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liquid interface
Journal of Electroanalytical Chemistry 671 (2012) 1–6
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Investigation of current oscillatory phenomena based on Fe3+/Fe2+ at the liquid/liquid interface Xiuhui Liu ⇑, Yijun Zhang, Yuehua He, Dafang Ji, Yongcheng Wang, Zhihua Wang, Xiaoquan Lu ⇑ Key Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 29 October 2011 Received in revised form 11 February 2012 Accepted 17 February 2012 Available online 25 February 2012 Keywords: Current oscillation Liquid/liquid interface Ion pair Specific adsorption DFT
a b s t r a c t A new oscillatory phenomenon based on Fe3+/Fe2+ was investigated systematically at the water/ 1,2-dichloroethane (W/DCE) interface by cyclic voltammetric technique. We focus our attention on study the concentrations of Fe3+/Fe2+ influence on oscillatory phenomena appearance in this paper. It was found that the current oscillation only occurred in the site of oxidation peak of Fe2+, and was related to the concentration of Fe3+ in the aqueous phase, indicating that the oscillation is caused by the specific adsorption of ion pairs at the liquid–liquid interface between Fe3+ in the aqueous phase and TPB in the organic phase. Furthermore, DFT theory was used to calculate the mechanism of ion pair formation for the first time. The result suggested that TPBFe3+TPB ion pair has the lowest-energy state, which provided qualitative support for ion pair embodied state. Combining experiment results and theoretical calculation, a specific adsorption of ion pair model on liquid/liquid interface was proposed and a mechanism for the observed current oscillation is also discussed in this paper. In addition, a spectrophotometric experiment was performed to evidence a strong attractive interaction between Fe3+ and TPB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Current oscillatory phenomena have been extensively investigated by research groups and are of great interest in those systems where the occurrence of oscillation is connected with chemical processes taking place at the electrode surface [1–8]. At the same time, some reports concerned the oscillatory phenomena occurrence in the interfaces between two immiscible electrolyte solutions (ITIES) [9–16]. Kakiuchi and co-workers first observed irregular current spikes in voltammetry of the transfer of anionic surfactants and alkyl sulfates across the 1,2-dichloroethane/water interface with a four-electrode [10,11]. Ohsaka studied the current oscillation at a hanging mercury drop electrode (HMDE) based on the redox reactions [12–14]. Although some reports have investigated the specific ion adsorption at liquid/liquid interfaces [17–19], systematically investigation of current oscillation have not been significantly developed till recently because the structure of ITIES interfaces is rather a controversial topic. Recently, our group has reported a new oscillation phenomenon [20,21] when we investigated the ion transfer across the liquid/liquid interface coupled to electrochemical redox reaction at the Pt electrode with a three electrode potentiostat. Many important results have already been obtained in studying this problem, using ⇑ Corresponding authors. Tel.: +86 0931 7975276; fax: +86 0931 7971323 (X. Liu), tel.: +86 0931 7971276; fax: +86 0931 7971323 (X. Lu). E-mail addresses: [email protected] (X. Liu), [email protected] (X. Lu). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2012.02.021
4 FeðCNÞ3 6 =FeðCNÞ6 redox couple as a representative at water/1,2DCE interface. It was found that the oscillation appearance depends on many experimental factors, such as the concentration of FeðCNÞ4 6 , the scan rate, and the size of the water drop. The occurrence of oscillation was thought to form the specific adsorption of þ ion pairs, FeðCNÞ4 6 TBAn , at the liquid–liquid interface. Therefore, a specific adsorption of ion pair model based on oscillatory phenomenon was proposed [21] which are significant for us to understand the structure of L/L interface better and to explain properties of the interesting nonlinear oscillation patterns. Following this strategy, we designed this experiment to investigate another specific adsorption of ion pairs between positive charged ion in the aqueous phase and negative charged ion in the organic phase in this paper. Fortunately, the current oscillation phenomenon can also be observed on the Fe3+/Fe2+ system at the L/L interface. We focus our attention on exploring the mechanism of oscillation with computational study in this paper.
2. Experimental 2.1. Reagents Ferric chloride (FeCl3, AR), ferrous chloride (FeCl2, AR), hydrochloric acid (HCl, AR) and 1,2-dichloroethane (DCE, AR) were purchased from Beijing Chemical Co., China. Tetra-n-pentylammonium tetraphenylborate (TPnATPB) was prepared from tetra-npentylammonium chloride (TPnACl, 99%, Aldrich) and sodium
X. Liu et al. / Journal of Electroanalytical Chemistry 671 (2012) 1–6
tetraphenylborate (99%, Aldrich) as described elsewhere [22]. As the supporting electrolytes in organic phase, tetra-n-butylammonium tetraphenylborate (TBATPB, 99%, Aldrich), tetra-n-butylammonium chloride (TBACl, 99%, Aldrich) and tetrabutylammonium perchlorate (TBAClO4, 99%, Aldrich) were used without purification. DCE was washed several times with deionized water before being used. All aqueous solutions were prepared with deionized water by a Milli-Q system (Milli-Q, Millipore Corp.).
