Use of properties of water to characterize the surface area of aqueous suspensions of pH-dependent solids

The particle morphology of amorphous carbonate-containing aluminum hydroxide was studied by low dose transmission electron microscopy. Primary particles, 25 to 50 A in diameter, were observed and found to be aggregated into larger secondary particles. Particle interactions in amorphous carbonate-containing aluminum hydroxide suspensions were studied by viscometry, fiber optic Doppler anemometry and freeze fracture/freeze etch scanning electron microscopy. The particle interactions were pH-dependent; however, they were not symmetrical in relation to the point of zero charge (PZC). Particle networks were present further from the PZC when the pH was above the PZC. It was hypothesized that negatively charged edge groups, which exist when the pH is above the PZC, interact with the hydroxyls on the faces of the sheet-like particles more strongly than do uncharged or positive edge groups, thus producing the observed asymmetrical behavior of pH-dependent properties. The equilibrium water retention, m$\sb{\rm w}$/m$\sb{\rm s},$ of suspensions having a pH-dependent surface charge appears to be due to water adsorbed by the surface as well as retained in pores. Due to increased pore water, maximal water retention occurred when the pH was equal to the PZC and attractive particle interactions predominated. When conditions were selected to favor closest packing of the particles, m$\sb{\rm w}$/m$\sb{\rm s}$ was observed to be linearily related to the BET surface area of fumed silica. The apparent specific isobaric expansibility of water in mixtures of water and freeze-dried amorphous carbonate-containing aluminum hydroxide powders was measured and found more expansible than pure water. The ratio of the measured value of this property in solid-water mixtures to its value in pure water was found to be exponentially related to the mass ratio of solid to water. Also, the surface charge characteristics of the solid did not appear to significantly affect this relationship. Therefore, a surface area of 500 to 600 m$\sp2$g$\sp{-1}$ was calculated for the samples in a manner reported previously. The results indicate that surface effects on properties of water can be used to characterize the surface area of pH-dependent solids