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W Pickering emulsion with oxygen plasma treated carbon nanotubes as surfactants
Journal of Industrial and Engineering Chemistry 17 (2011) 455–460
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of O/W Pickering emulsion with oxygen plasma treated carbon nanotubes as surfactants Wenbao Chen a, Xuyan Liu b, Yangshuo Liu a, Youngkil Bang a, Hyung-Il Kim a,* a b
Department of Industrial Chemistry, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, South Korea Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, South Korea
A R T I C L E I N F O
Article history: Received 21 July 2010 Accepted 7 October 2010 Available online 7 May 2011 Keywords: Surface modification Solid emulsifier Oil-in-water emulsion Stability
A B S T R A C T
The stable oil-in-water Pickering emulsions were prepared using oxygen plasma treated carbon nanotubes (CNTs) as stabilizers. Some hydrophilic groups such as hydroxyl and carboxyl groups were introduced on the surface of the CNTs by plasma treatment. The plasma treated CNTs showed the improvement in dispersion stability in water as well as cyclohexane for a long time. The Raman spectra showed that the original properties of CNTs were retained after the plasma treatment differently from other chemical modification methods. The plasma treated CNTs had a favorable interfacial interaction with water to form the stable O/W Pickering emulsions. The formation of stable O/W Pickering emulsion was dependent on the CNT concentration, the plasma treatment period, and the sonication time. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction It has long been known that the stable emulsions can be readily obtained by adding surfactants to immiscible liquids and providing mechanical energy to break up the mixture into micrometric droplets immersed in a continuous phase [1–5]. However, about a decade ago, a novel method called Pickering emulsion was proposed. Pickering [6] and Ramsden [7] reported the paraffin– water emulsions with solid colloids, which generated the selfassembly at the interface between the two immiscible phases, inhibiting the coalescence of the emulsion. Evidence has been gathered that emulsion stability does not necessarily require the amphiphilic surfactants in order to reduce the interfacial tension but can be efficiently promoted by the dispersed particles in the colloidal size range. The Pickering colloidal systems were recently studied by many researchers. Armes et al. [8,9] used the nanosilica particles as Pickering emulsifier and successfully prepared the raspberry-like organic–inorganic hybrid microspheres. Sacanna and Philipse [10] provided a single-step synthesis of monodisperse core/shell colloids based on the spontaneous Pickering emulsification. Pozzo et al. [11] built a small angle scattering model for Pickering emulsions and raspberry-like particles. It was well-accepted that the particle wetting properties played a major role for the evolving emulsion type. The particles should be partly wetted by both phases for an effective stabilization. In
* Corresponding author. Tel.: +82 42 821 6694; fax: +82 42 821 8999. E-mail address: [email protected] (H.-I. Kim).
general, hydrophilic colloids tended to stabilize the oil-in-water emulsions, while the water-in-oil emulsions were better stabilized by the hydrophobic particles [12]. If the particles were wetted too strongly by either water or oil, these particles were dispersed mostly in one of these phases, leading to the problem of inefficient stabilization. Among all the solid particles which are apt to work as the emulsifiers, carbon nanotubes (CNTs) have attracted the tremendous interests because of their unique optical [13], mechanical [14], and electrical properties [15]. However, the poor solubility of pristine CNTs in most solvents limited its application as emulsifier to make Pickering emulsion. Several W/O emulsions prepared with CNTs were reported recently. Hobbie et al. [16] prepared a W/O emulsion in toluene, with the single-walled CNTs residing at the interface between the immiscible fluids. Shen and Resasco [17] used silica modified CNTs as surfactant to make a W/O emulsion. Chen et al. [18] reported the large-scale fabrication of CNT capsules directly by a W/O emulsion technique with the acid-modified CNTs which were chopped due to the acid treatment. There was nearly no research reported with respect to the O/W emulsion prepared with CNTs owing to the hydrophobic property of CNTs. We introduced a novel approach toward fabricating O/W Pickering emulsion based on the oxygen plasma treated CNTs as the solid emulsifiers. In contrast to other conventional methods to modify the CNTs, the plasma modification of CNTs provided the efficient and low temperature method, which was valuable for the industrial production, because of non-polluting, little damage to the CNTs, and time-saving [19–21]. The plasma treatment is effective in introducing the polar functional groups on the
