Electron transfer at micro liquid–liquid interfaces

Electron transfer at micro liquid±liquid interfaces

Bernadette Quinn *, Riikka Lahtinen, Lasse MurtomaÈ ki, KyoÈ sti Kontturi

Laboratory of Physical Chemistry and Electrochemistry, Helsinki University of Technology, P.O. Box 6100, 02015 HUT, Finland

Received 10 October 1997; received in revised form 10 February 1998

Abstract

The novel use of micro ITIES to study electron transfer (ET) has been investigated. Micropipettes and microholeswere used to support the interface and the organic solvents considered were 1,2-DCE and o-NPOE. ET involvingferrocene, dimethylferrocene and tetracyanoquinodimethane as the organic redox couples was studied. The ferri/

ferrocyanide redox couple was used in all cases as the aqueous redox couple. Micro ITIES were found not to beamenable to kinetic determination. ET is complicated by reactions involving the base electrolytes used, partitioningof redox products, equilibrium reactions and interfacial precipitation, which will need to be considered if accurate

kinetic data is to be realised. # 1998 Elsevier Science Ltd. All rights reserved.

Keywords: Liquid±liquid interface; Micro ITIES; Electron transfer; W-NPOE interface; Phase formation

1. Introduction

Electron transfer (ET) reactions at the interface

between two immiscible electrolyte solutions (ITIES)

are considered to be of fundamental interest to the

understanding of biological reactions as these inter-

faces can be considered to be simple models for bio-

and arti®cial membranes. ET at these interfaces is also

of relevance to processes such as phase transfer

catalysis [1] and more recently interfacially generated

nanoparticles [2].

In comparison with ion transfer (IT) studies there

has been little progress in experimental ET studies

during the past 10 years. Ferrocene and its derivatives

have been studied extensively [3±7] and other organic

redox couples include tin and lutetium

biphthalocyanines [8], some metalloporphyrins [9] and

tetracyanoquinodimethane [10]. Techniques used are

cyclic voltammetry [7±9], ac impedance [10], current

scan polarography [11] and recently scanning electro-

chemical microscopy (SECM) [12, 13], TIR UV-VIS

spectroscopy [14] and some preliminary studies with

EPR [15]. However, the amount of reliable kinetic

data available is scarce. The SECM technique is one of

the most interesting developments to ITIES and Bard

proposes that this technique overcomes many of the

experimental problems encountered at liquid/liquid

interfaces. This group also introduced the use of

micropipette supported ITIES in place of the micro-

electrode normally used in SECM studies [12, 13].

The conventional use of cyclic voltammetry at

macro interfaces for the determination of kinetics is

questionable at ITIES because of the large IR drop

due to the low conductivity of the organic phase.

Kakiuchi and Teranishi [16] recently drew attention to

the fact that as little as 0.1% error in the level of posi-

tive feed back IR compensation employed for four

electrode cyclic voltammetry in a conventional cell can

introduce ca. 10% error in the determinations of the

apparent standard rate constant ks, ac impedance

measurements are generally believed to be more accu-

rate for determination of kinetic parameters, however

there is some debate as to the interpretation of the

spectra [17]. Theoretical studies are more numerous

though few have been applied to experimental results.

Recent predictions by Schmickler [18] indicate that

Butler±Volmer kinetics apply to ET, though the exper-

imental requirements for studying ET at these inter-

faces were simpli®ed.

Electrochimica Acta 44 (1998) 47±57

0013-4686/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved.

PII: S0013-4686(98 )00151-0

PERGAMON

* Corresponding author.

As the number of water±organic interfaces con-sidered in ET studies is very limited, it is of interest to

widen this range. o-Nitrophenyl octyl ether (NPOE) isone of the new solvents being used as the organicphase for ITIES. Its miscibility with water is by far the

lowest and its dielectric constant is intermediatebetween 1,2-dichloroethane (1,2-DCE) and nitroben-zene (NB) but to date only IT has been studied at the

w±NPOE interface [19]. Various means of obtainingmicro interfaces have been reported and the advan-tages of such interfaces are enhanced mass transfer

and a reduced IR contribution which should lead tomore reliable kinetic data. The determination of kineticdata has concentrated on IT at the w±1,2-DCEinterface [20±22], but surprisingly ET has not been

investigated.The purpose of this paper was to study ET at the

micro w±NPOE and w±1,2-DCE interfaces. Ferrocene

(Fc), dimethylferrocene (DmFc) and tetracyanoquino-dimethane (TCNQ) were chosen as the organic redoxcouples. The solubilities of others (e.g. lutetium

biphthalocyanine) were not su�cient in NPOE.Methods reported in the literature [23±26] for the de-termination of quasi-reversible kinetics were con-

sidered. Evidence of phase formation was discovered.

2. Experimental

o-NPOE was purchased from Fluka and passed

through a column of activated alumina prior to use.1,2-DCE was used as received (HPLC grade RathburnChemicals, Scotland). The organic redox couples were

ferrocene (Fluka), dimethylferrocene (Aldrich) and tet-racyanoquinodimethane (Sigma). In the aqueousphase, potassium hexacyanoferrates K3Fe(CN)6 (BakerUK) and K4Fe(CN)6 (Merck) were used without

further puri®cation. To increase the potential windowavailable, Li2SO4 (Merck, p.a.) was used as the aqu-eous base electrolyte as a salting out agent. The aqu-

eous phase was prepared from MQ1 treated water.Tetrabutylammonium tetrakis-4-chlorophenylborate(TBATPBCl) and tetraphenylarsonium tetrakis-4-

chlorophenylborate (TPAsTPBCl), were prepared asdescribed elsewhere [1]. All other reagents used were ofanalar grade.The experimental arrangements for the microhole

and micropipette assemblies are as in Refs. [22, 27].Microholes used in this study were a kind gift fromProfessor H.H. Girault, EPFL Switzerland. The polye-

ster ®lms containing a single microhole (12 mm thickMelinex, ICI, UK) were glued to a glass cylinder usinga solvent resistive glue (730 RTV Dow Coming, UK).

