Mixed anionic-nonionic micelle catalysed oxidation of aliphatic alcohol in aqueous medium

Article abstract of DOI:10.1016/j.molliq.2020.112655

Oxidation of isoamyl alcohol was carried out under pseudo 1st order reaction condition in aqueous medium by chromic acid. In addition to single micelle, mixed anionic-nonionic micelle (SDS-TX 100) was found to be effective catalyst. Promoters in presence of micelle catalyst showed almost million fold rate acceleration. The product was confirmed by IR and NMR study. UV and NMR analysis were carried out to establish the formation of mixed micelle. Formation of active oxidant was confirmed by fluorescence measurement. Interactions between surfactant and substrate were analysed by NMR spectra. In addition to SDS catalysed Bpy promoted reaction combination of mixed micelle with Bpy promoter was found to show higher rate of oxidation. For single micelle catalysed path the observed rate constants follow the order k_obs (SDS) > k_obs (TX 100) and k_obs (Phen) > k_obs (Bpy) > k_obs (PA) was observed for promoted reactions. In SDS micelle and mixed micelle k_obs (Bpy) > k_obs (Phen) > k_obs (PA) was observed while in TX 100 micelle k_obs (Phen) > k_obs (Bpy) > k_obs (PA) was found.

Full text of DOI:10.1016/j.molliq.2020.112655

Journal Pre-proof
Mixed anionic-nonionic micelle catalysed oxidation of aliphatic
alcohol in aqueous medium
Animesh Acharjee, Atanu Rakshit, Suman Chowdhury, Md.
Ansar Ali, Bula Singh, Bidyut Saha
PII:
S0167-7322(19)37129-6
DOI:
https://doi.org/10.1016/j.molliq.2020.112655
MOLLIQ 112655
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
26 December 2019
5 February 2020
6 February 2020
Please cite this article as: A. Acharjee, A. Rakshit, S. Chowdhury, et al., Mixed anionic-
nonionic micelle catalysed oxidation of aliphatic alcohol in aqueous medium, Journal of
Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112655
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2020 Published by Elsevier.

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Mixed Anionic-Nonionic Micelle Catalysed Oxidation of Aliphatic Alcohol in Aqueous
Medium
1
, 2
1
1
3
4
Animesh Acharjee , Atanu Rakshit , Suman Chowdhury , Md. Ansar Ali , Bula Singh , Bidyut
1
*
Saha
1
Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713104, India
Department of Chemistry, Hooghly Mohsin College, Chinsurah 712101, India
Department of Chemistry, Durgapur Government College, Durgapur 713214, India
Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India
2
3
4
*
Corresponding author: Dr. Bidyut Saha, Email: b_saha31@rediffmail.com
&
bsaha@chem.buruniv.ac.in
Abstract
st
Oxidation of isoamyl alcohol was carried out under pseudo 1 order reaction condition in
aqueous medium by chromic acid. In addition to single micelle, mixed anionic-nonionic micelle
(SDS-TX 100) was found to be effective catalyst. Promoters in presence of micelle catalyst
showed almost million fold rate acceleration. The product was confirmed by IR and NMR study.
UV and NMR analysis were carried out to establish the formation of mixed micelle. Formation
of active oxidant was confirmed by fluorescence measurement. Interactions between surfactant
and substrate were analysed by NMR spectra. In addition to SDS catalysed Bpy promoted
reaction combination of mixed micelle with Bpy promoter was found to show higher rate of
oxidation. For single micelle catalysed path the observed rate constants follow the order kobs
(SDS) > kobs (TX 100) and kobs (Phen) > kobs (Bpy) > kobs (PA) was observed for promoted
reactions. In SDS micelle and mixed micelle kobs (Bpy) > kobs (Phen) > kobs (PA) was observed
while in TX 100 micelle kobs (Phen) > kobs (Bpy) > kobs (PA) was found.
+
Keywords: Mixed micelle; catalysed; promoter; surfactant; active oxidant (AO ); rate constant.
Abbreviations:
CMC - Critical Micelle Concentration
SDS - Sodium dodecylsulphate
TX 100 - Triton X 100
Bpy - 2, 2´-Bipyridine
Phen - 1, 10-Phenanthroline
PA - 2-Picolinic acid/ Pyridine-2-carboxylic acid
1

