Probing Thermodynamic Aspects of Electrochemically Driven Ion

Aug 1, 2002 - Despite the high concentration of redox centers in the microdroplets, the redox potentials of the TAPDs are exclusively determined by th...
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J. Phys. Chem. B 2002, 106, 8697-8704

8697

Probing Thermodynamic Aspects of Electrochemically Driven Ion-Transfer Processes Across Liquid|Liquid Interfaces: Pure versus Diluted Redox Liquids Uwe Schro1 der* Institut fu¨ r Chemie und Biochemie, Ernst-Moritz-Arndt UniVersita¨ t, Soldmannstrasse 16, 17489 Greifswald, Germany

Jay Wadhawan, Russell G. Evans, and Richard G. Compton Physical & Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, U.K.

Bill Wood and David J. Walton School of Natural and EnVironmental Sciences, CoVentry UniVersity, Priory Street, CoVentry CV1 5FB, U.K.

Robert R. France Dyson Perrins Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, U.K.

Frank Marken, Philip C. Bulman Page, and Colin M. Hayman Department of Chemistry, Loughborough UniVersity, Loughborough LE11 3TU, U.K. ReceiVed: December 19, 2001; In Final Form: April 5, 2002

The voltammetry of the water-insoluble redox liquids para-N,N,N′,N′-tetraalkylphenylenediamines (TAPDs; A ) butyl, hexyl, heptyl, octyl, and nonyl) at the TAPD|electrode|electrolyte interface is studied. The iontransfer processes between the aqueous and the organic phasesconsequent of the necessity to maintain charge neutrality during electrochemical oxidation and reduction reactions in the organic phasesare elucidated as a function of the Gibbs free energy of transfer of the electrolyte anions, of the concentration of TAPD in the organic phase, and of the length of the alkyl chain of the phenylenediamines. Two conditions are considered: the voltammetry of microdroplets of the pure redox liquids and the voltammetry of the TAPDs dissolved in nitrobenzene, either deposited as microdroplets at an electrode surface. The oxidation of the neutral redox liquids, accompanied by the uptake of anions from the adjacent aqueous solution leads to the formation of ionic liquid phases, TAPD+X- and TAPD2+X-2 (X- ) monovalent electrolyte anions). Despite the high concentration of redox centers in the microdroplets, the redox potentials of the TAPDs are exclusively determined by the Gibbs free energy of the electrochemically driven transfer of anions between the aqueous and the redox liquid phase. There is no evidence for ion pair formation of the TAPD+ and TAPD2+ with the transferred anions X- either in the pure redox liquid phase or in the nitrobenzene matrix. Depending on the Gibbs energy of transfer of the electrolyte anions two different reaction mechanisms are observed: At low ∆G values, a reversible transfer of electrolyte anions across the liquid/liquid interface as a consequence of the redox process takes place. However, anions with high transfer energies do not enter the organic phase. Instead, upon oxidation TAPD mono- and dications are transferred from the organic into the aqueous phase, which leads to the diminution of the voltammetric signals due to the loss of material from the droplets. From the voltammetric data, the Gibbs energy of transfer of the different TAPD cations and of the hexafluoroarsenate anion were derived for the water|nitrobenzene interface. Whereas the oxidation of the TAPD in the nitrobenzene droplets due to its low concentration does not measurably affect the properties of the nitrobenzene|water interface of the droplet, the oxidation of the pure redox liquids leads to a considerable change of the liquid|liquid interface upon ionic liquid formation. This change depends on the nature of the inserted anion: the more hydrophilic the inserted anion is, the less hydrophobic the ionic liquid becomes.

Introduction Ion-transfer processes across liquid/liquid interfaces are of great interest for chemistry and biology. Much attention has been given to the study of facilitated/assisted ion-transfer reactions (e.g., see refs 1 and 2), which form the basis of a multitude of modern chemical technologies and which play a central role in the functioning of living cells. * To whom correspondence should be addressed. Tel: +49 3834 864330. Fax: +49 3834 864451. E-mail: [email protected].

