Extraction of Cations from an Ionic Liquid Droplet in a Dielectric Liquid

Aug 21, 2014 - KEPCO Research institute, 105 Moonji-Ro, Yusung-Gu, Daejeon, 305-760, South Korea. J. Phys. Chem. Lett. , 2014, 5 (17), pp 3021–3025...
0 downloads 0 Views 1MB Size
Download PDF
Letter pubs.acs.org/JPCL

Extraction of Cations from an Ionic Liquid Droplet in a Dielectric Liquid under Electric Field Myung Mo Ahn,† Do Jin Im,*,‡ Jang Gyu Kim,§ Dong Woog Lee,∥ and In Seok Kang*,† †

Department of Chemical Engineering, §Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, 790-784, South Korea ‡ Department of Chemical Engineering, Pukyong National University, 365 Sinseon-ro, Nam-Gu, Busan, 608-737, South Korea ∥ KEPCO Research institute, 105 Moonji-Ro, Yusung-Gu, Daejeon, 305-760, South Korea S Supporting Information *

ABSTRACT: Ionic liquids show great promise as excellent solvents or catalysts in energy and biological fields due to their unique chemical and physical properties. In this work, the characteristics of various ionic liquids are investigated with the electrophoresis of a charged droplet (ECD) method. Under normal situation, a charged droplet in a dielectric liquid shows back-and-forth bouncing motion between the positive and negative electrodes continuously. However, for some special ionic liquids, interesting retreating behavior of a charged droplet has been observed. This retreating behavior is due to the loss of positive charges of the droplet, and it suggests that only the positive ions are extracted from the droplet under the applied electric field. Based on this hypothesis of ion extraction, Fourier transform infrared (FTIR) spectroscopy analysis has been performed. The retreating behavior is also discussed from the intermolecular point of view according to the ion species. SECTION: Liquids; Chemical and Dynamical Processes in Solution

I

In this study, we experimentally investigate the charging characteristics of ionic liquids in the ECD system. The electrophoretic motions of ionic liquid droplets are investigated for combinations of various cations and anions. In the cases of some ionic liquids, it has been found that a charged ionic liquid droplet shows interesting retreating motion. Because the retreating motion of a liquid droplet has not been observed in ECD experiments for aqueous droplets, we focus on this phenomenon of ionic liquids. A hypothesis about this retreating motion is proposed and verified not only in macroscale but also at the molecular level. Finally, we discuss its implications for estimating the intermolecular force between the cation and the anion of an ionic liquid. A schematic view of the experimental setup is shown in Figure 1. A transparent plastic cuvette (internal dimensions: 10 × 10 × 45 mm3) was used for a test cell and located between a cold light source and a high speed camera. Two rectangular copper electrodes (8 × 60 mm2) were aligned parallel to each other at both sides of the cuvette. The electrodes are connected to a direct current (DC) high voltage power supply (Trek model 610E) and an electrometer (Keithley model 6514). To avoid thermal effects, a cooled light-emitting diode (LED) was used as a light source, and all experiments were performed at room temperature. Silicone oil (Shin-Etsu KF-96 1000 cSt) was

onic liquids are salt in liquid state. They have attracted significant attention as solvents or catalysts due to their unique physical and chemical properties, such as high thermal stability and low vapor pressure.1−3 Because ionic liquids have special electrochemical characteristics as well as unique physical and chemical properties,4−6 ionic liquids are used as electrolytes in the energy industry such as lithium ion batteries or fuel cells.7,8 Ionic liquids are also used as charge carrying media in electrowetting on dielectric (EWOD) systems or microchannel systems.9−13 However, despite the increasing need of ionic liquids in various applications, understanding of the charge transfer mechanisms of ionic liquids in a dielectric liquid is still much behind. Recently, a new method for droplet actuation called the electrophoresis of charged droplets (ECD) has been developed.14−16 Droplets, which are charged by contacting an electrode, are manipulated by the Coulomb force under electric field. One of the prominent advantages of this method is that charge transfer of a single droplet can be observed on the positive and negative electrodes separately because the ECD system is not a continuous current flow system but a discrete charge transfer system.15 Using this ECD method, the studies on the charging characteristics of various aqueous droplets have been conducted.14 Furthermore, the ECD method can be applied to measure the interfacial tension between any conducting liquid (including ionic liquid) and dielectric liquid.17 © XXXX American Chemical Society

