Membrane Form

The transmembrane flux is inversely proportional to the membrane thickness and is directly proportional to the membrane area and to the applied pressure differential across the membrane. Thus, a membrane manufacturer s primary objectives typically revolve around means for producing the thinnest possible membrane in a structural form that will accommodate the applied pressure while maximizing the membrane surface area. Thus, knowledge of the stractur-al and mechanical properties of the membrane material is of paramount importance. Unfortunately, many polymers of interest as gas-permeable membranes are rubbery materials with poor mechanical strength. Hence, many membranes require an underlying support material that can accommodate the appHed pressure load. 

Membrane properties, as discussed in the previous section are typically measured in dense polymer film approaching one millimeter in thickness, less than 100 cm area, and supported on a porous ceramic or metal backing plate. Scale up of flat-fihn membranes to commercial scale has been quite Hmited, typically to applications involving small quantities of gas, low-pressure differentials, and to high flux, low selectivity membranes. 

A number of researchers have produced capillary tubing and hollow-fibers from materials of sufficient strength to avoid the need for a porous support. These materials are typically melt spun into hair-size fibers having dense walls of sufficient strength to obviate the need for a supporting layer.. While the dense layer offers substantial resistance and limits the permeation flux, the hair-size of the hoUow-fibers enables designs that accommodate high membrane surface area densities (area per unit volume) [32]. 

Loeb and Sourirajan [16] introduced the unitary asymmetric membrane. These membranes consist of a microscopically thin skin on a porous support formed of the same material as the skin. By judicious selection of solvents, coagulants, and processing conditions, these researchers were able to precipitate polymer solutions to form both skin and porous support in a single processing 

The hkelihood that the specialty and commercial polymers can be from the same polymer class also increases the chances for comparable properties and thereby improves adhesion between the composite layers. Finally, co-spinning involves little additional processing facilities save the additional dope supply system. 

---

Another version of the urea electrode (Figure 11.17) immobilizes the enzyme in a polymer membrane formed directly on the tip of a glass pH electrode. In this case, the electrode s response is... 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Phase Inversion (Solution Precipitation). Phase inversion, also known as solution precipitation or polymer precipitation, is the most important asymmetric membrane preparation method. In this process, a clear polymer solution is precipitated into two phases a soHd polymer-rich phase that forms the matrix of the membrane, and a Hquid polymer-poor phase that forms the membrane pores. If precipitation is rapid, the pore-forming Hquid droplets tend to be small and the membranes formed are markedly asymmetric. If precipitation is slow, the pore-forming Hquid droplets tend to agglomerate while the casting solution is stiU fluid, so that the final pores are relatively large and the membrane stmcture is more symmetrical. Polymer precipitation from a solution can be achieved in several ways, such as cooling, solvent evaporation, precipitation by immersion in water, or imbibition of... 

Tubular Modules. Tubular modules are generally limited to ultrafiltration appHcations, for which the benefit of resistance to membrane fouling because of good fluid hydrodynamics overcomes the problem of their high capital cost. Typically, the tubes consist of a porous paper or fiber glass support with the membrane formed on the inside of the tubes, as shown in Figure 24. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

Lipids in model systems are often found in asymmetric clusters (see Figure 9.8). Such behavior is referred to as a phase separation, which arises either spontaneously or as the result of some extraneous influence. Phase separations can be induced in model membranes by divalent cations, which interact with negatively charged moieties on the surface of the bilayer. For example, Ca induces phase separations in membranes formed from phosphatidylserine (PS)... 

ITowever, membrane proteins can also be distributed in nonrandom ways across the surface of a membrane. This can occur for several reasons. Some proteins must interact intimately with certain other proteins, forming multisubunit complexes that perform specific functions in the membrane. A few integral membrane proteins are known to self-associate in the membrane, forming large multimeric clusters. Bacteriorhodopsin, a light-driven proton pump protein, forms such clusters, known as purple patches, in the membranes of Halobacterium halobium (Eigure 9.9). The bacteriorhodopsin protein in these purple patches forms highly ordered, two-dimensional crystals. 

Fig. 6. Proton-driven transport of alkali metal ions through a membrane formed from 12-crown-4 polymer (43 n = 1) (crown ether content of about 30%). M+], and (M+fc refer to metal ion concentrations at time - i and 0, respectively. (Cited from Ref.471)...

An oligomeric protein that spans a cell membrane forming a regulated pore through which Ca2+ can pass. Ca2+ channels differ considerably in their selectivity for Ca2+ over other cations For example DP3R are poorly selective, voltage-dependent Ca2+ channels are vety selective. 

SCF Due to splice variants there are soluble and membrane forms of SCF KIT/SCFR Hematopoiesis, gametogenesis, and me-lanogenesis... 

Additional exchange of ion pairs and solvent molecules as in any other membrane formed by polyelectrolytes. 

A solid emulsion is a suspension of a liquid or solid phase in a solid. For example, opals are solid emulsions formed when partly hydrated silica fills the interstices between close-packed microspheres of silica aggregates. Gelatin desserts are a type of solid emulsion called a gel, which is soft but holds its shape. Photographic emulsions are gels that also contain solid colloidal particles of light-sensitive materials such as silver bromide. Many liquid crystalline arrays can be considered colloids. Cell membranes form a two-dimensional colloidal structure (Fig. 8.44). 

Membranes are highly viscous, plastic structures. Plasma membranes form closed compartments around cellular protoplasm to separate one cell from another and thus permit cellular individuality. The plasma membrane has selective permeabilities and acts as a barrier, thereby maintaining differences in composition between the inside and outside of the cell. The selective permeabilities are provided mainly by channels and pumps for ions and substrates. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, and there are special areas of membrane strucmre—the gap junctions— through which adjacent cells exchange material. In addition, the plasma membrane plays key roles in cellcell interactions and in transmembrane signaling. 

Figure 41-5. Diagram of a section of a bilayer membrane formed from phospholipid molecules. The unsaturated fatty acid tails are kinked and lead to more spacing between the polar head groups, hence to more room for movement. This in turn results in increased membrane fluidity. (Slightly modified and reproduced, with permission, from Stryer L Biochemistry, 2nd ed. Freeman, 1981.)...

More than half of the total mass of the ATPase molecule is exposed on the cytoplasmic surface of the membrane, forming the 40-A x 60-A particles seen by negative staining electron microscopy [88 93]. 

In spite of this, the relatively low selectivity to gases associated with the lack of solubility in conventional membrane forming dipolar aprotic solvents prevented the facile preparation of PPO membrane systems for actual use in gas separations. 

Nonequilibrium thermodynamics provides a second approach to combined convection and diffusion problems. The Kedem-Katchalsky equations, originally developed to describe combined convection and diffusion in membranes, form the basis of this approach [6,7] ... 

III. Endocytosis Invagination of the plasma membrane forming an internalized membrane vesicle. 

Sujak, A., W. Okulski, and W.I. Gruszecki. 2000. Organisation of xanthophyll pigments lutein and zeaxanthin in lipid membranes formed with dipalmitoylphosphatidylcholine. Biochim. Biophys. Acta 1509 255-263. 

Subczynski, W. K., A. Wisniewska, J.-J. Yin, J. S. Hyde, and A. Kusumi. 1994. Hydrophobic barriers of lipid bilayer membranes formed by reduction of water penetration by alkyl chain unsaturation and cholesterol. Biochemistry 33 7670-7681. 

---