Selective polymeric membrane

This system utilizes specific membranes, between which the dmg reservoir is enclosed (Fig. 4). A tiny ehiptical disk, inserted into the cul-de-sac of the eye, releases pilocarpiae steadily. The dmg is deUvered through selected polymeric membranes. The dmg reservoir maintains a saturated solution between the membranes which acts osmoticaHy as the driving force for the dmg to diffuse through the rate-limiting membranes. 

Consider an equilibrium-limited esterification reaction. One way to drive the reaction to completion is to remove the water formed by the reaction selectively through a membrane. This can be an attractive strategy when higher temperatures are undesirable due to factors like colouration of the materials and formation of undesirable products even though these may be present at a low level. As another example, consider the air oxidation of cyclohexane or cyclododecane to cyclohexanone/-ol or cyclododecanone/-ol, where the product can undergo more facile oxidation to unwanted or much lower value products. Consequently, industrial processes operate at a level of less than 5% conversion. If a membrane can selectively remove cyclohexanone as it is formed, the problems mentioned above can be thwarted. However, selective polymeric membranes, which can work at oxidation temperature, have not yet been proved. 

The first and very simple solid contact polymeric sensors were proposed in the early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-selective polymeric membrane [94], These coated wire electrodes (CWEs) had similar sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but suffered from severe potential drifts, resulting in poor reproducibility. The origin of the CWE potential instabilities is now believed to be the formation of a thin aqueous layer between membrane and metal [95], The dominating redox process in the layer is likely the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and p02 changes in a sample. Additionally, the ionic composition of this layer may vary as a function of the sample composition, leading to additional potential instabilities. 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

This work was finally abandoned after the development of selective polymeric membranes in the 1980s. 

If the gas is cooled down, a C02-selective polymeric membrane may be used (cardopolymers and carrier membranes are especially interesting. Problem compression of exhaust gas may be needed see Section 4.3.1)... 

Zou, J. and Ho, W.S.W. C02-selective polymeric membranes containing amines in crosslinked poly(vinyl alcohol). Journal of Membrane Science, 2006, 286, 310. 

Gas separation membrane technologies are extensively used in industry. Typical applications include carbon dioxide separation from various gas streams, production of oxygen enriched air, hydrogen recovery from a variety of refinery and petrochemical streams, olefin separation such as ethylene-ethane or propylene-propane mixtures. However, membrane separation methods often do not allow reaching needed levels of performance and selectivity. Polymeric membrane materials with relatively high selectivities used so far show generally low permeabilities, which is referred to as trade-off or upper bound relationship for specific gas pairs [1]. 

Fig. 3. Fabrication of polymer membrane ISEs (a) dropwise addition of casting solution to glass ring resting on glass slide (b) ring loosely covered, THF evaporates (c) ion-selective polymeric membrane forms on the slide (d) membrane removed from slide and small disk cut out and (e) pasted onto a piece of PVC tubing (f) a Ag/AgCl wire and internal reference solution complete electrode.

C02-selective polymeric membranes (cardopolymers, FSCs), CMS, mixed matrix, or biomimetic materials are potential membrane materials for this application. The purified methane can then be compressed to 300 bar and stored in tanks for fuel in the transport sector or for conversion to methanol used for FCs. 

Franz, J. and V. Scherer, An evaluation of COj and Hj selective polymeric membranes for COj separation in IGCC processes. Journal of Membrane Science, 2010. 359(1) 173-183. 

Piletsky, S.A. Dubey, I.Y. Fedoryak, D.M. Kukhar, P.V. Substrate-selective polymeric membranes Selective transfer of nucleic acid components. Biopolim. Kletka... 

Table 5.5 CO2 and H2 Separation properties of C02-selective polymeric membranes...

Merkel et al. (2012) assessed such configuration considering an H2 membrane operating at 150 °C in which the retentate stream is treated in a low-temperature process in which high-purity CO2 is recovered by phase separation. Although the complete plant was not simulated, they estimated a reduction of both parasitic power consumptions and capital cost with respect to the benchmark process with CO2 separation by physical absorption. The optimal configuration proposed in Merkel et al. (2012) would also include a C02-selective polymeric membrane to recover the CO2 released with the vapors of the low-temperamre knockout drum. 

In attempts already made in the early 1970s, the ion-selective polymeric membrane was directly applied to a metal wire. From a theoretical point of view, this is an unsatisfactory situation because in all but a few exceptions these membranes contain neither a redox-active cation of the metal of which the wire is made nor an electron acceptor/donor pair that determines the redox potential. As a result, the phase boundary potential at the membrane/metal interface of most coated-wire ISEs is poorly defined. It is still not well understood why in real life freshly prepared coated-wire electrodes sometimes perform surprisingly well under circumstances when they can be recalibrated relatively frequently. However, formation of a water layer at the metal-membrane interface leads eventually to memory effects and, after delamination of the sensitive membrane, to catastrophic failure. 

S. C. Ma, N.A. Chaniotakis, M.E. Meyerhoff, Response properties of ion-selective polymeric membrane electrodes prepared with aminated and carboxylated poly(vinyl chloride). Anal Chem, 60, 2293-2299,1988. 

Cadogan, A.M., D. Diamond, M.R. Smyth, M. Deasy, M.A. McKervey, and S.J. Harris. 1989. Sodium-selective polymeric membrane electrodes based on calix[4]arene ionophores. Analyst 114 1551-1554. 

Bakker E (2014) Enhancing ion-selective polymeric membrane electrodes by instrumental control. Trends Anal Chem 53 98-105... 

This chapter reviews the recent developments of two types of facilitated transport membranes (1) supported liquid membranes (SLMs) with strip dispersion and (2) carbon-dioxide-selective polymeric membranes, for environmental, energy, and biochemical applications. 

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