Journal of Colloid and Interface Science 450 (2015) 174–181
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of fluorinated ceramic Janus particles via a Pickering emulsion method Arnaud Zenerino a,b, Claire Peyratout b, Anne Aimable a,⇑ a Centre Européen de la Céramique – Ecole Nationale Supérieure de Céramique Industrielle (ENSCI), Science des Procédés Céramiques et de Traitements de Surface, UMR 7315, 12 rue Atlantis, 87068 Limoges, France b Centre Européen de la Céramique – Ecole Nationale Supérieure de Céramique Industrielle (ENSCI), Groupe d’Etudes des Matériaux Hétérogènes, 12 rue Atlantis, 87068 Limoges, France
g r a p h i c a l a b s t r a c t
Hydrophilic side
Hydrophobic side R = -C(O)C8F17
a r t i c l e
i n f o
Article history: Received 23 December 2014 Accepted 8 March 2015 Available online 16 March 2015 Keywords: Janus particles Pickering emulsion Silica particles Fluorinated particles
a b s t r a c t Hypothesis: Dispersion or aggregation of ceramic nanoparticles in suspension is mainly influenced by their surface properties. The preparation of hydrophobic/hydrophilic nanoparticles was studied by synthesizing for the first time fluorinated Janus particles via a Pickering emulsion method. Experiments: Fluorinated silica Janus particles were synthesized with a ‘‘grafting to’’ method using silica/ wax emulsions. The parameters investigated to control the emulsion were (i) the SiO2 particles/wax ratio, (ii) the cetyltrimethylammonium bromide (CTAB)/SiO2 ratio and (iii) the ionic strength, i.e. NaCl concentration. Prior to grafting with fluorinated molecules, the partially wax-embedded silica particles were functionalized with an aminosilane. Fourier Transformed Infrared and Nuclear Magnetic Resonance spectroscopies, thermogravimetric and zeta potential analyzes were conducted to characterize the fluorinated Janus particles. Findings: Silica quantity has an influence on the silica nanoparticles distribution at the surface of the wax solid droplets. CTAB amount controls the size and shape of silica/wax solid droplets, whereas the ionic strength in the range 0–6.22 g L1 does not influence the silica/wax emulsion. Aminosilane functionalization was successfully employed on partially wax-embedded silica particles, followed by a grafting step with a carboxylic acid fluorinated compound. Ó 2015 Elsevier Inc. All rights reserved.
⇑ Corresponding author. Fax: +33 (0) 587 50 23 04. E-mail addresses: [email protected] (A. Zenerino), claire.peyratout@ unilim.fr (C. Peyratout), [email protected] (A. Aimable). http://dx.doi.org/10.1016/j.jcis.2015.03.011 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
A. Zenerino et al. / Journal of Colloid and Interface Science 450 (2015) 174–181
175
1. Introduction
2.2. Preparation of Janus particles
Anisotropic particles, called Janus particles [1], contain one hydrophilic and one hydrophobic side that confer different chemical compositions and special wetting properties. These particles can effectively replace surfactants in emulsion stabilization [2]. Moreover, Janus particles are relevant to several industrial domains, such as cosmetics, paint and ink development. For technical ceramics, they could be used to complete theoretical studies on aggregation phenomena [3,4] or in additive processes [5], to stabilize suspensions in order to obtain a good dispersion favouring a homogeneous deposition. Pickering emulsion has been previously employed to synthesize Janus particles based on the preparation of a wax/water emulsion [6,7]. One side of the particles was masked in wax (paraffin) at room temperature and the other side could be chemically modified. The immersion of particles into the wax depends on the three-phase contact angle at the emulsion interface [6]. After removing the wax, Janus particles with controllable geometry were obtained. In the emulsion, particles spontaneously accumulate at the oil–water interface and their adsorption is practically irreversible [8]. Adding low quantity of surfactant to the emulsion modifies the wettability of solid particles by adsorbing onto their surface and increases the adsorption at the oil–water interface [9]. Additionally, the adsorption of surfactant at both solid–liquid and liquid–liquid interfaces may be modified by the presence of additives in emulsion systems like electrolytes, which modify the particles charge and flocculation properties [10]. To obtain anisotropic particles, techniques such as ‘‘grafting to’’ and ‘‘grafting from’’ methods [11], surface-initiated free radical polymerization [12,13] or electrostatic interaction methodology [14] have been employed. In this work, the ‘‘grafting to’’ method was used to obtain new fluorinated Janus particles. Oil/water emulsion was realized with paraffin, which is liquid at 60 °C, and was stabilized by silica particles. Since silica particles are hydrophilic, it is necessary to use a surfactant to improve their hydrophobicity. After cooling the experimental set-up at ambient temperature, wax becomes solid and the silica particles are partly embedded in the solid paraffin. The objective of this work is to understand which factors are controlling the embedment of the silica particles into the hydrophobic phase. Thus, the influence of the [SiO2]/[wax] ratio, the [CTAB]/[SiO2] ratio and the ionic strength were studied to determine the best conditions to realize a timestable emulsion with a homogeneous droplet size. Following the emulsion preparation, the non-protected side of particles was functionalized by an aminosilane and then a fluorinated compound was grafted to the free amine functions to obtain partially hydrophobic silica particles.
