Chemical modification of the membrane

The separation of proteins can be improved by chemical modification of the membrane surface [94]. Coating a Carbosep M5 membrane (10,000 D) with quaternized polyvinylimidazole raises the retention of tetracycline from 25% towards 76%. Unfortunately the flux declines at the same time from 321/m h to 7.61/m h. 

Often preventive measures may be taken to avoid fouling the membrane. Prefilters or screens can be used to remove large particles which block thin channels or accumulate in stagnant areas of the module. High cross-flow velocities tend to sweep deposits away. Low pressures avoid compaction of gels on the membrane. Some polymers have a higher susceptibility to fouling and chemical modification of the membrane surface can have a profound effect on the propensity to foul. 

MBfRs depend on the ability of microorganisms to attach to the membrane surface. Several researchers have modified membranes to promote microbial attachment and retention. The methods can be classified as chemical modification of the membrane material or addition of support material onto the membrane surface. 

As a summary, it is possible to improve both, blood compatibility and the adsorption of dialysate contaminants, such as endotoxins, through a chemical modification of the membrane poly-mer, as realized in some polysuHone dialysis membranes. 

Modification of the membranes affects the properties. Cross-linking improves mechanical properties and chemical resistivity. Fixed-charge membranes are formed by incorporating polyelectrolytes into polymer solution and cross-linking after the membrane is precipitated (6), or by substituting ionic species onto the polymer chain (eg, sulfonation). Polymer grafting alters surface properties (7). Enzymes are added to react with permeable species (8—11) and reduce fouling (12,13). 

Another method used to vary the AG° of the recombination reaction without chemical modification of the centers, consists of placing the system in an electric field whose orientation and intensity are well defined [141]. However, the energy level shifts induced by the field also change the electronic factors, so that the interpretation of the experimental results is not straightforward. Bixon and Jortner have proposed using electric field effects to elucidate the nature of the primary electron step in bacterial photosystems [142], a problem that will be discussed in Sect. 3.5. One basic difficulty encountered in this method is the evaluation of the internal field effectively seen by the redox centers in the membrane. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Modification of the membranes affects the properties, Cross-linking improves mechanical properties and chemical resistivity, Fixed-charge membranes are formed by incorporadng polyelectrolytes into polymer solution and cross-linking after the membrane is precipitated, or by... 

In other words, it proves that it is possible to design any experiment in such a way that it exceeds the limit of its validity. Both the adhesion and the Severinghaus effect can be mitigated by proper design of the membrane (Li et al., 1988) or by chemical modification of the surface of the dielectric. 

Among the numerous approaches studied so far to minimize such phenomena in ED, it is worth citing pretreatment of the feed solution by coagulation (De Korosy et al., 1970) or microfiltration (MF) or ultrafiltration membrane processing (Ferrarini, 2001 Lewandowski et al., 1999 Pinacci et al., 2004), turbulence in the compartments, optimization of the process conditions, as well as modification of the membrane properties (Grebenyuk et al., 1998). However, all these methods are partially effective and hydraulic or chemical cleaning-in-place (CIP) is still needed today, thus... 

Chemical modification of a membrane surface can be used in combination with spacers and periodic applications of bioacids [70]. The paper by Redondo, however, is short on specifics (e.g., details of chemical modification of aromatic polyamides membrane surface), and therefore not very useful to those looking for insights into membrane fouling. 

The use of Schott s porous glass membremes (pore sizes from 10 to 90 nm) in the separation of proteins with molecular weights from 14,400 to 450,000 is illustrated by Langer and Schnabel [85] who show a decrease in retention with increasing pore size for different proteins. Due to the chemical nature of the membrane material, it lends itself to surface modifications, including the coupling of enzymes or the attachment of micro-organisms. 

Heat Treatment and Various Other Modifications. Heat treatment and other modifications, such as chemical treatment of the membrane, are well known as a last step to modify or "tailor" the membrane for its final use. 

The idea that chemical modification of the lipids of biological membranes could be achieved in situ was first demonstrated in 1976 [1]. The rationale underlying the work was that if the unsaturated double bonds were largely responsible for the fluid character of the membrane lipid matrix, their saturation would result in a reduction in fluidity. Although simple in concept, the practice required application of an entirely novel approach to the catalytic hydrogenation of lipids. 

Transport of newly synthesized PE to the plasma membrane. When an ethanolamine precursor is used, the primary site of PE synthesis is the ER (Chapter 8). The appearance of newly synthesized PE at the external leaflet of plasma membrane has been determined using chemical modification of the cell surface with TNBS at reduced temperature (R. Sleight,... 

Solute transport across the cytoplasmic membrane of bacteria occurs by two major mechanisms (i) Secondary transport systems transport by these systems is driven by electrochemical gradients and will lead to the translocation of solute in unmodified form (ii) Group translocation solute is substrate for a specific enzyme system in the membrane the enzyme reaction results in a chemical modification of the solute and release of the products at the cytoplasmic side. The only well-established group translocation system is the phosphoenolpyruvate phospho-transferase system (PTS) (see below). 

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