| Diffusivity-based separation of gas mixtures with microporous media is a topic of great technological and scientific importance. The separation of air in this mode is particularly challenging since oxygen and nitrogen molecules have very similar shapes and sizes. An important factor in the design of superior separation media is a knowledge of the molecular-level phenomena responsible for the sieving. In particular, the importance of understanding the relative roles of energetic and entropic (confinement) effects in soft (polymeric) and hard (inorganic) materials has been pointed out [Singh and Koros, Ind. Eng. Chem. Res. 35, 1231 (1996)]. This issue is difficult to resolve experimentally, as evidenced by the wide range in reported literature values reviewed here. We take a complementary approach based on molecular modeling, statistical mechanics, and transition-state theory.;First we study the separation of oxygen and nitrogen in model windows. Based on a fundamental derivation of expressions for separation selectivity, we calculate the energetic and entropic selectivities for a range of window sizes. Atomic-level flexibility (vibration) is also considered. The entropic selectivities are significantly lower than previously reported theoretical results, but are still consistent with experimental data. The energetic selectivity is seen to be very sensitive to the window dimensions and flexibility, but the entropic contribution is much less affected.;After gaining insight into this separation in simple models, we study the entropic and energetic selectivities in glassy polymers using transition-state theory. The selectivity behavior in rigid and flexible matrices is contrasted. Again, the entropic selectivities are not significantly affected by flexibility. Our findings indicate that the lower entropic selectivities offered by soft (polymeric) materials may be the result of geometric factors, rather than lack of rigidity per se.;We also use molecular dynamics to gain an understanding of gas separation in membranes comprised of a mixture of inorganic and polymeric materials. The permeation of small molecules (helium and neon) in simple membrane models made of long hydrocarbon chains in bulk and under confinement is studied. The effects of polymer loading and the state of the polymer (rubbery/glassy) on the membrane performance are investigated. |
