Micellar catalysis on the redox reaction of glycolic acid with chromium(VI)

Article abstract of DOI:10.1002/kin.1031

Chromium(VI) oxidation of glycolic acid in the absence and presence of cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) followed the same mechanism as shown by kinetic study. The reaction followed second-order kinetics, first-order in each reactant. The oxidation is strongly catalyzed by manganese(II) and cationic micelles of CTAB or CPB. The catalytic effect of micelles can be fitted to a model in which the reaction rate depends upon the concentration of both reactants in the micellar pseudophase. Some added inorganic salts (NaCl, NaBr, NaNO₃, and Na₂SO₄) reduce the micellar catalysis by excluding glycolic acid from the reaction site. The reactivity of glycolic acid towards chromium(VI) has been discussed and also compared with those obtained previously for the reaction between chromium(VI) and the reductants oxalic and lactic acids. On the basis of the observed results, probable mechanisms have been proposed.

Full text of DOI:10.1002/kin.1031

Micellar Catalysis on the
Redox Reaction of Glycolic
Acid with Chromium(VI)
KABIR-UD-DIN,1 KHALED HARTANI,1 ZAHEER KHAN2
1Chemistry Department, Aligarh Muslim University, Aligarh—202 002, India
2Chemistry Department, Jamia Millia Islamia, Jamia Nagar, New Delhi—110 025, India
Received 9 March 2000; accepted 26 January 2001
ABSTRACT: Chromium(VI) oxidation of glycolic acid in the absence and presence of cetyltri-
methylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) followed the same
mechanism as shown by kinetic study. The reaction followed second-order kinetics, first-order
in each reactant. The oxidation is strongly catalyzed by manganese(II) and cationic micelles
of CTAB or CPB. The catalytic effect of micelles can be fitted to a model in which the reaction
rate depends upon the concentration of both reactants in the micellar pseudophase. Some
added inorganic salts (NaCl, NaBr, NaNO3, and Na2SO4) reduce the micellar catalysis by ex-
cluding glycolic acid from the reaction site. The reactivity of glycolic acid towards chromium(VI)
has been discussed and also compared with those obtained previously for the reaction be-
tween chromium(VI) and the reductants oxalic and lactic acids. On the basis of the observed
results, probable mechanisms have been proposed. ᭧ 2001 John Wiley & Sons, Inc. Int J Chem
Kinet 33: 377–386, 2001
INTRODUCTION
strate between aqueous and micellar pseudophases is
generally written in terms of a Michaelis–Menten-
type equation [8,9].
Micelles are aggregates, containing hydrocarbon in-
teriors and polar surfaces, formed in aqueous solutions
by surfactants [1,2]. They provide different reactive
sites/microenvironments for different parts of reactant
molecules [3]. The hydrophobic nonpolar core is one
of the possible sites that can provide binding energy
for hydrophobic parts of the reactants. The micelle sur-
face (polar and usually charged) is another possible
site that can interact with the reactants’ polar groups.
Surfactants influence rate and equilibrium constants of
several chemical processes [4–6] because micelles
concentrate the reagents at the micelle–water inter-
face, that is, Stern layer [7]. Electrostatic and hydro-
phobic properties of the substrate are responsible for
the association/incorporation. The distribution of sub-
The incorporation of solutes into the micelles oc-
curs through the diffusion rates [10], and most thermal
reactions are too slow to perturbthis equilibrium trans-
fer. However, for very fast photochemical or electron-
transfer reactions, this generalization cannot be ap-
plied [11]. The study of electron-transfer reactions in
micellar systems can be considered as models to get
insight into electron transport occurring in biological
systems.
A large number of reports are available on the chro-
mic acid oxidation of organic acids with special focus
on the mechanisms and kinetics of the reactions [12].
Chromium(VI) oxidation of N-, O-, and S-containing
reductants in aqueous medium has been studied exten-
sively and reported from this laboratory [13–16]. De-
tailed micellar effect on the redox reactions of ␣-hy-
droxy acids and chromium(VI) has not been studied
Correspondence to: Kabir-ud-Din
᭧ 2001 John Wiley & Sons, Inc.

378
KABIR-UD-DIN ET AL.
so far except some reports by Panigrahi [17–19].
Therefore, we have initiated a systematic study on the
effect of organized assemblies (micelles) on the chro-
mium(VI) oxidation of ␣-hydroxy acids [20]. Srini-
vasan and his co-workers [21] have established that
involvement of chromium(IV) as an intermediate dur-
ing the reduction of chromium(VI) can be confirmed
by adding manganese(II). Thus, to understand the role
of both manganese(II) and surfactants on the chromic
acid oxidation of organic acids we present herein the
effect of both on the oxidation rate of glycolic acid.
