| We characterize the interactions of polymers and colloidal particles by studying the particle aggregation behavior in the presence of adsorbing polymers. Our approach is to start with a system containing particles only, and then proceed to a polymer-mediated particle suspension system.; In the bare-particle system, we make use of the development of CONTIN, a constrained inverse Laplace Transform routine, and dynamic light scattering (DLS) to monitor structures formed for both diffusion-limited (DLA) and reaction-limited (RLA) aggregation processes. We combine the light scattering theory with the Smoluchowski rate equation to model the intensity distribution of light scattered by clusters. A fit of the model prediction to the CONTIN output provides fractal dimensions, d{dollar}sb{lcub}rm f{rcub}{dollar}. We find that d{dollar}sb{lcub}rm f{rcub}{dollar} increases with aggregation time in DLA, suggesting reorganization of particles after they aggregate; this phenomenon does not appear in RLA. Early in the aggregation process, the polydispersity exponent {dollar}tau{dollar} for the power-law mass distribution for RLA clusters is 1.5.; In the flocculation of colloidal silica by pyrene end-tagged poly(ethylene glycol) (Py-PEG-Py), we use both DLS and fluorescence spectroscopy techniques to measure aggregate sizes and pyrene-pyrene interactions, respectively. DLS probes length scales from a few nanometers to several tenths of a micron, enabling us to study the overall aggregate dimensions. By contrast, fluorescence is sensitive to changes on a molecular level, allowing us to investigate the mechanism of polymer-induced aggregation. Our results demonstrate that DLS and fluorescence techniques provide an ideal combination to study particle/polymer interactions in solution.; The average aggregate size, {dollar}langle Rrangle{dollar}, grows with ionic strength, due to the anticipated charge screening effect. In addition, {dollar}langle Rrangle{dollar} increases with Py-PEG-Py concentration, indicating that the hydrophobic interaction enhances flocculation. Fluorescence excitation spectra provide evidence of intensified pyrene-pyrene interactions as the degree of flocculation increases. The emission spectrum, analyzed in terms of the excimer-to-monomer ratio {dollar}rm (Isb{lcub}e{rcub}/Isb{lcub}m{rcub}),{dollar} exhibits a consistent increase with ionic strength, implying enhanced excimer formation as a result of increasing polymer layer overlap. We compare our emission data with a qualitative model based on the chain segment density distribution developed by Ploehn and Russel. |
