- Email: [email protected]
Increasing the potential window of the interface between two immiscible electrolyte solutions to more than 1.2 V
Electrochemistry Communications 13 (2011) 1539–1541
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Increasing the potential window of the interface between two immiscible electrolyte solutions to more than 1.2 V Nico E.A. Cousens, Anthony R. Kucernak ⁎ Department of Chemistry, Imperial College, London SW7 2AZ, UK
a r t i c l e
i n f o
Article history: Received 2 August 2011 Received in revised form 17 October 2011 Accepted 17 October 2011 Available online 22 October 2011 Keywords: ITIES Ion transfer Salting out Hydrophobic ions Hydrophilic ions
a b s t r a c t The polarisation window of an interface between two immiscible electrolyte solutions (ITIES) was maximised by systematically investigating different electrolyte and solvent combinations. The bulky organic electrolyte bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and three inorganic electrolytes, LiF, NaCl and MgSO4 were studied. These electrolytes have high free energies of transfer between organic and aqueous phases, making them ideal choices. An unconventional solvent system produced by mixing 1,2-dichloroethane with cyclohexane was also used resulting in a 30% increase of the potential window. An ITIES system composed of 2 M MgSO4 in water and 1 mM bis(triphenylphosphoranylidene) ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in 1:1 DCE/cyclohexane achieves a potential window of 1256 mV. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last three decades, research into the interface between two immiscible electrolyte solutions (ITIES) has focussed on a wide range of topics including electroanalysis, ion transfer, electron transfer and biological models of cell membranes [1]. Recent developments have been boosted by the opportunity to self-assemble molecular, supramolecular and nano-architectures at the interface [2–7]. In addition, the use of ITIES for electrowetting has been shown to significantly reduce the potential requirement for an electrowetting response [8,9]. However, the potential drop over an ITIES is limited because at large potentials ions begin to cross the interface [10]. Therefore, it is desirable to maximise the potential window over which the ITIES is stable. Three fundamental aspects control the potential window: (1) the interaction between the ions and water, namely hydrophobicity of the “organic” ions and hydrophilicity of the “aqueous” ions; (2) interaction between the ions and the oil; and (3) ‘many-body’ interactions such as the salting out effect. Highly hydrophobic organic ions are more likely to provide large potential windows. The use of heavy ions, such bis(triphenylphosphoranylidene)ammonium and tetrakis(pentafluorophenyl)borate, to achieve this has been well established [11]. Similarly, the selection of inorganic electrolytes for the aqueous phase should be based on maximising hydrophilicity, e.g. by selecting small ions with highly localized charge. It has also been suggested that changing the solvent could be an effective way of increasing the potential window [12,13]. This will greatly
⁎ Corresponding author. Tel.: + 44 20 75945831; fax: + 44 20 75945804. E-mail address: [email protected] (A.R. Kucernak). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.10.015
