Interfacial displacement of nanoparticles by surfactant molecules in emulsions

Journal of Colloid and Interface Science 349 (2010) 537–543

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Interfacial displacement of nanoparticles by surfactant molecules in emulsions Charu Vashisth, Catherine P. Whitby *, Daniel Fornasiero, John Ralston Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia

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Article history: Received 27 February 2010 Accepted 27 May 2010 Available online 1 June 2010 Keywords: Particle-stabilised emulsions Pickering emulsions

a b s t r a c t The remarkable stability of nanoparticles attached to oil–water interfaces in macroemulsions hinders controlled detachment of these particles from emulsions. In this work it is shown that adding surfactant molecules which preferentially adsorb at the oil–water interface displaces nanoparticles from the interface. Surfactant adsorption at the oil–water interface is energetically favoured and readily occurs on mixing nanoparticle-stabilised oil-in-water emulsions with surfactant solutions. Depending on the surfactant concentration, there is a significant reduction in the interfacial tension. Hence there is substantial fragmentation of the oil droplets and foaming of the emulsion during mixing. Surfactant concentrations above the critical micelle concentration are required to achieve complete interfacial displacement and hence recovery of the nanoparticles from the emulsions. The effects of surfactant addition have important implications for tailoring the interfacial composition of emulsions. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Nanoparticles attach to the surfaces of drops and bubbles and impart remarkable stability to the interface [1]. Nanoparticles are trapped at fluid interfaces, providing there are attractive interactions between the particles and drops [2]. Particle wettability dictates the equilibrium position of the particles at liquid interfaces [3] and hence the interfacial curvature [4]. The trapped particles form compact networks, due to strong lateral attractions, that make the drop or bubble surfaces highly rigid and resistant to coalescence [5]. The presence of the rigid interfacial layer can significantly slow interfacial transfer of molecular species [6]. The attached nanoparticle layer alters drop and bubble flow behaviour [7,8]. Particle attachment to drop or bubble surfaces is a route for preparing hollow capsules [9] of controlled permeability and elasticity [10] that are biocompatible [11] and robust bicontinuous structures [12]. Particles stabilize metallic foams, lightweight materials with unique mechanical, thermal and electrical properties [13]. The selective attachment of hydrophobic particles to bubbles is used to separate valuable minerals from gangue in froth flotation [2]. The remarkable stability of particles attached to interfaces can be a problem. Particle-stabilised emulsions that form during the extraction of bitumen from oil sands reduce the volume of oil recovered and generate waste problems [14]. Strategies for recovering bioparticles selectively separated at liquid–liquid interfaces [15] may be limited by the stability of the particle–drop

* Corresponding author. Fax: +61 8 8302 3563. E-mail address: [email protected] (C.P. Whitby). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.05.089

aggregates. The efficient use of particles for the encapsulation and removal of non-aqueous phase contaminants from soils [16] requires displacement of the particles from the interface so they can be recycled. Displacing particles from interfaces in emulsions is a challenge. Yan and Masliyah [17] mixed solid-stabilized oil-in-water emulsions with large volumes of oil to induce macroscopic separation of the oil and water. Fujii et al. [18] demonstrated that pH-responsive particles form emulsions where a variation in the pH can induce separation of the oil and water. Detaching particles trapped at liquid–liquid interfaces is, however, rarely achieved efficiently. Separation of particles in biphasic solutions is typically hindered by particles remaining attached to the interface even after separation of the two liquid phases [19]. Particle coatings on water (or oil) drops in oil (water) buckle as the drops are deflated and remain attached rather than dispersing into the liquid [20]. We show here that mixing emulsions of nanoparticle-stabilised drops with surfactant which competitively adsorbs at the oil–water interface causes interfacial displacement of the nanoparticles. Typically surfactant molecules are mixed with nanoparticles in emulsions to enhance particle attachment [21–23]. Surfactant adsorption onto the particle surfaces alters the interactions between drops and nanoparticles. Tambe and Sharma [24] and Schulman and Leja [25] first linked the influence of surfactant adsorption on the oil–water contact angle of the particles to the type of emulsion formed. Lucassen-Reynders and van den Tempel [26] and Hassander et al. [27] linked improvements in emulsion stability (which depends on the extent of particle attachment) to changes in the extent of particle flocculation. Ravera et al. [28] found that surfactant adsorption affects the dynamics of particle transport to the interface. Wang et al. [29] observed that at very