Hitherto, most studies considered only ion-transfer processes in highly diluted systems, for instance, the transfer processes between an aqueous solution and a nitrobenzene phase. This has the advantage that one can rely on a wealth of information from the literature. In this paper, we make the attempt to thermodynamically describe the electrochemically facilitated ion-transfer processes across the interface between an aqueous solution and pure redox liquids. The studied processes, which involve the in situ formation of room-temperature ionic liquids,

10.1021/jp0146059 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/01/2002

8698 J. Phys. Chem. B, Vol. 106, No. 34, 2002 SCHEME 1

are accompanied by drastic changes of the phase properties during the electrochemical and ion-transfer reactions. Such an approach is challenging because reference data are not available from the literature and the liquid|liquid interface can no longer be considered to be a constant entity. The para-N,N,N′,N′-tetraalkylphenylenediamines, TAPDs, are very well suited for systematic voltammetric studies of electrochemically and chemically facilitated ion-transfer reactions for a number of reasons. First of all, the studied TAPDs and their oxidation products are liquids throughout all experimental conditions of our investigation. The length of the alkyl chains ensures a strong hydrophobic character of the compounds. The TAPDs can be oxidized in two reversible one-electron reaction steps (Scheme 1, part A). The product of the first oxidation step, the radical cation TAPD+, is electron paramagnetic resonance (EPR) visible,3 and because of its strong absorption in the visible range, it makes possible visual microscopic observation3 and spectroelectrochemical measurements.4 Second, the amine groups can undergo protonation reactions (Scheme 1, part B) giving the chance to learn about the interplay between electrochemical and chemical equilibria under biphasic conditions.54 As an experimental approach for this study, we have chosen the voltammetry of immobilized microdroplets. This technique, introduced by Marken et al.,3 opened up a convenient way for the study of electrochemically and chemically facilitated iontransfer processes across the liquid|liquid interfaces (e.g., see refs 6 and 7) Owing to the presence of the contact lines, liquid|water|electrode, the technique of depositing the organic phase in the form of microdroplets rather than as continuous films gives the unique opportunity to omit supporting electrolytes in the organic phase. This becomes possible because at the three-phase junctions charge compensation during electrochemical oxidation and reduction processes can always be provided, even in the case of ionically nonconducting organic phases. Experimental Section Chemical Reagents. Aqueous electrolyte solutions have been prepared with NaClO4, KAsF6, KPF6, KBF4, KSCN, KF, KI, KNO3, KCl, and Na2SO4 (Aldrich, Fluka, and Merck). All chemical reagents were purchased in the analytical or the purest commercially available grade. De-ionized water was taken from an Elgastat filter system (Elga, Bucks, U.K.) and had a resistivity of not less than 18 MΩ cm.

Schro¨der et al. THPD (hexyl) and TOPD (octyl) were prepared following a general literature procedure based on that reported by Furhop and Bartsch.3,8,9 The synthesis of TBPD (butyl), THePD (heptyl), and TNPD (nonyl) followed the same strategy. These materials were obtained in pure form after column chromatography with the following spectroscopic characteristics: TBPD 1H NMR (CDCl3) 0.8 (t, 12H (4 × NCH2(CH2)2CH3)), 1.4 (br m, 16H (4 × NCH2(CH2)2CH3)); 3.2 (m, 8H (4 × NCH2(CH2)2CH3)), 6.6 (s, 4H (Ar-H)); elemental analysis C 78.95, H 12.92, N 8.13; C22H40N2 requires C 79.45, H 12.12, N 8.42; THePD 1H NMR (CDCl3) 0.8 (br t, 12H (4 × NCH2(CH2)5CH3)), 1.6 (br m, 40H (4 × NCH2(CH2)5CH3)), 3.2 (br m, 8H (4 × NCH2(CH2)5CH3)), 6.6 (s, 4H (4 × Ar-H)); elemental analysis C 80.73, H 13.65, N 5.52; C34H64N2 requires C 81.53, H 12.88, N 5.59; TNPD 1H NMR 0.8 (br t, 12H (4 × NCH2(CH2)7CH3)), 1.5 (br m, 56H (4 × NCH2(CH2)7CH3)), 3.6 (br m, 8H ((4 × NCH2(CH2)7CH3)), 6.6 (s, 4H (Ar-H)); elemental analysis C 81.54, H 14.03, N 4.40; C42H80N2 requires C 82.28, H 13.15, N 4.57. Instrumentation. Electrochemical experiments were conducted with a µ-Autolab and with a PGSTAT 30 Autolab system (Ecochemie, Netherlands) in a conventional three-electrode cell. The electrodes used were a platinum wire as the counter electrode and a NaCl-saturated calomel electrode (Radiometer, Kopenhagen) as the reference. The working electrode was a 4.9 mm diameter basal plane pyrolytic graphite disk mounted in a Teflon holder. Before and after electrochemical experiments, the electrode surface was cleaned by rinsing with acetonitrile and occasionally renewed by polishing on a fine carborundum paper. The deposition of microdroplets of the pure redox liquids was achieved by solvent evaporation using a 0.5 mM solution of the organic precursors in acetonitrile. For the deposition of the TAPDs diluted in nitrobenzene, a different approach was chosen. First, solutions of the phenylenediamines in nitrobenzene were prepared. One drop of the solution was then transferred onto the electrode surface. To avoid the formation of a continuous film most of the drop was carefully swabbed off with filter paper, and the electrode was quickly immersed into the nitrobenzene-saturated aqueous solution to prevent evaporation of the nitrobenzene. Unless otherwise stated, experiments have been undertaken under an inert atmosphere of nitrogen and under thermostatic conditions at a temperature of 21 °C. Values for the activity of ionic species in the aqueous electrolyte solutions were taken from tabulated literature data.10 Values for the Gibbs energy of transfer ∆GX-(WfNB) were taken from printed and on-line databases.11,12 Results and Discussion Figure 1 shows the voltammetric behavior of para-N,N,N′,N′tetrahexylphenylenediamine (THPD) deposited onto an electrode surface in the form of microdroplets and immersed in a 0.1 M sodium perchlorate electrolyte solution. The voltammetry of THPD as a representative of the TAPDs under different pH conditions can be described with the following set of reactions. At neutral pH values, the oxidation of TAPD takes place accompanied by the charge-compensating uptake of anions from the aqueous phase into the organic phase, the rereduction being accompanied by the release of these ions into the aqueous solution.

