Pickering Emulsions Stabilized by a Lipophilic Surfactant and

Dec 18, 2009 - Shandong 250100, P. R. China, ‡Department of Oil and Gas ... China University of Petroleum, Qingdao, Shandong 266555, P. R. China, an...
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Pickering Emulsions Stabilized by a Lipophilic Surfactant and Hydrophilic Platelike Particles Jun Wang,† Fei Yang,‡ Junjun Tan,† Guopeng Liu,§ Jian Xu,† and Dejun Sun*,† †

Key Laboratory for Colloid and Interface Chemistry of the Education Ministry, Shandong University, Jinan, Shandong 250100, P. R. China, ‡Department of Oil and Gas Storage and Transportation Engineering, China University of Petroleum, Qingdao, Shandong 266555, P. R. China, and §Department of Chemistry, Shandong Institute of Education, Jinan, Shandong 250013, P. R. China Received October 8, 2009. Revised Manuscript Received November 25, 2009

Liquid paraffin-water emulsions were prepared by homogenizing oil phases containing sorbitan oleate (Span 80) and aqueous phases containing layered double hydroxide (LDH) particles or Laponite particles. While water-in-oil (w/o) emulsions are obtained by combining LDH with Span 80, the emulsions stabilized by Laponite-Span 80 are always o/w types regardless of the Span 80 concentration. Laser-induced fluorescent confocal micrographs indicate that particles are absorbed on the emulsion surfaces, suggesting all the emulsions are stabilized by the particles. The difference of the particle-stabilized emulsion type may be explained by comparing particle contact angles and the oil-water interfacial tensions, indicating that more Span 80 molecules are adsorbed on the LDH particles than on Laponite. Apparently, the LDH particles are rendered more hydrophobic by Span 80, resulting in the formation of w/o emulsions. The long-term stability of the emulsions was also compared. Emulsions stabilized by Span 80 alone completely separate into two bulk phases of oil and water after 3 months. However, emulsion stability is greatly enhanced with the addition of LDH or Laponite particles. This synergism was accounted for by an increase of the dilational viscoelasticity modulus of the oil-water interface after particles were added to the aqueous phase. This increase indicates that the gel-like particle layer stays at the oil-water interface and resists emulsion coalescence. Scanning electron microscope (SEM) images display the presence of a firm layer surrounding the emulsion droplets and a three-dimensional particle network which extends into the bulk phase aiding emulsion stability.

1. Introduction Pickering emulsions1 are of great practical interest due to their widespread use in many areas including cosmetics, food, pharmaceutics, oil recovery, and wastewater treatment. In most of these applications, the type and the long-term stability of Pickering emulsions are the two most important factors for producing emulsions with particular characteristics. It is well-known that the type and stability of Pickering emulsions can be controlled by tailoring the wettability of colloidal particles in the liquid media. Very stable emulsions are prepared with particles of intermediate wettability (contact angle θ approaches 90°), while the poor emulsion stability is observed when the particles are too hydrophilic (θ , 90°) or too hydrophobic (θ . 90°). When the contact angles of the particles are slightly smaller than 90° or slightly greater than 90°, stable oil-in-water (o/w) emulsions or water-inoil (w/o) emulsions will be respectively produced.2 In addition to particle wettability, the particle size, shape, and concentration can also influence the effective stability of an emulsion. The conventional model for Pickering emulsion stabilization assumes the formation of a densely packed particle layer at the oil-water interface3,4 which prevents droplet coalescence by a steric barrier mechanism. This mechanism is related to the interfacial viscoelasticity of the particle film. Other mechanisms of stability are *Corresponding author. E-mail [email protected]; Tel þ86 531 88364749; Fax þ86 531 88365437.

(1) Pickering, S. H. J. Chem. Soc. 1907, 91, 2001. (2) Akartuna, I.; Studart, A. R.; Tervoort, E.; Gonzenbach, U. T.; Gauckler, L. J. Langmuir 2008, 24, 7161. (3) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727. (4) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloid Interface Sci. 2003, 100-102, 503.

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related to the formation of particle bridges4-6 in a three-dimensional network throughout the emulsion to aid overall stability. In many cases, surfactants are usually added to tailor the particle wettability, so that the emulsion type can be changed or the emulsion stability can be improved. To date, many surfactants have been combined with nanoparticles to prepare Pickering emulsions, leading to the occurrence of many interesting phenomena in Pickering emulsions. Binks and Rodrigues7 have reported a double inversion of emulsions stabilized by silica nanoparticles and the double-chain cationic surfactant didecyldimethylammonium bromide. They attributed the double inversion to the variation of the wettability of silica particles from hydrophilic to hydrophobic and then back to hydrophilic following surfactant addition. The same emulsion double inversion has been reported by our group8 to be induced by the single-chain anionic surfactant sodium dodecyl sulfate (SDS) with layered double hydroxide (LDH) particles. Tsugita et al.9 have found a synergistic stabilization of emulsions by the combination of Na-Montmorillonite and polar organic compounds; this combination forms insoluble complexes that surround the oil droplets, thus stabilizing the emulsions. By combining silica particles with a nonionic poly(oxyethylene) surfactant, Midmore10 described a strong emulsification synergy between the particles and surfactant to stabilize o/w emulsions. Recently, Binks and co-workers have (5) Abend, N. B.; Gutschner, S. U.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 730. (6) Abend, S.; Lagaly, G. Clay Miner. 2001, 36, 557. (7) Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed. 2007, 46, 5389. (8) Wang, J.; Yang, F.; Li, C.; Liu, S.; Sun, D. Langmuir 2008, 24, 10054. (9) Tsugita, S. T. A.; Mori, K.; Yoneya, T.; Otani, Y. J. Colloid Interface Sci. 1983, 95, 551. (10) Midmore, B. R. Colloids Surf., A 1998, 145, 133.

