Synthetic membranes, solute

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. 

Many synthetic membranes are known to be useful for separation of water and various sizes of solutes from aqueous solutions by selective separation, for examples reverse osmosis, ultrafiltration, dialysis and so on 1 7). The permeability is much dependent on both of chemical and physical structures of the membranes. The choice of the barrier materials for membranes and the control of their morphology are important to get effective permselective membranes. 

Figure 3.20 Schematic of the interfacial polymerization process. The microporous film is first impregnated with an aqueous amine solution. The film is then treated with a multivalent crosslinking agent dissolved in a water-immiscible organic fluid, such as hexane or Freon-113. An extremely thin polymer film forms at the interface of the two solutions [47]. Reprinted from L.T. Rozelle, J.E. Cadotte, K.E. Cobian, and C.V. Knopp, Jr, Nonpolysaccharide Membranes for Reverse Osmosis NS-100 Membranes, in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (ed.), National Research Council Canada, Ottawa, Canada (1977) by permission from NRC Research Press...

C. Kleinstreuer and G. Belfort, Mathematical Modeling of Fluid Flow and Solute Distribution in Pressure-driven Membrane Modules, in Synthetic Membrane Processes,... 

The membrane must not be attacked by the solvent and must permit the solvent to permeate fast enough to achieve osmotic equilibrium in a reasonable time. If the membrane is too permeable, however, large leakage errors will result. Cellulose and cellulose acetate membranes are (he most widely used types with synthetic polymer solutions. Measurements at the relatively elevated temperatures needed to dissolve semicrystalline polymers are hampered by a general lack of membranes that are durable under these conditions. 

Another well-defined synthetic membrane is a planar bilayer membrane. This structure can be formed across a 1-mm hole in a partition between two aqueous compartments by dipping a fine paintbmsh into a membrane-forming solution, such as phosphatidyl choline in decane. The tip of the brush is then stroked across a hole (1 mm in diameter) in a partition between two aqueous media. The lipid film across the hole thins spontaneously into a lipid bilayer. The electrical conduction properties of this macroscopic bilayer membrane are readily studied by inserting electrodes into each aqueous compartment (Figure 12.14). For example, its permeability to ions is determined by measuring the current across the membrane as a function of the applied voltage. 

Photo-responsive Synthetic Membranes. Although the visual Information transduction Is too complexed to be realized in vitro, the photoeffects of retinal-containing synthetic membranes have been investigated. Alzawa et al. (74) prepared a photo-responsive membrane from a solution of 11-cis retinal, phosphatidyl choline and trlacetyl cellulose. The retinal was assumed to be incorporated into the molecular assemblies of phosphatidyl choline, which were dispersed In the trlacetyl cellulose membrane matrix. The membrane responded to visible light by showing a transmembrane potential in association with the photoisomerization of membrane-bound 11-cis retinal. On the other hand, a membrane Incorporating 11-cls retinal without phosphatidyl choline exhibited little light-induced transmembrane potential (75). 

Belfort, 6. Sinai, N. Sterling, D., "The State of Water in Synthetic Membranes and Aqueous Solutions." Second Annual Report, Jan., 1977 - Sept. 1977, project no. 848092, prepared for the NCRD, Prime Ministers Office, Jerusalem, Israel and GKSS, Baudesministerium fur Forshung und Technologie, West Germany, (March 1978). 

The development of synthetic membranes that can withstand high pressures has permitted the rapid filtration of even bacteria from a solution. Filtration is usually not considered to employ the Second Law, but gravity or higher pressures. These membranes are discussed in Chapter 37. However, once the topic of filtration is described, then related uses of it in addition to membrane filtration seem reasonable to mention. Therefore, not only regular filtering techniques but sieving techniques are described in this chapter. 

A striking example of the effects of a third component on the diffusive rate of a species is shown in Fig. 2.3-2 in which the acceleration or the flux of sodium sulfate in aqueous solution owing to the presence of acetone is shown. Soma of the most dramatic examples of malticomponent effects in diffusion occur in budt natural and synthetic membranes. 

An excellent review of the early history of noise studies of different ionic systems, such as single pores in thin dielectric films, microelectrodes, and synthetic membranes, is reference 3. The review by Weissman (48) describes several state-of-the-art fluctuation spectroscopy methods that include (1) determination of chemical kinetics from conductivity fluctuations in salt solutions, (2) observation of conductivity noise that arises from enthalpy fluctuations in the electrolyte with high temperature coefficient of resistivity, and (3) detection of large conductivity fluctuations in a binary mixture near its critical point. 

NMR measurement of solvent self-diffusion coefficients in polymer solutions NMR measurement of protein diffusion coefficients in solution and in synthetic membranes... 

G. S. Park, Transport principles - solution, diffusion and permeation in polymer membranes in Synthetic Membranes Science, Engineering and Applications (Eds. P. M. Bungay, H. K. Lonsdale, M. N. Pinho), NATO ASI Series, D. Reidel Publishing, Dordrecht, Holland, 1986, pp. 

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