affect the solvation energy of an ion. For example, the standard free energy of transfer of Mg2 + from water to oil changes from 64 kJ mol− 1 (nitrobenzene) to 114 kJ mol− 1 (DCE) [14]. There are two constraints on the organic solvent: it must be immiscible with water and must dissolve organic salts fairly well. This largely excludes very non-polar solvents such as alkanes, as they will not dissolve organic salts. However, as we shall see later, mixed solvents comprising alkanes and DCE—or similar oil—can be tailored to dissolve the desired salt whilst remaining relatively non-polar. Finally, ‘many-body’ interactions have an effect on the free energy of transfer. These include the salting out effect and the effect of ionic activity [15,16]. The purpose of the present work is to combine these methods in order to maximise the size of the ITIES window. This potentially broadens and improves the possible applications of ITIES. 2. Experimental A Gamry Reference 600 potentiostat was used for all electrochemical measurements unless otherwise stated. Experiments were performed in a custom built cell similar to that used by Samec et al. holding approximately 3 ml of aqueous phase and 2 ml of organic phase [17]. The surface area of the liquid/liquid interface was 28 mm2. The complete cell was: AgjAgCljAXðaqÞ5mMjBTPPATFPBðoilÞ1mMjBTPPATFClðaqÞ10mMjAgCljAg
where the oil phase was either DCE or a 1:1 mixture of DCE and cyclohexane. The Ag/AgCl reference was found to provide a stable reference potential even when used in sulphate or fluoride solutions. tetramethylammonium chloride (TMACl) was used as a reference potential scale for
N.E.A. Cousens, A.R. Kucernak / Electrochemistry Communications 13 (2011) 1539–1541
a 2
Current /μA cm-2
the Galvani potential difference (Δwoφ [TMA+] =160 mV)[18]. The window limits were arbitrarily measured as the potentials at which the ion transfer currents rose to ±1 μA cm− 2. At these very low currents Ohmic losses are insignificant. Capacitance curves were calculated from the fitting of a standard Randles circuit to impedance data. The frequency response was scanned from 2 to 100 Hz, with higher frequencies showing experimental artefacts as reported elsewhere [19]. The capacitance was calculated at 10 mV intervals. The organic electrolyte bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BTPPATFPB) was precipitated from two solutions of BTPPACl (≥98%, Fluka) and NaTFPB (supplied by Prof. T. Kakiuchi, Kyoto University) dissolved in a 1:2 mixture of methanol and water and then recrystallised from hot acetone. NaCl (≥ 99.8%, Sigma-Aldrich), LiF (≥99.995%, Aldrich), MgSO4.7H2O (analytical grade, VWR), TMACl (≥99.0%, Fluka), DCE (≥99.0%, Sigma-Aldrich) and cyclohexane (≥99%, Sigma-Aldrich) were used as supplied. Aqueous solutions were prepared with ultrapure water (resistivity 18.2 MΩ cm) from a Millipore Elix 5 water purification system. All experiments were conducted at 20 ±1 °C. The dielectric constant of the mixed solvent was measured using a homemade capacitance cell and a Solartron 1260 Impedance Analyser. The cell consisted of two parallel glass slides (75 mm × 25 mm) sputtered with gold, separated by 340 μm and calibrated against a range of solvents.
-
-
Xo↔Xw
1
0 NaCl 5 mM LiF 5 mM MgSO4 5 mM
-1
+
+
A w↔A o
-2 -800
-600
-400
-200
0
200
400
600
800
Δw o φ /mV
b 2 -
Current /μA cm-2
1
-
Xo ↔Xw
0 NaCl 5 mM LiF 5 mM MgSO4 5 mM
-1
MgSO4 2000 mM
-2 -800
3. Results and discussion
-600
-400
-200
0
200
+
+
Aw↔ Ao
400
600
800
Δ ow ϕ /mV
3.1. Hydrophilicity of the aqueous ions
c 100 75
2↔SO4 w
15
o
Current /μA cm-2
Windows for NaCl, LiF and MgSO4 are shown in Fig. 1a with results collected in Table 1. In accordance with the literature MgSO4 shows the largest window, however, it may not be suitable for all applications owing to its divalency [20]. BTPPATFPB (1 mM) was used as organic electrolyte in all cases, and pure DCE was used as organic solvent. All voltamograms are qualitatively similar. The negative peaks at low potentials arise due to transfer of anions from the aqueous phase into the organic solvent, while the positive peaks at higher potentials are a consequence of cation transfer. The point of zero charge (PZC) calculated from the capacitance minimum seen in Fig. 1c was the same as the PZC calculated from the transfer of TMA +.
