Role of cationic gemini surfactants (m-s-m type) on the oxidation of d-glucose by permanganate

Article abstract of DOI:10.1016/j.molliq.2016.01.055

Cationic gemini (m-s-m type; m = 16, s = 4,5,6) surfactants were used to determine the micelles assisted kinetics parameters, mechanism of permanganate-D-glucose redox system in an aqueous solution by means of UV-visible spectroscopy at 40 °C. Effects of different [gemini surfactant], [permanganate], [D-glucose] and temperature on the reaction rate were investigated. Various activation parameters such as activation energy (Ea), enthalpy of activation (ΔH^#), free energy of activation (ΔG^#), and entropy of activation (ΔS^#) have been evaluated. Menger-Portony pseudo-phase model modified by Bunton was used to analyze the role of gemini surfactant on the rate constant. Spacer chain length of surfactants has significant impact on the oxidation -reduction kinetics. A suitable mechanism reliable with the experimental results has been proposed and discussed. The cationic gemini surfactant micellar media are relatively more efficient than conventional monomeric surfactant i.e. cetyltrimethylammonium bromide (CTAB).

Full text of DOI:10.1016/j.molliq.2016.01.055

Journal of Molecular Liquids 216 (2016) 538–544
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Role of cationic gemini surfactants (m-s-m type) on the oxidation of
D-glucose by permanganate
Shaeel Ahmed Al-Thabaiti a, A.Y. Obaid a, Zaheer Khan a, Khloud Saeed Al-Thubaiti a,
Arshid Nabi b, Maqsood Ahmad Malik a,
⁎
a
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Cationic gemini (m-s-m type; m = 16, s = 4,5,6) surfactants were used to determine the micelles assisted kinet-
ics parameters, mechanism of permanganate-D-glucose redox system in an aqueous solution by means of UV–
visible spectroscopy at 40 °C. Effects of different [gemini surfactant], [permanganate], [D-glucose] and tempera-
ture on the reaction rate were investigated. Various activation parameters such as activation energy (Ea), enthal-
py of activation (ΔH#), free energy of activation (ΔG#), and entropy of activation (ΔS#) have been evaluated.
Menger-Portony pseudo-phase model modified by Bunton was used to analyze the role of gemini surfactant
on the rate constant. Spacer chain length of surfactants has significant impact on the oxidation -reduction kinet-
ics. A suitable mechanism reliable with the experimental results has been proposed and discussed. The cationic
gemini surfactant micellar media are relatively more efficient than conventional monomeric surfactant i.e.
cetyltrimethylammonium bromide (CTAB).
Received 25 September 2015
Received in revised form 31 December 2015
Accepted 16 January 2016
Available online xxxx
Keywords:
Cationic gemini surfactant
Micellar catalysis
D-Glucose
Permanganate
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
and two hydrophilic groups covalently bonded through a spacer,
which provides them special attention in both academic and industrial
Surfactant oriented research has become the topic of interest be-
cause of their physicochemical properties and potential application in
chemical and technological areas like mineral processing, petroleum,
pharmaceutical, food science and cosmetics [1–7]. They play a signifi-
cant role in resolving many hazardous environmental problems caused
by toxic polycyclic aromatic hydrocarbons. Polycyclic aromatic hydro-
carbons are formed by the pyrolysis of organic compounds during fossil
fuel utilization, forest fires, etc. Surfactant-enhanced remediation (SER)
process has been developed for the remediation of polycyclic aromatic
hydrocarbons from contaminated soil and water system [8]. The struc-
ture of the surfactant molecule under consideration determines the
physicochemical properties of surfactant molecules. For instance,
when two long alkyl chains with polar head groups are bonded cova-
lently by spacer of methylene units. Gemini surfactants are considered
a unique class of surfactant. The unique physicochemical properties
and enhanced performance in micellar catalysis have provided gemini
surfactants a wide range attention from different scientific fields.
