Abstract

We studied the dissolution of microcrystalline cellulose (MCC) at 60 °C in binary mixtures of dimethyl sulfoxide (DMSO) and the following quaternary ammonium acetates (QAAcOs): benzyl-trimethylammonium, N_BzMe3AcO; diallyl-benzyl-methylammonium, N_Al2BzMeAcO, tetra(n-butyl)ammonium (N_Bu4AcO), and tribenzyl-methylammonium, N_Bz3MeAcO (Al, Bu, Bz, Me, AcO refer to allyl, 1-butyl, benzyl, methyl and acetate group, respectively). We observed the following MCC dissolution order (given as wt%): N_Bu4AcO > N_Al2BzMeAcO > N_Bz3MeAcO ≫ N_BzMe3AcO. To explain this result we used the combined data of: (1) solvatochromism to calculate the following binary solvent (b-solvent) descriptors that are important for MCC dissolution: empirical polarity, Lewis acidity, Lewis basicity, dipolarity, polarizability; (2) isothermal titration micro-calorimetry to calculate the enthalpy of interaction (ΔH) of β-cyclodextrin (cellulose model) with QAAcO/DMSO-acetonitrile; (3) molecular dynamics simulations to calculate solvent-induced separation of cellulose chains; average number of hydrogen bonds between acetate ions and cellulose OH-groups, and average composition of the biopolymer solvation layer. We offer the following pieces of evidence to show the importance of hydrogen bonding for the efficiency of cellulose dissolution: the orders of b-solvent basicity and of |ΔH| are parallel to its cellulose dissolution efficiency; the biopolymer solvation layer of efficient b-solvents (e.g., N_Bu4AcO-DMSO) contains more ionic liquid ions and less DMSO molecules than that of N_Bz3MeAcO, leading to pronounced biopolymer chain separation, hence eventual dissolution. Our approach shows the power of the combined use of several techniques to rationalize the requirements for efficient cellulose solvents.

Graphic abstract

摘要

我们研究了微晶纤维素（MCC）在60°C下在二甲基亚砜（DMSO）和以下乙酸季铵盐（QAAcOs）的二元混合物中的溶解度：苄基三甲基铵，N _BzMe3 AcO; 二烯丙基苄基甲基铵N _Al2BzMe AcO，四（正丁基）铵（N _Bu4 AcO）和三苄基甲基铵N _Bz3Me AcO（Al，Bu，Bz，Me，AcO指烯丙基，1-丁基，苄基，甲基和乙酸酯基）。我们观察到以下MCC溶出顺序（给出的重量％）：N _BU4 ACO>Ñ _Al2BzMe ACO>Ñ _Bz3Me ACO»N _BzMe3醋酸 为了解释这一结果，我们使用以下组合数据：（1）溶剂变色法计算以下对MCC溶解很重要的二元溶剂（b-solvent）描述符：经验极性，Lewis酸度，Lewis碱度，双极性，极化率；（2）等温滴定微量热法计算相互作用的焓（ΔH）的β-环糊精（纤维素模型）与QAAcO / DMSO-乙腈；（3）分子动力学模拟以计算溶剂诱导的纤维素链分离；乙酸根离子和纤维素OH-基团之间的氢键的平均数目，以及生物聚合物溶剂化层的平均组成。我们提供以下证据来证明氢键对纤维素溶解效率的重要性：b溶剂碱性和| b的顺序。ΔH | 与其纤维素溶解效率平行；有效的b溶剂（例如N _Bu4 AcO-DMSO）的生物聚合物溶剂化层比N _Bz3Me包含更多的离子液体离子和更少的DMSO分子AcO，导致明显的生物聚合物链分离，因此最终溶解。我们的方法显示了多种技术结合使用对有效纤维素溶剂的要求合理化的力量。

图形摘要