Indian Journal of Chemistry
Vol. 50A, August 2011, pp. 1043-1049
Organized media as redox catalysts on the reaction rate of
CoIII-FeII redox couple
Mousumi Mukherjee & Ambikesh Mahapatra*
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Email: [email protected]/[email protected]
Received 1 March 2011; revised and accepted 14 July 2011
Micelles catalyze the [Co(NH3)5Cl](NO3)2 - Mohr’s salt redox reaction due to the binding of the substrates onto the
micellar surface by non-coulombic and coulombic interactions. This binding increases the encounter probability, leading to
a substantially accelerated reaction. The kinetic studies of the same redox reaction have also been carried out in the water
pools of non-ionic reverse micelles of Triton X-100/1-butanol in n-heptane. The findings indicate that probably substrate
solubilization sites in micelles and reverse micelles are the interfacial and Stern regions. The reaction rates decrease with the
water-to-surfactant mole ratio. The correct selection of surfactant for micelle, size of reverse micellar water pool cavity,
different additives and the catalyst connecting nanoparticles control the electron transfer reaction rate in solution. Copper-sol
induced catalysis occurs due to the availability of a suitable surface which acts as a mediator for the electron-transfer
reaction between [Co(NH3)5Cl](NO3)2 and Mohr’s salt.
Keywords: Solution chemistry, Kinetics, Electron transfer, Surfactants, Micelles, Reverse micelles, Organized media,
Cobalt-Iron redox reaction
Surfactants are amphiphiles which have clear regions
of hydrophobic and hydrophilic parts due to the
chemical structure of binding of the hydrophilic
moiety bound to the hydrophobic segment1. Micelles
are those in which the surfactant molecules
form aggregates at higher amphiphile concentrations
(> 10-4 mol L-1). The study of electron-transfer
reactions under different conditions in which one or
both reactants are forced to stay at the micellar
surface2 has been of increasing interest in recent times
since (i) the nature of charges of the reactants and
their increased or decreased local concentrations with
respect to their bulk concentrations can alter the
reaction rates, and, (ii) the non-coulombic interactions
between the substrate and the surfactant aggregates
can also influence the reaction rate. Generally, anionic
micelles enhance the rate of the reactions with
cationic reactants while cationic micelles retard the
rate in such reactions, while catalyzing reactions
involving anionic reactants3a. Several additives, e.g.,
salt and urea in presence of surfactant, alter the rate of
the reaction as polarity of the medium changes3b.
Reverse micelles consists of micro pools of water
lined by a monolayer of a surfactant, all dispersed in
an apolar solvent. The water present in a reverse
micelle is referred to as ‘water pool’4-6. It has been
reported that water pools in the reverse micellar
systems have much lower micro polarity7 which can
influence kinetic features. One of the several
advantages of a reverse Triton X-100 micellar system
is that the size of the water pool can be controlled
precisely at the nanometer level diameter through the
water-to-surfactant molar ratio. Therefore, size effect
on chemical and physical properties in nanometer
dimension can be studied by the use of Triton X-100
micelles. In n-heptane the radius (rw) of such a water
pool is about 2W (in Å), where W is the water-tosurfactant mole ratio. With this objective, the kinetics
of the reaction was investigated in the Triton X-100
reverse micellar system. The results are suggestive of
the low micro polarity of the water pools of this
system as compared to bulk water.
Metal particles of nanometer size exhibit size
dependent catalytic properties. This arises due to their
high surface-to-volume ratio and mostly due to sizedependent reactivity. For this reason, metal particles
can be treated as organized assemblies. A variety of
synthetic techniques for metallic nanostructures has
led to an explosion of interest in their application in
catalysis8,9.
Several kinetic studies on electron-transfer
processes (and other types of reactions10) have been
carried out in presence of these organized assemblies.
Many research groups including our group have
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INDIAN J CHEM, SEC A, AUGUST 2011
studied the kinetics and mechanism of reduction of
CoIII complexes by Mohr’s salt (FeII) in aqueous and
non-aqueous media11,12. The effects of nanoparticle
and nanoparticle-surfactant interaction on these
reactions have also been reported13-15. However,
report of systematic studies of a given reaction in
different organized media is rare. Considering the
easy incorporation of complexes into micelles, reverse
micelles and prospective catalysis by metal
nanoparticles, we have extended our studies to
different micellar, reverse micellar as well as
nanoparticle environments to examine their effects
on the reaction. Herein, we report the kinetics
of
the
electron-transfer
reaction
between
[Co(NH3)5Cl](NO3)2 and Mohr’s salt in a series of
organized reaction media of contemporary interest.
