Double Inversion of Emulsions Induced by Salt Concentration

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Double Inversion of Emulsions Induced by Salt Concentration Jingchun Zhang,†,§ Lu Li,†,§ Jun Wang,‡ Haigang Sun,† Jian Xu,† and Dejun Sun*,† †

Key Laboratory for Colloid and Interface Chemistry of the Ministry of Education, Shandong University, Jinan, Shandong 250100, People's Republic of China ‡ Shandong Provincial Key Laboratory of Test Technology for Material Chemical Safety, Jinan 250103, People's Republic of China S Supporting Information *

ABSTRACT: The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.

1. INTRODUCTION Emulsions are dispersions composed of two immiscible liquids and usually stabilized by emulsifiers, which have wide applications in fields such as cosmetics, foods, enhanced oil recovery, and templates for advanced materials fabrication. The emulsifiers can be molecular surfactants, solid particles, or a combination of surfactants and particles. Emulsions stabilized by surfactants have been known for a long time,1 and a thorough understanding of those stabilized by particles has also been attained recently.2 The hydrophile−lipophile balance number of surfactant molecules is one crucial parameter which determines the type and stability of emulsions. Likewise, the wettability of particles is decisive in the type and stability of Pickering emulsions.3 However, many emulsions applied in industries contain both surfactants and particles,4,5 so it is important to study the roles of each emulsifier and the interactions between them. Many reports have described the phase inversion induced by adding particles into a surfactant system or vice versa, as the wettability of particles is modified by the surfactant. Schulman and Leja6 found that water in oil (w/o) emulsions stabilized by oleic acid invert to oil in water (o/w) by adding barium sulfate particles. In contrast, the inversion of the emulsion from o/w to © 2012 American Chemical Society

w/o was observed by adding barium sulfate particles to emulsions stabilized by sodium dodecyl sulfate (SDS). Tambe and Sharma7 demonstrated inversion from o/w to w/o emulsions stabilized by calcium carbonate particles by increasing the addition of stearic acid surfactant. Binks and Rodrigues8 reported the first example of double inversion of emulsions. The emulsions containing silica particles can invert initially from o/w to w/o and subsequently back to o/w by increasing the concentration of surfactant didecyldimethylammonium bromide. Wang and co-workers9 reported that an emulsion double inversion is induced by adding SDS to an aqueous dispersion of layered double hydroxide (LDH) particles. Similarly double phase inversions of emulsions stabilized by CaCO3 nanoparticles with increasing concentrations of SDS, sodium carboxylates, and sodium 2-ethylhexyl sulfosuccinate in the aqueous phase was reported by Cui et al.10,11 In the reports mentioned above, the authors attributed the phase inversions to the great change of the particle wettability caused by the adsorption of surfactants. Received: February 17, 2012 Revised: April 1, 2012 Published: April 4, 2012 6769

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the suspensions were flocculated. However, for partially hydrophobic silica particle-stabilized o/w emulsions, stability against creaming could be dramatically improved by adding NaCl.25 Improvements of stability against coalescence of heptane-in-water emulsions stabilized by hydrophilic precipitated silica particles by adding divalent electrolyte have also been reported.26 Our previous work27 has also shown that the addition of salt promotes the adsorption of LDH particles onto the oil−water interface. The addition of salt can decrease the particle ζ potential, leading to the adsorption of particles to the interface and network formation of particles adsorbed at the interface, both of which promote the formation and stability of particle-stabilized emulsions. Here we report a novel double inversion of emulsions induced by simply changing the salt concentration for the first time to our knowledge. The emulsions are stabilized by Laponite particles and a lipophilic surfactant. We changed neither the concentration of particles nor the surfactant concentration in the system. The type and stability of the emulsions are affected by the salt concentration alone. In this process, the interactions between two emulsifiers are controlled by varying the salt concentrations, as different emulsifiers dominate the emulsion interface at different salt concentrations and the emulsions are inverted.

