Absolute Aggregation Rate Constants of Hematite Particles in Aqueous Suspensions: A Comparison of Two Different Surface Morphologies
✍ Scribed by M. Schudel; S.H. Behrens; H. Holthoff; R. Kretzschmar; M. Borkovec
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 758 KB
- Volume
- 196
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
one often remains satisfied with the more easily accessible Measurements of absolute aggregation rate constants were stability ratio (the ratio of the rate constant in the fast aggreperformed for two samples of well-characterized spheroidal hegation regime to the rate constant under the experimental matite particles with rather different surface morphology. At conditions of interest).
high ionic strength, the system is in the fast aggregation regime
The quantitative interpretation of the aggregation rate conwith pH independent rate constants around ( 1 -4 ) 1 10 018 stants in such systems continues to be a problem as well; m 3 / s. At low ionic strengths, the rate constant is a function of the classical theory due to Derjaguin, Verwey, Landau, and pH and goes through a flat maximum around the point of zero Overbeek (DLVO) usually underestimates the aggregation charge ( PZC ) where fast aggregation conditions are reached.
rate constants in the slow regime by many orders of magni-With increasing pH the rate constants increase gradually below the PZC while they decrease very rapidly above the PZC. Above tude (13). The origin of these discrepancies is still not enthe PZC the rate constants are rather well predicted by the tirely clear, but the most likely explanation for the observed classical DLVO theory. Below the PZC, however, it is necessary deviations must be sought in surface heterogeneities or partito assume a distribution in the surface potentials with a coefficle roughness (13,14). Such heterogeneity effects on the cient of variation around 30% to account for the observed behavaggregation rate constants are rather difficult to assess theoior. In spite of the pronounced differences in the surface retically (15-17). In a simplified fashion, their effect can roughness of both hematite samples, their aggregation rate conbe approximated by calculating the initial rate of heterocoagstants are rather similar.