Evidence of co-operativity in the pre-micellar region in the hydrolytic cleavage of phenyl salicylate in the presence of cationic surfactants of CTAB, TTAB and CPC

Article abstract of DOI:10.1016/j.molcata.2014.09.026

The effects of cationic surfactants of cetyl trimethyl ammonium bromide (CTAB), tetradecyl trimethylammonium bromide (TTAB) and cetyl pyridinium chloride (CPC) on the kinetics of intramolecular gen-eral base catalyzed hydrolysis ([OH^-] range 0.050.1 mol L^-1) of phenyl salicylate have been studied atdifferent temperatures. The rate is independent of [OH^-] in the studied range. The anionic surfactantsodium dodecyl sulphate (SDS) has no effect on the rate. The presence of small amount of any of thesecationic surfactants well below its critical micelle concentration markedly inhibits the rate of reactionsuggesting a pre-micellar aggregation between the substrate and surfactant monomers. The kinetic datahave been analyzed in terms of earlier reported models (Piszkiewicz's co-operativity model and Ragha-van and Srinivasan's model) for micellar catalysis. The binding constants between the substrate and thesurfactants evaluated from the two models are in good agreement. Three dimensional structure of thepre-micellar aggregate controls the approach of the nucleophile water molecule to the reaction center.The planar structure of the pyridinium head group of CPC provides less steric hindrance to the attackingwater molecule that leads to the least enthalpy of activation for CPC among the three surfactants. Theassociation between the negatively charged substrate and the cationic surfactant is favored owing toelectrostatic as well as hydrophobic interactions. The binding between the substrate and pre-micellesfollows the order: CPC > TTAB > CTAB.

Full text of DOI:10.1016/j.molcata.2014.09.026

Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Evidence of co-operativity in the pre-micellar region in the hydrolytic
cleavage of phenyl salicylate in the presence of cationic surfactants of
CTAB, TTAB and CPC
Pratik K. Sena,∗, Piyali Chatterjeea, Biswajit Palb
a Department of Chemistry, Jadavpur University, Kolkata 700032, India
b Department of Chemistry, St. Paul’s C.M. College, 33/1 Raja Rammohan Roy Sarani, Kolkata 700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 April 2014
Received in revised form 28 August 2014
Accepted 21 September 2014
Available online 28 September 2014
The effects of cationic surfactants of cetyl trimethyl ammonium bromide (CTAB), tetradecyl trimethyl
ammonium bromide (TTAB) and cetyl pyridinium chloride (CPC) on the kinetics of intramolecular gen-
eral base catalyzed hydrolysis ([OH−] range 0.05–0.1 mol L−1) of phenyl salicylate have been studied at
different temperatures. The rate is independent of [OH−] in the studied range. The anionic surfactant
sodium dodecyl sulphate (SDS) has no effect on the rate. The presence of small amount of any of these
cationic surfactants well below its critical micelle concentration markedly inhibits the rate of reaction
suggesting a pre-micellar aggregation between the substrate and surfactant monomers. The kinetic data
have been analyzed in terms of earlier reported models (Piszkiewicz’s co-operativity model and Ragha-
van and Srinivasan’s model) for micellar catalysis. The binding constants between the substrate and the
surfactants evaluated from the two models are in good agreement. Three dimensional structure of the
pre-micellar aggregate controls the approach of the nucleophile water molecule to the reaction center.
The planar structure of the pyridinium head group of CPC provides less steric hindrance to the attacking
water molecule that leads to the least enthalpy of activation for CPC among the three surfactants. The
association between the negatively charged substrate and the cationic surfactant is favored owing to
electrostatic as well as hydrophobic interactions. The binding between the substrate and pre-micelles
follows the order: CPC > TTAB > CTAB.
