Potentiometric properties and impedance spectroscopic data of poly(vinyl chloride) membranes containing quaternary ammonium salts of different chemical structure Original Research Article

The influence of the chemical structure of symmetrical and asymmetrical quaternary ammonium salts (QASs) on the potentiometric and impedance spectroscopic data of ion-selective poly(vinyl chloride) membranes plasticized with 2-nitrophenyl octyl ether (NPOE) and bis(2-ethylhexyl) sebacate (DOS) was investigated. It was found that the properties of the QAS, e.g. solubility in the membrane phase and, thus, the difference in the degree of association, influences the membrane characteristics. An increasing number of carbon atoms in symmetrical QASs diminishes the interference of lipophilic anions and enlarges the sensitivity to basic ones. The choice of plasticizer is important for the membrane properties as well, e.g. the influence of lipophilic ions is smaller for DOS membranes in comparison to NPOE ones. Results are discussed from the viewpoint of the difference in free enthalpy of hydration and basic properties of the anions. Additionally, the bulk properties of the membranes in dependence of QAS structure were studied by electrochemical impedance spectroscopy. The magnitude of the specific bulk resistance is governed by the dielectric properties of the plasticizer. Nevertheless, it is shown that QAS structure determines the long-term development of the bulk resistance.