Synergistic and competitive effects between particles and surfactants have also been proposed. In many cases, the emulsion stability is improved as a synergistic effect exists between particles and surfactants. Tsugita et al.12 found a synergistic stabilization of emulsions by the combination of sodium montmorillonite and polar organic compounds. Midmore13 described a strong emulsification synergy between the silica particles and nonionic poly(oxyethylene) surfactant to stabilize o/w emulsions. Havre and Sjöblom14 reported that the addition of asphaltene particles can enhance the stability of a w/o emulsion stabilized by carboxylic acids. They suggested that emulsions can be stabilized by a combination of naphthenic acids/naphthenates and asphaltenes in many acidic crude oil systems. Hannisdal and co-workers15 found that the stability of particle-stabilized emulsions could be improved by the adsorption of asphaltenes and resins onto silica particles. Binks and co-workers investigated the stability of emulsions using a combination of silica particles with cationic, anionic, and nonionic surfactants such as cetyltrimethylammonium bromide,16 SDS,17 and alkylpoly(oxyethylene) types,18 respectively. All the surfactants and silica particles have a synergistic effect in stabilizing emulsions, because of the increase of both particle hydrophobicity and particle flocculation after surfactant adsorption. However, emulsion inversion or stabilization is not always observed by the combination of particles with surfactants. Legrand et al.19 showed that adding silica particles to the continuous phase of a bitumen-in-water emulsion stabilized by cationic surfactant led to flocculation and partial coalescence of the emulsion droplets. Binks and co-workers16 also found that adding hydrophilic silica particles into an emulsion stabilized by alkylpoly(oxyethylene) surfactants caused emulsion coalescence. A competitive effect was reported when the surfactant was not adsorbed to particles. Wang et al.9 observed that, at very high surfactant concentrations, surfactant molecules can dominate emulsion formation and particle attachment does not occur. Whitby20 showed a complete displacement of silica particles from the water−oil interface by mixing particle-stabilized emulsions with sodium dodecyl sulfate solutions. When the concentration of surfactant is above the critical micelle concentration, the attached nanoparticles can be displaced and recovered from the emulsions. Pichot and co-workers21 reported that the silica particles adsorbed at the interface can be displaced into the aqueous phase as the surfactant concentration increases and an emulsion stabilized by a mixture of particles and surfactants would behave as a surfactant-only-stabilized emulsion when the concentration of surfactant in the system is high. The effect of the salt concentration on the formation, stability, and type of emulsions stabilized by various particles has been studied. Ashby and Binks22 investigated the effect of salt on the preparation of emulsions stabilized solely by Laponite particles and found that stable emulsions are only formed when the particles are flocculated by adding salt. Lagaly et al.23 reported that the electrostatic repulsion between clay particles can be reduced by adding CaCl2, which causes aggregation of the particles. Therefore, a dense film formed around the droplets, and the stability of the emulsions was improved. The effect of different electrolytes on the stability of toluene-in-water emulsions stabilized by hydrophilic fumed silica particles has been studied by Binks and co-workers.24 In the presence of NaCl, the emulsions were unstable to creaming at all salt concentrations (0−5 M) irrespective of the pH. Stable emulsions were obtained by adding LaCl3 at pH 10 only when