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2
, 4-DNP - 2, 4-Dinitrophenylhydrazine
+
AO - Active Oxidant
UV - Ultraviolet
IR - Infrared
NMR – Nuclear magnetic resonance
TMS – Tetramethylsilane
st
kobs – Pseudo 1 order rate constant.
2

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1
Introduction
The revolutionary observation of McBain in 1913 had introduced a paradigm shift in the field of
chemical science [1]. At present micelle is efficiently used to carry out organic reactions in water
[
2-6]. Many organic reactions are reconstructed in aqueous micellar medium to eliminate toxic,
volatile organic solvents and to mould the process in a green envelop [7-15]. Moreover micelle
has a unique property of catalysing chemical reaction due to close proximity of reactants in
micelle core or stern region depending on substrate [16-31]. Micelles are formed by surfactants.
Actually at critical micelle concentration or maximum aggregation number surfactant molecules
assemble to form different nano-like aggregates such as spherical micelle, vesicles, lamellar,
unilamellar, rodlike, disklike, threadlike micelle etc. and significantly alter different
physiochemical properties of the solution such as surface tension, viscosity, ionic conductance,
refractive index, molecular adsorption, diffusion coefficient etc. [32-34]. So physiochemical
properties study of daily life inevitable surfactant solution is an important research topic [35-40].
Sometimes better performance in practical application is achieved by using mixed micelle
instead of single one due to synergistic effect of constituent blocks and effectively used in
emulsification, dispersion, detergency, cleaning, targeted drug delivery, synthesis of advanced
nano-material, cosmetics, wastewater treatment, oil recovery and food industries [41-49].
Properties of mixed micelle solution are significantly different from single micelle and hence
their physiochemical aspects are interesting research topic for last two decades [50-53]. Though
properties of the mixed micelle are well studied by different techniques such as viscosity
measurement, surface tension measurement, conductometry, capillary electrophoresis, NMR,
small angle neutron scattering, dynamic light scattering, fluorescence anisotropy measurements,
2
5
D- NOESY etc., the application of such system in the field of oxidation kinetics is limited [54-
8]. Herein we report for the first time effect of mixed micelle on the oxidation kinetics and
catalysis from our end.
Oxidation by Cr(VI) is well established and has been used for years [59]. In spite of toxic and
carcinogenic effect, preference of chromic acid as oxidant is due to its definite aqueous
chemistry in sulfuric acid medium and formation of kinetically non-labile Cr(III) on completion
of reaction, which can be used as nutritional supplement in definite form such as chromium(III)
picolinate [60-62]. It has been seen for years that chromic acid oxidation of different organic
3

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compounds are catalysed by micelle and the rate can further be enhanced by using some
heteroaromatic bases [63-71]. These bases during the course of reaction are transformed as
Cr(III)-base complex and hence better called as promoter instead of catalyst [72-73].
st
In this paper we have reported oxidation of isoamyl alcohol in aqueous medium under pseudo 1
order reaction condition by chromic acid at 30°C. Isoamyl alcohol on oxidation produces
isovaleraldehyde which is used as precursor of 2, 3-dimethyl-2-butene and butizide [74]. The
reaction is catalysed by anionic, nonionic and mixed anionic-nonionic micelle. Additional
increment in rate is obtained by using promoters such as Bpy, Phen and PA. A tremendous
increment of rate is observed in SDS catalysed Bpy promoted reaction and mixed SDS-TX 100
micelle catalysed and Bpy promoted reaction.
2
Experimental
2
.1 Materials and reagents
Isoamyl alcohol (99.0 %, AR, Merck, India), K Cr O (99.9 %, AR, BDH, India), H SO (98%,
2
2
7
2
4
AR, Merck, India), Sodium dodecylsulfate ( >90%, AR, Merck, India), Triton X-100 (98%, AR,
SRL, India), 2-Picolinic acid (99%, AR, Sigma Aldrich, UK), 2, 2´-Bipyridine (99%, AR,
Spectrochem, India), 1, 10-Phenanthroline (99.5%, AR, Merck, India), Deuterium oxide (D O)
2
(
99.9%, AR, Sigma-Aldrich, USA), Chloroform-d (CDCl ) (99.8%, AR, Sigma-Aldrich, USA).
3
-1
Double distilled water (conductivity 2 μS cm ) was used for the preparation of required
solutions. All the chemicals were used as received without further purification.
2
.2 Instrumentation and Kinetic measurement
A double beam UV–Visible spectrophotometer, SHIMADZU UV-1800 (Japan) fitted with
temperature controller TCC 240A was employed to follow the progress of reactions using quartz
cuvettes of cell length 1 cm. Mettler Toledo ME204 chemical balance having readability of 0.1
mg and digital ultrasonic cleaner CD 4820 were used for preparation of solutions. Specific
-1,
conductances were measured by Eutech Instrument (CybarScan PCD 6500, cell constant 1 cm
accuracy 0.5%). Steady state fluorescence measurements were carried on Hitachi FL -7000
Spectroflurometer. IR spectra were recorded by Perkin Elmer Spectrum Two FT-IR
spectrometer, JEOL 400 MHz and Bruker Ascend 400 MHz spectrophotometers were utilized
1
for H-NMR study.
4