The kinetics of chromic acid oxidation of glycolic acid
has not been reported so far in the presence of man-
ganese(II) and surfactants.
EXPERIMENTAL
Glycolic acid, potassium dichromate, manganese(II)
chloride, surfactants (cetyltrimethylammonium bro-
mide, cetylpyridinium bromide, and sodium dodecyl
sulphate) were the purest available commercial spec-
imens. Inorganic salts (sodium chloride, sodium bro-
mide, sodium nitrate, and sodium sulphate) were also
commercially available samples. Aqueous solutions
were prepared in double-distilled, deionized, CO2-free
water.
Figure
1 Absorption spectra of reaction mixture
Ϫ3
Ϫ3
([Cr(VI)] ϭ 8.33 ϫ 10 mol dm and [glycolic acid] ϭ
Ϫ3
0.25 mol dm ) after completion of the reaction at 50ЊC.
Set I: In the absence of surfactants, reaction time: 30 min
(᭺), 1 hr (᭹). Set II: In the presence of surfactants (10 ϫ
Ϫ3
Ϫ3
10 mol dm ) CTAB (᭺) and CPB (᭹), Reaction time:
1 hr.
A digital pH meter model LI-122 (ELICO) was
used for the measurement of pH during the course of
reaction. A Bausch & LombSpectronic-20 spectro-
photometer was used for absorbance measurements.
The procedure used in determination of reaction rates
by the spectrophotometric method was essentially the
same as described in previous paper [13–16,20].
lonitrile was used to detect the formation of free rad-
icals during the course of oxidation. White precipitate
3
was observed in presence of 3.8 mol dmϪ acryloni-
3
3
trile, 3 ϫ 10Ϫ mol dmϪ chromium(VI) and 3.3 ϫ
10Ϫ mol dmϪ3 glycolic acid.
2
The overall reaction may be represented as eq. (1):
2HCrO4Ϫ ϩ 2 CH2COOH ϩ 8Hϩ
RESULTS AND DISCUSSION
OH
Product Analysis
2Cr(III) ϩ HCOOH ϩ HCHO ϩ 2CO2 ϩ 7H2O (1)
Under the experimental kinetic conditions used herein
([glycolic acid] ϭ 0.3 mol dmϪ , [Cr(VI)] ϭ 2.8 ϫ
3
Determination of cmc by Conductivity
Measurement
10Ϫ mol dmϪ , temp ϭ 50ЊC), the reaction product
(Cr(III)) shows no absorbance in the UV-visible range.
Therefore, the spectrum of the reaction mixture was
recorded in the presence of a higher concentration of
chromium(VI), which is an absorbing species, at dif-
ferent time intervals (Fig. 1). The two d-d bands, as-
4
3
The conductivity measurements of surfactants (CTAB
and CPB) solutions were made by a conductivity
bridge 305 (Systronics, India) using ELICO-type CC-
03 cell (cell constant ϭ 1.02). The cmc values of both
the surfactants in the presence and absence of glycolic
acid and chromic acid have been determined at break
points of nearly two straight-line portions of the spe-
cific conductivity vs. concentration plots. The esti-
cribed to chromium(III), were observed at ␭
ϭ
max
410 and 570 nm (Fig. 1). Unlike Hamm et al. [22], we
found no shift toward shorter wavelength. Thus, it is
confirmed that chromium(III) aqua ion is the product
under our experimental conditions.
Other oxidation products (formic acid and carbon
dioxide) were also detected qualitatively [23]. Acry-
4
3
mated cmc values are 6.3 ϫ 10Ϫ mol dmϪ
4
3
(water ϩ glycolic acid), 6.5 ϫ 10Ϫ mol dmϪ
(water ϩ chromic acid ϩ glycolic acid) and 6.3 ϫ

MICELLAR CATALYSIS ON THE REDOX REACTION OF GLYCOLIC ACID WITH CHROMIUM(VI)
379
4
3
4
10Ϫ mol dmϪ (water ϩ glycolic acid), 5.7 ϫ 10Ϫ
mol dmϪ3 (water ϩ chromic acid ϩ glycolic acid), re-
spectively, for CTAB and CPB.
