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Electrostatic Kerr effect at a potentiostatically controlled liquidvbliquid interface
JOUI~At
ELSEVIER
Journal of Electroanalytical Chemistry 396 (1995) 397-399
Electrostatic Kerr effect at a potentiostatically controlled liquid Iliquid interface 1 S. Nakabayashi a,b, K. Matumoto b, K. Uosaki b a PRESTO, Research Development Corporation of Japan, Japan b Physical Chemistry Laboratory, Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received 30 March 1995
Keywords: Electrostatic Kerr effect; Immiscible solutions; Interfaces
1. Introduction The interface between two immiscible electrolyte solutions is the one of the most controllable interphases which has been studied mainly by electrochemical methods [1-3]. In particular, the ion transport through this interface has a long history of extensive research [2,4-6]. Electrolyte solutions of 1,2-dichloroethane or nitrobenzene are commonly used as the non-aqueous phase, which contacts with the aqueous electrolyte solution. The Gibbs energy change of the ion between the two phases is compensated by the electrochemical potential difference [2]. At the interface, an electrochemical double layer is formed, and even in the potential region where the ion transport is not proceeding, the orientation of the solvent molecules might be changed as a function of the applied electrochemical potential [7-9]. However, there has not been any direct method to observe the structure change in the double layer. In this article we show that the immiscible nitrobenzene [water electrolyte interface rotates the polarization direction of the incident laser beam as a function of the potential difference between the interface and that the potential distribution of the interface such as the potential of zero charge (PZC) can be determined directly by this electrostatic Kerr effect.
2. Experimental The immiscible electrolyte solution used was nitrobenzene and water containing 0.02 M of tetrabuthylammo-
I Dedicated to Professors Honda, Matsuda and Tamamushi on the occasion of their 70th birthdays. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All fights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 0 5 3 - 6
nium tetraphenylborate and 0.02 M of lithium chloride as the supporting electrolyte. The potential was applied between two platinum electrodes placed in the nitrobenzene and the aqueous electrolyte solution. The electrochemical conditions were controlled by a conventional four-electrode system with two platinum electrodes as the quasi-reference electrodes in the two liquid phases. The potential difference between the interface was represented as Ark = thw - ~b°. The cross-sectional area of the interface was relatively large, approx. 20 cm 2; otherwise the shape of the interface was deformed as a function of the potential [2] and this deformation affected the optical experiments. The ion transfer cyclic voltammogram was measured by the combination of a potentiostat (Hokuto HA-151) and a waveform generator, (HB-111). The current and potential curve was recorded on an X - Y recorder (HP3036). A schematic representation for the optical experiment is shown in Fig. 1. The potential applied across the interface was modulated by the rectangular wave from a function generator (HP3314A). The potential modulation amplitude and the frequency were 0.3 V and 20 Hz respectively. The incident angle of the H e - N e laser beam was 5 ° 5:2 °. The two polarizers were so adjusted that no transmitted light was detected in the open-circuit condition, where the potential was not applied on the platinum electrodes. The transmitted light intensity through the interface and the two polarizers was detected by the photomultiplier tube. The signal was fed into a lock-in amplifier (NF LI-575) through the preamplifier. The synchronized intensity of the transmitted light with the potential modulation AI was measured as a function of the central d.c. potential (midpotential) with the constant modulation amplitude. In order to avoid the intensity fluctuation of the transmitted light by the deformation of the curvature of the interface, AI was normalized to the transmitted light intensity 10, without
398
S. Nakabayashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 397-399 12.$
Lock in A M t
tt.
12.0
~
11.$
~
11.0
a. O ¢J
10.5
Polarizer Nd filter
....
~ ....
I ....
~
Potentlostat
Polarizer
one of the polarizers near the photomultiplier tube. Thus, the relationship between A I / I o and mid-potential was obtained. The capacitance of the interface was measured as a function of the potential by the combination of the lock-in amplifier, the function generator and the potential sweeper. The amplitude and the frequency of the sinusoidal wave were 6 mV and 10 Hz respectively. The potential sweep rate was 0.2 mV s -I.
discussion
The ion transfer current across the nitrobenzene[water interface is measured as a function of the applied potential. The current is not observed from - 0 . 1 to 0.5 V, where the interface is polarized by the applied-potential. In this potential region, the optical measurement is conducted. The normalized transmitted light intensity A I / I o synchronized with the potential modulation is plotted in Fig. 2 as a function of mid-potential which is the potential at the centre of the potential modulation. The transmitted light intensity has a dip at the potential of 0.32 V. On both sides 4.0
3.5 3.0 2.S 2.0 oo 1.0
OOQ
@o
0.5 0.0 -0.2
-0.1
I ....