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.10.027
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W. Chen et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 455–460
substrates. The surface energy of CNTs was well known to increase by treating with plasma due to the increased polarity and hydrophilictiy [22]. After the plasma treatment, the strong interaction between CNTs and aqueous phase enabled the CNTs to stabilize the oil droplets without using any additional surfactant. The effects of plasma treatment of CNTs on the formation of O/W Pickering emulsion were investigated in terms of surface functionality and morphology. 2. Materials and methods 2.1. Materials Multi-walled CNTs, which were 110–170 nm in diameter and 5–9 mm in length with purity over 90%, were purchased from Aldrich and used as received. Sudan III as the oil soluble dye was purchased from Sigma–Aldrich. Cyclohexane (anhydrous, 99.5%) was purchased from Aldrich. Deionized water was used as an aqueous phase. 2.2. Plasma treatment of CNTs CNTs were treated with oxygen plasma (radio frequency of 13.56 MHz, Model EPPs 2000, Plasmart Inc., Korea) at a power of 100 W and a pressure of 200 mTorr for several different periods. Oxygen was introduced to the plasma instrument with the flow rate of 10 ml/min. 2.3. Preparation of O/W Pickering emulsions The oil soluble dye, sudan III, was dissolved in cyclohexane and sonicated for 10 min. Meanwhile, the plasma treated CNTs were sonicated in water for 30 min. The aqueous dispersions containing [(Fig._1)TD$IG]
the plasma treated CNTs were mixed with cycohexane dyed solution and sonicated for a given period to get the O/W Pickering emulsions. The emulsions were transferred to 20 ml volume vials with stopper for the following characterizations. 2.4. Measurement The Raman spectra were recorded on a SPEC 1404p single grating Raman spectrometer with a CCD detector. An argon laser source emitting at an excitation laser wavelength of 632.8 nm was used with an excitation power of 100 mW. The surface chemical properties of the samples were investigated using a MultiLab ESCA 2000 (VG Micro Tech Co.). The pressure inside the chamber was held at below 5 10 10 Torr during the analysis. The CNTs were mounted on the standard sample studs by means of double-sided adhesive tapes. The corelevel signals were obtained at the photoelectron takeoff angle of 458. Preliminary data analysis and quantification were performed using XPSPEAK 4.1 software. The dispersion stability of both pristine and plasma treated CNTs was tested for the dispersions in water and cyclohexane, respectively. 3 mg of CNTs were put in 10 ml of liquid medium and sonicated for 30 min. After standing for 24 h, photographs were taken for the CNT dispersions to evaluate the dispersion stability. The morphology of the Pickering emulsion was observed with a Leica DM 2000 microscope equipped with a Canon powershot A95 camera. The emulsions which were diluted with water were placed on the microscope slide and covered with a cover slip for the clear image under the microscope. The size of the droplets in Pickering emulsions was determined by dynamic light scattering (ELS-Z, Otsuka, Japan) at 25 8C using a He–Ne laser (633 nm) as a light source. The scattered light was measured at the angle of 908 and collected by the autocorrelator.
Fig. 1. XPS spectra of CNTs. (a) A: pristine CNTs, and B: O2 plasma-treated CNTs, (b) C1 s spectra of pristine CNTs, and (c) C1s spectra of O2 plasma-treated CNTs (plasma treatment time: 20 min).
[(Fig._2)TD$IG]
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457
Table 1 C1s surface compositions of the pristine and the plasma treated CNTs. Component
C1s
Peak position (eV) Concentration (%) Pristine CNTs Plasma treated CNTs
1
2
3
C–C
C–OH
C5 5O
4 COOH
284.5
285.8
287.5
289.0
90.0 76.7
10.0 17.5
– 3.8
– 2.0
3. Results and discussion 3.1. Characterizations of plasma treated CNTs 3.1.1. XPS analysis XPS analysis was carried out to investigate the surface functional groups on the pristine and the plasma-modified CNTs. As shown in Fig. 1, the obvious increase of the O1 s peak was observed in the plasma-treated CNTs due to the oxygen group introduced during the plasma treatment. For the plasma-treated CNTs, the main binding-energy peak was attributed to the C–C C1s (284.5 eV), while the three other peaks were assigned to –C–OH C1 s (285.8 eV), –C5 5O C1s (287.5 eV), and –COOH C1s (289 eV), respectively [23]. The relative contents of these different groups were calculated based on the relative area of each peak as shown in Table 1. Compared with the pristine CNTs, the apparent increase in the peak intensity at 285.8, 287.5, and 289 eV could be observed in the plasma-treated CNTs. The increased amount of surface hydrophilic groups played an important role in the dispersion stability the plasma-treated CNTs in water.
