Correlated Diffusion of Single Polyelectrolyte Molecules under Counterion’s Specificity

Counterions, as companions of polyelectrolytes, significantly determine the effective charges of the main chain, which is the central factor of the properties. In this study, the motion correlation between individual polyelectrolyte chains under different counterions is investigated by using dual-color fluorescence cross-correlation spectroscopy (DC-FCCS). Sodium polystyrenesulfonate (NaPSS) was chosen as the model polyelectrolyte, and it was separately labeled with fluorescent molecules of different colors. By discriminating the fluorescence emissions from these two fluorophores, the DC-FCCS makes measurements with molecular recognition. Six types of counterions (Na+, Cs+, Ca2+, Mg2+, Ba2+, and La3+) were selected to examine the effect of counterion’s specificity on the diffusion correlation of individual PSS– chains. Ions with higher valencies or ions of the same valency but with lower hydration capacity are discovered to bind more strongly and exert a bigger neutralization effect on the original charges of the main chain. Intra- and interchain aggregations are exposed with the presence of Ba2+ and La3+ ions. Both Ba2+ and La3+ induced precipitation at remarkably high salt-to-monomer ratios, but reentrant condensation is observed only for La3+, even under extreme dilution conditions. The results have shown that the profound effect of counterion type on their binding strength to the polyelectrolytes affects their mutual correlation of the diffusive motions. The results also demonstrated the promising capacity of DC-FCCS to access the details of the diffusive motions of interacting molecules.