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investigated the stability of emulsions using a combination of silica particles with a series of surfactants; they evaluated cationic, anionic, and nonionic surfactants such as cetyltrimethylammonium bromide,11 SDS,12 and alkylpoly(oxyethylene) types,13 respectively. In their work, all the surfactants and silica particles have a synergistic effect in stabilizing emulsions, which was linked to the increase of both particle hydrophobicity and particle flocculation after surfactant adsorption. However, emulsion stability is not always enhanced by the combination of particles with surfactants. Binks and co-workers13 have found that the addition of hydrophilic silica particles into an emulsion stabilized by alkylpoly(oxyethylene)-type surfactants caused emulsion coalescence. Legrand et al.14 have also showed that adding silica particles to the continuous phase of a bitumen-in-water emulsion stabilized by a cationic surfactant led to flocculation and partial coalescence of the emulsion droplets. It is to be noted that in the above work all the particles can well disperse in the aqueous phase and all the surfactants can be soluble in the water. Therefore, the emulsions have been prepared by mixing the particles and surfactants together in the same aqueous phase before emulsification. If the surfactants are lipophilic and the particles are hydrophilic, the two emulsifiers cannot be premixed together in one phase. Preparing emulsions by hydrophilic particles and lipophilic surfactants has been rarely reported, since it is difficult to investigate the interactions between the two dissimilar species at the oil-water interface. Despite the difficulties, Schulman and Leja15 in 1954 described the addition of BaSO4 powder in w/o emulsions stabilized by oleic acid (a lipophilic surfactant) which caused phase inversion to o/w emulsions. Later, Tambe and Sharma16 demonstrated the inversion from o/w to w/o emulsions stabilized by calcium carbonate particles with increasing concentrations of stearic acid as a surfactant. In these reports above, the authors attributed the emulsion phase inversion to an increase in particle hydrophobicity following surfactant adsorption onto the particle surfaces from the oil phase. Recently, Whitby et al.17 have prepared emulsions by homogenizing a Laponite aqueous dispersion with an oil solution containing lipophilic polar surfactants and found that a synergistic interaction existed between the particles and octadecylamine. In contrast, there was an antagonistic interaction caused by octadecanoic acid in the same emulsions. Finally, by emulsifying an aqueous silica dispersion and a monoolein/oil solution, Pichot et al.18 have obtained o/w emulsions and found that they possessed excellent long-term stability against coalescence. Despite some work investigating Pickering emulsions stabilized by a combination of hydrophilic particles and lipophilic surfactants, the interactions between surfactants, particles, and emulsion droplets are not fully understood. It is not entirely clear how lipophilic surfactants may be adsorbed onto particle surfaces via transfer across an oil-water interface from an oil phase. Typically, polar lipophilic surfactants were chosen as the surfactant in previous studies; however, nonionic surfactants have been investigated in relatively few reports. To extend this area of research, more combinations of agents and additional measurements are (11) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Langmuir 2007, 23, 3626. (12) Binks, B. P.; Rodrigues, J. A. Langmuir 2007, 23, 7436. (13) Binks, B. P.; Desforges, A.; Duff, D. G. Langmuir 2007, 23, 1098. (14) Legrand, J.; Chamerois, M.; Placin, F.; Poirier, J. E.; Bibette, J.; Leal-Calderon, F. Langmuir 2005, 21, 64. (15) Schulman, J. H.; Leja, J. Trans. Faraday Soc. 1954, 50, 598. (16) Tambe, D. E.; Sharma, M. M. J. Colloid Interface Sci. 1993, 157, 244. (17) Whitby, C. P.; Fornasiero, D.; Ralston, J. J. Colloid Interface Sci. 2008, 323, 410. (18) Pichot, R.; Spyropoulos, F.; Norton, I. T. J. Colloid Interface Sci. 2009, 329, 284.

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Figure 1. Structure of the Span 80 (sorbitan monoleate).

Figure 2. TEM image of Laponite particles (a) and LDH particles (b) in aqueous dispersion.

needed as well as systematic studies understand the properties of emulsions prepared with lipophilic surfactants and hydrophilic particles. In this context, we systemically investigated the type and long-term stability of emulsions prepared by homogenizing oil phases containing the lipophilic nonionic surfactant, sorbitan oleate (Span 80), and aqueous phases containing hydrophilic Laponite or LDH particles. The differences in surface properties between the two kinds of particles are anticipated to be able to influence the adsorption of surfactant molecules and hence to have an impact on emulsion properties.

