Emulsion liquid membranes preparation

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

A liquid membrane configuration, which is much used for technical applications, is the emulsion liquid membrane (ELM) systems where the acceptor phase is dispersed as a colloid phase, each colloid drop surrounded by a thin organic, surface active phase. This principle does not seem to have been used for analytical sample preparation, probably due to the difficulty of quantitatively recovering the disperse acceptor phase. 

Preparation of fine particles using emulsion liquid membrane... 

Liquid membranes can be prepared in two different configurations (see fig. 1). A liquid can be Impregnated in the pores of a porous solid for mechanical support. This form is commonly known as an immobilized liquid membrane (ILM). In the alternate configuration, the receiving phase is emulsified in an immiscible liquid membrane. This type of liquid membrane is known as a liquid surfactant, or emulsion liquid membrane (ELM). 

A recent study with biotechnology applications relates to amino acid extraction. Schugerl and co-workers (71 ) used a quaternary ammonium carrier in an emulsion liquid membrane system for enzyme catalyzed preparation of L-amino acids. Frankenfield et al. (72) discuss a wide variety of biomedical ELM applications including enzyme encapsulation, blood oxygenation, and treatment of chronic uremia. 

Hirai T, Orikoshi T, Komasawa 1. Preparation ofY203 Yb, Er infrared-to-visible conversion phosphor fine particles using an emulsion liquid membrane system. Chem Mater... 

Emulsion Liquid Membranes. The emulsion liquid membrane was invented by Li in 1968 (9, Chapter 2). Emulsion liquid membranes are prepared by dispersing an inner receiving phase in an inuniscible liquid membrane phase to form an emulsion. The preparation and transport mechanisms of ELMs are described in detail in the Chapter 15. The liquid membrane phase can be either aqueous or organic although the majority of work in the literature describes water-in-oil emulsions. Man and Kopp (27) created a set of quantitative guidelines for the formation of stable water-in-oil ELMs based on the surfactant HLB, surfactant concentration, organic viscosity, and volume ratios of the various phases. 

Draxler et al. describe the preparation and splitting of emulsions for emulsion liquid membrane systems in Chapter 7. An apparatus for the preparation of emulsions and various devices for the electrostatic splitting of emulsions are improved by investigation of the flow patterns using computational fluid field dynamics software. 

The preparation and splitting of emulsions are the key parameters in an emulsion liquid membrane process. A very stable emulsion which avoids any loss of emulsified droplets is a prior condition for the feasibility of the process. However, the more stable the emulsion, the more difficult to split it. So both steps are dependent on each other and have to be optimized, also with regard to cost optimization. In the present work, we try to calculate the flow pattern in the two steps using CFD (computational fluid dynamics) software in order to improve the design of the two steps. 

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. 

Draxler J, Weiss C, Marr R, and Rapaumbya GR. Preparation and spUtting of emulsions for Uquid membranes. In Bartsch RA, Way ID, eds. Chemical Separations with Liquid Membranes, Volume 642. Washington, DC American Chemical Society, ACS s3miposium series, 1996 103-114. 

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). 

In both purification and recovery applications the ELM must be demulsified into two immiscible phases after the extraction step of the process. This is commonly accomplished by heating, application of electric fields ( ), or centrifugation. The liquid membrane phase containing the surfactant and carrier will be recycled to the emulsion preparation step while the Internal phase of the emulsion containing the concentrated solute will undergo further purification in a recovery process or treatment and disposal in a purification process. Such a continuous process is shown in Figure 2. 

Various substances such as amino acids, organic acids, NaOH, NaCl, carbon dioxide, oxygen, metals, and various ions, such as Cd(II), Cu(II), Co(II), and Fe(III), can be separated by using suitable carrier agents in liquid or solid composite membranes. Liquid membranes behave like double liquid-liquid extraction systems where the usage of organic solvent is minimized. Such devices are generally prepared as bulk liquid, emulsion liquid, and supported liquid membranes. 

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