AGGREGATION FRAGMENTATION OF COLLOIDAL ALUMINA .2. SCALING LAWS OF FRAGMENTATION

This paper presents scaling laws for the fragmentation of colloidal aluminum oxide-polyacrylic acid complexes in an aqueous medium containing 10(-3) N KCl and 3 X 10(-4) N AlCl3 at pH 5.0. Since no polymer was detected in the supernatant liquid phase during aggregation/fragmentation, the evolution of the stability of the suspension could be attributed to reconformation and ion redistribution within the adsorbed polymer layer. Using the linear fragmentation model of Cheng and Redner (Phys. Rev. Lett. 60, 2450, 1988; J. Phys. A Math Gen. 23, 1233, 1990) to describe the mechanism of aggregate dispersion, the rate of polyelectrolyte-induced fragmentation of the oxide grain agglomerates was determined from the decrease in the weight S(t) and number N(t) average sizes of the aggregates. A particle counter technique was employed to obtain the size frequency curve c(n, t) of aggregates of oxide grains during the different phases of aggregation/fragmentation. Self-similarity of the fragment size distribution was verified and scaling laws were derived to describe the temporal variation of the average sizes. Deviation from the irreversible fragmentation model of Cheng and Redner could be attributed to the concomitant existence of coagulation and fragmentation, As the relative importance of these two processes was found to vary with time, the exponent z of the power law governing the decrease of S(t) with time over the last domain of enhanced fragmentation could be correlated to the exponent v of the rate of formation of small fragments. (C) 1995 Academic Press, Inc.