All processing methods of cellulosic materials have to deal with the unavoidable presence of water, therefore good understanding of cellulose-water interactions is critical. Even far below the saturation point, water will remain present, sorbed to specific sorption sites (so-called bound water). It has previously been shown that the presence of water on the fibril-fibril interfaces inside cellulose fibril aggregates is thermodynamically favourable. The objective of this study is to investigate the properties and role of the water confined between the nanoscale fibres, and to give insight into the thermodynamics of water removal.

Atomistic scale computer simulations are a popular tool for understanding nanoscale phenomena in cellulosic materials. This study uses all-atom molecular dynamics simulations of systems comprised of a thin layer of water between infinitely large sheets of crystalline cellulose, as well as simulations of a model fibril aggregate. Between the simulations, the distance between the confining cellulose planes is varied. The simulations allow for investigation of the properties of the water inside of cellulose fibril aggregates on a molecular level. Equilibrium simulations are combined with simulations of stepwise dehydration, where water is gradually removed.

Results show that water forms periodic structures on top of the cellulose surface. These structures can be quantified and compared by defining a spatial order parameter, and in these comparisons, the effect on confinement is revealed. Furthermore, from the translational and rotational diffusion of the water molecules, it is shown that water confined in cellulose has lower molecular mobility than bulk water. Both phenomena are connected to structural features of the cellulose fibril itself. Simulations of continous dehydration further uncover the ways the properties of the confined water shift, as well as the hydration forces at play during this process. These analyses are coupled with an analysis of the thermodynamics, providing the precise changes in the excess chemical potential of confined water molecules during drying.

Structure of the cellulose fibril aggregate

Jakob Wohlert

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