Membranes characteristics

HoUow fibers can be prepared from almost any spiunable material. The fiber can be spun directly as a membrane or as a substrate which is post-treated to achieve desired membrane characteristics. Analogous fibers have been spun in the textile industry and are employed for the production of high bulk, low density fabrics. The technology employed in the fabrication of synthetic fibers appUes also to the spinning of hoUow-fiber membranes from natural and synthetic polymers. 

All these attempts to improve the membrane characteristics by decreasing the methanol crossover and increasing their temperature resistance gave encouraging results. However, modification of the existing membranes (e.g., with barrier screens) without decreasing the electrical... 

BK Fritzinger, SK Bauman, DJ Lyman. Membrane characteristics, permeability parameters, and frictional coefficients for cupraphane. J Biomed Mater Res 5 3-16, 1971. 

The fluid mosaic model conveniently describes how the constituent molecules are ordered, and it correctly describes, in first order, some of the membrane s properties. However, it does not give explicit insight into why the biological membrane has a particular structure, and how this depends on the properties of the constituent molecules and the physicochemical conditions surrounding it. For this reason, only qualitative and no quantitative use can be made of this model as it pertains to permeation properties, for example. It is instructive to review the physicochemical principles that are responsible for typical membrane characteristics. In such a survey, it is necessary to discuss simplified cases of self-assembly first, before the complexity of the biological system may be understood. The focus of this quest for principles will therefore be more on the level of the molecular nature of the membrane, rather than viewing a... 

Miyoshi, H., Nishioka, T. and Fujita, T. (1986). Quantitative analysis of effects of substituted phenols on membrane characteristics of lecithin liposomes, Bull. Chem. Soc. Jpn., 59, 1099-1107. 

Surfactants have been explored widely for their effects on drug absorption, in particular using experimental animals (Gibaldi and Feldman, 1970 Gibaldi, 1976). Surfactants alter dissolution rates (of lipid materials), surface areas of particles and droplets, and membrane characteristics, all of which affect absorption. 

In a series of papers, Kennedy and co-workers studied the amphiphilic networks and their membrane characteristics. The use of poly(pentamethyl)cyclopentasiloxane)32 as an oxyphilic cross-linker in the novel tricontinuous membranes, consisting of hydrophilic and lipophilic polymers, was reported.466 The presence of polysiloxane brought about high oxygen permeability, which was studied in the water-swollen membranes.467 The use of polysiloxanes as sorbents,468 and as carriers for the receptor molecules in the ion-exchange membranes,469 470 was reviewed. 

Wang, E, Li, J., Chen, T. and Xu, J. 1999. Synthesis of poly(ether ether ketone) with high content of sodium sulfonate groups and its membrane characteristics. Polymer 40 795-799. 

Physical deterioration includes compaction by creeping and surface deteriorations by scratching and vibration. Creeping is accelerated at higher temperatures and pressures, resulting in the membrane compaction. This phenomenon is well analyzed and the membrane characteristics of compaction can be estimated in terms of m-value. Scratching and vibration can develop the microscopic defects in the surface structure of membranes, and give poor performances. We discussed this type of deterioration in Mexico in 1976 ( ). 

As membrane deteriorations can be seen in case of performance degradations or changes in membrane structure, we have investigated into these two aspects. Information about the relation between membrane characteristics and deterioration processes is useful for trouble-shooting. Even if operation records showed no implication of deteriorating process of a membrane, the analysis of the deteriorated membrane will reveal its own history. 

Our Investigation on durability and membrane characteristics changes under adverse conditions have much contributed to development of RO applications. Among these applications are those for ultra-pure water in electronic and pharmaceutical industries. Even under the circumstance of pH 7 and with 2 to 4 times per year of sterilization by H2O2 of as high as 1 %, the cellulose acetate membrane proved to show membrane life of more than 3 years. 

