Membrane transport crown ether carriers

Light-driven membrane transport. Cations may be transported through liquid membranes using crown ethers. For example, a typical system is of the type water-phase(I)/organic-phase/water-phase(II). The metal ion is added to water-phase(I) and the crown ether to the organic phase (to yield the liquid membrane). The crown acts as carrier for metal ions from water-phase(I) across the liquid membrane phase into water-phase(II). There have now been a very large number of studies of this type reported and a fuller discussion of this topic is given in Chapter 9. 

Lamb, J. D. Christensen, J. J. Izatt, S. R. Bedke, K. Astin, M. S. Izatt, R. M. "Effects of Salt Concentration and Anion on the Rate of Carrier-Facilitated Transport of Metal Cations through Bulk Liquid Membranes Containing Crown Ethers" J. Am. Chem. Soc., 1980 102 (10), p. 3399. 

In Chapter 3, P. Dzygiel and P. Wieezorek survey the applications of supported hquid membranes and their modifications (gel, polymer inclusion SLMs, integrated systems) in separations of metal ions, organics, gases, and contaminants in wastewater, in biochemical and biomedical processing. Choices of membrane support material, carriers and solvents which improve the transport kinetics and membrane stabihty in SLM system are discussed. The use of novel calix-his-crown ether carriers shows the potential for large-scale utilization in the future. 

The results provided above demonstrate that considerable selectivity can be achieved in alkali metal transport across bulk chloroform membranes by appropriately-structured ionizable crown ether carrier molecules. Although such bulk liquid membrane... 

J.D.Lamb, J.J.Christensen, S.R.Izatt, K.Bedke, M.S.Astin, R.M.Izatt, Effects of Salt Concentration and Anion on the Rate of Carrier Facilitated Transport of Metal Cations Through Bulk Liquid Membrane Containing Crown Ethers, J.Am.Chem.Soc., 102, 3399 (1980) H.Tsukube, Effects of Cation on Transport Efficiency and Selectivity of Amino Acid Derivative Anions, Bull.Chem.Soc.Jpn.,... 

Figure 12 illustrates an anion transport system with a lanthanide tris(p-diketonate) as the carrier. When the lipophilic lanthanide complex is present in Membrane, a highly coordinated complex is formed with the anion guest at the interface between Aq. I and Membrane, and K(I) cation is extracted into Membrane as the counter-cation. The resulting ternary complex moves across Membrane. At the interface between Membrane and Aq. II, the guest anion is released into Aq. II together with its counter-cation. Crown ether carrier mediates anion transport in a In Chemical Separations with Liquid Membranes Bartsch, R., et al. ... 

Addition of a chiral carrier can improve the enantioselective transport through the membrane by preferentially forming a complex with one enantiomer. Typically, chiral selectors such as cyclodextrins (e.g. (4)) and crown ethers (e.g. (5) [21]) are applied. Due to the apolar character of the inner surface and the hydrophilic external surface of cyclodextrins, these molecules are able to transport apolar compounds through an aqueous phase to an organic phase, whereas the opposite mechanism is valid for crown ethers. 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

The ability of calixarenes to bind large metal ions with high kinetic stability is important in the search for complexants for radionuclides such as Cs (ti/2 = 30.2 yr) and Sr (ti/2 = 65 d) from the reprocessing of exhausted nuclear fuel. There has been considerable interest in caesium-complexed calix[4]-bis-crowns as selective Cs-carriers. Transport isotherms of trace level Cs through supported liquid membranes containing calix[4]-bis-crowns have been determined as a function of the ionic concentration of the aqueous feeder solutions, and l,3-calix[4]-bis-o-benzo-crown-6 appears to be much more efficient in decontamination than mixtures of crown ethers and acidic exchangers, especially in highly acidic media. " ... 

It can be concluded that stereoselective (and particularly enantioselective) separation can proceed by the simple apphcation of a chiral organic phase or in most cases by incorporation of carriers in the membrane phase. Sometimes, their structure is very complex and these molecules can act as real receptors for the enantiomers. Different types of transport mechanisms are involved in the separation and the most popular one is cotransport. This is a result of the fact that most frequently used carriers are based on crown ethers structure. The stereoselectivities by apphcation of carrier-mediated SLM separation are very different and depend on the structure of the guest and host molecules. The magnitudes of the stereoselectivity are in most cases moderate but similar to other membrane-based separation techniques for stereoisomers. 

Also, Reinhoudt and co-workers reported the data on guanidinium (thiocyanate) transport with various crowns through bulk [43] and supported [44] liquid membranes. They developed corresponding mathematical models and found that not only the host-guest complexation constant but also the lipophilicity of the host per se control ionophoric properties. Omitting mathematics, the natural reason is the possibility of carrier leakage from the membrane phase (see related work on partition coefficients and their increments for crown ethers, K-octanol/water [127]). This interplay leads, for example, to better ionophoric properties of more lipophilic... 

Carrier Chemistry. The use of structurally modified macrocycllc polyethers (crown ethers) as CcU rlers In bulk, emulsion, and Immobilized liquid membranes Is the subject of the chapter by Bartsch et al. (111). They discuss the use of lonlzable crown ethers for the coupled transport of alkali metal cations. The lonlzable carboxylic and phosphonlc acid groups on the macrocycles eliminate the need for an anion to accompany the catlon-macrocycle complex across the liquid membrcuie or for an auxiliary complexlng agent In the receiving phase. The influence of carrier structure on the selectivity and performance of competitive alkali metal transport across several kinds of liquid membranes Is presented. 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

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