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On the stability of surfactant-free water-in-oil emulsions and synthesis of hollow SiO2 nanospheres
Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 55–60
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
On the stability of surfactant-free water-in-oil emulsions and synthesis of hollow SiO2 nanospheres Satoshi Horikoshi a,∗ , Yu Akao b , Taku Ogura c , Hideki Sakai b , Masahiko Abe b , Nick Serpone d,∗ a
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Functional Materials Research Laboratory, Research & Development Headquarters, Lion Co., 7-2-1 Hirai, Edogawa-ku, Tokyo 132-0035, Japan d Gruppo Fotochimico, Dipartimento di Chimica Organica, Universita di Pavia, Via Taramelli 10, Pavia 27100, Italy b c
a r t i c l e
i n f o
Article history: Received 19 August 2010 Received in revised form 19 September 2010 Accepted 21 September 2010 Available online 23 October 2010 Keywords: Hollow nanospheres Water-in-oil emulsion SiO2 Silica Surfactant-free emulsion
a b s t r a c t This article examines the relative stability of five surfactant-free water-in-oil (w/o) emulsions produced by 1-min irradiation of the mixture with 42-kHz ultrasounds; it also reports on the synthesis of hollow silica nanospheres in such emulsions. The oily phase in these w/o emulsions consisted of cyclohexane, dodecane, benzene, octane, and hexane, whereas the dispersed phase consisted of aqueous ammonia at pH 11. Light scattering experiments revealed that the size of the dispersed phase in the cyclohexane emulsion was fairly constant with time (ca. 0.97 m) and was the most stable emulsion, whereas for the least stable dodecane w/o emulsion the dispersed phase particles increased in size from ca. 1.5 m to 7.3 m after 48 min. The stability of the w/o emulsions decreased in the order cyclohexane > octane > benzene > hexane > dodecane. Hollow silica nanospheres were synthesized by a soft template method involving the five surfactant-free water-in-oil emulsions by hydrolysis of tetraethoxysilane (TEOS) at the water/oil interface (pH 11). Formation of these hollow SiO2 nanospheres is described in terms of the stability of the various emulsions and characterized by transmission electron microscopy, which showed well-defined hollow silica nanospheres of relatively uniform sizes (100 ± 20 nm) in the cyclohexane emulsion. The latter w/o emulsion was further examined for the pH dependence of the formation of the silica nanospheres. At pHs 6 and 7, respectively, highly aggregated hollow silica nanoparticles of ill-defined structure formed with relatively thin skin, whereas well-structured silica nanospheres were obtained at pHs 8 and 9 although the nanospheres also tended to aggregate. By contrast, at pH 10 well-defined hollow silica nanospheres were produced with a silica skin 3 times thinner (5 nm) than that of nanospheres produced at pH 11 (ca. 14 nm). Various stages in the overall process are described. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hollow spherical nanoparticles have attracted considerable attention in the last decade, which has led to a novel encapsulation technology and controlled release of drugs, dyes and inks, and to novel usage in catalysis and in acoustic insulation [1]. Many hollow particles with various diameters and wall thicknesses have been fabricated from a variety of inorganic materials: for example, silica [2], iron oxide [3], titania [4], and several others described in the review article of Fan et al. [5]. A solid template method involving a polymer such as polystyrene (PS) is a well-known method to prepare hollow nanoparticles [6]. One advantage of the solid template method is the ability to synthesize hollow nanoparticles of uniform size. A disadvantage, however, is that the polymer
∗ Corresponding authors. E-mail addresses: [email protected] (S. Horikoshi), [email protected] (N. Serpone). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.09.036
template (e.g. PS) must be removed either by dissolution with some organic solvent and/or by pyrolysis after the synthesis of the hollow nanoparticles has been achieved. Syntheses of hollow nanoparticles by a soft template method employing emulsions [7,8] and vesicles [9,10] have also been examined, although these also necessitate the removal of surfactants commonly added to stabilize the emulsions [11]. One such example is the direct preparation of silica hollow spheres in water-in-oil emulsions in which the dispersion medium was silicone oil and kerosene in the presence of cetyltrimethylammonium bromide (CTAB) (see Ref. [12] and references therein). The two key factors that contributed to the formation of stable and regular silica hollow particles in the latter case were the pH of the dispersed phase and the viscosity of the dispersion phase. The sonochemical method has also been applied effectively by Grigoriev et al. [13] to prepare aqueous dispersions of air-filled nanostructured quartz silica shells from surface-engineered amorphous silica nanoparticles in the presence of the hexadecyltrimethylammonium bromide (CTAB) surfactant; the non-equilibrium nature of the cavitation process
S. Horikoshi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 55–60
and high temperature and pressure in the cavitation microbubble led to partial crystallization of the amorphous silica nanoparticles producing the quartz phase and a high degree of interconnection between the silica nanoparticles in the microsphere shells. Properties of surfactants as emulsion stabilizers have been investigated actively for over three decades, with one such early study reported by Ogino and Ota [14] for the emulsification of oilin-water (o/w) mixtures. The group of Abe and coworkers [15] have examined surfactant-free o/w emulsions in which the oil consisted of oleic acid (OA) and four of its esters, with OA being regarded as an oil yielding benzene-like droplets of discrete sizes, albeit unstable in spite of the C18 chain length of OA, whereas the esters formed somewhat stable o/w emulsions. The stability of the latter emulsions was attributed to the formation of a particular structure around the carboxylate groups which, together with the high viscosities of the oil components, were the principal factors that caused the oil droplets in the surfactant-free emulsions to remain stable against flocculation and coalescence. Long-term (days) stable surfactant-free o/w (hexadecane in water) emulsions in alkaline media (pH 9–10) in the presence of 1 mM NaCl have been investigated by Beattie and Djerdjev [16]. Increasing the added electrolyte concentration to 5 mM caused the hexadecane/water emulsion to become unstable. Certain aspects and advances on oil-in-water emulsions were reviewed recently by Sakai [17]. Studies into the stability and optimization of water-in-oil surfactant-free emulsions have been rather scarce as they might apply to nanoparticle syntheses. The latter emulsions do not require removal of extraneous components necessary to achieve the preparation of hollow silica nanoparticles and thus may present some advantages and facilitate the overall process. This article reports first on the stability of surfactant-free water-in-oil (w/o) emulsions with cyclohexane, dodecane, benzene, octane or hexane serving as the oil dispersion medium, and then examines the synthesis of hollow silica nanospheres by a soft template method through the hydrolysis of the silica precursor TEOS (i.e. triethoxysilane) taking place at the water/oil interface with the dispersed phase at pH 11 by adding ammonium hydroxide (hereafter noted as aqueous ammonia). The synthesis was carried out in all the w/o emulsions with a particular focus on the w/o cyclohexane emulsion which proved to be the most stable.
