Types bulk liquid membranes

Liquid membranes can be of three types—bulk liquid membrane, immobilized on a solid supported hquid membrane, and liquid membrane as double emulsions. Of these three types, ELMs can achieve much higher mass transfer area than the other two membranes. ELMs were first used by Li [1] for separation of hydrocarbons. Since then, considerable work has been done to demonstrate qualitatively the feasibihty of performing separations with specific formulations. 

The carrier should not dissolve in the feed liquid or receptor phase in order to avoid leakage from the liquid membrane. In order to achieve sufficient selectivity, minimization of nonselective transport through the bulk of the membrane liquid is required. Liquid membranes can be divided into three basic types [6] emulsion supported and bulk liquid membranes, respectively (Fig. 5-2). 

Fig. 5-2. Three types of the liquid membrane eonfiguration (a) emulsion liquid membrane (b) supported liquid membrane (e) elassieal bulk liquid membrane set-up.

In the supported liquid membrane process, the liquid membrane phase impregnates a microporous solid support placed between the two bulk phases (Figure 15.1c). The liquid membrane is stabilized by capillary forces making unnecessary the addition of stabilizers to the membrane phase. Two types of support configurations are used hollow fiber or flat sheet membrane modules. These two types of liquid membrane configuration will be discussed in the following sections. 

Figure 1 shows several types of mass transfer or diffusion cells, which are of the simplest design for performing bulk liquid membrane (BLM) processes. Each of the devices is divided into two parts a common part containing the membrane liquid, M and a second part in which the donor solution F and acceptor solution R are separated by a solid impermeable barrier. The liquid, M contacts with the two other liquids and affects the transfer between them. All three liquids are stirred with an appropriate intensity avoiding mixing of the donor and acceptor solutions. For a liquid-ion exchange in a BLM system. Fig. 2 shows the transfer mechanism of cephalosporin anions, P , from donor (F) to acceptor (R) solution... 

In addition to SLM, which is the most commonly used three-phase extraction principle, at least in analytical chemistry, also other ways of placing an organic phase between two aqueous phases are known. In the classical bulk liquid membrane (BLM) setups, U-tubes or similar devices are used to confine bulk volumes of organic liquids between two aqueous phases. This type of devices is very little used for sample preparation in analytical chemistry, as the extraction process becomes slow and the enrichment factors possible are very limited. 

Broadly speaking, there are three different types of liquid membranes. Bulk liquid membrane (BLM) is a stirred organic phase of lower density than the aqueous phase positioned under it or vice versa. In emulsion liquid membrane (ELM), the receiver aqueous phase containing oil droplets is dispersed into the feed aqueous phase. The total volume of the receiving phase inside the oil droplets is at least ten times smaller than that of the source phase. The thickness of the membrane (organic film) is very small, while the surface area is enormous resulting in very fast separations. Though the efficiency of mass transfer in the liquid membranes is inversely proportional to the thickness of the membrane phase, too thin a film has poor stability due to low but finite solubility in F and R. It can also be disturbed by pressure differences created by the two aqueous phases. 

Table 7.3 shows a classification of the liquid membranes on the basis of the configuration and module types employed in gas separation. The liquid membranes can be divided in three main classes (i) supported liquid membrane (SLM), (ii) bulk liquid membrane (BLM), and (iii) supported ionic liquid membrane (SILM). 

In the present paper, we examine the influence of structural variation within series of crown ether carboxylic acid and crown ether phosphonic acid monoalkyl ester carriers upon the selectivity and efficiency of alkali metal transport across three types of liquid organic membranes. Structural variations within the carriers include the polyether ring size, the lipophilic group attachment site and the basicity of ethereal oxygens. The three membrane types are bulk liquid membranes, liquid surfactant (emulsion) membranes and polymer-supported liquid membranes. 

The liquid membrane (LM) concept combines solvent extraction (SX) and membrane-based technologies, enabling both extraction and back-extraction in a single step with reduced consumption of extractants and diluents. For these reasons, separation based on LMs can be viewed as a promising alternative to traditional SX. The LM separation approach involves mass transfer of a target chemical species between two solutions (i.e., feed and receiver solutions) separated by an immiscible LM [1]. The main types of LMs are bulk liquid membranes (BLMs), emulsion liquid membranes (ELMs), supported liquid membranes (SLMs), and polymer inclusion membranes (PIMs). 

In the recent past liquid membranes were employed for the separation and extraction of materials, and they can be conveniently employed for separating biological materials [129-137], Microemulsions of Winsor I (o/w) and Winsor II (w/o) types are considered dispersed liquid membranes that can augment the transfer of oil-soluble and water-soluble compounds, respectively, across them by trapping them in microdroplets for convenient uptake and subsequent release. The microemulsions (Winsor I and II) are called bulk liquid membranes. They are recent additions in the field of separation science and technology. This field has been fundamentally explored and advanced by Tondre and coworkers [138-147], who worked out the fundamentals of the transport process by studying the transfer of alkali metal picrates and other compounds across the w/o microemulsions [140-142], They also studied the transport of lipophilic compounds (pyrene, perylene, and anthracene) across o/w liquid membranes [138,139],... 

The vast majority of cation transport model studies have been conducted using crown ethers as ionophores and bulk liquid membranes as bilayer models. Much information has been obtained about carrier-mediated transport [1,7] but the mechanism for transport of molecules through biological membranes is predominantly of the channel type [8]. The synthesis of a cation-conducting channel is a daunting task. We felt that a suitable compound would require at least three basic features. First, it must be capable of insertion into a lipid bilayer. Second, it must span the membrane. Third it must have some residue integral to it that would... 