2. EXPERIMENTAL SECTION 2.1. Materials. Liquid paraffin (Sinopharm Chemical Reagent Co., China) was used as the oil phase. Its purity is greater than 99% (d420 = 0.835−0.855). The components of the liquid paraffin are mainly isoalkane, and the carbon number ranges from 16 to 26, measured with an Agilent 6820 gas chromatograph (Agilent Co.). The lipophilic nonionic surfactant was sorbitan monoleate (Span 80) (chemical pure; Shanghai Reagent Co., China). The surfactant was characterized by liquid chromatography/mass spectrometry (LC/MS; Agilent 1100 LC/MSD TRAP VL). Oleic acid, linoleic acid, and stearic acid are present in the LC/MS chromatogram, and most of the fatty acids are oleic acid. The content of carboxylic acids in the nonionic surfactants was determined by acid−base titration with potential indicating the end point, which was about 0.8 wt %. Laponite RD, a synthetic hectorite, was supplied by Rockwood Additives, Ltd. (United Kingdom) as a white powder. The average diameter of the particles is 30 nm, and the thickness is around 1 nm, measured by electron microscopy. The molecular formula of Laponite is Na0.7[(Si8Mg5.5Li0.4)O4(OH)20]. The structure of a single crystal of the particle is sandwich-like formed by a magnesium central layer and two silicate layers where Mg2+ cations are in octahedral coordination to oxygen atoms or hydroxyl groups and the silica atoms are in tetrahedral coordination to oxygen atoms. As some of the magnesium sites of the central layer are substituted by lithium cations, when the powders are dispersed in water, the Na+ ions on the particle surface are released and a strongly negative charge appears on the faces of the disks. On the other hand, because of the protonation of the hydroxyl groups with the hydrogen atoms of water, a weakly positive charge appears on the rim of the disks.9,28−30 Auramine O was purchased from Sigma-Aldrich. All the reagents were used as received. The water was deionized water purified by ion exchange. 2.2. Methods. 2.2.1. Preparation of Aqueous Dispersions of a Particle and Span 80 Oil Solution. A Laponite dispersion (2 wt %) was obtained by adding Laponite (8 g) to deionized water (392 g) using a multimixer (Baroid Co.). Then the suspensions were sealed and set aside for one week before use. A known volume of NaCl solution was added to 50 mL of the prepared suspensions. The concentrations of NaCl in the dispersions range from 0 to 0.5 M, and the concentration of particles in the solutions is 1 wt %. The solutions of Span 80 were obtained by dispersing a known mass of Span 80 into 100 mL of paraffin oil. 6770

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2.2.2. Preparation, Stability, and Characterization of the Emulsions. Batch emulsions were prepared by mixing equal volumes of the paraffin oil containing 5 mM Span 80 (or above) and dispersions of Laponite in aqueous NaCl using a homogenizer (Shanghai Forerunner M&E Co., China) operated at 5000 rpm for 5 min. After the homogenization, the emulsion type was immediately determined by conductivity measurement and also by drop tests. The emulsions were stored at 20 °C to investigate their stabilities by monitoring the volume of the released oil or water in a given time. Emulsification was also achieved by ultrasonification at 200 W for 1 min. The adsorption of particles at the emulsion surfaces was determined by laser-induced confocal microscopy (Olympus Fluoview 500, Japan). Auramine O was used as a fluorescent probe for labeling the negative Laponite particles through electrostatic attractive interactions. Auramine O was added to the Laponite dispersions, and the dye concentration was 1.0 × 10−5 M. Free dye in the bulk solution was removed by centrifugation. Emulsions were prepared by emulsifying 25 mL of the dispersions containing labeled particles and 25 mL of Span 80 oil solution. The fluorescent images of the emulsion were observed under the microscope. Cryo transmission electron microscopy (cryo-TEM) was used to observe the morphology of the emulsion droplets. The emulsion samples were first diluted 10 times with Span 80 oil solution or NaCl solution to make the emulsion droplets clear under the microscope. About 5 μL of the diluted sample suspensions was immediately transferred onto a TEM copper grid and quickly plunged into a reservoir of liquid ethane (cooled by the nitrogen) at −165 °C, causing vitrification. Then the vitrified sample was stored in liquid nitrogen and transferred to a cryogenic sample holder (Gatan 626). The examples were examined with a JEOL JEM-1400 transmission electron microscope (120 kV) at about −174 °C. 2.2.3. Interfacial Tension Measurements. Interfacial tension was measured by the pendant drop method using a rheology apparatus (Tracker, I.T.Concept, France). Prior to the measurement, the aqueous phases containing different concentrations of NaCl, without particles, were equilibrated with the oil phases containing 5 mM Span 80 by layering the oil phases on top of the aqueous phases and left to rest at 25 °C for 4 days. A sample 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. 2.2.4. Rheology Measurements. The rheology of aqueous dispersions of Laponite was performed at 25 ± 0.1 °C using an RS75 rheometer (Haake Co., Germany) with a cylindrical rotor in the controlled rate mode. The flow curves of Laponite dispersions with different salt concentrations were determined from the shear rate measurement. The shear rate was increased from 1 to 1 × 103 s−1.

