Silicon-containing membranes

However, there seems to be some drawback in the solubility or dispersibility of ion-sensing material in silicone rubber. This is mainly because silicone rubber does not contain a large quantity of plasticizer as the membrane solvent, in which neutral carriers can be dissolved easily, unlike in plasticized-PVC ion-sensing membranes. This issue is serious, especially with silicone-rubber membranes containing neutral carriers that show high crystallinity. Valinomycin, a typical ionophore, seems applicable to silicone-rubber-based K" -selec-tive electrodes [7,8,12-14]. Conventional crown-ether-based neutral carriers are also quite soluble in silicone rubber. 

Applicability in biological ion assay is an important factor for biocompatible potentio-metric ion sensors. Attempts were made to determine Na" " concentrations in human blood sera by using silicone-rubber membrane Na+-ISFETs based on (5) [Fig. 17(a)] [29]. The found values for Na concentration in undiluted, 10-fold diluted, and 100-fold diluted serum samples are in good agreement with the Na" " calibration plots. Even in the undiluted serum samples, only a slight potential shift was observed from the calibration. This indicates that the calixarene-based silicone-rubber-membrane Na+-ISFETs are reliable on serum Na assay. For comparison with the silicone-rubber membrane, Na -ISFETs with corresponding plasticized-PVC membrane containing (2) or (5) were also tested for the Na assay. The found values of Na" " concentration... 

The most widely applied commercial nano filtration membrane is the MPF series of Koch Int.[19] (Figure 4.9). The series contains polymeric silicon-derived membranes in the MWCO range of 400-700 Da. [20] The membranes are supplied in a 50% ethanol / water mixture, and should under no circumstances be allowed to dry out. Before use, the membranes must be conditioned with the solvent of choice. 

Figure 5.8 — Probe-type sensor based on continuous circulation of a stream containing an acid-base indicator for the batch determination of COj in sea water, (a) Reagent delivery capillary, (d) Reagent exit capillary, (c) Optical fibre from source, (d) Optical fibre to detector, (e) White silicone rubber membrane. (/) White silicone sealant, (g) Epoxy resin, (/i) 0-ring. (/) Sensor housing. (/) Optical cable. (Reproduced from [12] with permission of the American Chemical Society).

Figure 9.14 Pervaporation separation factor, /9pervap> as a function of the VOC evaporation separation factor, /3evaP- Data obtained with laboratory-scale spiral-wound modules containing a composite silicone rubber membrane and in laboratory cells with thick membranes...

Figure 8.12 Block diagram and photograph of a contained in the horizontal pressure vessels, membrane fuel-gas conditioning unit (FGCU) The unit produces 0.5-1.0 MMscfd of clean gas. used for a field gas compressor engine (the unit Reproduced with permission from Ind. Eng. uses silicone rubber membranes in spiral-wound Chem. Res. 2008, 47(7), 2109-2121. Copyright modules). The membrane modules are 2008 American Chemical Society [17].

Carbon dioxide sensor — An electrochemical device for measuring the -+ partial pressure of carbon dioxide, originally developed for measurements with blood by Sever-inghaus and Bradley [i]. It consists of a glass electrode as used for pH measurements covered with a membrane selectively permeable for CO2, e.g., silicone. Between membrane and glass electrode an electrolyte solution containing NaCl and Na2C03 is entrapped. 

Figure 3.48 Exploded schematic view of a flow-cell FPW liquid sensor. The silicon chip containing die thin silicon-nitride membrane, piezoelectric film and transducers is sandwiched between two etched silicon chips. The upper chip is a cap with fluid inlet and outlet fittings, b also provides vias for contact to a temperature-sensing polysilicon resistor deposited on the FPW chip below it. The lower chip introduces transducer contact leads and protects the underside of the membrane fitm contact with the fluid. (Hgwc courtesy of Beo Costello, Bokeley Microliulratitents, Inc.)...

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

The maximum HaS facilitated flux value corresponds to an HaS permeability of 332 x 10" cm STP)cm/ cm s kPa). Matson (8) reported a HaS permeability range of 2250-3000 x 10 cm cm/(cm s kPa) for an ILM containing aqueous solution of Ka(X), for a temperature range of 363-HO3 K. The feed gas HaS partial pressure In Matson s studies was approximately 20 kPa with a total feed pressure of 2.17 x 10 kPa. Robb (22) reported an ambient temperature HaS permeability of 638 x 10 cm cm/(cm s kPa) for a silicone rubber membrane. However, polymer membranes such as silicone rubber have much lower selectlvltles than facilitated transport membranes. 

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