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st
Under pseudo 1 order reaction condition kinetic data were collected by keeping concentration
of isoamyl alcohol excess (15 times) over the concentration of oxidant potassium dichromate in
-3
0
.40 mol dm H SO medium in absence and presence of surfactants (SDS, TX 100). All the
2 4
solutions were thermostated at 30°C (± 0.1°C) separately before the reaction and the oxidation
was initiated by mixing the required solutions. Promoters (PA, Bpy and Phen) were used in the
concentration range [oxidant] ≤ [promoter] ≥ [substrate] . The decrease in absorbance of
T
T
T
Cr(VI) at wavelength 450 nm was followed at regular time interval at 30°C. Each experiment
was carried out for three times. Completion of the reaction is indicated by appearance of pale
blue colour solution (unpromoted reaction) or pale violet colour solution (promoted reaction).
2
.3 Product analysis and stoichiometry
After completion of reaction solvent extraction with diethyl ether (thrice) followed by fractional
distillation resulted in two liquids with boiling points 132°C (isoamyl alcohol, literature value
1
31.5°C) and 93°C (isovaleraldehyde 92°C). 2, 4-DNP derivative was made with a small portion
of the lower boiling liquid in sulfuric acid medium [75]. The yellow coloured recrystallized
derivative showed melting point 123°C [75]. Product isovaleraldehyde was also confirmed by IR
1
and H- NMR spectral analysis.
st
As pseudo 1 order condition is employed so it is expected that Cr(VI) ion will be totally
reduced to Cr(III) at the end of reaction. The absorption maximum found at 580 nm is in
4
4
agreement with the existence of Cr(III) in the reaction mixture ( A (F) → T (F) ) [76-77]. The
2
g
2g
constant absorbances at 580 nm, over a period of three days after the completion of the reaction
in spectrophotometric titration with fixed concentrations of Cr(VI) and varying concentrations of
+
-3
isoamyl alcohol solutions at constant [H ] = 0.4 mol dm showed that 2 mol of chromic acid was
consumed by 3 mol of isoamyl alcohol (Equation 1).
+ 2 HCrO + 8 H+
3
3
HO
+ 2 Cr(III) + 8 H O
(1)
4
O
2
2
.3.1 IR spectral analysis
5

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IR spectrum of isovaleraldehyde shows characteristic carbonyl (C=O) stretching frequency of
-1
-1
aldehyde group at 1724 cm (Fig. 1(a)), its 2, 4-DNP derivative shows peaks at 1618 cm and
-1
3
293 cm for C=N and N–H symmetric stretching frequency respectively (Fig. 1(b)).
(a) (b)
Fig. 1 IR spectrum of (a) isovaleraldehyde and (b) 2, 4-DNP derivative of isovaleraldehyde
1
2
.3.2 H-NMR spectral analysis
1
H-NMR spectrum (Fig. 2) of isovaleraldehyde was recorded at 400 MHz in CDCl solvent.
3
Chemical shifts (ppm) were internally referenced to the TMS are reported in Table 1. Peak at
9
.39 ppm confirms the presence of aldehyde functionality.
1
Fig. 2 The H-NMR spectrum of isovaleraldehyde in CDCl solvent
3
6

