Intrinsic viscosity xanthan

When dissolved in more saline waters, xanthan gum produces a higher apparent viscosity than the same concentration of polyacrylamide (292). Prehydration of xanthan in fresh water followed by dilution in the saline injection water has been reported to provide higher viscosity than direct polymer dissolution in the same injection water. Optical rotation and intrinsic viscosity dependence on temperature indicate xanthan exists in a more ordered conformation in brine than in fresh water (293). 

The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. 

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... 

Young, S.L. and Shoemaker, C.F. 1991. Measurement of shear-dependent intrinsic viscosities of carboxymethyl cellulose and xanthan gum suspensions. J. Appl. Polymer Sci. 42 2405-2408. 

Launay, B., Cuvelier, G. and Martinez-Reyes, S. 1984. Xanthan gum in various solvent conditions intrinsic viscosity and flow properties. In Gums and Stabilisers for the Food Industry 2, ed. G.O. Phillips, D.J. Wedlock, and PA. Williams, pp. 79-98, Pergamon Press, London. 

Achayuthakan et al. (2006) studied vane yield stress of Xanthan gum-stareh dispersions. The intrinsic viscosity of Xanthan gum was determined to be 112.3 dl/g in distilled water at 25°C. In addition, the size of the granules in the dispersions of the studied starches waxy maize (WXM), cross-linked waxy maize (CWM), and cold water swelling (CWS) were determined. The values of yield stress of the starch-xanthan dispersions relative to those of the starch-water dispersions (YSA"S0) and relative mean granule diameters (D/DO) plotted against values of c[ ] of xanthan gum are shown in Figure 4-42. With the values of YS/YSO being less than 1.0, there was no synergism between CLWM starch and xanthan gum. 

The values for xanthan gum were also reported in an earlier work (7). The molecular sizes were obtained by using the Flory relation (8). There are alternate discussions as to what the configuration and the size of xanthan gum molecules in solution are. Whitcomb, Ek, and Macosko have presented an interpretation of the intrinsic viscosity data assuming a cylindrical rod conformation (9). The K-j and a values for Pusher are given by Lynch and Mac-Williams (10). It should be noted that a range of Kj and a values for polyacrylamides can be found in the literature (11). 

The intrinsic viscosity of Colloid is close to the value for xanthan gum. The Kj and a values are not available, hence the molecular weight and size are not calculated. These numbers are expected to be similar to that of xanthan gum. The actual... 

The first model has been explored for xanthan by Whitcomb and Macosko (1), who show that an undeformable ellipsoid of length 1.5 jm and midpoint diameter 19 A can fit data they observed for the variation of intrinsic viscosity with shear stress in distilled water. Rinaudo and Milas 2) have also adopted this model to fit their intrinsic viscosity and sedimentation data. 

In contrast, a recent study from this laboratory ( 3) concludes that native xanthan molecules are better viewed as stiff but wormlike chains. This conclusion follows from measurements of zero-shear intrinsic viscosity for a homologous series of xanthans of different molecular weight for native xanthan the exponent z in the relation [n ] = KM is only 0.96 rather than 1.8 as expected for rigid rods. It is the goal of this paper to explore whether a wormlike model is consistent with other experimental data, especially the dependence of intrinsic viscosity on shear stress (non-Newtonian behavior). 

What is the persistence length of xanthan The ellipsoid model of Whitcomb and Macosko (1) would imply a>1.5ijm. The ellipsoid model of Rinaudo and MiTas 2) impl ies 0.6nm. On the other hand, the close similarity between the intrinsic viscosity of xanthan and of DNA (3) for the same molecular weight suggests that the persistence lengths of the two polymers are similar, i.e. a vSO nm. 

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. 

From the data in Figure 1 and Table 1, parameter values for the Meter equation describing the intrinsic viscosity can be estimated [n]o 8000 ml/g [riL = 3000 ml/g, = 0.5 dyne cm 2, a-1 = 1.5 (geometric mean). Figure 2 shows a plot of [nl/Lnla shear stress in 0.5M NaCl, 0.04M PO4 pH 7 at 20°. Whitcomb and Macosko have previously reported the intrinsic viscosity of xanthan but in distilled water, where the shear stresses overlap, the data are very similar to those in 0.5M NaCl. There is thus remarkably little effect of ionic strength on the shear stress at which [n]/[n]o drops sharply. However, the value of [nJo obtained by Whitcomb and Macosko, 24700 ml/g, is substantially greater than the value obtained here, 8000 ml/g. 

