- Email: [email protected]
Edge-modified amphiphilic Laponite nano-discs for stabilizing Pickering emulsions
Journal of Colloid and Interface Science 410 (2013) 27–32
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Edge-modified amphiphilic Laponite nano-discs for stabilizing Pickering emulsions Ying Yang, Zhi Liu, Dayong Wu ⇑, Man Wu, Ye Tian, Zhongwei Niu ⇑, Yong Huang National Research Center of Engineering Plastics, Technical Institute of Physics and Chemistry, University of Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 29 March 2013 Accepted 25 July 2013 Available online 14 August 2013 Keywords: Laponite Amphiphilic Nano-discs Pickering emulsions
a b s t r a c t We investigated the effect of amphiphilic Laponite nano-discs, which were edge-modified by hydrophobic chains, on the properties of Pickering emulsions and Pickering emulsions polymerization. Comparing to unmodified Laponites, these amphiphilic nano-discs can greatly reduce the surface tension, resulting in very stable Pickering emulsions. These particles uniquely combine the Pickering effect with amphiphilic properties similar to the surfactant. Taking advantage of these amphiphilic Pickering emulsifiers, miniemulsion polymerization of styrene was performed. Homogeneous polystyrene nanoparticles with size around 150 nm could thus be prepared. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Pickering emulsion, which is the emulsion stabilized by solid particles, was first discovered by Ramsden and Pickering about a century ago [1,2]. Solid particles with intermediate wettability can be adsorbed irreversibly onto the oil–water interface, which attributes to high free energy of adsorption for particles, while surfactant molecules are usually in rapid dynamic equilibrium between the oil–water interface and the bulk phase [3]. This effectively irreversible adsorption leads to extreme stability for Pickering emulsions [4]. Besides, the solid particle-armored droplets were found to be mechanically robust, stable against coalescence during drop collision [5]. Up to now, this technique has led many applicable prospects [6–8]. For example, solid particles can accumulate and self-assemble at water–oil interface, which offers a unique way to fabricate novel structures [9,10], such as nanoparticle armored polymer latex [11,12], permeable hollow capsules [13], colloidosomes [14,15], biological capsules and films [16–18], and Janus structures [19,20]. Furthermore, Pickering emulsions stabilized with biological particles [21] are safer in medical, personal care and food applications than traditional emulsions which are stabilized by surfactants with tissue-irritancy [22]. When solid particles segregate at water–oil interface, they decrease the total free energy of system instead of changing the interfacial tension. The stability of Pickering emulsions is thus ⇑ Corresponding authors. Fax: +86 10 62554670. E-mail addresses: [email protected] (D. Wu), [email protected] (Z. Niu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.07.060
determined by the decreasing in total free energy of solid particles at interface [4]. Size of the solid particle is one main factor for decreasing of free energy [23]. Both theoretical prediction and experimental results showed that larger particles are normally more effective emulsifiers than nanoparticles, for they are more strongly confined to the interface [9]. For small nanoparticles, recent researches showed that dissolving surfactant or electrolyte in one phase [24–26], adsorbing polymers onto nanoparticles [27–30] or grafting them from a nanoparticle surface [31,32] makes the nanoparticles efficient emulsifiers. Wettability of the solid particle surface is another main factor for decreasing of free energy. Particles which have equal wettability to water phase and oil phase are mostly held in the interface firmly, so emulsions stabilized by these particles are mostly stable. For example, Binks et al. demonstrated that 10 nm silica nanoparticles with contact angle of 90° at oil–water interface could stabilize a toluene/water system, showing a maximum in desorption energy [33]. Janus particles or Gemini particles which have both hydrophobic part and hydrophilic part have aroused a lot of attentions recently. These particles have significantly higher adsorption energy than homogeneous particles, leading to extreme stability of emulsions [34,35]. Therefore, to get a stable Pickering emulsions, particles with inhomogeneous surface properties at the interface are in need. However, comparing to large colloidal particles with homogeneous surface, preparation of Janus or Gemini particles in nanoscale is very difficult. We realized that Laponite having nano-disc structure (25 nm in diameter, 1 nm in thickness) can be selectively modified only on the edge [36,37]. Hence, if hydrophobic molecules were covalently
28
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
Scheme 1. Schematic illustration for preparation of edge modified amphiphilic Laponite and Pickering emulsions.
grafted onto the edge of Laponite nanoparticle, the amphiphilic nanodisc structure could be prepared. With these edge modified Laponites as Pickering emulsifier, they would pack closely at the oil–water interface with the hydrophobic molecules on the edge pointing into the oil phase, leading to a stable emulsion (Scheme 1).
2. Materials and methods 2.1. Materials Laponite XLG, (Mg5.34Li0.66Si8O20(OH)4Na0.66) was purchased from Laporte Industries (UK), and washed with ethanol solution.
Fig. 1. Modification of Laponite: (1) Scheme of the two-step procedure used to modify the Laponite with hydrocarbons. (2) FTIR of Laponite (a), -SiH grafted Laponite (Laponite-SiH) (b), and alkane chain-grafted Laponite (Laponite-C18) (c). (3) TGA degradation profiles of Laponite (black), Laponite-SiH (red), and Laponite-C18 (blue). The inset table of Figs. 1–3 shows the weight loss and grafted amount of Laponite-SiH and Laponite-C18. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
29
Fig. 2. Characterization of emulsions stabilized by pure Laponite and Laponite-C18 at solid weight fraction of 2 wt% and oil volume fraction of 10% (a–f): Optical micrographs and digital photographs (insets) of Pickering emulsions stabilized by Laponite (a–c) and Laponite-C18 (d–f) with standing time of 0 h (a and d), 1 h (b and e) and 16 days (c and f). All scale bars of optical micrographs are 20 lm. Creaming process of emulsions is illustrated in g. The size distribution of emulsions stabilized by modified Laponite with standing time of 0 h is shown in (h). Digital photograph of Pickering emulsions stabilized by Laponite-C18 with standing time of 1 month is shown in i.