20 2SO4
50
10
25
5
0
0
C /μF cm-2
1540
-25 DCE Mixed solvent
-50
2+
Mg w↔Mg
-75 -100 -800
-600
-400
-200
0
200
400
2+ o
600
800
Δ ow ϕ /mV
3.2. Organic solvent As already explained, the potential window can be further increased if the solvation energy of the aqueous ions in the organic solvent is decreased (i.e., the energy of the aqueous ions in the organic solvent is increased). This can be achieved by lowering the dielectric constant of the organic solvent. As such, a 1:1 volume mixture of DCE and cyclohexane was used. This has a dielectric constant of 5.1 ± 0.1, a factor of two lower than the dielectric constant of DCE (10.4). Crucially, BTPPATFPB is soluble in this mixture (>10 mM), whereas it is insoluble in pure cyclohexane. Table 1 Size of voltage window for different electrolytes. In all cases the organic electrolyte was 1 mM BTPPATFPB. The limits are arbitrarily defined as the potential at which the current rises to ±1 μA cm– 2. Aqueous electrolyte
Organic solvent
Electrochemical window/mV
5 mM NaCl 5 mM LiF 5 mM MgSO4 5 mM NaCl 5 mM LiF 5 mM MgSO4 2000 mM MgSO4
DCE DCE DCE 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane
754 858 911 1044 1172 1208 1256
Fig. 1. Voltammograms for a variety of ITIES showing a progressive increase in electrochemical window. (a) 5 mM NaCl, LiF or MgSO4 in water with 1 mM BTPPATFPB in DCE as the organic phase; (b) 5 mM NaCl, LiF or MgSO4 and 2 M MgSO4 in water with 1 mM BTPPATFPB in 1:1 volume mixture of DCE and cyclohexane; (c) 10 mM MgSO4 in water with 10 mM BTPPATFPB in DCE and DCE/cyclohexane with larger current limits and capacitance curves about the PZC. Scan rate = 10 mV s– 1. Anions A+ and cations X– of the inorganic electrolytes transfer between the water and oil phases as denoted by subscripts w and o respectively. Δwoφ is the Galvani potential difference across the ITIES.
Fig. 1b shows the potential windows for the same electrolytes as in Fig. 1a but with the mixed solvent described earlier. The voltage window parameters are collected in Table 1. All three electrolytes show the same trends as for the pure DCE but the potential windows achievable with a low dielectric constant solvent are larger on an absolute scale. Taking MgSO4 as an example, the window increases from 911 mV to 1208 mV: an increase of over 30%. To the best of our knowledge, this is over 400 mV larger than any other polarisation window reported in the literature. It is important to note that at such small currents this increase cannot be attributed to Ohmic effects. Using the capacitance minimum to estimate the PZC the TMA + transfer potential for this electrolyte combination was calculated as 232 ± 10 mV. The error is associated with the particularly broad capacitance minimum.
N.E.A. Cousens, A.R. Kucernak / Electrochemistry Communications 13 (2011) 1539–1541
Fig. 1c shows the improvement in electrochemical window in moving from DCE to a mixed DCE/Cyclohexane solvent. Utilising the new solvent system a 300 mV improvement in the electrochemical window is seen (Table 1). Using the CVs in Fig. 1c the formal free enθ′, w → o ergies of transfer, ΔGtr , of the Mg 2+ and SO42− ions were calculated using the method developed by Shao, Stuart and Girault[18]. The CVs were corrected for Ohmic losses based on cell resistances of 300 Ω for the DCE cell and 4500 Ω for the mixed electrolyte cell as measured by impedance spectroscopy. The very sharp peaks following the return peak of the mixed electrolyte CV in Fig. 1c are seen elsewhere and most probably result from solvent transfer accompanyθ′, w → o ing ion transfer. Upon changing the solvent ΔGtr [Mg2 +] rose from −1 –1 θ′, w → o 114 kJ mol to 116 kJ mol while ΔGtr [SO42–] rose from 98 kJ mol– 1 to 113 kJ mol– 1, suggesting that the solvation energy of the sulphate is affected to a much greater extent. This corresponds to a combined increase of 17 kJ mol– 1 although