Organic reactions and nanoparticle preparation in Co2-induced water/P104/p-xylene microemulsions

Article abstract of DOI:10.1002/chem.200204496

Nanometer-sized gold particles are synthesized by the reduction of HAuCl₄ with KBH₄ in the CO₂-induced microemulsion of (EO)₂₇(PO)₆₁(EO)₂₇ (P104; EO = ethylene oxide, PO = propylene oxide)/p-xylene/CO₂/H₂O. The recovery of gold particles from the microemulsion can be easily accomplished by the venting of CO₂, while the surfactant remains in the organic phase. The effect of the molar ratio of the reductant to HAuCl₄, the concentration of the reactants, and the molar ratio of water to EO segments (W₀) in the reverse micelles on the size of the gold particles is studied. The hydrolysis of benzoyl chloride (BzCl) and p-nitrophenyl chloroformate (NPhCl) has also been carried out in the microemulsion. The results show that the observed rate constants k_obs of both substrates increase significantly with W₀, and that W₀ has a larger influence on the hydrolysis of BzCl. The different extents of the influence of W₀ on the two reactions can be ascribed to the different reaction mechanisms and the expected changes in nucleophilicity and polarity of water in the reverse micelles.

Full text of DOI:10.1002/chem.200204496

FULL PAPER
Organic Reactions and Nanoparticle Preparation in CO2-Induced
Water/P104/p-Xylene Microemulsions
Rui Zhang, Jun Liu, Jun He, Buxing Han,* Weize Wu, Tao Jiang, Zhimin Liu, and
Jimin Du[a]
Abstract: Nanometer-sized gold parti-
cles are synthesized by the reduction of
HAuCl4 with KBH4 in the CO2-induced
microemulsion of (EO)27(PO)61(EO)27
(P104; EO ˆ ethylene oxide, PO ˆ pro-
pylene oxide)/p-xylene/CO2/H2O. The
recovery of gold particles from the
microemulsion can be easily accom-
plished by the venting of CO2, while
the surfactant remains in the organic
phase. The effect of the molar ratio of
the reductant to HAuCl4, the concen-
tration of the reactants, and the molar
ratio of water to EO segments (W0) in
the reverse micelles on the size of the
gold particles is studied. The hydrolysis
of benzoyl chloride (BzCl) and p-nitro-
phenyl chloroformate (NPhCl) has also
been carried out in the microemulsion.
The results show that the observed rate
constants kobs of both substrates increase
significantly with W0, and that W0 has a
larger influence on the hydrolysis of
BzCl. The different extents of the influ-
ence of W0 on the two reactions can be
ascribed to the different reaction mech-
anisms and the expected changes in
nucleophilicity and polarity of water in
the reverse micelles.
Keywords: carbon dioxide
hydrolysis microemulsions
nanoparticles
¥ gold
¥
¥
¥
surfactant. In a microemulsion, high concentrations of both
hydrophilic and hydrophobic reactants can be dissolved
simultaneously. Also, the surface area between water and oil
can reach a value as high as 105 m2 LÀ1 of microemulsion.[24]
Solubilization of immiscible reactants, in the same region of a
surfactant assembly, can lead to an increase in reaction rates,
while the rates of reaction of segregated reactants are
retarded. The nanoparticles usually show novel electronic,
magnetic, optical, and chemical properties, that are signifi-
cantly different from those of the bulk materials, because of
their extremely small sizes and large specific surface
areas.[25 28] Therefore, they have various potential applications
in a diversity of areas, such as, electronic, mechanical devices,
engineering materials, superconductors, magnetic recording
media, catalysis, dyes, adhesives, drug delivery, and optical
and photographic suspensions.[28 32] As a result, a lot of effort
has been devoted to the preparation of nanoparticles, and to
the study of their properties in recent years. Many methods
Introduction
Microemulsions are often good solvents for both hydrophilic
3]
and hydrophobic substances.[1 Therefore, they have been
used as media for enhanced oil recovery,[4, 5] organic[6, 7] and
enzymatic reactions,[8, 9] and as a mobile phase in chromato-
graphic solutions.[10, 11] The surfactant-covered water pools in
the water oil microemulsions offer a unique microenviron-
ment for the formation of nanoparticles.[12
18]
Formation of
nanoparticles in such systems is controlled by the reactant
distribution in the droplets and by the dynamics of inter-
droplet exchange. The surfactant stabilized microcavities
provide a cagelike effect that limits particle nucleation,
growth, and agglomeration.[14, 19] As a result, the particles
obtained are generally very fine.[20]When performing synthet-
ic organic chemistry, one is often faced with the problem of
reacting a hydrophobic compound with one that is hydro-
philic, for example, water or a salt. One successful approach is
23]
[33]
to use microemulsions,[21
which are pseudohomogeneous
used to prepare nanoparticles include, the sol–gel,
mixtures of water insoluble organic compounds, water, and
coprecipitation,[34] hydrothermal,[35] gas-evaporation,[36] and
sputtering method.[37] Nanoparticle preparation in nanoreac-
18]
tors of microemulsions is one of the best methods.[12
Compressed CO2 is quite soluble in a number of organic
solvents and causes large expansion of the solvent.[38, 39] With
the CO2-expanded solvent medium, it is possible to achieve
optimum conditions for some processes. Sometimes the CO2-
expanded solvent may be better than either neat solvents or
supercritical CO2.[40] Compared to liquid solvent mixtures, the
[a] Prof. B. Han, Dr. R. Zhang, Dr. J. Liu, Dr. J. He, Dr. W. Wu, Dr. T.
