Shear-induced Counterion Redistribution of a Single Polyelectrolyte

The counterion distribution of a model polyelectrolyte, sodium polystyrene sulfonate (NaPSS), as a function of shear rate is investigated at single molecular level. The fluorescence resonance energy transfer (FRET) between the fluorescence donor attached at the charged PSS^- chain and the positively charged acceptor as the counterion probes shows the increase of average counterion-chain distance, indicating the expansion of counterion cloud by shear. Such a process is further verified by the emission spectra of the pH-responsive fluorophore labeled at PSS^- chain end, showing the local pH value is shifted to higher values by shear. The shear-induced counterion release or counterion cloud expansion is attributed to alternation of the electric filed distribution inside the solution and the overlap of electric fields of multiple PSS^- chains in the process of creating inhomogeneity in concentration inside the PSS^- solution by shear.