Polymeric liquid membranes

An important advance in ion-selective electrodes (ISEs) and related systems was based on the concept of polymeric liquid membranes developed by Eisenman [122]. The principle of this approach was to incorporate an organic compound as the ionophore into a polyvinyl chloride membrane... 

Jeong SH and Lee KH. Separation of CO2 from CO2/N2 mixture using supported polymeric liquid membranes at elevated-temperatures. Sep. Sci. Techn. 1999 34 2383. 

Dastgir, M. G., Peeva, L. G., Livingston, A. G. (2005). The performance of composite supported polymeric liquid membranes in the Membrane Aromatic Recovery System (MARS). Chem. Eng. Sci., 60, 7034-44. 

The solubilities of the various gases in [BMIM][PFg] suggests that this IL should be an excellent candidate for a wide variety of industrially important gas separations. There is also the possibility of performing higher-temperature gas separations, thanks to the high thermal stability of the ILs. For supported liquid membranes this would require the use of ceramic or metallic membranes rather than polymeric ones. Both water vapor and CO2 should be removed easily from natural gas since the ratios of Henry s law constants at 25 °C are -9950 and 32, respectively. It should be possible to scrub CO2 from stack gases composed of N2 and O2. Since we know of no measurements of H2S, SO, or NO solubility in [BMIM][PFg], we do not loiow if it would be possible to remove these contaminants as well. Nonetheless, there appears to be ample opportunity for use of ILs for gas separations on the basis of the widely varying gas solubilities measured thus far. 

Fig. 27 a and b. Schematic representation of the molecular structure of a side chain polymeric liquid crystals b polymer model membranes studied by 2H NMR... 

Membranes can be homogeneous, where the whole membrane participates in the permeation of a substance, or heterogeneous, where the active component is anchored in a suitable support (for solid membranes) or absorbed in a suitable diaphragm or acts as a plasticizer in a polymeric film. Both of the latter cases are connected with liquid membranes. Biological membranes show heterogeneity at a molecular level. 

Several manufacturers introduced products amenable for this solid-supported LLE and for supported liquid extraction (SLE). The most common support material is high-purity diatomaceous earth. Table 1.8 lists some commercial products and their suppliers. The most widely investigated membrane-based format is the supported liquid membrane (SLM) on a polymeric (usually polypropylene) porous hollow fiber. The tubular polypropylene fiber (short length, 5 to 10 cm) is dipped into an organic solvent such as nitrophenyl octylether or 1-octanol so that the liquid diffuses into the pores on the fiber wall. This liquid serves as the extraction solvent when the coated fiber is dipped... 

The principle of a three-phase membrane extraction is illustrated in Figure 1.28. An organic solvent is immobilized in the pores of a porous polymeric support consisting of a flat filter disc or a hollow fiber-shaped material. This supported liquid membrane (SLM) is formed by treating the support material with an organic solvent that diffuses into its pores. The SLM separates an aqueous... 

The above difficulties are removed in the new version of the liquid membrane, which employs a polymeric film with the ion-exchanger solution functioning as a plasticizer. Then it is much easier to prepare a membrane without leaks and using only a minute amount of the ion-exchanger solution. When the membrane ceases to function, it is simply replaced. For a survey of those electrodes see [109,111,112,113, 180] they are generally termed solvent-polymeric membranes [180] or polyvinyl chloride-matrix membranes [112]. 

The use of liquid membranes in analytical applications has increased in the last 20 years. As is described extensively elsewhere (Chapter 15), a liquid membrane consists of a water-immiscible organic solvent that includes a solvent extraction extractant, often with a diluent and phase modifier, impregnated in a microporous hydrophobic polymeric support and placed between two aqueous phases. One of these aqueous phases (donor phase) contains the analyte to be transported through the membrane to the second (acceptor) phase. The possibility of incorporating different specific reagents in the liquid membranes allows the separation of the analyte from the matrix to be improved and thus to achieve higher selectivity. 

