Simple electrochemical procedure for measuring the rates of electron transfer across liquid/liquid interfaces formed by coating graphite electrodes with thin layers of nitrobenzene

Article abstract of DOI:10.1021/jp982605b

A simple, new method is described that allows the rates of electron transfer across liquid/liquid interfaces to be measured with unprecedented ease. The method takes advantage of thin layers of organic liquids that adhere to the surface of pyrolytic graphite electrodes. The method was applied to the nitrobenzene/water interface. Redox reactants dissolved in the nitrobenzene included decamethylferrocene and zinc tetraphenyl-porphyrin. Reactants in the adjoining aqueous phase were multiply charged anions, including Fe(CN)₆^3-/4-, Ru(CN)₆^4-, Mo(CN)₈^4-, and IrCl₆^2-. Rate constants for cross-phase electron transfer were evaluated and compared with those obtained in recent studies that employed the scanning electrochemical microscope that was invented and developed by A. J. Bard and co-workers.

Full text of DOI:10.1021/jp982605b

9850
J. Phys. Chem. B 1998, 102, 9850-9854
Simple Electrochemical Procedure for Measuring the Rates of Electron Transfer across
Liquid/Liquid Interfaces Formed by Coating Graphite Electrodes with Thin Layers of
Nitrobenzene
Chunnian Shi and Fred C. Anson*
Arthur Amos Noyes Laboratories, DiVision of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, California 91125
ReceiVed: June 12, 1998; In Final Form: September 28, 1998
A simple, new method is described that allows the rates of electron transfer across liquid/liquid interfaces to
be measured with unprecedented ease. The method takes advantage of thin layers of organic liquids that
adhere to the surface of pyrolytic graphite electrodes. The method was applied to the nitrobenzene/water
interface. Redox reactants dissolved in the nitrobenzene included decamethylferrocene and zinc tetraphenyl-
3-/4-
porphyrin. Reactants in the adjoining aqueous phase were multiply charged anions, including Fe(CN)6
,
Ru(CN)64-, Mo(CN)84-, and IrCl62-. Rate constants for cross-phase electron transfer were evaluated and
compared with those obtained in recent studies that employed the scanning electrochemical microscope that
was invented and developed by A. J. Bard and co-workers.
The examination of electron transfer across interfaces separat-
ing two immiscible liquids has received increasing attention in
the past decade.1 One of the newest experimental methods that
has been used to measure the rates of such electron-transfer
processes is scanning electrochemical microscopy (SECM), a
technique invented and extensively applied by Bard and co-
workers.2 In recent applications of SECM to evaluate electron
transfer rates at liquid/liquid interfaces3 the advantages of the
technique compared with previously employed methods were
pointed out.3a Although SECM has many attractive features, it
requires a carefully constructed apparatus to control and measure
separations in the range of a few micrometers between a
microelectrode tip and the interface of interest. In addition, it
is usually necessary to resort to curve fitting procedures to obtain
kinetic parameters from the experimental data.3 We have found
that a very simple procedure for preparing thin layers of organic
Figure 1. Schematic diagram of the electrochemical cell and the EPG
liquids on the surface of graphite electrodes, which was used
electrode coated with a thin layer of nitrobenzene (NB) employed in
recently to study reactants adsorbed on graphite electrodes,4 can
this study. The diagram is not to scale. The diameter of the EPG
electrode was 0.64 cm, and the thickness of the NB layer was typically
20-30 µm.
