Inhibition effect of {surfactant-substrate} aggregation on the rate of oxidation of reducing sugars by alkaline hexacyanoferrate(III)

Article abstract of DOI:10.1002/kin.20271

The effect of cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium lauryl sulfate, NaLS), and nonionic (Brij-35) surfactants on the rate of oxidation of some reducing sugars (xylose, glucose, and fructose) by alkaline hexacyanoferrate(III) has been studied in the temperature range from 35 to 50°C. The rate of oxidation is strongly inhibited in the presence of surfactant. The inhibition effect of surfactant on the rate of reaction has been observed below critical micelle concentration (CMC) of CTAB. In case of NaLS and Brij-35, the inhibition effect was above CMC, at which the surfactant abruptly associates to form micelle. The kinetic data have been accounted for by the combination of surfactant molecule(s) with a substrate molecule in case of CTAB and distribution of substrate into micellar and aqueous pseudophase in case of NaLS and Brij-35. The binding parameters (binding constants, partition coefficients, and free-energy transfer from water to micelle) in case of NaLS and Brij-35 have been evaluated with the help of Menger and Portnoy model reported for micellar inhibition.

Full text of DOI:10.1002/kin.20271

Inhibition Effect of
{Surfactant–Substrate}
Aggregation on the Rate
of Oxidation of Reducing
Sugars by Alkaline
Hexacyanoferrate(III)
RATNA SHUKLA, SANTOSH K. UPADHYAY
Department of Chemistry, H. B. Technological Institute, Kanpur 208 002, India
Received 24 April 2006; revised 27 September 2006, 30 November 2006, 14 April 2007; accepted 13 May 2007
DOI 10.1002/kin.20271
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The effect of cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium
lauryl sulfate, NaLS), and nonionic (Brij-35) surfactants on the rate of oxidation of some reduc-
ing sugars (xylose, glucose, and fructose) by alkaline hexacyanoferrate(III) has been studied
in the temperature range from 35 to 50◦C. The rate of oxidation is strongly inhibited in the
presence of surfactant. The inhibition effect of surfactant on the rate of reaction has been
observed below critical micelle concentration (CMC) of CTAB. In case of NaLS and Brij-35, the
inhibition effect was above CMC, at which the surfactant abruptly associates to form micelle.
The kinetic data have been accounted for by the combination of surfactant molecule(s) with
a substrate molecule in case of CTAB and distribution of substrate into micellar and aqueous
pseudophase in case of NaLS and Brij-35. The binding parameters (binding constants, partition
coefficients, and free-energy transfer from water to micelle) in case of NaLS and Brij-35 have
been evaluated with the help of Menger and Portnoy model reported for micellar inhibition.
ꢀ
C
2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 595–604, 2007
well as premicellar catalysis in various redox reactions
is reported in the literature [7–12]. In some cases, pre-
micellar aggregation has also been observed. In prelim-
inary studies, it has been observed that a small amount
of cationic surfactant (even at below of its critical mi-
celle concentration, CMC), viz. cetyltrimethylammo-
nium bromide (CTAB) retarded the rate of oxidation
of reducing sugars by hexacyanoferrate(III), which is
a well-known one-electron oxidant [13] in alkaline
medium. The retarding effect of anionic (sodium lauryl
INTRODUCTION
Micelles are known to affect the rate of reaction by
partitioning the substrate between aqueous and mi-
celle pseudophase and also by perturbing the thermo-
dynamic parameters of the reaction [1–6]. Micellar as
Correspondence to: S. K. Upadhyay; e-mail: upadhyay s k@
rediffmail.com.
c
ꢀ
2007 Wiley Periodicals, Inc.

596
SHUKLA AND UPADHYAY
sulfate, NaLS) and nonionic (Brij-35) surfactants on
the rate of oxidation of reducing sugars by hexacyano-
ferrate(III) has also been observed but above CMC of
the surfactants. Therefore, in order to observe the mi-
cellar effect on the reaction mechanism, the detailed
kinetics of oxidation of some reducing sugars, viz. xy-
lose (aldopentose), glucose (aldohexose), and fructose
(ketohexose) by alkaline hexacyanoferrate(III) in the
presence of NaLS, CTAB, and Brij-35 has been in-
vestigated, and the results are reported in the present
communication.
of hexacyanoferrate(III) solution was kept within the
limits of Beer’s law.
