Membranes phases

Ionic liquid as active layer in supported liquid phase membranes for gas separation Melin, Wasserscheid, et al. 19, 20... 

On the materials side, there is considerable interest in developing novel membrane materials that are functionatized to selectively adsorb a specific component from a finid phase. Membrane materials that are environmentally stable and resistant to fonhng are also needed. Since... 

Figure 45-6. Interaction and synergism between antioxidant systems operating in the lipid phase (membranes) of the cell and the aqueous phase (cytosol). (R-,free radical PUFA-00-, peroxyl free radical of polyunsaturated fatty acid in membrane phospholipid PUFA-OOH, hydroperoxy polyunsaturated fatty acid in membrane phospholipid released as hydroperoxy free fatty acid into cytosol by the action of phospholipase Aj PUFA-OH, hydroxy polyunsaturated fatty acid TocOH, vitamin E (a-tocopherol) TocO, free radical of a-tocopherol Se, selenium GSH, reduced glutathione GS-SG, oxidized glutathione, which is returned to the reduced state after reaction with NADPH catalyzed by glutathione reductase PUFA-H, polyunsaturated fatty acid.)...

In another approach, the isotropic hyperflne coupling constant (A0 in Figure 10.2) of 16-SASL or 16-PC can be measured for fluid-phase membranes. A decrease in the A0 value indicates an increase in hydrophobicity at the 16-SASL position (Figure 10.5c). However, this constant reflects only the hydrophobicity of the membrane center. 

Another example of the high-temperature membrane technologies is a dual-phase membrane developed at Columbia University, which consists of solid oxide and molten carbonate. This membrane technology takes advantage of oxide and carbonate ionic transport. At the face of the membrane exposed to the high concentration of COz, carbonation dioxide... 

The principle of a three-phase membrane extraction is illustrated in Figure 1.28. An organic solvent is immobilized in the pores of a porous polymeric support consisting of a flat filter disc or a hollow fiber-shaped material. This supported liquid membrane (SLM) is formed by treating the support material with an organic solvent that diffuses into its pores. The SLM separates an aqueous... 

For combined hydrogen production and C02 capture several novel technologies are in development, most of them for the application in a pre-combustion C02 capture combined cycle. The main focus is to reduce the efficiency penalties and other associated costs of CO2 capture. The most important technologies in the R D phase, membrane reactors and sorption-enhanced reactors, are described below, with special attention paid to the catalytic aspects. 

Another way around the problem of pressure-driven flow in the single-phase membrane was presented by Meyers.He worked around the problem by allowing for a discontinuity in pressure at the membrane/solution interface, even though the electrochemical potential of all soluble species is continuous. He argued that additional mechanical stresses compressing the membrane should be indistinguishable from the thermodynamic pressure, and thus, the thermodynamic pressure might be discontinuous at the interface. 

Greaser, C. S. Lamarca, D. G. Brum, J. Werner, C. New, A. P dos Santos, L. M. E. Reversed-Phase Membrane Inlet Mass Spectrometry Applied to the Real-Time Monitoring of Low Molecular Weight Alcohols in Chloroform. Anal. Chem. 2002, 74, 300-304. 

Another example of a gas-phase membrane reactor is a palladium tube through which only hydrogen can permeate. This can be used to run the reaction... 

The reaction in a homogeneous solution with a polar organic solvent in which the enzymes and substrates are both soluble, occurs often at the expense of the enzyme stability [4, 5]. Besides immobilised enzymes in organic solvents [6], emulsion reactors, especially enzyme-membrane-reactors coupled with a product separation by membrane based extractive processes [7-9] and two-phase membrane reactors [10-12], are already established on a production scale. 

Membrane reactors can be considered passive or active according to whether the membrane plays the role of a simple physical barrier that retains the free enzyme molecules solubilized in the aqueous phase, or it acts as an immobilization matrix binding physically or chemically the enzyme molecules. Polymer- and ceramic-based micro- and ultrafiltration membranes are used, and particular attention has to be paid to the chemical compatibility between the solvent and the polymeric membranes. Careful, fine control of the transmembrane pressure during operation is also required in order to avoid phase breakthrough, a task that may sometimes prove difficult to perform, particularly when surface active materials are present or formed during biotransformahon. Sihcone-based dense-phase membranes have also been evaluated in whole-cell processes [55, 56], but... 

