Abstract--Photon correlation spectroscopy (PCS), a dynamic light-scattering technique for particle size measurement, was used to determine the coagulation rates of aqueous dispersions of relatively monodisperse South Carolina Peerless kaolinite, Silver Hill, Montana, illite, Wyoming montmorillonite, and Florida palygorskite. This technique allows quantitative measurement of the rate of coagulation for clay particles where the traditional turbidity method gives only a qualitative measure. The critical coagulation concentrations for KC1 at pH = 10.0 were: 0.199 M for kaolinite, 0.202 M for illite, 0.290 M for montmorillonite, and 0.034 M for palygorskite. The effective Hamaker constants, calculated using DerjaguinLandau-Verwey-Overbeek theory, were: 3.1 • 10 -20 J for kaolinite, 2.5 x 10 -2o J for illite, 2.2 x 10 -20 J for montmorillonite, and i.63 x 10 -19 J for palygorskite. Stern potentials at the critical coagulation concentration at pH 10.0 were: -42.7 mV for kaolinite, -40.7 mV for illite, -21.2 mV for montmorillonite, and -66.9 mV for palygorskite.
The time-dependent response to a shearing stress of a monolayer of dipalmitoylphosphatidylcholine (lecithin), which had been adsorbed on the surface of water, has been measured as a function of both pH and surface density using recently designed instrumentation. The substrate was buffered to the pH values of 4.5, 6, and 7.5 with a sodium phosphate/citric acid solution (Mcllvaine's buffer). At low surface density the monolayers are fluid. As the density increases the monolayers became viscoelastic; i.e., they begin to display an instantaneous or impact shear modulus, µ(°°), that decays with a characteristic time, r. Within the experimental uncertainty, the static modulus, g(0), vanishes, which implies that the films are not solidlike.
The self-beat intensity correlation spectra scattered from coagulating suspensions of illite platelets have been analysed to determine colloidal stability ratios. The stability ratio results as a function of KCl concentration were used to evaluate the effective Hamaker constant for the illite platelets. The effective Hamaker constant obtained by this method compares favourably with those obtained by direct measurement on crystallographically identical mica plates, either in KNO, solution or, after applying the geometric mixing rule, in a vacuum.
Colloidal processing has been shown to produce low defect and uniform ceramic microstructures from submicrometer ceramic powders. These concepts were applied to colloidal pressing to determine critical design relationships for uniaxial consolidation of dense and uniform green bodies from colloidal suspensions. Carefully controlled constant rate of strain consolidation experiments were carried out using alumina in water. The compression index decreased from 0.143 for a poorly dispersed alumina system to 0.077 for a welldispersed alumina suspension compression curve, indicating that the well-dispersed system is stiffer in consolidation. The compression curves showed that, as the degree of dispersion decreases, increased consolidation stresses are required to achieve a given particle packing density. The compression index increased with increasing strain rate for welldispersed alumina suspensions. Permeability through the sample ranged from 3 x lo-'' to 4 X iO-' cm/s, decreasing with decreasing void ratio during consolidation. Well-dispersed samples gave lower permeabilities than did poorly dispersed samples over a given consolidation increment. Coefficients of consolidation were nonconstant over the experimental effective stress range, invalidating the general solution to the linear consolidation equation. An approximate incremental solution was applied which indicated rapid pressing cycles are possible by starting with a suspension having a high solids concentration. Application of this consolidation data to nonlinear consolidation models is recommended for more exact prediction of consolidation time.
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