Shear-Induced Counterion Release of a Polyelectrolyte

The shear-induced changes in counterion distribution of a model polyelectrolyte, sodium polystyrene sulfonate (NaPSS), is investigated at single molecular level. Fluorescence resonance energy transfer (FRET) between the fluorescence donor attached at the PSS⁻ chain end and the cationic acceptor serving as the counterion probes in solution shows the increase of average counterion-chain distance. The emission spectra of the pH-responsive fluorophore labeled at PSS⁻ chain end show the dilution of hydronium ions near the chain by shear. The results indicate that the shear applied to the NaPSS solution makes the counterions released from the PSS⁻ chain. The mechanism of counterions' redistribution is attributed to the shear-induced transient overlap of electric fields of multiple PSS⁻ chains. The results indicate that the effective charge of a polyelectrolyte molecule increases with shear, in contrast to the conventional assumption of the invariable charges.