Osmotic coefficient of a synthetic rodlike polyelectrolyte in salt-free solution as a test of the Poisson-Boltzmann cell model

A comparison of the osmotic coefficient of a synthetic stiff-chain polyelec trolyte with the prediction of the Poisson-Boltzmann (PB) cell model is pre sented. The polyelectrolyte consists of a poly(p-phenylene) backbone with a ttached cationic groups (charge parameter 3.4) and either chloride or iodin e counterions. The osmotic coefficient phi was deduced from the osmotic pre ssure measured from dilute solutions (0.05-3.6 g/L) without added salt. The data demonstrate that the PB cell model captures the essential features of the experimental results without adjustable parameters. It fails to give a quantitative description, however, because the experimental phi is signifi cantly smaller than the value predicted by the PB cell model. Possible reas ons for this discrepancy as the neglect of the ion-ion correlations or spec ific interactions between the macroion and the counterions in the PB cell m odel are discussed.