Membrane properties

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. 

No membrane and no set of membrane properties has universal apphcability. Manufacturers who service multiple applications have a variety of commercial membranes. One firm hsts twenty different membranes having a broad spectrum of properties. 

The upper-bound hne connects discontinuous points, but polymers exist near the bound for separations of interest. Whether these will be available as membranes is a different matter. A useful membrane requires a polymer which can be fabricated into a device having an active layer around 50 nm thick. At this thickness, membrane properties may vary significantly from bulk properties, although not by a factor of 2. 

FIG. 22-75 Air fractionation by membrane. O2 in retentate as a function of feed fraction passed tbrougb tbe membrane (stage cut) showing tbe different result with changing process paths. Process has shell-side feed at 690 kPa (abs) and 298 K. Module comprised of hollow fibers, diameter 370 im od X 145 im id X 1500 mm long. Membrane properties (X = 5.7 (O2/N2), permeance for O2 = 3.75 X 10 Barrer/cm. Coutiesy Innovative Membrane Systems/ Fraxair)... 

Several authors have discussed the ion exchange potentials and membrane properties of grafted cellulose [135,136]. Radiation grafting of anionic and cationic monomers to impart ion exchange properties to polymer films and other structures is rather promising. Thus, grafting of acrylamide and acrylic acid onto polyethylene, polyethylene/ethylene vinyl acetate copolymer as a blend [98], and waste rubber powder [137,138], allows... 

The sensitivity of productivity or flux to transmembrane pressure (TMP) is referred to as the permeability L = flux/transmembrane pressure. TMP refers to a module average. Pure-component permeability (e.g., water permeability) refers to membrane properties while the more industrially relevant process permeability includes fouling and polarization effects. 

Membrane Types Key membrane properties include their size rating, selectivity, permeability, mechanical robustness (to allow module fabrication and withstand operating conditions), chemical robustness (to fabrication materials, process fluids, cleaners, and sanitizers), low extractibles, low fouling characteristics, high capacity, low cost, and consistency. 

Another device that finds frequent use is the stirred cell shown in Fig. 20-54. This device uses a membrane coupon at the bottom of the reservoir with a magnetic stir bar. Stirred cells use low fluid volumes and can be used in screening and R D studies to evaluate membrane types and membrane properties. The velocity profiles have been well defined (Schlichting, Boundary Layer Theory, 6th ed., McGraw-Hill, New York, 1968, pp. 93-99). 

The design of bioeompatible (blood compatible) potentiometric ion sensors was described in this chapter. Sensing membranes fabricated by crosslinked poly(dimethylsiloxane) (silicone rubber) and sol gel-derived materials are excellent for potentiometric ion sensors. Their sensor membrane properties are comparable to conventional plasticized-PVC membranes, and their thrombogenic properties are superior to the PVC-based membranes. Specifically, membranes modified chemically by neutral carriers and anion excluders are very promising, because the toxicity is alleviated drastically. The sensor properties are still excellent in spite of the chemical bonding of neutral carriers on membranes. 

E Pefferkorn, A Schmitt, R Varoqui. Helix-coil transition of poly(a,L-glutamic acid) at an interface Correlation with static and dynamic membrane properties. Biopolymers 21 1451-1463, 1982. 

Carotenoid molecules incorporated into the lipid membranes considerably interfere with both the structural and the dynamic membrane properties. Both effects are directly related to the chemical structure of carotenoid molecules. Importantly, it is the rigid, rod-like backbone of the carotenoids,... 

How Carotenoids Affect Membrane Properties (High Carotenoid Concentration).201... 

EPR Spin-Labeling Demonstrates Membrane Properties Significant for Chemical... 

HOW CAROTENOIDS AFFECT MEMBRANE PROPERTIES (HIGH CAROTENOID CONCENTRATION)... 

The hypothesis that polar carotenoids regulate membrane fluidity of prokaryotes (performing a function similar to cholesterol in eukaryotes) was postulated by Rohmer et al. (1979). Thus, the effects of polar carotenoids on membrane properties should be similar in many ways to the effects caused by cholesterol. These similarities were demonstrated using different EPR spin-labeling approaches in which the effects of dipolar, terminally dihydroxylated carotenoids such as lutein,... 

EPR SPIN-LABELING DEMONSTRATES MEMBRANE PROPERTIES SIGNIFICANT FOR CHEMICAL REACTIONS AND PHYSICAL PROCESSES INVOLVING CAROTENOIDS... 

Gudheti, M. V., Mlodzianoski, ML, and Ffess, S. T. (2007) Imaging and shape analysis of giant unilamellar vesicles (GUVs) as model plasma membranes Effect of fnms-DOPC (dielaidoyl phosphatidylcholine) on membrane properties. Biophys. J. 10.1529/biophysj.106.103374. 

Solvent polymeric membranes, conventionally prepared from a polymer that is highly plasticized with lipophilic organic esters or ethers, are the scope of the present chapter. Such membranes commonly contain various constituents such as an ionophore (or ion carrier), a highly selective complexing agent, and ionic additives (ion exchangers and lipophilic salts). The variety and chemical versatility of the available membrane components allow one to tune the membrane properties, ensuring the desired analytical characteristics. 

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