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Table 1 Chemical shift values for different protons of isovaleraldehyde
Isovaleraldehyde
Atom
Chemical shift (δ ppm)
d
a
b
c
d
9.39 (t)
H a
1.96 (dd)
1.80 (m)
0.64 (d)
c
O
d
b
2
.4 Precautions
As Cr(VI) is highly toxic and carcinogenic, hence special precaution been taken so that no
Cr(VI) is discarded through drainage. Due excessive usages of surfactants in different sectors
presently surfactant pollution becomes alarming, surfactants containing waste water discharge to
the water bodies causing serious harm to the aquatic animals and plants [78-79]. Hence, the used
surfactants were recycled after the completion of the reaction to eliminate such effect from
environment [21]. SDS was recycled using CaCl and precipitated as Ca-salt whereas TX 100
2
was separated using thermal method of precipitation [21].
3
. Result and Discussion
As previously mentioned, the progress of reactions was followed spectrophotometrically by
observing the decrease in absorbance (A ) of Cr(VI) at 450 nm at regular time interval (Fig. 3).
4
50
st
From the slope of the plot -ln(A ) vs. time in second pseudo 1 order rate constants (k ) were
4
50
obs
calculated and half life for each reaction was evaluated from the relation t1/2 = 0.693/ kobs (Fig.
S1), (Table 2). It is clearly seen in table 2 that micelle catalysed and promoted reactions
complete in much shorter time than uncatalysed reaction. Evidently the final species in
uncatalysed and micelle catalysed reaction is Cr(III) (Fig. S2) [80-81]. In promoted reaction the
final species is different from pure Cr(III), it is expected that Cr(III)-promoter complex are
4
4
formed [66]. Here, A (F) → T (F) transition shows a hypsochromic shift (λ = 546, 552
2
g
2g
max
and 565 nm for Bpy, Phen and PA-promoted reactions) due to the presence of strong field hetero
aromatic ligands. A single isobestic point in each reaction suggest that the probability of
formation of intermediate species like Cr(V) and Cr(IV) is of very low concentration (fig. S3)
[
82].
7

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0
0
0
0
0
0
.100
.080
.060
.040
.020
.000
5
min interval
3
50.00
400.00
500.00
nm.
600.00
650.00
Fig. 3 Scanned absorption spectra of uncatalysed reaction at regular time interval (5min):
-4
-3
-4
-3
-3
[
Isoamyl alcohol] = 75×10 mol dm , [Cr(VI)] = 5×10 mol dm , [H SO ] = 0.40 mol dm ,
T T 2 4
Temperature = 30°C.
Table 2 kobs and half life of the reaction in presence and absence of promoter and micellar
catalysts.
-3
-3
-3
4
4
-1
SDS [mol dm ] TX 100 [mol dm ] Promoter [mol dm ] ×10
10 × k [s ] t [h]
obs 1/2
3
3
×
10
× 10
None
None
None
None
None
0.209 ± 0.001
0.423 ± 0.003
0.479 ± 0.005
0.496 ± 0.005
0.589 ± 0.007
0.639 ± 0.006
0.321 ± 0.004
0.333 ± 0.004
0.365 ± 0.003
0.477 ± 0.002
0.525 ± 0.007
0.271 ± 0.001
0.480 ± 0.003
0.556 ± 0.003
0.651 ± .0004
0.739 ± 0.004
9.21
4.55
4.01
3.88
3.26
3.01
5.99
5.78
5.27
4.04
3.66
7.10
4.01
3.46
2.95
2.60
1
2
3
4
5
0
0
0
0
0
None
10
None
None
2
3
4
5
0
0
0
0
1
2
3
4
5
0
0
0
0
0
10
8

