Effect of ethylene glycol on micellization and micellar-catalyzed alkaline hydrolysis reaction of a cationic surfactant at 293-313 K

Article abstract of DOI:10.1134/S003602440913010X

Changes in critical micellar concentrations (CMC's) of gemini surfactant, α, ω-ethane bis(dimethyl cetyl ammonium bromide) (C ₁₆-2-C₁₆) with different concentrations of ethylene glycol (EG) addition have been investigated by electrical conductivity method. Subsequently, alkaline hydrolysis of ethyl acetate (EA) in the presence of C₁₆-2-C₁₆ and C₁₆-2-C₁₆-EG has been studied conductometrically at 303.2 and 313.2 K, respectively. It was found that an increase in concentrations of EG added to C₁₆-2-C₁₆ aqueous solutions caused an increase in CMC's of C₁₆-2-C ₁₆, provoked by the decrease in the interfacial Gibbs energy contribution to G_M° . The hydrolysis of EA showed catalytic and restrained dual behavior in the presence of surfactant, it may be related to higher microviscosity and change of morphology with increased surfactant for C₁₆-2-C₁₆ at higher concentration. Addition of EG did not change microenvironment in micellar interfacial region significantly, which had less effect on gemini C₁₆-2-C₁₆ micellar catalytic efficiency.

Full text of DOI:10.1134/S003602440913010X

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2009, Vol. 83, No. 13, pp. 2238–2242. © Pleiades Publishing, Ltd., 2009.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Effect of Ethylene Glycol on Micellization
and MicellarꢀCatalyzed Alkaline Hydrolysis Reaction
of a Cationic Surfactant at 293–313 K1
Sanxian Liu, Yuhua Shen, Anjian Xie, Xiufang Wang, Weiqiang Zhang, and Zhongwen Pan
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, P. R. China
eꢀmail address: anjx@163.com
Received January 19, 2009
Abstract—Changes in critical micellar concentrations (CMC’s) of gemini surfactant,
α
, ꢀethane bis(dimꢀ
ω
ethyl cetyl ammonium bromide) (C16–2ꢀC16) with different concentrations of ethylene glycol (EG) addition
have been investigated by electrical conductivity method. Subsequently, alkaline hydrolysis of ethyl acetate
(EA) in the presence of C16–2ꢀC16 and C16–2ꢀC16ꢀEG has been studied conductometrically at 303.2 and
313.2 K, respectively. It was found that an increase in concentrations of EG added to C16–2ꢀC16 aqueous
solutions caused an increase in CMC’s of C16–2ꢀC16, provoked by the decrease in the interfacial Gibbs
°
energy contribution to GM . The hydrolysis of EA showed catalytic and restrained dual behavior in the presꢀ
ence of surfactant, it may be related to higher microviscosity and change of morphology with increased surꢀ
factant for C16–2ꢀC16 at higher concentration. Addition of EG did not change microenvironment in micellar
interfacial region significantly, which had less effect on gemini C16–2ꢀC16 micellar catalytic efficiency.
DOI: 10.1134/S003602440913010X
INTRODUCTION
the reactants and providing a different reaction enviꢀ
ronment from the aqueous solution. As reported in
most of the available literatures on micellar catalysis,
effects of the micellar catalysis on hydrolysis of esters
have been studied in many conventional surfactant
solutions extensively. However, there have been few
investigations on micellarꢀcatalyzed alkaline hydrolyꢀ
sis reaction in gemini surfactant solutions until now
[8]. Herein, we describe the change of a gemini surfacꢀ
tant concentration at which aggregation occurs
(CMC) at the lower concentration range of EG, and
investigate the micellarꢀcatalyzed alkaline hydrolysis
of ethyl acetate (EA) in the presence of a cationic
gemini surfactant C16–2ꢀC16 and effect of ethylene
glycol (EG) on gemini micellar catalytic efficiency. It
was found that higher microviscosity and change of
morphology with increased surfactant for C16–2ꢀC16
at high concentration may remarkably influence the
hydrolysis reaction.
