Advances in Chemical Engi neering and Science , 2011, 1, 299-304
doi:10.4236/aces.2011.14041 Published Online October 2011 (http://www.SciRP.org/journal/aces)
Preparation of Highly Concentrated Silver Nanoparticles
in Reverse Micelles of Sucrose Fatty Acid Esters through
Solid-Liquid Extraction Method
Hidetaka Noritomi*, Yoshihiro Umezawa, Saori Miyagawa, Satoru Kato
Department of Applied Chemistry, Tokyo Metropolitan University, Tokyo, Japan
Recieved August 12, 2011; revised September 16, 2011; accepted September 20, 2011
Abstract
Silver nanoparticles were synthesized in reverse micelles consisting of sucrose fatty acid esters by dissolving
reactant powder in the water pool of reverse micelles through the solid-liquid extraction. Silver nanoparticles
having various sizes and shapes were obtained at high concentration. The size of silver nanoparticles was
controlled by reaction temperature. Moreover, the size of silver nanoparticles was dependent upon the aver-
age esterification degree of sucrose fatty acid esters forming reverse micelles. The wavelength in the peaks,
which corresponded upon the localized surface plasmon resonance of resultant silver nanoparticles, was cor-
related with their sizes.
Keywords: Reverse Micelle, Silver Nanoparticle, Size Control, Solid-Liquid Extraction, Sucrose Fatty Acid
Ester
1. Introduction
In recent years, the production and applications of metal
nanoparticles have rapidly been increased in the field of
nanotechnology, since their novel physical and chemical
properties are not only different from those of bulk sub-
stances due to their extremely small size and large spe-
cific surface area, but also metal nanoparticles exhibit
specific colors due to localized surface plasmon reso-
nance (LSPR) corresponding to the wavelength of light
that induces the largest electromagnetic field on the
nanopartices, when metal nanoparticles are impinged by
a beam of light [1]. Especially, the applications of silver
nanoparticles have widely been investigated, since they
exhibit some profitable properties such as catalysis [2],
antibacterial agent [3], nanoparticle colorant [4], nano-
paste for electrical circuits and electrodes [5], and sub-
strates for surface-enhanced Raman scattering [6]. The
colors of silver nanoparticles result from changes of
LSPR induced by their size and shape.
In order to produce metal nanoparticles having suit-
able sizes and/or shapes, many physical or chemical
techniques such as coprecipitation, gas-evaporation, sol-
gel method, and sputtering have been developed so far
[1]. Additionally, more attention has been paid on the
preparation of metal nanoparticles in reverse micelles [7].
Reverse micelles are thermodynamically stable nanome-
ter-sized aggregates of surfactant molecules dispersed in
a hydrophobic organic phase like octane, and can form
w/o type microemulsions containing a small amount of
water in their centers. The water phase in w/o type mi-
croemulsions is called water pool. Metal nanoparticles
are synthesized in the water pool, in which reactants are
dissolved. As the preparation of metal nanoparticles us-
ing reverse micelles does not require a special apparatus
and extreme conditions of temperature and pressure, it is
comparatively easy to expand the scale of reverse micel-
lar reaction system. However, the productivity of metal
nanoparticles per volume of reverse micellar system is
limited due to low overall concentration of reactants in
the reverse micellar system, since the volumetric ratio of
water phase playing a role as the dissolution of reactants
and the reaction field to the bulk organic phase is too
small [1].
In order to solubilize reactants in reverse micelles,
three solubilization methods are utilized: injection me-
thod; liquid-liquid extraction method; solid-liquid ex-
traction method [8]. First, the injection method is carried
out by injecting a few microliters of the concentrated
stock solution of reactants into the hydrocarbon solution

