Behaviour of polyelectrolytes in solutions of organic solvents: scattering properties and counterion condensation

We study the scattering, thermodynamic and charge transport properties of anionic and cationic polyelectrolytes in organic solvents over a wide range of dielectric constants (epsilon_r ≈ 8-180). The scattering data show a transition from extended conformations at high dielectric constants, to collapsed conformations for solvents with low permittivities (epsilon_ r<≈ 25).  This transition arises as the balance of forces switches between electrostatic repulsion between like-charged groups along the backbone in high dielectric media to attractive forces between condensed counterions in low dielectric media.

Conductivity and osmotic pressure measurements are used to evaluate the fraction of dissociated counterions in semidilute solutions in aqueous and organic media. The effects of counterion valence, size and solvophilicity are systematically studied. The data show that, for a fixed valence, the counterion type has a weak or null influence on the fraction of dissociated counterions. As predicted by the Oosawa-Manning theory, the fraction of free counterions decreases as the Bjerrum length of the solvent media increases. The charge fraction is found to be somewhat lower than the Manning condensation limit.