Table I Effect of Varying [Chromium(VI)] on the
Oxidation Rate of Glycolic Acid (0.3 mol dmϪ3) by
Chromium(VI) at 50ЊC in the Absence and Presence of
3
Cationic Micelles (10 ϫ 10Ϫ3 mol dmϪ
)
104[Chro-
108 ␥o
a
Ϫ
1
104 kobs/k (s )
Determination of Rate Constants
␺
Ϫ
3
mium(VI)]
(mol dm
Ϫ
3
Ϫ1
The reaction was followed by monitoring the decrease
in the [Cr(VI)] at 350 nm. Pseudo-first-order rate con-
(mol dm )
s )
Aqueous
CTAB
CPB
1.4
2.8
4.5
5.7
7.1
0.9
2.0
3.1
4.0
4.3
5.1
6.8
0.7
0.7
0.6
0.7
0.6
0.6
0.6
4.0
3.9
3.7
3.5
3.8
4.0
3.8
4.3
4.4
4.1
4.5
4.4
4.3
4.2
1
stants (kobs or k , sϪ ) were evaluated from linear plots
␺
(r Ն 0.994) of ln (Aϱ Ϫ A0)/(Aϱ Ϫ At) vs. times.
Dependence on [Oxidant]
8.5
11.4
The initial rate was directly proportional to the initial
concentration of chromium(VI) at constant [reductant]
a ␥o ϭ initial rate in aqueous medium (see Eq. (2)).
3
(ϭ0.30 mol dmϪ ) at fixed temperature of 50ЊC (Table
I). This confirms that the reaction is first-order with
respect to [oxidant]T , that is,
Ϫd[Cr(VI)]/dt ϭ kobs[Cr(VI)]T
(2)
Table II Effect of Varying [Glycolic Acid] and [Mn(II)]
on the Oxidation Rate of Glycolic Acid by
Dependence on [Reductant]
Chromium(VI) (2.8 ϫ 10Ϫ4 mol dmϪ3) at 50ЊC in the
The influence of the concentration of glycolic acid on
the reaction rate was studied at the other experimental
conditions remaining constant (Table II and Fig. 2).
3
Absence and Presence of Cationic Micelles (10 ϫ 10Ϫ
3
mol dmϪ
)
10[Glycolic
Acid]
(mol dm )
Ϫ
1
104 kobs/k (s )
␺
102 [Mn(II)]
(mol dm )
Ϫ
3
Ϫ3
Aqueous
CTAB
CPB
1.0
2.0
3.0
4.0
4.5
5.7
7.5
9.2
3.0
0.0
—
0.4
0.7
1.5
2.3
3.2
4.4
6.3
1.5
2.1
2.8
3.2
3.4
3.6
3.8
—
3.8
3.9
3.8
3.8
0.5
1.3
3.3
7.0
10.5
14.0
17.0
0.6
1.3
3.9
6.0
8.5
10.2
15.0
18.4
6.2
6.2
5.8
5.1
4.6
3.8
—
0.6
1.3
4.4
7.0
8.2
13.3
16.8
19.8
5.9
5.8
5.3
4.3
3.5
2.8
2.4
—
0.1
0.2
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
5.0
6.0
1.0
3.5
—
3.2
2.9
2.9
—
—
2.2
2.2
2.1
—
1.0
2.0
3.0
4.5
5.7
7.5
9.2
Figure 2 Variation of rate constants (kobs) with [glycolic
Ϫ3
acid] ([Mn(II)] ϭ 0.0 mol dm , ᭺) and [Mn(II)] ([glycolic
Ϫ3
acid] ϭ 0.3 mol dm , ᭹) for the oxidation of glycolic acid
Ϫ
Ϫ3
by chromium(VI) (ϭ2.8 ϫ 10 4 mol dm ) at 50ЊC.

380
KABIR-UD-DIN ET AL.
Linear variation of log kobs vs. log [glycolic acid] was
found with slope near unity (1.2, r ϭ 0.9995), which
indicates that the kinetic order for the glycolic acid is
unity.
Dependence on [Mn(II)]
The rate of disappearance of Cr(VI) was accelerated
in presence of Mn(II). The data are given in Table II
and Figure 2. The kobs values increase with the increase
in [Mn(II)], with a tendency to attain a limiting value
at high [Mn(II)]. The Burk–Lineweaver-type double-
reciprocal plot indicates association of the Mn(II) and
other reactants in some preequilibrium steps before the
electron transfer that also satisfies the Micaelis–Men-
ten reciprocal relationship (kinetic proof for complex
formation). Hence, a complex formation between
Mn(II) and reactive species of Cr(VI) and glycolic acid
occurs a priori.
Figure 3 Arrhenius plots for the oxidation of glycolic acid
Ϫ3
Ϫ4
(ϭ0.3 mol dm ) by chromium(VI) (ϭ2.8 ϫ 10 mol
Ϫ
3
Ϫ
3
Ϫ3
dm ) in aqueous (᭹), CTAB (10 ϫ 10 mol dm , ᭺)
and CPB (10 ϫ 10 3 mol dm , ٗ). Slope of the line equals
Ϫ
Ϫ3
Ea/R.
