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Pickering emulsions stabilized by surface-modified Fe3O4 nanoparticles
Journal of Colloid and Interface Science 367 (2012) 213–224
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Pickering emulsions stabilized by surface-modified Fe3O4 nanoparticles Jun Zhou a, Lijun Wang a, Xiuying Qiao a,⇑, Bernard P. Binks b,⇑, Kang Sun a a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK
a r t i c l e
i n f o
Article history: Received 21 July 2011 Accepted 1 November 2011 Available online 10 November 2011 Keywords: Pickering emulsion Fe3O4 Nanoparticles
a b s t r a c t Unmodified Fe3O4 nanoparticles do not stabilize Pickering emulsions of a polar oil like butyl butyrate. In order to obtain stable emulsions, the Fe3O4 nanoparticles were modified by either carboxylic acid (RCOOH) or silane coupling agents (RSi(OC2H5)3) to increase their hydrophobicity. The influence of such surface modification on the stability of the resultant Pickering emulsions was investigated in detail for both a non-polar oil (dodecane) and butyl butyrate in mixtures with water. The stability of dodecane-in-water emulsions in the presence of carboxylic acid-coated particles decreases as the length of the alkyl group (R) and the coating extent increase. However, such particles are incapable of stabilizing butyl butyrate-water emulsions even when the carboxylic acid length is decreased to two. However, the silane-coated Fe3O4 nanoparticles can stabilize butyl butyrate-in-water emulsions, and they also increase the stability of dodecane-in-water emulsions. Thermal gravimetric analysis indicates that the molar quantity of silane reagent is much higher than that of carboxylic acid on nanoparticle surfaces after modification, raising their hydrophobicity and enabling enhanced stability of the resultant polar oil–water emulsions. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Since Ramsden reported the stabilization of emulsions by particles alone early in the 20th century [1], the field experienced a slow development until the 1980s. Since then, due partly to advances in the synthesis and characterization of colloidal particles, there has been a resurgence of interest in Pickering emulsions. Commonly used nanoparticles for preparing Pickering emulsions include both spherical particles such as silica [2–5] and polystyrene [6–8] and flake-like particles [9–11] such as clay and kaolin. Through the study on these systems, it has been found that the emulsion type and stability depend, inter alia, on the oil polarity, on the wettability of the particles at the oil–water interface, on the pH of aqueous phase and on the electrolyte type. Moreover, some researchers [12–18] have put forward mechanisms for the stability of such emulsions. As for the applications of Pickering emulsions, some attention has been focused on the synthesis of microparticles or hollow particles by taking advantage of the self-assembly of these particles at the oil–water interface. Other applications are in the fields of environmental stimuli response and drug release and delivery. [19,20] Among the factors influencing the stability of Pickering emulsions, one of the most important is the hydrophobicity/hydrophi⇑ Corresponding authors. Fax: +86 21 34202745 (X.Y. Qiao), fax: +44 1482 466410. (B.P. Binks.) E-mail addresses: [email protected] (X. Qiao), [email protected] (B.P. Binks). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.001
licity of the particles, assessed by the contact angle how. The effects of oil polarity and the addition of surfactants on the behavior of Pickering emulsions can also be explained by changes in how. Originally, it was found that hydrophilic particles (how < 90°, e.g. barium sulfate, calcium carbonate, hydrophilic SiO2) could stabilize oil-in-water (o/w) emulsions while hydrophobic particles (how > 90°, e.g. polystyrene, PTFE, hydrophobic SiO2) could stabilize water-in-oil (w/o) emulsions. Later on, it was pointed out that emulsions of appreciable stability only formed when how was in an intermediate range around 90° [21]. In a model considering the capillary pressure within the liquid films between droplets, Kaptay [22] predicted that for an emulsion stabilized by a single layer of particles, for 15° < how < 90° an o/w emulsion will be stabilized whereas for 90° < how < 165° a w/o emulsion will be preferred. In addition, for an emulsion stabilized by a double layer of particles, it was predicted that if 15° < how < 129.3° then o/w emulsions will be stabilized, whereas if 50.7° < how < 160° w/o emulsions will occur. Experimental verification of these predictions remains undone since reliable determination of how for small particles is still a challenge. A number of ways exist to change and optimize the wettability (how) of particles. In situ modification, such as the use of surfactants or the adjustment of pH or concentration of electrolyte, is often utilized [23–26]. For example, on the addition of cationic surfactant to silica particle dispersions [25], flocculation of particles occurred improving the stability of the resultant Pickering emulsions. Alternatively, Akartuna et al. [27] added short chain carboxylic acids to
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alumina dispersions and noted that the chain length of the acid and its concentration needed optimizing in order to enhance the stability of o/w emulsions. Surface modification of particles by chemical reactions prior to mixing them with oil and water is the other means to change their wettability (how). Binks et al. [28,29] modified hydrophilic silica particles by a silanization reaction with (CH3)2SiCl2, producing a series of particles of increasing hydrophobicity given by a decrease in the relative percentage of silanol (SiOH) groups on their surfaces. Others modified polystyrene spheres by sulfonation [8,30], carboxylation [8] and aldehyde [7] reactions. Surface modification undoubtedly changes the wettability of the particles [8,30]. He et al. [30] found that when the sulfonation time was increased, how of the particles decreased from 122.5° to 84.5° and the stability and size of emulsion droplets varied. Stiller et al. [31] investigated the influence of organic and inorganic surface modification of TiO2 particles and found that the surface modification leading to how between 110° and 152° was required in order to yield stable Pickering emulsions of triglyceride oil and water. In recent years, super-paramagnetic Fe3O4 nanoparticles have attracted great attention in the biomedical field including their use in magnetic resonance imaging [32–34], hyperthermia therapy of cancers [35,36] and targeted drug delivery [37,38], due to their high magnetic saturation, negligible toxicity and easy surface modification. Taking advantage of Pickering emulsions, many kinds of functional materials based on Fe3O4 nanoparticles, such as magnetic hollow silica microspheres [39], Fe3O4/polystyrene/silica nanospheres [40], Fe3O4/polymer magnetic nanocomposites [41– 43] and Fe3O4/poly(N-isopropylacrylamide-co-methacrylic acid) thermoresponsive composite latex [44], have been successfully prepared. Actually, some groups have demonstrated that Pickering emulsions can be produced by both bare and modified Fe3O4 nanoparticles, such as trialkoxysilane–water emulsions stabilized by silane-modified Fe3O4 nanoparticles [45], dodecane–water and paraffin–water emulsions by oleic acid bilayer-coated Fe3O4 nanoparticles[46,47], and crude oil–water and hexane–water emulsions by Fe3O4 nanoparticles [48,49]. However, the influence of particle wettability and oil polarity on the stability of this kind of magnetic Pickering emulsion remains unstudied. In our previous work [50], we systematically investigated unmodified Fe3O4 nanoparticles as stabilizers of Pickering emulsions and found that they are relatively hydrophilic and cannot stabilize emulsions with highly polar oils such as butyl butyrate and decanol. In the present study, in order to increase the hydrophobicity of the particles, we pre-modify them with carboxylic acid or silane coupling agents and investigate the influence of the surface modification on the stability of the resultant magnetic Pickering emulsions. Ingram et al. [46] and Lan et al. [47] used oleic acid to pre-modify Fe3O4 nanoparticles and investigated bilayer oleic acid-coated particles as Pickering emulsion stabilizers. However, when the pH > 7, the outer layer of oleic acid may become oleate (a common surfactant), and the resultant relatively white Pickering emulsion may be stabilized mainly by oleate surfactant. Some Refs. [51–53] have mentioned that the outer layer of oleic acid is weakly linked to the particles via van der Waals forces with the chains of the first layer. In order to avoid interference from molecules of the modifier, it is proposed that monolayercoated particles should be utilized. Here, an easy washing method is introduced in order to obtain Fe3O4 nanoparticles coated with a single layer of carboxylic acid using either ethanoic, pentanoic, octanoic or decanoic acid (RCOOH, R@CH3A, CH3(CH2)3A, CH3(CH2)6A, CH3(CH2)8A). Moreover, the silane coupling agents [54,55] (RSi(C2H5O)3, R@CH3CH2A, (CH3)2CHCH2A, CH3(CH2)7A) were also chosen to modify the Fe3O4 nanoparticle surface to investigate the effects of particle wettability on the stability of the resultant magnetic Pickering emulsions.
Fig. 1. FTIR spectra and TGA curves of carboxylic acid-coated Fe3O4 nanoparticles. (a) FTIR spectra of carboxylic acid-coated Fe3O4 nanoparticles prepared with the same ratio of carboxylic acid to particles (0.06 mol/g) including a spectrum for unmodified particles. (b) TGA curves of octanoic acid-coated Fe3O4 nanoparticles prepared with the same ratio of octanoic acid to particles (0.09 mol/g) but at different temperatures. (c) TGA curves of carboxylic acid-coated Fe3O4 nanoparticles prepared with the ratio of carboxylic acid to particles of 0.06 mol/g.
2. Experimental 2.1. Materials The water used was purified by an Aquapro ultra-pure water system (China), and its conductivity is 0.25 lS/cm measured
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Fig. 2. (a) FTIR spectra and (b) TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles of 0.01 mol/g.
Fig. 3. TGA curves of (a) octanoic acid-coated and (b) octyltriethoxysilane-coated Fe3O4 nanoparticles prepared with different ratios of modifier to particles (given).
using an FE30/EL30 conductivity meter (Mettler-Toledo Instruments, Switzerland). FeSO47H2O (AR, 99%), FeCl36H2O (AR, 99%) and NaOH (AR, 95%) were purchased from the Sinopharm Chemical Reagent Co. (China) and used as received. 36–38% HCl (AR) was obtained from Shanghai Lingfeng Chemical Reagent Co. (China) and was used as received. In preparing magnetic Pickering emulsions, two oils of different polarity were chosen and used without further purification. These were dodecane (AR, 99%) and butyl butyrate (AR, 99%) purchased from Aladdin Reagent Co. (China). In pre-modifying Fe3O4 nanoparticles, 7 modifiers were chosen and used without further purification. These were ethanoic acid (AR, 99.5%) and decanoic acid (AR, 97%) purchased from Sinopharm (China), pentanoic acid (AR, 99%), octanoic acid (AR, 98%), ethyltriethoxysilane (AR, 97%), isobutyltriethoxysilane (AR, 96%) and octyltriethoxysilane (AR, 98%) purchased from Sigma–Aldrich. Toluene (AR, 99%) and ethanol (AR, 99.7%) were purchased from Sinopharm (China).
