Preparation and Characterization of Monodispersed Colloidal Particles
- PDF / 3,043,082 Bytes
- 6 Pages / 604.8 x 806.4 pts Page_size
- 94 Downloads / 243 Views
rticle Characteristics Size Distribution Colloidal p a r t i c l e s are n o r m a l l y formed through a sequential process of nucleation and growth of the nuclei. To obtain a monodisperse System, the two stages must be strictly separated and nucleation avoided during the growth period. Since the steady concentration of m o n o m e r s in the growth stage is determined by the balance between the rates of génération of monomers and their consumption by particle growth, the génération or introduction rate of monomers must be controlled so that it is low enough to keep the balanced monomer concentration below the critical supersaturation after the nucleation
MRS BULLETIN/DECEMBER1989
period. 1 2 Typically, the initial concentrations of métal sait, pH, and température are adjusted to meet this requirement for the formation of m o n o d i s p e r s e d métal hydrous oxide particles by forced hydrolysis of métal ions. An artificial séparation between nucleation and g r o w t h processes may be achieved by "seeding," in which foreign particles are introduced into the solution of monomers below the critical supersaturation. One may also lower the p H in hydrolysis of métal ions, dilute with solvent, add chelating agents, or suddenly change the température just after limited nucleation. AH thèse procédures could cause the monomer concentration above the critical supersaturation to plunge to a level below it. In a closed System, the m o n o m e r s contained as complexes or solid precursors must be released at a controlled rate in order for them to be présent below critical supersaturation during the growth period. Except for some spécial cases, particle coagulation must be avoided during ail stages of précipitation by lowering the ionic strength or by protecting the particles with surfactants or some lyophilic polymers. Typical well-defined monodisperse Systems reported in the literature by 1986 are summarized in Référence 1. Modal Size Monomer concentration is generally determined by the balance between the rates of monomer génération and the consumption by précipitation due to nucleation and growth. In the nucleation period, nucleation and growth of the nuclei occur simultaneously. If the
growth rate is extremely low, the balanced monomer concentration is determined mainly by the rates of monomer génération and the nucleation, so that it is kept nearly constant above the critical supersaturation for a long time. On the other hand, if the growth rate constant of the nuclei is great enough, the balanced concentration of monomers may soon be reduced to a level below the critical supersaturation. The monomers are consumed through the growth of the nuclei that increase in number and in individual surface areas. Hence, the particle number left after the nucleation period strongly dépends on the growth rate constant: i.e., the particle number concentration becomes greater with the slower rate. In other words, the resulting modal size gets smaller with the réduction of the growth rate constant, if the total amounts of the monom
Data Loading...
