Layers can be deceiving: A small molecule transport model in layered molecular crystals

Layered stacking is a common packing motif in molecular crystals and can induce significant anisotropy in material properties. Graphitic-like packing structures suggest the presence of nanoscopic channels between layers, but the permeability of these lattices to small molecules remains poorly understood. In this study, we employ steered molecular dynamics (MD) simulations to predict the diffusion behavior of H₂O and noble gases through a layered molecular crystal. Our findings reveal that small molecule transport occurs via a hopping mechanism characterized by distinct jumps between interstitial sites. Counterintuitively, we observe that transport is much faster perpendicular to the layers compared to between them. This is attributed to the relative stability of molecules residing within intralayer junctions between adjacent molecules, which results in lower energetic barriers for diffusion in directions normal to the layers. An empirical model derived from the MD data indicates that diffusion rates decrease exponentially with increasing molecular radius and is negligible for molecules larger than helium, including common atmospheric gases. These results have important implications for the interpretation of experiments measuring surface area, material response to extreme conditions, and aging processes.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It has been approved for unlimited release under document number LLNL-ABS-870582.