Polymers reverse osmosis

Osmosis is the flow of solvent through a semipermeable membrane into a solution the osmotic pressure is proportional to the molar concentration of the solute. Osmometry is used to determine the molar masses of compounds with large molecules, such as polymers reverse osmosis is used in water purification. 

In some applications, the interaction of the polymer with a specific "solvent" and/or with certain molecules carried by that solvent is not a detrimental event, but an essential aspect of the operation of the polymer. Reverse osmosis membranes and swollen hydrogels used in applications such as the desalination of water, kidney dialysis, soft contact lenses and surgical implants [17] are among such polymers. 

Cellulose acetate Loeb-Sourirajan reverse osmosis membranes were introduced commercially in the 1960s. Since then, many other polymers have been made into asymmetric membranes in attempts to improve membrane properties. In the reverse osmosis area, these attempts have had limited success, the only significant example being Du Font s polyamide membrane. For gas separation and ultrafUtration, a number of membranes with useful properties have been made. However, the early work on asymmetric membranes has spawned numerous other techniques in which a microporous membrane is used as a support to carry another thin, dense separating layer. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

In reverse osmosis membranes, the pores are so smaH, in the range 0.5— 2 nm in diameter, that they ate within the range of the thermal motion of the polymer chains. The most widely accepted theory of reverse osmosis transport considers the membrane to have no permanent pores at aH. Reverse osmosis membranes are used to separate dissolved microsolutes, such as salt, from water. The principal appHcation of reverse osmosis is the production of drinking water from brackish groundwater or seawater. Figure 25 shows the range of appHcabHity of reverse osmosis, ultrafiltration, microfiltration, and conventional filtration. 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Polymer Plasticizer. Nylon, cellulose, and cellulose esters can be plasticized using sulfolane to improve flexibiUty and to increase elongation of the polymer (130,131). More importantly, sulfolane is a preferred plasticizer for the synthesis of cellulose hoUow fibers, which are used as permeabiUty membranes in reverse osmosis (qv) cells (131—133) (see Hollow-FIBERMEMBRANEs). In the preparation of the hoUow fibers, a molten mixture of sulfolane and cellulose triacetate is extmded through a die to form the hoUow fiber. The sulfolane is subsequently extracted from the fiber with water to give a permeable, plasticizer-free, hoUow fiber. 

Cellulose acetate, the earhest reverse osmosis membrane, is still widely used. Asymmetric polyamide and thin-film composites of polyamide and several other polymers have also made gains in recent years whereas polysulfone is the most practical membrane material in ultrafiltration appHcations. 

In contrast to reverse osmosis, where cellulose acetate has occupied a dominant position, a variety of synthetic polymers has been employed for ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate. Polycarbonate resins, substituted olefins, and polyelectrolyte complexes have been employed among other polymers to form ultrafiltration membranes. 

H. Mehdizadeh, J. M. Dickson. Theoretical modifications of the finely porous model for reverse osmosis. J Appl Polym Sci 42 1143, 1991. 

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Amine sulfide terminal moieties can be imparted into vinyl polymers by using aminethiols as chain transfer agents in aqueous radical polymerization [1182]. The polymers are useful as mineral dispersants. Other uses are as water-treatment additives for boiler waters, cooling towers, reverse osmosis applications, and geothermal processes and oil wells and as detergent additives... 

Nomenclature, 17 384-413 basic scheme of, 17 384-385 biochemical, 17 401-402 computerized approaches to, 17 400-401 elastomer, 21 761t enzyme, 10 258-260 for ionic liquids, 26 840-841 glossaries related to, 17 404 inorganic, 17 387-394 macromolecular (polymers), 17 403 404 organic, 17 394-401 polymer, 20 390-395 pump, 21 88 quinone, 21 236-237 reactor technology, 21 358 related to mass transfer, 15 731-737 reverse osmosis, 21 674-676 Society of Rheology, 21 704 spray-related, 23 199t systematic, 17 394... 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

The synthesis of 5 lan thick Ti02 Si02 layers on a porous support can be performed using the procedure given below. First a mixed Ti[(OMe)3]4 alkoxide is synthesized by reacting partially hydrolyzed Si(OMe)4 with Ti-isopropoxide. This inorganic polymer is hydrolyzed at pH 11.0 and treated with 2-methyl-2-4-pentanediol and a binder. This solution is then slip-cast onto a porous support, dried and calcined at 700°C. The membrane can be useful in reverse osmosis applications. 

Geong and coworkers reported a new concept for the formation of zeolite/ polymer mixed-matrix reverse osmosis (RO) membranes by interfacial polymerization of mixed-matrix thin films in situ on porous polysulfone (PSF) supports [83]. The mixed-matrix films comprise NaA zeoHte nanoparticles dispersed within 50-200 nm polyamide films. It was found that the surface of the mixed-matrix films was smoother, more hydrophilic and more negatively charged than the surface of the neat polyamide RO membranes. These NaA/polyamide mixed-matrix membranes were tested for a water desalination application. It was demonstrated that the pure water permeability of the mixed-matrix membranes at the highest nanoparticle loadings was nearly doubled over that of the polyamide membranes with equivalent solute rejections. The authors also proved that the micropores of the NaA zeolites played an active role in water permeation and solute rejection. 

The solute-solvent-polymer (membrane material) interactions, similar to those governing the effect of structure on reactivity of molecules (20,21,22,23,24) arise in general from polar-, sterlc-, nonpolar-, and/or ionic-character of each one of the three components In the reverse osmosis system. The overall result of such interactions determines whether solvent, or solute, or neither is preferentially sorbed at the membrane-solution Interface. 

Nonpolar Parameters. In a reverse osmosis system involving cellulose acetate membranes and aqueous solutions of hydrocarbon solutes, the adsorption of water and that of solute on the polar and nonpolar sites of the membrane surface respectively may be expected to take place essentially independently. Further, since the polymer-solute interaction forces are attractive in nature for the above case, the mobility of the solute molecules through the membrane pore is retarded, and they also tend to agglomerate... 

Characteristics of Different Polymers as Reverse Osmosis Membrane Materials (53,56)... 

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