Membrane modifications

Because most of the established membrane polymers can not meet all the performance requirements for a membrane dedicated to a particular application, membrane modifications are gaining rapidly increasing importance. Membrane modification is aimed either to minimize undesired interactions, which reduce membrane performance (e.g., membrane fouling), or to introduce additional interactions (e.g., affinity, responsive or catalytic properties) for improving the selectivity or creating an entirely novel separation function [3]. Three general approaches can be distinguished  

In order to achieve membrane surface modification by chemical reactions, the following approaches have been proposed [3]  

Photografting technologies, that is, the control of chemical surface functionalization by highly selective excitation with UV light, can be used for grafting to and grafting from and has been intensively explored for controlled functionalization of polymeric membranes [29]. 

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T. Parassassi, F. Conti, E. Gratton, and O. Sapora, Membrane modification of differentiating proerythroblasts. Variation of l,6-diphenyl-l,3,5-hexatriene lifetime distributions by multifrequency phase and modulation fluorimetry, Biochim. Biophys. Acta 898, 196-201 (1987). 

In both situations the interaction of the medium inside the pore with the pore wall (1) is increased (2) or changed which affect the transport and separation properties (surface diffusion, multilayer adsorption) and/or help overcome equilibrium constraints in membrane reactors. Membrane modifications can be performed by depositing material in the internal pore structure from liquids (impregnation, adsorption) or gases. Several modification possibilities are schematically shown in Figure 2.3. Some results obtained by Burggraaf, Keizer and coworkers are summarized in Table 2.7. Composite structures on a scale of 1-5 nm were obtained. 

Coverage of thermal, chemical, surface, and mechanical properties of inorganic membranes includes discussion of pore diameter, thickness, and membrane morphology. You ll gain valuable insights into membrane modification, as well as the design and operation of membrane filtration units. 

Most recently, significant research efforts have been focused on materials compatibility and adhesion at the zeoHte/polymer interface of the mixed-matrix membranes in order to achieve enhanced separation property relative to their corresponding polymer membranes. Modification of the surface of the zeolite particles or modification of the polymer chains to improve the interfacial adhesion provide new opportunity for making successful zeolite/polymer mixed-matrix membranes with significantly improved separation performance. 

Miscellaneous membrane modification, film modification, antifouling compositions, analytical agents... 

This work deals with membrane modification to test... 

Wijers, M.C., Jin, M., Wessling, M. and Strathmann, H. (1998) Supported liquid membranes modification with sulfonated poly(ether ether ketone) - permeability selectivity and stability Journal of Membrane Science, 147, 117. 

Rintoul DA, Sklar LA, Simoni RD (1978), Membrane modification of Chinese hamster ovary cells, J. Biol. Chem. 253 7447-7452. 

Momchilova, A., Petkova, D., and Mechev, I., Sensitivity of 5 nuleotidase and phospholipase A2 towards liver plasma membrane modifications, Int. J. Biochem., 17, 787, 1985. 

Kawasaki, K., Ernst, R., Miller, S. Inhibition of Salmonella enterica serovar Typhimurium lipopolysaccharide deacylation by aminoarabinose membrane modification. J Bacteriol 187 (2005) 2448-2457. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

Based on these observations, Wang and Caruso [237] have described an effective method for the fabrication of robust zeolitic membranes with three-dimensional interconnected macroporous (1.2 pm in diameter) stmctures from mesoporous silica spheres previously seeded with silicalite-1 nanoparticles subjected to a conventional hydrothermal treatment. Subsequently, the zeolite membrane modification via the layer-by-layer electrostatic assembly of polyelectrolytes and catalase on the 3D macroporous stmcture results in a biomacromolecule-functionalized macroporous zeolitic membrane bioreactor suitable for biocatalysts investigations. The enzyme-modified membranes exhibit enhanced reaction stability and also display enzyme activities (for H2O2 decomposition) three orders of magnitude higher than their nonporous planar film counterparts assembled on silica substrates. Therefore, the potential of such structures as bioreactors is enormous. 

Development of better membranes and membrane modification methods... 

The overall energy conversion efficiency for the complete process of mathanol chemical energy conversion dc electric energy will be similar (43%) for a system with an RAFC operating at 0.70 V and a DMFC operating at 0.55 V, provided the fuel efficiency in the DMFC is raised to 90% (the latter requirement could be possibly achieved by combination of anodic effects and membrane modification). 

In order to enhance the overall performance of the membrane, it is necessary to modify the membrane material or the structure (41). The objectives for modification of the existing membranes are to increase flux, selectivity, and chemical resistance (solvent resistance, swelling resistance, and fouling resistance). Some of the most commonly practiced membrane modification methods are listed in Table 3. 

The first reported membrane modification method involved annealing of porous membranes by heat-treatment. Zsigmondy and Bachmann in 1922 demonstrated that... 

R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Polymeric-sUica based sols for membrane modification applications sol-gel synthesis and characterization with SAXS. /. Non-Cryst. Solids, 191 (1995) 1. 

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