Microencapsulation of Hydrophobic Liquids in Closed All-Silica

Article pubs.acs.org/Langmuir

Microencapsulation of Hydrophobic Liquids in Closed All-Silica Colloidosomes Yongliang Zhao, Yanqing Li, Dan E. Demco, Xiaomin Zhu,* and Martin Möller DWI − Leibniz Institute for Interactive Materials and Institute for Technical and Macromolecular Chemistry of RWTH Aachen University, Forckenbeckstraße 50, Aachen 52056, Germany ABSTRACT: A facile approach for the microencapsulation of hydrophobic liquids in closed solid all-silica colloidosomes is reported. The method is based on the formation of oil-in-water emulsions stabilized by silica nanoparticles and subsequent gluing of the particles at the water/oil interface by a silica precursor polymerhyperbranched polyethoxysiloxane. By this means different oils are successfully enclosed in hence formed all-silica colloidosomes with almost 100% efficiency. Via a systematical study it is demonstrated that this process is a delicate interplay between the emulsion stability, oil polarity, and sol−gel reaction kinetics. This approach allows fabricating microcapsules of hydrophobic liquid substances in a mechanically stable, chemically inert, and biocompatible matrix, silica, with high encapsulation capacity and a controlled release profile.

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INTRODUCTION Microencapsulation is a process, in which ultrasmall particles or droplets are surrounded by a coating in order to enhance material stability, reduce adverse or toxic effects, or extend material release for different applications in various fields of manufacturing.1−3 Organic polymers are the most common shell materials, and the microcapsules are usually formed by either physical methods such as coextrusion, spray-drying, coacervation, layer-by-layer assembly, or chemical processes like interfacial polymerization. Silica is another promising alternative for microencapsulation applications due to its chemical inertness, mechanical stability, biocompatibility, easy functionalization, and optical transparency.4 It can be used to encapsulate both aqueous and organic substances. Organic molecules are entrapped within the pores of silica matrix by adding them into the reaction mixtures during the sol−gel processes.5−7 The encapsulation capacity as well as efficiency of this approach is however pretty low, and it can hardly be employed for hydrophobic liquid substances. The combination of oil-in-water (o/w) emulsions and interfacial sol−gel technology is, however, well suitable to create a core−shell structure with the dopand molecules enclosed within a silica shell.7 Furthermore, because of the brittleness of silica, the incorporated compound can be released by breaking the thin shell under mechanical force. © 2014 American Chemical Society

To form a stable emulsion, a surfactant is usually required, which should normally be removed after the capsule formation. Besides classical surfactants, solid colloidal particles can also stabilize oil−water emulsions, which are often referred to as Pickering emulsions,8,9 due to their interfacial activity.10,11 In these emulsion systems the particle assemblies can be fixed at the interface to yield so-called colloidosomes with droplets encapsulated inside.12 Because of a facile control over the shell permeability by varying either the particle size or fusing degree, colloidosomes find a high potential in microencapsulation and controlled release applications. Strategies for gluing the interfacial particles involve electrostatic binding with polyelectrolytes,12−14 van der Waals interaction,12−14 sintering,12,15 gelation,16−18 cross-linking,19−23 polymerization,24 and polymer deposition.25 Recently, using water-in-oil (w/o) Pickering emulsions as templates, we succeeded in preparing new allsilica colloidosomes, where silica nanoparticles were linked by a silica precursor polymerhyperbranched polyethoxysiloxane (PEOS)at the water/oil interface.26 In contrast to most colloidosomes reported so far, these colloidosomes consist of almost pure silica and have a closed shell, which can be finetuned from a particle monolayer up to a bilayer bound with a Received: January 23, 2014 Revised: April 1, 2014 Published: April 1, 2014 4253

dx.doi.org/10.1021/la500311y | Langmuir 2014, 30, 4253−4261

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sandwiched thin silica film by increasing the weight ratio of silica nanoparticles to water (Rs/w).26 A laccase solution was encapsulated in these colloidosomes, and the encapsulated laccase exhibited catalytic activity and reusability that were controlled by Rs/w, i.e., by the shell structure.27 A question arises is whether this technique can be applied for o/w systems. So the aim of this work is to extend our approach to o/w Pickering emulsion systems in order to encapsulate hydrophobic liquids in the silica colloidosomes. The microencapsulation of organic liquids that include monomers, flavors, fragrances, pesticides, etc., is certainly of great scientific and industrial importance.1,2,28,29