RESULTS
Stoichiometry and Product Analysis
The stoichiometry of the reactions between sugar and
hexacyanoferrate(III) in the absence as well as in the
presence of surfactants has been studied. The reaction
mixtures containing a known excess of hexacyanofer-
rate(III) over reducing sugar in alkaline medium were
kept for 72 h at 40◦C until the reaction was complete.
Estimation of unreacted [Fe(CN)−6 3] showed that 1 mol
of reducing sugar (xylose, glucose, or fructose) con-
sumes nearly 2 mol of ferricyanide. The results may be
represented by the following stoichiometric equations:
EXPERIMENTAL
Material
Solution of hexacyanoferrate(III) was prepared by
dissolving potassium ferricyanide (GR grade; Loba,
Mumbai, India) in doubly distilled water. Freshly pre-
pared solutions of glucose (AR; s.d. fine, Mumbai,
India), fructose (AR; Thomas Baker, Mumbai, India),
and xylose (AR; Qualigens, Mumbai, India) in dou-
bly distilled water were used throughout the experi-
ments. The surfactants CTAB (AR; Thomas Baker),
NaLS (AR; s.d. fine), and Brij-35 (AR; Thomas
Baker) were purified until the CMC agreed with
their reported CMC as 9.8 × 10−4 mol dm−3, 8.0
× 10−3 mol dm−3, and 9.2 × 10−5 mol dm−3 of
CTAB [14a], NaLS [14b], and Brij-35 [14c], respec-
tively. The solutions of surfactants were prepared
in doubly distilled water just before the experiment
to avoid the aging. All other reagents, viz. NaOH,
NaClO4, and potassium ferrocyanide used were of
AR grade, and their solutions were prepared in doubly
distilled water.
3−
4−
6−
RCHOHCHO + 2OH− + 2Fe(CN) → 2Fe(CN)
6−
+ H2O + RCHOHCOOH
RCOCH2OH + 2OH− + 2Fe(CN)36− → 2Fe(CN)
4−
6−
+ H2O + RCOOH + HCHO
The presence of corresponding aldonic acid as the
oxidation product of aldose was identified by the spot
test [15]. The presence of formaldehyde as the oxi-
dation product in case of fructose was identified by
forming its 2,4-dinitrophenyl-hydrazone and compar-
ing it with an authentic sample [16]. The results are in
agreement with the earlier reported oxidation products
of reducing sugars [17–19].
Determination of Rate Constants
The reactions were studied at different initial concen-
trations of reactants in the absence as well as in the
presence of surfactants. Absorbance versus time plots
(Fig. 1) were found to be good a straight line upto 85%
of the reactions, suggesting a zero-order dependence of
rate with respect to hexacyanoferrate(III). Therefore,
the pseudo-zero-order rate constants in hexacyanofer-
rate(III) (kψ) have been evaluated from the slopes of
these straight lines. The reported rate constants data,
represented as an average of duplicate runs, were re-
producible to within 5%.