Dependence on Temperature
For determining activation parameters, the reaction
was studied at different temperatures (50–70ЊC), and
the data were found to follow the Arrhenius equation.
The energy of activation (Ea) was evaluated from the
slope of the ln kobs vs. 1/T plot (Fig. 3). The values of
Ea and corresponding activation parameters are shown
in Table IV.
The Mechanism. A detailed knowledge of the reac-
tive forms of the reactants is necessary for interpreta-
tion of the data. It was observed that the pH of the
reaction mixture containing chromium(VI) (ϭ2.8 ϫ
3
10Ϫ4 mol dmϪ ) and over the whole range of glycolic
3
acid (ϭ0.2 to 0.92 mol dmϪ ) remained constant
(Ϸ2.0). In this medium, the unionized form of gly-
colic acid is predominant and may be the reactive spe-
cies:
Dependence on [Surfactant] and [Salt]
The effect of both cationic and anionic surfactants
(CTAB, CPB, and SDS) on the observed rate constant
Ka
HO9CH2 9COOH E
RF
(k ) of the oxidation of glycolic acid by Cr(VI) was
␺
HO9CH2 9COOϪ ϩ Hϩ
(Ka ϭ 1.5 ϫ 10Ϫ )
4
3
studied at 0.3 mol dmϪ glycolic acid. The values of
k as summarized in Table III and graphically depicted
␺
With the decrease in [Hϩ] (increase in pH), the forward
reaction is favored, thereby increasing the concentra-
tion of HO9CH2 9COOϪ, which may not be a re-
active species.
in Figure 4 show an increase of nearly six-fold with
the increase in the cationic surfactant concentration
3
3
from 0.0 to 10.0 ϫ 10Ϫ mol dmϪ at 50ЊC. Further
increase in [surfactant] from 12.0 to 35.0 ϫ 10Ϫ3 mol
dmϪ3 resulted in a decrease of k .
In aqueous solution, various species of chromic
acid exist in equilibrium. The nature of these species
depends upon the pH and the concentration of potas-
sium dichromate [25]. At dilute solutions (as in the
␺
The cationic surfactants catalyses of the title reac-
tion are inhibited by added inorganic salts (NaCl,
NaBr, NaNO3 , and Na2SO4).1
Ϫ
present case), the monomeric (HCrO4 ) form predom-
Ϫ
inates. HCrO4 does not lose a proton until the pH is
1 In view of the following facts, changes in micellar shape and
size (that often come with increasing surfactant concentration and
salt addition) are ruled out under the conditions used herein; (a) the
raised to about 7, and it gains a second proton in the
pH Յ 1.0. In the present experimental conditions, the
3
3
highest [surfactant] used by us has been 35 ϫ 10Ϫ mol dmϪ
,
Ϫ
active oxidizing species is HCrO4 [26]. The proba-
whereas sphere-to-rod transition concentration for CTAB at 25ЊC is
2
Ϫ
bility of the existence of dimeric species (Cr2O7
)
3
3
270 ϫ 10Ϫ mol dmϪ [24], (b) our working temperature was
Ն50ЊC, which might check changes in micellar shape-size by salt
addition (increase in temperature works oppositely), and (c) viscos-
Ϫ
cannot be ruled out completely, but HCrO4 is more
2
Ϫ
reactive than Cr2O7
.
ity variations were small (all conditions remaining the same, ␩ ϭ
r
All the results mentioned above allow us to propose
the following mechanism:
1.02 (aqueous), 1.05 (with [CTAB] ϭ 40 ϫ 10Ϫ3 molϪ3), and 1.04
(with [CTAB] ϭ 40 ϫ 10Ϫ3 molϪ3, [NaNO3] ϭ 5 ϫ 10Ϫ2 mol dmϪ3)).

MICELLAR CATALYSIS ON THE REDOX REACTION OF GLYCOLIC ACID WITH CHROMIUM(VI)
381
O
Kes
HCrO4Ϫ ϩ HO9CH9COOH
ϪO9Cr9O9CH9COOH ϩ H2O
H
O
H
(A)
(B)
(C)
path I
path II
k1
kЈ1
O
и
Cr(III) ϩ CO2 ϩ CH2O
Cr(IV) ϩ HC9COOH
и
и
CH2O ϩ Cr(VI)
HCOOH ϩ Cr(V)
Cr(IV) ϩ B
Cr(III) ϩ CH2O ϩ COOH
Cr(V) ϩ CO2
и
COOH ϩ Cr(VI)
и
Cr(V) ϩ B
Cr(III) ϩ HCHO ϩ CO2
Scheme I
In Scheme I, paths I and II represent, repectively, a
one-step, three-electron and one-step, two-electron
transfer. Path II results in the formation of Cr(VI),
which can be demonstrated by the addition of Mn(II).
