Variable aggregation rates in colloidal gold: Kernel homogeneity dependence on aggregant concentration

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Dynamic light scattering is used to study the dependence of the aggregation kernel homogeneity {λ} on the aggregant concentration [HCl] for aqueous gold sols. We find the cluster growth kinetics are well described by a powerlaw, R_app~tz/D, where Rₐₚₚ is the measured apparent radius, D the cluster fractal dimension, and z=1/(1-{λ}) for all aggregant concentrations. The values for the dynamic exponent z, and hence the homogeneity {λ}, are functions of HCl concentration. We find the larger HCl concentrations yield a fast-aggregation regime characterized by {λ}{}-0.6. Smaller HCl concentrations yield a continuum of aggregation regimes characterized by homogeneities evolving from {λ}{}-0.6 towards 1.0. Our results do not support the view that aggregation in gold colloids is based on two limiting regimes, diffusion-limited and reaction-limited aggregation.