Owing to these excellent properties, gemini surfactants have been
used in industrial detergency, gene transfection, and also as corrosion
inhibitors [9,10]. The dimeric or gemini surfactants are different from
conventional surfactants because they have two hydrophobic chains
research fields [11–14]. The chemical structure of the two-headed,
two-tailed surfactant connected at the level of the head groups by a
spacer (s) is shown in Fig. 1. The various surface active properties of
gemini surfactant molecules are superior to those of corresponding con-
ventional surfactants having one hydrophilic and hydrophobic group.
They are the new generation surfactants with very low critical micelle
concentration, low Krafft points, unusual aggregation morphologies,
better wetting properties and have capability in lowering the surface
tension of water [15–21].
One of the widely used reaction media for various important organic
reactions is the micelle forming surfactant media and their catalytic be-
havior toward organic reactions is an interesting topic of discussion. The
esterolytic cleavage of phosphate and carboxylate esters in the presence
of cationic gemini surfactants is well documented in the literature [22].
The micellization kinetic effects and micellar growth of cationic dimeric
surfactant 12-s-12, 2Br− (spacer = 2–6) have been investigated in
aqueous and organic solvents [23,24]. The effect of gemini surfactants
on the chemical reaction rate has been the topic of interest for many re-
search groups from the last many years [25–27]. For understanding sev-
eral complex aspects about gemini micelles, kinetic investigations are
one of the best tools that can help for better observations [28–30]. Dif-
ferent kinetic and thermodynamic investigations have been made on
the micellization and micellar growth of different cationic dimeric sur-
factants both in aqueous and organic media [31–34]. The permanganate
ion is an important oxidizing agent in neutral, alkaline and acidic media.
⁎
Corresponding author at: Chemistry Department, Faculty of Science, King Abdulaziz
University, P.O. Box 80203, Jeddah 21589, Saudi Arabia.
E-mail address: maqsoodchem@gmail.com (M.A. Malik).
http://dx.doi.org/10.1016/j.molliq.2016.01.055
0167-7322/© 2016 Elsevier B.V. All rights reserved.

S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
539
Fig. 1. Chemical structure of gemini surfactant (16-s-16).
It is an eco-friendly and versatile oxidizing agent used for studying the
kinetics of oxidation of various organic and inorganic reactions. It has
achieved lot of importance in green chemistry as one of the most used
oxidants for water treatment [35]. Permanganate is stable in neutral
or slightly alkaline media but disproportionates in strongly alkaline
media to form manganese(V) (hypomanganate) or manganese(VI)
(manganate) [36]. The role of gemini surfactants in the oxidation of
mixture for two to three times till the purity of the compound
was established through TLC. The percentage yield of the synthe-
sized product was observed to be about 70–75%. The IR spectrum
of the gemini surfactants was recorded on a Bruker Tensor II FT-IR
Spectrometer. Perkin-Elmer series II analyzer was used for the ele-
mental analysis of the surfactants. 1H NMR spectra of the synthe-
sized cationic gemini surfactants were recorded on a 600-MHz
Bruker NMR spectrometer in CDCl3.
D-glucose by permanganate has not yet been reported. Therefore in
the present study we are reporting the oxidation of D-glucose by per-
manganate in the presence of three gemini surfactants.
2.3. Determination of critical micellar concentration (cmc)
2. Experimental
Conventional conductivity technique was used to determine the cmc
(the concentration over which monomeric surfactant molecules rapidly
aggregate to form micelles) of the gemini (16-s-16, s = 4, 5, 6) surfactants.
Aqueous KCl solutions in the proper concentration range were used for
the calibration of the conductivity cell (cell constant = 1.02). All conduc-
tivity measurements of the desired solution were carried out at 40 °C. The
cmc was achieved from the break points of nearly two straight lines of the
specific conductivity versus [surfactant] [39] and their values under differ-
2.1. Materials
The water (doubly distilled deionized with a conductivity of
(1–2) × 10−6
Ω
−1 cm−1) was used as solvent to the preparation of all
regents solutions. D-Glucose, potassium permanganate (KMnO4),
sulphuric acid (H2SO4), 1,4-dibromobutane, 1,5-dibromopentane,
1,6-dibromohexane, N,N-dimethylhexadecylamine, ethyl acetate
and ethanol absolute were purchased from Aldrich and used as re-
ceived. All the solvents and reagents used were of AR grade.
ent experimental conditions, i.e., gemini + water, gemini + MnO4−
gemini + glucose, and gemini + H2SO4 are summarized in Table 1.
