Chaotropic Monovalent Anion-Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

A nonintuitive observation of monovalent anion-induced ion current rectification inversion at polyimidazolium brush (PimB)-modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO₄^- ), the rectification inversion is easily observed at a low concentration (5 mm), while there is no rectification inversion observed for kosmotropic anions (Cl^- ) even at a high concentration (1 m). Moreover, at the specific concentration (for example, 10 mm), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl^- ≥NO₃^- >BF₄^- >ClO₄^- >PF₆^- >Tf₂ N^- ). Estimation of the electrokinetic charge density (σ^ek ) demonstrates that rectification inversion originates from the charge inversion owing to the over-adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration-dependent adsorption mechanism is proposed.