Permeability of a membrane to a specific permeant is an intrinsic property of the membrane's material because both the solubility and diffusivity of component i depend on the chemical and physical interactions with the membrane's material. For example, in the case of gas separation through dense polymer membranes, D always decreases with the increasing size of the gas molecule. The extent of this decrease is generally dependent on the flexibility of the polymer backbone. On the other hand, the solubility of gases generally increases with molecular size, because the intermolecular forces between gas and polymer increase. These are some general rules which can be useful for a first understanding, while a comprehensive review of the relationships between material structure and transport properties can be found in the work of Matteucci et al. and Wang et al. In porous membranes, transport occurs through the empty spaces in the membrane, as a consequence transport is governed in the first place by membrane morphology. Morphology includes surface and volume porosity, pore size distribution, and tortuosity. Depending on the pore size, different mechanisms will dominate molecular transport. These mechanisms include, broadly: Knudsen diffusion, surface diffusion, capillary condensation, and molecular sieving . For an exhaustive review of the permeation mechanisms in porous membranes, we like to refer to the work of Oyama et al. and Stratis et al. The properties used to estimate the separation performance of a porous membrane are permeance, retention, and molecular weight cut-off (MWCO). The permeance is defined as the net transport of constituents through the membrane 2 . Since transport is not an intrinsic membrane material property, permeance is not normalized for the membrane thickness.