Cellulose
placed at a distance so that they can stack on top of glucose rings along the cellulose backbone (Beckham et al. 2010; Ponyi et al. 2000). This suggests that the main mechanism for the adsorption is hydrophobic association by the release of interfacial water from both CBM and cellulose. At these less hydrophilic locations, there are no polar groups for water interac- tion, which means that the free energy of the hydrated state is high. Reducing the exposure of these areas to solvent thus results in an exothermic response upon release of water. Aromatic groups are relatively polar and interact strongly with water, compared to for example cyclohexane, which leads to a more favorable interface to water (Raschke and Levitt 2005). In addition, exchange of O-H O H-bonds to O-H N which have slightly lower energy (Pandey et al. 2017) would also lead to an exothermic response. Indeed, amines or amides are found in both CBMs and aromatic-rich molecules such as methylene blue (Lombardo and Thielemans 2019) that undergo exothermic adsorption.
release of (relatively) constrained water molecules, i.e. hydrophobic effects, is the main driving force for all hemicellulose adsorption to cellulose in water. The analogy with cellulose association is obvious. Although H-bonds do not drive adsorption (making) from an equilibrium thermodynamics point of view, they can still affect how easy it is to remove an adsorbed molecule (breaking), either by force or desorption, by contributing to the kinetic stability (Stuart and Fleer 1996), since the probability of many bonds being spontaneously broken at the same time becomes low. They can also contribute to specificity, namely how hemicelluloses organize on the cellulose surface. Specifically, recent experimental results in combination with MD simulation suggest that both xylan and glucomannan exhibit an abundance of structural motifs that allow them to adsorb to cellulose in conformations where they become almost seamless extensions to the cellulose crystal structure, including the H-bond network (Busse-Wicher et al. 2014, 2016; Martinez-Abad et al. 2017, 2020; Grantham et al. 2017; Simmons et al. 2016; Pereira et al. 2017). However, while the simulations generally show that hemicellulose molecules adsorbed in this manner can be remarkably stable against fluctuations, they also show that in terms of adsorption free energy, hemi- celluloses prefer the essentially non-H-bonded asso- ciation to hydrophobic (200) surfaces (Fig. 1), if such surfaces are present (Martinez-Abad et al. 2017; Pereira et al. 2017; Zhang et al. 2011). Nanocelluloses are suitable substrates for experi- mental determination of thermodynamic quantities of adsorption using isothermal titration calorimetry (ITC). A recent review (Lombardo and Thielemans 2019) that compiles the findings from ITC studies shows that adsorption of biomacromolecules, dyes, or drugs to cellulose or partially modified cellulose, is entropy driven and endothermic in most cases. In addition, MD simulations have shown that both urea (Chen et al. 2017) and the poorly soluble drug furosemide (Lombardo et al. 2018) adsorb to cellulose driven by a combination of dispersion interactions and translational entropy when they are exchanged with surface water. The exothermic exceptions, where H-bonding is a possibility, are the adsorption of cellulose binding modules (CBMs) of carbohydrate active enzymes or small and uncharged organic molecules. The binding site of CBMs is rich in aromatic amino acids such as tyrosine or tryptophane,
Fig. 6 The continuous interface between two crystalline domains (left) can be perturbed by molecular scale ‘‘defects’’ such as chemical surface modification (right), in this case surface acetylation. This leads to sub-nanometer sized cavities that can harbor water molecules. Cellulose is shown in black for carbon and yellow for oxygen, surface acetyl groups in green (carbon) and red (oxygen). Water is shown in light blue. Reproduced from Chen et al. (2020) with permission from the Royal Society of Chemistry
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