Study of association thermodynamics between crystal violet and sodium bis(2-ethylhexyl)-sulfosuccinate and kinetics of basic fading of crystal violet in microemulsions

Article abstract of DOI:10.1002/kin.20317

The thermodynamics of the association between 4,4′,4″- tris(dimethylamino)triphenylmethyl chloride (crystal violet or CV) and sodium bis(2-ethylhexyl)-sulfosuccinate (aerosol OT or AOT) in water/AOT/n-decane microemulsion and the kinetics of the basic hydrolysis of CV in a water-in-oil microemulsion were investigated by UV-vis spectroscopic measurements. An association model of CV and AOT was used to analyze the experimental data to obtain the association constants at various temperatures. By taking the association into account, the "actual" rate constants and the activation energies of the basic hydrolysis of CV in the media of water/AOT/oil were obtained. The difference in thermodynamics and kinetics between the two media of water/AOT/n-decane and water/AOT/isooctane is discussed.

Full text of DOI:10.1002/kin.20317

Study of Association
Thermodynamics between
Crystal Violet and Sodium
Bis(2-Ethylhexyl)-
Sulfosuccinate and Kinetics
of Basic Fading of Crystal
Violet in Microemulsions
ZHI YUN CHEN,1 JI HUA ZHAO,2 WEI HE,2 XUE QIN AN,2,3 WEI GUO SHEN1,2
1School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237,
People’s Republic of China
2College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China
3College of Chemistry and Environment Science, Nanjing Normal University, Nanjing 210097, People’s Republic of China
Received 14 September 2007; revised 30 November 2007; accepted 1 January 2008
DOI 10.1002/kin.20317
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The thermodynamics of the association between 4,4ꢀ,4ꢀꢀ-tris(dimethyl-
amino)triphenylmethyl chloride (crystal violet or CV) and sodium bis(2-ethylhexyl)-
sulfosuccinate (aerosol OT or AOT) in water/AOT/n-decane microemulsion and the kinetics
of the basic hydrolysis of CV in a water-in-oil microemulsion were investigated by UV–vis
spectroscopic measurements. An association model of CV and AOT was used to analyze the
experimental data to obtain the association constants at various temperatures. By taking the
association into account, the “actual” rate constants and the activation energies of the basic
hydrolysis of CV in the media of water/AOT/oil were obtained. The difference in thermodynam-
ics and kinetics between the two media of water/AOT/n-decane and water/AOT/isooctane is
discussed. ꢁ 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 294–300, 2008
C
Correspondence to: Wei Guo Shen; e-mail: shenwg@lzu.edu.cn.
Contract grant sponsor: National Natural Science Foundation of
Contract grant number: 0652nm010.
China.
Contract grant numbers: 20173024, 20473035, 20603014, and
20673059.
Contract grant sponsor: Chinese Ministry of Education.
Contract grant number: 105074.
Contract grant sponsor: Science and Technology Commission of
Shanghai Municipality.
“‘Actual’ second order rate constants k for the reaction of
the basic hydrolysis of CV in microemulsions formed with n-
decane and isooctane at various ω and at different temperatures”
are available as Supplementary Material on the journal’s homepage
at http://www.interscience.wiley.com/jpages/0538-8066/suppmat/.
c
ꢀ
2008 Wiley Periodicals, Inc.

