Cu(II) catalyzed oxidation of mercaptoacetic acid by methylene blue in acidic medium: Influence of solvent and morphology

Article abstract of DOI:10.1080/10426507.2015.1085039

Kinetic studies on the Cu(II) catalyzed oxidation of mercaptoacetic acid (thioglycolic acid, TGA) by the model electron receptor methylene blue in acidic medium show that the reaction follows a second order kinetics in TGA. The order in methylene blue is unity but at higher [TGA], the order is 3/4. The rate shows an inverse relationship with [H⁺] and a second order dependence on [Cu(II)]. The reaction conforms to Arrhenius behavior in aqueous medium but resorts to anti-Arrhenius behavior in aqueous methanol and in aqueous acetone media. The reaction involves the in situ participation of nanoparticles which has been confirmed by SEM, XRD, and FTIR techniques. In aqueous medium, cauliflower shaped nanoparticles (44.21-74.33 nm) are obtained but in the presence of acetone and methanol, nanogranules (38.96-70.73 nm) and nanodisks (47.30-72.16 nm), respectively, are produced at 35°C as revealed by SEM images and XRD data. The reaction is characterized by the participation of two transition states in aqueous acetone medium and the kinetics of the reaction, especially the temperature dependence, is governed by the adsorption of methylene blue on copper nanoparticles as influenced by morphology inter alia dielectric constant of the reaction medium.

Full text of DOI:10.1080/10426507.2015.1085039

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Cu(II) catalyzed oxidation of mercaptoacetic acid
by methylene blue in acidic medium: Influence of
solvent and morphology
Ranjana Sharma, Mahender Pal & K. K. Mishra
To cite this article: Ranjana Sharma, Mahender Pal & K. K. Mishra (2016) Cu(II) catalyzed
oxidation of mercaptoacetic acid by methylene blue in acidic medium: Influence of solvent
and morphology, Phosphorus, Sulfur, and Silicon and the Related Elements, 191:1, 41-47, DOI:
10.1080/10426507.2015.1085039
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Date: 14 March 2016, At: 07:25

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –
http://dx.doi.org/./..
Cu(II) catalyzed oxidation of mercaptoacetic acid by methylene blue in acidic medium:
Influence of solvent and morphology
Ranjana Sharma, Mahender Pal, and K. K. Mishra
Department of Chemistry, Rani Durgavati University, Jabalpur, India
ABSTRACT
ARTICLE HISTORY
Received  May 
Accepted  July 
Kinetic studies on the Cu(II) catalyzed oxidation of mercaptoacetic acid (thioglycolic acid, TGA) by the model
electron receptor methylene blue in acidic medium show that the reaction follows a second order kinetics
in TGA. The order in methylene blue is unity but at higher [TGA], the order is 3/4. The rate shows an inverse
