Sdt. Axford et Tm. Herrington, DETERMINATION OF AGGREGATE STRUCTURES BY COMBINED LIGHT-SCATTERING AND RHEOLOGICAL STUDIES, Journal of the Chemical Society. Faraday transactions, 90(14), 1994, pp. 2085-2093
Citations number
36
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The structures of aggregates of a clay, sodium bentonite, formed durin
g the rapid (diffusion-limited) aggregation of a destabilised colloida
l suspension have been studied using a combination of three experiment
al techniques. The first two of these were static and dynamic (quasi-e
lastic) light-scattering measurements, while the third involved rheolo
gical studies. The system was studied as a function of the pH of the a
ggregating suspension, between pH 2.3 and 10.2. Analysis of the light-
scattering results leads directly to a determination of the fractal di
mension, d(f), of the aggregates formed. The value of d(f) for the agg
regates showed a rapid transition from 3.0 below pH 4.3, to 1.8 above
pH 4.3. This implied a close-packed structure for aggregates formed un
der highly acidic conditions, but a more open one in less acidic and i
n slightly alkaline suspensions. Rheological measurements showed minim
a in the Bingham yield stresses for both aggregated and unaggregated b
entonite suspensions, at the cross-over point, pH 4.3, between the two
structures. In addition, the ratio of the storage modulus to the loss
modulus took only two values: a higher one below pH 4.3, and a lower
one above pH 4.3. It was concluded that hetero-flocculation is induced
in the aggregation of sodium bentonite below a certain acidity, resul
ting in aggregates which have a card-house, as opposed to band-like, s
tructure. Furthermore, these open aggregates showed more viscous, and
less elastic, behaviour than those with a higher fractal dimension.