Generally, the addition will lead to a decreased oxi-
dation rate, because Mn(II) removes the Cr(IV)
formed. The observed positive catalytic effect (vide
supra) rules out the possibility of formation of Cr(IV)
in the rate-determining step. Thus, we may safely con-
clude that the oxidation of glycolic acid by Cr(VI)
follows a one-step, three-electron transfer mechanism
(path I). This is in agreement with the suggestions of
Haight et al. [27].
In the presence of Mn(II), the proposed mechanism
is modified as Scheme II:
KI
HO9CH2COOH Mn(II) ϩ Hϩ
ϩ
HO9CH2COOH ϩ Mn(II)
(C1)
O
KII
Ϫ
ϩ
C1 ϩ HCrO4
ϪO9Cr9O9CH29COO Mn(II) ϩ H2O
O
(C2)
k2
и
C2
Cr(III) ϩ CH2O ϩ CO2 ϩ Mn(III)
2Mn(III) ϩ 2H2O
MnO2 ϩ Mn(II) ϩ 4Hϩ
и
(radical CH2O follows the same reaction as shown in Scheme I)
Scheme II
To assess the role of Mn(III), some experiments
Direct reduction of Cr(VI) by Mn(II) is thermody-
namically unfavorable [29]. Therefore, the reaction
proceeds through the formation of a termolecular com-
plex with HCrO4 , glycolic acid, and Mn(II). This is
indicative of Michaelis–Menten behavior (vide su-
pra).
were performed where a 470 nm wavelength (char-
acteristic of Mn(III)) [28] was used to monitor the for-
mation of Mn(III) during the course of oxidation in
presence of Mn(II), but they failed to detect any build-
up of Mn(III).
Ϫ

382
KABIR-UD-DIN ET AL.
Table III Effect of Ionic Micelles on the Oxidation Rate of Glycolic Acid (0.3 mol dmϪ3) by Chromium(VI)
(2.8 ϫ 10Ϫ4 mol dmϪ3) at 50ЊC
Ϫ
1
Ϫ1
104 k (s )
104 kcal (s )a
(k Ϫ kcal)/k
␺ ␺
␺
Ϫ
3
103[Surfactant] (mol dm )
CTAB
CPB
SDS
CTAB
CPB
CTAB
CPB
0.0
1.0
2.0
0.7
0.6
1.2
2.5
3.9
3.7
3.4
3.5
3.5
3.2
0.7
1.0
1.4
2.5
4.4
4.3
3.6
3.4
3.2
3.0
0.7
0.7
0.8
0.7
0.8
0.7
0.7
0.7
0.8
0.7
—
0.56
1.0
2.3
3.6
3.8
3.4
3.2
3.7
3.0
—
—
—
0.0
1.0
1.2
2.3
4.3
4.3
3.3
3.1
3.3
3.0
ϩ0.07
ϩ0.16
ϩ0.08
ϩ0.08
Ϫ0.03
0.0
ϩ0.09
Ϫ0.06
ϩ0.06
ϩ0.14
ϩ0.09
ϩ0.02
0.0
ϩ0.08
ϩ0.09
Ϫ0.03
0.0
5.0
10.0
12.0
15.0
20.0
25.0
35.0
a Calculated from Eq. (5) using kw, km, KS and cmc values recorded in Table IV. The error limits presented in the next column show good
agreement between k and kcal
.
␺
The rate laws that can be derived from the proposed
mechanisms are:
Analysis of Observed Data:
k ؊ [Surfactant]
␺
The reaction of HCrO4Ϫ and glycolic acid is catalyzed
by cationic micelles of CTAB and CPB (Fig. 4), sug-
gesting that HCrO4Ϫ binds to cationic micelles despite
insufficient hydrophobic character. At higher concen-
trations of the surfactant, the acceleration was low. It
could perhaps be due to (1) competition between the
kobs ϭ k1Kes [glycolic acid]
(in the absence of Mn(II)) (3)
ϭ k2KIKII [glycolic acid][Mn(II)]
(in the presence of Mn(II)) (4)
Ϫ
bromide and HCrO4 on the micellar surface and (2)
segregation of the reactants in different micelles as
they are diluted in an increasing number of aggregates.
In aqueous solution, HCrO4Ϫ has one absorption max-
ima at 350 nm. A change in ␭max from 350 to 360 nm
4
3
was observed in the presence of Ն10Ϫ mol dmϪ
CTAB or CPB. This indicates binding between
HCrO4Ϫ and positive head groups of cationic micelles.