,
2.2. Synthesis of dimeric gemini surfactant
2.4. Reaction product analysis
Alkanediyl-α,Ω-bis(dimethylhexadecylammonium bromide)
type of gemini surfactant, containing –N(CH3)2 head groups has
been prepared by the reported method [37,38] as shown in
Scheme 1. In a three necked round bottom flask a mixture of N,N-
dimethylhexadecylamine and α,Ω-dibromoalkane (molar ratio
2.1:1) was stirred in dry ethanol at 80 °C for about 48 h to ensure
the highest bisquaternization possible. The progress of the reaction
was scrutinized by thin layer chromatography. Hexane/ethyl ace-
tate mixture was used for washing the synthesized crude white
solid. The product was recrystallized from methanol/acetone
In order to identify the oxidation product of D-glucose, a series of ex-
periments were performed under different experimental conditions.
After the completion of the kinetic experiment, alkaline hydroxylamine
solution was added to the oxidized reaction mixture and the lactone
presence was examined by FeCl3–HCl blue test [40]. In order to neutral-
ize the reaction mixture barium carbonate was added to the reaction
mixture. On adding FeCl3 solution that had been colored violet with
phenol, turned bright-yellow, indicating that aldonic acid is formed as
the oxidation product. Apparently, the lactone, which formed in the
rate determining step, hydrolyzed to the aldonic acid in neutral medium
in a fast step [41]. In addition, lactone was identified against an authen-
tic sample (1,4-d-glucolactone) using 4:1:5 n-butanol-acetic acid-water
eluent. A three-stage dip of AgNO3, NaOH, and Na2S2O3 was used to vi-
sualize the paper chromatograms [42].
Table 1
The cmc values of gemini surfactants under different experimental conditions at 40 °C.
Reaction solutiona
16-4-16
16-5-16
16-6-16
104 cmc (mol dm−3
)
Water + 16-s-16
16-s-16 + MnO−4
16-s-16 + H2SO4
16-s-16 + glucose
0.41 (.30)
0.35
0.28
0.35 (0.34)
0.29
0.23
0.31 (0.39)
0.25
0.19
0.38
0.30
0.28
The literature values of cmc are given in the parenthesis.
a
Scheme 1. Synthetic route for synthesis of the gemini surfactant (16-s-16) (spacer
(s) = 4,5,6).
[Glucose] = 4.0 × 10−3 mol dm−3; [MnO−4] = 4.0 × 10−4 mol dm−3; [H2SO4] = 1.0
× 10−4 mol dm−3
.

540
S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
2.5. Kinetic measurements
interfacial ion exchange and the binding constant of the substrate are
the key factors for the efficiency of micellar catalysis. It seems reason-
able to expect that a factor of importance for the relative rates in the
aqueous and micellar pseudophase would be the orientation of the
substrate molecule within the surfactant aggregate. The overall kinetic
behavior of the surfactants is described in the framework of the
pseudo-phase model. It has been established that the rate enhancement
is mainly due to the reagent concentration in the micellar pseudo-
phase. In this system, substrate is distributed between the bulk and mi-
cellar phases (Scheme 2).
In Scheme 2, subscripts w and m represent aqueous and micellar
pseudo-phases. kw and km are the second order rate constants in
aqueous phase and micellar pseudophase, respectively. Kn and Ks are
the binding constants of D-glucose and MnO−4 to the micelles, respec-
tively and [Dn] = the concentration of the micellized surfactant =
([16-s-16]T-cmc).
The required volumes of MnO4−, 16-s-16 gemini surfactant and
H2SO4 (for maintaining the desired [H+]) were introduced into a
three necked reaction vessel fitted with a double-walled condenser to
arrest evaporation. The reaction vessel was kept in a thermostat main-
tained at a desired temperature (40 °C). The reaction was started by
adding the required and thermally equilibrated solution of D-glucose.
