Chirality and bound water in the hierarchical cellulose structure

Water binds strongly with cellulose and plays a seminal role in its hierarchical structure at ambient conditions. Macroscopic twisting of moistened paper has been earlier argued to originate from a coupling between the chirality and bound water of cellulose fibers. At the molecular level, the bound water is supposed to affect the microfibril network assembly. We studied these fundamental interrelations using molecular dynamics (MD) simulations of single microfibrils and macrofibrils formed by their bundles. The simulations were based on an amphiphilic 24-chain microfibril model, with 6 wt-% of loose chains representing hemicellulose in the macrofibrils. The large hierarchical system achieved in our simulations brings modeling to the same scale with microscopic characterization. This opens a way to compare directly structural features, such as macrofibril twist, between the model and reality.

The predictions were compared against measurements of bound water content, porosity and hierarchical twisting of wood fibers with low hemicellulose content. The simulations reproduce correctly the magnitude of the right-handed twist in both the single fibrils and the macrofibril. The dynamics of twisting is strongly coupled with an increase of conformational disorder in the constituent cellulose chains. Moreover, when the twist rate is extrapolated to the fiber level, the prediction agrees with our measurements on bleached birch kraft pulp. Mismatch in the twist rate at the different hierarchical levels prevents co-crystallization of a microfibril bundle, and thus plays an important role in water interactions. The openness of the bundle leaves routes for water molecules to enter. The molecular water layers found in the simulations correspond quite accurately to the measured amounts of non-freezing and freezing bound water. In summary, our work shows how chirality, specific surface area and bound water content are linked with one another, and how they underlie a surprising number of experimental features of cellulose.