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Table 1. Recipes for the Preparation of Silica Colloidosomes, Where the Weight of the Aqueous Solution of Silica Nanoparticles Is 10 g run

SiO2 (g)

PEOS (g)

1 2 3 4 5 6 7 8 9 10

0.12 0.06 0.18 0.12 0.12 0.12 0.12 0.12 0.12 0.12

0.20 0.20 0.20 0.10 0.30 0.20 0.20 0.20 0.20 0.20

11 12 13

0.12 0.12 0.12

0.20 0.20 0.20

14

0.12

0

EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS, GPR RECTAPUR, VWR), ammonia solution (28% EMSURE ACS, Reag. Ph. Eur. zur Analyze, VWR), absolute ethanol (EMSURE ACS, ISO, Reag. Ph. Eur. zur Analyze, VWR), toluene (AnalaR NORMAPUR ACS, ISO, Reag. Ph. Eur. zur Analyze, VWR), acetic anhydride (ACS reagent, ≥98.0%, Sigma-Aldrich), hexadecane (ReagentPlus, 99%, Sigma-Aldrich), hexyl acetate (99%, Aldrich), hexadecyltrimethoxysilane (90%, ABCR), and titanium trimethylsiloxide (ABCR) were used as received. Deionized water was used for all experiments. PEOS was synthesized according to the method published elsewhere.30 The resulting PEOS had the following characteristics: degree of branching 0.54, SiO2 content 49.2%, Mn 1740, and Mw/Mn 1.9 (measured by gel permeation chromatography in chloroform with evaporative light scattering detector calibrated using polystyrene standards). Preparation of Partially Hydrophobic Silica Nanoparticles. A 250 mL flask was charged with 5.7 mL of TEOS, 8.5 mL of ammonia aqueous solution, and 171 mL of absolute ethanol. The mixture was stirred on a magnetic stirrer at room temperature for 24 h. Afterward, 25 μL of hexadecyltrimethoxysilane was added in order to modify the surface of the resulting silica particles. The reaction mixture was stirred for an additional 48 h. The particles were separated by centrifugation at 11 000 rpm for 30 min on an Eppendorf centrifuge 5810 and then rinsed with water for three times. Finally, the particles were redispersed in water to yield a homogeneous silica aqueous dispersion. Formation of All-Silica Colloidosomes. 10 g of silica aqueous dispersion was mixed with 1.0 g of PEOS solution in different oils. The mixture was emulsified using ultrasonic irradiation for 15 min (Branson Sonifier 450 cell disrupter, 3 mm microtip, 0.9 time circle, 247 W output). The resulting o/w Pickering emulsion was gently stirred at room temperature for 1 day. The colloidosomes were isolated by centrifugation, rinsed 3 times with water, and then redispersed in water. The recipes for the colloidosome preparation are summarized in Table 1. Dynamic Light Scattering (DLS) Measurements. Hydrodynamic diameter of silica nanoparticles in water of pH 9 was measured with a Malvern Zetasizer Nano Series at a scattering angle of 147° at 25 °C. Before the measurements, the stock dispersions were diluted to a silica concentration of 1.5 wt ‰. Fluorescence Microscopy. Fluorescence microscopy was performed on a Zeiss Axioplan 2 microscope equipped with XBO 75 illuminating system (xenon lamp) and using a filter λex ≥ 470 nm and λem ≥ 500 nm. A fluorescence dye Nile red was dissolved in the oil phase. Field-Emission−Scanning Electron Microscopy (FE-SEM). FESEM measurements were performed on a Hitachi S4800 highresolution field emission scanning electron microscope with an accelerating voltage of 1.5 kV. Before measurements, the colloidosome dispersion was diluted to a desired concentration for spin-coating on the silicon wafer and air-dried under ambient conditions. Transmission Electron Microscopy (TEM). TEM measurements were carried out on a Zeiss Libra 120 transmission electron microscope. The accelerating voltage was set at 120 kV. The samples were prepared by placing a drop of the diluted colloidosome dispersion on a Formvar-carbon-coated copper grid with 200 meshes.