Iox/Ired:

+ TAPDO + X(rxn 1) W a TAPDO + XO + e

At pH values lower than 6, a chemically facilitated transfer of

Thermodynamic Aspects of Ion Transfer Processes

J. Phys. Chem. B, Vol. 106, No. 34, 2002 8699 + + TAPDO + XW + HW a TAPDHO + XO

(rxn 2)

It was shown in earlier contributions4,5 that the oxidation of this species now takes place coupled with the transfer of protons between the organic and the aqueous phase.

IIox/IIred:

+ + TAPDH+ O + XO a TAPDO + XO + HW + e (rxn 3)

As can be derived from rxns 1 and 3, the composition of the organic phase issindependent of the protonation equilibrium of the TAPDsvirtually identical after the first oxidation process. This explains why the second oxidation step (peaks IIIox/IIIred, Figure 1) is not affected by changing the pH conditions in the aqueous phase. Figure 1. Cyclic voltammograms for the oxidation of 3.5 nmol of THPD deposited on a 4.9 mm diameter basal plane pyrolytic graphite electrode and immersed in a 1 M NaClO4 electrolyte solution: (A) pH ) 8; (B) pH ) 5.9; (C) pH ) 3.7. Scan rate ) 10 mV s-1.

protons (accompanied by the charge-compensating uptake of anions) into the organic phase takes place, which leads to the protonation of one amine group of the TAPD molecules.

IIIox/IIIred:

2+ TAPD+ O + XW a TAPDO + XO + e (rxn 4)

Whereas the chemically facilitated ion-transfer processes connected with the biphasic protonation equilibria of the TAPDs (rxn 2) and their interaction with the redox equilibria will be subject of a forthcoming paper, the following discussion will be emphasized on the electrochemically induced ion-transfer reactions.

Figure 2. Cyclic voltammograms for the oxidation of 3.5 nmol of THPD deposited on a 4.9 mm diameter basal plane pyrolytic graphite electrode and immersed in a 0.1 M NaClO4 electrolyte solution at pH ) 8 and θ ) 42 °C: (A) 50; (B) 250; (C) 500; (D) 2000 mV s-1.

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Schro¨der et al. TABLE 1: Formal Potentials of the First Oxidation Step (Iox/Ired) for Microdroplets of the Pure Redox Liquids Measured in the Presence of Different Electrolyte Anionsa Ef, mV vs SSCE ((5 mV) TBPD (butyl) AsF6PF6ClO4SCNNO3ClFSO42-

Figure 3. Square-wave voltammograms for the oxidation of 3.5 nmol of THPD deposited on a 4.9 mm diameter basal plane pyrolytic graphite electrode and immersed in a 0.1 M NaClO4 electrolyte solution at pH ) 8, frequency ) 50 Hz, and amplitude ) 20 mV.