[(Fig._3)TD$IG]
[(Fig._4)TD$IG]
Fig. 2. Raman spectra of the pristine and the plasma treated CNTs.
3.1.2. Raman spectroscopy The Raman spectra of the pristine and the plasma-treated CNTs are shown in Fig. 2. The spectra were characterized by two intense peaks at 1324 cm 1 and 1570 cm 1. The peak around 1324 cm 1 is usually called D-band, which is associated with the vibration of sp3-bonded carbon atoms existing at the defects in the hexagonal graphitic layers. The intensity of the D-band was known to increase when CNTs were chemically functionalized [24]. The peak at 1570 cm 1 was generally called G-band, which was related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice. The ratio of the intensity of D to G bands was known to be correlated to the discord of the CNTs, which was used
Fig. 3. SEM images of (a) the pristine CNTs and (b) the plasma treated CNTs (inserts: SEM images of higher magnification).
Fig. 4. Photographs of CNTs dispersions (0.03 wt%) after 24 h from the preparation: (a) pristine CNTs dispersed in water, (b) pristine CNTs dispersed in cyclohexane, (c) plasma treated CNTs dispersed in water, and (d) plasma treated CNTs dispersed in cyclohexane.
[(Fig._5)TD$IG]
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surface functional groups introduced on the plasma treated CNTs seemed responsible for the uniform distribution of CNTs. Both the length and the diameter of the pristine CNTs and the plasmamodified CNTs were nearly same as shown in the inserted images. It was clear that the O2 plasma treatment did not break the CNTs but preserved the integrity of CNTs differently from the conventional acid-treatment which would result in the shorter and destructive CNTs [26].
Fig. 5. Photograph of CNTs dispersions (0.03 wt%) after 24 h from the preparation with various different periods of plasma treatment: (a) 5 min, (b) 10 min, (c) 20 min, and (d) 30 min.
to estimate the maintenance of the original properties of CNTs. The values of ID/IG for the pristine and the plasma treated CNTs did not show any noticeable change. Therefore the plasma treated CNTs kept the original graphite structures, which were related with the mechanical and conductive properties of CNTs [25]. 3.1.3. Morphology of plasma treated CNTs Scanning electron microscopy (SEM) was performed to evaluate the morphological change of CNTs after plasma treatment. As shown in Fig. 3, the pristine CNTs showed some agglomeration, which was due to the entanglement among the CNTs. However, the plasma treated CNTs showed a quite less agglomeration. The
[(Fig._6)TD$IG]
3.1.4. Dispersion stability of CNTs Fig. 4 shows the CNTs dispersions of 0.03 wt% in water and cyclohexane, respectively. Most of the pristine CNTs were settled down in the dispersion after 10 min due to the hydrophobicity of the CNTs meanwhile some CNTs remained as dispersed in cyclohexane. Therefore the pristine CNTs were more suitable as emulsifier for making W/O emulsion [27]. However, the plasma treated CNTs remained fairly dispersed in water after standing for 24 h, suggesting a favorable interactions formed between the water and modified CNTs. The hydrophilic groups such as hydroxyl and carboxyl groups on the plasma treated CNTs contributed a large share to these favorable interactions. Although the dispersion stability of CNTs in cyclohexane was also enhanced after plasma treatment, some sedimentation was observed after staying for 24 h. The plasma treated CNTs gave better dispersion stability in water rather than in cyclohexane. Plasma treatment period played a very important role in the degree of modification of various materials including the CNTs. Fig. 5 shows the aqueous dispersions of CNTs with various plasma treatment periods. For the plasma treatment for 10 min or less, the surface modification of CNTs was not enough to prepare the stable dispersion successfully. As the plasma treatment period increased, more hydroxyl and carboxyl groups were introduced on the surface of CNTs resulting in the formation of homogeneous and stable dispersions.
Fig. 6. Optical images of O/W Pickering emulsions prepared with plasma treated CNTs (treated for 20 min) depending on the various sonication times: (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min.
[(Fig._7)TD$IG]
W. Chen et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 455–460
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3.2. Characterizations of O/W Pickering emulsions prepared with plasma treated CNTs
[(Fig._8)TD$IG]
3.2.1. Effect of sonication time on morphology of emulsions The morphology of emulsion and the size of the droplets depend on several factors such as size, shape, concentration of solid particles, and sonication time. The effect of sonication time on the morphology of O/W Pickering emulsion was studied with the sonication power kept constant at 30 W. As shown in Fig. 6, the stable O/W Pickering emulsion was not formed when the sonication time was less than 10 min. As the sonication time increased, the stable O/W emulsion was prepared and nearly all the oil phase droplets kept their morphology very well. Therefore the plasma treated CNTs were qualified to be an effective emulsifier to prepare the stable O/W Pickering emulsions.