relationship with [H+] and a second order dependence on [Cu(II)]. The reaction conforms to Arrhenius
behavior in aqueous medium but resorts to anti-Arrhenius behavior in aqueous methanol and in aqueous
acetone media. The reaction involves the in situ participation of nanoparticles which has been confirmed
by SEM, XRD, and FTIR techniques. In aqueous medium, cauliflower shaped nanoparticles (44.21–74.33 nm)
are obtained but in the presence of acetone and methanol, nanogranules (38.96–70.73 nm) and nanodisks
(47.30–72.16 nm), respectively, are produced at 35°C as revealed by SEM images and XRD data. The reaction
is characterized by the participation of two transition states in aqueous acetone medium and the kinetics of
the reaction, especially the temperature dependence, is governed by the adsorption of methylene blue on
copper nanoparticles as influenced by morphology inter alia dielectric constant of the reaction medium.
KEYWORDS
Oxidation; kinetics;
mercaptoacetic acid;
methylene blue;
nanocatalysis; anti-Arrhenius
behavior
GRAPHICAL ABSTRACT
Due to the capability of changing their oxidation states,
transition metal ions often act as promoters of free radical reac-
tions3 and also act as catalysts involving nanocatalysis in some
cases. This has motivated us to investigate the oxidation of some
sulfydryl molecules by the model electron receptor MB in
the presence of metal ions such as Cu(II) and Ru(III).4-8 The
sulfydryl compounds act as reducing agents as well as the cap-
ping reagents in the synthesis of nanoparticles9,10 and it seems
that this has a bearing on Cu(II)-catalyzed oxidation of mercap-
toacetic acid (thioglycolic acid, TGA) by MB in acidic medium
reported presently. The reaction exhibits anti-Arrhenius behav-
ior in the presence of non-aqueous solvents such as methanol
Introduction
Sulfydryl substrates are known to play an important role in the
regulation of biochemical processes such as respiration and cell
division, and the sulfur cycle is regarded to be as indispensable
as the nitrogen and carbon cycles are in living systems.
The oxidation of thiols to disulfides is of paramount impor-
tance in providing stability to the protein structures and
RSH/RSSR couples are important in maintaining the redox
potential at biological sites.1 Methylene blue (MB), the oxidant
employed presently is a phenothiazine dye that has been widely
used as a photosensitizer in photodynamic therapy, e.g., in the
treatment of tumor and hepatitis C.2
CONTACT K. K. Mishra
kkmishra.chem@gmail.com
Department of Chemistry, Rani Durgavati University, Jabalpur, India
Supplemental data for this article can be accessed on the publisher’s website at http://dx.doi.org/./...
©  Taylor & Francis Group, LLC