Diameters varied from 5 to 50 mm.Four electrode measurements were carried out at a

conventionally sized interface (0.17 cm2) and the ap-

paratus and cell used are described in Ref. [7]. Positivefeedback IR compensation was employed. The conven-

tion used is that potentials are measured with respectto the organic phase, i.e. Do

wf = f(w)ÿf(o), where fis the Galvani potential. A positive current corre-

sponds to the transfer of negative charges from the or-ganic to the aqueous phase. The arrangement formicroelectrode measurements was as in Ref. [7] (diam-

eters used were 5, 10 and 25 mm). The cell used was asfollows

cell I: AgjAgCljXCl�w 0�jXTPBCl�o-NPOE or 1,2-DCE�� redox 1jjLi2SO4�w� � redox 2jPtwhere X = TPAs+ or TBA+. The organic base elec-

trolyte concentration was typically 10 mM and for theorganic redox couples 5 � 10ÿ5 and 5 � 10ÿ4 M for1,2-DCE and NPOE, respectively. The XCl concen-

tration in aqueous reference phase (w') was either 1 or10 mM. For ET involving Fc or DmFc, the concen-trations of Fe2+/Fe3+ used were 0.01 and 0.1 M in 1.5

M Li2SO4. For TCNQ, the concentrations of Fe2+/Fe3+ were 0.4 and 0.01 M in 1 M Li2SO4.The UV-VIS measurements were carried out in HP

8452A Diode Array Spectrophotometer and the IRspectra were recorded with Nicolet's Magna-IR2

System 750.

3. Results and discussion

3.1. Macro water±NPOE interface

Initially, electron transfer at the water-NPOE inter-face was studied with conventional four electrode cyc-lic voltammetry at the macro interface (Fig. 1(a)±(c))and compared to reported behaviour at the water±1,2-

DCE [7, 10]. Although the IR drop at the w±NPOEinterface is less signi®cant compared to the w±1,2-DCE [19], analysis of the resulting voltammograms

was complicated by the increased capacitance of w±NPOE interface, which enhanced the contribution ofthe capacitive current to the total current. Also,

measurements were hampered by the high viscosity ofNPOE which made it di�cult to maintain a ¯at inter-face. The di�erence between the concentration of aqu-eous and organic redox couples was several orders of

magnitude in order to avoid complications outlined byStewart et al. [28] and current was considered to belimited by the di�usion of the organic redox couple to

the interface. The high concentration of the aqueousbase electrolyte used (Li2SO4), salts out the transfer ofthe ferricenium and dimethylferricenium ions [7] shift-

ing the ionic transfer potentials in a positive direction,making it possible to observe ET without compli-cations from coupled IT.

B. Quinn et al. / Electrochimica Acta 44 (1998) 47±5748

The measured cyclic voltammograms show reversible

behaviour when DmFc is used as the organic redox

species, and quasi-reversible behaviour when Fc and

TCNQ are used. Prior to further analysis, sweeps were

corrected for the base electrolyte current. Di�usion

coe�cients were calculated using the Randles±SevcÏ ik

equation [29] for the peak current at the slowest sweep

rate (10 mV sÿ1) where the peak separation was 60 mV

for Fc and TCNQ. For DmFc, D was obtained from a

plot of Ip versus n1/2 where n is the sweep rate. The cal-

culated values are given in Table 1. where they are

compared with the reported values at the w±1,2-DCE

interface. Di�usion coe�cients for Fc and DmFc are

consistent with predictions by Samec [19] based on the

Stokes±Einstein equation (D1,2-DCE/DNPOE3 ca. 16)

since the ratio of the measured di�usion coe�cients to

those measured at the w±1,2-DCE interface should be

proportional to the inverse ratio of the viscosities of

the organic solvents (Z1,2-DCE/ZNPOE=16.06). For

TCNQ, the ratio was ca. 40. However, the dissolution

of TCNQ in NPOE was substantially slower than in

1,2-DCE and decomposition of TCNQ to its radical

anion cannot be ruled out.

Compensation by the positive feedback mode was

troublesome as the system was prone to oscillation,

thus values measured for the apparent standard rate

constant, ks using the Nicholson±Shain method [29]

can only be taken as a ®rst approximation (see

Table 1). However, it is interesting to note that values

obtained for TCNQ are in line with those measured at

the water±1,2-DCE interface using di�erential cyclic

voltabsorptometry [30] but are lower than those

measured by Cheng and Schi�rin [10] by ac impedance

for the same redox couple (ks=5.4 � 10ÿ3 cm sÿ1). ETinvolving Fc was quasi-reversible and the value was

