Abstract
Diluteโsolution hydrodynamic data for xanthan biopolymer in water suggest a rodlike molecule of dimensions 15,000 ร 20 ร
, and molecular weight 2.2 ร 10^6^ g/mol. Upon addition of NaCl to this system, the xanthan molecules selfโassociate to form stable aggregates. The native xanthan conformation can be thermally denatured to a disordered coil which can be stabilized at room temperature in 4__M__ urea. The transition to semidilute solutions is manifested by discrete changes in the concentration dependence of diffusion coefficient and zeroโshear viscosity at c โ 2.0 ร 10^โ4^ g/mL. At higher concentrations c โฅ 1.0 ร 10^โ3^ g/mL, the lightโscattering and shearโviscosity data are qualitatively but not quantitatively consistent with predictions of the dynamical theory of Doi and Edwards for an isotropic entangled solution of rigidโrod molecules. Measurements of latex sphere diffusion in xanthanโwater solution show a sudden retardation at c โ 1.0 ร 10^โ3^ g/mL, consistent with the cooperative formation of a motionally restricted network of long, thin, rigid fibers. At high shear rates, flow birefringence experiments indicate enhanced ordering of the xanthan chains in the semidilute regime.