Counterion Cloud Expansion of a Polyelectrolyte by Dilution
Kaikai Zheng, Kuo Chen, Weibin Ren, Jingfa Yang, Jiang Zhao*
Macromolecules 2018, 51, 4444.
It has long been documented that the reduced viscosity of polyelectrolyte has an anomalous dependence on its concentration, i.e. the Fuoss Law. To explore the molecular mechanism, the counterion distribution of sodium polystyrene sulfonate (NaPSS) as a function of concentration is investigated at single molecular level. By examination of the fluorescence resonance energy transfer (FRET) between a fluorescence donor on NaPSS chain and an acceptor in the counterions using single-molecule fluorescence spectroscopy, an increase of average counterion−chain distance is discovered upon dilution, indicating the expansion of counterion cloud. By photon counting histogram, an increase of effective charge of the NaPSS chain during dilution is exposed. The variation of these parameters agrees well with that of the reduced viscosity, helping to shed light on the molecular mechanism of the Fuoss law: the expansion of the counterion cloud increases hydrodynamic friction, and the increase of effective charges of NaPSS due to desorption of counterions brings about the stronger interchain coupling.