Membrane Porous Structure and Its Impact on Permeability and Selectivity

Membranes comprise complex porous structures, either as the actual separation medium, e.g. in the case of Dialysis, Micro- and Ultrafiltration or, in the case of composites used for gas separation, Nanofiltration and Reverse Osmosis, where an underlying porous support can impact the overall performance of the membrane. Such structural features have a wide impact on membrane performance, ultimately dictating the water permeability and solute selectivity as well as concentration polarization, propensity for and severity of fouling.

The porous structures can be characterized by their pore-size distribution as well as their spatial distribution, which can further be divided between the membrane surface distribution vs the depth distribution. Here, we theoretically probe the relative importance of these distributions for several illustrative cases: pressure-driven flow as impacted by membrane surface pore locations and depth morphologies, the obstruction presented by a support layer to diffusion through the overlying thin-film, and the selectivity of a porous membrane as affected by a porosity gradient. In particular, we examine the difference between periodic and random spatial distributions of pores.

Separation membranes come in many morphological forms, mostly dependent on the materials used as well as the fabrication method and conditions. Identifying advantageous porous structures would benefit design and motivate the development of better control over the formation of a given morphology.