300 H. NORITOMI ET AL.
of surfactants. This method is most commonly used to
prepare metal nanoparticles in reverse micelles. Second,
the liquid-liquid extraction method is carried out by
transferring solutes dissolved in an aqueous phase from
the aqueous phase into the hydrocarbon phase of surface-
tants. This method is relatively slow, and the concentra-
tion of reactants in the hydrocarbon phase is limited due
to the distribution between aqueous and reverse micellar
organic phases in thermodynamic equ ilibrium. Third, the
solid-liquid extraction method is carried out by mixing
solid reactants with the reverse micellar solution con-
taining already a certain amount of water. This method
can dissolve reactants until their dissolution reaches
saturation in water pool. Moreover, when using the
solid-liquid extraction instead of the conventional inject-
tion method to solubilize reactants into reverse micelles,
the following benefits are expected. First, the productive-
ity of metal nanoparticles is markedly promoted, since
reactants consumed for the formation of metal nanoparti-
cles are successively supplied from solid reactants al-
ready added in large excess, compared to the solubility
of the water pool of reverse micelles. Second, only a
small amount of water is needed for solid-liquid extrac-
tion. Consequently, the aggregation of resultant nanopar-
ticles is inhibited, since the lower water content is kept in
reverse micellar system, the more stably nanoparticles
are dispersed [9]. However, there have not been any re-
ports about the preparation of nanoparticles in reverse
micelles using the solid -liquid extraction method.
In our previous work, we have reported that silver
nanoparticles are prepared in reverse micelles of sucrose
fatty acid esters by using the conventional injection
method [10]. Sucrose fatty acid esters are commercial
food grade nonionic surfactants, and are biodegradable
and nonhazardous to the environment [11]. In the present
work, we examined the preparation of silver nanoparti-
cles in reverse micelles of sucrose fatty acid esters by
using solid-liquid extraction method to address how the
preparation conditions such as the reaction temperature
and the composition of surfactants affect the size and
shape of silver nanoparticles.
2. Experimental
Silver nitrate and sodium borohydride were the guaran-
teed reagents of Kanto Chemicals (Tokyo, Japan). DK-
SS (sucrose fatty acids of 99 wt% monoesters and 1 wt%
di- and triesters, fatty acid constituent consisting of 60
wt% stearic acid and 40 wt% palmitic acid) and DK-F-
20W (sucrose fatty acids of 11wt% monoesters, 36wt%
di- and triesters, and 53 wt% more than tetraesters, fatty
acid constituent con sisting of 70 wt% stearic acid and 30
wt% palmitic acid) were supplied from Dai-Ichi Kogyo
Seiyaku (Kyoto, Japan). The surfactant was used without
further purification. Isooctane and n-butanol were from
Kanto Chemicals (Tokyo, Japan), and were of analytical
grade.
The reverse micellar solutions were prepared by add-
ing the required amounts of sucrose fatty acid esters and
a small amount of water into the solution of n-butanol/
isooctane (3:7 (v/v)), and then were used for the prepa-
ration of nanoparticles within a few minutes.
The preparation of nanoparticles was carried out by
adding a certain amounts of AgNO3 powder and sodium
borohydride powder into the reverse micellar solution.
Since the preliminary study indicated that the synthe-
sis of particles finished at 25˚C after 3 h, and no residual
solid reactants existed in the reaction medium, the sam-
ples for various analyses were taken after 3 h. In the pre-
sent work, the concentrations of sucrose fatty acid esters
and water based on the overall volume of reverse micel-
lar solution were fixed at 50 g/L and 60 mM, respect-
tively. As a typical condition, the overall concentrations
of AgNO3 and sodium borohydride were 0.1 and 1.0 M,
respectively, and the temperature was fixed at 25˚C.
The TEM micrograph was obtained using a JEOL
JEM-2000FX electron microscope operating at 200 kV.
The sample for TEM was prepared by dilutin g a resultan t
colloidal solution one hundred-fold with n-butanol and
placing a drop of colloidal solution onto the standard
carbon-coated copper grids and drying it under vacuum.
The UV-vis spectra of the reverse micellar solutions
containing nanoparticles were measured by UV/vis spec-
trophotometer (Ubest-55, Japan Spectroscopic Co. Ltd.)
with a 10 mm quartz cell.