N2 atmosphere and using an oil bath at 80 °C. After the above mixture turned black, it was allowed to cool with continuous stirring, and the black particles obtained were washed three times with water and once with absolute ethanol.
2.2. Synthesis of unmodified Fe3O4 magnetic nanoparticles Fe3O4 magnetic nanoparticles were prepared by co-precipitation of aqueous ferrous and ferric ions. Before synthesis, aqueous phases of 0.5 M FeSO4 solution with 0.2 M HCl, 1 M FeCl3 solution with 0.2 M HCl and 1.5 M NaOH solution were prepared. During preparation, 100 mL NaOH solution was poured into a flask and heated to 80 °C, and then the mixture of 10 mL FeSO4 solution and 10 mL FeCl3 solution was added into the flask dropwise in an
2.3. Modification and characterization of Fe3O4 nanoparticles Fe3O4 nanoparticles as prepared above were dispersed in 50 mL absolute ethanol by a KQ100 ultrasonic cleaning machine (Kunshan Ultrasonic Instrument Co., China) using a power of 100 W for 5 min. and then poured into a flask. 100 mL toluene and some amount of modifier (carboxylic acid or alkyltriethoxysilane) were added and the flask heated to 110 °C in an N2 atmosphere. After holding the above mixture at 110 °C for 8 h, the black fluid was immediately poured into a beaker, and the modified particles were separated using a magnet. The solid black material was washed using absolute ethanol and water and then dried in an LGJ-10 common type freeze dryer (Beijing Huaxing Technology Development Co., China). In order to ensure only a monolayer of reagent on particle surfaces, washing (before drying) occurs at high temperature (75 °C) for carboxylic acid-modified particles and at low temperature (room) for silane-modified particles. In the experiments, an oil film can be seen on the surface of the solutions even after the carboxylic acid-modified particles have been washed by absolute ethanol 5 times and water for more than 10 times at room temperature. The protocol applied for these particles is thus to wash using
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Table 1 Measured amount of alkyl groups on Fe3O4 nanoparticle surfaces after modification with carboxylic acid or silane coupling agents and the volume fraction of the resultant stable emulsions prepared with either dodecane or butyl butyrate at different oil/water volume ratios. Particle codename
No. C atoms in alkyl group
Coating amount of modifier on particle surface per gram Fe3O4 (mmol/g)
Volume fraction of stable emulsion with dodecane (%)
Volume fraction of stable emulsion with butyl butyrate (%)
Oil/ water
Fe3O4 0.06EA-Fe3O4 0.06PA-Fe3O4 0.03OA-Fe3O4 0.06OA-Fe3O4 0.09OA-Fe3O4 0.06DA-Fe3O4 0.06PA-Fe3O4 0.06PA-Fe3O4
0 2 5 8 8 8 10 5 5
0 0.381 0.275 0.376 0.384 0.534 0.327 0.275 0.275
83.9 76.3 81.6 72.8 81.9 0 0 24.1 48.2
0 0 0 0 0 0 0 0 0
2:1 2:1 2:1 2:1 2:1 2:1 2:1 1:2 1:1
0.01ES-Fe3O4 0.01IBS-Fe3O4 0.005OS-Fe3O4 0.01OS-Fe3O4 0.015OS-Fe3O4 0.01OS-Fe3O4 0.01OS-Fe3O4
2 4 8 8 8 8 8
0.776 0.788 0.565 0.707 1.072 0.707 0.707
96.4 87.6 91.1 84.3 86.1 44.6 67.0
0 69.5 64.3 75.9 85.6 42.2 69.6
2:1 2:1 2:1 2:1 2:1 1:2 1:1
absolute ethanol at 75 °C for 3 times and then hot deionized water at 75 °C for 3 times; no oil film was then seen. For carboxylic acid-coated particles, the carboxylic acid/Fe3O4 ratio of 0.06 mol/g is chosen for ethanoic (0.06EA-Fe3O4), pentanoic (0.06PA-Fe3O4) and decanoic (0.06DA-Fe3O4) acids, and the ratios 0.03, 0.06 and 0.09 mol/g are chosen for octanoic acid (0.03OA-Fe3O4, 0.06OA-Fe3O4 and 0.09OA-Fe3O4). For silanecoated particles, the silane/Fe3O4 ratio of 0.01 mol/g is chosen for ethyltriethoxysilane (0.01ES-Fe3O4) and isobutyltriethoxysilane (0.01IBS-Fe3O4) and the ratios of 0.005, 0.01 and 0.015 mol/g for octyltriethoxysilane (0.005OS-Fe3O4, 0.01OS-Fe3O4 and 0.015OSFe3O4). In this case, both the effects of a different modifier and different extents of coating for the same modifier on the stability of Pickering emulsions stabilized by such modified particles can be investigated systematically. The morphology of the modified Fe3O4 nanoparticles was observed using transmission electron microscopy (TEM) employing a JEM-2100 microscope (JEOL, Japan) at an accelerating voltage of 200 kV. Before observation, the modified Fe3O4 nanoparticles were dispersed in water at a concentration of 0.4 wt.% using a KQ100 ultrasonic cleaning machine with a power of 10 W for 1 h. and then added dropwise onto a carbon-coated copper grid. About 500 nanoparticles were observed to determine their average diameter. In order to confirm that the modification is successful and the coating is a monolayer, Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA) were used to characterize the surface of the modified Fe3O4 nanoparticles. The composition of nanoparticles was determined by FTIR spectra covering the range from 400 cm 1 to 4500 cm 1 obtained from an Equinox 55 FTIR spectrometer (Vruker, Germany), and the disk shaped samples for measurements were obtained by compression molding with KBr. Using a TG209 F1 Iris Thermogravimetric Analyzer (NETZSCH, Germany), the weight change of modified Fe3O4 nanoparticles was investigated from 30 °C to 800 °C at a heating rate of 20 °C/min. in a nitrogen atmosphere, which can be used to evaluate the coating extent on the surface of the particles.
nanoparticles prepared above were investigated as stabilizers of water–butyl butyrate mixtures in this study. Thus, the carboxylic acid and silane-modified Fe3O4 nanoparticles were first dispersed at a concentration of 1 wt.% initially in oil (dodecane and butyl butyrate) by mixing for 1 h at a power of 100 W in a KQ100 ultrasonic cleaning machine (Kunshan Ultrasonic Instrument Co., China). After the addition of deionized water, emulsions with different oil/water volume ratios of 2:1, 1:1 and 1:2 were prepared using an FM200 high shear dispersing emulsifier (Fluko Equipment Shanghai, China) at a speed of 14,000 rpm for 3 min. Pickering emulsions stabilized by 1 wt.% of unmodified Fe3O4 nanoparticles initially in water were also prepared under the same conditions for comparison. After preparation, emulsions were immediately poured into cylindrical sample bottles, and their stability was investigated by recording the change in height with time of the emulsion–aqueous phase interface and then evaluating the volume fraction changes of the stable emulsion and resolved water. When no further changes occurred, photographs of the vessels were taken using a Samsung S760 digital camera. After this, the separated aqueous phase was dried to weigh the non-adsorbed Fe3O4 nanoparticles, allowing the amount of nanoparticles associated with the stable emulsion to be determined. The emulsion type was determined by measuring its conductivity (FE30/EL30 conductivity meter). Generally, the conductivity of the emulsion is mainly determined by that of the continuous phase. The oils used in this study have conductivities less than 0.1 lS/cm, and aqueous dispersions of 0.06PA-Fe3O4 nanoparticles have a conductivity of 24.6 lS/cm. If the conductivity is above 10 lS/cm, the emulsion type is O/W, and if the conductivity is below 0.1 lS/cm, the emulsion type is W/O. An XPS-8C optical microscope (Yongheng Optical Instruments, China) was used to image emulsion droplets. 3. Results and discussion 3.1. Characterization of modified Fe3O4 nanoparticles
2.4. Preparation of magnetic Pickering emulsions For unmodified Fe3O4 nanoparticles, stable Pickering emulsions can be prepared for the non-polar oil dodecane and the weakly polar oil poly(dimethylsiloxane), PDMS, but not for the highly polar oils butyl butyrate and decanol [50]. In order to obtain stable Pickering emulsions for these latter oils, the hydrophobized
3.1.1. Carboxylic acid-coated particles The unmodified Fe3O4 nanoparticles were prepared by co-precipitation of Fe2+ and Fe3+ in aqueous alkaline solution and then were modified by either carboxylic acid or silane coupling agents. In our study, the silane-coated nanoparticles were prepared by the same method as reported in previous Refs. [54,55], but the
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Fig. 4. TEM images of carboxylic acid- and silane-coated Fe3O4 nanoparticles. (a) 0.06DA-Fe3O4, (b) 0.06PA-Fe3O4, (c) 0.06EA-Fe3O4, (d) 0.01OS-Fe3O4, (e) 0.01IBS-Fe3O4, (f) 0.01ES-Fe3O4.