Keywords:
Phenyl salicylate
Hydrolytic cleavage
Cationic surfactants
Pre-micelles
Co-operativity
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
nature of the counterions and the head group bulk [12]. The micelles
play a critical role in the localization and/or dispersion of charges
Organized assemblies like micelles are being used extensively in
different fields such as cosmetics, preparation of photofunctional
dyes and sensors, targeted drug delivery, etc. [1–3]. Micelles can
act as microreactors where the reactants are partitioned by means
of electrostatic and/or hydrophobic interactions and thus modify
the reaction rates and equilibria [4–9]. Micellar catalyzed reac-
tions may be compared to those catalyzed by enzymes since the
hydrophobic interactions within the micelle have some similari-
ties with the complex reactions occurring in biological assemblies
[10,11]. Both micelles and enzymes bind substrates in a non-
covalent manner. Micellization takes place at the critical micelle
concentration, which in some systems shows the onset of the
increase in rate. Micellar effects on the rates are sensitive to the
on reactants and their activated states, and thus can easily influence
reaction rates, equilibrium and concentration or depletion of reac-
tants in the interfacial region [13–16]. However, in some cases it
has been found that the surfactants show accelerating or inhibiting
effect on reaction rates even at concentrations much lower than
CMC where normal micelles are not formed at all. The decarbox-
ylation of alkoxynitrobenzisoxazole-3-carboxylate ions was found
to be strongly accelerated [17] in the presence of a few cationic
surfactants even at very low amphiphile concentrations (much
below CMC). The oxidation of a few amino acids by chloramine-
T in HClO4 medium [18] was found to be markedly catalyzed by
the presence of a small amount of surfactant much below the CMC
of surfactant. These effects may be either due to complexation
of the substrate with monomeric surfactant or pre-micelles or to
substrate induced formation of pre-micelles [18–20]. In view of
the widespread domestic as well as industrial use of detergents,
the influence of such amphiphiles even in very dilute solution
∗
Corresponding author. Tel.: +91 33 2414 6666 2329.
E-mail addresses: senpk@yahoo.com (P.K. Sen), palbiswajit@yahoo.com (B. Pal).
http://dx.doi.org/10.1016/j.molcata.2014.09.026
1381-1169/© 2014 Elsevier B.V. All rights reserved.

24
P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
PhO
O
Na+
OH
Na+
O
O
O
OH
NaOH
+
H2O
+
Scheme 1. Alkaline hydrolysis of phenyl salicylate.
on the reactivity of substances should be of environmental con-
cern.
Phenyl salicylate finds use as an antiseptic owing to its antibac-
terial activity upon hydrolysis in the small intestine [21]. That
is why the mechanism of the hydrolytic cleavage of the ester
moiety followed by further biochemical reaction is of great impor-
tance [22]. A number of studies have been carried out on the
hydrolysis of methyl salicylate, methyl o-methoxybenzoate [23],
phenyl salicylate and its derivatives [24,25] in neutral and alka-
line medium. The effects of both cationic and anionic micelles on
the intramolecular general base catalyzed hydrolysis, alkanolysis,
aminolysis and hydrazinolysis of phenyl salicylate have already
been investigated [26–30]. It may be mentioned that all such stud-
ies were carried out in the post-micellar region. Nevertheless the
substrate–surfactant interaction in the pre-micellar region may
also be an important area of research which requires detailed inves-
tigations. However, no such studies on the reactions of phenyl
salicylate in the pre-micellar region are found in the literature.
The submicellar assemblies of surfactant monomers either formed
spontaneously or produced by interactions with reactants are gen-
erally known as “premicelles”. However, it is not always easy to
distinguish between reactant induced micellization and forma-
tion of premicelles [31]. Since phenyl salicylate exists as anion in
alkaline medium, an electrostatic interaction between the anionic
substrate and a cationic surfactant monomer is expected and thus
there is a possibility that the rate of hydrolysis may be influenced
even in the pre-micellar region. This prompted us to study the
kinetics of alkaline hydrolysis of phenyl salicylate (Scheme 1) in the
presence of different cationic surfactants like CTAB, TTAB and CPC
at concentrations much lower than their respective CMC’s in order
to see whether there is any evidence of co-operativity between the
substrate and the surfactant which may influence the reaction rate.
The chemical structures of the cationic surfactants used in
this study are shown in Scheme 2. The effect of surfactants on
the hydrolysis reaction in the pre-micellar region has been ana-
lyzed using Piszkiewicz’s model [19] and Raghavan and Srinivasan’s
model [32] and an attempt has been made to find out the binding
constants between the substrate and the surfactants as well as to
account for the thermodynamic parameters in terms of the elec-
trostatic and hydrophobic interactions between the substrate and
the surfactant monomers.
Fig. 1. UV–vis spectra of the reaction mixture at different time intervals;
[PhS] = 2.0 × 10−4 mol L−1, [OH−] = 0.01 mol L−1, 4% aqueous ethanol (v/v), T = 298 K
and scanning time interval: 150 s.
2. Materials and methods
2.1. Materials
Phenyl salicylate (PhS) (AR, CDH, India), absolute ethanol
(Merck, Germany), sodium hydroxide (AR, SRL, India) and surfac-
tants SDS (extrapure, SRL, India), CTAB (extrapure AR, SRL, India),
TTAB (extrapure, SRL, India) and CPC (extrapure, SRL, India) were
used without purification. For kinetic measurements, stock solu-
tions of PhS in 50% ethanol–water solvent and sodium hydroxide
in water were freshly prepared. The sodium hydroxide solution
was standardized against standard oxalic acid solution frequently.
