Examples silicone-rubber membranes

The principle of these electrodes is a little different, and is normally based on the measurement of the pH of a solution of electrolyte placed between a membrane and a glass electrode, the membrane being porous to the species it is desired to determine (Table 13.3). The dissolved gas conditions the pH of the solution behind the membrane. Membranes can be microporous (for example PTFE) or homogeneous (for example silicone rubber). 

Class Henry s Law coefficient (atm/mole frac.) Solubility in water (wt%) Pervaporation separation factor with silicone rubber membranes Ease of separation by pervaporation Examples... 

Example 6.3.12 Consider selective pervaporative transport of a VOC species i through a silicone rubber membrane in preference to water. The separation conditions are such that Ei is 1, of the order of 1000 further, the permeation rates are quite low with respect to the feed side mass-transfer coefficient (i.e. Vz ku). In this pervaporation process, it is known that the membrane resistance is quite low but the boundary layer resistance is quite high for the VOCs. On the other hand, for water, it may be safely assumed that the membrane resistance controls the water transport. Determine an expression for the separation factor, and find the selectivity of the membrane for VOC over water when the permeate pressure is negligible. Assume that the VOC concentration in water is low for this pervaporation process. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

Not only hydrocarbon systems, but also silicon rubbers (Lee 1986), can be pyrolyzed to obtain silicon-based membranes. Details of the pyrolysis are mainly reported for nonmembrane applications. A recent example is the paper of Boutique (1986) for the preparation of carbon fibers used in aeronautical or automobile constructions. 

Equation (2.79) expresses the driving force in pervaporation in terms of the vapor pressure. The driving force could equally well have been expressed in terms of concentration differences, as in Equation (2.83). However, in practice, the vapor pressure expression provides much more useful results and clearly shows the connection between pervaporation and gas separation, Equation (2.60). Also, the gas phase coefficient, is much less dependent on temperature than P L. The reliability of Equation (2.79) has been amply demonstrated experimentally [17,18], Figure 2.13, for example, shows data for the pervaporation of water as a function of permeate pressure. As the permeate pressure (p,e) increases, the water flux falls, reaching zero flux when the permeate pressure is equal to the feed-liquid vapor pressure (pIsal) at the temperature of the experiment. The straight lines in Figure 2.13 indicate that the permeability coefficient d f ) of water in silicone rubber is constant, as expected in this and similar systems in which the membrane material is a rubbery polymer and the permeant swells the polymer only moderately. 

A number of academic studies have produced rubbery hydrophobic membrane materials with far higher selectivities than silicone rubber [27], For example,... 

Many porous ion exchange membranes with high cation or anion selectivity have been described (B3, P13, S18). Sparingly soluble crystalline materials have been used as anion sensors, the membrane consisting of single crystals or pressed pellets often embedded in a vulcanized silicone rubber matrix. Examples include electrodes for fiuoride (LaF), sulfide (silver-silver sulfide), iodide, and sulfate. These probably function as... 

Hot vulcanizable silicone rubbers can be processed in two different ways either by extrusion to tubing or cables or by molding to so-called molded articles, such as, for example, crankshaft seals and membranes. High viscosity hot vulcanizable silicone rubber is exclusively used for extrusion articles, extruders typically used in the rubber industry being employed in their processing. 

Monsanto s Prism permeators for gas separation also employ composite membranes. Polyamide coatings are not used for the composite membrane in the Prism module. The Prism membrane consists of a coating of silicone rubber applied from an organic solvent on a porous polysulfone substrate. The Prism membrane is another good example of a composite membrane where Structure Level IV is used to obtain good membrane properties (22). 

A number of apphcations with polymeric membranes have been described. The most commonly used membrane material is silicon rubber or polyethylene. The possibility for both aq/polymer/ aq extraction (including trapping in the acceptor, very similar to SLM extraction) and also, for example, aq/polymer/org extraction (similar to MMLLE) has been danonstrated. 

Membrane nonporous membrane elastomer (example, rubbers and silicone rubbers) or glassy (example, polycarbonate, polyether imide, polyethersulfone, polyimide, polysulfone, polystyrene, polyvinyl chloride, polyvinyl fluoride and teflon.)... 

Micro-organisms replicate only in the presence of water. The activity of preservatives depends therefore on the concentration of the free and active form in the aqueous phase of the preparation. Free refers to the binding that some preservatives can have with active substances, excipients or packaging materials. Examples of the binding of preservatives are the adsorption of phenylmercuric compounds to rubber stoppers, the adsorption of benzalkonium chloride to silicon rubber tubes and to cellulose nitrate-membrane filters, the solubilisation of methyl parahydroxybenzoate by polysorbate 80 [49] and by sodium lauiyl sulfate [50] and the migration (distribution) towards the lipid phase of methyl parahydroxybenzoate in emulsions. 

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