Kinetics of a Hydrolysis Reaction in an Oil/Water Microemulsion System Near the Critical Point

Article abstract of DOI:10.1007/s10953-016-0467-9

We have constructed the pseudoternary phase diagram of surfynol465?+?n-butanol?+?cyclohexane?+?H₂O with k_m?=?2 (where k_m is the weight ratio of surfynol465 to n-butanol) by the water titration method. Electrical conductivity measurements were employed to investigate the microstructures of the single-phase region. In the oil/water microemulsion region, we have measured the hydrolysis reaction rate of 2-bromo-2-methylpropane near and far away from the critical point. It was found that the Arrhenius equation was valid for correlating experimental measurements far away from the critical point but a significant acceleration effect exists near the critical point, which is not consistent with thermodynamic interpretation of Griffiths and Wheeler.

Full text of DOI:10.1007/s10953-016-0467-9

J Solution Chem
DOI 10.1007/s10953-016-0467-9
Kinetics of a Hydrolysis Reaction in an Oil/Water
Microemulsion System Near the Critical Point
1
1
1,2
1
•
Ya Yang
Shenghui Zhang
•
Jing Jin
1
•
Jinshou Wang
•
Zhen Shi
Received: 3 May 2015 / Accepted: 28 February 2016
Ó Springer Science+Business Media New York 2016
Abstract We have constructed the pseudoternary phase diagram of surfynol465 ? n-
butanol ? cyclohexane ? H O with k = 2 (where km is the weight ratio of surfynol465
2
m
to n-butanol) by the water titration method. Electrical conductivity measurements were
employed to investigate the microstructures of the single-phase region. In the oil/water
microemulsion region, we have measured the hydrolysis reaction rate of 2-bromo-2-
methylpropane near and far away from the critical point. It was found that the Arrhenius
equation was valid for correlating experimental measurements far away from the critical
point but a significant acceleration effect exists near the critical point, which is not con-
sistent with thermodynamic interpretation of Griffiths and Wheeler.
Keywords Critical effect Á Critical point Á Microemulsion Á Phase diagram
1
Introduction
During the last four decades there have been extensive theoretical and experimental studies
of the critical behavior [1–6] and chemical reactions near the consolute point [7–11]. The
consolute point is an extremum in the temperature versus mole fraction phase diagram of a
constant pressure liquid–liquid mixture, where the homogeneous solution first begins to
separate into two immiscible layers. Near a consolute point, large fluctuations of the order
parameter dominate, resulting in behavior quite different from that far away from the
consolute point. Upon introduction of a reactant to the liquid, a chemical reaction can often
&
Jinshou Wang
wangjsh04@163.com
1
2
Department of Chemistry, Hubei University for Nationalities, Enshi 445000, Hubei, China
Key Laboratory of Biologic Resources of Protection and Utilization of Hubei Province,
Enshi 445000, Hubei, China
123

