Kinetics of competitive adsorption of PEO chains with different molecular weights

Molecular weight driven competitive adsorption of PEO [poly(ethylene oxide)] from aqueous solutions on silica was studied by employing total internal reflectance fluorescence (TIRF) and near-Brewster angle reflectivity. Coumarin tags on PEO chain ends facilitated the distinction between two competing populations differing only in molecular weight. In gentle shearing flow, coadsorption was found to include three steps: independent transport-limited adsorption of both species, exchange of short and long chains, and the ultimate equilibration of the long chains. When the short chains are on the order of 30K and contact the surface for 20 min or less, their displacement by longer chains is transport-limited and approaches completion. However, for short chains of higher MW (120K), their displacement by longer chains becomes slow and less complete, indicating an energy barrier for short chain displacement. A kinetic model for competitive adsorption kinetics was proposed on the basis of the Langmuir adsorption kinetics and allowed quantification of surface limited kinetics.