pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrochemical Reaction of Water-Insoluble Organic Droplets in Aqueous Electrolytes Using Acoustic Emulsification Mahito Atobe,* Shintaro Ikari, Koji Nakabayashi, Fumihiro Amemiya, and Toshio Fuchigami Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received December 25, 2009. Revised Manuscript Received February 4, 2010 Electrochemical oxidation of water-insoluble amines (n-octylamine and n-decylamine) was successfully accomplished in aqueous electrolytes using acoustic emulsification. Acoustically prepared emulsions of fumaric acid diethyl ester in aqueous electrolytes can be also electro-reduced very smoothly. Ultrasonication to the water-insoluble organic substrate/aqueous electrolyte mixtures allowed a formation of very stable emulsions having the characteristic of giving narrow monomer droplet size distributions in the submicrometer range in aqueous electrolytes without added surfactants, and the smooth electrochemical reaction in the emulsions took place via direct electron transfer between the electrode and the water-insoluble organic droplets. In this kind of electron-transfer system, supporting electrolyte should be dissolved not only in the aqueous phase but also in the organic droplets and should contribute to the formation of an electric bilayer inside the droplets.
Introduction Water is a most ideal solvent for electrolysis, but many organic materials are water-insoluble or only sparingly soluble. Therefore, organic solvents or special surfactants have been inevitably used for electrolysis of water-insoluble materials.1,2 However, the use of organic solvents or surfactants is practically disadvantageous from the aspects of operational costs, safety and environmental concerns. For electrolysis of insoluble liquid materials without organic solvents and surfactants, their mechanically stirred emulsions have been used,3 but generally, it is not possible to obtain a high electron transfer rate between the droplet and the electrode since the size of droplets is usually larger than the micrometer range.4-8 On the other hand, ultrasonic irradiation provides stable emulsions without surfactants simply by mechanical forces which arise at the liquid/liquid phase boundaries.9-11 This has been termed “acoustic emulsification”. Furthermore, it is also known that emulsions prepared by acoustic emulsification have the characteristic of giving narrow droplet size distributions in the submicrometer range.12,13 Hence, it can be expected that these droplets would allow good contact with the electrode, and
therefore, a practical reaction rate would be obtained even in aqueous electrolytes. Compton and co-workers reported that apparent diffusion rates of water-insoluble molecules in emulsion systems under ultrasonication were voltammetrically estimated to be large enough to perform their preparative electrolyses.14-17 In our previous work, we also demonstrated the use of ultrasound to emulsify a mixture of a water-insoluble monomer like 3,4-ethylenedioxythiophene and water-soluble electrolyte in the absence of a surfactant with the aim of carrying out electropolymerization in a clean medium.18,19 The technique worked very well. The emulsion was quite stable and the monomer droplets were very small (ca. 211 nm), which allowed good contact of the droplets with the electrode and, as a result, direct electron transfer from the droplet to the electrode. The successful results promoted us to perform a systematic study of the electrochemical reactions of a variety of water-insoluble organic materials. Herein, we wish to report the full details of electrochemical reactions of water-insoluble organic droplets in aqueous electrolytes using acoustic emulsification. As model reactions, the electrochemical oxidation of emulsified amines (n-octylamine and n-decylamine) and electrochemical reduction of emulsified fumaric acid diethyl ester in aqueous electrolytes were employed in this work (Schemes 1 and 2).20
*Corresponding author. Fax: þ81-45-924-5407. E-mail address: atobe@ echem.titech.ac.jp. (1) Gao, J.; Rusling, J. F.; Zhou, D.-L. J. Org. Chem. 1996, 61, 5992. (2) Rusling, J. F.; Zhou, D.-L. J. Electroanal. Chem. 1997, 439, 89. (3) Czerwinski, A.; Zimmer, H.; Van Pham, C.; Mark, H. B., Jr. J. Elctrochem. Soc. 1985, 132, 2669. (4) Vermeulen, T.; Williams, G. M.; Langlois, G. E. Chem. Eng. Sci. 1967, 22, 435. (5) Chatzi, E. G.; Boutris, C. J.; Kiparissides, C. Ind. Eng. Chem. Res. 1991, 30, 536. (6) Petela, R. Fuel 1994, 73, 557. (7) Shirota, D.; Kamiwano, M.; Kaminoyama, M.; Nishi, K.; Shimizu, K. Kagaku Kogaku Ronbunshu 1999, 25, 1015. (8) Nagy, E.; Hadik, P. Ind. Eng. Chem. Res. 2003, 42, 5363. (9) Li, M. K.; Fogler, H. S. J. Fluid Mech. 1978, 88, 499. (10) Li, M. K.; Fogler, H. S. J. Fluid Mech. 1978, 88, 513. (11) Reddy, S. R.; Fogler, H. S. J. Phys. Chem. 1980, 84, 1570. (12) Encyclopedia of Surface and Colloid Science; Kamogawa, K., Abe, M., Eds.; Marcel Dekker: New York, 2002. (13) Kamogawa, K.; Okudaira, G.; Matsumoto, M.; Sakai, T.; Sakai, H.; Abe, M. Langmuir 2004, 20, 2043.