the error in each value is estimated at ±1.9 kJmol – 1 because of the error in the position of the PZC. These values are in agreement with literature[20]. The capacitance at the PZC decreases from 3.57 μF cm– 2 for pure DCE to 3.25 μF cm– 2 for the mixed solvent. This decrease would be expected because of the lower dielectric constant of the mixed solvent. This simple method illustrates how the use of heavy organic salts allows the use of solvents with poorer ionic solubility. The result is an increase in the polarisability of the ITIES. Using even heavier salts and precisely tailored solvents it should be possible to make the ITIES even larger. 3.3. Many-body interactions Fig. 1b shows the effect of using a high concentration of ions in the aqueous phase. Increasing the MgSO4 concentration to 2 M has a small but significant effect on the size of the polarisation window. This cannot be caused by the salting out effect as the potential limits are defined by the transfer of aqueous ions rather than organic ions, as required for the salting out effect. The authors speculate that this increase results from a decrease in the activity coefficients of the MgSO4 ions. This interesting effect warrants further investigation and seems to provide another method of increasing the polarisation window. 4. Conclusions A range of electrolytes and solvent systems which maximise the size of the ITIES polarisation window have been reported. These
1541
windows are over 1.2 V in magnitude and larger than previously seen in the literature. Adoption of this technique or the use of new solvents will hopefully open new research possibilities in the field of ITIES such as electrowetting and nanoparticle assembly. Further work is in progress to find other solvents and solvent mixtures to further increase the size of the ITIES polarisation window. Acknowledgments The authors gratefully acknowledge the provision of the sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate by Prof. T. Kakiuchi, Kyoto University, Japan. References [1] H.H. Girault, Electrochimica Acta 32 (1987) 383–385. [2] N. Younan, M. Hojeij, L. Ribeaucourt, H.H. Girault, Electrochemistry Communications 12 (2010) 912–915. [3] P. Galletto, Journal of Physics: Condensed Matter 19 (2007) 375108. [4] Z. Samec, N. Eugster, D.J. FermIn, H.H. Girault, Journal of Electroanalytical Chemistry 577 (2005) 323–337. [5] B. Su, D.J. Fermín, J.-P. Abid, N. Eugster, H.H. Girault, Journal of Electroanalytical Chemistry 583 (2005) 241–247. [6] A. Kornyshev, Journal of Physics: Condensed Matter 19 (2007) 375111. [7] M. Flatte, A. Kornyshev, M. Urbakh, Journal of Physical Chemistry C 114 (2010) 1735–1747. [8] C.W. Monroe, Journal of Physics: Condensed Matter 18 (2006) 2837. [9] A. Kornyshev, A. Kucernak, M. Marinescu, C. Monroe, A. Sleightholme, M. Urbakh, Journal of Physical Chemistry C 114 (2010) 14885–14890. [10] A. Volkov, D. Deamer, Liquid-Liquid Interfaces: Theory and Methods, CRC Press, 1996. [11] A.A. Stewart, Y. Shao, C.M. Pereira, H.H. Girault, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 305 (1991) 135–139. [12] Z. Koczorowski, I. Paleska, G. Geblewicz, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 164 (1984) 201–204. [13] H. Katano, H. Tatsumi, M. Senda, H. Tatsumi, M. Senda, Talanta 63 (2004) 185–193. [14] Y. Yoshida, Z. Yoshida, H. Aoyagi, Y. Kitatsuji, A. Uehara, S. Kihara, Analytica Chimica Acta 452 (2002) 149–161. [15] G. Geblewicz, A. Kontturi, K. Kontturi, D. Schiffrin, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 217 (1987) 261–269. [16] A. Kontturi, K. Kontturi, L. Murtomaki, D. Schiffrin, Journal of the Chemical Society Faraday Transactions 86 (1990) 931–936. [17] Z. Samec, V. Marecek, D. Homolka, Faraday Discussions of the Chemical Society 77 (1984) 197–208. [18] Y. Shao, A.A. Stewart, H.H. Girault, Journal of the Chemical Society Faraday Transactions 87 (1991) 2593–2597. [19] M.C. Wiles, D.J. Schiffrin, T.J. Vandernoot, A.F. Silva, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 278 (1990) 151–159. [20] M. Zhou, S. Gan, L. Zhong, B. Su, L. Niu, Analytical Chemistry 82 (2010) 7857–7860.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Increasing the potential window of the interface between two immiscible electrolyte solutions to more than 1.2 V Nico E.A. Cousens, Anthony R. Kucernak ⁎ Department of Chemistry, Imperial College, London SW7 2AZ, UK