2.2.1. Amino-functionalization of silica/wax droplets The synthesis has been adapted from a previously published synthesis of Granick and coworkers [9,15]. Silica particles were dispersed in water with CTAB, wax was added and the mixture was heated at 90 °C. Emulsions were then realized by using Ultra Turrax stirring at 19,000 rpm for 1 min and then cooled to room temperature. The obtained silica/wax solid droplets were filtered and washed with deionized water to remove free particles and CTAB. After completely drying the silica/wax solid droplets in an oven at 50 °C, they were reacted with APTES (2 mM) and absolute methanol at ambient temperature for 45 min. Triethylamine (TEA) (2 mM) was used as a catalyst. After the reaction was completed, the mixture was filtered and washed with deionized water to remove the excess of silane and TEA. Amino group functionalized particles can be used as is or can further react with carboxylic acid groups of the fluorinated molecules.
2. Materials and methods 2.1. Chemicals Amorphous silica particles (Ref. SP03B) were purchased from the Fuso Chemical Company (d50 = 250 nm, specific surface area 12 m2 g1). Paraffin was purchased from Fisher Scientific. Triethylamine (TEA) (99%), (3-aminopropyl)triethoxysilane (APTES) (99%), N-hydroxysuccinimide (NHS) (99%), absolute methanol (99%) and cyclohexane (99%) were purchased from Alfa Aesar. Cetyltrimethylammonium bromide (99%) (CTAB), N-ethylcarbodiimide hydrochloride (EDCHCl) (98%) and heptadecafluorononanoic acid (97%) were purchased from Sigma Aldrich. All products have been used as received.
2.2.2. Coupling fluorinated molecules with silica/wax solid droplets A mixture of N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDCHCl) (1 eq) and N-hydroxysuccinimide (NHS) (2 eq) was added to a solution of heptadecafluorononanoic acid (1 eq) in water and was stirred during 2 h at ambient temperature. Amine functionalized silica/wax solid droplets (1 eq of –NH2) were then added and the reaction was maintained during five days at ambient temperature to obtain fluorinated-silica/wax solid droplets. The product was filtered and a white hydrophobic solid was obtained.
2.2.3. Wax dissolution Silica/wax solid droplets were introduced in cyclohexane and gently stirred at room temperature. After the complete paraffin dissolution in the solvent, silica functionalized particles were recovered by centrifugation at 11,000 rpm during 3 min.
2.3. Characterization methods Fourier Transformed Infrared (FTIR) spectra were carried out with a Nicolet 380 FTIR spectrometer from Thermo Scientific in the range 400–4000 cm1. The silica/wax solid droplets were imaged using a JEOL 7400 FEG-SEM Scanning Electron Microscope (SEM) or a Stereoscan S260 Cambridge microscope or using a Nikon Eclipse 50i optical microscope. The silica/wax solid droplets size distribution was determined by laser size measurement with a LA-950 from Horiba. Dynamic Light Scattering (DLS) with a Zetasizer Nano ZS equipped with a MPT-2 autotitrator from Malvern Instruments was used to determine zeta potential of particles. Aqueous suspensions of 0.015 wt% were prepared at a pH value around 10 with NaOH 0.1 M and titrated with a 0.01 M HCl solution. Three measurements were realized and an average value was determined. For amino-functionalized silica particles, the thermogravimetric measurements (TG) coupled with mass spectrometry were made on a TGA STA449F3 Jupiter with a graphite oven from Netzsch coupled with a Pfeiffer ThermoStar GSD 301T mass spectrometer. The samples were heated from 25 to 900 °C at 10 K min1 under argon flow of 20 mL min1. For fluorinated-silica particles, the thermogravimetric measurements were made on a TGA STA449F3 Jupiter with a silicon carbide oven from Netzsch. The samples were heated from 25 to 800 °C at 10 K min1 under argon flow of 20 mL min1. Nuclear Magnetic Resonance (NMR) spectrums were measured on a Brüker W-200 MHz.