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2.2. Apparatus and electrochemical measurements
3. Results and discussion
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3.1. Observed oscillation phenomena in the Fe /Fe W/DCE interface
2+
system on the
Fig. 1A depicts the typical cyclic voltammetric response of the Pt electrode in 8.0 mM FeCl3 + 8.0 mM FeCl2 + 0.5 M HCl aqueous solution. The reduction peak at +0.435 V and the oxidation peak at +0.517 V were observed, which is in good agreement with the literature report [24]. The peak-to-peak separation of Fig. 1A is about 82 mV, known as one-electron transfer reaction. Fig. 1B shows the cyclic voltammogram in following electrochemical cell.
Ptjx mM FeCl3 þ y mM FeCl2 þ 0:5 M HClj10 mM TBATPB þ DCEjAgTPBjAg
-6 0
Current/µA
A computer-controlled CHI-660C electrochemical workstation (CH Instruments Inc.) connected to a personal computer was used for all electrochemical measurements. Cyclic voltammograms were recorded using a three-electrode potentiostat. The aqueous and organic solutions were mutually saturated prior to each experiment. A drop of 1.5 lL of aqueous solution containing various molar ratios of the FeCl3/FeCl2 was transferred to the surface of a freshly polished platinum disk electrode (d = 2 mm) with a small syringe. When the droplet spread spontaneously across the surface of this platinum electrode and covered it completely, the electrode was turned over and immersed immediately into an organic phase solvent (about 1.5 mL) containing supporting electrolyte (TBATPB). A Pt wire (d = 1 mm) was used as the counter electrode, and an Ag wire coated with AgTPBCl (Ag/AgTPBCl) was used as the organic phase reference electrode, which was made according to the literature [23]. Prior to each experiment, the platinum electrode was polished sequentially with 0.3 and 0.05 lm alumina. Then, it was cleaned ultrasonically with ethanol and deionized water for 2 min, respectively. All experiments were performed at room temperature (20 ± 2 °C). Spectrophotometric measurements were carried out in aqueous solutions without a supporting electrolyte using an UV-2550 spectrophotometer. All of the calculations reported in this work were carried out using B3LYP/LANL2DZ level of DFT theory and the Gaussian-03 ab initio program package.
ðCell IÞ
A pair of current peaks in Fig. 1B belong to the redox peak of Fe3+/Fe2+ at the Pt electrode (x = y = 8 mM), and the peak-to-peak separation is about 746 mV in the fist circle, which is much larger than that in Fig. 1A. The reason is ascribed to the existence of W/DCE interfacial potential difference and the Ohmic drop [25]. Compared with Refs. [20,21], where both K+ ion transfer peak across the W/DCE interface and electron redox peak at the Pt electrode appeared, we do not observe K+ transfer peak across the W/DCE interface in this case because HCl was used as supporting electrolyte in aqueous solution. As we know, Cl transfer wave across the W/DCE interface was at the negative side of the poten-
-4 0
0 .8
0 .7
0 .6
0 .5
Potential/V Fig. 1. (A) Cyclic voltammogram obtained in the aqueous solution for 8 mM FeCl3/ FeCl2, and 0.5 M HCl. (B) Cyclic voltammogram obtained across the water/1,2-DCE interface, using (Cell I). (x = y = 8.0 mM). (C) Magnified view of the cyclic voltammogram B. Scan rate was 50 mV/s. Size of the drop was 1.5 lL.
tial according to [26]. Thus, only the redox peak of Fe3+/Fe2+ at the Pt electrode was observed in the potential range between 0 and 1.0 V. It is worth mentioning that the current oscillation can be observed in the oxidation peak of Fe2+ when the potential scan from 0 to 1.0 V in Fig. 1B. Meanwhile, the oscillation could shift towards the positive potential direction with increasing the concentrations of the Fe3+/Fe2+ redox couple (data are not shown). It is more clearly seen in Fig. 1C which is a magnified view of the voltammogram of Fig. 1B. Small current spikes that exceeded the noise level of the measurement were observed in the potential range between 0.7 and 1.0 V. The obvious irregular current oscillation is restorable in most of the cases and the appearance of oscillation is very reproducible in a series of successive potential scan processes as shown in Fig. 1B, similar to our previous studies [20,21]. Above results demonstrated that the L/L interface is one of the necessary conditions for the oscillation appearance.
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3.2. Influence of concentrations on the current oscillatory phenomena We carried out a series of experiments to further investigate the influence of concentrations on the current oscillatory phenomena in order to understand the oscillation clearly. First of all, a finding from Fig. 2A and B is that the oscillation is related to the concentrations of Fe3+/Fe2+. The current oscillation can only be observed when Fe3+ and Fe2+ concentrations are both larger than 5 mM. Second, we changed the concentrations of Fe3+ and Fe2+, respectively, and the result is very interest. The oscillatory phenomenon could be observed in Fig. 2C when concentrations of Fe3+changed from 5.0 to 10 mM while maintaining the concentration of Fe2+ as 4.0 mM. However, no oscillatory phenomenon occurred in Fig. 2D when concentrations of Fe2+ were changed from 5.0 to 10 mM but remained the concentration of Fe3+ as 4.0 mM. For further proved this result, we performed another experiment by adding Fe3+ or Fe2+ in the aqueous phase only. As shown in Fig. 2E and F,
the current oscillatory phenomenon appeared when Fe3+ concentration arrived at 10 mM, whereas, no current oscillation occurred even as the Fe2+ concentration was 10 mM. All above results illuminated that one of the necessary conditions for the oscillation appearing at the L/L interface was the concentration of Fe3+, indicating that Fe3+ ion played a key role in inducing oscillations. Finally, we also investigated the species and the concentrations of supporting electrolyte in organic phase affecting the appearance of the current oscillation. As shown in Fig. 3A–C, the current oscillation will occur when the concentration of TBATPB is 5.0 or 10 mM, meaning TBATPB concentration also influence oscillations appearance. On the other hand, when TBATPB in organic phase is replaced by TPnATPB (D), TBAClO4 (E) and TBACl (F), respectively, one can see clearly that the oscillation occurred only in process D, and absented in processes E and F, demonstrated further that the oscillation is related to the negatively charged TPB in the organic phase.