Recently, gemini surfactants with novel molecular
structures have attracted great interests owing to their
manifest lower critical micellar concentrations and
enhanced propensity for bringing down the oil–water
interfacial tension in comparison to their monovalent
single headgroup/single chain counterparts [1]. A
considerable number of investigations have been
reported on the synthesis of many gemini surfactants
and their unusual physicoꢀchemical properties,
including their high surface activity, unusual changes
of viscosity, unusual micelle structure, and aberrant
aggregation behavior [2–4]. Furthermore gemini surꢀ
factants are receiving extensive attention in both pure
and applied sciences, such as skin care, antibacterial
regimens, construction of highꢀporosity material,
analytical separations and transdermal drug delivery,
and so on [5].
Polar organic molecules added to aqueous micellar
solutions will alter the tendency of the amphiphile
molecules to avoid contact with the solvent, therefore,
it is expected to affect the value of surfactant concenꢀ
tration at which aggregation occurs as well as polarity
and solvent content in the interfacial region (“solvoꢀ
phobic effect”) [6]. The presence of polar organic solꢀ
vents is expected to alter the critical micellar concenꢀ
trations (CMC’s) of surfactants [7]. Surfactants that
generally form micelles in aqueous solutions above the
CMC’s can affect the rate of reaction by concentrating
EXPERIMENTAL
Materials. The gemini surfactant, C16–2ꢀC16 was
synthesized as described elsewhere [9]. Briefly, C16–2ꢀ
C16 was prepared through the reaction 1ꢀbromohexaꢀ
decane with N,N,N',N'ꢀtetramethylethylenediamine,
using a slight excess (about 10%) of the alkyl bromide,
under reflux in absolute ethanol for 48 h. After rotaꢀ
tory evaporation of the ethanol and the unreacted
compounds, the resulting solids were recrystallized
three times from ethyl acetateꢀethanol mixtures. The
1
The article is published in the original.
2238

EFFECT OF ETHYLENE GLYCOL ON MICELLIZATION
2239
3
1
κ × 10− ,
μ
S cm−
1
κ
,
S cm−
μ
1.6
1.4
1.2
3
12
8
2
1
4
0
2
4 6
[C16–2ꢀC16]
8
10
1.0
0
×
105, mol/l
20
40
60
t, min
Fig. 2. The curve of conductivity–[C –2ꢀC ] at (1) 20,
16
16
Fig. 1. The curve of reaction system conductivity κ–time t.
(2) 30, and (3) 40°C.
1
purity was confirmed by H NMR. EA and EG were As reaction carried out in the same container, there
analytical grade, which were purchased from Shanghai will be an equation:
Chemical Reagent Co, China and used without furꢀ
κt = (κ0 + akκ t)/(1 + akt).
(3)
∞
ther purification. All the other chemicals were also of
reagent grade and commercially available.
Dealing this equation with nonꢀlinear fitting, conseꢀ
quently kinetic expressions of alkaline hydrolysis of
EA can be obtained
Conductivity measurements. Conductivities of surꢀ
factants were measured at experimental temperature
in a water flow thermostat connected to the cell comꢀ
partment. Values of the CMC are obtained from the
breaks in plots of conductivity against [surfactant],
y = (B + Cx)/(1 + Ax).
onꢀlinear fitting parameters ak
On the basis of parameter and initial concentration
(4)
A
=
,
B
=
κ0,
C
=
akκ∞
.
and values of the ionization degree ( ) are estimated
α
A
from the ratios of slopes above and below the CMC.
(
a
), the rate constant of the alkaline hydrolysis can
k
Kinetic measurements. The hydrolysis of EA in the
presence of 0.04 M NaOH was studied by measuring
the change of reacting system conductivity with time.
The decrease in conductivity with the reaction was
recorded at intervals of two minutes with DDSJ–308A
conductivitimeter. The rate constants of reactions
were determined from the least square nonꢀliner fitꢀ
ting the curve of conductivity against reaction times.
The conductimeter was calibrated with KCl solutions
of the appropriate concentration range.
Based on the chemical kinetics theory, the alkaline
hydrolysis of EA is a typical secondꢀorder reaction.
