Interfacial fabrication of silica hollow particles in a reverse emulsion system

Article abstract of DOI:10.1246/cl.2005.1314

Silica hollow particles were prepared at room temperature in a reverse emulsion system (W/O emulsion) via interfacial strategy. The cationic surfactant cetyltrimethylammonium bromide (CTAB) and aliphatic amine were found to be critical factors for the production of hollow structure. Copyright

Full text of DOI:10.1246/cl.2005.1314

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Chemistry Letters Vol.34, No.10 (2005)
Interfacial Fabrication of Silica Hollow Particles in a Reverse Emulsion System
ꢀ
Linyong Song, Xuewu Ge, and Zhicheng Zhang
Department of Polymer Science and Engineering, University of Science & Technology of China,
Hefei, Anhui 230026, P. R. China
(Received June 14, 2005; CL-050764)
Silica hollow particles were prepared at room temperature in
a reverse emulsion system (W/O emulsion) via interfacial strat-
egy. The cationic surfactant cetyltrimethylammonium bromide
(CTAB) and aliphatic amine were found to be critical factors
for the production of hollow structure.
Over the past decade, extensive research has been undertak-
en in the preparation of inorganic hollow microspheres because
of their potential applications in cosmetics, coatings, inks, bio-
medicine, small containers for encapsulation process in cataly-
sis, drug delivery, development of artificial cells, and other
areas.1 On account of the well-controlled kinetics on hydrolysis
and condensation of silicon alkoxides in aqueous solution, silica
is by far the most popular inorganic matrix for hollow micro-
spheres.
,2
A previous method to produce silica hollow spheres is based
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–10
on the utilization of core–shell type templating materials.
In
these cases, polymer latexes, inorganic spheres, oil droplets, and
aggregates of polymers or surfactants are used as cores, and
silica microspheres are formed on the core templates. For most
processes using core templates, the templates need to be re-
moved by calcination or solvent treatments to create vacant inner
spaces. Another representative approach is to create silica hol-
1
1–14
low spheres using vesicle templates.
and base-condesation two-step preparation method is also
An acid-hydrolysis
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5
claimed to be effective for silica hollow sphere preparation.
Silica hollow spheres were also obtoined at the air–water inter-
face.1
6,17
However, this method was rather sensitive to the stir-
ring conditions, induction time, and time delays between the
dilution and neutralization steps in the preparation procedure.
In the present paper, we report a new facile and efficient
one-step method for the preparation of silica hollow particles
in reverse emulsion system (W/O emulsion). The procedure
was based on the hydrolysis and condensation of tetraethoxysi-
lane (TEOS) at the water/oil interface formed from a cationic
surfactant cetyltrimethylammonium bromide (CTAB) and alkyl-
phenol polyoxyethylene (n) ether (n ¼ 4, OP-4). Because silica
was deposited preferentially at the W/O interface, hollow parti-
cles were formed in situ. This method eliminates the need for
any template extraction steps. As the diameter and shell thick-
ness of hollow particles depend on the water droplet size, this
can be controlled to some extent by varying the ratio of
CTAB/water (see Figures 1A and 1B). The shell thickness
was ca. 60 and 20 nm, respectively.
Figure 1. TEM images of silica hollow particles at different con-
ꢃ3
ꢃ3
tent of CTAB (A) CATB ¼ 1 ꢂ 10 mol (B) CTAB ¼ 2 ꢂ 10
ꢃ3
mol (C) FESEM image of sample with CTAB ¼ 2 ꢂ 10 mol.
with outer diameters ranging from about 100 up to 250 nm (see
Figure 1). At the same time, few granular silica were enwrapped
in the hollow particles, which were due to the few hydrated silica
molecules penatrated into water phase and the condensation re-
action took place in the water. FESEM image showed silica par-
ticles of size similar to those observed with TEM and the frac-
tured particles confirmed the hollow structure. Similar images
were obtained after calcinations of the silica product at 873 K,
confirming that the spherical structures were composed of stable
silica. The diameter and shell thickness of silica hollow particles
were kept almost the same (see Supporting Information). While
the silica hollow particles aggregated obviously and this phe-
nomenon was due to the irreversible condensation of adjacent
surface hydroxyl groups among silica hollow particles at high
Silica transcription was carried out by the addition of the
silica precursor TEOS to a reverse emulsion in the presence of
