Phase Inversion of Emulsions Containing a Lipophilic Surfactant

Feb 28, 2013 - For the o/w emulsions prepared with 1.0 wt % Laponite particles and Span 80, fluorescently labeled Laponite particles (green) can be se...
1 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/Langmuir

Phase Inversion of Emulsions Containing a Lipophilic Surfactant Induced by Clay Concentration Jingchun Zhang,† Lu Li,† Jun Wang,‡ 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: Emulsions stabilized by clay particles and sorbitan monooleate (Span 80) were investigated, and an abnormal phase inversion was observed by increasing the concentration of clay particles in the aqueous phase. At a fixed concentration of Span 80 in the oil phase, the emulsions were oil-in-water (o/w) when the concentration of clay particles in the aqueous phase was low. Surprisingly, the emulsion inverted to water-in-oil (w/o) when the concentration of the hydrophilic clay particles was increased. On the basis of the results of rheological measurements and laser-induced fluorescent confocal microscopy observation, we suggest that this phase inversion is induced by the gel structures formed at high concentration of clay particles. The effects of clay concentration on the stability and the droplet size of these emulsions were also investigated.

1. INTRODUCTION Emulsions stabilized by solid particles have been studied extensively for their practical applications in fields such as cosmetics, food, pharmaceuticals, oil recovery, and templates for advanced materials fabrication. Equivalent to the hydrophile−lipophile balance (HLB) of surfactants, the wettability of particles, characterized by the contact angle θ, is decisive in the type and stability of the Pickering emulsions.1 Particles with θ slightly below and beyond 90° can produce highly stable oil-inwater (o/w) and water-in-oil (w/o) emulsions, respectively, due to the high adsorption energy.2,3 However, many raw particles in practice with extreme hydrophilicity or hydrophobicity cannot produce stable emulsions. Thus different methods have been applied to modify the particle wettability, for instance (1) in situ modification via the adsorption of amphiphilic molecules,4−22 (2) surface coating/grafting via chemical treatment.23−29 Comparatively, the in situ modification by adding amphiphilic molecules is much simpler and inexpensive. In fact, many emulsions applied in industry contain both surfactants and particles. Hence it is important to investigate the interaction between surfactants and particles and the stabilization mechanism of the emulsions containing particles and surfactants. Many reports have described the synergistic effect of particles and amphiphilic molecules in stabilizing emulsions, because of the increase of particle hydrophobicity and the flocculation of particles caused by the adsorption of amphiphiles.4−22 Phase inversion of emulsions from o/w to w/o would be induced, if © 2013 American Chemical Society

the wettability of the particles was changed greatly by the adsorption of surfactants.17,30−34 With further increasing the concentration of surfactants, if a bilayer of surfactant formed on the particle surface, then the particles would become hydrophilic again to stabilize o/w emulsions. Thus double phase inversions of emulsions were observed.17,31−34 However, if surfactants were not adsorbed on particles, then there would be a competitive adsorption between particles and surfactant at oil−water interface, and the emulsions behaved as surfactantonly stabilized emulsions at high surfactant concentration.35,36 In many reports, the particles and surfactants were dispersed in aqueous phase, thus the wettability of the particles could be modified before emulsification. Whereas, if the particles and surfactants are not in the same phase, the interactions between surfactants and particles occurs until emulsification, so it is difficult to investigate the interactions between particles and surfactants at the oil−water interface, and few investigations have been carried out. Tambe and Sharma5 demonstrated that emulsions stabilized by calcium carbonate particles could invert from o/w to w/o by increasing the concentration of stearic acid in the oil phase, as the surfactant could transfer across the interface and adsorb onto particles to increase the hydrophobicity of the particles. Whitby et al.37,38 investigated the interactions between nanoparticles dispersed in aqueous and oil Received: November 21, 2012 Revised: February 27, 2013 Published: February 28, 2013 3889

