Institute of Chemistry and Center for Computing in Engineering & Sciences, University of Campinas, Campinas, São Paulo 13084-862, Brazil.
Institute of Chemistry, Federal University of Rio de Janeiro, Rio de Janeiro, RJ 21941-909, Brazil.
Biomacromolecules. 2021 Oct 11;22(10):4251-4261. doi: 10.1021/acs.biomac.1c00801. Epub 2021 Sep 13.
Cellulose possesses considerable potential for a wide range of sustainable applications. Nanocellulose-based material properties are primarily dependent on the structural surface characteristics of its crystalline planes. Experimental measurements of the affinity of crystalline nanocellulose surfaces with water are scarce and challenging to obtain. Therefore, the relative hydrophilicity of different cellulose allomorphs crystalline planes is often inferred from qualitative assessments of their surface and the exposition of polar groups to the solvent. This work investigates the relative hydrophilicity of cellulose surfaces using molecular dynamics simulations. The behavior of a water droplet laid on different crystal planes was used to determine their relative hydrophilicity. The water molecules fully spread onto highly hydrophilic surfaces. However, a water droplet placed on less hydrophilic surfaces equilibrates as an oblate spheroidal cap allowing the measurement of a contact angle. The results indicate that the Iα (010), Iα (11̅0), Iβ (010), and Iβ (110) faces, as well as the faces of human-made celluloses II and III_I (100), (11̅0), (010), and (110) are all highly hydrophilic. They all have a contact angle value inferior to 11°. Not unexpectedly, the Iα (001) and Iβ (100) surfaces are less hydrophilic with contact angles of 48 and 34°, respectively. However, the Iβ (11̅0) plane, often referred to as a hydrophilic surface, forms a contact angle of about 32°. The results are rationalized in terms of structure, exposure of hydroxyl groups to the solvent, and degree of cellulose-cellulose versus cellulose-water hydrogen bonds on each face. The simulations also show that the surface oxidation degree tunes the surface hydrophilicity in a nonlinear manner due to cooperative effects involving water-cellulose interactions. Our study helps us to understand how the degree of hydrophilicity of cellulose emerges from specific structural features of each crystalline surface.
纤维素在广泛的可持续应用中具有相当大的潜力。纳米纤维素基材料的性质主要取决于其晶体平面的结构表面特性。结晶纳米纤维素表面与水亲和力的实验测量非常稀缺且难以获得。因此，不同纤维素同晶型变体晶体平面的相对亲水性通常是根据其表面的定性评估和极性基团对溶剂的暴露程度来推断的。本工作使用分子动力学模拟研究纤维素表面的相对亲水性。通过在不同晶体平面上放置水滴来确定它们的相对亲水性。水分子完全铺展在高度亲水的表面上。然而，放置在疏水性表面上的水滴会平衡为扁球形帽，从而可以测量接触角。结果表明，Iα(010)、Iα(11̅0)、Iβ(010)和 Iβ(110)面以及人造纤维素 II 和 III_I(100)、(11̅0)、(010)和(110)面均具有高度亲水性。它们的接触角值都小于 11°。不出所料，Iα(001)和 Iβ(100)表面的亲水性较低，接触角分别为 48°和 34°。然而，Iβ(11̅0)面通常被认为是亲水表面，其接触角约为 32°。这些结果可以根据结构、羟基对溶剂的暴露程度以及每个表面上纤维素-纤维素与纤维素-水氢键的程度来解释。模拟还表明，由于涉及水-纤维素相互作用的协同效应，表面氧化程度以非线性方式调节表面亲水性。我们的研究有助于我们理解纤维素的亲水性程度如何从每个晶体表面的特定结构特征中显现出来。
