Boron equilibria with high molecular weight guar: An NMR study

Equilibrium constants have been derived from ¹¹B-NMR spectra for water-soluble complexes between borate anion and high molecular weight guar (e.g., 2 × 10⁶ Da). The validity of constants was confirmed by their ability to predict the pH changes measured on mixing various combinations of sodium borate, guar, and caustic into aqueous salt solutions, by correlation of computed crosslink concentrations with the dynamic storage modulii measured on the same fluids, as well as published G′ data for borate/guar/HPG gels. A smooth correlation was also found between the computed crosslink concentrations and the viscosities of borate/guar gels (measured at constant shear). These equilibrium constants were also of the same order as those published for simple sugars forming six-member rings with borate anion. The stoichiometries of the complexes were obtained directly from the NMR peak areas rather than by analogy with simple, single-ring sugars, and at the low concentrations of chemicals used to produce moderately viscous fluids [e.g., 5700 CP at 1.5 mM boron, 10 g/L guar, and 4% salt]. The required analytical sensitivity was achieved with a 600 MHz NMR spectrometer. © 1996 John Wiley & Sons, Inc.