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The objective of the project has been to study ionic conductivity and capacitance of ionic liquid systems on a nanoscale. Ionic liquids – media that consist solely of anions and cations - exhibit unique properties that make them attractive materials for a variety of applications. They are thermally stable and do not have measurable vapor pressure, while other properties, such as viscosity, and electrochemical window size, can be tuned by the choice of ions. Ionic liquids are applied as non-volatile plasticizers, thermal and hydraulic fluids, lubricants, electrochemical devices as well as novel media in various reactions e.g. Friedel-Crafts acylation and alkylation, Diels-Alder reactions and many others. Ionic conductivity is one of the most important physical properties that determine applicability of ionic liquids in a number of laboratory and industrial processes. Many ionic liquids have been characterized in terms of their bulk ionic conductivities but to the best of our knowledge, ionic conductivity in restricted geometries such as nanopores has not yet been studied. In the project we examined ionic conductivity of ionic liquids restricted in single nanopores of known geometry and pore walls characteristics. We focused on studying how the pore diameter and chemistry of the pore walls influence transport of ionic liquids through nanopores.

Below we summarize the most important findings.

Finding 1: Ionic conductivity of ionic liquids in nanopores depends on the pore diameter.

Transport properties of ionic liquids in nanopores were studied in the conductivity cell in which a membrane containing a single pore separated two chambers filled with a given ionic liquid under study. A series of single pores with varying diameter between 5 nm and 400 nm was studied.

All experiments were performed in an argon atmosphere due to a high sensitivity of transport properties of ionic liquids to humidity. A Keithely 6487 picoammeter/voltage source was used for the current recordings. The current that was recorded in this system originated from the migration of anions and cations of a studied ionic liquid in electric field. Because the resistance of a single pore is significantly higher than the resistance of the bulk solution, the measured current was a direct measure of ionic transport through the nanopore without the contribution of the electrophoretic migration between the pore entrance and the electrodes.

The conductivity of ionic liquids was found to be a strong function of the pore opening. For pores with diameters less than 20 nm, the ionic conductivity was several times lower than the values for larger pores and the bulk solution. As an example, ionic conductivity of 1 - butyl – 3 methylimidazolium methyl sulfate [BMIM][CH₃SO₄] in the 5 nm pore was determined to be 0.03 S/m, while the conductivity in larger pores as well as in the bulk equaled 0.10 S/m.

A similar dependence of ionic conductivity on a pore diameter was found for 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethyl sulfate [BMIM][C₅H₁₁O₂SO₄]. The bulk ionic conductivity of this ionic liquid is 0.051 S/m as measured in a 370 nm diameter pore - approximately half the bulk value of conductivity for [BMIM][CH₃SO₄] (0.1 S/m). These two ionic liquids have the same cation but [BMIM][C₅H₁₁O₂SO₄] has a significantly larger anion. The dependence of ionic conductivity for this ionic liquid on pore diameter was also more pronounced, most probably due to the larger size of the anion. The conductivity value for [BMIM][C₅H₁₁O₂SO₄] in a 16 nm pore was found to be 8 times smaller than its corresponding bulk value.

The dependence of ionic conductivity on pore diameter points to the importance of steric interactions of constituent ions with the pore walls.

Finding 2: Ion currents carried by ionic liquids in nanopores with a diameter < 10 nm are rectified. Transport of ionic liquids was also studied in conically shaped nanopores with surface charges. Conical nanopores with negative surface charges transport primarily cations with the preferential direction of the flow from the small opening to the big opening of the cone. This feature is observed in current-voltage curves as ion current rectification and occurs only if the ions are influenced by the surface charges, or in other words if the thickness of the electrical double-layer in a given solution is comparable to the pore diameter. Checking for ion current rectification is therefore an indirect way of studying the thickness of the electrical double-layer. The structure of the electrical double-layer is very important for applications of ionic liquids in building capacitors, batteries, and fuel cells.

We observed rectifying current-voltage curves in nanopores with diameters equal or less than 10 nanometers. We also found that the rectification properties can be enhanced by formation of a surface pattern between a zone with negative charges and a zone with neutral walls. This system worked like a diode for ionic liquids transporting ions in one direction and blocking them in the other. It is the first ionic diode for ionic liquids. These experiments also indicate that the thickness of the electrical double-layer for ionic liquids is significantly larger than it is predicted from classical electrochemical theories.

The graduate student performing the experiments learned methods for characterization purity of ionic liquids as well as electrochemical techniques to study transport properties of ionic liquids. He also built the glove box where the experiments had to be performed, and designed an experimental set-up for computer controlled current recordings.