•
, , , i . . . .
• oa
I ....
o
J . . . .
i , ~
, Ipi ....
o.1 o.2 0.3 Potential / V
i ....
0.4
i , , ,
0.5
0.6
Fig. 3. The changein the capacitanceof the interfaceas a functionof the potential differenceacross the nitrobenzene [ water interface,
Fig. I. Schematicrepresentationof the experimentalset-up for measuring the electrostaticKerr effect at the electrochemicallycontrollednitrobenzene I water interface.
~
I ....
9.~ 9.0 -0.2
2
I ....
10.0
reL
and
r ....
•
X
3. Results
I ....
0
Ol
0.2
03
04
0.~
0.6
Mid-potential / V
Fig. 2. The change in the normalizedvalue A I / ! o throughthe nitrohenzene I water interfaceas a functionof the mid-potentialof the potential modulation.
of the dip, the intensity increased linearly. However, in the region more negative than 0.1 V, scattering from the linear relation is observed. In this region, a small amount of the ion transfer current is observed. The relationship between the capacitance of the interface and the applied potential difference is shown in Fig. 3. The potential of zero charge (PZC) can be approximated to the potential at the capacitance minimum which is located at 0.28 V. Thus, the potential at the dip in Fig. 2 is almost identical with the PZC which is determined by the impedance method. The experiments indicate clearly that the nitrobenzene ]water interface rotates the polarization direction of the incident laser beam because the transmitted light intensity through the crossed polarizers changes as a function of the applied potential as shown in Fig. 2. It is an electrostatic Kerr effect [10,11] that the polarization direction of the incident light is rotated by passing through an electrostatically polarized birefringent material. The shift in the phase angle of the incident light is expressed as [ 12]
,aq,=
foL#( x) 2 dx
(l)
where A~ is the phase shift resulting from passage through the material of length L which is electrostatically polarized by the electrostatic field ~ (x). K is the Kerr constant. A material having a large Kerr constant induces a large A~. Nitrobenzene is known to have a large electrostatic Kerr constant, 2.4 × 10 -12 m V -2 [12]. Thus, nitrobenzene has been used as the material for the Kerr shutter such as the Q-switch in the pulsed lasers [13]. However, in the case of a typical optoelectronic application, the potential difference is of the order of few kilovolts, which is applied between the two electrodes with the distance of a few millimetres dipped in the homogeneous nitrobenzene solution [14]. In this case, the electrostatic field sc is of the order of 10 5 V m -l. Because the electrochemical potential difference between the two immiscible electrolyte phases is screened within the Debye length in the solution [2], the electrostatic field strength in the electric double layer at the nitrobenzene and water interface is possibly larger than that used in conventional optoelectronic applications.
S. Nakabayashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 397-399
The Kerr constant K of water is 5.1 × 10 -14 c m V -2 i12]. As K of water is much smaller than that of nitroben~ene, the rotation of the polarization direction of the incident light into the nitrobenzene [water interface occurs mainly in the nitrobenzene phase. The electrostatic field at the interface can be estimated from the Poisson-Boltzman equation according to the Gouy-Chapman theory. The ~ynchronized intensity of the transmitted light as a func:ion of the mid-potential is obtained by the combination of Eq. (1) and ~ (x) which is estimated by the PoissonBoltzman equation. The relationship is obtained as
Al/lo = 27rK( n/2eeokT)l/2zelclgmid-- qbpzr I
(2)
where qbmid represents the mid-potential which is the potential at the centre of the modulation. Eq. (2) explains qualitatively that the observed A I / I o has a minimum at the PZC and the linear increase on both sides as shown in Fig. 2. Although Eq. (2) explains the results in Fig. 2 qualitatively, the calculated value of A I / I o does not agree with the observed value. A more detailed study considering the dielectric saturation and the orientation effect of the nitrobenzene molecules [I0] at the interface is under way in this laboratory.
