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Thermosensitive ZrP-PNIPAM Pickering Emulsifier and the Controlled-release Behavior Xuezhen Wang, Minxiang Zeng, Yi-Hsien Yu, Huiliang Wang, M. Sam Mannan, and Zhengdong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16690 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 15, 2017

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ACS Applied Materials & Interfaces

Thermosensitive ZrP-PNIPAM Pickering Emulsifier and the Controlled-release Behavior Xuezhen Wangabc, Minxiang Zenga, Yi-Hsien Yud, Huiliang Wange, M. Sam Mannanabd, Zhengdong Cheng*abd a Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 778433122, USA. b Mary Kay O'Connor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA. c Soft matter center, Guangdong Province Key Laboratory on Functional Soft Condensed Matter, School of materials and energy, Guangdong University of Technology, Guangzhou, 510006, China d Department of Materials Science and Engineering, Texas A&M University, College Station, TX 778433003, USA e College of Chemistry, Beijing Normal University, Beijing, 100875, China Abstract: Asymmetric Janus and Gemini ZrP-PNIPAM monolayer nanoplates were obtained by exfoliation of two-dimensional layered ZrP disks whose surface was covalently modified with thermosensitive polymer PNIPAM. The nanoplates largely reduced interfacial tension (IFT) of the oil/water interface so that they were able to produce stable oil/water emulsions, and the PNIPAM grafting either on the surface or the edge endowed the nanoplates rapid temperature response. The ZrP-PNIPAM nanoplates proved to be thermosensitive Pickering emulsifiers for controlled-release applications. Keywords: Pickering emulsifier, Thermosensitive, Asymmetric nanoplates, ZrP, PNIPAM, Controlledrelease INTRODUCTION Unlike the emulsions stabilized by molecular surfactants, Pickering emulsions stabilized by particles were usually more stable when exposed to “harsh” conditions such as high salt and high temperature. 1-2 For any Pickering stabilizer with intermediate contact angle value, it was widely believed that energy greater than kBT (kB, Boltzmann constant; T, temperature) was required to detach the particles from the oil/water interface, quantitatively depending on the particle size. The larger the particle, the more energy to detach particles from the oil/water interface. The greater energy requirement would contribute to the appealing stability of Pickering emulsions. 2 Asymmetric particles with both hydrophobic and hydrophilic properties on a single particle have proved to be better Pickering emulsifiers due to the high materialutilization efficiency and more surfactant-like behaviors compared with symmetric particles. 3-4 Janus and Gemini nanoplates are two types of asymmetric nanoplates being developed for emulsification. 5-6 By definition, Janus nanoplates are characterized by different surface modifications on either facial side, while Gemini nanoplates have different physical properties at the edge compared to facial sides. 5 In other words, the two unique physical properties— hydrophilicity and hydrophobicity—exist in one single particle. Other surface functionalizations developed on the asymmetric particles include magnetic, catalytic, optical, electrical, or responsive (temperature or pH). 4, 7 1 ACS Paragon Plus Environment


