Bypass secuencial axilo-fémoro-poplíteo como alternativa en la revascularización infrainguinal / Sequential axillo-femoral-popliteal bypass as an alternative in infrainguinal revascularisation

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On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances.

Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is retained against leaching losses in soils principally by sorption to organic matter, but the mechanism of sorption has been a matter of controversy. Conflicting evidence exists for proton transfer, electron transfer, and hydrophobic interactions between atrazine and soil humus, but no data are conclusive. In this paper we add to the database by investigating the role of (i) hydroxyatrazine (6-hydroxy-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and (ii) hydrophobicity in the sorption of atrazine by Brazilian soil humic substances. We demonstrate, apparently for the first time, that hydroxyatrazine readily forms electron-transfer complexes with humic substances. These complexes probably are the cause of the well-known strong adsorption by humic acids and they may be the undetected cause of apparent electron-transfer complexes between soil organic matter and atrazine, whose transformation to the hydroxy form is facile. We also present evidence that supports the important contribution of hydrophobic interactions to the pH-dependent sorption of atrazine by humic substances.

Flocculation Kinetics and Cluster Morphology in Illite/NaCl Suspensions.

Using static and dynamic light-scattering techniques, we investigated the flocculation of geologic specimen illite and soil illite colloids suspended in NaCl solution at pH 8. Our results show that the two universality classes proposed by Lin and co-workers to characterize the diffusion and reaction-limited régimes of cluster-cluster flocculation do not apply to illite colloids, which have nonspherical morphology. Static light-scattering measurements yielded mass fractal dimensions for the soil illite clusters that were significantly above those required by universality. Those for specimen illite were influenced by the initial colloid concentration, suggesting that cluster restructuring had occurred during flocculation. Dynamic light-scattering measurements revealed a significant departure from linear growth of the mean cluster size with time, as required by universality in the diffusion-limited régime, and, for specimen illite, an oscillatory dependence of the mean cluster radius on time was observed in the transition from diffusion-limited to reaction-limited flocculation. Overall, the morphology, interactions, and restructuring of primary particles are believed to be responsible for the departure from universality in the flocculation behavior of the illite colloids. Copyright 2000 Academic Press.

Surface Charge Density of Silica Suspended in Water-Acetone Mixtures.

The net proton surface charge density of silica suspended in water-acetone mixtures was studied by potentiometric titration. LiCl and NaCl were used as background electrolytes at concentrations of 10(-1), 10(-2), and 10(-3) mol L-1. The results showed that acetone lowers the net proton surface charge density of silica and that the greater the decrease, the greater the acetone concentration. The surface charge density of silica also is very sensitive to the nature of the background electrolyte, LiCl producing much lower surface charge densities than NaCl. The concept of free energy of transfer (DeltaG0t) of an electrolyte between two different solvents was applied to explain the results in a qualitative manner. Copyright 1999 Academic Press.

Surface geochemistry of the clay minerals.

Clay minerals are layer type aluminosilicates that figure in terrestrial biogeochemical cycles, in the buffering capacity of the oceans, and in the containment of toxic waste materials. They are also used as lubricants in petroleum extraction and as industrial catalysts for the synthesis of many organic compounds. These applications derive fundamentally from the colloidal size and permanent structural charge of clay mineral particles, which endow them with significant surface reactivity. Unraveling the surface geochemistry of hydrated clay minerals is an abiding, if difficult, topic in earth sciences research. Recent experimental and computational studies that take advantage of new methodologies and basic insights derived from the study of concentrated ionic solutions have begun to clarify the structure of electrical double layers formed on hydrated clay mineral surfaces, particularly those in the interlayer region of swelling 2:1 layer type clay minerals. One emerging trend is that the coordination of interlayer cations with water molecules and clay mineral surface oxygens is governed largely by cation size and charge, similarly to a concentrated ionic solution, but the location of structural charge within a clay layer and the existence of hydrophobic patches on its surface provide important modulations. The larger the interlayer cation, the greater the influence of clay mineral structure and hydrophobicity on the configurations of adsorbed water molecules. This picture extends readily to hydrophobic molecules adsorbed within an interlayer region, with important implications for clay-hydrocarbon interactions and the design of catalysts for organic synthesis.