Figure 1. Appearance of emulsions of liquid paraffin−water (1:1 by volume) stabilized by (a) Laponite particles (1.0 wt %) or (b) Span 80 (10 mM) alone at different salt concentrations (mM, given in the figure).

discrete particles, which reduced the coverage of the droplets by particles. The appearance of emulsions stabilized by Span 80 with different concentrations of NaCl after two weeks of preparation is shown in Figure 1b. Span 80 is a lipophilic nonionic surfactant with a hydrophile−lipophile balance value of 4.3 and a critical micelle concentration (cmc) value of 0.43 mM in paraffin oil,31 so Span 80 is well-known to form w/o emulsions. The stability of the w/o emulsions was not obviously affected by salt. The results above show that, by adding salt to the systems, the stability of emulsions stabilized by Laponite particles was improved, but stability enhancement of emulsions stabilized by Span 80 was not obvious. The type of both emulsions was not inverted by changing the salt concentration. 3.2. Effect of Salt on Emulsions Stabilized by a Mixture of Particles and Surfactant. According to our previous work,32 the type of emulsions stabilized by a mixture of Laponite and Span 80 is always o/w when the particle concentration is 1.0 wt % and the concentration of Span 80 ranges from 0 to 100 mM. This is mainly due to the hydrophilic nature of Laponite and low adsorption extent of Span 80 on Laponite. Here the effect of salt on the emulsions stabilized by Laponite particles and Span 80 was investigated. Interestingly, a double phase inversion was observed. The conductivities and the type of emulsions are shown in Figure 2. At fixed concentration of Laponite particles (1 wt %) in an aqueous dispersion and surfactant (5 mM, 10 or 100 mM) in paraffin oil, the conductivities of emulsions are high when the salt concentrations are low. The continuous phase is the aqueous phase, and the emulsions are o/w. Then the conductivities decrease to low values at intermediate salt concentrations, which indicates w/o emulsions are obtained and the oil phase is the continuous phase. Finally, the conductivities increase to much higher values at high salt concentrations as water becomes the new continuous phase. The stability and type of these emulsions prepared from Span 80 and Laponite after two weeks of preparation are shown in Figure 3. When the salt concentration is lower than 5 mM, the emulsions are o/w, which cream, and the released water is slightly turbid. The stability increases with the salt concen-

3. RESULTS AND DISCUSSION Different from previously reported double inversions induced by the adsorption of surfactants to the particles and the great change of the particle wettability, here we report a novel double inversion by changing the salt concentration alone, for the adsorption of particles and surfactants to the oil/water interface can be controlled by salt. 3.1. Effect of Salt on Emulsions Stabilized by Laponite Particles or Span 80 Alone. The effects of salt on the emulsions solely stabilized by Laponite particles (1 wt %) were investigated, and the appearance and stability of 1:1 paraffin− water emulsions stabilized by Laponite particles are shown in Figure 1a. The emulsions are all o/w, and the stability increases with the salt concentration. This is because the electrostatic repulsion between particles was reduced and the hydrophobicity of the particle surface was increased by salt.22 At higher salt concentrations, the decrease of the emulsion stability may be attributed to the flocculation and the reduction of 6771

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Figure 2. Influence of the NaCl concentration on the conductivity of paraffin−water (1:1 by volume) emulsions. The particle concentration is 1.0 wt % in the aqueous dispersion, and the surfactant concentrations in oil are 5, 10, and 100 mM.

Figure 4. Optical microscopy images of paraffin−water (1:1 by volume) emulsions stabilized by 1.0 wt % Laponite and 10 mM Span 80 at different salt concentrations immediately after preparation. The concentrations of NaCl (mM) are (a) 0.1, (b) 1, (c) 10, (d) 20, (e) 50, and (f) 100 mM. Scale bars represent 40 μm.