Method
To a reaction mixture containing appropriate quantities
of solutions of hexacyanoferrate(III), NaOH, and sur-
factant, required amount of doubly distilled water was
added so that the total volume of mixture was 50 mL
after adding substrate (reducing sugar). The reaction
mixture was then placed in a water bath maintained at
desired temperature 0.1◦C. The reaction mixture was
allowed to attain the bath temperature, and the reaction
was then initiated by adding requisite amount of sugar
solution placed separately in the same bath. The rates
were measured by monitoring the absorbance due to
hexacyanoferrate(III) as a function of time at 420 nm
(λmax of hexacyanoferrate(III)). The absorbance due
to hexacyanoferrate(II), substrate, and the surfactant
was negligible at this wavelength. The concentration
Kinetic Results in the Absence of Surfactant
The kinetics of oxidation of the reducing sugars by
alkaline hexacyanoferrate(III) in the absence of the
surfactant is reported in the literature [17,18]. The re-
action followed a first-order dependence of rate with
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INHIBITION EFFECT OF {SURFACTANT–SUBSTRATE} AGGREGATION ON RATE OF OXIDATION
597
The plot of kψ versus [substrate] was linear in the
absence of the surfactant, whereas that in the presence
of each surfactant deviated from linearity at higher
concentrations of the substrate, suggesting that in the
presence of surfactant the order of reaction in substrate
decreases at higher [substrate]. A plot of 1/kψ versus
1/[substrate] in the presence of the surfactant (Fig. 2)
was linear with a positive intercept, which further con-
firmed the suggested Michaelis–Menten-type kinetic
behavior.
The effect of alkali was studied at fixed ionic
strength maintained by NaClO4. The results of effect
of alkali on the rate of oxidation were identical in the
absence and in the presence of each of the surfactant.
The plot of kψ versus [OH−] (Fig. 3) was linear passing
through the origin in each case, suggesting a first-order
dependence of rate with respect to [OH−].
Figure 1 Plots of absorbance versus times (min) at 35◦C.
[Fe(CN)−6 3] = 10.0 × 10−4 mol dm−3, [substrate] = 10.0 ×
10−3 mol dm−3, [CTAB] = 0.55 × 10−4 mol dm−3, [OH−]
= 1, 2, 3, 4, 6, and 8 × 10−3 for a, b, c, d, e, and f, respectively,
µ = 0.8 × 10−2 mol dm−3 maintained by NaClO4.
Addition of hexacyanoferrate(II) and sodium per-
chlorate upto (0.002 mol dm−3 and 0.04 mol dm−3, re-
spectively) in a reaction mixture had a negligible effect
on the rate of oxidation in the presence of surfactant.
The activation parameters (Eact, ꢀH# and ꢀS#) in
the presence as well as in the absence of the surfac-
tant have been evaluated using Arrhenius and Eyring
equations and are reported in Table II. The large val-
ues of Eact and ꢀH# in the presence of surfactants
in comparison to those in aqueous medium are con-
sistent with the accepted view that the slow reaction
(in the presence of surfactant) would require a higher
Eact or ꢀH#. A comparison of ꢀS# values in aqueous
medium and in the presence of surfactants shows that
the entropy of activation in the presence of the surfac-
tants is less negative. The observed positive change in
entropy of activation in the presence of surfactant is in
contrast with the trend in the energy of activation. In
the absence of the surfactant, a negative value of ꢀS#
indicates that the activated complex in the transition
state has a more ordered or more rigid structure than
the reactants in the ground state. In the presence of the
surfactant, a positive change in entropy of activation
(less negative ꢀS#) suggests that the reactants become
respect to each OH− and reducing sugars and a zero-
order dependence of rate with respect to an oxidant. A
mechanism involving the formation of an intermediate
enediol anion of sugar in a slow and rate-determining
step and its subsequent reaction with ferricyanide in a
fast step to give product have been proposed for the
oxidation process.
Similar kinetic results have also been observed for
the oxidation of reducing sugars in the absence of the
surfactant during present investigations and, therefore,
have not been provided in the present communication.
Kinetic Results in the Presence
of Surfactants
The observed rate constants in the presence of each
surfactant (kψ) at various [Fe(CN)36−]0 remained iden-
tical (Table I), confirming an independent nature of the
rate with respect to hexacyanoferrate(III).