oil phase (g)

pH

toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.80 toluene 0.40 + hexadecane 0.40 hexadecane 0.80 hexyl acetate 0.80 hexyl acetate 0.40 + hexadecane 0.40 toluene 0.80

9.0 9.0 9.0 9.0 9.0 3.0 7.0 8.0 10.0 9.0 9.0 9.0 9.0

mean diameter of colloidosomesa (μm) 2.17 ± 0.35 1.91 ± 0.29 1.46 ± 0.58

2.23 ± 0.45 2.31 ± 0.37 0.96 ± 0.16

9.0

a

The mean diameter of colloidosomes was obtained by averaging diameter of 1000 colloidosomes estimated from scanning electron microscopy images.

Thermogravimetric Analysis (TGA). TGA measurements were conducted on a PerkinElmer STA 6000 unit operating under a nitrogen atmosphere with a flow rate of 20 mL min−1. The colloidosomes were isolated from the aqueous dispersions by centrifugation and air-dried overnight. 5−10 mg of the dried sample was then placed in a standard PerkinElmer alumina 85 μL crucible for the measurements. Fourier-Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra were recorded on a Nicolet 60 SXR FT-IR spectrometer using the KBr pellet technique. The colloidosome samples were dried at 45 °C overnight before measurements. 1 H NMR Spectroscopy. Solid-state 1H NMR spectra of free hexadecane and hexadecane encapsulated in all-silica colloidosomes were measured at 700.239 MHz on an AV700 Bruker NMR spectrometer. A CPMAS probe head was used for nonrotating samples at the temperature of 23 °C. For all measurements, the recycle delay was 7 s, the radio frequency pulse length was 1.9 μs, dwell time was 2 μs, and the number of scans was 4096. The dwell time of 2 μs, a dead time of 10 μs, and a receiver gain RG = 2 were used. The spectra were referenced ex situ to tetramethylsilane. The line widths of the spectral peaks were obtained by spectral decomposition using the DMFIT program.

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RESULTS AND DISCUSSION Our approach to encapsulate an organic liquid in silica colloidosomes using an o/w Pickering emulsion as a template is illustrated in Scheme 1. The interfacial activity of colloid particles is due to the decrease of the total free energy (ΔG) by placing them at the water/oil interface.31 For a single particle with an effective radius r, ΔG can be calculated using eq 1.32 ΔG = −πr 2γow(1 − |cos θow|)2

(1)

where γow is the interfacial tension arising from the oil/water interface and θow is the three-phase contact angle between the solid and the oil/water interface (Scheme 2). From eq 1 one can see that ΔG reaches the maximum, when the three-phase contact angle θow is 90°. The stabilization of different types of emulsions requires different surface wettability to adapt to differently curved oil/water inter4254


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Scheme 1. Schematic Illustration of the Formation Process of Silica Colloidosomes Templated by o/w Pickering Emulsions and the Chemical Structure of Hyperbranched Polyethoxysiloxane (PEOS)

Scheme 2. A Solid Spherical Particle with Contact Angle θow in Equilibrium at the Oil/Water Interface

faces.10,32,33 If θow is lower than 90°, o/w emulsions are more preferred. Meanwhile, particles with θow greater than 90° tend to stabilize w/o emulsions. In our previous work, the silica particles were modified with octadecyltrimethoxysilane in order to stabilize w/o Pickering emulsions, and in that case the modified silica particles were dispersible in toluene.26 In this work silica nanoparticles of 53.5 ± 3.9 nm were prepared by the Stöber method,34 and the surface of these particles was substituted with alkyl groups using a smaller amount of hexadecyltrimethoxysilane. The modification degree was delicately adjusted so that the particles could still be well dispersed in aqueous media and could effectively stabilize the o/w emulsions. The size distribution of the silica nanoparticles before and after hydrophobic modification measured by means of DLS technique is presented in Figure 1. The mean hydrodynamic diameter of the nonmodified silica particles was determined to be 100 nm that is nearly twice that measured by TEM (not shown). This deviation is possibly due to the formation of a hydration layer on the particle surface. After modification the particle size increases to 120 nm, implying no aggregation of the hydrophobized particles. As depicted in Scheme 1, for oil encapsulation in silica colloidosomes PEOS is dissolved in the oil phase and the

