SOLUTION STUDIES OF XANTHAN GUM EMPLOYING QUASI-ELASTIC LIGHT-SCATTERING

We have used quasielastic light scattering, circular dichroism spectroscopy, and viscosity measurements to investigate the thermal transition reported to occur for xanthan in aqueous solution at low ionic strength. The light-scattering data indicate that the diffusion coefficient of the macromolecule significantly increases with increasing temperature in the vicinity of the transition temperature. In addition there is an increase in macromolecular polydispersity near to the transition temperature. These observations suggest that the transition involves the breakdown of a multimolecular aggregate. Translational diffusion coefficients estimated from the light-scattering data for xanthan in low ionic strength solutions show different behavior in two concentration ranges, corresponding to dilute and entangled solutions. For the dilute solution range, extrapolation to zero concentration yields a z-average translational diffusion coefficient DT° = 1.66 × 10-8 cm2/s at 25 °C which corresponds to a z-average hydrodynamic radius of 146 nm obtained using the Stokes-Einstein equation. The variation of the diffusion coefficient with concentration in the region where chain entanglements are expected does not agree with recent theoretical predictions on solution behavior under these conditions. An explanation of this discrepancy is proposed based on the high chain stiffness inherent in the xanthan molecule. © 1979, American Chemical Society. All rights reserved.