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The objective of the project has been to measure ionic current carried by ionic liquids with a focus on transport at the nanoscale. Single nanopore in polymer films of well-defined geometry and surface chemistry served as our model nanosystem in which ionic liquids were enclosed. According to our hypothesis, ionic liquids with larger constituent ions were expected to exhibit stronger dependence of their conductivity on nanopore diameter originating from: (i) finite size of constituent ions, (ii) electrostatic interactions between ions and the pore walls, (iii) differences in the electrolyte structure in a nanopore.

Ionic conductivity is one of the most important physical properties that determine applicability of ionic liquids in a number of laboratory and industrial processes e.g. applying ionic liquids in fuel cells, solar cells and separation 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.

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 diameter, as determined for three ionic liquids: 1 - butyl – 3 methylimidazolium methyl sulfate [BMIM][CH₃SO₄], 1-butyl-3-methylimidazolium 2-(2-methoxyethoxy) ethyl sulfate [BMIM][C₅H₁₁O₂SO₄], and 1-ethyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl] imide [EMIM][Tf2N]. For pores with diameters less than 20 nm, the ionic conductivity was significantly 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 conductivity value for [BMIM][C₅H₁₁O₂SO₄] in a 16 nm pore was found to be 8 times smaller than its corresponding bulk value.

Interestingly, the effect of nanoscale on the [EMIM][Tf2N] conductance was much less pronounced. A conical nanopore with 8 nm diameter showed 70% of the bulk conductance. This ionic liquid had the largest bulk conductivity of 0.52 S/m, which points to the low viscosity of the compound.

CONCLUSION: Our results indicate that behavior of ionic liquids at the nanoscale is regulated by their density and size of the constituent ions.

FINDING 2: Ion currents carried by ionic liquids in nanopores with a diameter < 10 nm are rectified. Transport of ionic liquids was 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. Ion current rectification was observed with two studied ionic liquids and characterized by the rectification degree at 5 V of 1.5. It means that current for -5V was ~1.5 times higher than the current for +5V.

CONCLUSION: Transport of IL through conically shaped nanopores can be rectified.

FINDING 3: Ion current rectification can be enhanced in nanopores with surface charge patterns.

Rectification properties of nanopores can be enhanced by introducing surface charge patterns onto the pore walls of conical nanopores. Current-voltage curves of ionic liquids were recorded in pores with two surface charge patterns: (i) a nanopore contained a junction between a zone with negative surface charges and a neutral zone, (ii) a nanopore contained a zone with 12 nm long DNA molecules and a zone with carboxyl groups. Both these systems worked like a diode for ionic liquids transporting ions in one direction and blocking them in the other. These two systems are the first ionic diodes reported for ionic liquids.

CONCLUSION: Our results provide evidence of the importance of electrostatics in ionic liquids’ transport in nanopores, and much longer screening length than the values estimated by the classical electrochemistry Debye-Hueckel theory.

FINDING 4: Nanopores provide a natural nanoscale interface between two immiscible liquids.

Studying interfaces between an ionic liquid and an immiscible liquid is important for applications of ionic liquids in separation and extraction processes. We have tackled this issue for tributyl(tetradecylphosphonium) methyl sulfate in contact with KCl. We were interested in measuring the electric potential gradient established at the interface which results from different partition of ions into the two liquids. In order to localize the interface, a system of single conical nanopore with an integrated AgCl electrode was prepared (manuscript submitted to Nanotechnology).

CONCLUSION: Nanopores offer a system for studying phenomena occurring at the interface between two immiscible liquids.

 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. The glove box that the student built is currently equipped with electrical connections, which allows controlling the experiment by a computer.