The fast dropping oil water electrode

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Journal of Electroanalytical Chemistry 464 (1999) 128 132 Short Communication The fast dropping oil water electrode Antonie Baars 1, Koichi Aoki *, Jun Watanabe Department of Applied Physics, Fukui

1 Journal of Electroanalytical Chemistry 464 (1999) Short Communication The fast dropping oil water electrode Antonie Baars 1, Koichi Aoki *, Jun Watanabe Department of Applied Physics, Fukui Uni ersity, 3-9-1, Bunkyo, Fukui-shi, , Japan Received 13 August 1998; received in revised form 21 December 1998 Abstract A fast dropping oil water electrode with a small drop was constructed by dropping an aqueous electrolyte from a double capillary into a nitrobenzene phase, aiming at performances similar to microelectrodes. The double capillary was a combination of a thin capillary supported by the obliquely grounded capillary. This system produces drops with 0.07 s drop time and 0.8 mm radius. The voltammograms were obtained at the interface between the nitrobenzene+tetrabutylammonium tetraphenylborate (supporting electrolyte) and a LiCl aqueous solution. The current was ascribed to the charging of the interface rather than the ion-transfer because of the fast variation of the interface area. The differential capacity versus potential curves are in agreement with the literature curve Elsevier Science S.A. All rights reserved. Keywords: Oil water interface; Fast dropping electrode; Capacitive current; Differential capacity 1. Introduction Electrochemical processes at oil water interfaces have been gaining increasing attention [1,2] in conjunction with quantitative measurements of thermodynamic variables for solvent extraction as well as a model for ion or electron transfer at membranes of living cells. Polarizable interfaces can be created by extruding an organic phase (or an aqueous electrolyte phase) slowly through a capillary into an aqueous phase (or an organic phase). A very slow extrusion rate for a large area of the interface causes interfacial fluctuations yielding an unstable ion-transfer current. An interface with a smaller area at a well-controlled convection rate may give a more reproducible and reliable current. The * Corresponding author. Fax: ; d930099@icpc00.icpc.fukui-u.ac.jp. 1 Present address. ECO Chemie, Kanealweg 18/J, 3526 KL Utrecht, The Netherlands other advantage of the downsized interface is the possibility of reducing the ohmic potential drop in the organic phase. This is the case with ultramicroelectrode techniques [3,4]. An increase in the convection rate not only provides current-potential curves with a steady state current but also enables the study of rapid kinetics such as followup chemical complications and adsorption kinetics, as in hydrodynamic voltammetry [5]. A new technique associated with convection has recently been developed by using mercury microelectrodes [6 9] which rapidly drop into a solution. This technique, called dropping mercury microelectrodes (DM E), has the advantages of small size, fast convention and renewal of the electrode. If mercury is replaced by an aqueous solution, the technique of the DM E may be applied to the oil water interface in order to accomplish a small size and a rapid variation of the interface. In this paper, we construct the oil water interface by dropping aqueous electrolyte into nitrobenzene through a thin capillary /99/$ - see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S (99)

2 A. Baars et al. / Journal of Electroanalytical Chemistry 464 (1999) Table 1 Features of droplets at four capillaries Capillaries Inner diameter/mm Drop time/s Flow rate/mm 3 s 1 Radius of drop/mm A B C D 0.2* * The inner diameter of the double capillary. 2. Experimental 2.1. Construction of the electrochemical cell In the design of the dropping oil water interface the aqueous electrolyte solution replaces the mercury of the DM E [6]. The electronic and hydrodynamic properties of the aqueous electrolyte are, however, different from mercury, especially the conductivity as well as the interfacial tension. The interfacial tension at the nitrobenzene water interface is only 6% of that at the mercury water interface [10]. Thus, droplets of the aqueous electrolyte are likely to be larger than those of mercury. However, the smaller density of the aqueous electrolyte compared to that of mercury makes droplets smaller [11]. Since the momentum of the flow also participates in the drop time for rapid convection, it is further difficult to predict the size of droplets. Although a thinner capillary is expected to make smaller droplets, it decreases the flow rate. In contrast, the conductivity of the aqueous electrolyte, e.g. 12 S m 1 of 1 M LiCl, is much lower than that of mercury (10 6 Sm 1 ). Thus, the aqueous solution phase filled in the capillary shows an extremely high electric resistance when a capillary of the DM E is directly applied to the oil water interface. A large capillary is preferable for good conductance and a fast flow rate but it is not desirable for accomplishing short drop times. The capillary may have optimum conditions for practical use. We prepared four kinds of capillaries 0.4, 0.2, 0.13 and 0.03 mm in diameter. We measured drop time and flow rate when the reservoir tank of water was lifted up 60 cm from the tip of the capillary. The smaller the inner diameter of the capillary, the slower the flow rate, as shown in Table 1. However, the drop size and the drop time had no simple relation with the diameter. The higher the reservoir, the faster the flow rate. The capillary of 0.03 mm in diameter was too narrow for the solution to pass through. The drop time of the capillary of 0.13 mm diameter was too long for our purpose. Thus we chose the capillary with a diameter of 0.2 mm. This capillary demonstrated a large resistance, which proved impractical to compensate by use of the positive feedback circuit. In order to reduce the resistance, we cut off the capillary as short as possible (4 mm) and connected it to an additional large capillary with a help of silicon rubber, as shown in Fig. 1. The additional capillary was grounded obliquely on its surface in order to avoid accumulation of droplets. During our dropping processes, aqueous droplets got stuck on the orifice of the capillary and then interfered with the reproducibility of the current. This problem was solved by siliconization at the capillary. All experiments were made with siliconized capillaries. The capillary unit was mounted in a cell depicted in Fig. 2. The counter electrodes were Pt coils with a large surface (6 cm 2 ), one placed in the upper and the other placed in the lower water phase. The reference electrode, Re1, was an AgCl electrode in 0.1 M tetrabutylammonium chloride (TBACl) aqueous solution which was connected to the oil phase through a glass fritt. The Re2 reference electrode was an AgCl electrode filled with 1 M KCl solution, placed in the upper water reservoir via a plug. Fig. 1. Illustration of the capillary of the fast dropping system.

