Biocompatible natural sugar-based surfactant assisted oxidation of citric acid by MnO4- in absence and presence of SDS

Article abstract of DOI:10.1039/c6ra04242b

Crocin, a natural carotenoid with antioxidant properties, was used in the present investigation as a surfactant in the citric acid-MnO₄^- redox system for the first time. A conventional UV-Visible spectroscopic technique was used to determine the reaction rate with and without crocin. The reaction follows first-order kinetics with respect to [citric acid] under pseudo-first conditions. Crocin was found to catalyze the redox reaction, which was rationalized in terms of the solubilization and/or incorporation of reactants into the crocin aggregates. The micellar catalysis was explained using Menger-Portony pseudo-phase model modified by Bunton et al. The various parameters associated with the micellar catalysis and activation parameters were determined and discussed. Sodium dodecyl sulphate (SDS) inhibits citric acid oxidation. A mixture of non-ionic and anionic surfactant (crocin + SDS) also shows the inhibitory effect rather than catalytic effect. In the mix micellization, the SDS characters and electrostatic repulsion, dominate over the solubilization of reactants in to the Stern layer. On the basis of the observed results, probable mechanisms and reaction sites were proposed.

Full text of DOI:10.1039/c6ra04242b

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Biocompatible natural sugar-based surfaꢀctant
assisted oxidation of citric acid by MnO4 in
absence and presence of SDS†
Cite this: RSC Adv., 2016, 6, 45993
*
Zaheer Khan, Shaeel Ahmad Al-Thabaiti and Maqsood Ahmad Malik
Crocin, a natural carotenoid with antioxidant properties, was used in the present investigation as
ꢀ
a surfactant in the citric acid–MnO4 redox system for the first time. A conventional UV-Visible
spectroscopic technique was used to determine the reaction rate with and without crocin. The reaction
follows first-order kinetics with respect to [citric acid] under pseudo-first conditions. Crocin was found
to catalyze the redox reaction, which was rationalized in terms of the solubilization and/or incorporation
of reactants into the crocin aggregates. The micellar catalysis was explained using Menger–Portony
pseudo-phase model modified by Bunton et al. The various parameters associated with the micellar
catalysis and activation parameters were determined and discussed. Sodium dodecyl sulphate (SDS)
inhibits citric acid oxidation. A mixture of non-ionic and anionic surfactant (crocin + SDS) also shows the
inhibitory effect rather than catalytic effect. In the mix micellization, the SDS characters and electrostatic
repulsion, dominate over the solubilization of reactants in to the Stern layer. On the basis of the
observed results, probable mechanisms and reaction sites were proposed.
Received 16th February 2016
Accepted 22nd April 2016
DOI: 10.1039/c6ra04242b
www.rsc.org/advances
biocompatibility, sugar-based surfactants have a variety of
potential applications.7 Surfactants based on natural resources
1. Introduction
have attracted considerable interest rather than petrochemical
precursors because of the global demand for biocompatible
materials. The excellent biodegradable nature of sugar-based
surfactants has gained much attention as they are derived
from renewable sources and are dermatological compatible.8,9
Sugar-based surfactants are a comparatively new class of
surfactants with their increased use owing to their precious
properties and nontoxic effects. These surfactants may occur in
nature and can be synthesised chemically.10,11 The micelle
concentrations (cmc) for nonionic sugar-based surfactants have
the same order of magnitude as generally shown by conven-
tional non ionic surfactants.12 The aggregation behaviour of
sugar based surfactants is dependent on the hydrophobic chain
length and the head-group structure. It has been found that if
the hydrophilic head group is smaller and the hydrophobic part
is larger than the cmc can be reduced.13 Different types of
anionic, cationic, and nonionic sugar-based surfactants have
been reported.14–16 Alkyl polyglucosides are sugar based surfac-
tants that have attracted considerable attention17,18 owing to
their non-toxic nature to the environment19 and remarkable
physical properties such as low surface tension.20 Naess et al.21
reported the surface and aggregation properties of carotenoid
crocin, a naturally occurring bolaform non-ionic surfactant by
UV-Vis spectroscopy and surface tension studies. Khan et al.