ASSOCIATION THERMODYNAMICS BETWEEN CRYSTAL VIOLET AND SODIUM SULFOSUCCINATE
295
decrease of electrostatic attractions between CV+ and
the head groups of anionic AOT. Leis et al. suggest that
there exists some specific interaction between CV+ and
the surfactant head group [12], which might yield the
observed low reactivity of the basis hydrolysis of CV in
water/AOT/isooctane microemulsion.
INTRODUCTION
Water-in-oil microemulsions are thermodynamically
stable single-phase dispersions of water and surfac-
tant within a continuous oil medium, and are useful in
such areas as enhanced oil recovery, organic synthesis
reactions [1,2], preparation of nanoparticles [3–5] or
nanoparticle/polymer composite [6], and biochemical
reactions [7–9]. A microemulsion droplet of water in
oil can be considered as a microreactor that provides
special medium and enhances or retards reaction rates
[10–18]. Since the parameters such as the water pool
size, the nature and the concentration of surfactants,
and the pH of the water pool can be easily adjusted,
the interior water pool of reverse microemulsion is a
desirable microreactor where chemical reactions take
place.
Sodium bis(2-ethylhexyl)sulfosuccinate (aerosol
OT or AOT) is one of the prevalently investigated
anionic surfactants. Because of its double-tail nature,
the AOT molecule is almost structurally balanced with
respect to its hydrophilic and hydrophobic moieties
and is one of the few ionic surfactants that form mi-
croemulsions without the addition of a cosurfactant.
The ternary water–AOT–alkyl water-in-oil microemul-
sions are stable over a wide range of concentrations
and temperatures [19] and very well characterized.
The kinetic behavior of the alkaline fading (basic
hydrolysis) of the dye crystal violet (CV, a triphenyl-
methane dye) in aqueous media is well known [20,21],
and this reaction can be easily monitored by UV–vis
spectroscopic measurements. It is important to note
that CV has significant solubility in a water pool but
is insoluble in continuous oil. It was reported that the
rate of this reaction was slower in microemulsions with
AOT [11–13] or mixed surfactants formed with AOT
and nonionic surfactants [14] than in aqueous solu-
tions. This was explained by Izquierdo et al. from
aspects of electrostatic interaction between CV and
anionic surfactant head group and the change of wa-
ter activity in the microreactor [11]. Mukherjee et al.
found that the rate constants for the nonaqueous (ethy-
lene glycol/AOT/isooctane) microemulsion are greater
than for the aqueous microemulsion [13], and they
attributed this to the dependence of kinetics of ba-
sic hydrolysis of CV on dielectric constant and ionic
strength in water pool. Some authors [14] investigated
the changes of kinetics of the reaction after adding co-
surfactants, such as Brij30 (polyoxyethylene (4) lauryl
ether), polyoxyethylene nonylphenol, to AOT reverse
micelles for the sake of a better microreactor. Liu et
al. [14] found that the increase of nonionic surfactant
contents could be favorable to the reaction due to the
We have established association models between
CV and AOT in aqueous solutions and water/AOT/oil
microemulsions and determined the association con-
stants and the thermodynamic properties of associa-
tion in the medium of water/AOT/isooctane [10,22].
However, how the association and the kinetic behav-
ior after taking the association into account are af-
fected by the oil media remained unknown [10,22].
It is well known that the properties of a microemul-
sion are oil dependent. Therefore, the effect of the
oil on the thermodynamics and kinetics of basic fad-
ing of crystal violet in a water–oil microemulsion is
worthwhile to study. In this paper, we report the study
of the specific interaction between CV and AOT in
the medium of water/AOT/n-decane microemulsion at
various temperatures. By taking the association be-
tween AOT and CV into account, the “actual” rate
constants and the activation energies of the basic hy-
drolysis of CV in the media of water/AOT/n-decane
and water/AOT/isooctane are calculated from the mea-
surements of absorbance changes of CV with time at
various ω and temperatures. The difference in ther-
modynamics and kinetics between the two media is
investigated and discussed.
EXPERIMENTAL
Materials
AOT (sodium bis(2-ethylhexyl)sulfosuccinate) sup-
plied by Fluka (Buchs, Switzerland) was dried over
P2O5 in a desiccator for 2 weeks. Dye crystal vi-
olet, NaOH (AR-grade reagent, Beijing Chemical
Co., Beijing, People’s Republic of China), isooc-
tane (AR-grade reagent, Tianjin Second Chemi-
cal Co., Tianjin, People’s Republic of China), and
n-decane (mole fraction purity >0.99, Tianjin Guang
Fu Chemical Co., Tianjin, People’s Republic of China)
were used without further purification. The deionized
water with a conductivity less than 1.6 × 10−6 s m−1
was used throughout the study.
Sample Preparation
Aqueous solution with a CV concentration of 1 ×
10−3 mol L−1 was prepared. According to the required
International Journal of Chemical Kinetics DOI 10.1002/kin