J Solution Chem
be initiated. This permits critical mixtures to be used to study the critical effect on the net
rate of reaction.
The influence of the critical phenomena on chemical reactions has been reported by
Wheeler et al. [7–11]. However, not all the experimental results are consistent with these
theories, which support slowing down of chemical reactions near the critical point. Snyder
and Eckert [12] reported clear evidence of speeding up of the reaction of isoprene with
maleic anhydride near the liquid–liquid critical point. Kim and Baird [13–20] measured the
rates of thirteen different chemical reactions in three different critical binary liquid mix-
tures; in six cases [13–16], slowing down was found and in four cases [1–19], speeding up
was evident. In two cases [20], no effects were detected while in one case [20] the rate
constant oscillated in magnitude by about 10 %. Recently, Shen and colleagues [21, 22]
studied the kinetics of the oxidation of iodide ion by persulfate ion and the alkaline fading
reaction of crystal violet in the critical water/oil microemulsion of water/bis(2-ethylhexyl)
sodium sulfosuccinate/n-decane with the molar ratios of water to sodium bis(2-ethylhexyl)
sulfosuccinate being 35.0 and 40.8 and to 2-butoxyethanol ? water, respectively. It was
found that Arrhenius equations were valid for correlating experimental measurements far
away from their respective critical regions but there was significant slowing near their
respective critical regions. Obviously, several kinds of critical effects were found in dif-
ferent critical mixtures and even in the same one.
It should be pointed out that the hydrolysis reaction rate of 2-bromo-2-methylpropane in
the one-phase region of triethylamine ? H O [17] at temperatures near its critical point is
2
speeded up, which is different from six other hydrolysis reactions in liquid–liquid mixtures
at temperatures near their respective consolute points [14, 15]. The structure of these
reactants and reaction mechanism are similar; however, the critical effects are different.
These differences may be related to the reactants or the critical mixtures. To explore
whether the critical effects are related to the critical mixtures, recently we studied the
hydrolysis reaction rate of 2-bromo-2-methylpropane in a similar mixture of triethy-
lamine ? D O [23]. It was found that the reaction rate in the one phase region at tem-
2
peratures near the critical point slowed down, which is not consistent with the results of
Kim et al. [17].
In order to further explore whether the critical effects are related to the critical mixtures,
we have extended the studies of critical effect on the chemical reaction to more compli-
cated microemulsion systems [24]. To the best of our knowledge, the critical effect of the
oil/water (O/W) microemulsion on reaction rate is seldom explored. In this work, we have
measured the hydrolysis reaction rate of 2-bromo-2-methylpropane in the O/W
microemulsion region of surfynol465 ? n-butanol ? cyclohexane ? H O with k = 2
2
m
(where km is weight ratio of surfynol465 to n-butanol), near and far away from its critical
Fig. 1 Structure of surfynol465
H C
CH3
3
H C
CH3
3
H C
CH3
3
O
O
n
m
HO
OH
m + n = 10.4
1
23

J Solution Chem
point, by electrical conductivity and the experimental results are compared with those of
the same reaction in triethylamine ? H O and triethylamine ? D O.
2
2
2
Experimental
.1 Materials
-Bromo-2-methylpropane (C99 %) was provided by Aladdin Chemistry Co., Ltd.
2
2
Cyclohexane (C99.5 %) and n-butanol (99.5 %) were purchased from Sinopharm Chem-
ical Reagent Co., Ltd. Gemini surfactant surfynol465 was provided by Yongli Co., Ltd.
(C 98 %) and the structure is shown in Fig. 1. Twice distilled water was used throughout
the study.
2
.2 Construction of the Phase Diagram
The pseudoternary phase diagram was constructed to investigate the concentration ranges
of components for identifying the existence of microemulsion regions by the water titration
method. Cyclohexane was mixed with surfynol465/n-butanol mixture (2:1); water was then
added dropwise using a micropipet until the transparent and homogeneous dispersion
converted to a turbid mixture. The appearance of turbidity was considered as an indication
of phase separation. The microemulsion regions were identified as optically transparent
and isotropic mixtures. Finally, electrical conductivity measurements were employed to
investigate the gradual changes occurring in the microstructure of the microemulsion
system. The temperature was measured with a thermostat (SWQ-I , Sangli Electronic
A
Instrument Company) in which the temperature was controlled within ± 0.1 K.
2
.3 Determination of the Critical Composition and the Critical Temperature
of the System
The pseudoternary system has numerous double critical points; however, the critical
composition and critical temperature of a double critical point can be obtained when the
weight ratios of three components of a pseudoternary microemulsion system are deter-
mined. In this work, the critical composition was determined through the technique of
‘‘equal volume’’: adjusting the droplet concentration to achieve identical volumes of two
separating liquid phases at the phase separation temperature, and the concentration and
phase separation temperature were taken to be the critical ones [25]. Samples with the
critical composition were prepared in a test tube. It was placed inside a low temperature
thermostatic bath (GDH-0506, Ningbo Scientz Biotechnology Company) in which tem-
perature was controlled within ± 0.002 K. The temperature was measured with a platinum
resistance thermometer (SWC-II , Sangli Electronic Instrument Company). For a critical
D
reaction mixture, the critical composition and critical temperature always change slowly
during the reaction. In other words, the critical composition and the critical temperature
determined immediately after mixing are different from those of the reaction mixture that
has come to equilibrium. For a dilute critical mixture, the reaction has usually no mea-
surable effect on critical composition; however, it can cause a noticeable shift in the
critical temperature.
123