Table I Effect of [Hexacyanoferrate(III)] on the Rate Constant at 35◦C
kψ × 107 (mol dm−3 s−1
)
Fructose (a)
NaLS
Glucose (b)
Xylose (c)
NaLS
[Fe(CN)6]−3 × 104
(mol dm−3
)
CTAB
Brij-35
CTAB
NaLS
Brij-35
CTAB
Brij-35
6.0
8.0
10.0
12.0
2.5
2.6
2.5
2.5
2.7
2.7
2.8
2.8
3.0
3.1
3.0
3.0
1.7
1.7
1.7
1.8
2.0
2.1
2.1
2.2
2.0
2.2
2.0
2.0
2.8
2.8
2.8
2.7
2.9
2.9
2.7
2.9
2.9
2.7
2.9
2.9
[Substrate] = 10.0 × 10−3 mol dm−3, [OH−] = 2.0 × 10−3 mol dm−3 for fructose; 4.0 × 10−3 for glucose and xylose;
[CTAB] = 0.55 × 10−4 mol dm−3, [NaLS] = 13.86 × 10−3 mol dm−3, and [Brij-35] = 16.0 × 10−5 mol dm−3
International Journal of Chemical Kinetics DOI 10.1002/kin
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SHUKLA AND UPADHYAY
Figure 2 Plots of 1/kψ versus 1/[substrate] at 35◦C. [Fe(CN)6−3] = 10.0 × 10−4 mol dm−3, [OH−] = 2.0 × 10−3 mol dm−3
for a; 4.0 × 10−3 mol dm−3 for b and c, [CTAB] = 0.55 × 10−4 mol dm−3, [NaLS] = 13.86 × 10−3 mol dm−3, [Brij-35] =
16.0 × 10−5 mol dm−3. Plots a, b, and c represent fructose, glucose, and xylose, respectively.
relatively more rigid, which is not surprising in view
of the binding/association of the reactants (reducing
sugar) to the surfactant.
The effect of each surfactant on the rate of oxida-
tion has been studied at four different temperatures,
viz. 35, 40, 45, and 50◦C, and results are reported in
the form of the plots of kψ versus [surfactant] (Fig. 4).
The representative rate constants at different tempera-
tures for all the three reducing sugars in the absence of
surfactant (i.e., when [surfactant] = 0) are also given in
Fig. 4 in order to compare the inhibition action of the
surfactant on the rate. It is observed from these plots
that kψ decreases on increasing the [surfactant]. The
retarding effect of CTAB on the rate of oxidation (kψ)
has been observed even at below CMC (9.8 × 10−4 mol
dm−3). It was also observed that at very high [surfac-
tant], kψ attains a constant value (not shown in Fig. 4).
The intercepts of the plot of kψ versus [surfactant]
Figure 3 Plots of kψ versus [OH−] at 35◦C. [Substrate] = 10.0 × 10−3 mol dm−3. Other conditions are the same as in Fig. 2.
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INHIBITION EFFECT OF {SURFACTANT–SUBSTRATE} AGGREGATION ON RATE OF OXIDATION
599
Table II Activation Parameters for the Oxidation of Reducing Sugars in the Absence as well as in the Presence
of Different Surfactants
Substrate
Fructose
Surfactant
E
act
0.5 (kJ mol−1
)
ꢀH# 0.5 (kJ mol−1
)
−ꢀS# 1.0 (J K−1 mol−1
)
Absent
CTAB
NaLS
Brij-35
Absent
CTAB
NaLS
Brij-35
Absent
CTAB
NaLS
88.0
91.8
93.8
89.9
72.5
76.5
99.5
95.7
91.8
99.5
95.7
93.8
85.4
89.2
91.2
87.3
70.1
73.9
96.9
93.0
89.2
96.8
93.1
91.2
86.1
70.9
66.9
78.9
138.9
119.7
52.2
63.8
75.5
52.4
63.8
69.3
Glucose
Xylose
Brij-35
(Fig. 4) match with the observed rate constants at
[surfactant] = 0.
In the absence of other reactants, these anions un-
dergo epimerization and isomerization to form a mix-
ture of aldoses and ketoses (the Lobry de-Bruyn-
Alberda Van Ekenstien transformation). Aldoses and
ketoses generally yield a mixture of Z- and E-enediols,
the proposition of which differ from sugar to sugar and
experimental conditions, viz. strength and nature of al-
kali and temperature. In the presence of an oxidant or
a catalyst, the enediol anion has been considered as
the reactive species of the reducing sugar. The faster
rate of fructose enolization has been explained [20] on
the basis of an easy attack on the two active primary
H-atoms at C-2 of aldoses. It has been observed that
an aldohexose (glucose) reacts more slowly than an
aldopentose (xylose). A large group of aldohexose on
C-5 may retard the tendency of sugar to enolize.