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894

Langmuir

Article

aqueous phase. The purity of Laponite powder was also verified by Thermogravimetric Analysis (TGA), TEM and XRD. The solutions of Span 80 were obtained by dispersing a known mass of Span 80 into 100 mL paraffin oil. 2.2.2. Preparation and Characterization of Emulsions. Equal volumes of clay suspensions of different concentrations and solutions of Span 80 were mixed together and emulsified using a homogenizer (Shanghai Forerunner M&E Co., P.R. China) operated at 5000 rpm for 5 min. After the homogenization, the emulsion type was determined immediately by conductivity measurements and the drop tests. The emulsions were stored at 25 °C to investigate their stabilities by monitoring the volume of the released oil or water in a given time. The morphology of the emulsion droplets was observed with an Axioskop 40 optical microscope (ZEISS, Germany). 2.2.3. Adsorption of Particles at Droplet Surfaces. The adsorption of particles at the surfaces of emulsion droplet was observed by a laserinduced confocal microscope (Olympus Fluoview 500, Japan). Auramine O was used as fluorescent probes for labeling the negatively Laponite particles. Auramine O was added to the Laponite dispersions, and the dye concentration was 1.0 × 10−5 M. The fluorescence-labeled particle dispersions were centrifuged to remove the free dye in the bulk solution. Emulsions were prepared by emulsifying equal volumes of the dispersions of labeled particles and Span 80 oil solution. The fluorescent images of the emulsion were obtained by using the microscope. 2.2.4. Rheology Measurements. The rheology of aqueous dispersions of clay particles and emulsions was performed using a rheometer (Haake RS75, Germany) with a cylindrical rotor. For each sample, oscillatory rheological measurements were carried out first, then shear rate versus shear stress were applied to the sample over a shear rate range of 0.1−1000 s−1. All measurements were carried out at room temperature (25 ± 0.5 °C).

soluble surfactant at the oil−water interface, and found that a synergistic interaction existed between the particles and octadecylamine in stabilizing the emulsions. Okada and coworkers39 reported that stable emulsions could be prepared by hydroxyapatite nanoparticles dispersed in aqueous phase and polystyrene molecules with carboxyl end groups dissolved in the oil phase. Van den Dungen and Hartmann40 also reported that the oil soluble surfactant dimethyl-di(hydrogenated tallow)ammonium iodide promoted the adsorption of the Laponite RD at the oil−water interface. Here, we investigated the effects of clay concentration on the emulsions containing fixed concentrations of oil soluble surfactant sorbitan oleate (Span 80). Different from the abnormal phase inversion of emulsions41 or the anti-Finkle emulsions42,43 reported previously, a phase inversion of emulsions (o/w to w/o) was observed by increasing the initial concentrations of clay particles in the aqueous phase. At low particle concentrations, emulsions were o/w, but the type of the emulsions changed to w/o when the particle concentration was increased. In order to find out why the emulsions inverted to w/o at high clay concentrations, a systemically study was carried out.

2. EXPERIMENTAL SECTION 2.1. Materials. Liquid paraffin (Sinopharm Chemical Reagent Co., P.R. China) was used as the oil phase. The purity of it was 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 GC (Agilent Co.). The surfactant was sorbitan monooleate (Span 80) (Chemical pure; Sinopharm Chemical Reagent Co., P.R. China). Span 80 is a lipophilic nonionic surfactant with an hydrophile−lipophile balance value of 4.3, and a cmc value of 0.43 mM in paraffin oil.44 Auramine O was purchased from SigmaAldrich. The water was deionized and purified by ion exchange. Laponite RD was supplied by Rockwood Additives, Ltd. (UK, Batch NO.: 04/4338), as a white powder. 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 sheet and two silicate sheets. As some of the magnesium sites of the central layer are substituted with lithium cations, when the powders are dispersed in water, the sodium ions on the particle surface are released and a net negative charge appears on the faces of the disks. However, 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. The average diameter of the particles is 30 nm, and the thickness is around 1 nm.45 Sodium montmorillonite was extracted from Xinjiang Province, P.R. China. Raw clay was purified as the following steps. The sample was dispersed in deionized water with the concentration of 4 wt %. The dispersions were stirred for 24h, and then were centrifugated at 3000 rpm for 20 min to remove the impurities such as quartz and feldspar. The suspensions were collected, stirred for another 2h and centrifugated again. The collected suspensions were the purified montmorillonite dispersions, then the dispersions were dried at 105 °C and the purified montmorillonite powder was obtained. The average diameter of the particles is about 390 nm measured by dynamic light scattering (DLS) (Brookhaven, America). 2.2. Methods. 2.2.1. Preparation of Aqueous Dispersions of Particle and Span 80 Oil Solution. The Laponite or montmorillonite dispersions were prepared by dispersing a known mass of Laponite or montmorillonite particles into deionized water using a multimixer (Baroid Co.) to obtain a series of dispersions, which were then sealed and set aside for one week before use. In order to verify that there was no impurity in the aqueous phase, the surface tension of the Laponite RD and montmorillonite dispersion (0.5 wt %) was measured immediately after preparation. The surface tension is 72.3 mN/m. This value indicates that there is no surface active material in the

