Membrane continued field

The search for models of biological membranes among porous membranes continued in the twenties and thirties. Here, Michaelis [67] and Sollner (for a summary of his work, see [90] for development in the field, [89]) should be mentioned. The existence and characteristics of Donnan membrane equilibria could be confirmed using this type of membrane [20]. The theory of porous membranes with fixed charges of a certain sign was developed by Teorell [93], and Meyer and Sievers [65]. 

The membrane is treated as the same simple system as described above (Figure 1). We assume, besides the electroneutrality inside the membrane (constant field hypothesis), a continuity of the electrical potential ip from the bulk of the surrounding medium to the center of the cell. We are compelled, therefore, to assume that the potential at the outer Helmholtz plane, which is induced by the effective surface charge (dissociated functional groups or strongly adsorbed ions) is screened to a major extent by counterion-site pairing in the Nernst layer. We consider the three classes of transferable chemical species quoted in the introduction. Among the species not directly involved in chemical reactions, the calculation takes into account an anion (Cl ), two monovalent cations (K, Na+) and two divalent cations (Mg2+, Ca ). As for the species involved in chemical reactions, we assume that they do not interact inside the membrane (independent channels). However, their flux balance must take into account the chemical reactions in both lateral phases. We handle only here two types of equilibria the water dissociation and the first dissociation of the carbonic acid. The three flux balances of the five chemical species which are interconnected by the conditions for the equilibria are as follows ... 

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

Membrane Chemistry Three chemical families dominate the RO-NF membrane industry. Many other products are made on a small scale, and the field continues to attract significant R D resources. But three types command most of the market. 

Membrane-based separation techniques constitute nowadays well-established process methods for industrial treatments of fluids. Like SMB, membrane-based separations can be performed in continuous mode. In the field of preparative-scale enan-tiodiscrimination, much effort has been invested in this subject due its high potential [154, 155]. (Chapter 5 of this book is devoted to the subject, and further discusses the advantages and applications of membrane technologies.)... 

If we extend this vision to the biocatalysis at silicon, as reported in some of the recent papers,641 642 and continue learning from nature how to be bioinspired in making nanoporous silicas and other membranes, then, what seemed to be true in Goody s remark about liquid crystals, might also apply to the whole field of silicones new field lies open for the development of materials for which their design is only limited by human imagination. 256... 

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

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