J Solution Chem
2
.4 Kinetics Measurement
The electrical conductivity was measured using a conductivity meter (Model DDS-11A)
-
1
.
which had a repeatability of 0.001 mSÁcm
The overall reaction is:
ðCH3Þ CBr þ H2O ! ðCH3Þ COH þ HBr
3
3
Because the hydrolysis reaction produced the strong electrolyte HBr, we could follow the
progress of the reaction by making measurements of the electrical conductivity. The
instantaneous value of the conductance L is given by:
L ¼ L0 þ ðLe À L0Þðn=neÞ
ð1Þ
where L is the initial value and L is the value at equilibrium and n is the extent of
0
e
reaction. At time t = 0, the extent of reaction has the value n(0) = 0, while at chemical
equilibrium it has the value n = ne.
If the reaction is first order, the rate law is [17]:
dn
¼
Àkðn À neÞ
ð2Þ
ð3Þ
dt
For the initial condition, n = 0 at t = 0, the integral of Eq. 1 is:
n ¼ n ð1 À expðÀktÞÞ
e
where k is the rate constant.
If we substitute this into Eq. 1, we obtain:
L ¼ L0 þ ðLe À L0Þð1 À expðÀktÞÞ
By fitting the experimental value of L - L to time t, the reaction rate constant k of the
ð4Þ
0
hydrolysis reaction of 2-bromo-2-methylpropane can be obtained.
Fig. 2 Pseudoternary phase
diagram of surfynol465 ? n-
cyclohexane
butanol ? cyclohexane ? H
with k = 2 at 303.2 K
2
O
0
1
00
9
m
1
0
0
2
0
8
0
3
0
7
0
4
0
6
0
5
0
5
0
6
0
two phase
4
0
7
0
3
0
W/O
one phase
8
0
critical composition
2
0
9
0
1
0
1
00
0
water0
10
20 30 40 50 60 70 80 90 100
O/W bicontinuous surfynol465/n- butanol(2:1)
1
23

J Solution Chem
3
Result and Discussion
Figure 2 shows the pseudoternary phase diagram of surfynol465 ? n-butanol ? cyclo-
hexane ? H O with k = 2 at 303.2 K. A large single isotropic region extending from the
2
m
water corner to the cyclohexane one is observed. The region marked ‘‘one phase’’ is the
one phase microemulsion and the region marked ‘‘two phase’’ is a turbid region, that is, a
microemulsion in equilibrium with an excess cyclohexane or water phase. The composi-
tions are listed in Table 1. The large single phase microemulsion region has been differ-
entiated into discrete reverse micelle (water/oil), bicontinuous, and normal micelle (O/W)
regions according to the electrical conductivity measurements. The electrical conductivity
measurement is the most widely used simple technique for examining the microstructures
and their structural changes in surfactant based microemulsion systems. We have measured
electrical conductivity as a function of water weight fraction wH O% to identify the
microstructure type of the microemulsion phase in the present investigation. Figure 3 is a
representative electrical conductivity versus wH O% profile at 303.2 K for the studied
2
2
microemulsion system. The curve can be divided into four distinct segments. (1) The initial
nonlinear part may be attributed to inverse microdroplet aggregation; (2) the next linear
increase is due to the formation of aqueous microdomains, which results from the partial
fusion of clustered inverse microdroplets, suggesting that a water/oil microemulsion is
formed in this low water content region; (3) the third nonlinear increase of electrical
conductivity indicates that the medium undergoes a gradual structural transition and a
bicontinuous microstructure is formed due to the progressive growth and interconnection
of the aqueous microdomains; (4) the final decrease of electrical conductivity with increase
of water content corresponds to the appearance of water–continuous microemulsion. That
is, an O/W microemulsion is formed at high water content, and the decrease of electrical
conductivity results from the progressive decrease of the concentration of the O/W
microemulsion droplets. These results are similar to the observations made in an earlier
study on the electrical conductivity of microemulsions [26].
Table 1 Experimentally determined composition along the phase boundary of surfynol465 ? n-bu-
tanol ? cyclohexane ? H
03.2 K
2 m m
O with k = 2 (where k is the weight ratio of surfynol465 to n-butanol) at
3
Surfynol465/n–butanol (2:1) (wt%)
Cyclohexane (wt%)
Water (wt%)
8
1
2
3
3
4
5
5
5
5
3
2
1
1
.68
82.34
71.44
68.59
45.98
41.82
23.73
14.37
11.28
6.20
8.98
10.08
9.72
8.48
1.69
5.54
8.47
6.36
2.71
4.98
2.34
0.18
9.40
9.09
8.33
3.61
18.47
19.71
29.92
32.92
33.76
41.46
44.67
57.95
68.93
79.79
82.68
5.15
2.65
1.97
1.88
3.71
123