2. Experimental Section 2.1. Materials. Deionized water, purified by ion exchange, was used in all experiments. The oil phase was liquid paraffin (Sinopharm Chemical Reagent Co., China) with purity greater than 99% (d420 = 0.835-0.855). The composition of liquid paraffin is mainly isoalkane, and the main carbon number distribution measured with an Agilent 6820 GC (Agilent Co.) is between 16 and 26. The lipophilic nonionic surfactant used here was sorbitan monoleate (Span 80) (Chemical pure; Shanghai Reagent Co., China). The structure of Span 80 is shown in Figure 1. All the reagents were used as received. Laponite RD, a synthetic hectorite, was supplied by Rockwood Additives, Ltd. (UK), as a white powder. As shown in Figure 2a, Langmuir 2010, 26(8), 5397–5404

Wang et al. they are disklike, with an average diameter 30 nm and a thickness around 1 nm, as measured with a JEM-100CX II electron microscope. Within a single crystal of the Laponite particles, each sheet of octahedrally coordinated magnesium (as Mg oxide) is sandwiched between two silicate layers in a tetrahedral coordination environment with oxygen atoms. Some of the magnesium sites of the central layer are substituted with lithium cations, which can create negatively charged sheets.19 Layered double hydroxide (LDH) particles used here are Mg2Al-LDH, which were prepared according to the literature.20 As shown in Figure 2b, the hexagonal, platelike LDH particles have an average size of 100 nm, as measured with a Zetasizer 3000HS. The average thickness of LDH particles is reported to be 5 nm.21 They are structurally as containing magnesium hydroxide layers in which some magnesium cations are substituted by aluminum cations to form positively charged sheets.22 Hence, the LDH particles are easily dispersed in water.

2.2. Methods. 2.2.1. Preparation of Aqueous Dispersions of Two Kinds of Particles and Span 80 Oil Solution. The Laponite dispersion (1 wt %) was prepared by dispersing 4 g of Laponite particles into 396 g of deionized water using a multimixer (Baroid Co.). The prepared Laponite dispersion was sealed and set aside for 1 week before use. The aqueous dispersion of LDH (1 wt %) was obtained by dilution of a stock LDH suspension with a concentration 12 wt %. A known mass (from 0.0428 to 4.2800 g) of Span 80 was dissolved in 100 mL of paraffin oil to obtain a series of stock solutions of Span 80 ranging from 1.0 to 100 mM.

2.2.2. Preparation, Stability, and Characterization of Emulsions. All the emulsions were prepared by adding equal volumes of the oil phase containing different concentrations of Span 80 (1-100 mM) in the water phase containing LDH (1 wt %) or Laponite particles (1 wt %) and then emulsifying the mixed liquids using a homogenizer (Shanghai Forerunner M&E Co., China) operated at 6000 rpm for 5 min. Immediately after the homogenization, the emulsion type was determined by a conductivity measurement and also by observing the outcome of a drop of each emulsion added to either pure oil or pure water. The emulsions were transferred into stoppered glass tubes with an internal diameter 1.6 cm and length 15 cm and set aside for 3 months to evaluate the long-term stability of emulsions at 25 °C. The separation of the continuous phase (water for o/w emulsions or oil for w/o emulsions) was used to assess the creaming for o/w emulsions or sedimentation for w/o emulsions, whereas the division of the dispersed phase (oil for o/w emulsions or water for w/o emulsions) was an indicator of coalescence for both types of emulsions.23 The morphology of emulsion droplets was observed with an Axioskop 40 optical microscope (ZEISS, Germany). The emulsion droplet size was obtained by processing the image using the microscopic image analysis software.

2.2.3. Adsorption and Arrangement of Particles at the Emulsion Surfaces. A laser-induced confocal microscope (Olympus Fluoview 500, Japan) was used to observe the adsorption of particles at the surface of emulsion droplets. Auramine O and Rhodamine B were used as fluorescent probes for labeling the Laponite and LDH particles, respectively. First, the fluorescent dyes were added to 25 mL of the particle dispersions (1 wt %), and the dye concentrations were controlled to give a final concentration of 1.0  10-5 M. The fluorescence-labeled particle dispersions were washed with the deionized water using a centrifuge to remove the free dye in the bulk solution. Next, emulsions were prepared by emulsifying 25 mL of aqueous phases containing (19) Cummins, H. Z. J. Non-Cryst. Solids 2007, 353, 3891. (20) Wang, X. J.; Sun, D. J.; Liu, S. Y.; Wang, R. J. Colloid Interface Sci. 2005, 289, 410. (21) Wang, N.; Liu, S. Y.; Zhang, J.; Wu, Z. H.; Chen, J.; Sun, D. J. Soft Matter 2005, 1, 428. (22) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34, 883. (23) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 3748.

