Membranes polymeric, transport through

The mechanism of separation by non-porous membranes is different from that by porous membranes. The transport through nonporous polymeric membranes is usually described by a solution-diffusion mechanism (Figure 9.12a). The most current commercial polymeric membranes operate according to the solution-diffusion mechanism. The solution-diffusion mechanism has three steps (1) the absorption or adsorption at the... 

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

The use of liquid membranes in analytical applications has increased in the last 20 years. As is described extensively elsewhere (Chapter 15), a liquid membrane consists of a water-immiscible organic solvent that includes a solvent extraction extractant, often with a diluent and phase modifier, impregnated in a microporous hydrophobic polymeric support and placed between two aqueous phases. One of these aqueous phases (donor phase) contains the analyte to be transported through the membrane to the second (acceptor) phase. The possibility of incorporating different specific reagents in the liquid membranes allows the separation of the analyte from the matrix to be improved and thus to achieve higher selectivity. 

A superficial examination of experimental results obtained by using labeling techniques (electrodialytic transport through solvent polymeric membranes) indicates that there might be a substantial transport of water coupled with the carrier-mediated ion transport. This would be rather surprising because the cation in the carrier-cation complex is not hydrated. 

This concentration method uses a polymeric semipermeable membrane and principles of RO to effect separation of water from the organics in drinking water samples. In this process, a water sample is recirculated past the semipermeable membrane while hydraulic pressures exceeding the osmotic pressure are maintained. Water is transported through the membrane under these conditions (permeation). The concentration of solutes continues to build as water is removed from the system. 

The transport properties across an MIP membrane are controlled by both a sieving effect due to the membrane pore structure and a selective absorption effect due to the imprinted cavities [199, 200]. Therefore, different selective transport mechanisms across MIP membranes could be distinguished according to the porous structure of the polymeric material. Meso- and microporous imprinted membranes facilitate template transport through the membrane, in that preferential absorption of the template promotes its diffusion, whereas macroporous membranes act rather as membrane absorbers, in which selective template binding causes a diffusion delay. As a consequence, the separation performance depends not only on the efficiency of molecular recognition but also on the membrane morphology, especially on the barrier pore size and the thickness of the membrane. 

The permeation of a simple permeant, e.g., O2, through a polymeric membrane could occur, in principle, by two different mechanisms. One is the transport through pores, and the other is the transport through the free volume of polymer solid. The size of pore and its distribution is the most crucial parameter in the former case (porous membrane), and the value of a is determined by the molecular sizes of permeants A and B. The values of Ps are in the reverse order of the size of permeant, i.e., Pn2 > -P02 > -Pco2, and P can be dealt as a kinetic parameter. 

By inserting Henry s law (Equation 4.6) into Pick s law (Equation 4.1), integrating across the membrane and remembering the definition of the permeability coefficient (Equation 4.5), Equation 4.2 was developed as the standard equation for transport through a dense polymeric membrane. 

Pervaporation have been considered an interesting alternative process for the current industrial options for aroma recovery, distillation, partial condensation, solvent extraction, adsorption, or a combination thereof. It is considered a basic unit operation with significant potential for the solution of various environmental and energetic processes (moderate temperatures). This separation process is based on a selective transport through a dense membrane (polymeric or ceramic) associated with a recovery of the permeate from the vapour phase. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapour from the other side. Transport through... 

In this chapter, pressure driven processes involving porous inorganic or hybrid membranes are particularly examined. As recently shown by Bhave [1], differences with traditional organic elements mainly result from the structure and intrinsic properties of materials, either regarding flow (with ceramic membranes, the transport occiu-s through the intergranular spaces within the top layer, porous sublayers and support, while across polymeric barriers it develops... 

Similar measurements using either walljet electrochemistry or steady-state microelectrode voltammetry have been reported for layer-by-layer assembled and interfadally polymerized materials, respectively [24,28]. Additional measmements were made spectrophotometrically with polymerized porphyrin squares by using a U-tube. Results summarized in Fig. 5 revealed the following (a) After normalizing for differences in film thickness, transport through polymeric membranes is two to three orders of magnitude faster... 

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