Materials and Methods
All the reagents used were of either GR or AR
grade. The following reagents were obtained from
Merck (India) and used as such: sodium dodecyl
sulphate (SDS), polyoxyethylene (20) sorbitan
monolaurate (Tween 20), polyoxyethylene (23) lauryl
ether (Brij 35), copper sulphate (CuSO4), sodium
sulphate (Na2SO4), sodium perchlorate (NaClO4),
sodium chloride (NaCl), sodium nitrate (NaNO3),
sodium hydroxide (NaOH), Mohr’s salt, ammonium
sulphate, dextrose anhydrous, urea, sulphuric acid,
1-butanol and n-heptane. N-Cetyl-N,N,N-trimethyl
ammonium bromide (CTAB) of 98 % purity from
Loba Chemie (India), poly (oxyethylene) isooctyl
phenyl ether (Triton X-100 or TX-100) from
Qualigens
Fine
Chemicals
(India)
and
cetylpyridinium chloride (CPC) from SRL India were
used as received. The [Co(NH3)5Cl](NO3)2 complex
was prepared by a standard procedure. All aqueous
solutions were prepared in deionised and doubly
distilled water with second distillation being carried
out from alkaline permanganate in an all-Pyrex still.
All kinetic and spectral measurements were
recorded on an Agilent 8453E UV-visible
spectroscopic system and quartz cells (1.0 cm path
length) from Hellma. The dynamic light scattering
(DLS) results were obtained with a Nano ZX DLS
instrument supplied by Malvern Instruments, UK.
To prepare the reverse micellar stock solution
of strength 1.0 mol L-1, the required amount of
non-ionic surfactant, TX-100, was dissolved in the
apolar solvent, n-heptane. Solutions of different
concentrations were prepared by accurate dilution of
stock solution with n-heptane. Mohr’s salt solution
(in doubly distilled water) and the required amount of
water was added to the TX-100 solution and shaken to
obtain a transparent solution of the required
concentration of Mohr’s salt and W value. This can be
regarded as a reverse micellar system.
The copper nanoparticle sols were prepared from
CuSO4 by an earlier reported method16 in nitrogen
atmosphere with the help of the reducing agent,
glucose, in presence of NaOH. Three different
concentrations of glucose (5.78 × 10-3, 8.68 × 10-3 and
11.60 × 10-3 mol L-1 were used to obtain nanoparticles
of three different sizes. The particles included nano
rods (avg. length of ~116 nm and width 46 nm) and
nano spheres (dia. ~20 nm and ~ 4 nm).
Kinetic studies
Kinetics of the reaction between Mohr’s salt (FeII)
and [Co(NH3)5Cl](NO3)2 (CoIII complex) in the
presence of micelles, reverse micelles and nanometer
sized
metal
sol
have
been
studied
spectrophotometrically under pseudo-first order
conditions with an excess amount of FeII at 298 K.
Influence of addition of FeII solution on the spectra of
pure CoIII complex solution was recorded (Fig. 1). The
change proceeds through a maximum at ca. 534 nm.
The decrease in absorbance (At) of the reaction
mixture occurred at 534 nm. The integrated rate
equation used to determine the observed pseudo-first
order rate constant, kobs, values from the plot is:
At = A∞ + a1exp(-kobst); which is first order exponential
decay equation. The kobs values were reproducible
within ± 3 % for all kinetic runs.
Fig. 1—Absorption spectra of reaction mixture at different time
intervals. {[CoIII]0 = 7 × 10-3 mol L-1; [FeII]0 = 0.15 mol L-1;
ionic strength = 1.07 mol L-1; temp. = 298 K}.
MUKHERJEE & MAHAPATRA: EFFECT OF MEDIA ON REACTION RATE OF CoIII – FeII REDOX COUPLE
Results and Discussion
Kinetic studies in aqueous solution
The electron transfer reaction between the
oxidizing agent, CoIII complex salt, and reducing
agent, FeII salt in aqueous medium, proceeds through
a bimolecular chloro-bridged inner sphere13. An
electron can be transferred slowly from FeII to CoIII
through the chloro bridge. The kobs value for the
electron transfer reaction17 between [Co(NH3)5Cl]2+
and Fe(ClO4)2 with ionic strength of 1.7 mol L-1
corroborates with data of this reaction with ionic
strength 1.07 mol L-1. The rate of reaction at ionic
strength µ = 1.07 mol L-1 is less than that of the rate
of reaction13 at ionic strength µ = 1.4 mol L-1. This is
because both the complex and reducing agents are
positively charged. Thus, the rate decreases with
decrease in ionic strength.
