Quantitative analysis of dye decolourization reactions in mixed micellar systems of sodium dodecyl sulfate with tween-20, tween-80, and triton X-100

Article abstract of DOI:10.1071/CH11454

The reaction of triphenylmethane dye (ethyl violet) with hydroxyl ion has been investigated in absence and presence of micelles. In micellar solutions, the solubilization of dye carbocation is observed. The reaction rate constant follows pseudo-first order kinetics with respect to the nucleophile. In presence of sodium dodecyl sulfate (SDS) micelles, an inhibitory effect is observed due to repulsion of the nucleophile to the strongly bound dye carbocation in the negatively charged SDS aggregate. The presence of nonionic surfactant reduces the inhibitory effect of the anionic SDS micelles. Quantitative analysis of the micellar data obtained has been done by applying a positive cooperativity model of enzyme catalysis. The value of n (index of cooperativity) has been found to be greater than 1 for all systems under study. The presence of solvents such as ethanol, n-propanol, and n-butanol reduces the inhibitory effect of the micelles.

Full text of DOI:10.1071/CH11454

CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 153–159
Full Paper
http://dx.doi.org/10.1071/CH11454
Quantitative Analysis of Dye Decolourization Reactions
in Mixed Micellar Systems of Sodium Dodecyl Sulfate
with Tween-20, Tween-80, and Triton X-100
A
A
,
A B
Kimkholhing Lunkim, M. Niraj Luwang, and Sri K. Srivastava
A
Department of Chemistry, Manipur University, Imphal-795003, Manipur, India.
Corresponding author. Email: sksrivastava01@yahoo.co.in
B
The reaction of triphenylmethane dye (ethyl violet) with hydroxyl ion has been investigated in absence and presence of
micelles. In micellar solutions, the solubilization of dye carbocation is observed. The reaction rate constant follows
pseudo-first order kinetics with respect to the nucleophile. In presence of sodium dodecyl sulfate (SDS) micelles, an
inhibitory effect is observed due to repulsion of the nucleophile to the strongly bound dye carbocation in the negatively
charged SDS aggregate. The presence of nonionic surfactant reduces the inhibitory effect of the anionic SDS micelles.
Quantitative analysis of the micellar data obtained has been done by applying a positive cooperativity model of enzyme
catalysis. The value of n (index of cooperativity) has been found to be greater than 1 for all systems under study. The
presence of solvents such as ethanol, n-propanol, and n-butanol reduces the inhibitory effect of the micelles.
Manuscript received: 19 August 2011.
Manuscript accepted: 17 December 2011.
Published online: 27 January 2012.
Introduction
different from the solutions containing only the constituent
21]
single surfactants.
[
Micellar solutions have versatile uses in the fields of chemistry,
pharmacy, medicine, biochemistry, and industry. A range of
reactions, namely photochemical, oxidation, reduction, and
hydrolytic are influenced significantly in surfactant solutions
viz micellar and reversed micellar solutions.
micellar systems on the synthesis of various nanomaterials has
also been widely studied.
The rate constants of chemical reactions are known to be
altered or modified by micellar solutions.
show that surfactant aggregates can be utilized in controlling the
rate constant of chemical reactions and as probes for studying
reaction mechanisms.
tions can be attributed to electrostatic and hydrophobic
interactions.
The presence of solvent affects the reaction rate constant and
mechanism in various ways, such as changing the dielectric
constant, solvent polarity parameters, and electron transfer
[
1–3]
[22–24]
Applications of
reactions.
Although the effects of solvent on the reaction
rate constants have been studied by many, there is not much
work concerned directly with the effect of solvent on the
micellar inhibited reactions. Mixed alcohol-water systems have
been investigated due to their importance in the preparation of
[
4,5]
[
6–9]
Recent studies
[
25–27]
microemulsions.
These studies have shown that the incor-
poration of alcohols into micelles produces noticeable changes
in the micellar shape and in their transport properties. It also
causes micelle swelling and, in the case of ionic micelles, an
increase in the degree of ionization. All these structural changes
influence the reaction rate constants of micelle-modified pro-
[
10–14]
Effects of micelles on such reac-
Recent studies indicate that mixed surfactant solutions are
more active and have more potential from the viewpoints of
fundamental, technological, pharmaceutical, and biological
consideration. Mixed micelles contain more than one type of
surfactant, such as ionic-ionic, ionic-nonionic, nonionic-
[
28–30]
cesses through various effects.
