Membranes selectivity data

This short review cannot be comprehensive as of the exponential increase of the literature dealing with new procedures and applications. Table 2 summarizes selected data and lists fields of application. For a more inclusive view on the subject the interested reader is referred to existing monographs6-161 or to the critical biennial review in Analytical Chemistry. As the readers of this general volume on membranes most likely are acquainted with electro-analytical sensors, this article will be limited to the introduction of a theoretical approach which might be helpful also to researchers in the bio-membrane field. 

With respect to the size and charge selectivity of paracellular pathways, equivalent pore theory has been utilized to calculate an effective radius based on the membrane transport of uncharged hydrophilic molecules, while equivalent circuit theory has been used to separate mediated from paracellular membrane transport of small ions. The term equivalent should be emphasized, as selectivity parameters are obtained from membrane transport data, so phenomenological information is used to quantitate the magnitude of aqueous pathways... 

A C02-CH4 methane process gas stream, similar to a typical high CO2 natural gas has been under test by SEPAREX for CO2 removal in a 2-in. diameter element pilot plant since September 1981. The feed gas contains 30% CO2 and is delivered to the membrane test unit at 250-450 psig under ambient temperature conditions. The objective of the system is to reduce the CO2 level of the methane to less than 3.5%. The membrane system consists of 5 pressure tubes in series, each tube containing three 40-in. long elements. The gas is conditioned to maintain it at a minimum of 20°F above the dewpoint. The system was operated at a variety of flow rates, pressures, recoveries and temperatures. Selected data are presented in Figures 6 through 8. 

The dynamics of proton binding to the extra cellular and the cytoplasmic surfaces of the purple membranes were measured by the pH jump methods [125], The purple membranes selectively labeled by fluorescein Lys-129 of bacteri-orhodopsin were pulsed by protons released in the aqueous bulk from excited pyranine and the reaction of the protons with the indicators was measured. Kinetic analysis of the data implied that the two faces of the membrane differ in then-buffer capacities and in their rates of interaction with bulk protons. The extracellular surfaces of the purple membrane contains one anionic proton binding site per protein molecule with pA" 5.1. This site is within a Coulomb cage radius from Lys-129. The cytoplasmic surface of the purple membrane bears four to five pro-tonable moieties that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3 X 1010 M-1 sec ). The proximity between the elements is sufficiently high that even in 100 mM NaCl, they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. Quantitative evaluation of the dynamics of proton transfer from photoactivated bacteriorhodopsin to the bulk has been done by using numerical... 

The data in Figure 9.14 also allow determination of the relative contributions of the evaporative separation term /3eVap and the membrane selectivity term /3mem... 

Usually, to compare the selectivity of different membranes, the graph of the observed retention, denoted Robs, as function of the transmembrane pressure (AP) is used (see Fig. 9a). For the three studied membranes the data obtained at AP— 5 and 15 bar are reported in Table 5. 

Furthermore, membrane-selective functions were implicated by proteomic data of these secretory vesicles. The membrane fraction exclusively contained an extensive number of GTP nucleotide-binding proteins related to Rab, Rho, and Ras signaling molecules (83, 84), together with SNARE-related proteins and annexins that are involved in trafficking and exocytosis of secretory vesicle components (85, 86). Membranes also preferentially contained ATPases that regulate proton translocation... 

Note Data derived using stirred cells, +0.1 %-0.2% buffered solutions at 3.7 bar (55 psig). FIGURE 18.9 Membrane selection guide. (Data courtesy of Pall Corporation.)... 

The importance of the hairpin structure to membrane selectivity is illustrated by analogous transport data for membranes containing the linear-DNA transporter (Table 24.1). With this transporter, all the transport plots show only a single straight-line segment, and the fluxes for the single-mismatch DNAs are identical to the flux for the PC-DNA (Table 24.2), i.e., the singlebase mismatch selectivity coefficient for this linear (LN) DNA transporter is aLN,pc/i mm = 1 The linear-DNA transporter does, however, show some transport selectivity for the PC-DNA vs. the seven-mismatch DNA, aLN,pc/7MM = 5. 

The membrane characterization data reported in this section have been obtained by means of a home-made apparatus which is made of stainless steel and can operate from high vacuum up to 70 bars [17], It is characterized by the unique capability of performing a broad range of porous membrane characterization and evaluation measurements, namely equilibrium isotherms, absolute (integral and differential) and relative gas and condensed vapor permeabilities and selectivities. 

Figure 6.10 Comparison of model calculated and experimentally obtained dynamic selectivity data, obtained at treatment of industrial WPA, composed of 61 ppm Cd and 55 ppm Cu in the AHLM circulating module. Carrier 0.5 mol/kg PVSH in water. Membrane Tokayama Soda cation-exchange CM-2 Strip 2.0 mol/kg HCl. Feed carrier strip ratio = 2.5 1 2.5. Flow rates feed = carrier = 90 cm /min, strip = 60 cm /min. From Ref. [7] with permission.

