Glycerol is an attractive renewable building block for the synthesis of di‐ and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the selective etherification of glycerol to di‐ and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO
We report on the preparation of monodisperse, fluorescent hematite-silica core-shell ellipsoids, with adjustable shapes ranging from spindles to nearly spheres, that are suitable for optical rotational diffusion studies. Hematite cores are grafted with poly(vinylpyrrolidone) which ensures colloidal stability during the silica coating provided by the base-catalyzed hydrolysis and polymerization of tetraethoxysilane. Using tetramethylammonium hydroxide as base instead of the volatile ammonia facilitates continuous seeded growth of silica to colloids with a desired aspect ratio. A convenient feature of the hematite-silica particles is the rapid dissolution of the iron oxide core by acid, producing hollow silica ellipsoids that can be optically matched to near transparency. The control of shape and size of the silica ellipsoids, their optical properties, and the fairly high yield in comparison to other preparation methods for nonspherical model colloids make the ellipsoids very suitable for quantitative studies. As a case in point, we have measured the rotational diffusion coefficient of fluorescent ellipsoids with rotational fluorescence recovery after photobleaching. Dye-labeled ellipsoids can be imaged with confocal microscopy.
The dynamic magnetic susceptibility of relatively monodisperse iron ferrofluids was measured from 1 Hz to 100 kHz for different sizes of the iron particles (all with a 7-nm-thick organic surface layer, dispersed in Decalin). In the case of particles with an iron core of 6-nm radius, the orientation of the magnetic dipole moment thermally rotated inside the particles (Néel rotation). In the case of particles with a slightly larger iron core, the orientation of the magnetic dipole moment was blocked inside the particles but could still change by rotational diffusion of the particles themselves (Brownian rotation). With even larger particles (above 7-nm iron core radius), aggregates were formed: the rotational diffusion rate was lower than that of single particles by more than 1 order of magnitude. This sudden appearance of aggregates above a certain size of the iron particles agrees with previous observations in two dimensions, by cryogenic transmission electron microscopy of ultrathin ferrofluid films. Here, it is found that the threshold for aggregation is practically the same in three dimensions. Moreover, the rotational diffusion rate of the aggregates is seen to increase upon dilution, indicating a decrease in aggregate size. This suggests that a dynamic equilibrium exists between the sticking of particles to each other and unsticking, especially when the particles are sufficiently small so that the sticking energy is not more than a few times the thermal energy.
Atomic force microscopy (AFM) is used to study the size, shape, and polydispersity of a variety of magnetic and nonmagnetic model colloids, previously imaged by transmission electron microscopy (TEM) only. Both height and phase images are analyzed and special attention is given to 3D morphology and softness of particles, as well as structures and presence of secondary components in the colloid, difficult to investigate with TEM. Several methods of tip characterization followed by deconvolution were applied in order to improve the accuracy of lateral diameter determination. In the case of magnetite particles dispersed in conventional ferrofluids, we explore both experimentally and theoretically the possibility of using magnetic force microscopy (MFM). We propose and discuss several models which allow to estimate the magnetic moment of a single domain superparamagnetic sphere using MFM, which cannot be done with other techniques; alternatively the tip magnetization can be determined. C 2002 Elsevier Science (USA)
Langmuir's disjoining pressure between two flat, charged planes was calculated analytically for strongly overlapping double layers in the limit of zero electric field between the planes. The resulting repulsion has a long-range algebraic decay that stems from the thermodynamic equilibrium between homogeneously distributed interplate ions and ions in the surrounding electrolyte reservoir. Together with the van der Waals attraction, the repulsion forms the zero-field pendant of the exponentially screened DLVO potential, a pendant that is always repulsive at large plate-plate distances. The experimental occurrence of algebraic repulsions can be simply predicted from surface charge density and ionic strength.
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