recently examined the self aggregation behaviour of crocin
(natural sugar based bolaamphiphile) and its micellization with
the cationic surfactant and cetyltrimethylammonium bromide
Saffron is the dried stigma of a ower scientically known as
“Crocus sativus Linn”, a plant with four principal bioactive
components, namely, crocin, crocetin, picrocrocin and safra-
nal.1 Other components are carbohydrates, mineral, mucilage,
vitamin (riboavin, thiamine), anthocyanin, carotene, lycopene
and zeaxanthin.2 The bioactive metabolites, such as crocin,
picrocrocin and safranal, are responsible for the colour, taste
and avour of saffron, respectively. Crocins (red colored water
soluble carotenoids), glycosides of crocetin are the main bio-
logically active metabolites of saffron.3 The extraction, separa-
tion and purication process of these metabolites from saffron
has been investigated and discussed comprehensively.4,5
Surfactants (surface active agents) are unique class of
chemical compounds that are omnipresent in modern society
with a wide range of fascinating applications.6 Sugar-based
surfactants are a type of amphiphilic materials having sugar
moieties in their chemical structure. The name of sugar-based
surfactants indicates that their hydrophilic part is a carbohy-
drate, generally mono or oligosaccharide. The incorporation of
such natural saccharide into the amphiphile structure
produced interesting physicochemical and biological func-
tionality. In addition to the biodegradability, nontoxicity and
Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203,
Jeddah 21589, Saudi Arabia. E-mail: maqsoodchem@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra04242b
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(CTAB) and anionic sodium dodecyl sulfate (SDS) surfactant.22 slopes of the tangents to the plots of log(absorbance) versus time
The permanganate oxidation process is eco-friendly and it is with a xed-time method for all kinetic experiments.25,26 The
one of the most versatile and vigorous oxidants used for the values of the average linear regression coefficient, g $ 0.998,
oxidation of organic and inorganic compounds in acidic and were obtained for each kinetic run.
micellar media.23–25 Manganese(VII) is reduced to Mn(II) during
the oxidation processes via many manganese species having
different oxidation states such as Mn(VI), Mn(V), Mn(IV) and
Mn(III). The appearance of these intermediate oxidation states
depends on various reaction conditions, types of substrate and
2.3. Products identication and stoichiometry
For identication of the oxidation products of citric acid,
[MnO4ꢀ] ¼ 5.0 ꢁ 10ꢀ3 mol dmꢀ3 was added in an aqueous
solution of [citric acid] ¼ 5.0 ꢁ 10ꢀ2 mol dmꢀ3 at room
their stability. The importance of a sugar-based surfactant and
temperature. The formation of CO2 was estimated quantita-
sodium dodecyl sulfate (SDS) in the micellar catalysis of citric
ꢀ
tively. The evolved carbon dioxide was ushed out by passing
a continuous current of pure N2 gas (free from O2 and CO2),
absorbed in standard Ba(OH)2 solution and then titrated with
a standard HCl solution.27 On the other hand, a saturated
solution of 2,4-dinitrophenylhydrazine in 2 N HCl was added to
the reaction mixtures. The formation of yellow residue suggests
that reaction product has a carbonyl group. The precipitate was
ltered and washed with ethanol. The same method was
repeated with known [3-oxopentanedioic acid]. The FT-IR
spectra were obtained and identied as the 2,4-dini-
trophenylhydrazone of 3-oxopentanedioic acid. The oxidation
product of crocin was conrmed as follows: a solution of
[MnO4ꢀ] ¼ (5.0 ꢁ 10ꢀ3 mol dmꢀ3) was added to an aqueous
solution of [crocin] ¼ 5.0 ꢁ 10ꢀ3 mol dm3. Aer completion of
the reaction, a saturated solution of 2,4-dinitrophenylhydrazine
in 2 M HCl was added to the reaction mixture and was le
overnight in a refrigerator. The yellow precipitate was ltered,
washed and dried. The –CHO group was identied by the infra-
red spectrum of the yellow precipitate, which showed carbonyl
stretching at 1725 cmꢀ1. It is well known that one –OH group of
primary alcohol oxidizes to one –CHO. It was further observed
acid–MnO4 reaction system was investigated. In the present
investigation, we examine the usefulness of antioxidant crocin
as
a sugar-based surfactant in citric acid oxidation by
permanganate.
