Membrane permeation selectivity

Scheme 2 Functions of MIP films by various responses to specific template recognition in a receptor layer (i) direct or indirect monitoring of template binding (ii) monitoring of template binding via local changes of the MIP site (iii) monitoring of template binding via induced changes of the entire MIP film and (iv) coupled template binding and membrane permeation selectivity.

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]. 

This review addresses the issues of the chemical and physical processes whereby inorganic anions and cations are selectively retained by or passed through cell membranes. The channel and carrier mechanisms of membranes permeation are treated by means of model systems. The models are the planar lipid bilayer for the cell membrane, Gramicidin for the channel mechanism, and Valinomycin for the carrier mechanism. 

It should be apparent that the principles of selective ion transport are independent of the specific models being treated here and that many of these principles are at variance with what were traditional views on the basis of selective membrane permeation by inorganic ions. Thus, the concept of selectivity among monovalent cations being based on values of hydrated radii is replaced by the... 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

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. 

Membranes exhibiting selectivity for ion permeation are termed electrochemical membranes. These membranes must be distinguished from simple liquid junctions that are often formed in porous diaphragms (see Section 2.5.3) where they only prevent mixing of the two solutions by convection and have no effect on the mobility of the transported ions. It will be seen in Sections 6.2 and 6.3 that the interior of some thick membranes has properties analogous to those of liquid junctions, but that the mobilities of the transported ions are changed. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

An ion-exchange membrane consists of an ionomer, which contains fixed ions that are covalently bound to the polymer backbone. It is electrically neutral because of included counterions . If water-or probably another sufficiently polar solvent - is absorbed and if the fixed and counterions can be separately solvated to an adequate degree, the counterions become mobile and the ion-exchange membrane can work as an ion conductor. Owing to the electric field of the fixed ions coions with the same charge as the fixed ions are rejected and are typically absent inside the membrane. Thus the membrane is selective for the transfer of counterions ( permselectivity = permeation selectivity, e.g. [70]). 

Results of such single-molecule permeation experiments, using the MV +/ Ru(bpy)3 pair (Fig. 16), and membranes with four different nanotubule i.d.s, are shown in Fig. 17. The slopes of these permeation curves define the fluxes of and Ru(bpy)3 across the membrane. A permeation selectivity coefficient (ai)t can be obtained by dividing the flux by the Ru(bpy)3 flux. 

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. 

Demineralization of UF whey retentate as compared to UF whey permeate is generally slowed down by salt and/or proteins that build up onto and in the membranes (Perez et al., 1994). Finally, in the case of skim milk the aim of the ED process is not only to reduce the overall ash content, but also to increase the calcium/phosphate ratio to about 0.77 in skim milk powder used in infant formula (Batchelder, 1987), this goal being much easily achievable by replacing the conventional membranes with selective ones (Andres et al., 1995). 

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. 

Table 1.1 shows two developing industrial membrane separation processes gas separation with polymer membranes (Chapter 8) and pervaporation (Chapter 9). Gas separation with membranes is the more advanced of the two techniques at least 20 companies worldwide offer industrial, membrane-based gas separation systems for a variety of applications. Only a handful of companies currently offer industrial pervaporation systems. In gas separation, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the feed mixture the membrane permeate is enriched in this species. The basic process is illustrated in Figure 1.4. Major current applications... 

The cross-, co- and counter-flow schemes are illustrated in Figure 4.17, together with the concentration gradient across a median section of the membrane. It follows from Figure 4.17 that system performance can be improved by operating a module in an appropriate flow mode (generally counter-flow). However, such improvements require that the concentration at the membrane permeate surface equals the bulk concentration of the permeate at that point. This condition cannot be met with processes such as ultrafiltration or reverse osmosis in which the permeate is a liquid. In these processes, the selective side of the membrane faces the... 

Fundamental aspects of chemical membrane reactors (MRs) were introduced and discussed focusing on the peculiarity of MRs. Removal by membrane permeation is the novel term in the mass balance of these reactors. The permeation through the membrane is responsible for the improved performance of an MR in fact, higher (net) reaction rates, residence times, and hence improved conversions and selectivity versus the desired product are realized in these advanced systems. The permeation depends on the membranes and the related separation mechanism thus, some transport mechanisms were recalled in their principal aspects and no deep analysis of these mechanisms was proposed. 

Among the configurations described in that paper, the irradiation carried out on the recirculation batch seems very promising since it allows high irradiation efficiency and high membrane permeate flow rate to be obtained and also it is possible to select the membrane type depending on the photocatalytic process under study. 

Membranes used for separation are thin selective barriers. They may be selective on the basis of size and shape, chemical properties, or electrical charge of the materials to be separated. As discussed in previous sections, membranes that are microporous control separation predominantly by size discrimination, charge interaction, or a combination of both, while nonporous membranes rely on preferential sorption and molecular diffusion of individual species. This permeation selectivity may, in turn, originate from chemical similarity, specific complexation, and/or ionic interaction between the permeants and the membrane material, or specific recognition mechanisms such as bioaffinity. 

Many factors including partition characteristics, degree of ionization, molecular size etc. influence the transport of drugs across biological membranes. Permeation of intact mucosa may also involve passive diffusion, intercellular movement, transport through pores or other mechanisms. The objective of the studies reported here was to employ the dog model to investigate these factors in a systematic and experimentally well-controlled fashion. The non-steriodal anti-inflammatory drug, diclofenac sodium, was selected as a test compound in this evaluation process. 

A ternary system with a hyperbola-type PSPS is used to investigate the influence of membrane permeation (Fig. 4.32). The applied parameters (ct,A and Kg) and the corresponding eigenvalues of the matrix [A] are summarized in Tab. 4.3. For comparison, again the PSPS for the reactive distillation process is given in Fig. 4.32(a). The effect of a selective membrane with a diagonal [/e]-matrix is illustrated in Fig. 4.32(b, d). 

E. Kikuchi, Palladium/Ceramic Membranes for Selective Hydrogen Permeation and Their Application to Membrane Reactor , Catal. Today, 25 333-37 (1995). 

Figure 10.18 shows the output page. This screen shows the input variables such as the selected membrane, permeate flow rates, and recovery. Calculated outputs include ... 

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