Membrane-electrolyte interface

Earlier, Gavach et al. studied the superselectivity of Nafion 125 sulfonate membranes in contact with aqueous NaCl solutions using the methods of zero-current membrane potential, electrolyte desorption kinetics into pure water, co-ion and counterion selfdiffusion fluxes, co-ion fluxes under a constant current, and membrane electrical conductance. Superselectivity refers to a condition where anion transport is very small relative to cation transport. The exclusion of the anions in these systems is much greater than that as predicted by simple Donnan equilibrium theory that involves the equality of chemical potentials of cations and anions across the membrane—electrolyte interface as well as the principle of electroneutrality. The results showed the importance of membrane swelling there is a loss of superselectivity, in that there is a decrease in the counterion/co-ion mobility, with greater swelling. 

Equation (4.19) can be used only when (4.20) is valid. In simple systems (rapid processes at the membrane/electrolyte interface and a simple diffusion potential in the membrane) the apparent selectivity coefficient is a function of theflj/flK ratio alone, whereas in more complicated systems it also depends on the activities of J and K. 

This may work well if the process involves only electrically neutral species. However, when ions are discriminated on the basis of size, the partitioning process is affected by the Donnan potential. This potential, which we discuss more fully in Chapter 6, develops at the membrane/electrolyte interface. Another possibility is to discriminate on the basis of charge, as shown in Fig. 7.10 (see Chapter 7). Again, a porous barrier membrane is used, although here it would contain fixed, electrically charged moieties. When placed in front of the transducer, it rejects the like-charged species by electrostatic repulsion. In other words, it is a form of ion exchange membrane. 

Since the incorporation of a certain anion would take place for the system to reach equilibrium at the polymer coated electrode, the potential at the membrane-electrolyte interface would indicate the amount of anions incorporated. Indeed, a few groups of investigators realized that this was the case [274-276]. The potential at the PPy film coated electrode was shown to vary depending on the concentration of the Fe /Fe + pair [274], hydroxide ion [275], and many other anions and cations [276]. Recently, Hutchins and Bachas have developed a nitrate ion selective sensor using a PPy film coated electrode [277]. The electrode showed a near Nemstial slope of —56 1 mV per decade, good dynamic linearity, and a detection limit of (2 1) X 10 M for nitrate. The selectivity coefficients over CIO4 and 1 were 5.7 x 10 and 5.1 X 10 respectively. 

Electron transfer during photosynthesis, according to the tunnel mechanism, was examined in detail in Refs. 116-121. The most interesting of all for the modeling of electron transport in biological membranes is the case when the redox reaction at the membrane/electrolyte interface leads to ion permeability, and not only to electron permeability. Let us assume that ion B is insoluble in the membrane, and, therefore the membrane is impermeable to it. However, if at the interface the ion undergoes redox transformations ... 

Surface/Diffusion Potential Theory. " The transmembrane potential, E, is expressed as a difference between the electrical potential, Ei, and Eo of the two bulk phases in the two aqueous compartments separated by a membrane or as a sum of phase boundary potentials produced at the membrane-electrolyte interfaces and the diffusion potential within the membrane arising from the movement of ionic species through the membrane (Figure 28). 

Counterions around charged groups (phosphates in nucleic acids) play an important part because they induce the essential properties of the polyelectrolyte. Ions in the double layers at the membrane-electrolyte interface are also in a particular category and they introduce specific properties of the interface. If a biomolecule possesses a permanent dipole, the fluctuations of the dipole can create a special type of noise. 

We present in this section some interesting results concerning the membrane-electrolyte interface properties in the case of a collodion membrane.The membranes are collodion thick films (sslO m) made from a 4% collodion solution in diethylether these membranes contain fixed negative electrical charges inside. The electrolyte on both sides of the membrane is aqueous NaCl (concentration 2 x 10 -0.1 mol/liter), the active membrane surface is 1.2 cm, and the electrodes are made of platinized platinum. The power spectrum corresponding to the noise emitted by the total system is... 

We have determined the a and p exponents by varying the concentration of Nad concentration. The results obtained are shown in Figure 6. We can notice the particular behavior of a, which tends toward zero when the electrolyte concentration is zero. This reveals the disappearence of the membrane-electrolyte interface contribution in the case of very dilute electrolyte and the return to the thermal noise. 

We can also determine the power spectrum of the membrane-electrolyte interfaces ... 

Finally, the membrane-electrolyte interface noise generation can be divided into two parts ... 

The study of the membrane-electrolyte interfaces occurring in collodion membranes was focused on the corresponding complex power spectrum. The membrane-electrolyte interface power spectrum is not a simple 1/v noise but depends on electrolyte concentration. At infinite dilution of the electrolyte this power spectrum reverts to white noise. 

The occurrence of a redox reaction in the oil/water system is primarily established analytically by identification of the reaction products, often contained in different phases. Other indirect data, however, are also used, such as changes in the interfacial tension at the interface between immiscible hquids [22] or in the Volta potential [22-29]. A change in the transmembrane potential may also be an indication of a redox reaction at the membrane/electrolyte interface [6, 7, 26-28]. 

According to Ishibashi et al. [103,104] the composition of the region adjoining the liquid membrane-electrolyte interface differs considerably from that of a bulk solution. At the same time the composition of solutions at the interface determines the selectivity... 

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