Fabrication of hollow silica microspheres through the self-assembly behavior of polymers in W/O emulsion

Article abstract of DOI:10.1246/cl.2003.598

The major focus of this study is on understanding the mechanism of formation of the hollow silica microspheres through the self-assembly behavior of polymers (PEG and PVP) in W/O emulsion and studying their particle morphology, particle size, and pore str

Full text of DOI:10.1246/cl.2003.598

598
Chemistry Letters Vol.32, No.7 (2003)
Fabrication of Hollow Silica Microspheres through the Self-assembly Behavior of Polymers
in W/O Emulsion
Jae-Hyung Park, Seong-Youl Bae,y and Seong-Geun Ohꢀ
Department of Chemical Engineering, Hanyang Universtiy and CUPS, Seoul 133-791, Korea
yDepartment of Chemical Engineering, Hanyang University, Kyunggi-do, Ansan-si 425-791, Korea
(Received March 24, 2003; CL-030245)
The major focus of this study is on understanding the mech-
anism of formation of the hollow silica microspheres through
the self-assembly behavior of polymers (PEG and PVP) in
W/O emulsion and studying their particle morphology, particle
size, and pore structure depending on the experimental condi-
tions.
In order to remove the unreacted materials such as polymers,
octanol and surfactants, the particles obtained by centrifugation
were washed with ethanol. After this procedure was repeated
three times, the hollow silica particles were collected. Then,
particles were dried in an incubator at 40 ꢁC for 1 day and were
finally calcined in air at 600 ꢁC for 6 h to remove the remaining
polymers and water inside particles.
Recently, there has been intense interest about the fabrica-
tion of micro and nanometer-sized hollow particles. These hol-
low particles often exhibit properties that are substantially dif-
ferent from general particles (for example, low density, large
specific surface area, stability and surface permeability), which
make them attractive from both a scientific and a technological
viewpoints.1 Applications for such particles are cosmetics, in-
cluding capsule agents for drug delivery system (DDS), cataly-
sis, coatings, composite materials, dyes, inks, artificial cells, fil-
lers and protecting sensitive agents such as enzymes and
proteins.1{3
However, in case of hollow silica microspheres fabricated
in emulsion which has a lot of reaction factors and thermody-
namic instability, the yield of hollow silica microspheres is gen-
erally low.4{6 In this paper, we report a method to synthesize
hollow silica microspheres by sol-gel process of tetraethyl
orthosilicate (TEOS) in W/O emulsion containing polymers
and surfactants at high yield.
Scheme 1. Schematic illustrations of the key stages in the for-
mation of the W/O emulsion with polymers and optical micro-
scopic images of each stages. I) The molecular behavior of PEG
in W/O emulsion. II) The molecular behavior of PVP in W/O
emulsion. (the scale bar in images corresponds to 10 mm.)
The morphologies of particles preparaed through the
Scheme 1-I and Scheme 1-II are shown in Figure 1. SEM and
TEM images of particles shows the completely different
morphologies; mesoporous (Figures 1a and 1b) and hollow
structures (Figures 1c and 1d). In case of using PEG, both the
surface of silica particle and the inside have the mesoporous
structure (Figures 1a and 1b). The inset in Figure 1b is an ex-
panded TEM image of a single hollow microsphere with meso-
porous structure. But, the yield of hollow silica particles with
mesoporous shell was below 10%.
The morphologies of particles prepared using PVP are dif-
ferent from the those of using PEG in that they evidently have
hollow structure. Some broken microspheres are observed in the
SEM image (Figure 1c), which indicates that they have hollow
structure. This is further proven by the sharp contrast between
the dark surface and the pale core in the TEM image (Figure
1d). The shell wall thickness of particles had a broad distribu-
tion in the range of 100 to 400 nm, independent of the particle
size. This variable thickness of the shell wall could be the rea-
son for the formation of a closed structure below 400 nm parti-
cle size resulting in solid spheres. But, the particles prepared
using PVP have hollow structure more than 80%.
The overall procedure for the preparation of hollow parti-
cles in W/O emulsion with polymers is represented in Scheme 1.
External oil phase was prepared by dissolving hydroxypropyl
cellulose (HPC, average Mw ca. 370000) as a stabilizer of
emulsion structure in octanol (45 g), then held at 80 ꢁC for
4 h. Then oil phase was kept at 40 ꢁC. After 30 min, sorbitan
monooleate (3wt% Span 80) was added into oil phase. Water
phase was prepared with polymers: polyethylene glycol (PEG,
average Mw. 20000, 18 wt%) in Scheme 1-I or polyvinylpyrro-
lidone (PVP, average Mw. 40000, 28 wt%) in Scheme 1-II.
The polyoxyethylene(20) sorbitan monolaurate (5 wt%
Tween 20) and NH4OH as a catalyst were added in the water
phase. Water phase was added to an external oil phase to make
W/O emulsion. The weight ratio of water to oil phases in emul-
sion was kept as 1:9. To obtain a narrow and small droplet size
distribution, the mixtures were pulverized by using a homogeni-
zer at a rate of 12000 rpm for 5 min. Samples were prepared in
the experimental beakers with flat bottom and kept for 12 h with
soft agitation using small magnet stirrers. The sol–gel reaction
for the formation of silica particles was initated by adding
TEOS into oil phase. The molar ratio of water to TEOS was
kept as 10. After reaction was completed, the samples were cen-
trifuged at 2500 rpm for 15 min to obtain hollow silica particles.
The mechanism of formation of the hollow structure micro-
spheres can be explained as follows. First, the water–PEG inter-
action and the quantities of PEG dissolved in water have the im-
Copyright ꢀ 2003The Chemical Society of Japan