aliphatic amine. Powder X-ray diffraction profiles showed a sin-
ꢁ
gle broad peak centered at 2ꢀ ¼ 22 corresponding to the forma-
tion of hollow particles with amorphous silica shells. TEM
showed the presence of a large amount of silica hollow particles
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.10 (2005)
1315
shell structure was formed in suit. In Entries 1 and 3, when
NaOH was used as catalyst, the hydrolysis of TEOS was so fast
that SiOH-based molecule will become water-soluble and the
condensation will take place in the aqueous phase. On the other
hand, without cationic surfactant CTAB, the adsorption of non-
ionic surfactant on silicates was weaker than that of the CTAB
and the interface membrane of nonionic surfactant cannot pre-
vent the silicates from penetrating into the inner aqueous phase,
hence granular silica was also obtained.
In conclusion, we have demonstrated a new facile and effi-
cient sol–gel approach to create silica hollow particles at a W/O
interface. The diameter and thickness of the silica hollow parti-
cles can be controlled by varying the ratio of CTAB/water. The
porous shell wall of the spheres could have potential applications
as controlled release capsules for drugs, dyes, cosmetics, inks,
artificial cells, catalysts, and fillers. Furthermore, this approach
could be extended to the synthesis of other oxide composite
hollow spheres such as Ti–Si, Zr–Si, et al., as currently being
investigated in our laboratory.
Figure 2. Schematic representation of the formation of silica
hollow particles in W/O emulsion system.
temperature. The porosity of the shell was investigated using
nitrogen adsorption–desorption isotherms (see Supporting Infor-
mation). Typical value for the specific surface area according to
References and Notes
2
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1
2
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high special surface area of hollow particles may originate from
mesophases formed by the precursors and the mixed surfactants.
We proposed a formation mechanism as shown in Figure 2
to explain the hollow structure formation. The precursors of
TEOS, initially dissolved in the oil phase, penetrated slowly
into the surfactant membrane. Hydrolysis of TEOS occured as
a consequence of contact with the aqueous phase and catalysis
by dodecylamine. The hydrolyzed TEOS molecules were then
condensed at the W/O emulsion interface, leading to the forma-
tion of silica hollow particles.
3
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CTAB and the dodecylamine were found to play critical role
in the formation of stable silica hollow particles. To investigate
the role of cationic surfactant (CTAB) and aliphatic amine (do-
decylamine) and the proposed reaction mechanism, four recipes
were examined (see Table 1). Experiments Entries (1–3) resulted
in the exclusive formation of solid or granular silica (as observed
with TEM, see Supporting Information), indicating that indeed
both postively charged surfactant CTAB and aliphatic amine
need to be present in order to obtain silica hollow particles.
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ꢃ
ꢃ
of TEOS, the OH ion displaces of C2H5O and the higher
pH value would favor the hydrolysis reaction. It has been ob-
served that the hydrolysis of TEOS was obviously slower using
dodecylamine than NaOH as catalyst (by comparing the time of
the reverse emulsion from transparent to turbid), so the silicates
were attracted by the cationic surfactant and the condensation
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18 Experiments and Characterizaion: Reverse emulsion was
prepared by mixing 5 mL of aqueous solution of 0.37 g of CTAB
with 25.0 mL of kerosene solution of 2.0 g of OP-4 and 0.37 g of
dodecylamine under continually stirring. Hollow silica particles
were then prepared at room temperature by adding 9.32 g of
TEOS to the reverse emulsion and the reaction was continually
stirred 24 h. Finally, ethanol was added into the reverse emulsion
to precipitate the silica particles. The morphology of silica parti-
cles was studied by FESEM (JEOL JSM-6700) and TEM (Hitachi
H-800). The control experiments (Entries 1–3 in Table 1) were
conducted under the same conditions except the use of different
surfactants and basic catalysts.
Table 1. Results and conditions for the silica transcription
a
Entry
Surfactants
Base
NaOH
dodecylamine
NaOH
dodecylamine
Silica structure
1
2
3
4
OP-4+OP-10
OP-4+OP-10
OP-4+CTAB
OP-4+CTAB
solid silica
granular silica
granular silica
hollow partilce
a
In Entries 1 and 3, NaOH is 0.002 mol; in Entries 2 and 4,
dodecyl amine is 0.002 mol.
Published on the web (Advance View) August 28, 2005; DOI 10.1246/cl.2005.1314