A comparison of the kψ for various reducing sugars
under the similar experimental conditions (Table III)
indicate the reactivity of the order of
At high [CTAB], turbidity in the reaction mixture
was observed and, therefore, the rate constant (kψ) at
above CMC of CTAB could not be determined. It was
also observed that a small inhibition effect of NaLS and
Brij-35 was started even below of their CMC, but it was
more pronounced at above CMC of the surfactants.
There was also no turbidity in the reaction mixture
at high concentrations of the surfactant in these cases.
Therefore, in the case of NaLS and Brij-35, the kinetics
has been studied above the CMC of the surfactants.
A first-order dependence of the rate with respect
to each alkali and reducing sugar in the absence of
surfactant and observed first-order dependence of the
rate with respect to OH− in the presence of surfactants
indicate the enolization of reducing sugar as the rate-
determining step. In alkaline medium, the formation
of enediol anion of monosaccharides takes place as
follows:
kψ(fructose) > kψ(xylose) > kψ(glucose)
Thus, the observed reactivity order, i.e. fructose > xy-
lose > glucose, also confirms the enolization of reduc-
ing sugar as the rate-determining step.
Table III kψ for Different Reducing Sugars under Similar Experimental Conditions
kψ × 107 (mol dm−3 s−1
)
Surfactant
Fructose
Glucose
Xylose
[CTAB] = 0.55 × 10−4 (mol dm−3
)
)
)
5.5
5.8
6.0
1.7
2.1
2.0
2.8
2.7
2.9
[NaLS] = 13.86 × 10−3 (mol dm−3
[Brij-35] = 16.0 × 10−5 (mol dm−3
[OH−] = 4.0 × 10−3 mol dm−3, [substrate] = 10.0 × 10−3 mol dm−3, [Fe(CN)−6 3] = 10.0 × 10−4 mol dm−3, and temperature = 35◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin

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SHUKLA AND UPADHYAY
Figure 4 Plots of kψ versus [surfactant] at different temperatures. Other conditions are the same as in Figs. 2 and 3.
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INHIBITION EFFECT OF {SURFACTANT–SUBSTRATE} AGGREGATION ON RATE OF OXIDATION
601
The retarding effect of the surfactant on the rate
of oxidation and the observed Michaelis–Menten
behavior during the substrate effect in the presence of
the surfactants clearly indicate the binding/association
of the reducing sugar with surfactant and inactiveness
of the bounded/associated species toward the oxidant,
i.e. Fe(CN)36−. The inhibition effect by CTAB below
the CMC may be caused by the interaction between the
substrate and submicellar aggregates of the surfactant
that stabilize the initial state, or the substrate might pro-
mote micellization of the surfactant by the formation of
molecular complex between the substrate and surfac-
tant [21]. There is also evidence [22] for the formation
of small complexes between surfactant molecules and
substrate at the concentration of the surfactant below
CMC. In such instances, catalysis or for that matter
inhibition occurs at the surfactant concentration lower
than that for CMC.
In the absence of surfactants, considering steps (i)
and (ii) and by applying steady-state conditions with
respect to [S−], the rate of disappearance of Fe(CN)36−
may be given as
ꢀ
ꢁ
d Fe(CN)36−
ꢀ
ꢁ
−
= k2[S−] Fe(CN)36−
dt
ꢀ
3−ꢁ
k1k2[S][OH−] Fe(CN)6
=
ꢀ
ꢁ (1)
k
−1[H2O] + k2 Fe(CN)63−
Again, k2[Fe(CN)36−] ꢁ k−1[H2O] may be taken as
suitable approximation (step (ii) in fast step), the rate
law (1) reduces to
ꢀ
ꢁ
d Fe(CN)36−
−
= k1[S][OH−]
(2)
dt
To confirm association or complexation between
reducing sugar and surfactant, the spectrum of various
freshly prepared samples was analyzed on UV-double
beam spectrophotometer (SYSTRONICS-2203).