296
CHEN ET AL.
concentrations of CV and AOT, at the molar ratios ω of
water to AOT being 10, 15, 20, and 25, respectively, we
added appropriate amounts of AOT and the above CV
aqueous solution and water into volumetric flasks, and
diluted the mixture by n-decane to 5 mL. The mixtures
then were shaken to form a series of clear microemul-
sions. These microemulsions were used for study of
the association of CV and AOT.
DATA ANALYSIS
The Association of CV and AOT
in Microemulsions
The reaction between CV and AOT takes place in
the interfacial layer between water and AOT (Fig. 1).
Assuming a = nC/nAOT, where nAOT and nC are total
quantities of AOT and CV associated with AOT to
form the complexes, respectively. The equilibrium
constant K may be expressed as K = a/(cCV − x),
where x and cCV are the concentrations of CV
associated with AOT and the total concentration of
CV (the sum of free CV and CV associated with AOT)
in the water pool, respectively. Let Kꢀ = x/(cCV − x),
The samples for study of the kinetics of the reaction
were prepared as follows: An aqueous solution with
a concentration of sodium hydroxide of about 1 mol
L−1 was prepared, and its accurate concentration was
determined by titration with potassium hydrogen ph-
thalate. An appropriate amount of AOT was weighed
and dissolved by an appropriate amount of n-decane
or isooctane in a 5-mL volumetric flask, and then an
appropriate amount of crystal violet solution of 1 ×
10−3 mol L−1 and deionized water was injected with
a microsyringe, followed by diluting with n-decane or
isooctane to 5 mL to form the microemulsion as the
reaction medium. The overall concentrations of crys-
tal violet and AOT in the microemulsion were 1.5 ×
10−5 and 0.4 mol L−1, respectively. We added 25 μL
of the above sodium hydroxide solution into the mi-
croemulsion before the kinetics measurements. The
overall concentration of NaOH in the microemulsion
ꢀ
¯
¯
then K = K ωVw, where Vw is molar volume of water.
Taking the microemulsion without CV as a reference,
the measured absorbance A of sample is related to K
and the total concentration cCV
microemulsion by [22]:
of CV in overall
overall
K
A − εCVcCV
= (εC − εCV
)
cCV
overall
¯
overall
K + ωVw
= ScCV
(1)
overall
where εCV and εC are molar absorptivities of CV and
the complex, respectively. S may be expressed as
was about 5 × 10−3 mol L−1
.
¯
Vw
1
1
=
ω +
(2)
S
K(εC − εCV
)
εC − εCV
Fixing ω = 10, 15, 20, and 25, respectively, a
series of microemulsions with various concentrations
of CV were prepared and the absorbance of each
microemulsion was measured at various temperatures
and at the wavelength 580 nm, where the difference
between the values of εCV and εC is maximum. The
molar absorption coefficient of CV in the water at
580 nm and 298 K was determined to be (8.07
0.12) × 104 L mol−1 cm−1. Fitting the experimental
data to Eq. (1) gives S for each of ω at a certain
temperature, then 1/S for each of ω was fitted to
Eq. (2) to obtain the association constant K. The
values of K at various temperatures were then
used to calculate the Gibbs free energy of associa-
tion ꢀrGm through ꢀrGm = −RT ln(Kco), where
co = 1 mol L−1. A good linear relation between
ln(Kco) and 1/T indicated that ꢀrHm is an approx-
imately constant in the temperature range we studied.
UV–Vis Measurements
A series of water/AOT/n-decane microemulsions with
various concentrations of CV were prepared and set
in thermostats with required constant temperatures for
10 min to reach thermodynamic equilibria. The ab-
sorbance of each microemulsion was then measured
at the wavelength of 580 nm and various temperatures
with an Agilent 8453E diode array UV–vis spectropho-
tometer with a thermostated multiple cell holder.
Reaction kinetics of basic hydrolysis of CV
in microemulsions of water/AOT/n-decane and wa-
ter/AOT/isooctane with constant AOT concentration
being 0.4 mol L−1 was studied by kinetics mode of the
spectrophotometer at ω = (5), 8, 10, 13, 15, 18, 20,
23, and 25. The absorbance measurements of CV in
the microemulsion reaction media were carried out at
λmax = 600 nm and automatically with an interval of
about 30 s in a period of time of about 20 min after the
reaction was started. All the measurements were car-
ried out at constant temperatures of 298, 300, 303, 308,
310, 313, and 318 K, respectively, with the stability of
0.1 K.
The Kinetics Model of Basic Hydrolysis
of CV in Water/AOT/Oil Microemulsions
The reaction of basic hydrolysis of CV is illustrated
in Fig. 2, which may be taken as a pseudo-first-order
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ASSOCIATION THERMODYNAMICS BETWEEN CRYSTAL VIOLET AND SODIUM SULFOSUCCINATE
297
Figure 1 Scheme of interfacial association between crystal violet and sodium bis(2-ethylhexyl)sulfosuccinate.
reaction with respect to CV when the concentration of
NaOH is much higher than that of the CV:
where k1 = k2cOH is the pseudo-first-order rate con-
stant, k2 is the apparent second-order rate constant and
cOH is the concentration of NaOH in the water pool.
When the association between CV and AOT is taken
into account, the reaction rate may be expressed as
dcCV
r = −
= k2cCVcOH = k1cCV
(3)
dt
dcCV
r = −
= k(cCV − x)cOH
(4)
dt
where k is “actual” second-order rate constant of the
basic hydrolysis of CV. Substituting the constant Kꢀ =
x/(cCV − x) into Eq. (4) yields
dcCV
dt
Voverall
Vw 1 + Kꢀ
k
Figure 2 The equilibrium of basic hydrolysis of crystal
violet.
r = −
=
cCV cOH (5)
overall
International Journal of Chemical Kinetics DOI 10.1002/kin