a r t i c l e
i n f o
Article history: Received 2 August 2011 Received in revised form 17 October 2011 Accepted 17 October 2011 Available online 22 October 2011 Keywords: ITIES Ion transfer Salting out Hydrophobic ions Hydrophilic ions
a b s t r a c t The polarisation window of an interface between two immiscible electrolyte solutions (ITIES) was maximised by systematically investigating different electrolyte and solvent combinations. The bulky organic electrolyte bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and three inorganic electrolytes, LiF, NaCl and MgSO4 were studied. These electrolytes have high free energies of transfer between organic and aqueous phases, making them ideal choices. An unconventional solvent system produced by mixing 1,2-dichloroethane with cyclohexane was also used resulting in a 30% increase of the potential window. An ITIES system composed of 2 M MgSO4 in water and 1 mM bis(triphenylphosphoranylidene) ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate in 1:1 DCE/cyclohexane achieves a potential window of 1256 mV. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last three decades, research into the interface between two immiscible electrolyte solutions (ITIES) has focussed on a wide range of topics including electroanalysis, ion transfer, electron transfer and biological models of cell membranes [1]. Recent developments have been boosted by the opportunity to self-assemble molecular, supramolecular and nano-architectures at the interface [2–7]. In addition, the use of ITIES for electrowetting has been shown to significantly reduce the potential requirement for an electrowetting response [8,9]. However, the potential drop over an ITIES is limited because at large potentials ions begin to cross the interface [10]. Therefore, it is desirable to maximise the potential window over which the ITIES is stable. Three fundamental aspects control the potential window: (1) the interaction between the ions and water, namely hydrophobicity of the “organic” ions and hydrophilicity of the “aqueous” ions; (2) interaction between the ions and the oil; and (3) ‘many-body’ interactions such as the salting out effect. Highly hydrophobic organic ions are more likely to provide large potential windows. The use of heavy ions, such bis(triphenylphosphoranylidene)ammonium and tetrakis(pentafluorophenyl)borate, to achieve this has been well established [11]. Similarly, the selection of inorganic electrolytes for the aqueous phase should be based on maximising hydrophilicity, e.g. by selecting small ions with highly localized charge. It has also been suggested that changing the solvent could be an effective way of increasing the potential window [12,13]. This will greatly
⁎ Corresponding author. Tel.: + 44 20 75945831; fax: + 44 20 75945804. E-mail address: [email protected] (A.R. Kucernak). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.10.015