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Article fluorescence-labeled particles and 25 mL of the oil phase which could contain either 5 mM Span 80 or 10 mM Span 80 for emulsions containing Laponite or LDH, respectively. Finally, fluorescent images of the emulsions were obtained using the microscope as above. The arrangement of particles on the surfaces of emulsion microspheres was further investigated using scanning electron microscopy (SEM, Hitachi S-520, Japan). First, 1 mL of emulsion prepared using 1 wt % Laponite-30 mM Span 80 or using 1 wt % LDH-10 mM Span 80 was mixed with 15 mL of acetone and allowed to stand for 30 min. A small amount of flocs appeared at the bottom of the tube and were transferred to a clean glass slide. After being dried in an oven at 80 °C for 8 h, the samples were observed by SEM. 2.2.4. Measurement of Particle Contact Angles. The aqueous dispersions of particles were poured into a glass vessel and dried at 100 °C in an oven for 24 h. The white particles formed on the bottom of the vessel were collected and compressed to a pressure of 16 MPa into circular disks with a pellet press. The thickness of all disks was ∼1 mm. The contact angle of particles was measured by the compressed disk method described previously.24 Specifically, paraffin, containing different concentrations of Span 80, was poured into the vessel containing the particle disk. After 3 min, a drop of water was placed on the surface of the particle disk using a syringe, and the shape of the water drop was recorded with a digital camera. The data of contact angle θ were obtained using the image analysis software.

2.2.5. Interfacial Tension and Interfacial Dilational Viscoelasticity Measurements. Interfacial tension and dilational

viscoelasticity were measured by the pendant drop method25,26 using a rheology apparatus (Tracker, I.T. Concept, France). Prior to the measurement, the aqueous phases, with or without particles, were equilibrated with the oil phases containing different concentrations of Span 80 by layering the oil phases on the top of the aqueous phases and left at rest at 25 °C for 4 days. A sample (ca. 8 mL) of the oil phase was added to a glass cuvette, and a drop of the aqueous phase was formed on the end of a stainless steel needle immersed in the oil phase. The aging of the interface can be accurately characterized by measuring the dynamic interfacial tension of a drop having a constant interfacial area. These data were also used to evaluate the equilibrium interfacial tension values. Dilational viscoelasticity data were obtained by analyzing the interfacial tension response to the controlled harmonic perturbations of the interfacial area up to frequencies of 0.2 Hz.

3. Results and Discussion To determine whether a synergism exists between the lipophilic nonionic surfactant sorbitan oleate (Span 80) and hydrophilic particles in stabilizing emulsions, Span 80, Laponite particles, and LDH particles were first separately examined for their ability to stabilize 1:1 paraffin-water emulsions. Then by dissolving the surfactant Span 80 in the oil phase and dispersing LDH particles or Laponite particles in the aqueous phase, emulsions containing the mixed emulsifiers were obtained and examined. And then, the difference of the type of emulsions by the two mixed emulsifier systems is explained. Finally, the synergistic effect on the longterm stability of emulsions by particles-Span 80 is clarified. 3.1. Emulsions Stabilized by Particles or Span 80 Alone. Because of their highly charged and hydrophilic character, the Laponite or LDH particles always prefer to form o/w emulsions. Even though emulsions can be obtained shortly after emulsification, these emulsions are unstable against coalescence, with the (24) Yan, N.; Masliyah, J. H. J. Colloid Interface Sci. 1996, 181, 20. (25) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543. (26) Ravera, F.; Ferrari, M.; Liggieri, L.; Loglio, G.; Santini, E.; Zanobini, A. Colloids Surf., A 2008, 323, 99.

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Figure 3. Comparison of emulsion appearance stabilized by Span 80 alone (a, d), 1.0 wt % Laponite-Span 80 (b, e), and 1.0 wt % LDH-Span 80 (c, f). The top images (a-c) were obtained 24 h after preparation. The bottom images (d-f) were obtained after 3 months. The concentration of Span 80 in oil (mM) is indicated in the figure.

bulk oil phase released above. Also, the emulsions possess large drop diameters with a wide size distribution, suggesting that the hydrophilic particles do not stay at the oil-water interface due to their low attachment energy.4 The surfactant Span 80 is well-known to form w/o emulsions due to a low hydrophile-lipophile balance (HLB) value of 4.3. Figure 3a shows the appearance of emulsions stabilized by Span 80 after 24 h of preparation. At low Span 80 concentrations (1-2 mM), the prepared emulsions are o/w types with the water phase separated below, while at high concentrations (1050 mM), w/o emulsions are formed with the oil phase divided above. After 3 months of the emulsification, complete phase separation into bulk of oil and aqueous phases occurs for all emulsions at any Span 80 concentration, shown in Figure 3d. While the Span 80 surfactant stabilizes an emulsion initially, it does not offer long-term stability against both creaming and coalescence. The results above demonstrated that either surfactant or particles do not stabilize emulsions alone for a long time. 3.2. Emulsions Stabilized by the Mixed Emulsifier Systems. At a fixed particle concentration of 1.0 wt % Laponite in the aqueous phase, a series of emulsions were prepared with increasing concentrations of Span 80 in the oil phase (1100 mM). The appearance of these emulsions after 24 h of preparation is shown in Figure 3b. In this case the prepared emulsions are all o/w types, with the aqueous phase separated below. With an increase of Span 80 concentration, the volume of the separated aqueous phases gradually decreases, suggesting that o/w emulsion stability to creaming increases. Images of emulsions stabilized by Laponite-Span 80 are shown in Figure 4a-d, and the size distribution of emulsion droplets is shown in Figure 4e. Emulsion droplet size gradually decreases upon increasing Span 80 concentration. The droplet size distribution is wide at a low surfactant concentration (1 mM) and gradually becomes narrower from 5 to 100 mM surfactant concentration. The variations of the droplet size and size distribution are consistent with those of the emulsion stability against creaming. The decrease of the droplet size comes from the decrease of the oil-water interfacial tension as Span 80 concentration increases, which will be described in the next section. To investigate the long-term stability of Laponite-Span 80 emulsions, the emulsions were photographed after 3 months (shown in Figure 3e) to compare with the same emulsions at 24 h (Figure 3b). Although the volume of separated aqueous phases for the emulsions after 3 months of preparation becomes slightly larger, the long-term stability of emulsions against creaming is still considered excellent. The most noticeable effect is the absence of any coalescence (oil released) for all the o/w emulsions 5400 DOI: 10.1021/la903817b