When the spectrum of the solution consisting of 1.0
× 10−3 mol dm−3 fructose was analyzed in the range
200–400 nm, no peak was observed in the spectrum.
Again no peak was observed in the spectrum of the
solution consisting of 1.0 × 10−3 mol dm−3 fructose
and 5.5 × 10−4 mol dm−3 CTAB. When the solution
consisting 1.0 × 10−3 mol dm−3 fructose and 1.0
× 10−2 mol dm−3 OH− was analyzed, a peak was
observed at 206.4 nm. Furthermore, on adding 5.5
× 10−4 mol dm−3 CTAB in the above solution, the
peak shifted from 206.4 to 211 nm. An increase in
the absorbance of the solution on adding CTAB was
also noted. A shift in the peak and an increase in
the absorbance on addition of CTAB in the solution
of reducing sugar in the presence of OH− clearly
indicate the interaction between enediol anion of sugar
and surfactant.
The experimental results, i.e. the zero-order depen-
dence of rate with respect to oxidant and a first-order
dependence of rate with respect to each substrate and
OH− in the absence of surfactants, are in agreement
with the rate law (2). Observed negligible salt effect
supports the involvement of a neutral species in the
rate-determining step.
It is also observed that the rate of reaction depends
upon the formation of enediol anion of the reducing
sugar.
In the presence of surfactants, step (iii) should also
be considered. According to step (iii),
[X] = KS[S−][surfactant]
If [S−]0 and [surfactant]0 are the total concentration of
enediol anion and surfactant, respectively, then,
[S−]0 = [S−] + [X] or [S−] = [S−]0 − [X]
and
On the basis of above facts and kinetics results,
a common mechanism for the oxidation of reducing
sugars by hexacyanoferrate(III) in the presence of sur-
factants is proposed in Scheme 1.
[surfactant]0 = [surfactant] + [X]
or
[surfactant] = [surfactant]0 − [X]
Therefore, [X] becomes
[X] = Ks{[S−]0 − [X]}{[surfactant]0 − [X]}
where Ks = k/k . Therefore, the value of [S−], i.e.
−
concentration of enediol anion at any time, may be
obtained as
[S−]0{k + k[S−]0}
−
[S−] =
(3)
k + k[surfactant]0 + k[S−]0
Scheme 1
−
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SHUKLA AND UPADHYAY
where k[X]2 term has been neglected in comparison to
This equation has been observed to hold good for con-
tinuous inhibition in various reactions. The values of
km and Ks can be evaluated with the help of slope and
intercept of the plot.
The binding constant (Ks) of the substrate with sur-
factant can be related [24–27] to the partition coeffi-
cient (P) by relation
other terms.
Furthermore, k [S−]0 ꢁ k[S−]2 may be taken as
−
0
suitable approximation and therefore, Eq. (3) reduces
to
k [S−]0
k + k[surfactant]0 + k[S−]0
−
[S−] =
(4)
−
¯
Ks = PV
According to Eq. (4), the formation of enediol anion
at any time will be affected by the total concentra-
tions of surfactant and enediol anion. Since the rate
of reaction depends upon formation of enediol anion
that is proportional to {[reducing sugar] [OH−]}, the
order of reaction in the substrate at higher [substrate]
will decrease (due to binding/association of substrate
with surfactant), which is expected from Eq. (4). The
retarding effect of surfactant on the rate of reaction
can also be explained on the basis of Eq. (4). A de-
crease in order of reaction in OH− is also expected from
Eq. (4), although it has not been observed experimen-
tally because the concentration of OH− was very low
(10−3 mol dm−3) under the experimental conditions.