Therefore, the emulsions could be stabilized by different emulsifiers at different salt concentrations. 3.3. Double Inversion of Emulsions. We conjecture that there exists a competitive adsorption between Laponite particles and Span 80 at the water−oil interface and different emulsifiers dominate the composition of the adsorption layer of emulsion droplets at different salt concentrations. The o/w emulsions are stabilized mainly by the hydrophilic particle, but the w/o emulsions are mainly stabilized by the hydrophobic surfactant. To confirm this hypothesis, laser-induced confocal scanning microscopy and cryo-TEM were used to observe the emulsion droplets. The fluorescently labeled Laponite particles (green) can be seen adsorbed at the surface of droplets of the o/w emulsion (Figure. 5a,c). However, for the w/o emulsions, the

Figure 3. Appearance of emulsions of liquid paraffin−water (1:1 by volume) stabilized by a mixture of Laponite (1.0 wt % in an aqueous dispersion) and Span 80 (10 mM in oil) at different NaCl concentrations (mM, given in the figure).

tration as the electrostatic repulsion between the particles is partially screened, and more particles are adsorbed at the interface. From 5 to 20 mM NaCl, w/o emulsions are obtained, which sediment, releasing a clear oil phase above with time. The effect of salt on the stability of w/o emulsions is not obvious, which is consistent with the effect of salt on the emulsions stabilized by Span 80 alone discussed in the above section and may be because the lipophilic surfactant Span 80 is not sensitive to salt. Finally, above 50 mM NaCl, the emulsions are o/w again, and the stability decreases with the salt concentration. The reason for the decreased stability may be the flocculation of particles increasing with the salt concentration, which reduces the coverage of the droplets by particles at high salt concentration. The appearance of the droplets for emulsions of different types was observed by microscopy. Optical microscopy images of these emulsions are shown in Figure 4. The droplet sizes of the initial o/w emulsions (Figure 4a,b) and the final o/w emulsions (Figure 4e,f) are much larger than those of the w/o emulsions (Figure 4c,d). The droplet sizes of the w/o emulsions shown in Figure 4c,d are approximately the same as the diameter of emulsions prepared from Span 80 alone. The droplet size of the emulsions stabilized by surfactants alone is always reported to be smaller than that of the emulsions by solid particles alone.9 The concentrations of particles and surfactants in these emulsions are constant, and the double inversion was induced by just increasing the salt concentration.

Figure 5. Confocal fluorescence microscopy images of liquid paraffin− water (1:1 by volume) emulsions stabilized by 1.0 wt % Laponite and 5 mM Span 80: (a) o/w at 1 mM NaCl, (b) w/o at 10 mM NaCl, (c) o/w at 100 mM NaCl. The fluorescent probe is Auramine O (1.0 × 10−5 M). Scale bars represent 20 μm.

accumulation of particles at the droplet surface was not observed, where the particles dispersed evenly in the inner phase (Figure 5b). In these emulsions, we can find that if the accumulation of the hydrophilic Laponite particles is observed at the surfaces of the droplets, the type of emulsion is o/w. If most of the particles disperse in the aqueous phase, the emulsions should be mainly stabilized by the hydrophobic surfactants; therefore, the emulsions are w/o. Cryo-TEM observations of emulsion droplets in the intermediate w/o emulsions and the final o/w emulsions are 6772

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shown in Figure 6. For the w/o emulsion (Figure 6a), the adsorption layer is thin and few particles are adsorbed at the

Figure 6. Cryo-TEM images of the emulsions obtained at 10 mM NaCl (a) and 100 mM NaCl (b).

