Novel liquid crystalline materials for potentiometric analysis
The aim of this project was to investigate a range of novel liquid crystalline pyrazine based compounds as potential neutral ionophores, for use in ion selective electrodes and to investigate the effect of their liquid crystal properties on ionophoric behaviour. The ionophores were initially incorporated into poly (vinyl chloride) matrix membranes and tested for their analytical usefulness as ion-selective electrodes, for the potentiometric response of a range of monovalent and divalent cations. Two series of electrodes were fabricated, Series A and Series B. Series A incorporated o- nitrophenyl octyl ether as membrane plasticiser, which is used extensively in PVC membrane ion-selective electrodes commercially and for research purposes. The potentiometric results found the ionophore 2-n-hexyloxy-5-(4-bromophenyl)pyrazine to exhibit a near-Nernstian response for caesium ions over a concentration range of 10[sup]-1-10[sup]-5M. Selectivity coefficient values suggest this electrode is more responsive towards caesium ions in the presence of interfering ions, but this electrode is not comparable with or better than the current commercially available one which incorporates calix arenehexakis( carboxymethylether )-hexakis( ethylester ) as ionophoric material. The Series B set of electrodes incorporated a liquid crystal mixture, known commercially as E7, as a membrane placticiser. This was also used to generate an ionophoric system in the liquid crystal phase at room temperature. E7 or any liquid crystal material has not been used as plasticiser or ionophoric material for use in ion-selective electrodes to date. The potentiometric results found the electrode, incorporating the ionophore 2-n-hexyloxy-5-(4- bromophenyl)pyrazine, to exhibit a near-Nernstian response of 55.18mV decade[sup]-1 over a concentration range 10[sup]-1-10[sup]-5M. The selectivity of this electrode was much improved in comparison to that from Series A and was comparable with the current commercially available one. By changing the internal reference solution from 10[sup]-1M caesium chloride solution to 10[sup]-1M ammonium chloride solution this electrode can be used as an ammonium sensor in the absence of caesium ions. The influence of the liquid crystal properties of the selected pyrazine ionophore on the potentiometric behaviour, was firstly attempted by immersing the electrode in a heated sample of the test solution. and then allowing the solution to cool to the ionophores liquid crystal monotropic smectic A phase temperature. With the high temperatures involved it was found the PVC membrane blistered. Secondly, with the influence of E7 exhibiting plasticising properties, and showing a nematic liquid crystal phase at room temperature, it was possible to overwhelm the pyrazine ionophore with E7 resulting in converting the smectic A phase to a nematic mixture at room temperature. The added advantage of using E7 is that the nematic phase is easily influenced by the application of an external electric field. This would enable to electronically reorientate the molecular structure and hence see what influence this would have on the performance of the electrode. A number of alternative polymer matrices were investigated to see which allowed the best dispersion of E7 and the ionophore, as PVC did not show a response to the molecular switching at low voltages. Norland Optical Adhesive 65, a photocurable polymer, and photopolymerised decyl methacrylate both provided good matrices for the dispersion of E7 and the ionophore with switching occurring at low voltages.