The binding constant between the micelle and the
substrates in case of NaLS and Brij-35 has been cal-
culated by the Menger and Portnoy model [23] re-
ported for miceller inhibition. According to the Menger
and Portnoy model, the substrate “S” is distributed be-
tween the aqueous and micellar pseudophase as given
in Scheme 2.
¯
where V is the partial molar volume of the sur-
factant monomer and the partition coefficient P =
[substrate]micelle/[substrate]water
.
The molar volume [28] of the surfactant has been
calculated using the relation
x1M1 + x2M2
¯
V =
d
where x1 and x2 are mole fraction of surfactant and
water (solvent), respectively and M1 and M2 are their
molecular weights.
The binding constant is also related to standard
transfer free energy per mol (ꢀµ◦) of a solute from
water to micelle [29,30] by the following equation:
ꢀµ◦ = −RT ln(55.5Ks)
The validity of the Menger and Portnoy model in
case of NaLS and Brij-35 has been tested by the plot
In the scheme, kω, km, and kψ are rate constants
in aqueous phase, micellar media, and observed rate
constant, respectively; Ks is the binding or associa-
tion constant of substrate with surfactant. Dn, S, and
SDn represent micellar surfactant, free substrate, and
associated substrate, respectively.
−
of 1/(kω kψ) versus 1/([D] − CMC). The observed
−
linearity in the 1/(kω kψ) versus 1/([D] − CMC) plots
(Fig. 5) indicates the applicability of the Menger and
Portnoy model in the systems. The value of km and
binding constant (Ks) were obtained and are reported
in Table IV. The value of partition coefficients and ꢀµ◦
have also been evaluated in each case and are reported
in Table V.
According to Scheme 2, the observed rate constant
kψ may be given as
Micellar aggregation/binding between the surfac-
tant and the substrate is well reported in the literature.
The effect of organized structure on the rate of chem-
ical reactions has been attributed to the hydrophobic
as well as electrostatic interactions. The electrostatic
surface potential at micellar surface can attract or repel
the reaction species, and hydrophobic interactions can
bring about the incorporation into the micelle even of
the reagent that bears the same charge or neutral as
ionic micelle. Thus, the rate and mechanism of chemi-
cal reactions may be affected by means of electrostatic
and/or hydrophobic interactions.
kω + kmKs{[D] − CMC}
kψ
=
1 + KS{[D] − CMC}
The above equation may be rearranged in the form
1
1
1
=
+
−
−
−
(kω kψ)
(kω km) (kω km)Ks{[D] − CMC}
The retardation of the rate of disappearance of fer-
ricyanide with an increase in surfactant concentration
indicates the inactiveness of the bounded/associated
(surfactant–substrate) species toward the oxidant, i.e.,
Scheme 2
International Journal of Chemical Kinetics DOI 10.1002/kin

INHIBITION EFFECT OF {SURFACTANT–SUBSTRATE} AGGREGATION ON RATE OF OXIDATION
603
Figure 5 Plots of 1/(kω – kψ) versus 1/([D] – CMC) at different temperatures in case of NaLS and Brij-35. Other conditions
are the same as in Figs. 2 and 3.