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Layered alpha-zirconium phosphate (ZrP hereafter) disks, with chemical formula of Zr(HPO4)2·H2O, have lots of hydroxide groups on the surface. Layered ZrP disks can be easily exfoliated into monolayer ZrP nanoplates using interactional amines, such as propyl amine, tetrabutylammonium, amino alcohols or long chain amine. The exfoliated ZrP nanoplates are very stable in water, which enable them to be good material for discotic liquid crystal study8-9. After surface modification with hydrophobic alkane, ZrP disks were proved to be useful in stabilization of Pickering emulsions 10 and Pickering encapsulations,11 especially after being exfoliated into monolayers. Poly (N-isopropylacrylamide) (PNIPAM) is a temperature-responsive material with a lower critical solution temperature (LCST) of about 32 °C in water; it has been investigated for possible applications in tissue engineering and drug delivery.12 PNIPAM gels showed useful applications in the controlled-release areas due to its thermo-responsibility12-14. The thermos-responsibility was caused by the swelling and shrinking of polymer chains at temperatures below and above LCST. The ability of decreasing air-water and oil-water interface tension of PNIPAM have been studied although it didn’t show thermo-responsive property at air-water interface. 15-18 Pure PNIPAM microgel,19-21particles of PNIPAM with copolymers,22 or organic particle with PNIPAM grafting have been used as Pickering emulsifiers to stabilize thermosensitive water/oil emulsions.21-23 Until now, only limited research in inorganic-based thermosensitive Pickering emulsifiers has been reported.18, 23-24 Various studies were focused on modification of silica particle with PNIPAM. Usually, hydrophobic silica was treated with PNIPAM to become the Pickering emulsifier. 25 Covalently bonding between the inorganic particle and PNIPAM was reported less often in the literature. Yang’s group prepared a Janus polymer (PNIPAM)/inorganic (silica) composite nanosheets as Pickering emulsifier to stabilize thermosensitive O/W emulsions with a high polydispersity. 26 Disk-like thermosensitive Pickering emulsifiers, however, have rarely been reported. Here we demonstrate ZrP nanoplates whose surface was partially grafted with PNIPAM as a thermosensitive Pickering emulsifier. Layered ZrP disks were first modified with PNIPAM using a preirradiated polymerization27 technique and then exfoliated into monolayers using tetrabutylammonium. ZrP-PNIPAM monolayer nanoplates obtained after exfoliation were then used to stabilize oil/water emulsions and to fabricate polystyrene (PS) particles as Pickering emulsifiers. The controlled-release behavior was studied using a model material, hydrophobic molecular liquid crystal, which was emulsified by ZrP-PNIPAM nanoplates to form oil in water droplets. By controlling temperature, the release of the droplets was observed under a crossed polarized microscope. This proved that asymmetric thermosensitive ZrP-PNIPAM nanoplates served well in the controlled-release areas as a Pickering emulsifier. RESULTS AND DISCUSSIONS Layered ZrP disks with diameters around were synthesized using a typical hydrothermal method with 3 M phosphorus acid together with zirconium (IV) oxychloride octahydrate and reacted for 5 hours at 200 C.28 Surface modification of layered ZrP with PNIPAM was achieved via gamma ray pre-irradiation procedure, as we reported in previous paper.27 As indicated in Figure 1, the pristine ZrP and ZrP-PNIPAM were different from each other obviously. Individual disk could be identified from the SEM image of pristine ZrP sample based on their clear edges. The modified layered ZrP-PNIPAM showed much smoother surface which was due to the polymer on the surface.

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Figure 1. Pristine ZrP (a) and the layered ZrP-PNIPAM (b) after surface modification. Layered ZrP-PNIPAM was exfoliated into Janus and Gemini monolayers using an intercalation agent, tetrabutylammonium hydroxide (TBA+OH-). As shown in Schematic 1, for each layered ZrP-PNIPAM, two Janus nanoplates and several Gemini nanoplates, depending on the number of layers, were obtained after exfoliation. The tetrabutylammonium ion (TBA +), as well as PNIPAM, was attached to the surface of ZrP via electrostatic interaction due to the hydroxide (−OH) groups between the ZrP layers, enabling layered ZrP-PNIPAM to be exfoliated. The layered ZrP-PNIPAM suspension was white, and it became transparent right after adding TBA+OH- solution. The pictures showed different color of ZrP-PNIPAM suspension before and after exfoliation (Figure S1). It was obvious that the layered ZrP-PNIPAM had been successfully exfoliated by observing the color change. Since it was not challenging for TBA + to intercalate into the ZrP disks, we believed the ZrP-PNIPAM had been exfoliated into monolayers. TEM proved that ZrP-PNIPAM nanoplates remained disk shape after exfoliation (Figure S2). The thickness of the ZrP layers used here was about 10 nm;28 about 15 nanoplate monolayers were produced from each layered ZrP, since the monolayer ZrP was 0.68 nm. 29 Hence, the ZrP-PNIPAM suspension contained about 13% Janus and 87% Gemini nanoplates after exfoliation. As we discussed in previous paper, the thickness of PNIPAM layer was 118 nm. The intercalation agent TBA + thickness was about 1 nm,29 which was much smaller than the PNIPAM chains.