Ϫ
The binding constant (KS) of HCrO4 to the micelles
was determined spectrophotometrically. This method
requires that the substrate has a different absorbance
in water and in the micelles. An increase in absorbance
at 360 nm with an increase in [CTAB] or [CPB] from
3
3
0.0 to 9.0 ϫ 10Ϫ mol dmϪ was observed, and the
binding constants were calculated by Bunton’s method
[30], which were 1412 and 108 molϪ1 dm3 for CTAB
Ϫ
and CPB, respectively. The associated HCrO4 may
exist either inside the Stern layer or at the interface
between the micellar surface and the bulk solvent. In
the Cordes’ [31] review the activity of water at the
surface of ionic micelles is not very different from that
of the bulk solvent. In terms of these findings, it is
concluded that HCrO4Ϫ molecules exist at the micellar
surface rather than in the Stern layer, because the water
activity decreases with increasing distance from the
exterior to the interior region of the Stern layer.
Figure 4 Effect of [surfactant] (CTAB, ᭺; CPB, ᭹) on the
rate constant for the oxidation of glycolic acid by chro-
mium(VI). The conditions were the same as in Figure 2. Set
Ϫ3
I: without Mn(II); Set II: with Mn(II) (ϭ1.0 ϫ 10 mol
Ϫ
3
dm ).

MICELLAR CATALYSIS ON THE REDOX REACTION OF GLYCOLIC ACID WITH CHROMIUM(VI)
383
To see the effect of cationic surfactants on the ox-
Ϫ
idation path of the HCrO4 and glycolic acid, the re-
action was carried out under pseudo-first-order con-
Ϫ
ditions of [glycolic acid] ϾϾ [HCrO4 ] as a function
Ϫ
of [HCrO4 ], [Mn(II)], [glycolic acid], and tempera-
3
ture at constant [surfactant] (ϭ10 ϫ 10Ϫ mol
3
dmϪ ). The values are incorporated in Tables I and II.
The order of each in [Cr(VI)], [Mn(II)], and [glycolic
acid] is the same as observed in the absence of micelles
(Fig. 5). Although it has been shown that micelles can
cause a change in the aqueous reaction mechanism,
the aqueous- and micellar-mediated oxidation of gly-
Ϫ
colic acid by HCrO4 followed the same reaction
mechanism (Schemes I and II).
The oxidative cleavage of glycolic acid by
HCrO4Ϫ in the presence of micelles may be easily dis-
cussed in terms of the micellar pseudo-phase model.
The reaction scheme for the oxidation, in the presence
of cationic micelles, may be given as shown in Scheme
III.
Figure 5 Plots of rate constants vs. [Mn(II)] and [glycolic
acid] for the oxidation of glycolic acid by chromium(VI) in
Ϫ
3
Ϫ3
presence of CTAB (10 ϫ 10 mol dm , ᭺) and CPB
(10 ϫ 10 3 mol dm , ᭹). The conditions were the same as
in Figure 2. Rate acceleration is evident at quite low [Mn(II)]
(see inset).
Ϫ
Ϫ3
KS
(kw Ϫ k )
Ϫ
Ϫ
␺
(HCrO4 )w ϩ Dn ERF (HCrO4 )m
ϭ KS [surfactant] Ϫ KS ϫ cmc (7)
(k Ϫ km)
␺
kw
Ϫ
(HCrO4 )w 9
9: Products
The plot of (kw Ϫ k )/(k Ϫ km) vs. [surfactant] is lin-
␺
␺
km
ear for both the surfactants CTAB and CPB. However,
this equation is useful only to explain the rate con-
Ϫ
(HCrO4 )m 99: Products
Scheme III
stants in the steeply rising part of the k Ϫ
␺
[surfactant] profile because there is large uncertainty
Ϫ
in the value of left-hand side of eq. (7) when k Ϸ
where the subscripts w and m denote HCrO4 in
water and the micelles, respectively, and Dn
␺
kw or k Ϸ km.
ϭ
␺
The k Ϫ surfactant profiles show maxima, which
[surfactant] Ϫ cmc. The observed rate law, ␥ ϭ k
␺
␺
is characteristic of bimolecular reactions [32]. Hence,
the presence of glycolic acid at the surface of micelles
cannot be completely ruled out. The distribution of
glycolic acid in micellar and aqueous pseudo-phases
is as follows:
[Cr(VI)]T , and Scheme III yield eq. (5):
kw ϩ kmKS [Dn]
k ϭ
␺
(5)
1 ϩ KS [Dn]
Equation (5) can be rearranged to give eq. (6), which
may be used to calculate KS and km .