Water obtained from Millipore Synergy was used throughout the
experiments.
2.2. Kinetic measurements
The kinetics of the hydrolytic cleavage of phenyl salicylate
under weakly alkaline medium (0.005–0.1 mol L−1 NaOH) in 4%
aqueous ethanol solvent in the presence and in the absence of sur-
factants were studied spectrophotometrically by using UV–visible
Shimadzu 1800 spectrophotometer with thermostatted cell com-
partments and quartz cells with 1.0 cm path lengths. Appropriate
quantities of solutions of PhS, aqueous ethanol and Millipore
water were placed in the cuvette. After ensuring thorough mix-
ing and temperature equilibration, the reaction was initiated by
adding requisite amount of NaOH solution. The time-resolved
spectra (scanning time interval: 150 s) of the reaction mixture
([PhS] = 2.0 × 10−4 mol L−1, [OH−] = 0.01 mol L−1, 4% (v/v) aqueous
ethanol) show maximum absorbance at 340 nm (Fig. 1). There is
no apparent shift of the absorption peak near ꢀmax, indicating that
the concentration of any reaction intermediate during the reaction
course would be very low. The absorbance at the ꢀmax decreases
gradually with time indicating the progress of the reaction.
Thus, the kinetics of the reaction was followed by monitor-
ing the absorbance of the reaction mixture at 340 nm [33]. The
reaction was studied under pseudo-first order conditions, where
[OH−] ꢀ [PhS]. Under such conditions, the observed pseudo-first
order rate constant (kobs) was calculated from Eq. (1) using the
non-linear least-square technique.
Na+
Me
O
O
O
Br
Me N C16H33
Me
S
O C
H
12 25
Cetyl trimethyl ammonium
bromide (CTAB)
Sodium dodecyl sulphate
(SDS)
Me
Cl
N
Br
Me N C14H29
Me
C
H
16 33
Cetyl pyridinium Chloride
(CPC)
Tetradecyl trimethyl ammonium
bromide (TTAB)
At = A∞ + εS0 exp(−kobst)
(1)
Scheme 2. The chemical structures of different anionic and cationic surfactants.

P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
25
The hydrolysis reaction under the experimental condition
was studied at three different temperatures viz., 305, 310 and
315 K and the activation parameters for the uncatalyzed reaction
in the absence of surfactants have been determined using the
Eyring equation. The enthalpy of activation (ꢂH#) and entropy
of activation (ꢂS#) were found to be 68.60 6.21 kJ mol−1 and
−73.21 5.84 J K−1 mol−1, respectively.
The pKa value of phenol is 9.89 [34]. However, in case of salicylic
acid, due to the intra molecular H-bonding between COO− and
the OH group as well as +R effect of the COO− group, the pKa for
the phenolic OH (13.6) in salicylic acid [34] becomes much higher
compared to that of phenol. But in phenyl salicylate the R effect of
COOPh group will increase the acidity of the phenolic OH group
and the pKa value should be much lower than that of salicylic acid
(mentioned above) and it is expected to be slightly less than that
of phenol. Literature report [35] shows that the pKa of phenyl sali-
cylate is 9.25. Therefore, under the condition of the experiments
([OH−] = 0.05–0.1 mol L−1), salicylate ion happens to be the pre-
dominating reactive species of the ester molecule according to the
following dissociation taking place:
Fig. 2. Typical pseudo-first order exponential decay curves in the presence of
different concentrations of CTAB at 315 K. Plots of absorbance versus time;
[PhS] = 0.75 × 10−4 mol L−1, [OH−] = 0.05 mol L−1, [CTAB] = (A) 0.0, (B) 2.0 × 10−4, (C)
COOPh
OH
COOPh
O
+
H+
4.0 × 10−4 and (D) 7.0 × 10−4 mol L−1
.
(2)
The pH-independent hydrolytic cleavage of the reaction is sug-
gested to take place through the participation of the neighboring
hydroxyl group which acts as a general base catalyst [23]. The pre-
ferred mechanism is shown in Scheme 3, where a water molecule
is presumed to be involved in the rate determining step.