carboxylic acid-coated nanoparticles were prepared by a different method from those in the literature. In the latter, the nanoparticles were usually synthesized in one step with the addition of alkali to the mixed solution of carboxylic acid, Fe2+ and Fe3+. However, in order to investigate the influence of the surface modification in more detail, the carboxylic acid was added after the Fe3O4 nanoparticles were prepared in order to achieve monolayer modification by a novel washing protocol. FTIR spectroscopy was used to determine the surface composition of the modified particles. Fig. 1a shows the FTIR spectra of the
carboxylic acid-coated and unmodified Fe3O4 nanoparticles. Due to the low content of organic modifier used in the synthesis, the characteristic absorbance peaks of alkyl groups between 2800 and 3000 cm 1 are unobservable, but the peaks in the fingerprint regions of 1333–650 cm 1 and 1700–1333 cm 1 are regular and can be observed clearly. Similar to the FTIR results of monolayer and bilayer oleic acid-coated Fe3O4 nanoparticles [56,57], for our carboxylic acid-coated Fe3O4 nanoparticles, the characteristic C@O stretch band of the carboxyl group at 1710 cm 1 disappears, while three new peaks at 1428, 1523 and 1631 cm 1 originating
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Table 2 Conductivity (in lS cm 1) 20 days after preparation of Pickering emulsions stabilized by 1 wt.% carboxylic acid- or silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios. Particles
Codename
Carboxylic 0.06EA-Fe3O4 acid-coated 0.06PA-Fe3O4 0.03OA-Fe3O4 0.06OA-Fe3O4 0.09OA-Fe3O4 0.06DA-Fe3O4 0.06PA-Fe3O4 0.06PA-Fe3O4 Silane-coated
Dodecane Butyl butyrate 18.7 21.31 10.53 8.74 – – 7.87 10.46
0.01ES-Fe3O4 22.1 0.01IBS-Fe3O4 23.9 0.005OS-Fe3O4 23.3 0.01OS-Fe3O4 8.66 0.015OS-Fe3O4 7.78 0.01OS-Fe3O4 8.28 0.01OS-Fe3O4 8.63
Oil/ water
2:1 2:1 2:1 Complete phase separation 2:1 2:1 2:1 1:1 1:2 – 10.3 10.13 6.30 7.88 7.45 5.16
2:1 2:1 2:1 2:1 2:1 1:1 1:2
from the asymmetric and symmetric stretching vibration of COOand one peak at 1052 cm 1 from the CAO single bond stretching appear. These results indicate that the carboxylic acid group is chemisorbed onto the Fe3O4 nanoparticle surface as carboxylate, and the monolayer-modified Fe3O4 nanoparticles have been successfully prepared by the new washing technique. Moreover, for these modified particles, the characteristic absorbance band of magnetite, Fe3O4, at 587 cm 1 is found but the band of maghemite, Fe2O3, at 630 cm 1 is not detected, confirming the purity of Fe3O4 in these nanoparticles without further oxidation. TGA curves (Fig. 1b and c) support the above conclusion. Fig. 1b gives the weight loss curves for octanoic acid-coated particles prepared at different temperatures. For bilayer carboxylic acid-coated Fe3O4 nanoparticles, the TGA curve has two stages of weight loss, because the interaction between the inner layer and outer layer is totally different to that between the inner layer and the particle surface. Zhang et al. [57] prepared bilayer oleic acid-coated Fe3O4 nanoparticles, and TGA results showed that their first weight loss is near 260 °C and the second weight loss is near 390 °C. When Fu et al. [51] prepared Fe3O4 nanoparticles coated with lauric acid as a first layer and decanoic acid as a second layer, they found that monolayer-coated lauric acid nanoparticles had only one weight loss at 150–360 °C while the bilayer-coated ones had a first weight loss at 150–250 °C and a second weight loss at 250–650 °C. Shen et al. [52] got similar results for the Fe3O4 nanoparticles coated with decanoic acid as the inner layer and different acids as the outer layer. From these references, it can be concluded that for bilayer carboxylic acid-coated Fe3O4 nanoparticles, there are two weight loss peaks with the first one at a temperature lower than the boiling point of the acid and the second one at a temperature about 100 °C higher than the boiling point of the acid. For monolayer carboxylic acid-coated Fe3O4 nanoparticles however, there is only one weight loss peak at a temperature higher than the boiling point of the acid. Obviously, in Fig. 1b and c, the TGA curves of all the carboxylic acid-coated Fe3O4 nanoparticles prepared at different temperatures or with different modifiers exhibit only one weight loss at 300 °C, higher than the boiling points of the carboxylic acids used in the synthesis. Thus, it can be concluded that the carboxylic acid-coated Fe3O4 nanoparticles prepared by the new washing technique here are monolayer modified only. The influence of the reaction temperature was investigated in the case of octanoic acid. From Fig. 1b, it can be noted that the amount of octanoic acid adsorbed as a monolayer on the surface of Fe3O4 nanoparticles increases with the modifying temperature, from about 3 wt.% to 7 wt.% when the modifying temperature is
raised from 40 °C to 110 °C. The temperature dependence of the monolayer density could be attributed to the competition of two interactions between carboxylic acid and Fe3O4 nanoparticles – chemical bonding and hydrogen bonding. High reaction temperature favors chemical bonding of carboxylic acid onto the particle surface, which is much more stable than the hydrogen bonding occurring during the following solvent washing at high temperature. Octanoic acid hydrogen bonded to the Fe3O4 nanoparticles can be easily washed away by absolute ethanol and water at 75 °C, which leads to the decrease in the coating amount remaining on particle surfaces. From Fig. 1c showing the TGA curves of carboxylic acid-coated Fe3O4 nanoparticles prepared with the same ratio of acid to particles (0.06 mol/g), it can be deduced from the weight loss that the adsorbed amount of carboxylic acid is 2.2, 2.7, 5.2 and 5.3 wt.% (corresponding to the coating amount of 0.381, 0.275, 0.384 and 0.327 mmol/g with respect to the dry particles) for 0.06EA-Fe3O4, 0.06PA-Fe3O4, 0.06OA-Fe3O4 and 0.06DAFe3O4 nanoparticles, respectively. 3.1.2. Silane-coated particles For silane-modified Fe3O4 nanoparticles, the method and mechanism are the same as those for c-aminopropyltriethoxysilane (APTES)-modified Fe3O4 nanoparticles [54,55,58]. The above references mention that in the absence of water, SiAOC2H5 reacts with AOH on the surface of Fe3O4 nanoparticles with the formation of SiAOAFe bonds. The FTIR spectra and TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles (0.01 mol/g) are given in Fig. 2. The characteristic absorbance peaks of AOH (near 3400 cm 1) and Fe3O4 (471, 587, 1621 cm 1) [53] are very clear in Fig. 2a, which reveals that not all Si atoms link to Fe3O4 nanoparticles and superfluous SiAOC2H5 groups hydrolyze to SiAOH groups. The absorbance peak at 2925 cm 1 is related to the stretching vibration of the CH2 group, whose intensity is the weakest for ethyltriethoxysilane-coated particles because the content of CH2 groups in ethyltriethoxysilane is the least among those of the silane coupling agents used. In addition, the absorbance peak at 1043 cm 1 originates from the stretching vibration of the SiAO bond, and the weak absorbance peak at 870 cm 1 may originate from the stretching vibration of the CASi bond. The appearance of all the characteristic absorbance bands of silane coupling agents confirms the successful coating of silane coupling agent on the surface of Fe3O4 nanoparticles in this study. The TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles (0.01 mol/g) are shown in Fig. 2b. In the measured temperature range from 150 °C to 550 °C, the weight loss is 2.2, 4.3 and 7.4 wt.% (corresponding to the coating amount of 0.776, 0.788 and 0.707 mmol/g with respect to particles) for 0.01ES-Fe3O4, 0.01IBS-Fe3O4 and 0.01OS-Fe3O4 nanoparticles, respectively. The weight loss mechanism of silanecoated Fe3O4 nanoparticles is very different from that of carboxylic acid-coated particles. It is well known that the bond energy of the SiAO bond (460 kJ/mol) is much higher than that of the CASi (347 kJ/mol) and CAC bonds (332 kJ/mol). Therefore, the weight loss of silane-coated particles results from the decomposition of the alkyl group (ethyl, butyl or octyl), while that of carboxylic acid-coated particles results from the decomposition of the carboxylic acid (ethanoic, pentanoic, octanoic or decanoic). In this case, the coating percent of silane coupling agents should be much higher than that of carboxylic acids for modifiers with the same alkyl chain length, although the modifying ratio of carboxylic acid to particles is six times that of silane coupling agents. For example, for octyl chains, the molecular mass ratio of CH3(CH2)7A to CH3(CH2)7Si(O-)3 is 113/189 (59.8%), and the coating percent of octyltriethoxysilane should be 7.4 wt.%/59.8% = 12.9 wt.%, but the coating percent of octanoic acid should be equal to the weight loss of 5.2 wt.%, much lower than that of octyltriethoxysilane. For alkyl
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Fig. 5. Microscope images 20 days after preparation of dodecane-in-water Pickering emulsions stabilized by 1 wt.% carboxylic acid- or silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (given). (a) 0.06EA-Fe3O4, 2:1, (b) 0.06OA-Fe3O4, 2:1, (c) 0.06PA-Fe3O4, 2:1, (d) 0.06PA-Fe3O4, 1:1, (e) 0.06PA-Fe3O4, 1:2, (f) 0.01ES-Fe3O4, 2:1, (g) 0.01IBS-Fe3O4, 2:1, (h) 0.01OS-Fe3O4, 2:1, (i) 0.01OS-Fe3O4, 1:1, (j) 0.01OS-Fe3O4, 1:2.
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Fig. 6. Microscope images 20 days after preparation of butyl butyrate-in-water Pickering emulsions stabilized by 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (given). (a) 0.01IBS-Fe3O4, 2:1, (b) 0.005OS-Fe3O4, 2:1, (c) 0.015OS-Fe3O4, 2:1, (d) 0.01OS-Fe3O4, 2:1, (e) 0.01OS-Fe3O4, 1:1, (f) 0.01OS-Fe3O4, 1:2.
groups adsorbed on the particle surfaces, their amount is higher for silane-coated particles than for carboxylic acid-coated particles with the same number of carbon atoms in the modifier, especially for modifiers having a greater number of carbon atoms. Moreover, with an increase in the amount of modifier used in the synthesis, the amount of alkyl groups on the particle surface is obviously increased for both silane- and carboxylic acid-coated Fe3O4 particles, as expected and reflected in Fig. 3 and Table 1. Fig. 4 contains TEM images of the carboxylic acid- and silanecoated Fe3O4 nanoparticles. Compared with the morphology of the unmodified Fe3O4 nanoparticles [50], it can be seen that modification does not change the size or morphology of the particles, but changes the extent of aggregation, as expected if particles become more hydrophobic. Some degree of flocculation can lead to an increase in the stability of Pickering emulsions [29,59,60]. 3.2. Type of Pickering emulsion stabilized by modified Fe3O4 nanoparticles It is known that the –OH groups on the surface of Fe3O4 nanoparticles can be substituted by alkyl groups. Many Refs. [54,61– 64] have mentioned that increasing the percent of alkyl groups on the particle surface can increase the hydrophobicity of the particles. Park et al. [54] investigated the influence of alkyl chain
length on the hydrophobicity of organically modified silica gels and found that the amount of absorbed water decreases as the modified alkyl chain is increased from methyl through to octyl. Horozov et al. [61] reported that the contact angle of SiO2 nanoparticles at the air–water interface is increased from 51° to 110° with an increase in the percent of alkyl groups substituting the AOH groups on particle surfaces. It is very difficult to determine the contact angle of these modified Fe3O4 nanoparticles however, because these particles cannot be compressed due to their brittleness or used in powder form with the Washburn method due to their hydrophobicity and unknown material constant. In order to qualitatively assess the hydrophobicity of the modified Fe3O4 nanoparticles, we designed an immersion test by carefully placing the particles on the surface of methanol–water solutions containing different methanol content (from 0 to 100 wt.%) and recorded the time for the particles to immerse in the solutions. Most of the mass of these modified Fe3O4 nanoparticles sedimented to the bottom of the solution immediately, but a small percentage floated on the solution surface as the length of the alkyl chain in the modifier increased or the methanol content in solution decreased. By contrast, unmodified Fe3O4 nanoparticles sedimented immediately without any particles remaining on the surface for all the solutions. The immersion time could not be determined for these particles, unlike the case for SiO2 particles, probably
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Fig. 7. Digital photographs of the vessels taken 20 days after emulsion preparation containing magnetic Pickering dodecane-in-water emulsions stabilized by: (a) either 1 wt.% Fe3O4 initially in water or 1 wt.% carboxylic acid-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 (from left Fe3O4, 0.06EA-Fe3O4, 0.06PAFe3O4, 0.03OA-Fe3O4, 0.06OA-Fe3O4, 0.09OA-Fe3O4 and 0.06DA-Fe3O4) and (b) 1 wt.% 0.06PA-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left 1:2, 1:1 and 2:1). Change in the volume fraction of residual emulsion versus time for the dodecane-in-water emulsions stabilized by (c) 1 wt.% Fe3O4 initially in water or 1 wt.% carboxylic acid-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.06PA-Fe3O4 nanoparticles initially in oil with different oil/ water volume ratios.