Jiang, Dr. Z. Liu, J. Du
Center for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100080 (P. R. China)
Fax : (‡86)10-62559373
E-mail: hanbx@infoc3.icas.ac.cn
Chem. Eur. J. 2003, 9, 2167 2172
DOI: 10.1002/chem.200204496
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B. Han et al.
FULL PAPER
the stirrer was started, the solution became hazy and milky, and CO2 was
charged into the cell slowly. With the addition of CO2, the hazy and milky
solution gradually became transparent. The pressure at which the solution
became completely clear was recorded. Generally, one experiment
required about two hours. W0 was calculated easily on the basis of the
masses of the polymer and water added.[41]
easy and complete separation of CO2 from the liquid solvent
after depressurizing is also an advantage.
Recently, we published a short communication to report a
novel finding,[41] that compressed CO2 could induce the
formation of reverse micelles of block (EO)27(PO)61(EO)27
(P104; EO ˆ ethylene oxide, PO ˆ propylene oxide) in p-
xylene. The copolymer could not form reverse micelles in the
solvent at 408C. By using compressed CO2 to tune the solvent
properties, the reverse micelles were formed. The CO2-
induced reverse micelles could solubilize polar and ionic
chemicals like methyl orange and cobalt chloride. The unique
advantage of these kind of reverse micelles is that the
formation and breaking of them could be repeated easily by
controlling the pressure. We are very interested in the
applications of these kinds of microemulsions. In this work,
we study the hydrolysis of benzoyl chloride (BzCl) (Sche-
me 1a), p-nitrophenyl chloroformate (NPhCl) (Scheme 1b),
and explore the possibility to prepare Au nanoparticles in the
CO2-induced microemulsion. The results indicate that the
microemulsion can be used to carry out the reactions and to
prepare nanoparticles with potential advantages. These stud-
ies in turn give some information on the properties of the
reverse micellar solution.
Apparatus and procedures used for the hydrolysis reaction: The apparatus
used in the hydrolysis reaction was similar to that for determining the phase
behavior described above. The difference being, the usage of a six-port
valve, equipped with a sample loop, which was used to charge the
hydrophobic reactants. In a typical reaction, the air in the high-pressure
view cell was replaced by CO2. P104 (5 g) in p-xylene (15 wt%) and a
known amount of water were loaded into the reaction cell. The cell was
then placed into the constant temperature (40.08C) water bath. After
thermal equilibrium had been reached, the magnetic stirrer was started and
CO2 was compressed into the cell to the desired pressure. After the
addition of CO2, the hazy and milky solution became homogeneous and
transparent; this indicated the formation of the microemulsion. The
hydrophobic reactant (BzCl or NPhCl dissolved in p-xylene) was injected
into the high-pressure view cell through the sample loop of the six-port
valve and the reaction was then started. After stirring for 10 hours, the
reaction mixture was subjected to column chromatography to remove the
surfactant P104. The products were then analyzed by HPLC (DuPont
instrument, model 8800).
Apparatus and procedures used to synthesize and recover Au nano-
particles: The schematic diagram of the apparatus used for the synthesis of
the Au nanoparticles in microemulsion is shown in Figure 1. The constant
temperature water bath, high-pressure syringe pump (DB-80), pressure
Scheme 1. Hydrolysis reaction.
Figure 1. Schematic diagram of the apparatus for the reaction and phase
behavior study. 1) gas cylinder, 2) high-pressure syringe pump, 3) pressure
gauge, 4) temperature controller, 5) high-pressure cell, 6) water bath,
7) magnetic stirrer, a and b. valves.