Received: July 18, 2014 Accepted: August 21, 2014

3021

dx.doi.org/10.1021/jz501511z | J. Phys. Chem. Lett. 2014, 5, 3021−3025

The Journal of Physical Chemistry Letters

Letter

Figure 1. Schematic view of the experimental setup and the detailed image of a test cell.

used for the dielectric fluid medium and a micropipette was used to dispense an ionic liquid droplet in the test cell. The motion of a droplet was recorded by a high speed camera (Photron Fastcam 1024 PCI model 100 K), and the images of droplet motion were analyzed by a LabVIEW Vision Assistant tool kit. The experimental procedure is as follows. Two bare copper electrodes were polished with sandpaper and washed with nitric acid and isopropyl alcohol (IPA) for the removal of any surface contaminants to minimize the influence of electrode surface condition. After the test cell was filled with silicone oil, two electrodes were fixed in the test cell. The test cell containing silicone oil was placed on a stage, and a DC high voltage power supply, which was connected to the two electrodes, was turned on. A single ionic liquid droplet was dispensed from a micropipette between the two electrodes in the test cell, and the droplet was pulled toward an electrode due to initial charges of the dispensed droplet.18 Once the droplet contacted an electrode, the droplet was charged and moved due to the electric field between the two electrodes. The electrophoretic motions of droplets were investigated with 1-alkyl-3-methylimidazolium methylsulfate ([Cnmin]+ [CH 3 SO 4 ] − ), 1-alkyl-3-methylimidazolium ethylsulfate ([Cnmin]+ [C2H5SO4]−) and 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmin]+ [(CF3SO2)2N]−). The numbers of carbons in the alkyl chain of the cation are 2, 4 and 6; thus, total nine kinds of ionic liquids were used for the droplet phase with three different cations and three different anions. (The material properties of the ionic liquids used in the experiments are shown in Table 1 of the Supporting Information). Typical electrophoretic motion of a deionized (DI) water droplet between the two electrodes in the ECD experimental system is shown in Figure 2a. Once the DI water droplet contacts an electrode due to its initial charge, it moves backand-forth horizontally between the two electrodes continuously. Ionic liquid droplets, which have methylsulfate (MS) or ethylsulfate (ES) as their anions, show the typical electrophoretic motion as shown in Figure 2b. Because ionic liquids are heavier than water, the ionic liquid droplets move slightly downward during the bouncing motion. However, not all ionic liquid droplets show regular bouncing motion between the two electrodes. Ionic liquid droplets with bis(trifluoromethylsulfonyl)imide (NTf2) anion show interesting and unique motion as shown in Figure 2c. After touching the

Figure 2. (a) Time-lapse images of a 300 nL DI water droplet under 3.0 kV/cm. (b) Time-lapse images of a 400 nL BMIM-MS droplet under 3.0 kV/cm. (c) Time-lapse images of a 400 nL EMIM-NTf2 droplet under 3.0 kV/cm. + and − symbols in the droplet indicate the net charge of the droplet.

positive high voltage electrode, the droplet moves toward the negative electrode. However, it cannot arrive at the negative electrode and retreats back to the positive electrode again. The droplet always shows the retreating motion to the positive electrode continuously even though the polarities of electrodes are changed. If the droplet starts from the negative electrode, it moves all the way to touch the positive electrode and shows the retreating motion mentioned above. With different cations, the maximum distances from the positive electrode were different even under the same electric field. The results of the electrophoretic motions of ionic liquids are summarized in Figure S1 in the Supporting Information. (Real movies of the droplet motions are also in the Supporting Information.) As shown in Figure 2c, the droplet with NTf2 anion on the positive electrode is charged positively and moves to the negative electrode. As the droplet translates, the droplet is continuously slowing down. After some point, droplet stops and goes back to the positive electrode. This means that the net charge of the droplet is changing from positive to neutral and then to negative. There are two possibilities of this phenomenon. One is that the droplet obtains negative charge from outer medium and the other is that the droplet loses positive charge to outer medium. The first hypothesis can be neglected because the charge leakage from silicone oil into a charged ionic liquid droplet hardly occurs due to the extremely low conductivity of the silicone oil (order of 10−13 S/m) and the very long characteristic charge relaxation time (order of 100 s). The typical bouncing time of the droplet is order of 1 s. Furthermore, if the charge leakage is the reason for the retreating motion, all other ionic liquids should show the same phenomenon as well. Therefore, it is reasonable that the droplet loses the positive charges to outer medium continuously, that is, the cations of ionic liquid droplet with NTf2 anion are extracted into silicone oil. The NTf2 anion is less polarized and bigger than ES and MS anions (The molecular structures of anions are shown in Figure S2 of the Supporting Information); thus, the interaction between cation and anion is 3022