3 130 A. Baars et al. / Journal of Electroanalytical Chemistry 464 (1999) except for the addition of the stage connecting to Re2. When the switch SW2 is closed, the potential across the oil water interface becomes V in = (water) (oil). Since the counter electrode Ce2 is kept at virtual ground, the current can be measured with a current follower. This is an advantage for getting a high dynamic performance with low noise. This design might cause problems of oscillations or instability when the SW1 is closed to the positive feedback mode. The instability is, however, avoided by the capacitor C3, that damps oscillations. It should be noted that the oscillation can be damped quite severely without affecting the dynamic performance of the potentiostat, because the voltage at Re2 is more or less stable with respect to ground (Ce2) Chemicals and instruments Fig. 2. The electrochemical cell; (A) the capillary that produces the aqueous droplets, (Re1) the AgCl 0.1 M TBACl in water reference electrode, (Re2) the AgCl 1.0 M KCl in water reference electrode, (Ce1 and Ce2) the platinum coils as a counter electrode, (W) the water phase with 1M LiCl, (O) the oil phase with x M TBATPB in 1 M TBACl+nitrobenzene, (B) drain for excess of the aqueous electrolyte Design of the 4-electrode potentiostat Although some 4-electrode potentiostats have already been devised [12,13], they are primarily used for static oil water interfaces. Since they have drawbacks for rapid responses such as in ac voltammetry, we designed a rapid 4-electrode potentiostat. The circuit shown in Fig. 3 is a variant of the conventional adder potentiostat [14], The aqueous solution was prepared from Aquarius TU-100 (Advantec) deionized water with 1 M LiCl of analytical grade (Wako). Nitrobenzene was also of analytical grade (Wako). The electrolyte in nitrobenzene was tetrabutylammonium tetraphenylborate (TBATPB), which was synthesized from tetrabutylammonium bromide and sodium tetraphenylborate. Chemicals used for the reference electrode were TBACl and KCl of analytical grade (Wako). All measurements were made at 25. The dc voltammograms were recorded with a personal computer system equipped with a A/D and D/A converter interface card, PCI-812 (Advantech). The dc potentials were supplied through the DAC to the V in of the potentiostat (Fig. 3). The currents were measured from the damped I out by the ADC, and averaged over 1 s. The averaged currents, I were recorded in dc voltammograms. 3. Results and discussion Fig. 3. Circuit of the 4-electrode potentiostat. C1=20 pf, C2=2 nf, C3=C5=4.7 nf, C4=C6=100 pf, R1=100 k, R2=10 k, R3=1 k, R4=1 k, operational amplifiers: LF356. When the upper reservoir including 1 M LiCl was lifted up 60 cm above the water level of the lower reservoir, the aqueous electrolyte flowed out from the capillary in a series of droplets at the rate of 15 drops s 1 at any potential. Although this drop rate is rather lower than the 500 drops s 1 of the DM E, it is still much faster than that of the conventional dropping oil water interface. The small droplet (radius 0.8 mm) can decrease the IR drop more than the conventional oil water interface. We measured the resistance between Re1 and Re2 by the ac impedance technique with 1 khz, and obtained 45 k. This was close to the value (40 k ) calculated from the molar conductance of LiCl, the concentration of LiCl, the radii (0.2 and 0.4 mm) and lengths (4 and 16 mm) of the capillaries. The difference, 5 k, may be ascribed to the resistance of the oil phase. The resistance was corrected with the IR feedback by adjusting SW1 and R4 in the circuit.