42
R. SHARMA ET AL.
Table . Effect of variation of [TGA] on the rate constant [MB] = . × − M;
[HCl] = . M; [CuSO] = . × − M; I = . M; Temp. = °C.
Table . Effect of variation of initial concentration of methylene blue [TGA] =
. × − M; [HCl] = . M; [CuSO] = . × − M; I = . M; Temp = °C.
[MB] ×  M
k ×  s−
k M− s−
[TGA] ×  M
k ×  s−
k/ × M/ s−
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
were also verified by van’t Hoff differential and graphical meth-
ods (Figure 1). The statistical treatment of kinetic data also con-
firms this conclusion.
and acetone which is ascribed to the influence of morphology of
Cu nanoparticles on the rate of reaction.
Geben Sie hier eine Formel ein.
Results and discussion
The values of the rate constant k3/4 were calculated (Table S1)
by using the equation for nth order rate constant14 and substi-
tuting n = 3/4 where n gives the order in MB (Equation SI).
The order of reaction in TGA is 2 as verified by drawing plots
between the logarithm of the rate constant and [TGA] which
give a straight line with a slope equal to 2.4.
Stoichiometry and the reaction products
The principal reactants interact in a molar ratio of 2:1 form-
ing the end products disulfide and dihydromethylene blue as
was determined spectrophotometrically. It was experimentally
observed that MB is completely reduced to give dihydromethy-
lene blue (leucobase, H2MB) showing maximum absorbance at
257 nm11 whereas the formation of the disulfide RSSR was quali-
tatively demonstrated by recording the FTIR spectra of the reac-
tion mixture after completion of reaction which shows the char-
The rate constant remains almost unchanged on increasing
the initial concentration of MB. However, the reaction shows a
first-order kinetics in MB at low [MB] while the order changes
from 1 to 2 on increasing [MB] beyond 2.0 ×10−5 M (Table 2).
The rate decreases linearly with increasing the concentration
of hydrogen ions, which influences the kinetics in MB in a com-
plex manner (Table S2). The plots of log k3/4 against log [H+]
give a straight line with a negative slope (0.80) (Figure 2).
The ionic strength of the system was maintained at a fixed
level (ca. 0.35M) in these variations by adding the requisite
amount of KCl to the reaction system. Incidentally, the relation
between the rate constant and [H+] in the oxidation of thiols is
generally found to be complex.15 The rate increases on increas-
ing the concentration of KCl and NaCl, while it remains almost
unaffected on adding K2SO4 to the reaction mixture. The rate
constant decreases, however, on addition of KNO3 and attains a
limiting value at higher concentrations (ca. > 0.10M). The ionic
strength was also calculated in terms of molality but on calcu-
acteristic S−S linkage near 443.6 cm−1 12
. Therefore, the chem-
ical reaction is represented as:
2RSH + MB −→ RSSR + H2MB
Kinetic features of the reaction
The order of reaction in MB was determined by employing the
Ostwald isolation method13 and the runs with 200- to 500-fold
excess [TGA] over [MB] were followed. At low concentrations
of mercaptoacetic acid (TGA) (ca. ࣘ 5.0 × 10−3M), the order
of reaction in MB is 1 but at higher concentrations, the order
changes from 1 to 3/4 as shown in Table 1. These conclusions
lating the former on the basis of Debye-Hückel limiting law,16
a
Figure . Plot of log(a−x) versus time t at different [TGA]. [TGA] = . × − M (ꢀ),
Figure . A plot of log k/ versus log [HCl] [TGA] = . × − M; [MB] = . × −
M; [CuSO] = . × − M; I = .M; Temp. = °C.
. × − M (ꢁ), . × − M (×), . × − M (ꢂ), . × − M, ( ) [HCl] =
. M, [Cu(II)] = . × − M; [MB] = . × − M, Temp. = °C.