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1
0
20
None
0.285 ± 0.004
0.304 ± 0.005
0.336 ± 0.007
0.392 ± 0.005
0.410 ± 0.005
0.840 ± 0.009
1.186 ± 0.006
1.490 ± 0.013
1.058 ± 0.009
3.783 ± 0.030
7.526 ± 0.070
10.80 ± 0.061
0.909 ± 0.007
3.296 ± 0.010
5.094 ± 0.040
7.604 ± 0.015
0.802 ± 0.006
2.151 ± 0.010
3.423 ± 0.023
4.569 ± 0.030
0.630 ± 0.006
1.918 ± 0.021
3.031 ± 0.024
4.624 ± 0.050
0.617 ± 0.007
1.612 ± 0.010
2.790 ± 0.025
4.010 ± 0.070
0.852 ± 0.001
3.062 ± 0.009
7.058 ± 0.060
1.232 ± 0.011
4.991 ± 0.060
10.60 ± 0.055
16.10 ± 0.020
0.997 ± 0.001
4.362 ± 0.010
6.360 ± 0.027
9.094 ± 0.050
28.40 ± 0.070
191.2 ± 0.100
1.181 ± 0.016
5.312 ± 0.020
10.10 ± 0.091
6.75
6.33
5.73
4.91
4.69
2.29
1.62
1.29
1.82
0.51
0.25
0.18
2.11
0.58
0.38
0.25
2.40
0.89
0.56
0.42
3.05
1.00
0.64
0.42
3.12
1.19
0.69
0.48
2.26
0.63
0.27
1.56
0.39
0.18
0.12
1.93
0.44
0.30
0.21
0.07
0.01
1.63
0.36
0.19
3
4
5
0
0
0
None
None
2-Picolinic acid
5
2
5
7
5
2
5
7
5
2
5
7
5
2
5
7
5
2
5
7
5
2
5
7
5
2
5
5
2
5
7
5
2
5
7
5
2
5
2
5
5
0
5
1
2
, 10-Phenanthroline
, 2´-Bipyridine
5
0
5
5
0
5
1
0
None
10
2-Picolinic acid
2-Picolinic acid
2-Picolinic acid
5
0
5
None
5
0
5
1
1
0
0
10
5
0
5
None
10
1, 10-Phenanthroline
1, 10-Phenanthroline
5
0
None
5
0
5
1
0
10
1, 10-Phenanthroline
5
0
5
1
0
None
10
2, 2´-Bipyridine
2, 2´-Bipyridine
5
None
5
0
9

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7
5
2
5
7
5
15.80 ± 0.010
6.355 ± 0.009
21.10 ±0.010
40.80 ± 0.030
68.00 ± 0.090
0.12
0.30
0.09
0.05
0.03
1
0
10
2, 2´-Bipyridine
5
0
5
-4
-3
-4
-3
-3
[
Isoamyl alcohol] = 75×10 mol dm , [Cr(VI)] = 5×10 mol dm , [H SO ] = 0.40 mol dm ,
T
T
2
4
Temperature = 30°C.
Data in table 2 suggest that Phen promoted reactions are faster than other two promoters, while
SDS shows highest rate among the micelle. Bpy promoted SDS catalysed reaction is the fastest
one and it is so fast that at higher concentration of Bpy the reaction cannot be studied. Mixed
micelle shows an intermediate rate of oxidation but with increase in concentration rate increases
and combination of mixed micelle-Bpy is suitable one.
3
.1 Mechanism of the reaction.
Product analysis shows that isovaleraldehyde is the product of chromic acid oxidation of isoamyl
alcohol. The mechanism of the reaction can be divided into four categories (i) unpromoted and
uncatalysed path, (ii) promoted path, (iii) micelle catalysed path and (iv) micelle catalysed
promoted path.
3
.1.1 Unpromoted and uncatalysed path
-
Chromic acid in aqueous medium predominately exists as HCrO (B) as the protonation constant
4
st
of B is very low [83]. Under pseudo 1 order reaction condition this species reacts reversibly
with the substrate to form neutral chromate ester (1). In the subsequent steps the so formed
neutral ester undergoes acid catalysed decomposition to form isovaleraldehyde and Cr(III)
(Scheme S1).
3
.1.2 Promoted path
Rate of reactions in promoted path are greater than unpromoted one. The observed rate constant
values vary in the order k (Phen) > k (Bpy) > k (PA) (Fig. 4). Completion spectra of
obs
obs
obs
promoted reactions indicate that some Cr(III)-promoter complexes are formed at the end of the
1
0