Table IV Binding Constants (Ks) and Rate Constants in Micellar Media (km) at Different Temperatures
K × 102
k
× 107 (mol dm−3 s−1
)
s
m
Surfactant
NaLS
Substrate
35◦
40◦
45◦
50◦
35◦
40◦
45◦
50◦
Fructose
Glucose
Xylose
Fructose
Glucose
Xylose
2.3
3.0
2.8
175
300
350
2.0
2.6
2.2
160
250
275
1.5
1.8
1.0
150
220
150
1.0
0.8
0.5
75
125
50
2.4
1.9
2.2
2.3
1.6
2.5
4.1
2.2
2.3
4.6
2.2
2.6
7.1
4.1
2.6
6.8
4.7
4.3
11.4
7.3
5.2
9.8
9.9
5.2
Brij-35
Table V Partition Coefficients (P) and Free-Energy Transfer from Water to Micelle (−ꢀµ) at Different Temperatures
P
−ꢀµ (kJ mol−1
)
Surfactant
NaLS
Substrate
35◦
40◦
45◦
50◦
35◦
40◦
45◦
50◦
Fructose
Glucose
Xylose
Fructose
Glucose
Xylose
9.2
12.3
11.3
878
1506
1757
8.3
10.7
8.3
803
1255
1318
6.2
7.2
4.1
753
1104
753
4.1
3.0
2.0
376
627
251
24.1
24.8
24.6
35.3
36.6
37.0
24.2
24.9
24.2
35.6
36.7
37.0
23.8
24.2
22.7
36.0
37.0
36.0
23.1
22.3
21.2
34.7
36.1
33.6
Brij-35
International Journal of Chemical Kinetics DOI 10.1002/kin

604
SHUKLA AND UPADHYAY
Fe(CN)36−. When surfactant is present in the reac-
tion mixture, it incorporates/binds the substrate by hy-
drophobic interactions, leading to a decrease in the
concentration of the substrate in aqueous phase, and
thus a retarding effect of the surfactant on the rate of
reaction is observed.
There are no electrostatic interactions with polar
head groups of nonionic surfactant, i.e. Brij-35. The
poly(oxyethylene) head groups of nonionic surfactant
play a significant role in favoring the incorporation or
solubilization of the substrate in the micelle. The high
values of Ks and P in case of Brij-35 are in favor of
the absence of the electrostatic forces.
In the case of NaLS, the electrostatic repulsion be-
tween negatively charged substrate (enediol anion of
sugar) and ionic surfactant opposes the binding be-
tween the micelle and substrate. Thus, in case of NaLS,
the hydrophobic interactions favor the binding whereas
the electrostatic repulsion opposes it. Consequently,
the binding between the micelle and substrate should
be much less. The low values of Ks and P are in fa-
vor of domination of electrostatic repulsions in case of
NaLS.
In the case of CTAB, the electrostatic attraction
between negatively charged substrates (enediol anion
of sugar) and positively charged surfactant favors the
binding between the surfactant and substrate in addi-
tion to hydrophobic interactions. The domination of
electrostatic attractions may be responsible for the as-
sociation of CTAB and substrate even below CMC of
the surfactant. Since the Menger and Portnoy model
is applicable for the binding of the substrate with mi-
celle, it has not been applied in case of CTAB, where
the substrate is associated with surfactant molecule(s)
and not with the micelle.
4. Hong Jun Quin; Rong Guo, Acta Phys Chim Sinica
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8. Sanchez, F.; Moya, M. L.; Jimenez, R.; Gomez, H. C.;
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Trans 1997, 93, 1281.
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1191.
10. Gemeay, A. H.; Mansour, I. A.; El-Sharkawy, G. R.;
Zaki, B. A. J Colloid Interface Sci 2003, 263, 228.
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269, 7.
12. Kambo, N.; Upadhyay, S. K. J Dispersion Sci Technol
2006, 27(6) 887.
13. Mushran, S. P.; Agrawal, M. C., Rasayanik Samiksha
1974, 4, 347.
14. (a) Harley, C. S.; Collie, B.; Samis, C. S. Trans Faraday
Soc 1936, 32, 795; (b) Menger, F. M.; Portnoy, C. E. J
Am Chem Soc 1967, 89, 4698; (c) Zheng, Z.; Obbard,
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15. Iyenger, T. A.; Puttaswamy; Mahadevappa, D. S. Car-
bohydr Res 1990, 197, 119.
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D.C.H.: Boston, 1955, 284.
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The transfer of free-energy charge per mole from
water to micelle (ꢀµ◦) in case of NaLS and Brij-35 is
also in accordance with the above result. A decrease
in the value of Ks or P on increasing the temperature
suggests that the binding is an exothermic process.
23. Menger, F. M.; Portnoy, C. E. J Am Chem Soc 1967, 89,
4698.
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We wish to thank professor R. P. Singh, Director, HBTI,
Kanpur, for his keen interest in the work.
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International Journal of Chemical Kinetics DOI 10.1002/kin