2. Experimental 2.1. Chemical reagents and preparation of hollow SiO2 nanospheres The precursor system to SiO2 was tetraethoxysilane (also known as tetraethyl orthosilicate; TEOS; Kojundo Chemical Laboratories Co. Ltd.; 99.9999% pure). The aqueous phase of the w/o emulsion consisted of a highly pure aqueous ammonia solution (concentration, 27% v/v). The oil phase of the w/o emulsion consisted of either cyclohexane (C6 H12 ), dodecane (C12 H26 ), benzene (C6 H6 ), octane (C8 H18 ), or hexane (C6 H14 ) as the dispersion medium. Unless noted otherwise, the water-in-oil emulsion was prepared by mixing the dispersion medium (organic solvent; 15 mL) and an aqueous ammonia solution (0.25 mL; 1% v/v; pH 11) using ultrasonic irradiation for 1 min with 42-kHz ultrasounds. The emulsion system was then allowed to stand for 60 min under atmospheric air in the closed vessel. Concomitantly, a solution of TEOS (0.25 mL; 1% v/v) was prepared in each of the dispersion media (10 mL) and, unless noted otherwise, was subsequently added slowly to the 60-min rested w/o emulsion and then allowed to rest for 5 min in the closed vessel to allow the hydrolysis and condensation reactions to complete.
2.2. Characterization Particle size distributions of the aqueous ammonia dispersed phase in the oil medium (pure organic solvents containing no TEOS) were determined by dynamic light scattering techniques using an Otsuka Electrics Co. Ltd., ELSZ-2plus apparatus. Note that the light scattering experiments were carried out every 3–5 min on emulsion samples that remained undisturbed for the whole series of measurements up to 60 min. The viscosities of the pure dispersion media were assessed with a Carri-Med Ltd. ARG2 Rheometer. The size distributions and morphologies of the silica hollow nanoparticles were characterized by transmission electron microscopy (TEM) using a Hitachi High-Technologies Co. H-7650 electron microscope. TEM sampling of the silica nanospheres was achieved by placing a portion of the emulsion onto the TEM grid, which was then “dried” under dynamic vacuum for 2 days. 3. Results and discussion 3.1. Stability of the w/o emulsions The stabilities of the water-in-oil emulsions with the various dispersion media were examined first in the absence of the TEOS precursor. Fig. 1 displays the temporal changes in the mean sizes of the aqueous ammonia dispersed phase in the w/o emulsions monitored by dynamic light scattering. The initial size of the dispersed phase particles in the w/o cyclohexane emulsion changed relatively little with standing time (mean size, 0.97 m). By contrast, for dodecane the temporal increases in the mean size of the aqueous dispersed phase were considerable changing from an initial size of ca. 1.5 m to nearly 7.3 m after 48 min standing time. With octane as the dispersion medium, the mean sizes of the dispersed phase remained fairly steady at ca. 3.4 m. For benzene the mean size was relatively constant at ca. 2.8 m up to 44 min after which the dispersed phase tended to decrease in size to ∼1.6 m, whereas for hexane emulsions the initial size decreased fairly rapidly in the first 22 min following which the size of the dispersed phase remained constant at ca. 1.3 m. The standard deviations of the mean sizes of the dispersed aqueous phase in the various w/o emulsions are listed in Table 1, from which we deduce that the overall stability of the emulsions, based on the overall size changes of Fig. 1 relative to the initial aqueous pool size, decreased in the order cyclohexane > octane > benzene > hexane > dodecane. Except for octane, the emulsion stability seems to depend to some extent on the size and nature of the oily phase component. The stability of the w/o emulsions was further examined by assessing the sedimentation rates vS calculated from Stokes’ law
8 dodecane
Size of aqueous pool (μm)
56
6
octane
4
2
hexane
benzene
cyclohexane
0 0
10
20
30
40
50
60
Standing time (min) Fig. 1. Time changes in the mean sizes of the aqueous ammonia dispersed phase in w/o emulsions as a function of the time that the emulsions were left standing as monitored by light scattering methods.
S. Horikoshi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 55–60
57
Table 1 Standard deviations of the experimental mean particle sizes of the aqueous droplets in the w/o emulsions, difference in densities between the dispersion media and the aqueous ammonia dispersed phase, and measured viscosities of the dispersion media.
Standard deviations of the mean sizes (m) of the dispersed phase in the w/o emulsions Difference in densities between dispersion medium and dispersed phase (g cm−3 ) [18] Measured viscosities of pure dispersion media at 25 ◦ C (cP)
(Eq. (1) [18]) at the various standing times (see Fig. 2),
vS =
a2 (dp
− dm )g
(1)
18
where a is the mean size (diameter) of the dispersed phase particles in the emulsion; (dp − dm ) is the difference between the mass density of the dispersed phase particles and the mass density of the dispersion medium (see Ref. [19]), respectively; g is the gravitational acceleration; and is the viscosity of the dispersion medium. The sedimentation rate of the cyclohexane dispersion media was remarkably slow (ca. 0.074 × 10−8 m s−1 at 0 min) and remained so throughout with little change compared with other dispersion media. Hexane dispersion media showed the fastest initial sedimentation rate (22 × 10−8 m s−1 at 0 min), decreasing rapidly with time to ca. 2.2 × 10−8 m s−1 at 60 min, whereas for the dodecane dispersion media the sedimentation rate increased with standing time from ca. 0.14 × 10−8 m s−1 (0 min) to ca. 3.4 × 10−8 m s−1 (48 min). Aggregation and coalescence of the dispersed phase in a w/o emulsion are highly dependent on the sedimentation dynamics. Therefore, a stable template to synthesize hollow silica nanospheres of fairly uniform sizes was unlikely under the unstable emulsion conditions for dodecane, octane, benzene, and hexane dispersion media. However, a stable soft template for the synthesis of hollow silica nanospheres with an aqueous ammonia dispersed phase in a w/o cyclohexane emulsion was possible with stable and relatively small dispersed phase pools as the sedimentation rate was significantly slow. 3.2. Synthesis of the SiO2 hollow nanospheres in the w/o emulsions Despite the instability or stability of the surfactant-free w/o emulsions, we nonetheless proceeded to prepare the hollow silica nanospheres in the five dispersion media. Fig. 3 illustrates the TEM images of the hollow silica nanoparticles prepared in the surfactant-free w/o emulsions of (a) cyclohexane, (b) dodecane, (c) benzene, (d) octane, and (e) hexane as the dispersion media. Hollow silica nanospheres of reasonably uniform size (ca. 100 ± 20 nm) with a skin ca. 14 nm thick were clearly formed in the cyclohexane
Sedimentation rate (10-8 m s-1)
10 hexane 8 6
dodecane
octane
4 2
benzene
0
cyclohexane 0
10
20
30
40
50
60
Standing time (min) Fig. 2. Calculated sedimentation rates (Eq. (1)) of the aqueous ammonia dispersed phase in w/o emulsions at various standing times with cyclohexane, dodecane, benzene, octane, and hexane acting as the dispersion media.