Thus when phenyl salicylate undergoes hydrolytic cleavage, the
rate determining step involves the nucleophilic attack by a water
molecule which is assisted by the neighboring O− group of the
phenolate ion. Since water (nucleophile) is present in large excess
compared to PhS, the rate law may be expressed as:
At and A are the absorbance values at time t and at infinite time
∞
respectively. S0 is the initial concentration of PhS and ε is the appar-
ent molar extinction coefficient. The fitting of the observed data
points to Eq. (1) was found to be reasonably good for all kinetic
runs. The kobs values were reproducible to within 5%. Typical
pseudo-first order plots for the reaction in the presence of differ-
ent concentrations of CTAB [(0.0–7.0) × 10−4 mol L−1] are shown in
Fig. 2.
2.3. Tensiometric measurements of CMC
d[PhS]
dt
v = −
= kobs[PhS]
(3)
In order to find out the critical micelle concentration of CTAB,
TTAB and CPC, tensiometric measurements were performed in the
presence of 0.01 mol L−1 NaOH and 0.0002 mol L−1 PhS separately.
A plot of surface tension (ꢁ) versus log [surfactant] was drawn for
each of the surfactants. The intersection of two straight lines indi-
cated the value of CMC.
where kobs is the pseudo-first order rate constant.
3.2. Hydrolysis reaction in the presence of surfactants
From tensiometric measurements the CMC values of CTAB were
found to be 0.81 × 10−3 mol L−1 and 0.95 × 10−3 mol L−1 respe−c1-
tively in the presence of 0.01 mol L−1 NaOH and 2.0 × 10−4 mol L
PhS separately. Similar CMC values are 0.32 × 10−3 mol L−1
and 0.40 × 10−3 mol L−1 for TTAB and 1.10 × 10−3 mol L−1 and
1.31 × 10−3 mol L−1 for CPC respectively in the presence of NaOH
and PhS. Thus, it will be evident that our kinetic experiments were
carried out at detergent concentrations much below the respective
CMC’s.
The reaction was investigated in the presence of an anionic sur-
factant SDS. The rate was studied at varying concentrations of SDS
((2.0–20.0) × 10−4 mol L−1) keeping [PhS], [OH−] and temperature
constant at 7.5 × 10−5 mol L−1, 0.05 mol L−1 and 305 K, respectively.
The rate was found to remain unaffected by SDS (Table S1).
The influence of three cationic surfactants, viz., CTAB, TTAB and
CPC on the reaction rate was then studied by varying surfactant
3. Results and discussion
3.1. Hydrolysis reaction in the absence of surfactants
The hydrolysis of phenyl salicylate in the absence of surfac-
tants was studied at various hydroxyl ion concentrations in the
range (0.05–0.1) mol L−1 keeping [PhS] and temperature constant
at 7.5 × 10−5 mol L−1 and 305 K, respectively. No attempt was made
to keep the ionic strength constant since the reaction was found
to be independent of ionic strength as studied by the variation of
[NaClO4] in the range (0.01–0.5) mol L−1. In this range of alkali con-
centration, the rate was found to be independent of [OH−]. The
observed pseudo-first order rate constant (kobs) values are pre-
sented in Table 1.
O
O
O
PhO
O
PhO OH
OH
OH
O
slow
OH
fast
H2O
+
PhO
Scheme 3. Mechanism of the hydrolytic cleavage of phenyl salicylate in the absence of surfactant.

26
P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
Table 1
Observed pseudo-first order rate constants for the alkaline hydrolysis of phenyl salicylate in the absence and presence of cationic and anionic surfactants at 305 K;
[PhS] = 7.5 × 10−5 mol L−1
.
[NaOH] (mol L−1
)
Aqueous medium, 104 kobs (s−1
)
Micellar medium
104 (s−1) (SDS)a
*
k
104
k
(s−1) (CTAB)b
104
k
(s−1) (TTAB)c
104 (s−1 (CPC)d
k )
0.05
0.07
0.10
16.8 0.7
17.0 0.8
16.7 0.7
17.4 0.8
17.1 0.7
17.2 0.6
10.8 0.5
11.0 0.5
10.7 0.4
13.7 0.6
13.4 0.7
13.6 0.6
13.9 0.7
14.0 0.8
14.2 0.7
a
[SDS] = 4.0 × 10−4 mol L−1
.
b
c
d
*
[CTAB] = 1.0 × 10−4 mol L−1
.
[TTAB] = 3.33 × 10−5 mol L−1
.
[CPC] = 1.0 × 10−4 mol L−1
.