also be applied to the measurement of rates of electron ransfer
at liquid/liquid interfaces. In this report we describe the
application of the method to the interface between water and
nitrobenzene phases. The simplicity of the method should prove
advantageous in cases where the instrumentation required for
SECM is not available. The method offers a supplement to
the more complex experimental methods employed in earlier
studies of electron transfer at liquid/liquid interfaces.1,5
The new method is based on the introduction of a thin layer
of one phase (nitrobenzene in this report) between the surface
of an electrode and a second phase in which the first phase is
immiscible (an aqueous electrolyte in this report). The resulting
configuration is depicted schematically in Figure 1. Dry,
polished, pyrolytic graphite electrodes exhibit sufficient hydro-
phobic character to cause small volumes of organic liquids such
as nitrobenzene (NB) to spread evenly across their surface to
produce a thin, adherent layer of the organic phase.4 Immersion
of such electrodes in aqueous electrolyte solutions leads to a
configuration like the one depicted in Figure 1. With NB as
the organic phase, thin layers with thicknesses of 25-30 µm
were easily and reproducibly formed. These thin layers proved
to be quite stable and were shown to insulate the electrode
surface from species in the aqueous phase that do not partition
into the organic phase.4
If a suitable electroactive reactant is dissolved in the organic
liquid used to form the thin layer, it becomes possible to measure
steady-state currents at the electrode with magnitudes that can
be limited by the rate of electron transfer across the nitroben-
zene/water interface. A particularly useful advantage of mea-
suring steady-state currents with the electrode configuration
shown in Figure 1 is that no ion transport across the interface
accompanies the cross-phase electron transfer. At steady state,
electroneutrality in both phases is maintained without ion
transport because electrons injected or removed at the electrode/
* Corresponding author. Telephone: 626 395 6000. FAX: 626 405 0454.
E-mail: fanson@cco.caltech.edu.
10.1021/jp982605b CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/10/1998

Liquid/Liquid Interface Electron-Transfer Rates
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9851
NB interface are removed or injected at exactly the same rate
at the NB/H2O interface and at the H2O/auxiliary electrode
interface. Thus, extraction of electron-transfer rates at the NB/
H2O interface from the measured steady-state currents is
unusually straightforward. There is no need for additional
experimental manipulations or iterative curve fitting to obtain
rate constants. Some examples of the application of this simple
procedure are provided in what follows.
Experimental Section
Materials. Cobalt and zinc tetraphenylporphyrins from
Porphyrin Products, Inc. (Logan, UT) were purified by column
chromatography on neutral alumina. Other chemicals and
solvents were of high purity and were used as received. The
cylindrical pyrolytic graphite electrodes with 0.32 cm2 of the
edges of the graphitic planes exposed (Union Carbide) were
mounted and polished as previously described.4
Apparatus and Procedures. Conventional electrochemical
cells and instrumentation were employed. Thin layers of NB
solutions containing reactants of interest were applied to graphite
electrodes held in an upside-down position by transferring 0.8-
1.5 µL aliquots of the solutions to the electrode surface with a
microsyringe. The organic liquid spread spontaneously across
the surface of the graphite. The electrode was then turned over
and immediately immersed in the aqueous supporting electrolyte.
The stability of the resulting, thin, adherent film of NB was
demonstrated previously.4 Supporting electrolyte in the NB film
can be acquired by partitioning of electrolyte from the aqueous
phase. From conductance measurements we estimated that 1-3
mM NaClO4 dissolved in NB layers that were equilibrated with
aqueous solutions of NaClO4. Even when the concentration of
electrolyte in the NB phase was small, the uncompensated
resistance introduced by the thin layers of NB was small enough
to prevent significant distortions in the shapes of cyclic
voltammetric responses from reactants dissolved in the NB.4
In most cases, a suitable salt, e.g., tetra-N-hexylammonium
perchlorate (THAP), was dissolved in the nitrobenzene before
the thin layer was formed to produce a more conducting organic
phase which contained a known concentration of supporting
electrolyte.
Figure 2. Voltammetric observation of electron-transfer from DMFc
in a thin layer of NB to Fe(CN)63- in water. (A) Cyclic voltammogram
3-
for 0.47 mM Fe(CN)6 at an uncoated EPG electrode: supporting
electrolyte, 0.1 M NaClO4 + 0.1 M NaCl; scan rate, 5 mV s-1
throughout. (B) Repeat of A after the electrode surface was covered
with 1 µL of NB. (C) Cyclic voltammogram with the electrode covered
with 1 µL of NB containing 0.55 mM DMFc+ (generated by oxidation
of DMFc at the initial potential). The aqueous solution contained only
supporting electrolyte (0.1 M NaClO4 + 0.1 M NaCl). (D) Solid line:
3-
Repeat of C with 1.7 mM Fe(CN)6 present in the aqueous phase.