Cyclohexane
Dodecane
Benzene
Octane
Hexane
0.17 0.133 0.722
1.65 0.159 1.364
0.61 0.130 0.401
0.31 0.207 0.311
0.73 0.254 0.085
emulsion (Fig. 3a). With benzene as the dispersion medium the synthesis produced smaller silica nanoparticles but highly aggregated into a lump (Fig. 3c). Aggregation of silica nanospheres also occurred to some extent in dodecane, octane and hexane as the dispersion media (Fig. 3b, d, and e). In addition, when dodecane was the dispersion medium for the w/o emulsion the synthesis led to expanded hollow silica nanospheres producing a thin silica film. The hollow particles were synthesized twice on different days so as to verify and confirm by TEM techniques the reproducibility of the syntheses.
3.3. Optimization of the synthesis of hollow SiO2 nanospheres in the cyclohexane emulsion The amount of the aqueous ammonia dispersed phase added to the w/o cyclohexane emulsion was decreased from 1% v/v to 0.1% v/v (representing 0.025% v/v in the emulsion). The hollow silica nanospheres were synthesized under otherwise similar conditions as above, namely the TEOS was added after the emulsion had been at rest for 60 min. At this lower concentration of the dispersed phase formation of the hollow silica nanospheres was hardly noticeable as attested by the TEM image of Fig. 4a. In fact whatever the structure of the silica particles produced under such conditions, they were highly coalesced silica particles. Fig. 4b displays the TEM image of silica nanoparticles produced in the w/o cyclohexane emulsion in the absence of aqueous ammonia. Evidently, under near-neutral pH conditions slow hydrolysis [12] of the TEOS silica precursor yielded strongly coalesced and aggregated silica particles. Next we proceeded to repeat the synthesis of the silica nanoparticles with the 1% (v/v) of the aqueous ammonia dispersed phase by the addition of the TEOS precursor solution to the w/o emulsion at standing times of 0, 20, 30, 40 and 50 min. The TEM images displayed in Fig. 4c and d illustrate the resulting albeit aggregated hollow silica nanospheres produced on addition of the TEOS precursor at the 0 and 30 min standing time, respectively. Addition of TEOS immediately after the formation of the w/o cyclohexane emulsion generated significantly smaller (ca. 20–30 nm) hollow silica nanospheres. Hollow silica nanospheres produced in the 30min rested emulsion were somewhat larger (ca. 40–45 nm). Clearly, a standing time of 60 min after the formation of the w/o emulsion secures a stable emulsion, and leads to an optimal synthesis of hollow silica nanospheres that seem neither to coalesce nor to aggregate (see Fig. 3a). Using the latter standing time conditions, we also examined the hydrolysis rate of TEOS in the emulsion immediately (0 time) after the addition of this precursor and after 1 and 3 min. Fig. 4e illustrates the hollow silica nanoparticles formed after 1 min of hydrolysis and condensation time. TEM data showed hollow silica nanospheres with a considerably thicker skin (ca. 25–30 nm) in comparison with the thinner skin (ca. 14 nm) of the hollow nanospheres produced by allowing the hydrolysis/condensation process to occur for 5 min (compare with Figs. 3a and 4e). Syntheses of the hollow silica nanospheres were also carried under various other conditions of standing times, ratios of added TEOS precursor, and concentration of the aqueous ammonia dispersed phase in all the w/o emulsions examined. No visible improvements were evidenced on the formation of hollow silica nanospheres from those displayed in Fig. 3a.
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Fig. 3. TEM image of hollow silica nanoparticles prepared in w/o emulsion systems with the dispersion media being (a) cyclohexane, (b) dodecane, (c) benzene, (d) octane, and (e) hexane.
It was rather curious that the mean sizes of the aqueous dispersed phase particles obtained by the light scattering technique (Fig. 1) were far greater than the sizes of the hollow silica nanospheres produced. For example, in the cyclohexane w/o emulsion the dispersed phase particles were some 8–9 times greater than the sizes of the silica nanospheres. This suggests that the dispersed phase likely consisted of clusters of the aqueous phase droplets, which on addition of the tetraethoxy orthosilicate (TEOS)
precursor molecules to the emulsions and initiation of hydrolysis at the water/oil interface caused the clusters to breakup (Scheme 1). Overall then, formation of hollow silica nanospheres under our conditions likely implicated no less than six principal stages: (stage 1) TEOS precursor molecules interact with the aqueous dispersed phase clusters containing the base catalyst (OH− ) which initiates hydrolysis of the TEOS at the interface producing silanols and silicates [20]; (stage 2) formation of the latter species leads to the
Fig. 4. TEM images of hollow silica nanospheres in w/o emulsions with cyclohexane as the dispersion medium under various conditions: (a) with 0.1% v/v quantity of aqueous ammonia dispersed phase; (b) no aqueous ammonia dispersed phase present; (c) with the 1% (v/v) of the aqueous ammonia dispersed phase and on addition of TEOS to emulsion at 0 min; (d) with the 1% (v/v) of the aqueous ammonia dispersed phase and after 30 min standing time with TEOS present; (e) with the 1% (v/v) of the aqueous ammonia dispersed phase and formation of silica nanoparticles after 1 min hydrolysis of TEOS. For further details see text.
S. Horikoshi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 55–60
59
Scheme 1. Cartoons illustrating the proposed stages in the formation of the hollow silica nanospheres in the water-in-oil emulsions at pH 11.
