Synthetic membranes, types

Liquid membrane type ion-seleetive electrodes (ISEs) provide one of the most versatile sensing methods because it is possible to customize the sensory elements to suit the structure of the analyte. A wealth of different synthetic and natural ionophores has been developed, in the past 30 years, for use in liquid membrane type ISEs for various inorganic and organic ions [1], In extensive studies [2-4], the response mechanism of these ISEs has been interpreted in terms of thermodynamics and kinetics. However, there have been few achievements in the characterization of the processes occurring at the surface of ISEs at molecular level. 

With either type of dialysis, studies suggest that recovery of renal function is decreased in ARF patients who undergo dialysis compared with those not requiring dialysis. Decreased recovery of renal function may be due to hemodialysis-induced hypotension causing additional ischemic injury to the kidney. Also, exposure of a patient s blood to bioincompatible dialysis membranes (cuprophane or cellulose acetate) results in complement and leukocyte activation which can lead to neutrophil infiltration into the kidney and release of vasoconstrictive substances that can prolong renal dysfunction.26 Synthetic membranes composed of substances such as polysulfone, polyacrylonitrile, and polymethylmethacrylate are considered to be more biocompatible and would be less likely to activate complement. Synthetic membranes are generally more expensive than cellulose-based membranes. Several recent meta-analyses found no difference in mortality between biocompatible and bioincompatible membranes. Whether biocompatible membranes lead to better patient outcomes continues to be debated. 

Central to the osmosis phenomenon is the semipermeable membrane (SPM), whose physical properties and species-selectivity directly govern the kinetics and thermodynamics of osmotic flow. Naturally occurring biomembranes of high selectivity, permeable to water but not to other solutes, are ubiquitous, for example, in macroscopic stomach linings and blood vessels, as well as in the microscopic cell membranes that encapsulate all known cell types. Some common synthetic membranes, such as Gore-Tex and cellophane, also exhibit selective permeability and osmotic activity. 

Such materials are known as semipermeable membranes. They are essential components of nearly all living things, and the development of new materials of this type is an important component of biomedical research. The control of diffusion of molecules through a membrane can be accomplished by variations in the hydrophilicity of the polymer molecules that constitute the membrane. As in biological membranes, hydrophobic molecules are more likely to pass through the hydrophobic domains of a synthetic membrane than through the hydrophilic regions, and vice versa. 

Figure 3.15 Polypropylene structures, (a) Type I open cell structure formed at low cooling rates, (b) Type II fine structure formed at high cooling rates [37]. Reprinted with permission from W.C. Hiatt, G.H. Vitzthum, K.B. Wagener, K. Gerlach and C. Josefiak, Microporous Membranes via Upper Critical Temperature Phase Separation, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, Washington, DC. Copyright 1985, American Chemical Society and American Pharmaceutical Association...

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). 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

Electrodialysis. In electrodialysis, separation of an aqueous stream is achieved through the use of synthetic membranes and an electric field. The membrane allows only one type of ions to pass through and may be chosen to remove other ions that move in the opposite direction. Therefore, it produces one stream rich in particular ions and another stream depleted of those ions. The two streams can be recycled or disposed off. This technique is commonly used in the desalination of brackish water. The other uses are in acid mine drainage treatment, the desalting of sewage-plant effluents, and in sulfite-liquor recovery. 

The range of available membrane materials used in water and wastewater treatment is quite broad, but most of them are synthetic membranes. Synthetic membranes can be organic or inorganic however, the most important class of membrane materials is organic or polymer membrane. The choice of a given polymer as a membrane material is not arbitrary (13). Inorganic materials generally possess superior chemical and thermal stability relative to polymeric materials. However, both types of membranes have different applications. A list of common membranes is shown in Table 2. 

In discussing the architecture and properties of aromatic polyamide membranes, it is convenient to refer to four levels of structure. Broadly speaking, these levels of structure are useful for understanding the properties of any synthetic membrane, irrespective of what type of polymer is used to make the membrane or whether the membrane is intended for RO, gas separation or ultrafiltration. The levels of structure as used in this paper are defined in Table II. 

