Aqueous liquid membranes preparation

Applications of liquid membrane technology, 110-122 Aqueous liquid membranes, preparation, 141,142/... 

Audunsson [29] reported on a sandwich-type extraction module equipped with liquid membranes, prepared by immersing hydrophobic microporous membranes (e.g. PTFE membranes) in organic solvents for about 15 min. The inert men ranes then act as supports for the immobilized solvent. When an aqueous sample passes by the membrane, non-ionic components in the sample are extracted into the hydrophobic liquid film and transferred into an appropriate acceptor solution on the other side of the membrane. When the acceptor remains stagnant while the sample flows continuous ) for a defined period, a preconcentration is effected in the acceptor solution, which is subsequently transferred to the detector. The procedure is equivalent to extraction and back-extraction in a single step. More details on such a system used for sample cleanup in gas-liquid chromatography is presented in Sec. 3.7. 

In this study, experiments on the simultaneous permeation of CO2 and CH4 through aqueous MEA, DEA and ethylenediamine hydrochloride (EDAHCl) membranes are performed and the permeation rates of CO2 are quantitatively discussed on the basis of the approximate solution of facilitated transport (77). A method is also proposed for applying the facilitated transport theory developed for a liquid membrane consisting of a liquid membrane phase alone to the analysis of facilitated transport through a supported liquid membrane prepared by impregnating a microporous polymer support having tortuous pore structure with a carrier solution. 

A liquid membrane can be prepared by emulsifying an aqueous solution in an organic liquid, then adding the emulsion to another aqueous solution. In this way, the organic liquid segregates the solutions but allows selective diffusion of solutes across it. Similarly, oil/water/oil type emulsions can be formed in which the liquid membrane is the water encapsulation layer. Very high rates of mass transfer can be achieved because of the large effective membrane surface area represented by the emulsion droplets. 

Ultrasound-assisted emulsification in aqueous samples is the basis for the so-called liquid membrane process (LMP). This has been used mostly for the concentration and separation of metallic elements or other species such as weak acids and bases, hydrocarbons, gas mixtures and biologically important compounds such as amino acids [61-64]. LMP has aroused much interest as an alternative to conventional LLE. An LMP involves the previous preparation of the emulsion and its addition to the aqueous liquid sample. In this way, the continuous phase acts as a membrane between both the aqueous phases viz. those constituting the droplets and the sample). The separation principle is the diffusion of the target analytes from the sample to the droplets of the dispersed phase through the continuous phase. In comparison to conventional LLE, the emulsion-based method always affords easier, faster extraction and separation of the extract — which is sometimes mandatory in order to remove interferences from the organic solvents prior to detection. The formation and destruction of o/w or w/o emulsions by sonication have proved an effective method for extracting target species. 

Even if the problems of poor crystal intergrowth due to local exhaustion of reactants in the autoclave and synthesis of zeolite material in the bulk of the solution were solved, an important problem remains, related to the fact that several batch synthesis cycles (with their associated heating and cooling processes) are often required to achieve a zeolite membrane of good quality. Thus, a synthesis procedure in which reactants are continuously supplied to the synthesis vessel while this is maintained at a constant temperature would clearly be desirable not only for performance but also for the feasibility of the scale-up. This type of approaches has already been tested for inner MFI and NaA zeolite membranes [33-35], and the results obtained indicate that the formation of concomitant phases and the amount of crystals forming in the liquid phase are greatly reduced. Similarly, the continuous seeding of tubular supports by cross-flow filtration of aqueous suspensions [36-37] has been carried out for zeolite NaA membrane preparation. 

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. 

Abstract Two types of membrane are presented free-standing films which are formed from aqueous polyelectrolyte solutions and membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Layer-by-layer assemblies represent versatile membranes suitable for dehydration of organic solvents and ion separation in aqueous solution. The results show that the structuring of the polyelectrolytes in the liquid films and the permeability of the multilayer membranes depends on different internal and environmental parameters, for example molecular weight, polymer charge density, ionic strength, and temperature. 

The second section refers to polyelectrolyte membranes prepared by alternating electrostatic layer-by-layer assembly of cationic and anionic polyelectrolytes on porous supports. Mass transport across ultrathin polyelectrolyte multilayer membranes is described. The permeation of gas molecules, liquid mixtures, and ions in aqueous solution has been investigated. The studies indicate that the membranes are excellently suited for separation of alcohol/water mixtures under pervaporation conditions and for ion separation, e.g. under nanofiltration conditions. 