2. Experimental and methods
2.1. Materials
Citric acid (99%, BDH), potassium permanganate (99%, BDH),
sodiumdodecyl sulphate (98%, Fluka), crocin (99%, Sigma
Aldrich) were used as received. Double distilled deionized water
(conductivity of (1–2) ꢁ 10ꢀ6 ohmꢀ1 cmꢀ1), previously subjected
to deionization, followed by distillation in alkaline KMnO4, was
used as a solvent for the preparation of stock solutions. A
solution of KMnO4 was standardized with sodium thiosulphate
and stored in an amber glass bottle.
2.2. Kinetic procedure
To determine the reaction rates, a series of kinetic runs were
performed under different experimental conditions (crocin +
MnO4ꢀ, citric acid + MnO4ꢀ, crocin + citric acid + MnO4ꢀ, crocin
+ citric acid + SDS + MnO4ꢀ, and citric acid + SDS + MnO4ꢀ) at
a xed temperature. In a typiꢀcal experiment, all required reac-
tants solutions except MnO4 were taken in a separate three
neck reaction vessel tted with a double walled condenser to
arrest evaporation and kept in a water bath at the desired
temperature to attain equilibrium for each kinetic run. Both
ꢀ
that the aldehyde does not undergo oxidation with MnO4
under the present kinetic conditions because the spot test for
carboxylic acid was negative.
3. Results and discussion
3.1. General considerations
reactants were thermostated separately at the desired temper- Crocin, a diester of disaccharide gentiobiose and the dicar-
ature before mixing. The required solution of MnO4ꢀ was added boxylic acid crocetin, is a natural sugar-based, highly unsatu-
to theꢀreaction vessel. The zero time was taken when half of the rated bola form bio surfactant, responsible for the color of
MnO4 solution had been added. The reaction volume was saffron, and forms an orange color in water (Fig. 1). The
always 50 cm3. The progress of the reaction was ꢀfollowed by oxidation kinetics of carbohydrates–MnO4ꢀ redox systems have
measuring the absorbance of the remaining MnO4 at denite been the subject of various investigations. In general, mineral
time intervals at 525 nm (lmax for MnO4ꢀ) on a UV-Visible acids were used as a source of [H+].28–30 Before attempting to
spectrophotometer (UV-260 Shimadzu, with 1 cm quartz discuss the role of crocin as a surfactant to the oxidation of
cuvette). Pseudo-rst-order conditions ([citric acid] $ [MnO4ꢀ]) citric acid by MnO4ꢀ, it is necessary to discuss the stability of
ꢀ
were used to determine the rate constants (kobs, sꢀ1). Duplicate the crocin with MnO4 (oxidant). Tondre and his co-workers
runs obtained results that were reproducible to within ꢂ5%. To suggested avoiding the use of even buffer solutions to main-
see the formation of water-soluble colloidal MnO2 as an inter- tain the pH of micellar solutions because the control of pH and
mediate, some kinetic experiments were also conducted at 410 ionic strength is not as straightforward in micellar solutions as
nm (permanganate has negligible absorbance at this wave- in ordinary solvents.31 We did not use any mineral acid to
length). The pH of the reaction mixture was also measured at control the pH of the working solutions.31 The pH was found to
the end of each kinetic experiment and the pH dri during the be constant with increasing [crocin] (ESI Table T1†). To gain
course of the reaction was very small (within 0.03 unit). The insight into the stability of crocin, a series of kinetic experi-
pseudo-rst-order rate constants were calculated from the ments were carried with different [crocin] (varying from 0.0 to
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Fig. 1 Most active constituents of saffron (Crocus sativus Linn).
