Particle concentration hydrosols

The nature of stability of disperse systems with solid dispersed phase and liquid continuous phase against coagulation is determined by phase composition, particle size and particle concentration. The stability of hydrosols at low electrolyte concentrations is usually related to the electrostatic component of disjoining pressure (Chapter VII), arising from the overlapping diffuse parts of electrical double layers. 

This chapter has presented a theoretical derivation of continuous particle size distributions for a coagulating and settling hydrosol. The assumptions required in the analysis are not overly severe and appear to hold true in oceanic waters with low biological productivity and in digested sewage sludge. Further support of this approach is the prediction of increased particle concentration at oceanic thermoclines, as has been observed. This analysis has possible applications to particle dynamics in more complex systems namely, estuaries and water and waste-water treatment processes. Experimental verification of the predicted size distribution is required, and the dimensionless coeflBcients must be evaluated before the theory can be applied quantitatively. 

The preparation of stable, concentrated hydrosols containing nanometer particles is well known. An aqueous iron-oxidhydratsol (Graham sol) is a typical example. 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid ... 

Churaev, Nikologorodskaya, and co-workers (33) investigated the Brownian and electrophoretic motion of silica hydrosol particles in aqueous solutions of an electrolyte at different concentrations of poly(ethylene oxide) (PEO) in the disperse medium. The adsorption isotherms of PEO on the surface of silica particles were obtained. The thickness of the adsorption layers of PEO was determined as a function of the electrolyte concentration and the pH of the dispersed medium. The results can be used in an analysis of the flocculation and stabilization conditions for colloidal dispersions of silica (with non-ionogenic water-soluble polymers of the PEO type). 

TEM images of particles deposited from the standard sol yielded an (number) average diameter of 1.5 nm (mass-weighted mean 2.2 nm), and a coefficient of variation (o/) of 38 %, but some cluster coalescence within the spedmen was observed. This caused bands of much coarser particles (<10 nm) at locally high concentrations to be present in the specimen. Only the well separated clusters were measured for the distribution presented here. The addition of the colloid-stabilising agent gelatin to the hydrosol prevented the formation of these bands of presumably... 

The view that the structure of gel made from dilute hydrosol is better described as fibrillar matter rather than as a series of independent particles was expresed by Scott, Hockey, and Barby (120). The fact that the sol gelled at a concentration of only 1-3% SiOj and was made at pH 3-7 by deionizing sodium silicate indicates that the particles were less than 3 nm in size and that these must have chained together into rather long chain segments between branch points. Evidently such particles formed chains and coalesced into rods or fibers until they essentially lost their particulate identity. 

---