Publications

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.

Abstract A sensitive novel approach of using an amperometric ion detector for the now injection analysis of salts has been developed. The detection methodology is based on measuring the-current associated with the transfer of ions across polarized microinterfaces between the aqueous sample solution and a 2-nitrophenyloctyl ether-poly(vinyl chloride) gel phase, referred to as ionodes, Different sodium salts of fluoride, chloride, bromide, nitrate, and sulfate were investigated. It was found that by employing an amperometric pulse detection mode and pure water as eluent; the detection limit of the ionode detector could be lowered to ppt level of salt concentrations under flowing conditions.

Abstract The standard (p degrees = 0.1MPa) molar enthalpies of formation for 1,2-, 1,3-, and 1,4-dimethoxybenzene and 1,2,3- and 1,3,5-trimethoxybenzene in the gaseous phase were derived from the standard molar enthalpies of combustion, in oxygen, at T = 298.15 K, measured by static bomb combustion calorimetry and the standard molar enthalpies of evaporation at T = 298.15 K, measured by Calvet microcalorimetry: 1,2-dimethoxybenzene, -(202.4 +/- 3.4) kJ.mol(-1); 1,3-dimethoxybenzene -(221.8 +/- 2.4) kJ.mol(-1); I,4-dimethoxybenzene, -(211.5 +/- 3.0) kJ.mol(-1); I,2,3-trimethoxybenzene, -(339.0 +/- 2.2) kJ.mol(-1); 1,3,5-trimethoxybenzene: -(371.4 +/- 3.0) kJ.mol(-1). Ab initio geometry optimizations for the three dimethoxybenzene and the three trimethoxybenzene isomers were performed using the 3-21G* and the 6-31G* basis sets. Single-point MP2/6-31G* and DFT energy calculations allowed the estimation of the enthalpies of formation of all methoxy-substituted benzenes.

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.