Selective barrier structure

Selective barrier structure. Transport through porous membranes is possible by viscous flow or diffusion, and the selectivity is based on size exclusion (sieving mechanism). This means that permeability and selectivity are mainly influenced by membrane pore size and the (effective) size of the components ofthe feed Molecules... 

The ability of the membrane to act as a selective barrier is related to the membrane s normal structure... 

Synthetic membranes for molecular liquid separation can be classified according to their selective barrier, their structure and morphology and the membrane material. The selective barrier- porous, nonporous, charged or with special chemical affinity -dictates the mechanism of permeation and separation. In combination with the applied driving force for transport through the membrane, different types of membrane processes can be distinguished (Table 2.1). 

Selective barrier Typical structure Concentration difference Pressure difference Electrical potential... 

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]. 

Because the mechanisms are based on pore flow and size exclusion (cf. Section 2.2), the polymer material itself does not have direct influence on flux and selectivity in U F. The U F membranes usually have an integrally asymmetric structure, obtained via the NIPS technique, and the porous selective barrier (pore size and thickness ranges are 2-50 nm and 0.1-1 (im, respectively) is located at the top (skin) surface supported by a macroporous sublayer (cf. Section 2.4.2). However, the pore-size distribution in that porous barrier is typically rather broad (Figure 2.6), resulting in limited size selectivity. 

Entry of molecules to the brain is regulated by a selective barrier that exists between the brain and the blood, known as the blood-brain barrier (BBB). This chapter reviews the structure and physiology of this barrier and the new and emerging technologies to overcome this barrier and achieve drag delivery to the CNS. 

Figure 15.2. Location of xenobiotic transporters in selected barrier and excretory tissues. For simplicity, the tissues are arranged along a structure representing the vascular space. Arrows indicated direction of transport under normal conditions. This figure is not meant to be comprehensive not all transporters expressed in a tissue are shown. Transporters driven by ATP pump substrates out of cells (efflux). Other transporters are capable of supporting substrate uptake or efflux. Which of these processes predominates depends on available driving forces—for example, substrate concentration gradient and the capability to couple transport to sources of potential energy.

Preparation and Deposition of the Selective Barrier Layer on the Micro-porous Support. The techniques used for the preparation of composite structures may be grouped into four general types 91 " s... 

The detailed mechanism and relative energetics for the loss of the HD in the reaction of PtAuCD2" with NH3 are shown in Fig. 22. Selected optimized structures of relevant species in the reaction are collected in Fig. 23. NBO analyses indicate that the N-Pt bond in 12 arises from strong donor-acceptor interactions of the lone pair of NH3 with an antibonding orbital of the PtC subunit. The N-C bond coupling through migration of NH3 to C produces an intermediate 14 with a barrier of... 

A membrane is a layer of material that serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force. Some components are allowed passage by the membrane into a permeate stream, whereas others are retained by it and accumulate in the retentate stream. Membranes can be of various thicknesses, with homogeneous or heterogeneous structure. Membranes can also be classified according to their pore diameter. According to the International Union of Pure and Applied Chemistry (lUPAC), there are three different pore diameter (dp) size classifications microporous (dp < 2 nm), mesoporous (2 nm < dp < 50 nm), andmacroporous (dp > 50 mn). Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated... 

The exact definition of a membrane is comphcated, but according to Mulder [11], a general definition could be a selective barrier between two phases, the term selective being inherent to a membrane or a membrane process. However, the definition says nothing about membrane structure or membrane function. Membrane science arbitrarily can be divided into seven categories material selection, material characterization and evaluation, membrane preparation, membrane characterization and evaluation, membrane transport phenomena, membrane module design, and process performance [12]. 

Membranes can be defined as selective barriers between two phases and have a plethora of apphcations, including water purification, desalination, drug development, and chemical sensing. BCP SA can be a useful approach for membrane fabrication because tunable structure and narrow size control of membrane pores is possible. 

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