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Anisotropic membranes

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. [Pg.89]

Membranes made by the Loeb-Sourirajan process consist of a single membrane material, but the porosity and pore size change in different layers of the membrane. Anisotropic membranes made by other techniques and used on a large scale often consist of layers of different materials which serve different functions. Important examples are membranes made by the interfacial polymerization process discovered by Cadotte [15] and the solution-coating processes developed by Ward [16], Francis [17] and Riley [18], The following sections cover four types of anisotropic membranes ... [Pg.97]

Most of the membranes listed in Table 5.20 are formed through phase separation processes, i.e., melt extrusion or coagulation of a polymer solution by a nonsolvent. In melt extrusion, a polymer melt is extruded into a cooler atmosphere which induces phase transition. The melt extrusion of a single polymer usually gives a dense, isotropic membrane. However, the presence of a compound (latent solvent) that is miscible with the polymer at the extrusion temperature but not at the ambient temperature, may lead to a secondary phase separation upon cooling. Removal of the solvent then yields a porous isotropic membrane. Anisotropic membranes may result from melt extrusion of a dope mixture of polymers containing plasticizers. [Pg.649]

Our first exploration of property space was focused on acetylcholine. This molecule was chosen for its interesting structure, major biological role, and the abundant data available on its conformational properties [15]. The behavior of acetylcholine was analyzed by MD simulations in vacuum, in isotropic media (water and chloroform) [16] and in an anisotropic medium, i.e. a membrane model [17]. Hydrated n-octanol (Imol water/4mol octanol) was also used to represent a medium structurally intermediate between a membrane and the isotropic solvents [17]. [Pg.11]

Conversely, in a membrane model, acetylcholine showed mean log P values very similar to those exhibited in water. This was due to the compound remaining in the vicinity of the polar phospholipid heads, but the disappearance of extended forms decreased the average log P value somewhat. This suggests that an anisotropic environment can heavily modify the conformational profile of a solute, thus selecting the conformational clusters more suitable for optimal interactions. In other words, isotropic media select the conformers, whereas anisotropic media select the conformational clusters. The difference in conformational behavior in isotropic versus anisotropic environments can be explained considering that the physicochemical effects induced by an isotropic medium are homogeneously uniform around the solute so that all conformers are equally influenced by them. In contrast, the physicochemical effects induced by an anisotropic medium are not homogeneously distributed and only some conformational clusters can adapt to them. [Pg.14]

The I term is of particular relevance since, in anisotropic media such as liposomes and artiflcial membranes in chromatographic processes, ionic charges are located on the polar head of phospholipids (see Section 12.1.2) and thus able to form ionic bonds with ionized solutes, which are therefore forced to remain in the nonaqueous phase in certain preferred orientations. Conversely, in isotropic systems, the charges fluctuate in the organic phase and, in general, there are no preferred orientations for the solute. Given this difference in the I term (but also the variation in polar contributions, less evident but nevertheless present), it becomes clear that log P in anisotropic systems could be very different from the value obtained in isotropic systems. [Pg.324]

Commercial interest in RO began with the first high-flux, high-NaCl-retention Loeb-Sourirajan anisotropic cellulose acetate membrane. Practical application began with the thin film composite (TFC) membrane and implementation for seawater desalination at Jeddah, Saudi Arabia [Muhurji et ak. Desalination, 76, 75 (1975)]. [Pg.45]

The rate of the active transport of sodium ion across frog skin depends both on the electrochemical potential difference between the two sides of this complex membrane (or, more exactly, membrane system) and also on the affinity of the chemical reaction occurring in the membrane. This combination of material flux, a vector, and chemical flux (see Eq. 2.3.26), which is scalar in nature, is possible according to the Curie principle only when the medium in which the chemical reaction occurs is not homogeneous but anisotropic (i.e. has an oriented structure in the direction perpendicular to the surface of the membrane or, as is sometimes stated, has a vectorial character). [Pg.461]

Although continuum solvation models do appear to reproduce the structural and spectroscopic properties of many molecules in solution, parameterization remains an issue in studies involving solvents other than water. In addition, the extension of these approaches to study proteins embedded in anisotropic environments, such as cell membranes, is clearly a difficult undertaking96. As a result, several theoretical studies have been undertaken to develop semi-empirical methods that can calculate the electronic properties of very large systems, such as proteins28,97 98. The principal problem in describing systems comprised of many basis functions is the method for solving the semi-empirical SCF equations ... [Pg.35]

Such an ordered organization of biological photoreceptor molecules appears to be a common feature (which, of course, is hardly detected in more opaque, highly scattering biological material). In the vertebrate rod maximum absorption lies in membranes (discs) perpendicular to the long axis of the rods, but anisotropically within these planes47). [Pg.31]

A second approach with respect to anisotropic flavin (photo-)chemistry has been described by Trissl 18°) and Frehland and Trissl61). These authors anchored flavins in artificial lipid bilayers by means of C18-hydrocarbon chains at various positions of the chromophore. From fluorescence polarization analysis and model calculations they conclude, that the rotational relaxation time of the chromophore within the membrane is small compared to the fluorescence lifetime (about 2 ns74)). They further obtain the surprising result that the chromophore is localized within the water/lipid interface, with a tilt angle of about 30° (long axis of the chromophore against the normal of the membrane), irrespective of the position where the hydrocarbon chain is bound to the flavin nucleus. They estimate an upper limit of the microviscosity of the membrane of 1 Poise. [Pg.40]

Besides these shortcomings the bluelight receptor and sensory transduction problem is recently being attacked on the basis of completely artificial flavin/membrane systems. These appear to provide well-defined model systems to study anisotropic flavin (photo-) chemistry. This, in turn, is an essential prerequisite which allows the primary photo-events of physiological bluelight reception to be imitated and elucidated. [Pg.41]

Several local labels need to be measured, usually one-by-one in individual samples in the case of 19F-NMR. The combined set of anisotropic NMR parameters then allows one to re-construct the geometry of the entire peptide and to determine its alignment in the membrane, as illustrated in Fig. 1 [35-37, 47, 48]. The only prerequisite is that the 19F-labelled moiety has to be rigidly attached to the peptide backbone, and that the peptide assumes a well-defined secondary structure. Provided that a sufficient number of local orientational constraints can be measured... [Pg.95]


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See also in sourсe #XX -- [ Pg.228 , Pg.232 ]




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