We have studied oil-in-water emulsions stabilized by monodisperse, fluorescent silica colloids presenting either a smooth or a rough surface. The presence of the fluorescent core allows for direct visualization of the colloids on the surface of the emulsion droplets. Droplet interfacial tension, measured by micropipet tensiometry, is not modified by particle adsorption at the interface, suggesting a purely steric stabilization mechanism. Surface roughness is shown to considerably lessen the ability of particles to stabilize droplets. At variance with what is commonly assumed, no straightforward relation exists between the extent of particle interfacial adsorption and emulsion macroscopic stability; stable emulsions can be obtained even with very low droplet surface coverage. Finally, we directly monitor the Brownian motion of the adsorbed particles, showing that their surface diffusion coefficient is very close to the bulk value. Evidence of a possible role of particle surface dynamics on the stabilization of poorly covered droplets is presented.

New and published l3C)|, |V|) and tin-hydrogen (|2/(ll9Sn,1H)|, |2/|) /coupling data for 25 methyltin(IV) compounds (several in a variety of solvents) have been collected. From a relationship between |'y| and the Me-Sn-Me angle, , described previously, |2J|,0 data pairs have been derived. A plot of these data reveals that and |2J| are related by a smooth curve described by (deg) = 0.0161|2J|2 -1.32|27| + 133.4; data for most methyltin(IV) compounds lie within 4°of this empirical line. Data for dimethyltin dichloride and dibromide in solvents of varying coordinating ability are described by a somewhat different relationship: (deg) = 0.0105|2J)2 -0.799|2/| + 122.4. Several applications of the former equation for determining the structures of methyltin(IV) compounds in solution are briefly described, including its use in the assignment of tin coordination number.NMR spectroscopy is an important tool for investigating mo-and coupling constants, however, is generally based on crystal lecular structure in solution. The interpretation of chemical shifts structure data (X-ray) and is consequently subject to uncertainties This article not subject to U.S.

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A waxy crude oil which gels below a threshold temperature has been investigated under static and dynamic conditions, using a combination of rheological methods, optical microscopy, and DSC. Particular attention is given in this work to the influence of the mechanical history on gel strength and to describing the time-dependent rheological behavior. The gels display a strong dependence of the yield stress and moduli on the shear history, cooling rate, and stress loading rate. Of particular interest is the partial recovery of the gel structure after application of small stress or strain (much smaller than the critical values needed for flow onset) during cooling, which can be used to reduce the ultimate strength of the crude oil gel formed below the pour point. A second focus of this study is to further develop the physical interpretation of the mechanism by which wax crystallization produces gelation. Gelation of the waxy crude oil studied is suggested to be the result of the association between wax crystals, which produces an extended network structure, and it is shown that the system displays features common to attractive colloidal gels, for one of which, fumed silica (Aerosil 200) in paraffin oil, rheological data are reported. The colloidal gel model provides a simple and economical basis for explaining the response of the gelled oil to various mechanical perturbations and constitutes a fruitful basis from which to develop technologies for controlling the gelation phenomenon, as suggested by the rheological results reported.

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The observation of CIDNP, coupled with the reactivity exhibited by ~Z and the other two intermediates, implicate a biradical description of these molecules.

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