due to their much higher density (4.25 cf. 2.2 g/cm3). Nevertheless, it seems that the more hydrophobic the particles are, the more easily they float on the surface of the solutions with lower methanol content. According to these observations, it can be concluded that the Fe3O4 nanoparticles are more hydrophobic after modification and the modified particles are more hydrophobic for those with a greater amount of alkyl groups on their surfaces (see Table 1). It is expected that the higher hydrophobicity of silane-coated nanoparticles compared with carboxylic acid-coated ones will result in higher stability of the Pickering emulsion with a polar oil phase. The type of Pickering emulsion stabilized by modified Fe3O4 nanoparticles was determined by the conductivity of the emulsions, which are listed in Table 2. It can be seen in Table 2 that the conductivity of all the Pickering emulsions stabilized by modified Fe3O4 nanoparticles is much larger than 0.1 lS/cm, so all these Pickering emulsions are dodecane-in-water or butyl butyrate-in-water emulsions. For these modified Fe3O4 nanoparticles, the emulsion type is independent of the type or coating extent of the modifier on the particle surface, the oil phase or the oil/water volume ratio used for emulsion preparation. Microscopic images 20 days after the preparation of Pickering emulsions stabilized by both carboxylic acid- and silane-coated Fe3O4 nanoparticles are shown in Fig. 5 for dodecane and in
Fig. 6 for butyl butyrate. The droplets of the emulsions are spherical, and their mean size varies from 40 to 70 lm. For dodecane-inwater emulsions, the size of the droplets increases slightly as the length of the alkyl group in carboxylic acid-modified particles increases, while the size of droplets hardly changes with the alkyl chain length in silane-coated particles. For butyl butyrate-in-water emulsions, the size of droplets hardly changes with the coating percent of octyltriethoxysilane. Similar to Pickering emulsions stabilized by unmodified Fe3O4 nanoparticles, the average droplet size increases with an increase in the oil volume fraction for emulsions stabilized by modified Fe3O4 nanoparticles. 3.3. Stability of Pickering emulsions stabilized by modified Fe3O4 nanoparticles The stability of Pickering emulsions stabilized by unmodified Fe3O4 nanoparticles was investigated in our previous work [50], where we found that such particles can stabilize emulsions of a non-polar oil like dodecane and a weakly polar oil such as PDMS, but cannot stabilize emulsions of highly polar oils such as butyl butyrate and decanol. When using carboxylic acid-coated nanoparticles, it is found that these modified particles cannot stabilize emulsions of butyl butyrate but some of these particles can stabilize emulsions of dodecane. For dodecane-in-water emulsions,
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Fig. 8. Digital photographs of the vessels containing magnetic Pickering dodecane-in-water emulsions stabilized by (a) 1 wt.% Fe3O4 initially in water or silane-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 (from left, Fe3O4, 0.01ES-Fe3O4, 0.01IBS-Fe3O4, 0.005OS-Fe3O4, 0.01OS-Fe3O4, 0.015OS-Fe3O4) and (b) 1 wt.% 0.01OS-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left, 1:2, 1:1, 2:1). Change in the volume fraction of residual emulsion versus time for the dodecane-in-water emulsions stabilized by (c) 1 wt.% Fe3O4 initially in water or 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.01OS-Fe3O4 nanoparticles with different oil/water volume ratios.
digital photographs of the emulsions taken 20 days after preparation and the changes in the volume fraction of residual emulsion (ratio of emulsion volume to total liquid volume) as a function of time are given in Fig. 7 along with the long-term stability listed in Table 1. Obviously, no emulsion can be obtained when the 0.09OA-Fe3O4 nanoparticles and 0.06DA-Fe3O4 nanoparticles were used as the stabilizer. Emulsions stabilized by carboxylic acid-coated particles exhibit both creaming and coalescence to different extents. From Fig. 1c, it is known that when the ratio of carboxylic acid to particles is 0.06 mol/g, the amount of carboxylic acid adsorbed on nanoparticle surfaces is 0.381, 0.275, 0.384 and 0.327 mmol/g with respect to the dry particles for EA, PA, OA and DA, respectively. The stable emulsion fraction at long time for these four particle types is 76.3%, 81.6%, 81.9% and 0% in order. When changing the ratio of octanoic acid to particles from 0.03 to 0.09 mol/g, the amount of adsorbed octanoic acid increases from 0.376 to 0.534 mmol/g, and the stable emulsion fraction attained is 72.8%, 81.9% and 0% for the 0.03OA-Fe3O4, 0.06OA-Fe3O4 and 0.09OAFe3O4 nanoparticles, respectively, lower than that for the unmodified Fe3O4 nanoparticles. Thus, it can be seen that once the particle hydrophobicity becomes too high, emulsions of dodecane and water cannot be formed. Moreover, similar to the unmodified Fe3O4 nanoparticles, the stable emulsion fraction increases with an increase in the volume fraction of oil, from 24.1% to 48.2% to 81.6% for 0.06PA-Fe3O4 nanoparticles at oil/water volume ratios of 1:2, 1:1 and 2:1, as seen in Fig. 7d and Table 1.
Pickering emulsions stabilized by silane-coated Fe3O4 nanoparticles are investigated too. All the coated particles can stabilize emulsions of dodecane, and interestingly, some of these particles can now stabilize emulsion of butyl butyrate. The digital photographs of these emulsions taken 20 days after preparation and the change in the volume fraction of residual emulsion versus time are given in Fig. 8 for dodecane as oil and in Fig. 9 for butyl butyrate. Emulsions are stable to coalescence and only exhibit creaming. When the ratio of silane to particles is 0.01 mol/g, the amount of silane adsorbed on nanoparticle surfaces is 0.776, 0.788 and 0.707 mmol/g with respect to particles for ES-, IBS- and OS-coated particles, respectively. Correspondingly, the stable emulsion fraction of the butyl butyrate-in-water emulsions is 0%, 69.5% and 75.9% in that order. Although Pickering emulsions of dodecane can be stabilized by all the silane-coated Fe3O4 nanoparticles, emulsions of butyl butyrate can only be stabilized by 0.01IBSFe3O4 and 0.01OS-Fe3O4 and not by 0.01ES-Fe3O4 nanoparticles. The reason is that increasing the length of the alkyl group on the particle surface will increase the hydrophobicity of the particles. For octyltriethoxysilane-coated particles, different ratios of silane to particles used in the synthesis also affect the wettability of the modified particles and the stability of the resultant emulsions. The TGA curves in Fig. 3b shows that the amount of OS adsorbed on nanoparticle surfaces is 0.565, 0.707 and 1.072 mmol/g with respect to particles for 0.005OS-Fe3O4, 0.01OS-Fe3O4 and 0.015OS-Fe3O4 nanoparticles. Varying the coating extent of octyltriethoxysilane has little effect on emulsion sta-
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Fig. 9. Digital photographs of the vessels taken 20 days after emulsion preparation containing magnetic Pickering butyl butyrate-in-water emulsions stabilized by (a) 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with an oil/water volume ratio of 2:1 (from left, 0.01ES-Fe3O4, 0.01IBS-Fe3O4, 0.005OS-Fe3O4, 0.01OS-Fe3O4, 0.015OS-Fe3O4) and (b) 1 wt.% 0.01OS-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left, 1:2, 1:1, 2:1). Change in the volume fraction of residual emulsion versus time for the butyl butyrate-in-water emulsions stabilized by (c) 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.01OS-Fe3O4 nanoparticles with different oil/water volume ratios.
