Surfactant-Free Polyurethane Nanocapsules via Inverse Pickering

Article pubs.acs.org/Langmuir

Surfactant-Free Polyurethane Nanocapsules via Inverse Pickering Miniemulsion Alexander Schoth, Katharina Landfester, and Rafael Muñoz-Espí* Max Planck Institute for Polymer Research, Mainz 55128, Germany S Supporting Information *

ABSTRACT: We report on a surfactant-free synthesis of Pickering-stabilized submicrometer-sized capsules in inverse miniemulsion. Functionalized silica nanoparticles are able to stabilize water-in-cyclohexane miniemulsions to form stable polyurethane shells via interfacial polyaddition. The effect of the type of silica functionalization on the stabilizing properties is demonstrated by varying the hydrophobicity and, therefore, the contact angle between silica and the two liquid phases. Addition of small amounts of salt leads to a reduction of the capsule size and to a narrow size distribution. The impermeability of the formed capsule shell is proven by encapsulation of an organic fluorescent dye and release studies in aqueous environment. In addition, we show the possibility to encapsulate large amounts of inorganic salts without negative effects concerning the stability of the emulsion, which enables the application for phase-change materials.

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mechanically stable film, which is able to prevent droplet coalescence. A very important variable is the wetting ability of the stabilizing particles. The particles need a contact angle close to 90° to both liquids in order to favor the formation of a particle film at the liquid/liquid interface.3,4 Pickering stabilization is a well-established technique in emulsion and miniemulsion polymerization.5 Nanoparticles have been synthesized with a large number of differently functionalized inorganic stabilizers, such as clay,6−8 silica,9−13 or ceria.14 The synthesis of nanocapsule morphologies in direct miniemulsion has also been successfully achieved by different groups.10,13,15,16 In general, capsules surrounded by smaller particles are often referred to as colloidosomes. Among the inorganic nanoparticles used in the stabilization of such systems, silica is by far the most common one. Especially in direct systems, the chemical environment (like pH value and salt concentration) plays an important role, as it influences the surface properties and, therefore, the stabilizing ability of the inorganic particles.17−19 However, the synthesis of Pickering stabilized nanomaterials in water-in-oil systems is still a challenge. Up to now, only a few synthetic routes for nanoparticles have been successful.6,20,21 Pickering-stabilized capsule morphologies with a liquid hydrophilic core are, to the best of our knowledge, only known with diameters above 10 μm.22,23 Herein we demonstrate the synthesis of polyurethane nanocapsules that are stabilized solely by functionalized silica nanoparticles. We investigate the influence of the silica

INTRODUCTION The encapsulation of water-soluble substances is currently an ongoing challenge in colloid science. Miniemulsion polymerization offers the possibility to encapsulate a broad variety of hydrophilic substances in many different shell materials. These systems can be used for a wide range of applications, including drug delivery, self-healing materials, or contrast agents for biomedical applications. While in direct miniemulsions (oil-inwater) the capsule shells are built by phase separation and selfassembly, the method of choice in inverse systems (water-inoil) is the interfacial polymerization. A big challenge for stabilizing inverse miniemulsions is the choice of the surfactant. Mostly, block copolymers with a hydrophilic and a lipophilic block are used. The capsule shell is formed by the reaction of a water-soluble compound, such as diols, glycerol or sugars, with an oil-soluble reactant. This oil-soluble reactant is usually an isocyanate compound, which reacts with many polar groups, such as acids, amines or hydroxyl groups. As a consequence, the hydrophilic blocks of the surfactants offer many possibilities for side reactions, leading to a covalent fixation of the surfactant in the capsule shell. This effect is problematic in many aspects. The surface properties of the capsules become undefined and less controllable, what makes physical measurements difficult. Biological applications are also problematic, as many surfactants can cause cell damage. To avoid these disadvantages of surfactants, a synthetic approach for surfactant-free nanocapsules would be desirable. A possible alternative to surfactants is the stabilization of emulsions solely by nanoparticles. These systems are known as Pickering emulsions, named after Spencer U. Pickering.1,2 Here, the particles adsorb to the liquid/liquid interface and form a © XXXX American Chemical Society

Received: December 3, 2014 Revised: March 16, 2015

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DOI: 10.1021/acs.langmuir.5b00442 Langmuir XXXX, XXX, XXX−XXX

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Langmuir functionalization on the stability of the final miniemulsion and present strategies to reach a narrow size distribution. Additionally, we demonstrate the encapsulation of an organic dye and of large amounts of an inorganic salt, in order to introduce two possible fields of application.