3. RESULTS AND DISCUSSION Emulsions stabilized by Laponite in the present of Span 80 of different concentrations have been investigated in our previous work.45 The type of the emulsions is always o/w, when the particle concentration is 1.0 wt % and the concentration of Span 80 ranges from 0 to 100 mM, as shown in Figure 1. The

Figure 1. The appearance of o/w emulsions of liquid paraffin−water (1:1 by volume) stabilized by Laponite (1.0 wt %) and Span 80, 24 h after preparation. The initial concentrations of Span 80 in oil (mM) are shown.

emulsions stabilized by montmorillonite and Span 80 were investigated in this work, and a similar result was attained (Figure S1 of the Supporting Information, SI). The emulsions prepared from clay particles (1.0 wt %) and Span 80 of different concentrations are always o/w because of the hydrophilic nature of clay particles and low adsorption extent of Span 80 on clays.45 Here, we studied the effect of clay particles concentration on the properties of the emulsions stabilized by fixed concentrations of Span 80. An interesting phenomenon was observed: w/o emulsions were prepared with higher concentration of hydrophilic clay particles (Figure 2). 3890

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894

Langmuir

Article

This indicates that the stability of the emulsions is dramatically improved for the formation of a three-dimensional network of clay particles in the aqueous phase.46 However, above 2.0 wt % particles concentration, the emulsions change into w/o and the stability increases with Laponite particle concentration. We also investigated the type and stability of emulsions prepared with montmorillonite and Span 80 (Figure S2 of the SI), similarly, w/o emulsions were obtained at high concentrations of montmorillonite particles. The appearance of the droplets in the emulsions of different types observed by microscopy is shown in Figure 4 and Figure Figure 2. Photograph of vessels containing emulsions of liquid paraffin−water (1:1 by volume) stabilized by Span 80 (10 mM) and Laponite particles of different concentrations (wt %, given), 24 h after preparation.

3.1. Characterization of the Emulsions. At fixed concentrations of Span 80 (10, 50, or 100 mM) in the oil phase, a series of emulsions were prepared with different concentrations of Laponite or montmorillonite particles in aqueous phase, and the variation of the conductivities and the type of the emulsions are shown in Figure 3. For both Laponite

Figure 4. Optical microscope images of paraffin−water (1:1 by volume) emulsions stabilized by 10 mM Span 80 and (a) 0.5, (b) 1.0, (c) 1.5, (d) 2.0, (e) 2.5, and (f) 3.0 wt % Laponite particles, immediately after preparation. Scale bars represent 40 μm.

S3 of the SI. The droplet size of the o/w emulsions (Figure 4a− c) is much larger than that of the w/o emulsions (Figure 4d−f). The droplet size of the w/o emulsions is approximately 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.32 Some multiple emulsion droplets are observed at the concentrations near the phase inversion (Figure 4c, marked by arrows), although the proportion is low. These multiple emulsions should be stabilized by the hydrophilic particles and lipophilic surfactant. Thus, we hypothesis that the w/o emulsions are mainly stabilized by lipophilic Span 80 surfactant and the o/w emulsions are stabilized by the combination of clay particles and Span 80. 3.2. The Mechanism of This Phase Inversion. The main question arising is why o/w emulsions invert to w/o at high clay concentrations. As clay particles are hydrophilic, the emulsions should be always o/w when the clay concentration is increased. One possibility is that the amount of Span 80 adsorbed on the clay is different at different clay concentrations, and the change of the wettability of the clay particles induces this abnormal phase inversion. If this inversion is induced by the adsorption of surfactant on the clay and the variation of the wettability of the particles, then phase inversion of emulsions should also be observed by changing surfactant concentration

