Asymmetric membranes, synthetic

Another means of classifying membranes is by morphology or structure. This is also a very illustrative route because the membrane structure determines the separation mechanism and hence the application. If we confine ourselves to solid synthetic membranes, two tyf>es of membrane may be distinguished, i.e. symmetric or asymmetric membranes. The two classes can be subdivided further as shown schematically in figure I -5. The thicknesses of symmetric membranes (porous or nonporous) range roughly from 10 to 200 )xm, the resistance to mass transfer being determined by the total membrane thickness. A decrease in membrane thickness results in an increased permeation rate. 

The asymmetric membrane consists of a skin layer, which is 1 p,m thick, and a support layer, which comprises the rest of the wall thickness (Sugaya and Sakai, 1999). The skin layer is in contact with the blood and controls the solute removal. The pore structure of the support layer is much more open and varies among the various synthetic membranes, and this layer dictates the thermal and mechanical properties of the membranes. The average pore size in the skin layer for low-flux membranes is around 10 A and for the high-flux membranes is around 30-50 A. The pore size in the support layer is greater... 

The use of synthetic polymeric membranes in separation processes [19,20] took off with the breakthrough of asymmetric membrane formation first developed by Loeb and Sourirajan [21]. These membranes have a thin dense polymer layer that governs the separation on top of a much thicker porous layer that provides mechanical support. They were first produced in the laboratory by spreading... 

A further problem was the presumed lack of, or at least a low degree of, stereospecificity of cannabinoid action. As all receptors are asymmetric, it is conceivable that their interactions with asymmetric ligands will be limited to one enantiomer only. However, synthetic (+)-zl9-THC (2) showed canna-bimimetic activity of 5-10% compared with that of the natural (-)-d9-THC (1) [19]. This observation cast doubt over the existence of a specific cannabinoid receptor and hence of a cannabinoid mediator. This presumed low degree of stereoselectivity and the above described suggestions and data on the actions of cannabinoids on membranes, delayed research aimed at the identification of a receptor-mediator cannabinoid system. 

Reverse osmosis membranes are usually synthetic membranes and are made of polymers. Depending on their structures, RO membranes can be classified as either asymmetric or composite. 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Synthetic membranes can further be classified as symmetric or asymmetric (Figure 9.1)... 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The pore size, shape, and electrostatic surface charge are fundamental to particle removal. Synthetic polymers (cellulose acetate, polyamides, etc.) and inorganic materials (ceramics, metals) are generally the principal materials of construction. Membranes may be formed with symmetric or asymmetric pores, or formed as composites of ultra thin layers attached to coarser support material. Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration relate to separation of ions, macromolecules, and particles in the 0.001 to 10 pm range (Rushton et al. 1996). 

Transbilayer movement of lipid at the endoplasmic reticulum In eukaryotic systems a detailed pattern of synthetic asymmetry has emerged with respect to the topology of the enzymes of phospholipid synthesis in rat liver microsomal membranes. Protease mapping experiments (D.E. Vance, 1977 R. Bell, 1981) have indicated that the active sites of the phospholipid synthetic enzymes are located on the cytosolic face of the ER. Thus, in both prokaryotic and eukaryotic systems, it appears that the site of synthesis of the bulk of cellular phospholipid is the cytosolic side of the membrane. This asymmetric localization of synthetic enzymes strongly implicates transbilayer movement of phospholipids as a necessary and important event in membrane assembly that is required for the equal expansion of both leaflets of the bilayer [13]. 

Asymmetric synthetic hollow fiber membranes designed for use in ultra-filtration/dialysis processes can provide an interesting support for immobilizing enzymes. 

The search for biologically active compounds (natural and synthetic) continued to be a significant inspiration for organic synthesis. Asymmetric catalysis, new synthetic reactions, and advances in separation techniques (HPLC) and analytical techniques (NMR and mass spectrometry) support these advances. In 1995, Kyriacos Costa (K.C.) Nicolaou (1946- ) at Scripps Research Institute reported the total synthesis of brevetoxin B (see the figure on page 375). This toxic substance is produced by algae in red tide and is very deadly to fish. It binds to sodium channels in membranes of muscle and nerve cells producing an excessive influx of Na. ... 

FIGURE 3.2 Asymmetric structure of the membrane. (From Matsuura, T., Synthetic Membranes and Membrane Separation Processes, CRC Press, Boca Raton, FL, 1994.)... 

Most commercial membrane separations use natural or synthetic, glassy or rubbery polymers. To achieve high permeability and selectivity, nonporous materials are preferred, with thicknesses ranging from 0.1 to 1.0 micron, either as a surface layer or film onto or as part of much thicker asymmetric or composite membrane materials, which are fabricated primarily into spiral-wound and hollow-fiber-type modules to achieve a high ratio of membrane surface area to module volume. 

Smolders CA, Vugteveen E (1985) New characterization methods for asymmetric ultrafiltration membranes. In Lloyd DR (ed) Materials science of synthetic membranes. ACS Symposium Series 269. American Chemical Society, Washington, DC, p 327... 

Synthetic polymers are made by polymerization of one monomer or by the co-polymerization of two different monomers. A broad range of structures has been produced, from linear chain polymers, snch as polyethylene, to cross-linked structnres, snch as bntyl mbber [167]. Polymer membranes can have symmetric or asymmetric strnctnres. The former, which is considered the less important type, can be porons or microporons [168]. Dense membranes, such as silicone rubber, are non-porous on a macroscopic scale [124] therefore, permeating species mnst dissolve into the polymer and then diffuse through the membrane, making them highly selective. However, mass transfer rates were much lower than those observed in porons membrane, due to the dominating solution-diffusion mechanism [167]. 

Flat-sheet asymmetric-skinned membranes made from synthetic polymers (also copolymers and blends), track-etched polymer membranes, inorganic membranes with inorganic porous supports and inorganic colloids such as Zr02 or alumina with appropriate binders, and melt-spun thermal inversion membranes (e.g., hollow-fiber membranes) are in current use. The great majority of analytically important UF membranes belong to the first type. They are usually made of polycarbonate, cellulose (esters), polyamide, polysulfone, poly(ethylene terephtha-late), etc. 

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