lower than that measured at the water±1,2-DCE

interface [7] whereas at the water±NB and water±ben-

zene interfaces ET with Fc is reported to be

reversible [31, 32]. There is however some controversy

as to the reversibility of ET involving Fc at the water±

1,2-DCE interface with con¯icting reports from di�er-

ent groups [5, 7]. The apparent reversibility of ET

involving DmFc was surprising as during the measure-

ment, the interface became blue in colour. This

Fig. 1. Cyclic voltammograms for interfacial ET at the macro

w±NPOE interface. (a) 5 � 10ÿ4 M DmFc in contact with

0.01 M [Fe(CN)6]4ÿ and 0.1 M [Fe(CN)6]

3ÿ in 1.5 M Li2SO4

(Re: cell 1: X = TPAs+). Sweep rates: 0.01, 0.025, 0.05,

0.075, 0.1 and 0.125 V sÿ1. (b) 5 � 10ÿ4 M Fc in contact with

0.01 M [Fe(CN)6]4ÿ and 0.1 M [Fe(CN)6]

3ÿ in 1.5 M Li2SO4

(Re: cell 1: X = TBA+). Sweep rates: 0.025, 0.05, 0.075, 0.1,

0.125 and 0.15 V sÿ1. (c) 5 � 10ÿ4 M TCNQ in contact with

0.4 M [Fe(CN)6]4ÿ and 0.01 M [Fe(CN)6]

3ÿ in 1 M Li2SO4

(Re: Cell 1: X = TPAs+). Sweep rates: 0.01, 0.025, 0.05,

0.075, 0.1 and 0.125 V sÿ1.

Fig. 1

B. Quinn et al. / Electrochimica Acta 44 (1998) 47±57 49

phenomenon was also noted for Fc, but at higher con-

centrations compared to DmFc. Similar precipitation

was observed at the w±1,2-DCE interface for both Fc

and DmFc, but at higher concentrations relative to the

w±NPOE interface. Such behaviour has not been pre-

viously reported and will be discussed in more detail in

Section 3.4.

Cyclic voltammograms at microelectrodes were con-

sistent with those predicted for one electron transfer

and di�usion coe�cients were calculated from the lim-

iting current (see Table 1). However, as previously

noted for Fc and DmFc from microelectrode measure-

ments in 1,2-DCE, values for D are too large taking

into account the size of the molecules [7]. It is interest-

ing to note that the coe�cients obtained from micro-

electrode studies are also consistent with those

predicted from the ratio of solvent viscosities from the

Stokes±Einstein equation (D1,2-DCE/DNPOE=17).

Cunnane et al. [7] believe that the enhanced di�usion

coe�cients are due to the rapid decomposition of the

ferricenium (and dimethylferricenium) ions to ferrocene

(dimethylferrocene) in a catalytic cycle with the organic

base electrolyte anion. Taking this into account and

solving the transport problem for a microdisk electrode

assuming equality of di�usion coe�cients for the oxi-

dised and reduced species yields:

Id � 4nFDc*r

�1� pr

��kp

4��Dp

��1�

where Id is the di�usion limited current, n the number

of electrons transferred, r the radius of the microelec-

trode, c* the bulk concentration of the limiting species

(Fc or DmFc), D its di�usion coe�cient and k the

®rst-order rate constant for the chemical reaction (see

Appendix A for details). From Eq. (1) it can be easily

seen that limiting current is enhanced compared to the

case where this kind of decomposition reaction is

absent. Values for the chemical rate constant can be

calculated from the measured limiting current using

di�usion coe�cients determined at the macro liquid-

liquid interface. The decomposition was noted to be

more signi®cant in 1,2-DCE than in NPOE.

Dimethylferricenium appears to be more susceptible to

degradation compared to ferricenium.

For TCNQ, the measured di�usion coe�cient wasthe same as at the macro w±NPOE interface within ex-perimental error and in line with that measured in 1,2-

DCE taking the solvent viscosity into account.

3.2. Micropipette supported ITIES

The concept of micropipette supported ITIES wasdeveloped by Taylor and Girault [33] and has been

used to study kinetics of ion transfer and facilitatedion transfer [20±22]. It is a particularly useful tech-nique for determining what species is transferringacross the interface [34]. Due to the asymmetric di�u-

sion regime at these interfaces, di�usion within themicropipette yields a peak-shaped voltammogram (pla-nar di�usion) whereas di�usion outside yields a

pseudo-steady state wave (spherical di�usion). Anexample of a slow potential sweep for ET is shown inFig. 2 where the observed current is consistent with

transfer limited by the di�usion of Fc to the w±1,2-DCE interface yielding a steady state wave similar tothat observed with microelectrodes. This con®rms the

Table 1

Results from conventional four electrode cyclic voltammetry at the macro water±NPOE interface. Values obtained are compared

to reported values at the water±1,2-DCE interface

DNPOE (107 cm2 sÿ1) D1,2-DCE (106 cm2 sÿ1) ks NPOE (103 cm sÿ1) ks 1,2-DCE (103 cm sÿ1)

DmFc 2.9 (10.0) 3.5 [7] >50 reversible [7]

Fc 3.5 (6.9) 6.7 [7] 2.5 reversible [7]

TCNQ 4.0 (5.7) 16.0 [10] 2.7 5.4 [10]

Results in brackets are from microelectrode measurements in NPOE.

Fig. 2. Cyclic voltammogram of ET between 2 � 10ÿ4 M Fc

and 0.01 M [Fe(CN)6]4ÿ/0.1 M [Fe(CN)6]

3ÿ in 1.5 M Li2SO4

at the micropipette supported w±1,2-DCE interface. Sweep

rate 25 mV sÿ1 and r = 15 mm. (Re: cell 1: X = TPAs+).


heterogeneous nature of ET for the redox couples stu-

died. The semi-logarithmic plot of E versus log[(IdÿI)/I] was linear with the slope of 61 mV decadeÿ1 (Fig. 3).For reversible transfer the predicted value is 59 mV

decadeÿ1. For quasi-reversible transfer at microelec-

trodes kinetic data should be attained from steady

state measurements [23±26]. This is dependent on the

accurate determination of E1/2 as the interfacial area is

decreased, (discussed in detail in the section on micro-

holes) and also on the assumption of a negligible IR

contribution to the measured current. However, in this

case the IR drop is non-negligible due to the high re-

sistance of the glass tip, which can be the order of

mega ohms. The other organic redox couples studied

at the water±1,2-DCE interface gave a similar re-

sponse.