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Experimental Section Materials. All chemicals were used without further purification. n-Octylamine, n-decylamine, and fumaric acid diethyl ester (14) Marken, F.; Compton, R. G.; Bull, S. D.; Davies, S. G. Chem. Commun. 1997, 995. (15) Wadhawan, J. D.; Del Campo, F. J.; Compton, R. G.; Foord, J. S.; Marken, F.; Bull, S. D.; Davies, S. G.; Walton, D. J.; Ryley, S. J. Electroanal. Chem. 2001, 507, 135. (16) Banks, C. E.; Rees, N. V.; Compton, R. G. J. Electroanal. Chem. 2002, 535, 41. (17) Davies, T. J.; Banks, C. E.; Nuthakki, B.; Rusling, J. F.; France, R. R.; Wadhawana, J. D.; Compton, R. G. Green Chem. 2002, 4, 570. (18) Asami, R.; Atobe, M.; Fuchigami, T. J. Am. Chem. Soc. 2005, 127, 13160. (19) Asami, R.; Fuchigami, T.; Atobe, M. Langmuir 2006, 22, 10258. (20) Kunugi, Y.; Chen, P. O.; Nonaka, T.; Chong, Y.-B.; Watanabe, N. J. Electrochem. Soc. 1993, 140, 2833.
Published on Web 02/17/2010
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Scheme 1. Anodic Oxidation of n-Octylamine and n-Decylamine
Scheme 2. Cathodic Reduction of Fumaric Acid Diethyl Ester
Figure 1. Linear sweep voltammograms (a) without and with 2.0 mmol n-octylamine (b) under mechanical stirring at 1400 rpm and (c) under ultrasonication at 248 W cm-2 in 4.0 M KOH aq. Table 1. Current Efficiency in the Anodic Oxidation of Emulsified Amines
were purchased from Tokyo Kasei Kogyo Co. and used as received. KOH was purchased from Wako Pure Chemical Industries, Ltd. Distilled and deionized water was used as a solvent for voltammetry and electrochemical reactions. Measurement of Ultrasound Intensity. The intensity of ultrasound was determined by adiabatic measurement of the temperature rise of sonicated water (calorimetry).21 Linear Sweep Voltammetry. Linear sweep voltammograms for oxidation of n-octylamine (2 mmol) were measured with a three-electrode system using a nickel plate (1 1 cm2) working electrode, a platinum plate (2 2 cm2) counter electrode, and a saturated calomel electrode (SCE) as a reference electrode in 10 mL of 4.0 M KOH aqueous solution under ultrasonication with 248 W cm-2 intensity by an ultrasonic stepped horn (3.2 mm diameter) connected with a 20 kHz oscillator (SONIFIER-250D, Branson Ultrasonics Co.) at a solution temperature of 25 ( 1 °C. Linear sweep voltammograms for reduction of fumaric acid diethyl ester (2 mmol) were recorded with a three-electrode system using a glassy carbon disk (3 mm diameter) working electrode, a platinum plate (2 2 cm2) counter electrode, and a saturated calomel electrode (SCE) as a reference electrode in 10 mL of pH 7.0 phosphate buffer aqueous solution under ultrasonication (106 W cm-2 intensity, 20 kHz frequency) at a solution temperature of 25 ( 1 °C. For comparison, voltammograms were also measured under mechanical stirring at 1400 rpm (Magnestir, MGP-306 Sibata Scientific Technology Ltd.).