PHOSPHORUS, SULFUR, AND SILICON
43
Table . Rate constants at different temperatures in aqueous and aqueous
methanol (%v/v) media [TGA] = . × − M; [MB] = . × − M; [HCl] =
. M; [CuSO] = . × − M; I = . M.
(B) Aqueous
methanol
(A) Aqueous
Medium
k/
×
Medium
temperature (K) k ×  s−  M− s− Temperature (K) k ×  s−





.



.
.
.
.
.
.
.
Temperature in Kelvin (K).
is found that anti-Arrhenius behavior is more pronounced in
aqueous acetone medium (36%v/v) (Table 5) where the rate con-
stant decreases on increasing the temperature.
Figure . A plot of logk versus log [CuSO]. [TGA] = . × − M; [MB] = . ×
− M; [HCl] = . M; I = .M; Temp. = °C.
The Eyring plot between log (kh/KT) (where k and K denote
the rate constant and Boltzmann constant, respectively) and 1/T
for the reaction studied in aqueous medium is found to be linear
(Figure 4a) but these plots for aqueous acetone medium are non-
linear (Figure 4b) and the two linear segments on extrapolation
intersect at a particular temperature (Tinv). These segments cor-
respond to low temperature and high temperature zones,17 and
Tinv was found to be 303 K for the reaction in aqueous acetone
medium. According to Gypser et al., this indicates two transi-
tion states for the reaction.18 At temperatures below Tinv, ꢀH∗ is
found to be −89.5 kJ mol−1 while for a higher temperature zone,
its value is −25.1 kJ mol−1. The activation entropy ꢀS∗ for the
corresponding medium is found to be −604 JK−1 mol−1 at 20°C
and −391.2 JK−1 mol−1 at 35°C. In aqueous acetone medium, it
is evident that a larger negative value of ꢀH∗ for the reaction at
lower temperature compensates a larger negative value of ꢀS∗
negligible difference of 6.17 × 10−3 was noticed in the value of
the activity coefficient. The rate increases on increasing the con-
centration of Cu(II) and the order with respect to the catalyst is
nearly 2. The plots of log k1 against log [CuSO4] give a straight
line with a slope of 1.8 (Figure 3).
The influence of the dielectric constant was studied by adding
varying volumes of methanol to the system and it was observed
that the rate initially increases on decreasing the dielectric con-
stant of the medium but subsequent variations in the dielectric
constant did not influence the rate (Table 3). The rate is not
affected on adding dithiodiglycolic acid (disulfide) externally to
the reaction system. The addition of leucobase (H2MB) slightly
retards the rate at higher concentrations and leads to zero-order
kinetics in MB (Table S3). In order to account for the kinetic
influence of the Sn/HCl couple used to reduce MB; identical
blank runs were made by adding the solution of tin dissolved
in HCl so that an equal amount of the metal could be added to
the reaction system.
It was found that the rate attains a limiting value at lower
dielectric constant (D ࣘ 75.0). Thus the runs were made at dif-
ferent temperatures in aqueous and aqueous methanol media.
The rate constants are summarized in Table 4.
The reaction adheres to Arrhenius behavior in aqueous
medium but exhibits anti-Arrhenius behavior in aqueous
methanol medium where the rate constant remains unaffected
on varying the temperature. The energy of activation, entropy
of activation and free energy of activation in aqueous medium
are found to be 34.8 kJ mol−1, −199.9 JK−1mol−1, and 96.3
kJ mol−1 respectively. In order to verify this and to study the
influence of the dielectric constant on the temperature depen-
dence of the reaction, acetone was introduced to the system. It
making the process more favorable under these conditions.17
A
larger negative value of ꢀS∗ also indicates the prominence of
adsorption at 20°C.
The FTIR spectra of TGA (Figure S1a) and the reaction mix-
ture after completion of the reaction suggest the participation of
S-H stretch (2500 cm−1), C-H stretch for aliphatic compounds
near 2850–3000 cm−1, symmetric COOH stretch (1400 cm−1
)
and asymmetric COOH stretch (1700 cm−1).19,20 The spectra
of the reaction system recorded after completion of the reaction
also show that the S–H stretching vibration which occurs near
2500 cm−1 is feebly present in aqueous and aqueous methanol
media but is absent in aqueous acetone medium (Figure S1.b–
d) indicating a specific involvement of the sulfydryl group
under these conditions. Moreover, a larger negative ꢀS∗ in aque-
ous acetone medium implies that at lower temperature, perhaps
adsorption of MB on a reacting species occurs while at higher
temperature, desorption is favored leading to a retardation in
rate.
Table . Rate constant at different dielectric constant [TGA] = . × −; [MB] =
. × − M; [CuSO] = . × − M; [HCl] = . M; [I] = . M; Temp = °C.
Table . Rate constant obtained at different temperatures in aqueous-acetone
medium (%v/v) [TGA] = . × − M; [MB] = . × − M; [HCl] = . × −
M; [KCl] = . M; [CuSO] = . × − M; I = . M.
D
k ×  s−
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Temperature (K)
k ×  s−





.
.
.
.
.

