Cellulose
direction, removal of inter-chain H-bonds would likely have a large effect on the stiffness. However, the specific effect of H-bonds on the stiffness, in any direction, is not easy to isolate, both because removal of the H-bonds in simulations simultaneously changes other parameters as well (chain packing, etc.) and because different contributions to the stiffness (H- bonds, dispersion interactions, covalent bonds, etc.) are not necessarily additive.
Both dynamic FTIR and Raman spectroscopy (Hinterstoisser and Salme´n 1999; Kong and Eichhorn 2005) have shown that intramolecular H-bonds shift their frequencies during axial deformation of cellu- lose, and there was also evidence that they may disassociate if the strain was sufficiently high. How- ever, a H-bond is approximately ten times easier to stretch than a C-O-C bond angle and hundred times less stiff than an ordinary C-O covalent bond. Glucan chains in cellulose are both aligned and extended, which leads to a dominant part of the axial deforma- tion being associated with covalent degrees of free- dom (Djahedi et al. 2015). Completely removing hydrogen bonds within any cellulose crystalline allomorph (Wohlert et al. 2012; Eichhorn and Davies 2006) causes a significant reduction of its stiffness, but also a loss in structural organization of the chains. Therefore, cooperative effects between hydrogen- and covalent bonding were suggested, and also investi- gated spectroscopically by Altaner et al. (Altaner et al. 2014). They proposed a molecular-scale leverage mechanism by which the deformation of the O3H3 O5 H-bond was enhanced since FTIR spec- troscopy showed that the O2H2 O6 bond on the opposite side of the glycosidic linkage was not deforming during chain extension (Fig. 4). This effect was analyzed in a simplified spring model with parameters based on MD data (Djahedi et al. 2016), which showed that the leverage effect was indeed present within MD simulations, although mitigated by the fact that a large part of the total deformation took place in the sugar rings. Nevertheless, this study showed a relative effect of intramolecular hydrogen bonds of about 15-20% of the total stiffness, similar to what has been seen in MD simulations of crystalline cellulose where H-bonds were artificially turned off (Wu et al. 2013). However, analysis of the respective energy contributions to the total MD potential showed that the major part of the axial stiffness comes from deformations of bonds, angles and dihedrals in com- bination with a large contribution from dispersion interactions (Djahedi et al. 2015). In the transverse directions on the other hand, the situation is different, since covalent interactions can be expected to con- tribute less. DFT calculations (Chen et al. 2021) show that dispersion interactions contribute around 50% of the stiffness in the direction perpendicular to the H-bonded planes, and about 30% in the direction parallel to the H-bonded planes (Fig. 2). In that
At the surfaces
Accessible OH groups are abundant on cellulose crystallite surfaces. The ‘‘hydrophilic’’ (110) and (1-10) crystallographic planes (Fig. 1) are most likely the dominant planes exposed in native fibrils from wood (Nishiyama 2009; Daicho et al. 2018), although alternative models have larger exposure of the (010) or the ‘‘hydrophobic’’ (200) planes (Fernandes et al. 2011; Yang and Kubicki 2020). The two hydrophilic surfaces are similar, exposing about 5.4 OH per nm 2 that are potentially accessible to the environment. However, both modeling (Heiner and Teleman 1997; Heiner et al. 1998) and experiments (Lindh et al. 2016) show that the intra-chain H-bond O3H3 O5 is so stable that, in practice, the O3H3 hydroxyl does not act as a donor. This leaves 3.6 OH groups per nm 2 that are available for H-bonding with other fibrils, macro- molecules, or solvent molecules. In addition, their high reactivity makes them serve as points for chemical surface modification such as acetylation (Sassi and Chanzy 1995), TEMPO-mediated oxidation (Saito et al. 2006), or polymer grafting (Rol et al. 2019), which naturally affect the fibril’s H-bonding capability.
Adsorption to cellulose
To explain adsorption, it is common to look for favorable specific interactions, such as hydrogen bonds, electrostatic attraction, or p p interactions, and seeking explanations in the chemical structure of the surface and the adsorbing molecule. Since both cellulose and molecules that adsorb to cellulose are often decorated with polar groups, such as hydroxyl groups, it is assumed that hydrogen bonds are impor- tant. This explanation is so common in the literature
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