Determination of the Formal Potentials and the Role of Intermolecular Interactions in the TAPD Droplets. As has been already described in previous papers13 and as can be seen in Figure 1, the voltammetric oxidation of microdroplets of the TAPDs is affected by intermolecular interactions within the TAPD phase. They find their expression in the very narrow peak shape and the unusual separation of the oxidation and reduction peaks of redox system I and in the partially very large peak separation of the oxidation and reduction peak of the redox system III (see Figure 1). For the latter redox pair (rxn 4), one can speculatively attribute changes in the geometrical configuration of the TAPD molecules from the TAPD+ to the TAPD2+ cation to be responsible for this behavior. In the TAPD2+ molecule, the rotation of the amine/ammonium groups might be abolished and a planar molecule body is formed (see Scheme 1). The planar phenylenediamine body is likely to cause reorientations of the TAPD2+X-2 ionic liquid phase toward a higher packing density, which could explain the overpotential of the rereduction necessary to return to the less-dense state of the TAPD+X- ionic liquid. Experimental support for this assumption is given by the following observations. In cyclic voltammetry, the reduction of the ionic liquid TAPD2+X-2 is sensitive toward scan rate: At scan rates lower than 100 mV s-1(see Figure 2A), one can find the reduction signal (IIIred) with the described peak separation. With increasing scan rates, this signal is replaced by a reversible reduction peak (III′red) (Figure 2B-D). This can be interpreted taking the rate of the reorientation into account: at higher scan rates, the TAPD2+X-2 liquid is rereduced before the reorientation of the liquid takes place. This phase transformation aspect is also supported by the finding that the described peak separations cannot be observed for the oxidation of the TAPDs dissolved in droplets of nitrobenzeneshere, the voltammetric responses are fully reversible. Certainly, such conditions make it very difficult to use cyclic voltammetry to obtain exact values for the formal potential of the electrochemical oxidation of the studied redox liquids. Consequently, it was an important task to find a technique that enables eliminating the described effects to gain access to the thermodynamic data of the redox and ion-transfer processes. For this, the square-wave voltammetry was found to be a powerful tool. The fast time scale of square-wave voltammetric experiments in comparison to cyclic voltammetry, based on the rapid transition between the forward sampling and the reverse sampling, allows suppressing effects caused by slower physical and chemical transformations of the reaction products. This in

-15 68 134

THPD (hexyl)

THepPD (heptyl)

TOPD (octyl)

TNPD (nonyl)

-22 31 114 183 303 376 470 449

46 131 193 318 388 494 472

9 60 140 205 327 397 513 496

69 151 206 332 388 518 486

a Derived from square-wave voltammetric experiments; frequency ) 20 Hz, amplitude ) 20 mV.

Figure 4. Plot of the formal potentials of microdroplets of THPD (9) and TOPD (b) versus the reciprocal of the radii of the electrolyte anions measured in 0.1 M electrolyte solutions at pH 8.

connection with the elimination of capacitive currents is the reason for the well-defined and in most cases electrochemically reversible voltammetric signals that could be obtained, as shown in Figure 3 for the example of microdroplets of THPD in the presence of 1 M NaClO4 aqueous electrolyte solution. From these voltammograms, the desired reversible potential values could be extracted. Table 1 shows the formal potentials for the oxidation process Iox/Ired of microdroplets of different TAPDs in the presence of 0.1 M solutions of different anion species. Because of the high concentration of redox centers in the organic phase and the strong intermolecular interactions (narrow peak shape and peak separation), these formal potentials were at first expected to be possibly governed by specific ion-pairing interactions between the TAPD cations and the transferred anions. However, as Figure 4 shows, there is a strong linear relationship between the formal potentials and the reciprocal of the radii of the electrolyte anions. Following the classical electrostatic Born theory,14 such dependence is an indication for a dominating influence of the free Gibbs energy of the transfer of the anions between the aqueous and the organic phase:

∆G°,OfW )tr

(

NAz2e2 1 1 8π0r O W

)

( represents the elementary charge, NA is the Avogadro constant, 0 is the permittivity of vacuum, O and W are the relative permittivities of the organic and the aqueous phase.) Because there are no data available for the transfer of ions across the water/TAPD interface, we used Gibbs free energy data for

Thermodynamic Aspects of Ion Transfer Processes

Figure 5. Plot of the formal potentials of microdroplets of THPD nitrobenzene 0 versus the standard membrane potential, ∆water φX- of the electrolyte anions measured in 0.1 M electrolyte solutions at pH 9.

