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Ion transfer reactions across a liquid—liquid interface supported on a micropipette tip
179
J. Electroanal. Chem., 208 (1986) 179-183 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note ION TRANSFER REACTIONS ACROSS A LIQUID-LIQUID SUPPORTED ON A MICROPIPE’ITE TIP
G. TAYLOR Department
INTERFACE
and H.H.J. GIRAULT of Chemistry,
Umversrty
of Edinburgh, West Mains Roacl, Edmburgh EH9 3JJ (Great Britazn)
(Received 12th May 1986) INTRODUCTION
Electrochemical studies of ion transfer reactions across liquid-liquid interfaces have shown that the experimental determination of the kinetic parameters of these processes using classical techniques (ac voltammetry [l], convolution sweep voltammetry [2]) is difficult for the following reasons: (a) The large IR drop present mainly in the organic phase (nitrobenzene, l,Zdichloroethane), cannot be fully compensated or accounted for, leading consequently to a distortion of the measurements. (b) The presence of a supporting electrolyte at a relatively high concentration (10e3 M to 10-l M) in the organic phase used to minimize the ZR drop usually perturbs the experiment because the ion studied is subject to a strong ion-pairing effect with a counterion of the base electrolyte. However, despite the inaccuracy of the measurements, these investigations show clearly that ion transfer reactions are relatively fast processes. For example, the rate constant for the transfer of the tetraalkyl ammonium ion (C,-C,) from water to nitrobenzene was estimated to be higher than lop2 cm s-l [1,3]. In order to determine the kinetic parameters of such fast reactions, it is therefore necessary to enhance the mass transport of the ion to the interface. Hydrodynamic methods, which are often used to increase and control mass transport on solid electrodes, have already been used to study the kinetics of facilitated ion transport across supported liquid membranes [4]. However, due to the lack of mechanical stability of a liquid-liquid interface, these techniques cannot be used to establish two well characterised diffusion layers on each side of an unsupported interface? Transient techniques, on the other hand, where the rate of mass transport varies with time and frequency, do not lead to accurate measurements because of the low polarity of the organic phase (see above). The present communication aims to show that the use of very small liquid-liquid interfaces (< 100 pm2) provides the same advantageous properties as observed with 0022-0728/86/%03.50
0 1986 Elsevier Sequoia S.A.
180
solid microelectrodes.
The most important features of these small surface areas are
[51: (a) The formation of a spherical mass transfer pattern, leading to high steady-state conditions of mass transport. (b) The low value of the IR drop permits the investigation of charge transfer reactions in low polar media. EXPERIMENTAL
Using the technology developed to manufacture glass micropipettes for physiological intracellular measurements [6], liquid microelectrodes have been prepared from “Kwik-Fil” capillaries with inner filament (Clark Electromedical Instruments, Reading, England) pulled with a vertical pipette puller (Kopf 720, Tujunga, CA, USA). In order to obtain pipettes with “fine tips” and “short shanks”, the puller was used with a fine platinum-iridium (90%, 10%) heater element to localise the melting zone as much as possible. The tip of the pulled micropipette was then cut squarely with a pair of scissors. The pipettes were filled through the tip by capillary rise assisted by pumping through the stem. The diameter of the micropipette was measured with a microscope equipped with a micrometer eyepiece. (It is easier to measure the internal diameter of filled than of empty pipettes.) The cell assembly is composed of a U-tube where the non-aqueous solution is sandwiched between the “working” aqueous solution, which is similar to that located in the micropipette, and the “reference” aqueous solution. The purpose of the top aqueous layer is to maintain a partition equilibrium between the two immiscible electrolyte solutions, and prevent evaporation of the non-aqueous solvent. Two silver-silver chloride reference electrode wires were used, as shown in Fig. 1. The current follower, built with a high input impedance FET operational amplifier (Burr Brown OAP 104), was battery powered and located near the electrochemical cell in a Faraday cage.
Fig. 1. Circuit diagram for an ITIES supportedon a micropipette tip. (1) Working aqueous electrolyte, e.g. LiCI; (2) organic salt in 1,2dichloroethane, e.g. CVTPB; (3) reference aqueous electrolyte, e.g. CVCl. The (2)/(3) interface in unpolarisable because both solutions contain a common cation.