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EXPERIMENTAL SECTION

Materials. The aqueous silica dispersion Ludox TMA (34 wt %, diameter of 22 nm), propyl trimethoxysilane (PTMS, 97%), octadecyl trimethoxysilane (ODTMS, 90%), sodium chloride (98%), 1,6hexanediol (99%), toluene-2,4-diisocyanate (TDI, 95%), sodium sulfate decahydrate (98%), and the fluorescence dye sulforhodamine 101 (SR101) were purchased from Sigma-Aldrich. Ethanol (p.a. grade), concentrated ammonia solution, and cyclohexane (p.a. grade) were purchased from Fisher Scientific, and sodium dodecyl sulfate (SDS, 98%) from Alfa Aesar. All chemicals were used without further purification. Characterization. Transmission electron microscopy (TEM) was carried out with a JEOL JEM-1400 electron microscope operating at an acceleration voltage of 120 kV. The samples were prepared by diluting the capsule dispersion in cyclohexane; then one droplet of the sample was placed on a 300 mesh carbon-coated copper grid and left to dry at room temperature. The size distributions of the capsules were determined by statistical treatment of transmission electron micrographs of at least 200 capsules per sample. Scanning electron microscopy (SEM) was conducted in a Leo Gemini 1530 field emission microscope, operated with an extractor voltage of 0.6 kV. Samples for SEM observation were prepared by drop-casting of diluted dispersions on silicon wafers. The amount of hydrophobization agent on the silica surface was determined by thermogravimetric analysis (TGA) using a MettlerToledo TGA/SDTA-851 thermobalance (40 to 1000 °C, heating rate of 10 K min-1). The thermal properties of the nanocapsules were investigated by differential scanning calorimetry (DSC) with a PerkinElmer DSC 823 instrument. The sample was heated up from −50 °C to 90 °C and again cooled down at a heat rate of 10 K min−1. This procedure was repeated four times. Fluorescence spectroscopy was measured in a Tecan M1000 Microplate Reader at an excitation wavelength of 585 nm. For sample preparation, 1 mL of the nanocapsule dispersion was dispersed in 5 mL of a 0.1 wt % solution of SDS in water. For evaporation of the cyclohexane and to enable the release of the dye, the dispersion was allowed to stir for 24 h. The nanocapsules were then removed in a centrifuge at 4000 rpm for 2 min. The supernatant solutions as well as the noncentrifuged dispersions were measured in the plate reader. Functionalization of Silica. For the functionalization of Ludox TMA silica particles, a modified method from that reported by Bourgéat-Lami et al.24 was applied, as described in a previous publication.25 A mixture of the Ludox TMA suspension (50 mL), ethanol (50 mL), and SDS (50 mg) was prepared. The pH of this dispersion was set to 9.5, using concentrated ammonia solution. After dropwise addition of the trimethoxysilane compound (PTMS or ODTMS, 0.02 mol), the dispersion was stirred for 24 h at room temperature to allow equilibration. Afterward, it was refluxed for 2 h. The modified particles were centrifuged at 4000 rpm for 30 min, washed several times with ethanol/water, and dried under vacuum. Preparation of Nanocapsules in Inverse Miniemulsion. The silica nanoparticles (450 mg) were dispersed in cyclohexane (9 mL). The aqueous phase consisting of 1,6-hexanediol (45 mg) and water (850 μL) was prepared separately. For some samples, varying amounts of sodium chloride, sodium sulfate, or SR101 were added. The two phases were combined, stirred, and sonicated in the ultrasound bath until the cyclohexane phase was clear. Ultrasonication took place for 180 s at 70% intensity with a pulse/pause interval of 30 and 20 s (Branson W 450 digital sonifier; 1/4″ tip). Afterward, a solution of TDI (90 mg) in cyclohexane (2 mL) was added dropwise under stirring.