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reaction (Fig. S2). Cr(III) is kinetically inert, so such type complex formation is difficult. Hence,
+
we may expect that during the reaction at first Cr(VI)-promoter complex (AO ) is formed which
then reduced to Cr(III)-promoter complex. In fact, absorption and emission spectral analysis with
Cr(VI) and promoters suggest such type of complex formation (Fig. S4 and S5). Previous NMR
analysis also supports Cr(VI)-promoter complex formation [84]. In the proposed mechanism
(Scheme 2, 3) active oxidant forms a neutral ester which undergoes acid catalysed decomposition
to the product. The observed rate variation with different promoters can be explained if we
consider the formation of active oxidants. All the three-promoter used here are chelating agents
with N, N or N, O donors and can form chelated complex with Cr(VI). Rate of formation of
+
active oxidant (AO ) is higher in case of Phen as in 1, 10-Phenanthroline both the donating N
atom are always cis with three stable aromatic ring. This may be attributed to the enhanced rate
of oxidation in case of Phen. Stable conformer for 2, 2´-Bipyridine is s-trans and hence an
activation barrier is there to change the conformation to s-cis prior to the formation of active
oxidant. This may results a relative reduction in the rate of reaction. 2-picolinic acid is depriving
of more aromatic stabilization (in Bpy two aromatic rings and in Phen three aromatic rings) and
the presence of more electronegative O atom may affect the rate.
Fig. 4 Dependence of kobs on [Promoter] for Cr(VI) oxidation of isoamyl alcohol (in absence of
T
-4
-3
surfactant) in aqueous H SO media at 30°C. [Cr(VI)] = 5×10 mol dm , [H SO ] = 0.4 mol
2
4
T
2
4
-3
-4
-3
dm , [Isoamyl alcohol] = 75×10 mol dm .
T
1
1

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3
.1.3 Micelle catalysed path
Micelle catalysed path can be subdivided in to two categories (i) single micelle catalysed path
and (ii) mixed micelle catalysed path.
3
.1.3.1 Single micelle catalysed path
Both SDS and TX 100 used here are at post CMC and the observed rate constants follow the
order k (SDS) > k (TX 100) (Fig. 5). To explain this trend we must look at the mechanism
obs
obs
part (Scheme S4), in proposed mechanism once again a neutral ester is formed which undergoes
acid catalysed decomposition to form the product. Unlike the unpromoted path here in presence
of micelle, to facilitate binding of Cr(VI) through ion-dipole interaction and H bonding
interaction to the micellar surface the active species of chromic acid is H CrO [83]. UV
2
4
spectroscopic analysis of Cr(VI)-micelle system also suggests a definite interaction between
Cr(VI) and micelle surface (stern layer) as broadening of Cr(VI) absorbance band occurs with
red shift of λmax (Fig. 6) [85]. Interaction between substrate (alcohol) and micelle was also
investigated by NMR. Modulated chemical shifts of surfactant in presence of substrate strongly
suggest a definite interaction between them (Fig. S6). Upfield chemical shifts of head group
region protons of either surfactant in presence of alcohol indicates that polar -OH group of
alcohol orients itself toward the stern layer of micelle, while hydrophobic chain buried in
palisade layer. The upfield shift may be due to the increased electron density by the presence of
alcoholic -OH group in vicinity of head group protons of surfactant. The upfield shift of other
protons can be explained considering hydrophobic interaction between hydrocarbon chains of
alcohol and surfactant [21]. The observed greater slope of rate constant in SDS can be explained
+
considering the acid catalysed decomposition of neutral chromate ester. H can approach neutral
ester present in the negatively charged SDS micelle with greater ease (attractive interaction)
compare to neutral TX 100. This may lead to observed higher rate in SDS micelle (Scheme S5).
1
2

