroxyquinoxaline, 2,3-dihydroxyquinoxaline, and 2-hydroxy-3-methylquinoxaline were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry. The standard molar enthalpies of sublimation, at T = 298.15 K, of the three compounds were measured by Calvet microcalorimetry. The derived standard molar enthalpies of formation in the gaseous phase are 45.9 +/- 4.3 kJ.mol(-1) for 2-hydroxyquinoxaline, -(179.2 +/- 5.3) kJ.mol(-1) for 2,3-dihydroxyquinoxaline, and -(8.8+/- 4.9) kJ.mol(-1) for 2-hydroxy-3-methylquinoxaline. In addition, theoretical calculations using the density functional theory and the B3LYP/6-311G** hybrid exchange-correlation energy functional were performed for these molecules in order to obtain the most stable geometries and to access their relative stability. The theoretical results are in general good agreement with experimental findings.