The properties of high-pH suspensions of mixtures of silica with low-molecular-weight samples of the water-soluble polymer polyethylenimine (PEI) have been studied. At pH > 10 and low ionic strength, silica nanoparticles are stabilized by a negative surface charge, and PEI has only a very low positive charge. The adsorption of PEI induces a localized positive charge on the segments of polymer closest to the silica surface. The parts of the molecule furthest away from the surface have little charge because of the high pH of the medium. The polymer-covered particle remains negatively charged, imparting some electrostatic stabilization. Suspensions of silica and low-molecular-weight PEI are low-viscosity fluids immediately after mixing, but aggregation occurs leading to the eventual gelation (or sedimentation at lower concentrations) of these mixtures, indicating colloidal instability. The gelation time passes through a minimum with increasing surface coverage. The rate of gelation increases exponentially with molecular weight: for molecular weight> or = 10,000 Da PEI, the instability is so severe that uniform suspensions cannot be produced using simple mixing techniques. The gelation rates increase rapidly with temperature, ionic strength, and reduction in pH. The rate of gelation increases with increasing particle concentration at low surface coverage but decreases at high coverage as a consequence of a small increase in pH. Gels are broken by application of high shear into aggregates that re-gel more rapidly than the original discrete coated particles.

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Enantiomeric analysis and empirical determination of the absolute configuration of amines and amino acids can be easily performed using acetyl-d(3) chloride as a nonchiral derivatizing agent (deuterium probe) and deuterium NMR in a chiral solvent (Courtieu's method). In the case of amino acids, derivatization to amido esters, performed with methanol-d(4) and acetyl-d(3) chloride, gives a double opportunity for enantiomeric analysis.

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Si les paysages viticoles participent à la valorisation esthétique d'un territoire donné, ils contribuent aussi au façonnement d'un environnement singulier et caractéristique. Ils offrent, de ce fait, une possible forme de distinction, d'identification et d'identité aux individus. Au sein d'une région qui ne présente pas de paysages exclusivement viticoles, la vigne détient-elle une place significative et éminente dans les représentations mentales des sociétés locales ? Cet article rend compte d'un corpus de données issues de déclarations obtenues par des entretiens menés auprès des habitants d'une petite région rurale, le Vic-Bilh. Le but est ici de comprendre les représentations socio-spatiales que les individus se font du paysage de vignes en tant que vecteur et élément constitutif de leur propre identité.