Interface between monoclinic crystalline cellulose and water: Breakdown of the odd/even duplicity

The interface between the (110) crystal face of cellulose I beta and water was studied by molecular dynamics simulation with cellulose coordinates refined from electron diffraction data as a starting point. Potential energies, pucker parameters, torsion angles, and hydrogen bonding have been used for the characterization. Only the topmost layer in the cellulose differs in terms of structure and dynamics from the crystal bulk, but even these difference are small. At the surface approximately half of the cellulose intermolecular hydrogen bonding is lost, but this is compensated by hydrogen bonds with water molecules. Much of the difference between even and odd (200) planes disappears at the interface, except for the orientation of the glucose ring plane. Water dynamics is retarded by a factor of 2-3 close to the surface. The potential energy of water molecules in the first hydration layer is lower by 2 kJ/mol. The cellulose surface contains about five exposed hydroxyl groups per square nanometer, which accounts for the good hydration of the surface.