Figure 1. Hydrodynamic diameter distribution of silica nanoparticles with and without modification with hexadecyltrimethoxysilane in water of pH 9.

resulting hydrophobic liquid is emulsified in water containing the hexadecyl-substituted silica nanoparticles to form an o/w emulsion. The emulsion is then gently stirred at room temperature for 1 day. The colloidosomes formed in this process can be isolated by centrifugation and redispersed in water. The influence of silica concentration in water, pH value of the aqueous phase, PEOS content, and oil phase was systematically investigated in this work (Table 1). When toluene was used as the oil phase, we succeeded in preparing silica colloidosomes with the following recipe (run 1 in Table 1). 0.2 g of PEOS was dissolved in 0.8 g of toluene, and this solution was emulsified in 10 g of water (pH 9) with 0.12 g of silica nanoparticles dispersed inside. The fluorescence optical micrograph in Figure 2 depicts water-dispersed colloidosomes by dissolving a fluorescence dye Nile red in toluene, and the capsular structure is clearly confirmed. According to the FE-SEM and TEM data shown in Figure 3, most of the colloidosomes are perfectly spherical with an 4255


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confirmed by the FE-SEM image of an “opened” colloidosome (Figure 3d). In Figure 3d the detailed structure of the colloidosome shell is revealed. The particles are observed on both outer and inner surfaces of the colloidosome shell, so it seems that they are glued together with PEOS in a particle monolayer. It is unlike the silica colloidosomes templated by w/o Pickering emulsions, where a continuous silica layer either glues a particles monolayer from the inner surface or is sandwiched between two particle layers. When the colloidosome shell comprises a monolayer of particles, its radius can be estimated using eq 2.24 ⎛ W ⎞⎛ ρp ⎞ R o = Coverage ·π ⎜⎜ o ⎟⎟⎜⎜ ⎟⎟R p ⎝ Wp ⎠⎝ ρo ⎠

(2)

where Coverage is the coverage of the area of an oil droplet by particles; ρp and ρo are the density of particles and oil, respectively; Rp and Ro are the radius of a particle and an oil droplet; and Wp and Wo are the weight of particles and oil. Assuming the area of the oil droplet is fully covered by particles (Coverage = 1) and ρp is 2.2 g/cm3 for amorphous silica, the radius of a colloidosome particle encapsulating toluene is calculated to be 1.3 μm, which is in good agreement with the experimental data. By comparing the FT-IR spectra of the prepared colloidosome in dry state with PEOS and the initial silica nanoparticles (Figure 4), it is clear that PEOS is completely converted and the colloidosomes consist almost entirely of silica.

Figure 2. Fluorescence micrograph of an aqueous dispersion of colloidosomes with an encapsulated toluene solution of a fluorescence dye Nile red (Table 1, run 1).

average diameter of 2.17 μm (Figure 3a). The shell consists of closely packed silica nanoparticles (Figure 3b,c), and no holes are observed between the particles (Figure 3d). Importantly, the colloidosomes in the electron images can keep the spherical morphology without collapse under high vacuum and high voltage electron irradiation, indicating their outstanding mechanical strength. Further, the capsular structure is clearly

Figure 3. (a, b) FE-SEM and (c) TEM images of typical colloidosomes prepared with the optimal recipe (cf. Table 1, run 1); (d) FE-SEM image of an “opened” colloidosome. 4256