dx.doi.org/10.1021/jz501511z | J. Phys. Chem. Lett. 2014, 5, 3021−3025

The Journal of Physical Chemistry Letters

Letter

weaker.19−21 The weaker interaction between cation and anion of ionic liquid with NTf2 can result in the extraction of cations by the external electric force. To verify our hypothesis, conductivity change of silicone oil was examined after experiments. If cations are extracted into the silicone oil, the conductivity of the silicone oil must be changed. In order to see the conductivity change of the silicone oil, the same experiment was performed 10 times for each case (a 400 nL droplet was dispensed under 3.0 kV/cm electric field and waited until the droplet arrived at the bottom of the cuvette). After experiments, the 10 droplets at the bottom were removed and the current signals on the silicone oil were measured to estimate the electric conductivity of the silicone oil. The average current on the silicone oil is plotted in Figure 3a. Using the average current data (the number of measure-

that the influence of the solubility of EMIM-NTf2 in silicone oil is negligible in these experiments. More direct verification is conducted at the molecular level with Fourier transform infrared (FTIR) spectroscopy analysis. The silicone oil was analyzed after experiments with EMIM-MS and EMIM-NTf2. As shown in Figure 3b, the bands appear at 2800−3100 cm−1, indicating the existence of an alkyl group in the silicone oil in the case of the EMIM-NTf2 experiment. This is a direct proof of the cation extraction because there is no CH3 in the NTf2 anion. With these verifications, we may explain why the maximum distance of each retreating ionic liquid droplets from the positive electrode are different. Although the retreating motion occurs when the anion is NTf2, the extraction rate is dependent on the cation because the cations are extracted into the silicone oil. The extraction process occurs in two steps. As shown in Figure 4a, the cations on the droplet surface near the pole,

Figure 4. (a) Schematic image of the cation extraction process. (b) Cation loss rates according to the carbon chain length of cations under the same experimental condition (2.75 kV/cm, 400 nL).

Figure 3. (a) The average electric current on the silicone oils (1000 cSt) in the cuvette. Each silicone oil sample was prepared after 10-fold dispensing of droplets (400 nL, 3.0 kV/cm). The relative standard deviation ranges 0.5−12.3%. (b) FTIR spectroscopy analysis of the silicone oils after experiments with EMIM-MS and EMIM-NTf2.

where the electric field is strongest, are first extracted into the silicone oil phase. Then for continuous extraction, the cations inside the droplet must move to the surface region. Since the electric field inside the droplet is negligible compared to the external electric field, the movement of the cations are mainly due to diffusion and it is the rate limiting step. According to the Stokes−Einstein formula, the diffusivity is inversely proportional to the cation radius. As shown in Figure 4b, the ionic liquid droplet with smaller size cations loses its positive charges more quickly; as a result, the maximum distance from electrode is shorter. This tendency shows qualitatively good agreement with the numerical and experimental results of the cation diffusivity.21 However, it should be noted that the above argument is only a plausible explanation. The accurate mechanism should be explained by a more detailed and systematic analysis such as MD simulation.

ments n = 5 for each electric field), the electric conductivity of the silicone oil could be obtained. Before the experiment, the conductivity of the silicone oil was 5.06 × 10−14 S/m. After the ECD experiment of EMIM-MS, the conductivity of the oil was 8.97 × 10−14 S/m. However, after the ECD experiment of EMIM-NTf2, the conductivity of the oil was increased to 3.60 × 10−13 S/m. It means that the conductivity of silicone oil was changed significantly during EMIM-NTf2 experiments. To make sure that this conductivity change was due to the cation extraction, the solubility of the ionic liquid in the silicone oil also was examined. After the same experiment of dispensing EMIM-NTf2 droplets (10 times dispensing) without electric field, the conductivity of the silicone oil (7.35 × 10−14 S/m) was found to be similar to that of pure silicone oil. This means 3023

