Two-layer membrane

By changing the enzyme and mediator, the amperometric sensor in Figure 11.39 is easily extended to the analysis of other substrates. Other bioselective materials may be incorporated into amperometric sensors. For example, a CO2 sensor has been developed using an amperometric O2 sensor with a two-layer membrane, one of which contains an immobilized preparation of autotrophic bacteria. As CO2 diffuses through the membranes, it is converted to O2 by the bacteria, increasing the concentration of O2 at the Pt cathode. 

In the third simulation example, we carried out an analysis of some of the aspects that characterize the case of the mass transfer of species through a membrane which is composed of two layers (the separative and the support layers) with the same thickness but with different diffusion coefficients of each entity or species. To answer this new problem the early model has been modified as follows (i) the term corresponding to the source has been eliminated (u) different conditions for bottom and top surfaces have been used for example, at the bottom surface, the dimensionless concentration of species is considered to present a unitary value while it is zero at the top surface (iii) a new initial condition is used in accordance with this case of mass transport through a two-layer membrane (iv) the values of the four thermal diffusion coefficients from the original model are replaced by the mass diffusion coefficients of each entity for both membrane layers (v) the model is extended in order to respond correctly to the high value of the geometric parameter 1/L. 

In porous composite membranes, the support layer(s) can play an important role in the reactor performance. This, for example, is the case with consecutive reactions such as partial oxidations where intermediate products are desirable. Harold et al. [1992] presented a concept in which two reactants are introduced to a two-layer membrane system from opposite sides ethylene on the membrane side while oxygen on the support side. The mass transfer resistance of the support layer lowers the oxygen concentration in the catalytic zone and directs the preferred intermediate product, acetaldehyde, toward the membrane side. Thus the support layer structure enhances the yield of acetaldehyde. 

Figure 2.15. The two-layer membrane for butane dehydrogenation. From Alfonso et al. [2.289], with permission from the American Chemical Society.

Gharsallaoui, A. Roudaut, G. Beney, L. Chambin, O. Voilley, A. Saurel, R., Properties of spray-dried food flavours microencapsulated with two-layered membranes Roles of interfacial interactions and water. Food Chemistry (2012) 132, 1713-1720. 

Figure 19.15 The two-layer membrane designed by De La Iglesia and co-workers. Reprinted with permission from De La Iglesia et al. (2006). Copyright 2006 Elsevier.

Moreover, when two-layer membranes (asymmetric or composite) are studied, the total system is electrode/solution (c)/membrane (dense/porous layers)/solution (c)/ electrode and the impedance plots can then present three relaxation processes (two at the lowest frequencies which are associated with the membrane itself plus the contribution of the electrolyte solution at high frequencies with fmax 10 Hz), as can observed in Figure 9.4. Note, equivalent circuit (ReCe)-(RiQi)-(R2C2). 

The process was started in the early 1970s with development of the perfluorosul-fonate membrane Nation by DuPont [130]. In 1975, a perfluorocarboxylate membrane capable of producing 35 wt% caustic soda became available, from Asahi Glass in Japan [131]. In 1978 the first two-layer membrane was developed, with low electrical resistance and high current efficiency [132]. 

Hild N, Schneider OD, Mohn D, Luechinger NA, Koehler FM, Hofmann S, et al. Two-layer membranes of calcium phosphate/collagen/PLGA nanofibres in vitro biominer-alisation and osteogenic differentiation of human mesenchymal stem cells. Nanoscale 2011 3(2) 401-9. 

Lipid bilayer (Section 26 4) Arrangement of two layers of phospholipids that constitutes cell membranes The polar termini are located at the inner and outer membrane-water interfaces and the lipophilic hydrocarbon tails cluster on the inside... 

The anode and cathode chambers are separated by a cation-permeable fluoropolymer-based membrane (see Membrane technology). Platinum-electroplated high surface area electrodes sold under the trade name of TySAR (Olin) (85,86) were used as the anode the cathode was formed from a two-layer HasteUoy (Cabot Corp.) C-22-mesh stmcture having a fine outer 60-mesh stmcture supported on a coarse inner mesh layer welded to a backplate. The cell voltage was 3.3 V at 8 kA/m, resulting ia a 40% current efficiency. The steady-state perchloric acid concentration was about 21% by weight. 

The simulations to investigate electro-osmosis were carried out using the molecular dynamics method of Murad and Powles [22] described earher. For nonionic polar fluids the solvent molecule was modeled as a rigid homo-nuclear diatomic with charges q and —q on the two active LJ sites. The solute molecules were modeled as spherical LJ particles [26], as were the molecules that constituted the single molecular layer membrane. The effect of uniform external fields with directions either perpendicular to the membrane or along the diagonal direction (i.e. Ex = Ey = E ) was monitored. The simulation system is shown in Fig. 2. The density profiles, mean squared displacement, and movement of the solvent molecules across the membrane were examined, with and without an external held, to establish whether electro-osmosis can take place in polar systems. The results clearly estab-hshed that electro-osmosis can indeed take place in such solutions. 