3. Results and Discussion
3.1. Productivity of Silvar Nanoparticles through
Solid-Liquid Extraction Method
In order to successively supply reactants consumed for
the formation of silver nanoparticles to the water poo ls of
reverse micelles, which are reaction fields, and produce
silver nanoparticles at high concentration, we have em-
ployed solid-liquid extraction method instead of the
conventional injection method [8]. The colloidal solution
of silver nanoparticles prepared by solid-liqu id ex traction
method was much concentrated, compared to that pre-
pared by the injection method examined in our previous
method [10]. The concentration of silver nanoparticles
obtained by the present system was about one hundred
and thirty times larger than that obtained by the injection
method. After diluting the resultant colloidal solution
one hundred-fold with n-butanol, the measurement of
UV-visible absorption spectrum was carried out. The

H. NORITOMI ET AL.
301
color of the solution after dilution was yello w, similar to
the case of injection method, and, as shown in Figure 1,
its UV-visible absorption spectrum exhibited the peak
around 403 nm, which corresponds upon the LSPR of
silver nanoparticles [12]. Figure 2 shows the relationship
between surfactant concentration and the absorbance at
403 nm of colloidal solutions of silver na noparticles pre-
liminarily diluted one handred-fold with n-butanol. Any
absorbance at 403 nm was not observed without surfac-
tant. On the other hand, the absorbance at 403 nm in-
creased above 20 g/L of surfactant concentration. We
have reported that the apparent critical micelle concen-
tration in sucrose fatty acid/n-butanol/isooctane system
was observed around 10 g/L [13]. Consequently, it is
suggested that the preparation of silver nanoparticles
proceeds as follows. First, empty reverse micelles con-
taining a small amount of water approach solid reactants
added in large excess, compared to the solubility of the
water pool of reverse micelles. When the water pools of
reverse micelles contact solid reactants, reactants are
solubilized into the water pools. Then, silver nanoparti-
cles are synthesized from reactants solubilized in reverse
micelles. Resultant silver nanoparticles are coated by
surfactants, and are dispersed in organic solvents. Emp-
Figure 1. Absorption spectrum of silver nanoparticles pre-
pared by solid-liquid extraction method.
Figure 2. Effect of surfactant concentration on absorbance
at 403 nm of colloidal solution dispersing silver nanoparti-
cles prepared by solid-liquid extraction method: [AgNO3]ov
= 0.1 M; [NaBH4]ov = 1.0 M; n-butanol/isooctane containing
50 g/L DK-SS and 60 mM H2O; reaction time = 3 h; reac-
tion temperature = 25˚C.
tied reverse micelles approach solid reactants again.
Thus, the production of silver nanoparticles is promoted
by repeating the process composed of the extraction of
reactants, the synthesis of silver nanoparticles, and the
dispersion of resultant silver nanoparticles.
3.2. Dependence of Reaction Temperature upon
Size of Silver Nanopartices
Figure 3 shows the typical transmission electron micro-
graphs and size distributions of silver nanoparticles when
silver nanoparticles were prepared in the reverse micellar
system of DK-SS for 3 h at different reaction tempera-
tures. The TEM images showed that the obtained silver
nanoparticles displayed a wide variety of shapes. The
size of resultant silver nanoparticles was almost similar
to that prepared in DK-SS or AOT (sodium bis(2-ethyl-
hexyl) sulfosuccinate) reverse micelles by the conven-
tional injection method [10,14,15]. The size of silver na-
noparticles was strongly dependent upon reaction tem-
perature. Figure 4 shows the plots of the mean diameter
of silver nanoparticles against reaction temperature. As
seen in the figure, the mean diameter of silver nanoparti-
cles gradually increased with an increase in reaction
temperature. The line in the figure represents the fitting
curve with the correlation coefficient of 0.99 as
0.079 6.2dt

 (1)
where d is the mean diameter of silver nanoparticles, and
t is the reaction temperature. This tendency was similar
to the case using the conventional injection method [10].
The solubilization of reactants, the exchange of reactants
between reverse micelles, the reduction reaction, and the
growth of nanoparticles increase with increasing the re-
action temperature [16-19].
Figure 5 shows the plots of the mean diameter of sil-
ver nanoparticles obtained at different reaction tempera-
ture against the wavelength in the peak of UV-visible
absorption spectrum due to the LSPR of silver nanopar-
ticles. The standard deviation concerning the size distri-
bution of silver nanoparticles depended upon reaction
temperature. The wavelength in the peaks increases with
an increase in the mean diameter of silver nanoparticles.