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Fig. 5 Dependence of kobs on [Surfactant] for Cr(VI) oxidation of isoamyl alcohol (in absence of
T
-4
-3
promoter) in aqueous H SO media at 30°C. [Cr(VI)] = 5×10 mol dm , [H SO ] = 0.40 mol
2
4
T
2
4
-3
-4
-3
dm , [Isoamyl alcohol] = 75×10 mol dm .
T
1
1
0
0
.500
.000
.500
.000
(
(
(
(
i) Cr(VI)
ii) Cr(VI) + TX 100
iii) Cr(VI) + SDS + TX100
iv) Cr(VI) + SDS
(
iv)
(
iii)
(
ii)
(i)
2
00.00
300.00
400.00
nm.
500.00
600.00
1
3

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Fig. 6 UV-Vis study of interaction between Cr(VI) and different surfactant above CMC.
-4
-3
-3
-3
[
Cr(VI)] = 5×10 mol dm , [SDS] = [TX 100] = 20×10 mol dm in aqueous media at 30°C.
T T T
3
.1.3.2 Mixed micelle catalysed path
UV spectrum of SDS + TX 100 with Cr(VI) suggests that in solution they do not exist as discrete
unit rather a mixed micelle may form (Fig. 6). In fact, UV spectral analysis at 276 nm (λ max of
TX 100 absorbance) with varying concentration of TX 100 and fixed post CMC of SDS suggest
a definite interaction between them as hypochromic shift is observed (Fig. 7). We further
determine CMC of SDS in presence of TX 100 with constant mole fraction (0.5) of both the
surfactants by conductometry method. CMC was found to be 0.15 mM, which is much lower
than CMC of SDS (8.2 mM) and higher than CMC of TX 100 (0.24 mM) (Fig. S7) but is
consistent with literature value [58, 86]. This observation once again suggests that discrete single
micelle is not present in the solution rather true mixed micelle is present. NMR study with 1:1
surfactant solution shows following type of modulated chemical shifts with δ values for SDS are
shifted to upfield while that of TX 100 are downfield (Table 3) (Fig. 8 and S8). This observation
is quite similar in trend with available data in literature [55]. So, we may conclude in present
case too that methylene group of SDS (alpha protons) are situated near the phenoxy ring of TX
1
00 in mixed micelle, the hydrophobic part of SDS does not extended much into the micellar
core while hydrophilic part of TX 100 remain coiled in the mixed micellar solution.
0
0
0
0
0
.800
.600
.400
.200
.000
TX 100 (0.01- 0.5) mM
TX 100(0.01- 0.5) mM + SDS 0.01M
CTX 100
2
50.00
260.00
270.00
nm.
280.00
290.00 295.00
1
4

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Fig. 7 Interaction between TX 100 and SDS micelle at 30°C in aqueous medium, [SDS] = 0.01
T
-3
mol dm , [TX 100] = 0.01- 0.5 mM.
T
1
Table 3 H chemical shifts (ppm) of mixed SDS and TX 100 in D O.
2
α - β-
Bulk Termina a-H’s b-
c-
d-H’s e-H’s f-
g-
h-
H’s H’s -H’s l-H’s
H’s
H’s
H’s H’s H’s
SDS(0.01M)
3.91 1.54 1.15 0.735
SDS (0.01M) + 3.81 1.45 1.05 0.730
TX100(0.01M)
0.565 1.52 1.15 7.055 6.655 3.92 3.66 3.45-
3.55
TX
0.535 1.47 1.11 7.05 6.65 3.85 3.62 3.4-
3.6
1
00(0.01M)
-
+
ONa
O S
c
O
a
Terminal

d
O
e

f
h
b
O
O
(CH CH O) H
2 2 8
Bulk
g
1
Fig. 8 Selected region of SDS-TX 100 interaction HNMR.
1
5