4 A. Baars et al. / Journal of Electroanalytical Chemistry 464 (1999) Fig. 4. Voltammograms at the nitrobenzene (x M TBATPB) water (1 M LiCl) interface for x=0.1 ( ), 0.05 ( ) and M ( ). Dc voltammograms, which were obtained at different concentrations (0.10, 0.05, M) of TBATPB in nitrobenzene, are shown in Fig. 4. The voltammograms in Fig. 4 mostly consist of the charging current, partly because the potential at zero current (pzc) was invariant to the concentration, and partly because the current was proportional to the rate of the surface production. Indeed, the mercury DM E gave a large charging current rather than a Faradaic current [6,7] because of the large rate of the surface production. Values of the charge density were calculated from It 1 /A, where t 1 is the drop time, and A is the surface area of the interface. The differentiation, d /de, gives the differential capacity, C d. Fig. 5 showed the plot of the numerical differentiation against the potential. The curve for 0.1 M TBATPB agreed with the literature values obtained for 0.1 M TBATPB at a flat interface with an area of 19 mm 2 [15]. Thus the present system enables the determination of the double layer charge from measurements of the charging current at a fast dropping rate. The integration of the charge density with respect to the potential gives the surface tension,, according to Lippman s equation. Fig. 6 shows the potential dependence of for three concentrations of TBATPB. They demonstrate the expected shape [16]. Fig. 5. Potential dependence of differential capacity at the oil water interface under the same conditions as in Fig. 4. The curves were drawn by the differentiation of the plots in Fig. 4 without curve fitting or data selection, whereas the symbols were from literature values [15] for 0.1 M TBATPB in nitrobenzene. capillaries, and to reduce the ohmic drop of the solution in the capillary. The obliquely ground siliconated surface was useful for avoiding accumulation of droplets. The short drop time increased the charging current more than the ion transfer current. The dropping behaviour is virtually independent of surface tension and thus the potential. Instead, the dropping behaviour depends on hydrodynamic phenomena such as flow rate and viscosity. The fast dropping interface is 4. Conclusions A fast dropping oil water electrode with a drop time of 0.07 s and a radius of 0.81 mm has been created by dropping an aqueous solution into an oil phase via a double capillary. Requirements of creating fast and small droplets are to find an optimum radius for the Fig. 6. Potential dependence of the surface tension, corresponding to the curves in Fig. 5. The potential applied ranged from 1 to1v.

5 132 A. Baars et al. / Journal of Electroanalytical Chemistry 464 (1999) helpful for investigating diffuse double layers and adsorption at the oil water interface. Acknowledgements We sincerely thank Dr H. Katano, Fukui Prefecture University for his guidance with the synthesis of the organic electrolytes. References [1] H.H. Girault, D.J. Schiffrin, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 15, Marcel Dekker, NY, 1989, p. 1. [2] M. Senda, T. Kakinchi, T. Osakai, Electrochim. Acta 36 (1991) 253. [3] R.M. Wightman, D.O. Wipf, in: A.J. Bard (Ed.), Electroanalytical Chemistry, vol. 15, Marcel Dekker, NY, 1989, pp [4] K. Aoki, Electroanalysis 5 (1993) 627. [5] A.J. Bard, L. Faulkner, Electrochemical Methods, Wiley, NY, 1980, pp [6] A. Baars, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 283 (1990) 99. [7] A. Baars, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 329 (1992) 171. [8] A. Baars, J.W. Knapen, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 368 (1994) 293. [9] A. Baars, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 364 (1994) 189. [10] A.W. Adamson, Physical Chemistry of Surfaces, 3rd Edition, Wiley, NY, 1976, p. 39. [11] A.W. Adamson, Physical Chemistry of Surfaces, 3rd Edition, Wiley, NY, 1976, p. 20. [12] T. Osakai, T. Nuno, Y. Yamamoto, A. Saito, M. Senda, Bunseki Kagaku 38 (1989) 479. [13] Z. Samec, V. Marecek, J. Electroanal. Chem. 100 (1979) 841. [14] A.J. Bard, L. Faulkner, Electrochemical Methods, Wiley, NY, 1980, p [15] T. Wandlowski, K. Holub, V. Marecek, Z. Samec, Electrochim. Acta 40 (1995) [16] T. Kakinchi, J. Nokuchi, M. Senda, J. Electroanal. Chem. 336 (1992)

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Electronic Supplementary Material (ESI) for RSC Advances. This journal is The Royal Society of Chemistry 2014 Journal Name Chemicals Electronic Supplementary Information Two kinds of commercial anion exchange