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high surfactant concentrations, surfactant molecules can dominate emulsion formation and particle attachment does not occur. In contrast, we show how surfactant molecules mixed with nanoparticle-stabilised emulsions cause particle detachment. This alternative way of mixing surfactant molecules and particles favours competitive adsorption of the surfactant molecules at the oil–water interface. The macroscopic and microscopic changes to emulsion structure caused by mixing particle-stabilised emulsions with surfactant solutions reveal the extent to which the surfactant molecules adsorb at, and displace particles from, the interface. The novel outcome is a method for recovering particles strongly attached to liquid interfaces. 2. Experimental section 2.1. Preparation and characterisation of particle dispersions Silica nanoparticles (12 nm in diameter) modified by reaction with hexadecylsilane were supplied by Degussa (Aerosil R816). The particle contact angle (h) was 23° at the air–water interface and 60° at the toluene–water interface [30]. The BET surface area was 190 ± 20 m2 g 1. Dispersions of nanoparticles in solutions of sodium chloride (Chem Supply, 99%) in water (ultrapure with a resistivity not less than 18.2 MX cm) were sonicated in an ultrasound bath (Soniclean 160T, 70 W power, 44 kHz operating frequency) for 30 min. TEM images revealed that the powder broke down into 20–50 nm size particles. The zeta potentials of the nanoparticles were 38 mV in 0.001 M NaCl and 13 mV in 0.1 M NaCl at pH 5.8 (measured by Laser Doppler Velocimetry using a Malvern Zetasizer Nano).

particles were separated from the water layer by centrifugation (the foam and cream layers were removed prior to centrifugation). Emulsions mixed with aqueous salt solutions alone were not altered by rehomogenisation. 2.4. Interfacial structure Confocal fluorescence microscopy (CFM) images of the emulsions were obtained using a Leica SP5 spectra scanning confocal microscope. Rhodamine B (Sigma, 97%, 10 5 M solution) was used to stain the aqueous phase of the emulsions. The dye adsorbs onto silica particles due to electrostatic interactions. The samples were excited at a wavelength of 514 nm and the fluorescence emission intensity collected over 555–655 nm. Cryo-scanning electron microscopy (cryo-SEM) images of the emulsion structure were obtained using a Philips XL30 Field Emission Scanning Electron Microscope fitted with a CT1500 HR Low Temperature Cryo system. A few drops of the emulsion were frozen in nitrogen slush. The frozen emulsion was fractured under ultra high vacuum and then etched at 95 °C for 60 s. The fractured surface was coated in platinum (10 mA, 105 s, 1 mbar Argon) before being transferred to the cold stage ( 170 °C) of the SEM. Images were captured using the Microscope Control software and processed using CorelDraw 9. 3. Results and discussion Rehomogenising (mixing) the nanoparticle-stabilised emulsions with solutions of SDS alters the average drop size in the

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2.2. Particle-stabilised emulsions

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Emulsions were prepared by homogenising dodecane (Sigma Aldrich, 99%, passed through chromatographic alumina twice to remove polar impurities) with aqueous nanoparticle dispersions using a rotor–stator mixer (Ingenieubüro CAT X1030D, M. Zipperer GmbH) with a 10 mm head operated at 13,000 rpm for 2 min while fully immersed in the liquids. The oil volume fraction was 0.3 and the particle concentration in the aqueous phase was 2 wt.%. The emulsions did not phase separate for several months. The emulsions were characterised by light scattering (Mastersizer 2000) and optical microscopy (Olympus BH2 Research microscope). The drop size distributions consisted of a single population with a volume-weighted mean diameter (D(4, 3)) of about 70 lm. The average drop size did not change with salt concentration.

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2.3. Mixing emulsions with surfactant The emulsions were mixed with aqueous solutions of sodium dodecyl sulfate (SDS, Sigma Aldrich, 99%) at 13,000 rpm for 2 min. The final oil volume fraction was (0.08) and the particle concentration was 0.54 wt.%. The surfactant concentration varied between 10 5 and 10 2 M. The critical micelle concentration (cmc) of SDS was 0.008 M in 0.001 M NaCl and 0.001 M in 0.1 M NaCl (by drop shape tensiometry using a DataPhysics Instruments Contact Angle System OCAH 200). Some emulsions were prepared directly at the final oil volume fraction in the absence of particles. After mixing, the emulsions separated into layers of foam, cream, water and particles. Foam was produced in the presence of surfactant, because the rotor–stator homogenizer aerates as well as fragmenting liquids being mixed. Although simple, this method produced reproducible foams (incorporation of the same volume of air, drop size and stability over time). Changes in the heights of the layers were monitored over time at 23 °C. The released

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Fig. 1. (a) Variation in the average drop diameter (D(4, 3)) of 8 vol.% dodecane-inwater emulsions stabilised by 0.54 wt.% silica particles with the concentration of SDS in the aqueous phase for a background salt concentration of 0.1 M NaCl. The line is shown simply to guide the eye. Below are optical micrographs of an emulsion after mixing at 0.1 M SDS (b) and an emulsion stabilised by 0.1 M SDS alone (c). The scale bar corresponds to 100 lm.

