Supported membranes

There has been a surge of research activity in the physical chemistry of membranes, bilayers, and vesicles. In addition to the fundamental interest in cell membranes and phospholipid bilayers, there is tremendous motivation for the design of supported membrane biosensors for medical and pharmaceutical applications (see the recent review by Sackmann [64]). This subject, in particular its biochemical aspects, is too vast for full development here we will only briefly discuss some of the more physical aspects of these systems. The reader is referred to the general references and some additional reviews [65-69]. 

In order to maintain a definite contact area, soHd supports for the solvent membrane can be introduced (85). Those typically consist of hydrophobic polymeric films having pore sizes between 0.02 and 1 p.m. Figure 9c illustrates a hoUow fiber membrane where the feed solution flows around the fiber, the solvent—extractant phase is supported on the fiber wall, and the strip solution flows within the fiber. Supported membranes can also be used in conventional extraction where the supported phase is continuously fed and removed. This technique is known as dispersion-free solvent extraction (86,87). The level of research interest in membrane extraction is reflected by the fact that the 1990 International Solvent Extraction Conference (20) featured over 50 papers on this area, mainly as appHed to metals extraction. Pilot-scale studies of treatment of metal waste streams by Hquid membrane extraction have been reported (88). The developments in membrane technology have been reviewed (89). Despite the research interest and potential, membranes have yet to be appHed at an industrial production scale (90). 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

Finally, to evaluate the membranes, analysis such as X-ray diffraction (XRD), SEM, TEM and light scattering were performed at the School of Mineral and Material Engineering, Universiti Sains Malaysia. The last part of the work, testing the produced membrane to remove emulsifier oil from domestic wastewater, was accomplished on a limited budget. An experimental rig and membrane module were required. Also the need for experimental data for the application of the supported membrane may show the real success of this project. 

In the case of supported membranes also, the support can play an important role in the separation performance of the membrane in the gas as well as in the Hquid phase [101-103]. Transport in these support pores can be accurately described by the Dusty Gas Model [100, 104] although it is put forward by Kerkhof and Geboers that their Binary Friction Model is physically more correct [105]. 

Sackmann, E. (1996) Supported membranes scientific and practical applications. Science, 271, 43—48. 

Tanaka, M. and Sackmann, E. (2005) Polymer-supported membranes as models of the cell surface. Nature, 437, 656-663. 

Mossman, K. and Groves, J. (2007) Micropatterned supported membranes as tools for quantitative studies of ihe immunological synapse. Chem. Soc. Rev., 36. 46-54. 

Kawasaki et /. (1996) have used a supported membrane catalyst for extraction of erythromycin from its dilute, slightly alkaline aqueous solutions. 1-Decanol was used as an intermediate fluid membrane phase and a buffered acidic aqueous solution was used to strip the organic membrane. 

In 1886, Brown11 discovered an organism which formed extremely tough membranes when cultivated m suitable nutrient solutions containing carbohydrates such as D-fructose, D-mannitol or D-glucose ethanol, sucrose or starch did not support membrane formation by this organism which Brown called Bacterium xylinum ) (Acetobacter xylinum). The membranes were readily soluble in cuprammonium hydroxide solution and yielded a dextrorotatory sugar upon acid hydrolysis. These properties and the results of combustion analysis led him to believe that the membrane was cellulose. 

Leenars, A.EM. and A.J. Burggraaf, The preparation and characterization of alumina membranes with ultra-fine pores, Part 2. The formation of supported membranes, ]. Coll. Interface Sci., 105(1), 27-40, 1985. 

A very narrow light path must be used in order to ensure that closely adjacent bands are resolved. If the instrument measures transmitted light it is necessary to make the supporting membrane translucent. This can be done by either impregnating the strip with an oil with a high refractive index or, for... 

S.-F. Sui, T. Urumow, and E. Sackmann, Interaction of insulin receptors with lipid bilayers and specific and nonspecific binding of insulin to supported membranes, Biochemistry 27, 7463-7469 (1988). 

In the discussions to follow two types of membranes must be distinguished (1) nonsupported and (2) supported ones. Nonsupported membranes are produced by pouring a slip onto a very smooth, dense substrate on which gelling takes place by slow evaporation of the dispersion liquid. In this way rather thick, crack-free films can be obtained. They are especially suitable for characterization purposes and structural investigations. These are evaluated to determine whether or not the structures obtained are similar or comparable with those of the supported ones, made from the same slip and the same material. The next section will also focus on the supported membranes. 

The term separation side or permeation side of the membrane refers to the side of the membrane opposite to the side from which the feed is supplied. For example, for the supported membrane systems shown in Figure 7.2 with the feed entering from the side of the top layer, the separation or permeation side is the side of the support. 

Porous 7-AI2O3 supported membranes (plate-shaped), thickness 5-8 fitn, mean pore diameter 4-5 nm. Reactants enter the reactor from opposite membrane sides. 

Zeolite membranes are generally synthesized as a thin, continuous film about 2-20 xm thick on either metallic or ceramic porous supports (e.g., alumina, zirco-nia, quartz, siHcon, stainless steel) to enhance their mechanical strength. Typical supported membrane synthesis follows one of two common growth methods (i) in situ crystallization or (ii) secondary growth. Figure 10.2 shows the general experimental procedure for both approaches. 

For the purposes of review. Figure 1 illustrates the basic function of the cathode in a solid oxide fuel cell. Whether acting alone or as part of a stack of cells, each cell consist of a free-standing or supported membrane of an oxygen-ion-conducting electrolyte, often yttria-stabilized zirconia (YSZ). Oxygen, which is fed (usually as air) to one side of the membrane, is reduced by the cathode to oxygen ions via the overall half-cell reaction... 

Irregularly hyperbranched grafts provide a useful way to modify surfaces. A variety of chemistry can be used and a wide variety of grafts can be prepared. The hyperbranched grafts can serve as supported membranes, as catalyst supports or as substrates for further covalent graft chemistry. Functional groups within these interfaces can be readily modified by solution-state chemistry. The interfaces themselves can be used as media for further chemistry within the interface or as substrates in molecular recognition and self assembly of other macromolecules. 

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