Aggregation in colloidal gold

Static and dynamic light scattering are used to study the aggregation kinetics and structure of colloidal gold. Fast aggregation is achieved by either addition of pyridine, lowering of the pH, or addition of salt. The kinetics of rapid growth is followed by measurement of the polarized relaxation time ${\tau }_{\mathrm{vv}}$ (proportional to the mean cluster radius). We observe a power law, ${\tau }_{\mathrm{vv}}$∝(Ct${)}^x$, with x=0.38±0.04, where t is the time and C is the initial sol concentration. Measurements of the depolarized relaxation time ${\tau }_{\mathrm{vh}}$ as Q→0 (Q is the wave vector) are found to be proportional to the third power of the cluster radius, typical of rotational diffusion. Such measurements allow independent confirmation of the kinetics as ${\tau }_{\mathrm{vh}}$=(Ct${)}^{3x}$, with x=0.38±0.04. The static light scattering intensity I from these rapidly grown clusters obeys a power law, I∝$Q^{\mathrm{-}y}$ with y=1.74±0.1, at a wavelength of λ=633 nm (optical resonance is 680 nm), with the value of y increasing away from resonance to a value of 2.0±0.1 at λ=457 nm. Under diluted conditions the scattering behavior is identical on and off resonance with y=2.0±0.1. These results are inconsistent with the diffusion-limited cluster aggregation model which predicts y≊1.75 and x≊0.56. Finally, it is shown that slow pyridine-induced aggregation of diluted gold sols results in linear growth of the cluster radius with time, but the resulting aggregates still obey I∝$Q^{\mathrm{-}y}$, with y=1.8±0.1. Simple models of reaction-limited aggregation cannot account for the latter observations.