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even after 3 months. This example is in sharp contrast to the complete instability for emulsions with Span 80 alone after 3 months (compare Figure 3d with Figure 3a). For emulsions prepared with LDH-Span 80, we also investigated their type and long-term stability. At a fixed LDH concentration of 1.0 wt %, the appearance of emulsions after 24 h and 3 months is shown in parts c and f of Figure 3, respectively. In contrast with Laponite-Span 80 (Figure 3b,e), the emulsions prepared by combining LDH and Span 80 were all w/o types above a Span 80 concentration of 1 mM, at which an o/w type emulsion is obtained. The volume of the released oil phases from every system of w/o emulsions gradually decreases with adding Span 80 surfactant, indicating the stability of w/o emulsions against sedimentation (oil divided) is concomitantly enhanced. Figure 5a-d shows optical microscopy images of these emulsions, and Figure 5e denotes their size distribution. The initial o/w droplet has a large size (Figure 5a) and a broad size distribution (1 mM in Figure 5e), corresponding to an unstable emulsion. The droplet size of the w/o emulsions gradually decreases as Span 80 concentration increases (Figure 5b-d), and the size distribution becomes much narrower (from 5 to 30 mM, Figure 5e), which agrees well with the better stability of the w/o emulsions against sedimentation shown in Figure 3c. In addition, the decrease of the oil-water interfacial tension (section 3.3) on increasing Span 80 concentration may account for the decrease of emulsion droplet size. The long-term stability of these emulsions was examined by comparing the emulsions after 24 h (Figure 3c) with those after 3 months (Figure 3f). Although the volume of the released oil phases appreciably increases after 3 months, the long-term stability of emulsions is greatly improved with the presence of LDH particles, compared with those in the absence of particles (Figure 3d). In addition, the stability to coalescence (water separated) of all the LDH-Span 80 w/o emulsions is excellent regardless of how long they are laid (24 h or 3 months). The type and long-term stability of the emulsions described above may be summarized as follows: (1) emulsions stabilized by LDH-Span 80 present a w/o type above a low Span 80 concentration of 1 mM, whereas those stabilized by Laponite-Span 80 always are of an o/w type; (2) although the long-term stability of emulsions is poor with Span 80 or particles alone, emulsion stability can be remarkably improved by the combination of either particle with surfactant. We propose a strong synergistic effect is exhibited between Laponite or LDH particles and Span 80 in stabilizing emulsions. 3.3. Effects of Particles with Different Surface Properties on the Type of Emulsions Stabilized by the Mixed Emulsifiers. To confirm the adsorption of particles at the surface of emulsion droplet, laser-induced confocal scanning microscopy was conducted for emulsions stabilized by Laponite-Span 80 (Figure 6) or by LDH-Span 80 (Figure 7). Auramine O-labeled Laponite particles (green) can be seen at the surfaces of o/w emulsion droplets (Figure 6a), and rhodamine B-labeled LDH particles (red) may also be observed at the w/o emulsion surfaces (Figure 7a). The optical images of these emulsions exhibited in Figures 6b and 7b correspond well to the confocal fluorescence microscope images shown in Figures 6a and 7a, respectively. The results indicate that the Laponite or LDH particles do adsorb at the emulsion droplet surfaces; i.e., the emulsions are stabilized by the presence of the solid particles. The wettability of solid particles at the oil-water interface is crucial to both the preferred type and stability of Pickering emulsions.27 While an exact method for the determination of (27) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 8622.

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Figure 4. Optical microscope images of paraffin-water (1:1 by volume) emulsions stabilized by 1.0 wt % Laponite-Span 80 immediately

after preparation. The initial concentrations of Span 80 in oil are 1 (a), 5 (b), 30 (c), 100 mM (d). The scale bars in the figure are 40 μm. (e) Size distributions of emulsion droplets at varied concentrations.

Figure 5. Optical microscope images of paraffin-water (1:1 by volume) emulsions stabilized by 1.0 wt % LDH-Span 80, immediately after

preparation. The initial concentrations of Span 80 in oil are 1 (a), 5 (b), 10 (c), 30 mM (d). The scale bars in the figure are 40 μm. (e) Size distributions of emulsion droplets at varied Span 80 concentrations.