298
CHEN ET AL.
rate constant k2 and “actual” second-order rate constant
k:
ꢂ
ꢃ
K
k = k2 1 +
(8)
¯
ωVw
RESULTS AND DISCUSSION
Thermodynamic Parameters of Association
between AOT and CV
The values of association constant, Gibbs free energy,
enthalpy, and entropy for association of CV and AOT
in water/AOT/n-decane microemulsion together with
that in water/AOT/isooctane microemulsion reported
previously [22] are listed in Table I. It is found that in
both systems the increase of temperature leads to an
increase of the association constant K, whereas a de-
crease of the Gibbs free energy ꢀrGm. There are higher
association enthalpy and higher association entropy for
isooctane (ꢀrHm = (35 2) kJ mol−1, ꢀrSm = (105
Figure 3 Plot of ln A against t with various ω at 300 K.
where Voverall is the overall volume of microemulsion
and Vw is the volume of water in microemulsion. Ac-
cording to Eq. (1), the derivative of absorbance with
respect to time gives
8) J mol−1
K
−1) than that for n-decane (ꢀrHm
=
ꢀ
ꢁ
(25 1) kJ mol−1, ꢀrSm = (71 3) J mol−1
K
−1).
dA
dt
K
Vw dcCV
= εCV + (εC − εCV
)
¯
K + ωVw Voverall dt
The positive association enthalpy and entropy indicate
that the association reaction of CV and AOT is en-
dothermic and entropy driven.
(6)
(7)
A combination of Eqs. (1), (5), and (6) yields
The properties of association complex CVAOT
formed by CV+ and surfactant AOT depend on the
conformation of AOT. It is known that in trans con-
formation, both of the ester groups connected with
C-1ꢀ (see Fig. 1) and the sulfo-group direct toward the
polar side; thus, there exist not only the interaction
between CV+ and the sulfo-group but also the interac-
tion between CV+ and carbonyl of the ester bond. In
the gauche conformation, its ester group moves from
the polar region toward the hydrocarbon apolar re-
gion of the surfactant monolayer [23,24], there exists
only the interaction between CV+ and the sulfo-group.
The penetration of nonpolar solvent molecules into the
dA
dt
k
= −
cOHA
1 + Kꢀ
According to Eq. (7), a plot of ln A against t yields a
k
straight line with the slope being 1+Kꢀ cOH. As a demon-
stration, Fig. 3 shows such linear relationships for the
samples with various ω at 300 K. We were unable to
obtain the rate constants for the samples with ω < 8
or ω < 5 because the changes of absorbance with time
were too fast.
ꢀ
¯
A combination of Eqs. (3), (7), and K = K ωVw
gives the relation between the apparent second-order
Table I Values of Association Thermodynamic Parameters of Crystal Violet and Sodium
Bis(2-Ethylhexyl)Sulfosuccinate in Water-in-Oil Microemulsions Formed with n-Decane and Isooctane
K(L mol−1
n-Decane
Isooctanea
)
ꢀ G (kJ mol−1
)
ꢀ H (kJ mol−1
)
ꢀ S (J mol−1 K−1
)
r
m
r
m
r m
T (K)
n-Decane
Isooctanea
n-Decane
Isooctanea
n-Decane
Isooctanea
298
300
303
308
310
313
318
0.21 0.03 0.18 0.02
0.24 0.01 0.20 0.03
0.26 0.03 0.25 0.03
0.29 0.01 0.27 0.03
0.33 0.04 0.32 0.03
0.35 0.03 0.35 0.02
0.40 0.03 0.47 0.02
3.9 0.3
3.6 0.1
3.4 0.3
3.1 0.1
2.9 0.3
2.7 0.2
2.4 0.2
4.2 0.3
4.0 0.4
3.5 0.3
3.3 0.3
2.9 0.2
2.7 0.1
2.0 0.1
25
1
35
2
71
3
105
8
a From Chen et al. [22].
International Journal of Chemical Kinetics DOI 10.1002/kin