399
References [1] S. Kihara and K. Maeda, Prog. Surf. Sci., 47 (1994) 1. [2] J.D. Reid, O.R. Melroy, W.E. Bronner, H.C. Hughes, P. Vanysek and R.P. Buck, in A.F. Silva (Ed.), Trends in Interfacial Electrochemistry, Reidel, New York, 1986, p103. [3] R. Aveyard and B. Vincent, Prog. Surf. Sci., 8 (1977) 59. [4] Z. Samec, V. Marecek, J. Koryta and M.W. Khalil, J. Electroanal. Chem., 83 (1977) 393. [5] Z. Samec, V. Marecek, J. Weber and D. Homolka, J. Electroanal. Chem., 126 (1981) 115. [6] T. Kakutani, T.Osakai and M Senda, Bull. Chem. Soc. Jpn., 56 (1983) 991. [7] T. Kakiuchi and M. Senda, Bull. Chem. Soc. Jpn., 56 (1983) 1753. [8] C. Gavach, P. Seta and B. D'Epenoux, J. Electroanal. Chem., 83 (1977) 225. [9] D.A. Higgins and R.M. Corn, J. Phys. Chem., 97 (1993) 489. [10] H. Watanabe and A. Morita, in I. Prigogine and S.A. Rice (Eds.), Advances in Chemical Physics, Vol. LV1, Wiley, New York, 1984, p255. [11] C.G. LeFevre and R.J.W. LeFevre, Rev. Pure Appl. Chem., 5 (1955) 261. [12] F.A. Jenkins and H.E. White, Fundamentals of Optics, London, 1987, p. 693. [13] D.C. O'Shea, W.R. Callen and W.T. Rhodes, Introduction to Lasers and Their Applications, Addison-Wesley, Menlo Park CA, 1978, p.l14. [14] S.M. Lee and SM. Hauser, Rev. Sci. Instrum., 35 (1964) 1679.
ELSEVIER
Journal of Electroanalytical Chemistry 396 (1995) 397-399
Electrostatic Kerr effect at a potentiostatically controlled liquid Iliquid interface 1 S. Nakabayashi a,b, K. Matumoto b, K. Uosaki b a PRESTO, Research Development Corporation of Japan, Japan b Physical Chemistry Laboratory, Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Received 30 March 1995
Keywords: Electrostatic Kerr effect; Immiscible solutions; Interfaces
1. Introduction The interface between two immiscible electrolyte solutions is the one of the most controllable interphases which has been studied mainly by electrochemical methods [1-3]. In particular, the ion transport through this interface has a long history of extensive research [2,4-6]. Electrolyte solutions of 1,2-dichloroethane or nitrobenzene are commonly used as the non-aqueous phase, which contacts with the aqueous electrolyte solution. The Gibbs energy change of the ion between the two phases is compensated by the electrochemical potential difference [2]. At the interface, an electrochemical double layer is formed, and even in the potential region where the ion transport is not proceeding, the orientation of the solvent molecules might be changed as a function of the applied electrochemical potential [7-9]. However, there has not been any direct method to observe the structure change in the double layer. In this article we show that the immiscible nitrobenzene [water electrolyte interface rotates the polarization direction of the incident laser beam as a function of the potential difference between the interface and that the potential distribution of the interface such as the potential of zero charge (PZC) can be determined directly by this electrostatic Kerr effect.