The zero time was taken when half of the D-glucose solution has been
added. The reaction progress was followed at 525 nm (λmax of MnO−4 )
using a sampling technique and UV/vis spectrophotometer (UV-260
Shimadzu, with 1 cm quartz cuvettes) was used for absorbance mea-
surements at specific time intervals. A control dynamic pH-meter fitted
with a combination electrode was used to measure the pH of the work-
ing solutions. Pseudo-first-order conditions were used to determine the
rate constants (kobs, s−1) by using a large excess of D-glucose over
MnO−4 in all kinetic runs. Duplicate runs gave results that were repro-
ducible to within 4%. Other details of the kinetic measurements
were the same as described earlier [43].
Corresponding to the Scheme 2, the following rate equation can be
obtained as Eq. (1) and modified as Eq. (2):
kw þ kmKs½Dnꢀ
kobs
¼
ð1Þ
ð2Þ
3. Results and discussion
ð1 þ Ks½DnꢀÞ
À
Á
k0 ½D‐glucoseꢀ þ Ksk0 ‐k0w MGS½Dnꢀ
3.1. Effect of [16-s-16] on reaction rate
w
T
m
kobs
kw0
¼
ð1 þ Ks½DnꢀÞ
The effect of [16-s-16] (s = 4,5,6) on the reaction rate was observed
by varying different concentrations of gemini surfactants (from
1.0 × 10−4 to 22.0 × 10−4 mol dm−3) and keeping all other reaction
conditions constant, i.e. [MnO−4 = 4.0 × 10−4 mol dm−3, [D-glucose] =
4.0 × 10−3 mol dm−3, [H2SO4] = 1.00 × 10−4 mol dm−3 and temper-
ature = 40 °C. It was observed that reaction rate first increases with
[surfactant], remains constant up to certain concentration and increases
sharply at higher concentrations (Fig. 2). Inspection of data clearly indi-
cates that used surfactant has pre- and post-micellar catalytic effects on
the redox reaction, which might be due to the preponement of micelli-
zation by reactants and/or the presence of premicelles [44]. Generally,
micellar catalysis has been interpreted in terms of the pseudophase
ion exchange model and pseudophase model [45]. The catalytic effect
of gemini surfactants on the reaction rate is explained in terms of the
Menger and Portnoy [46] pseudo-phase model (Scheme 2), later devel-
oped by Bunton [47]. Micelles are dynamic structures which are influ-
enced by counterions, ionic strength, polarity of the medium and
temperature, etc. [48]. The effects of structural variation of the surfac-
tants have been analyzed by various models and theories [49–51]. The
kw
½D‐glucoseꢀ
km
½D‐glucoseꢀ
½Dnꢀ
¼
km0
¼
MGS
¼
MGS
The following equilibrium was used to obtain the value MGS.
Using Eq. (4) and the mass balance of glucose, [D-glucose]T
=
[(D-glucose)w] + [(D-glucose)m], a quadratic Eq. (5) can be obtained
which is solved for [(D-glucose)m] with the help of a computer program
with Kn as an adjustable parameter [52]. The cmc values were deter-
mined under experimental conditions and a non-linear least-square
technique was used for the calculation of k'm and Kn. These values are
summarized in Table 2.
