Colloid formation particle diameters

The composition of the insoluble compound (precipitate) obtained from the analyte must be known and stable. Poorly soluble substances may form colloidal suspensions (particle diameters from 10 7 to 10 4 cm). The formation of a colloidal suspension can be minimized or prevented by carrying out the precipitation from a dilute solution of the analyte, at a temperature close to the boiling point of water and with constant stirring. The relative supersaturation affects the particle size and is expressed as Q - S/S, where Q is the instantaneous concentration of the added species and S is the equilibrium solubility of the compound that precipitates. Particle size seems to be inversely proportional to relative supersaturation. The electric double layer formed during precipitation keeps the colloidal precipitate particles from coming into contact with each other, thus preventing further coagulation. 

The inter-relationship between colloid and polymer chemistries is completed by colloidal polymer particles. The formation of 50-nm-diameter, 100- to 200-nm-long polyaniline fibrils in a poly(acrylic acid)-template-guided polymerization, similar in many ways to those produced from polymerized SUVs (see above), provides a recent example of polymer colloids [449], The use of poly(styenesulfonic acid) as a template yielded globular polyaniline particles which were found to be quite different morphologically from those observed in the regular chemical synthesis of polyaniline [449]. 

The critical parameter governing the initial layer formation was found to be the ratio of the particle diameter (in the colloidal suspension) to the pore diameter of the support. Given a certain colloidal suspension (characterised by its alumina concentration, kind and concentration of peptising acid used and ageing time) gel films could be formed on a support with pores below a critical diameter as shown in Table 8.1 [2,3]. 

Percy and coworkers [39,40] synthesized colloidal dispersions of polymer-silica nanocomposite particles by homopolymerizing 4-vinylpyridine or copolymerizing 4-vinylpyridine with either methyl methacrylate, styrene, n-butyl acrylate or n-butyl methacrylate in the presence of fine-particle silica sols using a free-radical in aqueous media at 60°C. No surfactants were used and a strong acid-based interaction was assumed to be a prerequisite for nanocomposite formation. The nanocomposite particles had comparatively narrow size distributions with mean particle diameters of 150-250 nm and silica contents between 8 and 54 wt.%. The colloidal dispersions were stable at solids contents above 20 wt.%. 

Finally, I should mention the formation of monodisperse colloidal C02B particles in the microemulsion Triton X-lOO-decanol-water. The diameter of C02B particles is shown in Table 14 as a function of micellar composition. 

In one example, colloidal Pd particles are obtained by the electrolysis of aqueous solutions of palladium chloride at pH 1 in a two-layer bath in the presence of a hydrocarbon solvent and epoxy dianic resin or PVA. Electrolysis results in flie formation of colloidal palladium organosols stabilized by the chemisorption of tiie polymer. Metal-lopolymers containing up to 90-95% of Pd remain after the removal of solvent and residual electrolyte. They are formed under high cathode polarization where concomitant elimination of hydrogen adsorbed on the nanoparticles (5.5-7.S nm in diameter) occurs. 

For most dilute silica sols around pH 2, where there is little ionic charge on the particle, no coagulation by electrolyte is observed, presumably because of the hydration layer. However, Harding (237) has called attention to the fact that relatively large colloidal silica particles 50-100 nm or more in diameter flocculate at low pH, whereas small particles do not. It remains to be determined whether the flocculation Is due to the van der Waals attractive energy between the particles or to the formation of multiple hydrogen bonds between the silanol-covered surfaces over the area of contact at collision. 

Colloid Formation. Several platinum colloids prepared by the alcohol reduction method are listed in Tables I (nonionic polymers) and II (cationic polyelectrolytes). Examples of particles diameters as determined by TEM are given in Table m. In all cases colloids were formed, and most were stable for several weeks, even months. The TEM investigations showed that the particles were in most cases evenly distributed and about 1 - 5 nm in diameter. Depending on the polymer, a range of particle sizes and narrow size distributions were obtained. 

If 3> > 0.001, formation of colloidal crystals is observed [94,96-101] in form of regular bcc or fee lattices with a lattice constant as large as several particle diameters. 

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