Dimethylethoxysilane(DMES) was purchased from J&K Scientific Ltd. 1-octadecene and Karstedt Catalyst were purchased from Aladdin Industrial Corporation. Paraffin, styrene and toluene were commercially available and used without further purification. Azodiisobutyronitrile (AIBN) was obtained from Sigma–Aldrich and purified by crystallization from methanol. 2.2. Methods 2.2.1. Modification of Laponite In a typical procedure, Laponite was dried in vacuum for 24 h at 60 °C before reaction. Laponite (4.5 g), DMES (4.0 mL, 13.5 mmol), and toluene (300 mL) were mixed in a 500-mL flask and stirred at ambient temperature for 3 days. 4.6 mL of 1-octadecene and 40 lL of Karftedt catalyst were then drop-wise added into the flask. After stirring for another 6 days at room temperature, the product
was separated by centrifugation (300 rpm for 5 min) and further washed with toluene for removing excess alkane. Finally, the product Laponite-C18 was dried in vacuum at 40 °C for 24 h. 2.2.2. Preparation of Pickering emulsions The Pickering emulsions were prepared through classic method. The Laponite dispersion was first prepared by dispersing 2.5 g of Laponite particles into 100 mL of deionized water and stirring for 4 h. Then, by adding oil phase into the Laponite dispersion and ultrasonicating at 400 W for 2 min using ultrasonic cell crusher, the emulsions were obtained. 2.2.3. Emulsion polymerization of styrene Pickering emulsions was prepared with styrene (AIBN was dissolved before used) as oil phase according to the former procedure. The polymerization was then carried out at 70 °C for 4 h under
30
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
Fig. 3. The surface tension of pure Laponite (black square) and Laponite-C18 (red cycle) aqueous at different solid particles content. The insets are the pictures of contact angle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Thermo-gravimetric analysis (TGA) was performed on a thermo-gravimetric analyzer Q50 from TA instruments. Samples were heated from ambient temperature to 600 °C at a rate of 10 °C/ min under nitrogen flow. Digital graphs of emulsions were obtained with digital camera (IXUS 130, Canon). The morphology of emulsion droplets was observed with an optical microscope (Observer D1, Carl ZEISS). The size and size distribution of emulsion droplets were recorded by Zetasizer Nano from Malvern. Films were formed by pouring aqueous dispersions of Laponite and modified Laponite on the face of glass disc and dried at 60 °C overnight, then the contact angles of these films were measured at ambient temperature. Surface tensions of Laponite and modified Laponite dispersions were measured by the pendant drop method at 22 °C. Both measurements of contact angle and surface intension were carried on a DataPhysics OCA 20 contact angle system. Morphologies of the polystyrene (PS) nanoparticles were observed with a scanning electron microscope (SEM, S-4300), operated at an accelerating voltage of 10 kV. The visualized TEM images for dispersion of Laponite on the face of the PS nanoparticles were obtained from a JEM-2100F electron microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 120 kV.
stirring. Polystyrene particles were centrifuged and washed with water, and were obtained after lyophilization.
3. Results and discussions
2.2.4. Characterizations Fourier transform infrared (FTIR) was recorded by an Excalibur 3100 spectrometer on power-pressed KBr pellets.
It is known that the chemically accessed SiOH groups are only present on the edge of the Laponite particle, which has great promise to render it with different properties on the face and edge [36].
3.1. Modification of laponite
Fig. 4. SEM (a and b) and TEM (c and d) images of polystyrene particles using Laponite (a and c) and modified Laponite (b and d) as stabilizers. The insets in d is the zoom in image.
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
A two-step procedure was used here to modify the Laponite with hydrocarbons. As shown in Fig. 1-1, Si–H bond is firstly grafted onto the edge of Laponite by reacting Si–OH on the edge of Laponite with DMES. Secondly, hydroponic chains are grafted onto the edge of Laponite by hydrosilylation of Si–H bond and 1-Octadecene. By this approach, the prepared modified Laponite (LaponiteC18) particles are amphiphilic: edge is hydrophobic and face is hydrophilic. After purification with excess toluene, the grafting reaction was analyzed by FTIR and TGA. 3.1.1. Fourier transform infrared (FTIR) Qualitative evidence of grafting was provided by Fourier transform infrared. The FTIR spectra of raw Laponite, Laponite-SiH and Laponite-C18 are shown in Figs. 1 and 2. The FTIR spectrum of Laponite (Figs. 1 and 2a) shows the characteristic adsorption band at 1000 cm1 and 3400 cm1 ascribed to Si–O stretching and H–OH. Comparing the spectrum of Laponite, the spectrum of LaponiteSiH (Figs. 1 and 2b) presents the characteristic vibrations of the silicon-hydrogen bond (dSi–H, 876 cm1), the Si–CH3 (dsy(Si–)CH3, 1263 cm1) and the aliphatic CH3 groups (mC–H, 2976 cm1). The presence of Si–H and aliphatic groups indicates that silanols of the Laponite reacted with siloxy groups of DMES and resulted in grafting of Si–H on the Laponite particle edge. In the spectrum of Laponite-C18 (Figs. 1 and 2c), the disappearance of Si–H bonds (dSi–H, 876 cm1) and the appearance of CH2 bonds (cCH2 796 cm1) prove that hydroponic chains have been successfully grafted onto the Laponite particle through reaction between Si–H bond on the edge of Laponite and 1-octadecene. 3.1.2. Thermo gravimetric analysis (TGA) To give quantitative evidences of alkane chains grafted on the Laponite particle edge, thermo gravimetric analysis was used to determine the amount of alkane chains chemically anchored on the Laponite edge. Figs. 1 and 3 show the TGA curves of Laponite before and after grafting modification. The region between 200 and 600 °C, which corresponding to the thermal decomposition of the organic molecule, was considered for quantitative determination of the silane coverage. The grafted amount can be determined by Eq. 1 [37].