Kn
ðD‐glucoseÞw þ Dn ⇌ ðD‐glucoseÞm
ð3Þ
ð4Þ
Â
Ã
ðD‐glucoseÞm
Â
Ã
À
Â
ÃÁ
Kn
¼
ðD‐glucoseÞw þ ½Dnꢀ‐ ðD‐glucoseÞm
Â
Ã
À
Á
Kn ðD‐glucoseÞm 2‐ 1 þ Kn½Dnꢀ þ Kn½D‐glucoseꢀ
ꢁ ðD‐glucoseÞm þ Kn ½Dnꢀ ðD‐glucoseÞT ¼ 0 T
Â
Ã
À
Â
ÃÁ
ð5Þ
Under the present experimental condition it was observed that the
rate constant values are more at 16-4-16 as compared to 16-5-16 and
16-6-16 and follow the order 16-4-16 N 16-5-16 N 16-6-16 among the
gemini molecules. It is well documented in the literature that the length
of the spacer and the type of the moiety dictate the conformation of the
gemini surfactant molecule [53]. Because of shorter spacer length the
micelle formation is more in 16-4-16 as compared to 16-5-16 and 16-
6-16 gemini molecules. The shorter spacer is the reason for the increas-
ing geometrical constraints in the formation of aggregates with decreas-
ing the spacer unit length [54]. The cmc values decrease with increased
in hydrophobic chain length of the surfactant molecule. The micellar
morphology tends to be less ellipsoidal with increasing spacer in gemini
surfactants is well supported by microviscosity and SANS data [55]. As a
result the spacer greatly controls the surfactant morphology and the
rate constant values obtained in the present study are consistent with
the expectation being maximum at spacer = 4, beyond which looping
of the spacer (to minimize its contact with water [56]) will progressive-
ly make the Stern layer more wet with the resultant rate constant de-
crease. Thus, increasing the hydrophobicity of spacer from 4 to 6
entirely changes the whole scenario of the reaction kinetics.
Fig. 2. Plot showing the effects [16-s-16] on kobs of the oxidation of D-glucose by
permanganate. Reaction conditions: [MnO−4 ] = 4.0 × 10−4 mol dm−3, [D-glucose] =
4.0 × 10−3 mol dm−3, [H2SO4] = 1.00 × 10−4 mol dm−3, temperature = 40 °C.

S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
541
Scheme 2. Schematic representation for the distribution of D-glucose and MnO−4 in micelles according to pseudophase model.
Inspection of Table 1 clearly suggests that the corresponding cmc
values of 16-s-16 surfactants in aqueous solution are in close agreement
with the values reported in the literature [57]. The small change in the
cmc with MnO−4 could be associated to the formation of ion-pair
between permanganate ion and surfactant molecules through
electrostatic interactions. This behavior indicates the electrostatic inter-
action between the positive head group of 16-s-16 surfactants and per-
manganate ion or the association of the MnO−4 into the stern layer of the
16-s-16 micelles, which results in the formation of ion-pair complex be-
tween MnO−4 and cationic gemini surfactant. In the present study,
H2SO4 was used to maintain [H+] constant. Therefore, cmc values
were also determined in the presence of H2SO4. Table 1 also indicates
that the cmc values decreases in presence of H2SO4, which might be at-
tributed due to the electrostatic interactions and/or association of HSO4−
and SO24− ions with the positive head group of 16-s-16 cationic micelles.
shortest spacer (s = 4). It is due to the fact that a decrease in spacer
chain length increases the surface charge density of the micelles. Thus,
16-4-16 interacts more strongly with the MnO−4 as compared to the
other largest spacer geminis (16-5-16 and 16-6-16). Reactions take
place between the micellar solubilized MnO−4 and D-glucose and the
bound counter ions in the Stern and Gouy-Chapman layers' junctural re-
gion. Micellization increases the reaction rate because on micellization
counterions can be attracted or repelled more effectively. Micellar sur-
faces are water-rich and do not provide a uniform reaction medium be-
cause a micelle is a porous cluster with a rough surface and deep-water-
filled cavities. Therefore, it is not possible to precisely locate the exact
site to the micelle-assisted reactions. However, localization of the reac-
tants can be considered. A highly schematic (possible solubilization
and/or incorporation) could be that as shown in Scheme 3.
