Fluorescence studies of polymer adsorption. 4. pH effect on the adsorption of pyrene-end-tagged poly(ethylene glycol) on colloidal silica

Abstract

The effect of pH on the adsorption of pyrene-end-labeled poly(ethy1ene glycol) (Py-PEG-Py) and its subsequent rearrangement and displacement upon adsorption of untagged PEG were investigated with photostationary excimer fluorescence. A slight pH effect was detected for Py-PEG-Py of molecular weight 8650 (Py-PEG-Py(8650)) upon adsorption of PEG(22 OOO). The fluorescence results support the picture that the probe chains adsorb more strongly to silica at pH 4.2 than at pH 7.1. No pH effect for Py-PEG-Py(4250) was detected, presumably due to the dominance of the entropic effect on exchange with untagged PEG(22 OOO) over the enthalpic interaction between the tracer chain and the surface. Comparison with predictions from a model taking into account the rearrangement of a tracer chain on the surface shows qualitative agreement for both Py-PEG-Py(4250) and Py-PEG-Py(8650).