Chemistry Letters Vol.32, No.7 (2003)
599
portant role in the mechanism of Scheme 1-I. The water–PEG
interaction was believed to be through a hydrogen bonding be-
tween the isolated hydroxyl groups of water and the ether oxy-
gen in PEG polymer. Hydrogen bonding has generally been as-
sumed to be a ubiquitous mechanism of polymer adsorption on
hydrophilic surfaces.7;8 Also, in case of the addition of PEG in
aqueous phase, unstable multiple emulsion droplets are ob-
served, and both Ostwald ripening and release of the internal
oil phase into the outer oil phase occur. This is the coalescence
between the smaller inner oil gloubles within the water droplet
and coalescence of the small inner oil droplets with the outer
droplets interface.5 Thus, the molecular behavior of PEG in
the aqueous droplet is just the same as shown in Scheme 1-I.
of PEG and water. After the reaction with TEOS, the lower part
of the droplet was only remained (Figure 2b). But, due to the
thin water layer, the shape of the upper part was disrupted.
By means of this experiment, it was inferred that the particles
of different mophology were formed by the different molecular
behavior of PEG and PVP in water.
Figure 2. A) optical microscopic image of emulsion droplets;
Inset modifided illustration indicated droplets structures. B)
SEM image of silica particles produced from W/O emulsions
containing PEG and PVP in aqueous phase.
In conclusion, we have succeeded in preparation of hollow
microspheres through the self-assembly behavior of polymers in
W/O emulsion. The molecular behavior of PVP led to the for-
mation of emulsion droplets possessing a hydrophobic part of
PVP core surrounded by a hydrophilic part of PVP shell with
aqueous layer. The central layer, composed of alkyl chain in
PVP, fix the inner oil droplets to core. Also, the molecular be-
havior of PEG led to the formation of multiple emulsion posses-
sing a hydrogen bonding with aqueous phase. Because this route
is based on common and nonexpensive polymer products and
the processing is simple, it may have great potential for further
applications in fabricating supramolecuar materials for various
fields such as cosmetics, catalysis, composite materials, drug
encapulation and controlled release.
This research was supported by the Korea Research Foun-
dation through the project number 2001-041-E00372.
Figure 1. SEM(A,C) and TEM(B,D) images of silica parti-
cles produced from W/O emulsions containing PEG (A,B)
and PVP (C,D) in aqueous phase; The inset of figure B is an
expanded TEM image of a single hollow microsphere with
mesoporous shell wall.
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Published on the web (Advance View) June 17, 2003; DOI 10.1246/cl.2003.598