44
R. SHARMA ET AL.
Figure . The Eyring plot between log(kh/KT) and /T for reaction system in (a) aqueous medium and (b) aqueous acetone medium (% (v/v)).
Reaction scheme
Looking to the possibility of adsorption of MB as explained
earlier and since a thiol itself is a reducing agent it can reduce
metal ions to nanoparticles9 and in some cases, metal ion catal-
ysis is associated with the participation of nanoparticles; such a
possibility in the present case was checked under these condi-
tions by recording SEM images and XRD spectra of the reaction
system after completion of the reaction. These studies suggest
the participation of Cu nanoparticles which may be formed by
the involvement of species C3 during the course of reaction as
shown below.
The kinetic results suggest that Cu(II) interacts with TGA lead-
ing to deprotonation of the substrate molecule to form com-
plexes with varying compositions,21,22 represented as C1 and C2,
respectively (Equations 1 and 2).
K1
Cu(II) + RSH ꢃ [CuII(RS−)]+ + H+
(1)
(2)
(C1 )
K2
[CuII(RS−)]+ + RSH ꢃ [CuII(RS−) ] + H+
2
(C2 )
k4ꢀ
[CuI(RS−)] −→ Cu(0) + RS·
(8)
A hypsochromic shift from 222 nm in methanol to 204 nm
in aqueous medium in the UV spectra of the Cu(II)-TGA sys-
tem (Figure S2a and b) confirms the formation of species like
C2 which is expected to be less polar than the reactants. It
is very likely that an electron is transferred in situ from one
of the ligated thiolate anions to the central CuII metal ion to
form [CuI(RS−)] (C3) and a thiyl radical as reported earlier.21
(Equation 3).
(C3 )
Thus, on presuming step (5) as the rate determining step
and assuming steady state for C3 and substituting [C1], [C2]
and [MBH+] from the respective equilibria; the following rate
expression is obtained (for complete derivation of the rate
expression, please see supporting information Equation SII):
k1ꢀ
d[MB]
dt
kꢀ1kꢀ2K1K2K3[Cu(II)][RSH]2[MB]
[H+](k2ꢀ K3[MB][H+] + k4ꢀ )
[CuII(RS−) ] −→ [CuI(RS−)] + RS·
(3)
2
−
=
(9)
(C3 )
It may be pointed out here that Cu+ ion in presence of
excess RS− ion leads to the formation of species [CuI(RS−)]23
If kꢀ4 > kꢀ2 K3 [MB] [H+]; the rate is given as
and although Cu(I) is not stable in aqueous solutions,24
a
d[MB]
dt
kꢀ1kꢀ2K1K2K3[Cu(II)][RSH]2[MB]
kꢀ4[H+]
−
=
(10)
number of stable cationic and anionic Cu(I) complexes are
reported to be stable under such conditions.25, 26 In fact, Cu(I)-
glutathione complexes are known to be the intermediates in cel-
lular metabolism and storage of Cu(I).27
The complex C3 may be oxidized by protonated methylene
blue (MBH+) to produce the complex C1 along with the half-
reduced MB radical (HM.) as given by Equations 4 and 5.
Equations (9) and (10) explain a second order kinetics in TGA
(RSH) and a first-order and fractional order kinetics in MB as
well as an inverse relationship of rate with [H+]. The rate expres-
sion, however, does not explain the second-order kinetics behav-
ior in Cu(II) which may perhaps be attributed to the partic-
ipation of copper nanoparticles in the reaction system. From
(4) Equation (10), the pseudo first order rate constant (kobs) will be
given as
k3
MB + H+ ꢃ MBH+
[CuI(RS−)] + MBH+ −→ [CuII(RS−)]+ + HM· (5)
K2ꢀ
(C1 )
kꢀ1kꢀ2K1K2K3[Cu(II)][RSH]2
kobs
=
=
(11)
(12)
kꢀ4[H+]
kꢀ4[H+]
The half-reduced MB radical subsequently interacts with the
substrate molecule RSH to give the end products viz., leucobase
and the disulfide (Equations 6 and 7).
1
kobs
kꢀ1kꢀ2K1K2K3[Cu(II)][RSH]2
k3ꢀ
HM· + RSH −→ H2MB + RS·
(6)
It is thus evident that a plot of 1/kobs against 1/[RSH]2 should
(7) give a straight line passing through the origin as has been
fast
2RS· −→ RSSR