Beattie et al. [22] have shown that the micropipette

supported interface exhibits behaviour between that of

a microsphere and a microhemisphere:

Id � 3:35pnFDc*r �2�where Id is the di�usion limited current, r the radius of

the micropipette, c* the bulk concentration of the lim-

iting species and D its di�usion coe�cient. The di�u-

sion coe�cients calculated from Eq. (2) for Fc and

DmFc at the water±1,2-DCE interface were up to an

order of magnitude greater than measured at the

macro interface. Despite the obvious di�culties in

determining the exact radius of the tips used, it was

interesting to note that D values were in the same

range as measured using Pt microelectrodes. Again, the

high values of D estimated can be explained in terms

of the decomposition reaction discussed in the preced-

ing section (see Eq. (1)). The di�usion coe�cient for

TCNQ however was in quite good agreement with the

value measured at the macro interface taking into

account the experimental di�culties encountered at the

micropipette water±1,2-DCE interface. It would have

been interesting to study ET utilising TCNQ at the

micropipette supported w±1,2-DCE interface using ac

impedance and compare values measured with those

reported by Cheng and Schi�rin [10] at the macro

interface, however, the stability of the interface was

such that repeatable results could not be obtained.

In NPOE, the voltammograms showed more pro-

nounced capacitive behaviour (Fig. 4) and semi-logar-

ithmic analysis was not possible. It can be seen from

Fig. 4 that as the sweep rate is increased, the capacitive

current distorts the faradaic current making the limit-

ing current di�cult to extract. This was unlike the

micropipette supported w±1,2-DCE where the shape of

the voltammogram was invariant up to 0.2 V sÿ1. Forthe case of DmFc, Kakiuchi's semi-derivative

method [35] for reversible charge transfer was applied

to obtain the limiting current and half wave potential.

The method was limited by noise and the di�usion

coe�cient calculated from the limiting current was

again up to an order of magnitude greater than that

measured at the macro interface. This was consistent

with D determined from microelectrode measurements

in NPOE and as already stated, a similar phenomena

was noted for DDmFc1,2-DCE measured from micropipette

and microelectrode studies in 1,2-DCE.

Previously, the fast potential step technique has had

some success in determining the kinetics of ion transfer

across phospholipid monolayers adsorbed at micropip-

ette ITIES [27]. Here, a similar approach was con-

sidered to study the chosen model case of Fc ET at the

water±1,2-DCE interface (see Fig. 2). Short potential

steps (1±400 ms) were performed stepping from a zeroFig. 3. Semi-logarithmic analysis of the current±potential

curve in Fig. 2 showing a slope of 61 mV decadeÿ1.

Fig. 4. ET at the micropipette supported w±NPOE interface

involving 5 � 10ÿ4 M DmFc and 0.01 M [Fe(CN)6]4ÿ/0.1 M

[Fe(CN)6]3ÿ in 1.5 M Li2SO4 (Re: cell 1: X = TPAs+). Sweep

rates = 1, 10, 25 and 50 mV sÿ1 and r= 10 mm.


current potential region to one where signi®cant cur-

rent is ¯owing. The expected falling transient for fara-

daic decay was not observed. A typical set of current

transients is illustrated in Fig. 5 showing evidence of

adsorption and phase formation mostly due to the

base electrolyte system. The high resistance associated

with the tip (order of MO) makes the RC time con-

stant, which characterises the capacitive current decay,

in this case large and capacitive current is observed

within the time frame of the pulse applied. Analysis of

the transients incorporating the e�ects of adsorption

and capacitance was unsuccessful as it introduced too

many free parameters to the ®tting procedure.

Kakiuchi [36] recently considered equilibrium ET±IT

coupling where the partition of an indi�erent electro-

lyte can drive the redox reaction in the absence of an

external applied potential. The e�ect of the volume

ratio between the organic and aqueous phases can also

signi®cantly alter the equilibrium potential distribution.

In this case, as Vw<<Vo, the e�ect of ion partition at

equilibrium (i.e. on contact) will alter the potential

scale and alters the initial conditions.

For micropipette ITIES, repeatability was not high

due to the di�culty in pulling exactly similar pipettes

and also there was evidence that the tips became

blocked during the experiment whereby the measured

current increased and the shape of the voltammogram

became more distorted until eventually a limiting cur-

rent could not be observed. As the micropipettes are

®lled externally and then introduced to the cell, it is

also possible that the highly concentrated aqueous

phase crystallised in contact with air. There is some

debate as to whether micropipettes can be treated ana-

logously to a microelectrode as the micropipette ITIESis not shrouded in an semi in®nite insulating plane. At

microelectrodes, where this condition is not ful®lled,Shoup and Szabo [37] have shown how the currentscan be increased by up to 40%.

3.3. Microhole supported ITIES

The use of a microhole supported ITIES overcomes

some of the short comings of micropipettes, e.g. thelack of in®nite insulating plane, problems with dura-bility and high resistivity. Beattie et al. [21, 22] have

utilised microhole supported w±1,2-DCE interface fordetermining IT kinetics using steady state voltammetryand ac impedance. Osborne et al. [20] have shown that

a microhole supported ITIES behaves analogously toan inlaid microdisk electrode of similar dimensions,thus:

Id � 4nFDc*r �3�where the symbols are as in Eq. (2). In this case, theradius of the hole was taken to be the laser entrancehole as previously taken for the w±1,2-DCE

interface [20]. Slow sweep cyclic voltammograms forthe three organic redox couples at both the w±NPOEand w±1,2-DCE interfaces were recorded.