General Procedure for Electrochemical Oxidation of Amines. A divided H-type glass cell with a glass frit diaphragm in
a cooling bath was equipped with a nickel mesh anode (ca. 11 cm2), a Pt mesh cathode (ca. 35 cm2), and an ultrasonic stepped horn (3.2 mm diameter) connected with a 20 kHz oscillator (SONIFIER-250D, Branson Ultrasonics Co.). The top of the horn was positioned 3 cm apart from the cell bottom. Two mmol of a substrate (n-octylamine or n-decylamine) was galvanostatically electrolyzed at 55 mA in 4.0 M KOH aqueous solution (10 mL) by passing 0.5 F mol-1 of charge under ultrasonication with 248 W cm-2 intensity at a solution temperature of 25 ( 1 °C unless otherwise stated. Mechanical stirring at various stirring speeds (Magnestir, MGP-306 Sibata Scientific Technology Ltd.) was also used instead of ultrasonication. After electrolysis, products were extracted with diethyl ether (15 5 mL) and analyzed by gas chromatography (GC-8A with PEG 6000, 2 m column, Shimadzu Co., Japan).
General Procedure for Electrochemical Reduction of Fumaric Acid Diethyl Ester. A divided H-type glass cell with a glass frit diaphragm in a cooling bath was equipped with a glassy carbon plate cathode (6.5 cm2), a Pt mesh anode (ca. 35 cm2), and an ultrasonic stepped horn (3.2 mm diameter) connected with a 20 kHz oscillator (SONIFIER-250D, Branson Ultrasonics Co.). The top of the horn was positioned 3 cm apart from the cell bottom. Two mmol of fumaric acid diethyl ester was galvanostatically electrolyzed at 66 mA in pH 7.0 phosphate buffer aqueous (21) Mason, T. J.; Lorimer, J. P.; Bates, D. M. Ultrasonics 1992, 30, 40.
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entry
substrate
1 2 3 4 5 6 7 8
n-octylamine n-octylamine n-octylamine n-octylamine n-octylamine n-octylamine n-decylamine n-decylamine
mechanical stirring (rpm)
ultrasonication (W cm-2)
current efficiency (%)
500 1000 1400
2.5 31 33 44 68 71 17 46
106 177 248 1400 248
Figure 2. Linear sweep voltammograms (a) without and with 2.0 mmol fumaric acid diethyl ester (b) under mechanical stirring at 1400 rpm and (c) ultrasonication at 106 W cm-2 in phosphate buffer solution (pH 7). Table 2. Current Efficiency in the Cathodic Reduction of Emulsified Fumaric Acid Diethyl Ester current efficiency (%)
entry 1 2
mechanical stirring (rpm)
ultrasonication (W cm-2)
HM
HD
total
106
trace 53
trace 17
trace 70
1400
solution (10 mL) by passing 1.0 F mol-1 of charge under ultrasonication with 106 W cm-2 intensity at a solution temperature of 25 ( 1 °C unless otherwise stated. Mechanical stirring at 1400 rpm (Magnestir, MGP-306 Sibata Scientific Technology Ltd.) was also used instead of ultrasonication. After electrolysis, products were extracted with diethyl ether (15 5 mL) and analyzed by gas chromatography (GC-8A with Polyester FF, 2 m column, Shimadzu Co., Japan). Demulsification Studies. The overall stability of the emulsion including creaming was monitored with photographic recording. The photograph was taken with a digital camera (IXY 300, Cannon Co.). Measurement of Droplet Size and Distribution. Droplet size and distribution were determined by the dynamic light scattering (DLS) method at 25 °C with a light scattering photometer (DLS-7000, Otsuka Electronics Co.) without diluting the mixture.
Measurement of Supporting Electrolyte Concentration in Organic Phase. Two mmol of n-octylamine was added to 10 mL Langmuir 2010, 26(11), 9111–9115
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Figure 3. Observations of emulsification and demulsification of n-octylamine (2.0 mmol) in 4.0 M KOH aqueous solution.
