The effect of aluminum hydroxide dissolution on the bleeding of aluminum lake dyes.

The effect of pH on the bleeding of FD&C yellow No. 5 aluminum lake and FD&C red No. 40 aluminum lake was investigated. The pH-bleeding profiles corresponded to the pH-solubility profile of aluminum hydroxide. The similarity of the bleeding profiles of both lake dyes and the pH-solubility profile of aluminum hydroxide indicates that pH related bleeding, other than that occurring by competition with anions, is a result of dissolution of the aluminum hydroxide substrate. This dissolution is related to the properties of the substrate rather than to the structure of adsorbed dye.

Role of water in the aging of aluminum hydroxide suspensions.

The rate of acid neutralization of anhydrous suspensions of amorphous aluminum hydroxide in methanol did not change during aging. At low water content, when all of the water was adsorbed, an initial decrease in the rate of acid neutralization was observed but no further change occurred during aging. When enough water was present to exceed the adsorptive capacity and produce water in the bulk methanol, the initial rate of acid neutralization decreased and a decrease in the rate of acid neutralization also occurred during aging. The decrease in rate of acid neutralization during aging is believed to be due to polymorphic transformation of the amorphous aluminum hydroxide and aggregation. The change caused by aggregation could be reversed by dehydration or sonication, but the change due to polymorphic transformation could not be reversed.

Effect of shear on the apparent viscosity of amorphous carbonate ion containing aluminum hydroxide suspensions.

The application of shear to carbonate ion containing aluminum hydroxide suspensions caused a change in the apparent viscosity by two possible mechanisms: change in the surface charge because of desorption of specifically adsorbed carbonate ion, and aggregate dispersal and formation of more extensive particle networks. The desorption of specifically adsorbed carbonate ion is related to the expansion of the air-liquid interface during shear. Shear-inducing processing equipment which generates a minimal amount of new air-liquid interface was found to produce the least change in pH and, consequently, in surface charge. However, viscosity increases caused by aggregate dispersal and formation of more extensive particle networks may occur without a shear-induced change in surface charge.

Mechanism of freeze-thaw instability of aluminum hydroxycarbonate and magnesium hydroxide gels.

The effect of freeze-thaw cycles on the physical stability of aluminum hydroxycarbonate and magnesium hydroxide gels was studied. Coagulation following a freeze-thaw cycle, leading to the formation of visible aggregates, affected the content uniformity of both gels. The freeze-thaw cycles did not affect the crystal form or surface characteristics of the gels as determined by X-ray powder diffraction and point of zero charge, but caused a slight reduction in the rate of acid neutralization and a large increase in the rate of sedimentation. The greatest effect was observed after the first freeze-thaw cycle. While the duration of freezing was not a factor, the rate of freezing was important and was inversely related to the aggregate size. The aggregates which formed following a freeze-thaw cycle were not redispersed by shaking, but were reversed by ultrasonic treatment or homogenization. The adsorption of polymers or surface-active agents prior to freezing reduced and, in some cases, prevented the formation of aggregates. The physical instability produced by a freeze-thaw cycle was explained by the modified DLVO theory. The force exerted on the particles by the growing ice crystals forced the particles into the primary minimum, producing strong interparticle attraction. On thawing, simple agitation did not provide enough force to overcome the attractive force of the primary minimum. Adsorption of polymers or surface-active agents increased the steric repulsive force and prevented the particles from reaching the primary minimum.