bility for both dodecane and butyl butyrate, all emulsions appearing stable. The volume fraction of the stable emulsion of butyl butyrate changes more than that of dodecane upon increasing the extent of modifier coating. One of the aims of our research is to obtain stable polar oil– water Pickering emulsions stabilized by Fe3O4 nanoparticles. Silanes and carboxylic acids were chosen to modify the particle surface to increase their hydrophobicity. However, carboxylic acidcoated particles do not yield stable butyl butyrate-in-water emulsions even if the ratio of carboxylic acid to particles is six times that of silane reagent and the number of carbon atoms in the alkyl groups is increased to 10. The silane-coated particles can achieve this even when the number of carbon atoms is as low as 4. The TGA results reveal that the amount of adsorbed alkyl groups is much higher for silane-coated Fe3O4 nanoparticles than for carboxylic acid-coated Fe3O4 nanoparticles. 4. Conclusions In order to obtain stable magnetic Pickering emulsions with polar oils, carboxylic acid and silane coupling agents with different chain lengths were used to prepare monolayer-modified Fe3O4 nanoparticles of increased particle hydrophobicity. The extent of adsorption of silane reagent on particle surfaces is higher than that of carboxylic acid. Carboxylic acid-modified Fe3O4 nanoparticles cannot stabilize emulsions of butyl butyrate even when the num-
ber of carbon atoms in their alkyl chain is increased to 10, while the silane-coated particles achieve this even when the number of carbon atoms is as low as 4. All emulsions are of the o/w type and their conductivity and droplet sizes are influenced weakly by the coating type, coating extent and modifier alkyl chain length. Similar to unmodified Fe3O4 nanoparticles, the average droplet size and the volume fraction of stable emulsion in the presence of modified Fe3O4 nanoparticles increase with an increase in the oil/water ratio. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 20803047) and the Chen Xing Young Scholar Award Program of Shanghai Jiao Tong University (Grant No. T241460617). The authors also thank the Instrumental Analysis Center of Shanghai Jiao Tong University for assistance with the measurements. References [1] [2] [3] [4]
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Pickering emulsions stabilized by surface-modified Fe3O4 nanoparticles Jun Zhou a, Lijun Wang a, Xiuying Qiao a,⇑, Bernard P. Binks b,⇑, Kang Sun a a b
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Surfactant and Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK
a r t i c l e
i n f o
Article history: Received 21 July 2011 Accepted 1 November 2011 Available online 10 November 2011 Keywords: Pickering emulsion Fe3O4 Nanoparticles
a b s t r a c t Unmodified Fe3O4 nanoparticles do not stabilize Pickering emulsions of a polar oil like butyl butyrate. In order to obtain stable emulsions, the Fe3O4 nanoparticles were modified by either carboxylic acid (RCOOH) or silane coupling agents (RSi(OC2H5)3) to increase their hydrophobicity. The influence of such surface modification on the stability of the resultant Pickering emulsions was investigated in detail for both a non-polar oil (dodecane) and butyl butyrate in mixtures with water. The stability of dodecane-in-water emulsions in the presence of carboxylic acid-coated particles decreases as the length of the alkyl group (R) and the coating extent increase. However, such particles are incapable of stabilizing butyl butyrate-water emulsions even when the carboxylic acid length is decreased to two. However, the silane-coated Fe3O4 nanoparticles can stabilize butyl butyrate-in-water emulsions, and they also increase the stability of dodecane-in-water emulsions. Thermal gravimetric analysis indicates that the molar quantity of silane reagent is much higher than that of carboxylic acid on nanoparticle surfaces after modification, raising their hydrophobicity and enabling enhanced stability of the resultant polar oil–water emulsions. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Since Ramsden reported the stabilization of emulsions by particles alone early in the 20th century [1], the field experienced a slow development until the 1980s. Since then, due partly to advances in the synthesis and characterization of colloidal particles, there has been a resurgence of interest in Pickering emulsions. Commonly used nanoparticles for preparing Pickering emulsions include both spherical particles such as silica [2–5] and polystyrene [6–8] and flake-like particles [9–11] such as clay and kaolin. Through the study on these systems, it has been found that the emulsion type and stability depend, inter alia, on the oil polarity, on the wettability of the particles at the oil–water interface, on the pH of aqueous phase and on the electrolyte type. Moreover, some researchers [12–18] have put forward mechanisms for the stability of such emulsions. As for the applications of Pickering emulsions, some attention has been focused on the synthesis of microparticles or hollow particles by taking advantage of the self-assembly of these particles at the oil–water interface. Other applications are in the fields of environmental stimuli response and drug release and delivery. [19,20] Among the factors influencing the stability of Pickering emulsions, one of the most important is the hydrophobicity/hydrophi⇑ Corresponding authors. Fax: +86 21 34202745 (X.Y. Qiao), fax: +44 1482 466410. (B.P. Binks.) E-mail addresses: [email protected] (X. Qiao), [email protected] (B.P. Binks). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.001
licity of the particles, assessed by the contact angle how. The effects of oil polarity and the addition of surfactants on the behavior of Pickering emulsions can also be explained by changes in how. Originally, it was found that hydrophilic particles (how < 90°, e.g. barium sulfate, calcium carbonate, hydrophilic SiO2) could stabilize oil-in-water (o/w) emulsions while hydrophobic particles (how > 90°, e.g. polystyrene, PTFE, hydrophobic SiO2) could stabilize water-in-oil (w/o) emulsions. Later on, it was pointed out that emulsions of appreciable stability only formed when how was in an intermediate range around 90° [21]. In a model considering the capillary pressure within the liquid films between droplets, Kaptay [22] predicted that for an emulsion stabilized by a single layer of particles, for 15° < how < 90° an o/w emulsion will be stabilized whereas for 90° < how < 165° a w/o emulsion will be preferred. In addition, for an emulsion stabilized by a double layer of particles, it was predicted that if 15° < how < 129.3° then o/w emulsions will be stabilized, whereas if 50.7° < how < 160° w/o emulsions will occur. Experimental verification of these predictions remains undone since reliable determination of how for small particles is still a challenge. A number of ways exist to change and optimize the wettability (how) of particles. In situ modification, such as the use of surfactants or the adjustment of pH or concentration of electrolyte, is often utilized [23–26]. For example, on the addition of cationic surfactant to silica particle dispersions [25], flocculation of particles occurred improving the stability of the resultant Pickering emulsions. Alternatively, Akartuna et al. [27] added short chain carboxylic acids to
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alumina dispersions and noted that the chain length of the acid and its concentration needed optimizing in order to enhance the stability of o/w emulsions. Surface modification of particles by chemical reactions prior to mixing them with oil and water is the other means to change their wettability (how). Binks et al. [28,29] modified hydrophilic silica particles by a silanization reaction with (CH3)2SiCl2, producing a series of particles of increasing hydrophobicity given by a decrease in the relative percentage of silanol (SiOH) groups on their surfaces. Others modified polystyrene spheres by sulfonation [8,30], carboxylation [8] and aldehyde [7] reactions. Surface modification undoubtedly changes the wettability of the particles [8,30]. He et al. [30] found that when the sulfonation time was increased, how of the particles decreased from 122.5° to 84.5° and the stability and size of emulsion droplets varied. Stiller et al. [31] investigated the influence of organic and inorganic surface modification of TiO2 particles and found that the surface modification leading to how between 110° and 152° was required in order to yield stable Pickering emulsions of triglyceride oil and water. In recent years, super-paramagnetic Fe3O4 nanoparticles have attracted great attention in the biomedical field including their use in magnetic resonance imaging [32–34], hyperthermia therapy of cancers [35,36] and targeted drug delivery [37,38], due to their high magnetic saturation, negligible toxicity and easy surface modification. Taking advantage of Pickering emulsions, many kinds of functional materials based on Fe3O4 nanoparticles, such as magnetic hollow silica microspheres [39], Fe3O4/polystyrene/silica nanospheres [40], Fe3O4/polymer magnetic nanocomposites [41– 43] and Fe3O4/poly(N-isopropylacrylamide-co-methacrylic acid) thermoresponsive composite latex [44], have been successfully prepared. Actually, some groups have demonstrated that Pickering emulsions can be produced by both bare and modified Fe3O4 nanoparticles, such as trialkoxysilane–water emulsions stabilized by silane-modified Fe3O4 nanoparticles [45], dodecane–water and paraffin–water emulsions by oleic acid bilayer-coated Fe3O4 nanoparticles[46,47], and crude oil–water and hexane–water emulsions by Fe3O4 nanoparticles [48,49]. However, the influence of particle wettability and oil polarity on the stability of this kind of magnetic Pickering emulsion remains unstudied. In our previous work [50], we systematically investigated unmodified Fe3O4 nanoparticles as stabilizers of Pickering emulsions and found that they are relatively hydrophilic and cannot stabilize emulsions with highly polar oils such as butyl butyrate and decanol. In the present study, in order to increase the hydrophobicity of the particles, we pre-modify them with carboxylic acid or silane coupling agents and investigate the influence of the surface modification on the stability of the resultant magnetic Pickering emulsions. Ingram et al. [46] and Lan et al. [47] used oleic acid to pre-modify Fe3O4 nanoparticles and investigated bilayer oleic acid-coated particles as Pickering emulsion stabilizers. However, when the pH > 7, the outer layer of oleic acid may become oleate (a common surfactant), and the resultant relatively white Pickering emulsion may be stabilized mainly by oleate surfactant. Some Refs. [51–53] have mentioned that the outer layer of oleic acid is weakly linked to the particles via van der Waals forces with the chains of the first layer. In order to avoid interference from molecules of the modifier, it is proposed that monolayercoated particles should be utilized. Here, an easy washing method is introduced in order to obtain Fe3O4 nanoparticles coated with a single layer of carboxylic acid using either ethanoic, pentanoic, octanoic or decanoic acid (RCOOH, R@CH3A, CH3(CH2)3A, CH3(CH2)6A, CH3(CH2)8A). Moreover, the silane coupling agents [54,55] (RSi(C2H5O)3, R@CH3CH2A, (CH3)2CHCH2A, CH3(CH2)7A) were also chosen to modify the Fe3O4 nanoparticle surface to investigate the effects of particle wettability on the stability of the resultant magnetic Pickering emulsions.
Fig. 1. FTIR spectra and TGA curves of carboxylic acid-coated Fe3O4 nanoparticles. (a) FTIR spectra of carboxylic acid-coated Fe3O4 nanoparticles prepared with the same ratio of carboxylic acid to particles (0.06 mol/g) including a spectrum for unmodified particles. (b) TGA curves of octanoic acid-coated Fe3O4 nanoparticles prepared with the same ratio of octanoic acid to particles (0.09 mol/g) but at different temperatures. (c) TGA curves of carboxylic acid-coated Fe3O4 nanoparticles prepared with the ratio of carboxylic acid to particles of 0.06 mol/g.
2. Experimental 2.1. Materials The water used was purified by an Aquapro ultra-pure water system (China), and its conductivity is 0.25 lS/cm measured
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Fig. 2. (a) FTIR spectra and (b) TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles of 0.01 mol/g.
Fig. 3. TGA curves of (a) octanoic acid-coated and (b) octyltriethoxysilane-coated Fe3O4 nanoparticles prepared with different ratios of modifier to particles (given).
using an FE30/EL30 conductivity meter (Mettler-Toledo Instruments, Switzerland). FeSO47H2O (AR, 99%), FeCl36H2O (AR, 99%) and NaOH (AR, 95%) were purchased from the Sinopharm Chemical Reagent Co. (China) and used as received. 36–38% HCl (AR) was obtained from Shanghai Lingfeng Chemical Reagent Co. (China) and was used as received. In preparing magnetic Pickering emulsions, two oils of different polarity were chosen and used without further purification. These were dodecane (AR, 99%) and butyl butyrate (AR, 99%) purchased from Aladdin Reagent Co. (China). In pre-modifying Fe3O4 nanoparticles, 7 modifiers were chosen and used without further purification. These were ethanoic acid (AR, 99.5%) and decanoic acid (AR, 97%) purchased from Sinopharm (China), pentanoic acid (AR, 99%), octanoic acid (AR, 98%), ethyltriethoxysilane (AR, 97%), isobutyltriethoxysilane (AR, 96%) and octyltriethoxysilane (AR, 98%) purchased from Sigma–Aldrich. Toluene (AR, 99%) and ethanol (AR, 99.7%) were purchased from Sinopharm (China).
N2 atmosphere and using an oil bath at 80 °C. After the above mixture turned black, it was allowed to cool with continuous stirring, and the black particles obtained were washed three times with water and once with absolute ethanol.