181 RESULTS AND DISCUSSION
Figure 2a shows the potential window obtained by the system 10e2 M LiCl/10-2 M CVTPB/10-2 M CVCl with a 25 pm inner diameter pipette. Usually, with larger interfacial areas, the polarisation curve is symmetrical and the current waves at both ends have the same shape as that observed in this case at negative potentials, corresponding to the transfer of TPB- ions from DCE to water and back. However, it can be seen that the current wave is in the steady state at positive potentials. This wave corresponds to the transfer of Cl-- ions from water to DCE and back. Examination of the potential window for other base electrolyte systems (e.g. Fig. 4) shows that if the return scan corresponds to the transfer of an ion from oil to water, then the current wave is in the steady state. Figure 2b shows the voltammogram obtained when 200 ,uM of TEATPB are added to the oil phase. The transfer of TEA+ ion from 12-DCE to water is in the steady state, showing that enhanced mass transport occurs due to spherical diffusion of the ion to the micropipette tip, as is usually observed with solid microelectrodes. On the other hand, the peculiarity of the liquid microelectrode is that the return scan is no longer in the steady state but has a peak shape showing that the return transfer is controlled by linear-diffusion mass transport. This asymmetry between the two diffusion regimes has been observed for every ion transfer reaction studied and is an interesting characteristic of liquid micropipette electrodes. This phenomenon is illustrated in Fig. 3. I i nA
.lO
Fig. 2. (a) Potential window for the system Ag/AgCI/10-2 M L1C1//10-3 M CVTPB/lO-’ $4 CVCl/AgCl/Ag. Sweep rate = 25 mV/s; internal radius = 25 pm; external radius - 60 pm. (b) As (a) after addition of 200 CM of TEATPB in phase (2).
182 10
Izs -0
-5 07
-06
-05
-04
-03
-02
-01
E/V
Fig. 3. Cyclic voltammogram for the transfer of TEA + ion across a 10 mM LiCl/l interface. Sweep rate = 25 mV/s; internal radius = 25 pm; external radius = 60 pm.
If this ITIES at the tip of the micropipette steady-state current could be calculated by
were
a perfect
mM
microdisc,
CVTPB
the
I = 4FD&,+c&A+r
where F is the faraday constant, D&,+ and C& + are the diffusion coefficient and concentration of TEA+ in dichloro ethane respectively and r represents the radius of the disc. Using D&+ = 7 x 10e6 cm” s-i [7] and r = 12.5 pm, the theoretical value for the current is 0.7 nA, which is less than the measured value of about 5 nA. This discrepancy between theoretical and observed values can be attributed to the fact that the ITIES is not a perfect disc and consequently the interfacial area may be larger than expected, especially if the aqueous electrolyte does not wet the glass properly. Although very fine micropipette tips (< 1 pm) can be made with automatic pipette pullers, we observed that it is very difficult to use such fine microelectrodes for electrochemical studies of ion transfer reactions across an ITIES. Since it is not possible to pull such a small tip with a short shank, the liquid microelectrode obtained is very resistive. The resistance of a cone section having a length I and
Fig. 4. Potential TBABr/AgCl/Ag (- - - - - -) Internal
wmdow for the system: Ag/AgC1/10-2 LiCl//10-3 M TBATPB/10m2 M ) Internal radius = 25 pm; outside radius = 60 pm: 5 mm long shank. for: (radius = 10 pm; outside radius = 30 pm; 2 cm long shank.
183
terminated
at one end by a disc of radius r,, and at the other end by a disc of radius
r,, is given by
R = pl,/?rr,rz For a fine tip of radius rl = 0.5 pm and a 2 cm long shank, the leading capillary having an internal diameter of 0.4 mm, the resistance of the microelectrode for a lo-’ M LiCl filling solution is equal to 250 Ma. Figure 4 shows the influence of the tip radius on the potential window. It can be seen that the system becomes very resistive and hinders any electrochemical investigation. ACKNOWLEDGEMENTS
The authors wish to thank the Nuffield Foundation grant, and the University of Edinburgh for financial
of England support.
for an equipment
REFERENCES 1 2 3 4 5 6
T. Osakat, T. KakuJam and M. Senda, Buil. Chem. Sot. Jpn.. 57 (1984) 370. Z. Samec. V. Marecek. J. Weber and D. Homolka, J. Electroanal. Chem., 126 (1981) 105. Z. Samec and V. Marecek, J. Electroanal. Chem., 200 (1986) 17. W.J. Albery, R.A. Choudery and P.R. Fisk, Faraday Discuss. Chem. Sot.. 77 (1984) 53. M.I. Montenegro. Port. Electrochim. Ada, 3 (1985) 165. R.D. Purves, Mlcr~l~trode Methods for Intracelluhar Recording, Academrc Press, New York. London, 1981. 7 H.H.J. Girault and D.J. Schiffrin in M.J. Allen and P.N.R. Usherwood (Eds.), Charge and Field Effects in Biosystems, Abacus Press, Tunbridge Wells, 1984, pp. 171-178.