20.0 ꢁ 10ꢀ4 mol dmꢀ3) at a xed [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4 mol increasing [crocin]. The plot of log kobs versus log[crocin] is
dmꢀ3, and temperature ¼ 20 C (ESI Table T1†). The observed linear with slope ¼ 0.99, indicating a rst-order dependence
ꢃ
results are depicted graphically in Fig. 2 as a kobs–[crocin] with [crocin] under our experimental conditions. The reducing
prole. The crocin oxidation rates were found to increase with nature of crocin might be due to the presence of gentiobiose
sugar moieties (two units of D-glucose joined with b1–6 linkage).
One polar head group of gentiobiose residue has one primary
–OH and 6 secondary –OH groups. It is well known that the
oxidation of secondary –OH is not possible under normal
reaction conditions.32 Thus, only primary –OH of gentiobiose is
ꢀ
responsible for the reduction of MnO4 (Scheme 1).
In Scheme 1, the reaction proceeds through the formation of
ꢀ
a complex between the gentiobiose unit of crocin and MnO4
.
In the next step of this reaction, which is the rate determining
step, the complex under goes one-step two electrons oxidation–
reduction mechanism leading to the formation of Mn(V)
(HMnO3) and the oxidation product of the gentiobiose unit, i.e.,
corresponding aldehyde.
ꢀ
3.2. Citric acid–MnO4 with crocin
To establish the role of [citric acid] and [MnO4ꢀ], a series of
kinetic experiments were performed for different [MnO4ꢀ] (3.3
to 6.6 ꢁ 10ꢀ4 mol dmꢀ3) at constant [citric acid] ¼ 16.6 ꢁ 10ꢀ3
mol dmꢀ3 and for different [citric acid] (from 0.0 to 20.0 ꢁ 10ꢀ3
mol dmꢀ3) at xed [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4 mol dmꢀ3 (ESI Table
T2†). The pseudo-rst-order rate constants are also depicted
Fig. 2 Plot of [crocin] versus kobs. Reaction conditions: [MnO4ꢀ] ¼ 6.6
ꢁ 10ꢀ4 mol dmꢀ3, temperature ¼ 20 ꢃC.
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ꢀ
Scheme 1 Oxidation of crocin by MnO4
.
Table 1 Values of the cmc and otherꢀpaarameters for the crocin-
graphically (Fig. 3). The effect of [citric acid] indicates that the
rst-order kinetics at low citric acid concentrations shis to
higher-order at higher concentrations. To gain insight into the
role of crocin (bola-forꢀm surfactant) on the oxidation kinetics of
the citric acid–MnO4 redox system, the effects of [crocin]
assisted oxidation of citric acid by MnO4
104cmc
Reaction conditions (mol dmꢀ3
)
Parameters Values
Crocin only
Crocin + MnO4
Crocin + citric acid
SDS only
SDS + MnO4
8.4
7.9
8.2
80.3
80.1
80.3
104kw
102kw
Kc
12.9 sꢀ1
8.0 molꢀ1 dm3 sꢀ1
50 molꢀ1 dm3
45 molꢀ1 dm3
9.2 sꢀ1
ꢀ
Ks
103km
ꢀ
SDS + citric acid
Activation parameters
Ea ¼ 32 (18) kJ molꢀ1
DH# ¼ 35 (21) kJ molꢀ1
DS# ¼ ꢀ87 (ꢀ130) J Kꢀ1 molꢀ1
DG# ¼ 61 (56) kJ molꢀ1
a
Reaction Conditions: [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4 mol dmꢀ3, [citric acid] ¼
16.6 ꢁ 10ꢀ3 mol dmꢀ3, and temperature ¼ 25 ꢃC. For the crocin
catalysed reaction, the activation parameters are given in the
parenthesis.
