Asymmetric anisotropic membrane

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. 

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 first membrane separation was performed with nitrocellulose in 1855. During the following 100 years, the technology played a limited role as a research tool in analytical chemistry. A major breakthrough occurred in 1958-1961 when the anisotropic or asymmetric membrane was developed. While membranes employed previously were uniform throughout, the upper portion of anisotropic membranes represents only 1% of the total film and is the actual filter, the other 99% acting as a support. The thinness of the membrane and the very fine pore structure promote excellent throughput for UF. 

Symmetric polymeric membranes possess a uniform pore structure over the entire thickness. These membranes can be porous or dense with a constant permeability from one surface to the other. Asymmetric (also sometimes referred to as anisotropic) membranes, on the other hand, typically show a dense (nonporous) structure with a thin (0.1-0.5 pm) surface layer supported on a porous substrate. The thin surface layer maximizes the flux and performs the separation. The microporous support structure provides the mechanical strengdi. 

A membrane that has the same chemical and physical structure throughout its thickness in the direction of travel of the separating species is called a symmetric or isotropic membrane. If it has a different chemical and physical structure in the direction of its thickness it is called an asymmetric or anisotropic membrane (see Figure 2.20). The most common form of asymmetric membrane has a very thin skin of highly selective material, supported on a much thicker substrate, usually made from the same material, but quite possibly of different materials. If the materials are the same, then the asymmetric structure is usually created in one piece with the thin active skin. 

The structure of the so-called "composite" membranes used in reverse osmosis is also much more complex than the conventional, simplistic description of the ultrathin semipermeable film deposited on and supported by a porous substrate. Most of these membranes which exhibit high flux and separation are composed of an anisotropic, porous substrate topped by an anisotropic, ultrathin permselective dense layer which is either highly crosslinked, or exhibits a progressively decreased hydrophilicity toward the surface. The basic difference between the conventional anisotropic (asymmetric) membrane and the thin film composite is that the latter might be... 

The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, Reverse Osmosis, Academic, N.Y., 1970). It is made... 

Figure 4.3 shows a cross section of a CA membrane. The structure is asymmetric or "anisotropic," with a solute-rejecting layer on top of a microporous support, all made of the one polymeric material. 

Membranes for Reverse Osmosis. The first commercially successful membrane was the anisotropic or asymmetric structure invented by Loeb and Sourirajan (1960 cited by Sourirajan, 1970). It is made of cellulose acetate and consists of a dense layer 0.2-0.5 jjim diameter. The thin film has the desired solute retention property while offering little resistance to flow, and the porous substructure offers little resistance to flow but provides support for the skin. The characteristics of available membranes for reverse osmosis and ultrafiltration are listed in Tables 19.2 through 19.4. 

Matsuyama H, Berghmans S, Batarseh MT, Lloyd DR (1999) Formation of anisotropic and asymmetric membranes via thermally-induced phase separation. In Piimau I, Freeman BD (eds) Membrane formation and modification. American Chemical Society, Washington,... 

Membrane cross section. Isotropic (symmetric], integrally anisotropic (asymmetric], bi- or multilayer, thin-layer, or mixed matrix composite. 

The first breakthrough came in 1959 when Sourirajan and Loeb discovered a method to make a very thin cellulose acetate (CA) membrane using the phase inversion method [4]. This technique produces homogenous membranes with an asymmetric (or anisotropic) structure. The membranes were subsequently found to be skinned when examined under an electron microscope by Riley in 1964 [3]. The membranes consisted of a very thin, porous salt-rejecting barrier of CA, integrally supported by a fine CA porous substrate. Pictures of asymmetric membranes are shown in Figures 1.2 and 1.3. These early Loeb-Sourirajan (L-S) membranes exhibited water fluxes that were lOtimes higher than those observed by Reid, and with comparable salt rejection [5]. The membrane flux was 8—18 1/m /h (knh) with 0.05% NaCl product water from a 5.25% NaCl feedwater... 

Anisotropic (asymmetric) Defines a particular type of ultrastructure of microporous membranes. The surface of the membrane where separation occurs is more dense than the rest of the membrane body. The pore diameter increases in a direction perpendicular to the membrane surface, with the pore opening near the separation surface being smaller than the pore opening on the bottom of the membrane. This skin layer is typically present in polymeric membranes made by the phase-inversion process. Some asymmetry is also present in many inorganic membranes. 

For pervaporation and gas sep ation, nonporous membranes are required preferably with an anisotropic morphology, an asymmetric structure with a dense top layer and an open porous sublayer, as found in asymmetric and composite membranes. The requirements for the substructure are in fact the same as for gas separation membranes ... 

Cellulose acetate was the first material used to make RO membranes. This kind of material was first used by Loeb and Sourirajan in 1963. Nowadays the use of these membranes is limited due to their lower performance than composite membranes. Cellulose acetate membranes are inexpensive, very easy to prepare, resistant against oxidants and mechanically tough in nature. The membrane has an asymmetric or an anisotropic structure and consists of a thin active layer on a coarse supportive layer. However, this kind of membrane is very sensitive towards the pH and temperature of the feed water. Thus, it is better to maintain the feed water pH between 4 and 6 because the membranes are slowly hydrolyzed with time and above 35°C, the properties of the membrane change (Vos et a/., 1966). Moreover, these types of membranes are very susceptible to biological attack. 

The basic structure of present-day membranes is illustrated in Figure 8.5. It consists of a selective polymeric film and a much thicker but more porous sublayer, which provides the necessary structural support but otherwise does not actively participate in the separation. Such membranes composed of a dual layer of different materials are referred to as anisotropic or asymmetric. 

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