Membrane surface electrical properties

Although most membrane technologists now recognize the important contribution of membrane surface electrical properties in defining the separation characteristics, there remains confusion as to how best to describe and quantify such interactions. There is an unfortunate tendency in the applied membrane... 

Fortunately, AFM in conjunction with the colloid probe technique offers an alternative means of membrane surface electrical properties characterization. If a colloid probe is approached towards a surface it is possible to quantify the force of interaction. Figure 6.15 shows typical data for a Desal DK membrane, which is one of the least rough membranes [7]. 

W. R. Bowen, T. A. Doneva, and J. A. G. Stoton, Colloids Surf. A, 201,73 (2002). The Use of Atomic Force Microscopy to Quantify Membrane Surface Electrical Properties. 

However, the separation characteristics of membrane interfaces do not depend solely on the physical form of surface features. In liquids, surface electrical properties and the adhesion of solutes to membrane surfaces may also have profound effects on separation performance. It is thus exceedingly fortunate that an atomic force microscope may also be used to determine both of these additional controlling factors. Finally, means may be devised to quantify all of these controlling factors in liquid environments that match those of process streams. 

Atomic force microscopy can determine the key properties of synthetic membranes pore size distribution, surface morphology and surface roughness, surface electrical properties, surface adhesion. 

We report here preliminary experiments by which we examine whether heterocyst differentiation entails changes in the surface electric properties of thylakoids. For this purpose, we have used the method of Chow, Barber (1980) which permits the estimation of the surface electric charge density (a) and potential (iIJq) from pairs of mono- and divalent metal cation concentrations that equally reverse the quenching of 9-AA fluorescence, caused by membrane particles. 

BIOELECTROCHEMISTRY. Application of the principles and techniques of electrochemistry to biological and medical problems. It includes such surface and interfacial phenomena as the electrical properties of membrane systems and processes, ion adsorption, enzymatic clotting, transmembrane pH and electrical gradients, protein phosphorylation, cells, and tissues. 

Fig. 17. a A scanning electron micrograph of square pores etched in a 3 micrometer thick silicon membrane. The pores were produced by anisotropic etching and their width on this side of the membrane is 6 pm. Cells (fibroblasts 3T3) attach to the surface and migrate over the pores, b Electrodes are placed on either side of the membrane and a constant current passed through it (mainly through the pores). The presence of cells is easily detected and movements of cell filopodia of less than 100 nm and the passive electric properties of the cell body can be determined by analysis of the signal fluctuations and impedance... 

The fact that the majority of in vivo processes occur on the surface of or within the membrane and that electrical phenomena are very important in membranes such as those found in the chloroplast, muscle fibres, nerve fibres, mitochondria, etc., has recently led to intensive study of the electrical properties of bilayer lipid membranes (BLM) in an attempt to reproduce a model of the cell membrane. Membranes of 5-10 nm thickness have been studied, the membranes consisting of two parallel sheets of lipids with a hydrophobic environment in the interior of the membrane and the hydrophilic groups directed to the exterior aqueous medium. 

Another less-utilized transduction mechanism for biosensors involves the acoustoelectric effect. In principle, any biochemical process that produces a change in the electrical properties of the solution, can be monitored by observing changes in the frequency and/or attenuation of the device if its surface is not metallized. For example, a SH-SAW device has been reported for the detection of pH changes associated with the enzyme-catalyzed hydrolysis of urea [235]. Using an immobilized urease membrane on the sensor surface, it was anticipated that urea concentrations as small as 3 /u.M could be reliably detected. 

Microbial cells are surrounded by a cell wall, which retains the cell contents, and is the primary barrier between the cell surface and the environment in which it exists. The quality of the cell wall, in terms of selective permeability, maintains the necessary levels of nutrients, trace elements, and cell internal pH. The cell membrane is the site of transfer processes water is able to pass through this membrane, in or out of the cell, depending on the thrust of the osmotic pressure. The chemistry of the cell wall affects its properties in terms of surface electric charge and the availability of binding ions. 

Information is conveyed within individual neurons by electrical means - generating nerve impulses which can be recorded and displayed on an oscilloscope. In the twentieth century it was shown that information is conveyed between neurons by chemical means. When a nerve impulse (an action potential) travelling along the long extension or axon of a neuron arrives at a junction (or synapse) with another neuron, it causes the release of a neurotransmitter chemical from the presynaptic cell. This chemical affects the electrical properties of the postsynaptic neuron through its interaction with specialized receptor proteins embedded in the surface membrane of the postsynaptic cell. It has been shown that there are numerous kinds of neurotransmitter chemical which, depending on the specific... 

Special Property Membranes. In the literature, there are numerous methods reported for the preparation of ion-exchange membranes with special properties,87-89 for instance, for use as battery separators, ion-selective electrodes, or in the chlor-alkali process. Especially membranes recently developed for the chlor-alkali industry are of commercial significance. These membranes are based on polytetrafluoroethylene and carry sulfone groups in the bulk of the membrane phase and carboxyl-groups on the surface as the charged moiety. They combine good chemical stability with high selectivity and low electric resistance. 

Figure 5.5 Transport properties of a cation exchange membrane having a cationic polyelectrolyte layer formed by electrodeposition. (A) PNaCa ( ) current efficiency (%) ( ) electrical resistance of the membrane during electrodialysis for 1 h. After solutions containing 0.0416N sodium chloride and poly(3-methylene-N, N-dimethylcyclohexylammonium chloride) of various concentrations had been electrodialyzed, for 60 min at a current density of 10 mA cm 2, as anolyte to electrodeposit the polyelectrolyte on the membrane surface (catholyte was 0.0416N sodium chloride), a 1 1 mixed solution of 0.208N calcium chloride and 0.208 N sodium chloride was electrodialyzed at a current density of 10 mA cmr1 for 60 min (cation exchange membrane NEOSEPTA CH-45T).

Membrane structures that contain the visual receptor protein rhodopsin were formed by detergent dialysis on platinum, silicon oxide, titanium oxide, and indium—tin oxide electrodes. Electrochemical impedance spectroscopy was used to evaluate the biomembrane structures and their electrical properties. A model equivalent circuit is proposed to describe the membrane-electrode interface. The data suggest that the surface structure is a relatively complete single-membrane bilayer with a coverage of 0.97 and with long-term stability/... 

Electrochemical impedance spectroscopy provides a sensitive means for characterizing the structure and electrical properties of the surface-bound membranes. The results from impedance analysis are consistent with a single biomembrane-mimetic structure being assembled on metal and semiconductor electrode surfaces. The structures formed by detergent dialysis may consist of a hydrophobic alkyl layer as one leaflet of a bilayer and the lipid deposited by dialysis as the other. Proteins surrounded by a bound lipid layer may simultaneously incorporate into pores in the alkylsilane layer by hydrophobic interactions during deposition of the lipid layer. This model is further supported by the composition of the surface-bound membranes and by Fourier transform infrared analyses (9). 

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