| The adsorption of the fluorescent dye Rhodamine 6G onto a model polystyrene latex was investigated as a potential tool for characterizing wet surfaces, in particular the density of surface charges and their distribution. R6G adsorption was found to be charge-driven in the dilute regime, and hydrophobically driven in the concentrated regime, with the crossover occurring near the neutralization of surface charge. For negatively charged particles, adsorbed dyes were almost completely quenched near the point of charge neutralization, where the dispersions were also unstable. At higher R6G loadings, the surface fluorescence was recovered and the dispersions restabilized.; The second part of this work investigated the relaxation and exchange characteristics of two water soluble polymers on silica. After homopolymers adsorb onto a solid surface from solution, they typically reconfigure, increasing the number of physical contacts, changing their footprint and entangling with neighboring molecules. While the initial adsorption is typically fast and transport limited, the subsequent relaxation process is often quite slow, on the order of several hours, and has been reported to be influenced by the polymer molecular weight. Our work demonstrates the influence of relaxation time, labeling, intermolecular entanglements, osmotic pressure, backbone stiffness and polydispersity on the dynamics of practical model systems consisting of hydroxyethyl cellulose and polyethylene oxide layers adsorbed on silica from aqueous solution. The results reveal that prolonged relaxation times and stiffer polymers result in strongly entangled, more perseverant layers. The stabilization process is requires a critical surface concentration to be reached, which is independent of molecular weight. For chains in excess of 100,000 g/mol adsorption energy and entanglement formation play an increasingly important role in layer stabilization. Polydisperse samples display molecular weight driven self exchange, increasing the fraction of larger adsorbed chains on the surface and thereby enhancing entanglement formation. Further analysis of the data suggests a dynamic equilibrium between the adsorbed layer and the surrounding solution with train formation at the expense of loops over time. Experimental results are compared with a simple reversible model for polymer adsorption and exchange, which successfully predicts the initial stages (first 50%) of the exchange dynamics. |
