Membrane resistance, specific

Specific cake resistance a ranges from 10 m/kg for easily filtered large particles to lo m/kg for gelatinous cakes. The membrane resistance Rp typically equals 10 to lO" m. Both a and Rp depend on the filtering pressure. 

Tortuosity is a long-range property of a porous medium, which qualitatively describes the average pore conductivity of the solid. It is usual to define x by electrical conductivity measurements. With knowledge of the specific resistance of the electrolyte and from a measurement of the sample membrane resistance, thickness, area, and porosity, the membrane tortuosity can be calculated from eq 3. 

Here, Rmem is the specific membrane resistance in units of wcm2. For lipid bilayers Rmem is of the order of 108 flcm2. If we built a membrane of similar thickness ( 4 nm) of a good insulator like porcelaine (specific resistivity 1014 flcm) its membrane resistance would only be 1014 Qcrn. 10-7 cm = 107 flcm2. In addition, a bilayer can stand potentials of typically 200 mV, which results in an enormous electric field strength of 108 V/m. 

Although a preindustrial project of ethanol production with immobilized cells goes back to the 1980s (8), its application is still limited because of technical problems related to the gel stability and even more to the mass-transfer resistance of the gel membrane (9). Specifically, both substrate and product counterdiffusion (in addition to the presence of cells inside the beads) could reduce the diffusion coefficients of glucose and ethanol up to 13.7 and 28.1%, respectively (10). 

Now, considering the resistance of the solute, designate the combined effect of compressibility, membrane resistance R, and solute resistance as. Analogous to cake filtration, call a as specific membrane resistance. Hence,... 

In practice, it is not feasible to test the derived equations experimentally by varying all the forces and fluxes independently. Usually some simplification is gained by allowing the system to develop to a specific steady state. A useful steady state for illuminated bacteriorhodopsin liposomes is that of electroneutral total flow, i.e., the condition in which the net movement across the membrane of all chemical species adds up to no charge movement. It can be derived and shown that this condition is attained within seconds, considering the membrane resistance and electrical capacity in the usual salt media [28], Electroneutral total flow is mathematically expressed as ... 

In this equation, R (=P -i- R is the intrinsic membrane resistance that includes the fouling layer resistance (Pp due to specific membrane-solute interactions. The polarization layer resistance, Rp, consists of two resistances R due to gel-polarized layer and R due to associate boundary layer. The intrinsic membrane resistance is unaffected by operating parameters, whereas the polarization layer resistance is a function of applied pressure. When R is negligible, the filtrate flux is given by (49)... 

Chemical Abstracts Service Registry Number CAS 14277-97-5. Domoic acid, a glutamic acid analog that is resistant to temperature extremes, is an excitatory neurotoxin produced by a diatom and concentrated in shellfish. Ingestion leads to amnesic shellfish poisoning, which can also include seizures. Its relevance to use in warfare and terrorism, apart from its being unfamiliar to most disaster-response personnel, is that it is also easily absorbed by inhalation and across mucous membranes. No specific antitoxin is available, and treatment is supportive. 

The action of deltamethrin on the GABA response was more difficult to determine, because low concentrations of deltamethrin caused depolarization and rapid firing of action potentials, accompanied by a decrease in membrane resistance. Deltamethrin at 0.1 nM caused a massive increase in firing frequency, and a depolarization of 30mV, after perfusion for ten minutes. Application of TTX, which specifically blocks voltage dependent sodium channels, abolished the action potentials, and also reversed the pyrethroid-induced depolarization and decrease in membrane resistance. 

With increasing temperature, electrolyte and membrane resistance tend to decrease while water vapor pressure tends to increase, and minimum operating voltage occurs at a specific temperature. 

Much work has been done with polymers to create biocompatible surfaces that resist protein fouling, i.e., non specific adsorption. A membrane composed of modified polyethersulfone (PES) (Omega Membrane) resisted albumin (ALB) absorption 24 times better than unmodified PES, and three times better than regenerated cellulose, a material known to resist biofouling. We compared porous SiC, both n-type and p-type, to the Omega Membrane [14] and found a very similar resistance to ALB absorption (Figure 12.3). 

Table 9.1 provides the values of membrane resistance (/ ), capacitance (Cm), and thickness d) of artificial BLMs and natural cell membranes [11,18]. The resistance of artificial membranes is much higher than that of biological membranes. This results from the presence of translocators such as peptides and proteins in the cell membranes. The resistance of artificial membranes can however be reduced to the levels of natural cell membranes when ion translocators are inserted. Specific capacitance (C ) is the primary criterion to distinguish between solventless BLMs and black lipid films. Table 9.1 exhibits that the specific capacitance of the solventless BLMs (about 0.9 /itF cm ) approaches the values measured for natural cell membranes, and is almost twice the magnitude observed for black lipid membranes. These values of specific capacitance can be used to estimate the hydrocarbon thickness, d, of membranes using the equation... 

To model the electrodialysis stack, we assume that since there are many cells in a stack the behaviors in different pairs of adjacent dialysate and concentrate channels are the same. If we neglect the potential drop in the electrode cells adjacent to the electrodes as small compared with that in the rest of the system, the potential drop across a channel pair is constant and equal to the total applied voltage divided by the number of channel pairs. The dialysate and concentrate channels are taken to have the same separation 2h (Fig. 6.2.1). Since there is symmetry about the center plane of each channel, we may model the electrodialysis cell pair of Fig. 6.2.1 by one half of the dialysate channel and one half of the adjacent concentrate channel separated by a membrane, as shown in Fig. 6.2.4. For specificity we choose the cation exchange membrane. Both types of membranes are assumed to have the same resistances and thicknesses and to be perfectly selective. To simplify the problem somewhat further, we take the membrane resistance to be small so that the ohmic drop within the membranes may be neglected. 

The limitation of membrane is that the presence of a membrane itself consists of another resistance to mass transfer, which is not encountered in absorption columns. This resistance could negatively affect the overall mass transfer and significantly lower the selectivity. This membrane resistance can be minimized by reducing the membrane thickness or by increasing its intrinsic gas permeability. Many authors [1,15,16] have specifically investigated this point. 

The phenomenon of fouling is very complex and difficult to describe theoretically. Even for a given solution, fouling will depend on physical and chemical parameters such as concentration, temperature, pH, ionic strength and specific interactions (hydrogen bonding, dipole-dipole interactions). However, reliable values of flux decline are necessary for process design. The flux may also be described by a resistances-in-series model, in which a resistance of a cake layer is in series with the membrane resistance. The flux can be described by... 

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