Porous membranes functions

As the cell is discharged, Zn2+ ions are produced at the anode while Cu2+ ions are used up at the cathode. To maintain electrical neutrality, SO4- ions must migrate through the porous membrane,dd which serves to keep the two solutions from mixing. The result of this migration is a potential difference across the membrane. This junction potential works in opposition to the cell voltage E and affects the value obtained. Junction potentials are usually small, and in some cases, corrections can be made to E if the transference numbers of the ions are known as a function of concentration.ee It is difficult to accurately make these corrections, and, if possible, cells with transference should be avoided when using cell measurements to obtain thermodynamic data. 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

A separator is a porous membrane placed between electrodes of opposite polarity, permeable to ionic flow but preventing electric contact of the electrodes. A variety of separators have been used in batteries over the years. Starting with cedar shingles and sausage casing, separators have been manufactured from cellulosic papers and cellophane to nonwoven fabrics, foams, ion exchange membranes, and microporous flat sheet membranes made from polymeric materials. As batteries have become more sophisticated, separator function has also become more demanding and complex. 

Kinoshita et alJ107,l0x used poly(L-glutamic acid) containing 12-14 mol% azobenzene units in the side chains (Scheme 3, Structure III) to prepare membranes obtained by coating a porous Millipore filter with a 0.2 % chloroform solution of III. Irradiation at 350 nm was found to increase the membrane potential and crossmembrane permeability. The photoinduced alterations of the membrane functions were completely reversible and could be controlled by irradiation and dark-adaptation, in correlation with the trans-cis photoisomerization of the azobenzene units. 

In order to predict correctly the fluxes of multicomponent mixtures in porous membranes, simplified models based solely on Fields law should be used with care [28]. Often, combinations of several mechanisms control the fluxes, and more sophisticated models are required. A well-known example is the Dusty Gas Model which takes into account contributions of molecular diffusion, Knudsen diffusion, and permeation [29]. This model describes the coupled fluxes of N gaseous components, Ji, as a function of the pressure and total pressure gradients with the following equation ... 

Fig. 8. Activity of immobilized antibody by the radiation polymerization method as a function of antigen concentration. Antigen a-fetoprotein. Antibody anti-a-fetoprotein. Immobilization method polymerization of HEMA in a thin porous membrane O polymerization of HEMA in particle form...

Other electrodes functioning in a similar way have been developed for other dissolved gases, with important clinical applications (Table 14.1). Since the porous membrane does not let past species that can poison the electrode, these electrodes are ideal for measurements in biological fluids. 

Fig. 7.7. Transient current response, 1 - exp(-kEi), as a function of time, and the biological layer thickness, Itq. The data indicate that the response is relatively insensitive to the thickness of the porous membrane (mtq = 0, top ntlq = 60, bottom), but sensitive to the diffusion barrier presented by the biological layer (DIDf= 1, left DIDf= 10, right). D =...

Figure 34.18 Water flux and salt rejection as functions of deposition time, in minutes, for composite membranes of acetylene/C0/H20 (CNCA porous membrane used as substrate).

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. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Membranes have also been used in reactors where their permselective properties are not important. Instead their well-engineered porous matrix provides a well-controlled catalytic zone for those reactions requiring strict stoichiomeuic feed rates of reactants or a clear interface for multiphase reactions (e.g., a gas and a liquid reactant fed from opposing sides of the membrane). Functional models for these types of membrane reactors have also been developed. The conditions under which these reactors provide performance advantages have been identified. 

Figure 11.18 Comparison of methane conversion as a function of the molar fraction of oxygen in the total feed in two reactors using porous membranes with a uniform permeability and a decreasing permeability profile at 750 C and a total flowrate of 172 standard cm /min (open circles represent the membrane with a decreasing permeability along its length and open squares represent the membrane with a uniform permeability) (Coronas et al., 1994J...

Since chromatographic membranes consist of a substrate to which the interactive ligand is coupled, three main steps are usually involved in their preparation (i) basic membrane preparation (ii) functionalization (activation) of the basic membranes and (iii) spacer arms and ligand molecules coupling on the activated porous membrane surface [9]. The preparation of basic materials is essential for the performances of the separation process. 

Most commercial microporous membranes are hydrophobic and relatively inert. If the selected basic membrane does not possesses the functional groups necessary for spacers arms and hgand couphng, it can be activated to acquire reactive groups such as hydroxyl, carboxyl, halogen, or amine groups using similar methods as for particulate materials. The major methods employed to modify the porous membranes (some of them aheady mentioned) are based on... 

A system that more closely resembles the in vivo state is the porous membrane system. Because cells can receive nutrient from both sides of the membrane, the cell layer has an active basal and apical (or luminal) surface that allows the selective uptake and transport of nutrients and secreted products. A variety of biological functions can now be studied, such as active transport, better differentiation and co-culture of different cell types (Pitt Gabriels, 1986 Millipore, 1990 Halleux Schneider, 1991). 

It must be recognized that these channels are not empty but have poly(acrylic acid) (PAA) chains tethered to their inner walls. However, since the relative volume occupied by these chains is significantly lower than the parent PtBA, these channels are rendered substantially porous. A very interesting feature of these porous membranes is the chemical valving effect , wherein the relative permeability of the membranes was shown to vary by almost two orders of magnitude as a function of pH, with the lowest permeability being witnessed at a pH of 3 [41]. The explana-... 

Note that diffusion models and hydraulic permeation models have their own caveats the membrane is neither a homogeneous acid solution, nor is it the well-structured porous rock. Critical comparison of the results of the two approaches with each other and with experiments, is of crucial importance for understanding the membrane functioning within the cell and developing the strategies on water management and optimized membrane properties. 

Epithelial barrier models for the skin [48,49], respiratory tract [50], BBB, and intestine [39] are constructed to study and predict the absorption, penetration, and metabolism of drugs or environmental toxins through these barriers. All the models are physically tight structures, and generally involve cells cultured at the air-liquid interface on porous membrane support, such as a porous polycarbonate filter. The use of a permeable support allows cells to be grown in a polarized state under more natural conditions promote Cell differentiation and enhance cell functions. 

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