Reentrant Variation of Single-Chain Elasticity of Polyelectrolyte Induced by Monovalent Salt

The interactions between monovalent counterions and polyelectrolyte are important in chemical and biological systems. The condensation and screening effect of counterions complicate the polyelectrolyte solutions. By means of single-molecule AFM, the single-chain mechanics of a strong polyelectrolyte, poly(sodium styrenesulfonate) (PSSNa), in KCl aqueous solutions over almost whole concentration range have been studied. The M-FJC model has been used to describe the single-chain elasticity of PSSNa in KCl solutions with a parameter of single-chain modulus (K₀). Along with the increase of the concentration of KCl from zero to almost the saturation concentration, a reentrant variation of K₀ of single PSSNa chain can be observed. When [K⁺] is between 0.01 to 3 M, the charges on the PSSNa backbone are almost completely screened, i.e., the PSSNa chain is virtually neutral in this case. Because K₀ has a positive correlation with the net charge of the polymer chain, the increased K₀ at very high KCl concentrations (≥3.5 M) indicates that the chain is charged again. Due to the negative charges on the backbone of PSSNa, only the positively charged counterions (K⁺) can be adsorbed on the chain. Thus, the PSSNa chain should be positively charged when KCl concentrations ≥3.5 M. That is, the charge inversion occurs in this case, which is induced by a monovalent salt. This finding may lay the foundation for the future applications of drug delivery and gene therapy.