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Fig. 2. Cyclic voltammograms obtained for using (Cell I). (A) x = y = 4.0 mM, (B) x = y = 5 mM, (C) x = 10, y = 4.0 mM, (D) x = 4.0, y = 10 mM, (E) x = 10, y = 0 mM, (F) x = 0, y = 10 mM. Scan rate was 50 mV/s. size of the drop was 1.5 lL.
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Fig. 3. Cyclic voltammograms obtained for (Cell I) (x = y = 8 mM) with the concentrations of TBATPB are (A) 1 mM, (B) 5 mM and (C) 10 mM; and with the supporting electrolyte (10 mM) in organic phase as (D) TPnATPB, (E) TBAClO4 and (F) TBACl, respectively. Scan rate was 50 mV/s, and size of the drop was 1.5 lL.
3þ
Fe
3þ
þ e Fe
2þ
ðPt=WÞ
ð1Þ 3þ
Fe ðwÞ þ 2TPB ð0Þ TPB Fe TPB ðadsÞ
ðO=WÞ
ð2Þ
when TPB in organic phase is replaced by other species, such as ClO4 and Cl, we consider that Fe3+ may transfer across the interface from W to O or ClO4 and Cl transfer across the interface from O to W when the oxidation process of Fe2+ occurred at the Pt electrode in or der to maintain the electro-neutrality. Because ClO4 and Cl are hydrophilic anion ions, they tend to transfer into the aqueous phase easily, rather than adsorb in the L/L interface, which is consistent with the experiment results in Fig. 3E and F (no oscillation
0.5
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3
0.3
Absorbance
We have achieved the results from Ref. [2] that the electrochemical process couples the electron transfer at the Pt/W with the ion transfer at the W/O interface based on electro-neutrality requirement. In this case, both the oxidation process of Fe2+ at the Pt electrode and Fe3+ transfer across the interface from W to O proceed simultaneously as shown in reactions 1 and 2. Because Fe3+ is hydrophilic ion, it tends to remain at the L/L interface and forms ion pairs with TPB.
2
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0.1 1
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Wavelength/nm Fig. 4. UV–vis spectra in organic phase. (1) 15 105 mol/L TBATPB, (2) 15 105 mol/L FeCl3, (3) 15 105 mol/L TBATPB and FeCl3.
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Fe3++3TPB-
0.00 -200 643
G (kcal / mol)
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Fe +2TPB
B -
3+
-
Fe TPB +TPB
C -
-
3+
-
TPB Fe TPB +TPB
-
Fe
3+
(TPB-)3
Fig. 5. Potential energy profiles of the reaction Fe3+ with TPB calculated with B3LYP/LANL2DZ level of DFT theory.
appearance). On the other hand, the redox peak current of Fe3+/Fe2+ in Fig. 3E and F are 40 lA and 36 lA, respectively, much larger than that of in Fig. 3D (about 11 lA). This result proved further that there are no ion pairs formed at the L/L interface in Fig. 3E and F so that the Fe3+ concentrations in the aqueous phase are larger than that in processes C and D. That is why no current oscillation appearing in Fig. 3E and F. Therefore, from above experiments, we can draw the conclusion that the oscillation is caused by the adsorption of ion-pair at the interface between the positively charged Fe3+ in the aqueous side and the negatively charged TPB in the organic side. In addition, the optical properties of TBATPB and FeCl3 were further characterized by UV spectra. Fig. 4 shows the UV spectra of the TBATPB (1), FeCl3 (2), and a mixture of both (3) in the organic phase at the same concentration (15 105 mol/L). It can be seen that the molar absorptivity of TBATPB + FeCl3 is higher than that of FeCl3, which is an important feature of the strong interaction between Fe3+ and TPB and is an indication of an ion pairing between them [27,28]. 3.3. Investigation the mechanism of oscillation at the W/DCE interface with theoretical calculation combined a specific adsorption of ion pair model Above results showed the oscillation is related to L/L interface, concentration of Fe3+, and the species and concentrations of supporting electrolyte in organic phase. In order to investigate the mechanism of ion pair formation in the L/L interface, computations were carried out by using the Gaussian-03 ab initio program package [29] for the first time. Firstly, we create and optimize the structure of each ion pair from scratch, where all the possible structures need to be calculated and compared. Fig. 5A and B displayed the optimized ion pair structures the Fe3+TPB and TPBFe3+TPB using B3LYP/LANL2DZ level of DFT theory [30–32]. The orange1 ball represents B of the TPB and blue ball represents Fe3+. The bond lengths of Fe–B of the Fe3+TPB and TPBFe3+TPB are 3.2 Å and 3.7 Å, respectively. As shown in Fig. 5B, Fe3+ placed centrally between two TPB, similar to the sandwich structure, 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