Fig. 7. Variation of the oil phase droplet size as a function of plasma treated CNTs concentration.
3.2.2. Effect of plasma treated CNTs content on morphology of emulsion To investigate the effect of CNTs concentration on the droplet size several emulsions were prepared with various concentrations of CNTs (0.01–0.05 wt%) with a constant oil to water ratio. The
Fig. 8. Optical images of O/W Pickering emulsions prepared with CNTs which were plasma treated for various different periods: (a) 5 min, (b) 10 min, (c) 20 min, (d) 30 min, and (e) non-treated.
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droplet size was measured with dynamic light scattering. As shown in Fig. 7, O/W emulsions prepared with lower CNTs concentration resulted in the relatively bigger size and wider distribution of droplets. As the CNTs concentration increased, both the size and the distribution of droplets decreased fast due to the decrease in the oil–water interfacial tension. 3.2.3. Effect of plasma treatment period on morphology of emulsion It is well known that solid particles should spontaneously be localized at the liquid–liquid interfaces to act as stabilizers for Pickering emulsions. Hydrophilic particles tended to form O/W emulsions whereas hydrophobic particles form W/O emulsions. However, if the particles were excessively wetted by water or oil, they remained dispersed in either phase and no stable emulsions were formed. In our research, five different kinds of CNTs were prepared with various plasma treatment periods from 0 min to 30 min in order to control the surface wettability of the CNTs. O/W Pickering emulsions were prepared successfully with these plasma treated CNTs using a constant oil-to-water ratio and kept for 1 week as shown in Fig. 8. However, no intact droplets were observed when using the CNTs without plasma treatment. The droplets had a wide size distribution accompanied with some agglomerations when the plasma treatment period was not long enough. There were no sufficient hydrophilic groups formed on the surface of CNTs to make O/W emulsions after the short period of plasma treatment. As the plasma treatment period increased, nearly all the droplets kept their integrity and had a narrow size distribution. However, the plasma treatment over 30 min caused the preferential wetting between CNTs and water due to the more hydrophilic nature of the plasma treated CNTs resulting in the more CNTs located in the aqueous phase and the less stable O/W emulsions. 4. Conclusions The stable O/W Pickering emulsions were prepared based on the oxygen plasma treated CNTs as stabilizers. The CNTs having
different wettability with water could be produced by altering the plasma treatment period. The plasma treatment did not give any noticeable damage to CNTs differently from other chemical methods. The size and size distribution of oil phase droplets were controlled by changing the sonication time, the CNTs concentration, and the plasma treatment periods. The functional composites, microspheres, and microcapsules could be synthesized easily with oil soluble monomers base on the O/W Pickering emulsions using the plasma treated CNTs as the solid emulsifiers.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
M.E. Karaman, B.W. Ninham, R.M. Pashley, J. Phys. Chem. 100 (1996) 15503. P. Meneghetti, S. Qutubuddin, Langmuir 20 (2004) 3424. H.J. Yang, W.G. Cho, S.N. Park, J. Ind. Eng. Chem. 15 (2009) 331. Y. Doi, Y. Kawashima, K. Matsuoka, Y. Moroi, J. Phys. Chem. B 108 (2004) 2594. G. Otieno, J.Y. Kim, J. Ind. Eng. Chem. 14 (2008) 187. S.U. Pickering, CXCVI emulsions, J. Chem. Soc. 91 (1907) 2001. W. Ramsden, Proc. R. Soc. Lond. 72 (1904) 156. C. Barthet, A.J. Hickey, D.B. Cairns, S.P. Armes, Adv. Mater. 11 (1999) 408. M.J. Percy, C. Barthet, J.C. Lobb, M.A. Khan, S.F. Lascelles, M. Vamvakaki, S.P. Armes, Langmuir 16 (2000) 6913. S. Sacanna, A.P. Philipse, Adv. Mater. 