Divalent counterion effect on the electrostatic persistence length of polyelectrolyte chains: An electric birefringence study

We prepared salt-free polystyrenesulfonate (PSS) solutions with various rat ios alpha of univalent and divalent counterions, i.e., X0.5 alphaNa1-alphaP SS (X=Mg,Ca), and investigated them in the dilute region by means of the el ectrostatic birefringence spectroscopy that detects the rotational relaxati ons of polyelectrolyte chains. Thc obtained relaxation spectra show that, w ith replacing univalent counterions by divalent ones, the relaxation streng ths decrease and the relaxation times gradually shift to higher values, Fro m the rotational relaxation times, the electrostatic persistence length L-p for 0 less than or equal to alpha less than or equal to 1 is also evaluate d using the worm-like chain model, which reveals that L-p with divalent cou nterions (i.e., alpha = 1) becomes approximately half of that with univalen t ones (alpha = 0). The result suggests that the divalent counterion effect on conformations of intrinsically flexible polyelectrolyte is weaker than that expected from theoretical considerations.