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The NH3-catalyzed formation of colloidal silica particles from tetra-ethyl-orthosilicate (TEOS) in methanol and ethanol is studied by means of light scattering and Raman spectroscopy. We find that the growth is characterized by an incubation period after which no significant nucleation takes place. The particles have uniform, non-fractal structure and show low polydispersity. In the presence of excess water, the rate-limiting step is the hydrolysis, which is a first-order process in the orthosilicate concentration.

Particles embedded in a plasma acquire a net charge as a result of collisions with electrons and ions. Due to the stochastic nature of encounters between particle and charged species, the instantaneous charge fluctuates. The static properties of the charge fluctuations are quantified for particles surrounded by an undisturbed plasma in orbital motion limit. For particles that satisfy the condition e2/4πε0RkTe≪1 the charge distribution is a Gaussian function whose average and variance is related to the ion and electron currents toward the particle. For a Maxwellian plasma, in particular, analytical solutions are developed for the average charge and the variance as a function of the parameters of the plasma ne/ni, Te/Ti, and Me/Mi. Finally, the methodology is extended to non-Maxwellian plasmas using the Druyvesteyn as an example.

A simplified monomer-addition model with a first-order activation step is developed to describe the dynamics of growth of silica particles from alkoxides. In the fimit of slow hydrolysis, we obtain expressions for the evolution of the particle mass and particle polydispersity, as well as an expression for the particle size as a function of the hydrolysis rate constant, the polymerization rate constant, and the initial concentration of the orthosilicate. We find that the formation of the particles is adequately modeled by a reaction limited growth.

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We report on the formation of titania (TiO2) colloids via the hydrolysis and condensation of alkoxides under a large excess of water. This process is characterized by a rapid precipitation of large aggregates, followed by a slow peptization (deaggregation) induced by the presence of nitric acid. We find that the hydrolysis temperature and the length of the alkoxy group have a minor effect on the size of the peptized colloid. In contrast, the particle size is sensitive to the peptization temperature and exhibits a minimum at 50|SDC. The presence of alcohols inhibits peptization and results in both larger colloids and longer peptization treatments. The smallest size (~20 nm in diameter) is obtained when no alcohol is added to the reaction mixture. The results suggest that the formation of TiO2 nanoparticles is controlled by colloidal interactions, whereas chemical factors (the rate of hydrolysis and condensation) have a secondary role.

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