Determination of kinetics from steady state voltam-mograms for quasi-reversible reactions at microelec-trodes is based on a comparison of the reversible

system with the experimental system as the size of theelectrode is varied. For large values of radius r thereaction behaves reversibly, so the semi-logarithmic

plot of E versus log[(IdÿI)I] is linear with a slope of59/n mV decadeÿ1 and an intercept equal to E1/2.Decreasing the radius of the interface enhances masstransport and charge transfer increasingly limits cur-

rent, which shifts the apparent half wave potential tomore positive potentials for oxidation and to morenegative for reduction (potential of the water phase

with respect to the organic phase) and increases theslope and decreases the linearity of the plot.Examples of cyclic voltammograms for di�erent

microhole radii at the w±NPOE interface, whereDmFc was used as the organic redox couple, areshown in Fig. 6. The sweeps were corrected for e�ectsfrom the base electrolyte and capacitance to the

measured current. From values of the limiting currentobtained, it could be ascertained that the interfaceradii were not as predicted from the laser entrance,

thus the position of the interface within the porewould appear to be ambiguous. The true interfaceradius was determined from Id using Eq. (3) and

DDmFcNPOE=2.9 � 10ÿ7 cm2 sÿ1 evaluated at the macro

interface, though the observed current may beenhanced as for the case of the micropipette supported

Fig. 5. Current transients resulting from fast potential steps at

the micropipette w±1,2-DCE interface where the potential was

stepped from 360 mV to 560, 660 and 760 mV. Experimental

details as in Fig. 2.


interface. Values are given in Table 2. The largest radii

for the microholes in the case studied are at the upper

limit to be considered micro and there may be some

deviation from steady state.

Voltammograms were analysed according to the

Tomes criteria [29], which for reversible charge transfer

predicts vE3/4ÿE1/4v = 56.4/n mV at 258C, where n is

the number of electrons transferred and E3/4 and E1/4

refer to the three-quarter and one-quarter wave poten-

tials respectively. To observe a kinetic shift, the Tomes

criteria should increase from the reversible value as the

hole size is decreased [22±24]. The variation can be

used to estimate ks and a and is based on the linear re-

lationship between vE3/4ÿE1/4v versus 1/r where r is the

radius of the interface [24]. Semi-logarithmic analysis

of E versus log[(IdÿI)/I] was also performed and an

example of a typical plot is given in Fig. 7. The plots

for radii considered were linear (in all cases R2>0.995)

and the slopes and values obtained for vE3/4ÿE1/4v andE1/2 are given in Table 2.

However, the slopes in Table 2 show deviations

from expected behaviour. The slope should increase

going from reversible to irreversible, thus slopes of the

order of 59/n mV decadeÿ1 and greater were predicted

from analysis of cyclic voltammograms at the macro

interface. As can be seen from Table 2, this was notobserved and the slopes were 4323 mV decadeÿ1,thus methods proposed by Oldham et al. [23, 24] and

others [25, 26] for determining kinetics at microelec-

trodes are inapplicable. The analysis of two other

redox couples studied yielded similar results; for

TCNQ 4023 mV decadeÿ1 and for Fc ca. 40 mV dec-

adeÿ1. Electron transfer for Fc occurs quite close to

the edge of the potential window, making determi-

nation of E1/2, and Id in this case problematic, thus the

slope reported is quite approximate. Moving the elec-

tron transfer to the middle of the potential window

was complicated by the low di�usion coe�cients in the

organic phase making the useful concentration range

quite restricted (the concentration of one redox pair

should be several orders of magnitude greater than the

Fig. 6. Steady state voltammograms showing hole radius

dependence for electron transfer between 5 � 10ÿ4 M DmFc

and 0.01 M [Fe(CN)6]4ÿ/0.1 M [Fe(CN)6]

3ÿ in 1.5 M Li2SO4

at the w±NPOE interface. Sweep rate = 10 mV sÿ1 and

r= 2.5, 5 and 15 mm (Re: cell 1: X = TPAs+).

Table 2

Results obtained from semi-logarithmic plots of E versus log[(IdÿI)/I] for steady state voltammograms at the water±NPOE inter-

face where the organic redox couple used was DmFc (5 � 10ÿ4 M)

rappa (mm) rcal

b (mm) Id (nA) E1/2 (mV) Slope (mV decadeÿ1) vE3/4ÿE1/4v (mV)

2.5 2.1 0.012 390 44 43

4 3 0.018 391 41 40

5 4 0.023 392 42 41

10 14 0.078 395 43 40

15 23 0.130 409 46 44

25 63 0.354 401 44 42

a Hole radius taken at laser entrance.b Hole radius calculated using Eq. (3).

Fig. 7. Semi-logarithmic analysis of the current±potential

curve in Fig. 6. showing a slope of 42 mV decadeÿ1.


other to simplify analysis). The unexpectedly small

slope obtained is not an artifact due the nature of the

interface formed within the pore as previous studies onIT at microhole supported w±NPOE interface yielded

the expected slope of 5825 mV decadeÿ1 [38]. The

value of the slope for the case of ET was surprising.Samec et al. [19] have noted that there is signi®cant

interfacial ion pairing at the w±NPOE interface and

Bard et al. [32] report ®lm formation due to ion pair-ing between the organic base electrolyte cation and the

aqueous hexacyanoferrates. Also, a high level of or-ganisation for NPOE was observed at the air±water

interface using Langmuir techniques [39], suggesting

that the dielectric constant in the vicinity of the inter-face is less than in the bulk solution which would

enhance ion pairing. Thus, it is possible that a pre-

equilibrium involving the formation of an interfacialion pair between the organic base electrolyte cation

and the hexacyanoferrate is the rate determining step,

preceding the electron transfer, resulting in thedecrease of the slope [40]. Theoretically, any potential

dependent pre-equilibrium would explain the slope de-

viation observed.

