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 (σek ) 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.