Fig. 5. TEM images of hollow silica nanospheres using the aqueous ammonia dispersed phase at pH (a) 6, (b) 7, (c) 8, (d) 9, and (e) pH 10 in w/o emulsions with cyclohexane as the dispersion medium.
breakup of the clusters; (stage 3) hydrolytic processes continue at the interface of the declustered particles, ultimately causing the silicate species to condense (stage 4) with concomitant hydrolytic depolymerization (stage 5), followed by interfacial self-assembly of the silicate species (stage 6), which eventually form the silica skin of the hollow nanospheres.
nanoparticles of ill-defined structure with relatively thin skin. Hollow silica nanospheres were better structurally defined when the synthesis was carried out at pHs 8 and 9 (see Fig. 5c and d), albeit the nanospheres tended to aggregate. By contrast, at pH 10 the synthesis produced well-defined hollow silica nanospheres (Fig. 5e) with a skin that was thinner (ca. 5 nm) than that of the nanospheres produced at pH 11 (ca. 14 nm; see Fig. 3a).
3.4. Influence of pH in the synthesis of hollow SiO2 nanospheres in w/o cyclohexane emulsions
4. Concluding remarks
The influence of pH in the hydrolysis of TEOS was reported by Song et al. [12] and Feng et al. [21] who showed that the growth rate of silica changes significantly with pH. That is, the rate of hydrolysis of the tetraethoxysilane (TEOS) increases with increase of pH of the aqueous dispersed phase, whereas the rate of condensation of the silicate species formed at the w/o interface decreases with increase in pH and the rate of hydrolytic depolymerization of the condensate appears to be rather independent of pH above pH 7 [12,20]. Accordingly, we also examined the influence of pH on the formation of the hollow SiO2 nanospheres under our conditions (60 min standing time, cyclohexane as the dispersion medium, and aqueous ammonia as the dispersed phase) at pHs 6, 7, 8, 9 and 10. Fig. 5a and 5b depicts the silica particles produced at pHs 6 and 7, respectively, which show some aggregated and hollow silica
This article has examined the relative stability of waterin-oil emulsions using various oily components and aqueous ammonia at pH 11 as the dispersed aqueous phase. The stability of these w/o emulsions decreased in the order cyclohexane > octane > benzene > hexane > dodecane. The size of the dispersed phase in the cyclohexane emulsion was fairly constant with time (ca. 0.97 m) and was the most stable emulsion, whereas for the least stable dodecane w/o emulsion the dispersed phase particles increased in size from ca. 1.5 m to 7.3 m after 48 min. Hollow silica nanospheres have been synthesized by the hydrolysis of tetraethoxysilane (TEOS) at the water/oil interface (pH 11) in these five surfactant-free water-in-oil emulsions that are reasonably stable even in the absence of stabilizers normally used in such soft template syntheses. Transmission electron
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microscopy revealed well-defined hollow silica nanospheres of relatively uniform sizes (100 ± 20 nm) in the cyclohexane emulsion. Optimal conditions for the synthesis of well-defined hollow silica nanospheres in the cyclohexane w/o emulsion proved to be the pH (either 10 or 11), the standing time of 60 min for the emulsion to stabilize, the ratio of dispersed phase to dispersion medium (1% v/v), and the concentration of the aqueous ammonia in the dispersed phase. At pHs 6 and 7, respectively, highly aggregated hollow silica nanoparticles of ill-defined structure formed with relatively thin skin, whereas well-structured silica nanospheres were obtained at pHs 8 and 9 although the nanospheres also tended to aggregate. By contrast, at pH 10 well-defined hollow silica nanospheres were produced with a silica skin 3 times thinner (5 nm) than that of nanospheres produced at pH 11 (ca. 14 nm). Acknowledgments Financial support to S.H. from the Japan Society for the Promotion of Science (JSPS) through a Grant-in-aid for young scientists (No. B-21750210) is gratefully appreciated. One of us (N.S.) thanks Prof. Albini and his group at the Universita di Pavia, Italy, for their continued kind hospitality during the many semesters spent in their laboratory since 2002. We are also grateful to the personnel of Otsuka Electrics Co. Ltd. for technical assistance. References [1] F. Caruso, Hollow capsule processing through colloidal templating and selfassembly, Chem. Eur. J. 6 (2000) 413–419. [2] Q. Zhao, Y. Xie, T. Dong, Z. Zhang, Oxidation-crystallization process of colloids: an effective approach for the morphology controllable synthesis of SnO2 hollow spheres and rod bundles, J. Phys. Chem. C 111 (2007) 11598–11603. [3] S.-W. Cao, Y.-J. Zhu, Iron oxide hollow spheres: microwave-hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties, Acta Mater. 57 (2009) 2154–2165.