J Solution Chem
Fig. 3 Conductivity as a
function of water concentration
with weight ratio of
surfynol465 ? n- butanol (2:1) to
cyclohexane being 27.62 at
T 303.2 K
0.05
0.04
0.03
0.02
0.01
0.00
O/W
W/O bicon-
tinuous
0.0
0.2
0.4
0.6
0.8
1.0
w %
H O
2
Using electrical conductivity as a measure of the extent of reaction in the critical
microemulsion system, the critical composition should be located in the O/W or bicon-
tinuous microemulsion region. It was found that the critical composition could not be
located in the bicontinuous microemulsion region for surfynol465 ? n-butanol ? cyclo-
hexane ? H O, so we only studied the hydrolysis reaction of 2-bromo-2-methylpropane in
2
the O/W microemulsion region. It is known that 2-bromo-2-methylpropane is insoluble in
water, so the hydrolysis reaction may take place at the outer interface of the O/W
microemulsion structure. The microemulsion system can provide a high interfacial area of
?
-
contact for the reaction and the products of H and Br will go into the continuous
aqueous phase. So, in the O/W microemulsion region, this reaction can be followed by
making measurements of the electrical conductivity. The concept of the hydrolysis reaction
in the O/W microemulsion region is shown in the inset of Fig. 4.
The critical mass fraction of surfynol465 for the mixture of surfynol465 ? n-bu-
tanol ? cyclohexane ? H O with k = 2 and k = 4 (where k is weight ratio of surfy-
2
m
n
n
nol465 to cyclohexane) was found to be 0.0976, which is located in the O/W
microemulsion region shown in Fig. 2. 2-Bromo-2-methylpropane was then added to the
critical mixture using a micropipette with stirring. After about 60 s, the stirrer was stopped
and the experimental data were recorded at the same time. The initial concentration of
-
1
2
-bromo-2-methylpropane is 0.0045 molÁkg . The critical temperatures determined
immediately after mixing is 303.92 ± 0.05 K. Because the mixture has a lower critical
solution temperature, it can be maintained in the one phase region during the entire course
of the reaction simply by holding the thermostat at a temperature of 303.92 K or lower.
The electrical conductivity of the critical reaction mixture of surfynol465 ? n-bu-
tanol ? cyclohexane ? H O ? 2-bromo-2-methylpropane
2
(initial
concentration
-
1
m = 0.0045 molÁkg ) with km = 2 and k = 4 was measured as a function of time at
n
different temperatures. Figure 4 shows a plot of Eq. 4 for a run at 303.375 K. By fitting the
experimental value of L - L at 303.375 K to time t, the rate constant k can be obtained.
0
1
23