19 (2007) 3824. K. Larson-Smith, K. Jackson, D.C. Pozzo, J. Colloid Interface Sci. 343 (2010) 36. B.P. Binks, J.H. Clint, Langmuir 18 (2002) 1270. S.D. Tan, H.A. Lopez, C.W. Cai, Y.G. Zhang, Nano Lett. 4 (2004) 1415. J.W. Shin, J.P. Jeun, P.H. Kang, J. Ind. Eng. Chem. 15 (2009) 555. J.S. Im, S.J. Kim, P.H. Kang, Y.S. Lee, J. Ind. Eng. Chem. 15 (2009) 699. H. Wang, E.K. Hobbie, Langmuir 19 (2003) 3091. M. Shen, D.E. Resasco, Langmuir 25 (2009) 10843. H.Y. Yi, H.H. Song, X.H. Chen, Langmuir 23 (2007) 3199. I. Bica, J. Ind. Eng. Chem. 14 (2008) 230. K. Yu, Z.Q. Zhu, M. Xu, Q. Li, W. Lu, Chem. Phys. Lett. 373 (2003) 109. W.G. Lee, Y.J. Kwon, J. Ind. Eng. Chem. 14 (2008) 89. O.J. Yoon, H.W. Kim, D.J. Kim, H.J. Lee, J.Y. Yun, Y.H. Noh, D.Y. Lee, D.H. Kim, S.S. Kim, N.E. Lee, Plasma Process. Polym. 6 (2009) 101. Y. Peng, H.W. Liu, Ind. Eng. Chem. Res. 45 (2006) 6483. M.F. Zhang, M. Yudasaka, A. Koshio, S. Iijima, Chem. Phys. Lett. 364 (2002) 420. J.W. Klett, A.D. Mcmilian, N.C. Gallego, C.A. Walls, J. Mater. Sci. 39 (2004) 3659. S.M. Kwon, H.S. Kim, S.J. Myung, H.J. Jin, J. Polym. Sci. Polym. Phys. 46 (2008) 182. Y.J. Zhang, Y.F. Shen, D. Kuehner, S.X. Wu, Z.M. Su, S. Ye, L. Niu, Chem. Commun. 36 (2008) 4273.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of O/W Pickering emulsion with oxygen plasma treated carbon nanotubes as surfactants Wenbao Chen a, Xuyan Liu b, Yangshuo Liu a, Youngkil Bang a, Hyung-Il Kim a,* a b
Department of Industrial Chemistry, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, South Korea Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong, Yuseong-Gu, Daejeon 305-764, South Korea
A R T I C L E I N F O
Article history: Received 21 July 2010 Accepted 7 October 2010 Available online 7 May 2011 Keywords: Surface modification Solid emulsifier Oil-in-water emulsion Stability
A B S T R A C T
The stable oil-in-water Pickering emulsions were prepared using oxygen plasma treated carbon nanotubes (CNTs) as stabilizers. Some hydrophilic groups such as hydroxyl and carboxyl groups were introduced on the surface of the CNTs by plasma treatment. The plasma treated CNTs showed the improvement in dispersion stability in water as well as cyclohexane for a long time. The Raman spectra showed that the original properties of CNTs were retained after the plasma treatment differently from other chemical modification methods. The plasma treated CNTs had a favorable interfacial interaction with water to form the stable O/W Pickering emulsions. The formation of stable O/W Pickering emulsion was dependent on the CNT concentration, the plasma treatment period, and the sonication time. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction It has long been known that the stable emulsions can be readily obtained by adding surfactants to immiscible liquids and providing mechanical energy to break up the mixture into micrometric droplets immersed in a continuous phase [1–5]. However, about a decade ago, a novel method called Pickering emulsion was proposed. Pickering [6] and Ramsden [7] reported the paraffin– water emulsions with solid colloids, which generated the selfassembly at the interface between the two immiscible phases, inhibiting the coalescence of the emulsion. Evidence has been gathered that emulsion stability does not necessarily require the amphiphilic surfactants in order to reduce the interfacial tension but can be efficiently promoted by the dispersed particles in the colloidal size range. The Pickering colloidal systems were recently studied by many researchers. Armes et al. [8,9] used the nanosilica particles as Pickering emulsifier and successfully prepared the raspberry-like organic–inorganic hybrid microspheres. Sacanna and Philipse [10] provided a single-step synthesis of monodisperse core/shell colloids based on the spontaneous Pickering emulsification. Pozzo et al. [11] built a small angle scattering model for Pickering emulsions and raspberry-like particles. It was well-accepted that the particle wetting properties played a major role for the evolving emulsion type. The particles should be partly wetted by both phases for an effective stabilization. In
* Corresponding author. Tel.: +82 42 821 6694; fax: +82 42 821 8999. E-mail address: [email protected] (H.-I. Kim).