2.2. Synthesis of unmodified Fe3O4 magnetic nanoparticles Fe3O4 magnetic nanoparticles were prepared by co-precipitation of aqueous ferrous and ferric ions. Before synthesis, aqueous phases of 0.5 M FeSO4 solution with 0.2 M HCl, 1 M FeCl3 solution with 0.2 M HCl and 1.5 M NaOH solution were prepared. During preparation, 100 mL NaOH solution was poured into a flask and heated to 80 °C, and then the mixture of 10 mL FeSO4 solution and 10 mL FeCl3 solution was added into the flask dropwise in an
2.3. Modification and characterization of Fe3O4 nanoparticles Fe3O4 nanoparticles as prepared above were dispersed in 50 mL absolute ethanol by a KQ100 ultrasonic cleaning machine (Kunshan Ultrasonic Instrument Co., China) using a power of 100 W for 5 min. and then poured into a flask. 100 mL toluene and some amount of modifier (carboxylic acid or alkyltriethoxysilane) were added and the flask heated to 110 °C in an N2 atmosphere. After holding the above mixture at 110 °C for 8 h, the black fluid was immediately poured into a beaker, and the modified particles were separated using a magnet. The solid black material was washed using absolute ethanol and water and then dried in an LGJ-10 common type freeze dryer (Beijing Huaxing Technology Development Co., China). In order to ensure only a monolayer of reagent on particle surfaces, washing (before drying) occurs at high temperature (75 °C) for carboxylic acid-modified particles and at low temperature (room) for silane-modified particles. In the experiments, an oil film can be seen on the surface of the solutions even after the carboxylic acid-modified particles have been washed by absolute ethanol 5 times and water for more than 10 times at room temperature. The protocol applied for these particles is thus to wash using
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Table 1 Measured amount of alkyl groups on Fe3O4 nanoparticle surfaces after modification with carboxylic acid or silane coupling agents and the volume fraction of the resultant stable emulsions prepared with either dodecane or butyl butyrate at different oil/water volume ratios. Particle codename
No. C atoms in alkyl group
Coating amount of modifier on particle surface per gram Fe3O4 (mmol/g)
Volume fraction of stable emulsion with dodecane (%)
Volume fraction of stable emulsion with butyl butyrate (%)
Oil/ water
Fe3O4 0.06EA-Fe3O4 0.06PA-Fe3O4 0.03OA-Fe3O4 0.06OA-Fe3O4 0.09OA-Fe3O4 0.06DA-Fe3O4 0.06PA-Fe3O4 0.06PA-Fe3O4
0 2 5 8 8 8 10 5 5
0 0.381 0.275 0.376 0.384 0.534 0.327 0.275 0.275
83.9 76.3 81.6 72.8 81.9 0 0 24.1 48.2
0 0 0 0 0 0 0 0 0
2:1 2:1 2:1 2:1 2:1 2:1 2:1 1:2 1:1
0.01ES-Fe3O4 0.01IBS-Fe3O4 0.005OS-Fe3O4 0.01OS-Fe3O4 0.015OS-Fe3O4 0.01OS-Fe3O4 0.01OS-Fe3O4
2 4 8 8 8 8 8
0.776 0.788 0.565 0.707 1.072 0.707 0.707
96.4 87.6 91.1 84.3 86.1 44.6 67.0
0 69.5 64.3 75.9 85.6 42.2 69.6
2:1 2:1 2:1 2:1 2:1 1:2 1:1
absolute ethanol at 75 °C for 3 times and then hot deionized water at 75 °C for 3 times; no oil film was then seen. For carboxylic acid-coated particles, the carboxylic acid/Fe3O4 ratio of 0.06 mol/g is chosen for ethanoic (0.06EA-Fe3O4), pentanoic (0.06PA-Fe3O4) and decanoic (0.06DA-Fe3O4) acids, and the ratios 0.03, 0.06 and 0.09 mol/g are chosen for octanoic acid (0.03OA-Fe3O4, 0.06OA-Fe3O4 and 0.09OA-Fe3O4). For silanecoated particles, the silane/Fe3O4 ratio of 0.01 mol/g is chosen for ethyltriethoxysilane (0.01ES-Fe3O4) and isobutyltriethoxysilane (0.01IBS-Fe3O4) and the ratios of 0.005, 0.01 and 0.015 mol/g for octyltriethoxysilane (0.005OS-Fe3O4, 0.01OS-Fe3O4 and 0.015OSFe3O4). In this case, both the effects of a different modifier and different extents of coating for the same modifier on the stability of Pickering emulsions stabilized by such modified particles can be investigated systematically. The morphology of the modified Fe3O4 nanoparticles was observed using transmission electron microscopy (TEM) employing a JEM-2100 microscope (JEOL, Japan) at an accelerating voltage of 200 kV. Before observation, the modified Fe3O4 nanoparticles were dispersed in water at a concentration of 0.4 wt.% using a KQ100 ultrasonic cleaning machine with a power of 10 W for 1 h. and then added dropwise onto a carbon-coated copper grid. About 500 nanoparticles were observed to determine their average diameter. In order to confirm that the modification is successful and the coating is a monolayer, Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA) were used to characterize the surface of the modified Fe3O4 nanoparticles. The composition of nanoparticles was determined by FTIR spectra covering the range from 400 cm 1 to 4500 cm 1 obtained from an Equinox 55 FTIR spectrometer (Vruker, Germany), and the disk shaped samples for measurements were obtained by compression molding with KBr. Using a TG209 F1 Iris Thermogravimetric Analyzer (NETZSCH, Germany), the weight change of modified Fe3O4 nanoparticles was investigated from 30 °C to 800 °C at a heating rate of 20 °C/min. in a nitrogen atmosphere, which can be used to evaluate the coating extent on the surface of the particles.
nanoparticles prepared above were investigated as stabilizers of water–butyl butyrate mixtures in this study. Thus, the carboxylic acid and silane-modified Fe3O4 nanoparticles were first dispersed at a concentration of 1 wt.% initially in oil (dodecane and butyl butyrate) by mixing for 1 h at a power of 100 W in a KQ100 ultrasonic cleaning machine (Kunshan Ultrasonic Instrument Co., China). After the addition of deionized water, emulsions with different oil/water volume ratios of 2:1, 1:1 and 1:2 were prepared using an FM200 high shear dispersing emulsifier (Fluko Equipment Shanghai, China) at a speed of 14,000 rpm for 3 min. Pickering emulsions stabilized by 1 wt.% of unmodified Fe3O4 nanoparticles initially in water were also prepared under the same conditions for comparison. After preparation, emulsions were immediately poured into cylindrical sample bottles, and their stability was investigated by recording the change in height with time of the emulsion–aqueous phase interface and then evaluating the volume fraction changes of the stable emulsion and resolved water. When no further changes occurred, photographs of the vessels were taken using a Samsung S760 digital camera. After this, the separated aqueous phase was dried to weigh the non-adsorbed Fe3O4 nanoparticles, allowing the amount of nanoparticles associated with the stable emulsion to be determined. The emulsion type was determined by measuring its conductivity (FE30/EL30 conductivity meter). Generally, the conductivity of the emulsion is mainly determined by that of the continuous phase. The oils used in this study have conductivities less than 0.1 lS/cm, and aqueous dispersions of 0.06PA-Fe3O4 nanoparticles have a conductivity of 24.6 lS/cm. If the conductivity is above 10 lS/cm, the emulsion type is O/W, and if the conductivity is below 0.1 lS/cm, the emulsion type is W/O. An XPS-8C optical microscope (Yongheng Optical Instruments, China) was used to image emulsion droplets. 3. Results and discussion 3.1. Characterization of modified Fe3O4 nanoparticles
2.4. Preparation of magnetic Pickering emulsions For unmodified Fe3O4 nanoparticles, stable Pickering emulsions can be prepared for the non-polar oil dodecane and the weakly polar oil poly(dimethylsiloxane), PDMS, but not for the highly polar oils butyl butyrate and decanol [50]. In order to obtain stable Pickering emulsions for these latter oils, the hydrophobized
3.1.1. Carboxylic acid-coated particles The unmodified Fe3O4 nanoparticles were prepared by co-precipitation of Fe2+ and Fe3+ in aqueous alkaline solution and then were modified by either carboxylic acid or silane coupling agents. In our study, the silane-coated nanoparticles were prepared by the same method as reported in previous Refs. [54,55], but the
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Fig. 4. TEM images of carboxylic acid- and silane-coated Fe3O4 nanoparticles. (a) 0.06DA-Fe3O4, (b) 0.06PA-Fe3O4, (c) 0.06EA-Fe3O4, (d) 0.01OS-Fe3O4, (e) 0.01IBS-Fe3O4, (f) 0.01ES-Fe3O4.
