Abstract
Quasi-elastic light-scattering studies of the polysaccharide xanthan in aqueous solution reveal the existence of two different structural forms characterized by different values of the translational diffusion coefficient D ▪. Xanthan dissolved in deionized water yields a value of D ▪ 2.40 × 10 -8 cm 2/s, provided that the solution is centrifuged or filtered immediately prior to data collection. However, if this solution is allowed to stand for one week, D ▪ decreases to 1.10 × 10 -8 cm 2/s. In contrast, D ▪ 2.75 × 10 -8 cm 2/s for solutions of xanthan dissolved in 4 m urea, and this value does not change with time. These results suggest that xanthan undergoes a self-association in aqueous solution and that this association is inhibited in 4 m urea solution, which points to the role of hydrogen bonding in the process. Insertion of our measured values of D ▪ and the zero-shear intrinsic viscosity into the Flory-Mandelkern equation yields an estimated molecular weight of M 2.16 × 10 6 for xanthan in 4 m urea. The aggregation observed in aqueous solutions probably explains the wide variation in molecular weights reported previously for xanthan.
Published Version
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