Viscosity of xanthan solutions

The viscosity of xanthan solutions in calcium bromide brines was also measured as a function of temperature. We found that the transition temperature fell rapidly as the concentration of salt was increased, so much so that above 2 molar it fell below that of succinoglycan. 

The viscosity of xanthan solutions is also distinct from that of flexible polyelectrolyte solutions which generally shows a strong Cs dependence [141]. In this connection, we refer to Sho et al. [142] and Liu et al. [143], who measured the intrinsic viscosity and radius of gyration of Na salt xanthan at infinite dilution which were quite insensitive to Cs ( > 0.005 mol/1). Their finding can be attributed to the xanthan double helix which is so stiff that its conformation is hardly perturbed by the intramolecular electrostatic interactions. In fact, it has been shown that the electrostatic persistence length contributes only 10% to the total persistence length even at as low a Cs as 0.005 mol/1 [142]. Therefore, the difference in viscosity behavior between xanthan and flexible polyelectrolyte... 

Is such a deformable chain model inconsistent with the non-Newtonian intrinsic viscosity Finding an answer to this question is the goal of this paper. To this end, the viscosity of xanthan solutions was measured over a broad range of shear stress, including especially the low-shear Newtonian limit which has not been measured by Whitcomb and Macosko. The intrinsic viscosity at various shear stresses was then determined and the resultant experimental curve was compared to theoretical expectations for a flexible chain (bead-and-spring) model. 

A few studies considered the effect of pH on the viscosity of xanthan solutions. Jeanes et al. observed a rapid increase in the viscosity of xanthan solution at pH 9-11 [28]. Whitcomb and Macosko [29] and Philips et al. [30] found the viscosity of xanthan to be independent of pH. Szabo examined the stability of various EOR polymers in caustic solutions at room temperature, including Kelzan MF (a biopolymer) [6]. He found a fast initial drop in the viscosity of a xanthan solution containing 2 wt% sodium chloride and 5 wt% sodium hydroxide, at 12.5 s", which virtually stopped after 10 days. Krumrine and Falcone found that the effect of alkali (sodium silicates) on the viscosity of xanthan solution depended on the concentration of sodium and calcium ions present [31]. Ryles examined the thermal stability of bio-polymers in alkaline conditions [16]. He found that xanthan was totally degraded (in anaerobic conditions) upon the addition of 0.8 wt% sodium hydroxide at temperatures from 50 to 90°C (in a 1 wt% sodium chloride brine). Seright and Henrici observed total biopolymer degradation at pH > 8 and a temperature of 120°C [26]. 

The effect of sodium chloride on the viscosity of xanthan solutions shown in Figures 24 and 25 can be explained as follows In deionized water, the xanthan... 

Calcium ion is more detrimental to the viscosity of xanthan solutions than sodium ion. This is especially true for Flocon 4800 solutions. The high pyruvate content of Flocon 4800 gives the polymer chains more ionic character (negative charges). This makes the polymer more sensitive to salts, especially divalent cations. 

Alkalis can modify the viscosity of xanthan solutions in many ways. First, alkalis provide cations into the polymer solution. These cations can reduce the viscosity of polymer solutions through the charge-shielding mechanism explained earlier. Secondly, the acetyl groups in the side chains of xanthan can be removed by strong alkalis [83]. The elimination of the acetyl groups may have no effect on viscosity [84] or may increase the viscosity of xanthan solutions [28]. Finally, alkalis can hydrolyze the xanthan backbone, which can be very detrimental to the solution viscosity. 

The most important aspect of Figure 33 is the dramatic effect of strong alkalis on the viscosity of xanthan solutions. Sodium carbonate, on the other hand, is less detrimental to the viscosity of xanthan solutions. Therefore, in alkali/polymer or alkali/surfactant/polymer processes whereby xanthan gum is used, it would be extremely beneficial to use a buffered alkali rather than a strong alkali. 

Auerbach, M. H., Prediction of Viscosity of Xanthan Solutions in Brines, SPE 13591, presented at the International Symposium on Oilfield and Geothermal Chemistry held in Phoenix, Arizona, April 9-11 (1985). 

Since cellulases are commercially available, we looked at this reaction further. A number of several commercial cellulases were screened for their ability to reduce the Brookfield viscosity of xanthan solution. Mannosidase and glucosidase were also used in combination with cellulase with the hope of exposing the cellulose backbone by removing the side chains. All of the cellulases tested were found to be somewhat active toward r ucing the molecular weight of xanthan gum (Table 3). 

