Xanthan gels

Carico(139-140) has published comparative information about the various gelling agents. Xanthan has been adapted to foam fracturing gels. Xanthan gels can be broken by the addition of lithium hypochlorite(141). 

Sanchez, C., Zuniga-Lopez, R., Sehmitt, C., Despond, S., and Hardy, J. (2000). Microstructure of acid-induced skim milk-locust bean gum xanthan gels. Ini Dairy J. 10, 199-212. 

Rodriguez-Hemandez, A. I. and Tecante, A. 1999. Dynamic viscoelastic behavior of gellan-iota-carrageenan and gellan-xanthan gels. Food Hydrocolloids 13 59-64. 

Nevertheless, simpler suspending and flavouring agents can also be prepared at the dispensary (e.g. methylcellulose 1%, or other celluloses such as hypromellose, microcrystalline cellulose, sodium carboxy-methylcellulose at 1-2%, xanthan gels, pharmacopoeial syrups, or mixtures of the above). In some formulations suspending agents may not be required, for example, when it is known that the drug is soluble... 

M. R. Avery, L. A. Burkholder, and M. A. Gruenenf elder, Use of Crosslinked Xanthan Gels in Actual Profile Modification Field Projects, Paper SPE 14114, Presented at the SPE 1986 International Meeting on Petroleum Engineering, Beijing, China, March 17-20 (1986). 

Fig. 5.79—Effect of brine Injection on permeability of a sand-pack/gel system treated with 100-ppm-Cr(lll)/2,000-ppm-xanthan gel. ...

Concentrations above 0.3% form a gel with borate which is reversible upon the subsequent addition of mannitol (a sequestrant for borate) or of acid. Usefiil combinations are formed with carrageenan (63) and xanthan gum (64) and agar. In many appHcations, it is used in combination with these gums at considerable cost savings. 

The intermolecular interactions stabilise the helices and greatly influence the properties of exopolysaccharides in solution, ie solubility, viscosity and gel-formation. A strong interaction or good-fit between molecules will lead to insolubility, whereas poor interaction will lead to solubility of exopolysaccharides. The interactions between molecules is influenced by the presence of side-chains. For example, cellulose is insoluble but introduction of a three monosaccharide side-chain into the cellulose chain gives the soluble xanthan. Small changes in the structure of the side-chains can alter the molecular interactions and thus properties of the exopolysaccharide. 

Xanthan does not in itself form gels, despite the strong intermolecular interactions which occur in solution. However, some of the rheological properties of xanthan have... 

Less polysaccharide can also be used in products by taking advantage of the synergistic gelling of xanthan/galactomannan mixtures which forms thermoreversible gels at lower concentrations than if each is used separately. 

Xanthan gum is a slimy gel produced by the bacterium Xan-thomonas campestris, which causes black rot on cruciferous vegetables, such as cauliflower and broccoli. The slime protects the bacterium from viruses and prevents it from drying out. 

Pedersen, J.K.. Carrageenan, pectin and xanthan/locust bean gum gels. Trends in their food uses.Food Chem. 6 (1980) 77-88. 

Xanthan has several undesirable properties. It ordinarily forms microgel particles that can plug permeability(137), it is expensive relative to other natural polysaccharide gel agents, and it resists degradation by ordinary gel breaker additives. These features have kept xanthan applications in fracturing gels to a minimum. Some literature(138) on xanthan has appeared. 

The gel time of a 2000 ppm Flocon 4800 (a Pfizer xanthan polymer) in 2% NaCl solution was measured with various Cr(III) crosslinkers at room temperature (Table II). In this series of experiments Cr(III) concentration was 90 ppm. The most reactive Cr(III) species were dates derived from Cr(N0 )g with one and two equivalents of NaOH. Gels formed within 5 minutes and the reaction rate appeared to be diffusion-controlled. Cr(N03)3 without NaOH required 48 hours to gel the polymer solution. This reflects the time needed to hydrolyze CrCNOg) in Equation 3. 

Entanglement and weak gel formation are characteristic of some oilfield polysaccharides such as guar and starch, but are present only weakly, if at all, in both xanthan and succinoglycan solutions. Solutions of xanthan and succinoglycan are thus able to pass through porous media such as rock, while guar and starch cannot because of their gel-like nature. Hence the different uses of these polymers in the oilfield. 

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