The plotted points are the steady-state currents recorded at a scan rate
of 0 mV s-1 as the potential was stepped to more negative (O) and to
more positive (b) values. (E) Cathodic plateau currents such as that in
D as a function of the concentration of Fe(CN)63- in the aqueous phase.
The NB phase contained 0.55 mM DMFc+. (F) Reciprocal plateau
3- -1
currents vs [Fe(CN)6
mM.
]
with [DMFc+]NB ) 0.55 (b) or 0.20 (O)
of DMFc+ from the NB layer by partitioning into the aqueous
phase became more significant in the absence of ClO4- anions,
which appear to diminish the solubility of DMFc+ in the aqueous
3-
phase. When Fe(CN)6 was added to the aqueous phase, the
voltammetric response changed, as shown in Figure 2D. The
enhanced cathodic current at the potential where the DMFc+ is
reduced to DMFc indicates that DMFc engages in cross-phase
electron transfer to Fe(CN)63- anions in the aqueous phase. The
appearance of a cathodic plateau instead of a current peak
indicates that a steady-state concentration profile is generated
within the NB layer. With a sufficiently high concentration of
Fe(CN)63- in the aqueous phase, the concentration of DMFc at
the NB/H2O interface becomes negligibly small and the corre-
sponding cathodic plateau current, iD, is limited by the diffusion
of DMFc and DMFc+ across the NB layer. (In the absence of
sufficient supporting electrolyte in the NB phase, migration of
the DMFc+ could also become important. We have neglected
this possible complication in this report.) Under these conditions,
the magnitude of the plateau current can be described by eq 1,
Results
To demonstrate the simplicity and effectiveness of the use
of thin layers of immiscible solvents on graphite electrode
surfaces to evaluate electron-transfer rates at the interface
between the thin layer and aqueous supporting electrolytes, we
examined several such reactions.
Electron Transfer from Decamethylferrocene to Fe(CN)6
across the NB/H2O Interface. Shown in Figure 2A is a cyclic
voltammogram recorded at a bare edge plane pyrolytic graphite
(EPG) electrode in a 0.5 mM aqueous solution of Fe(CN)6
with 0.1 M NaClO4 + 0.1 M NaCl as the supporting electrolyte.
When a thin layer of NB was placed on the electrode surface
(using the procedure described in the Experimental Section),
the voltammetric response from the Fe(CN)63-/4- couple disap-
peared because the Fe(CN)63- anions could no longer reach the
electrode surface (Figure 2B). In Figure 2C is shown the
response obtained when decamethylferrocene (DMFc) was
dissolved in the NB before the thin layer was formed on the
electrode. The aqueous solution contained only supporting
electrolyte. The reversible couple near -0.2 V corresponds to
the DMFc+/DMFc couple.6 This response was quite stable:
Repetitive scanning of the potential between +0.3 and -0.4 V
produced no significant decrease in the peak currents. The loss
3-
3-
iD ) nFACNBD/δ
(1)
where n ) 1, F is Faraday’s constant, A is the electrode area,
CNB is the concentration of DMFc dissolved in the NB layer,
D is the diffusion coefficient of DMFc in NB, and δ is the
thickness of the layer. The observed cathodic plateau currents,
iobs, for a series of Fe(CN)63- concentrations are shown in Figure
2E. The currents become independent of the concentrations of

9852 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Shi and Anson
TABLE 1: Bimolecular Rate Constants for Cross-Phase Electron Transfer between Pairs of Redox Reactants in Adjoining
NB/H2O Phases
entry no.
