Membrane pores constricted

Cake layer formation builds on the membrane surface and extends outward into the feed channel. The constituents of the foulant layer may be smaller than the pores of the membrane. A gel layer can result from denaturation of some proteins. Internal pore fouling occurs inside the membrane. The size of the pore is reduced and pore flow is constricted. Internal pore fouling is usually difficult to clean. 

Figure 11.7. Transient configuration of M2 helices around the open pore, (a) A barrel of a-helical segments, having a pronounced twist, forms in the cytoplasmic leaflet of the bilayer, constricting the pore maximally at the cytoplasmic membrane surface. The bend in the rods is at the same level as for the closed pore, but instead of pointing inward has rotated over to the side, (b) Schematic representation of the most distant three rods. A tentative alignment of the amino acid sequence with the densities suggests that a line of polar residues (serines and threonine see Fig. 11.3) should be facing the open pore. [From Unwin (31).]...

Figure 6.3.27. (a) UF membrane as a bundle of size-distributed tortuous capillaries, (b) Macrosolute retention behavior of two types of UF membranes having a narrow or a broad pore size distribution, (c) Macrosolute retention via smaller membrane pore size, pore mouth adsorption, pore blockage due to pore constriction. 

Figure 2.2 Schematic representation of the main types of membrane pores (a) isolated (b) dead-end (c) straight cylindrical (d) constricted (e) conical.

FIGURE 8.38 Models and mechanisms of induced polarization. Top halves of figures without field lower halves with applied field (a) electrode polarization (metallic polarization), (b) membrane polarization, (c) polarization by constrictivity of pores. 

Fig. 26. Screen filters contain pores of a uniform size and retain all particulates greater than the pore diameter at the surface of the membrane. Depth filters contain a distribution of pore sizes. Particulates entering the membrane are trapped at constrictions within the membrane. Both types of filters are rated 10...

However, if the particulates or solutes accumulated on the surface can be dispersed back into the bulk fluid, these membranes can be used to great advantage since there is relatively little if any "internal-fouling" of the membrane structure. There is a high probability that a molecule or particle which penetrates the skin will not be trapped within the filter structure but will pass through into the filtrate. Schematically, the pores may be represented by ever-widening cones with no internal constrictions to restrain molecules or particles. 

The second category of microporous membranes is the depth filter (b), which captures the particles to be removed in the interior of the membrane. The average pore diameter of a depth filter is often 10 times the diameter of the smallest particle able to permeate the membrane. Some particles are captured at small constrictions within the membrane, others by adsorption as they permeate the membrane by a tortuous path. Depth filters are usually isotropic, with a similar pore structure throughout the membrane. Most microfiltration membranes are depth filters. 

The mechanism of particle capture by depth filtration is more complex than for screen filtration. Simple capture of particles by sieving at pore constructions in the interior of the membrane occurs, but adsorption of particles on the interior surface of the membrane is usually at least as important. Figure 2.34 shows four mechanisms that contribute to particle capture in depth membrane filters. The most obvious mechanism, simple sieving and capture of particles at constrictions in the membrane, is often a minor contributor to the total separation. The three other mechanisms, which capture particles by adsorption, are inertial capture, Brownian diffusion and electrostatic adsorption [53,54], In all cases, particles smaller than the diameter of the pore are captured by adsorption onto the internal surface of the membrane. 

VanGelder, R, Saint, N., van Boxtel, R., Rosenbusch, J. R, and Tommassen, J. (1997). Pore functioning of outer membrane protein PhoE of Escherichia coli mutagenesis of the constriction loop L3. Protein Eng. 10, 699-706. 

As expected, the potassium channel is a tetramer of identical subunits, each of which includes two membrane-spanning a helices. The four subunits come together to form a pore in the shape of a cone that runs through the center of the structure (Figure 13.22). Beginning from the inside of the cell, the pore starts with a diameter of approximately 10 A and then constricts to a smaller cavity with a diameter of 8 A. Both the opening to the outside and the central cavity of the... 

With conventional sol-gel routes, the pore size distribution is usually broad and the tortuosity of the pore network is important with the presence of constrictions. Thus ordered interconnected pore networks with constant pore size are strongly attractive. Hierarchical porosity and adaptive porosity are also fascinating approaches to increase or manage the permeability of ceramic membranes. 

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