Xanthan Gum—A Lyotropic, Liquid Crystalline Polymer and its Properties as a Suspending Agent

Abstract A variety of polysaccharides have been used in drilling fluids,1–4 and some of these polymers have been studied in tertiary recovery. Of particular interest in both these areas has been xanthan gum, the extracellular polysaccharide produced by Xanthomonas campestris, because of its relatively stable viscosity properties as a function of salt concentration, pH, temperature, and shear degradation. Indeed, no synthetic polymer or any other commercially available, naturally occurring polymer, or polymer derivative has been able to duplicate the solution behavior of xanthan gum. During a study of xanthan gum's ability to suspend barium sulfate, it appeared that xanthan gum in aqueous solution at relatively low concentrations could have lyotropic liquid crystalline behavior. Behavior of this type has been noted for other biopolymers5 and recently has been demonstrated for various cellulose derivatives.6–10 An anisotropically ordered polymer solution would be extremely difficult to reproduce synthetically but could explain most of the unique properties of xanthan solutions. To test the possibility that xanthan gum could have liquid crystalline order, a variety of xanthan solutions of various polymer concentrations in the presence and absence of various salts were studied under a polarized light microscope (100X). With solutions from 2 to 10% (wt/vol) xanthan gum in distilled water at room temperature, birefringent, ordered domains were observed at 10% concentration, with a decrease in birefringence as the polymer concentration decreased. At 2% concentration, no birefringence was observed, indicating the apparent area of change from an isotropic to an anisotropic phase. This point appears to occur at 2.9% by optical determination. In fact, this value could be considerably lower as revealed by other techniques for anisotropic xanthan solutions.8–10 For the 10% solution of xanthan gum, heating in a roller oven at 150°F (66°C) for 2 hours did not diminish the birefringence. Similarly, when a 10% xanthan gum solution was prepared in 10% KCI and in 33% CaCl2 solutions, birefringence was not decreased.