Supported liquid membranes polymeric support

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. 

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 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. 

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,... 

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. 

Membrane Techniques The interest in membrane techniques for sample preparation arose in the 1980s. Extraction selectivity makes membrane techniques an alternative to the typical sample enrichment methods of the 1990s. Different membrane systems were designed and introduced into analytical practice some more prominent examples are polymeric membrane extraction (PME), microporous membrane liquid-liquid extraction (MMLLE), and supported liquid membrane extraction (SEME) [106, 107]. Membrane-assisted solvent extraction (MASE) coupled with GC-MS is another example of a system that allows analysis of organic pollutants in environmental samples [108-111] ... 

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. 

However, ELMs are quite difficult to prepare and after transport, the oil droplets have to be separated and broken up to recover the receiving phase. Compared to the ELM, the BLMs are easier to operate. The supported liquid membranes (SLM) are categorized into two types of supports, namely, a flat-sheet supported liquid membrane (FSSLM) or a hollow fiber supported liquid membrane (HFSLM). Here a polymeric filter with its pores filled with the organic phase acts as membrane. The three different types of liquid membranes have already been schematically represented in Chapter 29. A schematic representation of a hollow fiber semp is shown in Figure 31.2. 

Supported liquid membranes comprised the bulk of the published literature on the transport studies of metal ions across thin polymeric films [16,56-59]. Several literature reports on actinide transport across supported liquid membranes using various types of extractants viz., acidic extractants, neutral extractants and amine extractants are discussed below. 

A novel polymeric bicontinuous microemulsion (PBM) membrane, consisting of an interconnecting network of nanometer pore size water channels, was employed as liquid membrane support [13] for the immobilization of new porphyrin carrier [14] for facilitated oxygen transport. Although the membrane resulted to be stable due to the nanoporous structure, a modest (2.3-2.4) O2/N2 selectivity was achieved. 

Liquid-liquid methods include solvent extraction with immiscible liquid-liquid systems in which a suitable ligand is dissolved in an organic phase and contacted with a metal ion containing an aqueous phase and liquid membranes. Separations can also be achieved with pseudo-phase systems such as micelles, microemulsions, and vesicles. Such separations can be solid-liquid or liquid-liquid and include separations with normal- and reversed-phase silica, and polymeric supports where the mobile phase contains the organized molecular assembly (OMA) of micelles, microemulsions, or vesicles. Separation of metal ions using the pseudo-phase systems is stiU in its infancy and a brief account will be provided here. 

Meanwhile, the science of chemical sensors was developing fast. The technology of polymer-supported liquid membranes was already a mature science, as it was more than 15 years since it was first reported. The use of plasticized PVC allowed for the construction of membrane-based sensors with great ease, and gave sensor technology a new boost. It was the same basic technology that was subsequently used for the development of liquid-polymeric-based ISEs. 

A supported liquid membrane (SLM) process has been considered, among other possible options, for the reiiK>val of contaminants from groundwater, because of the following advantages of SLM s over competing techniques (solvent extraction, ion exchange, polymeric membrane processes, etc.) ... 

In the biomedical applications outlined by Ward et al. (7 ), more so than in any other separation application of synthetic polymeric membranes, the goal is to mimic natural membranes. Similarly, the development of liquid membranes and biofunctional membranes represent attempts by man to imitate nature. Liquid membranes were first proposed for liquid separation applications by Li (46-48). These liquid membranes were comprised of a thin liquid film stabilized by a surfactant in an emulsion-type mixture. Wtille these membranes never attained widespread commercial success, the concept did lead to immobilized or supported liquid membranes. In... 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

By judicious choice of the membrane liquid, complexation agent and support, immobilized liquid membranes (ILM) can have both high selectivity and high permeant fluxes. Liquid membranes have the additional advantage that diffusion coefficients in liquids are several orders of magnitude larger than in polymeric membranes. Previously reported ILM research in the literature includes purification and recovery processes in both gas and liquid phases ( ). This variety of applications creates different requirements for supports for ILMs. This paper discusses criteria which influence selection of ILM support materials. 

According to configuration definition, three groups of hquid membranes are usually considered (see Fig. 1.1) bulk (BLM), supported or immobilized (SLM or ILM), and emulsion (ELM) liquid membrane transport. Some authors add to these definitions polymeric inclusion membranes, gel membranes, dual module hollow-fiber membranes, but, to my opinion, the first two types are the modifications of the SLM and the third is the modification of BLM. It will be discussed in detail in the respective chapters. 

Nevertheless, there are some problems limiting the practical apphcation of SLMs. The main problem is the stability of the liquid membrane, caused by leakage and/or losses of membrane phase components during transport process. However, by proper choice of the porous polymeric support, using organic solvents used as a membrane phase and membrane phase components, the instability can be significantly reduced. 

In the SLM process, like in all membrane processes, the membrane plays a key role in the transport and separation efficiency. The permeation rate and separation efficiency depends strongly on the type of liquids and supports used for SLM construction. However, the transport properties depend on the type of liquids used as a membrane phase the hquid membrane stability and mechanical stability depend, to a large extent, on the microstructure like pore shape, size, and tortuosity of the membrane used as a support. Therefore, many types of polymeric and inorganic microporous membrane supports are studied for the liquid membrane phase immobilization. 

Table 3.1 Characteristic of some commercially available membranes used as a polymeric support in flat-sheet supported liquid membranes (FS-SLM)... 

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. 

PBMM polymerized bicontinuous microemulsion membrane SLM supported liquid membrane... 

In this context, facilitated transport of a specific gas molecule through modified polymeric membranes or liquid membranes containing mobile carriers can be employed to improve single bulk material (polymer) properties. Ceramic material is not traditionally employed as hquid membrane support due to their high cost, use of not aggressive compounds to be separated and mild operating conditions. 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

AU the techniques used to increase the stabihty of the SLM, such as the geUed SLM techniques [10, 11] (Fig. 7.5B) and the addition of thin top-layer by interfacial polymerization reaction on the SLM (Fig. 7.5C) [12], are essentiaUy applied in the removal of (metal) ions from solution. The stability of liquid membranes used for the separation of gases is more comphcated. Here, the addition of a top-layer on the macroporous support can negatively influence the permeabihty of gases through the membrane. Therefore, a careful choice of the layer material is important because it has to be impermeable to the solvent and should posses a high permeabihty for the gas molecules considered. In addition, the thickness of the top-layer as weU as that of the whole liquid membrane has to be minimized. 

The nature of the supporting membrane also plays an important role in the performance of supporting ionic liquid membranes. In this context, de los Rios et al. [3] nsed two polymeric membranes, nylon and mitex, as supporting membranes. Nylon membrane was a hydrophilic polyamide membrane with a pore size of 0.45 pm and a thickness of 170 pm. Mitex membrane was a hydrophobic polytetrafluoroethylene membrane with a pore size of 10 pm and a thickness of 130 pm. It was observed that less ionic liquid was absorbed into the mitex membranes, which was explained by the different textural properties and the high hydrophobic character of these membranes, which probably restrict interaction with the hydrophilic ionic liquids used [27]. 

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