Figure 4. Observations of emulsification and demulsification of n-decylamine (2.0 mmol) in 4.0 M KOH aqueous solution.
Figure 5. Observations of emulsification and demulsification of fumaric acid diethyl ester (2.0 mmol) in phosphate buffer solution (pH 7). of aqueous solution containing 4.0 M KOH. Next, the solution was emulsified by ultrasonication, and then the mixture was centrifuged at 2000 rpm for 30 min. After the centrifuge, supernatant (organic phase) was subjected to ion chromatography (PIA-1000, Shimadzu Co.) to measure the concentration of the potassium ion in the organic phase.
Results and Discussion Electrochemical Oxidation of Amines. At first, we measured linear sweep voltammograms for oxidation of emulsified n-octylamine in an aqueous electrolyte at 50 mV s-1 of scan rate Langmuir 2010, 26(11), 9111–9115
under mechanical stirring and ultrasonication (Figure 1). As shown in Figure 1a, the voltammogram recorded under mechanical stirring showed an oxidation peak at 0.32 V vs SCE. In contrast, by recording the voltammogram under ultrasonication, the anodic current was increased, and moreover the voltammogram showed the limiting current at oxidation potentials higher than 0.32 V vs SCE (Figure 1b). As mentioned in the introducing part, ultrasonication provides stable emulsions having the characteristic of giving narrow droplet size distributions in the submicrometer range.12,13 In addition, it is well-known that ultrasonication promotes the mass transport of a substrate to the electrode surface DOI: 10.1021/la904875g
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Figure 6. DLS number size distribution of (a) n-octylamine, (b) n-decylamine, and (c) fumaric acid diethyl ester droplets from acoustically emulsified aqueous solutions. The mean sizes of n-octylamine, n-decylamine, and fumaric acid diethyl ester droplets were 370, 3500, and 150 nm, respectively.
from the bulk solution.22-25 Hence, it can be stated that the higher anodic current measured is due to the smaller droplets and the higher limiting current (defined by the rate of diffusion to the electrode surface) is due to the greater mass transport under ultrasonication. We then carried out preparative scale of the anodic oxidation of emulsified amines in an aqueous electrolyte. As shown in entries 1-3 of Table 1, the current efficiency for the oxidation of n-octylamine was increased with an increase in the stirring speed but the efficiency was reached a saturated value (ca. 30%) at 1000 rpm. However, the efficiencies obtained under ultrasonication were higher than this saturated value and at an intensity of 248 W cm-2 reached 71% (see entries 4-6). Similarly to the case of n-octylamine oxidation, the efficiency for the n-decylamine oxidation was increased by ultrasonication (see entries 7 and 8). However, the magnitude of the increase was less than that for the n-octylamine oxidation. This observation will be discussed in more detail below. Electrochemical Reduction of Fumaric Acid Diethyl Ester. To demonstrate the generality of this methodology, the electrochemical reduction of fumaric acid diethyl ester was also (22) (23) (24) (25)
Atobe, M.; Matsuda, K.; Nonaka, T. Electroanalysis 1996, 8, 784. Atobe, M.; Nonaka, T. Chem. Lett. 1997, 26, 323. Atobe, M.; Fuchigami, T.; Nonaka, T. J. Electroanal. Chem. 2002, 523, 106. Atobe, M.; Shen, Y.; Fuchigami, T. Org. Lett. 2004, 6, 2441.
