Uniform Particles of Pure and Silica Coated Cholesterol

Uniform crystalline colloidal cholesterol particles of narrow size distribution were obtained by precipitation. The method consisted of adding a miscible non-solvent (water) into cholesterol solutions of different alcohols and acetone, without any additives. The properties of the resulting particles depended in a sensitive way on the concentration of all reactants, temperature, pH, ionic strength, and aging time. The major observed effects were due to the solubility of cholesterol, which was strongly affected by the solvent mixture and temperature. Precipitation in 1-propanol/water system yielded stable dispersions of well-defined particles, which were used to evaluate the effects of different experimental parameters on their properties. Aging of stable dispersions resulted in multi-layered aggregation of the primary platelets, the degree and rate of which process was strongly affected by temperature. Finally, it was shown that the colloidal cholesterol particles could be coated with homogeneous silica layers in order to alter their surface characteristics

Obituaries

Mladen Krajčinović (1920.-1952.) Slavko Miletić (1927.-1952.

Kinetics of Diffusional Detachment of Colloidal Particles From Surfaces

Experimental and theoretical problems related to the detachment of spherical particles from plane surfaces are reviewed. The diffusional kinetics is considered only by taking electrostatic, van der Waals, and short range repulsion energies into account. The rate of the detachment was treated in terms of the first order kinetic process, using a single or a multipopulation model

Kinetics of Deposition of Colloidal Metal Oxide Particles on a Steel Surface

The deposition of spherical a-Fe20 3 and rod-like B-FeOOH. particles on steel from aqueous dispersions of different pH and ionic strength using a packed column technique has been studied. A procedure was developed by which the coagulation of the suspended particles was minimized even in the presence of high electrolyte concentrations during the duration of an experiment. In the absence of the potential barrier the results agree with the theoretical expectations based on the convective diffusion model. The same interpretation applies to the effects of the pH and of the indifferent electrolyte if the influence of electrostatic forces is taken into consideration. A batch technique was used to investigate the equilibration phenomena in the same system at room temperature and at 210 °c

Formation of Monodispersed Cadmium Sulfide Particles by Aggregation of Nanosize Precursors

Monodispersed spherical cadmium sulfide particles were used as a model system in order to explain the size selection in the formation of colloids by aggregation of nanosize subunits. Several procedures of mixing the reactants were employed to precipitate these solids and follow the kinetics of particle growth. Efficient numerical simulation techniques for the model rate equations were developed to fit the experimental results. Our results have confirmed the recently proposed mechanism of two-stage growth by nucleation of nanosize crystalline primary particles and their subsequent aggregation into polycrystalline secondary colloids.Comment: 18 pages (with 6 figures) in PD

Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

It has been experimentally established in numerous cases that precipitation of monodispersed colloids from homogeneous solutions is a complex process. Specifically, it was found that in many systems nuclei, produced rapidly in a supersaturated solution, grow to nanosize primary particles (singlets), which then coagulate to form much larger final colloids in a process dominated by irreversible capture of these singlets. This paper describes a kinetic model that explains the formation of dispersions of narrow size distribution in such systems. Numerical simulations of the kinetic equations, with experimental model parameter values, are reported. The model was tested for a system involving formation of uniform spherical gold particles by reduction of auric chloride in aqueous solutions. The calculated average size, the width of the particle size distribution, and the time scale of the process, agreed reasonably well with the experimental values.Comment: 38 pages in plain TeX and 7 JPG figure