ASSOCIATION THERMODYNAMICS BETWEEN CRYSTAL VIOLET AND SODIUM SULFOSUCCINATE
299
domain of hydrocarbon tails of surfactants aligned at
an interface possibly drives more ester groups in AOT
toward polar region. The penetration effect depends
on the sizes and structures of the solvent molecules
[25,26]. The effect of the penetration of isooctane
seems larger than n-decane. This increases the pro-
portion of trans conformation of AOT molecules and
further affects the conformation of association com-
plex CVAOT. This causes larger association enthalpy
and larger association entropy for isooctane than that
for n-decane. Therefore, the association reaction be-
tween CV and AOT in water/AOT/isooctane is more
favorable than that in water/AOT/n-decane microemul-
sion at higher temperature, which is consistent with the
Figure 4 “Actual” rate constants k for the basic hydrolysis
of crystal violet at various ω.
entropy-driven process. However, the above discussion
involving the assumption of the change of AOT confor-
mations needs to be conformed by more experimental
results.
negative group of AOT, which counteracts the negative
charges of AOT. This is favorable for OH− to approach
the CV+, which located near the AOT interface. The
characteristics of water molecules incorporated to mi-
croemulsion strongly depend on the ω value and the
nature of the surfactant head groups [27]. The pro-
portion of bound water decreases with the increase of
ω; therefore, the electrostatic repulsion between OH−
and AOT head groups becomes more significant, which
reduces the effective concentration of OH− near the in-
terface where the reaction of basic hydrolysis of CV
takes place, which causes the decrease of reaction rate
constant and the increase of activation energy men-
tioned in the following text.
The activation energies Ea and the natural loga-
rithm of preexponential factors ln A of the basic hy-
drolysis kinetics of CV for various ω were estimated
by processing the temperature effect on the “actual”
rate constants k according to the Arrhenius formula,
ln k = −(Ea/RT ) + ln A, which are listed in Table II,
Kinetics of Basic Hydrolysis of CV in
Water-in-Oil Microemulsions
According to Eq. (8), the apparent second-rate constant
of basic hydrolysis of CV is less than the “actual” rate
constant k in the water-in-oil microemulsion owing
to the existence of association of CV and AOT. The
“actual” second-order rate constants for various ω and
temperatures are shown in Fig. 4.
From Fig. 4, we can observe that the value of k
decreases with the increase of ω at a constant tem-
perature and increases with temperature in the range
from 298 K to 318 K at a constant ω. It has been
observed that the “actual” rate constants k are lower
in water/AOT/n-decane than in water/AOT/isooctane
medium at higher temperature in evidence, which ac-
corded with the second-order rate constants k2 [13].
It is well known that the bound water molecules lo-
cate near the interface and are hydrogen bonded to the
Table II Values of the Arrhenius Activation Energy Ea and the Natural Logarithm of Preexponential Factor ln A of
Basic Hydrolysis of Crystal Violet in Microemulsions Formed with n-Decane and Isooctane for Various Molar Ratios ω
E (kJ mol−1
)
ln A (L mol−1 s−1
)
a
ω
n-Decane
Isooctane
n-Decane
Isooctane
5
8
63.1 1.8
64.4 1.8
66.8 2.1
70.4 1.9
72.4 1.4
74.2 1.3
76.1 1.1
76.7 1.4
22.6 0.7
22.8 0.7
23.5 0.8
24.7 0.8
25.4 0.6
25.9 0.5
26.6 0.5
26.7 0.5
62.7 0.9
63.7 0.6
64.0 0.8
64.9 0.6
67.1 0.9
68.5 0.8
69.3 0.5
22.0 0.4
22.2 0.2
22.2 0.3
22.4 0.2
23.1 0.3
23.6 0.3
23.8 0.2
10
13
15
18
20
23
International Journal of Chemical Kinetics DOI 10.1002/kin

300
CHEN ET AL.
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International Journal of Chemical Kinetics DOI 10.1002/kin