Molecular viscoelasticity of xanthan polysaccharide

Abstract

Previous studies of xanthan polysaccharide using molecular probes of local chain geometry such as optical rotation and n.m.r. have demonstrated a cooperative disorder–order transition in aqueous solution on cooling or on addition of salt. In the present work we have investigated chain geometry of the ordered species at a ‘macromolecular level’ using quasi-elastic light scattering, transient electric birefringence and elongational flow birefringence, and have used viscoelastic measurements to probe the ‘supramolecular’ organisation responsible for the ‘weak-gel’ properties of xanthan solutions.

Solution viscoelasticity at fixed xanthan concentration was drastically modified by changing the counterion to the polyelectrolyte and by treatment with urea, although in all cases the local ordered structure was unaffected. ‘Macromolecular’ studies under conditions which minimise intermolecular interactions (Na⁺ salt form in the presence of urea) and at substantially lower concentrations indicate a persistence length comparable to that of other highly persistent biopolymers, such as double helical DNA and the triple helical polysaccharide schizophyllan.

We conclude that in aqueous solution xanthan may be regarded as a highly extended worm-like chain interacting by non-covalent association to develop a weak-gel network, which is readily reversible under shear.