Influence of Boundaries of Nanoporous Crystals on Molecular Exchange under the Conditions of Single-File Diffusion

We study the tracer exchange of molecules between the phase adsorbed in one-dimensional channels and the
surrounding gas phase by molecular dynamics simulations. Under the conditions of single-file diffusion, a
novel boundary effect is observed. The shape of the tracer-exchange concentration profiles deviates from
those obtained under the conditions of normal diffusion. Compared to the profiles for normal diffusion, which
correspond to the same degree of exchange, the equilibrium concentration is reached faster at the boundaries
and slower in the middle part of the channel in the case of single-file diffusion. This boundary effect is
observed for the system neopentane in AlPO4-5 (which was chosen as a reference system), as well as for
modified systems. The effect can be understood considering two diffusion mechanisms which occur in parallel.
First, the diffusion of the whole chain of particles, that is, the center-of-mass diffusion, obeying the laws of
normal diffusion. Second, the individual movement of the particles relative to the center of mass of the
chain. The second mechanism admits additional displacements which, on average, lead to an accelerated
exchange of the marginal particles.