Fig. 23.4 Organophilic pervaporation (PV) for in situ recovery of volatile flavour compounds from bioreactors. The principle of PV can be viewed as a vacuum distillation across a polymeric barrier (membrane) dividing the liquid feed phase from the gaseous permeate phase. A highly aroma enriched permeate is recovered by freezing the target compounds out of the gas stream. As a typical silicone membrane, an asymmetric poly(octylsiloxane) (POMS) membrane is exemplarily depicted. Here, the selective barrier is a thin POMS layer on a polypropylene (PP)/poly(ether imide) (PEI) support material. Several investigations of PV for the recovery of different microbially produced flavours, e.g. 2-phenylethanol [119], benzaldehyde [264], 6-pentyl-a-pyrone [239], acetone/buta-nol/ethanol [265] and citronellol/geraniol/short-chain esters [266], have been published...

For similar solvent polymeric membranes (78 wt.% dicresyl butyl phosphate in polyvinyl chloride) self-diffusion coefficients of the order of 10-7 cm2s 1 have been reported.12 These diffusion coefficients, as well as measurements of rotational mobilities,14 indicate that the solvent polymeric membranes studied here are indeed liquid membranes. This liquid phase is so viscous, however, that convective flow is virtually absent. This contrasts with pure solvent membranes where an organic solvent is interposed between two aqueous solutions either by sandwiching it between two cellophane sheets or by fixing it in a hole of a Teflon sheet separating the aqueous solutions.15 The extremely high convective flow is one of the reasons why the term membrane for extraction systems... 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

Stabilized liquid membrane device (SLMD) A water-insoluble organic complexing mixture diffuses to the exterior surface of the sampler through a polymeric membrane Divalent metal ions Preconcentration, in situ sampling, determination of labile metal ions in grab samples Days to several weeks Extraction with acid 73... 

The polymeric membrane extraction systems, that is, MIMS, MESI, and PIME, have been extensively applied for sampling VOCs and SVOCs. In contrast, liquid membrane extraction... 

By far the majority of polymeric membranes, including UF membranes and porous supports for RO, NF or PV composite membranes, are produced via phase separation. The TIPS process is typically used to prepare membranes with a macroporous barrier, that is, for MF, or as support for liquid membranes and as gas-liquid contactors. In technical manufacturing, the NIPS process is most frequently applied, and membranes with anisotropic cross-section are obtained. Often,... 

Pertraction (PT) can be realized through a liquid membrane, but also through a nonporous polymeric membrane that was applied also industrially [10-12]. Apart from various types of SLM and BLM emulsion liquid membranes (ELM) were also widely studied just at the beginning of liquid membrane research. For example, an emulsion of stripping solution in organic phase, stabilized by surfactant, is dispersed in the aqueous feed. The continuous phase of emulsion forms ELM. Emulsion and feed are usually contacted in mixed column or mixer-settlers as in extraction. EML were applied industrially in zinc recovery from waste solution and in several pilot-plant trials [13,14], but the complexity of the process reduced interest in this system. More information on ELM and related processes can be found in refs. [8, 13-16]. 

Schlosser, S. and Kossaczky, E. (1987) Liquid membranes stabilized by polymeric surfactants, in Synthetic Polymeric Membranes (ed. B. Sedlacek), W. de Gruyter, Berlin, p. 571. 

Kemperman, A.J.B., Rolevink, H.H.M., Bargeman, D., Vandenboomgaard, T. and Strathmann, H. (1998) Stabilization of supported liquid membranes by interfadal polymerization top layers. Journal of Membrane Science, 138, 43. 

Most liquid membranes are less stable than their polymeric counterparts. Although the thin liquid film in the membrane corresponds to a short diffusion path and hence a high mass transfer rate, small amounts of the immobilized liquid can be displaced under pressure. Also, the immobilized liquid may slowly dissolve in the external phases, eventually leading to discontinuities in the liquid... 

Liquid-membrane-type ISEs, based on water-immiscible liquid substances impregnated in a polymeric membrane, are widely used for direct potentio-... 

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