and the B–Fe–B angle of TPBFe3+TPB is 176.8°. Secondly, the Gibbs Free Energy of forming ion pair from Fe3+ and TPB were also calculated by B3LYP/LANL2DZ level of DFT theory [33,34] for considering the solvation effect. Fig. 5 displayed the optimized ion pair structures corresponding to the lowest energy states of Fe3+TPB, TPBFe3+TPB and Fe3+(TPB)3, respectively. On the basis of theoretical calculation, we can postulate the following processes stages at the interface:
Fe3þ ðwÞ þ TPB ð0Þ Fe3þ TPB ðadsÞ
DG1 ¼ 1198 kcal=mol ð3Þ
Fe3þ TPB ðadsÞ þ TPB ð0Þ TPB Fe3þ TPB ðadsÞ
DG2 ¼ 378 kcal=mol
ð4Þ
when one Fe3+ ion and one TPB ion form Fe3+TPB ion pair, its energy lie almost 1198 kcal/mol, lower than energy relative to the corresponding ground state (Fe3+ + 3TPB), indicating efficient interaction between Fe3+ and TPB ions. Furthermore, one Fe3+TPB combined one TPB will produce the lowest energy state, whose energy lie almost 1576 kcal/mol compared to the corresponding ground state (Fe3+ + 3TPB), meaning that TPBFe3+TPB ion pair is more stable than Fe3+TPB ion pair. Whereas, the next step of the reaction of TPBFe3+TPB and TPB ion forming Fe3+(TPB)3 ion pair is hampered because its energy lie almost 933 kcal/mol higher than that of TPBFe3+TPB, indicating TPBFe3+TPB is the most stable ion pair. From the Gouy–Chapman theory, we know that the Galvani potential difference between the two phases was spread entirely within the two back to back double layers. The polarized structure of the L/L interface [35–37] can be regarded as a non-homogeneous two-component liquid phase, known as a mixed solvent layer with two or three molecular diameter thick. The penetration of the ions in the interfacial region depends on their hydrophobicity or hydrophilicity. The diffuse double layers are separated by an inner layer which contains no ion except specifically adsorbed [38]. Thus, a specific adsorption of ion pair model was proposed in Fig. 6 for explain the mechanism of oscillation better. Fig. 6A shows the profile about Fe3+/Fe2+ solution drop at the Pt electrode, and Fig. 6B is the model of a specific adsorption of ion pair. In this model, we considered the L/L interface as two space charge regions separated by a mixed solvent layer (MSL) [28].
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(a)
Pt
MSL
Pt -eFe2+ Fe3+
+
Fe3+
-
TPB-
(b)
MSL
=
- +-
ion pair
A
+
+
-
+ W
- + - + -
-
+
-
-
O
-
+
+
- +-
+
+
- +-
O W
+
+
+
+
W
-O
- -
B
Fig. 6. Schematic drawing for a special adsorption of ion pair model at the L/L interface.
We have discussed that the electrochemical process couples the electron transfer at the Pt/W with the ion transfer at the W/O interface based on electro-neutrality requirement in (Cell I). At first, the positively charged Fe3+ and the negatively charged TPB will form the ion pair (TPBFe3+TPB) at the mixed solvent layer (shown in Fig. 6Ba) due to electrostatic interaction between them. Then, when the potential scan starts from 0.2 V towards positive potential in Fig. 1B, the oxidation process of Fe2+ will occur and the ion pains, TPBFe3+TPB, are accumulating at the interface with increasing concentration of Fe3+. When the potential sweep to the oxidation peak of Fe2+ (near 0.8 V), electrode reaction reaches a maximum rate, driving Fe3+ transfer from droplet to the mixed solvent layer to form ion pairs with TPB. Thus, the interfacial tension on such a small water drop (1.5 lL) is drastically changed due to the diffusion of Fe3+ ions, resulting in oscillation occurence. 4. Conclusions A new current oscillatory phenomenon based on the system of Fe3+/Fe2+ was studied on the W/DCE interface in this paper. It was found that the current oscillation phenomenon only occur in the site of oxidation peak of Fe3+, and related to the concentrations of both Fe3+ in the aqueous phase and TPB in the organic phase, which indicates that the oscillation feature is the specific adsorption of ion pair forming mechanism in W/DCE interface. Moreover, the mechanism of ion pair formation is calculated by DFT theory for the first time. The result suggested that TPBFe3+TPB ion pair has the lowest-energy state, which provided qualitative support for ion pair embodied state. Acknowledgments This work was supported by the Natural Science Foundation of China (Nos. 20875077, 20965007, and 20927004), and the Key Laboratory of Polymer Materials of Gansu Province. References [1] Q.-K. Yu, Y. Miyakita, S. Nakabayashi, R. Baba, Electrochem. Commun. 5 (2003) 321–324. [2] Y. Yuan, Z. Gao, Y. Shao, J. Electroanal. Chem. 526 (2002) 85–91.