Experimental Section
Materials: P104 was provided by BASF Corporation with a composition of
(EO)27(PO)61(EO)27. BzCl, p-xylene, hydrogen tetrachloroaurate
(HAuCl4), and potassium borohydride (KBH4) were AR Grade, and were
supplied by Beijing Chemical Reagent Factory. NPhCl, ꢀ97% purity, was
obtained from Aldrich and used as received. CO2 (99.995% purity) was
supplied by Beijing Analytical Instrument Factory. Double distilled water
was used throughout the experiments.
gauge, and magnetic stirrer were the same as those used for the phase
behavior study, but the high-pressure cell was different. The key function of
the cell was that it could mix the two solutions under pressure if necessary.
It consisted mainly of a stainless steel body, a stainless steel baffle with a
teflon seal on the edge, and a handle. The baffle could divide the cell into
two chambers (Chamber A and Chamber B shown in Figure 1). The
solutions in the two chambers could be mixed by adjusting the position of
the handle under pressure. In the experiment, the aqueous solutions of
HAuCl4 and KBH4 of desired concentrations were freshly prepared
separately. The cell was flushed with CO2 to remove the air. Chamber A
and Chamber B of the cell were charged with the solution of P104 in p-
xylene. Desired amounts of HAuCl4 and KBH4 aqueous solutions were
loaded into Chamber A and Chamber B, respectively. The cell was placed
in a water bath at constant temperature. After thermal equilibrium had
been reached, CO2 was compressed into the cell to the desired pressure and
the stirrers in the two chambers were started to accelerate the formation of
microemulsions. The two micellar solutions, containing HAuCl4 and KBH4
were mixed by turning the baffle using the handle. Au nanoparticles were
then synthesized by reducing HAuCl4 with KBH4 in the reverse micelles.
After the reaction, CO2 was slowly vented for about half an hour, and the
Au nanoparticles were collected as a precipitate, as the reverse micelles
began to break after releasing CO2. The organic solution was decanted and
the precipitated Au particles were collected and washed with water. The
size and shape of the obtained Au particles were determined by TEM with
a HITACHI H-600A electron microscope. The maximum resolution of the
Apparatus and procedures to study the phase behavior: The apparatus for
studying the phase behavior of KBH4/H2O/P104/p-xylene/CO2 and
HAuCl4/H2O/P104/p-xylene/CO2 microemulsions was the same as that
previously used to investigate the phase behavior of P104/H2O/p-xylene/
CO2 system.[41] Briefly, it consisted of a high-pressure view cell with a
volume of 40 cm3, a constant temperature water bath, a high-pressure
syringe pump (DB-80), a pressure gauge, a magnetic stirrer, and a gas
cylinder. The temperature of the water bath was controlled by HAA-
KE D8 temperature controller, and the temperature fluctuation of the
water bath was less than Æ0.038C. The pressure gauge was composed of a
pressure transducer (FOXBORO/ICT, Model 93) and an indicator, which
was accurate to Æ0.025 MPa in the range of 0 20 MPa. There were
graduations on the high-pressure view cell so that the volume of the liquid-
phase in the cell could be known easily. The experiment was based on the
fact that the solution is clear and transparent if the water is all solubilized,
otherwise, the solution is hazy or milky.[42 43] In a typical experiment, the air
in the view cell was replaced by CO2, P104 (5 g) in p-xylene (15 wt%), and
the desired amount of aqueous KBH4 or HAuCl4 solution was loaded into
the high-pressure view cell. The cell was placed into the constant
temperature water bath. After thermal equilibrium had been reached,
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CO2-induced Microemulsions
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microscope was 0.5 nm. Particles were sonicated for 1 minute in ethanol
and then directly deposited on the copper grid before the measurement.
is nonionic, and the microemulsion is not sensitive to the ionic
strength. Figure 2 also shows that the reverse micelles can be
formed as the pressure of CO2 reaches over 5.5 MPa, because
W0 increases sharply with increasing pressure. The data in
Figure 2 allow us to select suitable conditions to prepare Au
nanoparticles. In all the following sections, the results are
obtained at 40.08C, and the concentration of the polymer in
the solution is 15 wt% (based on p-xylene).
Apparatus and procedures for the UV study: UV-visible spectroscopy was
used to monitor the synthesis of Au nanoparticles in the microemulsion.
