Thin liquid membrane

Membrane Extraction. An extraction lechnique which uses a thin liquid membrane or film has been introduced. The principal advantages of liquid-membrane extraction are that the inventory of solvent and extractant is extremely small and the specific inlerfacial area can be increased without the problems which accompany fine drop dispersions. 

An alternative method for producing thin liquid membranes is to separate the source and receiving aqueous phases with an organic liquid which is held within the pores of an inert porous polymer. 

The second problem with immobilized liquid membranes is that they are relatively thidr (25-150 1) conqnied to ultra-thin supported polymer membranes (0.1-2 p). Since flux is inversely proportional to membrane Uiickness, this reduces the total flux achievable. Although methods of fabricating suppcnled ultra-thin polymer membranes has been known for some time, there has been no method of preparing similar ultra-thin liquid membranes. 

In the following two sections approaches for overcoming the limitations of immobilized liquid membranes are discussed In the first section a method of reducing the evaporation of the liquid phase via the Kelvin effect is investigated. The second section reports on methods to fabricate suppwted ultra-thin liquid membranes. 

An Example of an Ultra-Thin Liquid Membrane. An idtra-thin version of the aqueous/IiBr membrane was developed. The membrane was fabricated by floating a 47 mm diameter asymmetric cellulose acetate membrane (Amicon) on a 6.9 M LiBr solution for 24 hours. The membrane was then exposed to a vacuum for S hours to remove the water while leaving the lithium bromide. It was then exposed to a nitrogen/oxygen mixture containing approximately 10 mmHg water vapor until the pores filled. 

Ultra-thin liquid membranes can be fabricated down to 1 pL in thickness. Ultra-thin liquid membranes can be formed in the skin layer of an porous asymmetric polymer membrane by methods in which the liquid is selectively deposited in the skin rather than the backing support. The wide variety of pore sizes and membrane configurations available in asymmetric membranes allows for good flexibility in the design of ultra-thin liquid membrane systems. 

Thin liquid membrane - Fiber bore has a high vacuum... 

As the main disadvantage of liquid membrane systems is the instability over a longer period of time, another approach would be to perform separation through a solid membrane [22]. Enantioselective polymer membranes typically consist of a nonse-lective porous support coated with a thin layer of an enantioselective polymer. This... 

ILs, on the other hand, are uniquely suited for use as solvents for gas separations. Since they are non-volatile, they cannot evaporate to cause contamination of the gas stream. This is important when selective solvents are used in conventional absorbers, or when they are used in supported liquid membranes. For conventional absorbers, the ability to separate one gas from another depends entirely on the relative solubilities (ratio of Henry s law constants) of the gases. In addition, ILs are particularly promising for supported liquid membranes, because they have the potential to be incredibly stable. Supported liquid membranes that incorporate conventional liquids eventually deteriorate because the liquid slowly evaporates. Moreover, this finite evaporation rate limits how thin one can make the membrane. This... 

Foams consist of bubbles separated from each other by thin—liquid or solid—membranes. The bubbles must have no connection with their neighbors if such a connection exists—i.e., both phases (the gaseous and the condensed) are continuous—the system is a sponge rather than a foam. Bread is the most important kind of food that is chiefly sponge and a little foam. 

A high specific interfacial area and a direct spectroscopic observation of the interface were attained by the centrifugal liquid membrane (CLM) method shown in Fig. 2. A two-phase system of about 100/rL in each volume is introduced into a cylindrical glass cell with a diameter of 19 mm. The cell is rotated at a speed of 5000-10,000 rpm. By this procedure, a two-phase liquid membrane with a thickness of 50-100 fim. is produced inside the cell wall which attains the specific interfacial area over 100 cm. UV/VIS spectrometry, spectro-fluorometry, and other spectroscopic methods can be used for the measurement of the interfacial species and its concentration as well as those in the thin bulk phases. This is an excellent method for determining interfacial reaction rates on the order of seconds. 

The voltammetric method and concept are expected to be applicable also to the analysis of the oscillation at a very thin membrane such as a bilayer lipid membrane, since even the ion transfer through a bilayer lipid membrane can be observed as a vol-tammogram and interpreted by the way similar to that for a liquid membrane [20,21]. 

Farago and Chigier 2l() found that at similar aerodynamic Weber numbers, the disintegration modes of a thin liquid sheet in air streams are similar to those of a round liquid jet in a coaxial air stream (Table 3.2). At high aerodynamic Weber numbers, Membrane-Type or Fiber-Type breakup mode may set in. 

As discussed by Frankemfeld and Li(28) and del Cerro and Boey(29), liquid membrane extraction 28,29) involves the transport of solutes across thin layers of liquid interposed between two otherwise miscible liquid phases. There are two types of liquid membranes, emulsion liquid membranes (ELM) and supported liquid membranes (SLM). They are conceptually similar, but substantially different in their engineering. 

Solvents used in liquid membranes should have special characteristics such as low aqueous solubility, as a thin film of solvent is in contact with large volumes of aqueous solutions, and low viscosity to provide large diffusion coefficients in the liquid membrane. Furthermore, the analyte should have large partition coefficients between the donor and the membrane phase to give good extraction recovery and, at the same time, interfering substances in the sample should have low partition coefficients for efficient cleanup. 

The formulation of the three phases must be such that the liquid membrane extracts the solute from one of the phases and the third phase strips it from the membrane. Thus extraction and stripping take place in the same contactor, and the stripping phase is where the solute is accumulated, instead of the organic phase as in the case of conventional solvent extraction. This allows for a middle phase of small volume that, being thin, behaves like a membrane. 

A glass electrode which takes advantage of rapid and reproducible ion exchange across a glass membrane located as a thin-walled bulb at the tip of the electrode. This membrane has the following liquid-membrane junction ... 

Reusch and Cussler (48) have developed liquid membranes across which diffusion is highly selective. They suggest that these membranes may form the basis for industrial cation separations. The liquid membranes used consist of a thin layer of organic liquid separating two aqueous electrolyte solutions. Under ordinary conditions, the movement of electrolytes across the membranes would be extremely small because... 

The method of impregnating liquid membranes has become more and more popular. By impregnating fine-pore polymer films with a suitable membrane liquid, relatively stable heterogeneous solid-liquid membranes are obtained. These membranes are shaped as thin, flat barriers or hollow fibers. Usually they are manufactured from oleophilic polymers, wettable by membrane liquid. The two interfaces, F/M and M/R, have equal or close areas which can be made very large by employing modules of spirally wounded flat membrane or bundles of hollow fibers. 

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