(HO9CH2 9COOH)w ϩ Dn
(A)w
KG
1
1
1
ERF (HO9CH2 9COOH)m (8)
ϭ
ϩ
(6)
(kw Ϫ k ) (kw Ϫ km) (kw Ϫ km)KS [Dn]
␺
(A)m
According to eq. (6), the plot of 1/(kw Ϫ k ) vs. 1/[Dn]
which can be fitted to a mass action model as
␺
should be linear. The plot of k vs. [surfactant] showed
␺
[(A)m]
KG ϭ
a maximum, implying that the model (one reactant into
the micellar phase [3]) is inadequate for the present
reaction. However, eq. (5) may also be modified to eq.
(7), which can be used to treat the data and cmc, and
KS can be calculated. The value of km is assumed to be
given by the value of k at the maximum of the k Ϫ
(9)
[(A)w] ([Dn] Ϫ [(A)m])
which can be written as
KG[(A)m]2 Ϫ (KG[Dn] ϩ KG[A]T ϩ 1) [(A)m]
␺
␺
[surfactant] profile.
ϩ KG [Dn] [A]T ϭ 0 (10)

384
KABIR-UD-DIN ET AL.
Table V Comparison between Oxidation Rates of
Different Reductants in Absence and Presence of
Cationic Micelles
We estimated the values of KG , KS , and km with the
help of a computer program (using cmc ϭ 6.5 ϫ
4
4
3
10Ϫ and 5.7 ϫ 10Ϫ mol dmϪ for CTAB and CPB,
respectively) developed for eq. (10); the values are
summarized in Table IV.
103 k
Ϫ
1
Ϫ1
(mol dm3 s )b
Ϫ
1
104 kobs/k (s )a
␺
It is widely accepted [12] that redox reactions of
alcohols, oxalic acid, and ␣-hydroxy acids with chro-
mium(VI) have the characteristic of forming ester-like
species. The stability and reducing properties of such
species depend upon the nature of the reductant as well
as the side-chain of the species C:
Aque-
Aque-
Reductant
COOH
ous CTAB CPB ous CTAB CPB
6.8 10.0
—
11.9 17.5
—
2.9 3.8
1.3 1.5
COOH
CH39CH9COOH
9.5 11.8 15.2 2.3
O
OH
H9CH9COOH
OH
ϪO9Cr9O9R
0.7
3.9 4.4 0.2
O
(C)
a The respective reductant, oxidant and surfactant concentrations
were 5.7 ϫ 10Ϫ2, 7.1 ϫ 10Ϫ4, 10.0 ϫ 10Ϫ4; 4.0 ϫ 10Ϫ1, 2.8 ϫ
3
O
10Ϫ4, 10.0 ϫ 10Ϫ4; 3.0 ϫ 10Ϫ1, 2.8 ϫ 10Ϫ4, 10.0 ϫ 10Ϫ mol
3
dmϪ
.
b The second-order rate constants, k, were obtained by dividing
the observed pseudo-first-order rate constants with the respective
[reductant].
(R ϭ 9(CH2)n 9CH3 ,
,
and
9C9COOH
9CH(R)9COOH
alcohols, oxalic acid, and ␣-hy-
droxy acids, respectively). A group which is electron-
donating will increase the ease of protonation of the
species but decrease the tendency of the chro-
mium(VI) species to accept electrons from the reduc-
ing agent, and electron-withdrawing groups have the
opposite effect. Therefore, the decreasing order
O
ment is due to the presence of 9COOH group despite
the electronegative character of this group, which
should have reduced the activity of the alcoholic group
towards oxidation. The reducing property is oxalic
acid Ͼ lactic acid Ͼ glycolic acid (Table V). We may
thus conclude that the presence of 9CH3 and 9H in
the case of lactic and glycolic acids, respectively, at
the site appears to inhibit, but not totally prevent, the
oxidation.
of side-chain (R) in species C is
Ͼ
9C9COOH
9CH(CH3)9COOH Ͼ 9CH(H)9COOH for ox-
alic, lactic, and glycolic acids, respectively. This has
been found to be the case (Table V). Thus, it may be
concluded that the presence of 9COOH (electron
withdrawing) moeity in the reductant increases the
tendency of chromium(VI) to accept the electron from
the side-chain of species C. The overall rate enhance-
Interestingly, the same sequence of reactivity of
these acids is found in the presence of cationic surfac-
tants. Chromium(VI) appears to form an ion pair with
the positive head groups of CTAB and CPB micelles.
