Membrane permselective cellulose acetate

Peroxyni trite Biological samples Dialysis (for sampling) CL 1 x 1CU11 and 1 x 1CT10 mol L 1 for systems without and with dialysis Flow injection system permselective cellulose acetate membrane sampler [544]... 

The requirement of hydrophilicity in barrier materials has been widely accepted, but the mechanism by which it affects membrane performance, especially for the permselectivity, is not fully understood. Cellulose acetate and some kinds of polyamides and their analogues featured in the present review have both hydraulic permeability and permselectivity, while most highly hydrophilic materials have high permeability for water and show unselective permeation for ions and organic solutes. 

Except poly(dimethylsiloxane), all of the conventional polymers such as cellulose acetate, poly(phenylene oxide), nylon, polyethylene, and nafion are known to permeate water preferentially against ethanol111,112). However, in order to separate ethanol from dilute aqueous solution, it is favorable to use a membrane which permeates ethanol preferentially (ethanol-permselective membrane). 

The MF membranes are usually made from natural or synthetic polymers such as cellulose acetate (CA), polyvinylidene difiuoride, polyamides, polysulfone, polycarbonate, polypropylene, and polytetrafiuoroethylene (FIFE) (13). Some of the newer MF membranes are ceramic membranes based on alumina, membranes formed during the anodizing of aluminium, and carbon membrane. Glass is being used as a membrane material. Zirconium oxide can also be deposited onto a porous carbon tube. Sintered metal membranes are fabricated from stainless steel, silver, gold, platinum, and nickel, in disks and tubes. The properties of membrane materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability, permselectivity and the cost of membrane material as well as manufacturing process. 

A continuous effect is the decrease in water content and void volume with Increasing temperature. Water is lost from the primary gel during annealing, both because of the formation of virtual crosslinks and because of the decrease in hydrogen bonding and cluster size in the water Itself. An example of a discontinuous effect is the dramatic increase in permselectivity (salt rejection) observed when cellulose acetate membranes are heated above the glass transition temperature 68.6 C. In fact, not one but two... 

Virtually the entire membrane manufacture today is based on laminate structures comprising a thin barrier layer deployed upon a much thicker, highly permeable support. Most are formed of compositionaUy homogeneous polysulfone, cellulose acetate, polyamides, and various fluoropolymers by phase inversion techniques in which ultrathin films of suitably permselective material are deposited on prefabricated porous support structures. Hydrophobic polymers as polyethylene, polypropylene, or polysulfone are often used as supports. A fairly comprehensive hst of microporous and ultrafiltration commercial membranes and produced companies are presented in Refs [107-109]. A review on inorganic membranes has been given in Ref. [110]. 

Although the original membranes were asymmetric which are cast by phase inversion, most modern membranes are made in composite form. Asymmetric membranes consist of a skin layer 0.1 - 1 um thick which is permselective with pore size of 10 - 1000 A, supported by a porous layer of the same material with a thickness of 100 - 500 im. Composite membranes are made of two different materials, an ultra thin ( <0.1 pm) skin layer of polyamide, cellulose acetate, polyvinylalcohol, or other materials, supported by an ultrafiltration porous layer such as polysulfone. 

Selectivity in these devices can be optimized through selection of moderate operating potentials, the use of enzymes that will selectively oxidize or reduce the analyte, or through selective control of mass transport via an additional outer permselective membrane. Mechanisms that have been used for this purpose include size (hydrolyzed cellulose acetate) and charge (Nafion ) exclusion. 

Permselective membranes Dialysis membranes Cellulose acetate Small molecules... 

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