Cation permselectivity

When the extraction of the hydrophilic counteranion from the aqueous solution into the membrane bulk is negligible (cation permselectivity preserved), the concentration of the complex cation in the membrane bulk Cb, is equal to that of the fixed anionic sites, X, in the membrane matrix, due to the electroneutrality condition within the membrane bulk ... 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

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. 

Data obtained from this concentration cell (Fig. 11) show that these membranes can show ideal cation-permselective behavior and that the re-... 

To illustrate this point, consider the membrane plated for 60 minutes. The tubules in this membrane average —9.4 nm in inside radius. At low concentrations of salt, the electrical double should be thicker than this tubule radius. Anions are excluded from the tubes, and ideal cation permselectivity is observed. At high salt concentrations, the electrical double layer is thin relative to the tubule radius. Anions can now enter the tubules, and ideal cation permselectivity is lost (Fig. 11). Finally, the membrane plated for 180 minute shows cation permselectivity almost identical to that of the ionomer Nation [92], which is a highly cation-permselective polymer used in industrial electrolytic processes [93],... 

The dashed lines at the top and bottom of Fig. 12 are the values that would be achieved if the nanotubule membrane showed ideal cation and ideal anion permselectivity, respectively [Eq. (1)]. At negative applied potentials, the nanotubule membrane shows ideal cation permselectivity, whereas at positive applied potentials the membrane shows ideal anion permselectivity. This selectivity occurs because at negative applied potentials, excess electrons are present on the walls of the tubes and excess posi-... 

Such anion adsorption can be prevented by chemisorbing a mono-layer of a strongly adherent thiol molecule to the Au surfaces [97,98]. 1-Propanethiol (PT) was used here because the gold nanotubules can still be wetted with water after chemisorption of the PT monolayer [97].t The Em versus applied potential curves for an untreated and PT-treated gold nanotubule membrane, with KBr solutions present on either side of the membrane, are shown in Fig. 13. The untreated membrane shows only cation permselectivity, but the permselectivity of the PT-treated membrane can be switched, exactly as was the case with the nonadsorbing electrolyte (Fig. 12). 

We have demonstrated that these Au nanotubule membranes can be cation permselective, anion permselective, or nonselective, depending on the potential applied to the membrane [99]. These membranes can be as permse-... 

Ion exchange in LbL films has been also studied using radiolabeled + ions [104] by Schlenoff and coworkers. In that case, poly(butylviologen)/PSS multilayers showed the exchange of Ca in a stoichometric ratio with the charge passed through the electrode, an indication of cation permselectivity. 

However, when the membranes were exposed to aqueous solutions of an intermediate to high concentration of primary cation thiocyanate (M SCN ), IR bands of the complex as well as the X could be observed. At very high analyte concentrations, no preferential permeation for either M or SCN occurred and the slope of the electrode response was considerably decreased as compared to a Nemstian response. Cation permselectivity for M SCN" was observed only for the 84-incorporated membranes at low analyte concentrations. 

The question hits the big unknown in explaining the cation permselectivity of the neutral carrier systems mentioned in my report. In the solvent polymeric membranes we studied, the contribution of anions to the electrical current (anion transport number) is usually negligible if hydrophilic anions (e.g., Cl ) are involved. In the presence of lipophilic anions (e.g., SCN ) there exists some contribution of anions to the total ion flux across the membrane [see Anal. Chem., 48, 1031 (1976)]. The reasons for such a behavior may be ... 

Locally electro-neutral concentration polarization of a binary electrolyte at an ideally cation-permselective homogeneous interface. Consider a unity thick unstirred layer of a univalent electrolyte adjacent to an ideally cation-permselective homogeneous flat interface. Let us direct the x-axis normally to this interface with the origin x = 0 coinciding with the outer (bulk) edge of the unstirred layer. Let a unity electrolyte concentration be maintained in the bulk. 

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

A perfect prototype of an ideally cation-permselective interface is a cathode upon which the cations of a dissolved salt are reduced. Experimental polarization curves measured on metal electrodes fit the theory very closely. Since in dimensional units the limiting current is proportional to the bulk concentration, the polarization measurements on electrodes may serve for determining the former. This is the essence of the electrochemical analytical method named polarography. (For the theory of polarographical methods see [28]—[30].)... 

Another prototype of an ideally cation-permselective interface would be a cation-exchange membrane (C-membrane). Most practically employed C-membranes are extremely permselective, so that their polarization curves would be expected to coincide with those at electrodes (given the same... 

Fig. 15. Cluster network model for highly cation-permselective Nafion membranes126). Counterions are largely concentrated in the high-charge shaded regions which provide somewhat tortuous, but continuous (low activation energy), diffusion pathways. Coions are largely confined to the central cluster regions and must, therefore, overcome a high electrical barrier, in order to diffuse from one cluster to the next...

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]. 

For cation permselectivity anionic sites must be present, and vice versa. 

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]. 

This is evidenced by the amount of literature on ionomers and by the appearance of two monographs devoted to the subject (J, ). Most of the research effort on the ionomers has focused on only a small number of materials, notably ethylenes (3-9 ), styrenes (10,11), rubbers (12-16) and recently aromatic (17) and fluorocarbon-based ionomers (18). The last material is known for its high water permeability and cation permselectivity. Because of its unique properties, it has been employed as an ion-exchange membrane in chlor-alkali cell operations in electrochemical industries. Perfluorinated ion-exchange membranes are the subject of the present chapter. 

Limiting Current Density. In the membrane process, boundary layers form at both sides of the membrane due to its cation permselectivity. Such boundary layers do not occur in the diaphragm process. For the boundary layer at the surface of the membrane facing the anolyte, the following basic equation is established (73). 

In the diaphragm process, these phenomena do not occur because the diaphragm has no cation permselectivity. 

To restore electroneutrality, sodium ions are transported selectively in the electrochemical field gradient across the cation-exchange membrane from the anode to the cathode chamber. Ideally the membrane should be 100% cation permselective, therefore excluding any hydroxyl ion transport but in practice this is not the case and current efficiencies are always less than 100%. This current inefficiency is represented by the reaction of hydroxyl ions with chlorine. Patent applications for this method of chlor-alkali production appeared as early as 1949 (2). 

W.G. Grot, Heat-treated fluorocarbon sulfonylamine cation permselectivity, USP 3,969,285 (July 13, 1976). Jpn. Pat. JP 52-13228 (examined application). 

Z. Ogumi, Y. Uchimoto, M. Tsujikawa, K. Yasuda and Z. Takehara, Modification of ion-exchange membrane surface by plasma process. Part 2. Monovalent cation permselective membrane from perfluorosulfonate cation exchange membrane, J. Membr. Sci., 1990, 54, 163-174. 

W.G. Grot, Reinforced cation permselective membrane and preparation method thereof, Jpn. Pat. application JP 57-20328. 

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