carboxylic acid-coated nanoparticles were prepared by a different method from those in the literature. In the latter, the nanoparticles were usually synthesized in one step with the addition of alkali to the mixed solution of carboxylic acid, Fe2+ and Fe3+. However, in order to investigate the influence of the surface modification in more detail, the carboxylic acid was added after the Fe3O4 nanoparticles were prepared in order to achieve monolayer modification by a novel washing protocol. FTIR spectroscopy was used to determine the surface composition of the modified particles. Fig. 1a shows the FTIR spectra of the
carboxylic acid-coated and unmodified Fe3O4 nanoparticles. Due to the low content of organic modifier used in the synthesis, the characteristic absorbance peaks of alkyl groups between 2800 and 3000 cm 1 are unobservable, but the peaks in the fingerprint regions of 1333–650 cm 1 and 1700–1333 cm 1 are regular and can be observed clearly. Similar to the FTIR results of monolayer and bilayer oleic acid-coated Fe3O4 nanoparticles [56,57], for our carboxylic acid-coated Fe3O4 nanoparticles, the characteristic C@O stretch band of the carboxyl group at 1710 cm 1 disappears, while three new peaks at 1428, 1523 and 1631 cm 1 originating
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Table 2 Conductivity (in lS cm 1) 20 days after preparation of Pickering emulsions stabilized by 1 wt.% carboxylic acid- or silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios. Particles
Codename
Carboxylic 0.06EA-Fe3O4 acid-coated 0.06PA-Fe3O4 0.03OA-Fe3O4 0.06OA-Fe3O4 0.09OA-Fe3O4 0.06DA-Fe3O4 0.06PA-Fe3O4 0.06PA-Fe3O4 Silane-coated
Dodecane Butyl butyrate 18.7 21.31 10.53 8.74 – – 7.87 10.46
0.01ES-Fe3O4 22.1 0.01IBS-Fe3O4 23.9 0.005OS-Fe3O4 23.3 0.01OS-Fe3O4 8.66 0.015OS-Fe3O4 7.78 0.01OS-Fe3O4 8.28 0.01OS-Fe3O4 8.63
Oil/ water
2:1 2:1 2:1 Complete phase separation 2:1 2:1 2:1 1:1 1:2 – 10.3 10.13 6.30 7.88 7.45 5.16
2:1 2:1 2:1 2:1 2:1 1:1 1:2
from the asymmetric and symmetric stretching vibration of COOand one peak at 1052 cm 1 from the CAO single bond stretching appear. These results indicate that the carboxylic acid group is chemisorbed onto the Fe3O4 nanoparticle surface as carboxylate, and the monolayer-modified Fe3O4 nanoparticles have been successfully prepared by the new washing technique. Moreover, for these modified particles, the characteristic absorbance band of magnetite, Fe3O4, at 587 cm 1 is found but the band of maghemite, Fe2O3, at 630 cm 1 is not detected, confirming the purity of Fe3O4 in these nanoparticles without further oxidation. TGA curves (Fig. 1b and c) support the above conclusion. Fig. 1b gives the weight loss curves for octanoic acid-coated particles prepared at different temperatures. For bilayer carboxylic acid-coated Fe3O4 nanoparticles, the TGA curve has two stages of weight loss, because the interaction between the inner layer and outer layer is totally different to that between the inner layer and the particle surface. Zhang et al. [57] prepared bilayer oleic acid-coated Fe3O4 nanoparticles, and TGA results showed that their first weight loss is near 260 °C and the second weight loss is near 390 °C. When Fu et al. [51] prepared Fe3O4 nanoparticles coated with lauric acid as a first layer and decanoic acid as a second layer, they found that monolayer-coated lauric acid nanoparticles had only one weight loss at 150–360 °C while the bilayer-coated ones had a first weight loss at 150–250 °C and a second weight loss at 250–650 °C. Shen et al. [52] got similar results for the Fe3O4 nanoparticles coated with decanoic acid as the inner layer and different acids as the outer layer. From these references, it can be concluded that for bilayer carboxylic acid-coated Fe3O4 nanoparticles, there are two weight loss peaks with the first one at a temperature lower than the boiling point of the acid and the second one at a temperature about 100 °C higher than the boiling point of the acid. For monolayer carboxylic acid-coated Fe3O4 nanoparticles however, there is only one weight loss peak at a temperature higher than the boiling point of the acid. Obviously, in Fig. 1b and c, the TGA curves of all the carboxylic acid-coated Fe3O4 nanoparticles prepared at different temperatures or with different modifiers exhibit only one weight loss at 300 °C, higher than the boiling points of the carboxylic acids used in the synthesis. Thus, it can be concluded that the carboxylic acid-coated Fe3O4 nanoparticles prepared by the new washing technique here are monolayer modified only. The influence of the reaction temperature was investigated in the case of octanoic acid. From Fig. 1b, it can be noted that the amount of octanoic acid adsorbed as a monolayer on the surface of Fe3O4 nanoparticles increases with the modifying temperature, from about 3 wt.% to 7 wt.% when the modifying temperature is
raised from 40 °C to 110 °C. The temperature dependence of the monolayer density could be attributed to the competition of two interactions between carboxylic acid and Fe3O4 nanoparticles – chemical bonding and hydrogen bonding. High reaction temperature favors chemical bonding of carboxylic acid onto the particle surface, which is much more stable than the hydrogen bonding occurring during the following solvent washing at high temperature. Octanoic acid hydrogen bonded to the Fe3O4 nanoparticles can be easily washed away by absolute ethanol and water at 75 °C, which leads to the decrease in the coating amount remaining on particle surfaces. From Fig. 1c showing the TGA curves of carboxylic acid-coated Fe3O4 nanoparticles prepared with the same ratio of acid to particles (0.06 mol/g), it can be deduced from the weight loss that the adsorbed amount of carboxylic acid is 2.2, 2.7, 5.2 and 5.3 wt.% (corresponding to the coating amount of 0.381, 0.275, 0.384 and 0.327 mmol/g with respect to the dry particles) for 0.06EA-Fe3O4, 0.06PA-Fe3O4, 0.06OA-Fe3O4 and 0.06DAFe3O4 nanoparticles, respectively. 3.1.2. Silane-coated particles For silane-modified Fe3O4 nanoparticles, the method and mechanism are the same as those for c-aminopropyltriethoxysilane (APTES)-modified Fe3O4 nanoparticles [54,55,58]. The above references mention that in the absence of water, SiAOC2H5 reacts with AOH on the surface of Fe3O4 nanoparticles with the formation of SiAOAFe bonds. The FTIR spectra and TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles (0.01 mol/g) are given in Fig. 2. The characteristic absorbance peaks of AOH (near 3400 cm 1) and Fe3O4 (471, 587, 1621 cm 1) [53] are very clear in Fig. 2a, which reveals that not all Si atoms link to Fe3O4 nanoparticles and superfluous SiAOC2H5 groups hydrolyze to SiAOH groups. The absorbance peak at 2925 cm 1 is related to the stretching vibration of the CH2 group, whose intensity is the weakest for ethyltriethoxysilane-coated particles because the content of CH2 groups in ethyltriethoxysilane is the least among those of the silane coupling agents used. In addition, the absorbance peak at 1043 cm 1 originates from the stretching vibration of the SiAO bond, and the weak absorbance peak at 870 cm 1 may originate from the stretching vibration of the CASi bond. The appearance of all the characteristic absorbance bands of silane coupling agents confirms the successful coating of silane coupling agent on the surface of Fe3O4 nanoparticles in this study. The TGA curves of silane-coated Fe3O4 nanoparticles prepared with the same ratio of silane to particles (0.01 mol/g) are shown in Fig. 2b. In the measured temperature range from 150 °C to 550 °C, the weight loss is 2.2, 4.3 and 7.4 wt.% (corresponding to the coating amount of 0.776, 0.788 and 0.707 mmol/g with respect to particles) for 0.01ES-Fe3O4, 0.01IBS-Fe3O4 and 0.01OS-Fe3O4 nanoparticles, respectively. The weight loss mechanism of silanecoated Fe3O4 nanoparticles is very different from that of carboxylic acid-coated particles. It is well known that the bond energy of the SiAO bond (460 kJ/mol) is much higher than that of the CASi (347 kJ/mol) and CAC bonds (332 kJ/mol). Therefore, the weight loss of silane-coated particles results from the decomposition of the alkyl group (ethyl, butyl or octyl), while that of carboxylic acid-coated particles results from the decomposition of the carboxylic acid (ethanoic, pentanoic, octanoic or decanoic). In this case, the coating percent of silane coupling agents should be much higher than that of carboxylic acids for modifiers with the same alkyl chain length, although the modifying ratio of carboxylic acid to particles is six times that of silane coupling agents. For example, for octyl chains, the molecular mass ratio of CH3(CH2)7A to CH3(CH2)7Si(O-)3 is 113/189 (59.8%), and the coating percent of octyltriethoxysilane should be 7.4 wt.%/59.8% = 12.9 wt.%, but the coating percent of octanoic acid should be equal to the weight loss of 5.2 wt.%, much lower than that of octyltriethoxysilane. For alkyl
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Fig. 5. Microscope images 20 days after preparation of dodecane-in-water Pickering emulsions stabilized by 1 wt.% carboxylic acid- or silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (given). (a) 0.06EA-Fe3O4, 2:1, (b) 0.06OA-Fe3O4, 2:1, (c) 0.06PA-Fe3O4, 2:1, (d) 0.06PA-Fe3O4, 1:1, (e) 0.06PA-Fe3O4, 1:2, (f) 0.01ES-Fe3O4, 2:1, (g) 0.01IBS-Fe3O4, 2:1, (h) 0.01OS-Fe3O4, 2:1, (i) 0.01OS-Fe3O4, 1:1, (j) 0.01OS-Fe3O4, 1:2.
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Fig. 6. Microscope images 20 days after preparation of butyl butyrate-in-water Pickering emulsions stabilized by 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (given). (a) 0.01IBS-Fe3O4, 2:1, (b) 0.005OS-Fe3O4, 2:1, (c) 0.015OS-Fe3O4, 2:1, (d) 0.01OS-Fe3O4, 2:1, (e) 0.01OS-Fe3O4, 1:1, (f) 0.01OS-Fe3O4, 1:2.
groups adsorbed on the particle surfaces, their amount is higher for silane-coated particles than for carboxylic acid-coated particles with the same number of carbon atoms in the modifier, especially for modifiers having a greater number of carbon atoms. Moreover, with an increase in the amount of modifier used in the synthesis, the amount of alkyl groups on the particle surface is obviously increased for both silane- and carboxylic acid-coated Fe3O4 particles, as expected and reflected in Fig. 3 and Table 1. Fig. 4 contains TEM images of the carboxylic acid- and silanecoated Fe3O4 nanoparticles. Compared with the morphology of the unmodified Fe3O4 nanoparticles [50], it can be seen that modification does not change the size or morphology of the particles, but changes the extent of aggregation, as expected if particles become more hydrophobic. Some degree of flocculation can lead to an increase in the stability of Pickering emulsions [29,59,60]. 3.2. Type of Pickering emulsion stabilized by modified Fe3O4 nanoparticles It is known that the –OH groups on the surface of Fe3O4 nanoparticles can be substituted by alkyl groups. Many Refs. [54,61– 64] have mentioned that increasing the percent of alkyl groups on the particle surface can increase the hydrophobicity of the particles. Park et al. [54] investigated the influence of alkyl chain
length on the hydrophobicity of organically modified silica gels and found that the amount of absorbed water decreases as the modified alkyl chain is increased from methyl through to octyl. Horozov et al. [61] reported that the contact angle of SiO2 nanoparticles at the air–water interface is increased from 51° to 110° with an increase in the percent of alkyl groups substituting the AOH groups on particle surfaces. It is very difficult to determine the contact angle of these modified Fe3O4 nanoparticles however, because these particles cannot be compressed due to their brittleness or used in powder form with the Washburn method due to their hydrophobicity and unknown material constant. In order to qualitatively assess the hydrophobicity of the modified Fe3O4 nanoparticles, we designed an immersion test by carefully placing the particles on the surface of methanol–water solutions containing different methanol content (from 0 to 100 wt.%) and recorded the time for the particles to immerse in the solutions. Most of the mass of these modified Fe3O4 nanoparticles sedimented to the bottom of the solution immediately, but a small percentage floated on the solution surface as the length of the alkyl chain in the modifier increased or the methanol content in solution decreased. By contrast, unmodified Fe3O4 nanoparticles sedimented immediately without any particles remaining on the surface for all the solutions. The immersion time could not be determined for these particles, unlike the case for SiO2 particles, probably
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Fig. 7. Digital photographs of the vessels taken 20 days after emulsion preparation containing magnetic Pickering dodecane-in-water emulsions stabilized by: (a) either 1 wt.% Fe3O4 initially in water or 1 wt.% carboxylic acid-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 (from left Fe3O4, 0.06EA-Fe3O4, 0.06PAFe3O4, 0.03OA-Fe3O4, 0.06OA-Fe3O4, 0.09OA-Fe3O4 and 0.06DA-Fe3O4) and (b) 1 wt.% 0.06PA-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left 1:2, 1:1 and 2:1). Change in the volume fraction of residual emulsion versus time for the dodecane-in-water emulsions stabilized by (c) 1 wt.% Fe3O4 initially in water or 1 wt.% carboxylic acid-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.06PA-Fe3O4 nanoparticles initially in oil with different oil/ water volume ratios.
