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Introduction

A variety of polysaccharides have been used in drilling fluids, 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 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 polymer derivative has been able to duplicate the solution behavior of xanthan gum.

Behavior of Xanthan Gum in Solution

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 biopolymers and recently has been demonstrated for various cellulose derivatives. 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 19% by optical determination. In fact, this value could be considerably lower as revealed by other techniques for anisotropic xanthan solutions. For the 10% solution of xanthan gum, heating in a roller oven at 150 deg. F (66 deg. C) for 2 hours did not diminish the birefringence. Similarly, when a 10% xanthan gum solution was prepared in 10% KCl and in 33% CaCl solutions, birefringence was not decreased. In addition, when the xanthan solution is sheared between a glass slide and a cover slip, the optic axis (chain direction) aligns using the shear direction (as determined by the colors, displayed using a first-order red plate). Preferential alignment also is observed at interfaces Preferential alignment also is observed at interfaces - e.g., at an air/solution interface, the polymer molecules tend to align parallel to the surface. Moreover, when barium sulfate particles are suspended in the solution, the observed colors show that the polymer chains tend to align parallel to the barium polymer chains tend to align parallel to the barium sulfate/solution interface, encompassing the entire insoluble particle. This could be a principal reason for the unique suspending properties of xanthan gum. Preliminary small-angle light scattering is observed from a 5% xanthan solution placed between crossed polarizers. This is interpreted as being caused by polarizers. This is interpreted as being caused by fluctuations in orientation by regions consisting of ordered chains.

Behavior of Other Polymers

Since xanthan gum has unique suspending properties and appears to have liquid crystalline ordering in solution, a study of the liquid crystalline behavior of other naturally occurring polymers was undertaken. In relatively dilute solutions, this behavior also was observed for Unialgan PA TM at 3.8%, and Unialgan PC TM at 7.4% (unicellular PA TM at 3.8%, and Unialgan PC TM at 7.4% (unicellular algae from Marine Colloids), and i-carrageenan at 5.7%, though each sample showed less birefringence than that of xanthan gum.

SPEJ

P. 555

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