The effect of lipophilic salts on surface charge in polymeric ion-selective electrodes

Abstract

The effect of quaternary ammonium salts (QAS) structure and concentration in polymeric membranes on their performance in potentiometric Ion Selective Electrodes (ISEs) without ionophore is studied. The membranes with both symmetric: tetraoctylammonium bromide (TOAB), tetradodecylammonium bromide (TDDAB), tetraoctadecylammonium bromide (TODAB), and asymmetric: tridodecylmethylammonium chloride (TDMAC) and dimethyldioctadecylammonium chloride (DODMAC) quaternary ammonium salts were tested in very broad concentration range (1–10 −4%). The observed ISEs responses could not be explained within the framework of the Phase Boundary Model, but could be easily understood semi-qualitatively with help of the simple Gouy–Chapman electrical double layer theory. The latter provides a straightforward link between the electrical potential on the membrane's surface and the corresponding surface charge density. Assuming that the electrical potential difference measured in ISEs originates from adsorption of surface active ions (tetraalkylammonium cations, QA +) and co-adsorption of anions present in the sample, the surface charge density was calculated using the Grahame equation. We postulate that the positive charge provided by adsorption of QA + at the membrane's surface provides a constant positive charge density, dependent on the QAS surface activity and concentration. This positive charge is partially neutralised by anions originating either from QAS in the membrane, or from inorganic electrolyte in the contacting aqueous phase (sample). This process could explain the dependence of electrical potential difference measured in ISEs on the concentration and type of both QAS and inorganic anions present in the sample (the latter known as “Hofmeister pattern” in potentiometry). Also the pH sensitivity of ISEs with low-QAS content membranes can be explained in the framework of the proposed model by a competition for adsorption sites on the membrane surface between QAS and high surface activity of hydroxide ions.