The UV-visible apparatus was similar to that reported previously.[44] It
consisted of a gas cylinder, a high-pressure pump, a pressure gauge, a UV-
visible spectrometer, a temperature-controlled high-pressure UV sample
cell, valves, and fittings. The UV-visible spectrophotometer was produced
by Beijing General Instrument Company (model TU-1201, resolution:
0.1 nm). The sample cell was composed mainly of a stainless steel body, two
quartz windows, a stirrer, and a temperature controlling system. The optical
path length and the inner volume of the sample cell were 1.32 cm and
1.74 cm3, respectively.
Synthesis and recovery of Au nanoparticles
UV-visible spectroscopy of the Au nanoparticles: Colloidal
dispersions of Au exhibit absorption bands in the UV-visible
range, that are due to the resonant excitation of surface
plasmons. Therefore, the Au nanoparticles, stabilized in the
In the experiment, a UV sample cell was connected to the two-chamber cell
(see Figure 1, the connecting tube and the valve are not shown in the
Figure). The air in the UV sample cell was removed by a vacuum pump.
The temperature of the cell was maintained at 408C. The microemulsion
with Au nanoparticles was then synthesized and transferred into the UV
sample cell at constant temperature and pressure. The UV spectrum of the
solution was then recorded every ten minutes until it was unchanged.
reverse micelles, can be analyzed in situ by their UV-visible
47]
spectra.[45
It is well known that the plasmon absorption
peak at 525 nm is characteristic of Au particles larger than
ꢁ3 nm. This peak is red-shifted and is substantially broad-
ened upon particle aggregation.[45] In this work, the spectrum
of a ruby-colored solution of Au nanoparticles in the P104/p-
xylene/CO2 microemulsion system shows an absorption peak
at 525 nm (Figure 3).
Results and discussion
The phase behavior of a system with and without HAuCl4 and
KBH4: Au nanoparticles can be synthesized in the water core
of microemulsions by the reduction of HAuCl4 with KBH4.[45]
We have studied the phase behavior of P104/p-xylene/CO2/
H2O microemulsion (Figure 1).[41] The results indicate that at
408C and at an ambient pressure of CO2, the P104/p-xylene/
CO2 cannot solubilize water. As the pressure of CO2 increases
to a certain value, the water solubilization capability of P104/
p-xylene/CO2 solution increases abruptly. This indicates the
formation of reverse micelles.
The addition of inorganic salts may affect the phase
behavior of the microemulsion.[7] Therefore, in this work we
first investigate the phase behavior in the presence of HAuCl4
and KBH4; results of which are illustrated in Figure 2. At a
Figure 3. The UV-visible spectrum of Au nanoparticles in P104/p-xylene/
CO2/H2O microemulsion. P ˆ 5.75 MPa, W0 ˆ 2.9, [KBH4] ˆ [HAuCl4] ˆ
0.05 molLÀ1
.
The recovery of nanoparticles: The conventional techniques to
recover nanoparticles from the reverse micelles, such as super-
rate centrifugation and filtration, are generally to precipitate
nanoparticles together with the surfactants. It is difficult to
remove the surfactant completely as the obtained particles are
always embedded in the surfactant.[48] Without a doubt, the
best way to recover the nanoparticles from the reverse
micelles is by precipitation of the particles, while the
surfactants remain in the solvent. In this work, the reverse
micelle formation is induced by compressed CO2. The break-
ing of the reverse micelles can be accomplished simply by the
venting of CO2. Therefore, solubilized Au nanoparticles in
reverse micelles can be precipitated after releasing the CO2.
Our experiment shows that the upper organic phase has no
UV absorbance band for Au after CO2 is released; this
indicates the precipitation of the Au particles. The reversi-
bility of the reverse micelle system is a unique feature and it is
Figure 2. The effect of the salt concentration and pressure of CO2 on the
maximum W0 for the solution of P104 in p-xylene (15 wt%) at 40.08C.
given temperature and pressure, p-xylene/CO2 mixed solvent
can dissolve small amounts of water, which have been
subtracted from the total amount of water when calculating
W0 (molar ratio of water to EO segments).[41] Figure 2 shows
that HAuCl4 and KBH4 have no considerable effect on the
phase behavior in the salt concentration range in which we are
interested. This maybe ascribed to the fact that the surfactant
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B. Han et al.
FULL PAPER
advantageous for the recovery of the nanoparticles synthe-
sized in the reverse micelles.