One important constitnent of cell membranes is a class of molecules— the phosphohpids— that spontaneously form two-layer films in a... 

The lipid bilayer of a cell membrane contains two layers of a phospholipid such as lecithin, arranged tail-to-tail. 

The design of the Pd-membrane reactor was based on the chip design of reactor [R 10]. The membrane is a composite of three layers, silicon nitride, silicon oxide and palladium. The first two layers are perforated and function as structural support for the latter. They serve also for electrical insulation of the Pd film from the integrated temperature-sensing and heater element. The latter is needed to set the temperature as one parameter that determines the hydrogen flow. 

Cell membranes consist of two layers of oriented lipid molecules (lipid bilayer membranes). The molecules of these two layers have their hydrocarbon tails toward each other, while the hydrophilic heads are outside (Fig. 30.1a). The mean distance between lipid heads is 5 to 6mn. Various protein molecules having a size commensurate with layer thickness float in the lipid layer. Part of the protein molecules are located on the surface of the lipid layer others thread through the layer (Fig. 30.1fc). Thus, the membrane as a whole is heterogeneous and has a mosaic structure. 

The cells of all contemporary living organisms are surrounded by cell membranes, which normally consist of a phospholipid bilayer, consisting of two layers of lipid molecules, into which various amounts of proteins are incorporated. The basis for the formation of mono- or bilayers is the physicochemical character of the molecules involved these are amphipathic (bifunctional) molecules, i.e., molecules which have both a polar and also a non-polar group of atoms. Examples are the amino acid phenylalanine (a) or the phospholipid phosphatidylcholine (b), which is important in membrane formation. In each case, the polar group leads to hydrophilic, and the non-polar group to hydrophobic character. 

Extramembrane water Membrane is a bilayer sandwiched between two layers of water Water located within the bilayer which is attached to or in approximate contact with the expanses of membrane constituents... 

Modem membranes usually consist of at least two layers one from sulfonic type copolymer and another from carboxylic type copolymer. The membranes are usually reinforced by fluoropolymer fabric to provide better mechanical properties and long lifetimes. The most important properties are considered in detail in the reviews mentioned above10,11 and in a basic text by Seko et al.6... 

In Gram-negative bacteria which are characterised by a rather complex cell envelope, the CM is also referred to as inner membrane to distinguish it from a second lipid bilayer, termed outer membrane (OM). The space between these two layers is called the periplasm (PP). In the periplasmic space, many proteins are found with a variety of functions. Some are involved in biosynthesis and/or export of cell wall components and surface structures (e.g. pili, flagellae,... 

Ion-exchange membranes for chlor-alkali electrolysis generally contain a sulphonic layer which faces the anode and a carboxylic layer which faces the cathode, joined by lamination. The Na+ transport number is higher in the carboxylic layer than in the sulphonic layer, and a region of low Na+ concentration therefore tends to form at the interface between the two layers during electrolysis, as shown in Fig. 17.5. 

The quality of the support is especially critical if the formation of the top layer is mainly determined by capillary action on the support (see Section 2.3.2). Then, besides a narrow pore size distribution the wettability of the support system plays a role (see Equation 2.1). An example of the synthesis of a two-layer support and ultrafUtration membrane is given in the French Patent 2,463,636 (Auriol and Trittcn 1973). In many cases an intermediate layer, whose pore sizes and thickness lie between those of the main support and the top layer (see Figure 2.2), is used. This intermediate layer can be used to improve the quality of the support system. If large capillary pressures are used to form such an intermediate layer, defects (pinholes) in the support will be transferred to this layer. This can be avoided by decreasing the acting capillary pressures or even by eliminating them. This can be done in several ways. 

Figure 3.6. Pore size distribution by mercury porosimetry of a two-layered zirconia membrane composite.

Another possible solution to the problem of analyzing multiple-layered membrane composites is a newly developed method using NMR spin-lattice relaxation measurements (Glaves 1989). In this method, which allows a wide range of pore sizes to be studied (from less than 1 nm to greater than 10 microns), the moisture content of the composite membrane is controlled so that the fine pores in the membrane film of a two-layered composite are saturated with water, but only a small quantity of adsorbed water is present in the large pores of the support. It has been found that the spin-lattice relaxation decay time of a fluid (such as water) in a pore is shorter than that for the same fluid in the bulk. From the relaxation data the pore volume distribution can be calculated. Thus, the NMR spin-lattice relaxation data of a properly prepared membrane composite sample can be used to derive the pore size distribution that conventional pore structure analysis techniques... 

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