reactant in H2O phasea
EfH O,b mV
reactant in NB phase c
EfNB,d mV
∆E,e mV
ket,f cm s-1 M-1
k′et,g cm s-1 M-1
2
0.55 mM DMFc h
0.49 mM DMFc
0.47 mM DMFc
0.40 mM DMFc
0.37 mM ZnTPP+
0.22 mM ZnTPP+
0.22 mM ZnTPP+
0.39 mM CoTPP2+ l
0.18 mM Fc l
-240
-90
-145
-175
690
690
690
862
185
730
425
265
330
395
-60
139
505
302
530
280
0.9
0.8
0.9
1.1
0.8
1.7
1.8
0.2
1.4
0.05
3-
1
2
3
4
5
6
7
8
9
Fe(CN)6
Fe(CN)6
Fe(CN)6
Fe(CN)6
185
175
185
220
750
551
185
560
715
450
3- i
3-
3- j
4-
Ru(CN)6
0.02
0.5
4-
Mo(CN)8
4-
Fe(CN)6
4-
Fe(CN)6
2- k
IrCl6
10
Fe2+ m
0.32 mM ZnTPP+
0.06
a Supporting electrolyte: 0.1 M NaClO4 + 0.1 M NaCl except where indicated. b Formal potential of the reactant redox couple in the H2O phase
vs a saturated calomel electrode as measured by cyclic voltammetry at the bare EPG electrode in the H2O phase. c Supporting electrolyte: 0.25 M
THAP except where indicated. d Apparent formal potential of the reactant redox couple in the NB phase vs a saturated calomel electrode in the H2O
phase containing the indicated supporting electrolyte as measured by cyclic voltammetry. e The driving force of the reaction: (EHf O - ENf B) with
2
the oxidant in the H2O phase; (EfNB - EHf O) with the oxidant in the NB phase. f Evaluated from the slope of plots such as the one in Figure 2F using
2
eqs 2 and 3. g Evaluated from data in ref 3c for the H2O/benzene interface extrapolated to the same driving force. h No supporting electrolyte was
added to the NB phase. The difference between EfNB for entries 1 and 2 indicates that NaClO4 partitions into the NB phase to produce a
concentration of ca. 0.3 mM. i Supporting electrolyte: 0.01 M NaClO4 + 0.1 M NaCl. j Supporting electrolyte: 1 M NaClO4 + 0.1 M NaCl.
k Supporting electrolyte: 2 M HClO4. l No supporting electrolyte was added to the NB phase. pH titration showed that the NB phase contained ca.
2 mM HClO4. m Supporting electrolyte: 0.01 M NaClO4 + 0.1 M NaCl + 0.5 M H2SO4.
where ket is a bimolecular rate constant (with units of cm s-1
Fe(CN)63- above about 6 mM. The limiting value of the plateau
M-1) governing electron transfer between the two reactants
current at this point, 33 µA, compares favorably with the value
of 31 µA calculated from eq 1 using A ) 0.32 cm2, CNB
)
present at concentrations CNB and CH O in the NB and aqueous
2
0.55 mM, D ) 6 × 10-6 cm2 s-1,4 and δ ) 3.2 × 10-3 cm
(calculated from the volume of NB used to prepare the thin
layer.4)
phases, respectively, and the reaction rate is assumed to be first
order in both reactants. The observed plateau current, iobs
,
should then be described by eq 3,7 from which iet can be obtained
The reasonable agreement between the calculated and ob-
served values of the limiting plateau current is evidence that
the current is controlled by the diffusion of DMFc in the NB
phase. The plateau currents remained essentially the same when
the concentrations of the supporting electrolyte were varied
between 0.01 and 1 M NaClO4 in the aqueous phase and
between 0 and 0.25 M THAP in the NB phase. This behavior
was consistent with the fact that no ion transfer across NB/
H2O takes place when the currents are measured at steady state.
In the absence of added electrolyte in the NB phase, NaClO4 is
presumed to partition from the aqueous phase into the NB layer
(perhaps driven by the oxidation of DMFc to DMFc+ at the
initial electrode potential). The quantity of NaClO4 present in
the NB phase was sufficient to prevent distortion of the
responses by ohmic drops in the thin layer of NB.