2. Experimental The immiscible electrolyte solution used was nitrobenzene and water containing 0.02 M of tetrabuthylammo-
I Dedicated to Professors Honda, Matsuda and Tamamushi on the occasion of their 70th birthdays. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All fights reserved SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 0 5 3 - 6
nium tetraphenylborate and 0.02 M of lithium chloride as the supporting electrolyte. The potential was applied between two platinum electrodes placed in the nitrobenzene and the aqueous electrolyte solution. The electrochemical conditions were controlled by a conventional four-electrode system with two platinum electrodes as the quasi-reference electrodes in the two liquid phases. The potential difference between the interface was represented as Ark = thw - ~b°. The cross-sectional area of the interface was relatively large, approx. 20 cm 2; otherwise the shape of the interface was deformed as a function of the potential [2] and this deformation affected the optical experiments. The ion transfer cyclic voltammogram was measured by the combination of a potentiostat (Hokuto HA-151) and a waveform generator, (HB-111). The current and potential curve was recorded on an X - Y recorder (HP3036). A schematic representation for the optical experiment is shown in Fig. 1. The potential applied across the interface was modulated by the rectangular wave from a function generator (HP3314A). The potential modulation amplitude and the frequency were 0.3 V and 20 Hz respectively. The incident angle of the H e - N e laser beam was 5 ° 5:2 °. The two polarizers were so adjusted that no transmitted light was detected in the open-circuit condition, where the potential was not applied on the platinum electrodes. The transmitted light intensity through the interface and the two polarizers was detected by the photomultiplier tube. The signal was fed into a lock-in amplifier (NF LI-575) through the preamplifier. The synchronized intensity of the transmitted light with the potential modulation AI was measured as a function of the central d.c. potential (midpotential) with the constant modulation amplitude. In order to avoid the intensity fluctuation of the transmitted light by the deformation of the curvature of the interface, AI was normalized to the transmitted light intensity 10, without
398
S. Nakabayashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 397-399 12.$
Lock in A M t
tt.
12.0
~
11.$
~
11.0
a. O ¢J
10.5
Polarizer Nd filter
....
~ ....
I ....
~
Potentlostat
Polarizer
one of the polarizers near the photomultiplier tube. Thus, the relationship between A I / I o and mid-potential was obtained. The capacitance of the interface was measured as a function of the potential by the combination of the lock-in amplifier, the function generator and the potential sweeper. The amplitude and the frequency of the sinusoidal wave were 6 mV and 10 Hz respectively. The potential sweep rate was 0.2 mV s -I.
discussion
The ion transfer current across the nitrobenzene[water interface is measured as a function of the applied potential. The current is not observed from - 0 . 1 to 0.5 V, where the interface is polarized by the applied-potential. In this potential region, the optical measurement is conducted. The normalized transmitted light intensity A I / I o synchronized with the potential modulation is plotted in Fig. 2 as a function of mid-potential which is the potential at the centre of the potential modulation. The transmitted light intensity has a dip at the potential of 0.32 V. On both sides 4.0
3.5 3.0 2.S 2.0 oo 1.0
OOQ
@o
0.5 0.0 -0.2
-0.1
I ....
•
, , , i . . . .
• oa
I ....
o
J . . . .
i , ~
, Ipi ....
o.1 o.2 0.3 Potential / V
i ....
0.4
i , , ,
0.5
0.6
Fig. 3. The changein the capacitanceof the interfaceas a functionof the potential differenceacross the nitrobenzene [ water interface,
Fig. I. Schematicrepresentationof the experimentalset-up for measuring the electrostaticKerr effect at the electrochemicallycontrollednitrobenzene I water interface.
~
I ....
9.~ 9.0 -0.2
2
I ....
10.0
reL
and
r ....
•
X
3. Results
I ....
0
Ol
0.2
03
04
0.~
0.6
Mid-potential / V
Fig. 2. The change in the normalizedvalue A I / ! o throughthe nitrohenzene I water interfaceas a functionof the mid-potentialof the potential modulation.