grafted amount ðmequiv=gÞ ¼
103 W 200600 ð100 W200600 ÞM
ð1Þ
where M (g/mol) is the molecular weight of the grafted molecules, W200–600 is the net weight loss between 200 and 600 °C, which is W200–600 = Wmodified Laponite–WLaponite. The grafted amount is shown in the inset table of Figs. 1 and 3. It can be seen that the grafted amount of Laponite-C18 (0.48 mequiv/ g) is less than that of Laponite-SiH (0.75 mequiv/g). This indicates that hydrosilylation reaction is not complete due to partial hydrolysis of Si–H bonds. 3.2. Pickering emulsions stabilized by Laponite and Laponite-C18 Pickering emulsions with 2 wt% of Laponite or Laponite-C18 as stabilizer and 10 wt% of paraffin as oil phase were prepared. These emulsions were both oil-in-water type confirmed by dilution test and dye method, but had different stability. It should be noted that the Pickering emulsions stabilized by Laponite with addition of electrolyte or surfactant can be as stable as several months [25,26,38]. Here, we only compare the emulsions stabilized by Laponite and amphiphilic Laponite in the same condition without any additives. The inset digital photographs in Fig. 2a, b, d, and f show that the emulsions stabilized by Laponite-C18 have higher stability than that of Laponite stabilized. After 1 h of standing, the emulsions stabilized by Laponite are aged due to creaming (Fig. 2b inset), while
31
the emulsions stabilized by Laponite-C18 are still in their uniform phase (Fig. 3e inset) and are stable for up to months. The droplet size and size distribution are also differed. Comparing with emulsions stabilized by Laponite, the emulsions stabilized by Laponite-C18 have much smaller and more stable droplets. The droplet size of the Laponite stabilized emulsions changed greatly along with time (Fig. 2a–c), while the droplet size in Laponite-C18 stabilized emulsions kept consistent along with time (Fig. 2d–f). We assume that Laponite-C18 particles with amphiphilic property and improvable wettability could be adsorbed effectively at oil–water interface and form solid film to prevent from coalescence, which lead to stable emulsions with narrow size distribution (Fig. 2h). For Laponite particle with the same disc-like structure, both of its side and face are hydrophilic. When these nanodiscs are confined at interfaces, the energy reduction of nanodisc is comparable to the thermal energy, which leads to the coalescence of emulsions droplets and become a larger drop (Scheme g in Fig. 2). 3.3. Contact angle and surface tension To confirm the amphiphilic character of Laponite-C18, the surface tension of Laponite and Laponite-C18 dispersion was measured. As shown in Fig. 3, surface tension continuously decreases with increasing of amount of Laponite-C18 and reaches to around 62 mN/m when the mass fraction of solid particles is 2.5 wt%. However no significant variation is found for Laponite dispersion. The results prove that Laponite-C18 has a significant influence on surface tension, which indicates the Laponite-C18 particles have surfactant-like activity. Furthermore, the contact angle of Laponite-C18 (54.1 ± 0.6°) is larger than that of Laponite (26.2 ± 0.1°), which indicates that the wettability of Laponite particle is improved after grafting hydrophobic chains onto the edge of the particle. 3.4. Pickering emulsion polymerization of styrene One advantage of these amphiphilic Laponite is used as stabilizer for Pickering miniemulsion polymerization. Previous studies have shown Laponite-armored polystyrene (PS) nanoparticles can be prepared by Laponite stabilized emulsion polymerization with the addition of salt [12,26]. However, these armored PS latex were connected with each other with inhomogeneous surface and wide size distribution [12,26]. It is still a challenge to prepare homogeneous polymer latex nanoparticles by Pickering emulsion polymerization method, which is comparable to traditional surfactant stabilized emulsion polymerization. This can be realized by amphiphilic Laponites stabilized Pickering emulsion polymerization. As shown in Fig. 4a and c, the size of PS particles with Laponite as stabilizer alters from hundreds of nanometers to several micrometers with inhomogeneous surfaces. As for Laponite-C18 stabilized Pickering emulsion polymerization of styrene, the nearly monodispersed PS nanoparticles with size around 150 nm were prepared, and no aggregations were found on the surface of PS (Fig. 4b and d). This can be greatly attributed by the amphiphilic properties of Laponite-C18, which acts as more like a surfactant. It should be mentioned that the process of polymerization was carried out under agitation, leading to a difference of shear force between formation of polystyrene particles and formation of emulsion particles. This is why the size of polystyrene particles differs from the size of Pickering emulsion droplets. 4. Conclusions We successfully fabricated amphiphilic Laponite with hydrophobic edge and hydrophilic face, which has surfactant activity
32
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
