Anion selectivity at the aqueous/polymeric membrane interface: A streaming current study of potentiometric Hofmeister effect

The anion-selectivity of model polymeric membranes as used in potentiometric sensors (ion-selective electrodes, ISE) was studied using ζ-potential measurements. For this purpose the streaming current measurements of plasticized poly(vinyl chloride) were performed by pushing an aqueous electrolyte through a thin channel with walls lined with the polymer. Despite the literature reports on accumulation of plasticizer on the PVC membrane surface, no significant effect of plasticizer on the ζ-potential vs pH curve was observed. A cationic surfactant, cetyltrimethylammonium bromide (CTAB) was chosen as a water-soluble analogue of tetraalkylammonium salts used in anion-selective ISE displaying a characteristic Hofmeister selectivity pattern. The effect of anions on ζ-potential was studied at fixed concentration of CTAB (10−4 mol L−1 ≈ 0.1 cmc), by varying the concentration and nature of anion in the added 1:1 electrolytes (NaCl, NaBr, NaF, NaClO4, NaI and NaNO3). The anions chosen cover the whole Hofmeister series, and their influence on the measured ζ-potential parallels that of the Hofmeister effect in potentiometry, as well as in colloid/interface science. A significant deviation was observed only for fluoride ions. The effect of anion selectivity on ζ-potential is clearly notable, but rather weak, due to a large distance between the location of the Stern layer (where the ion-specific interactions take place), and that of the hydrodynamic slip plane (where the ζ-potential is actually measured).

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► Hofmeister effects at polymer–water interface are studied with zeta potential.
► Streaming current measurements at plasticized PVC–aqueous interface were performed.
► Cetyltrimethylammonium bromide (CTAB) was used as a water soluble analogue of lipophilic salts in ISE.
► Plasticized PVC behaves hydrodynamically as a hard material.
► Anion-specific effects are observable in zeta potential.