[PhS] = 1.12 × 10−4 mol L−1
.
the reaction scheme [17–20]. If the reaction scheme remains unal-
tered the variation of rate may arise owing to partitioning of
the reactants between the bulk aqueous phase and the micellar
pseudo-phase which may either increase or decrease the local
concentration of the reactants thereby enhancing or diminishing
the reaction rate [20]. Actually electrostatic and/or hydrophobic
interaction between the reactants and the surfactant aggregates
are responsible for the change of reaction rates [5]. Sometimes
change in reaction rate is attributed to a change in the dielectric
constant or the pH of the medium at the micellar surface [36,37].
In the present case the reactions were studied at surfactant
concentrations much below the CMC region. Hence the effect of
micellar aggregates on the reaction rate is not applicable in our
present study. The decrease in reaction rate in the presence of
cationic surfactants in this region of surfactant concentration may
be attributed to the formation of pre-micelles. The negatively
charged salicylate ion possibly aggregates with a small number
(n ≈ 2–4) of positively charged surfactant monomers to form pre-
micelles which are different from normal micelles (formed by the
aggregation of a large number of surfactant molecules only). A num-
ber of different studies have been reported where the reaction rates
are found to be accelerated by the surfactants in the pre-micellar
region [20,38,39] and this enhancement of reaction rate has been
explained by the formation of a catalytic micelle by the surfactant
monomers with the substrate. It has been proposed by Piszkiewicz
[19] that this small number, n (approx. 1–6) of detergent monomers
which aggregate with the substrate molecule, may be viewed as
an index of positive co-operativity. However, there are instances
[17,40] where the inhibition of reaction rates also are attributed to
the formation of such small assemblies of substrate and surfactant
monomers known as pre-micelles.
The interesting feature observed for the hydrolytic cleavage of
phenyl salicylate in the presence of three cationic surfactants is the
decrease of rate of reaction at low surfactant concentrations (well
below CMC of respective surfactant). This observation indicates
either substrate induced micellization or premicellar aggregates
[17]. The decrease in rate in the presence of cationic surfactants
may be explained in the following manner. In the absence of any
surfactant the hydrolysis of phenyl salicylate takes place through
nucleophilic attack by a water molecule in the slow step. In the pre-
micellar aggregates, since the substrate is surrounded by/bound to
surfactant monomers, the entry of water molecule to the reaction
site gets restricted and the reaction rate becomes slower. A possible
reaction mechanism for the hydrolytic cleavage of phenyl salicylate
in the presence of surfactant occurring through pre-micellar cluster
is shown in Scheme 4.
Fig. 3. Dependence of pseudo-first order rate constant on [CTAB]. Plots of k« versus
[CTAB] at different temperatures; [PhS] = 7.5 × 10−5 mol L−1, [OH−] = 0.05 mol L−1
.
concentrations (below their respective CMC’s) while [PhS] and
[OH−] were kept constant at 7.5 × 10−5 mol L−1 and 0.05 mol L−1
respectively at a constant temperature (Table S1). It has been
observed that in each case the rate constant (k ) decreases with an
increase in [surfactant] at lower concentration range (below CMC)
and tends to reach a limiting value (Fig. 3 and Figs. S1, S2).
The alkaline hydrolysis of phenyl salicylate in the presence
of surfactant was also studied at different NaOH concentrations
((0.05–0.1) mol L−1) keeping [PhS], [surfactant] and temperature
constant. The pseudo-first order rate constant (k ) was found to
remain more or less unchanged with change of alkali concentra-
tion (Table 1). The effect of variation of surfactant concentrations
on the reaction rate was studied for all the three cationic surfac-
tants at three different temperatures. The activation parameters
for the reaction in the presence of cationic surfactants have been
determined using the Eyring equation and the values are presented
in Table 2.
Surfactants are often found to accelerate or inhibit the reaction
rates either by changing the reaction environments or by modifying
Table 2
Activation parameters for the alkaline hydrolysis of phenyl salicylate in the absence
and presence of different surfactants.
In earlier investigations [19,32,41] several models have been
proposed to account for such enhancement or inhibition of reaction
rate in the presence of surfactant at concentrations much below the
In presence of
ꢂH# (kJ mol−1
)
−ꢂS# (J K−1 mol−1
)
No surfactant
CTAB
TTAB
68.60 6.21
73.21 5.84
6.85 2.75
42.35 6.70
82.58 9.67
CMC. In the present case, the data of rate constant (k ) vs. [surfac-
91.93 10.23
78.91 2.09
67.50 7.44
tant] plots at various temperatures have been analyzed in terms of
those models of pre-micellar aggregates reported earlier.