In this study, the effect of nonionic surfactants (Tween-20,
Tween-80, and TX-100) on the anionic sodium dodecyl sulphate
(SDS) inhibited reactions of ethyl violet (EV) with hydroxyl
ions has been investigated. Further, the alkaline fading of EV in
the presence of SDS micelles in aqueous solutions containing
various amounts of solvents such as ethanol, n-propanol, and
n-butanol are studied.
[
15–19]
nonionic etc.
From a fundamental point of view, mixtures
of ionic-nonionic surfactants are more interesting because
they often exhibit highly non-ideal behaviour. Adding a non-
ionic surfactant into ionic surfactant micelles can reduce the
electrostatic repulsion between the charged surfactant heads and
greatly facilitates mixed micelle formation. Non-ideal behav-
iour of an ionic/nonionic surfactant mixture can also be
influenced by other structural characteristics of the two
surfactants, such as differences in the size of the surfactant
Experimental Section
Reagents and Materials
Ethyl violet, sodium hydroxide, sodium dodecyl sulfate, Tween-
20, Tween-80, Triton X-100 (TX-100), ethanol, n-propanol, and
n-butanol were used as received from HIMEDIA without further
[
20]
heads or length of the surfactant tails. In mixed micelles the
tendency to form aggregated structures can be substantially
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

1
54
K. Lunkim, M. N. Luwang, and S. K. Srivastava
(
a)
Table 1. Absorbance of ethyl violet in absence and presence of various
micellar systems
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
No surfactant
SDS
TX-100
Entry
Dye in solution
lmax [nm]
Tween-20
Tween-80
1
2
3
4
5
6
7
8
Aqueous
594
601
602
600
603
600
602
603
SDS
Tween-20
Tween-80
TX-100
Mixed SDS þ Tween-20
Mixed SDS þ Tween-80
Mixed SDS þ TX-100
400
450
500
550
600
650
700
ꢂ5
systems. The cationic dye, EV (1 ꢁ 10 M) exhibits a maxi-
Wavelength [nm]
mum absorption, l at 594 (ꢀ0.5) nm. In presence of different
max
surfactant systems, the l
value is shifted to a longer wave-
max
(
b)
length. The red shift in the lmax value of ethyl violet indicates the
solubilization of the dye carbocation in the micellar systems. In
all surfactant systems analyzed, the lmax value is shifted to the
range of 600–603 nm. The lmax value for EV in absence and
presence of various surfactant systems are summarized in
Table 1.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
No surfactant
TX-100:SDS
TX-100:Tween-80
TX-100:Tween-20
The reaction of EV with the hydroxide ion brings about
fading of the colour of the EV resulting in the formation of the
colourless carbinol. The reaction scheme is given in Scheme 1.
ꢂ
The decolourization of dye carbocations by nucleophile (OH )
ions were monitored by observing the change in the absorbance
as a function of time at the respective absorption maxima. In the
initial step, the substrate (dye) ionizes in an aqueous solution
giving positively charged dye carbocation. The positive charge
is localized over the whole molecule due to resonance stabiliza-
tion of the molecule. The nucleophile attacks the dye carboca-
tion forming a colourless product.
4
00
450
500
550
600
650
700
Wavelength [nm]
Fig. 1. Absorption spectra of the dye, EV in (a) SDS, TX-100, Tween-20,
and Tween-80 and (b) in mixed surfactant systems of SDS with TX-100,
Tween-20, and Tween-80.