How well do predicted and observed non-Newtonian intrinsic viscosity agree for a wormlike model of xanthan Fixman (Ref. J2 Fig. 4) gives the non-Newtonian intrinsic viscosity for a flexible chain model at various values of the excluded volume parameter a, as a function of the normalized shear rate parameter The parameter Kn, which incorporates the effects of molecular weight and chain stiffness, equals 1.71[n]oMnog/RT where [nJo is the polymer intrinsic viscosity at zero shear stress, o is the solvent viscosity, g is the shear rate in sec"i and the other symbols have their usual meaning. 

The experimental parameters required in the theory to model the chain and its stiffness are M, [n]o, and an excluded volume parameter. For xanthan we take a log-normal distribution in M between 1x10 and 30x10, with peak at 10x10 and width approximating the observed distribution (3). The intrinsic viscosity at each molecular weight is fixed By the relation [n] = 4.76xl0 ... 

Summary. The non-Newtonian intrinsic viscosity of xanthan can be explained either by a bead-spring model or by a rigid rod model with appropriate parameters. A Kuhn-equivalent chain with about 200 repeating units per link and about 50 links per molecule is in my view more consistent with all the data than is a rigid rod model. 

Khouryieh H. A., Herald T. J., Aramouni R, Alavi S. 2007. Intrinsic viscosity and viscoelastic properties of xanthan/guar mixtures in dilute solutions Effect of salt concentration on the polymer interactions. Fond Res. Int. 40, 883-893. 

The main aim of this study was to investigate the effects of the origin and amount of proteins on the aggregation of xanthan chains in solution by means of rheological measurements. Modifications of fermentation conditions led to variations of intrinsic viscosity, Huggins constant and pyruvate content of the xanthan produced. 

Viscosimetric determinations. The Newtonian intrinsic viscosity of the xanthan molecule was determined by measuring the viscosities of several dilute polymer solutions with a Contraves Low-Shear viscometer. Extrapolation at zero polymer concentration of the reduced specific viscosity gave the value of the intrinsic viscosity, and the Huggins constant was calculated from the slope of the curve. 

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. 

Addition to purified and diluted solutions of xanthan. For this study, it was necessary to prepare a non-aggregated xanthan solution which was obtained by extensive ultrafiltration of a commercial xanthan sample which was initially non-aggregated. The absence of aggregation was confirmed by the Huggins constant which was 0.4 and the intrinsic viscosity which was 6.7 m kg. This corresponds to a molecular weight of 4.8x10 daltons. This xanthan solution was adjusted at a polymer concentration of 0.4 g.l"l in a protein-rich solution such as com steep liquor (CSL). Before use, the com steep solution was centrifuged and only the clear supernatant was added to the xanthan solution. The solvent was 0.1 M sodium chloride and the ratio of protein to xanthan was 10% (w/w). 

Intrinsic viscosity determinations (Fig.l) were carried out with xanthan solutions prepared in the presence or not of com steep liquor. The main results are summarized in Table III. 

Xanthan type Fermentation conditions Intrinsic viscosity (m3 kg-1) Huggins constant k DS Pyravale (%)... 

The main differences between xanthan C and the other products were the degree of pyruvate substitution of xanthan, the nature of the fermentation medium, the protein concentration and the intrinsic viscosity (see Table II). 

Finally, as the intrinsic viscosity of xanthan C was higher, its molecular weight can also be higher. It is quite possible that interactions between xanthan and bivalent ions and proteins can be different according to the polysaccharide chain length. 

Figure 5. The zero shear intrinsic viscosity of xanthan and welan versus 1/Vl.

Fig. 5.10. Intrinsic viscosity [q] determined at high shear rates Y with a capillary viscosimeter and at lower shear rates with a Zimm-Crothers viscosimeter for different xanthan gums in 0.1 mol/l sodium chloride (NaCI) solution at 25 C. Data from [93]. For strongly shear thinning polymer solutions, only low shear viscosimeters reach the shear rate independent viscosity region...

The viscosity of a polymer solution is related to the size and extension of the polymer molecule in that particular solution larger molecular species are generally associated with higher solution viscosities. In this section, the issue of the molecular size of the polymer is discussed mainly in the light of viscosity-related properties of the polymer solution. Relationships are developed that apply to both random coil molecules, such as HPAM, and more rigid macromolecules like xanthan. A number of other quantities are related to viscosity these include the relative viscosity, specific viscosity, reduced viscosity and inherent viscosity, which are defined in Table 3.1. (Billmeyer, 1971 Rodriguez, 1983). Clearly, all of these quantities are related to the polymer concentration in solution, and a more fundamental quantity which will be defined is the intrinsic viscosity, [ ]. The intrinsic viscosity is the limit of the reduced viscosity or inherent viscosity as the solution concentration of polymer tends to zero as shown below. 

Figure 3.4. Determination of intrinsic viscosity for a xanthan sample using the same laboratory data plotted as reduced and inherent viscosity versus concentration (unpublished data).

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