CPC

P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
27
Me
N+
Br-
Me
Phenyl salicylate
+
Me
CTAB monomer
H2O
Me
N+
Me
Br-
Me
OPh
O
Products
H2O
O
Me
N+
Br-
Me
Me
Pre-micelle Cluster
Scheme 4. Schematic representation of hydrolysis of phenyl salicylate in the pres-
ence of cationic surfactant in the pre-micellar region.
Fig. 4. Plot of log{(kw − k« )/ (k« − km)} versus log[CTAB] at different temperatures;
[PhS] = 7.5 × 10−5 mol L−1, [OH−] = 0.05 mol L−1
.
3.3. Piszkiewicz’s model
A kinetic model, which is very much similar to the Hill model
[42] for enzymatic reactions, was developed by Piszkiewicz [19].
This model accounts for the dependence of the rate constants on the
surfactant concentration especially at low surfactant concentration
and the data may be treated without reference to CMC. According
to this model, a small number (n) of detergent molecules (D) aggre-
gates/associates with the substrate (S) to form a pre-micelle (DnS)
which may react at a different rate to yield the product in addition
to the normal reaction occurring in the aqueous phase as given in
Scheme 5.
The km was taken as the limiting or minimum rate constant
in the presence of surfactant (i.e. constant value of rate constant
at higher surfactant concentration but below CMC) in the plot of
k« vs. [surfactant] (Fig. 3). The plots at three different temper-
atures were fairly linear. The values of co-operativity index (n),
log[D]50, [D]50 and KD at three different temperatures have been
determined with the help of these plots and are summarized in
Table 3.
It appears that for a given reaction the nature of the surfactant
produces an effect on both [D]50 and ‘n’. In the present inves-
tigation, the observed values of ‘n’ greater than one (1 < n < 4)
are in agreement with earlier observations of Piszkiewicz and
are viewed as indices of co-operativity. These values are far less
than the number of surfactant molecules found in the normal
micelle and previously been interpreted as indicative of pre-
micellar aggregates. [D]50 values for the three surfactants are
also in good agreement with the experimental observations. The
KD values are very small indicating that dissociation of the pre-
micelle (DnS) back to its free components is negligible. Such small
KD values were also obtained in earlier studies [18]. It may be
plausible that before the formation of pre-micelles, both phenyl
salicylate and the surfactant monomer remain in hydrated forms.
Again in the pre-micelle these molecules are bound through
hydrophobic as well as electrostatic interactions which are pos-
sibly stronger than the hydration interactions of the individual
species. This may account for the small values of KD for all the three
surfactants.
Here, KD is the dissociation constant of the pre-micelle (DnS) [19]
back to its free components and km and kw are the rate constants
in micellar medium and aqueous phase respectively.
Following this model the observed rate constant (k ) comes out
to be:
km[D]n + kwKD
k
=
(4)
KD + [D]n
On rearranging, the above equation takes the form:
ꢀ
ꢁ
kw − k
log
= n log[D] − log KD
(5)
k
− km
Thus, according to the above scheme of reaction, a plot of
log{(kw − k« )/(k« − km)} versus log[D] is expected to be linear. The
linear plot will have a slope value of ‘n’ which is also known as
co-operativity index. At log{(kw − k« )/(k« − km)} = 0, k becomes
equal to the average of kw and km. Thus at this point the effect of sur-
factant shows one half of its maximum effect on rate constant. The
value of log[D] at this point has been designed as log[D]50 [19]. The
log{(kw − k« )/(k« − km)} versus log [surfactant] plots are shown in
Fig. 4 and Figs. S3, S4.
Table 3
Piskiewicz’s co-operativity values.
Temperature
Surfactant
n
log[D]50
[D]50 (mol L−1
)
KD
CTAB
TTAB
CPC
1.83
2.29
3.26
−4.0
1.0 × 10−4
2.88 × 10−5
1.58 × 10−4
0.93 × 10−4
2.95 × 10−5
1.94 × 10−4
1.20 × 10−4
3.31 × 10−5
1.91 × 10−4
4.90 × 10−8
4.28 × 10−11
4.22 × 10−13
2.32 × 10−8
3.68 × 10−12
2.01 × 10−13
1.82 × 10−8
2.21 × 10−12
0.34 × 10−13
DnS
nD
+
S
KD
km
−4.54
−3.80
305 K
CTAB
TTAB
CPC
1.89
2.52
3.42
−4.03
−4.53
−3.71
Product
Product
DnS
S
310 K
315 K
kw
CTAB
TTAB
CPC
1.97
2.60
3.62
−3.92
−4.48
−3.72
Scheme 5. Reaction steps involved in the Piszkiewicz model.