[4] J. Yang, J.U. Lind, W.C. Trogler, Synthesis of hollow silica and titania nanospheres, Chem. Mater. 20 (2008) 2875–2877. [5] H.J. Fan, U. Gc¸sele, M. Zacharias, Formation of nanotubes and hollow nanoparticles based on kirkendall and diffusion processes: a review, Small 3 (2007) 1660–1671. [6] H. Zou, S. Wu, J. Shen, Polymer/silica nanocomposites: preparation, characterization, properties, and applications, Chem. Rev. 108 (2008) 3893–3957. [7] W. Li, X. Sha, W. Dong, Z. Wang, Synthesis of stable hollow silica microspheres with mesoporous shell in nonionic W/O emulsion, Chem. Commun. (2002) 2434–2435. [8] C. Zimmermann, C. Feldmann, M. Wanner, D. Gerthsen, Nanoscale gold hollow spheres through a emulsion approach, Small 3 (2007) 1347–1349. [9] T. Ogura, H. Shibata, K. Sakai, H. Sakai, M. Abe, Direct preparation of highly ordered multilayer-type silica nanocapsules using spontaneously formed vesicles as templates, Chem. Lett. 38 (2009) 120–121. [10] D.H.W. Hubert, Martin Jung, Peter M. Frederik, P.H.H. Bomans, J. Meuldijk, A.L. German, Vesicle-directed growth of silica, Adv. Mater. 12 (2000) 1286–1290. [11] D. Langevin, Emulsions, Acc. Chem. Res. 21 (1988) 255–260. [12] L. Song, X. Ge, M. Wang, Z. Zhang, Direct preparation of silica hollow spheres in a water in oil emulsion system: the effect of pH and viscosity, J. Non-Cryst. Solids 352 (2006) 2230–2235. [13] D. Grigoriev, R. Miller, D. Shchukin, H. Möhwald, Interfacial assembly of partially hydrophobic silica nanoparticles induced by ultrasonic treatment, Small 3 (2007) 665–671. [14] K. Ogino, M. Ota, Studies of spontaneous emulsification. II. The effect of nonionic surfactants, Bull. Chem. Soc. Jpn. 49 (1975) 1187–1190. [15] K. Kamogawa, H. Akatsuka, M. Matsumoto, S. Yokoyama, T. Sakai, H. Sakai, M. Abe, Surfactant-free O/W emulsion formation of oleic acid and its esters with ultrasonic dispersion, Colloids Surf. A: Physicochem. Eng. Aspects 180 (2001) 41–53. [16] J.K. Beattie, A.M. Djerdjev, The pristine oil/water interface: surfactant-free hydroxide-charged emulsions, Angew. Chem. Int. Ed. 43 (2004) 3568–3571. [17] T. Sakai, Surfactant-free emulsions, Curr. Opin. Colloid Interface Sci. 13 (2008) 228–235. [18] H. Lamb, Hydrodynamics, 6th ed., Cambridge University Press, Cambridge, UK, 1994. [19] K. Benran, A Handbook of Chemistry, Maruzen Co. Ltd., Tokyo, 2004. [20] C.K. Brinker, Hydrolysis and condensation of silicates: effects on structure, J. Non-Cryst. Solids 100 (1988) 31–50. [21] Z. Feng, Y. Li, D. Niu, L. Li, W. Zhao, H. Chen, L. Li, J. Gao, M. Ruan, J. Shi, A facile route to hollow nanospheres of mesoporous silica with tunable size, Chem. Commun. (2008) 2629–2631.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
On the stability of surfactant-free water-in-oil emulsions and synthesis of hollow SiO2 nanospheres Satoshi Horikoshi a,∗ , Yu Akao b , Taku Ogura c , Hideki Sakai b , Masahiko Abe b , Nick Serpone d,∗ a
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Functional Materials Research Laboratory, Research & Development Headquarters, Lion Co., 7-2-1 Hirai, Edogawa-ku, Tokyo 132-0035, Japan d Gruppo Fotochimico, Dipartimento di Chimica Organica, Universita di Pavia, Via Taramelli 10, Pavia 27100, Italy b c
a r t i c l e
i n f o
Article history: Received 19 August 2010 Received in revised form 19 September 2010 Accepted 21 September 2010 Available online 23 October 2010 Keywords: Hollow nanospheres Water-in-oil emulsion SiO2 Silica Surfactant-free emulsion
a b s t r a c t This article examines the relative stability of five surfactant-free water-in-oil (w/o) emulsions produced by 1-min irradiation of the mixture with 42-kHz ultrasounds; it also reports on the synthesis of hollow silica nanospheres in such emulsions. The oily phase in these w/o emulsions consisted of cyclohexane, dodecane, benzene, octane, and hexane, whereas the dispersed phase consisted of aqueous ammonia at pH 11. Light scattering experiments revealed that the size of the dispersed phase in the cyclohexane emulsion was fairly constant with time (ca. 0.97 m) and was the most stable emulsion, whereas for the least stable dodecane w/o emulsion the dispersed phase particles increased in size from ca. 1.5 m to 7.3 m after 48 min. The stability of the w/o emulsions decreased in the order cyclohexane > octane > benzene > hexane > dodecane. Hollow silica nanospheres were synthesized by a soft template method involving the five surfactant-free water-in-oil emulsions by hydrolysis of tetraethoxysilane (TEOS) at the water/oil interface (pH 11). Formation of these hollow SiO2 nanospheres is described in terms of the stability of the various emulsions and characterized by transmission electron microscopy, which showed well-defined hollow silica nanospheres of relatively uniform sizes (100 ± 20 nm) in the cyclohexane emulsion. The latter w/o emulsion was further examined for the pH dependence of the formation of the silica nanospheres. At pHs 6 and 7, respectively, highly aggregated hollow silica nanoparticles of ill-defined structure formed with relatively thin skin, whereas well-structured silica nanospheres were obtained at pHs 8 and 9 although the nanospheres also tended to aggregate. By contrast, at pH 10 well-defined hollow silica nanospheres were produced with a silica skin 3 times thinner (5 nm) than that of nanospheres produced at pH 11 (ca. 14 nm). Various stages in the overall process are described. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Hollow spherical nanoparticles have attracted considerable attention in the last decade, which has led to a novel encapsulation technology and controlled release of drugs, dyes and inks, and to novel usage in catalysis and in acoustic insulation [1]. Many hollow particles with various diameters and wall thicknesses have been fabricated from a variety of inorganic materials: for example, silica [2], iron oxide [3], titania [4], and several others described in the review article of Fan et al. [5]. A solid template method involving a polymer such as polystyrene (PS) is a well-known method to prepare hollow nanoparticles [6]. One advantage of the solid template method is the ability to synthesize hollow nanoparticles of uniform size. A disadvantage, however, is that the polymer
∗ Corresponding authors. E-mail addresses: [email protected] (S. Horikoshi), [email protected] (N. Serpone). 0927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2010.09.036
template (e.g. PS) must be removed either by dissolution with some organic solvent and/or by pyrolysis after the synthesis of the hollow nanoparticles has been achieved. Syntheses of hollow nanoparticles by a soft template method employing emulsions [7,8] and vesicles [9,10] have also been examined, although these also necessitate the removal of surfactants commonly added to stabilize the emulsions [11]. One such example is the direct preparation of silica hollow spheres in water-in-oil emulsions in which the dispersion medium was silicone oil and kerosene in the presence of cetyltrimethylammonium bromide (CTAB) (see Ref. [12] and references therein). The two key factors that contributed to the formation of stable and regular silica hollow particles in the latter case were the pH of the dispersed phase and the viscosity of the dispersion phase. The sonochemical method has also been applied effectively by Grigoriev et al. [13] to prepare aqueous dispersions of air-filled nanostructured quartz silica shells from surface-engineered amorphous silica nanoparticles in the presence of the hexadecyltrimethylammonium bromide (CTAB) surfactant; the non-equilibrium nature of the cavitation process
