CONTIN ANALYSIS OF COLLOIDAL AGGREGATES

We make use of the development Of CONTIN, a constrained inverse Laplace transform routine, to monitor structures formed during the aggregation of colloidal particles. This approach, based on numerical inversion of the autocorrelation function measured in a dynamic light scattering experiment, provides a means to monitor both diffusion-limited (DLA) and reaction-limited (RLA) cluster aggregation processes. We combine the theory of light scattering with the Smoluchowski rate equation to model the intensity distribution of light scattered by colloidal aggregates. We track the cluster intensity distribution of Ludox HS and SM silica colloids as a function of time. A fit of the model prediction to the output from CONTIN provides fractal dimensions, d(f), agreeing with those from static light scattering measurements for both DLA and RLA. We observe an increase of d(f) with aggregation time in DLA, indicating a possible reorganization of clusters after they aggregate; this phenomenon does not appear in RLA. Early in the aggregation process, the polydispersity exponent tau for the power-law mass distribution for RLA clusters is 1.5. At a later stage of the RLA experiment, the clusters become large enough that rotational diffusion contributes significantly to the measured autocorrelation functions.