28
P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
K1
K2
DnS
nD
+
S
DnSW
DnS
+
W
km
Products
Products
DnSW
kw
S
+ W
Scheme 6. Reaction steps involved in the Raghavan and Srinivasan’s model.
3.4. Raghavan and Srinivasan’s model
Further attempt has been made to explain the inhibition effect
of the surfactant monomers on the reaction kinetics with the help
of another kinetic model known as Raghavan and Srinivasan’s
model [32]. This model describes the distribution of the reactant
and the nucleophile in both the aqueous and pre-micellar pseudo
phases. It predicts the formation of a ternary complex involving
substrate, nucleophile and surfactant monomers. This ternary com-
plex is assumed to be the pre-micelle. Similar to Piszkiewicz model,
this model also predicts the constancy in pseudo-first order rate
constant (k« ) at high surfactant concentrations (below CMC) and
may be used for evaluating the binding constants of reactants. The
model is represented in Scheme 6.In Scheme 6, D, S and W repre-
sent the surfactant monomer, the substrate and water, respectively.
Here water is the attacking reagent and thus has been consid-
ered as the nucleophile. DnS and DnSW are the binary and ternary
complexes, respectively. The reaction occurs in both the aqueous
and pre-micellar pseudo phases. Following the above scheme, the
pseudo-first order rate constant comes out to be:
Fig. 5. Plots of {(kw − k« )/ k« }(1/[D]n) versus (kM /k« ) for CTAB at different temper-
atures.
remains mostly in the bulk phase. Thus one may suggest that
the reaction takes place between the substrate solubilized into
a catalytic micelle and the nucleophile water residing at the
interface.
Since SDS is an anionic surfactant, the electrostatic repulsion
between the anionic substrate (Phenyl salicylate) and SDS possi-
bly does not favor the co-operative binding between the substrate
and SDS and thus SDS has no effect on the reaction rate. The bind-
ing between the negatively charged substrate and the cationic
surfactant may arise due to both electrostatic and hydrophobic
interactions. In case of CPC, possibly the similar nature of aromatic
rings in both the substrate and the surfactant favors the binding
more because of better hydrophobic interactions. Hence the val-
ues of the binding constant K1 are highest for CPC among the three
surfactants.
kw + kmK1K2[D]n
k
=
(6)
1 + K1[D]n{1 + K2[S]T }
Similar type of rate equation has also been reported in earlier
studies (Martinek et al. [43], Berezin [44] and Bunton et al. [36]).
On rearranging the equation takes the form:
In the pre-micelle the electrostatic interaction operates
between the phenyl salicylate anion and the cationic head group
of the surfactant monomers. Also there are hydrophobic interac-
tions present in the pre-micelle between the hydrocarbon tail of the
surfactant and the aromatic ring of the substrate. The pre-micelle
undergoes nucleophilic attack by the water molecule in the rate
determining step which ultimately leads to the reaction products.
The entry of the water molecule is controlled by the three dimen-
sional structure of the catalytic micelle at the reaction center. In
case of CPC, due to the planner structure of the pyridinium head
group, the nucleophile water molecule experiences less steric hin-
drance and the attack becomes easier. That is why the enthalpy
of activation (ꢂH#) is least in case of CPC (Table 2). For CTAB and
TTAB the three dimensional structure of the trimethyl ammonium
(−N(CH3)+) head group bound to the salicylate provides greater
steric hindrance which makes it more difficult for the entry of the
water molecule. As a result the enthalpy of activation (ꢂH#) is
higher for these two surfactants (Table 2).
When phenyl salicylate undergoes hydrolytic cleavage, the
rate determining step involves the nucleophilic attack by a water
molecule which is assisted by the neighboring O− group of the phe-
nolate ion. The resulting intermediate containing two OH groups,
one oxo anion (O−) and another phenoxy group ( OPh) is highly
polar and thus will be surrounded by a large number of water
molecules (Electrostriction). Due to this highly water-structured
intermediate compared to salicylate, a moderate decrease in
entropy is observed.
ꢂ
ꢃ
ꢂ
ꢃ
kw − k
1
[D]n
km
= K1{1 + K2[S]T } − (K1K2)
(7)
k
k
Thus, if a plot of {(kw − k« )/k« }(1/[D]n) versus (km/k« ) is drawn
at a particular temperature a straight line with a negative slope is
expected. Actually such linear plots were obtained at three different
temperatures for all the three surfactants (Fig. 5 and Figs. S5, S6).