dx.doi.org/10.1021/jz501511z | J. Phys. Chem. Lett. 2014, 5, 3021−3025

The Journal of Physical Chemistry Letters

■

The cation loss ratio can also be estimated from total net charge of the droplet on the electrode and number of total molecules in the droplet. In the case of experiment in Figure 2c, the total net surface charge of the droplet on the positive electrode is 3.8 × 10−11 C, and this charge amount corresponds to 2.4 × 108 cations.22 This means that the droplet loses 2.4 × 108 cations until the droplet is neutralized. The total number of cations in the droplet (400 nL) is 9.3 × 1017. By dividing the number of cation loss by the total number of cations in the droplet, the cation loss ratio is obtained and is about 2.6 × 10−8 %. It is a very small portion of the total cations in the droplet. This result has significant meaning from the application point of view because the tiny quantity of cation loss does not affect the physical properties of the ionic liquid. Otherwise, consistent actuation is impossible and it may cause problems in many applications. On the other hand, the cation extraction phenomenon can be used as an extraction method of specific ions from ionic liquids. Even the electro-spraying or electro-jetting methods cannot extract individual specific ions from ionic liquids but they make submicron droplets, which have both cations and anions, from the mother droplet.23,24 However, specific cations can be extracted individually without any anions in the ECD system if the applied electric force on the ions in the ionic liquid droplet is greater than the intermolecular force between cations and anions. One important implication of the current study is that the intermolecular force between the cation and the anion of an ionic liquid with NTf2 anion can be estimated by using the bulk scale ECD system. When the external electric force is greater than the intermolecular force between cation and anion, the cations can be extracted into silicone oil. If the critical applied electric voltage to make the retreating motion of an ionic liquid droplet is given, the intermolecular force can be estimated with the force balance between the electric force on the cation and intermolecular force of ions. Through an in-depth study together with MD simulation, we expect that the intermolecular force between the cation and anion of an ionic liquid can be estimated precisely without expensive equipment in our future work. The charging characteristics of ionic liquids with various cations and anions have been investigated from experimental studies with the ECD system. While the ionic liquid droplets with NTf2 anion show the retreating motion back to the positive electrode, the ionic liquid droplets with ES and MS anions show regular back-and-forth bouncing motion between the positive and negative electrodes. The interaction between the cation and the anion is the key parameter to the electrophoretic motion of the ionic liquid droplets under electric field. The weaker interaction between cations and anions of the ionic liquids with NTf2 anion results in the retreating motion of the droplet. The current findings about the retreating motion of the ionic liquids with NTf2 anion suggest a simple way for estimating the intermolecular force between the cation and the anion through the bulk scale electrophoretic experiments together with MD simulation.

■

Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program grant No. 2013R1A1A2010483 funded by the Ministry of Education, Science and Technology (MEST) and also supported by Career Scientist Program grant No. 2013R1A1A2011956 funded by the Ministry of Science, ICT and Future Planning (MSIP) through the National Research Foundation of Korea (NRF).

■

REFERENCES

(1) Freemantle, M. W. T.; Rogers, R. D. An Introduction to Ionic liquids; Royal Society of Chemistry: London, 2009. (2) Buzzeo, M. C.; Evans, R. G.; Compton, R. G. Non-haloaluminate Room-Temperature Ionic Liquids in Electrochemistry - A Review. ChemPhysChem 2004, 5, 1106−1120. (3) Rogers, R. D.; Seddon, K. R. Ionic Liquids as Green Solvents: Progress and Prospects; American Chemical Society: Washington, DC, 2003. (4) Shamsipur, M.; Beigi, A. A. M.; Teymouri, M.; Pourmortazavi, S. M.; Irandoust, M. Physical and Electrochemical Properties of Ionic Liquids 1-Ethyl-3-methylimidazolium tetrafluoroborate, 1-Butyl-3methylimidazolium Trifluoromethanesulfonate and 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide. J. Mol. Liq. 2010, 157, 43−50. (5) Montanino, M.; Moreno, M.; Alessandrini, F.; Appetecchi, G. B.; Passerini, S.; Zhou, Q.; Henderson, W. A. Physical and Electrochemical Properties of Binary Ionic Liquid Mixtures: (1−x) PYR14TFSI-(x) PYR14IM14. Electrochim. Acta 2012, 60, 163−169. (6) Torriero, A. A. J.; Siriwardana, A. I.; Bond, A. M.; Burgar, I. M.; Dunlop, N. F.; Deacon, G. B.; MacFarlane, D. R. Physical and Electrochemical Properties of Thioether-Functionalized Ionic Liquids. J. Phys. Chem. B 2009, 113, 11222−11231. (7) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621−629. (8) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (9) Barikbin, Z.; Rahman, M. T.; Khan, S. A. Fireflies-On-A-Chip: (Ionic Liquid)-Aqueous Microdroplets for Biphasic Chemical Analysis. Small 2012, 8, 2152−2157. (10) Chatterjee, D.; Hetayothin, B.; Wheeler, A. R.; King, D. J.; Garrell, R. L. Droplet-Based Microfluidics with Nonaqueous Solvents and Solutions. Lab Chip 2006, 6, 199−206. (11) Dubois, P.; Marchand, G.; Fouillet, Y.; Berthier, J.; Douki, T.; Hassine, F.; Gmouh, S.; Vaultier, M. Ionic Liquid Droplet as EMicroreactor. Anal. Chem. 2006, 78, 4909−4917. (12) Barikbin, Z.; Rahman, M. T.; Parthiban, P.; Rane, A. S.; Jain, V.; Duraiswamy, S.; Lee, S. H. S.; Khan, S. A. Ionic Liquid-based Compound Droplet Microfluidics for “On-Drop” Separations and Sensing. Lab Chip 2010, 10, 2458−2463. (13) Wijethunga, P. A. L.; Nanayakkara, Y. S.; Kunchala, P.; Armstrong, D. W.; Moon, H. On-Chip Drop-to-Drop Liquid Microextraction Coupled with Real-Time Concentration Monitoring Technique. Anal. Chem. 2011, 83, 1658−1664. (14) Im, D. J.; Noh, J.; Moon, D.; Kang, I. S. Electrophoresis of a Charged Droplet in a Dielectric Liquid for Droplet Actuation. Anal. Chem. 2011, 83, 5168−5174. (15) Im, D. J.; Ahn, M. M.; Yoo, B. S.; Moon, D.; Lee, D. W.; Kang, I. S. Discrete Electrostatic Charge Transfer by the Electrophoresis of a

