Membrane electric parameters measurements

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. 

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. 

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. 

As was previously indicated, IS measurements can also be used to determined membrane modifications and Figure 9.13 shows Nyquist and Bode plots for PS-Uf and PS-Uf/BSA fouled membranes in contact with a NaCl solution. Here a significant increase in electrical resistance due to membrane fouling can be observed, but the electrolyte contribution hardly differs in both systems. In both cases, the equivalent circuit for the membrane-electrolyte system is given by (R,C,)-(RM that is, a series association of the electrolyte part, formed by a resistance in parallel with a capacitor and the membrane part, which consists of a parallel association of a resistance and a CPE or non-ideal capacitor (RmQm). Fitting the experimental data allows determination of the electrical parameters (resistance, capacitance) for the different NaCl solutions studied and their variation with electrolyte concentration is shown in Figure 9.13c, d, respectively. 

Theoretical and experimental descriptions of three different types of electrical measurements commonly used for membrane characterization have been presented. Streaming potential, membrane potential, and impedance spectroscopy are nondestructive techniques and they can be carried out with the membranes in contact with electrolyte solutions, which allows characterization of membranes in working conditions , but they also permit us to established changes in the membrane characteristic parameters related to solution chemistry (ion size, charge, concentration, pH) and/or membrane structure. 

Streaming potential measurements or tangential streaming potential in the case of dense membranes, but important transport information associated with hydro-dynamic permeation is missing in the latter cases. Other information related to different characteristic membrane material parameters (electrical/adsorption parameters) can also be obtained from these measurements. 

A parameter (usually symbolized by P, and often containing a subscript to indicate the specific ion) that is a measure of the ease with which an ion can cross a unit area of membrane by simple (or passive) diffusion through a membrane experiencing a 1.0 M concentration gradient. For a particular biological membrane, the permeabilities are dependent on the concentration and activity of various channel or transporter proteins. In an electrically active cell (e.g., a neuron), increasing the permeability of K+ or CF will usually result in hyperpolarization of the membrane. Increasing will cause depolarization. 

In another type, mammalian cells or plasma membranes are used as electrical capacitors. Electrical impedance (El) uses the inherent electrical properties of cells to measure the parameters related to the tissue environment (Kyle et al., 1999). The mechanical contact between cell-cell and cell-substrates is measured via conductivity or El (Deng et al., 2003 ... 

In the geometric representation of the composition of the membrane, the volume fraction of each component in each layer is described quantitatively by corresponding geometric parameters. In the electrochemical part of the model, each layer is treated as a set of two resistors and all sets, whose number equals the number of layers, are arranged in series forming an equivalent electrical circuit. Summation of the resistances of the layers expressed with appropriate equations leads to the final formula on specific conductivity of the membrane. Calculations based on the model require measurements of the conductivity of the membrane in contact with electrolyte solutions of different concentration. 

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