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

Abstract 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, ClO 4 − ), the rectification inversion is easily observed at a low concentration (5 m m ), 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 m m ), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl − ≥NO 3 − >BF 4 − >ClO 4 − >PF 6 − >Tf 2 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.