Ionic site additives

This type of initiation is known as monofunctional initiation as it produces one active (ionic) site for propagation. Propagation takes place by the addition of monomer to the ionic site ... 

On the basis of this mechanism, a great number of ionophore-based ISEs were developed. It should also he noted here that polyvinyl chloride (PVC) has been used since the early 1970s to support liquid membranes, and the term plasticized liquid membrane" has been coined to describe these systems. In those days, the addition of ionic sites was not explicitly realized, but instead, impure ionic sites in the PVC (anionic sites) played virtually the same role. However, the membrane including such an impurity does not allow quantitative control of the optimized amounts of ionic sites, and therefore, it was supplemented later by deliberately added ionic sites. 

Eaker and Porath studied the elution of amino acids on Sephadex 6-10 with particular focus on the eluant ionic strength and composition (e.g. acetic acid or pyridine content) (11). They described the gel as a "weak exchanger," thus accounting for effective separations of mixtures of up to seven amino acids. They noted that K ec increases with ionic strength, and attributed this effect to variations in the "effective size" of the ion, which includes the "electrical double layer and the hydration layer." In addition to discussing the effects of ionic sites on gel and solute, Eaker and Porath also discussed the "aromatic adsorption" of solutes with coplanar II electron systems, and the hydrophobic adsorption of partly apolar solutes (accompanied by a positive enthalpy change). They pointed out that such "short range" effects require "intimate contact" between solute and gel and are unlikely to occur in the presence of electrical repulsion. 

For ionomeric systems in which the strong interactions between ionic sites and the penetrant (water) result in concentration dependent diffusion coefficients, the Fickian diffusion model, which assumes the solubility coefficient is independent of the concentration, is not valid. The commonly used Fickian diffusion constants actually contain mobility and solubility gradient contributions [19], In addition, the concentration dependent solubilities lead to nonlinear concentration profiles during steady state diffusion. Therefore, mobility measurements which generate average diffusion coefficients are generally not satisfactory. 

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). 

Facilitated transport of ethylene through Ag -containing perfluorosulfonic add ion-exchange membranes results in high separation factors for ethylene over ethane. Ethylene of higher than 99 percent purity was obtained from a 50 50 mbcture of ethylene and ethane at 25 T and feed and permeate pressures of 1 atmosphere. An ethylene permeability of over 2000 Barrer was obtained. Two different types of perfluorosulfonic acid membranes were studied to determine the importance of ionic site dendty and water content on membrane performance. The effect of temperature on the transport mechanism was also studied over the range of 5-35 ""C. In addition, the transport data obtained at high pressure show carrier-saturation and membrane compaction phenomena. 

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