Permeation, selective

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Possible applications of MIP membranes are in the field of sensor systems and separation technology. With respect to MIP membrane-based sensors, selective ligand binding to the membrane or selective permeation through the membrane can be used for the generation of a specific signal. Practical chiral separation by MIP membranes still faces reproducibility problems in the preparation methods, as well as mass transfer limitations inside the membrane. To overcome mass transfer limitations, MIP nanoparticles embedded in liquid membranes could be an alternative approach to develop chiral membrane separation by molecular imprinting [44]. 

Traditionally concepts of ion selective permeation of biological membranes have centered on differences in the effective radii of hydrated nuclei. An example of that perspective derives from consideration of the resting membrane potential, E, which in the squid axon is approximated by the Nernst equation... 

In many bioconversions, product inhibition is critical and this forces the transformations to relatively low concentrations, so if a membrane that selectively permeates the desired product could work in a dirty environment, this would be very useful. 

A membrane reactor offers the possibility of combining two individual processes in the same unit operation. (1) Selective permeation (thus separation) can be coupled directly with the reaction by means of either a catalytically active membrane or of a passive membrane placed next to the... 

An overview of the decomposition and dehydrogenation reactions that have been investigated using semipermeable membranes for selective permeation of one of the reaction products is given in Tables 7.1 and 7.3, respectively. An overview of the most interesting studies is given in Tables 7.2 and 7.4. 

Zeolites used for the preparation of mixed-matrix membranes not only should have suitable pore size to allow selective permeation of a particular molecular component, but also should have appropriate particle size in the nanometer range... 

Many nitrogen generator devices are commercially available to produce high purity gas in small amounts. In these, nitrogen is obtained from compressed air. It is separated from other air components by selective permeation through polymeric hollow fiber membranes after prefiltration. 

I would like to extend Prof. Simon s characterizations of these beautiful new molecules to include a description of the effects on lipid bilayers of his Na+ selective compound number 11, which my post-doctoral student, Kun-Hung Kuo, and I have found to induce an Na+ selective permeation across lipid bilayer membranes [K.-H. Kuo and G. Eisenman, Naf Selective Permeation of Lipid Bilayers, mediated by a Neutral Ionophore, Abstracts 21st Nat. Biophysical Society meeting (Biophys. J., 17, 212a (1977))]. This is the first example, to my knowledge, of the successful reconstitution of an Na+ selective permeation in an artificial bilayer system. (Presumably the previous failure of such well known lipophilic, Na+ complexing molecules as antamanide, perhydroan-tamanide, or Lehn s cryptates to render bilayers selectively permeable to Na+ is due to kinetic limitations on their rate of complexation and decomplexation). 

I would also like to reemphasize the noncyclic character of these molecules, which I believe provides the way to circumvent the kinetic limitations on the rates of complexing and decomplexing such strongly hydrated ions as Na+, Li+, Ca2+, which I suspect has stood in the way previously of translating the complexing selectivity of macrocyclic com-plexones into effective mediators for the selective permeation of these ions across the bilayer membrane. This may already offer a clue to the architecture of the Na+ selective sites of the cell membrane. 

Molecular exclusion chromatography. The stationary phase in molecular exclusion chromatography is a material containing pores, the dimensions of which are chosen to separate the solutes present in the sample based on their molecular size. This can be perceived as a molecular sieve allowing selective permeation. This technique is known as gel filtration or gel permeation, depending on the nature of the mobile phase, which is either aqueous or organic. The distribution coefficient in this technique is called the coefficient of diffusion. 

The ILM (Figure 24.1 and Figure 24.3) borders the neuroretina from the vitreous (internal) side. Its outer portion consists mostly of the basement membrane of Muller s cells, whereas the inner portion is formed by vitreous fibrils and mucopolysaccharides. This basement membrane covers the entire inner surface of the retina. Anteriorly it is around 50 nm in thickness, and posteriorly it thickens to around 2000 nm. ILM acts as a selective permeation barrier between the intercellular space of the retina and the vitreous [21]. 

PE, being a commodity polymer, is used in its different physical forms viz. fibres, sheets, membranes, moulds with different backbone chemical configurations (LPE, LLDPE, LDPE, HDPE, UHMWPE, UHSPE etc). Each of these forms of PE requires surface modification at some stage of application. The surfaces of PE fibres are often modified to make them compatible in the composites, whereas PE sheets/tapes are modified to achieve adhesion. Moulds are frequently surface-modified for probability and membranes for selective permeation. In the same way, different chemical configurations of PE, by the virtue of their properties, are used for different applications after surface modification. 

In the case of reverse osmosis, the enrichment factors (E and Ea) are less than 1.0, typically about 0.01, because the membrane rejects salt and permeates water. For other processes, such as dehydration of aqueous ethanol by pervaporation, the enrichment factor for water will be greater than 1.0 because the membrane selectively permeates the water. 

Equation (9.4) shows that the separation achieved in pervaporation is equal to the product of the separation achieved by evaporation of the liquid and the separation achieved by selective permeation through the membrane.1... 

J. Jagur-Grodzinski, S. Marian and D. Vofsi, The Mechanism of a Selective Permeation of Ions Through Solvent Polymer Membranes , Sep. Sci. 8, 33 (1973). 

Most polymers that have been of interest as membrane materials for gas or vapor separations are amorphous and have a single phase structure. Such polymers are converted into membranes that have a very thin dense layer or skin since pores or defects severely compromise selectivity. Permeation through this dense layer, which ideally is defect free, occurs by a solution-diffusion mechanism, which can lead to useful levels of selectivity. Each component in the gas or vapor feed dissolves in the membrane polymer at its upstream surface, much like gases dissolve in liquids, then diffuse through the polymer layer along a concentration gradient to the opposite surface where they evaporate into the downstream gas phase. In ideal cases, the sorption and diffusion process of one gas component does not alter that of another component, that is, the species permeate independently. 

In another study, Tsum et al. [80] reported the use of porous Ti02 membranes having pores of several nanometers for a gas-phase photocatalytic reaction of methanol as a model of volatile organic component (VOC). In this system, the titanium dioxide is immobilized in the form of a porous membrane that is capable of selective permeation and also a photocatalytic oxidation that occurs both on the surface and inside the porous Ti02 membranes. In this way, it is possible to obtain a permeate stream oxidized with OH radicals after one-pass permeation through the Ti02 membranes. 

Kim JH, Ha SY, Nam SY et al (2001) Selective permeation of C02 through pore-filled polyacrylonitrile membrane with poly(ethylene glycol). J Membr Sci 186(1) 97-107... 

FIGURE 35 Emulsified liquid membrane separation mechanisms (A) selective permeation (B) chemical reaction inside emulsion droplet and (C) chemical conversion in liquid membrane and further conversion inside droplet. Both (B) and (C) provide quasi-infinite sink conditions for extraction from the feed solution. 

---