Permselectivity, evidence

The PVA/PSSNa membranes evidence a high permselectivity, comparable with the one of commercial ion exchange membrane as it can see in table 14, where were presented the permeability coefficient (P) and the ratio P to D (diffusion coefficient) that express the effect of porosity and of the electrolyte exclusion. 

When the feed solution is 1 mM in KCl and 0.5 mM in the cationic dye methylene blue and the receiver solution is 1 mM KCl, the initially colorless receiver solution turns blue due to transport of the cationic dye across the membrane [71]. In contrast, when the feed solution is 1 mM in KCl and 5 mM in KMn04 (Mn04 is red) and the receiver solution is 1 mM KCl, the receiver solution remains colorless [71]. These experiments provide simple visual evidence that this membrane transports a large cation but does not transport a much smaller anion. We have used potentiometric measurements to explore the nature of this cation permselectivity. 

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

All spectroscopic evidence on the composition of a relatively thin layer at the membrane surface was thus found to be in agreement with the interpretation of permselectivity as being due to the exclusion of counteranions from the membrane phase. However, the depth accessible to ATR-IR is of the order of 0.1 1.0 pm and is thus too large for the observation of phenomena in the region in closest proximity to the membrane/aqueous interface in which charge separation is assumed to take place. Optical second harmonic generation (SHG), which has an even more pronounced surface sensitivity than ATR-IR, was recently shown to be very suitable for the investigation of the interface between ISE membranes and sample solutions."" ... 

In summary, there is evidence that the skin presents a weak cation permselectivity [25,76,77,80,93,125], which can be reversed by acidifying the pH of the solutions bathing the skin [10,23,76,77]. At pH>p/, the skin is negatively charged and electroosmotic flow proceeds in the anode-to-cathode direction. At pH < pi, the skin becomes positively charged and electroosmotic flow reverses to the cathode-to-anode direction. Under the application of an electric field, counterions (cations at physiological pH) are preferentially admitted into the skin. As a consequence, the sodium and chloride transport numbers are 0.6 and 0.4, respectively, during transdermal iontophoresis (in contrast to their values in a neutral membrane tNa = 0.45 rCi = 0.55) [126]. 

One side is filled tvith 1 mM KCl while the other side is filled with either 0.5 mM solution of a cationic dye, methylene blue or 1 mM of KMn04 solution. In the former case, the transport is observed from one half of the cell to the other while the latter does not penetrate through the membrane. This is clear evidence to show that the membranes transport a large cation while a small anion does not pass through. This cation permselectivity or charge based selectivity is further explored by potentiometry [75]. The authors go on to demonstrate that the selectivity towards cations or anions can be controlled by the potential applied to the membrane [75]. 

Immunoisolation may also have an advantage in preventing the autoimmune destruction of the transplanted islets (27) by separating them from the host (T-cells or autoantibodies) by the permselective membrane. Although the detailed mechanism of islet cell damage is unknown, evidence from spontaneously diabetic BB rats indicates that the rapid failure of islet transplants, even in twins (28), may be due to a recurrence of autoimmune disease rather than the conventional transplant rejection phenomenon (29). 

Microporous silica hydrogen permselective membranes have been extensively studied as a potentially more practical alternative to Pd membranes. Very recently, a comprehensive review was published, tackling various aspects of silica membrane synthesis, application and economics [63]. It was made evident that state-of-the-art silica membranes have good hydrogen flux and separation, as well as respectable thermal stability. However, the hydrothermal stability of a silica hydrogen permselective membrane is a key factor in determining its suitability for a commercial apphcation of membrane-assisted processes. 

Hillman et al. examined the mass transport accompanying the redox of polybithiophene in acetonitrile containing tetraalky-lammonium salts [70], The mass change is similar, but not identical to the value anticipated when a permselective anion transport is assumed. Detailed consideration as well as several pieces of experimental evidence show the involvement of the salt and solvent transport in opposite directions. The anion transport accompanying the solvent was reported for poly(3,4-ethylenedioxthiophene) films in 0.1 M TEAPFe/acetonitrile [71]. 

The subunits defined in the model are listed in Table I. Also shown are the group molar volume and the group contribution to the COj permeability and the CO2/CH4 permselectivity the best fit solution to the matrix of linear equations. The gas permeability of each polymer in the dataset was calculated firom the resultant subunit permeability indicated in Table I and the normalized structural equation for each polymer. The CO2 and CH4 permeability (in Barrers) predicted by the group contribution model is compared to experimental values in Figure 1. An excellent correlation is evident for both gases. The correlation between model predicted and experimental CO2/CH4 selectivity is shown in Figure 2. 

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