Figure 20 Types of liquid membranes (a) bulk liquid membrane (b) emulsion liquid membrane. (Reproduced from Ref. 80. lUPAC, 1986.) (c) supported liquid membrane. (Reproduced from Ref. 80. lUPAC, 1986.) and (d) pol)mer inclusion membrane.

In contrast to osmotic, dialysis, filtration or size-exclusion type membranes, PIMs as other liquid membranes (i.e., bulk liquid membranes, emulsion liquid membranes and supported liquid membranes) rely on the action of a chemical agent to extract the solute of interest from an aqueous phase (Kolev, 2005). The action of this chemical agent is the most important factor in the performance of any PIM and its behavior shares considerable similarities with SX apphcations (e.g., hydrometaUurgy). 

Figure 4. Four Types of Cells Used to Study Transport Across Bulk Liquid Membranes.

Rgure 8. Frequently Used Membrane Types. A, B, C, and D are bulk liquid membrane, emulsion liquid membrane, supported liquid membrane, and dual module hollow fiber membrane configurations respectively. (Reproduced with permission from ref. 47. Copyright 1990 CRC Press.)... 

Figure 1. Different Types of Liquid Membranes BLM = Bulk Liquid Membrane ELM = Emulsion Liquid Membrane and SLM = Supported Liquid Membrane with a) Flat Membrane and b) Hollow Fiber Membrane Configurations. (F = feed solution M = membrane phase S = stripping solution)...

Representations of the three general types of liquid membranes, e.g., bulk liquid membranes, emulsion liquid membranes, and supported or immobilized liquid membranes, are presented in Figure 1. 

Bulk Liquid Membranes (BLM). This is the simplest type of liquid membrane (2-8) and is utilized for fundamental studies of certain aspects of liquid membrane transport processes. In one such process, a beaker-in-a-beaker cell (Figure 1) consists of inner and outer compartments which contain the aqueous feed (F) and strip (S) solutions, respectively. The inner beaker contains the stripping solution and is surroimded by the feed solution. Both aqueous solutions contact the upper organic layer, which is the liquid membrane. Mass transfer takes place from the feed solution through the liquid membrane and into the strip solution. Bartsch et aL studied the transport of alkali metal cations across bulk liquid membranes in which a crown ether carboxylic acid in the organic layer served as the carrier (2,3). 

Carrier-facilitated transport of actinides across bulk, supported, and emulsion liquid membranes, as well as plasticized membranes and recently developed emulsion-free liquid membranes, are reviewed. The discussion includes the effects of important experimental variables upon the solute flux for various types of liquid membranes. Applications of liquid membranes in the recovery and removal of radiotoxic actinides from the nitric acid wastes generated during reprocessing of spent fuel by the PUREX process and wastes produced by other radiochemical operations are surveyed. 

Synthetic bulk liquid membranes consist of a cyhndrical glass vessel and a coaxially arranged glass cylinder, which separates the two aqueous phases (Fig. 6a) [332,333]. In the U-shaped cell (Fig. 6b), the separation of the receiving and the source aqueous phase is accomplished by means of the two side arms [334,335]. H-type cells (Fig. 6c) represent a modification of the U-shaped cell and also work with a less dense solvent (e.g., 1-hexanol) as the liquid membrane component [336]. 

The customary type of a liquid membrane electrode is a design in which the sensitive membrane is composed of a water-immiscible organic solvent containing the ion of interest in the form of ion associate. The membrane is interposed between a standard (internal) and a test (external) ion solution.""" In early types of liquid membrane electrodes, an organic phase as a membrane is placed between two aqueous phases in bulk or with the support of a thin, porous cellulose sheet, sintered glass, or similar lamellas. Nitrobenzene is the popular membrane solvent other organic solvents are also applied such as o-nitrotoluene, 4-ethylnitrobenzene, 4-nitro-m-xylene, and p-nitrocoumarin. 

We recently synthesized several reasonably surface-active crown-ether-based ionophores. This type of ionophore in fact gave Nernstian slopes for corresponding primary ions with its ionophore of one order or less concentrations than the lowest allowable concentrations for Nernstian slopes with conventional counterpart ionophores. Furthermore, the detection limit was relatively improved with increased offset potentials due to the efficient and increased primary ion uptake into the vicinity of the membrane interface by surfactant ionophores selectively located there. These results were again well explained by the derived model essentially based on the Gouy-Chapman theory. Just like other interfacial phenomena, the surface and bulk phase of the ionophore incorporated liquid membrane may naturally be speculated to be more or less different. The SHG results presented here is one of strong evidence indicating that this is in fact true rather than speculation. 

Much effort has been expended in attempting to use membranes for separations. Reverse osmosis membranes are used worldwide for water purification. These membranes are based on size selectivity depending on the pores used. They do not have the ability to selectively separate target species other than by size. Incorporation of carrier molecules into liquid membrane systems of various types has resulted in achievement of highly selective separations on a laboratory scale. Reviews of the extensive literature on the use of liquid membrane systems for carrier-mediated ion separations have been published [15-20]. A variety of liquid membranes has been studied including bulk (BLM), emulsion (ELM), thin sheet supported (TSSLM), hollow fiber supported (HFSLM), and two module hollow fiber supported (TMHFSLM) types. Of these liquid membranes, only the ELM and TMHFSLM types are likely to be commercialized. Inadequacies of the remaining... 

Teramoto M, Matsuyama H, Nakai K, Uesaka T, and Ohnishi N. Eacilitated uphill transport of eicosapentaenoic acid ethyl ester through bulk and supported liquid membranes containing silver nitrate as carrier A new type of uphill transport. J Mem Sci, 1994 91(1-2) 209-213. Teramoto M, Matsuyama H, and Ohnishi N. Selective facilitated transport of benzene across supported and flowing liquid membranes containing silver nitrate as a carrier. J Mem Sci, 1990 50 269-278. 

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. 

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