The dependence of the pseudo-first order rate constant on
ꢂ
nucleophile (OH ) concentration was studied at a fixed concen-
tration of dye by varying the concentration of the nucleophile
(
Fig. 2). The rate constants were found to increase linearly with
purification. Deionized, double distilled water was used
throughout the experiments.
increasing concentration of the nucleophile. However, the rate
constant of the reaction is independent of the dye concentration
as there is no observable change on the rate constant of the
reaction with increasing dye concentration. The dye concentra-
Procedure
ꢂ5
Kinetic measurements were made spectrophotometrically, with
a matched pair of quartz cuvettes having an internal thickness of
tion was varied from 1.5 to 4.5 ꢁ 10 M at a fixed concentration
ꢂ2
of the nucleophile (3 ꢁ 10 M). The reaction follows pseudo-
1
0 mm, at 25 ꢀ 0.18C, employing a Shimadzu 2450 UV-Visible
first order kinetics with respect to the nucleophile.
spectrophotometer through which water from a high precision
refrigerate constant thermostatic bath (HAAKE DC-10) was
continuously circulated. The alkaline fading of the dye, EV
was followed by observing the change in absorbance as a
Effect of SDS Anionic Micelles
þ
The effect of the surfactant SDS on the rate constant of the
ꢂ
þ
reaction of EV with OH is shown in Fig. 3. The rate constant
of the reaction decreases with increase of the SDS concentration
initially and reaches saturation at higher concentration. The
function of time at 594 nm with an initial EV concentration of
ꢂ5
1
mental condition was observed throughout.
.5 ꢁ 10 M. Pseudo-first order kinetics under the experi-
2
overall inhibition factor, k (k /k ) is found to be ,9.3, where
rel
w m
The observed pseudo-first order rate constants, k , were
C
obtained from slopes of ln(A) versus time, following Eqn 1:
kw and km is the rate constant at aqueous and micellar phase.
The value of rate constant in aqueous (k ) and micellar phases
w
ꢂ3 ꢂ1
and 1.05 ꢀ 0.01 ꢁ 10ꢂ3
ꢂ1
lnðAÞ ¼ ꢂkt þ lnðAoÞ
where A and A are the absorbances of the dye, EV at initial and
ð1Þ
(k ) are 9.81 ꢀ 0.06 ꢁ 10
s
s
m
respectively.
Considering the reaction system between substrate constant
dye carbocation) and anionic SDS micelles, both hydrophobic
o
time t, respectively.
(
and electrostatic interactions are favourable to each other and
dye carbocations are strongly bound to the SDS micelles. The
core SDS micelles are hydrocarbon like and have a smaller
dielectric constant than water. Due to the hydrophobicity of the
Results and Discussion
Fig. 1 shows the absorbance spectra of the triphenylmethane
dye, ethyl violet in absence and presence of various surfactant

Reactions in Mixed Micellar Systems
155
H C
C H
H C
C H
H C
C H
2 5
5
2
2
5
5
2
2
5
5
2
ϩ
N
N
N
Ϫ
HO
slow)
(
ϩ
C
H C
H C
C H
H C
C H
5
5
2
OH
C2H5
5
2
2
5
5
2
2
N
N
N
N
N
N
C H
C H
C H
C H
C H
C H
2 5
2
5
2
5
2
5
2
5
2
5
þ
ꢂ
Scheme 1. Reaction mechanism of the dye, EV with the nucleophile, OH .
(
a) 8
EV] ϭ 1.5 × 10Ϫ5
M
[EV] ϭ 1.5 × 10Ϫ5
M
[
10
8
7
6
5
4
3
2
1
[OH] ϭ 0.2 M
6
4
2
0
0
2
4
6
8
10 12
14
16 18
0
2
4
6
8
Ϫ
2
[
OH ] ×10 [M]
5
[
SDS] × 10 [M]
(
b)
6
5
4
3
2
1
0
Fig. 3. Effect of SDS concentration on the rate constant of the reaction.
[
OH] ϭ 0.03 M
SDS : Tween-20
1
0
8
6
4
1:0
1
1
1
:0.33
:2
:5
2
0
1
2
3
4
5
ϩ
5
[
EV ] ×10 [M]
ꢂ
þ
Fig. 2. Effect of concentration of (a) OH and (b) EV on the rate constant
0
2
4
6
8
5
of the reaction.
[Total surfactant] × 10 [M]
Fig. 4. Effect of Tween-20 on the rate constant of the reaction.
dye carbocation, the positively charged dye carbocations are
solubilized on the micellar aggregate. It is also favoured by the
electrostatic interaction between the positively charged dye
carbocation and negatively charged SDS micelles. The nucleo-
phile, which lacks hydrophobicity and bears a charge similar to
that of the SDS micellar surface (or head group) is excluded
from the micellar aggregates containing the dye carbocation.
