The importance of pore exit effects on the diffusion of molecules in AlPO4-
5 pores is evaluated using two molecular modeling techniques. In the first
approach, a dual control volume grand canonical molecular dynamics techniqu
e is used to obtain molecular fluxes of methane out of the truncated crysta
l as a function of temperature and sorbate loading. The simulation results
indicate the presence of a low-temperature surface barrier for diffusion, w
hich retards the flux of methane relative to its apparent flux in the intra
crystalline regions of the material. This pore exit barrier tends to dimini
sh as temperature and loading increase. An explanation based on clustering
phenomena is proposed to explain the latter. Next, a simple activated trans
port model is proposed to predict the relative importance of the surface ba
rrier on the transport of sorbates in AlPO4-5. The potential of mean force
for a single sorbate molecule along the pore axis of a truncated crystal pr
ovides the required activation energy barriers for the model. The model cor
rectly predicts the reduction in the importance of exit effects with an inc
rease in the temperature. It is also observed that exit effects become more
important as the ratio of the size of the sorbate molecule to the pore siz
e approaches unity. In particular, exit effects are significant in micromet
er-thick AlPO4-5 crystals in the case of large molecules such as SnBr4 and
CCl4 at room temperature.