J. Electroanal. Chem., 208 (1986) 179-183 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note ION TRANSFER REACTIONS ACROSS A LIQUID-LIQUID SUPPORTED ON A MICROPIPE’ITE TIP
G. TAYLOR Department
INTERFACE
and H.H.J. GIRAULT of Chemistry,
Umversrty
of Edinburgh, West Mains Roacl, Edmburgh EH9 3JJ (Great Britazn)
(Received 12th May 1986) INTRODUCTION
Electrochemical studies of ion transfer reactions across liquid-liquid interfaces have shown that the experimental determination of the kinetic parameters of these processes using classical techniques (ac voltammetry [l], convolution sweep voltammetry [2]) is difficult for the following reasons: (a) The large IR drop present mainly in the organic phase (nitrobenzene, l,Zdichloroethane), cannot be fully compensated or accounted for, leading consequently to a distortion of the measurements. (b) The presence of a supporting electrolyte at a relatively high concentration (10e3 M to 10-l M) in the organic phase used to minimize the ZR drop usually perturbs the experiment because the ion studied is subject to a strong ion-pairing effect with a counterion of the base electrolyte. However, despite the inaccuracy of the measurements, these investigations show clearly that ion transfer reactions are relatively fast processes. For example, the rate constant for the transfer of the tetraalkyl ammonium ion (C,-C,) from water to nitrobenzene was estimated to be higher than lop2 cm s-l [1,3]. In order to determine the kinetic parameters of such fast reactions, it is therefore necessary to enhance the mass transport of the ion to the interface. Hydrodynamic methods, which are often used to increase and control mass transport on solid electrodes, have already been used to study the kinetics of facilitated ion transport across supported liquid membranes [4]. However, due to the lack of mechanical stability of a liquid-liquid interface, these techniques cannot be used to establish two well characterised diffusion layers on each side of an unsupported interface? Transient techniques, on the other hand, where the rate of mass transport varies with time and frequency, do not lead to accurate measurements because of the low polarity of the organic phase (see above). The present communication aims to show that the use of very small liquid-liquid interfaces (< 100 pm2) provides the same advantageous properties as observed with 0022-0728/86/%03.50
0 1986 Elsevier Sequoia S.A.
180
solid microelectrodes.
The most important features of these small surface areas are
[51: (a) The formation of a spherical mass transfer pattern, leading to high steady-state conditions of mass transport. (b) The low value of the IR drop permits the investigation of charge transfer reactions in low polar media. EXPERIMENTAL
Using the technology developed to manufacture glass micropipettes for physiological intracellular measurements [6], liquid microelectrodes have been prepared from “Kwik-Fil” capillaries with inner filament (Clark Electromedical Instruments, Reading, England) pulled with a vertical pipette puller (Kopf 720, Tujunga, CA, USA). In order to obtain pipettes with “fine tips” and “short shanks”, the puller was used with a fine platinum-iridium (90%, 10%) heater element to localise the melting zone as much as possible. The tip of the pulled micropipette was then cut squarely with a pair of scissors. The pipettes were filled through the tip by capillary rise assisted by pumping through the stem. The diameter of the micropipette was measured with a microscope equipped with a micrometer eyepiece. (It is easier to measure the internal diameter of filled than of empty pipettes.) The cell assembly is composed of a U-tube where the non-aqueous solution is sandwiched between the “working” aqueous solution, which is similar to that located in the micropipette, and the “reference” aqueous solution. The purpose of the top aqueous layer is to maintain a partition equilibrium between the two immiscible electrolyte solutions, and prevent evaporation of the non-aqueous solvent. Two silver-silver chloride reference electrode wires were used, as shown in Fig. 1. The current follower, built with a high input impedance FET operational amplifier (Burr Brown OAP 104), was battery powered and located near the electrochemical cell in a Faraday cage.
Fig. 1. Circuit diagram for an ITIES supportedon a micropipette tip. (1) Working aqueous electrolyte, e.g. LiCI; (2) organic salt in 1,2dichloroethane, e.g. CVTPB; (3) reference aqueous electrolyte, e.g. CVCl. The (2)/(3) interface in unpolarisable because both solutions contain a common cation.