Similar measurements were carried out at the water±

1,2ÿDCE interface (see Fig. 8 for example). The

slopes of the semi-logarithmic plots for Fc, DmFc and

TCNQ were 5225, 5825 and 5823 mV decadeÿ1,respectively (in all cases R2>0.999). In this case, the

slopes for all were as expected for reversible to quasi-

reversible transfer. However, the E1/2 values obtained

and values for the Tomes criteria as a function of hole

radius were scattered (see Table 3 as an example of

results for dimethylferrocene), which prevented further

analysis. Also, repeatability was problematic as the

hole size appeared to decrease with repeated use.

Again, the true radius could be calculated using

Eq. (3).

The following complication for the microhole sup-

ported interfaces to the determination of E1/2 values

should be considered. The interface is unlike a micro-

electrode metal±solution interface in that the polyester

®lm has a ®nite thickness (d= 12 mm). Solving the

transport problem, assuming a cylindrical geometry

with equal entrance and exit radii for the pore support-

ing the interface and taking the positional dependence

and the ®nite thickness of the ®lm into account, the

current±voltage characteristic for a steady state wave is

as follows [38]:

Eÿ E 0 0 �RT

nFln

�Id ÿ I

I

�� RT

nFln

Do

Dw

� RT

nFln

�r0 � x 0

r0 ÿ x 0 � d

��4�

where d is the thickness of the ®lm, r0 is the radius of

the pore and x0 the interfacial position within the

pore. There are two limiting cases: (i) when the inter-

face is located at the aqueous side of the pore

(entrance) x0=0 and (ii) when it is located at the

NPOE side of the pore (exit) x0=12 mm. The contri-

bution from the last term in Eq. (4) is the order of220

mV depending on the value of x0. Thus, the position

of the interface within the hole can a�ect the value of

E1/2 determined from the intercept of the semi-logarith-

mic plot. For the w±NPOE interface, the assumption

that the organic solvent ®lls the hole and the interface

Fig. 8. Steady state voltammograms showing hole radius

dependence for electron transfer between 5 � 10ÿ4 M TCNQ

and 0.4 M [Fe(CN)6]4ÿ/0.01 M [Fe(CN)6]

3ÿ in 1 M Li2SO4 at

the w±1,2-DCE interface (Re: cell 1: X = TPAs+). Sweep

rate 10 mV sÿ1 and r= 2.5, 5 and 15 mm.

Table 3

Results obtained from semi-logarithmic plots of E versus log[(IdÿI)/I] for steady state voltammograms at the water±1,2-DCE inter-

face where the organic redox couple used was DmFc (5 � 10ÿ4 M)

rappa (mm) rcal

b (mm) Id (nA) E1/2 (mV) Slope (mV decadeÿ1) vE3/4ÿE1/4v (mV)

2.5 0.4 0.07 521 53 52

5 1 0.15 519 67 62

25 16 2.45 531 58 56

140 � 2 96 18.62 474 55 54

Where superscripts a and b are as in Table 2.


is located at the entrance hole on the aqueous side isperhaps not valid since NPOE is much more viscous

than 1,2-DCE. The position can also be dependent onthe radius.To summarise, while microhole and micropipette

supported w±1,2-DCE and w±NPOE interface giveuseful qualitative information on ET, they appear tobe of limited use for quantitative determinations.

Repeatability is a serious problem as there was evi-dence that, in particular the micropipette and micro-hole w±1,2-DCE interfaces, became blocked. Methods

developed for microelectrodes based on variations ofparameters such as the Tomes criteria, as a function ofinterface radius, cannot be applied successfully,because micro ITIES are not amenable to the accuracy

level required. Fc and DmFc, though widely studiedwere non-ideal redox couples for ET at ITIES and inaddition, anomalous behaviour, previously unreported

was observed. This is discussed in Section 3.4.

3.4. Anomalous behaviour of Fc and DmFc at w±1,2-DCE and w±NPOE interfaces

As stated earlier, precipitation was noted at both thew±NPOE and w±1,2-DCE interfaces for ET reactionsinvolving either Fc or DmFc. This behaviour was most

apparent during conventional CV measurements at themacro interface. For example, for ET involving DmFc,it was noted that during successive sweeping the peakcurrent decreased, while the peak potential remained

invariant (see Fig. 9). Renewing the interfacial regionresulted in the re-establishment of the initial peak cur-rent values. This would appear to indicate an inter-

facial process where the product (precipitate) does not

interfere with the kinetics of ET reaction, while appar-ently decreasing the interfacial area. Precipitation wasconcentration dependent and potential independent

and, for higher concentrations of Fc or DmFc used inthe organic phase, occurred spontaneously when theaqueous and organic phases came into contact. It

appeared to be faster in NPOE than in 1,2-DCE.The phenomena was studied in more detail in a sep-

arate measurement where the organic phase (10 mMDmFc in either 1,2-DCE or NPOE) and aqueousphase (1.5 M Li2SO4, 0.1 M K3Fe(CN)6 and 0.01 M

K4Fe(CN)6) were placed in contact under equilibriumconditions (i.e. in the absence of an applied potential)

and left for a period of 24 h. A blue colour developedalmost immediately in the aqueous phase and after 24h, the aqueous phase was dark blue and precipitation

was obvious at the interface and on the walls of theglass. Using UV-VIS spectroscopy, the concentrationof DmFc in the organic phase (1,2-DCE) was moni-

tored and had decreased substantially after 24 h (to ca.20% of the original value). Filtering the aqueous phase

yielded a clear yellow supernatant and a ®ne dark blueprecipitate. The precipitate was insoluble in 1,2-DCEand acetone, however it dissolved in water. Using a

micropipette supported w±1,2-DCE interface to testthe aqueous solution (2.5 mg mlÿ1 of the precipitate)for transferable ions indicated that the complex is not

transferred in the usual potential region employed forET studies.

