Characterization of Membrane Charge

TABLE 32.2 Contact Angles of Commercial Nanoflltration and Tight Ultrafiltration Membranes Measured with the Sessile Drop Method 

Streaming potential measurements can be used to determine the potential of a membrane. The potential is the potential at the plane of shear between a charged surface and a liquid 

In electrochemical impedance spectroscopy (EIS) measurements a membrane is placed in an electrolyte solution of specified concentration. Two electrodes are located on the membrane surface, and a small alternating current of known frequency and with small amplitude is applied to the system. The phase difference and the amplitude of the simultaneous electrical potential that develops across the membrane are measured. A wide frequency range is used in the measurements, and the membrane is assumed to be a plane capacitor. As a result, the resistances and capacitances of the membrane are obtained and the thickness of the membrane can be evaluated. By using knowledge of the dielectric constant of water and the membrane material, the porosity of the membrane can be evaluated from the capacitance values. Impedance can be calculated from the values for resistance and 

In membrane potential measurements, a membrane is positioned between two half-cells. The cells are filled with electrolyte solutions the solution of higher electrolyte concentration is in contact with the active side of the membrane, and the solution of lower electrolyte concentration is in contact with the membrane support layer. The electrical potential difference is measured by inserting electrodes directly into the bulk solutions (Fig. 32.7a). The membrane potential measurements evaluate the amount of charge inside the membrane (Canas and Benavente, 2002 Nakagaki and Takagi, 1986 Schaep and Vandecasteele, 2001 Takagi et al., 2000). 

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