J Solution Chem
Fig. 4 Plot of instantaneous
value of the electrical
conductance L versus time t at
1
.00
.95
0
303.375 K for hydrolysis
reaction of 2-bromo-2-
methylpropane in
surfynol465 ? n-
0.90
2
butanol ? cyclohexane ? H O
0
.85
.80
m n
with k = 2 and k = 4 at its
critical composition. The points
are the experimental data and the
solid line is the fitting curve of
the experimental value according
to Eq. 1. The inset shows the
concept of the hydrolysis reaction
in the O/W microemulsion region
0
0.75
0
.70
.65
0
0
5
10
15
20
25
t/min
In Fig. 4 the points are the experimental data and the solid line is the fitting curve of the
experimental value according to Eq. 4. It can be seen that the fitting curve is in agreement
with the experimental data, which suggests that the progress of the hydrolysis reaction of
2
-bromo-2-methylpropane in the O/W microemulsion region can be followed by making
measurements of the conductance and the concept of the hydrolysis reaction in the O/W
microemulsion region shown in the inset of Fig. 4 is reasonable. With the same method,
the rate constants at other temperatures were obtained and are listed in Table 2. In Fig. 5,
we show an Arrhenius plot of the temperature dependence of k. The straight line, which
was fitted to the data collected at temperatures outside the critical region, has the equation
lnðkÞ ¼ ð35:05 Æ 1:07Þ À ð98;440 Æ 2671Þ=RT
ð5Þ
where the uncertainty is the standard error. This line can serve as the ‘‘background’’ for
determining the existence of a critical effect. As mentioned by Griffiths and Wheeler [27],
the observation of critical effects at the consolute point of a multicomponent mixture
Table 2 Rate constant k versus temperature T for the hydrolysis of 2-bromo-2-methylpropane in surfy-
= wsurfynol465/wn-butanol and
nol465 ? n-butanol ? cyclohexane ? H
k
2
O with k
m
= 2 and k
n
= 4 (where k
m
n
= wsurfynol465/wcyclohexane) at its critical composition
-
k (s
1
)
-1
)
-1
-1
k (s )
T (K)
T (K)
k (s
T (K)
k (s
)
T (K)
2
3
3
3
3
3
3
98.159
03.275
03.466
03.655
03.809
03.915
03.961
0.00943
0.0184
0.02237
0.0228
0.0236
0.02192
0.0212
299.15
0.0109
0.0209
0.0210
0.0225
0.0227
0.0203
300.948
303.408
303.575
303.796
303.875
303.951
0.0135
0.0197
0.0230
0.0224
0.0220
0.0220
301.949
303.608
303.702
303.779
303.855
303.963
0.0159
0.0214
0.0211
0.0228
0.0207
0.0208
303.435
303.559
303.815
303.850
303.935
-
1
The initial concentration of 2-bromo-2-methylpropane is 0.0045 molÁkg . The critical temperature T
c
is
3
03.92 ± 0.05 K
123