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and high temperature and pressure in the cavitation microbubble led to partial crystallization of the amorphous silica nanoparticles producing the quartz phase and a high degree of interconnection between the silica nanoparticles in the microsphere shells. Properties of surfactants as emulsion stabilizers have been investigated actively for over three decades, with one such early study reported by Ogino and Ota [14] for the emulsification of oilin-water (o/w) mixtures. The group of Abe and coworkers [15] have examined surfactant-free o/w emulsions in which the oil consisted of oleic acid (OA) and four of its esters, with OA being regarded as an oil yielding benzene-like droplets of discrete sizes, albeit unstable in spite of the C18 chain length of OA, whereas the esters formed somewhat stable o/w emulsions. The stability of the latter emulsions was attributed to the formation of a particular structure around the carboxylate groups which, together with the high viscosities of the oil components, were the principal factors that caused the oil droplets in the surfactant-free emulsions to remain stable against flocculation and coalescence. Long-term (days) stable surfactant-free o/w (hexadecane in water) emulsions in alkaline media (pH 9–10) in the presence of 1 mM NaCl have been investigated by Beattie and Djerdjev [16]. Increasing the added electrolyte concentration to 5 mM caused the hexadecane/water emulsion to become unstable. Certain aspects and advances on oil-in-water emulsions were reviewed recently by Sakai [17]. Studies into the stability and optimization of water-in-oil surfactant-free emulsions have been rather scarce as they might apply to nanoparticle syntheses. The latter emulsions do not require removal of extraneous components necessary to achieve the preparation of hollow silica nanoparticles and thus may present some advantages and facilitate the overall process. This article reports first on the stability of surfactant-free water-in-oil (w/o) emulsions with cyclohexane, dodecane, benzene, octane or hexane serving as the oil dispersion medium, and then examines the synthesis of hollow silica nanospheres by a soft template method through the hydrolysis of the silica precursor TEOS (i.e. triethoxysilane) taking place at the water/oil interface with the dispersed phase at pH 11 by adding ammonium hydroxide (hereafter noted as aqueous ammonia). The synthesis was carried out in all the w/o emulsions with a particular focus on the w/o cyclohexane emulsion which proved to be the most stable.
2. Experimental 2.1. Chemical reagents and preparation of hollow SiO2 nanospheres The precursor system to SiO2 was tetraethoxysilane (also known as tetraethyl orthosilicate; TEOS; Kojundo Chemical Laboratories Co. Ltd.; 99.9999% pure). The aqueous phase of the w/o emulsion consisted of a highly pure aqueous ammonia solution (concentration, 27% v/v). The oil phase of the w/o emulsion consisted of either cyclohexane (C6 H12 ), dodecane (C12 H26 ), benzene (C6 H6 ), octane (C8 H18 ), or hexane (C6 H14 ) as the dispersion medium. Unless noted otherwise, the water-in-oil emulsion was prepared by mixing the dispersion medium (organic solvent; 15 mL) and an aqueous ammonia solution (0.25 mL; 1% v/v; pH 11) using ultrasonic irradiation for 1 min with 42-kHz ultrasounds. The emulsion system was then allowed to stand for 60 min under atmospheric air in the closed vessel. Concomitantly, a solution of TEOS (0.25 mL; 1% v/v) was prepared in each of the dispersion media (10 mL) and, unless noted otherwise, was subsequently added slowly to the 60-min rested w/o emulsion and then allowed to rest for 5 min in the closed vessel to allow the hydrolysis and condensation reactions to complete.
2.2. Characterization Particle size distributions of the aqueous ammonia dispersed phase in the oil medium (pure organic solvents containing no TEOS) were determined by dynamic light scattering techniques using an Otsuka Electrics Co. Ltd., ELSZ-2plus apparatus. Note that the light scattering experiments were carried out every 3–5 min on emulsion samples that remained undisturbed for the whole series of measurements up to 60 min. The viscosities of the pure dispersion media were assessed with a Carri-Med Ltd. ARG2 Rheometer. The size distributions and morphologies of the silica hollow nanoparticles were characterized by transmission electron microscopy (TEM) using a Hitachi High-Technologies Co. H-7650 electron microscope. TEM sampling of the silica nanospheres was achieved by placing a portion of the emulsion onto the TEM grid, which was then “dried” under dynamic vacuum for 2 days. 3. Results and discussion 3.1. Stability of the w/o emulsions The stabilities of the water-in-oil emulsions with the various dispersion media were examined first in the absence of the TEOS precursor. Fig. 1 displays the temporal changes in the mean sizes of the aqueous ammonia dispersed phase in the w/o emulsions monitored by dynamic light scattering. The initial size of the dispersed phase particles in the w/o cyclohexane emulsion changed relatively little with standing time (mean size, 0.97 m). By contrast, for dodecane the temporal increases in the mean size of the aqueous dispersed phase were considerable changing from an initial size of ca. 1.5 m to nearly 7.3 m after 48 min standing time. With octane as the dispersion medium, the mean sizes of the dispersed phase remained fairly steady at ca. 3.4 m. For benzene the mean size was relatively constant at ca. 2.8 m up to 44 min after which the dispersed phase tended to decrease in size to ∼1.6 m, whereas for hexane emulsions the initial size decreased fairly rapidly in the first 22 min following which the size of the dispersed phase remained constant at ca. 1.3 m. The standard deviations of the mean sizes of the dispersed aqueous phase in the various w/o emulsions are listed in Table 1, from which we deduce that the overall stability of the emulsions, based on the overall size changes of Fig. 1 relative to the initial aqueous pool size, decreased in the order cyclohexane > octane > benzene > hexane > dodecane. Except for octane, the emulsion stability seems to depend to some extent on the size and nature of the oily phase component. The stability of the w/o emulsions was further examined by assessing the sedimentation rates vS calculated from Stokes’ law
8 dodecane
Size of aqueous pool (μm)
56
6
octane
4
2
hexane
benzene
cyclohexane
0 0
10
20
30
40
50
60
Standing time (min) Fig. 1. Time changes in the mean sizes of the aqueous ammonia dispersed phase in w/o emulsions as a function of the time that the emulsions were left standing as monitored by light scattering methods.
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Table 1 Standard deviations of the experimental mean particle sizes of the aqueous droplets in the w/o emulsions, difference in densities between the dispersion media and the aqueous ammonia dispersed phase, and measured viscosities of the dispersion media.