181 RESULTS AND DISCUSSION
Figure 2a shows the potential window obtained by the system 10e2 M LiCl/10-2 M CVTPB/10-2 M CVCl with a 25 pm inner diameter pipette. Usually, with larger interfacial areas, the polarisation curve is symmetrical and the current waves at both ends have the same shape as that observed in this case at negative potentials, corresponding to the transfer of TPB- ions from DCE to water and back. However, it can be seen that the current wave is in the steady state at positive potentials. This wave corresponds to the transfer of Cl-- ions from water to DCE and back. Examination of the potential window for other base electrolyte systems (e.g. Fig. 4) shows that if the return scan corresponds to the transfer of an ion from oil to water, then the current wave is in the steady state. Figure 2b shows the voltammogram obtained when 200 ,uM of TEATPB are added to the oil phase. The transfer of TEA+ ion from 12-DCE to water is in the steady state, showing that enhanced mass transport occurs due to spherical diffusion of the ion to the micropipette tip, as is usually observed with solid microelectrodes. On the other hand, the peculiarity of the liquid microelectrode is that the return scan is no longer in the steady state but has a peak shape showing that the return transfer is controlled by linear-diffusion mass transport. This asymmetry between the two diffusion regimes has been observed for every ion transfer reaction studied and is an interesting characteristic of liquid micropipette electrodes. This phenomenon is illustrated in Fig. 3. I i nA
.lO
Fig. 2. (a) Potential window for the system Ag/AgCI/10-2 M L1C1//10-3 M CVTPB/lO-’ $4 CVCl/AgCl/Ag. Sweep rate = 25 mV/s; internal radius = 25 pm; external radius - 60 pm. (b) As (a) after addition of 200 CM of TEATPB in phase (2).
182 10
Izs -0
-5 07
-06
-05
-04
-03
-02
-01
E/V
Fig. 3. Cyclic voltammogram for the transfer of TEA + ion across a 10 mM LiCl/l interface. Sweep rate = 25 mV/s; internal radius = 25 pm; external radius = 60 pm.
If this ITIES at the tip of the micropipette steady-state current could be calculated by
were
a perfect
mM
microdisc,
CVTPB
the
I = 4FD&,+c&A+r
where F is the faraday constant, D&,+ and C& + are the diffusion coefficient and concentration of TEA+ in dichloro ethane respectively and r represents the radius of the disc. Using D&+ = 7 x 10e6 cm” s-i [7] and r = 12.5 pm, the theoretical value for the current is 0.7 nA, which is less than the measured value of about 5 nA. This discrepancy between theoretical and observed values can be attributed to the fact that the ITIES is not a perfect disc and consequently the interfacial area may be larger than expected, especially if the aqueous electrolyte does not wet the glass properly. Although very fine micropipette tips (< 1 pm) can be made with automatic pipette pullers, we observed that it is very difficult to use such fine microelectrodes for electrochemical studies of ion transfer reactions across an ITIES. Since it is not possible to pull such a small tip with a short shank, the liquid microelectrode obtained is very resistive. The resistance of a cone section having a length I and
Fig. 4. Potential TBABr/AgCl/Ag (- - - - - -) Internal
wmdow for the system: Ag/AgC1/10-2 LiCl//10-3 M TBATPB/10m2 M ) Internal radius = 25 pm; outside radius = 60 pm: 5 mm long shank. for: (radius = 10 pm; outside radius = 30 pm; 2 cm long shank.
183
terminated
at one end by a disc of radius r,, and at the other end by a disc of radius
r,, is given by
R = pl,/?rr,rz For a fine tip of radius rl = 0.5 pm and a 2 cm long shank, the leading capillary having an internal diameter of 0.4 mm, the resistance of the microelectrode for a lo-’ M LiCl filling solution is equal to 250 Ma. Figure 4 shows the influence of the tip radius on the potential window. It can be seen that the system becomes very resistive and hinders any electrochemical investigation. ACKNOWLEDGEMENTS
The authors wish to thank the Nuffield Foundation grant, and the University of Edinburgh for financial
of England support.
for an equipment
REFERENCES 1 2 3 4 5 6
T. Osakat, T. KakuJam and M. Senda, Buil. Chem. Sot. Jpn.. 57 (1984) 370. Z. Samec. V. Marecek. J. Weber and D. Homolka, J. Electroanal. Chem., 126 (1981) 105. Z. Samec and V. Marecek, J. Electroanal. Chem., 200 (1986) 17. W.J. Albery, R.A. Choudery and P.R. Fisk, Faraday Discuss. Chem. Sot.. 77 (1984) 53. M.I. Montenegro. Port. Electrochim. Ada, 3 (1985) 165. R.D. Purves, Mlcr~l~trode Methods for Intracelluhar Recording, Academrc Press, New York. London, 1981. 7 H.H.J. Girault and D.J. Schiffrin in M.J. Allen and P.N.R. Usherwood (Eds.), Charge and Field Effects in Biosystems, Abacus Press, Tunbridge Wells, 1984, pp. 171-178.