Xanthan gum includes D-glucosyl, D-mannosyl, and D-glucuronyl acid residues together with 0-acetyl and pyruvyl residues [4]. The content of pyruvic acid varies substantially based on the species, which in turn leads to varying viscosities of xanthan solutions. Xanthan is considered to assume rigid double-strand helical conformation in its native form [85], with side chains positioned parallel to the helix axis and stabilizing the structure [24]. In solutions, the helical structure turns into flexible coils [85]. 

As well as the physical mechanism which may affect the degradation of biopolymer at elevated temperatures. Ash et al. (1983) discuss a number of other mechanisms including hydrolytic, free-radical and enzymatic mechanisms. With reference to the hydrolytic stability of xanthan. Ash et al. (1983) noted that the acetate group in xanthan is removed very readily at pH 12 even at room temperature. They also noted that a lower acetate content of xanthan has the effect of lowering the transition temperature. It has previously been reported that the viscosity of xanthan solutions is either increased (Jeanes et al, 1961) or is almost unaffected (Nisbet et al, 1982) by this loss of acetate groups. Ash et al (1983) concluded that the effect on solution viscosity of the loss of acetate may indeed be small but the... 

Compared with solutions of partially hydrolyzed polyacrylamides, viscosities of xanthan solutions are much less affected by changes in salinity or divalent-ion content. Figs. 5.17 and 5.1836 illustrate this by plotting the solution viscosity at various shear rates vs. salinity and divalent-ion content. 

Under many conditions, the viscosity of HEC solutions falls about 100 times faster than that of xanthan solutions. The rate of viscosity loss of Shellflo-S solutions, on the other hand is comparable with that of xanthan solutions at temperatures below the transition temperature, and comparable with that of HEC solutions above. 

The viscosity of xanthan gum solutions is increased in the presence of ceratonia. This interaction is used synergistically in controlled-release drug delivery systems. 

The viscosity of xanthan gum solutions is considerably increased, or gelation occurs, in the presence of some materials such as ceratonia, guar gum, and magnesium aluminum silicate. This effect is most pronounced in deionized water and is reduced by the presence of salt. This interaction may be desirable in some instances and can be exploited to reduce the amount of xanthan gum used in a formulation see Section 7. 

FIGURE 6.6 Dependence of the apparent viscosity >/a of xanthan solutions (concentration indicated) on shear rate low ionic strength. (Approximate results from various sources.)... 

The viscosities of xanthan gum solutions can be maintained quite well for 6 or 8 months if the solutions are mixed properly and filtered prior to aging. The degradation affects the injec-tability of these solutions rather than the viscosity. 

Exposure to temperatures as high as 80 C for extended periods has little effect on the viscosity of xanthan gum solutions. This resistance to thermal degradation is enhanced by the presence of... 

The viscosity of aqueous solutions of xanthan gum is essentially independent of pH between pH 6 and pH 9, and shows only small changes in viscosity over the pH range of pH 1 to pH 11. 

Figure 7. Viscosity of xanthan gum, locust bean gum solutions (aO.1% gum...

It is important to note that the typical application of xanthan uses a semi-dilute, not dilute solution. Interactions between the chains, which are not considered in this study, play an important role in the viscosity of these solutions. 

The first step of this study was undertaken in order to correlate the fermentation conditions to the properties in solution of the xanthan produced. Some of the characteristics of xanthan solutions A, B and C obtained in different conditions are presented in Table II. The analyses presented in this Table were carried out on the final samples in the case of batch cultures, i.e. at the moment when the initial glucose was totally consumed. The doublet or triplet data of Table II resulted from repeated fermentations. We have verified that, during the culture in the two batch fermentations, the intrinsic viscosity remained nearly constant. 

Figure 25 Shear-dependent viscosity of xanthan gum solutions. (From KC Tam and C Tiu, Water-soluble polymers (rheological properties) in Polymeric Materials Encyclopedia, JC Salamone, ed. Boca Raton, FL CRC Press, 1996, p. 8655.)...

Zatz JL, Knapp S. Viscosity of xanthan gum solutions at low shear rates. J Pharm Sci 1984 73 468-471. 

Figure 6.5. Influence of pore size on apparent viscosity/shear rate curves for the flow of xanthan solutions through glass beads (from Chauveteau and Zaitoun, 1981).

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