The conductivity property of additive and ions are
directly proportional to its concentration in dilute
solution, and the conductivity of CH3COOC2H5 and
C2H5OH can be negligible in reaction system, as well
as the conductivity of Na+ will not change with the
reaction time, furthermore the migration rate of OH–
is greater than that of RCOO–, therefore, the value of
the conductivity of the system depends mainly on the
concentration of OH–. If the initial concentrations
be calculated. As can be seen from Fig. 1 the conducꢀ
tivity of reaction system decreases with time.
RESULTS AND DISCUSSION
Effects of temperature and EG on gemini surfacꢀ
tant’s CMC. Temperature range selected in experiꢀ
ments is from 293.15 to 313.15 K, which step is 10 K.
It was found that the CMC of C16–2ꢀC16 increased
slightly with temperature increasing (showed in Fig. 2
and Table 1). This behavior can be explained by the
work of Ramadan et al. [10] on conventional surfacꢀ
tants. Their reasoning also applies to gemini surfacꢀ
tants. The authors showed that micellization is made
up of two contributions. The first one is related to the
destruction of the structured water surrounding alkyl
chains. The second contribution is associated to the
condensation of alkyl chains into a micelle. Increasing
temperature results in a decrease of the first contribuꢀ
tion, as the structured water severely breaks down, but
affects only little the second contribution. As a result
the possibility of micellization decreases and becomes
more difficult, and the CMC of C16–2ꢀC16 increases.
(we denote a) of reactants were equivalent, the relaꢀ
tionship of the system conductance and rate constant
was as follows:
In general, it is believed that the interaction
between alcohol and water molecules forms hydrogen
bonds. Such strong interaction impacts on the formaꢀ
tion of the water structure (iceberg structure) near the
micelle, which weakens the surfactant hydrophobic
(G0 – Gt)/(Gt – G ) = akt.
∞
(1)
Changing (1) into another form to be
Gt = (G0 + akG t)/(1 + akt).
∞
(2)
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2240
SANXIAN LIU et al.
Table 1. CMC’s of C16–2ꢀC16 in alcohol–water mixtures and at a variety of temperatures
303.2 K
T, K
CMC ×
105, M
[EG]
×
104, M
CMC
×
105, M
[PEG]
×
105, M
CMC ×
105, M
1
1
0
2.86
1.99
2.31
3.15
0
2.86
2.91
2.98
3.05
293.2
303.2
313.2
2.39
2.86
4.35
0.41
0.83
1.65
0.80
1.60
2.40
effect and the ability to form micelles, leading to the
increase in CMC.
°
The ΔGM values are summarized in Table 2 together
with the Gibbs energy of micellization per alkyl tail,
Therefore, addition of PEG which compared with
EG makes slight increase in CMC’s of C16–2ꢀC16.
However, as an exception, EG is highly waterꢀsoluble,
and has less impact on the structure of water, which
inserts ionic micelle will cause a decrease in cationic
headgroup repulsion at micellar Stern layer [11].
Accordingly, a decrease in CMC’s of C16–2ꢀC16 at
lower concentration of EG can be found from Table 1.
°
°
= ΔGM /2. To estimate the effect of the presꢀ
ΔGM, tail
ence of the organic polar solvent on the micellizaꢀ
tion process, the Gibbs energy of transfer values,
° °
Gtrans = ΔGM (water–EG or water–PEG) – ΔGM
(pure water), are also listed in Table 2. Table 2 shows
that ΔGM value (–62.86 kJ mol–1) for the gemini
surfactant is larger in magnitude than that for
CTAB (–30.5 kJ mol–1) [13]. However, ΔGM, tail values
are quite similar for C16–2ꢀC16, 2Br–, and CTAB. For
Δ
°
Gibbs free energy of micellization. C16–2ꢀC16ꢀ alcoꢀ
hol systems had conductivityꢀ[surfactant] profiles
consisting of two linears segments meeting at the critꢀ
ical micelle concentration (CMC); the ratio between
°
ΔGM, tail = –30.5 kJ mol–1)
°
=
°
singleꢀchain CTAB (ΔGM
their slopes (k1
, k2) was taken as the degree of micellar
surfactant in water or water–EG/PEG mixtures.