general, hydrophilic colloids tended to stabilize the oil-in-water emulsions, while the water-in-oil emulsions were better stabilized by the hydrophobic particles [12]. If the particles were wetted too strongly by either water or oil, these particles were dispersed mostly in one of these phases, leading to the problem of inefficient stabilization. Among all the solid particles which are apt to work as the emulsifiers, carbon nanotubes (CNTs) have attracted the tremendous interests because of their unique optical [13], mechanical [14], and electrical properties [15]. However, the poor solubility of pristine CNTs in most solvents limited its application as emulsifier to make Pickering emulsion. Several W/O emulsions prepared with CNTs were reported recently. Hobbie et al. [16] prepared a W/O emulsion in toluene, with the single-walled CNTs residing at the interface between the immiscible fluids. Shen and Resasco [17] used silica modified CNTs as surfactant to make a W/O emulsion. Chen et al. [18] reported the large-scale fabrication of CNT capsules directly by a W/O emulsion technique with the acid-modified CNTs which were chopped due to the acid treatment. There was nearly no research reported with respect to the O/W emulsion prepared with CNTs owing to the hydrophobic property of CNTs. We introduced a novel approach toward fabricating O/W Pickering emulsion based on the oxygen plasma treated CNTs as the solid emulsifiers. In contrast to other conventional methods to modify the CNTs, the plasma modification of CNTs provided the efficient and low temperature method, which was valuable for the industrial production, because of non-polluting, little damage to the CNTs, and time-saving [19–21]. The plasma treatment is effective in introducing the polar functional groups on the
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.10.027
456
W. Chen et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 455–460
substrates. The surface energy of CNTs was well known to increase by treating with plasma due to the increased polarity and hydrophilictiy [22]. After the plasma treatment, the strong interaction between CNTs and aqueous phase enabled the CNTs to stabilize the oil droplets without using any additional surfactant. The effects of plasma treatment of CNTs on the formation of O/W Pickering emulsion were investigated in terms of surface functionality and morphology. 2. Materials and methods 2.1. Materials Multi-walled CNTs, which were 110–170 nm in diameter and 5–9 mm in length with purity over 90%, were purchased from Aldrich and used as received. Sudan III as the oil soluble dye was purchased from Sigma–Aldrich. Cyclohexane (anhydrous, 99.5%) was purchased from Aldrich. Deionized water was used as an aqueous phase. 2.2. Plasma treatment of CNTs CNTs were treated with oxygen plasma (radio frequency of 13.56 MHz, Model EPPs 2000, Plasmart Inc., Korea) at a power of 100 W and a pressure of 200 mTorr for several different periods. Oxygen was introduced to the plasma instrument with the flow rate of 10 ml/min. 2.3. Preparation of O/W Pickering emulsions The oil soluble dye, sudan III, was dissolved in cyclohexane and sonicated for 10 min. Meanwhile, the plasma treated CNTs were sonicated in water for 30 min. The aqueous dispersions containing [(Fig._1)TD$IG]
the plasma treated CNTs were mixed with cycohexane dyed solution and sonicated for a given period to get the O/W Pickering emulsions. The emulsions were transferred to 20 ml volume vials with stopper for the following characterizations. 2.4. Measurement The Raman spectra were recorded on a SPEC 1404p single grating Raman spectrometer with a CCD detector. An argon laser source emitting at an excitation laser wavelength of 632.8 nm was used with an excitation power of 100 mW. The surface chemical properties of the samples were investigated using a MultiLab ESCA 2000 (VG Micro Tech Co.). The pressure inside the chamber was held at below 5 10 10 Torr during the analysis. The CNTs were mounted on the standard sample studs by means of double-sided adhesive tapes. The corelevel signals were obtained at the photoelectron takeoff angle of 458. Preliminary data analysis and quantification were performed using XPSPEAK 4.1 software. The dispersion stability of both pristine and plasma treated CNTs was tested for the dispersions in water and cyclohexane, respectively. 3 mg of CNTs were put in 10 ml of liquid medium and sonicated for 30 min. After standing for 24 h, photographs were taken for the CNT dispersions to evaluate the dispersion stability. The morphology of the Pickering emulsion was observed with a Leica DM 2000 microscope equipped with a Canon powershot A95 camera. The emulsions which were diluted with water were placed on the microscope slide and covered with a cover slip for the clear image under the microscope. The size of the droplets in Pickering emulsions was determined by dynamic light scattering (ELS-Z, Otsuka, Japan) at 25 8C using a He–Ne laser (633 nm) as a light source. The scattered light was measured at the angle of 908 and collected by the autocorrelator.
