On the origin of the Hofmeister effect in anion-selective potentiometric electrodes with tetraalkylammonium salts

A new, adsorption-based mechanism of potentiometric response of the plasticized PVC membranes is proposed. In contrast to previous mechanistic studies of the membrane-based Ion-Selective Electrodes (ISE), the membranes in this study did not contain any purposely-added additives: lipophilic salt or ionophore. Cationic or anionic response of such “blank” ISEs was observed upon contact with ionic surfactants: cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS), respectively. Such behavior cannot be explained using a classical Interface between Two Immiscible Electrolyte Solutions (ITIES) formalism, based on the Nernst equation for ion partitioning between the aqueous and the membrane phases. Moreover, the “blank” membranes responded anionically (with a negative slope) to the presence of sodium salts of bromide, nitrate and fluoride at constant concentration of CTAB. Also in this case, the theoretical Galvani potential difference vs. added salt concentration, calculated with the ITIES formalism did not fit well the measured potential differences. Better agreement was obtained when assuming that the potentiometric response stems from partitioning between the aqueous phase and the interface (adsorption), and not between the aqueous phase and the membrane phase (bulk partition), as in the classical approach. The Stern layer potentials were calculated from interfacial tension isotherms for CTAB, both alone and in the presence of sodium bromide, nitrate and fluoride using the Surface quasi-Two Dimensional Electrolyte model (STDE). The calculated Stern potentials compare well with those obtained experimentally using ISE.