Xanthan stiffness

Both polysaccharide molecules are relatively stiff, stiffer even than simple cellulosics such as HEC, and have a molecular masses in excess of two million. Recent work by Rinaudo and coworkers (Personal communication) and Crecenzi and colleagues (Int. J. Biol. Macromol., submitted) has shown that succinoglycan molecules are also stiffer than those of xanthan. 

More than half a century ago, Bawden and Pirie [77] found that aqueous solutions of tobacco mosaic virus (TMV), a charged rodlike virus, formed a liquid crystal phase at as very low a concentration as 2%. To explain such remarkable liquid crystallinity was one of the central themes in the famous 1949 paper of Onsager [2], However, systematic experimental studies on the phase behavior in stiff polyelectrolyte solutions have begun only recently. At present, phase equilibrium data on aqueous solutions qualified for quantitative discussion are available for four stiff polyelectrolytes, TMV, DNA, xanthan (a double helical polysaccharide), and fd-virus. 

Fig. lla-c. Phase boundary mass concentrations for aqueous solutions of three stiff polyions a xanthan (a double-helical polysaccharide) [78] b fd-virus [24] c tobacco mosaic virus (TMV) [23], Circles, experimental results curves, predictions by the first-order perturbation theory (see text)... 

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

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... 

The solid curves in the figure represent the molecular weight dependence of r)0 for quasi-binary system consisting of a fractionated xanthan sample and 0.1 mol/1 aqueous NaCl. The circles for quasi-ternary solutions almost follow them at the same c, except at small 2. Thus, to a first approximation, r)o of stiff polymer solutions is independent of molecular weight distribution, and may be treated as a function of Mw or Mv and c. 

Recently Sato et al. [144,145] have extended the viscosity equation, Eq. (74), to multicomponent solution containing stiff-chain polymer species with different lengths. They showed a favorable comparison of the extended theory with the viscosity data for the quasi-ternary xanthan solutions presented in Fig. 21. 

Xanthan forms the most pseudoplastic (instantaneous, reversible shear thinning) solutions of all the gums. This property is due to the stiffness of its molecules and/or intermolecular associations of two or more molecules. In plots of viscosity vs. concentration, there is a Newtonian (non-pseudoplastic) plateau at very low shear rates, which at least, makes its solutions appear to have a yield value (a yield value being the force required to initiate flow). As a result, xanthan is an excellent stabilizer for suspensions and emulsions. 

The relatively small region of allowable values of tp and j/ in polysaccharides linked between rings makes the wormlike chain model realistic for them. The polymer behaves as a random flight one over contour lengths S Lp, but as a stiff rod if S Lp. Persistence lengths can be fairly large 350 50 A for xanthan gum, 80-100 A for alginate or hyaluronate. ... 

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). 

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 ... 

Electrostatic polyelectrolyte complexes (PEC) are mentioned in the literature involving chitosan and synthetic or natural polymers such as PAA and CMC [155,156], xanthan, carrageenan, alginate [157-162], pectin [163,164], heparin, hyaluronan (HA) [165-169], sulfated cellulose, dextran sulfate, or N-acylated chitosan/chondroitin sulfate. Many systems were cited in the literature [2, 170]. The electrostatic interaction was discussed in relation with the stiffness of the backbone and the nature of the ionic groups involved. Especially, with alginate or HA, a pH-dependent complex is formed, whose stability depends on the ionic concentration. The complex formation was investigated in dilute solution by potentiometry (pH) and conductivity to determine the fraction of ion pairs (-COO + NH3 —) formed in dependence of the experimental conditions [164, 166]. 

The above theory was developed basically for flexible molecules which may have some degree of stiffness. However, if it is assumed that xanthan molecules may be modelled as rigid rods to a first approximation, then a theory has been developed which allows the determination of molecular dimensions from viscometric measurements in a similar way to the flexible coil case. The relationship between intrinsic viscosity and macromolecular shape has been derived theoretically for rigid rods (Layec and Wolff, 1974). When the ratio of the length to the diameter of the molecule, p, is large (p 50), the model gives a relationship between the zero shear intrinsic viscosity, Mq, and p of the form ... 

Examples of shear-dependent viscosity, (a) Red blood cells (normal human, Hb 37% Mills et al., 1980). (b) A 0.35 wt % aqueous solution of xanthan gum, a stiff biopolymer (Whitcomb and Macosko, 1978). (c) A commercial yogurt (deKee et al., 1980). (d) Polystyrene-ethylacrylate latex spheres Dw 0.5 m at various concentrations in diethylene glycol, r = 23° C (Uun, 1988). The lines in (a)-using models described in this chapter. 

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