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Investigation of current oscillatory phenomena based on Fe3+/Fe2+ at the liquid/liquid interface Xiuhui Liu ⇑, Yijun Zhang, Yuehua He, Dafang Ji, Yongcheng Wang, Zhihua Wang, Xiaoquan Lu ⇑ Key Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 29 October 2011 Received in revised form 11 February 2012 Accepted 17 February 2012 Available online 25 February 2012 Keywords: Current oscillation Liquid/liquid interface Ion pair Specific adsorption DFT
a b s t r a c t A new oscillatory phenomenon based on Fe3+/Fe2+ was investigated systematically at the water/ 1,2-dichloroethane (W/DCE) interface by cyclic voltammetric technique. We focus our attention on study the concentrations of Fe3+/Fe2+ influence on oscillatory phenomena appearance in this paper. It was found that the current oscillation only occurred in the site of oxidation peak of Fe2+, and was related to the concentration of Fe3+ in the aqueous phase, indicating that the oscillation is caused by the specific adsorption of ion pairs at the liquid–liquid interface between Fe3+ in the aqueous phase and TPB in the organic phase. Furthermore, DFT theory was used to calculate the mechanism of ion pair formation for the first time. The result suggested that TPBFe3+TPB ion pair has the lowest-energy state, which provided qualitative support for ion pair embodied state. Combining experiment results and theoretical calculation, a specific adsorption of ion pair model on liquid/liquid interface was proposed and a mechanism for the observed current oscillation is also discussed in this paper. In addition, a spectrophotometric experiment was performed to evidence a strong attractive interaction between Fe3+ and TPB. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Current oscillatory phenomena have been extensively investigated by research groups and are of great interest in those systems where the occurrence of oscillation is connected with chemical processes taking place at the electrode surface [1–8]. At the same time, some reports concerned the oscillatory phenomena occurrence in the interfaces between two immiscible electrolyte solutions (ITIES) [9–16]. Kakiuchi and co-workers first observed irregular current spikes in voltammetry of the transfer of anionic surfactants and alkyl sulfates across the 1,2-dichloroethane/water interface with a four-electrode [10,11]. Ohsaka studied the current oscillation at a hanging mercury drop electrode (HMDE) based on the redox reactions [12–14]. Although some reports have investigated the specific ion adsorption at liquid/liquid interfaces [17–19], systematically investigation of current oscillation have not been significantly developed till recently because the structure of ITIES interfaces is rather a controversial topic. Recently, our group has reported a new oscillation phenomenon [20,21] when we investigated the ion transfer across the liquid/liquid interface coupled to electrochemical redox reaction at the Pt electrode with a three electrode potentiostat. Many important results have already been obtained in studying this problem, using ⇑ Corresponding authors. Tel.: +86 0931 7975276; fax: +86 0931 7971323 (X. Liu), tel.: +86 0931 7971276; fax: +86 0931 7971323 (X. Lu). E-mail addresses: [email protected] (X. Liu), [email protected] (X. Lu). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2012.02.021
4 FeðCNÞ3 6 =FeðCNÞ6 redox couple as a representative at water/1,2DCE interface. It was found that the oscillation appearance depends on many experimental factors, such as the concentration of FeðCNÞ4 6 , the scan rate, and the size of the water drop. The occurrence of oscillation was thought to form the specific adsorption of þ ion pairs, FeðCNÞ4 6 TBAn , at the liquid–liquid interface. Therefore, a specific adsorption of ion pair model based on oscillatory phenomenon was proposed [21] which are significant for us to understand the structure of L/L interface better and to explain properties of the interesting nonlinear oscillation patterns. Following this strategy, we designed this experiment to investigate another specific adsorption of ion pairs between positive charged ion in the aqueous phase and negative charged ion in the organic phase in this paper. Fortunately, the current oscillation phenomenon can also be observed on the Fe3+/Fe2+ system at the L/L interface. We focus our attention on exploring the mechanism of oscillation with computational study in this paper.
2. Experimental 2.1. Reagents Ferric chloride (FeCl3, AR), ferrous chloride (FeCl2, AR), hydrochloric acid (HCl, AR) and 1,2-dichloroethane (DCE, AR) were purchased from Beijing Chemical Co., China. Tetra-n-pentylammonium tetraphenylborate (TPnATPB) was prepared from tetra-npentylammonium chloride (TPnACl, 99%, Aldrich) and sodium
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tetraphenylborate (99%, Aldrich) as described elsewhere [22]. As the supporting electrolytes in organic phase, tetra-n-butylammonium tetraphenylborate (TBATPB, 99%, Aldrich), tetra-n-butylammonium chloride (TBACl, 99%, Aldrich) and tetrabutylammonium perchlorate (TBAClO4, 99%, Aldrich) were used without purification. DCE was washed several times with deionized water before being used. All aqueous solutions were prepared with deionized water by a Milli-Q system (Milli-Q, Millipore Corp.).
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2.2. Apparatus and electrochemical measurements
3. Results and discussion
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C
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3+
3.1. Observed oscillation phenomena in the Fe /Fe W/DCE interface
2+
system on the
Fig. 1A depicts the typical cyclic voltammetric response of the Pt electrode in 8.0 mM FeCl3 + 8.0 mM FeCl2 + 0.5 M HCl aqueous solution. The reduction peak at +0.435 V and the oxidation peak at +0.517 V were observed, which is in good agreement with the literature report [24]. The peak-to-peak separation of Fig. 1A is about 82 mV, known as one-electron transfer reaction. Fig. 1B shows the cyclic voltammogram in following electrochemical cell.