(varying from 0.0 to 20.0 ꢁ 10ꢀ4 mol dmꢀ3) in the presence of
xed [citric acid] ¼ 16.6 ꢁ 10ꢀ4 mol dmꢀ3, and [citric acid]
(varying from 0.0 to 20.0 ꢁ 10ꢀ4 mol dmꢀ3) at constant [crocin]
Fig. 3 Plot of [citric acid] versus kobs. Reaction conditions: [MnO4ꢀ] ¼
6.6 ꢁ 10ꢀ4 mol dmꢀ3, temperature ¼ 20 ꢃC.
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Table 2 Values of pH at different [citric acid], [crocin] and [SDS]
103 [SDS]
103 [citric acid]
104 [crocin]
(mol dmꢀ3
)
(mol dmꢀ3
)
(mol dmꢀ3
)
pH
0.0
0.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
4.0
8.0
10.0
15.0
20.0
0.0
3.3
0.0
3.3
5.0
0.0
0.0
0.0
0.0
0.0
0.0
5.0
10.0
15.0
17.5
5.0
5.0
5.0
5.0
5.0
6.7
3.2
2.9
3.3
3.4
3.3
3.4
3.3
3.4
3.2
3.2
3.4
3.4
3.4
3.3
3.4
5.5
10.0
16.6
16.6
16.6
16.6
16.6
16.6
16.6
16.6
16.6
16.6
Fig. 5 Plot of [crocin] versus kobs. Reaction conditions: [MnO4ꢀ] ¼ 6.6
ꢁ 10ꢀ4 mol dmꢀ3, [citric acid] ¼ 16.6 ꢁ 10ꢀ3 mol dmꢀ3, temperature ¼
20 ꢃC.
¼ 5.0 ꢁ 10ꢀ4 mol dmꢀ3, respectively, studied at [MnO4ꢀ] ¼ 6.6
ꢁ 10ꢀ4 mol dmꢀ3 and temperature ¼ 20 ꢃC (Table 2). The
observed results are summarized in Fig. 4 and 5. An inspection
of Fig. 4 indicates that the plot has an intercept on the y-axis,
whichꢀmight be due to the modest self oxidation of crocin by
MnO4 (Fig. 2). The rate constants increase rapidly with [cro-
cin]. The increase in kobs was observed even at [crocin] < cmc (¼
8.2 ꢁ 10ꢀ4 mol dmꢀ3).21 This fact is usually interpreted as the
reactants inducing micelle formation (surfactant molecules
start aggregating below cmc). The catalysis observed under the
experimental conditions is in agreement with similar bimolec-
ular reactions in the presence of a surface active agent.33,34 It is
further probable that crocin molecules display a certain cata-
lytic effect, which may be rationalized in terms of increasing
concentrations of the reactants (citric acid and MnO4ꢀ) in the
micellar pseudo phases through solubilization, incorporation,
and/or association with hydrophobic interactions. The mecha-
nism to the oxidation of citric acid by MnO4 is presented in
ꢀ
Scheme 2.
In Scheme 2, eqn (1) is a one-step two electrons transfer
oxidation–reduction mechanism (rate determining step), which
leads to the formation of Mn(V) and 3-oxopentanedioic acid.
Mn(V) is unstable, and is immediately converted to the stable
form of Mn, i.e., MnO2 (eqn (2)). Water soluble colloidaꢀl MnO2,
generally, formed as an intermediate in the MnO4 redox
reactions.24 Finally, colloidal MnO2 reacts with another mole-
cule of citric acid and formed Mn(II) as the reduction product of
ꢀ
MnO4
.
