Renewable Materials and Nanotechnology Research Group, Department of Chemical Engineering , KU Leuven , Campus Kulak Kortrijk, Etienne Sabbelaan 53 , Box 7659, 8500 Kortrijk , Belgium.
Biomacromolecules. 2019 Aug 12;20(8):3181-3190. doi: 10.1021/acs.biomac.9b00755. Epub 2019 Jul 24.
The thermodynamics of interactions between cations of the second group of the periodic table and differently negatively charged cellulose nanocrystals was investigated using isothermal titration calorimetry (ITC). The interaction of cations with the negatively charged CNCs was found to be endothermic and driven by an increase in entropy upon adsorption of the ions, due to an increase in degrees of freedom gained by the surface bound water upon ion adsorption. The effect was pH-dependent, showing an increase in enthalpy for cellulose suspensions at near-neutral pH (6.5) when compared to acidic pH (2). Sulfated cellulose nanoparticles were found to readily interact with divalent ions at both pH levels. The adsorption on carboxylate nanocrystals was found to be pH dependent, showing that the carboxylic group needs to be in the deprotonated form to interact with divalent ions. For the combined system (sulfate and carboxylate present at the same time), at neutral pH, the adsorption enthalpy was higher than the value obtained from cellulose nanocrystals containing a single functional group, while the association constant was higher due to an increased favorable entropic contribution. The higher entropic contribution indicates a more restricted surface-bound water layer when multiple functionalities are present. The stoichiometric number was nearly constant for all systems, showing that the adsorption depends almost completely on the ion valency and on the amount of ionic groups on the CNC surface, independent of the type of functional group on the CNC surface as long as it is deprotonated. In addition, we showed that the reduction in Gibbs free energy drives the ionotropic gelation of nanocellulose suspensions, and we show that ITC is able to detect gel formation at the same time as determining the critical association concentration.
采用等温滴定微量热法（ITC）研究了第二周期阳离子与不同带负电荷的纤维素纳米晶体之间相互作用的热力学。研究发现，阳离子与带负电荷的 CNCs 的相互作用是吸热的，并且吸附离子会导致熵增加，这是由于离子吸附时表面结合水的自由度增加。这种效应依赖于 pH 值，与酸性 pH（2）相比，在接近中性 pH（6.5）时，纤维素悬浮液的焓增加。硫酸化纤维素纳米颗粒在两种 pH 值下都很容易与二价离子相互作用。发现羧酸盐纳米晶体的吸附依赖于 pH 值，表明只有在羧酸基团去质子化的情况下才能与二价离子相互作用。对于组合系统（同时存在硫酸盐和羧酸盐），在中性 pH 下，吸附焓高于仅含有单一官能团的纤维素纳米晶体的值，而由于有利的熵贡献增加，缔合常数更高。较高的熵贡献表明，当存在多个官能团时，表面结合水层的限制更大。对于所有系统，化学计量数几乎保持不变，这表明吸附几乎完全取决于离子价态和 CNC 表面上离子基团的数量，而与 CNC 表面上的官能团类型无关，只要它是去质子化的。此外，我们表明，吉布斯自由能的降低驱动了纳米纤维素悬浮液的离子凝胶化，并且我们表明 ITC 能够在确定临界缔合浓度的同时检测到凝胶形成。