The effect of operating conditions on particle size and
distribution: The particle size and distribution data are
obtained by measuring the diameter of the particles in the
transmission electron microscopy (TEM) photographs. The
operating conditions such as W0, the molar ratio of reductant
to HAuCl4, have large influences on the nucleation and
growth of the nanoparticles, and consequently on the size and
50]
shape of the particles.[49
To study the effect of operating
Figure 5. Effect of the molar ratio of KBH4 to HAuCl4 on the size of Au
nanoparticles. W0 is 1.5. The concentration of HAuCl4 is 0.05 molLÀ1 in
aqueous solution. The concentration of KBH4 is: a) 0.01 molLÀ1 and
b) 0.125 molLÀ1 in aqueous solution.
conditions on the particle size and distribution, the synthesis
of Au nanoparticles was carried out by changing the following
experimental conditions: W0 of the microemulsion, molar
ratio of KBH4 to HAuCl4, and the concentration of both
reactants. Figure 4 illustrates the TEM photographs of Au
concentation of the reactants increased from 0.05
0.12 molLÀ1, the particle size decreased from 20 6.5 nm
(Figure 4b, Figure 6). Figure 6 also shows a distinct bimodal
size distribution. The larger particles have a diameter of about
17 nm. The smaller particles, at a higher concentration of
HAuCl4, can be attributed to more nuclei forming at the very
beginning of the reduction. To form a stable nucleus a
minimum number of atoms are
53]
required.[18, 52
For nuclea-
tion, several atoms must collide
at the same time.[17] Nn (Num-
ber of nuclei formed) is ex-
pected to increase with the
reactant concentration so that
at larger concentrations of
HAuCl4, Nn will be larger;
therefore smaller particles are
formed.
Figure 4. TEM photographs of Au particles synthesized in P104/p-xylene/
CO2/H2O microemulsion with different W0. a) W0 ˆ 0.9; b) W0 ˆ 1.5;
c) W0 ˆ 2.5. The concentration of KBH4 and HAuCl4 are both 0.05 molLÀ1
in aqueous solution.
Figure 6. TEM photographs of
Au particles synthesized in
P104/p-xylene/CO2/H2O micro-
emulsion with higher concen-
tration of reactant. W0 is 1.5.
The concentration of HAuCl4
and KBH4 is 0.12 mol lÀ1 in
aqueous solution.
nanoparticles synthesized in the microemulsion of P104/p-
xylene/CO2/H2O with different W0. The diameter of the Au
particle increases with increasing W0. As the W0 increases
from 0.9 2.5, Au particle size increases from 6 30 nm. This
may be ascribed mainly to the fact that the average number of
reactant ions per droplet increases with increasing W0.[50]
Also, the number of nuclei decreases with increasing W0,
because the rearrangement rate of the microemulsion de-
creases with the amount of water; [17] hence, less nuclei result
in larger particles.
The synthesis of Au particles has been performed at various
molar ratios of KBH4 to HAuCl4 (R). The concentration of
HAuCl4 was fixed at 0.05 molLÀ1 and R was changed from
0.2 2.5 (Figure 5a, R ˆ 0.2, Figure 4b, R ˆ 1, Figure 5b, R ˆ
2.5). With increasing R, the particle size decreases from 23
5 nm. This is mainly due to the fact that increasing the amount
Hydrolysis reactions: The hy-
drolysis of BzCl and NPhCl
(Scheme 1) is performed in the
P104/H2O/p-xylene/CO2 mi-
croemulsions. Both of these
substrates are water-insoluble
and known to be sensitive to the physical properties of the
reaction medium.[22] The water is in large excess relative to the
substrate; hence, the hydrolysis can be treated with pseudo-
first-order kinetics. The observed pseudo-first-order rate
constants kobs ˆ k[H2O], determined for the hydrolysis reac-
tions at different W0 values, are listed in Tables 1 and 2, and
Figure 7 illustrates the dependence of kobs on W0. The data in
Tables 1 and 2, and Figure 7 indicate that kobs increases with
the increase of W0 for both substrates. For example, the kobs at
of reducing agent per droplet will accelerate the nucleation
51]
process.[49
Therefore, smaller particles are formed in the
Table 1. Hydrolysis of benzoyl chloride (BzCl) in P104/p-xylene/CO2/H2O
microemulsions.[a]
microemulsion with a larger R value. Figure 5b also shows
that the particles have a bimodal size distribution. The larger
particles have a diameter of about 14 nm. The bimodal
distribution of particle diameter also appears in other micro-
emulsion systems.[17] The presence of bigger particles shows
particle aggregation.
Entry
W0
Mol H2O/MolÀ1 BzCl
kobs  105 [sÀ1
]
1
2
3
4
1.03
1.62
2.25
3.08
9.9
1.12
2.16
6.06
10.6
15.5
21.5
29.5
[a] The concentration of BzCl is 0.096 molLÀ1 based on the volume of the
CO2 expanded solution.