Figure 6. Confocal fluorescence microscope image (a) and the corresponding optical microscope image (b) of an o/w emulsion stabilized by 1.0 wt % Laponite-5 mM Span 80, immediately after preparation.

nanoparticle contact angle (θ) does not yet exist, we have recently approximated their values using a compressed disk method.8 Figure 8 shows the variation of contact angles of the two kinds of particles with increasing initial Span 80 concentration. In the absence of Span 80, θ is ∼35° for both types of particles. As Span 80 concentration increases, particle contact angle rapidly increases at first and then more gently. A plateau value of around 100° is reached at 20 mM for Laponite, but θ still increases slightly even up to around 180° at 50 mM for LDH, much larger than that of Laponite. Since θ exceeds 90° for both particles at high surfactant concentrations, it is predicted that emulsions should invert into w/o types, which in fact occurs for the emulsions stabilized by LDH-Span 80 but does not occur for those stabilized by Laponite-Span 80. Langmuir 2010, 26(8), 5397–5404

Figure 7. Confocal fluorescence microscope image (a) and the corresponding optical microscope image (b) of a w/o emulsion stabilized by 1 wt % LDH-10 mM Span 80, immediately after preparation.

It should be pointed out that measured values for contact angle may not exactly reflect the actual angles. A possible reason may be derived from the process differences between the emulsification and the contact angle measurement. In the emulsification process, the particle surface first contacts the water and then the oil, but in the contact angle measurement, the particle surface first contacts the oil and subsequently the water. In this process, the water droplet should break through the oil film around the particle surface to make contact, which would make the measured values larger than the real values exhibited in the emulsification process.11 Other reasons are related to the great differences in the oil/ water ratio and the solid surface areas in the two processes. DOI: 10.1021/la903817b

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Figure 8. Variation of contact angles (θ) of Laponite particles (0) and of LDH particles (O) versus initial concentration of Span 80 in paraffin oil.

Previously, Binks et al.7 reported that the result of contact angle measurement was not consistent with the occurrence of a double phase inversion of emulsions that were stabilized by silica particles and dichain cationic surfactants. In another emulsion system containing silica particles and CTAB,11 surfactant adsorption on the particle surfaces led to maximum contact angles of 110°, but the emulsions did not invert into a w/o type. Although the experimental values of contact angle may not exactly match the actual angles, the results reflect the trend of particle wettability that can be attributed to the adsorption of Span 80 onto the particles across the oil-water interface. At the same Span 80 concentration, the values of contact angle of LDH particles are much larger than those of Laponite, suggesting that more Span 80 may be adsorbed on the LDH particle surface in comparison with Laponite particles. The LDH particles may become much more hydrophobic than Laponite particles at the same surfactant concentration; hence, the w/o emulsions are obtained in LDH-Span 80 emulsions, whereas o/w emulsions are always formed in Laponite-Span 80 emulsions. Schulman15 and Tambe16 have attributed phase inversions in Pickering emulsions to the variations of particle wettability, which is induced by the adsorption of a lipophilic surfactant onto particle surfaces via transfer across the oil-water interface. Span 80 adsorption on particles via transfer across oil-water interface can be further confirmed by measuring the equilibrium tension of oil-water interface. This involves measuring the oilwater interfacial tension (γ) as a function of the initial surfactant concentration with and without particles, as shown in Figure 9. In the absence of particles, the oil-water interfacial tension gradually decreases upon increasing initial Span 80 concentration (Figure 9a); this observation is due to the adsorption of more Span 80 molecules at the oil-water interface. The addition of particles into the aqueous phase increases γ at every surfactant concentration (curves b and c in Figure 9). Given a fixed surfactant concentration, the depletion of surfactant due to the adsorption of a part of the Span 80 makes the adsorbed surfactant density at the oil-water interface decrease, leading to an increase of γ. Although particles modified by Span 80 also decrease γ, the extent is less than that by surfactant alone. The overall effect is the increase of γ with particle addition in the aqueous phase. Previous work25 has showed that CTAB-coated silica particles can be adsorbed at the oil-water interface to decrease the water-hexane interfacial tension, but the silica particles have less effect than the surfactant CTAB. It is known that if the interfacial tension of the particle-containing system rises above the critical micelle concentration (cmc) of a surfactant, the analysis is not exact. 5402 DOI: 10.1021/la903817b

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Figure 9. Equilibrium interfacial tension (γ) versus concentration of Span 80 in paraffin oil: (a, 2) pure water-Span 80 oil solution; (b, 0) 0.5 wt % Laponite aqueous dispersion-Span 80 oil solution; (c, O) 0.1 wt % LDH aqueous dispersion-Span 80 oil solution.