Figure 1. Preparation of Pickering miniemulsions consisting of water, NaCl, silica particles, and cyclohexane. The silica particles are nonfunctionalized (left vial, blue), propyl-functionalized (central vial, red), and octadecyl-functionalized (right vial, green) Ludox TMA.

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RESULTS AND DISCUSSION The stabilization of emulsions with particles via the Pickering mechanism depends on the contact angles of the continuous phase, the dispersed phase, and the stabilizing particles. As a general rule, hydrophilic particles stabilize direct emulsions, while hydrophobic particles stabilize inverse emulsion.3 Therefore, the surface functionalization of the particles is crucial for this system, as it determines the hydrophobicity of the particle surface. We used three different types of silica: Ludox TMA particles without modification, functionalized with propyl B

DOI: 10.1021/acs.langmuir.5b00442 Langmuir XXXX, XXX, XXX−XXX

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Figure 2. Transmission (A, B) and scanning (C) electron micrographs of PU nanocapsules containing water (A) without and (B, C) with 2.5 wt % sodium chloride. Scale bars are 1 μm.

Figure 3. Size distribution of salt-free PU capsules compared to capsules containing 2.5 wt % NaCl.

Figure 5. Transmission (A) and scanning (B) electron micrographs of PU nanocapsules containing 5 wt % sodium sulfate. Scale bars are 5 μm.

Figure 4. Fluorescence intensities of the fluorescent dye SR101 in the nanocapsule dispersion and the supernatant solution after centrifugation.

of aggregates. After addition of the aqueous phase consisting of water and 2.5 wt % NaCl and stirring for 10 min, the hydrophilic, unfunctionalized particles clearly do not stabilize the emulsion. As the further steps do not lead to significant changes, this sample is neglected in the upcoming considerations. ODTMS-silica is able to stabilize the pre-emulsion, as can be seen in the second picture. PTMS-silica shows an unexpected behavior. After stirring, the organic phase becomes completely clear, while the aqueous part is turbid. Presumably, the silica has assembled around the water droplets without stabilizing the emulsion sufficiently. After ultrasonication, PTMS- and ODTMS-silica give stable miniemulsions. However, after 30 min and, more clearly, after 24 h, the ODTMSsilica precipitates and the emulsion is no longer stable. PTMSsilica gives a stable miniemulsion, which does not separate even after 24 h. The clear oil phase that evolves on top of the red vial can be explained by the high density difference between cyclohexane and the silica-loaded water droplets. In this case,

trimethoxysilane (PTMS), and with octadecyl trimethoxysilane (ODTMS) (see thermogravimetric analysis in Figure S1, Supporting Information). Figure 1 shows the preparation of a miniemulsion with the three types of silica as stabilizers. The left vial (marked in blue) contains unfunctionalized Ludox TMA, the silica in the central vial (marked in red) is functionalized with PTMS, while the right one (marked in green) contains ODTMS-functionalized silica. The first picture shows the silica particles in cyclohexane. While the nonfunctionalized particles are too hydophilic and, therefore, precipitate immediately, the functionalized particles can be dispersed in cyclohexane to a certain extent. ODTMS-silica in the green vial gives an opaque suspension, whereas the dispersion of PTMS-silica is turbid, indicating a larger amount C

DOI: 10.1021/acs.langmuir.5b00442 Langmuir XXXX, XXX, XXX−XXX

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Figure 6. Transmission electron micrograph and size distribution of PU nanocapsules containing 20 wt % sodium sulfate. Scale bar is 500 nm.

