Icing in the Cake: Water in Nanoscopic Confinement by Cellulose

Area of Science:

• Materials Science
• Biophysics
• Computational Chemistry

Background:

• Water plays a crucial role in the structure and function of cellulosic materials and plant cell walls.
• Water molecules have been observed to occupy fibril-fibril interfaces within cellulose aggregates, suggesting a structural function.

Purpose of the Study:

• To investigate the properties of water confined between cellulose surfaces using molecular dynamics simulations.
• To analyze the impact of varying confinement distances (L) on water behavior, from submonolayer to bulk conditions.
• To explore interactions between different crystalline faces of cellulose and water molecules.

Main Methods:

• Molecular dynamics simulations were employed to model water confined by cellulose surfaces.
• Simulations varied the confinement distance (L) to represent different water layer thicknesses.
• Different crystalline faces of cellulose were studied to understand surface interactions.

Main Results:

• Confinement significantly hinders molecular motion, with translational self-diffusion slowing by up to three orders of magnitude in defective monolayers.
• Water molecules exhibit strong preferential orientation relative to confining cellulose surfaces under confinement.
• Water layer mass density shows non-monotonic behavior with decreasing pore size, increasing towards ice-like densities at monolayer separation and sharply dropping in submonolayer regimes.

Conclusions:

• Confined water in cellulose displays unique properties, including reduced mobility and anisotropic reorientation, especially in defective layers.
• The observed phenomena highlight the intricate atomistic-scale interactions between cellulose and water.
• Findings provide insights into the structural role of water in cellulosic materials.