Thus, the concentration of Span 80 in this study is controlled to be lower or near its cmc value, reported as 0.43 mM in paraffin oil.28 At the same surfactant concentration, the difference in the interfacial tension of the particle-containing (curves b and c in Figure 9) and particle-free system (curve a in Figure 9) allows us to evaluate the adsorption extent of Span 80 on the particles. By comparing γ of the LDH-containing system (curve c in Figure 9) with Laponite-containing system (curve b in Figure 9), the former produces a much higher value than the latter at the same surfactant concentration. This suggests that much less surfactant molecules are packed on the oil-water interface in the presence of LDH, since more molecules have adsorbed on the LDH surface than the Laponite surface, in line with the result of the contact angle measurement. In other words, the LDH particles can be turned more hydrophobic to form a w/o emulsion, consistent with the result of the emulsion type referred above. In addition, as the Span 80 concentration in the oil phase increases, γ for both systems of particle aqueous dispersion-oil phase (curves b and c in Figure 9) gradually decreases, which can favor the breakage of larger droplets into small ones, contributing to the decrease of the droplet size either for o/w emulsions by Laponite-Span 80 (shown in Figure 4) or for w/o emulsions by LDH-Span 80 (shown in Figure 5) discussed above. The difference in Span 80 adsorption on LDH versus Laponite particles may originate from the difference in surface properties of the two kinds of particles. Considering the Span 80 molecular structure, we propose that H-bonding interactions may be the main contributing factor to adsorption, which may occur between the three available hydroxyl groups or oxygen atoms of Span 80 and hydroxyl groups or oxygen atoms on particle surfaces. As described above, the crystal structure of LDH particles indicates that many hydroxyl groups are exposed on their exterior surfaces.22 However, in the case of Laponite particles, the hydroxyl groups are located inside the basal hexagonal cavities of particle structure; few groups are exposed on the exterior surfaces.19 More H-bonding interactions with Span 80 molecules may occur for the LDH particles than Laponite particles, resulting in more Span 80 adsorbed on the LDH surface than on the Laponite surface. Additionally, the adsorption may also involve weak dipolar interactions between the polar groups of Span 80 and the particle charges. Since the charge density of the Mg2Al-LDH particles29 is around 4 e/nm2, compared with Laponite30 (1.4 e/nm2), the highly charged LDH particles should adsorb more Span 80. (28) Santini, E.; Liggieri, L.; Sacca, L.; Clausse, D.; Ravera, F. Colloids Surf., A 2007, 309, 270. (29) Oh, J. M.; Kwak, S. Y.; Choy, J. H. J. Phys. Chem. Solids 2006, 67, 1028. (30) Nicolai, T.; Cocard, S. Eur. Phys. J. E 2001, 5, 221.

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Figure 10. Modulus (ε) of the dilational viscoelasticity versus concentration of Span 80 in paraffin oil: (a, 2) pure water-Span 80 oil solution; (b, 0) 0.5 wt % Laponite aqueous dispersion-Span 80 oil solution; (c, O) 0.1 wt % LDH aqueous dispersion-Span 80 oil solution.

Grillo et al.31 have reported that the adsorption of the nonionic surfactant, polyoxyethylene alkyl ether, on Laponite particles can be attributed to hydrogen bonding between the ethylene oxide groups of the surfactant and hydroxyl groups located inside the basal hexagonal cavities and also due to dipolar interactions between the polar chains of adsorbed molecules and the negative charge of the particles. Also, Wang et al.20 have attributed the adsorption of the nonionic block copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (EPE1100), on LDH particles to H-bonding interactions between ethylene oxide groups of the copolymers and hydroxyl groups on LDH surfaces. 3.4. Enhanced Long-Term Stability of Emulsions by the Mixed Emulsifiers. Emulsions stabilized by Span 80 alone completely separate into two bulks of oil and water after 3 months of preparation (section 3.2), while the addition of particles into the aqueous phase greatly improve the long-term stability of emulsions (Figure 3, above). To investigate the effect of particles in stabilizing emulsions, dilational interfacial rheology measurements were completed for each phase-separated system (oil/surfactant and water/ particle) before emulsification. Since the dilational interfacial rheology is always related to the capability of interfaces to dampen the external disturbance, it can provide important information on the stability of films, foams, and emulsions. Interfacial dilational viscoelasticity was investigated here as a function of the initial surfactant concentration (Figure 10). In the absence of particles, the modulus of interfacial dilational viscoelasticity (ε) for a system of pure water-oil solution containing Span 80 is lower than 40 mN m-1 over the whole range of Span 80 concentrations. By comparison, at any given surfactant concentration, the addition of particles into the aqueous phase increases ε by about 10 mN m-1 for Laponite and even by 20-30 mN m-1 for LDH. Ravera et al.25 have studied the properties of mixed SiO2/CTAB interfacial layers through the measurement of effective interfacial tension and interfacial rheology (in a limited frequency range) and shown that the presence of particles increases ε, similar to the present result. An increase of the modulus of the dilational viscoelasticity indicates that a firm particle layer at the oil-water interface confers a strongly viscoelastic character to the interfacial layer and hence resists emulsion droplet coalescence. This correlation between the modulus of dilational viscoelasticity and the emulsion stability has been shown for the emulsions stabilized by silica/ CTAB.26 In their study, results from optical observations of emulsion droplets were combined with interfacial rheological (31) Grillo, I.; Levitz, P.; Zemb, T. Eur. Phys. J. B 1999, 10, 29.