and improvable wettability. The Laponite-C18 nanoparticles were obtained by grafting hydrophobic chains onto the edge of Laponite particles. The chemical structures of Laponite-C18 were confirmed via FTIR and TGA measurements. The results of contact angle and surface intension proved that modified Laponite has improvable wettability and surface activity. Using these Laponite-C18 nanoparticles as stabilizer, stable Pickering emulsions with uniform size were prepared. Polystyrene nanoparticles with uniform size were synthesized by miniemulsion polymerization of styrene in the Laponite-C18 stabilized Pickering emulsions. We anticipate that these amphiphilic nanodiscs could have great potentials in fabricating complex structures at interfaces and find applications in the field of food, medical and personal care. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21074143, 51172247, 21272243), and Hundred Talents Program of the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
W. Ramsden, Proc. R. Soc. London 72 (1903) 156–164. S.U. Pickering, J. Chem. Soc. 91 (1907) 2001–2021. B.P. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21–41. R. Aveyard, B.P. Binks, Adv. Colloid Interface Sci. 100–102 (2003) 503–546. S.Y.R.F. Tabor, L. Ong, G.W. Stevens, R.R. Dagastine, Soft Matter 8 (2012) 3112– 3121. V. Alvarado, X. Wang, M. Moradi, Energies 4 (2011) 1058–1086. E. Dickinson, Trends Food Sci. Technol. 24 (2012) 4–12. B.P. Nair, C.P. Sharma, Langmuir 28 (2012) 4559–4564. Z. Niu, J. He, T.P. Russell, Q. Wang, Angew. Chem. Int. Ed. 49 (2010) 10052– 10066. C. Zeng, H. Bissig, A.D. Dinsmore, Solid State Commun. 139 (2006) 547–556.
[11] N. Zgheib, J.L. Putaux, A. Thill, F. D’Agosto, M. Lansalot, E. Bourgeat-Lami, Langmuir 28 (2012) 6163–6174. [12] S. Cauvin, P.J. Colver Patrick, A.F. Stefan, Macromolecules 38 (2005) 7887– 7889. [13] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Science 298 (2002) 1006–1009. [14] M. Williams, S.P. Armes, D.W. York, Langmuir (2012) 1142–1148. [15] Z. Wang, M.C. van Oers, F.P. Rutjes, J.C. van Hest, Angew. Chem. Int. Ed. 5 (2012) 1–6. [16] Y. Zhao, G. Yin, Z. Zheng, H. Wang, Q. Du, J. Polym, Part A: Polym. Chem. 49 (2011) 5257–5269. [17] M. Amirkhani, N. Berger, M. Abdelmohsen, F. Zocholl, M.R. Gonçalves, O. Marti, J. Polym. Sci., Part B: Polym. Phys. 49 (2011) 1430–1436. [18] A.F. Bon Stefan, T. Chen, Langmuir 23 (2007) 9527–9530. [19] N.P. Pardhy, B.M. Budhlall, Langmuir 26 (2010) 13130–13141. [20] V.N. Paunov, O.J. Cayre, Adv. Mater. 16 (2004) 788–791. [21] C. Li, P. Sun, C. Yang, Starch 64 (2012) 497–502. [22] I. Nicander, S. Ollmar, A. EeK, B.L. Rozell, L. Emtestam, Br. J. Dermato. 134 (1996) 221–228. [23] B.P. Binks, S.O. Lumsdon, Langmuir 17 (2001) 4540–4547. [24] E.T.A. van den Dungen, P.C. Hartmann, Appl. Clay Sci. 55 (2012) 120–124. [25] J. Wang, F. Yang, J. Tan, G. Liu, J. Xu, D. Sun, Langmuir 26 (2010) 5397–5404. [26] N.P. Ashby, B.P. Binks, PCCP 2 (2000) 5640–5646. [27] J. Wang, G. Liu, L. Wang, C. Li, J. Xu, D. Sun, Colloids Surf. A 353 (2010) 117–124. [28] M. Reger, H. Hoffmann, J. Colloid Interface Sci. 368 (2012) 378–386. [29] M. Reger, T. Sekine, T. Okamoto, K. Watanabe, H. Hoffmann, Soft Matter 7 (2011) 11021–11030. [30] Z. Wei, C. Wang, H. Liu, S. Zou, Z. Tong, J. Appl. Polym. Sci. 125 (2012). pp. E358–E368. [31] W. Li, L. Yu, G. Liu, J. Tan, S. Liu, D. Sun, Colloids Surf. A 400 (2012) 44–51. [32] T. Saigal, H. Dong, K. Matyjaszewski, R.D. Tilton, Langmuir 26 (2010) 15200– 15209. [33] B.P. Binks, S.O. Lumsdon, Langmuir 16 (2000) 8622–8631. [34] A. Walther, M. Hoffmann, A.H.E. Müller, Angew. Chem. 120 (2008) 723–726. [35] A.F. Mejia, A. Diaz, S. Pullela, Y.W. Chang, M.C. Carpenter, J.D. Batteas, M.S. Mannan, A. Clearfield, Z. Cheng, Soft Matter 8 (2012) 10245–10254. [36] P.A. Wheeler, J. Wang, J. Bake, L.J. Mathias, Chem. Mater. 17 (2005) 3012–3018. [37] N.N. Herrera, J.M. Letoffe, J.L. Putaux, L. David Laurent, E. Bourgeat-Lami, Langmuir 20 (2004) 1564–1571. [38] J. Zhang, L. Li, J. Wang, Langmuir 28 (2012) 6769–6775.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Edge-modified amphiphilic Laponite nano-discs for stabilizing Pickering emulsions Ying Yang, Zhi Liu, Dayong Wu ⇑, Man Wu, Ye Tian, Zhongwei Niu ⇑, Yong Huang National Research Center of Engineering Plastics, Technical Institute of Physics and Chemistry, University of Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 29 March 2013 Accepted 25 July 2013 Available online 14 August 2013 Keywords: Laponite Amphiphilic Nano-discs Pickering emulsions
a b s t r a c t We investigated the effect of amphiphilic Laponite nano-discs, which were edge-modified by hydrophobic chains, on the properties of Pickering emulsions and Pickering emulsions polymerization. Comparing to unmodified Laponites, these amphiphilic nano-discs can greatly reduce the surface tension, resulting in very stable Pickering emulsions. These particles uniquely combine the Pickering effect with amphiphilic properties similar to the surfactant. Taking advantage of these amphiphilic Pickering emulsifiers, miniemulsion polymerization of styrene was performed. Homogeneous polystyrene nanoparticles with size around 150 nm could thus be prepared. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Pickering emulsion, which is the emulsion stabilized by solid particles, was first discovered by Ramsden and Pickering about a century ago [1,2]. Solid particles with intermediate wettability can be adsorbed irreversibly onto the oil–water interface, which attributes to high free energy of adsorption for particles, while surfactant molecules are usually in rapid dynamic equilibrium between the oil–water interface and the bulk phase [3]. This effectively irreversible adsorption leads to extreme stability for Pickering emulsions [4]. Besides, the solid particle-armored droplets were found to be mechanically robust, stable against coalescence during drop collision [5]. Up to now, this technique has led many applicable prospects [6–8]. For example, solid particles can accumulate and self-assemble at water–oil interface, which offers a unique way to fabricate novel structures [9,10], such as nanoparticle armored polymer latex [11,12], permeable hollow capsules [13], colloidosomes [14,15], biological capsules and films [16–18], and Janus structures [19,20]. Furthermore, Pickering emulsions stabilized with biological particles [21] are safer in medical, personal care and food applications than traditional emulsions which are stabilized by surfactants with tissue-irritancy [22]. When solid particles segregate at water–oil interface, they decrease the total free energy of system instead of changing the interfacial tension. The stability of Pickering emulsions is thus ⇑ Corresponding authors. Fax: +86 10 62554670. E-mail addresses: [email protected] (D. Wu), [email protected] (Z. Niu). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.07.060
determined by the decreasing in total free energy of solid particles at interface [4]. Size of the solid particle is one main factor for decreasing of free energy [23]. Both theoretical prediction and experimental results showed that larger particles are normally more effective emulsifiers than nanoparticles, for they are more strongly confined to the interface [9]. For small nanoparticles, recent researches showed that dissolving surfactant or electrolyte in one phase [24–26], adsorbing polymers onto nanoparticles [27–30] or grafting them from a nanoparticle surface [31,32] makes the nanoparticles efficient emulsifiers. Wettability of the solid particle surface is another main factor for decreasing of free energy. Particles which have equal wettability to water phase and oil phase are mostly held in the interface firmly, so emulsions stabilized by these particles are mostly stable. For example, Binks et al. demonstrated that 10 nm silica nanoparticles with contact angle of 90° at oil–water interface could stabilize a toluene/water system, showing a maximum in desorption energy [33]. Janus particles or Gemini particles which have both hydrophobic part and hydrophilic part have aroused a lot of attentions recently. These particles have significantly higher adsorption energy than homogeneous particles, leading to extreme stability of emulsions [34,35]. Therefore, to get a stable Pickering emulsions, particles with inhomogeneous surface properties at the interface are in need. However, comparing to large colloidal particles with homogeneous surface, preparation of Janus or Gemini particles in nanoscale is very difficult. We realized that Laponite having nano-disc structure (25 nm in diameter, 1 nm in thickness) can be selectively modified only on the edge [36,37]. Hence, if hydrophobic molecules were covalently
28
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
Scheme 1. Schematic illustration for preparation of edge modified amphiphilic Laponite and Pickering emulsions.
grafted onto the edge of Laponite nanoparticle, the amphiphilic nanodisc structure could be prepared. With these edge modified Laponites as Pickering emulsifier, they would pack closely at the oil–water interface with the hydrophobic molecules on the edge pointing into the oil phase, leading to a stable emulsion (Scheme 1).
2. Materials and methods 2.1. Materials Laponite XLG, (Mg5.34Li0.66Si8O20(OH)4Na0.66) was purchased from Laporte Industries (UK), and washed with ethanol solution.
Fig. 1. Modification of Laponite: (1) Scheme of the two-step procedure used to modify the Laponite with hydrocarbons. (2) FTIR of Laponite (a), -SiH grafted Laponite (Laponite-SiH) (b), and alkane chain-grafted Laponite (Laponite-C18) (c). (3) TGA degradation profiles of Laponite (black), Laponite-SiH (red), and Laponite-C18 (blue). The inset table of Figs. 1–3 shows the weight loss and grafted amount of Laponite-SiH and Laponite-C18. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
29
Fig. 2. Characterization of emulsions stabilized by pure Laponite and Laponite-C18 at solid weight fraction of 2 wt% and oil volume fraction of 10% (a–f): Optical micrographs and digital photographs (insets) of Pickering emulsions stabilized by Laponite (a–c) and Laponite-C18 (d–f) with standing time of 0 h (a and d), 1 h (b and e) and 16 days (c and f). All scale bars of optical micrographs are 20 lm. Creaming process of emulsions is illustrated in g. The size distribution of emulsions stabilized by modified Laponite with standing time of 0 h is shown in (h). Digital photograph of Pickering emulsions stabilized by Laponite-C18 with standing time of 1 month is shown in i.