The value of the co-operativity index (n) applied in this
model has been obtained from Piszkiewicz’s co-operativity model
(Table 3). From the intercepts and slopes of these linear plots, the
values of the binding constants (K1 and K2) and KD (reciprocal of K1)
have been evaluated. It is evident that KD, the dissociation constant
of the binary complex (DnS) in the Piszkiewicz model is actually the
reciprocal of the value of K1 in Raghavan and Srinivasan’s Model.
The evaluated KD values from this model corroborate with those
obtained from the Piszkiewicz’s model. The values of the bind-
ing constants and KD for all the three surfactants at three different
temperatures are shown in Table 4.
The high values of K1 indicate that the equilibrium favors the
formation of the binary complex. Initially phenyl salicylate and the
surfactant monomers are in the environment of water molecules,
but stronger electrostatic and hydrophobic interactions between
them play a role such that the substrate, in a co-operative man-
ner, binds to a small number (n) of surfactant monomers forming
binary complex or the catalytic micelle (DnS). Again the low val-
ues of K2 indicate that the nucleophile, i.e., the water molecule

P.K. Sen et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 23–30
29
Table 4
Binding constants using Raghavan and Srinivasan model and comparison with Piskiewicz’s co-operativity values.
Temperature
Surfactant
Piszkiewicz model KD
Raghavan and Srinivasan model
KD
K1
K2
CTAB
TTAB
CPC
4.90 × 10−8
4.28 × 10−11
4.22 × 10−13
2.32 × 10−8
3.68 × 10−12
2.01 × 10−13
1.82 × 10−8
2.21 × 10−12
0.34 × 10−13
4.34 × 10−8
4.96 × 10−11
3.37 × 10−13
2.33 × 10−8
3.54 × 10−12
1.83 × 10−13
1.80 × 10−8
2.08 × 10−12
0.41 × 10−13
2.31 × 107
2.01 × 1010
2.97 × 1012
4.28 × 107
2.83 × 1011
5.45 × 1012
5.56 × 107
4.80 × 1011
2.42 × 1013
1.028
0.982
1.022
305 K
310 K
315 K
CTAB
TTAB
CPC
0.990
1.010
1.020
CTAB
TTAB
CPC
1.012
1.009
0.973
When the hydrolysis takes place in the presence of surfactants,
the electrostatic attraction between the substrate and the surfac-
tant monomer in the pre-micelle decreases the polarity of the entity
and thus it is much less hydrated. The nucleophilic attack by the
water molecule to the pre-micelle in the rate determining step
gives rise to the final products which are very much polar. Thus
it may be expected that the transition state will be much more
polar compared to the pre-micelle cluster. Therefore, the transition
state will be appreciably hydrated (Electrostriction) [45], result-
ing in a decrease in entropy (−ve ꢂS). As the reaction proceeds,
in the transition state the surfactant monomers tend to become
free with the breaking of the pre-micellar structure, there should
be an increase in entropy (+ve ꢂS), but these monomers will get
hydrated again with a decrease in entropy (−ve ꢂS). It may be plau-
sible that all these −ve ꢂS terms overbalance the +ve ꢂS term and
the resultant entropy of activation (ꢂS#) becomes negative. But
the magnitude is different due to different extent of aggregation
in the pre-micelle for different surfactants and different amount
of hydration in the transition state. The greater the number of
monomers coming out of the pre-micelle cluster, the greater will
be their hydration and the larger the decrease in entropy. In case of
CTAB, less than two monomers (Table 3) aggregate with the sub-
strate in the pre-micelle and so when they become free and get
hydrated, the decrease in entropy is comparatively less resulting
in a small negative ꢂS# (Table 2). But for TTAB more than two
surfactant monomers (Table 3) aggregate with the substrate and
consequently when they become free, decrease in entropy due to
hydration is higher than in case of CTAB leading to a higher negative
ꢂS# (Table 2). In a similar way, the higher negative value of ꢂS#
(Table 2) for CPC compared to TTAB can be explained due to higher
co-operativity index (n) value for CPC (n > 3) (Table 3). Such type of
large difference in ꢂS# value for a CTAB catalyzed reaction com-
pared to uncatalyzed reaction has been observed for the oxidation
of D-fructose by Ce(IV) in sulfuric acid medium [15].
manner owing to the similar aromatic rings in the substrate and the
surfactant.
Acknowledgement
Financial assistance from DST through PURSE program to
the Department of Chemistry, Jadavpur University is gratefully
acknowledged.
Appendix A. Supplementary data
Supplementary material related to this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2014.09.026.
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