The particle size decreases with the concentration of the
reactants, while W0 and R are kept constant. When the
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CO2-induced Microemulsions
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Table 2. Hydrolysis of p-nitrophenyl chloroformate (NPhCl) in P104/p-
xylene/CO2/H2O microemulsions.[a]
Entry
W0
Mol H2O/MolÀ1 NPhCl
kobs  105 [sÀ1
]
1
2
3
4
5
0.61
1.01
1.66
2.21
3.02
10.1
16.7
27.6
36.7
50.1
2.61
3.26
4.21
5.32
6.98
[a] The concentration of NPhCl is 0.055 molLÀ1 based on the volume of the
CO2-expanded solution.
Scheme 2. Mechanism of hydrolysis reaction.
associative or dissociative character (Scheme 2b), and
c) associative (SN2) or addition elimination, with a tetrahe-
dral addition intermediate (Scheme 2c). The dissociative
pathway will be favored by a polar solvent and a conjugative
electron release effect that will stabilize the carbocationic
intermediate. The associative pathway is more sensitive to the
nucleophilicity of the nucleophilic agent and is favored by an
electron attractive substitute. The hydrolysis of BzCl takes
place predominately through a dissociative (SN1) path at
larger W0, and the reaction takes place fundamentally through
an associative (SN2) pathway at small W0 in the sodium bis(2-
ethylhexyl) sulfosuccinate (AOT)/isooctane/water microe-
mulsion.[54] Due to conjugative electron release by the aryl
À
group which assists the C Cl bond breaking, in the transition
Figure 7. The dependence of kobs on W0.
À
state of BzCl hydrolysis, the C Cl bond breaking progresses
further relative to that found in the hydrolysis of NPhCl.[21, 22]
In other words, the hydrolysis of BzCl proceeds by a
mechanism more like SN1, which involves the formation of
W0 ˆ 3.08 is almost ten times faster than that at W0 ˆ 1.03 for
the hydrolysis of BzCl. It can also be seen from Figure 7 that
the effect of W0 on the rate constant of NPhCl is smaller than
that of BzCl. The kobs of NPhCl hydrolysis is larger than that of
BzCl at W0 < 2.2, and becomes smaller at W0 > 2.2. This is
consistent with earlier published results by other authors,[21]
who conducted the reactions in other microemulsions. This
can be explained by the difference in mechanism of the two
substrates. Both substrates are poorly soluble in water;
therefore, they preferentially solubilize in the hydrophobic
acylium and ClÀ ions.[22, 23]Therefore, the kobs of BzCl
‡
hydrolysis increases with increasing W0 as a result of the
increased polarity of the water. Furthermore, the kobs in-
creases slowly when the W0 is very small, and in this case the
reaction may take place through an associative pathway in
which decreased nucleophilicity is unfavorable to the reac-
tion. In the case of NPhCl, the hydrolysis takes place through
a SN2 path, owing to the inductive electron attraction effect of
p-nitrophenoxy group. The nucleophilic attack is the rate-
determining step, and the reaction is sensitive to the
nucleophilicity of water. As the W0 increases, the nucleophi-
licity of the interfacial water decreases; this is unfavorable for
hydrolysis, even though the increase of water polarity is
favorable. Therefore, there are two opposite factors that
affect the reaction rate with increasing W0. As a result, the
rate constant is less sensitive to W0.
phase (p-xylene/CO2) and interphase of the microemul-
22, 54]
sions.[21
As a result, the reactions only occur in the
interphase where the substrates and water can meet. The
structure of the interfacial water in reverse micelles changes
with the increase of W0.[55] In the case of a surfactant, such as
P104, added water initially hydrates the EO segments. Further
addition of water is incorporated into the micellar cores and
causes the reverse micelles to swell. Water in the periphery is
more structured and has different physical properties, such as
lower polarity, as compared to the bulk water. Only in large
water droplets do the physical properties resemble those of
bulk water.[56] The polarity of water in the micelles increases
with W0. On the other hand, an increase in the droplet size (W0)
leads to a decrease in interaction between water molecules and
hydrophilic EO segments in the polymer surfactant,[57] thereby
reducing the nucleophilicity of the interfacial water. As the
hydrolysis of the substrates is sensitive to the polarity and
nucleophilicity of the water, the variation of W0 will affect the
kinetics of the hydrolysis. The acyl transfer reactions are
commonly classified into three groups:[54, 58]a) dissociative
mechanism (SN1), with an acylium ion intermediate (Sche-
me 2a); b) concerted displacement, which can have an
Conclusion
In conclusion, the CO2-induced microemulsion (P104)/
p-xylene/CO2/H2O can be used to carry out the hydrolysis
reactions of BzCl and NPhCl, and to prepare Au nano-
particles by the reduction of HAuCl4 with KBH4 in the reverse
micelles. The observed rate constants kobs of both hydrolysis
reactions increase with W0, and W0 has a larger influence on
the hydrolysis of BzCl. The Au nanoparticles can be
precipitated and recovered after the venting of CO2, while
the copolymer remains in the organic phase.