surface of the droplets, so the type of these emulsions is determined by the surfactant. However, a thick layer composed of particles is clearly observed at the interface for the final o/w emulsion (Figure 6b), which shows a large amount of particles are adsorbed at the surface of the droplet and the emulsion is mainly stabilized by particles. These results are very consistent with the observation of laser-induced confocal scanning microscopy. How does the salt concentration affect the composition of the interfacial films and the emulsion type? We anticipate that the salt can influence the interactions between particles and surfactant by changing the states of Laponite particles. Figure 7 shows the rheology of the particle dispersions in NaCl solutions. When the NaCl concentration is very low or there is no NaCl, the viscosity is low. As the salt concentration is increased, the solution viscosity increases, but after reaching a maximum, the viscosity decreases. This variation agrees with the phase diagram for a Laponite dispersion with different concentrations of salt in the literature.30,33−37 At low salt concentrations, the particles are discrete22 and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, gel-like dispersions are formed, the viscosity of the dispersions increases dramatically, and the transition of the particles from the aqueous phase to the interface is inhibited.38 The emulsions are mainly stabilized by the lipophilic surfactant, and w/o emulsions are obtained. For high salt concentrations, the viscosity of the dispersion is low. Flocculation occurs as the electrostatic repulsion between particles is reduced and the size of the particles is larger; thus, the adsorption of particles is promoted, and the emulsions invert to the o/w type. A thick layer of particles is observed at the surface of the droplets. This mechanism is illustrated in Figure 8. The influence of salt on the surfactant was also considered. If the adsorption of surfactant to the particle surfaces increases dramatically with the salt concentration, the particles should become more hydrophobic and w/o emulsions should be obtained. However, this cannot explain the second inversion. The interfacial tension decreased slightly with an increase of the salt concentration (Figure 9), indicating that the adsorption of surfactant to the interface was not dramatically affected by the salt concentration. Therefore, we believe that the double inversion of the emulsion is mainly caused by the variation of the amount of particles adsorbed to the interface.

Figure 7. (a) Apparent viscosity versus shear rate of a Laponite dispersion (1 wt %) for different concentrations of NaCl (mM, given in the figure) at 25 °C. (b) Apparent viscosity versus NaCl concentration at 25 °C for a Laponite dispersion at share rates of 10 (■), 30 (●), and 100 (▲) s−1.

Figure 8. Schematic of emulsions formed at different salt concentrations.

Additional experiments were carried out to support our understanding. Since the energy density input into the system by ultrasound is higher than that of the homogenizer,39,40 ultrasound would disrupt the gel structure, and the particles can easily transfer from the aqueous bulk to the interface. Therefore, emulsions should be o/w at all salt concentrations. 6773

