Competitive adsorption of poly(ethylene oxide) chains with and without charged end groups

Competitive adsorption between species of the same backbone chemistry and chain length, but differing in their end groups, has been studied employing the model system of fluorescein dianion terminated PEO (F-PEO) and the corresponding native PEO [poly(ethylene oxide)] onto negatively charged silica glass. Here, one end of each F-PEO chain is electrostatically repelled from the surface. Kinetic and equilibrium measurements in gentle shearing flow were accomplished with a combination of total internal reflectance fluorescence and optical reflectivity. Pure samples of F-PEO and native PEO exhibited identical transport-limited adsorption kinetics and high-affinity isotherms, but remarkable differences became apparent during competitive adsorption from mixtures of the two. The surface selectivity, which favored adsorption of native PEG, was screened as the ionic strength was increased. The selectivity was also reduced for chains of high molecular weight, though the molecular weight effect was not as intense as the influence of ionic strength. Coadsorption kinetics over a variety of bulk solution concentrations were in excellent agreement with predictions from a binary mixture treatment that involved Langmuir isotherms for each species and allowed the relative rates of adsorption and diffusion from solution to be varied. Comparison of theory and experiment revealed that the entire competitive adsorption process including the initial coadsorption and late-stage exchange was completely controlled by the diffusion rate of the chains to the surface.