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[25] G. A. Ozin, Science 1996, 271, 920.
[26] G. A. Ozin, Adv. Mater. 1992, 4, 612.
[27] R. W. Cahn, Nature 1992, 359, 591.
[28] A. Henglein, Chem. Rev. 1989, 89, 1861.
[29] V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Nature 1994, 370, 354.
[30] B. C. Gates, Chem. Rev. 1995, 95, 511.
Acknowledgement
The authors are grateful to the National Natural Science Foundation of
China (20133030).
[31] L. L. Beecroft, C. K. Ober, Chem. Mater. 1997, 9, 1302.
[32] A. F. Lee, C. J. Baddeley, C. Hardacre, R. M. Ormerod, R. M.
Lambert, G. Schmid, H. West, J. Phys. Chem. 1995, 99, 6096.
[33] B. Fegley, Jr., P.White, H. K. Bowen, Am. Ceram. Soc. Bull. 1985, 64,
1115.
[1] R. Schom‰cker, Prog. Colloid Polym. Sci. 1990, 81, 131.
[2] J. H. Fendler, Membrane Mimetic Chemistry, Wiley-Interscience, New
York, 1982.
[3] J. K. Thomas, Chem. Rev. 1980, 80, 283.
[4] D. Langevin in Reverse Micelles (Eds.: P. L. Luisi, B. E. Straub),
Plenum, New York, 1984, pp. 287 303.
[34] Z. X. Tang, C. M. Sorensen, K. J. Klabunde, G. C. Hadjipanayis, J.
Colloid Interface. Sci. 1991, 146, 38.
[5] K. A. Bartscherer, M. Minier, H. Renon, Fluid Phase Equilib. 1995,
107, 93.
[35] S. Komarneni, R. Roy, E. Breval, M. Ollinen, Y. Suwa, Adv. Ceram.
Mater. 1986, 1, 87.
[6] C. A. Bunton, L. S. Romsted in Handbook of Microoemulsion Science
and Technology (Eds.: P. Kumar, K. L. Mittal), Marcel Dekker, New
York, 1999, pp. 457 482.
[36] R. W. Siegel, S. Ramasamy, H. Hahn, Z. Q. Li, T. Lu, R. Gronsky, J.
Mater. Res. 1988, 3, 1367.
[37] P. Fayet, L. Z. Woste, Phys. D 1986, 3, 177.
[7] F. Candau in Handbook of Microoemulsion Science and Technology
(Eds.: P. Kumar, K. L. Mittal), Marcel Dekker, New York, 1999,
pp. 679 712.
[8] Y. L. Khmelnitsky, I. N. Neverova, V. I. Polyakov, V. Y. Grinberg,
A. V. Levashov, K. Martinek, Eur. J. Biochem. 1990, 190, 155 159.
[9] K. Holmberg in Handbook of Microoemulsion Science and Technol-
ogy (Eds.: P. Kumar, K. L. Mittal), Marcel Dekker, New York, 1999,
pp. 713 742.
[10] M. A. Hernandez-Torres, J. S. Landy, J. G. Dorsey, Anal. Chem. 1986,
58, 744.
[11] R. W. Gale, R. D. Smith, J. L. Fulton, Anal. Chem. 1987, 59, 1977.
[12] M. P. Pileni, J. Phys. Chem. 1993, 97, 6961.
[13] V. Pillai, P. Kumar, M. J. Hou, P. Ayyub, D. O. Shah, Adv. Colloid
Interface Sci. 1995, 55, 241.
[38] M. A. McHugh, V. J. Krukonis, Supercritical Fluid Extraction: Prin-
ciples and Practice, 2nd ed., Butterworth-Heinemann, Boston, 1994.
[39] C. A. Eckert, L. K. Barbara, P. G. Debenedetti, Nature 1996, 383, 313.