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decreased to 1.91 ± 0.29 μm (Figure 5b) as predicted by eq 2. It should be mentioned that the shell structure (not shown) was not affected by the silica amount; i.e., only the particle monolayer was observed. With the further increase of the silica particle content the colloidosome size could not be further reduced. PEOS is another crucial component for the formation of silica colloidosomes, and its amount is also expected to have an influence on the colloidosome formation. Without PEOS (run 14 in Table 1) the Pickering emulsion can be stable for approximately 3 days. Since the nanoparticles were not chemically linked together, upon drying the emulsion droplets collapsed and only separated silica nanoparticles were observed by SEM. We found that at a lower PEOS concentration (0.1 g in 0.8 g of toluene; cf. run 4 in Table 1), as shown in Figure 5c, most colloidosomes had an “opened mouth”. Apparently, the amount of PEOS was too low to link all interfacial silica particles together. With a higher PEOS concentration (0.3 g in 0.8 g of toluene, cf. run 5 in Table 1), the gluing silica layer became thicker, as judged by the embedded depth of the silica particles into this layer from the inner surface (Figure 5d), and the colloidosome size was decreased due to the reduced amount of the oil phase. Instead of PEOS the monomeric silica precursor, TEOS, was also tried, but no colloidosomes were formed. The reason might be the relatively high water solubility and bad film formation properties of the hydrolyzed TEOS. As we have shown for the w/o Pickering emulsions,26 the interfacial sol−gel reaction of PEOS, which was controlled by the pH value, played a crucial role in the formation of silica colloidosomes. Colloidosomes with a well-defined shell structure were obtained in the pH range of 1−4, where liquid hydrolyzed PEOS formed at the w/o interface. However, in the case of o/w emulsions, the silica colloidosomes were obtained only at pH 9. At acidic pH the Pickering emulsion was not stable due to the low surface charge density of the silica particles. Therefore, no formation of complete colloidosomes

Figure 4. FT-IR spectra of PEOS, silica nanoparticles, and dried silica colloidosomes (Table 1, run 1).

In the case of the silica colloidosomes prepared in the w/o Pickering emulsions, the structure of the colloidosome shell can be fine-tuned from a particle monolayer up to a bilayer by varying the Rs/w value. In this work we also varied the silica nanoparticle amount by keeping the oil, water, and PEOS content constant. When the silica amount was reduced to 0.06 g, a stable emulsion could still be formed. However, toluene could not be efficiently encapsulated, and it was separated from the aqueous phase after 1 day of stirring. FE-SEM images show the formation of broken particle fragments rather than complete capsules (Figure 5a). We ascribe this to the loose coverage of particles on the o/w interface that could not serve as a strong scaffold for the capsule formation. Upon increasing the amount of silica particles to 0.18 g, the silica colloidosomes were obtained as well and the colloidosome size was slightly

Figure 5. FE-SEM images of colloidosomes prepared with different contents of silica nanoparticles: (a) 0.06 g and (b) 0.18 g (cf. Table 1, runs 2 and 3) and different PEOS amounts: (c) 0.1 g and (b) 0.3 g (cf. Table 1, runs 4 and 5). The scale bars represent 2 μm. 4257


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Figure 6. FE-SEM images of colloidosomes formed at different pH values: (a) 3, (b) 7, (c) 8, and (d) 10 (cf. Table 1, runs 6−9). The scale bars represent 1 μm.

Figure 7. FE-SEM images of colloidosomes prepared with different liquid cores: (a) toluene and hexadecane, (b) hexadecane, (c) hexyl acetate, (d) hexyl acetate and hexadecane (cf. Table 1, runs 10−13). The scale bars represent 2 μm.

were shown to have low affinity toward the gluing silica layer (Figure 6d), most probably because of the repulsion force caused by too high surface charge. pH 9 seems to be the optimal pH value for the oil encapsulation in the silica colloidosomes, where PEOS can act as an effective glue for silica particles due to the high stability of the Pickering emulsions, proper hydrolysis/condensation rate of PEOS, and good affinity between PEOS and the silica particles. So far we demonstrated the successful microencapsulation of toluene in the silica colloidosomes; it is therefore questionable

was observed (Figure 6a). At neutral pH the oil phase could still not be encapsulated. The FE-SEM images showed that the colloidosomes were collapsed due to the low mechanical strength of the glued particle layer (Figures 6b,c). Under neutral pH conditions the Pickering emulsions were stable for about 2 days after ultrasonic emulsification and the sol−gel reaction was known to be pretty slow, so probably the emulsions were broken before the mechanically strong silica layer was formed from PEOS. At too high pH, silica colloidosomes could not be prepared, either. The particles 4258