of the dip, the intensity increased linearly. However, in the region more negative than 0.1 V, scattering from the linear relation is observed. In this region, a small amount of the ion transfer current is observed. The relationship between the capacitance of the interface and the applied potential difference is shown in Fig. 3. The potential of zero charge (PZC) can be approximated to the potential at the capacitance minimum which is located at 0.28 V. Thus, the potential at the dip in Fig. 2 is almost identical with the PZC which is determined by the impedance method. The experiments indicate clearly that the nitrobenzene ]water interface rotates the polarization direction of the incident laser beam because the transmitted light intensity through the crossed polarizers changes as a function of the applied potential as shown in Fig. 2. It is an electrostatic Kerr effect [10,11] that the polarization direction of the incident light is rotated by passing through an electrostatically polarized birefringent material. The shift in the phase angle of the incident light is expressed as [ 12]
,aq,=
foL#( x) 2 dx
(l)
where A~ is the phase shift resulting from passage through the material of length L which is electrostatically polarized by the electrostatic field ~ (x). K is the Kerr constant. A material having a large Kerr constant induces a large A~. Nitrobenzene is known to have a large electrostatic Kerr constant, 2.4 × 10 -12 m V -2 [12]. Thus, nitrobenzene has been used as the material for the Kerr shutter such as the Q-switch in the pulsed lasers [13]. However, in the case of a typical optoelectronic application, the potential difference is of the order of few kilovolts, which is applied between the two electrodes with the distance of a few millimetres dipped in the homogeneous nitrobenzene solution [14]. In this case, the electrostatic field sc is of the order of 10 5 V m -l. Because the electrochemical potential difference between the two immiscible electrolyte phases is screened within the Debye length in the solution [2], the electrostatic field strength in the electric double layer at the nitrobenzene and water interface is possibly larger than that used in conventional optoelectronic applications.
S. Nakabayashi et al. / Journal of Electroanalytical Chemistry 396 (1995) 397-399
The Kerr constant K of water is 5.1 × 10 -14 c m V -2 i12]. As K of water is much smaller than that of nitroben~ene, the rotation of the polarization direction of the incident light into the nitrobenzene [water interface occurs mainly in the nitrobenzene phase. The electrostatic field at the interface can be estimated from the Poisson-Boltzman equation according to the Gouy-Chapman theory. The ~ynchronized intensity of the transmitted light as a func:ion of the mid-potential is obtained by the combination of Eq. (1) and ~ (x) which is estimated by the PoissonBoltzman equation. The relationship is obtained as
Al/lo = 27rK( n/2eeokT)l/2zelclgmid-- qbpzr I
(2)
where qbmid represents the mid-potential which is the potential at the centre of the modulation. Eq. (2) explains qualitatively that the observed A I / I o has a minimum at the PZC and the linear increase on both sides as shown in Fig. 2. Although Eq. (2) explains the results in Fig. 2 qualitatively, the calculated value of A I / I o does not agree with the observed value. A more detailed study considering the dielectric saturation and the orientation effect of the nitrobenzene molecules [I0] at the interface is under way in this laboratory.
399
References [1] S. Kihara and K. Maeda, Prog. Surf. Sci., 47 (1994) 1. [2] J.D. Reid, O.R. Melroy, W.E. Bronner, H.C. Hughes, P. Vanysek and R.P. Buck, in A.F. Silva (Ed.), Trends in Interfacial Electrochemistry, Reidel, New York, 1986, p103. [3] R. Aveyard and B. Vincent, Prog. Surf. Sci., 8 (1977) 59. [4] Z. Samec, V. Marecek, J. Koryta and M.W. Khalil, J. Electroanal. Chem., 83 (1977) 393. [5] Z. Samec, V. Marecek, J. Weber and D. Homolka, J. Electroanal. Chem., 126 (1981) 115. [6] T. Kakutani, T.Osakai and M Senda, Bull. Chem. Soc. Jpn., 56 (1983) 991. [7] T. Kakiuchi and M. Senda, Bull. Chem. Soc. Jpn., 56 (1983) 1753. [8] C. Gavach, P. Seta and B. D'Epenoux, J. Electroanal. Chem., 83 (1977) 225. [9] D.A. Higgins and R.M. Corn, J. Phys. Chem., 97 (1993) 489. [10] H. Watanabe and A. Morita, in I. Prigogine and S.A. Rice (Eds.), Advances in Chemical Physics, Vol. LV1, Wiley, New York, 1984, p255. [11] C.G. LeFevre and R.J.W. LeFevre, Rev. Pure Appl. Chem., 5 (1955) 261. [12] F.A. Jenkins and H.E. White, Fundamentals of Optics, London, 1987, p. 693. [13] D.C. O'Shea, W.R. Callen and W.T. Rhodes, Introduction to Lasers and Their Applications, Addison-Wesley, Menlo Park CA, 1978, p.l14. [14] S.M. Lee and SM. Hauser, Rev. Sci. Instrum., 35 (1964) 1679.