These results point out that changes in the magnitude
of the Gibbs energy contribution associated with the
transfer of the hexadecyl chains from the bulk phase
into the micellar interior are the main factor controlꢀ
ionization,
α
(
Williams method) [12]:
α = k1/k2.
(5)
The data in Table 1 can be used to obtain the Gibbs
free energy of micellization, ΔGM , by considering
equation [3]:
°
ling the value of ΔGM as well as its variations upon
°
changing concentration of EG and PEG. The increase
in the CMC by increasing concentration of EG and
PEG can be explained by taking into account that the
transfer of the surfactant tail (one or more) and the
alkyl chains in the headgroup from the bulk phase into
the micellar core are less spontaneous.
Alkaline hydrolysis of EA in the presence of
C16 ⎯ 2ꢀC16. By the approach of nonꢀlinear fitting, the
kinetic experiment data of EA alkaline hydrolysis in
micellar solution was showed in Table 3. According to
the catalysis theory, we measure the catalytic or prohibꢀ
°
ΔGM = 2RT(1.5 – α)lnCMC.
(6)
°
Table 2. Gibbs energy of micellization (ΔGM ), Gibbs energy
°
of micellization per alkyl tail (ΔGM, tail ), and Gibbs energy of
°
transfer (ΔGtrans ) for C16–2ꢀC16, 2Br–water–ethylene glyꢀ
col or polyethylene glycol solutions at 303.2 K (kJ mol–1)
ited behavior (
lar solution and water, respectively) by kc
kc k0 < 1. The hydrolysis of EA showed catalytic and
k
c and
k
0 are the rate constants in micelꢀ
c, mol l–1
°
°
°
–ΔGM
–ΔGM, tail
ΔGtrans
/k0 > 1 or
/
0
62.86
31.43
prohibited dual behavior in the presence of surfactant.
At the lower concentration, it can be found that the
rate constant for the catalyzed hydrolysis of EA
increases slowly with increasing C16–2ꢀC16 concenꢀ
trations until a maximum point is reached, but further
increase in the surfactant concentration caused a
gradual decrease in the rate constants, and ultimately
the reaction was restrained.
Obviously, the micellar concentration and electroꢀ
static effects have played a dominant role in the alkaꢀ
line hydrolysis of EA. The mechanism of EA hydrolyꢀ
sis promoted by C16–2ꢀC16 is shown in Fig. 3. On the
EG
4.13 × 10–4
8.26 × 10–4
1.65 × 10–3
PEG
8.00 × 10–6
1.60 × 10–5
2.40 × 10–5
63.98
65.66
61.86
31.99
32.83
30.93
–1.12
–2.80
1.00
62.48
61.26
60.26
31.24
30.63
30.13
0.38
1.60
2.60
Note: c is the concentration of EG or PEG.
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EFFECT OF ETHYLENE GLYCOL ON MICELLIZATION
c, l mol–1 min–1) of Table 4. Kinetic experimental data (
2241
Table 3. Kinetic experimental data (
k
k
c, l mol–1 min–1) of
EA hydrolysis in C16–2ꢀC16 in micellar solution at 303.2 ester hydrolysis in C16–2ꢀC16/EG system at 303.2 K
and 313.2 K
0% EG
27.5% EG
[C16–2ꢀC16] ×
104, mol/l
303.2 K
313.2 K
[C16–2ꢀC16] ×
kc
kc/k0
kc
kc/k0
104, mol l–1
kc
kc/k0
kc
kc/k0
0
5.485
6.494
6.256
7.405
7.425
5.864
5.407
1.000
1.184
1.140
1.350
1.357
1.069
0.986
3.562
3.537
3.911
4.548
5.528
4.501
3.498
1.000
0.993
1.098
1.276
1.552
1.264
0.982
0
5.485
6.494
6.256
7.405
7.425
5.864
5.407
5.263
1.000
1.084
1.140
1.350
1.357
1.069
0.986
0.960
10.547
11.649
11.711
12.236
13.185
10.549
10.521
10.372
1.000
1.105
1.110
1.160
1.250
1.002
0.997
0.983
0.050
0.101
0.302
0.504
1.007
2.014
0.050
0.101
0.302
0.504
1.007
2.014
3.021
one hand, hydrophobic substrates can be solubilized in
micellar solution and the reactive counterions (OH–)
are effectively concentrated at interfacial region (Stern
layer) of cationic micelles, as a result, the probability
of contact of substrates and OH– significantly
increased. On the other hand, cationic micelle can
also bring stability in the negative charge of the transiꢀ
tion state, so that the reaction rate increases. However,
further increase in the surfactant concentration may
make substrates enter the interior of the micelles, the
probability of contact of substrates and OH–
decreased, which would lead to a decrease of rate.
