Membrane relative selectivity

Open circuit voltage (OCV) has been used as an indication of cell performance, although it is mainly determined by the electrochemical reversibility of the catalyst and it does not account for ohmic loss in the membrane and alcohol crossover. Thus, is not surprising to find a poor correlation between OCV of DMFC and membrane relative selectivity [4]. Other criteria used to compare DMFC performance is to compare current density at a fixed voltage [17], which is only practical if comparison is performed at the same temperature and feed methanol concentration. 

In spite the reduce amount of information, the correlation between membrane relative selectivity and power density is poor, as already noted for other PEM. 

In general, activation of alpha-1 adrenergic receptors causes a contraction of smooth muscle and of blood vessels, pilomotor muscles, dilator pupillae, vas deferens, nictitating membrane, splenic capsule, and sphincters of the intestine and urinary bladder and of the bile duct. An exception is the relaxation of the smooth muscle of the intestine. Prazosin [19216-56-9] indoramin [26844-12-2] and WB-4101 are relatively selective antagonists of these receptors. 

VMATs are not inhibited by drugs such as cocaine, tricyclic antidqnessants and selective serotonin reuptake inhibitors that affect plasma membrane monoamine transport. Amphetamines have relatively selective effects on monoaminergic cells due to selective uptake by plasma membrane monoamine transporters, but their effect appears to be mediated by their ability as weak bases to reduce ApH, the driving force for vesicular monoamine transport that leads to efflux of the vesicular contents into the cytoplasm. 

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. 

The geometries for asymmetric mixed-matrix membranes include flat sheets, hollow fibers and thin-fihn composites. The flat sheet asymmetric mixed-matrix membranes are formed into spirally wound modules and the hollow fiber asymmetric mixed-matrix membranes are formed into hollow fiber modules. The thin-film composite mixed-matrix membranes can be fabricated into either spirally wound or hollow fiber modules. The thin-film composite geometry of mixed-matrix membranes enables selection of different membrane materials for the support layer and low-cost production of asymmetric mixed-matrix membranes utilizing a relatively high-cost zeolite/polymer separating layer on the support layer. 

Several of the polymer membrane anion-selective electrodes described in the literature use quaternary ammonium salts as ion carriers (ionophores) (7). These electrodes respond according to the Hofmeister series (CIO4 > SCN > I > NO3 > Br - N3 > NC>2 > Cl > HCO3 acetate) (2, 5), which is the order of relative lipophilicity of the anions. Therefore, in strict terms, electrodes that respond according to this series could be considered "nonselective". 

In practice, ideal Nemstian behavior often cannot be attained in the presence of interfering ions. The degree of interference caused by other cations is determined by the selectivity of the ion receptor for the primary ion, by the partition coefficients of the different ions over the membrane and aqueous phase, and by the relative activities of the ions in the sample solution. This imposes a second important role to the ion receptor for obtaining ion selectivity in the membrane. By selective complexation of the primary ions i, other cations j are largely excluded from the membrane, with the result that the primary ion becomes the potential determining species. When interfering ions are not completely excluded from the membrane, their contribution to the membrane potential can be treated in terms of an apparent increase of the activity of the primary ion. This is expressed in the semi-empirical Nickolsky-Eisemnan equation (Equation 4) ... 

Shown in Table 8.6 arc some literature data on the use of dense membrane reactors for liquid- or multi-phase catalytic reactions. Compared to gas/vapor phase application studies, these investigations are relatively few in number. Most of them involve hydrogenation reactions of various chemicals such as acetylenic or ethylenic alcohols, acetone, butynediol, cyclohexane, dehydrolinalool, phenylacetylene and quinone. As expected, the majority of the materials adopted as membrane reactors are palladium alloy membranes. High selectivities or yields are observed in many cases. A higher conversion than that in a conventional reactor is found in a few cases. 

It is generally accepted that AmB-induced injury to cells is due to its binding to sterols in the cell membrane ergosterol in the case of fungal cells and cholesterol in mammalian cells [14]. This binding is more avid to ergosterol than to cholesterol, which explains AmB s relatively selective toxicity to fungal cells [51, 52]. 

Amphoteridn is a complex amphoteric polyene antibiotic that binds to cell membranes and forms a pore through which ions can pass, with consequences that include loss of potassium ions from within the cell. Since the antibiotic binds more readily to fungal cell membranes than mammalian, its action is relatively selective. It can potentiate the action of certain other antifungals. and it may be used with flucytosine. Also, it confers antifungal activity on rifampicin (normally antibacterial). As it has an appreciable renal toxicity, it needs to be used with caution in some patients. Nystatin is a polyene antibiotic similar in structure to amphotericin, often used for local treatment. 

P2X7 (P2Z ATP ) receptors show an order of potency where A TP is active, but ADP or AMP is inactive dBz-ATP (2, 3 -0-di (benzoyl)-adenosine triphosphate) is a relatively selective agonist. Many immune and inflammatory cells express these receptors which appear to be coupled in some way to plasma membrane pores. It was not initially evident that these receptors fell into the P2X ionotropic group, and... 

---