The effect of viscosity is important in the production of liquid membranes. These are, to a limited extent, employed in the extraction of non-ferrous metal salts (particularly Zn, Ni, Cu) from process efluents. In their manufacture a prepared water/oil emulsion (e.g. 1/3 kerosene with 2% of a surfactant and 2/3 aqueous NiSO4 with a homogenizing agent) is stirred into the non-ferrous metal salt containing effluent in a ratio of ca. 1 5. It emerged [404], that it is by no means unimportant, how the prepared water/oil emulsion is stirred into this solution. It can be carefully added layer-wise over the aqueous solution and then the stirrer switched on (A), or immediately added to the rotating stirrer (B). 

Ability of WSP to interact with enzymes, drugs, selective and chelating properties of many polyelectrolytes make them very perspective in the development of liquid membrane bioreactors. It is known [103] that formation of a ternary metal ion-carrier-chelator complex at the inner vesicle waU can enhance the overaU selectivity in accordance with a multiplicative, rather than additive, function of equihbrium metal-hgand binding constants. Enzyme-containing lipid vesicles (liposomes), which are hpid dispersions that contain water-soluble enzymes in the trapped aqueous space, may be named as hquid membrane micro- or nanobioreactors with intraped WSP. Preparation and properties of hpid vesicles are described in [104] review. 

Synthesis of Phosphoric Acids and Their Derivatives. - A series of monoalkyl and dialkyl phosphorus acid chiral esters have been synthesised for use as carriers for the transport of aromatic amino acids through supported liquid membranes. The compounds acted as effective carriers but enantio-separation was at best moderate. A range of phosphono- and phosphoro-fluoridates have been prepared by treatment of the corresponding thio- or seleno- phosphorus acids with aqueous silver fluoride at room temperature (Scheme 1). In some cases oxidation rather than fluorination occurred. Stereospecifically deuterium-labelled allylic isoprenoid diphosphates, e.g. (1), have been synthesised from the corresponding deuterium-labelled aldehyde by asymmetric reduction, phosphorylation and Sn2 displacement with pyrophosphate (Scheme 2). ... 

Sample preparation by means of liquid membrane extraction is a technique that in essence contains two LLE extractions in one step. The setup is easily automated, and sample preparation is performed in a closed system, thus minimizing the risk for contamination and losses during the process. Because the extraction is made from an aqueous phase (donor) to a second, also aqueous phase (acceptor), further enrichment on a precolumn is possible before injection into the LC apparatus. Liquid membranes were used for enrichment of metsulfuron-methyl and chlorsulfuron from clean aqueous samples and natural waters. A similar device was developed by a Chinese... 

Liquid membranes are prepared from immiscible, liquid ion exchangers, which are retained in a porous inert. solid support. As. shown schematically in Figure 23-8. a porous, hydrophobic (that is. water-repelling), plastic disk (typical dimensions 3 X 0.15 mm) holds the organic layer between the two aqueous solutions. For divalent cation determinations, the inner tube contains an aqueous standard soittt ion of MCI, where M is the cation whose activity is to be determined. This solution is also saturated with AgCI to lorm a Ag-AgCI reference electrode with the silver lead wire. 

In studies of NO facilitated transport, Ward ( ) Immobilized a formamlde solution of Fe Ions between two silicone rubber membranes. Ward s analysis of the mass transfer data from the liquid membrane cell showed that the resistance of the silicone rubber supporting membranes was negligible compalred to the resistance of the 0.1 cm formamlde liquid membrane. Ward (26) used an Identical membrane configuration to study electrically Induced facilitated gas transport. A similar Immobilization technique was used by Otto and Quinn (27) to prepare an ILM for COj transport. An aqueous bicarbonate solution was Immobilized between silicone copolymer membranes formulated to have high COj permeability (28). 

Deetz (115) describes several experimental methods to overcome the well known stability problems with ILMs for selective transport of gases. He introduces methods to prepare ultra-thin (.1 to 2 pm) stable, aqueous, Immobilized liquid membranes. The problem of volatilization of the liquid membrane can be reduced or eliminated by Immobilizing the liquid phase in pores small enough to significantly reduce the molar free energy of the solution via the Kelvin effect. Ultra-thin ILMs can be produced by selective immobilization of the liquid membrane in the skin layer of a mlcroporous asymmetric polymer support. 

The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263-305°C. The polymers show an outstanding chemical stability. They are soluble only in 98% H2SO4. Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect. ... 

Instability is the most common problem of SLMs, including the loss of solvents and carriers through evaporation and decomposition or the breaking through of the solvent by too high pressure difference across the membranes. SLM stability can be affected by the type of polymeric support and its pore radius, solvents used in the liquid membrane, interfacial tension between the liquid and membrane phase, flow velocity of the aqueous phases, and method of preparation [1]. 

Supported liquid membrane extraction (SLME) is emerging as a fast and efficient sample preparation technique. Aromatic aminophosphonate isolation from water samples based on SLME allowed the identification and study of the operational parameters (pEI and ionic strength of the aqueous phase, composition of the membrane phase, and concentration of analytes) as well as the structure-extraction efficiency relationship. 

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