[40] G. Musie, M. Wei, B. Subramaniam, D. H. Busch, Coord. Chem. Rev.
2001, 219 221, 789.
[41] R. Zhang, J. Liu, J. He, B. X. Han, X. G. Zhang, Z. M. Liu, T. Jiang,
G. H. Hu, Macromolecules, 2002, 35, 7869.
[42] P. Alexandridis, K. Andersson, J. Phys. Chem. B 1997, 101, 8103.
[43] P. Alexandridis, K. Andersson, J. Colloid Interface Sci. 1997, 194, 166.
[44] J. Lu, B. X. Han, H. K. Yan, Phys. Chem. Chem. Phys. 1999, 1, 3269.
[45] M. Brust, D. Bethell, C. J. Kiely, D. J. Schiffrin, Langmuir 1998, 14,
5425.
[46] Y. Mizukoshi, K. Okitsu, Y. Maeda, T. A. Yamamoto, R. Oshima, Y.
Nagata, J. Phys. Chem. B 1997, 101, 7033.
[14] V. Pillai, D. O. Shah in Industrial Applications of Microemulsions
(Eds.: C. Solans, H. Kunieda), Marcel Dekker, New York, 1997,
pp. 227.
[47] T. Fujimoto, Y. Mizukoshi, Y. Nagate, Y. Maeda, R. Oshima, Scr.
Mater. 2001, 44, 2183.
[48] M. L. Steigerwald, A. P. Alivisatos, J. M. Gibson, T. D. Harris, R.
Kortan, A. J. Muller, A. M. Thayer, T. M. Duncan, D. C. Douglas,
L. E. Brus, J. Am. Chem. Soc. 1988, 110, 3046.
[49] C. L. Chiang, J. Colloid Interface Sci. 2001, 239, 334.
[50] C. L. Chiang, J. Colloid Interface Sci. 2000, 230, 60.
[51] J. L. Zhang, B. X. Han, J. C. Liu, X. G. Zhang, J. He, Z. M. Liu, T.
Jiang, G. Y. Yang, Chem. Eur. J. 2002, 8, 3879.
[52] P. C. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel
Dekker, New York, 1977, p. 234.
¬
¬
[15] M. A. Lopez-Quintela, J. Quiben-Solla, J. Rivas in Industrial Appli-
cations of Microemulsions (Eds.: C. Solans, H. Kunieda), Marcel
Dekker, New York, 1997.
[16] L. Jeunieau, F. Debuigne, J. B. Nagy in Reactions and Synthesis in
Surfactant Systems (Ed.: J. Texter), Marcel Dekker, New York, 2001.
[17] J. B. Nagy in Handbook of Microoemulsion Science and Technology
(Eds.: P. Kumar, K. L. Mittal), Marcel Dekker, New York, 1999,
pp. 499 548.
[18] K. Osseo-Asare in Handbook of Microoemulsion Science and
Technology (Eds.: P. Kumar, K. L. Mittal), Marcel Dekker, New
York, 1999, pp. 549 604.
[53] J. A. Dirksen, T. A. Ring, Chem. Eng. Sci. 1991, 46, 2389.
[54] L. GarcÌa-Rio, J. R. Leis, J. A. Moreira, J. Am. Chem. Soc. 2000, 122,
10325.
[19] C. C.Wang, D. H. Chen, T. C. Huang, Colloids Surf. A. 2001, 189, 145.
[20] M. P. Pileni, Structure and Reactivity in Reverse Micelles, Elsevier,
Amsterdam, 1989.
[55] S. L. Regen, J. Org. Chem. 1977, 42, 875.
[56] P. Mukerjee in Solution Chemistry of Surfactants, Vol 1 (Ed.:K. L.
Mittal), Plenum, New York, 1979, pp. 153.
[21] G. B. Jacobson, C. T. Lee, K. P. Johnston, J. Org. Chem. 1999, 64, 1201.
[22] L. GarcÌa-RÌo, J. R. Leis, E. Iglesias, J. Phys. Chem. 1995, 99, 12318.
[23] H. Al-Lohedan, C. A. Bunton, M. M. Mhala, J. Am. Chem. Soc. 1982,
104, 6654.
[57] C. Guo, H. Z. Liu, J. Y. Chen, Colloids Surf. A 2000, 175, 193.
[58] B. D. Song, W. P. Jencks, J. Am. Chem. Soc. 1989, 111, 8470.
[24] M. J. Schwuger, K. Stickdorn, R. Schom‰cker, Chem. Rev. 1995, 95,
849 864.
Received: October 10, 2002 [F4496]
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