Silicon rubber, membrane

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. 

FIG. 4 Selectivity comparison among silicone-rubber-membrane Na -ISFETs based on calixarene neutral carriers (1), (2), and (5). 

FIG. 6 Time-course changes of potential response for silicone-rubber-membrane Na+-selective electrodes based on neutral carriers (5), (2), and bis(12-crown-4) on changing Na concentration from 1 X 10 to 3 X 10 M. (From Ref. 22.)... 

Foly(dimethylsiloxane) (silicone rubber) membranes are fabricated by hydrolysis of alkox-ysilyl terminal groups of silicone-rubber precursors [oligo(dimethylsiloxane) derivatives and crosslinking agents], followed by condensation. Covalent bonding of neutral carriers carrying an alkoxysilyl group to silicone rubber is, therefore, feasible by simple reaction of the silicone-rubber precursor with alkoxysilylated neutral carriers, as schematically shown in Scheme 1 [44]. 

SCHEME 1 Mechanism for chemical modification of poly(dimethysiloxane) (silicone rubber) membranes with ion-sensing active material. 

FIG. 11 Potential response of Na+-selective electrodes based on silicone-rubber membranes modified chemically by triethoxysilylated calix[4]arene (8) (O) without anion excluder ( ) with TFPB ( ) modified chemically by triethoxysilylated tetraphenylborate (9) as well. (From Ref. 44.)... 

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 Na -selective electrodes based on silicone-rubber membranes modified chemically by (8) and (9), were also investigated for Na assay in control serum and urine [22]. The found values for the Na concentrations in both of the serum and urine samples are in good agreement with their corresponding actual values with a relative standard deviation of about 1%. These results suggest that the Na -selective electrodes based on silicone-rubber membranes modified chemically by calix[4]arene neutral carrier (8) are reliable on assay in human body fluid. 

The heat of permeation of chlorine in a silicone rubber membrane is —3 to — 5 kcal mol-1 while that of nitrogen is 1.0-1.5 kcal mol-1. Separation of these gases therefore is helped by low temperature. The flux of chlorine actually increases as temperature is lowered, reflecting the increased sorption. The flux of nitrogen decreases, reflecting the lower diffusivity. 

J. Pick, K. Toth, E. Pungor, M. Vasak, and W. Simon, A potassium-selective silicone-rubber membrane electrode based on neutral carrier, Anal Chim Acta 64, 477-480 (1973). 

Zeolite-filled silicone rubber membranes part 1. Membrane preparation and pervaporation results. 

Jia and coworkers prepared thin-film composite zeolite-filled silicone rubber membranes by a dip-coating method [82]. The membranes have a thin silicalite-1/ silicone rubber mixed-matrix selective layer on top of a porous polyetherimide support. 

Hennepe, H.j.C., Bargeman, D., Mulder, M.H.V., and Smolders, C.A. (1987) Zeolite-filled silicone rubber membranes part 1. membrane pervaporation and pervaporation results. J. Membr. Sd., 35 (1), 39-55. 

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).

Recently developed blood oxygenators are disposable, used only once, and can be presterilized and coated with anticoagulant (e.g., heparin) when they are constructed. Normally, membranes with high gas permeabilities, such as silicone rubber membranes, are used. In the case of microporous membranes, which are also used widely, the membrane materials themselves are not gas permeable, but gas-liquid interfaces are formed in the pores of the membrane. The blood does not leak from the pores for at least several hours, due to its surface tension. Composite membranes consisting of microporous polypropylene and silicone rubber have also been developed. 

T. Uragami, H. Shinomiya, Concentration of aqueous alcoholic solutions through a modified silicone rubber membrane by pervaporation and evaporation, Makromol. Chem. 192 (1991) 2293-2305. 

Figure 3.23 Schematic of the apparatus developed by Ward et al. [52] to prepare water-cast composite membranes. Reprinted from J. Membr. Sci., 1, W.J. Ward, HI, W.R. Browall and R.M. Salemme, Ultrathin Silicone Rubber Membranes for Gas Separations, p. 99, Copyright 1976, with permission from Elsevier...

A second method of determining the coefficient ( >,/5) and the intrinsic enrichment of the membrane Ea is to use Equation (4.11). The term ln(l — 1/E) is plotted against the permeate flux measured at constant feed solution flow rates but different permeate pressures or feed solution temperatures. This type of plot is shown in Figure 4.10 for data obtained with aqueous trichloroethane solutions in pervaporation experiments with silicone rubber membranes. 

Figure 4.10 Trichloroethane enrichment [ln(l — 1/E)] as a function of permeate flux Jv in pervaporation experiments with silicone rubber membranes in spiral-wound modules using solutions of 100 ppm trichloromethane in water [15]. Feed solution flow rates are shown...

In the case of pervaporation of dissolved volatile organic compounds (VOCs) from water, the magnitude of the concentration polarization effect is a function of the enrichment factor. The selectivity of pervaporation membranes to different VOCs varies widely, so the intrinsic enrichment and the magnitude of concentration polarization effects depend strongly on the solute. Table 4.2 shows experimentally measured enrichment values for a series of dilute VOC solutions treated with silicone rubber membranes in spiral-wound modules [15], When these values are superimposed on the Wijmans plot as shown in Figure 4.12, the concentration polarization modulus varies from 1.0, that is, no concentration polarization, for isopropanol, to 0.1 for trichloroethane, which has an enrichment of 5700. 

The competitiveness of membrane systems in this application is very sensitive to the selectivity of the membranes for propane, butane and higher hydrocarbons over methane. If the membranes are very selective (propane/methane selectivity of 5-7, butane/methane selectivity of 10-15), the permeate stream from the main set of modules will be small and concentrated, minimizing the cost of the recompressor. Currently, silicone rubber membranes are being considered for this application, but other, more selective materials have been reported [55],... 

H.J.C. te Hennepe, D. Bargeman, M.H.V. Mulder and C.A. Smolders, Zeolite-filled Silicone Rubber Membranes Part I Membrane Preparation and Pervaporation Results, J. Membr. Sci. 35, 39 (1987). 

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