ASSOCIATED CONTENT

* Supporting Information S

The properties, structural formulas, and experimental motion movies of the ionic liquids are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 3024

dx.doi.org/10.1021/jz501511z | J. Phys. Chem. Lett. 2014, 5, 3021−3025

The Journal of Physical Chemistry Letters

Letter

Charged Droplet in a Dielectric Liquid. Langmuir 2012, 28, 11656− 11661. (16) Im, D. J.; Yoo, B. S.; Ahn, M. M.; Moon, D.; Kang, I. S. Digital Electrophoresis of Charged Droplets. Anal. Chem. 2013, 85, 4038− 4044. (17) Lee, D. W.; Im, D. J.; Kang, I. S. Measurement of the Interfacial Tension in an Ionic Liquid−Dielectric Liquid System Using an Electrically Deformed Droplet. J. Phys. Chem. C 2013, 117, 3426− 3430. (18) Choi, D.; Lee, H.; Im, D. J.; Kang, I. S.; Lim, G.; Kim, D. S.; Kang, K. H. Spontaneous Electrical Charging of Droplets by Conventional Pipetting. Sci. Rep. 2013, 3, 2037. (19) Babarao, R.; Dai, S.; Jiang, D. E. Understanding the High Solubility of CO2 in an Ionic Liquid with the Tetracyanoborate Anion. J. Phys. Chem. B 2011, 115, 9789−9794. (20) Borodin, O. Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids. J. Phys. Chem. B 2009, 113, 11463−11478. (21) Koddermann, T.; Paschek, D.; Ludwig, R. Molecular Dynamic Simulations of Ionic Liquids: A Reliable Description of Structure, Thermodynamics and Dynamics. ChemPhysChem 2007, 8, 2464− 2470. (22) Ahn, M. M.; Im, D. J.; Kang, I. S. Geometric Characterization of Optimal Electrode Designs for Improved Droplet Charging and Actuation. Analyst 2013, 138, 7362−7368. (23) Garoz, D.; Bueno, C.; Larriba, C.; Castro, S.; Romero-Sanz, I.; de la Mora, J. F.; Yoshida, Y.; Saito, G. Taylor Cones of Ionic Liquids from Capillary Tubes as Sources of Pure Ions: The Role of Surface Tension and Electrical Conductivity. J. Appl. Phys. 2007, 102, 064913. (24) Romero-Sanz, I.; Bocanegra, R.; de la Mora, J. F.; GameroCastano, M. Source of Heavy Molecular Ions Based on Taylor Cones of Ionic Liquids Operating in the Pure Ion Evaporation Regime. J. Appl. Phys. 2003, 94, 3599−3605.

3025

dx.doi.org/10.1021/jz501511z | J. Phys. Chem. Lett. 2014, 5, 3021−3025