1
1
1
)
+
(3)
iobs iD iet
because iD can be calculated from eq 1 or measured as in Figure
2E. According to eqs 1 and 3, a plot of (iobs
-1
-1
)
vs (CH O)
2
should be linear with a slope inversely proportional to ketCNB
and an intercept equal to (iD) -1. Shown in Figure 2F are two
3-
such plots for the DMFc/Fe(CN)6 reactant pair. The plots
are reasonably linear, and the slopes of the lines correspond to
values of ket that are in reasonable agreement, 0.9 and 0.8 cm
. The intercept of the lower line in Figure 2F
corresponds to a value of 30 µA for iD, which is in good
s-1 M-1
agreement with the limiting value of the current at the highest
3-
concentration of Fe(CN)6 in Figure 2E. Thus, the behavior
The forward and reverse traces in Figure 2D would be
expected to be superimposable if true steady-state currents were
involved. The hysterysis in the solid curve in Figure 2D resulted
from the finite scan rate (5 mV s-1) at which the curve was
recorded. When the currents were recorded point-by-point, i.e.,
at a scan rate of 0 mV s-1, they were independent of the
direction of the changes in potential (plotted points in Figure
2D).
shown in Figure 2F supports the use of eq 3 to analyze data
like those in Figure 2E.
Experiments such as those in Figure 2 were repeated with
differing concentrations of supporting electrolyte in the aqueous
phase and a fixed high concentration of THAP in the NB phase
in order to inspect the sensitivity of ket to changes in the driving
force of the electron-transfer reaction produced by changes in
the Galvani potential difference at the NB/H2O interface, as
elaborated in detail by Wei et al.3a The experimental data,
available in the Supporting Information, were used to obtain
values of the rate constants that are included in the summary
compiled in Table 1.
With concentrations of Fe(CN)63- less than ∼6 mM, smaller
plateau currents are obtained (Figure 2E) with magnitudes that
are likely to be influenced by the rate of electron transfer from
3-
DMFc molecules in the NB phase to Fe(CN)6 anions in the
3-
aqueous phase. The diffusion of Fe(CN)6 anions in the
Electron Transfer from Reductants in the Aqueous Phase
to ZnTPP+ in the NB Phase. To compare rate constants
evaluated with the thin layer and SECM techniques, some of
the reactant couples examined previously by Bard and co-
workers3 were also examined. For this purpose, we reacted zinc
tetraphenylporphyrin radical cation (ZnTPP+) generated in the
aqueous phase was neglected in the present experiments because,
especially at higher concentrations, the magnitudes of the plateau
currents were smaller than the peak currents for the diffusion-
3-
limited reduction of Fe(CN)6 at the uncoated electrode.
It is convenient to use a characteristic current to describe the
rate of cross-phase electron transfer at the NB/H2O interface3a,7
4-
NB phase with Fe(CN)64-, Mo(CN)84-, or Ru(CN)6 anions
in the aqueous phase. A typical set of results is shown in Figure
3, which is entirely analogous to Figure 2 except that anodic
iet ) nFAketCNBCH O
(2)
2

Liquid/Liquid Interface Electron-Transfer Rates
J. Phys. Chem. B, Vol. 102, No. 49, 1998 9853
4-
Figure 3. Electron transfer from Mo(CN)8 in the aqueous phase to
the radical cation of zinc tetraphenylporphyrin (ZnTPP+) in the NB
phase. (A) Cyclic voltammogram of 0.5 mM Mo(CN)84- at an uncoated
EPG electrode: supporting electrolyte, 0.1 M NaClO4 + 0.1 M NaCl;
scan rate, 5 mV s-1 throughout. (B) Repeat of A after the electrode
was covered with a thin layer of NB. (C) Cyclic voltammogram for an
electrode covered with 1 µL of NB containing 0.45 mM ZnTPP and
0.25 M THAP. The aqueous solution contained only supporting
electrolyte (0.1 M NaClO4 + 0.1 M NaCl). (D) Repeat of C except the
NB phase contained 0.1 mM ZnTPP and the aqueous phase contained
3.9 mM Mo(CN)84-. (E) Anodic plateau currents such as that in D as
4-
Figure 4. Electron transfer from Fe(CN)6 in the aqueous phase to
CoTPP2+ in the thin layer of NB. (A) Cyclic voltammogram of 0.7
mM Fe(CN)64-at an uncoated EPG electrode: supporting electrolyte,
2 M HClO4; scan rate, 5 mV s-1 throughout. (B) Repeat of A after the
electrode was covered with a thin layer of NB. (C) Cyclic voltammo-
gram for an electrode covered with 1 µL of NB containing 0.39 mM
CoTPP. The aqueous phase contained only supporting electrolyte (2
M HClO4). (D) Repeat of C with 5.7 mM Fe(CN)6 present in the
aqueous phase. (E) Anodic plateau currents such as that in D as a
function of the concentration of Fe(CN)6 in the aqueous phase. The
4-
4-
a function of the concentration of Mo(CN)8 in the aqueous phase.