ative largely, this may be due to the formation of
premicelle, which possessed some catalysis in hydrolꢀ
ysis reaction. The maximum rate present to concenꢀ
tration of surfactants in the CMC (2.86
×
10–5, 4.35
×
10–5 mol l–1) or more, it also indicates that the surfacꢀ
tant that formed micelles is a better catalyst in the
hydrolysis of EA.
Effect of EG on catalytic hydrolysis of EA in gemini
micellar solution. The hydrolysis rate constant (kc
=
3.562 l mol–1 min–1) in EGꢀwater system was smaller
than the rate constant (
k
c = 5.485 l mol–1 min–1) in the
pure water solution, namely akaline hydrolysis of ethyl
acetate had been inhibited by EG (Table 4). This is due
to higher viscosity of EG (30°C, ηEG = 16.5 mPa s)
Rheological properties and microꢀ morphology of
gemini surfactant aggregation may also affect the
hydrolysis. Previous studies on gemini surfactants
have clearly demonstrated that the microviscosity
increased strongly from the singleꢀchained surfactant
(monomer) to the gemini surfactant (dimer) [4, 14].
Accordingly, higher microviscosity of the gemini surꢀ
factant will cause a decrease in diffusion coefficient of
the reactants in aqueous solution of the gemini surfacꢀ
tant, thus resulting in a decrease in reaction rate of the
hydrolysis reaction. In addition, with an increase in
surfactant concentration gemini surfactant showed a
remarkable increase in aggregation number and exhibꢀ
ited a strong tendency to micelle growth, significant
changes of micelle shape with an increase in surfactant
concentration as following sequence of structures [4]:
spherical micelles, cylindrical micelles, wormꢀlike
micelles, branched threadꢀlike micelles, closed ring
cylindrical micelles. Melhado and Gutsche [15] found
that spherical micelles of smaller aggregation number
have higher catalytic activity than rodꢀlike micelles of
larger aggregation number in the micellar catalysis
research, accordingly the apparent rate constant
decreased when a sphereꢀtoꢀrod transition occurs in
surfactant solution.
than that of water (30°C, ηH O = 0.801 mPa s), the
2
viscosity of EG is about 20 times larger than that of
water [16]. EG added to the reaction system will lead
to the increase in the viscosity of the solution, conseꢀ
quently cause a decrease in diffusion coefficient of the
reactants in aqueous solution of the gemini surfactant
Bulk phase
OH−
OH−
H2O
Br−
H2O
Br−
Br−
+
OH−
Br−
+
+
N
N
+
Stern layer
H2O
H2O
Micelle core
Besides, as can be seen from the experimental data,
when gemini surfactant concentrations are close to its
CMC, the apparent hydrolysis rate of EA increase relꢀ
Fig. 3. The sketch map of EA catalytic hydrolysis in the
stern layer of gemini micelles.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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SANXIAN LIU et al.
C16–2ꢀC16, thus resulting in a decrease in reaction rate Education of China (20070357002), the Key Laboraꢀ
tory of Environmentꢀfriendly Polymer Materials of
Anhui Province, and Anhui Key Laboratory of Funcꢀ
tional Material of Inorganic Chemistry.
of the hydrolysis reaction.
The value of kc k0 indicated that ethyl glycol had
/
little influence on micellar catalysed hydrolysis of EA
(Table 4). It has been reported that EG addition
induced the spontaneity of micellization to decrease
(see above), the aggregation number of micelle
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ACKNOWLEDGMENTS
This work is supported by the National Science
Foundation of China (grants 20871001, 20671001,
20731001), the Major Program of Anhui Provincial
Education Department (grant ZD2007004ꢀ1), the
Research Fund for the Doctoral Program of Higher
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