2ND VIRIAL-COEFFICIENT FOR CHARGED RODS - SODIUM XANTHAN IN AQUEOUS SODIUM-CHLORIDE

Second and third virial coefficients A2 and A3 and z-average radii of gyration for a narrow-distribution sample of sodium xanthan, a charged rodlike polysaccharide, with a molecular weight of 2.27 x 10(5) in aqueous sodium chloride were determined by light scattering at 25-degrees-C as functions of salt concentration C(s) in a range from 0.005 to 1 M. The radius of gyration was essentially independent of C(s) as expected for charged rods, while A2 and A3 increased markedly with decreasing C(s). It was found that this C(s) dependence of A2 is almost quantitatively described by the current theory for long charged rods over the entire C(s) range studied if use is made of the effective linear charge density calculated from the solution of the linearized Poisson-Boltzmann equation for a uniformly charged cylinder with the ion condensation hypothesis and if intermolecular attractive interactions are incorporated. The effective charge density obtained from the Poisson-Boltzmann equation led to a poor agreement with the present data.