Kinetic studies in presence of micelles
The kobs values of the reduction rate in presence of
different micelles are depicted in (Fig. 2). The rate of
reduction is highest in presence of anionic micelles
and lowest in presence of cationic micelles. The order
of rate is as follows: kobs (in presence of anionic
micelles) > kobs (in presence of non ionic micelles)
> kobs (in presence of cationic micelles). In presence
of the non-ionic surfactants the order of reduction rate
is as follows: kobs (in presence of TX-100) > kobs (in
presence of Tw 20) > kobs (in presence of Brij 35).
This order of rate can be explained with the
concept of encounter probability18,19 and the fractal
Fig. 2—Plots of kobs versus [surfactant]0 for the studied reaction.
{[CoIII]0 = 7 × 10-3 mol L-1; [FeII]0 = 0.15 mol L-1; ionic strength =
1.07 mol L-1; temp. = 298 K. 1, SDS; 2, TX-100; 3, Tw 20;
4, Brij 35; 5, CTAB; 6, CPC}.
1045
nature of the micellar surface20-22. Micellar surfaces
bind many organic/inorganic compounds by
coulombic and/or non coulombic interactions23. Due
to non-coulombic interaction, the surfactants form
micelles of colloidal size24 and occupy a large space
in the reaction volume which ultimately increases the
local concentration of the reacting species, CoIII
complex and hydrated FeII ion as well as the rate.
Secondly, due to non-coulombic interaction, the
complex is hydrophilic in nature12. Thus, the reaction
occurs in the Stern layer of the micelles. The noncoulombic interaction plays an important role for both
SDS and TX-100 micelles and a less important role
for CTAB due to strong coulombic repulsion. In the
case of the cationic complex, the complex is bound to
SDS by both coulombic and non-coulombic
interactions. The stronger force of attraction between
SDS and complex (i.e., immobilization of complex)
explains qualitatively the faster rate of reduction of
CoIII-complex in SDS micelles. Anionic micelles
indirectly help to increase the collision probability
between the complex and Mohr’s salt through a
physical factor, i.e., through their incorporation in the
micellar Stern layer which is the compact region of
the charged head groups and the relatively small
counter ions of the ionic micelle25,26 (Scheme 1). The
reduction rate involving cationic complex in a
cationic micellar solution does not increase much
because of the low collision probability between
complex and Mohr’s salt.
The rate of the reaction is lowest in presence of
CTAB micelles. The positively charged micellar
1046
INDIAN J CHEM, SEC A, AUGUST 2011
surface of CTAB prevents the incorporation of the
cationic complex into the Stern layer due to
coulombic repulsion but only non-coulombic
interaction is operative to catalyze the rate slightly.
Therefore, the reaction rate follows the order:
RateSDS > RateTX-100 > RateCTAB.
The retarding effect of NaCl on the reaction rate in
SDS micellar medium supports the above explanation.
The micellar inhibition on reduction rate arises due to
displacement of the cationic complex, Co(NH3)5Cl2+,
from the stern layer of the anionic micelle on addition
of salt.
TX-100 micelles are the most effective catalyst
when compared with other non-ionic micelles with
higher hydrophobic property. It provides a well
defined micellar surface area for the catalysis to
take place. The order of reaction in this case is:
RateTX-100 > RateTw 20 > RateBrij 35.
CTAB micelles are more effective catalyst as
compared with the other cationic micelle i.e., CPC.
This is because of the bulk and flatness of the
surfactant ‘head’; the micelles of CPC are unusually
loose structures containing considerable amounts of
water. Thus, RateCTAB > RateCPC.
DLS study
The incorporation of Mohr’s salt in the micelle can
be well understood from the dynamic light scattering
(DLS) studies which show that the micelle size
increases gradually with the addition of Mohr’s salt.
The size of the micelle of SDS (10-1 mol L-1) was
1.29 nm at 25 °C and after incorporation of 1 mL of
0.15 mol L-1 Mohr’s salt, the size increased to 4.85 nm;
the size of the micelles gradually increases with the
addition of Mohr’s salt (5 mL of 0.15 mol L-1).