Dimethylethoxysilane(DMES) was purchased from J&K Scientific Ltd. 1-octadecene and Karstedt Catalyst were purchased from Aladdin Industrial Corporation. Paraffin, styrene and toluene were commercially available and used without further purification. Azodiisobutyronitrile (AIBN) was obtained from Sigma–Aldrich and purified by crystallization from methanol. 2.2. Methods 2.2.1. Modification of Laponite In a typical procedure, Laponite was dried in vacuum for 24 h at 60 °C before reaction. Laponite (4.5 g), DMES (4.0 mL, 13.5 mmol), and toluene (300 mL) were mixed in a 500-mL flask and stirred at ambient temperature for 3 days. 4.6 mL of 1-octadecene and 40 lL of Karftedt catalyst were then drop-wise added into the flask. After stirring for another 6 days at room temperature, the product
was separated by centrifugation (300 rpm for 5 min) and further washed with toluene for removing excess alkane. Finally, the product Laponite-C18 was dried in vacuum at 40 °C for 24 h. 2.2.2. Preparation of Pickering emulsions The Pickering emulsions were prepared through classic method. The Laponite dispersion was first prepared by dispersing 2.5 g of Laponite particles into 100 mL of deionized water and stirring for 4 h. Then, by adding oil phase into the Laponite dispersion and ultrasonicating at 400 W for 2 min using ultrasonic cell crusher, the emulsions were obtained. 2.2.3. Emulsion polymerization of styrene Pickering emulsions was prepared with styrene (AIBN was dissolved before used) as oil phase according to the former procedure. The polymerization was then carried out at 70 °C for 4 h under
30
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
Fig. 3. The surface tension of pure Laponite (black square) and Laponite-C18 (red cycle) aqueous at different solid particles content. The insets are the pictures of contact angle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Thermo-gravimetric analysis (TGA) was performed on a thermo-gravimetric analyzer Q50 from TA instruments. Samples were heated from ambient temperature to 600 °C at a rate of 10 °C/ min under nitrogen flow. Digital graphs of emulsions were obtained with digital camera (IXUS 130, Canon). The morphology of emulsion droplets was observed with an optical microscope (Observer D1, Carl ZEISS). The size and size distribution of emulsion droplets were recorded by Zetasizer Nano from Malvern. Films were formed by pouring aqueous dispersions of Laponite and modified Laponite on the face of glass disc and dried at 60 °C overnight, then the contact angles of these films were measured at ambient temperature. Surface tensions of Laponite and modified Laponite dispersions were measured by the pendant drop method at 22 °C. Both measurements of contact angle and surface intension were carried on a DataPhysics OCA 20 contact angle system. Morphologies of the polystyrene (PS) nanoparticles were observed with a scanning electron microscope (SEM, S-4300), operated at an accelerating voltage of 10 kV. The visualized TEM images for dispersion of Laponite on the face of the PS nanoparticles were obtained from a JEM-2100F electron microscope (JEOL, Tokyo, Japan) with an accelerating voltage of 120 kV.
stirring. Polystyrene particles were centrifuged and washed with water, and were obtained after lyophilization.
3. Results and discussions
2.2.4. Characterizations Fourier transform infrared (FTIR) was recorded by an Excalibur 3100 spectrometer on power-pressed KBr pellets.
It is known that the chemically accessed SiOH groups are only present on the edge of the Laponite particle, which has great promise to render it with different properties on the face and edge [36].
3.1. Modification of laponite
Fig. 4. SEM (a and b) and TEM (c and d) images of polystyrene particles using Laponite (a and c) and modified Laponite (b and d) as stabilizers. The insets in d is the zoom in image.
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
A two-step procedure was used here to modify the Laponite with hydrocarbons. As shown in Fig. 1-1, Si–H bond is firstly grafted onto the edge of Laponite by reacting Si–OH on the edge of Laponite with DMES. Secondly, hydroponic chains are grafted onto the edge of Laponite by hydrosilylation of Si–H bond and 1-Octadecene. By this approach, the prepared modified Laponite (LaponiteC18) particles are amphiphilic: edge is hydrophobic and face is hydrophilic. After purification with excess toluene, the grafting reaction was analyzed by FTIR and TGA. 3.1.1. Fourier transform infrared (FTIR) Qualitative evidence of grafting was provided by Fourier transform infrared. The FTIR spectra of raw Laponite, Laponite-SiH and Laponite-C18 are shown in Figs. 1 and 2. The FTIR spectrum of Laponite (Figs. 1 and 2a) shows the characteristic adsorption band at 1000 cm1 and 3400 cm1 ascribed to Si–O stretching and H–OH. Comparing the spectrum of Laponite, the spectrum of LaponiteSiH (Figs. 1 and 2b) presents the characteristic vibrations of the silicon-hydrogen bond (dSi–H, 876 cm1), the Si–CH3 (dsy(Si–)CH3, 1263 cm1) and the aliphatic CH3 groups (mC–H, 2976 cm1). The presence of Si–H and aliphatic groups indicates that silanols of the Laponite reacted with siloxy groups of DMES and resulted in grafting of Si–H on the Laponite particle edge. In the spectrum of Laponite-C18 (Figs. 1 and 2c), the disappearance of Si–H bonds (dSi–H, 876 cm1) and the appearance of CH2 bonds (cCH2 796 cm1) prove that hydroponic chains have been successfully grafted onto the Laponite particle through reaction between Si–H bond on the edge of Laponite and 1-octadecene. 3.1.2. Thermo gravimetric analysis (TGA) To give quantitative evidences of alkane chains grafted on the Laponite particle edge, thermo gravimetric analysis was used to determine the amount of alkane chains chemically anchored on the Laponite edge. Figs. 1 and 3 show the TGA curves of Laponite before and after grafting modification. The region between 200 and 600 °C, which corresponding to the thermal decomposition of the organic molecule, was considered for quantitative determination of the silane coverage. The grafted amount can be determined by Eq. 1 [37].