Figure 3. Variation of conductivity and type of paraffin−water emulsion with the concentrations of Laponite (solid symbols) and montmorillonite (open symbols). The concentrations of Span 80 are shown.

and montmorillonite, when the concentrations are low, the conductivities of emulsions are high indicating water as the continuous phase. Interestingly, as the concentrations of particles are increased, the conductivities fall to low values indicating oil become the continuous phase and w/o emulsions are prepared. The type of the emulsions was also inferred by observing what happened when a drop of each emulsion was added to either pure oil or water. Here, we increased the concentrations of the hydrophilic clay particles, and the emulsions should retain o/w according to the theories that hydrophilic particles prefer to stabilize o/w emulsion. The appearance of these emulsions 24 h after preparation is shown in Figure 2. When the initial concentration of Laponite in the aqueous phase is lower than 2.0 wt %, the emulsions are o/w, which cream with an aqueous phase separated below. The stability to creaming of the o/w emulsions is not obviously improved by increasing particle concentration from 0.1 to 1.0 wt %. Then, an abrupt change of the creaming behavior occurs in a narrow range of particle concentration (1.0 to 1.5 wt %). 3891

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894

Langmuir

Article

with a fixed clay particles concentration. However, when the particle concentration is fixed at 1.0 wt %, inversion of emulsions was not observed by only changing the concentration of Span 80 in a wide range as shown in Figure 1. In addition, it cannot explain that this inversion occurs at a fixed particle concentration either, when the initial concentration of Span 80 in oil phase is greatly varied (form 10 mM to 100 mM) as shown in Figure 3. It seems that the inversion is determined by the clay concentration, thus the rheological properties of clay suspensions were investigated (Figures 5 and S4 of the SI).

(o/w−w/o) is due to the gelation of the clay dispersions at high particle concentrations. The gelled aqueous phase easily becomes fragments during emulsification,47 which makes it likely to be the inner phase. The gel structures could also inhibit the transition of particles from aqueous phase to the interface and the emulsions are stabilized mainly by lipophilic surfactant. The proposed mechanism is shown in Figure 6.

Figure 6. Schematic of emulsions formed at different particles concentrations.

To further confirm this mechanism, the distribution of clay particles in the emulsions was investigated by laser-induced confocal scanning microscopy (Figure 7). For the o/w

Figure 7. Confocal fluorescence microscopy images of liquid paraffin− water (1:1 by volume) emulsions stabilized by Laponite and 5 mM Span 80: (a) o/w at 1.0 wt % Laponite, (b) w/o at 2.5 wt % Laponite. The fluorescent probe is Auramine O (1.0 × 10−5 M). Scale bars represent 10 μm.

Figure 5. (a) Viscosity versus share rate of Laponite dispersions at different concentrations. (b) Storage (G′, solid symbols) and loss (G″, open symbols) modulus for Laponite dispersions. The concentrations (wt %) of Laponite in the suspensions are given in the figure.

emulsions prepared with 1.0 wt % Laponite particles and Span 80, fluorescently labeled Laponite particles (green) can be seen adsorbed at the surface of droplets (Figure 7a), whereas the accumulation of particles at oil−water interface was not observed in the w/o emulsions (Figure 7b) and the particles distributed evenly in the inner phase. The distribution of particles in the emulsions of different types was also investigated by rheological measurements. The relationship between emulsion viscosity and particle concentration is shown in Figures 8 and S5 of the SI. For the o/w emulsions, the viscosity increases with the initial particle concentration, and the emulsions change from Newtonian fluids to non-Newtonian fluids, indicating the formation of structures by the unadsorbed particles and emulsion droplets. However, the variation of the viscosity of the w/o emulsions as a function of the shear rate may be caused by the partial

