Ion-permeable membranes

We illustrate the coulometric procedure in Figure 17-28, in which the main compartment contains anode solution plus unknown. The smaller compartment at the left has an internal Pt electrode immersed in cathode solution and an external Pt electrode immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane. Two Pt electrodes are used for end-point detection. 

Beginn developed Percec-type dendrimers, which are known to form supramolecu-lar channels, with polymerizable acrylate groups in order to obtain ion-permeable membranes [97-99]. First, the dendron 78 (Scheme 40) was dissolved in a polymerizable acrylate mixture that does not shrink on polymerization. The second step was the thermo-reversible gelation of the acrylate mixture, which was followed by the last step, polymerization to fix the supramolecular channel structure (Scheme 40). In the first experiments, compounds with only one polymerizable group were used but it turned out that the gelating properties were not sufficient [100, 101] so threefold modified 78 had to be developed. 

When two electrolyte solutions at different concentrations are separated by an ion--permeable membrane, a potential difference is generally established between the two solutions. This potential difference, known as membrane potential, plays an important role in electrochemical phenomena observed in various biomembrane systems. In the stationary state, the membrane potential arises from both the diffusion potential [1,2] and the membrane boundary potential [3-6]. To calculate the membrane potential, one must simultaneously solve the Nernst-Planck equation and the Poisson equation. Analytic formulas for the membrane potential can be derived only if the electric held within the membrane is assumed to be constant [1,2]. In this chapter, we remove this constant held assumption and numerically solve the above-mentioned nonlinear equations to calculate the membrane potential [7]. 

Davies AT, Genders JD, and Fletcher D. Ion Permeable Membranes. The Electrochemical Consultancy, Romsey, England, 1997. 

Early experiments in the development of isoelectric focusing, a high-resolution steady-state electrophoresis method, occurred in 1912, with an electrolytic cell that was used to isolate glutamic acid from a mixture of its salts.1 A simple U-shaped cell, such as that used for moving-boundary electrophoresis (Chapter 9), with two ion-permeable membranes equidistant from the center, created a central compartment that separated anodic and cathodic chambers, as shown in Figure 11.1. Redox reactions occurring in the anodic (Eq. 11.1) and cathodic (Eq. 11.2) electrolyte compartments generated H+ and OH ions in the respective chambers ... 

Figure 3.15. Change in solution concentration of Na and after a Na -saturated smectite suspension is immersed in a 1.0-mM KCI solution. The solution phase is separated from the clay by an ion-permeable membrane. The broken line depicts the concentration of Na" that would be in solution if all of the Na were exchanged from the clay.

T. Sata, Y. Shimokawa and K. Matsusaki, Preparation of ion-permeable membranes having an azobenzene moiety and their transport properties in electrodialysis, J. Membr. Sci., 2000, 171, 31. 

Some fundamental studies relating to methanol transport in membranes has also been reported. A study of the transport of formaldehyde and ethylene glycol through ion permeable membranes in electrolytic ceils has also been investigated. The water and methanol uptake from methanol-water... 

FIGURE 51.8 Schematic diagram of using PCSLs to interface microfluidics with ion-permeable membranes, (a) APCSL-protected microchannel substrate is bonded to a PMMA cover piece having a membrane reservoir, (b) Prepolymer solution is poured into the membrane reservoir, (c) An ion-permeable hydrogel is photopolymerized. (d) The PCSL is melted and removed from the channel. (Adapted from Kelly, R. T., et al., Anal. Chem., 78, 2565, 2006. Copyright 2006. With permission from American Chemical Society.)... 

Using PCSLs to integrate microfabricated channels with ion-permeable membranes improves EFGF experiments. The smaller cross-sectional channel dimensions in the microchip EFGF devices reduce band width and improve the resolution of protein peaks. Importantly, the fabrication protocols are flexible and easy to adapt as needed for different device designs. 

Kelly, R. T., Li, Y, and Woolley, A. T., Phase-changing sacrificial materials for interfacing microfluidics with ion-permeable membranes to create on-chip preconcentrators and electric field gradient focusing microchips. Ana/. Chem., 78, 2565, 2006. 

The chemical structures of ion-permeable membranes as well as ion-exchange resins are three-dimensionally crosslinked polymers with ionic groups attached. The structural units of the most common ion-exchange membrane are shown in Fig. 6.1. 

Ion-permeable membranes are also made by swelling existing films with styrene and DVB, which can then be post-treated to add functional groups, or by grafting of ion-exchange functional groups directly onto the polymer matrix of existing films. For example, free radicals formed by radiation of polyethylene or fluoropolymer films become sites for addition of vinyl sulfonic acid, acrylic acid or vinyl amines [30]. 

The ion-permeable membrane properties vital for the electrodialysis efficiency are discussed below. 

Ion-permeable membranes are changing ionic forms during eleclrodialysis. The electrical conductivity of ion-permeable membrane containing two ion species 1 and 2 can be calculated as the average between two extreme values. One of them is based on the model of parallel independent movement of two types of ions /m) and another one is based on the model of successive ion movement from one fixed charge to another [2] ... 

Selectivity demonstrates the relative transport property divergence between real and ideal membranes. Commercial ion-permeable membranes typically have selectivity from 0.93 to 0.99. 

During exposure of ion-permeable membranes to radiation, their mechanical firmness and flexibihty decreases. Ion-exchange capacity, crosshnking, conductivity and selectivity decrease as well. 

Electrodialysis (ED) is used to remove ionized substance from hquids through selective ion-permeable membranes. ED is the most widely commercialized electromembrane technology. Desalination of brackish water is the area of electrodialysis application with the largest number of installations. This chemical-free technology competes with reverse osmosis. Electrodialysis shows better resistance to fouling and scaling. It also has an economical advantage in desalination of low-salinity solutions [13]. Also, it should be kept in mind that because of small material consumption ED is the most environmental friendly process for solution desalination [14]. 

Fouling of ion-permeable membranes is often connected to the presence of weakly ionizable organic substances in the process solution. The organic ions produced by these substances have low mobility in the membrane phase, so they concentrate at the membrane/solution interface. Eventually these ions are absorbed by the membrane, which leads to membrane poisoning [7j. 

First Course in Ion-permeable membranes. The Electrochemical Consultancy, 255 pages. 

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