grafted amount ðmequiv=gÞ ¼
103 W 200600 ð100 W200600 ÞM
ð1Þ
where M (g/mol) is the molecular weight of the grafted molecules, W200–600 is the net weight loss between 200 and 600 °C, which is W200–600 = Wmodified Laponite–WLaponite. The grafted amount is shown in the inset table of Figs. 1 and 3. It can be seen that the grafted amount of Laponite-C18 (0.48 mequiv/ g) is less than that of Laponite-SiH (0.75 mequiv/g). This indicates that hydrosilylation reaction is not complete due to partial hydrolysis of Si–H bonds. 3.2. Pickering emulsions stabilized by Laponite and Laponite-C18 Pickering emulsions with 2 wt% of Laponite or Laponite-C18 as stabilizer and 10 wt% of paraffin as oil phase were prepared. These emulsions were both oil-in-water type confirmed by dilution test and dye method, but had different stability. It should be noted that the Pickering emulsions stabilized by Laponite with addition of electrolyte or surfactant can be as stable as several months [25,26,38]. Here, we only compare the emulsions stabilized by Laponite and amphiphilic Laponite in the same condition without any additives. The inset digital photographs in Fig. 2a, b, d, and f show that the emulsions stabilized by Laponite-C18 have higher stability than that of Laponite stabilized. After 1 h of standing, the emulsions stabilized by Laponite are aged due to creaming (Fig. 2b inset), while
31
the emulsions stabilized by Laponite-C18 are still in their uniform phase (Fig. 3e inset) and are stable for up to months. The droplet size and size distribution are also differed. Comparing with emulsions stabilized by Laponite, the emulsions stabilized by Laponite-C18 have much smaller and more stable droplets. The droplet size of the Laponite stabilized emulsions changed greatly along with time (Fig. 2a–c), while the droplet size in Laponite-C18 stabilized emulsions kept consistent along with time (Fig. 2d–f). We assume that Laponite-C18 particles with amphiphilic property and improvable wettability could be adsorbed effectively at oil–water interface and form solid film to prevent from coalescence, which lead to stable emulsions with narrow size distribution (Fig. 2h). For Laponite particle with the same disc-like structure, both of its side and face are hydrophilic. When these nanodiscs are confined at interfaces, the energy reduction of nanodisc is comparable to the thermal energy, which leads to the coalescence of emulsions droplets and become a larger drop (Scheme g in Fig. 2). 3.3. Contact angle and surface tension To confirm the amphiphilic character of Laponite-C18, the surface tension of Laponite and Laponite-C18 dispersion was measured. As shown in Fig. 3, surface tension continuously decreases with increasing of amount of Laponite-C18 and reaches to around 62 mN/m when the mass fraction of solid particles is 2.5 wt%. However no significant variation is found for Laponite dispersion. The results prove that Laponite-C18 has a significant influence on surface tension, which indicates the Laponite-C18 particles have surfactant-like activity. Furthermore, the contact angle of Laponite-C18 (54.1 ± 0.6°) is larger than that of Laponite (26.2 ± 0.1°), which indicates that the wettability of Laponite particle is improved after grafting hydrophobic chains onto the edge of the particle. 3.4. Pickering emulsion polymerization of styrene One advantage of these amphiphilic Laponite is used as stabilizer for Pickering miniemulsion polymerization. Previous studies have shown Laponite-armored polystyrene (PS) nanoparticles can be prepared by Laponite stabilized emulsion polymerization with the addition of salt [12,26]. However, these armored PS latex were connected with each other with inhomogeneous surface and wide size distribution [12,26]. It is still a challenge to prepare homogeneous polymer latex nanoparticles by Pickering emulsion polymerization method, which is comparable to traditional surfactant stabilized emulsion polymerization. This can be realized by amphiphilic Laponites stabilized Pickering emulsion polymerization. As shown in Fig. 4a and c, the size of PS particles with Laponite as stabilizer alters from hundreds of nanometers to several micrometers with inhomogeneous surfaces. As for Laponite-C18 stabilized Pickering emulsion polymerization of styrene, the nearly monodispersed PS nanoparticles with size around 150 nm were prepared, and no aggregations were found on the surface of PS (Fig. 4b and d). This can be greatly attributed by the amphiphilic properties of Laponite-C18, which acts as more like a surfactant. It should be mentioned that the process of polymerization was carried out under agitation, leading to a difference of shear force between formation of polystyrene particles and formation of emulsion particles. This is why the size of polystyrene particles differs from the size of Pickering emulsion droplets. 4. Conclusions We successfully fabricated amphiphilic Laponite with hydrophobic edge and hydrophilic face, which has surfactant activity