3.3. Effect of [reactants] on reaction rate
3.2. Probable reaction site
To find the effect of [MnO−4 ] on the reaction rate, [MnO−4 ] was varied
from 2.0 to 8.0 × 10−4 mol dm−3 at fixed [D-glucose] = 4.0 × 10−3
mol dm−3, [H2SO4] = 1.00 × 10−4 mol dm−3 and temperature =
40 °C in the presence of constant gemini surfactants [16-s-16] =
3.0 × 10−4 mol dm−3. The reaction rate remains constant and did not
show any significant effect, indicating pseudo-first kinetics with respect
to [MnO−4 ]. To investigate the effect of [D-glucose] on the reaction rate,
[D-glucose] was varied from 1.0 × 10−3 to 10.0 × 10−3 mol dm−3 at
It has been established that electrostatic, hydrophobic, and hydro-
gen bonding were the main driving force for the solubilization and/or
incorporation of reactants (MnO−4 and glucose) into the micellar
pseudo-phases. Due to the electrostatic interactions, MnO−4 formed
ion-pair with the positive head group (−N+(CH3)2) of surfactant. As
the spacer chain length increases between the two cationic head groups
of used gemini surfactants, surface area of the solubilized reactants de-
creases, which in turn, decreases the reaction rates (Table 2). In the
present case, the maximum rate enhancement is found with the
constant [MnO−4
]
=
4.0
×
10−4 mol dm−3
,
[H2SO4]
=
1.0 × 10−4 mol dm−3, [16-s-16] = 3.0 × 10−4 mol dm−3 and temper-
ature = 40 °C. The reaction rate increases with [D-glucose] (Fig. 3). The
plot of log kobs versus log [D-glucose] gave a straight line with a slope of
0.93, 0.86, 0.72, in the presence of gemini surfactants 16-6-16, 16-5-16
and 16-4-16 respectively, shows that the oxidation reaction was frac-
tional order with respect to [D-glucose]. The double reciprocal plot of
1/kobs versus 1/[D-glucose] gave a straight line with a definite intercept,
indicating the reaction follow Michaelis–Menten type of kinetic. Initially
the kinetic runs were carried out in absence of H2SO4 and it was
observed that the reaction does not take place. This suggests that the
reaction is highly dependent on [H+]. The rate constant, obtained as a
Table 2
Activation parameters, rate and binding constant values for the D-glucose oxidation by
permanganate. Reaction conditions: [MnO−4 ] = 4.0 × 10−4 mol dm−3, [D-glucose] = 4.0
× 10−3 mol dm−3, [16-s-16] = 3.0 × 10−4 mol dm−3 and [H2SO4] = 1.00 × 10−4 mol
dm−3
.
Temperature (°C)
104 kobs (s−1
16-6-16
)
16-5-16
16-4-16
function of [H2SO4] at constant [MnO−4 ] = 4.0 × 10−4 mol dm−3
10−3 mol dm−3
[D-glucose] 4.0 [16-s-16] =
,
40
45
50
55
60
1.02
1.40
1.95
2.65
3.40
1.25
1.70
2.25
3.0
1.50
2.0
2.55
3.55
4.35
=
×
,
3.0 × 10−4 mol dm−3 and temperature = 40 °C, was found to
reach a maximum at 5.0 × 10−4 mol dm−3 H2SO4, and then fall dras-
tically. The plot between log[H2SO4] and logkobs was found to be lin-
ear with positive slopes = 0.87, 0.82, 0.79 for 16-6-16, 16-5-16 and
16-4-16, respectively. Thus the reaction follows fractional-order ki-
netics with respect to [H2SO4]. Interestingly, the kinetics and mech-
anism of D-glucose-MnO−4 redox system is the same in the absence
and presence of gemini surfactants. It is also concluded from the ob-
served results that the order of reaction in the presence of cationic
surfactants do not differ in any way as that of aqueous medium.
3.90
Parameters
Ea (kJ mol−1
)
52
49
48
45
44
41
−199
103
8.1
ΔH# (kJ mol−1
ΔS# (JK−1 mol−1
ΔG# (kJ mol−1
)
)
−213
115
9.2
110
88.2
−205
109
7.3
105
76.7
)
103 km (s−1
)
Ks (mol−1 dm3)
Kn (mol−1 dm3)
92
85.2

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S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
Scheme 3. Schematic model showing probable location of reactants for the micellar-catalyzed reaction between D-glucose and MnO4−
.