3.3. Probable role of crocin
Crocin is a bola-form non-ionic sugar based bio surfactant. The
rate enhancement of citric acid from 20.0 ꢁ 10ꢀ4 to 60.0 ꢁ 10ꢀ4
sꢀ1 with increasing [crocin] from 4.0 ꢁ 10ꢀ4 to 20.0 ꢁ 10ꢀ4 mol
dmꢀ3 (ca. 60 folds) clearly suggests that crocin aggregates,
micelles, solubilize the reactants into its small volume. It was
Fig. 4 Plot of [citric acid] versus kobs. Reaction conditions: [MnO4ꢀ] ¼
6.6 ꢁ 10ꢀ4 mol dmꢀ3, [crocin] ¼ 5.0 ꢁ 10ꢀ4 mol dmꢀ3, temperature ¼
20 ꢃC.
ꢀ
Scheme 2 Oxidation of citric acid by MnO4
.
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observed that the reaction rate increases with [crocin] (Fig. 3), the reactants decreases, which in turn increases the reaction
which clearly demonstrate the crocin catalytic effect not only rates. It is not possible to precisely locate the exact site for the
above but even below the cmc (i.e., micellar as well as pre- micelle-mediated reactions, but at least, localization of the
micellar catalyses are observed).35,36 It is well known that reactants can be considered.
A possible understanding
surfactant aggregates, micelles, change the reaction rates by (although highly schematic) could be that as shown in
incorporating the reactants into the micellar pseudo-phases. Scheme 3.
Various factors, such as the nature of the reactants, nature of
micelles, and interactions (hydrophobic, electrostatic, hydrogen
bonding, and van der Walls forces) are involved in the solubi-
lization processes.37 Micelles are a porous cluster with a rough
surface, deep-water-lled cavities, and water-rich surfaces, and
do not provide a uniform reaction medium.38 The water activity
in the Stern layer in no different compared to the bulk medium.
Micellar surfaces are water rich and the polarities of micelle-
water are lower than those of bulk water. Citric acid is water
solublꢀe. In the present case, the incorporation of citric acid and
MnO4 into the micellar palisade- and Stern-layer of crocin
micelles cannot be ruled out. The Stern-layer surface area of
crocin aggregates is higher due to the presence of two polar
gentiobiose sugar moieties at the head groups.21 As a result, the
chances of MnO4ꢀ and citric acid solubilization in the reaction
site, i.e., Stern-layer, increases.39,40 Therefore, the surface area of
3.4. Analysis of crocin catalyzed kinetic data
The catalytic behavior of crocin on the present study can be
explain in terms of the pseudo phase model of micelles
proposed by Menger and Portony41 and modied by Bunton
et al. (Scheme 4).34–36
kw½citric acidꢄ þ ðKskm ꢀ kwÞMCS½Dnꢄ
kobs
¼
(5)
1 þ Ks½Dnꢄ
where kw and km are the second-order rate constant (rst-order
rate constant/[(citric acid)w]) and second-order rate constant
(rst-order rate constant), respectively, in aqueous and micellar
pseudo phases. MSC ¼ mole ratio of citric acid bound to the
micellar head group (MSC ¼ [(citric acid)m]/[Dn]). To evaluate the
value of MSC, the following equilibrium is considered.
ꢀ
Scheme 3 Schematic model showing the probable reaction site for sugar-based surfactant mediated oxidation reaction for citric acid–MnO4
redox system.
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Scheme 4 Schematic of a micellar catalyzed reaction according to
the pseudo-phase model, where Ks, Kc, kw, km and [Dn] have their usual
significance. According to eqn (5) the following rate-law was derived
for the bimolecular reaction (both the reactants are considered to be
incorporated in the micellar phase) and Bunton considered the total
volume of the micelles as a separate phase.