Standard deviations of the mean sizes (m) of the dispersed phase in the w/o emulsions Difference in densities between dispersion medium and dispersed phase (g cm−3 ) [18] Measured viscosities of pure dispersion media at 25 ◦ C (cP)
(Eq. (1) [18]) at the various standing times (see Fig. 2),
vS =
a2 (dp
− dm )g
(1)
18
where a is the mean size (diameter) of the dispersed phase particles in the emulsion; (dp − dm ) is the difference between the mass density of the dispersed phase particles and the mass density of the dispersion medium (see Ref. [19]), respectively; g is the gravitational acceleration; and is the viscosity of the dispersion medium. The sedimentation rate of the cyclohexane dispersion media was remarkably slow (ca. 0.074 × 10−8 m s−1 at 0 min) and remained so throughout with little change compared with other dispersion media. Hexane dispersion media showed the fastest initial sedimentation rate (22 × 10−8 m s−1 at 0 min), decreasing rapidly with time to ca. 2.2 × 10−8 m s−1 at 60 min, whereas for the dodecane dispersion media the sedimentation rate increased with standing time from ca. 0.14 × 10−8 m s−1 (0 min) to ca. 3.4 × 10−8 m s−1 (48 min). Aggregation and coalescence of the dispersed phase in a w/o emulsion are highly dependent on the sedimentation dynamics. Therefore, a stable template to synthesize hollow silica nanospheres of fairly uniform sizes was unlikely under the unstable emulsion conditions for dodecane, octane, benzene, and hexane dispersion media. However, a stable soft template for the synthesis of hollow silica nanospheres with an aqueous ammonia dispersed phase in a w/o cyclohexane emulsion was possible with stable and relatively small dispersed phase pools as the sedimentation rate was significantly slow. 3.2. Synthesis of the SiO2 hollow nanospheres in the w/o emulsions Despite the instability or stability of the surfactant-free w/o emulsions, we nonetheless proceeded to prepare the hollow silica nanospheres in the five dispersion media. Fig. 3 illustrates the TEM images of the hollow silica nanoparticles prepared in the surfactant-free w/o emulsions of (a) cyclohexane, (b) dodecane, (c) benzene, (d) octane, and (e) hexane as the dispersion media. Hollow silica nanospheres of reasonably uniform size (ca. 100 ± 20 nm) with a skin ca. 14 nm thick were clearly formed in the cyclohexane
Sedimentation rate (10-8 m s-1)
10 hexane 8 6
dodecane
octane
4 2
benzene
0
cyclohexane 0
10
20
30
40
50
60
Standing time (min) Fig. 2. Calculated sedimentation rates (Eq. (1)) of the aqueous ammonia dispersed phase in w/o emulsions at various standing times with cyclohexane, dodecane, benzene, octane, and hexane acting as the dispersion media.
Cyclohexane
Dodecane
Benzene
Octane
Hexane
0.17 0.133 0.722
1.65 0.159 1.364
0.61 0.130 0.401
0.31 0.207 0.311
0.73 0.254 0.085
emulsion (Fig. 3a). With benzene as the dispersion medium the synthesis produced smaller silica nanoparticles but highly aggregated into a lump (Fig. 3c). Aggregation of silica nanospheres also occurred to some extent in dodecane, octane and hexane as the dispersion media (Fig. 3b, d, and e). In addition, when dodecane was the dispersion medium for the w/o emulsion the synthesis led to expanded hollow silica nanospheres producing a thin silica film. The hollow particles were synthesized twice on different days so as to verify and confirm by TEM techniques the reproducibility of the syntheses.
3.3. Optimization of the synthesis of hollow SiO2 nanospheres in the cyclohexane emulsion The amount of the aqueous ammonia dispersed phase added to the w/o cyclohexane emulsion was decreased from 1% v/v to 0.1% v/v (representing 0.025% v/v in the emulsion). The hollow silica nanospheres were synthesized under otherwise similar conditions as above, namely the TEOS was added after the emulsion had been at rest for 60 min. At this lower concentration of the dispersed phase formation of the hollow silica nanospheres was hardly noticeable as attested by the TEM image of Fig. 4a. In fact whatever the structure of the silica particles produced under such conditions, they were highly coalesced silica particles. Fig. 4b displays the TEM image of silica nanoparticles produced in the w/o cyclohexane emulsion in the absence of aqueous ammonia. Evidently, under near-neutral pH conditions slow hydrolysis [12] of the TEOS silica precursor yielded strongly coalesced and aggregated silica particles. Next we proceeded to repeat the synthesis of the silica nanoparticles with the 1% (v/v) of the aqueous ammonia dispersed phase by the addition of the TEOS precursor solution to the w/o emulsion at standing times of 0, 20, 30, 40 and 50 min. The TEM images displayed in Fig. 4c and d illustrate the resulting albeit aggregated hollow silica nanospheres produced on addition of the TEOS precursor at the 0 and 30 min standing time, respectively. Addition of TEOS immediately after the formation of the w/o cyclohexane emulsion generated significantly smaller (ca. 20–30 nm) hollow silica nanospheres. Hollow silica nanospheres produced in the 30min rested emulsion were somewhat larger (ca. 40–45 nm). Clearly, a standing time of 60 min after the formation of the w/o emulsion secures a stable emulsion, and leads to an optimal synthesis of hollow silica nanospheres that seem neither to coalesce nor to aggregate (see Fig. 3a). Using the latter standing time conditions, we also examined the hydrolysis rate of TEOS in the emulsion immediately (0 time) after the addition of this precursor and after 1 and 3 min. Fig. 4e illustrates the hollow silica nanoparticles formed after 1 min of hydrolysis and condensation time. TEM data showed hollow silica nanospheres with a considerably thicker skin (ca. 25–30 nm) in comparison with the thinner skin (ca. 14 nm) of the hollow nanospheres produced by allowing the hydrolysis/condensation process to occur for 5 min (compare with Figs. 3a and 4e). Syntheses of the hollow silica nanospheres were also carried under various other conditions of standing times, ratios of added TEOS precursor, and concentration of the aqueous ammonia dispersed phase in all the w/o emulsions examined. No visible improvements were evidenced on the formation of hollow silica nanospheres from those displayed in Fig. 3a.
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Fig. 3. TEM image of hollow silica nanoparticles prepared in w/o emulsion systems with the dispersion media being (a) cyclohexane, (b) dodecane, (c) benzene, (d) octane, and (e) hexane.