Thus in presence of SDS micelles, an inhibitory effect is
observed due to repulsion of the nucleophile to the strongly
bound dye carbocation in the negatively charged SDS
Effect of Mixed Micelles on the Rate Constant
of the Reaction
Fig. 4 shows the effect of the nonionic surfactant Tween-20 on a
SDS inhibited reaction of EV with the nucleophile. The con-
centration of the nonionic surfactant with respect to the anionic
surfactant SDS was varied in the range (SDS : Tween-20 ¼ 1 : 0,
1 : 0.33, 1 : 2, and 1 : 5). With increasing concentration of the
nonionic surfactant, there is a reduction of the inhibitory effect
of the anionic SDS micelles. The possible reason for the
[
31]
aggregate.

1
56
K. Lunkim, M. N. Luwang, and S. K. Srivastava
ϪOH
ϪOH
ϪOH
ϪOH
ϪOH
ϪOH
ϪOH
ϩ Ϫ
Rϩ
ϪOH
Rϩ Ϫ Rϩ
R
Ϫ
Ϫ
Ϫ
Rϩ
Ϫ
Rϩ
Rϩ
Rϩ
Ϫ
Ϫ
Ϫ
Ϫ
Rϩ
ϪOH
Rϩ
Ϫ
ϪOH
Rϩ
Ϫ
ϪOH
Rϩ
Ϫ
ϪOH
Ϫ
Rϩ
Rϩ
Rϩ
Rϩ
Ϫ
Ϫ
Ϫ
Ϫ
Rϩ
Rϩ
ϪOH
ϪOH
ϪOH
ϪOH
ϪOH
ϪOH
ϩ
+
Ϫ
ϭ Anionic surfactant (SDS)
ϭ Nonionic surfactant
R
ϭ
D
y
e
c
a
r
b
o
c
a
t
i
o
n
(
E
V
)
ϪOH ϭ Nucleophile
Scheme 2. Schematic diagram for the interaction of the dye with the nucleophile in micellar and mixed micellar systems.
SDS: TX-100
1:0
SDS: Tween-80
1
0
10
8
1
1
1
1
:0
:0.33
:2
:5
1:0.33
1:0.5
1:2
8
6
4
2
0
6
4
2
0
0
2
4
6
8
5
0
2
4
6
5
8
[
Total surfactant] × 10 [M]
[
Total surfactant] × 10 [M]
Fig. 5. Effect of Tween-80 on the rate constant of the reaction.
Fig. 6. Effect of TX-100 on the rate constant of the reaction.
reduction of the inhibitory effect of the SDS might be due to
incorporation of the nonionic surfactant to form mixed micelles.
This model favours approach of the nucleophile towards the dye
carbocation, which were initially repelled by the negatively
charged SDS head group. Thus, in the mixed micellar system of
SDS and Tween-20, the inhibitory effect of the SDS micelles is
reduced with increasing concentration of the nonionic surfactant
Tween-20. A schematic diagram for the interaction of the dye
carbocation with the nucleophile in single and mixed surfactant
system is shown in Scheme 2.
D S
nD ϩ S
n
KD
kw
km
Product
Scheme 3. Formation of micellar aggregate and product.
For the mixed surfactant systems of SDS with Tween-80
Fig. 5) and TX-100 (Fig. 6), similar observations of the
reduction of the inhibitory effect of the SDS micelles with
increasing concentration of the nonionic surfactants were found.
and enzymes are structurally and functionally similar. Both
micelles and enzymes have hydrophobic cores with polar
regions on their surfaces, and bind a substrate in a non-covalent
manner. The kinetics of micellar catalysis also resembles that of
(
[
34–38]
enzyme catalysis as described in various investigations.
A kinetic model analogous to the Hill Model which describes
the positive cooperativity in enzyme catalyzed reactions has
Quantitative Analysis of Micellar Data
In order to understand the mechanism by which enzymes cata-
lyze reactions, many efforts have been made in studying
[
39–41]
been used for quantitative analysis of micellar data.