Equation (2.65) is not commonly used as an expression for gas-phase membrane permeability, but is of interest because it shows that large permeability coefficients are obtained for compounds with a large diffusion coefficient (A)> a limited affinity for the gas phase (large yf ), a high affinity for the membrane material (small and a low saturation vapor pressure (p,tM). The molar gas... 

In an early study, the author s group synthesised the lipophilic, tri-branched ligand 15 for use in two-phase solvent extraction as well as three phase membrane transport studies [33] the parent ring system in this case has been well documented to interact with a range of first-row transition and post transition ions [34],... 

Related to the experimental studies performed in our laboratory, in this review packed-bed membrane reactors were discussed. It should be mentioned that there are significant investigational activities devoted to study catalytically active membranes where the catalyst is deposited in either the membrane pores or on the inner or outer surface of the tubes [11]. Another similarly interesting and promising principle is based on using the Contactor type of membrane reactors, where the reactants are fed from different sides and react within the membrane [79]. Significant efforts have been made to exploit this principle for heterogeneously catalyzed gas-liquid reactions (three-phase membrane reactors) [80, 81]. 

Kholpanov, L.P., Avetisyan, K.V., Malysov, V.A. Fluid mechanics and mass transfer during three-phase membrane extraction. Theor. Found. Chem. Eng., Vol. 22, 299-305, 1988. 

The classification of separations should reflect the patterns of component transport and equilibrium that develop in the physical space of the system. The transport equations show that we have two broad manipulative controls that can be structured variously in space to affect separative transport. First is the chemical potential which controls both relative transport and the state of equilibrium. Chemical potential, of course, can be varied as desired in space by placing different phases, membrane barriers, and applied fields in appropriate locations. A second means of transport control is flow, which can be variously oriented with respect to the phase boundaries, membranes, and applied fields—that is, with respect to the structure of the chemical potential profile. 

Viewed in this way, chemical potential profiles (along with flow) govern separation different phases, membranes, and applied fields are simply convenient media for imposing the desired profiles. The media are selected on pragmatic grounds chemical compatibility with the components and the system, selectivity between components, noninterference with detectability, ease of solvent removal (another separation process), facilitation of rapid transport, and so on. 

It has been suggested that these ceramides form a gel-phase membrane domain within the skin. Straight fatty acid chains as well as the small polar head groups on the ceramides are thought to produce a tightly packed domain which is less fluid and thereby less permeable than other liquid crystalline domains which are also present. Recent evidence using differential scanning calorimetery (DSC) and infrared absorption spectroscopy analyses verifies the presence of gel phases within the stratum comeum. 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

La liquid-ordered phase bilayers prepared from binary mixtures of the same phosphatidylcholine and cholesterol but is much slower ( 10 s ) in Lx liquid-ordered phase membranes prepared from sphingomyelin and cholesterol (55). The activation free energy for the process, which corresponds to the energy necessary to put the translocating lipid molecule at the bilayer mid-plane, is 100kJmol In contrast, the rate constant for transmembrane translocation of cholesterol may be 1 s (56). 

In the case of nonequilibrium spatial distribution of a certain species, characterized by a nonuniform distribution of chemical potential, diffusion of this species will occur in accordance with the gradient of its chemical potential. If a gradient exists in the chemical potential for one of the species, then a statistical force will be exerted on the particle distribution. The average velocity of a particle will be given by the product of its mobility and the sum of the forces acting on it (4). During the transport of a substance across a membrane, it must move through phase boundaries, such as aqueous phase/membrane/aqueous phase. As mentioned before, the substance has different affinities to each phase encountered. In most cases, the diffusion within the membrane is much slower than in the liquid phase. 

Cholesterol affects a large variety of membrane properties in animal cells (39). It is involved in modifying dynamical membrane properties by reducing passive permeation, slowing down the lateral diffusion of molecules in fluid-like membranes, and speeding up diffusion in gel-phase membranes. It also affects bilayer properties by condensing the bilayer, which changes its elastic properties and promotes the order of phospholipid acyl chains in the hydrophobic membrane core. In this manner, cholesterol develops the formation of the liquid-ordered... 

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