Schematic 1. Exfoliation of layered ZrP-PNIPAM into Janus and Gemini nanoplates. 3 ACS Paragon Plus Environment


The size (hydrodynamic diameter) of monolayer ZrP-PNIPAM was measured using dynamic light scattering (DLS, Malvern Instruments Ltd, UK), as shown in Figure 2, the error bar indicated the error during measurement. The ZrP-PNIPAM nanoplate size was around 500 nm at temperatures below 30 °C. The size suddenly decreased to around 300 nm as temperature increased from 30 °C to 35 °C. The sudden size decrease of ZrP-PNIPAM nanoplates as temperature come up was due to the attached PNIPAM experiencing a lower critical solution temperature (LCST), around 32 °C, as shown in Schematic 1. As temperature climbed higher than LCST, the PNIPAM chains suddenly shrinked and dehydrated to a smaller volume. The monolayer ZrP-PNIPAM had a temperature-responsive property in range of 30 °C to 35 °C. At temperatures above 35 °C, PNIPAM grafting did not increase the size of ZrP-PNIPAM nanoplates due to the shrinking of PNIPAM. 600

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Figure 2. Average hydrodynamic diameter of ZrP-PNIPAM nanoplates as a function of temperature measured by DLS. Error bars reflect the relative peak width from DLS.

The ability to stabilize oil/water emulsion of layered ZrP-PNIPAM, exfoliated ZrP-PNIPAM nanoplates as well as pure PNIPAM was studied using dyed dodecane as oil. As shown in Figure 3, all of the stabilizers were able to emulsify dodecane/water emulsion. The emulsions produced by layered ZrP-PNIPAM were larger in size compared to those by exfoliated ZrP-PNIPAM and pure PNIPAM polymer. The emulsions stabilized by PNIPAM polymer were not stable, after 24 hours, most of them were broken. Both layered ZrP-PNIPAM and exfoliated ZrP-PNIPAM produced stable emulsions which didn’t change much after 24 hours. However, some of the layered ZrP-PNIPAM particles were settled down to the bottom of the glass vial. The reason was that the layered ZrP particle were higher density compared to the suspension. The ZrP-PNIPAM settling down would lead to low material utilization. Hence, we concluded that exfoliated ZrP-PNIPAM nanoplates were better Pickering emulsifier compared to the layered ZrP-PNIPAM and pure PNIPAM in terms of making smaller and more stable oil/water emulsions. Both PNIPAM chains and the monolayer structure of ZrP-PNIPAM nanoplates enable the asymmetric nanoplates to be better Pickering emulsifiers.


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Figure 3. Nonadecane/water emulsions stabilized by layered ZrP-PNIPAM, exfoliated ZrP-PNIPAM nanoplates and pure PNIPAM. ZrP-PNIPAM nanoplates served well as thermosensitive Pickering emulsifiers too. Oil/water emulsions have been stabilized using ZrP-PNIPAM nanoplates alone (Supporting Information, Figure S3); for instance, a dodecane (dyed)/water emulsion can be easily emulsified by gentle manual shaking using ZrP-PNIPAM as emulsifiers. A temperature-responsive capacity was also obtained by noting the visible change from the photos: the dodecane/water emulsion was totally coalesced after sitting in an oven at 65 °C oven for just 5 minutes. This was due to the temperature reaction of PNIPAM chains grafting on the Pickering emulsifiers. As the temperature exceeded LCST, the change of PNIPAM would lead to the coalescence of emulsions, as already indicated by the increasing IFT as discussed above. Dodecane/water emulsions were also produced using ZrP nanoplates for comparison. Compared to emulsions emulsified using ZrP-PNIPAM nanoplates, we found that: (1) Less emulsions were produced by ZrP nanoplates; (2) the size of emulsions by ZrP nanoplates were much larger and; (3) the emulsions didn’t show the temperature dependent property. Hence, we concluded that ZrP-PNIPAM nanoplates was better thermosensitive emulsifier compared to ZrP nanoplates, not only due to the thermos-sensitivity but also the ability of producing more stable emulsions. PNIPAM on ZrP nanoplates surface made them become better Pickering emulsifiers. To better understand this property, interfacial tension (IFT) of ZrP-PNIPAM suspension compared to ZrP without PNIPAM were studied. The reduction of oil/water IFT is one of the most important parameters characterizing surfactants, especially in the water flooding of enhanced oil recovery (EOR). 30 Usually, for surfactants used for producing oil in water emulsion, the smaller the IFT achievable by a surfactant, the better the emulsifier performance of the surfactant. As to the Pickering emulsifiers, the ability to reduce IFT was still a useful parameter to determine emulsifier capability.31 The interfacial tension between cyclohexane and water in the presence of ZrP-PNIPAM or exfoliated ZrP nanoplates was measured by spinning drop tensiometer (Grace Instrument M6500, USA) at various temperatures (Figure 4). The concentration of both nanoplates was 0.09 g/mL. The IFT of pure 5 ACS Paragon Plus Environment