PHOSPHORUS, SULFUR, AND SILICON
45
(Figure S4). The morphology of the nanoparticles changed
to nanodisks in aqueous methanol as shown by SEM images
of the reaction system (Figure 6b). Incidentally, the aggrega-
tion of suspended metal oxide and oxyhydroxide nanoparti-
cles is reported to change drastically as a function of sol-
vent properties such as dielectric constant and coordinative
ability.32
This is again revealed in the present case because in aque-
ous acetone medium (36%v/v), nanogranules are obtained at
35°C whereas nanowires are obtained at 20°C (Figures 7a and
7b). XRD analysis of the reaction mixture in aqueous acetone
medium showed that the size of the particles varied between
39.96 and 70.33 nm at 25°C and lay in the range of 66.81–
105.21 nm at 40°C (Figure S5a and S5b and Table S4). It clearly
demonstrates that the temperature dependence of the reaction is
governed by the dielectric constant of the medium as well as by
the morphology of nanoparticles. It may be mentioned here that
the morphology of ZnO nanoparticles is reported to be influ-
enced by solvents such as acetone, water and ethanol.33 In the
present case also, the solvents appear to exert a specific influ-
ence on the morphology of the Cu nanoparticles. The reaction
rate is highly influenced by the morphology of the nanoparti-
cles as illustrated by the fact that in aqueous acetone medium the
nanowires obtained at 20°C were catalytically more active than
nanogranules obtained at higher temperatures. This contention
is also supported by the observations of Suib et al. on the influ-
ence of different morphologies of ZnO nanoparticles on their
photocatalytic activity.34
The extent of adsorption decreases on increasing the polar-
ity of the reaction medium35 and in view of this it seems that
aqueous methanol plays an intermediary role between water and
aqueous acetone. These conclusions are also confirmed by com-
paring the values of Gibbs free energy ꢀG∗ which is found to be
87.5 kJmol−1 at 20°C and 95.5 kJmol−1 at 35°C in aqueous ace-
tone medium indicating the prominence of adsorption at lower
temperature. In aqueous methanol medium, however, the values
of ꢀH∗ and ꢀS∗ seem to counterbalance each other to nullify
the influence of temperature on the rate of reaction.
Figure  A plot of /kobs against /[RSH].
observed (Figure 5). From Equation (12)
kꢀ4[H+]
Slope =
(13)
kꢀ1kꢀ2K1K2K3[Cu(II)]
From Figure 5, the slope = 0.025 and since K1 = 109 and K3 =
kꢀ1kꢀ2K2
0.6,28,29 the value of
is found to be 3.33 × 10−3
.
k4ꢀ
It seems that Cu nanoparticles (Equation 8) are oxidized by
MBH+ to give Cu(II):
Cu(0) + MBH+ + H+ −→ Cu(II) + H2MB
(14)
In spite of the participation of copper nanoparticles, the
reaction system remains homogeneous because the particles
are an order of magnitude smaller than the wavelength of vis-
ible light as explained by El-Sayed in nanocatalysis.30 Inci-
dentally, the reaction represented by Equation (14) has been
shown to occur by Lopez-Quintela and coworkers in the
reduction of MB by hydrazine catalyzed by size-selected Cu
nanoclusters.31
SEM images of the reaction system recorded in aqueous
medium after completion of the reaction reveal the participa-
tion of cauliflower shaped nanoparticles at 35°C (Figure 6a). The
size of the particles as determined by XRD analysis in aque-
ous medium was found to vary between 44.21 and 74.33 nm
(Figure S3) while in aqueous methanol medium (30%v/v) the
particle size was found in the range 47.30–72.16 nm at 35°C
These explanations justify the mechanistic approach
advanced presently and also highlight the complex chem-
istry of sulfydryl substrates vis-à-vis the nature of metal ion
acting as a catalyst, the nature of the reaction medium and
temperature.
Figure . SEM images of the reaction system at °C (a) in aqueous medium (b) in aqueous methanol medium (%v/v).