176
A. Zenerino et al. / Journal of Colloid and Interface Science 450 (2015) 174–181
3. Results and discussion 3.1. Influence of the silica/wax ratio on the silica-decorated wax solid droplet size The silica-decorated wax solid droplet diameter was studied as a function of the surface ratio R between the silica particles surface (SSiO2) and the solid wax droplet surface (SWax). Results are presented in Table 1 and Fig. 1. When R increases from 1.21 until 1.54, by decreasing the amount of silica particles, the obtained wax droplets size increases from 40 lm to 140 lm respectively. As shown in Fig. 1, the droplet size growth is not linear with respect of R. In this regime, there are not enough silica particles to cover the wax droplet surface and a coalescence phenomenon is thus observed. Moreover silica particles bi- or multilayers were obtained for these ratios. When R is increased above 1.21 by adding silica particles, the droplet size is then stabilized at around 40 lm. Silica monolayer are obtained for R values equal or above 1.21. Microscopic observations showed that the optimal covering ratio R = 1 cannot be obtained with our experimental conditions and that a R value of 1.21 leads to the optimal coverage of wax droplets with silica particles. Moreover, the wax droplet coverage seems to be better organized and exhibits less defaults with R = 1.21. The surface coverage parameter, C, i.e. the packing density of the particles at the interface, is determined by the following relationship (Eq. (1)) in Pickering emulsion systems [16]:
1 mp ¼ D 4C qp dp V d
Fig. 1. Ratio of silica surface (S(SiO2))/wax surface (S(wax)) as a function of wax droplet diameter and SEM images. Scale bar = 2 lm.
Table 2 Weight of compounds according to the [CTAB]/[SiO2] ratio, diameter (D) and spherical coefficient of wax droplets. Ratio [CTAB]/[SiO2] Wax SiO2 [SiO2] (g L1) CTAB (mg) [CTAB] (mg L1) Total weight D (lm) Number of wax droplets measured Percentage of non spherical particles (when spherical coefficient is
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis of fluorinated ceramic Janus particles via a Pickering emulsion method Arnaud Zenerino a,b, Claire Peyratout b, Anne Aimable a,⇑ a Centre Européen de la Céramique – Ecole Nationale Supérieure de Céramique Industrielle (ENSCI), Science des Procédés Céramiques et de Traitements de Surface, UMR 7315, 12 rue Atlantis, 87068 Limoges, France b Centre Européen de la Céramique – Ecole Nationale Supérieure de Céramique Industrielle (ENSCI), Groupe d’Etudes des Matériaux Hétérogènes, 12 rue Atlantis, 87068 Limoges, France
g r a p h i c a l a b s t r a c t
Hydrophilic side
Hydrophobic side R = -C(O)C8F17
a r t i c l e
i n f o
Article history: Received 23 December 2014 Accepted 8 March 2015 Available online 16 March 2015 Keywords: Janus particles Pickering emulsion Silica particles Fluorinated particles
a b s t r a c t Hypothesis: Dispersion or aggregation of ceramic nanoparticles in suspension is mainly influenced by their surface properties. The preparation of hydrophobic/hydrophilic nanoparticles was studied by synthesizing for the first time fluorinated Janus particles via a Pickering emulsion method. Experiments: Fluorinated silica Janus particles were synthesized with a ‘‘grafting to’’ method using silica/ wax emulsions. The parameters investigated to control the emulsion were (i) the SiO2 particles/wax ratio, (ii) the cetyltrimethylammonium bromide (CTAB)/SiO2 ratio and (iii) the ionic strength, i.e. NaCl concentration. Prior to grafting with fluorinated molecules, the partially wax-embedded silica particles were functionalized with an aminosilane. Fourier Transformed Infrared and Nuclear Magnetic Resonance spectroscopies, thermogravimetric and zeta potential analyzes were conducted to characterize the fluorinated Janus particles. Findings: Silica quantity has an influence on the silica nanoparticles distribution at the surface of the wax solid droplets. CTAB amount controls the size and shape of silica/wax solid droplets, whereas the ionic strength in the range 0–6.22 g L1 does not influence the silica/wax emulsion. Aminosilane functionalization was successfully employed on partially wax-embedded silica particles, followed by a grafting step with a carboxylic acid fluorinated compound. Ó 2015 Elsevier Inc. All rights reserved.