J Solution Chem
Fig. 5 Arrhenius plot of the rate
constant for hydrolysis reaction
of 2-bromo-2-methylpropane in
surfynol465 ? n-
-
-
-
3.6
3.8
4.0
butanol ? cyclohexane ? H
with k = 2 and k = 4 (where
2
O
m
n
k
k
m
n
= wsurfynol465/wn-butanol and
= wsurfynol465/wcyclohexane) at
its critical composition. The
initial concentration of 2-bromo-
2
0
-methylpropane is
-4.2
-4.4
-1
.0045 molÁkg . The critical
is
03.92 ± 0.05 K
temperature T
3
c
-
-
4.6
4.8
Tc
0
.00328 0.00330 0.00332 0.00334 0.00336
1/T(1/K)
depends on a count of the number of inert components. A component can be considered to
be inert if it is not involved in any chemical reaction. If the mixture contains only one inert
component, critical slowing down will be found. If the mixture contains two or more inert
components, no critical effect is expected. In our reaction mixture, neither the reactants,
2
-bromo-2-methylpropane and water, nor the products, hydrogen bromide and tert-butanol,
can be considered to be inert because they are involved in the main reaction. However,
surfynol465, n-butanol and cyclohexane can be assumed to be three inert components
whose mass fractions are constant independent of T and p, and thus, no critical effect is
expected in the mixture according to Griffiths and Wheeler. However, the net rate of the
reaction in the one phase region at temperatures near the critical point shown in Fig. 5
speeds up, which is not consistent with thermodynamic interpretation of Griffiths and
Wheeler. As mentioned above, the structure and mechanism of these hydrolysis reactions
[
14, 15, 17, 23] are similar; however the critical effects are different. It seems that the
critical effects depend on the critical mixtures or reactants. Therefore it is worth comparing
the critical effect of the same hydrolysis reactant of 2-bromo-2-methylpropane in our
system with that in different critical mixtures of triethylamine ? H O and triethy-
2
lamine ? D O. In triethylamine ? D O, the net reaction rate is slowed down. However,
2
2
critical speeding up is found in our mixture and triethylamine ? H O, which is not con-
2
sistent with thermodynamic interpretation of Griffiths and Wheeler. These phenomena
indicate that critical effect on the net rate of the hydrolysis reaction of 2-bromo-2-
methylpropane may depend on the specific property of the critical mixture.
It should be pointed out that Shen and colleagues [28] recently repeated the experiment
of the hydrolysis reaction of 2-bromo-2-methylpropane in the critical mixture of triethy-
lamine ? H O. However, a critical slowing down rather than a speeding up was observed
2
both in one phase region and the two phase region. These disagreements were attributed to
improper use of the background Arrhenius equation according to Shen. Obviously, the
critical effect on the hydrolysis reaction rate of 2-bromo-2-methylpropane is controversial,
1
23

J Solution Chem
even in the same critical mixture. We consider that the controversy needs more experi-
mental and theoretical studies. For our critical mixture, the critical speeding up may be
related to the specific property normal micelle (O/W). As mentioned above, the hydrolysis
reaction of 2-bromo-2-methylpropane may take place at the outer interface of the O/W
?
-
microemulsion structure. Near the critical point, the products of H and Br may go into
the continuous aqueous phase more easily due to the critical concentration fluctuation [29,
3
0], which will promote the equilibrium process in favor of the products side and accel-
erate the hydrolysis reaction. Although specific properties of the critical mixture of solid–
solvent clustering mechanism [31] and aggregates of ionic compounds [32] have been used
to explain the critical speeding up, up to now there is no satisfactory universal theory for
critical speeding. In other words, the contrast between the theory of critical slowing down
and the experimental observation of speeding-up still remains one of the central puzzles in
understanding the kinetics of chemical reactions at the critical solution point.
4
Conclusion
We have extended the studies of the critical effect on the chemical reaction to more com-
plicated O/W microemulsion media, which is seldom explored. Using the water titration
method, we have constructed the pseudoternary phase diagram of surfynol465 ? n-bu-
tanol ? cyclohexane ? H O with k = 2. Electrical conductivity measurements were
2
m
employed to investigate the microstructures of the single-phase region. In the O/W
microemulsion region, we have measured the rate of hydrolysis reaction of 2-bromo-2-
methylpropane near and far away from the critical point. It was found that the hydrolysis
reaction rate of 2-bromo-2-methylpropane speeded at temperatures near the critical point,
which is not consistent with thermodynamic interpretation of Griffiths and Wheeler. More-
over, the experimental results were compared with those of the same reaction in different
critical mixtures, which indicated the critical effect on the hydrolysis reaction rate of
2
-bromo-2-methylpropane might depend on the specific property of the critical mixture.
Acknowledgments This work was supported by The National Natural Science Foundation of China (Nos.
2
2
1363007, 21361009 and 21175033), and the Natural Science Foundation of Hubei Province (No.
012FFC02401).
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