Membrane transport background information

The remainder of this overview chapter provides fimdamental background information related to transport of small molecules in polymers and then describes materials design strategies to prepare polymers with excellent permeability and selectivity properties for both supercriticd gas separations and vapor separations. In addition to high permeability and selectivity, membranes must also be stable in industrial process environments, which may be chemically and/or thermally challenging. For example, due to chemical stability and thermal transition temperatures of polymers used in gas separations, these materials are typically used at or near ambient temperatures. The chapter by Bayer et al. in this book describes the use of selective crosslinking of polyimides to prepare high performance membrane materials that are stable to 300°C. 

Transport of protons and water are the two phenomena of prime interest. Prior to examining the mechanisms that govern their transport, it is useful to review some of the background briefly that informs model formulation, including relevant aspects of membrane morphology, hydration behaviour and sorption isotherms. 

With the above-mentioned background, it is informative to describe an application of the N- P equation as applied to the nerve axon, which is surrounded by a plasma membrane. One of the main functions of the plasma membrane is to control the passage of ions and molecules into and out of the cell. For most biomembranes, the intracellular [K+]i greatly exceeds extracellular [K+]o, and the opposite is true for the extracellular [Na+]o and [Cl ]o. These concentration differences are due to the active transport system embedded in the lipid bilayer of the plasma membrane [3]. 

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