Similarly, the size of the micelle of TX-100 and
CTAB (10-1 mol L-1) was 1.27 nm and 1.25 nm at
25 °C respectively and after incorporation of 1 mL
of 0.15 mol L-1 Mohr’s salt, the size increased to
4.62 nm and 4.23 nm respectively; the size of the
micelles gradually increased with the addition of
Mohr’s salt (5 mL of 0.15 mol L-1). These results
show that Mohr’s salt is incorporated in the micelles
and hence can easily reduce micelle-bound
CoIII-complex in the Stern layer.
aqueous solution of TX-100 above its CMC
(3 × 10-4 mol L-1). Urea decreases the polarity of the
aqueous phase where the transition state is less
stabilized, and hence decreases the rate of the ionic
reaction. However, addition of NaCl increases the
reduction rate. NaCl increases polarity of the medium,
thus enhancing the rate of the reaction since the
transition state is positively charged. So is the case
with the other salts Na2SO4, NaNO3 and NaClO4; the
rate of the reaction increases due to increase in the
polarity of the medium. Bronsted and Bjerrum models
also support these observations.
Kinetic studies in presence of reverse micelles
Reverse micelles are the accumulation of surfactant
in an apolar solvent27. They are small-sized (in the
nanometer range) water droplets encompassed by a
layer of surfactant molecules, such as TX-100, with
the non polar solvent acting as the dispersion
medium27,28. The alkanes, iso-octane or n-heptane,
permit maximum solubilization of water at room
temperature and thus are normally used to stabilize
the reverse micelle. Increase in the amount of
surfactant-entrapped water results in the formation of
water-in-oil micro emulsions.
The effect of varying concentrations of TX-100 on
reaction the rate has been studied at different values
of W (W = 3-10) (Fig. 4). The variation of TX-100
(0.1 - 0.4 mol L-1) at a low value of W (=3) has a
significant effect on the rate. The reactants being ionic
cannot exist in the bulk organic phase; the reaction
can take place either in the water pool or in the
Effect of urea and salts on the reaction in micellar medium
Micellar catalysis is affected in the presence of
additives, such as urea, NaClO4, NaNO3, Na2SO4 and
NaCl (Fig. 3). It was found that the reduction rate was
diminished when urea was added to a 10-3 mol L-1
Fig. 3—Plots of kobs versus [reagent]0 for the studied reaction.
{[CoIII]0 = 7 × 10-3 mol L-1; [FeII]0 = 0.15 mol L-1; ionic strength =
1.07 mol L-1; [TX-100]0 = 10-3 mol L-1; temp. = 298 K. 1, NaCl;
2, Na2SO4; 3, NaNO3; 4, NaClO4; 5, urea}.
MUKHERJEE & MAHAPATRA: EFFECT OF MEDIA ON REACTION RATE OF CoIII – FeII REDOX COUPLE
1047
Scheme 2
Fig. 4—Plot of kobs versus [TX 100]0 for the reaction in reverse
micellar medium of TX-100 at varying W values at 298 K.
[W= 1, 4.16; 2, 5.55; 3, 6.94; 4, 8.33; 5, 9.72].
micellar interphase. It is known29 that at constant
W the increase in surfactant concentration results in
increase in the micellar concentration and hence the
area of the interface, although there is no change in
the micellar composition or other properties. Thus,
increase of rate with TX-100 concentration shows that
the reaction is taking place on the micellar interface
(Scheme 2) at this low value of W. At high value of
W (=10) the effect of TX-100 concentration is
insignificant. The increase of the surfactant
concentration at constant W increases the interfacial
area at high W value and thus the reactants are
displaced from micellar interface to bulk of large
sized water pool. In this case, the reaction is taking
place mainly in the bulk of entrapped water pool30.
The effect of variation of W on the rate of reaction,
at fixed TX-100 surfactant concentration and other
variables, has been studied (Fig. 4). On increasing
W as well as entrapped water pool size, the reactants
are displaced into the bulk of water pool and at
W = 12.63, the reaction takes place mostly in the bulk
of water pool. Thus, at high W value the reverse
micelle has very little effect on the rate. In other
words, as W increases, the properties of entrapped
water pool are similar to those of the continuous bulk
water phase.
Effect of copper nanoparticles on the reaction kinetics
The copper nanoparticles were prepared in the
present study from CuSO4 with glucose in the
presence of NaOH, to give well defined nano rods
(avg. length of ~ 116 nm and width 46 nm) and
nano spheres (dia. ~20 nm and ~ 4 nm). The use of
three different concentrations of glucose gave
nanoparticles of three different sizes. The particles
thus formed are resistant to oxidation16. The
stabilization of copper nanoparticles is due to the
capping of particles by glucose and/or gluconic acid
in alkaline condition. Always freshly prepared
nanoparticles solutions were used in the present
kinetic studies. As oxidation of copper
nanoparticles is slow and sets in after a few days
with disappearance of absorption peaks31, it does
not affect the kinetic studies.
