43 research outputs found
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
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 Ā°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