Filling Porous Membranes

This method is based on the measurement of pressure necessary to blow air through a water-filled porous membrane [9,85]. This technique is also called the bubble point method. This method is only able to determine maximum pore size present in the pore distribution, corresponding to the minimum pressure necessary to blow the air bubble first observed. Figure 2.8 shows a schematic drawing of the test apparatus. The top of the filter is placed in contact with water. When the membrane is wet, all the pores are filled with liquid. The bottom of the filter is in contact with air. When the air pressure is gradually increased, bubbles of air penetrate through the membrane at a certain pressure. 

Figure 3.4.8. Solute concentration profile in diffusive transport through a liquid-filled porous membrane from a feed solution to a permeate solution.

The use of porous membranes as templates for electrode structures was pioneered by Martin and coworkers nearly 20 years ago, and this approach has since been extended to include numerous electrode compositions and geometries " and applications beyond energy storage, including sensing and separations. In this approach, chemical and electrochemical routes are used to fill in the cylindrical, uniform, unidirectional pores of a free-standing membrane with electrochemically active materials and... 

Figure 10. Schematic showing the conversion of a porous membrane into a template in which the pores are filled or coated to form random ensembles or ordered arrays of nanometer- and micrometer-scale cylinders or tubes. (Derived with permission from ref 87. Copyright i997 Electrochemical Society.)...

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

FIGURE 5.8 Schematic of the injection tee and porous membrane structure is shown in (a). Porous membrane region width is 7 pm. CCD images of analyte concentrated for (b) 2 min, and (c) 3 min. Injection of concentrated analyte plug is depicted in (d). All channels are filled with 3% LPA in lx TBE buffer. DNA sample 25 pg/mL ( )X 114-IIaelll digest with 6.0 pM TO-PRO dye added [590]. Reprinted with permission from the American Chemical Society. 

Evaluation of the absorptive features of different powders these were quantified according to Enslin as previously described.8,9 Briefly, a thermostated glass cylinder with a porous membrane on one end was filled with water and connected to a graduated capillary tube at the other end. The membrane was covered with the powder. Water absorption through the membrane was measured by the variation in the liquid level in the capillary tube. 

In the simplest cells, the barrier between the two solutions can be a porous membrane, but for precise measurements, a more compbcated arrangement, known as a salt bridge, is used. The salt bridge consists of an intermediate compartment filled with a concentrated solution of KC1 and fitted with porous barriers at each end. The purpose of the salt bridge is to minimize the natural potential difference, known as the junction potential, that develops (as mentioned in the previous section) when any two phases (such as the two solutions) are in contact. This potential difference would combine with the two half-cell potentials so as introduce a degree of uncertainty into any measurement of the cell potential. With the... 

Separative flow often occurs in a packed bed—typically a tube filled with a granular material. Chromatography in packed columns is the most important example of packed-bed flow. Similar flow is found in porous membranes used for membrane separation. The fluid flowing through such media can be a gas, a liquid, or a supercritical fluid. 

In microporous reservoir systems, drug molecules are released by diffusion through the micropores, which are usually filled with either water or oil (e.g. silicone, castor and olive oil). Solvent-loading of a porous membrane device is achieved simply by immersing the device in the solvent. When this technique presents some difficulty, the implantable device is placed inside a pressure vessel and pressure is then applied to facilitate the filling of the solvent into pores. The transport of drug molecules across such porous... 

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Gaseous permeation can be used for the characterization of porous membranes using an apparatus working with the technique of fixed volume-variable pressure as shown in Fig. 3.70. The technique, which was initially developed for dense polymer membranes, is based on the recording of the pressure evolution with time of a downstream compartment, which is separated from an upstream compartment filled with a pure gas by a flat membrane. Before starting the experiments, both compartments are put under very low pressure and, at the initial time of the measurements, a relatively high pressured pure gas is introduced into the upstream compartment [3.59]. 

The water transport mechanism changes from the flow mechanism in porous membrane to the diffusive transport in nonporous homogeneous membrane due to the deposition of a homogeneous LCVD layer that fills the pore, i.e., water transport changes from bulk flow to diffusive flow when pores are covered by LCVD film. 

Capillary condensation provides the possibility of blocking pores of a certain size with the liquid condensate simply by adjusting the vapor pressure. A permporometry lest usually begins at a relative pressure of 1, thus all pores filled and no unhindered gas transport. As the pressure is reduced, pores with a size corresponding to the vapor pressure applied become emptied and available for gas transport. The gas flow through the open mesopores is dominated by Knudsen diffusion as will be discussed in Section 4.3.2 under Transport Mechanisms of Porous Membranes. The flow rate of the noncondensable gas is measured as a function of the relative pressure of the vapor. Thus it is possible to express the membrane permeability as a function of the pore radius and construct the size distribution of the active pores. Although the adsorption procedure can be used instead of the above desorption procedure, the equilibrium of the adsorption process is not as easy to attain and therefore is not preferred. 

In the case of porous membranes, where solute diffusion takes place inside the hquid filling the pores, it is usual defining an effective diffusion coefficient D. ... 

Basic adsorption isotherms have been described in this chapter. For micro-porous membranes, the use of the DR equation to describe micropore filling has been shown to be quite adequate. Techniques for the determination of surface area and pore size distribution have ben presented. The use of potential functions for the determination of pore size distribution in microporous materials has been described. Although the potential function techniques give consistent and satisfactory results, caution must be exerted in using these techniques for the calculation of the pore size distribution, due to the uncertainty involved in the values of the parameters used in the calculation and the simplifying assumptions employed in the derivation of the model equations. 

In this work, ac electrochemical deposition has been applied to nanowire formation in PAA. The advantage of the ac electrochemical deposition is that the membrane remains on the A1 substrate and that the barrier layer does not avoid the deposition. In this case, the fabrication of ordered and metal-filled porous alumina structures is not limited by the thickness and size of the PAA layer. These structures may be used in high temperature technological operations. 

SLM is a practical design to utilize liquid membranes with mobile carriers for the facilitated transport of certain gases, that is, CO2. Essentially, this is a porous membrane with the pores filled with solvent and carriers by capillary force. 

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