3.4. Effect of temperature and activation parameters
adsorption of MnO−4 and D-glucose on micellar surface but also through
stabilization of transition state. The decline in the ΔS# proposes that the
transition state is well structured in case of gemini surfactant micelles
(spacer = 4) in comparison to gemini surfactants (spacer = 05 and
spacer = 6). However, a more meaningful mechanistic explanation of
the apparent values of ΔH# and ΔS# is not possible because the rate con-
stant does not represent a single elementary kinetic step as it is a com-
plex function of true rate, binding and ionization constants.
Activation parameters are believed to provide useful information re-
garding the environment in which chemical reactions take place. In
order to determine the activation parameters, a series of kinetic exper-
iments for oxidation of D-glucose by MnO4‾ were carried out at four dif-
ferent temperatures (40–60 °C) and other reactant concentration
constant (i.e. [MnO4−
]
=
4.0
1.0 × 10−4 mol dm−3 and [D-glucose] = 4.0 × 10−3 mol dm−3
in presence of m-s-m type gemini surfactant micelles
× , [H2SO4] =
10−4 mol dm−3
)
=
3.5. Mechanism and rate law
(3.0 × 10−4 mol dm−4). Arrhenius plots (lnkobs and 1/T) are shown in
Fig. 4. The values of activation energy (Ea) were calculated from the
slopes of Fig. 4. Activation enthalpy (ΔH#), activation entropy (ΔS#)
and the free energy (ΔG#) are determined by using Eyring equations
(Table 2). It is observed from the derived results that gemini surfactants
lower the values of activation enthalpy (ΔH#) and activation entropy
(ΔS#) than aqueous. This lowering may occur not only through the
Kinetic method is the most important one, which establishes the
most refined mechanism at the molecular level for any reaction. The
most predominant form of D-glucose is α-D-glucopyranose followed
by β-D-glucopyranose. Among these β-D-glucopyranose is considered
as the reaction species of D-glucose. Glucopyranose possess three differ-
ent alcohol functionalities including a primary alcohol, three secondary
Fig. 3. Plot showing the effect of [D-glucose] on kobs. Reaction conditions: [MnO−4 ] =
Fig. 4. Arrhenius plots for D-glucose oxidation by permanganate. Reaction conditions:
4.0
×
10−4 mol dm−3
,
[16-s-16]
=
3.0
×
10−4 mol dm−3
,
[H2SO4]
=
[MnO−4 ] = 4.0 × 10−4 mol dm−3, [16-s-16] = 3.0 × 10−4 mol dm−3, [H2SO4] =
1.00 × 10−4 mol dm−3, temperature = 40 °C.
1.00 × 10−4 mol dm−3, [D-glucose] = 4.0 × 10−3 mol dm−3
.

S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
543
Scheme 4. Oxidation of D-glucose by permanganate.
alcohol groups and one hemiacetal hydroxyl group. Each of these OH
groups can be oxidized to corresponding sugar derivatives as shown
below.
The reduction of colloidal MnO2 to Mn(III) by Mn(II) has also been
reported on several occasions [60].
A rate law consistent with Scheme 4 may be expressed as Eq. (12).
Â
ÃÂ
Ã
Â
Ã
k K Ka Hþ MnO‐4 ½D‐glucoseꢀ
‐d MnO‐4
T
À
Â
Ã
Â
Ã
Á
¼
ð12Þ
ð13Þ
1 þ Ka Hþ þ K Ka Hþ ½D‐glucoseꢀ
dt
and
Â
Ã
k K Ka Hþ ½D‐glucoseꢀ
In the light of above observations and observed results, the follow-
ing mechanism given in Scheme 4 has been proposed for the oxidation
of D-glucose by permanganate.
Â
Ã
Â
Ã
kobs
¼
1 þ Ka Hþ þ K Ka Hþ ½D‐glucoseꢀ
In Scheme 4, Eq. (6) represents the protonation of MnO−4 to HMnO4.