32
Y. Yang et al. / Journal of Colloid and Interface Science 410 (2013) 27–32
and improvable wettability. The Laponite-C18 nanoparticles were obtained by grafting hydrophobic chains onto the edge of Laponite particles. The chemical structures of Laponite-C18 were confirmed via FTIR and TGA measurements. The results of contact angle and surface intension proved that modified Laponite has improvable wettability and surface activity. Using these Laponite-C18 nanoparticles as stabilizer, stable Pickering emulsions with uniform size were prepared. Polystyrene nanoparticles with uniform size were synthesized by miniemulsion polymerization of styrene in the Laponite-C18 stabilized Pickering emulsions. We anticipate that these amphiphilic nanodiscs could have great potentials in fabricating complex structures at interfaces and find applications in the field of food, medical and personal care. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21074143, 51172247, 21272243), and Hundred Talents Program of the Chinese Academy of Sciences. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
W. Ramsden, Proc. R. Soc. London 72 (1903) 156–164. S.U. Pickering, J. Chem. Soc. 91 (1907) 2001–2021. B.P. Binks, Curr. Opin. Colloid Interface Sci. 7 (2002) 21–41. R. Aveyard, B.P. Binks, Adv. Colloid Interface Sci. 100–102 (2003) 503–546. S.Y.R.F. Tabor, L. Ong, G.W. Stevens, R.R. Dagastine, Soft Matter 8 (2012) 3112– 3121. V. Alvarado, X. Wang, M. Moradi, Energies 4 (2011) 1058–1086. E. Dickinson, Trends Food Sci. Technol. 24 (2012) 4–12. B.P. Nair, C.P. Sharma, Langmuir 28 (2012) 4559–4564. Z. Niu, J. He, T.P. Russell, Q. Wang, Angew. Chem. Int. Ed. 49 (2010) 10052– 10066. C. Zeng, H. Bissig, A.D. Dinsmore, Solid State Commun. 139 (2006) 547–556.
[11] N. Zgheib, J.L. Putaux, A. Thill, F. D’Agosto, M. Lansalot, E. Bourgeat-Lami, Langmuir 28 (2012) 6163–6174. [12] S. Cauvin, P.J. Colver Patrick, A.F. Stefan, Macromolecules 38 (2005) 7887– 7889. [13] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Science 298 (2002) 1006–1009. [14] M. Williams, S.P. Armes, D.W. York, Langmuir (2012) 1142–1148. [15] Z. Wang, M.C. van Oers, F.P. Rutjes, J.C. van Hest, Angew. Chem. Int. Ed. 5 (2012) 1–6. [16] Y. Zhao, G. Yin, Z. Zheng, H. Wang, Q. Du, J. Polym, Part A: Polym. Chem. 49 (2011) 5257–5269. [17] M. Amirkhani, N. Berger, M. Abdelmohsen, F. Zocholl, M.R. Gonçalves, O. Marti, J. Polym. Sci., Part B: Polym. Phys. 49 (2011) 1430–1436. [18] A.F. Bon Stefan, T. Chen, Langmuir 23 (2007) 9527–9530. [19] N.P. Pardhy, B.M. Budhlall, Langmuir 26 (2010) 13130–13141. [20] V.N. Paunov, O.J. Cayre, Adv. Mater. 16 (2004) 788–791. [21] C. Li, P. Sun, C. Yang, Starch 64 (2012) 497–502. [22] I. Nicander, S. Ollmar, A. EeK, B.L. Rozell, L. Emtestam, Br. J. Dermato. 134 (1996) 221–228. [23] B.P. Binks, S.O. Lumsdon, Langmuir 17 (2001) 4540–4547. [24] E.T.A. van den Dungen, P.C. Hartmann, Appl. Clay Sci. 55 (2012) 120–124. [25] J. Wang, F. Yang, J. Tan, G. Liu, J. Xu, D. Sun, Langmuir 26 (2010) 5397–5404. [26] N.P. Ashby, B.P. Binks, PCCP 2 (2000) 5640–5646. [27] J. Wang, G. Liu, L. Wang, C. Li, J. Xu, D. Sun, Colloids Surf. A 353 (2010) 117–124. [28] M. Reger, H. Hoffmann, J. Colloid Interface Sci. 368 (2012) 378–386. [29] M. Reger, T. Sekine, T. Okamoto, K. Watanabe, H. Hoffmann, Soft Matter 7 (2011) 11021–11030. [30] Z. Wei, C. Wang, H. Liu, S. Zou, Z. Tong, J. Appl. Polym. Sci. 125 (2012). pp. E358–E368. [31] W. Li, L. Yu, G. Liu, J. Tan, S. Liu, D. Sun, Colloids Surf. A 400 (2012) 44–51. [32] T. Saigal, H. Dong, K. Matyjaszewski, R.D. Tilton, Langmuir 26 (2010) 15200– 15209. [33] B.P. Binks, S.O. Lumsdon, Langmuir 16 (2000) 8622–8631. [34] A. Walther, M. Hoffmann, A.H.E. Müller, Angew. Chem. 120 (2008) 723–726. [35] A.F. Mejia, A. Diaz, S. Pullela, Y.W. Chang, M.C. Carpenter, J.D. Batteas, M.S. Mannan, A. Clearfield, Z. Cheng, Soft Matter 8 (2012) 10245–10254. [36] P.A. Wheeler, J. Wang, J. Bake, L.J. Mathias, Chem. Mater. 17 (2005) 3012–3018. [37] N.N. Herrera, J.M. Letoffe, J.L. Putaux, L. David Laurent, E. Bourgeat-Lami, Langmuir 20 (2004) 1564–1571. [38] J. Zhang, L. Li, J. Wang, Langmuir 28 (2012) 6769–6775.