Ptjx mM FeCl3 þ y mM FeCl2 þ 0:5 M HClj10 mM TBATPB þ DCEjAgTPBjAg
-6 0
Current/µA
A computer-controlled CHI-660C electrochemical workstation (CH Instruments Inc.) connected to a personal computer was used for all electrochemical measurements. Cyclic voltammograms were recorded using a three-electrode potentiostat. The aqueous and organic solutions were mutually saturated prior to each experiment. A drop of 1.5 lL of aqueous solution containing various molar ratios of the FeCl3/FeCl2 was transferred to the surface of a freshly polished platinum disk electrode (d = 2 mm) with a small syringe. When the droplet spread spontaneously across the surface of this platinum electrode and covered it completely, the electrode was turned over and immersed immediately into an organic phase solvent (about 1.5 mL) containing supporting electrolyte (TBATPB). A Pt wire (d = 1 mm) was used as the counter electrode, and an Ag wire coated with AgTPBCl (Ag/AgTPBCl) was used as the organic phase reference electrode, which was made according to the literature [23]. Prior to each experiment, the platinum electrode was polished sequentially with 0.3 and 0.05 lm alumina. Then, it was cleaned ultrasonically with ethanol and deionized water for 2 min, respectively. All experiments were performed at room temperature (20 ± 2 °C). Spectrophotometric measurements were carried out in aqueous solutions without a supporting electrolyte using an UV-2550 spectrophotometer. All of the calculations reported in this work were carried out using B3LYP/LANL2DZ level of DFT theory and the Gaussian-03 ab initio program package.
ðCell IÞ
A pair of current peaks in Fig. 1B belong to the redox peak of Fe3+/Fe2+ at the Pt electrode (x = y = 8 mM), and the peak-to-peak separation is about 746 mV in the fist circle, which is much larger than that in Fig. 1A. The reason is ascribed to the existence of W/DCE interfacial potential difference and the Ohmic drop [25]. Compared with Refs. [20,21], where both K+ ion transfer peak across the W/DCE interface and electron redox peak at the Pt electrode appeared, we do not observe K+ transfer peak across the W/DCE interface in this case because HCl was used as supporting electrolyte in aqueous solution. As we know, Cl transfer wave across the W/DCE interface was at the negative side of the poten-
-4 0
0 .8
0 .7
0 .6
0 .5
Potential/V Fig. 1. (A) Cyclic voltammogram obtained in the aqueous solution for 8 mM FeCl3/ FeCl2, and 0.5 M HCl. (B) Cyclic voltammogram obtained across the water/1,2-DCE interface, using (Cell I). (x = y = 8.0 mM). (C) Magnified view of the cyclic voltammogram B. Scan rate was 50 mV/s. Size of the drop was 1.5 lL.
tial according to [26]. Thus, only the redox peak of Fe3+/Fe2+ at the Pt electrode was observed in the potential range between 0 and 1.0 V. It is worth mentioning that the current oscillation can be observed in the oxidation peak of Fe2+ when the potential scan from 0 to 1.0 V in Fig. 1B. Meanwhile, the oscillation could shift towards the positive potential direction with increasing the concentrations of the Fe3+/Fe2+ redox couple (data are not shown). It is more clearly seen in Fig. 1C which is a magnified view of the voltammogram of Fig. 1B. Small current spikes that exceeded the noise level of the measurement were observed in the potential range between 0.7 and 1.0 V. The obvious irregular current oscillation is restorable in most of the cases and the appearance of oscillation is very reproducible in a series of successive potential scan processes as shown in Fig. 1B, similar to our previous studies [20,21]. Above results demonstrated that the L/L interface is one of the necessary conditions for the oscillation appearance.
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3.2. Influence of concentrations on the current oscillatory phenomena We carried out a series of experiments to further investigate the influence of concentrations on the current oscillatory phenomena in order to understand the oscillation clearly. First of all, a finding from Fig. 2A and B is that the oscillation is related to the concentrations of Fe3+/Fe2+. The current oscillation can only be observed when Fe3+ and Fe2+ concentrations are both larger than 5 mM. Second, we changed the concentrations of Fe3+ and Fe2+, respectively, and the result is very interest. The oscillatory phenomenon could be observed in Fig. 2C when concentrations of Fe3+changed from 5.0 to 10 mM while maintaining the concentration of Fe2+ as 4.0 mM. However, no oscillatory phenomenon occurred in Fig. 2D when concentrations of Fe2+ were changed from 5.0 to 10 mM but remained the concentration of Fe3+ as 4.0 mM. For further proved this result, we performed another experiment by adding Fe3+ or Fe2+ in the aqueous phase only. As shown in Fig. 2E and F,
the current oscillatory phenomenon appeared when Fe3+ concentration arrived at 10 mM, whereas, no current oscillation occurred even as the Fe2+ concentration was 10 mM. All above results illuminated that one of the necessary conditions for the oscillation appearing at the L/L interface was the concentration of Fe3+, indicating that Fe3+ ion played a key role in inducing oscillations. Finally, we also investigated the species and the concentrations of supporting electrolyte in organic phase affecting the appearance of the current oscillation. As shown in Fig. 3A–C, the current oscillation will occur when the concentration of TBATPB is 5.0 or 10 mM, meaning TBATPB concentration also influence oscillations appearance. On the other hand, when TBATPB in organic phase is replaced by TPnATPB (D), TBAClO4 (E) and TBACl (F), respectively, one can see clearly that the oscillation occurred only in process D, and absented in processes E and F, demonstrated further that the oscillation is related to the negatively charged TPB in the organic phase.
