Anomalous counterion condensation in salt-free hydrophobic polyelectrolyte solutions: Osmotic pressure measurements

The effective charge fraction f eff of a highly charged hydrophobic polyelectrolyte (poly(styrene)-co-styrene sulphonate, sodium salt) has been measured as a function of the bare chemical charge fraction f between 0.38 and 1 by osmotic-pressure measurements complemented by cryoscopy. It was found that f eff is highly reduced, compared to the prediction of counterion condensation theory and the measurements we made on a model hydrophilic polyelectrolyte. Furtheremore, f eff varies linearly with the chemical charge fraction and extrapolates to zero effective charge at a chemical charge fraction of 0.18, a value close to the limit of solubility of the polyelectrolyte in water. We relate this anomalous behaviour to the variations of the local dielectric constant, measured previously with the same polyelectrolyte, a parameter which is not considered in the structural models for hydrophobic polyelectrolytes.