46
R. SHARMA ET AL.
Figure . SEM image of reaction system in aqueous acetone (%v/v) medium at (a) °C and (b) °C.
nanoparticles was calculated from XRD spectra using the Scher-
rer equation:40
Experimental
Reagents
K.λ
β. cos θ
D =
The samples of mercaptoacetic acid (TGA, RSH) (E. Merck, Ger-
many; assay 80%), MB (E. Merck, Germany) and CuSO4·5H2O
(Qualigens, AnalaR) were used as such and their solutions were
prepared afresh in doubly distilled water by dissolving an exactly
weighed quantity of the samples. The solution of TGA was sub-
sequently standardized iodimetrically.36 The solution of MB was
stored in the dark to avoid photochemical effects. The solution
of CuSO4·5H2O (Qualigens, AnalaR) was prepared in doubly
distilled water in acidic medium in conformity with the stan-
dard procedure.37 The oxidation product viz. the corresponding
disulfide was prepared by oxidizing the respective substrate with
hydrogen peroxide (30%v/v) and recrystallizing it from ether.38
Dihydromethylene blue was prepared by reducing MB with Sn-
HCl couple.39 All reagents were E. Merck’s GR grade samples.
Doubly distilled water was used throughout these investigations.
The Supplemental Materials contain sample spectra of reactions
(Figures S1–S4).
where K is the shape factor (normally, the value is taken as 0.9),
˚
λ is the wavelength of the X-ray used (1.5418 A), β is the line
broadening at half of the maximum intensity (FWHM) in radi-
ans, and θ is the Bragg angle.
The reaction mixtures after completion of the reaction were
allowed to stand at room temperature, and after evaporation of
the solvent the residue was dried and subjected to SEM, XRD,
and FTIR analysis. Kinetics were studied by making the runs
in presence of a large excess of TGA over MB and by measur-
ing a decrease in absorbance of MB with time at 664 nm at a
particular temperature and at a fixed ionic strength. The reac-
tion mixture was thermostatted in steam washed reaction ves-
sel (Pyrex, England) coated with Black Japan and the solution of
MB was subsequently added to the reaction system. The aliquots
were withdrawn and the concentration of MB was determined
at different time intervals by using Thermospectronic Unicam
UV-530 Spectrophotometer.
Instrumentation
Conclusions
The progress of the reaction was followed on a Thermospec-
The kinetics of the reaction between mercaptoacetic acid (TGA)
and MB in acidic medium catalyzed by Cu(II) are very much
influenced by the solvent and a diverse influence has been
exerted by water, methanol and acetone vis-à-vis temperature.
The reaction adheres to Arrhenius behavior in aqueous medium
but in aqueous methanol and aqueous acetone medium an anti-
Arrhenius behavior has been noticed. The rate is not influenced
on varying the temperature in aqueous methanol medium while
in aqueous acetone, the rate decreases on increasing tempera-
ture. Our observations confirm the participation and regulatory
role of Cu-TGA nanoparticles and entropy controlled adsorp-
tion of MB on nanoparticles is found to cause a deviation in
reaction path mediated by the solvent and temperature.
tronic Unicam UV-530 spectrophotometer by measuring the
depletion in absorbance of MB (ε
= 6.76 × 104 M−1 cm−1
max
at 664 nm) with time. Since the rate of reaction was practically
unchanged in the presence and in absence of nitrogen under pre-
vailing conditions the inert atmosphere was normally not main-
tained in these kinetic investigations. The ingredients of the
reaction mixture were thermostatted (Julabo, Germany; Model
No.F-34E; temperature stability
reaction vessels (Pyrex, England) coated with Black Japan. The
precision of kinetic data are within 5%.
The IR spectra of the samples were recorded on Bruker
(Model Alpha-T) and Shimadzu FTIR spectrometers (Model.
No.8400 S) using KBr pellets in the range 4000–500 cm−1. SEM
images were obtained on a JEOL/EO JSM-6390 Scanning Elec-
0.03°C) in steam washed
tron Microscope while XRD spectra were recorded on a PANa- Acknowledgments
lytical Empyrean X-ray diffractometer. The wavelength of X-ray
The authors are grateful to Maulana Azad National Institute of Technol-
ogy (MANIT), Bhopal for availing SEM facility, Model Government Sci-
ence College, Jabalpur for FTIR and Indian Institute of Science Education
˚
used was 1.5418 A and d-spacing was recorded for different val-
ues of 2θ (Bragg angle). The step size (2θ) was 0.06° and the
generator setting parameters were 40 mA, 45 kV. The size of the Research (IISER), Bhopal for XRD analysis, respectively.

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