It was rather curious that the mean sizes of the aqueous dispersed phase particles obtained by the light scattering technique (Fig. 1) were far greater than the sizes of the hollow silica nanospheres produced. For example, in the cyclohexane w/o emulsion the dispersed phase particles were some 8–9 times greater than the sizes of the silica nanospheres. This suggests that the dispersed phase likely consisted of clusters of the aqueous phase droplets, which on addition of the tetraethoxy orthosilicate (TEOS)
precursor molecules to the emulsions and initiation of hydrolysis at the water/oil interface caused the clusters to breakup (Scheme 1). Overall then, formation of hollow silica nanospheres under our conditions likely implicated no less than six principal stages: (stage 1) TEOS precursor molecules interact with the aqueous dispersed phase clusters containing the base catalyst (OH− ) which initiates hydrolysis of the TEOS at the interface producing silanols and silicates [20]; (stage 2) formation of the latter species leads to the
Fig. 4. TEM images of hollow silica nanospheres in w/o emulsions with cyclohexane as the dispersion medium under various conditions: (a) with 0.1% v/v quantity of aqueous ammonia dispersed phase; (b) no aqueous ammonia dispersed phase present; (c) with the 1% (v/v) of the aqueous ammonia dispersed phase and on addition of TEOS to emulsion at 0 min; (d) with the 1% (v/v) of the aqueous ammonia dispersed phase and after 30 min standing time with TEOS present; (e) with the 1% (v/v) of the aqueous ammonia dispersed phase and formation of silica nanoparticles after 1 min hydrolysis of TEOS. For further details see text.
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Scheme 1. Cartoons illustrating the proposed stages in the formation of the hollow silica nanospheres in the water-in-oil emulsions at pH 11.
Fig. 5. TEM images of hollow silica nanospheres using the aqueous ammonia dispersed phase at pH (a) 6, (b) 7, (c) 8, (d) 9, and (e) pH 10 in w/o emulsions with cyclohexane as the dispersion medium.
breakup of the clusters; (stage 3) hydrolytic processes continue at the interface of the declustered particles, ultimately causing the silicate species to condense (stage 4) with concomitant hydrolytic depolymerization (stage 5), followed by interfacial self-assembly of the silicate species (stage 6), which eventually form the silica skin of the hollow nanospheres.
nanoparticles of ill-defined structure with relatively thin skin. Hollow silica nanospheres were better structurally defined when the synthesis was carried out at pHs 8 and 9 (see Fig. 5c and d), albeit the nanospheres tended to aggregate. By contrast, at pH 10 the synthesis produced well-defined hollow silica nanospheres (Fig. 5e) with a skin that was thinner (ca. 5 nm) than that of the nanospheres produced at pH 11 (ca. 14 nm; see Fig. 3a).
3.4. Influence of pH in the synthesis of hollow SiO2 nanospheres in w/o cyclohexane emulsions
4. Concluding remarks
The influence of pH in the hydrolysis of TEOS was reported by Song et al. [12] and Feng et al. [21] who showed that the growth rate of silica changes significantly with pH. That is, the rate of hydrolysis of the tetraethoxysilane (TEOS) increases with increase of pH of the aqueous dispersed phase, whereas the rate of condensation of the silicate species formed at the w/o interface decreases with increase in pH and the rate of hydrolytic depolymerization of the condensate appears to be rather independent of pH above pH 7 [12,20]. Accordingly, we also examined the influence of pH on the formation of the hollow SiO2 nanospheres under our conditions (60 min standing time, cyclohexane as the dispersion medium, and aqueous ammonia as the dispersed phase) at pHs 6, 7, 8, 9 and 10. Fig. 5a and 5b depicts the silica particles produced at pHs 6 and 7, respectively, which show some aggregated and hollow silica
This article has examined the relative stability of waterin-oil emulsions using various oily components and aqueous ammonia at pH 11 as the dispersed aqueous phase. The stability of these w/o emulsions decreased in the order cyclohexane > octane > benzene > hexane > dodecane. The size of the dispersed phase in the cyclohexane emulsion was fairly constant with time (ca. 0.97 m) and was the most stable emulsion, whereas for the least stable dodecane w/o emulsion the dispersed phase particles increased in size from ca. 1.5 m to 7.3 m after 48 min. Hollow silica nanospheres have been synthesized by the hydrolysis of tetraethoxysilane (TEOS) at the water/oil interface (pH 11) in these five surfactant-free water-in-oil emulsions that are reasonably stable even in the absence of stabilizers normally used in such soft template syntheses. Transmission electron
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microscopy revealed well-defined hollow silica nanospheres of relatively uniform sizes (100 ± 20 nm) in the cyclohexane emulsion. Optimal conditions for the synthesis of well-defined hollow silica nanospheres in the cyclohexane w/o emulsion proved to be the pH (either 10 or 11), the standing time of 60 min for the emulsion to stabilize, the ratio of dispersed phase to dispersion medium (1% v/v), and the concentration of the aqueous ammonia in the dispersed phase. At pHs 6 and 7, respectively, highly aggregated hollow silica nanoparticles of ill-defined structure formed with relatively thin skin, whereas well-structured silica nanospheres were obtained at pHs 8 and 9 although the nanospheres also tended to aggregate. By contrast, at pH 10 well-defined hollow silica nanospheres were produced with a silica skin 3 times thinner (5 nm) than that of nanospheres produced at pH 11 (ca. 14 nm). Acknowledgments Financial support to S.H. from the Japan Society for the Promotion of Science (JSPS) through a Grant-in-aid for young scientists (No. B-21750210) is gratefully appreciated. One of us (N.S.) thanks Prof. Albini and his group at the Universita di Pavia, Italy, for their continued kind hospitality during the many semesters spent in their laboratory since 2002. We are also grateful to the personnel of Otsuka Electrics Co. Ltd. for technical assistance. References [1] F. Caruso, Hollow capsule processing through colloidal templating and selfassembly, Chem. Eur. J. 6 (2000) 413–419. [2] Q. Zhao, Y. Xie, T. Dong, Z. Zhang, Oxidation-crystallization process of colloids: an effective approach for the morphology controllable synthesis of SnO2 hollow spheres and rod bundles, J. Phys. Chem. C 111 (2007) 11598–11603. [3] S.-W. Cao, Y.-J. Zhu, Iron oxide hollow spheres: microwave-hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties, Acta Mater. 57 (2009) 2154–2165.
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