⇑ Corresponding author. Fax: +33 (0) 587 50 23 04. E-mail addresses: [email protected] (A. Zenerino), claire.peyratout@ unilim.fr (C. Peyratout), [email protected] (A. Aimable). http://dx.doi.org/10.1016/j.jcis.2015.03.011 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
A. Zenerino et al. / Journal of Colloid and Interface Science 450 (2015) 174–181
175
1. Introduction
2.2. Preparation of Janus particles
Anisotropic particles, called Janus particles [1], contain one hydrophilic and one hydrophobic side that confer different chemical compositions and special wetting properties. These particles can effectively replace surfactants in emulsion stabilization [2]. Moreover, Janus particles are relevant to several industrial domains, such as cosmetics, paint and ink development. For technical ceramics, they could be used to complete theoretical studies on aggregation phenomena [3,4] or in additive processes [5], to stabilize suspensions in order to obtain a good dispersion favouring a homogeneous deposition. Pickering emulsion has been previously employed to synthesize Janus particles based on the preparation of a wax/water emulsion [6,7]. One side of the particles was masked in wax (paraffin) at room temperature and the other side could be chemically modified. The immersion of particles into the wax depends on the three-phase contact angle at the emulsion interface [6]. After removing the wax, Janus particles with controllable geometry were obtained. In the emulsion, particles spontaneously accumulate at the oil–water interface and their adsorption is practically irreversible [8]. Adding low quantity of surfactant to the emulsion modifies the wettability of solid particles by adsorbing onto their surface and increases the adsorption at the oil–water interface [9]. Additionally, the adsorption of surfactant at both solid–liquid and liquid–liquid interfaces may be modified by the presence of additives in emulsion systems like electrolytes, which modify the particles charge and flocculation properties [10]. To obtain anisotropic particles, techniques such as ‘‘grafting to’’ and ‘‘grafting from’’ methods [11], surface-initiated free radical polymerization [12,13] or electrostatic interaction methodology [14] have been employed. In this work, the ‘‘grafting to’’ method was used to obtain new fluorinated Janus particles. Oil/water emulsion was realized with paraffin, which is liquid at 60 °C, and was stabilized by silica particles. Since silica particles are hydrophilic, it is necessary to use a surfactant to improve their hydrophobicity. After cooling the experimental set-up at ambient temperature, wax becomes solid and the silica particles are partly embedded in the solid paraffin. The objective of this work is to understand which factors are controlling the embedment of the silica particles into the hydrophobic phase. Thus, the influence of the [SiO2]/[wax] ratio, the [CTAB]/[SiO2] ratio and the ionic strength were studied to determine the best conditions to realize a timestable emulsion with a homogeneous droplet size. Following the emulsion preparation, the non-protected side of particles was functionalized by an aminosilane and then a fluorinated compound was grafted to the free amine functions to obtain partially hydrophobic silica particles.
2.2.1. Amino-functionalization of silica/wax droplets The synthesis has been adapted from a previously published synthesis of Granick and coworkers [9,15]. Silica particles were dispersed in water with CTAB, wax was added and the mixture was heated at 90 °C. Emulsions were then realized by using Ultra Turrax stirring at 19,000 rpm for 1 min and then cooled to room temperature. The obtained silica/wax solid droplets were filtered and washed with deionized water to remove free particles and CTAB. After completely drying the silica/wax solid droplets in an oven at 50 °C, they were reacted with APTES (2 mM) and absolute methanol at ambient temperature for 45 min. Triethylamine (TEA) (2 mM) was used as a catalyst. After the reaction was completed, the mixture was filtered and washed with deionized water to remove the excess of silane and TEA. Amino group functionalized particles can be used as is or can further react with carboxylic acid groups of the fluorinated molecules.
2. Materials and methods 2.1. Chemicals Amorphous silica particles (Ref. SP03B) were purchased from the Fuso Chemical Company (d50 = 250 nm, specific surface area 12 m2 g1). Paraffin was purchased from Fisher Scientific. Triethylamine (TEA) (99%), (3-aminopropyl)triethoxysilane (APTES) (99%), N-hydroxysuccinimide (NHS) (99%), absolute methanol (99%) and cyclohexane (99%) were purchased from Alfa Aesar. Cetyltrimethylammonium bromide (99%) (CTAB), N-ethylcarbodiimide hydrochloride (EDCHCl) (98%) and heptadecafluorononanoic acid (97%) were purchased from Sigma Aldrich. All products have been used as received.