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Fig. 2. Cyclic voltammograms obtained for using (Cell I). (A) x = y = 4.0 mM, (B) x = y = 5 mM, (C) x = 10, y = 4.0 mM, (D) x = 4.0, y = 10 mM, (E) x = 10, y = 0 mM, (F) x = 0, y = 10 mM. Scan rate was 50 mV/s. size of the drop was 1.5 lL.
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Fig. 3. Cyclic voltammograms obtained for (Cell I) (x = y = 8 mM) with the concentrations of TBATPB are (A) 1 mM, (B) 5 mM and (C) 10 mM; and with the supporting electrolyte (10 mM) in organic phase as (D) TPnATPB, (E) TBAClO4 and (F) TBACl, respectively. Scan rate was 50 mV/s, and size of the drop was 1.5 lL.
3þ
Fe
3þ
þ e Fe
2þ
ðPt=WÞ
ð1Þ 3þ
Fe ðwÞ þ 2TPB ð0Þ TPB Fe TPB ðadsÞ
ðO=WÞ
ð2Þ
when TPB in organic phase is replaced by other species, such as ClO4 and Cl, we consider that Fe3+ may transfer across the interface from W to O or ClO4 and Cl transfer across the interface from O to W when the oxidation process of Fe2+ occurred at the Pt electrode in or der to maintain the electro-neutrality. Because ClO4 and Cl are hydrophilic anion ions, they tend to transfer into the aqueous phase easily, rather than adsorb in the L/L interface, which is consistent with the experiment results in Fig. 3E and F (no oscillation
0.5
0.4
3
0.3
Absorbance
We have achieved the results from Ref. [2] that the electrochemical process couples the electron transfer at the Pt/W with the ion transfer at the W/O interface based on electro-neutrality requirement. In this case, both the oxidation process of Fe2+ at the Pt electrode and Fe3+ transfer across the interface from W to O proceed simultaneously as shown in reactions 1 and 2. Because Fe3+ is hydrophilic ion, it tends to remain at the L/L interface and forms ion pairs with TPB.
2
0.2
0.1 1
0.0
-0.1 350
400
450
500
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600
Wavelength/nm Fig. 4. UV–vis spectra in organic phase. (1) 15 105 mol/L TBATPB, (2) 15 105 mol/L FeCl3, (3) 15 105 mol/L TBATPB and FeCl3.
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Fe3++3TPB-
0.00 -200 643
G (kcal / mol)
-400 1198
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1576
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Fe3+(TPB-)3
-1000 -1200
Fe3+TPB-+TPB-
-1400 -1600 TPB-Fe3+ TPB-
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A 3+
Fe +2TPB
B -
3+
-
Fe TPB +TPB
C -
-
3+
-
TPB Fe TPB +TPB
-
Fe
3+
(TPB-)3
Fig. 5. Potential energy profiles of the reaction Fe3+ with TPB calculated with B3LYP/LANL2DZ level of DFT theory.
appearance). On the other hand, the redox peak current of Fe3+/Fe2+ in Fig. 3E and F are 40 lA and 36 lA, respectively, much larger than that of in Fig. 3D (about 11 lA). This result proved further that there are no ion pairs formed at the L/L interface in Fig. 3E and F so that the Fe3+ concentrations in the aqueous phase are larger than that in processes C and D. That is why no current oscillation appearing in Fig. 3E and F. Therefore, from above experiments, we can draw the conclusion that the oscillation is caused by the adsorption of ion-pair at the interface between the positively charged Fe3+ in the aqueous side and the negatively charged TPB in the organic side. In addition, the optical properties of TBATPB and FeCl3 were further characterized by UV spectra. Fig. 4 shows the UV spectra of the TBATPB (1), FeCl3 (2), and a mixture of both (3) in the organic phase at the same concentration (15 105 mol/L). It can be seen that the molar absorptivity of TBATPB + FeCl3 is higher than that of FeCl3, which is an important feature of the strong interaction between Fe3+ and TPB and is an indication of an ion pairing between them [27,28]. 3.3. Investigation the mechanism of oscillation at the W/DCE interface with theoretical calculation combined a specific adsorption of ion pair model Above results showed the oscillation is related to L/L interface, concentration of Fe3+, and the species and concentrations of supporting electrolyte in organic phase. In order to investigate the mechanism of ion pair formation in the L/L interface, computations were carried out by using the Gaussian-03 ab initio program package [29] for the first time. Firstly, we create and optimize the structure of each ion pair from scratch, where all the possible structures need to be calculated and compared. Fig. 5A and B displayed the optimized ion pair structures the Fe3+TPB and TPBFe3+TPB using B3LYP/LANL2DZ level of DFT theory [30–32]. The orange1 ball represents B of the TPB and blue ball represents Fe3+. The bond lengths of Fe–B of the Fe3+TPB and TPBFe3+TPB are 3.2 Å and 3.7 Å, respectively. As shown in Fig. 5B, Fe3+ placed centrally between two TPB, similar to the sandwich structure, 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.