From control measurements, it was determined thatthe precipitation was dependent on the presence of Fc(DmFc), the ferri/ferro couple and that the presence of

Li2SO4 promoted the reaction. In the absence of theorganic base electrolyte, the precipitation was thesame, which would preclude reactions involving the de-

composition of organic base electrolyte anion. As pre-viously discussed, Bard et al. [32] propose ®lm

formation at the interface due to ion pairing betweenthe organic base electrolyte cation and anions of theaqueous redox couple where again precipitation occurs

when the solubility product is exceeded. For the abovecase, such ®lm formation is also possible, however theblue precipitate cannot be ascribed to this kind of

interfacial ion pairing, as it was independent of thepresence of the organic base electrolyte. Equilibrium

ET driven IT is one explanation for the observedbehaviour [36]. This would involve electron transfer tothe aqueous phase driving ferricenium (dimethylferrice-

nium) ion transfer from the oil to water phase to main-tain electroneutrality. It is possible that Fc+ andDmFc+ form ion pairs with the hexacyanoferrate com-

plexes and precipitation occurred when the solubilityproduct of the compound was exceeded.

The di�culties in working with the ferri/ferro redoxpair are well documented at metallic electrodes and in

Fig. 9. ET involving DmFc at the macro w±NPOE interface

illustrating the decrease in the current observed with succes-

sive sweeps (- - -) compared to the initial current (ÐÐÐ).

Cell as in Fig. 1(a) and sweep rate = 25 mV sÿ1.


some instances, the formation of Prussian blue likedeposits has been reported [41, 42]. The stability of the

redox pair is a�ected by the choice of the supportingelectrolyte and anomalous behaviour has been noted inthe presence of high concentrations of Li+ [43]. The

formation of Prussian blue in acidic solutions may bedue to the partial labilisation of the normally inert cya-nide complexes of iron in the course of the electrode

reaction under the in¯uence of the electrode ®eld andthe resulting poorly soluble compound may beadsorbed at the electrode/interface [42]. FTIR spectra

of the precipitate indicated the presence of the CNgroup at 2090 cmÿ1 which is characteristic for Prussianblue [44]. The UV spectra of the compound in aqueoussolution had an absorption band at 700 nm which is

also believed to be characteristic of Prussian blue [45]and which disappeared when concentrated HCl wasadded. Thus, formation of a water soluble Prussian

blue like deposit is possible. ET involving Fc and itsderivatives, while apparently heterogeneous in nature,is complicated by competing chemical reactions which

should naturally be taken into account if accurate kin-etic data is to be realised.

4. Conclusions

Micropipette measurements support the hetero-geneous nature of ET reactions studied, though suchinterfaces were found not to be amenable to quantitat-ive determinations. Measurements at the macro w±

NPOE interface provided a useful estimate for thedegree of charge transfer reversibility. ET involvingDmFc was found to be reversible whereas that invol-

ving Fc or TCNQ was quasi-reversible. Di�usion coef-®cients determined in NPOE for Fc and DmFc were inline with predictions based on the Stokes±Einstein

equation. Steady state measurements at the microholesupported ITIES did not have the accuracy or repeat-ability levels required for the reliable extraction of kin-

etic parameters using methods proposed for solidmicroelectrodes. For the redox couples studied at thew±NPOE interface, the slopes of semi-logarithmicplots of E versus log[(IdÿI)/I] deviated from expected

values. The obtained value of ca. 40 mV decadeÿ1

could be due to a pre-equilibrium prior to electrontransfer. Anomalous phase formation was observed for

studies involving Fc or DmFc at both interfaces. Thiswas noted to be independent of reactions involving theorganic base electrolyte.

Acknowledgements

The support of the Neste Foundation, Finland(R.L.) and The Academy of Finland (B.Q.) is grate-

fully acknowledged. The kind gift of microholes usedin this study from Professor H.H. Girault, EPFL,

Lausanne, Switzerland was appreciated.

Appendix A

The di�usion equations at steady-state are (assuming

D1=D2=D):

@c1@ t� Dr2c1 � kc2 � 0 �A:1�

@c1@ t� Dr2c2 ÿ kc2 � 0 �A:2�

where c1 and c2 are the concentrations of ferrocene

and ferricenium respectively, k is the chemical rateconstant for the autocatalytic reaction and H2 forspherical coordinates is

r2 � @ 2

@r2� 2

r

@

@r

Current density is given by

i

nF� kfc

s1 ÿ kbc

s2 �A:3�

where the superscript s refers to values at the surfaceand kf and kb are the electrochemical rate

constants [29].The di�usion equations can be solved with the aid

of the boundary conditions

ÿ�@c1@ r

�ro

��@c2@r

�ro

� i

nFD�A:4�

where ro denotes the radius of the sphere. FromEq. (A.2), the surface concentration of ferriceniumbecomes

Fig. A1. Schematic illustrating the decomposition reaction of

Fc+ leading to an increased concentration of Fc in the vicin-

ity of the electrode surface.


cs2 �1

nFD�1=ro ��k=D

p � �A:5�

Utilising Eq. (A.3), the steady-state current density isobtained as

i

nFc*� 1

�1=kf� � �1� kb=kf�=�D�1=ro ��k=D

p �� A:6�

where c* is the bulk concentration of ferrocene. At thelimit kf41 taking into account that equations for themicrosphere can be modi®ed to apply to the microdisk

by the substitution ro=pa/4 where a is the radius ofthe disk, the di�usion limited current Id is given by

Id � 4nFDc*a

�1� pa

��kp

4��Dp

��A:7�

Note: in the body of the article, the radius of the diska is denoted as r for clarity.