cyclohexane/water was reported to be in the range of 48 mN/m to 51 mN/m,32 decreasing inversely with temperature.33 By addition of exfoliated ZrP nanoplates into the aqueous phase, the IFT was greatly reduced to 12.2 ± 0.3 mN/m, and again, the IFT value decreased as temperature increased. The situation was much different when ZrP-PNIPAM nanoplates were added into the suspension; in this case, the IFT was further reduced overall to about 2.1 ± 0.3 mN/m. More importantly, the trends were quite different in the ZrP-PNIPAM suspension. From 25 °C to 30 °C, the IFT slightly decreased. As the temperature increased further, the IFT suddenly increased at 35 °C, and the IFT kept increasing with temperature. The IFT of pure PNIPAM with dilute concentration was also measured using same method, the IFT was greatly reduced by pure PNIPAM (Figure S4) as well. This helped to explain why pure PNIPAM behaves well in terms of emulsification (Figure 3). The IFT of the ZrP-PNIPAM suspension was less than that of ZrP suspension at same concentration, indicating that the PNIPAM modification endowed the ZrP nanoplates with more hydrophobicity. Hence, we concluded that PNIPAM chains would be more likely to stay in the oil phase while TBA+ groups preferred to stay in the aqueous phase. This conclusion could be supported by the emulsions we obtained in Figure S3 as well, some emulsions stabilized by ZrP were stick to the hydrophilic glass wall while none of those by ZrP-PNIPAM was found on glass wall, it seemed true that ZrP-PNIPAM increased the hydrophobicity of the emulsions. The sudden IFT increase at 35 °C was consistent with the LCST of PNIAM, so we concluded that the PNIPAM shrinking at temperatures above its LCST caused an increase in IFT of the ZrP-PNIPAM suspension. Quantitative measurement of an increasing IFT across the LCST of PNIPAM was also obtained from PNIPAM microgels15. The ability to reduce IFT indicated that ZrP-PNIPAM nanoplates can be used as a Pickering emulsifier.

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Figure 4. IFT between oil and aqueous ZrP-PNIPAM and ZrP suspensions as a function of temperature. To further study the behavior of oil/water emulsion stabilized by ZrP-PNIPAM nanoplates, polystyrene (PS) particles were fabricated and imaged under the scanning electron microscope. First, styrene together 6 ACS Paragon Plus Environment

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with initiator in water emulsions were obtained by gently manual shaking using ZrP-PNIPAM as Pickering emulsifier. The whole specimen was then heated to 65 °C to allow the polymerization of styrene. The morphology of PS particles would somehow indicate the oil/water emulsions stabilized by ZrP-PNIPAM nanoplates. The total volume used for the emulsification procedure here was 1 mL, and the sample was set in a 65 °C oven right after emulsification, which would enable instant decomposition of the initiator, azobisisobutyronitrile (AIBN). Hence, the polymerization occurred right after emulsification. In this case, the styrene/water emulsions were “frozen” so that their morphology could be well studied. The PS particle precursor (monomer together with initiator in water droplets) was emulsified at temperatures below and above LCST of PNIPAM, room temperature (around 20 °C) and 65 °C, respectively, to study the temperature-responsive performance of ZrP-PNIPAM emulsifiers. As shown in Figure 5, individual PS particles were fabricated in both temperatures. Instead of high energy input like sonication which was common for Pickering emulsification15, it was obvious that the oil/water emulsions were emulsified successfully by just gently manually shaking. Low energy input for the emulsification indicated that ZrP-PNIPAM nanoplates could serve as good emulsifiers which was competitive to lots of molecular surfactants. The PS particles emulsified at 65 °C (Figure 5c, d) were much larger and much more polydispersed overall than PS particles emulsified at room temperature (Figure 5a, b). The PS particles emulsified at room temperature were less than 1 µm in diameter while the size of PS particles emulsified at 65 °C ranged widely, from several micrometers to tens of micrometers.