2.2.2. Coupling fluorinated molecules with silica/wax solid droplets A mixture of N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDCHCl) (1 eq) and N-hydroxysuccinimide (NHS) (2 eq) was added to a solution of heptadecafluorononanoic acid (1 eq) in water and was stirred during 2 h at ambient temperature. Amine functionalized silica/wax solid droplets (1 eq of –NH2) were then added and the reaction was maintained during five days at ambient temperature to obtain fluorinated-silica/wax solid droplets. The product was filtered and a white hydrophobic solid was obtained.
2.2.3. Wax dissolution Silica/wax solid droplets were introduced in cyclohexane and gently stirred at room temperature. After the complete paraffin dissolution in the solvent, silica functionalized particles were recovered by centrifugation at 11,000 rpm during 3 min.
2.3. Characterization methods Fourier Transformed Infrared (FTIR) spectra were carried out with a Nicolet 380 FTIR spectrometer from Thermo Scientific in the range 400–4000 cm1. The silica/wax solid droplets were imaged using a JEOL 7400 FEG-SEM Scanning Electron Microscope (SEM) or a Stereoscan S260 Cambridge microscope or using a Nikon Eclipse 50i optical microscope. The silica/wax solid droplets size distribution was determined by laser size measurement with a LA-950 from Horiba. Dynamic Light Scattering (DLS) with a Zetasizer Nano ZS equipped with a MPT-2 autotitrator from Malvern Instruments was used to determine zeta potential of particles. Aqueous suspensions of 0.015 wt% were prepared at a pH value around 10 with NaOH 0.1 M and titrated with a 0.01 M HCl solution. Three measurements were realized and an average value was determined. For amino-functionalized silica particles, the thermogravimetric measurements (TG) coupled with mass spectrometry were made on a TGA STA449F3 Jupiter with a graphite oven from Netzsch coupled with a Pfeiffer ThermoStar GSD 301T mass spectrometer. The samples were heated from 25 to 900 °C at 10 K min1 under argon flow of 20 mL min1. For fluorinated-silica particles, the thermogravimetric measurements were made on a TGA STA449F3 Jupiter with a silicon carbide oven from Netzsch. The samples were heated from 25 to 800 °C at 10 K min1 under argon flow of 20 mL min1. Nuclear Magnetic Resonance (NMR) spectrums were measured on a Brüker W-200 MHz.
176
A. Zenerino et al. / Journal of Colloid and Interface Science 450 (2015) 174–181
3. Results and discussion 3.1. Influence of the silica/wax ratio on the silica-decorated wax solid droplet size The silica-decorated wax solid droplet diameter was studied as a function of the surface ratio R between the silica particles surface (SSiO2) and the solid wax droplet surface (SWax). Results are presented in Table 1 and Fig. 1. When R increases from 1.21 until 1.54, by decreasing the amount of silica particles, the obtained wax droplets size increases from 40 lm to 140 lm respectively. As shown in Fig. 1, the droplet size growth is not linear with respect of R. In this regime, there are not enough silica particles to cover the wax droplet surface and a coalescence phenomenon is thus observed. Moreover silica particles bi- or multilayers were obtained for these ratios. When R is increased above 1.21 by adding silica particles, the droplet size is then stabilized at around 40 lm. Silica monolayer are obtained for R values equal or above 1.21. Microscopic observations showed that the optimal covering ratio R = 1 cannot be obtained with our experimental conditions and that a R value of 1.21 leads to the optimal coverage of wax droplets with silica particles. Moreover, the wax droplet coverage seems to be better organized and exhibits less defaults with R = 1.21. The surface coverage parameter, C, i.e. the packing density of the particles at the interface, is determined by the following relationship (Eq. (1)) in Pickering emulsion systems [16]:
1 mp ¼ D 4C qp dp V d
Fig. 1. Ratio of silica surface (S(SiO2))/wax surface (S(wax)) as a function of wax droplet diameter and SEM images. Scale bar = 2 lm.
Table 2 Weight of compounds according to the [CTAB]/[SiO2] ratio, diameter (D) and spherical coefficient of wax droplets. Ratio [CTAB]/[SiO2] Wax SiO2 [SiO2] (g L1) CTAB (mg) [CTAB] (mg L1) Total weight D (lm) Number of wax droplets measured Percentage of non spherical particles (when spherical coefficient is
