Crosslinking mechanism of boric acid with diols revisited

Complex formation of boric acid with diol functional groups is well known and many papers have been published assuming borate is the reactive species. Based on the high reactivity of boric acid towards nucleophiles, this paper assumes boric acid is the reactant. We prove this hypothesis by presenting a study of the reaction of the polymer guaran with boric acid under various conditions. A thermodynamic analysis of the underlying reactions does not show any differences between either crosslinking species. Thermodynamics shows that the concentration of the 2:1 crosslinks, that give rise to the enhanced viscosity, is proportional to [B], the boric acid concentration, [X-2], the concentration of available crosslink sites and inversely proportional to [H+]. These predictions were all confirmed by experiment. However, a difference between the mechanisms is demonstrated in a kinetic analysis: Longer gel times are predicted with the mechanism based on boric acid as the pH of the fluid increases, that is exactly what is observed in practice. Evidence is provided from visco-elastic measurements. A new approach to describe inter- and intramolecular crosslink formation is introduced and evidence for the correctness of this description is provided. (c) Wiley-VCH Verlag GmbH C Co.