Kc
*
ðcitric acidÞ þ Dn )
ðcitric acidÞ
(6)
(7)
w
m
Â
Ã
ðcitric acidÞ
m
Â
ÃÀ
Â
ÃÁ
kobs
¼
ðcitric acidÞ Dn ꢀ ðcitric acidÞ
w
m
Fig. 6 Plot of [SDS] versus kobs. Reaction conditions: [MnO4ꢀ] ¼ 6.6 ꢁ
10ꢀ4 mol dmꢀ3, [citric acid] ¼ 16.6 ꢁ 10ꢀ3 mol dmꢀ3, temperature
20 ꢃC.
Using eqn (8) and (9), a quadratic eqn (10) can be obtained,
which is solved for [(citric acid)m], with the help of a computer
programme (where Kc ¼ an adjustable parameter).42
Kc
to 66.6 ꢁ 10ꢀ3 mol dmꢀ3) at a constant [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4
mol dmꢀ3, [citric acid] ¼ 16.6 ꢁ 10ꢀ4 mol dmꢀ3, and temper-
ature 20 ꢃC. Interestingly, the reaction rate rst increases (from
12.9 ꢁ 10ꢀ4 to 22.1 ꢁ 10ꢀ4 sꢀ1) at lower [SDS] ¼ 3.3 ꢁ 10ꢀ3 mol
dmꢀ3 and then decreases with increasing [SDS]. At higher [SDS]
$ 16.6 ꢁ 10ꢀ3 mol dmꢀ3, the SDS has no signicant impact on
the reaction rates (remain constant). Fig. 6 clearly indicates that
the overall reaction rate decreases with increasing [SDS], which
might be due to the electrostatic repulsion between the negative
head group of SDS (–OSO3ꢀ) and MnO4ꢀ. Such type of repulsion
can not be ruled completely with –OSO3ꢀ and lone-pairs of citric
acid –OH and –COOH groups. Surprisingly, the crocin behav-
iour has been changed entirely from catalysis (Fig. 6) to inhi-
bition (Fig. 7) in the presence of SDS.
*
ðcitric acidÞ þ Dn )
ðcitric acidÞ
(8)
w
m
Â
Ã
ðcitric acidÞ
m
Â
ÃÀ
Â
ÃÁ
Kc ¼
(9)
ðcitric acidÞ Dn ꢀ ðcitric acidÞ
w
m
Kc[(citric acid)m]2 ꢀ (1 + Kc[Dn] + Kc[citric acid]T)[(citric acid)m]
+ Kc[Dn][(citric acid)T] ¼ 0
(10)
For the calculation, the cmc values of crocin were calculated
using the surface tension method under different experimental
conditions (Table 1). A nonlinear least-square technique was
used to determine Kc for which the value of Sdi2 turned out to be
a minimum and was taken as the best value of Kc (Table 1).
ꢀ
3.5. Citric acid–MnO4 with SDS and crocin + SDS
It has been established that SDS surfactant is unstable with
permanganate in an acidic solution of perchloric acid. The rst-
order kinetics with respect to [SDS] at low concentrations shif-
ꢀ
ted to second-order at higher concentrations. The –O–SO3
groupꢀwas responsible for the oxidative degradation of SDS by
43
MnO4
.
Therefore, the SDS stability is a crucial problem for
the kinetic experiments that we address rst. In the rst set of
experiments, a solution of SDS (5.0 cm3, 0.01 mol dmꢀ3) was
added to a solutioꢀn of MnO4ꢀ (5.0 cm3, 0.001 mol dmꢀ3) and the
spectra of MnO4 were obtained at different time intervals.
Interestingly, the absorbance of the reaction mixture remains
constant for ca. 2 h, indiꢀcating that SDS is not oxidized and/or
decomposed with MnO4 under our experimental conditions.
The shape and position of the MnO4ꢀ spectra also does not alter
in the presence of SDS. Thus, we may condently state that SDS
ꢀ
and MnO4 do not have any type of interactions due to elec-
trostatic repulsion.