It should be mentioned that alternative possibilities to prepare similar membranes include the use of a porous alumina membrane as matrix, with the titania nanotubes grown in the channels. Nanoporous alumina membranes are commercial products, also synthesized by anodic oxidation. The commercial Whatman Corporation anodic membrane has holes of about 20-nm diameter at the top of the membrane and about 200-nm diameter at the bottom of membrane. Within these pores Ti02 nanotubes fabricated by template synthesis and water vapour hydrolysis could be grown, but non-uniform membrane characteristics are obtained due to the non-uniform pores of the commercial alumina... 

The boundary conditions used make no reference to membrane characteristics. 

In a very thoughtful investigation of solvent systems to model membrane characteristics, Leahy et al. (1989, 1992) have argued that two receptors sited in different tissues (or membranes) could exist in environments that are very different in hydrogen bonding character one may be surrounded by amphiprotic groups (as in a protein) or by proton donors the other may be surrounded by proton acceptors (as in a phospholipid membrane). 

The lipid solubility, degree of ionization, pKa of the drug, pH of the drug solution, presence of saliva and the membrane characteristics, molecular weight and size of the drug, various physicochemical properties of the formulation, and the presence or absence of permeation enhancers, all affect the absorption and the permeation of drugs through the oral mucosa. 

Bosnian GJ, Bartholomeus IG, de Man AJ, van Kalmthout PJ, de Grip WJ. Erythrocyte membrane characteristics indicate abnormal cellular aging in patients with Alzheimer s disease. Neurobiol Aging 1991 12(1) 13 18. 

Table 2.1 Overview of main polymer membrane characteristics and membrane-based processes for molecular separations in liquid phase.

Le-Clech, P., Jefferson, B. and Judd, S.J. (2003) Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor. Journal of Membrane Science, 218,117—129. Le-Clech, P., Jefferson, B. and Judd, S.J. (2005) Comparison of submerged and side-stream tubular membrane bioreactor configurations. Desalination, 173, 113-122. 

Table 2. NF and LPRO membrane characteristics from Filmtec (DOW)...

The conventional model developed to explain cell membrane characteristics influencing drug permeability is routinely referred to as the fluid-mosaic model (Figures 2.1 and 2.2). In this model the main components, for our purposes, are a phospholipid (e.g., sphingomyelin and phosphatidylcholine) bilayer (8 nm), with polar moieties at both the external and internal surfaces, and with proteins periodically traversing the phospholipid plane perpendicularly. 

Figure 5.32. Impedance plots for single cells at ambient temperature, a Nation 117. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A) 0.70 V (0.76 V) ( ) 0.6 V (0.74 V) ( ) 0.5 V (0.74 V). b Nafion 112. Cell voltage and ohmic drop corrected potential (in parenthesis) ( ) 0.9 V (0.9 V) ( ) 0.8 V (0.81 V) (A ) 0.70 V (0.73 V) ( ) 0.6 V (0.67 V) ( ) 0.5 V (0.61 V). Plots were corrected for the high-frequency resistances. Left detail of the high-frequency regions [29]. (Reprinted from Journal of Electroanalytical Chemistry, 503, Freire TJP, Gonzalez ER. Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells, 57-68, 2001, with permission from Elsevier.)...

Freire TJP, Gonzalez ER (2001) Effect of membrane characteristics and humidification conditions on the impedance response of polymer electrolyte fuel cells. J Electroanal Chem 503 57-68... 

The constituent particles, various binders and other additives used for making porous supports or membranes have been shown to affect the plastic properties of the extrusion paste. The plastic properties, in turn, have significant impacts on the final support and membrane characteristics [Shkrabina et al., 1995a and 1995b]. 

Macroscopic features such as the shapes of membrane elements, modules and systems and general application characteristics will be covered in Chapter 5. Good understanding of the basic membrane characteristics should provide the much needed background and guidelines for design and operation of the membrane systems. 

Possible transport mechanisms in a fluid system through the membrane pores are multiple. They vary to a great extent with the membrane pore size and, to a less extent, with chemical interaction between the transported species and the membrane material. Under the driving force of a pressure gra nt, permeants (whether in the form of solvents, solutes or gases) can transport across a membrane by one or more of the mechanisms to be discussed below. The degree by which they affect permeability and permselectivity depends on the operating conditions, membrane characteristics and membrane-permeating species interactions in the application environment. 

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