Figure 5. SEM images of PS particles stabilized by ZrP-PNIPAM nanoplates at room temperature (a, b) and 65 oC (c, d). The great difference apparent between the PS particles prepared at room temperature and those prepared at 65 °C indicates the different emulsification capability of ZrP-PNIPAM nanoplates at temperatures below and above LCST of PNIPAM. The results are consistent with their IFT values as well. 7 ACS Paragon Plus Environment


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At room temperature, the IFT of the mixture was greatly reduced by addition of Pickering emulsifier ZrPPNIPAM nanoplates, and the styrene/water emulsions were small. Even after heating procedure, the PS particles were still relatively uniform. The IFT of ZrP-PNIPAM suspension increased, however, and the emulsions more easily coalesced, producing larger-size emulsions, which resulted in larger PS particles. Due to the temperature responsive property, PNIPAM has been applied in various controlled-release fields.12 A controlled-release behavior of the emulsions stabilized by ZrP-PNIPAM nanoplates was carried out using 4-cyano-4'-pentylbiphenyl (5CB) as model material. 5CB, a hydrophobic material with birefringence property under crossed polarized microscopy, was used as the oil phase. After mixing 5CB, ZrP-PNIPAM nanoplates, and water, the mixture was shaken manually for emulsification, resulting in 5CB in water emulsion droplets. The emulsions were filled into a capillary and then sealed with epoxy. The capillary was observed under crossed polarized microscopy with a temperature-controlled stage. As shown in Figure 6a, once the temperature was switched to 40 C, the droplets began to coalesce, merge, and enlarge with time. Finally, all droplets were broken and coalesced into a bulk oil phase after 240 seconds. In other words, by switching temperature, from below to above LCST, the model material was fully released after 4 minutes. If the droplets were saved under room temperature, however, they were quite stable. As shown in Figure 6b, the 5CB emulsions did not show significant difference even after 20 days’ aging.

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Figure 6. Controlled-release test of ZrP-PNIPAM using model material. The droplets stabilized by ZrPPNIPAM nanoplates were investigated at 40 °C (a) and room temperature (b). The (a) group images were taken at the same location at 40 °C. 8 ACS Paragon Plus Environment

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As discussed above, ZrP-PNIPAM nanoplates could serve as a good Pickering emulsifier candidate. The oil/water emulsion stabilized by ZrP-PNIPAM remained stable for a long period of time. The emulsions, however, were coalesced immediately as temperature rose, and totally broken in 4 minutes. Hence, we believe that ZrP-PNIPAM is an ideal emulsifier for controlled-release applications since it is able to fabricate stable oil droplets with fast response. CONCLUSIONS Fabricated asymmetric ZrP-PNIPAM nanoplates have been fabricated and proved to be thermosensitive Pickering emulsifiers. The nanoplates first showed the capability to reduce IFT of cyclohexane/water interface, and then the ability to produce stable oil/water emulsions. The nanoplates also proved to be temperature-responsive. Quantitative measurement showed IFT increasing with temperature above LCST of PNIPAM. In additional, the PS particles emulsified by ZrP-PNIPAM were much larger and far more polydispersed when the emulsified temperature was above the LCST of PNIPAM. A controlled-release experiment was realized by investigating the coalescence behavior of oil droplets stabilized by ZrP-PNIPAM nanoplates, when being heated to 40 °C. The results indicated that the droplets were stable at room temperature but would be “released” easily at 40 °C. Hence, we conclude that ZrPPNIPAM nanoplates can be a very good candidate as thermosensitive Pickering emulsifier for controlledrelease areas, for example, in drug delivery. MATERIALS AND METHODS ZrP-PNIPAM Pickering Emulsifiers Preparation. ZrP used in this paper were synthesized by the hydrothermal method.28 Following a typical preparation method, H3PO4 (3 M) and zirconium (IV) oxychloride octahydrate (ZrOCl2·8H2O, 10 wt%) were reacted at 200 C for 5 h to yield a white product. The product was centrifuged, washed three times with deionized water (DI), and dried in the oven overnight at 65 C. It was then ground into a powder and saved for future use. Layered ZrP-PNIPAM was prepared through the pre-irradiation polymerization detailed procedure reported in our previous publication.27 Generally speaking, peroxide groups were induced onto the layered ZrP by gamma ray radiation. The peroxide groups were then decomposed into free radicals to initiate polymerization of NIPAM upon heating. After ZrP-PNIPAM was synthesized, the sample was centrifuged, washed three times using DI water, and then was lyophilized using a freezing dryer (FD-1A50, Beijing Boyikang Laboratory Instruments Co., Ltd, China) at 50 °C (∽15 Pa) for 48 hours after preparation. The layered ZrP-PNIPAM aqueous suspension, which was held in a plastic tube, was dipped into liquid nitrogen to allow the water to be frozen suddenly, and the frozen sample was transferred into the freezer dryer where the water was sublimated into gas under vacuum. A white puff-like sample was obtained after lyophilization. ZrP-PNIPAM monolayer Pickering emulsifiers were obtained by exfoliating 1 g of ZrP-PNIPAM in 30mL water using 2.2 mL 40 wt% tetrabutylammonium hydroxide (TBAOH). The molar ratio of ZrP to TBAOH is 1:1; hence, the concentration of ZrP-PNIPAM was about 3.1 wt%. The ZrP nanoplate suspension used in this manuscript for comparison was obtained by exfoliating 1g of ZrP without PNIPAM surface modification following the same procedure. IFT Measurement. IFT was measured using a spinning drop tensiometer (Grace Instrument M6500, USA), using cyclohexane as the oil droplet. For the aqueous phase, both ZrP-PNIPAM and ZrP nanoplates were 9 ACS Paragon Plus Environment