Fig. 7 Plot of [SDS] versus kobs in presence of crocin. Reaction
conditions: [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4 mol dmꢀ3, [citric acid] ¼ 16.6 ꢁ
In the second set of experiments, a series of kinetic experi-
ments were performed in the presence of [SDS] (from 1.3 ꢁ 10ꢀ3 10ꢀ3 mol dmꢀ3, [crocin] ¼ 5.0 ꢁ 10ꢀ4 mol dmꢀ3, temperature ¼ 20 ꢃC.
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It is well known that pH shis by about plus and/or minus MnO4ꢀ is catalysed by the nonionic micelles of sugar based bola
two units are expected at the surface of CTAB and/or SDS crocin surfactant. This lowering occurs not only through the
micelles with respect to the bulk pH.44–46 However, a series of solubilization of both reactants (citric acid and MnO4ꢀ) into the
experiments were also performed to see any change in the polar region of aggregates, i.e., Stern-layer, but also through
macroscopic pH of the working solution in presence of different stabilization of the transition state (Scheme 2: eqn (1)).34 Thus,
[SDS], [citric acid], and/or [crocin]. The pH was found nearly Arrhenius and Eyring equations are applicable to the micellar
constant with increasing [SDS], [crocin], and [citric acid] (weak media, and the sensitivity of the micelle structure to tempera-
acid; pK1 ¼ 3.13, pK2 ¼ 4.76 and pK3 ¼ 6.39). The observed ture is kinetically unimportant.
values of pH are given in Table 2. This is not surprising because
ionic micelles show a marked difference in the effective local pH
to exist at its micellar surface over that in the bulk aqueous
4. Conclusions
solvent. The SDS aggregates concentrate hydrogen ions into the In this study, we demonstrate the use of sugar-based crocin as
Stern layer so that there is extensive build up of the non ionic a surfactant in the redox kinetics of citric acid by MnO4ꢀ. Crocin
species of citric acid. On the other hand, crocin is a neutral non- is unstable in the presence of MnO4ꢀ. Its reaction rate is
ionic species, which does not inuence the pH of the working negligible compared to citric acid. In the presence of crocin, the
solutions.
citric acid oxidation rate increases from 12.9 ꢁ 10ꢀ4 sꢀ1 to 62.9
Mahapatro et al. and Hasan and Rocek in their pioneering ꢁ 10ꢀ4 sꢀ1 (nearly 60 fold). The catalytic behaviour has been
study proposed a one step three-electron oxidation mechanism explained in terms of the incorporation and/or solubilization of
for a-hydroxy acids and suggested that the oxidation of alcohols reactants into the micellar aggregates through hydrogen
is accelerated by the presence of a carboxyl group (despite the bonding. SDS has an inhibitory effect on citric acid oxidation,
electronegative character of this group, which should reduce which might be due to the electrostatic repulsion between the
the activity of the alcoholic group towards oxidation).47,48 In negative head group of SDS and the reactants. The presence of
ꢀ
a reaction mixture containing MnO4 + crocin + citric acid, two surfactants (anionic SDS + non-ionic crocin) also inhibits
there is competition between crocin and citric acid to react with the reaction rate. During the whole processes, electrostatic
MnO4ꢀ rst. The rate constants were found to be 1.0 ꢁ 10ꢀ4 sꢀ1
,
forces play a more important role compared to hydrogen
12.9 ꢁ 10ꢀ4 sꢀ1 and 21.2 ꢁ 10ꢀ4 sꢀ1 for crocin (5.0 ꢁ 10ꢀ4 mol bonding. The activation energy increases and decreases with
dmꢀ3), citric acid (16.6 ꢁ 10ꢀ3 mol dmꢀ3), and crocin + citric SDS and crocin, respectively.
acid (5.0 ꢁ 10ꢀ4 mol dmꢀ3 + 16.6 ꢁ 10ꢀ4 mol dmꢀ3; ESI Tables
T1 and T2†), at [MnO4ꢀ] ¼ 6.6 ꢁ 10ꢀ4 mol dmꢀ3. The citric acid
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