Fig. 1. XPS spectra of CNTs. (a) A: pristine CNTs, and B: O2 plasma-treated CNTs, (b) C1 s spectra of pristine CNTs, and (c) C1s spectra of O2 plasma-treated CNTs (plasma treatment time: 20 min).
[(Fig._2)TD$IG]
W. Chen et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 455–460
457
Table 1 C1s surface compositions of the pristine and the plasma treated CNTs. Component
C1s
Peak position (eV) Concentration (%) Pristine CNTs Plasma treated CNTs
1
2
3
C–C
C–OH
C5 5O
4 COOH
284.5
285.8
287.5
289.0
90.0 76.7
10.0 17.5
– 3.8
– 2.0
3. Results and discussion 3.1. Characterizations of plasma treated CNTs 3.1.1. XPS analysis XPS analysis was carried out to investigate the surface functional groups on the pristine and the plasma-modified CNTs. As shown in Fig. 1, the obvious increase of the O1 s peak was observed in the plasma-treated CNTs due to the oxygen group introduced during the plasma treatment. For the plasma-treated CNTs, the main binding-energy peak was attributed to the C–C C1s (284.5 eV), while the three other peaks were assigned to –C–OH C1 s (285.8 eV), –C5 5O C1s (287.5 eV), and –COOH C1s (289 eV), respectively [23]. The relative contents of these different groups were calculated based on the relative area of each peak as shown in Table 1. Compared with the pristine CNTs, the apparent increase in the peak intensity at 285.8, 287.5, and 289 eV could be observed in the plasma-treated CNTs. The increased amount of surface hydrophilic groups played an important role in the dispersion stability the plasma-treated CNTs in water.
[(Fig._3)TD$IG]
[(Fig._4)TD$IG]
Fig. 2. Raman spectra of the pristine and the plasma treated CNTs.
3.1.2. Raman spectroscopy The Raman spectra of the pristine and the plasma-treated CNTs are shown in Fig. 2. The spectra were characterized by two intense peaks at 1324 cm 1 and 1570 cm 1. The peak around 1324 cm 1 is usually called D-band, which is associated with the vibration of sp3-bonded carbon atoms existing at the defects in the hexagonal graphitic layers. The intensity of the D-band was known to increase when CNTs were chemically functionalized [24]. The peak at 1570 cm 1 was generally called G-band, which was related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice. The ratio of the intensity of D to G bands was known to be correlated to the discord of the CNTs, which was used
Fig. 3. SEM images of (a) the pristine CNTs and (b) the plasma treated CNTs (inserts: SEM images of higher magnification).
Fig. 4. Photographs of CNTs dispersions (0.03 wt%) after 24 h from the preparation: (a) pristine CNTs dispersed in water, (b) pristine CNTs dispersed in cyclohexane, (c) plasma treated CNTs dispersed in water, and (d) plasma treated CNTs dispersed in cyclohexane.
[(Fig._5)TD$IG]
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surface functional groups introduced on the plasma treated CNTs seemed responsible for the uniform distribution of CNTs. Both the length and the diameter of the pristine CNTs and the plasmamodified CNTs were nearly same as shown in the inserted images. It was clear that the O2 plasma treatment did not break the CNTs but preserved the integrity of CNTs differently from the conventional acid-treatment which would result in the shorter and destructive CNTs [26].
Fig. 5. Photograph of CNTs dispersions (0.03 wt%) after 24 h from the preparation with various different periods of plasma treatment: (a) 5 min, (b) 10 min, (c) 20 min, and (d) 30 min.
to estimate the maintenance of the original properties of CNTs. The values of ID/IG for the pristine and the plasma treated CNTs did not show any noticeable change. Therefore the plasma treated CNTs kept the original graphite structures, which were related with the mechanical and conductive properties of CNTs [25]. 3.1.3. Morphology of plasma treated CNTs Scanning electron microscopy (SEM) was performed to evaluate the morphological change of CNTs after plasma treatment. As shown in Fig. 3, the pristine CNTs showed some agglomeration, which was due to the entanglement among the CNTs. However, the plasma treated CNTs showed a quite less agglomeration. The
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3.1.4. Dispersion stability of CNTs Fig. 4 shows the CNTs dispersions of 0.03 wt% in water and cyclohexane, respectively. Most of the pristine CNTs were settled down in the dispersion after 10 min due to the hydrophobicity of the CNTs meanwhile some CNTs remained as dispersed in cyclohexane. Therefore the pristine CNTs were more suitable as emulsifier for making W/O emulsion [27]. However, the plasma treated CNTs remained fairly dispersed in water after standing for 24 h, suggesting a favorable interactions formed between the water and modified CNTs. The hydrophilic groups such as hydroxyl and carboxyl groups on the plasma treated CNTs contributed a large share to these favorable interactions. Although the dispersion stability of CNTs in cyclohexane was also enhanced after plasma treatment, some sedimentation was observed after staying for 24 h. The plasma treated CNTs gave better dispersion stability in water rather than in cyclohexane. Plasma treatment period played a very important role in the degree of modification of various materials including the CNTs. Fig. 5 shows the aqueous dispersions of CNTs with various plasma treatment periods. For the plasma treatment for 10 min or less, the surface modification of CNTs was not enough to prepare the stable dispersion successfully. As the plasma treatment period increased, more hydroxyl and carboxyl groups were introduced on the surface of CNTs resulting in the formation of homogeneous and stable dispersions.