The line in the figure represents th e fitting curve with the
correlation coefficient of 0.99 as
λ = 8.2d + 340 (2)
where λ is the wavelength of peaks, and d is the mean
diameter of silver nanoparticles. The position of the
plasmon adsorption peak depends upon the particle size
and shape, and especially tends to be red-shifted with
ncreasing the particle size [14,16,20,21]. i

H. NORITOMI ET AL.
302
Figure 3. Transmission electron micrographs and particle size distributions of silver nanoparticles synthesized in DK-SS re-
verse micelles: reaction temperature = (a) 5˚C, (b) 15˚C, (c) 25˚C, and (d) 40˚C.
3.3. Dependence of Average Esterification
Degree of Surfactants upon Size of Silver
Nanoparticles
The formation and stability of micelles are due to the
structure and/or HLB of surfactants [22-24]. The struc-
ture and HLB of sucrose fatty acid esters alter with the
esterification degree [11]. In order to elucidate the effect
of those factors on the formation of silver nanoparticles,
we have examined the synthesis of silver nanoparticles in
the reverse micelles of sucrose fatty acid esters by mix-
ing DK-SS (average esterification degree = 1.01, HLB =
19) with DK-F-20W (average esterification degree = 3.1,
HLB = 2). The more weight fraction of DK-F-20W is,
the larger the average esterification degree is. Figure 6
shows the typical transmission electron micrographs and
size distributions of silver nanoparticles when silver
nanoparticles were prepared in the reverse micellar sys-
tem of sucrose fatty acid esters at different average es-
terification degrees for 3 h. The silver nanoparticles in
various shapes were obtained in those reverse micellar
systems. The standard deviation concerning the size dis-
tribution of silver nanoparticles was strongly dependent
upon the average esterification degree of surfactants.
When the average esterification degree was 1.85, the
minimal standard deviation was obtained. This result
indicated that at that average esterification degree, mono-
disperse silver nanoparticles were formed. As shown in
Figure 7, the mean diameter of silver nanoparticles was
dependent upon the average sterification degree, similar
Figure 4. Effect of reaction temperature on the mean di-
ameters of silver nanoparticles in reverse micelles of DK-SS:
[AgNO3]ov = 0.1 M; [NaBH4]ov = 1.0 M; n-butanol/ isooctane
containing 50 g/L DK-SS and 60 mM H2O; reaction time =
3 h.
Figure 5. Relationship of the mean diameter of silver nano-
particles obtained at different reaction temperature with
the wavelength in the peak of UV-visible absorption spec-
trum due to the LSPR of silver nanoparticles. e

H. NORITOMI ET AL.
303
Figure 6. Transmission electron micrographs and particle size distributions of silver nanoparticles synthesized in reverse
micelles of DK-SS and DK-F-20W: average esterification degree = (a) 1.01, (b) 1.43, (c) 1.85, (d) 2.68, and (e) 3.10.
Figure 7. Effect of average esterification degree of surfac-
tants on the size of silver nanoparticles synthesized in re-
verse micelles of DK-SS and DK-F-20W: [AgNO3]ov= 0.1 M;
[NaBH4]ov = 1.0 M; n-butanol/isooctane containing 50 g/L
DK-SS/DK-F-20W and 60 mM H2O; reaction temperature
= 25 ˚C; reaction time = 3 h.
to the case of injection method [10,25]. The size of silver
nanoparticles gradually increased with an increase in the
average esterification degree. Thus, when the bulkiness
of surfactants increased or the HLB value decreased, the
size of silver nanoparticles tended to increase.
4. Conclusions
We have demonstrated that the synthesis of silver
nanoparticles is drastically promoted in the sucrose fatty
acid ester/n-butanol/isooctane system by supplying reac-
tants to the water pool through solid-liquid extraction.
The size of silver nanoparticles decreased with a de-
crease in reaction temperature. The wavelength in the
peaks corresponding upon the LSPR was linearly cor-
related with the size of silver nanoparticles prepared by
the present method. The size of silver nanoparticles
tended to depend upon the average esterification degree
of sucrose fatty acid esters.
5. Acknowledgements
Authors thank Mr. Misaki for taking TEM micrographs,
and Dai-Ichi Kogyo Seiyaku Co., Ltd. for supplying su-
crose fatty acid esters.
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