Ϫ
Therefore, associated HCrO4 , due to the character-
Table IV Values of Different Parameters for the
istic of ester formation, may form a complex with the
reductant. As a result, effective concentrations of hy-
drophilic molecules (oxalic, lactic, or glycolic acids)
increase in the reactive region of the micelle. Presence
of 9CH3 in lactic acid may also help the acid to con-
centrate at the reaction site. Taking into account the
hydrophilic nature of the ␣-hydroxy acid (glycolic)
and the presence of micellar effect on its oxidation,
we are tempted to say that the reaction probably occurs
at interfacial region of the Gouy–Chapman and Stern
layers. Only a few reports have so far appeared that
reveal the occurrence of reactions at this junctural re-
gion [33–39].
Oxidation of Glycolic Acid by Chromium(VI) in the
3
Presence of Micelles (10 ϫ 10Ϫ3 mol dmϪ
)
Parameters
Aqueous
CTAB
CPB
5.7
104 cmc (mol dm )
6.5
52
29
1.3
2.3
Ϫ
3
Ϫ
KS (mol 1 dm3)
42
6
2.4
2.3
37.1
34.7
Ϫ
KG (mol 1 dm3)
Ϫ
1
103 km (s )
104 kw (mol 1 dm3 s )
Ϫ
Ϫ1
Ϫ
1
Ea (kJ mol )
69.5
67.0
Ϫ100.2
96.8
53.6
51.1
Ϫ170
100
⌬H# (kJ mol )
Ϫ
1
Ϫ
Ϫ1
⌬S# (J K 1 mol )
Ϫ203
100
⌬G# (kJ mol )
Ϫ
1

MICELLAR CATALYSIS ON THE REDOX REACTION OF GLYCOLIC ACID WITH CHROMIUM(VI)
385
Activation Parameters
coulombic attraction, whereas both specific and cou-
lombic interactions control the binding of halide ions
[43]. Therefore, SO4 has a much more profound ef-
fect than the BrϪ and ClϪ.
Comparison between the activation parameters in
CTAB and CPB micelles with that in aqueous medium
(Table IV) clearly indicates that the micelles act as
catalysts and provide a new reaction path that has a
lower value of Ea . The micellar structure is sensitive
to temperature. A complete account of all the factors
that influence ⌬H# and ⌬S# is not possible because the
2
Ϫ
Effect of Anionic SDS Micelles
To see the effect of anionic surfactant, the reaction of
3
glycolic acid (ϭ0.3 mol dmϪ ) and chromium(VI)
rate constant, k , does not represent a single elemen-
␺
4
3
(ϭ2.8 ϫ 10Ϫ mol dmϪ ) was also investigated at
tary kinetic step, as it is a complex function of kw , km ,
KS , and KG .
50ЊC in the presence of varying amounts of SDS. The
values of k were found to be constant with increasing
␺
[SDS] (Table III). It is not surprising, because there is
Ϫ
an electrostatic repulsion between HCrO4 and the
Effect of Salts
negative head groups of the SDS micelles. As a result,
Ϫ
HCrO4 exists in the bulk aqueous medium and the
Added salts generally reduce the reaction rate of bi-
molecular reactions [40,41] due to the exclusion of
reactants from the reaction site, that is, Stern layer
(where most of the micellar reactions are believed to
occur [42]). The salt effects have been found to depend
on the nature of the counterions to the micelle. In the
present case, the added salt effects generally follow
the expected pattern (Fig. 6). The inhibition in the re-
action rate by added NaCl, NaBr, NaNO3 , and Na2SO4
are caused by the exclusion of glycolic acid from the
rate remains unaffected.
CONCLUSION
For the redox reaction of glycolic acid with chro-
mium(VI), it is interesting to note that a micro amount
of manganese(II) is found to enhance the rate, which
also confirms the path of reduction of chromium(VI)
to chromium(III) without passing through chro-
mium(IV) as intermediate. Cationic micelles catalyze
the reaction, but anionic micelles produce no effect.
Obviously, electrostatic attraction/repulsion plays an
important role during the course of the reaction in the
presence of ionic surfactants. The catalytic effect of
CTAB/CPB micelles suggests that the reaction prob-
ably occurs at the interfacial region of the Stern and
Gouy–Chapman layers. Thus, our results join the
group of the limited number of reports that reveal the
occurrence of reactions at the Stern and Gouy–Chap-
man layers’ junctural region vis-a-vis inside the Stern
layer.
reaction site. Values of k depend upon the hydropho-
␺
bicity or decreasing polarizability of the ClϪ, BrϪ,
Ϫ
2
NO3 , and SO4 Ϫ, that is, they follow the sequence
2
Ϫ
Ϫ
2Ϫ
SO4 Ͼ NO3 Ͼ BrϪ Ͼ ClϪ (Fig. 6). SO4 should
interact with cationic micelles largely by nonspecific,
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