References

[1] V. Cunnane, D.J. Schi�rin, C. Beltran, G. Geblewicz, T.

Solomon, J. Electroanal. Chem. 247 (1988) 203.

[2] D. Bethell, M. Brust, D.J. Schi�rin, C. Kiely, J.

Electroanal. Chem. 409 (1996) 137.

[3] Z. Samec, V. MarecÏ ek, J. Weber, J. Electroanal. Chem.

96 (1977) 245.

[4] Z. Samec, V. MarecÏ ek, J. Weber, D. Homolka, J.


[5] J. HanzlõÂ k, Z. Samec, J. Hovorka, J. Electroanal. Chem.

216 (1987) 303.

[6] J. HanzlõÂ k, J. Hovorka, Z. Samec, S. Toma, Coll. Czech.

Chem. Comm. 53 (1988) 903.

[7] V. Cunnane, G. Geblewicz, D.J. Schi�rin, Electrochim.

Acta 40 (1995) 3005.

[8] G. Geblewicz, D.J. Schi�rin, J. Electroanal. Chem. 244

(1988) 27.

[9] Y. Cheng, D.J. Schi�rin, J. Electroanal. Chem. 314

(1991) 153.

[10] Y. Cheng, D.J. Schi�rin, J. Chem. Soc. Faraday Trans.

89 (1993) 199.

[11] S. Kihara, M. Suzuki, K. Maeda, K. Ogura, M. Matsui,

Z. Yoshida, J. Electroanal. Chem. 271 (1989) 107.

[12] T. Solomon, A.J. Bard, Anal. Chem. 67 (1995) 2728.

[13] T. Solomon, A.J. Bard, J. Phys. Chem. 99 (1995) 17487.

[14] Z. Ding, P. Brevet, H.H. Girault, Chem. Commun. (in

press).

[15] R. Dryfe, R. Webster, B. Coles, R. Compton, Chem.

Commun. 8 (1997) 779.

[16] T. Kakiuchi, Y. Teranishi, J. Electroanal. Chem. 396

(1995) 401.

[17] A. Watts, T. Vandernoot, in: D.W. Deamer, A.G.

Volkov (Eds.), Liquid/liquid Interfaces: Theory and

Methods, CRC Press, Boca Raton, FL, 1996, ch. 5.

[18] W. Schmickler, J. Electroanal. Chem. 428 (1997) 123.

[19] Z. Samec, J. Langmaier, A. TrojaÂ nek, J. Electroanal.

Chem. 426 (1997) 37.

[20] M.C. Osborne, Y. Shao, C.M. Pereira, H.H. Girault, J.


[21] P.D. Beattie, A. Delay, H.H. Girault, Electrochim. Acta

40 (1995) 2961.

[22] P.D. Beattie, A. Delay, H.H. Girault, J. Electroanal.

Chem. 380 (1995) 167.

[23] K. Oldham, C. Zoski, A.M. Bond, J. Electroanal. Chem.

248 (1988) 467.

[24] K. Oldham, J. Myland, C. Zoski, A.M. Bond, J.


[25] Z. Galus, J. Golas, J. Osteryoung, J. Phys. Chem. 92

(1988) 1103.

[26] M.V. Mirkin, A.J. Bard, Anal. Chem. 64 (1992) 2293.

[27] A.K. Kontturi, K. Kontturi, L. MurtomaÈ ki, B. Quinn,

V.J. Cunnane, J. Electroanal. Chem. 424 (1997) 69.

[28] A.A. Stewart, J.A. Campbell, H.H. Girault, Ber.

Bunsenges. Phys. Chem. 94 (1990) 83.

[29] A.J. Bard, L.R. Faulkner, Electrochemical Methods,

John Wiley, New York, 1980.

[30] A. Ding, private communication.

[31] C. Wei, A.J. Bard, M.V. Mirkin, J. Phys. Chem. 99

(1995) 16033.

[32] M. Tsionsky, A.J. Bard, M.V. Mirkin, J. Phys. Chem.

100 (1996) 1788.

[33] G. Taylor, H.H. Girault, J. Electroanal. Chem. 208

(1986) 179.

[34] A.A. Stewart, Y. Shao, C.M. Pereira, H.H. Girault, J.


[35] T. Kakiuchi, J. Electroanal. Chem. 385 (1995) 135.

[36] T. Kakiuchi, Electrochim. Acta 40 (1995) 2999.

[37] D. Shoup, A. Szabo, J. Electroanal. Chem. 160 (1984)

19.

[38] B. Quinn, R. Lahtinen, K. Kontturi, J. Electroanal.

Chem. 436 (1997) 285.

[39] D. Grandell, unpublished results.

[40] A.K. Kontturi, K. Kontturi, D.J. Schi�rin, J.


[41] J. Kawiak, P.J. Kulesza, Z. Galus, J. Electroanal. Chem.

226 (1987) 305.

[42] J. Kawiak, T. Jedral, Z. Galus, J. Electroanal. Chem.

145 (1983) 163.

[43] L.M. Peter, W. DuÈ rr, P. Bindra, H. Gerischer, J.


[44] K. Niwa, K. Dobhofer, Electrochim. Acta 31 (1986) 439.

[45] D. Ellis, M. Eckho�, V.D. Ne�, J. Phys. Chem. 85

(1981) 1225.


Documents

Electron transfer at micro liquid–liquid interfaces