Ligands ionophores

Polymers with low glass transition do not require plasticizers. However, these compounds are often unpolar (Table 4) and, consequently, they are unsatisfactory solvents for polar ligands, ionophores, dyes and analytes. 

How can we create such a membrane for a wider range of analytes The most successful approach is to use ion-selective liquid membranes (2, 3). The liquid membranes are hydrophobic and immiscible with water, and most commonly made of plasticized poly(vinyl chloride). The selectivity is achieved by doping the membranes with a hydrophobic ion (ionic site) and a hydrophobic ligand (ionophore or carrier) that selectively and reversibly forms complexes with the analyte (Figure 7.1). Whereas the technique has been well established experimentally since the 1960s, it is only recently that the response mechanisms are fully understood. In this chapter, principles of liquid membrane ISEs will be introduced using simple concepts of ion-transfer equilibrium at water/liquid membrane interfaces. Non-equilibrium effects on the selectivity and detection limits will also be discussed. This information will enable practitioners of ISEs to better optimize experimental conditions and also to interpret data. Additionally, examples of ISEs based on commercially available ionophores are listed. More comprehensive lists of ionophore-based ISEs developed so far are available in recent lUPAC reports (4-6). 

Heterocycles as ligands in ionophores for potentiometric and optical sensors 98CRV1593. 

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,... 

Lee, S. S. Yoon, I. Park, K.-M. Jung, J. H. Lindoy, L. F. Nezhadali, A. Rounaghi, G. Competitive bulk membrane transport and solvent extraction of transition and post transition metal ions using mixed-donor acyclic ligands as ionophores. J. Chem. Soc.-Dalton Trans. 2002, 2180-2184. 

Lachowicz, E., Rozanska, B., Teixidor, F., Meliani, H., Barboiu, M. and Hovnanian, N. (2002) Comparison of sulphur and sulphur-oxygen ligands as ionophores for liquid-liquid extraction and facilitated transport. Journal of Membrane Science, 210, 279—290. 

Most indicator chemistry is adapted to aqueous solution (for titration in water). Therefore, the molecules are water-soluble and if dissolved in lipophilic polymers, they are washed out immediately. In order to make dyes, ionophores and ligands soluble in polymers and to avoid leaching of the components into the sample solution, they have to be made lipophilic6. 

L neutral ligand C neutral chromo-ionophore R- lipophilic anionic site I analyte cation... 

L neutral ligand C- charged chromo-ionophore R+ lipophilic cationic site X- analyte anion... 

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. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Some synthetic ionophores form complexes with metallic ions containing a varying number of ligands per metal ion, usually 1 or 2. It then holds for the overall concentration of ion J in the membrane that... 

If the ligand (neutral carrier) is capable of completely enveloping the cations, the terms Ky and Eq of the membrane depend only on the ratio of the complex stability constants K JK of the ions with the ionophore. 

Mixed donor macrocycles have been employed in a number of applications involving the separation or analysis of manganesetll). These include examples of use of such a ligand as the extractant in solvent extraction processes " and as the ionophore in membrane transport studies. 

---