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carried out in its emulsion. Figure 2 shows linear sweep voltammograms for reduction of emulsified fumaric acid diethyl ester in an aqueous electrolyte at 10 mV s-1 of scan rate under mechanical stirring and ultrasonication. As in the oxidation of n-octylamine, the current measured for the reduction under ultrasonication was much higher than that recorded under mechanical stirring at 1400 rpm. We then carried out preparative scale of the electrochemical reduction (Table 2) of emulsified ester in an aqueous electrolyte. In general, this reaction gives the corresponding hydromonomer (HM) and hydrodimer (HD) as shown in Scheme 2, along with hydrogen evolution.26,27 However, only a trace amount of HM was obtained in the electroreduction of mechanically emulsified ester. In sharp contrast, the electroreduction in acoustically emulsified solution proceeded very smoothly. A total current efficiency of 70% was reached for formation of the two products. From this fact, it can be stated that acoustic emulsification is effective for the electrochemical reaction of various waterinsoluble organic substrates in aqueous electrolytes. Emulsification and Demulsification. The demulsification of n-octylamine emulsified in aqueous media containing 4.0 M KOH was monitored at 25 ( 2 °C with photographic recording. As shown in Figure 3, the demulsification was observed immediately after emulsification by mechanical stirring and consequently, emulsion droplets were hardly found in the beaker after 30 min. In sharp contrast, the turbidity still remained even for 30 min after the ultrasonic treatment. Stable droplets of n-decylamine and fumaric acid diethyl ester were also obtained by the ultrasonic treatment as shown in Figures 4 and 5. In particular, the turbidity of the later sample remained even for 6 h after the ultrasonic treatment. The mean size of emulsion droplets formed by the acoustic emulsification was measured by the dynamic light scattering (DLS) method. However, the size of emulsion droplets formed by the mechanical stirring could not be measured due to their low stability. As shown in Figure 6b, the mean size of n-decylamine droplets formed by the acoustic emulsification was biggest in all three samples, and it was found to be 3.5 μm. On the other hand, n-octylamine and fumaric acid diethyl ester emulsions showed the characteristic of giving narrow droplet size distributions in the submicrometer range, and their mean sizes were 370 and 150 nm, respectively (see Figure 6, parts a and c). It should be noted here that the remarkable ultrasonic effect on the current efficiency was observed in both n-octylamine oxidation and fumaric acid diethyl ester reduction. In contrast, the ultrasonication resulted in a smaller increase in the efficiency in the case of n-decylamine oxidation. From these facts, stable and small droplets can be allowed a good contact with the electrode and, as a result, smooth electron transfer to the electrode. Influence of Electrolyte Concentration in Organic Phase. For the direct electron transfer between the electrode and droplets as illustrated in Figure 7, the electrolyte should be dissolved not only in the aqueous phase but also in the droplets, and contribute to the formation of an electric bilayer inside the droplets when the droplets are contacted with the electrode. To provide support for this suggestion, the concentration of KOH in the n-octylamine droplets in 4.0 M KOH aqueous solution was measured by using ion chromatography, and it was found to be 63 mM. This leads to the n-octylamine droplets being ionically conductive, a situation that leads to a high current efficiency for the oxidation reaction (see entry 6 of Table 1). (26) Parker, V. D. Acta Chem. Scand. 1981, 35B, 149. (27) Parker, V. D. Acta Chem. Scand. 1981, 35B, 274.
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Figure 7. Schematic illustration of the electrochemical reaction of a water-insoluble substrate in an aqueous solution.
As expected the concentration of KOH in the n-octylamine droplets decreased with a decrease in the concentration of KOH in the aqueous phase. The lower concentration of KOH in the droplets leads to a decrease in the electroactivity of the droplets. For example, the current efficiency for the oxidation of n-octylamine droplets in 0.1 M KOH was very low (4%), and oxygen evolution was mainly occurred.
reaction in aqueous electrolytes via the direct electron transfer between the electrode and the water-insoluble organic droplets; (c) the ultrasonic promotion of the mass transport of organic droplets to the electrode surface. We believe that the present methodology will be applied to various electrochemical reactions that are useful for organic syntheses and can be contribute to develop the novel emulsion electrolysis system.
Conclusions We have developed a novel electrochemical reaction of waterinsoluble organic droplets in aqueous electrolytes using acoustic emulsification. This new methodology has many practical advantages and characteristics: (a) the formation of stable and small organic droplets in aqueous electrolytes without added surfactants using ultrasonic treatment; (b) very smooth electrochemical
Acknowledgment. We thank Professor Dr. Takakazu Yamamoto and Dr. Hiroki Fukumoto (Tokyo Institute of Technology) for measurement of DLS. This work was partially supported by the Global COE program (Tokyo Institute of Technology) and the Grant-in-Aid for Scientific Research (20350046 and 21656205) from The Japanese Ministry of Education, Science, Culture. and Sports.
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