In presence of alkali, α-glucose transforms into
β-glucose through the open chain structure, which is
very reactive. In open chain form, the available free
–CHO group reduces the Cu(II) to Cu(0) at alkaline
pH. As a consequence of the reduction, the –CHO
group oxidizes to the corresponding–COOH, which
offers stability to the generated particles under
alkaline condition. The basic copper hydroxide
formed is responsible for nucleation and successive
growth. The available amount of –OH is different due
to different amounts of glucose in the three sets. The
–
OH directs the growth of copper nanoparticles into
different shapes. Increased co-ordination ability of the
sugar molecules in alkaline medium has been well
documented due to the formation of deprotonated
sugar anions. The co-ordinatively unsaturated surface
atoms of copper particles get stabilized by the anionic
sugar moiety.
Copper nanoparticles enhance this electron transfer
rate. Initially, both the reactants are adsorbed on the
surface of the copper metal particles. Subsequently,
the reductant transfers electron to CoIII complex
through the surface of the metal particle32 and
consequently CoIII complex is reduced. In absence of
the metal particles, the reacting species exhibit a rapid
diffusion19, lowering the probability of fruitful
encounters as well as rate.
1048
INDIAN J CHEM, SEC A, AUGUST 2011
Table 1—Pseudo first order rate constants (kobs) per unit mass of the catalyst at different concentrations of Cu nanoparticles for
the studied reaction. {[CoIII]0 = 7 × 10-3 mol L-1; [FeII]0 = 0.15 mol L-1; ionic strength = 1.07 mol L-1; temp. = 298 K}
105[Cu]0 (mol L-1)
0.00
3.07
7.68
12.29
16.90
21.50
kobs /mass (s-1 g-1)
Cu (4 nm)
1.33 × 10-3 (±0.04)
383.67 (±0.06)
236.07 (±0.09)
168.72 (±0.10)
138.43(±0.11)
129.03 (±0.13)
The rate of electron transfer reaction increases with
increase in nanoparticle concentration. The rate
constant per unit mass of the catalyst with change in
nanoparticle concentration is given in Table 1. As the
concentration of nanoparticle increases, the surface
area of heterogeneous catalyst increases and thus the
electron transfer reaction becomes more facile.
It was found that the smaller copper nanoparticles
(avg. dia. 4 nm) are more efficient than the larger
copper particles. This is because the smaller particles
have a more negative potential33 and a larger surface
area that provides necessary conditions for electron
relay for the complex reduction. It has been found that
the rate increases with decrease of copper particle size
and rate in presence of copper nanoparticles of avg.
dia. 4 nm is about four times the rate in absence of the
copper nanoparticles. The surface of the nanoparticles
facilitates the formation of a bimolecular chlorobridged inner sphere complex (formed by both
cationic reacting species) and thus the rate of the
reaction increases in presence of nanoparticles.
Cu (20 nm)
1.33 × 10-3 (±0.04)
310.20 (±0.05)
180.33 (±0.07)
133.33 (±0.08)
114.55 (±0.09)
95.31 (±0.10)
Micelles provide a method of organizing the reactants
on a molecular scale and enhancing the reaction
rate. Coulombic and non-coulombic forces exert
a profound influence on the encounter probability.
A delicate balance of both gives rise to an optimum
condition for catalysis.
Acknowledgement
Financial help from Center for Advanced Studies,
Department of Chemistry, Jadavpur University,
Jadavpur, and research fellowship to one of the
authors (MM) by UGC, New Delhi, are gratefully
acknowledged. The authors thank Prof. A Dasgupta,
Department of Biochemistry, University of Calcutta,
Kolkata, for DLS instrumental facility.
References
1
2
3
Effect of ionic strength on reaction rate
The ionic strength of the reaction mixture in
presence of either micelles or copper nanoparticle
may be maintained constant by adding the required
volume of thermostated ammonium sulphate solution.
However, it was not practically possible to maintain
the ionic strength constant in the present study in
presence of reverse micelles of TX-100. It is well
known that increasing of ionic strength makes the
Stern layer more compact, which ultimately increases
the rate of the reaction. Due to the significant catalytic
effect of either micelles or copper nanoparticles, the
effect of ionic strength on rate is masked and is
insignificant. Hence, in the present study, the ionic
strength was not maintained constant.
Conclusions
The present study reports the catalytic effect of the
typical environments of various organized assemblies
on the well known redox reaction of CoIII complex.
Cu (146 nm)
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273.47 (±0.04)
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78.30 (±0.08)
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