When the particle concentration is lower than 1.5 wt %, the viscosity of the suspensions is independent of the share rate, whereas shear-thinning behavior is observed at particle concentration above 1.5 wt % (Figure 5a). The shear-thinning indicates the presence of networks in the suspensions. The formation of the gel structures can also be inferred from oscillatory rheological measurements, since the more developed the network the more the system demonstrates an elastic response to shear.13 In Figure 5b, we can see that the storage modulus (G′) are larger than the loss modulus (G″) when the particle concentration is higher than 2.0 wt %, showing that gel structures are formed at this particles concentration and the dispersion is more solid-like. It should be noted that the inversion of the emulsions from o/w to w/o occurs just at this particle concentration. Thus we conjecture that the inversion 3892

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894

Langmuir

Article

phases on the type of emulsions. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support was provided by the Key Project of Chinese National Programs for Fundamental Research and Development (973 Program, NO. 2009CB930103).

■

Figure 8. Rheological characterization liquid paraffin−water (1:1 by volume) emulsions prepared with 10 mM Span 80 and Laponite. Open symbols represent o/w emulsions, solid symbols represent w/o emulsions. The concentrations of Laponite in the aqueous phase are indicated in the figure (wt %).

flocculation of the water droplets in the emulsions and the redispersion of the flocculates during shear.48−50 The viscosity of the w/o emulsions is similar to the emulsion stabilized by Span 80 alone, and changes little with the variation of the particles concentration because the particles are mainly in the inner phase. We proposed that the inversion is caused by the formation of gel structures in the aqueous phase. If the gel structures are destroyed, then the emulsions prepared with high concentration of Laponite particles (>2.0 wt %) should be o/w type. Thus, adequate NaCl was added into the dispersion to make the salt concentration in the aqueous phase is 1.0 mol/L. According to the phase diagram of Laponite, the particles are flocculated and the gel structures do not exist at this salt concentration.51,52 The experimental results did show that the emulsions obtained are all o/w in the range of particle concentration from 2% to 3%. On the contrary, gel structure could also be formed with low concentrations of Laponite (1.0 wt %) and salt (10 mM). Emulsions of w/o type were also prepared with this aqueous dispersion and solution of Span 80 in paraffin oil in our previous work.53 In conclusion, gel formation induced by increasing particle concentration or changing salt concentration is the reason for the inversion of the emulsions from o/w to w/o.

4. CONCLUSIONS For the emulsions stabilized by Span 80 and clay particles, an abnormal inversion from o/w to w/o was observed by increasing the initial concentration of hydrophilic clay (Laponite or montmorillonite) particles. When gels were formed at high clay concentrations, the gelled aqueous phase easily became fragments during emulsification which make it likely to be the inner phase. The gel structures also inhibit the transition of particles from aqueous phase to the interface. Thus, w/o emulsions mainly stabilized by lipophilic surfactant were prepared with high concentrations of clay.

■

REFERENCES

(1) Binks, B. P. Particles as Surfactants-Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (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) Kaptay, G. On the Equation of the Maximum Capillary Pressure Induced by Solid Particles to Stabilize Emulsions and Foams and on the Emulsion Stability Diagrams. Colloids Surf., A 2006, 282−283, 387−401. (4) 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. (5) 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. (6) Midmore, B. R. Synergy between Silica and Polyoxyethylene Surfactants in the Formation of O/W Emulsions. Colloids Surf., A 1998, 145, 133−143. (7) 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. (8) 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. (9) Binks, B. P.; Rodrigues, J. A. Enhanced Stabilization of Emulsions Due to Surfactant-Induced Nanoparticle Flocculation. Langmuir 2007, 23, 7436−7439. (10) 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. (11) Eskandar, N. G.; Simovic, S.; Prestidge, C. A. Synergistic Effect of Silica Nanoparticles and Charged Surfactants in the Formation and Stability of Submicron Oil-in-Water Emulsions. Phys. Chem. Chem. Phys. 2007, 9, 6426−6434. (12) Lan, Q.; Yang, F.; Zhang, S.; Liu, S.; Xu, J.; Sun, D. Synergistic Effect of Silica Nanoparticle and Cetyltrimethyl Ammonium Bromide on the Stabilization of O/W Emulsions. Colloids Surf., A 2007, 302, 126−135. (13) Torres, L. G.; Iturbe, R.; Snowdenb, M. J.; Chowdhry, B. Z.; Leharne, S. A. Preparation of O/W Emulsions Stabilized by Solid Particles and Their Characterization by Oscillatory Rheology. Colloids Surf., A 2007, 302, 439−448. (14) Akartuna, I.; Studart, A. R.; Tervoort, E.; Gonzenbach, U. T.; Gauckler, L. J. Stabilization of Oil-in-Water Emulsions by Colloidal Particles Modified with Short Amphiphiles. Langmuir 2008, 24, 7161− 7168. (15) Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of Adding Anionic Surfactant on the Stability of Pickering Emulsions. J. Colloid Interface Sci. 2009, 329, 173−181. (16) Wang, J.; Liu, G.; Wang, L.; Li, C.; Xu, J.; Sun, D. Synergistic Stabilization of Emulsions by Poly(oxypropylene)diamine and Laponite Particles. Colloids Surf., A 2010, 353, 117−124.