J. Phys. Chem. B, Vol. 106, No. 34, 2002 8701

Figure 7. Plot of the formal potentials of the first oxidation step (Iox/ Ired) for THPD dissolved in nitrobenzene and for the pure redox liquid measured in the presence of different electrolyte anions versus the standard membrane potentials of the electrolyte anions at the water/ nitrobenzene interface at pH ) 9.

mainly caused by the different concentrations of THPD in the droplets, both plots are very similar. Clearly, two sections are visible. Section A. As expected, the redox process is accompanied by the transfer of anions between the aqueous and the organic phase. The Nernst equation for the reaction can be formulated as follows:

E ) E° + ∆OWφX0 - -

Figure 6. Cyclic voltammograms for the oxidation of THPD dissolved in microdroplets of nitrobenzene (cTHPD ) 25 mM) deposited at a basal plane pyrolytic graphite electrode and immersed in a 0.1 M NaClO4 electrolyte solution: (A) pH ) 8; (B) pH ) 5.7; (C) pH ) 3.5. Scan rate ) 100 mV s-1.

tabulated water|solvent interfaces to investigate such a relationship. Figure 5 shows a plot of the mid-peak potentials as a function of the standard membrane potential, ∆OWφ°X- (with ∆OWφ°X- ) ∆GX-(WfO)/F), of a number of different electrolyte anions for the example of the system water|nitrobenzene. The linear dependence with a slope of 1.0 confirms the dominating role of the free energy of transfer. This is a contradiction to the expectation mentioned above. It shows that the transfer of the anions from the aqueous phase into the organic phase clearly overrules the energetic effects of interactions in the organic phase. To elucidate this phenomenon, the voltammetry of immobilized microdroplets of the pure tetraalkylphenylenediamine redox liquids was compared with the voltammetry of the TAPDs dissolved in nitrobenzene and deposited as microdroplets at an electrode surface. The rxns 1-4 have already been phenomenologically described in earlier papers for microdroplets of the pure redox liquids. As shown in Figure 6, they were found to take place for the case the TAPDs being dissolved in droplets of nitrobenzene. The formal potentials of redox system I (Figures 1 and 6) measured for droplets of pure THPD, as well as for droplets of THPD dissolved in nitrobenzene (cTHPD ) 0.025 M), are presented in Figure 7 as a function of the standard potential of transfer of the electrolyte anions. Apart from a potential shift

RT RT aTAPD+OaX-O (1) ln aX-W + ln aTAPDO F F

Because of the very small diffusion length within the microdroplets, the voltammetry of the immobilized microdroplets shows typical thin-film characteristics. Thus, equilibrium conditions could be assumed to apply for the entire organic phase, which allows us to eliminate the diffusion term as a part of the Nernstian equations. Using the electroneutrality condition aTAPD+O ) aX-O and the condition for the formal potential aTAPD+O ) aTAPDO, we can express the formal potential by

EF ) E° + ∆OWφX0 - -

RT RT aTAPD0,abs ln aX-W + ln F F 2

(2)

where aTAPD0,abs is the total phenylenediamine concentration in the nitrobenzene phase. This reaction type is characterized by a high chemical reversibility of the voltammetric oxidation and rereduction. What Can Be Deduced from These Data? Because tabulated values for the Gibbs energy of transfer of a number of anions for the water|nitrobenzene system are available,11,12 one can determine the standard potential of the oxidation of the TAPDs in nitrobenzene (using eq 2) without the need to use internal voltammetric standards such as decamethylferrocene. The procedure is similar to the approach of Scholz et al.6 and Komorsky-Lovric´ et al.7 After deriving E°, ∆G values of the transfer for anions that are not yet tabulated can be determined. As an example, the standard potentials of the studied TAPDs (Table 2) have been derived from the formal potentials measured in the presence of 0.1 M NaClO4 (∆GX-(WfNB) of ClO4- of 9400 kJ mol-1).11 Using these standard potential values, we determined the ∆GX-(WfNB) values for other anions (e.g., NO3- expt ) 26 kJ mol-1, lit ) 26 kJ mol-1; PF6- expt ) 0.3 kJ mol-1, lit ) 0.6 kJ mol-1). As demonstrated for nitrate and hexafluorophosphate,

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Schro¨der et al.