4- -1
(F) Reciprocal plateau currents vs [Mo(CN)8
] .
4-
NB phase contained 0.39 mM CoTPP. (F) Reciprocal plateau currents
rather than cathodic plateau currents are obtained. The formal
potential of the ZnTPP+/ZnTPP couple in NB obtained from
Figure 3C was 0.69 V. This value is somewhat less positive
than the value obtained by Tsionsky et al.,3b who used benzene
as the organic phase. Experimental data like those in Figure 3
were also obtained for the other two reductants and are included
in the Supporting Information. The values of ket for all three
reactant pairs are given in Table 1.
The concentrations of ZnTPP in the NB layers used in
experiments such as those in Figure 3 were purposefully
maintained at relatively low values. The higher currents that
resulted with higher concentrations of ZnTPP sometimes
produced distortions in the steady-state current plateaus. Tsion-
sky et al. reported similar anomalous behavior in their SECM
measurements which they attributed to the formation of
precipitates at the benzene/H2O interface when the concentration
of ZnTPP+ in the organic phase was increased.3b Under similar
conditions we sometimes observed peak-shaped current-
potential curves with current magnitudes that exceeded by
severalfold the calculated values of iD for ZnTPP+ in the NB
phase. The phenomenon could always be eliminated by
diminishing the concentration of ZnTPP+ in the NB, and no
current anomalies were observed when the reactant in the NB
was neutral instead of cationic (i.e., ferrocene or DMFc). We
did not examine the phenomenon in more detail, but we suspect
that the formation of precipitates at the NB/H2O interface may
induce convective transport within the NB phase.
4- -1
vs [Fe(CN)6
] .
two sets of voltammetric responses from CoTPP in the NB phase
(Figure 4C) corresponding to the CoTPP+/CoTPP couple near
0.4 V and the CoTPP2+/CoTPP+ couple near 0.9 V. When
4-
Fe(CN)6 was present in the aqueous phase (Figure 4D), the
current in the vicinity of the CoTPP/CoTPP+ couple was not
enhanced because CoTPP+ is too weak an oxidant to oxidize
Fe(CN)64-. However, the more strongly oxidizing CoTPP2+
cation is capable of accepting an electron from Fe(CN)64-, and
an anodic current plateau is obtained near 0.9 V (Figure 4D).
The value of ket calculated from the slope of the line in Figure
4E is given in Table 1.
Analogous experiments with ferrocene in the NB phase and
IrCl62- in the aqueous phase were also carried out (Supporting
Information) to obtain the value of ket for this reactant pair given
in Table 1.
Discussion
Method. The simplicity of the method described in this
report for evaluating rates of electron transfer between reactants
located in different immiscible phases is its most attractive
feature. Wei et al. pointed out the variety of potential problems
that can be encountered in using conventional electrochemical
methods for studying charge-transfer kinetics at the interface
between two immiscible liquids.3a The alternative SECM
method utilized by them has a number of advantages compared
with the conventional methods. However, the actual charge-
transfer process that is monitored in SECM experiments is more
complex because it involves simultaneous electron transfer and
ion transfer, while only electron transfer is involved with the
Additional Systems. An interesting set of results was
obtained with cobalt tetraphenylporphyrin radical cation (Co-
TPP2+) as the electron acceptor in the NB phase. Figure 4
summarizes the experimental data. The noteworthy difference
from the behavior of ZnTPP in Figure 3C is the presence of

9854 J. Phys. Chem. B, Vol. 102, No. 49, 1998
Shi and Anson
3-
steady-state thin layer procedure described in this report. The
method is considerably simpler than previously employed
methods because it requires neither a bipotentiostat nor comput-
ers for positioning microelectrodes or fitting experimental data
to calculated curves. In addition, the steady-state plateau
currents used in the evaluation of the rate constants in the thin
layer method are not influenced by the kinetics of heterogeneous
electron transfer at the electrode surface.
