Cation liquid membranes

In supported liquid membranes, a chiral liquid is immobilized in the pores of a membrane by capillary and interfacial tension forces. The immobilized film can keep apart two miscible liquids that do not wet the porous membrane. Vaidya et al. [10] reported the effects of membrane type (structure and wettability) on the stability of solvents in the pores of the membrane. Examples of chiral separation by a supported liquid membrane are extraction of chiral ammonium cations by a supported (micro-porous polypropylene film) membrane [11] and the enantiomeric separation of propranolol (2) and bupranolol (3) by a nitrate membrane with a A/ -hexadecyl-L-hydroxy proline carrier [12]. 

Cholanic acid also possesses the ability of transporting cations across a lipophilic membrane but the selectivity is not observed because it contains no recognition sites for specific cations. In the basic region, monensin forms a lipophilic complex with Na+, which is the counter ion of the carboxylate, by taking a pseudo-cyclic structure based on the effective coordination of the polyether moiety. The lipophilic complex taken up in the liquid membrane is transferred to the active region by diffusion. In the acidic region, the sodium cation is released by the neutralization reaction. The cycle is completed by the reverse transport of the free carboxylic ionophore. 

Table 4. Amounts of cation transported by the synthetic ionophores through chloroform liquid membrane after 2 days...

On the other hand, Bartsch et al. have studied cation transports using crown ether carboxylic acids, which are ascertained to be effective and selective extractants for alkali metal and alkaline earth metal cations 33-42>. In a proton-driven passive transport system (HC1) using a chloroform liquid membrane, ionophore 31 selectively transports Li+, whereas 32-36 and 37 are effective for selective transport of Na+ and K+, respectively, corresponding to the compatible sizes of the ring cavity and the cation. By increasing the lipophilicity from 33 to 36, the transport rate is gradually... 

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

To evaluate the contribution of the SHG active oriented cation complexes to the ISE potential, the SHG responses were analyzed on the basis of a space-charge model [30,31]. This model, which was proposed to explain the permselectivity behavior of electrically neutral ionophore-based liquid membranes, assumes that a space charge region exists at the membrane boundary the primary function of lipophilic ionophores is to solubilize cations in the boundary region of the membrane, whereas hydrophilic counteranions are excluded from the membrane phase. Theoretical treatments of this model reported so far were essentially based on the assumption of a double-diffuse layer at the organic-aqueous solution interface and used a description of the diffuse double layer based on the classical Gouy-Chapman theory [31,34]. 

Figure 16(a) (O) shows the EMF responses of a 1,2-dichloroethane membrane containing anionic sites (KT/ C1PB). A Nernstian response was obtained. An SHG response to KCl was observed at activities of the latter above 10 M [Fig. 16(b), O]-These results can be interpreted in the same way as for ionophore-incorporated PVC liquid membranes, for which we have shown that the concentration of oriented cation complexes at the liquid-liquid interface can explain both the observed SHG signal and EMF response. The present SHG responses thus suggest primary ion concentration dependent charge separation at the interface of the 1,2-dichloroethane membranes incorporated with ionic sites. 

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. 

With the larger racemic cr-hydroxy-l-naphthaleneacetic acid too, extraction takes place in the presence of 1309] and the appropriate cations but enantiomeric differentiation is not observed. These results were confirmed in transport experiments in which the alkali mandelate is carried through a liquid membrane of [309] dissolved in chloroform. Lehn et al. (1978) explain these observations in terms of an ion pair included in the cavity of the crown ether. The reversal of chiral recognitions between potassium and cesium mandelate of 25% indicates that the structures of the two complexes are different. 

Scheme 2 Transport of alkali metal cations across liquid membranes using [1] as a...

There are two general types of liquid-membrane ISEs, namely one which involves liquid-phase ion exchange, with the response being selective to the anion or cation under scrutiny (generally polyvalent ions), while the other type involves... 

Other applications of supported liquid membranes have been related to metal speciation. For example, recently a system for chromium speciation has been developed based on the selective extraction and enrichment of anionic Cr(VI) and cationic Cr(III) species in two SLM units connected in series. Aliquat 336 and DEHPA were used respectively as carriers for the two species and graphite furnace atomic absorption spectrometry used for final metal determination. With this process, it was possible to determine chromium in its different oxidation states [103]. 

If a liquid membrane contains electrically neutral ligands, for example carrier antibiotics, that are capable of completely enveloping monovalent cations, then ... 

Fig. 11. Comparison of the electrochemical alkali ion selectivity of neutral antibiotics in liquid membranes (log for macrotetrolides (85), log K lM for valinomycin (86)) with the stability of the complexes (log K from Table 2) between these antibiotics and alkali cations in methanol...

For liquid membranes, the exchange reaction has to be modified insofar as every cation in the membrane, free or complexed, assumes the status of an individual particle (27). No detailed investigation is necessary, therefore, to recognize that the selectivity parameter Kyot is variable in this case, quite contrary to the definition in Eq. (16). 

If the liquid membrane contains carrier molecules that form 1 1 complexes with all cations, and if the concentration of the free ligand within the membrane is assumed to be constant, then Eq. (18) can be formulated as ... 

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... 

Figure 3. Schematic representations of models based on charge separation for membrane potential changes induced by host-guest complexation at liquid membrane surfaces, (a) Membrane potential change induced by a cationic guest, (b) Membrane potential change induced by an anionic guest (reproduced with permission of Elsevier Science Ltd. from Comprehensive Supramolecular Chemistry, 1996, Vol. 2,p. 176).

Figure 8. Potentiometric responses to 5 -CMP (o) and 5 -AMP ( ) by PVC matrix liquid membranes (DOP/PVC = 2 1 wt/wt) containing (a) lipophilic cation 9 (3.1 wt%) and (b) lipophilic cation 9 (3.1 wt%) plus neutral cytosine derivative 10a (1.3 wt%) (150 mol% 9 relative to 10a). Measured at pH 6.8 (0.1 M HEPES-NaOH buffer) (adapted with permission from Anal. Chim. Acta 1997, 341, 134. Copyright 1997 Elsevier Science Ltd.).

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