and the B–Fe–B angle of TPBFe3+TPB is 176.8°. Secondly, the Gibbs Free Energy of forming ion pair from Fe3+ and TPB were also calculated by B3LYP/LANL2DZ level of DFT theory [33,34] for considering the solvation effect. Fig. 5 displayed the optimized ion pair structures corresponding to the lowest energy states of Fe3+TPB, TPBFe3+TPB and Fe3+(TPB)3, respectively. On the basis of theoretical calculation, we can postulate the following processes stages at the interface:
Fe3þ ðwÞ þ TPB ð0Þ Fe3þ TPB ðadsÞ
DG1 ¼ 1198 kcal=mol ð3Þ
Fe3þ TPB ðadsÞ þ TPB ð0Þ TPB Fe3þ TPB ðadsÞ
DG2 ¼ 378 kcal=mol
ð4Þ
when one Fe3+ ion and one TPB ion form Fe3+TPB ion pair, its energy lie almost 1198 kcal/mol, lower than energy relative to the corresponding ground state (Fe3+ + 3TPB), indicating efficient interaction between Fe3+ and TPB ions. Furthermore, one Fe3+TPB combined one TPB will produce the lowest energy state, whose energy lie almost 1576 kcal/mol compared to the corresponding ground state (Fe3+ + 3TPB), meaning that TPBFe3+TPB ion pair is more stable than Fe3+TPB ion pair. Whereas, the next step of the reaction of TPBFe3+TPB and TPB ion forming Fe3+(TPB)3 ion pair is hampered because its energy lie almost 933 kcal/mol higher than that of TPBFe3+TPB, indicating TPBFe3+TPB is the most stable ion pair. From the Gouy–Chapman theory, we know that the Galvani potential difference between the two phases was spread entirely within the two back to back double layers. The polarized structure of the L/L interface [35–37] can be regarded as a non-homogeneous two-component liquid phase, known as a mixed solvent layer with two or three molecular diameter thick. The penetration of the ions in the interfacial region depends on their hydrophobicity or hydrophilicity. The diffuse double layers are separated by an inner layer which contains no ion except specifically adsorbed [38]. Thus, a specific adsorption of ion pair model was proposed in Fig. 6 for explain the mechanism of oscillation better. Fig. 6A shows the profile about Fe3+/Fe2+ solution drop at the Pt electrode, and Fig. 6B is the model of a specific adsorption of ion pair. In this model, we considered the L/L interface as two space charge regions separated by a mixed solvent layer (MSL) [28].
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(a)
Pt
MSL
Pt -eFe2+ Fe3+
+
Fe3+
-
TPB-
(b)
MSL
=
- +-
ion pair
A
+
+
-
+ W
- + - + -
-
+
-
-
O
-
+
+
- +-
+
+
- +-
O W
+
+
+
+
W
-O
- -
B
Fig. 6. Schematic drawing for a special adsorption of ion pair model at the L/L interface.
We have discussed that the electrochemical process couples the electron transfer at the Pt/W with the ion transfer at the W/O interface based on electro-neutrality requirement in (Cell I). At first, the positively charged Fe3+ and the negatively charged TPB will form the ion pair (TPBFe3+TPB) at the mixed solvent layer (shown in Fig. 6Ba) due to electrostatic interaction between them. Then, when the potential scan starts from 0.2 V towards positive potential in Fig. 1B, the oxidation process of Fe2+ will occur and the ion pains, TPBFe3+TPB, are accumulating at the interface with increasing concentration of Fe3+. When the potential sweep to the oxidation peak of Fe2+ (near 0.8 V), electrode reaction reaches a maximum rate, driving Fe3+ transfer from droplet to the mixed solvent layer to form ion pairs with TPB. Thus, the interfacial tension on such a small water drop (1.5 lL) is drastically changed due to the diffusion of Fe3+ ions, resulting in oscillation occurence. 4. Conclusions A new current oscillatory phenomenon based on the system of Fe3+/Fe2+ was studied on the W/DCE interface in this paper. It was found that the current oscillation phenomenon only occur in the site of oxidation peak of Fe3+, and related to the concentrations of both Fe3+ in the aqueous phase and TPB in the organic phase, which indicates that the oscillation feature is the specific adsorption of ion pair forming mechanism in W/DCE interface. Moreover, the mechanism of ion pair formation is calculated by DFT theory for the first time. The result suggested that TPBFe3+TPB ion pair has the lowest-energy state, which provided qualitative support for ion pair embodied state. Acknowledgments This work was supported by the Natural Science Foundation of China (Nos. 20875077, 20965007, and 20927004), and the Key Laboratory of Polymer Materials of Gansu Province. References [1] Q.-K. Yu, Y. Miyakita, S. Nakabayashi, R. Baba, Electrochem. Commun. 5 (2003) 321–324. [2] Y. Yuan, Z. Gao, Y. Shao, J. Electroanal. Chem. 526 (2002) 85–91.
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