Layered support structure

VIII. S-LAYER AS SUPPORTING STRUCTURE FOR FUNCTIONAL LIPID MEMBRANES... 

FIG. 14 Schematic illustration of an archaeal cell envelope structure (a) composed of the cytoplasmic membrane with associated and integral membrane proteins and an S-layer lattice, integrated into the cytoplasmic membrane, (b) Using this supramolecular construction principle, biomimetic membranes can be generated. The cytoplasmic membrane is replaced by a phospholipid or tetraether hpid monolayer, and bacterial S-layer proteins are crystallized to form a coherent lattice on the lipid film. Subsequently, integral model membrane proteins can be reconstituted in the composite S-layer-supported lipid membrane. (Modified from Ref. 124.)... 

The use of Upid bilayers as a relevant model of biological membranes has provided important information on the structure and function of cell membranes. To utilize the function of cell membrane components for practical applications, a stabilization of Upid bilayers is imperative, because free-standing bilayer lipid membranes (BLMs) typically survive for minutes to hours and are very sensitive to vibration and mechanical shocks [156,157]. The following concept introduces S-layer proteins as supporting structures for BLMs (Fig. 15c) with largely retained physical features (e.g., thickness of the bilayer, fluidity). Electrophysical and spectroscopical studies have been performed to assess the appUcation potential of S-layer-supported lipid membranes. The S-layer protein used in aU studies on planar BLMs was isolated fromB. coagulans E38/vl. 

Figure 5.32. Illustrating common configurations of elements that may be quantified by AES analysis (a) homogeneous (b) large, well-defined structures varying laterally and with depth (c) small, well-defined structures varying laterally and with depth, (d) monolayer segregants and absorbed layers (e) layered composition gradients and (f) a catalyst promoter on a layered support. (After Seah 1986,...

For Eurodif and for Pierrelatte, the supports were made by private industrial companies, the final separating layer by SPEC and the CEA developed the process and had the overall technical responsibility. A handful of companies were competing to manufacture the membrane support structure. Finally, two companies proposing ceramic oxide based supports, Ceraver (the new name of CGEC) and Euroceral (a 50/50 joint venture between Norton and Desmarquest) each won 50% of the market. This happened in 1975. Within a matter of 6 years, each company had to deliver more than 2,000,000 m of supports which SPEC would convert into more than 4,000,000 m of membranes (Charpin and Rigny 1990). Special plants were built at a very rapid pace. These were close to Tarbes for Ceraver, close to Montpellier for Euroceral and close to the Eurodif site for SPEC. 

With anodic oxidation very controlled and narrow pore size distributions can be obtained. These membranes mounted in a small module may be suitable for ultrafiltration, gas separation with Knudsen diffusion and in biological applications. At present one of the main disadvantages is that the layer has to be supported by a separate layer to produce the complete membrane/support structure. Thus, presently applications are limited to laboratory-scale separations since large surface area modules of such membranes are unavailable. 

It is evident, that in any membrane reactor operation mode there are important parameters which determine the performance of the process (Shah, Remmen and Chiang 1970). These are (1) the total and partial pressures on both sides of the membrane, (2) the total and partial pressure differences across the membrane, (3) the diffusion mechanism through the support and the membrane layer (membrane structure), (4) the thickness of the membrane, (5) the reactant configuration (i.e. whether the reactants are supplied from the same or from opposite sides of the membrane, in counter or co-current flow) and (6) the catalyst distribution. 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

Matsuura (130) measured the ESR spectra of bismuth molybdate catalysts both before and after reduction of butene. A board signal of high intensity was observed for the y phase of bismuth molybdate. The authors proposed that in the layered Bi2MoOs structure, a (Mo042 ) layer shifts with respect to the nearest layers and causes the formation of Mo-Bi-Mo sites. The results supported the earlier proposed reaction site model, based on adsorption measurements, which consisted of an A site (Bi) and two B sites (Mo). 

The dermis can be divided into two layers, the papillary layer adjacent to the epidermis and the deeper reticular layer. Blood vessels and sensory nerve endings are present in this layer. The reticular layer is made up of collagen and elastin lattice. Papillary and reticular layers provide structural and nutritional support.45... 

This so-called "active" layer has characteristics similar to those of cellulose acetate films but with a thickness of the order of 0.1 micrometer (jjm) or less, whereas the total membrane thickness may range from approximately 75 to 125 ym (see Figure 1). The major portion of the membrane is an open-pore sponge-like support structure through which the gases flow without restriction. The permeability and selectivity characteristics of these asymmetric membranes are functions of casting solution composition, film casting conditions and post-treatment, and are relatively independent of total membrane thickness. 

The calculated phase behavior for asymmetric surface fields, where the interactions between the components and the top (cmi) and bottom (rM2) interface may vary freely, is even much richer (see Fig. 12 for two layers of structure) than for the symmetric case. A particular feature is the appearance of hybrid structures, which are either a coexistence of different surface reconstructions close to each of the interfaces or interconnected structures. Nevertheless, the general features of Fig. 12 can still be explained in terms of surface field and confinement effects, noting that each of the surface fields now supports its own surface reconstruction, which may interconnect for combinations of perpendicular and parallel structures. As in the symmetric case, this behavior is modulated by the film thickness via interference and confinement effects in a very complicated manner. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Anisotropic membranes are layered structures in which the porosity, pore size, or even membrane composition change from the top to the bottom surface of the membrane. Usually anisotropic membranes have a thin, selective layer supported on a much thicker, highly permeable microporous substrate. Because the selective layer is very thin, membrane fluxes are high. The microporous substrate... 

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]. 

The fabrication of polyimide based microstructures with a single metallization layer is described. The silicon wafer is merely used as a supporting structure to perform the photolithographic steps and thin film procedures in the cleanroom. The wafer is removed at the end of the process. 

Good quality RO membranes can reject >95-99% of the NaCl from aqueous feed streams (Baker, Cussler, Eykamp et al., 1991 Scott, 1981). The morphologies of these membranes are typically asymmetric with a thin highly selective polymer layer on top of an open support structure. Two rather different approaches have been used to describe the transport processes in such membranes the solution-diffusion (Merten, 1966) and surface force capillary flow model (Matsuura and Sourirajan, 1981). In the solution-diffusion model, the solute moves within the essentially homogeneously solvent swollen polymer matrix. The solute has a mobility that is dependent upon the free volume of the solvent, solute, and polymer. In the capillary pore diffusion model, it is assumed that separation occurs due to surface and fluid transport phenomena within an actual nanopore. The pore surface is seen as promoting preferential sorption of the solvent and repulsion of the solutes. The model envisions a more or less pure solvent layer on the pore walls that is forced through the membrane capillary pores under pressure. 

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