Fig. 6. Optical images of O/W Pickering emulsions prepared with plasma treated CNTs (treated for 20 min) depending on the various sonication times: (a) 5 min, (b) 10 min, (c) 15 min, and (d) 20 min.
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3.2. Characterizations of O/W Pickering emulsions prepared with plasma treated CNTs
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3.2.1. Effect of sonication time on morphology of emulsions The morphology of emulsion and the size of the droplets depend on several factors such as size, shape, concentration of solid particles, and sonication time. The effect of sonication time on the morphology of O/W Pickering emulsion was studied with the sonication power kept constant at 30 W. As shown in Fig. 6, the stable O/W Pickering emulsion was not formed when the sonication time was less than 10 min. As the sonication time increased, the stable O/W emulsion was prepared and nearly all the oil phase droplets kept their morphology very well. Therefore the plasma treated CNTs were qualified to be an effective emulsifier to prepare the stable O/W Pickering emulsions.
Fig. 7. Variation of the oil phase droplet size as a function of plasma treated CNTs concentration.
3.2.2. Effect of plasma treated CNTs content on morphology of emulsion To investigate the effect of CNTs concentration on the droplet size several emulsions were prepared with various concentrations of CNTs (0.01–0.05 wt%) with a constant oil to water ratio. The
Fig. 8. Optical images of O/W Pickering emulsions prepared with CNTs which were plasma treated for various different periods: (a) 5 min, (b) 10 min, (c) 20 min, (d) 30 min, and (e) non-treated.
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droplet size was measured with dynamic light scattering. As shown in Fig. 7, O/W emulsions prepared with lower CNTs concentration resulted in the relatively bigger size and wider distribution of droplets. As the CNTs concentration increased, both the size and the distribution of droplets decreased fast due to the decrease in the oil–water interfacial tension. 3.2.3. Effect of plasma treatment period on morphology of emulsion It is well known that solid particles should spontaneously be localized at the liquid–liquid interfaces to act as stabilizers for Pickering emulsions. Hydrophilic particles tended to form O/W emulsions whereas hydrophobic particles form W/O emulsions. However, if the particles were excessively wetted by water or oil, they remained dispersed in either phase and no stable emulsions were formed. In our research, five different kinds of CNTs were prepared with various plasma treatment periods from 0 min to 30 min in order to control the surface wettability of the CNTs. O/W Pickering emulsions were prepared successfully with these plasma treated CNTs using a constant oil-to-water ratio and kept for 1 week as shown in Fig. 8. However, no intact droplets were observed when using the CNTs without plasma treatment. The droplets had a wide size distribution accompanied with some agglomerations when the plasma treatment period was not long enough. There were no sufficient hydrophilic groups formed on the surface of CNTs to make O/W emulsions after the short period of plasma treatment. As the plasma treatment period increased, nearly all the droplets kept their integrity and had a narrow size distribution. However, the plasma treatment over 30 min caused the preferential wetting between CNTs and water due to the more hydrophilic nature of the plasma treated CNTs resulting in the more CNTs located in the aqueous phase and the less stable O/W emulsions. 4. Conclusions The stable O/W Pickering emulsions were prepared based on the oxygen plasma treated CNTs as stabilizers. The CNTs having
different wettability with water could be produced by altering the plasma treatment period. The plasma treatment did not give any noticeable damage to CNTs differently from other chemical methods. The size and size distribution of oil phase droplets were controlled by changing the sonication time, the CNTs concentration, and the plasma treatment periods. The functional composites, microspheres, and microcapsules could be synthesized easily with oil soluble monomers base on the O/W Pickering emulsions using the plasma treated CNTs as the solid emulsifiers.
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