due to their much higher density (4.25 cf. 2.2 g/cm3). Nevertheless, it seems that the more hydrophobic the particles are, the more easily they float on the surface of the solutions with lower methanol content. According to these observations, it can be concluded that the Fe3O4 nanoparticles are more hydrophobic after modification and the modified particles are more hydrophobic for those with a greater amount of alkyl groups on their surfaces (see Table 1). It is expected that the higher hydrophobicity of silane-coated nanoparticles compared with carboxylic acid-coated ones will result in higher stability of the Pickering emulsion with a polar oil phase. The type of Pickering emulsion stabilized by modified Fe3O4 nanoparticles was determined by the conductivity of the emulsions, which are listed in Table 2. It can be seen in Table 2 that the conductivity of all the Pickering emulsions stabilized by modified Fe3O4 nanoparticles is much larger than 0.1 lS/cm, so all these Pickering emulsions are dodecane-in-water or butyl butyrate-in-water emulsions. For these modified Fe3O4 nanoparticles, the emulsion type is independent of the type or coating extent of the modifier on the particle surface, the oil phase or the oil/water volume ratio used for emulsion preparation. Microscopic images 20 days after the preparation of Pickering emulsions stabilized by both carboxylic acid- and silane-coated Fe3O4 nanoparticles are shown in Fig. 5 for dodecane and in
Fig. 6 for butyl butyrate. The droplets of the emulsions are spherical, and their mean size varies from 40 to 70 lm. For dodecane-inwater emulsions, the size of the droplets increases slightly as the length of the alkyl group in carboxylic acid-modified particles increases, while the size of droplets hardly changes with the alkyl chain length in silane-coated particles. For butyl butyrate-in-water emulsions, the size of droplets hardly changes with the coating percent of octyltriethoxysilane. Similar to Pickering emulsions stabilized by unmodified Fe3O4 nanoparticles, the average droplet size increases with an increase in the oil volume fraction for emulsions stabilized by modified Fe3O4 nanoparticles. 3.3. Stability of Pickering emulsions stabilized by modified Fe3O4 nanoparticles The stability of Pickering emulsions stabilized by unmodified Fe3O4 nanoparticles was investigated in our previous work [50], where we found that such particles can stabilize emulsions of a non-polar oil like dodecane and a weakly polar oil such as PDMS, but cannot stabilize emulsions of highly polar oils such as butyl butyrate and decanol. When using carboxylic acid-coated nanoparticles, it is found that these modified particles cannot stabilize emulsions of butyl butyrate but some of these particles can stabilize emulsions of dodecane. For dodecane-in-water emulsions,
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Fig. 8. Digital photographs of the vessels containing magnetic Pickering dodecane-in-water emulsions stabilized by (a) 1 wt.% Fe3O4 initially in water or silane-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 (from left, Fe3O4, 0.01ES-Fe3O4, 0.01IBS-Fe3O4, 0.005OS-Fe3O4, 0.01OS-Fe3O4, 0.015OS-Fe3O4) and (b) 1 wt.% 0.01OS-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left, 1:2, 1:1, 2:1). Change in the volume fraction of residual emulsion versus time for the dodecane-in-water emulsions stabilized by (c) 1 wt.% Fe3O4 initially in water or 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.01OS-Fe3O4 nanoparticles with different oil/water volume ratios.
digital photographs of the emulsions taken 20 days after preparation and the changes in the volume fraction of residual emulsion (ratio of emulsion volume to total liquid volume) as a function of time are given in Fig. 7 along with the long-term stability listed in Table 1. Obviously, no emulsion can be obtained when the 0.09OA-Fe3O4 nanoparticles and 0.06DA-Fe3O4 nanoparticles were used as the stabilizer. Emulsions stabilized by carboxylic acid-coated particles exhibit both creaming and coalescence to different extents. From Fig. 1c, it is known that when the ratio of carboxylic acid to particles is 0.06 mol/g, the amount of carboxylic acid adsorbed on nanoparticle surfaces is 0.381, 0.275, 0.384 and 0.327 mmol/g with respect to the dry particles for EA, PA, OA and DA, respectively. The stable emulsion fraction at long time for these four particle types is 76.3%, 81.6%, 81.9% and 0% in order. When changing the ratio of octanoic acid to particles from 0.03 to 0.09 mol/g, the amount of adsorbed octanoic acid increases from 0.376 to 0.534 mmol/g, and the stable emulsion fraction attained is 72.8%, 81.9% and 0% for the 0.03OA-Fe3O4, 0.06OA-Fe3O4 and 0.09OAFe3O4 nanoparticles, respectively, lower than that for the unmodified Fe3O4 nanoparticles. Thus, it can be seen that once the particle hydrophobicity becomes too high, emulsions of dodecane and water cannot be formed. Moreover, similar to the unmodified Fe3O4 nanoparticles, the stable emulsion fraction increases with an increase in the volume fraction of oil, from 24.1% to 48.2% to 81.6% for 0.06PA-Fe3O4 nanoparticles at oil/water volume ratios of 1:2, 1:1 and 2:1, as seen in Fig. 7d and Table 1.
Pickering emulsions stabilized by silane-coated Fe3O4 nanoparticles are investigated too. All the coated particles can stabilize emulsions of dodecane, and interestingly, some of these particles can now stabilize emulsion of butyl butyrate. The digital photographs of these emulsions taken 20 days after preparation and the change in the volume fraction of residual emulsion versus time are given in Fig. 8 for dodecane as oil and in Fig. 9 for butyl butyrate. Emulsions are stable to coalescence and only exhibit creaming. When the ratio of silane to particles is 0.01 mol/g, the amount of silane adsorbed on nanoparticle surfaces is 0.776, 0.788 and 0.707 mmol/g with respect to particles for ES-, IBS- and OS-coated particles, respectively. Correspondingly, the stable emulsion fraction of the butyl butyrate-in-water emulsions is 0%, 69.5% and 75.9% in that order. Although Pickering emulsions of dodecane can be stabilized by all the silane-coated Fe3O4 nanoparticles, emulsions of butyl butyrate can only be stabilized by 0.01IBSFe3O4 and 0.01OS-Fe3O4 and not by 0.01ES-Fe3O4 nanoparticles. The reason is that increasing the length of the alkyl group on the particle surface will increase the hydrophobicity of the particles. For octyltriethoxysilane-coated particles, different ratios of silane to particles used in the synthesis also affect the wettability of the modified particles and the stability of the resultant emulsions. The TGA curves in Fig. 3b shows that the amount of OS adsorbed on nanoparticle surfaces is 0.565, 0.707 and 1.072 mmol/g with respect to particles for 0.005OS-Fe3O4, 0.01OS-Fe3O4 and 0.015OS-Fe3O4 nanoparticles. Varying the coating extent of octyltriethoxysilane has little effect on emulsion sta-
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Fig. 9. Digital photographs of the vessels taken 20 days after emulsion preparation containing magnetic Pickering butyl butyrate-in-water emulsions stabilized by (a) 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil with an oil/water volume ratio of 2:1 (from left, 0.01ES-Fe3O4, 0.01IBS-Fe3O4, 0.005OS-Fe3O4, 0.01OS-Fe3O4, 0.015OS-Fe3O4) and (b) 1 wt.% 0.01OS-Fe3O4 nanoparticles initially in oil with different oil/water volume ratios (from left, 1:2, 1:1, 2:1). Change in the volume fraction of residual emulsion versus time for the butyl butyrate-in-water emulsions stabilized by (c) 1 wt.% silane-coated Fe3O4 nanoparticles initially in oil at an oil/water volume ratio of 2:1 and (d) 1 wt.% 0.01OS-Fe3O4 nanoparticles with different oil/water volume ratios.
bility for both dodecane and butyl butyrate, all emulsions appearing stable. The volume fraction of the stable emulsion of butyl butyrate changes more than that of dodecane upon increasing the extent of modifier coating. One of the aims of our research is to obtain stable polar oil– water Pickering emulsions stabilized by Fe3O4 nanoparticles. Silanes and carboxylic acids were chosen to modify the particle surface to increase their hydrophobicity. However, carboxylic acidcoated particles do not yield stable butyl butyrate-in-water emulsions even if the ratio of carboxylic acid to particles is six times that of silane reagent and the number of carbon atoms in the alkyl groups is increased to 10. The silane-coated particles can achieve this even when the number of carbon atoms is as low as 4. The TGA results reveal that the amount of adsorbed alkyl groups is much higher for silane-coated Fe3O4 nanoparticles than for carboxylic acid-coated Fe3O4 nanoparticles. 4. Conclusions In order to obtain stable magnetic Pickering emulsions with polar oils, carboxylic acid and silane coupling agents with different chain lengths were used to prepare monolayer-modified Fe3O4 nanoparticles of increased particle hydrophobicity. The extent of adsorption of silane reagent on particle surfaces is higher than that of carboxylic acid. Carboxylic acid-modified Fe3O4 nanoparticles cannot stabilize emulsions of butyl butyrate even when the num-
ber of carbon atoms in their alkyl chain is increased to 10, while the silane-coated particles achieve this even when the number of carbon atoms is as low as 4. All emulsions are of the o/w type and their conductivity and droplet sizes are influenced weakly by the coating type, coating extent and modifier alkyl chain length. Similar to unmodified Fe3O4 nanoparticles, the average droplet size and the volume fraction of stable emulsion in the presence of modified Fe3O4 nanoparticles increase with an increase in the oil/water ratio. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 20803047) and the Chen Xing Young Scholar Award Program of Shanghai Jiao Tong University (Grant No. T241460617). The authors also thank the Instrumental Analysis Center of Shanghai Jiao Tong University for assistance with the measurements. References [1] [2] [3] [4]
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