High-salt stabilization of Laponite clay particles.

Colloidal radioactive transuranic wastes are currently buried in large tanks in the form of dense colloids in high salt, high pH aqueous media. These facilities are beginning to fail, so it is necessary to transport and "package" them for more permanent disposal, processes requiring understanding of the microstructure that develops under such conditions. Laponite RD clay is believed to be a good simulant for the colloids in the waste tanks, and the present study addresses their behavior under high salt conditions, where previous studies have frequently observed the phenomenon of "restabilization," i.e., the attainment of aggregation stability at high electrolyte conditions. Specifically, the aggregation kinetics and the resulting cluster structure (fractal dimension) of Laponite RD clay colloids at high concentrations of BaCl2 (an electrolyte previously shown to lead to restabilization) are investigated. At low-to-intermediate electrolyte concentrations, the clay is found to behave in accord with DLVO theory, i.e., low salt conditions yield slow aggregation into densely-packed aggregates, whereas intermediate salt concentrations, sufficient to cause double layer collapse, produce rapid aggregation into open aggregates. High salt concentrations, however, show slow rates of aggregation. The aggregate structure under these conditions is found to mimic that found for very low electrolyte concentrations, i.e., high fractal dimension. Further experiments show that a sudden increase in salt concentration in a system containing young open aggregates produced under intermediate salt concentrations causes them to reform into more compact structures.

Aggregate restructuring by polymer solvency effects.

This study investigates the aggregation in cyclohexane of silica particles initially stabilized by grafted polystyrene and destabilized by temperature reduction. It complements an earlier study by Zhu and Napper (P.W. Zhu, D.H. Napper, Phys. Rev. E 50 (1994) 1360) in which the aggregation of polystyrene latex particles with tethered poly(N-isopropyl acrylamide) (PNIPAM) in water was investigated. Their dynamic light scattering results showed that both the rate of aggregation and the aggregate fractal dimension increased with a sufficient decrease in the PNIPAM adlayer solvency, achieved by means of either salt (NaNO3) addition or temperature rise. This result stands in contrast to those obtained when an electrostatically stabilized colloid is destabilized, i.e., that the more rapidly aggregates are formed, the lower the resulting fractal dimension. The authors explained their results in terms of the effects of both salt effects and increased temperature on the extent of the hydrophobic interactions between the adlayer-covered surfaces in the water. The present study examines a sterically-stabilized colloid in a nonaqueous solvent, where neither salt effects nor hydrophobic effects play a role. Temperature is decreased to bring the system from better-than-theta-conditions to worse-than-theta-conditions. Power-law aggregation kinetics are observed at 15.7 degrees C by dynamic light scattering. The particles first undergo reduced rate aggregation, producing low-fractal-dimension aggregates, which after some time, restructure into more compact aged clusters. The fractal dimension of these aged clusters increases with increasing initial aggregation rate, consistent with results seen by Zhu and Napper, but without the presence of hydrophobic effects. The ability of the polymer-grafted particles to rearrange suggests aggregation into a secondary minimum, with the ability to slide over one another to achieve a more energetically favorable, denser configuration. The reversible nature of the aggregation is verified by additional experiments gradually bringing the system from worse-than-theta-conditions back to better-than-theta-conditions, with an attendant decrease in aggregate fractal dimension, and ultimately full redispersion.

Transient aggregation during dry-down of solvent-borne dispersions.

The present work investigates the role of changing solvency conditions on the stability of sterically stabilized colloids in evaporating solvent-borne dispersions and the recovery time for the redispersion of aggregates if destabilization occurs. Process conditions during the conversion of the coated fluid dispersion to a solid film must be carefully controlled to ensure that aggregation, leading to uneven pigment distribution, does not occur. Although a polymeric binder serves as a retention aid in a solvent-borne coating, it also act as a steric stabilizer during dry-down or curing of the coating. Because the dispersion medium in the coatings often is a mixture of solvents and nonsolvents, the binder's interactions with the dispersion medium govern whether aggregation is likely to occur. Tracking such an occurrence may be difficult due to rapidly changing solvent compositions. Light scattering is utilized herein to indicate if the conditions of a high speed coating and drying process (in this case, the production of magnetic data storage media) may lead to aggregation and, if aggregation is detected, the ability of aggregates to fully redisperse. Conformational changes of free polymeric binder chains in solution are followed to gauge the polymer-solvent interactions and, therefore, the binder's ability to prevent aggregation. A "danger zone" (conditions where aggregation is probable) is constructed from these measurements, and further dynamic light scattering measurements on binder-grafted-oxide particles detect aggregation in this zone. The aggregates are redispersed in a series of "good" solvents, and redispersion is found not to be instantaneous, suggesting that high-speed drying processes may not allow the flocculated colloids enough recovery time.