Ionic Liquids Transport through Single Nanopores

Room temperature ionic liquids are media consisting only of ions and are liquids at temperatures below 100 C. Constituent ions of ionic liquids are bulky and reach a size of 1 nm and larger. The finite size of the ions, which often is comparable to the diameter of the nanopores through which they get transported, makes application of classical electrochemical and electrostatic theories questionable. It is because the existing continuum theories for simplicity treat ions as point charges. A similar situation of a tight fit between transporting ions and pore diameter exists in the biological channels of a cell membrane. Using single nanopores prepared in polyethylene terephthalate (PET) by the track-etching technique, we investigated how ion current through nanopores and screening of surface charges on the pore walls were influenced by the size of the transported ions. Experiments were performed with aqueous solutions of KCl and the ionic liquids 1-Butyl-3-methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium chloride. The screening was evaluated using the rectification properties of homogeneously charged nanopores, as well as pores with a surface charge pattern that enables them to function as ionic diodes. Interactions of ions with surface charges were also described using reversal potential measurements in conjunction with Goldman Katz theory. Our experiments indicate that aqueous solution of ionic liquids screen surface charges over smaller distances than KCl.