The next reaction shows formation of complex between D-glucose and
HMnO4 (Eq. (7)). It has been established that different species of manga-
nese (Mn(VI), Mn(V), Mn(IV) and Mn(III)) are formed as an
intermediate(s) during the reduction of permanganate. The stability of
these species strongly depended on the experimental conditions and pH
of the working media, i.e., acidic, alkaline and neutral. Out of these,
Mn(IV) species is commonly involved in the permanganate oxidation of
various reductants [58]. In analogy with previous studies [59], we assume
that it decomposes in a one-step, two electron oxidation–reduction
mechanism to MnO34− (Mn(V)) and radical (Eq. (8)). Mn(V) is highly un-
stable in an acidic medium [60] with respect to disproportionation and
immediately gets converted into MnO2 (Mn(IV)). Other oxidation states
of Mn are obviously involved in the reaction; they are extremely unstable
under the experimental conditions used in this study.
After the slow steps, the following fast reactions may also take place.
ð10Þ
Fig. 5. Plot of log 1/kobs vs. 1/[D-glucose] for the oxidation of D-glucose by permanganate.
Reaction conditions: [MnO4−
3.0 [H2SO4]
10−4 mol dm−3
4.0 × 10−3 mol dm−3, temperature = 40 °C.
]
=
4.0
×
10−4 mol dm−3
10−4 mol dm−3
,
,
[16-s-16]
[D-glucose]
=
=
×
,
=
1.00
×
Radical þ MnðIIIÞ F→ast Lactone þ MnðIIÞ
ð11Þ

544
S.A. Al-Thabaiti et al. / Journal of Molecular Liquids 216 (2016) 538–544
Table 3
Effect of [D-glucose] on the oxidation of D-glucose by permanganate and comparison of the kobs and kcal for the oxidation of D-glucose by permanganate. Reaction conditions:
[MnO4−] = 4.0 × 10−4 mol dm−3, [16-s-16] = 3.0 × 10−4 mol dm−3, [H2SO4] = 1.00 × 10−4 mol dm−3, temperature = 40 °C.
103 [D-glucose] (mol dm−3
)
104 kobs (s−1
16-6-16
)
104 kcal (s−1
16-6-16
)
(kobs − kcal / kobs)
16-5-16
16-4-16
16-5-16
16-4-16
16-6-16
16-5-16
16-4-16
1
2
3
4
5
6
7
8
9
0.24
0.49
0.77
1.02
1.21
1.40
1.61
1.79
1.97
2.08
0.32
0.64
0.92
1.25
1.47
1.67
1.86
2.04
2.20
2.34
0.47
0.82
1.15
1.50
1.70
1.86
2.05
2.20
2.36
2.48
0.22
0.48
0.75
1.02
1.18
1.37
1.60
1.81
2.00
2.04
0.30
0.60
0.94
1.27
1.50
1.67
1.88
2.02
2.24
2.28
0.47
0.79
1.12
1.53
1.74
1.81
2.07
2.15
2.30
2.54
0.08
0.02
0.02
0.00
0.02
0.02
0.00
−0.01
−0.01
0.01
0.06
0.06
-0.02
-0.01
−0.02
0.00
−0.01
0.01
−0.01
0.02
0.00
0.03
0.00
0.02
−0.02
0.02
−0.01
0.02
0.02
10
−0.02
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Eq. (14).
Â
Ã
1 þ Ka Hþ
1
kobs
1
k
Â
Ã
¼
þ
ð14Þ
k K Ka Hþ ½D‐glucoseꢀ
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The micellar catalyzed oxidation of D-glucose by MnO4− was studied
in the presence of different gemini surfactant (16-s-16) concentrations
at 40 °C. The observed results clearly demonstrate that the reaction rate
is enhanced in the presence of gemini surfactants as compared to cat-
ionic surfactant cetyltrimethylammonium bromide. A plausible mecha-
nism and related rate law have been derived on the basis of observed
kinetic results. The effect of different reactant concentrations was also
carried out to provide the complete information regarding the order of
the reaction. It seems that the increased hydrophobicity was responsi-
ble for higher concentration of D-glucose in the Stern layer of the mi-
celles. Activation parameters were also evaluated at different
temperatures. The activation energy (Ea), activation enthalpy (ΔH#),
activation entropy (ΔS#) and free energy (ΔG#) values have been calcu-
lated and interpreted accordingly.
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