This
[
32,33]
mechanisms of simpler model chemical reactions.
tions catalyzed or inhibited by micelles have thus been viewed
Reac-
assumes that a constant, S (dye carbocation) and a number ‘n’ of
surfactant (or detergent) molecules D, aggregate to form a
catalytically functional micellar aggregate D S, which may
n
[
34,35]
as models for enzyme catalyzed reactions. This analogy,
though far from perfect, is based on the observation that micelles
undergo reaction to yield products (Scheme 3).

Reactions in Mixed Micellar Systems
157
(
a)
1.0
0.5
0.0
K is the dissociation constant of the micelle (its reciprocal
D
SDS : Tween-20
1
1
:0
is binding constant), km and kw are the rate constant of the
reaction in micellar and bulk (aqueous) phases, respectively. For
this reaction scheme, pseudo-first order rate constant k is
:0.33
1:2
1:5
C
expressed as a function of concentration of the detergent D,
[
36,37,39]
by Eqn 2
:
n
km½Dꢃ þ kwKD
kC ¼
ð2Þ
ð3Þ
n
–
–
0.5
1.0
KD þ ½Dꢃ
This equation can be re-written according to Eqn 3:
ꢀ
ꢁ
–5.0
–4.8
–4.6
–4.4
ðkC ꢂ kwÞ
log
¼ n log ½Dꢃ ꢂ log KD
log [D]
ðk ꢂ kCÞ
m
(
b) 1.0
SDS : Tween-80
1:0
From Eqn 3, a plot of log [(k ꢂ k )/(k ꢂ k )] versus
C
w
m
C
1
:0.33
1:2
:5
log [D] should be linear with slope n and intercept (ꢂlog K ).
D
0
0
.5
.0
The value of [D] or log [D] at this point of half maximal
inhibition is designated as [D]50 or log [D]50 and when the left
hand side of Eqn 3 is zero, we have
1
log KD
log ½Dꢃ50
¼
ð4Þ
n
–
–
0.5
1.0
Thus, in this model, slope (n) of the double log plot describes
the stoichiometry of the reaction scheme. The value of n . 1
reflects positive cooperativity.
–
5.0
–4.9 –4.8
–4.7
log [D]
–4.6 –4.5
–4.4
The applicability of Eqn 3 was tested by using the micellar
data for dye carbocation (EV) in mixed micellar systems as
shown in Figs 4–6. The Hill-type double log plots and the
quantitative analysis according to Eqns 2 and 3, for the reactions
under study are shown in Fig. 7 and Table 2 respectively. It was
found that the binding constant in the mixed micellar system
decreases with increase in the concentration of the nonionic
surfactants. This indicates that the presence of the nonionic
surfactants reduces the binding capacity of the dye carbocation
to the micelles, thereby allowing greater interaction of dye
carbocation with the nucleophile. This further leads to a reduc-
tion of the inhibitory effect of the anionic SDS micelles, thereby
increasing the overall rate constant in the mixed micelles. We
note that the value of n was found to be greater than 1 for all the
systems under study, showing the positive cooperativity in the
mixed micellar system. The applicability of Piszkiewicz’s
cooperativity model has also been reported for other micellar
(c)
0
0
.8
.6
SDS : TX-100
1
:0
1:0.33
1
1
:0.5
:2
0.4
0
.2
.0
0
–0.2
–
–
–
0.4
0.6
0.8
–
5.0
–4.9
–4.8 –4.7
–4.6 –4.5
–4.4
log [D]
[
31,42]
reactions.
Fig. 7. Quantitative analysis of the effect of mixed micellar solutions of
SDS with (a) Tween-20, (b) Tween-80, and (c) TX-100 in different mole
þ
Effect of Solvents (Ethanol, n-Propanol, n-Butanol)
fractions, on the reaction of dye carbocation (EV ) with OH ion, applying
positive cooperativity model of enzyme catalysis (Hill-type plots).
