FTIR-ATR spectroscopic analysis of bis-crown ether based PVC-membranes

Summary

In order to get more information about the complexation reaction between potassium-selective electrode membranes and aqueous solutions of KCl, KSCN, NaCl, NaSCN, NH₄Cl and NH₄SCN, model membranes have been analyzed by the molecular-specific surface technique Attenuated Total Reflection (ATR) in the infrared region, thus providing an information depth between 0.1 and 3 μm depending on the angle of incidence of the radiation. The membranes have been prepared by dissolving PVC, dioctylsebacate and crown ether BME-44 in differing amounts in THF and casting in a glass ring on a glass plate. It has been found that DOS is slightly enriched at the surface but the membranes are sufficiently stable. It has further been found that complex formation occurs in the surface region for SCN⁻ as well as Cl⁻. Pretreated and subsequently rinsed samples have shown that Cl⁻ is almost totally washed away. But in the case of the lipophilic SCN⁻¹ anion, this diffuses into the bulk; rinsing results in a loss at the near-surface region only. A special diffusion experiment, in which only one side of the membrane was in contact with the solution, has shown that diffusion of KSCN through the membrane occurs at a very low rate with a diffusion constant of appr. 10⁻⁸ cm²/s.

For better understanding of the transport phenomena in situ measurements will be carried out.