In addition to blocatalytlc, energy-transducing and Information transducing membranes, there are, of course, other types of blofunctlonal synthetic membranes. However, this review concentrates on these three Important blofunctlonal membranes. The historical background of their development, the molecular mechanism In biological membranes on which blofunctlonal synthetic membranes are modelled, the methodology of membrane preparation and current trends In the research and development are described. 

The experimental methods based on electrokinetic phenomena (and especially electrophoresis) have found very widespread application for routine characterization of electrical surface properties of solid particles, liquid droplets, porous media, synthetic membranes, etc. A systematic presentation of the main results obtained on different types of systems is given in chapters 6 to 8 of Reference 716, and in chapters 8 to 33 of Reference 718. A glance at the books " and review articles " " " in the field, however, shows that the properties of air-water and oil-water interfaces are either not considered at all or only briefly mentioned. This fact is surprising, as a number of studies " " (the first performed more than 70 years ago) have convincingly demonstrated a substantial negative potential at bare (without any surfactant) air-water and oil-water interfaces. This spontaneous charging cannot be explained in a trivial way — it requires the specific preferential adsorption of some kind of ion, because from a purely electrostatic viewpoint the approach of an ion to the... 

Valinomycin, a cyclic 12-depsipeptide with the sequence cyc/o(LVal-DHyv-DVal-LLac)3, selectively transports K ions across natural and synthetic membranes. The conformations of the K complex and the uncomplexed form of valinomycin are different, but not as markedly different as the com-plexed and uncomplexed forms of antamanide. The molecular formula of valinomycin exhibits a threefold symmetry, which is maintained in the crystalline state for the K complex. If the differences in the side chains are overlooked, the approximate symmetry is raised to An early crystal structure determination of the KAUCI4 complex (Pinkerton et al, 1969) established that the ion is octahedrally coordinated to the carbonyl oxygen atoms of the six ester groups and that the carbonyl oxygen atoms from the six amide groups form hydrogen bonds with the six NH moieties (4 1 type... 

M. Taniguchi and G. Belfort. Low protein fouling synthetic membranes by UV-assisted surface grafting modification Varying monomer type. J. Membr. Set, 231(1-2) 147-157, March 2004. 

FIGURE 41.2 Basic principle of artificial cells Artificial cells are prepared to have some of the properties of biological cells. Like biological cells, artificial cells contain biologically active materials (I). The enclosed material (I) can be retained and separated from undesirable external materials, such as antibodies, leukocytes, and destructive substances. The large surface area and the ultra-thin membrane allow selected substrates (X) and products (Y) to permeate rapidly. Mass transfer across 100 mL of artificial cells can be 100 times higher than that for a standard hemodialysis machine. The synthetic membranes are usually made of ultrathin synthetic polymer membranes for this type of artificial cell. (From Chang, T.M.S., Artif. Cells Blood Substit. ImmobU. Biotechnol., 22(1), vii, 1994.)... 

Membranes can be natural or synthetic. Regarding the type of material, synthetic membranes can be divided into organic, made of various polymers (Figure 23.4), aud iuor-ganic, composed of ceramic or metal (Figure 23.5). 

There are four main types of polymeric membranes (a) Loeb—Sourirajan phase separation RO, UF and MF membranes, (b) interfacial composite RO and NF membranes, (c) solution-coated composite GS membranes, and (d) other anisotropic membranes such as plasma polymerisation coated. Several methods of manufacturing synthetic membranes are given in Table 1.5. Each method produces different membrane morphology porosity, pore size distribution, and ultrastructure. Membrane formulation techniques are discussed in detail in several texts [8, 16—18]. 

As of 1995, more than 30 different polymer blends were being used in the manufacture of membranes for hemodialysis and hemofiltration (Klinkmann and Vienken, 1995). The various membrane types used for renal replacement therapy can be divided into membranes derived from cellulose (83 percent of 1991 worldwide total) and from synthetic materials (the remaining 17 percent) (Klinkmann and Vienken, 1995). Synthetic membranes have been constructed from such materials as polyacrylonitrile (PAN), polysulfone, polyamide, polymethylmethacrylate, polycarbonate, and ethyl-vinylalchohol copolymer (Klinkmann and Vienken, 1995). In the United States, use of cellulosic materials for membrane construction predominates at around 95 percent of the total number of membranes used (Klinkmann and Vienken, 1995). 

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