affect the solvation energy of an ion. For example, the standard free energy of transfer of Mg2 + from water to oil changes from 64 kJ mol− 1 (nitrobenzene) to 114 kJ mol− 1 (DCE) [14]. There are two constraints on the organic solvent: it must be immiscible with water and must dissolve organic salts fairly well. This largely excludes very non-polar solvents such as alkanes, as they will not dissolve organic salts. However, as we shall see later, mixed solvents comprising alkanes and DCE—or similar oil—can be tailored to dissolve the desired salt whilst remaining relatively non-polar. Finally, ‘many-body’ interactions have an effect on the free energy of transfer. These include the salting out effect and the effect of ionic activity [15,16]. The purpose of the present work is to combine these methods in order to maximise the size of the ITIES window. This potentially broadens and improves the possible applications of ITIES. 2. Experimental A Gamry Reference 600 potentiostat was used for all electrochemical measurements unless otherwise stated. Experiments were performed in a custom built cell similar to that used by Samec et al. holding approximately 3 ml of aqueous phase and 2 ml of organic phase [17]. The surface area of the liquid/liquid interface was 28 mm2. The complete cell was: AgjAgCljAXðaqÞ5mMjBTPPATFPBðoilÞ1mMjBTPPATFClðaqÞ10mMjAgCljAg
where the oil phase was either DCE or a 1:1 mixture of DCE and cyclohexane. The Ag/AgCl reference was found to provide a stable reference potential even when used in sulphate or fluoride solutions. tetramethylammonium chloride (TMACl) was used as a reference potential scale for
N.E.A. Cousens, A.R. Kucernak / Electrochemistry Communications 13 (2011) 1539–1541
a 2
Current /μA cm-2
the Galvani potential difference (Δwoφ [TMA+] =160 mV)[18]. The window limits were arbitrarily measured as the potentials at which the ion transfer currents rose to ±1 μA cm− 2. At these very low currents Ohmic losses are insignificant. Capacitance curves were calculated from the fitting of a standard Randles circuit to impedance data. The frequency response was scanned from 2 to 100 Hz, with higher frequencies showing experimental artefacts as reported elsewhere [19]. The capacitance was calculated at 10 mV intervals. The organic electrolyte bis(triphenylphosphoranylidene)ammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BTPPATFPB) was precipitated from two solutions of BTPPACl (≥98%, Fluka) and NaTFPB (supplied by Prof. T. Kakiuchi, Kyoto University) dissolved in a 1:2 mixture of methanol and water and then recrystallised from hot acetone. NaCl (≥ 99.8%, Sigma-Aldrich), LiF (≥99.995%, Aldrich), MgSO4.7H2O (analytical grade, VWR), TMACl (≥99.0%, Fluka), DCE (≥99.0%, Sigma-Aldrich) and cyclohexane (≥99%, Sigma-Aldrich) were used as supplied. Aqueous solutions were prepared with ultrapure water (resistivity 18.2 MΩ cm) from a Millipore Elix 5 water purification system. All experiments were conducted at 20 ±1 °C. The dielectric constant of the mixed solvent was measured using a homemade capacitance cell and a Solartron 1260 Impedance Analyser. The cell consisted of two parallel glass slides (75 mm × 25 mm) sputtered with gold, separated by 340 μm and calibrated against a range of solvents.
-
-
Xo↔Xw
1
0 NaCl 5 mM LiF 5 mM MgSO4 5 mM
-1
+
+
A w↔A o
-2 -800
-600
-400
-200
0
200
400
600
800
Δw o φ /mV
b 2 -
Current /μA cm-2
1
-
Xo ↔Xw
0 NaCl 5 mM LiF 5 mM MgSO4 5 mM
-1
MgSO4 2000 mM
-2 -800
3. Results and discussion
-600
-400
-200
0
200
+
+
Aw↔ Ao
400
600
800
Δ ow ϕ /mV
3.1. Hydrophilicity of the aqueous ions
c 100 75
2↔SO4 w
15
o
Current /μA cm-2
Windows for NaCl, LiF and MgSO4 are shown in Fig. 1a with results collected in Table 1. In accordance with the literature MgSO4 shows the largest window, however, it may not be suitable for all applications owing to its divalency [20]. BTPPATFPB (1 mM) was used as organic electrolyte in all cases, and pure DCE was used as organic solvent. All voltamograms are qualitatively similar. The negative peaks at low potentials arise due to transfer of anions from the aqueous phase into the organic solvent, while the positive peaks at higher potentials are a consequence of cation transfer. The point of zero charge (PZC) calculated from the capacitance minimum seen in Fig. 1c was the same as the PZC calculated from the transfer of TMA +.