Fig. 8 shows the effect of the solvents (ethanol, n-propanol, and
n-butanol) on the reaction of EV with the nucleophile in a SDS
surfactant system. There is an increase on the rate constant of the
reaction with increasing concentration for all solvents. Due to
very low solubility in n-butanol, it was not possible to study the
effect at higher concentrations; however a significant effect is
observed even at lower concentrations. The increase in the rate
constant of the reaction is observed, as mixed solvents have a
lower dielectric constant than water and the reaction under study
takes place between charged species. There are many factors
responsible for the influence of solvents on the rate constant of
the reaction, such as changing water structure, preferential sol-
vation of reactants, changing dielectric constant of the reaction
system, and changing cohesive energy. The effectiveness of
solvent as an accelerator of the reaction rate constant is in the
following order:
n-butanol > n-propanol > ethanol > water
The change in the overall rate constant of the reaction on
addition of solvents might be due to the displacement of a part of
the reactants from the micellar phase into the bulk phase where
the reaction is comparatively fast, which evidently shows up as
an increase in the rate constant of the reaction in presence of
micelles. The efficiency of alcohols in reducing the micellar

1
58
K. Lunkim, M. N. Luwang, and S. K. Srivastava
1
2
Table 2. Values of parameters from Hill-type plots for SDS inhibited reaction of dye carbocations (EV ) with nucleophile (OH ) in various mixed
micellar systems
System
Rate constant,
3
m
Slope, n
[D]50
Intercept (–log k
D
)
K
D
Binding constant
(1/K
ꢂ1
k
[ꢁ10 s
]
(index of cooperativity)
D
)
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ5
ꢂ10
ꢂ8
ꢂ9
ꢂ7
ꢂ12
ꢂ7
ꢂ5
ꢂ8
10
SDS
1.05 ꢀ 0.01
1.97 ꢀ 0.02
4.77 ꢀ 0.04
4.67 ꢀ 0.04
2.54 ꢀ 0.02
3.83 ꢀ 0.03
5.20 ꢀ 0.04
1.62 ꢀ 0.01
2.85 ꢀ 0.02
5.99 ꢀ 0.05
1.87 ꢀ 0.11
1.62 ꢀ 0.14
1.81 ꢀ 0.22
1.54 ꢀ 0.09
2.38 ꢀ 0.15
1.46 ꢀ 0.12
1.01 ꢀ 0.22
1.58 ꢀ 0.09
2.37 ꢀ 0.34
1.05 ꢀ 0.19
1.43 ꢁ 10
3.39 ꢁ 10
3.18 ꢁ 10
3.02 ꢁ 10
2.23 ꢁ 10
2.38 ꢁ 10
2.54 ꢁ 10
5.41 ꢁ 10
2.34 ꢁ 10
2.11 ꢁ 10
9.06 ꢀ 0.49
7.24 ꢀ 0.63
8.14 ꢀ 1.02
6.96 ꢀ 0.44
11.07 ꢀ 0.72
6.75 ꢀ 0.57
4.64 ꢀ 0.72
7.39 ꢀ 0.46
11.02 ꢀ 1.58
4.91 ꢀ 0.88
8.71 ꢁ 10
1.15 ꢁ 10
8
SDS : T20
1 : 0.33
5.75 ꢁ 10
7.24 ꢁ 10
1.09 ꢁ 10
1.73 ꢁ 10
1.38 ꢁ 10
1.92 ꢁ 10
1.17 ꢁ 10
5.62 ꢁ 10
4.37 ꢁ 10
2.45 ꢁ 10
9
7
9
7
4
8
1
1
: 2
: 5
SDS : T80
1 : 0.33
8.51 ꢁ 10
1
1
: 2
: 5
1.77 ꢁ 10
2.29 ꢁ 10
4.07 ꢁ 10
SDS : TX-100
1 : 0.33
ꢂ12
ꢂ5
11
1
1
: 0.5
: 2
9.54 ꢁ 10
1.04 ꢁ 10
5
1.23 ꢁ 10
8.12 ꢁ 10
n-propanol, and n-butanol reduces the inhibitory effect of the
micelles.
[
[
[
EV] ϭ 1.5 × 10Ϫ5
OH] ϭ 0.02 M
M
M
4
3
2
1
0
0
0
0
0
SDS] ϭ 5 × 10Ϫ3
References
[
[
[
[
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1
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Ethanol
n-propanol
n-butanol
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