When the driving force for the reduction of Fe(CN)6 in
the aqueous phase by DMFc in the NB phase was varied by
changing the concentration of NaClO4 in the aqueous phase,
there was very little change in the values of ket (entries 2-4 in
Table 1). This behavior could be an indication that the actual
electron transfer was not cross-phase but involved a small
quantity of DMFc that had partitioned into the aqueous phase.
Hanzlik et al. contemplated the same possibility for the
ferrocene/Fe(CN)63- reactant pair.9 Experiments such as those
in entries 1-4 in Table 1 may provide a useful means for
distinguishing between cross-phase and in-phase electron trans-
fer.
The maximum value of the rate constant, ket, that may be
measured with the thin layer method is determined by the
relative values of iD (eq 1) and iet (eq 2). If it is assumed that
the slopes of plots based on eq 3 would allow iet to be evaluated
as long as iet < ∼10iD, the maximum value of ket would be
Additional measurements will clearly be needed to identify
with certainty the reasons for the trends in the rate constants
collected in Table 1. The use of thin layers of organic solvents
as described in this report may facilitate such measurements.
given by 10D/CH Oδ. With a 20 µm layer of nitrobenzene, a
2
diffusion coefficient of the reactant in NB of 6 × 10-6 cm2
s-1, and a reactant concentration of 10-3 M in the aqueous
phase, the calculated upper limit on measurable values of ket is
30 cm s-1 M- 1, which compares very favorably with upper
limits available from previously employed procedures. This
upper limit could be raised if it proved possible to employ
thinner layers of the organic phase.
Acknowledgment. This work was supported by the National
Science Foundation. It is a pleasure to draw attention to the
fact that the work described in the present paper, as has so much
other research on charge transfer at interfaces, benefited
significantly from the stimulating ideas and experimental
innovations which have emanated from Prof. Allen J. Bard and
his research group.
In their report, Wei et al.3a discussed the possibility of
measuring interfacial charge-transfer rates by using thin layers
of one solvent trapped at the surface of microelectrode tips that
were immersed in a second, immiscible solvent, but no
experimental examples were presented. The discovery that
stable, thin layers of organic solvents are easy to deposit and
retain on conventional graphite electrodes was the key to the
development of the approach described in this report.
Supporting Information Available: Figures similar to
Figure 3 containing the experimental data that were used to
complete entries 2-4, 5, 7, 9, and 10 in Table 1 (7 pages).
Ordering information is given on any current masthead page.
Results. Values of bimolecular rate constants for the cross-
phase electron-transfer processes examined in this study are
summarized in Table 1. Some of the same reactant pairs were
examined previously by Bard and co-workers using benzene
instead of nitrobenzene as the organic phase.3 In most cases
they did not evaluate bimolecular rate constants explicitly, but
values estimated from their data are also given in Table 1. The
dielectric constants of the two solvents, 2.2 and 34 for benzene
and NB, respectively, are so different that some of the
differences in the values of the rate constants obtained in the
two studies might be attributed to this factor. The data in Table
References and Notes
(1) (a) Girault, H. H.; Schiffrin, D. J. In Electroanalytical Chemistry;
Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 1. (b) Girault,
H. H. In Modern Aspects of Electrochemistry; Bockris, J. O’M., Conway,
B. E., White, R. E., Eds.; Plenum Press: New York, 1993; Vol. 25, p 1.
(2) (a) Bard, A. J.; Denuault, C.; Lee, C.; Mandler, D.; Wipf, D. O.
Acc. Chem. Res. 1990, 23, 357. (b) Bard, A. J.; Fan, F.-R. F.; Mirkin, M.
F. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New
York, 1994; Vol. 18. (c) Bard, A. J.; Fan, F.-R. F.; Mirkin, M. V. Physical
Electrochemistry: Principles, Methods and Applications; Rubinstein, I., Ed.;
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