Carboxymethyl hydroxypropyl guar gum physicochemical properties in dilute aqueous media.

We investigate carboxymethyl hydroxypropyl guar gum (CMHPG) solution properties in water and NaCl, KCl, and CaCl2 aqueous solutions. The Huggins, Kraemer, and Rao models were applied by fitting specific and relative viscosity of CMHPG/water and CMHPG/salt/water to determine the intrinsic viscosity [η]. The Rao models yielded better results (R2 = 0.779-0.999) than Huggins and Kraemer equations. [η] decreased up to 84% in salt solution over the range 0.9-100 mM compared to water. Salt effects screened the CMHPG charged side groups chains leading to a compacted structure. In 0.9 mM NaCl(aq), the hydrodynamic coil radius (Rcoil) was 28% smaller and 45% smaller in 100 mM NaCl solution relative to water. Similar decreases were seen in KCl and CaCl2 solutions. KCl and CaCl2 were more effective than NaCl. CMHPG is salt-tolerant and shows comparatively less viscosity change than native guar gum, with modest reduced viscosity increases with CMHPG dilution at all salt concentrations. The electrostatic interactions were effective up to 100 mM salt. The activation energy of viscous flow for CMHPG solutions was computed and compared to measured xanthan gum and several literature values. These data show that the barrier to CMHPG flow is higher than for xanthan gum.

Salt and Temperature Effects on Xanthan Gum Polysaccharide in Aqueous Solutions.

Xanthan gum (XG) is a carbohydrate polymer with anionic properties that is widely used as a rheology modifier in various applications, including foods and petroleum extraction. The aim was to investigate the effect of Na+, K+, and Ca2+ on the physicochemical properties of XG in an aqueous solution as a function of temperature. Huggins, Kraemer, and Rao models were applied to determine intrinsic viscosity, [η], by fitting the relative viscosity (ηrel) or specific viscosity (ηsp) of XG/water and XG/salt/water solutions. With increasing temperature in water, Rao 1 gave [η] the closest to the Huggins and Kraemer values. In water, [η] was more sensitive to temperature increase (~30% increase in [η], 20-50 °C) compared to salt solutions (~15-25% increase). At a constant temperature, salt counterions screened the XG side-chain-charged groups and decreased [η] by up to 60% over 0.05-100 mM salt. Overall, Ca2+ was much more effective than the monovalent cations in screening charge. As the salt valency and concentration increased, the XG coil radius decreased, making evident the effect of shielding the intramolecular and intermolecular XG anionic charge. The reduction in repulsive forces caused XG structural contraction. Further, higher temperatures led to chain expansion that facilitated increased intermolecular interactions, which worked against the salt effect.

Xanthan gum in aqueous solutions: Fundamentals and applications.

Xanthan gum is a naturally occurring polysaccharide obtained from Xanthomonas campestris. The xanthan gum backbone consists of ß-d-glucose linked like in cellulose. The trisaccharide ß-d-mannose-(1-4)-α-d-glucuronic acid-(1-2)-α-d-mannose is linked to O(3) position of every other glucose residue. Ketal bonds link pyruvic acid residues to approximately half of the terminal mannose residues. The terminal mannose residues also carry acetate groups. Xanthan gum is used as a thickening, stabilizing, or suspending agent in various applications, e.g., food, pharmaceutical, cosmetic, and petroleum extraction. The performance of xanthan gum is based on its macromolecular conformation and association in solution and at interfaces. In water, xanthan gum undergoes conformational transitions from helix to random coil, in response to stimuli such as pH, ionic strength, temperature, and shear. This review presents fundamental information on the behavior of xanthan gum in aqueous media, at conditions and in the presence of additives which are of interest to applications that benefit from viscosity changes such as in oil and gas extraction. Effects on xanthan gum aqueous solutions of pH, electrolytes, changes in temperature, and added natural polysaccharides or synthetic polymers are highlighted. Such information is useful in the formulation of products and the design of processes involving xanthan gum and related polysaccharide polymers.