for the dye and did not release it within the regarded time frame. Consequently, our system is a possible candidate for the safe encapsulation of water-soluble compounds, such as dyes or drugs. Encapsulation of Hydrophilic Substances. Besides organic compounds, the presented system also offers the possibility to encapsulate inorganic salts. As already shown in Figure 2, the salts act as ultralipophobes and lead to a stabilization of the miniemulsion. Furthermore, salts with special properties can add functions to the nanocapsules. An example for functional salts are the phase change materials (PCMs). This class of materials shows a high enthalpy of fusion, which makes them appropriate for heat storage. The two main groups of materials are paraffins and salt hydrates.26,27 The encapsulation of PCMs in submicrometer-sized capsules can offer advantages regarding the homogeneity of distribution and the mechanical stability of the capsules in the carrier material. In our experiments, we encapsulated sodium sulfate, whose decahydrate melts at 32 °C.28 Figure 5 shows electron micrographs of the nanocapsules with 5 wt % sodium sulfate. The encapsulation was successful and the capsules show a homogeneous size distribution of 1050 ± 290 nm. For applications as PCM storage material, a higher amount of salt is necessary. We were able to increase the salt content up to 20 wt %, as can be seen in Figure 6. The salt is inside the capsule, and the capsule size stays in the same range with 860 ± 230 nm. Salt contents above 20 wt % were not possible due to the limited solubility in water. Another reason is that with a higher salt concentration the viscosity of the aqueous phase increases. This increase hinders droplet breakup, which could lead to larger droplets and a much broader droplet size distribution. DSC of the dried capsules is shown in Figure 7. In the first cycle, recrystallization to the decahydrate occurs. The melting enthalpy of −58 J g−1 stays constant in the following measurement cycles and shows a stable condition of the dried capsules. Considering the salt content of 20 wt %, this result fits perfectly the literature value28 of −254 J g−1 for the pure salt.

Figure 7. Thermal analyses of PU nanocapsules containing 20 wt % sodium sulfate.

shaking for a few seconds is already sufficient to regain a homogeneous emulsion. Capsule Formation. With a stable miniemulsion system, the synthesis of a capsule shell via the interfacial polymerization reaction of 1,6-hexanediol and toluene diisocyanate (TDI) is possible. The capsule morphology with a shell thickness of around 35 nm is shown in Figure 2A. The size distribution of the capsules, determined by statistical treatment of electron micrographs, with a diameter of 760 ± 430 nm can be seen in Figure 3. This is, to the best of our knowledge, the first Pickering-stabilized nanocapsule synthesis in inverse miniemulsion. The broad size distribution of the particles can be improved by the addition of an ultralipophobe. In this case, sodium chloride (2.5 wt %) was added, which led to a more homogeneous size distribution and an average size of 960 ± 320 nm, as seen in Figures 2B/C and 3 (for the exact composition of the capsules, see also Figure S2, Supporting Information). Encapsulation of Hydrophobic Substances. The permeability of the polyurethane shell is of great importance for potential applications. As a control compound, the watersoluble dye SR101 was encapsulated, and its release in an aqueous surfactant solution was studied by fluorescence spectroscopy. Therefore, the capsule dispersion was dispersed in a 0.1 wt % solution of SDS in water and stirred for 24 h to enable the release of the dye. These aqueous dispersions were sufficiently stable to perform the following sprectroscopical analyses. The fluorescence intensity of the supernatant solution after centrifugation is close to zero, as shown in Figure 4. This observation indicates that the nanocapsules were impermeable

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CONCLUSIONS In conclusion, we have demonstrated that the well-established Pickering stabilization mechanism can be extended to the synthesis of nanocapsules with a narrow size distribution in inverse miniemulsion. The wettability of inorganic particles can be tuned by changing the type of surface functionalization, making them capable to stabilize the emulsion. The synthesized capsules can be redispersed in aqueous surfactant solutions to D

DOI: 10.1021/acs.langmuir.5b00442 Langmuir XXXX, XXX, XXX−XXX

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make them suitable for a wide range of uses, from industrial to biomedical applications. The safe encapsulation of organic water-soluble compounds like the fluorescent dye sulforhodamine 101 is easily transferable to other dyes or drugs. With the encapsulation of inorganic salts, we demonstrated that also dissociated ions have no negative effect on the stability of the miniemulsion. As a consequence, our approach opens the door toward the surfactant-free encapsulation of any water-soluble substance in inverse miniemulsion.

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ASSOCIATED CONTENT

S Supporting Information *

Thermogravimetric analyses of the functionalized silica particles as well as of the different hybrid nanocapsules. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 (0) 6131 379 410. Fax: +49 (0) 6131 379 100. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Petra Räder for the DSC measurements. REFERENCES

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DOI: 10.1021/acs.langmuir.5b00442 Langmuir XXXX, XXX, XXX−XXX