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whether this approach can be employed for other organic liquids. We tried to encapsulate hydrophobic liquids of different polarity (Table 1, runs 10−13). By adding hexadecane to toluene, merging of two colloidosomes was observed (Figure 7a), which was clearly induced by coalescence of two emulsion droplets. When toluene was completely replaced by hexadecane, colloidosomes of bigger size were obtained (Figure 7b). Hexadecane has a lower density than toluene, so according to eq 2 the radius of the resulting oil droplets should be bigger. Furthermore, due to the lower polarity of hexadecane as compared with toluene, the three phase contact angle θow should be lower, leading to a smaller curvature and bigger oil droplets.10 Hexyl acetate is used as a flavoring because of its fruity odor. Unfortunately, pure hexyl acetate could not be encapsulated in the silica colloidosomes under the conditions established for toluene (Figure 7c). Its mixture with hexadecane, however, was successfully enclosed into the silica colloidosomes (Figure 7d), and the capsule size was much smaller than that with toluene and hexadecane. According to eq 1, the drop of free energy by placing a particle at the interface becomes less significant with the decrease of the oil/water interfacial tension γow, so the Pickering emulsion becomes less stable when γow is lower. Because hexyl acetate is more polar than toluene, the hexyl acetate/water interfacial tension is lower than that arising from the toluene/water interface, thus resulting in a less stable emulsion. By adding hexadecane to hexyl acetate the Pickering emulsion becomes more stable due to the increased interfacial tension. Since the hexadecane/hexyl acetate mixture has a higher polarity than toluene, a higher θow and thus smaller oil droplets are observed. Another approach to stabilize a polar oil-in-water emulsion is to use more hydrophobic particles.10 From the presented data one can see that the formation of silica colloidosomes templated by o/w Pickering emulsions is a delicate interplay between the emulsion stability, oil polarity, and sol−gel reaction kinetics. TGA was used to study the encapsulation capacity of the silica colloidosomes and the release of the encapsulated liquids under isothermal conditions. As an example we show the data obtained from the colloidosomes prepared with hexadecane (Table 1, run 11). The colloidosomes were isolated from the aqueous dispersion by centrifugation and air-dried overnight at room temperature. Figure 8 shows the isothermal weight loss of the colloidosomes at different temperatures in comparison with free hexadecane. Importantly, the total weight loss upon heating is close to the amount of hexadecane used in the preparation of the colloidosomes, indicating almost 100% encapsulation efficiency. The isothermal release of hexadecane encapsulated in the silica colloidosomes occurs almost linearly, i.e. in a one-step process. In contrast, the organic molecules entrapped in sol−gel xerogels and coatings were found to release in a two-step process: low activated evaporation from open pores at the surface and a much slower process from the internal porosity.6 According to the TGA data, the substance enclosed in the colloidosomes has a slower release rate than the free one (Figure 8). The activation energy of evaporation of hexadecane encapsulated in the colloidosomes is calculated using the Arrhenius equation to be 66.7 kJ/mol, which is higher than that of free hexadecane (59.8 kJ/mol), thus highlighting the retardation of the oil release. Figure 9 compares FT-IR spectra of the colloidosome sample before and after evaporation of hexadecane, which differ solely in characteristic bands of hexadecane e.g. at 2957, 2924, 2853, and 1467 cm−1,

Figure 8. Isothermal evaporation of free hexadecane and hexadecane encapsulated in silica colloidosomes (cf. Table 1, run 11) at different temperatures.

Figure 9. FT-IR spectra of air-dried silica colloidosomes prepared with hexadecane before (down) and after (up) evaporation of hexadecane (cf. Table 1, run 11).

indicating clearly that hexadecane is indeed encapsulated in the silica colloidosomes and is released upon heating. The 1H NMR spectrum of free hexadecane (Figure 10) shows two peaks corresponding to CH3 (δ = 0.95 ppm) and CH2 (δ = 1.35 ppm) protons. The line width at the half-height of the CH2 protons obtained by the spectral decomposition is 77 Hz. An intensive peak at δ = 1.25 ppm is detected on the 1H NMR spectrum of hexadecane encapsulated in the silica colloidosomes, and its line width at the half-height is much higher, close to 1500 Hz. Besides, a very broad peak is also present, and it can be attributed to the OH groups and water molecules on the silica surface. The significant broadening of the alkyl proton peak in this case is most probably related to the presence of large magnetic susceptibility heterogeneity inside the silica colloidosomes, providing a clear evidence for the encapsulation of liquid hexadecane. The FE-SEM images of the residue after the evaporation of hexadecane (not shown) reveal no change in the morphology, implying that the organic liquids are also released from colloidosomes through the nanopores of 4259


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AUTHOR INFORMATION

Corresponding Author

*Tel +49-241-8023341; fax +49-241-8023301; e-mail zhu@ dwi.rwth-aachen.de (X.Z.). Present Address

Y.L.: Institut für Anorganische Chemie der RWTH Aachen, Landoltweg 1, 52074 Aachen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was performed within the Interreg Euregio MeuseRhine IV-A project “BioMiMedics” (2011−2014) financially supported by European Union and the government of North Rhine-Westphalia (Germany). The authors thank Dr. Walter Tillmann for performing the FT-IR measurements.