The ion selectivity data clearly indicate that veiy specific ion-solvent interactions take place in the vicinal water and that these bear little resemblance to expectations from bulk-phase observations. This, in turn, means that such quantities as the classical standard ion activity coefficients are not applicable, and hence osmotic coefficients must also differ from the expected values. In fact, the use of the generally accepted osmotic coefficients is simply inappropriate, and unusual osmotic behavior must be anticipated. Likewise, if the activity coefficients display anomalous behavior so must cell membrane potentials. In other words, ion distribution and the osmotic behavior of cells must be influenced by vicinal water, and models of cell volume regulation must anticipate and take into account this aspect (see also Wiggins, 1979). 

The method is relatively simple and especially suitable to compare the selectivity of novel ion-selective electrodes prepared with different membranes. The data thus calculated may not be representative for mixed sample solutions. For further details, see Ref. 62. 

Asher ei alalso studied the slow release of other medicinals through liquid membranes. The data shown in Table 19.4-7 demonstrates the selective release of up to 64% of encapsulated sodium salicylate from a liquid membrane while 80% of the sucrose solution, encapsulated in the same membrane, was retained. 

Fluvoxamine is a highly selective inhibitor of 5-HT reuptake at the presynaptic membrane. Potency data from in vitro affinity studies suggest that fluvoxamine is less potent than the other SSRIs (e.g., paroxetine, sertraline, and citalopram). Its mechanism of action is similar to that of the other SSRIs. Fluvoxamine appears to have little or no effect on the reuptake of NE or dopamine. In vitro studies have demonstrated that fluvoxamine possesses virtually no affinity for other neuroreceptors. Its onset of action is similar to the other SSRIs (2-4 weeks). 

For comparison purposes, selectivity data is also presented for membranes prepared with a typical quaternary ammonium exchanger, tridodecylmethylammonium chloride (TDMA-Cl) (1). All the porphyrin species tested yield selectivities which clearly deviate from that observed with TDMA-Cl. More importantly, two compounds, Mn[TPPP]Cl (i) and Sn[TPP]Cl2 (5.), show extraordinary selectivity toward thiocyanate and salicylate, respectively. In the case of Mn[TPPP]Cl, adding conjugated and bul)cy substituents in the form of twelve phenyl groups to the basic tetraphenylporphyrin [TPP] structure dramatically enhances the response and selectivity toward thiocyanate... 

Membrane permeance and selectivity data are based on those of polymeric membranes obtained from Faiz and Li (2012). This is due to a lack of studies on CMS membranes for the separation of olefins/parafifins. The assumption of the use of polymeric membrane data is conservative since CMS membranes have been shown to give better permeance and selectivity than polymeric membranes for olefln/paraffin separation (Xu et al., 2012)... 

Basic process data are available from the development of organic vapor separation. The real challenge is the transformation of the available knowledge into high-pressure applications. Several drawbacks such as the compaction of the substructure of composite membranes and the influence of the boundary layer On the membrane selectivity have to be overcome. Pour structure and polymer compositions have to be suited to the high operating pressure in the presence of higher hydrocarbons [22, 23]. 

Olefin/paraffin selectivities of 5 to 10 appear quite modest compared to the claims of some reports [32, 33]. However, much of the literature selectivity data has been calculated from the ratio of the permeabilities of pure olefin to pure paraffin. Olefin/paraffin selectivities measured with gas mixtures under conditions likely in a real process show that using pure gas permeabilities overestimates the membrane selectivity by a factor of 2 to 10. Therefore, it will be some time before olefin/paraffin-selective membranes are used in ethylene plants, although some nearer-term applications exist in petrochemical and refinery operations. 

Therefore, an attempt will be made in this chapter to find some relationships between the surface characterization parameters obtained by AFM and the membrane performance data. Most obviously, the pore size and the pore size distribution will have a direct influence on the selectivity and the permeation rate of NF, UF, and MF membranes, where pores are most visible. 

Attention should be focused on the extremely high selectivity data of the PPO membrane that was cast from the PPO solution in carbon disulfide (PPO-CS2). On the surface of this membrane it was observed that several nodular aggregates merged... 

Membrane performance is a trade-offbetween membrane selectivity and membrane productivity. Membrane selectivity, a (=A/B), is defined by the ratio of permeability of components through the membrane where A is the water permeability coefficient and B is the solute permeability coefficient. In the case of RO and NF membranes, water/NaCl selectivity for seawater RO membranes is about 10,000. The higher the selectivity, the lower the permeate flux or productivity. This relationship for various RO membranes used with dilute NaCl solution is shown in Figure 1.6. The shaded regions refer to different feed concentrations and to different types of membranes. The data is fairly independent of the feed concentration but is a function of the physical and chemical properties of the membrane. 

PV Flux and Separation Selectivity Data for Different Mass% of Water in the Feed Mixture at 30°C for Different Membranes... 

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