ASSOCIATED CONTENT

S Supporting Information *

The properties of the emulsions prepared with montmorillonite and Span 80. The effect of the volume ratio of the oil and water 3893

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894

Langmuir

Article

(17) Binks, B. P.; Rodrigues, J. A. Influence of Surfactant Structure on the Double Inversion of Emulsions in the Presence of Nanoparticles. Colloids Surf., A 2009, 345, 195−201. (18) Tigges, B.; Dederichs, T.; Möller, M.; Liu, T.; Richtering, W.; Weichold, O. Interfacial Properties of Emulsions Stabilized with Surfactant and Nonsurfactant Coated Boehmite Nanoparticles. Langmuir 2010, 26, 17913−17918. (19) Cui, Y.; Threlfall, M.; van Duijneveldt, J. S. Optimizing Organoclay Stabilized Pickering Emulsions. J. Colloid Interface Sci. 2011, 356, 665−671. (20) Regera, M.; Sekineb, T.; Hoffmann, H. Pickering Emulsions Stabilized by Amphiphile Covered Clays. Colloids Surf., A 2012, 413, 25−32. (21) Williams, M.; Armes, S. P.; York, D. Clay-Based Colloidosomes. Langmuir 2012, 28, 1142−1148. (22) Binks, B. P.; Lumsdon, S. O. Stability of Oil-in-Water Emulsions Stabilised by Silica Particles. Phys. Chem. Chem. Phys. 1999, 1, 3007− 3016. (23) Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16, 8622−8631. (24) Saleh, N.; Sarbu, T.; Sirk, K.; Lowry, G. V.; Matyjaszewski, K.; Tilton, R. D. Oil-in-Water Emulsions Stabilized by Highly Charged Polyelectrolyte-Grafted Silica Nanoparticles. Langmuir 2005, 21, 9873−9878. (25) Fujii, S.; Read, E. S.; Armes, S. P.; Binks, B. P. StimulusResponsive Emulsifiers Based on Nanocomposite Microgel Particles. Adv. Mater. 2005, 17, 1014−1018. (26) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. TemperatureInduced Inversion of Nanoparticle-Stabilized Emulsions. Angew. Chem., Int. Ed. 2005, 44, 4795−4798. (27) Horozov, T. S.; Binks, B. P. Particle-Stabilized Emulsions: A Bilayer or a Bridging Monolayer? Angew. Chem., Int. Ed. 2006, 45, 773−776. (28) Saigal, T.; Dong, H. C.; Matyjaszewski, K.; Tilton, R. D. Pickering Emulsions Stabilized by Nanoparticles with Thermally Responsive Grafted Polymer Brushes. Langmuir 2010, 26, 15200− 15209. (29) Alvarez, N. J.; Anna, S. L.; Saigal, T.; Tilton, R. D.; Walker, L. M. Interfacial Dynamics and Rheology of Polymer-Grafted Nanoparticles at Air−Water and Xylene−Water Interfaces. Langmuir 2012, 28, 8052−8063. (30) 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. (31) Binks, B. P.; Rodrigues, J. A. Double Inversion of Emulsions by Using Nanoparticles and a Di-Chain Surfactant. Angew. Chem., Int. Ed. 2007, 46, 5389−5392. (32) 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. (33) 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. (34) 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. (35) 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. (36) 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. (37) Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of Oil Soluble Surfactant in Emulsions Stabilised by Clay Particles. J. Colloid Interface Sci. 2008, 323, 410−419.