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ABBREVIATIONS

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REFERENCES

PEOS, hyperbranched polyethoxysiloxane; TEOS, tetraethoxysilane; DLS, dynamic light scattering; FE-SEM, fieldemission−scanning electron microscopy; TEM, transmission electron microscopy; TGA, thermogravimetric analysis; FT-IR, Fourier-transform infrared.

(1) Benita, S. Microencapsulation: Methods and Industrial Applications, 2nd ed.; Taylor & Francis: New York, 2006. (2) Ghosh, S. K. Functional Coatings: By Polymer Microencapsulation; Wiley-VCH: Weinheim, 2006. (3) Lakkis, J. M. Encapsulation and Controlled Release Technologies in Food Systems, 1st ed.; Blackwell: Ames, 2007. (4) Pagliaro, M. Silica-Based Materials for Advanced Chemical Applications; RSC: Cambridge, 2009. (5) Avnir, D. Organic Chemistry within Ceramic Matrices: Doped Sol-Gel Materials. Acc. Chem. Res. 1995, 28, 328−334. (6) Böttcher, H.; Kallies, K. H.; Haufe, H.; Seidel, J. Silica Sol-Gel Glasses with Embedded Organic Liquids. Adv. Mater. 1999, 11, 138− 141. (7) Ciriminna, R.; Sciortino, M.; Alonzo, G.; de Schrijver, A.; Pagliaro, M. From Molecules to Systems: Sol-Gel Microencapsulation in Silica-Based Materials. Chem. Rev. 2011, 111, 765−789. (8) Ramsden, W. Separation of Solids in the Surface-Layers of Solutions and ‘Suspensions’ (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation). Prelim. Acc. Proc. R. Soc. London 1903, 72, 156−164. (9) Pickering, S. U. CXCVI.−Emulsions. J. Chem. Soc., Trans. 1907, 91, 2001−2021. (10) Binks, B. P. Particles as Surfactants: Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (11) Binks, B. P.; Horozov, T. S. Colloidal Particles at Liquid Interfaces; Cambridge University Press: Cambridge, UK, 2006. (12) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles. Science 2002, 298, 1006− 1009. (13) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of Latex Particles by Using Emulsion Droplets as Templates. 1. Microstructured Hollow Spheres. Langmuir 1996, 12, 2374−2384. (14) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of Latex Particles by Using Emulsion Droplets as Templates. 2. Ball-like and Composite Aggregates. Langmuir 1996, 12, 2385−2391. (15) Yow, H. N.; Routh, A. F. Release Profiles of Encapsulated Actives from Colloidosomes Sintered for Various Durations. Langmuir 2009, 25, 159−166.

Figure 10. Solid-state 1H NMR spectra of free hexadecane (a) and hexadecane encapsulated in silica colloidosomes (b).

the silica, similar as for the silica colloidosomes enclosing water.26

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CONCLUSIONS

In this work we have presented a simple approach for microencapsulation of organic liquids in solid silica. The microcapsules were fabricated using o/w Pickering emulsions as templates in combination with a hydrophobic liquid silica precursor polymer PEOS as an interfacial binder for silica particles. The influence of reaction parameters including concentration of silica particles and PEOS, pH value of the aqueous phase, and the polarity of the oil on the capsule formation was systematically studied. The encapsulation efficiency of this method was very high and reached almost 100%. It was demonstrated that the encapsulation of oils in silica colloidosomes reduced their evaporation rate and increased the activation energy of the evaporation process. Furthermore, due to brittleness of the shell, the oil-loaded silica colloidosomes can be considered as promising controlled release systems, where the substances are released under mechanical force. This technique opens a new gateway for the design of functional inorganic microcapsules and microencapsulation of hydrophobic substances. 4260


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Microencapsulation of Hydrophobic Liquids in Closed All-Silica

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