TABLE 2: Standard Potentials of the Tetraalkylphenylenediamines (mV vs SSCE) Derived from the Formal Potentials of the TAPDs Dissolved in Microdroplets of Nitrobenzene and Measured in the Presence of a 0.1 M Aqueous NaClO4 Solution and Gibbs Energies of Transfer (kJ mol-1) of the TAPD Cation Species for the Water-Nitrobenzene Interface Calculated after the Eqs 2, 4, and 6

TBPD THPD THepPD TOPD TNPD

E°(I) ((5 mV)

E°(III) ((5 mV)

∆GTAPD+(WfNB) ((1 kJ)

∆GTAPD2+(WfNB) ((1 kJ)

-78 -98 -78 -103 -77

680 662 675 654 695

-31.0 -43.6

-12.6 -13.6 -16.6 -17.3 -18.2

the experimental values are in good agreement with literature data. Additionally, the value for untabulated hexafluoroarsenate ion, AsF6-, was determined with ∆GX-(WfNB) ) -4.1 kJ mol-1. For the pure redox liquids, these derivations are not possible because the term of the standard potential of the anion transfer and the standard potential of the oxidation of the TAPD cannot be separated. One could, of course, make the assumption that the standard potential E° of the oxidation of TAPD in the redox liquid is the same as the one determined for the TAPDs dissolved in nitrobenzene. From the literature, a number of examples of this approach are known. We believe that data based upon such an assumption are likely to be rather unreliable. Instead, in a forthcoming paper, we will discuss an approach that utilizes chemically facilitated ion-transfer processes in combination with the electrochemical equilibrium to gain access to the individual terms of the potential of the anion transfer and the standard potential of the oxidation. Section B. The high Gibbs energy of transfer for the small ions such as F- or highly charged ions such as sulfate makes the transfer of these ions energetically unfavorable compared to the opposing transfer of the formed TAPD radical cation TAPDO a TAPD+ W+e

0 E ) E° - ∆OWφTAPD + +

aTAPD + W RT ln F aTAPDO

(rxn 5) (3)

species (square-wave voltammograms in Figure 9a,b). As was found for both conditionssthe oxidation of the TAPDs in the pure phase and diluted in nitrobenzenesthe oxidation takes place accompanied by a transfer of anions from the aqueous phase into the organic droplet (∆Ef/(∆log(aX-aqu) ) 49 mV) according to rxn 4. In the case of the nitrobenzene droplets, this as expected leads to a shift of the formal potential as a function of the Gibbs free energy of transfer of the transferred ions. Here, analogous to redox system I (Figure 6), two different reaction mechanisms can be found (Figure 10). In the presence of anions with a low Gibbs energy of transfer, the oxidation takes place according to rxn 4. It can be expressed by the Nernst equation

E ) E° + ∆OWφX0 - -

RT RT ln aX - + ln F F W

aTAPD

aX

2+

O

aTAPD

-

O

(5)

+

O

In the presence of anions with a high Gibbs energy of transfer, the transfer of TAPD2+ cations from the droplet into the aqueous phase becomes energetically favored: 2+ TAPD+ O a TAPDW + e

with the formal potential being a function of the standard redox potential and the standard potential of the transfer of the radical 0 cation (∆OWφTAPD + ) ∆GTAPD+(WfO)/F): 0 EF ) E° - ∆OWφTAPD +

Figure 8. Cyclic voltammograms for the oxidation of 3.5 nmol of THPD deposited on a 4.9 mm diameter basal plane pyrolytic graphite electrode and immersed in a 0.1 M NaF electrolyte solution.