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Figure 11. (a) Low- and (b) high-magnification SEM images of solid phases after removing the oil phase from an o/w paraffinwater (1:1 by volume) emulsion stabilized by 1 wt % Laponite30 mM Span 80. SEM image (c) of the solid phase left from precursor w/o paraffin-water (1:1 by volume) emulsion stabilized by 1 wt % LDH-10 mM Span 80.

measurements; the authors concluded that the attachment of nanoparticles with a certain degree of hydrophobicity at the interface is an irreversible process, and a nearly solid film forms at the fluid interface, preventing droplets coalescence and improving emulsion stability. Scanning electron microscopy (SEM) was used to observe the emulsion droplets stabilized by both mixed emulsifier systems (Figure 11). For the SEM measurement, the oil phase was removed from the emulsions with acetone as described in the Experimental Section. Although care was taken in the experimental process, the shells in most emulsion droplets are still partially broken down and opened cavities are left. However, the nearly spherical morphology is preserved. Comparison of the average cavity size with droplet diameter of the precursor emulsions shows that there is a close correlation between the two, suggesting that the cavities result from loss of the oil phase. In the case of emulsions stabilized by Laponite-Span 80 (Figure 11a), all the spherical cavities obtained from the precursor emulsions are surrounded with a thick Laponite particle shell conferring the droplet interface with mechanical integrity, which can protect the droplets against coalescence. Figure 11b illustrates an opened cavity covered with a thick gel-like particle shell, whose thickness is around 2 μm. This observation supports the results of the interfacial dilational viscoelasticity measurements, namely, that a gel-like particle layer exists at the oil-water interface to resist emulsion droplet coalescence. In addition, many particles are observed to aggregate into flocs among the droplets, and droplets are connected with each other through a particle network (Figure 11a). The close-packed particles adsorbed on the droplet surfaces extend into the aqueous phase serving as bridges between neighboring droplets; the bridges thus form a three-dimensional network;a further aid to stability. For the w/o emulsions stabilized by LDH-Span 80, the SEM image of one emulsion droplet is shown in Figure 11c. A hemisphere of the w/o emulsion droplet with a diameter of around DOI: 10.1021/la903817b

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10 μm is observed. The interior surface of the droplet is covered with particle aggregates that are connected to each other forming a close-packed multilayer surrounding the droplet like a bird0 s nest. The shells composed of LDH particle aggregates also provide the w/o emulsions with added stability. The SEM results in the present work are similar to those reported by Binks et al.4 By evaporation of oil and water from silica-stabilized emulsions, they observed the cavity structure formed from particle/surfactant emulsions. Recently, SEM32 and cryogenic scanning electron microscopy7,11-13 (cryo-SEM) have been used to probe the arrangement of particles around droplets. Through these observations, a multilayer composed of close-packed particles has been observed to encapsulate emulsion droplets, improving their stability. On the basis of the above results, a mechanism for the longterm stability of the emulsions is inferred. First, for a particlestabilized emulsion, the mechanical integrity of the oil-water interface may be strengthened by the interactions between close-packed particles, significantly contributing to the stability. Second, the increased stability is related to the presence of particles (or aggregates) in the liquid phase among the neighboring droplets. In this case, the existence of particles (or aggregates) provides a resistance to film thinning. Finally, the increased stability is related to the formation of a three-dimensional network among the droplets and particles, in which the emulsion droplets would be particularly stable.

4. Conclusions In contrast to Pickering emulsions obtained with surfactants and particles premixed in the aqueous phase, we have systematically investigated emulsifying oil phases containing the lipophilic nonionic surfactant Span 80 and aqueous phases containing LDH or Laponite particles. A strong synergistic effect is exhibited between particles and Span 80 in stabilizing Pickering emulsions. (32) Jiang, Y.; Wan, P.; Xu, H.; Wang, Z.; Zhang, X.; Smet, M. Langmuir 2009, 25, 10134.

5404 DOI: 10.1021/la903817b

For the emulsions stabilized by LDH-Span 80, w/o emulsions are obtained when a small amount of Span 80 is added (2 mM); in contrast, Laponite-Span 80 emulsions are always o/w type regardless of Span 80 concentration. The difference in the emulsion type can be derived from the different adsorption extent of Span 80 on the two kinds of particles with different surface properties. More Span 80 are absorbed on the LDH than on Laponite particles via transfer across the oil-water interface from oil phase, indicating that the LDH particles are rendered more hydrophobic. Additionally, the stability of the emulsions containing lipophilic nonionic surfactant Span 80 is greatly enhanced with the addition of particles. The synergy exhibited in these emulsions may be explained by an increase of the dilational viscoelasticity modulus of the oil-water interface when particles are added, indicating that a gel-like particle layer stays at the oil-water interface to resist emulsion coalescence. Particle aggregates are shown to adsorb at droplet interfaces and extend themselves into the liquid continuous phase serving as bridges between neighboring droplets, which aids emulsion stability. On the basis of these results, a mechanism for the long-term stability of emulsions is inferred. First, the attachment of closepacked particles at the droplet surfaces significantly contributes to the emulsion stability. Second, the particles (or aggregates) present in the liquid phase between approaching droplets resist liquid film thinning, further assisting in emulsion stability. Finally, the increased stability is also related to the formation of a three-dimensional network of particle flocs among emulsion droplets. Acknowledgment. This work was financially supported by a grant from the National Natural Science Foundation of China (No. 20603020) and the Doctoral Programs Foundation of the Ministry of Education of China (20060422021). The authors thank Professors Xusheng Feng (Shandong University) and J. David Van Horn (Visiting Professor, Shandong University) for help in preparation of the manuscript.

Langmuir 2010, 26(8), 5397–5404