Membrane, liquid complexes

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. 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

In supported liquid membranes, a microporous support impregnated with the liquid complexing agent separates the feed and product solutions. In coupled... 

The analyte in the sample can be neutralized by adjusting the sample pH, or by adding an ion pairing or complexing agent in the sample or in the membrane liquid if the analytes happen to be permanently charged. Hence, the analytes in the sample diffuse through the membrane liquid, and, on the membrane/acceptor interface, are instantaneously trapped on the acceptor side by chemical means. 

In all liquid membrane extraction techniques, the membrane is an organic liquid, which is in contact with the aqueous sample. Analytes are extracted either by simple partitioning of uncharged species into the organic phase or by the action of some extractant, a compound present in the membrane liquid which can form complexes with the analyte, thereby facilitating its transport into the membrane liquid. So far, this is in principle the same as classical LLE. The difference between the various liquid membrane extraction techniques refers mainly to the acceptor side of the membrane. 

A possible mechanism for the transport of the extracted species across the manbrane can be based on Fickian diffusion of its complex or ion pair with the carrier through the membrane liquid phase. Another mechanism that can be responsible for the bulk membrane transport of the extracted species is the so-called chained carrier mechanism proposed by Cussler et al. [36]. This mechanism applies to ion-exchange membranes with ionic sites covalently bound to... 

This kind of interface is found in some specific electrodes with a liquid (or polymer) membrane containing complexes. Valinomycin is a complexing compound which is selective to K ions. It is used in membranes of selective electrodes based on PVC. 

Key words wastewater, membrane operations, supported liquid membranes, pervaporation, membrane bioreactors, complexation... 

The prosthetic groups of many membrane enzyme systems contain metalloporphyrins. Therefore, it is not surprising that an attempt has been made to reveal and study the catalytic effect of porphyrins during redox processes at the interface between immiscible liquids. Complexes of different metals with different porphyrins exhibited catalytic activity in the oil/water system [50]. First of all, let us consider the transformations of metal complexes of porphyrins in the octane/water system. In wet octane, as a result of the hydrolysis and dimerization reactions, FeEP changes to a //-complex and this results in a change in the absorption spectrum (Fig. 5). With a strong acidification of the aqueous phase, the equihbrium of Eqs. (12) and (13) shifts to the left and at pH 1 the positions of the maxima on the absorption spectra of ethioporphyrin in dry and wet octane coincide. 

The general types of liquid membranes are reviewed by Peterson and Lamb in Chapter 4, and factors which influence the effectiveness of a membrane separation system are summarized. These factors include the complexation/decomplexation kinetics, membrane thickness, complex diffusivity, anion type, solvent type, and the use of ionic additives. A novel membrane type, the polymeric inclusion membrane, is introduced. 

The strain tensor must conform to the symmetry of the liquid crystal phase, and as a result, for nonpolar, nonchiral uniaxial phases there are ten nonzero components of kij, of which four are independent ( i i, 22> A 33 and 24)- These material constants are known as torsional elastic constants for splay (k, 1), twist ( 22) bend ( 33) and saddle-splay ( 24) terms in 24 do not contribute to the free energy for configurations in which the director is constant within a plane, or parallel to a plane. The simplest torsional strains considered for liquid crystals are one dimensional, and so neglect of 24 is reasonable, but for more complex director configurations and at surfaces, k24 can contribute to the free energy [7]. In particular 24 is important for curved interfaces of liquid crystals, and so must be included in the description of lyotropic and membrane liquid crystals [8]. Evaluation of Eq. (16) making the stated assumptions, leads to [9] ... 

The third liquid-membrane technique is the hollow fiber contained liquid membrane (HFCLM). In the SLM/ ILM technique, the liquid membrane is in contact with the feed liquid/feed gas and the strip liquid/permeate gas. The membrane liquid may be lost by solubilization/volatiliza-tion in addition, there may be irreversible reactions with extractants, complexing agents inside the liquid membrane, which could reduce the performance over time. The HFCLM structure can take care of such problems. In this structure (Figure 8.1.50), the membrane module (cylindrical or otherwise) has two sets of porous hollow fiber membranes. The shell-side interstitial space between the fibers is filled with a liquid acting as the membrane. If this membrane liquid wets the membrane pores and is therefore present in the pores, the pressure conditions in the feed liquid and the sweep/strip fluid should be such (higher) that the membrane liquid is not dispersed in the feed/sweep/stilp fluids. If the membrane liquid does not wet the membrane pores, but the feed/sweep fluids do, then the membrane liquid pressure should be higher than the feed/sweep fluid pressures. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

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