E ) E° -

0 ∆OWφTAPD 2+

(rxn 6)

aTAPD 2+ RT W + ln F aTAPD +

(6)

O

(4)

By determining the formal potential of the oxidation (see Table 2), we could determine the Gibbs transfer energy of the TBPD and the THPD radical cations. For the heptyl, octyl, and nonyl compounds, an apparent decrease of the transfer energy values was observed because of a possible association of the radical cations and the electrolyte anions in the aqueous phase facilitating the transfer of these ions into the aqueous phase. A consequence of rxn 5 is the chemical irreversibility of the oxidation of the TAPDs in the presence of anions with a high Gibbs energy of transfer (Figure 8). The transfer of TAPD+ into the aqueous phase leads to a loss of material from the electrode and thus to a diminution of the voltammetric response. Also, the organic phase remains ionically nonconducting, there is no ionic liquid formation in the case of the undiluted TAPDs. A peculiar phenomenon can be observed comparing the formal potentials of the redox system III, rxn 4, for the condition of droplets of the pure redox liquids and for the diluted redox

Using eq 6, we determined the Gibbs energies of transfer of the TAPD2+ cations (see Table 2). In comparison to the ∆G values of the TAPD+ radical cations, these transfer energies are less negative owing to the higher charge of the dications. As expected, ∆GX-(WfNB) increases with increasing length of the alkyl chain of the phenylenediamine dication (Figure 11). A different situation was found for the oxidation of the pure TAPD droplets (see the voltammograms in Figures 9b, 10). Although from the shift of the formal potential as a function of the activity of the anions (∆EF/log aX- ) 51 mV (ClO4-), 48 mV (PF6-)) in the aqueous phase the reaction was found to follow rxn 4, the value of the redox potential appears to be practically independent of the kind of transferred anion. This is a contradiction to eq 5 from which it follows that the involved anion-transfer process causes a linear dependence of the formal potentials as a function of the free energy of transfer of the involved anions. The finding can be explained taking the high concentration of redox centers in the TAPD microdroplets and

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J. Phys. Chem. B, Vol. 106, No. 34, 2002 8703

Figure 9. Square voltammograms for the oxidation of (A) THPD dissolved in microdroplets of nitrobenzene (cTHPD ) 25 mM) and (B) microdroplets of pure THPD deposited at a basal plane pyrolytic graphite electrode and immersed in a 0.1 M electrolyte solution at pH ) 8, frequency ) 50 Hz, and amplitude ) 20 mV.

uptake of anions into the pure TAPD droplets upon oxidation process I (rxn 1) forms an ionic liquid phase that represents an entirely new compound. Because of its ionic composition, this liquid is likely to be much less hydrophobic than the reduced, neutral redox liquid. The more hydrophilic the anion X- in the 0 ionic liquid TAPD+X- is, the smaller ∆G(WfTAPD +X-) will be. That means that, in the case of the second oxidation process (formation of TAPD2+X-2, rxn 4), an increased hydrophilicity of the anion X- is compensated by the increased hydrophility of the organic phase thus leading to the practically constant formal potential of this redox process. Conclusions

Figure 10. Plot of the formal potentials of the second oxidation step (IIIox/IIIred) for THPD dissolved in nitrobenzene and for the pure redox liquid measured in the presence of different electrolyte anions versus the standard membrane potentials of the electrolyte anions at the water/ nitrobenzene interface at pH ) 9.

Figure 11. Plot of the Gibbs energy of transfer of the TAPD dications versus the number of carbon atoms of their alkyl chains.

the precursory oxidation, rxn 1, into account. Whereas in the case of the phenylenediamines dissolved in nitrobenzene the uptake of anions into the organic phase practically does not change the properties of the nitrobenzene|water interface, the

In the present study, we compared the electrochemically facilitated ion-transfer processes across the interface between aqueous electrolyte solutions containing different anion species and organic phases consisting either of the pure redox liquid tetraalkylphenylenediamines or the TAPDs dissolved in a nitrobenzene matrix. The oxidation of the pure redox liquids, accompanied by the charge-compensating uptake of anions from the aqueous phase, leads to the formation of ionic liquids. The ion transfer is dominated by the Gibbs energies of transfer of the electrolyte anions. Specific ion-pairing interactions could not be observed. The formation of the ionic liquid phase is accompanied by a change of the properties of the liquid|liquid interface due to a declining hydrophobicity of the organic layer. From the voltammetric measurements, the free energies of transfer of a number of TAPD+ radical cations and of the TAPD2+ cations could be determined for the water|nitrobenzene interface. For the undiluted redox liquids, this was not possible because the separation of the term of the standard potential of the oxidation and of the ion transfer could not be achieved because of a lack of reference data. For this, a new approach will be presented in a forthcoming paper. Acknowledgment. U.S. gratefully acknowledges support from the Alexander von Humboldt foundation (Feodor Lynen Program). F.M. thanks the Royal Society for a University Research Fellowship and New College, Oxford, for a Stipendiary Lectureship.

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