Membrane electric parameters

These examples clearly show how the EIS measurements allow the estimation of the membrane electrical parameters (R and C however, they also indicate the possibility of obtaining qualitative information on the membrane structure, which can also be of great interest. In any case, it should be pointed out that impedance is an extensive magnitude (it depends on the sample area), and for that reason, comparisons of the type of curves and the concentration dependence instead of particular values are usually made. In addition, IS measurements with the dry and wet membranes, but without an electrolyte solution between the electrode and the membrane surface, can also be performed and complementary information, mainly related to the membrane material itself or the interfacial (electrode/membrane) effects, can be obtained. 

Because of the nature of electroporation, virtually any molecule can be introduced into cells. For transfer of DNA, the electroporation forces are important. An electrophoretic effect of the field causes the polyanion DNA to travel toward the positive electrode. Fluorescence studies have shown that DNA enters the cell through the pole facing the negative electrode, where the membrane is more destabilized and where the field will drive the DNA towards the center of the cell (245). Membrane resealing occurs after pore formation. Whereas pore formation happens in the microsecond time frame, membrane resealing happens over a range of minutes with variations depending on electrical parameters and temperature (246). 

Wu S-Y, Robinson JR (1999) Transcellular lipophilic complex-enhanced intestinal absorption of human growth hormone. Pharm Res 16 1266-1272 Yamashita S, Saitoh H, Nakanishi K, Masada M, Nadai T, Kimura T (1987) Effects of diclofenac sodium and disodium ethylenediaminetetraacetate on electrical parameters of the mucosal membrane and their relation to the permeability enhancing effects in the rat jejunum. J Pharm Pharmacol 39 621-626... 

Marszalek, P. Zielinsky, J. J. Fikus, M. Tsong, T. Y. Determination of electric parameters of cell membranes by a dielectrophoresis method. Biophys. J. 59, 982-987. 

The electrical parameters of varieties supplied with a liquid solution are lower than those of the varieties supplied with gases, but research in direct methanol fuel cells was done mainly with liquid supply. This type of fuel cell is much simpler to design and operate, inasmuch as neither a special evaporator nor dual temperature control (for the evaporator and for the reaction zone of the fuel cell) is needed. With the supply of a liquid methanol solution, all risk of the membrane drying out close to the anode side is eliminated. The elimination of heat is also easier with cells having liquid solution supply. All subsequent information on direct methanol fuel cells in the present chapter refers to the variety supplied with liquid water-methanol solution. 

The differences fonnd when the individual layers or the composite structures were measured are attributed to the hydrophobic character of the ETFE film, which is more significant when the RC70PP membrane partially isolates one of its surfaces from the aqueous NaCl solution and, consequently, it allows its separation from the external solution. These results are a clear example of the strong influence that the material layer may have on the electrical response of a composite layered structure. As already indicated, the electrical parameters for the different samples at the studied NaCl concentrations can be determined by nonlinear analysis of the data shown in Figure 2.7a in the case of the membrane RC70PP and the ETFE film, these results also include the electrolyte contribution, but individual electrical resistance values, R or R, respectively, can be determined by subtracting those obtained for the electrolyte solntion (R) at the same concentration. 

In addition to the membrane voltage V or Em), there are other electrical parameters associated with the membrane. Prominent among these are membrane capacitance (Cm), resistance Rm), current (/m), and breakdown voltage Vy,). From basic physics, V ccq (the fundamental charge associated with an electron), one can write... 

Let us look at the example of interactions between phosphatidylethanolamine and a-tocopherol in bilayer membranes [515]. In this case, the formation of lipid domains consisting of phosphatidylethanolamine, 1, and phosphatidylethanolami-ne-a-tocopherol, 3, of a certain composition is observed. Assuming that only these two domains exist in the membrane and that the electrical parameters are additive, one can write an equation for the capacitance and resistance of the membrane ... 

According to these results, the carrier hardly affects the membrane/NaCl-solution interface but it seems to smooth the membrane surface irregularities however, transport across the membrane could be modified since other non-electrical parameters such as ion/molecule size or diffusivity could be important. 

This chapter presents a general description of the three kinds of measurements (SP, MP, IS) used for electrical characterization of membrane in working conditions , that is, with the membranes in contact with dectrolyte solutions and the information achieved from them is briefly indicated. The main attention focuses on the characterization of those membranes commonly used in traditional separation processes (from diverse materials and with different structures), and IS measurements are considered in more detail. Membrane and matrix material electrical parameters are obtained, but the measurements also provide thermodynamics and geometrical/structural information. 

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