Determination of the mode and efficacy of the cross-linking of guar by borate using MAS 11B NMR of borate cross-linked guar in combination with solution 11B NMR of model systems
Determination of the mode and efficacy of the cross-linking of guar by borate using MAS 11B NMR of borate cross-linked guar in combination with solution 11B NMR of model systems
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DOI：
10.1039/b406952h
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发表时间：
2004-01-01
影响因子：
4
通讯作者：
Barron, AR
中科院分区：
文献类型：
--
作者：
Bishop, M;Shahid, N;Barron, AR
硼酸和多糖瓜尔胶（瓜尔豆胶的主要成分）的反应产物已通过 B-11 NMR 光谱进行了研究。通过与 1,2-二醇 (HOCMe2CMe2OH、cis-C6H10(OH)(2)、trans-C6H10(OH)(2)、o-C6H4(OH)(2))、1,3-二醇 (neol-H-2)、单糖（L-岩藻糖、甘露糖和半乳糖）和二糖 （纤维蛋白和蔗糖）发现瓜尔胶聚合物通过两个1,2-二醇的硼酸盐络合物交联，两者都形成螯合五元环环（[B5(2)]），这与之前的提议形成对比。基于空间限制，我们提出瓜尔胶聚合物的优先交联是通过半乳糖侧链的 3,4-二醇发生的。硼酸与顺式和反式 1,2-环己二醇络合的 DeltaH 和 DeltaS 已根据适当平衡常数的温度依赖性确定，并与模型化合物的从头计算结合使用，以了解先前关于瓜尔兰-硼酸相互作用的相互矛盾的建议。 B-11 NMR 衍生的 pH 依赖性平衡常数和从头计算已用于了解硼酸交联瓜尔胶效率低下的原因（适合作为压裂液的粘性凝胶每 3 个单糖重复单元需要几乎 2 个硼酸根离子）：糖组分（甘露糖和半乳糖）上最具反应性的位点由于其性质而无法反应。 瓜尔胶结构；剩余瓜尔胶醇取代基和水溶剂的酸度（pK(a)）相当，导致交联和硼酸盐形成之间的竞争；由于适度的平衡（K-eq），很大一部分硼酸在交联瓜尔胶中无效。与之前的工作相比，我们提供了醇与硼酸而不是硼酸根阴离子反应的证据。根据苯基硼酸获得的结果，提出了替代交联剂。
The reaction product of boric acid and the polysaccharide guaran (the major component of guar gum) has been investigated by B-11 NMR spectroscopy. By comparison with the B-11 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-H-2), monosaccharides (L-fucose, mannose and galactose) and disaccharides (celloboise 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. B-11 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 (pK(a)) 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 (K-eq). 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.