Membrane asymmetrical

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

Asymmetric membranes have a tight, low-permeability, retentive zone that performs the desired separation and a more open, high-permeability zone that provides mechanical strength to the overall membrane. This structure is particularly critical to the economic viability of reverse-osmosis membranes. Asymmetric membranes operated in TFF mode must have the tight side facing the feed channel so that particles are retained on its surface and can be acted upon by the tangential flow. Asymmetric membranes operated in NFF mode can... 

Terms such as symmetric and asymmetric, as well as microporous, meso-porous and macroporous materials will be introduced. Symmetric membranes are systems with a homogeneous structure throughout the membrane. Examples can be found in capillary glass membranes or anodized alumina membranes. Asymmetric membranes have a gradual change in structure throughout the membrane. In most cases these are composite membranes... 

In general the sphingolipids are located on the exterior face of a membrane while the phospholipids make up the inner face. This is understandable when we recall that sphingolipids include the gangliosides and cerebrosides whose polar ends contain carbohydrates or complex carbohydrate derivatives. The large number of chiral centers in carbohydrate molecules offer a complex pattern on the surface of the membrane which can impart a large degree of specificity for a particular cell type. The composition and limited fluidity of the bilayer make the entire membrane asymmetric, that is, different on the inner and outer layers or leaflets. 

Fig. 16 SEM image of the cross section of a theophylline-imprinted membrane asymmetric structure of imprinted membrane prepared by the wet phase inversion process. Reproduced with permission from [217]...

Lipase immobilized hollow-fiber membranes Asymmetric dihydroxylation Asymmetric epoxidation... 

Membrane asymmetric homogeneous, or microporous (cellulose acetate, polyamide, polysulfone, polyacrylonitrile) composite of a homogeneous polymer film on microporous substructure (cellulose acetate, polyamide, polysulfone, polyimide, polyvinyl alcohol). Usual pore 0.1 to 0.2 jm. 

Membrane asymmetric homogeneous or microporous active dense 20- to 50- jm layer of cellulose acetate with total thickness 100 pm composite of a homogeneous polymer him on microporous substructure. 

Membrane asymmetric thin film, noncellulose membrane membrane usually negatively charged to reject anions. 

Separation, CO2/C2 hydrocarbons Separation of mixtures Separation, membrane Separation, membrane, asymmetric Separation, propane/propylene Separation, xylenes SFe-Xe diffusion in BOG, modelling Shape selectivity 11-P-24 25-P-07 25-... 

ABSTRACT Electrical phenomena in artificial cells are described. The constituent material of the cells, referred to as proteinoid or as thermal protein, have been extensively studied in the context of the origin of life, which led to the finding of excitability as one of the biofunctions. The activities found in proteinoid cells are such as to make them useful models for modern excitable cells as well as for protocells. For example, the proteinoid cells display double membrane, asymmetric permeability, membrane potentials, action potentials, and photoactivity. 

Ulbricht and Schwarz (1997) applied for the first time the pore-filling heterogeneous graft copolymerization concept to asymmetric membranes. Asymmetric PAN membranes (average pore sizes of 7 or 12 nm) were used as matrix of polymeric PV separation phases, which were prepared in situ by heterogeneous photo-initiated graft copolymerization. By this means, defect-free and stable layers were synthesized from moderately hydrophilic methacrylates such as poly(ethylene glycol methacrylates). 

Membrane Asymmetric membranes AI2O3 or HO2 on AI2O3 support (Larbot. 1986 Cot, 1988)... 

Moreover, when two-layer membranes (asymmetric or composite) are studied, the total system is electrode/solution (c)/membrane (dense/porous layers)/solution (c)/ electrode and the impedance plots can then present three relaxation processes (two at the lowest frequencies which are associated with the membrane itself plus the contribution of the electrolyte solution at high frequencies with fmax 10 Hz), as can observed in Figure 9.4. Note, equivalent circuit (ReCe)-(RiQi)-(R2C2). 

The dope formulation for dense mixed-matrix films must be revisited for making asymmetric membranes as the structure of the membrane is controlled by phase separation kinetics. Different solvents used in dense film formulation may be used in asymmetric formulations to control the phase separation rate of the membrane within the nonsolvent (quench) bath and obtain membranes of varying porosities." Further, nonsolvents and additives can be added to fine tone the morphology of the membrane to control pore sizes and suppress the formation of large voids within the membrane. """ Asymmetric skinned membranes can also contain a second more volatile solvent that can evaporate and form a high solids concentration layer (skin) on the exposed surface... 

Recent studies by Katz et al. (1977) have served to demonstrate a likely method for membrane glycoprotein synthesis—specifically, the insertion of the G protein of vesicular stomatitus virus (VSV) into pancreatic endoplasmic reticulum (PER). Their results are consistent with a scheme of membrane glycoprotein synthesis which postulates that the growing protein is extruded across the ER membrane with the amino terminus extruded first and is then translocated across the membrane. Asymmetric insertion, as well as glycosylation, requires the presence of membranes during protein synthesis, and glycosylation is restricted to the luminal surface of the ER. 

The selectivity (permeance ratio) of PPO membrane (asymmetric) tends to increase as the size of the nodule observed at the top surface of the membrane becomes smaller. 

All the above effects indicate that the selectivity (permeance ratio) of the PPO membrane (asymmetric) increases as the close-packing of the nodules and polymer entanglement between nodules are enhanced. In fact, it was observed that the membrane selectivities increased when nodules merged and roughness parameter decreased. 

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