20 2SO4
50
10
25
5
0
0
C /μF cm-2
1540
-25 DCE Mixed solvent
-50
2+
Mg w↔Mg
-75 -100 -800
-600
-400
-200
0
200
400
2+ o
600
800
Δ ow ϕ /mV
3.2. Organic solvent As already explained, the potential window can be further increased if the solvation energy of the aqueous ions in the organic solvent is decreased (i.e., the energy of the aqueous ions in the organic solvent is increased). This can be achieved by lowering the dielectric constant of the organic solvent. As such, a 1:1 volume mixture of DCE and cyclohexane was used. This has a dielectric constant of 5.1 ± 0.1, a factor of two lower than the dielectric constant of DCE (10.4). Crucially, BTPPATFPB is soluble in this mixture (>10 mM), whereas it is insoluble in pure cyclohexane. Table 1 Size of voltage window for different electrolytes. In all cases the organic electrolyte was 1 mM BTPPATFPB. The limits are arbitrarily defined as the potential at which the current rises to ±1 μA cm– 2. Aqueous electrolyte
Organic solvent
Electrochemical window/mV
5 mM NaCl 5 mM LiF 5 mM MgSO4 5 mM NaCl 5 mM LiF 5 mM MgSO4 2000 mM MgSO4
DCE DCE DCE 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane 1:1 DCE:cyclohexane
754 858 911 1044 1172 1208 1256
Fig. 1. Voltammograms for a variety of ITIES showing a progressive increase in electrochemical window. (a) 5 mM NaCl, LiF or MgSO4 in water with 1 mM BTPPATFPB in DCE as the organic phase; (b) 5 mM NaCl, LiF or MgSO4 and 2 M MgSO4 in water with 1 mM BTPPATFPB in 1:1 volume mixture of DCE and cyclohexane; (c) 10 mM MgSO4 in water with 10 mM BTPPATFPB in DCE and DCE/cyclohexane with larger current limits and capacitance curves about the PZC. Scan rate = 10 mV s– 1. Anions A+ and cations X– of the inorganic electrolytes transfer between the water and oil phases as denoted by subscripts w and o respectively. Δwoφ is the Galvani potential difference across the ITIES.
Fig. 1b shows the potential windows for the same electrolytes as in Fig. 1a but with the mixed solvent described earlier. The voltage window parameters are collected in Table 1. All three electrolytes show the same trends as for the pure DCE but the potential windows achievable with a low dielectric constant solvent are larger on an absolute scale. Taking MgSO4 as an example, the window increases from 911 mV to 1208 mV: an increase of over 30%. To the best of our knowledge, this is over 400 mV larger than any other polarisation window reported in the literature. It is important to note that at such small currents this increase cannot be attributed to Ohmic effects. Using the capacitance minimum to estimate the PZC the TMA + transfer potential for this electrolyte combination was calculated as 232 ± 10 mV. The error is associated with the particularly broad capacitance minimum.
N.E.A. Cousens, A.R. Kucernak / Electrochemistry Communications 13 (2011) 1539–1541
Fig. 1c shows the improvement in electrochemical window in moving from DCE to a mixed DCE/Cyclohexane solvent. Utilising the new solvent system a 300 mV improvement in the electrochemical window is seen (Table 1). Using the CVs in Fig. 1c the formal free enθ′, w → o ergies of transfer, ΔGtr , of the Mg 2+ and SO42− ions were calculated using the method developed by Shao, Stuart and Girault[18]. The CVs were corrected for Ohmic losses based on cell resistances of 300 Ω for the DCE cell and 4500 Ω for the mixed electrolyte cell as measured by impedance spectroscopy. The very sharp peaks following the return peak of the mixed electrolyte CV in Fig. 1c are seen elsewhere and most probably result from solvent transfer accompanyθ′, w → o ing ion transfer. Upon changing the solvent ΔGtr [Mg2 +] rose from −1 –1 θ′, w → o 114 kJ mol to 116 kJ mol while ΔGtr [SO42–] rose from 98 kJ mol– 1 to 113 kJ mol– 1, suggesting that the solvation energy of the sulphate is affected to a much greater extent. This corresponds to a combined increase of 17 kJ mol– 1 although the error in each value is estimated at ±1.9 kJmol – 1 because of the error in the position of the PZC. These values are in agreement with literature[20]. The capacitance at the PZC decreases from 3.57 μF cm– 2 for pure DCE to 3.25 μF cm– 2 for the mixed solvent. This decrease would be expected because of the lower dielectric constant of the mixed solvent. This simple method illustrates how the use of heavy organic salts allows the use of solvents with poorer ionic solubility. The result is an increase in the polarisability of the ITIES. Using even heavier salts and precisely tailored solvents it should be possible to make the ITIES even larger. 