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(2) Aveyard, R.; Binks, B. P.; Clint, J. H. Emulsions Stabilised Solely by Colloidal Particles. Adv. Colloid Interface Sci. 2003, 100−102, 503− 546. (3) Binks, B. P. Particles as Surfactants-Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (4) Mulqueen, P. Recent Advances in Agrochemical Formulation. Adv. Colloid Interface Sci. 2003, 106, 83−107. (5) Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16, 8622−8631. (6) Schulman, J. H.; Leja, J. Control of Contact Angles at the OilWater-Solid Interfaces. Emulsions Stabilized by Solid Particles (BaSO4). Trans. Faraday Soc. 1954, 50, 598−605. (7) Tambe, D. E.; Sharma, M. M. Factors Controlling the Stability of Colloid-Stabilized Emulsions: I. An Experimental Investigation. J. Colloid Interface Sci. 1993, 157, 244−253. (8) Binks, B. P.; Rodrigues, J. A. Double Inversion of Emulsions by Using Nanoparticles and a Di-Chain Surfactant. Angew. Chem. 2007, 119, 5485−5488. (9) Wang, J.; Yang, F.; Li, C.; Liu, S.; Sun, D. Double Phase Inversion of Emulsions Containing Layered Double Hydroxide Particles Induced by Adsorption of Sodium Dodecyl Sulfate. Langmuir 2008, 24, 10054−10061. (10) Cui, Z.; Shi, K.; Cui, Y.; Binks, B. P. Double Phase Inversion of Emulsions Stabilized by a Mixture of CaCO3 Nanoparticles and Sodium Dodecyl Sulphate. Colloids Surf., A 2008, 329, 67−74. (11) Cui, Z.; Cui, C.; Zhu, Y.; Binks, B. P. Multiple Phase Inversion of Emulsions Stabilized by in Situ Surface Activation of CaCO3 Nanoparticles via Adsorption of Fatty Acids. Langmuir 2012, 28, 314−320. (12) Tsugita, S. T. A.; Mori, K.; Yoneya, T.; Otani, Y. Studies on O/ W Emulsions Stabilized with Insoluble Montmorillonite-Organic Complexes. J. Colloid Interface Sci. 1983, 95, 551−560. (13) Midmore, B. R. Synergy between Silica and Polyoxyethylene Surfactants in the Formation of O/W Emulsions. Colloids Surf., A 1998, 145, 133−143. (14) Havre, T. E.; Sjöblom, J. Emulsion Stabilization by Means of Combined Surfactant Multilayer (D-Phase) and Asphaltene Particles. Colloids Surf., A 2003, 228, 131−142. (15) Hannisdal, A.; Ese, M.; Hemmingsen, P. V.; Sjöblom., J. ParticleStabilized Emulsions: Effect of Heavy Crude Oil Components PreAdsorbed onto Stabilizing Solids. Colloids Surf., A 2006, 276, 45−58. (16) Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Synergistic Interaction in Emulsions Stabilized by a Mixture of Silica Nanoparticles and Cationic Surfactant. Langmuir 2007, 23, 3626−3636. (17) Binks, B. P.; Rodrigues, J. A. Enhanced Stabilization of Emulsions Due to Surfactant-Induced Nanoparticle Flocculation. Langmuir 2007, 23, 7436−7439. (18) Binks, B. P.; Desforges, A.; Duff, D. G. Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant. Langmuir 2007, 23, 1098−1106. (19) Legrand, J.; Chamerois, M.; Placin, F.; Poirier, J. E.; Bibette, J.; Leal-Calderon, F. Solid Colloidal Particles Inducing Coalescence in Bitumen-in-Water Emulsions. Langmuir 2005, 21, 64−70. (20) Vashisth, C.; Whitby, C. P.; Fornasiero, D.; Ralston, J. Interfacial Displacement of Nanoparticles by Surfactant Molecules in Emulsions. J. Colloid Interface Sci. 2010, 349, 537−543. (21) Pichot, R.; Spyropoulos, F.; Norton, I. T. O/W Emulsions Stabilised by Both Low Molecular Weight Surfactants and Colloidal Particles: The Effect of Surfactant Type and Concentration. J. Colloid Interface Sci. 2010, 352, 128−135. (22) Ashby, N. P.; Binks, B. P. Pickering Emulsions Stabilised by Laponite Clay Particles. Phys. Chem. Chem. Phys. 2000, 2, 5640−5646. (23) Lagaly, G.; Reese, M.; Abend, S. Smectites as Colloidal Stabilizers of Emulsions: I. Preparation and Properties of Emulsions with Smectites and Nonionic Surfactants. Appl. Clay Sci. 1999, 14, 83− 103.

Figure 9. Effect of the NaCl concentration on the Span 80 oil solution−water equilibrium interfacial tension. The concentration of Span 80 in paraffin is 5 mM.

The experimental result does show that the emulsions prepared by using ultrasound are all o/w. This result further confirms our hypothesis that the gel structure is the reason for the formation of the w/o emulsions at intermediate salt concentrations.

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CONCLUSIONS In conclusion, we have shown the adsorption of surfactants and particles at an oil−water interface can be controlled, and a novel double phase inversion of emulsions was achieved without variation of the emulsifier contents just by changing the salt concentration. The salt concentration influences the dispersing states of the particles, which affects the adsorption of particles to the interface. Therefore, when the salt concentration is varied, the composition of the interfacial film is greatly changed and the emulsions types are inverted. This new way of achieving double inversion of emulsions offers the potential to easily select the emulsion type for special applications.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of the ultrasound study. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-531-88365437. Phone: +86-531-88364749. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program, Grant 2009CB930100) is gratefully acknowledged.

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REFERENCES

(1) Binks, B. P., Ed. Modern Aspects of Emulsion Science; The Royal Society of Chemistry: Cambridge, U.K., 1998. 6774

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dx.doi.org/10.1021/la300695v | Langmuir 2012, 28, 6769−6775