Determination of the mode and efficacy of the cross-linking of guar by borate using MAS¹¹B NMR of borate cross-linked guar in combination with solution¹¹B NMR of model systems

Abstract

The reaction product of boric acid and the polysaccharide guaran (the major component of guar gum) has been investigated by 11B NMR spectroscopy. By comparison with the 11B NMR of boric acid and phenylboronic acid complexes of 1,2-diols (HOCMe2CMe2OH, cis-C6H10(OH)2, trans-C6H10(OH)2, o-C6H4(OH)2), 1,3-diols (neol-H2), monosaccharides (L-fucose, mannose and galactose) and disaccharides (cellobiose and sucrose) it is found that the guaran polymer is cross-linked via a borate complex of two 1,2-diols both forming chelate 5-membered ring cycles ([B5(2)]), this contrasts with previous proposals. Based upon steric constraints we propose that preferential cross-linking the guaran polymer occurs via the 3,4-diols of the galactose side chain. The DeltaH and DeltaS for complexation of boric acid to cis- and trans-1,2-cyclohexanediol have been determined, from the temperature dependence of the appropriate equilibrium constants, and used in conjunction with ab initio calculations on model compounds, to understand prior conflicting proposals for guaran-boric acid interactions. 11B NMR derived pH dependent equilibrium constants and ab initio calculations have been used to understand the reasons for the inefficiency of boric acid to cross-link guaran (almost 2 borate ions per 3 monosaccharide repeat units are required for a viscous gel suitable as a fracturing fluid): the most reactive sites on the component saccharides (mannose and galactose) are precluded from reaction by the nature of the guar structure; the comparable acidity (pKa) of the remaining guaran alcohol substituents and the water solvent, results in a competition between cross-linking and borate formation; a significant fraction of the boric acid is ineffective in cross-linking guar due to the modest equilibrium (Keq). In contrast to prior work, we present evidence for the reaction of alcohols with boric acid, rather than the borate anion. Based upon the results obtained for phenylboronic acid, alternative cross-linking agents are proposed.

Complex Fluids and Hydraulic Fracturing

“…Xanthan is less-common owing to its higher cost, and materials modified from cellulose (which is itself not soluble in water) are also used since they have fewer impurities than guar (192) and easily form gels when derivitized (e.g., hydroxyethyl cellulose or HEC) (200) (190,197), resulting in a dynamic ionic bond in which the borate ion connects cis-diols on the galactose. The dynamics of these crosslinks have been extensively studied (191,203), and they give rise to a classical Maxwell-like linear viscoelastic response of the material (190) and a more viscous solution (as compared to the uncrosslinked case) in steady shear that can also display shear thickening above a critical shear rate (204). Crosslinking with metal ions is performed over a larger range of pH (depending on the ion) (192), and gels crosslinked with zirconate can tolerate a higher downhole temperature than with borate (197).…”

Innovative Concepts for the Shaping and Modification of Cellulose

“…For guar gum it was shown that a borate complexation is also possible along a single polymer chain. [36] A large variety of these negatively charged species may be formed during these reactions as shown in Figure 9. Both with monosaccharide and with polysaccharides five and six-membered rings are observed.…”

Nutrient Solutions for Hydroponic Systems

“…4. Transformation of boric acid (black circles) and anion forms of boric acid (white circles) as a function of pH (Bishop et al, 2004).…”