3.3. Many-body interactions Fig. 1b shows the effect of using a high concentration of ions in the aqueous phase. Increasing the MgSO4 concentration to 2 M has a small but significant effect on the size of the polarisation window. This cannot be caused by the salting out effect as the potential limits are defined by the transfer of aqueous ions rather than organic ions, as required for the salting out effect. The authors speculate that this increase results from a decrease in the activity coefficients of the MgSO4 ions. This interesting effect warrants further investigation and seems to provide another method of increasing the polarisation window. 4. Conclusions A range of electrolytes and solvent systems which maximise the size of the ITIES polarisation window have been reported. These
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windows are over 1.2 V in magnitude and larger than previously seen in the literature. Adoption of this technique or the use of new solvents will hopefully open new research possibilities in the field of ITIES such as electrowetting and nanoparticle assembly. Further work is in progress to find other solvents and solvent mixtures to further increase the size of the ITIES polarisation window. Acknowledgments The authors gratefully acknowledge the provision of the sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate by Prof. T. Kakiuchi, Kyoto University, Japan. References [1] H.H. Girault, Electrochimica Acta 32 (1987) 383–385. [2] N. Younan, M. Hojeij, L. Ribeaucourt, H.H. Girault, Electrochemistry Communications 12 (2010) 912–915. [3] P. Galletto, Journal of Physics: Condensed Matter 19 (2007) 375108. [4] Z. Samec, N. Eugster, D.J. FermIn, H.H. Girault, Journal of Electroanalytical Chemistry 577 (2005) 323–337. [5] B. Su, D.J. Fermín, J.-P. Abid, N. Eugster, H.H. Girault, Journal of Electroanalytical Chemistry 583 (2005) 241–247. [6] A. Kornyshev, Journal of Physics: Condensed Matter 19 (2007) 375111. [7] M. Flatte, A. Kornyshev, M. Urbakh, Journal of Physical Chemistry C 114 (2010) 1735–1747. [8] C.W. Monroe, Journal of Physics: Condensed Matter 18 (2006) 2837. [9] A. Kornyshev, A. Kucernak, M. Marinescu, C. Monroe, A. Sleightholme, M. Urbakh, Journal of Physical Chemistry C 114 (2010) 14885–14890. [10] A. Volkov, D. Deamer, Liquid-Liquid Interfaces: Theory and Methods, CRC Press, 1996. [11] A.A. Stewart, Y. Shao, C.M. Pereira, H.H. Girault, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 305 (1991) 135–139. [12] Z. Koczorowski, I. Paleska, G. Geblewicz, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 164 (1984) 201–204. [13] H. Katano, H. Tatsumi, M. Senda, H. Tatsumi, M. Senda, Talanta 63 (2004) 185–193. [14] Y. Yoshida, Z. Yoshida, H. Aoyagi, Y. Kitatsuji, A. Uehara, S. Kihara, Analytica Chimica Acta 452 (2002) 149–161. [15] G. Geblewicz, A. Kontturi, K. Kontturi, D. Schiffrin, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 217 (1987) 261–269. [16] A. Kontturi, K. Kontturi, L. Murtomaki, D. Schiffrin, Journal of the Chemical Society Faraday Transactions 86 (1990) 931–936. [17] Z. Samec, V. Marecek, D. Homolka, Faraday Discussions of the Chemical Society 77 (1984) 197–208. [18] Y. Shao, A.A. Stewart, H.H. Girault, Journal of the Chemical Society Faraday Transactions 87 (1991) 2593–2597. [19] M.C. Wiles, D.J. Schiffrin, T.J. Vandernoot, A.F. Silva, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 278 (1990) 151–159. [20] M. Zhou, S. Gan, L. Zhong, B. Su, L. Niu, Analytical Chemistry 82 (2010) 7857–7860.