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The observed rate with 1:1 SDS-TX 100 mixed micelle is somewhat lower than that observed in
single micelle catalysed reaction (Table 2). With increase in concentration of either the surfactant
keeping the other surfactant constant at post CMC showed rate increment and at higher
concentration rate are comparable to that of single micelle catalysed reaction (Fig. 9). The
mechanism of reaction here is same as that of single micelle catalysed path. The observed result
can be explained if we consider the above mentioned fact with observation that in mixed micelle
of SDS-TX 100 at first TX 100 micelle is formed then with increase in total micelle
concentration SDS micelle shows aggregation behavior [58]. As TX 100 micelle is populated by
SDS at lower concentration so available space for alcohol to penetrate into micelle is reduced
(steric factor) and hence a slow rate is observed. With increase in concentration of either the
surfactants single micellar aggregate may form in addition to the mixed micelle, resulting higher
rate of oxidation. NMR investigation of mixed micelle (1:1) alcohol interaction shows hardly any
change in chemical shift values. This suggests that alcohol could not effectively penetrate the
mixed micelle at specified concentration to modulate the chemical shift values (Fig. S9).
Fig. 9 Dependence of kobs on mixed micelle for Cr(VI) oxidation of isoamyl alcohol (in absence
-4
-3
of promoter) in aqueous H SO media at 30°C. [Cr(VI)] = 5×10 mol dm , [H SO ] = 0.40 mol
2
4
T
2
4
-3
-4
-3
dm , [Isoamyl alcohol] = 75×10 mol dm .
T
1
6

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3
.1.4 Micelle catalysed promoted path
We have already seen the order of rate constant in promoted path with reasoning and the order is
k (Phen) > k (Bpy) > k (PA). Here in the micelle catalysed promoted path the mechanism is
obs
obs
obs
+
same as that of promoted path and the observed rate depends on how easily active oxidant (AO )
reacts with substrate present in micelle to form chromate ester (Scheme S6). In TX 100 micelle
catalysed path there exists pi-pi interaction in between aromatic sextet of TX 100 and the active
oxidants. This facilitates congregation of active oxidants in stern layer leading to higher rate of
oxidation. The number of pi electrons is highest in Phen so it shows higher rate followed by the
other two. So the observed rate follow the order k (Phen) > k (Bpy) > k (PA) in TX 100
obs
obs
obs
medium (Fig. S10a). In SDS medium there is no pi-pi interaction present, rather attractive
electrostatic interaction is present between positively charged active oxidant and negatively
charged SDS micelle. The active oxidant formed by Phen and Bpy are bipositive while that of
PA is unipositive. So, considering the attractive interaction Phen and Bpy promoted reaction
should show same rate followed by PA promoted one (k (Bpy) ≈ k (Phen) > k (PA)). Beside
obs
obs
obs
+
attractive interaction there exists repulsive interaction between pi electron cloud of AO and
negative charge surface of SDS micelle. Pi electron density is highest in Phen and Lowest in PA.
So the repulsive interaction is highest in Phen promoted reaction and lowest in PA promoted
reaction. Hence considering repulsive interaction k (PA) > k (Bpy) > k (Phen) should be
obs
obs
obs
observed. The actual order of rate constant in SDS micelle is k (Bpy) > k (Phen) > k (PA),
obs
obs
obs
can be explained considering both the attractive and repulsive interaction simultaneously (Fig.
S10b). The observed order of rate constants k (Bpy) > k (Phen) > k (PA) in mixed micellar
obs
obs
obs
system can be explained considering above discussed factors with steric factor discussed in the
mixed micelle catalysed path (Fig. S10c.). Variation of micellar system for a fixed promoter is
shown in fig. S 11.
4
Conclusions
In this study we have presented a green approach of alcohol oxidation with utilisation of mixed
micelle and single micelle. Use of promoters in combination with micellar catalyst completes the
reaction almost instantly. In addition to SDS-Bpy combination mixed SDS-TX 100 micelle in
combination with Bpy found to be efficient catalyst. Use of relatively low cost mixed micelle
may open new avenue and suppose to be beneficial for industry [58]. In future such type of study
1
7

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with cationic-neutral and anionic-cationic micelle may lead to some new findings. Beside the
effect of mixed micelle on oxidation some physiochemical properties study such as shape and
size of mixed micelle system may be of immense importance for scientific community.
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Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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Graphical Abstract
2
5

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Highlights
1
2
3
. Greener efficient method of oxidation of isoamyl alcohol.
. SDS catalysed reactions are faster than TX 100 catalysed reactions.
. Mixed micelle (SDS + TX 100) found to be efficient catalyst.
4
5
. Rate constants in promoted path follow the order, kobs (Phen)> kobs (Bpy)> kobs (PA).
. SDS catalysed and Bpy promoted reaction showed million folds rate acceleration.
2
6