(38) Whitby, C. P.; Fornasiero, D.; Ralston, J.; Liggieri, L.; Ravera, F. Properties of Fatty Amine−Silica Nanoparticle Interfacial Layers at the Hexane−Water Interface. J. Phys. Chem. C 2012, 116, 3050−3058. (39) Okada, M.; Maeda, H.; Fujii, S.; Nakamura, Y.; Furuzono, T. Formation of Pickering Emulsions Stabilized via Interaction between Nanoparticles Dispersed in Aqueous Phase and Polymer End Groups Dissolved in Oil Phase. Langmuir 2012, 28, 9405−9412. (40) Van den Dungen, E. T. A.; Hartmann, P. C. Synergistic Effect of Laponite RD and Oil Soluble Surfactant in Stabilization of Miniemulsions. Appl. Clay Sci. 2012, 55, 120−124. (41) Binks, B. P.; Philip, J.; Rodrigues, J. A. Inversion of SilicaStabilized Emulsions Induced by Particle Concentration. Langmuir 2005, 21, 3296−3302. (42) Golemanov, K.; Tcholakova, S.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Latex-Particle-Stabilized Emulsions of Anti-Bancroft Type. Langmuir 2006, 22, 4968−4977. (43) Destribats, M.; Lapeyre, V.; Sellier, E.; Leal-Calderon, F.; Schmitt, V.; Ravaine, V. Water-in-Oil Emulsions Stabilized by WaterDispersible Poly(N-isopropylacrylamide) Microgels: Understanding Anti-Finkle Behavior. Langmuir 2011, 27, 14096−14107. (44) Santini, E.; Liggieri, L.; Sacca, L.; Clausse, D.; Ravera, F. Interfacial Rheology of Span 80 Adsorbed Layers at Paraffin Oil− Water Interface and Correlation with the Corresponding Emulsion Properties. Colloids Surf., A 2007, 309, 270−279. (45) Wang, J.; Yang, F.; Tan, J. J.; Liu, G. P.; Xu, J.; Sun, D. J. Pickering Emulsions Stabilized by a Lipophilic Surfactant and Hydrophilic Platelike Particles. Langmuir 2010, 26, 5397−5404. (46) Nciri, H.; Benna-Zayani, M.; Stambouli, M.; Kbir-Ariguib, N.; Trabelsi-Ayadi, M.; Rosilio, V.; Grossiord, J.-L. Influence of Clay Addition on the Properties of Olive Oil in Water Emulsions. Appl. Clay Sci. 2009, 43, 383−391. (47) Gibaud, T.; Barentin, C.; Taberleta, N.; Manneville, S. ShearInduced Fragmentation of Laponite Suspensions. Soft Matter 2011, 5, 3026−3037. (48) Barnes, H. A. Rheology of Emulsions-A Review. Colloids Surf., A 1994, 91, 89−95. (49) Pal, R. Effect of Droplet Size on the Rheology of Emulsions. AIChE J. 1996, 42, 3181−3190. (50) Lequeux, F. Emulsion Rheology. Curr. Opin. Colloid Interface Sci. 1998, 3, 408−411. (51) Mourchid, A.; Delville, A.; Lambard, J.; Levitz, P. Sol-Gel Transition of Colloidal Suspensions of Anisotropic Particles of Laponite. Faraday Discuss. 1995, 101, 275−285. (52) Ruzicka, B.; Zaccarelli, E. A. Fresh Look at the Laponite Phase Diagram. Soft Matter 2011, 7, 1268−1286. (53) Zhang, J.; Li, L.; Wang, J.; Sun, H.; Xu, J.; Sun, D. Double Inversion of Emulsions Induced by Salt Concentration. Langmuir 2012, 28, 6769−6775.

3894

dx.doi.org/10.1021/la304642m | Langmuir 2013, 29, 3889−3894