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suspended into DI water at a concentration of 0.09 g/mL. The temperature was set to the target temperature and held for 5 minutes before the values were recorded. Fabrication of PS Particles. 50 µL of ZrP-PNIPAM suspension as prepared above was diluted into 950 µL DI water. 100 µL styrene and 0.016g AIBN was added into the diluted ZrP-PNIPAM suspension. Styrenein-water emulsions were fabricated by manually shaking in a manner similar to that used for the dodecane/water emulsion. The whole specimen was heated to 65 °C and the polymerization of styrene proceeded for 5 hours. The morphology of PS particle was obtained using a scanning electron microscope (FEI Quanta 600 FE-SEM). Controlled-release Test. A controlled-release test was carried out using 4-cyano-4'-pentylbiphenyl (5CB) as oil phase. 100 µL of 5 CB, 900 µL DI water and 50 µL of ZrP-PNIPAM suspension were mixed together and manually shaken for about 30s. The resulting emulsion suspension were filled into a capillary (diameter: 0.2×4 mm) by capillary force, then the capillary sealed with epoxy. Then the capillary was investigated under a microscop with a temperature-controlled stage. The temperature was set to 40 °C and allowed to rest for about 10 minutes until the temperature stabilized. The capillary was then put on a glass slide for investigation under the microscope. ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org/. The exfoliation of ZrP-PNIPAM, photos of dodecane in water emulsion emulsified by ZrP-PNIPAM nanoplates and the responsive to temperature as well as the IFT of PNIPAM were included. AUTHOR INFORMATION Corresponding Author *Phone: 979-845-3413. E-mail: [email protected]. 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. ACKNOWLEDGMENTS This work is supported by NASA (NASA-NNX13AQ60G). X.Z. Wang acknowledges support from the Mary Kay O'Connor Process Safety Center (MKOPSC) at Texas A&M University. REFERENCES (1) Tcholakova, S.; Denkov, N. D.; Lips, A. Comparison of Solid Particles, Globular Proteins and Surfactants as Emulsifiers. Phys. Chem. Chem. Phys. 2008, 10 (12), 1608-1627. (2) Binks, B. P. Particles as Surfactants—similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7 (1–2), 21-41. (3) Creighton, M. A.; Ohata, Y.; Miyawaki, J.; Bose, A.; Hurt, R. H. Two-Dimensional Materials as Emulsion Stabilizers: Interfacial Thermodynamics and Molecular Barrier Properties. Langmuir 2014, 30 (13), 36873696. (4) Kaewsaneha, C.; Tangboriboonrat, P.; Polpanich, D.; Eissa, M.; Elaissari, A. Janus Colloidal Particles: Preparation, Properties, and Biomedical Applications. ACS Appl. Mater. Interfaces 2013, 5 (6), 1857-1869. (5) de Gennes, P. G. Soft Matter. Rev. Mod. Phys. 1992, 64 (3), 645-648. 10 ACS Paragon Plus Environment

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