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emulsions, generates foam and causes the release of particles. Fig. 1a shows the variation in the average drop size as the concentration of surfactant added to the emulsion increases. In the absence of surfactant, the emulsion drops are spherical in shape and discrete. As the surfactant concentration increases, the emulsion drop size population shifts to smaller sizes. The decrease in drop size indicates that surfactant molecules adsorb at the oil– water interface, reducing the interfacial tension and enabling the drops to be fragmented. An important observation is that surfactant concentrations above the cmc are required to stabilise drops (or bubbles) in the absence of particles. This is consistent with earlier observations of a transition in emulsion stability over a narrow range of surfactant concentrations just below the cmc [31]. Since the drops do not coalesce at low surfactant concentrations, presumably there are nanoparticles attached to the interface. Above the cmc, the average drop size does not change significantly with the surfactant concentration and is comparable to the average drop size of emulsions stabilised by surfactant alone (13 lm). Fig. 1b and c is images of drops after mixing with 0.1 M SDS and of drops in an emulsion stabilised by 0.1 M SDS alone, respectively, for comparison. 10000

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[SDS](M) Fig. 2. (a) Variation in the volume of foam initially formed (Vf) in emulsions in the absence (s) and presence (d) of silica particles with the SDS concentration. (b) Variation in the time taken for the foam volume to decrease by half (t1/2) in the same emulsions with the SDS concentration. (c) Variation in the percentage of particles (mp) released from the emulsions with the concentration of SDS in the aqueous phase for a background salt concentration of 0.1 M NaCl. All lines are shown simply to guide the eye.

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Fig. 2a shows the variation in the volume of foam initially formed as air is incorporated into the emulsions during mixing. The foaming is an important guide to the role of surfactant adsorption at the different interfaces in the emulsions. So, for comparison, Fig. 2a shows the volume of foam generated by mixing an emulsion in the absence of particles. There is a sharp increase in the foamability of the (surfactant-stabilised) emulsions at surfactant concentrations near the cmc. In contrast, the foamability of the nanoparticle-stabilised emulsions gradually increases with the surfactant concentration. The air bubbles are millimetres in size and the lamellae are white, indicating the presence of oil drops in the lamellae and Plateau borders. The foams are stabilized by surfactant molecules adsorbed on the bubble surfaces and by oil drops trapped in the aqueous phase between the bubbles. The stability of aqueous foams containing oil droplets depends on whether the oil drops can enter and rupture the foam lamellae [32]. We found that where oil drops are stabilised (by nanoparticles, surfactant or both species), they accumulate in the Plateau borders rather than penetrating and rupturing the air–water interface created during mixing. Thus foams are stabilised at low SDS concentrations in the presence of particles, because stable drops form and provide additional stability. In the absence of particles, drops are not stabilized at low SDS concentrations and hence foam does not form. At high concentrations of surfactant, the volume of foam formed by agitating the nanoparticle-stabilised emulsions is lower than that generated in emulsions containing surfactant alone. It is also lower than the volume of foam generated by mixing aqueous surfactant solutions or nanoparticle dispersions containing surfactant at the same surfactant concentrations. So mixing particle-stabilised oil drops with surfactant destroys some of the foam that is typically stabilised by this concentration of surfactant molecules. Once the mixing halts, water drains out of the foams and the bubbles coarsen due to coalescence and disproportionation. The foams completely destabilise over 24 h. Fig. 2b (inset) shows how the time taken for the foam volume to decrease by half varies with the surfactant concentration. The long term stability of the foams is similar to that of foams generated during the formation of surfactant-stabilised emulsions. So, in all cases, the emulsified oil drops remaining in the foam are easily displaced as the water drains out. Over time, particles sediment out of the foamy emulsions. The concentration of salt in the aqueous phase affects the particle release. In the absence of surfactant, unattached particles tend to aggregate with the drops as the salt concentration increases [33]. Significant particle release is only observed after mixing with surfactant. The mass of particles released is affected by the extent of heteroaggregation and homoaggregation in the emulsions. Fig. 2c shows that for emulsions with a high concentration of NaCl (0.1 M) in the aqueous phase the mass of released particles increases abruptly near the surfactant concentration where the average drop size in the emulsion is minimised and the maximum volume of foam produced. About three quarters of the nanoparticles present in the emulsions are released. The remaining nanoparticles are loosely aggregated with the drops in the emulsion creams. At lower salt concentrations (0.001 M, not shown), particle homoaggregation tends to dominate. Particles are released and sediment out from the emulsions at lower surfactant concentrations. The mass of released particles increases gradually with the surfactant concentration. To further investigate the changes caused to the nanoparticlestabilised emulsions by homogenisation with surfactant solutions, the mixing time was varied. Fig. 3a shows that the reduction in the average drop size in the emulsion occurs within the first 100 s of mixing. In contrast, the minimum drop size in emulsions stabilised by surfactant alone is achieved within just 10 s of homogenisation.

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Fig. 3. (a) Variation in the volume of foam initially produced (Vf, s) and the mean drop diameter (D(4, 3), j) of emulsions stabilised by silica particles and 0.01 M SDS with the homogenisation time, t. The lines were added simply to guide the eye. (b) Examples of drop size distributions in the same emulsions after homogenising for (from right to left) 0 ( ), 60 ( , red), 240 ( , blue), and 360 s ( , green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The maximum foam volume is produced in about 120 s of homogenisation. The volume of foam generated decreases a little for longer mixing times. Foams can be destabilised by the addition of particles [34,35] and it is likely that the particles displaced from the emulsions break some of the foam bubbles. Examples of the drop size distribution in the emulsions at different mixing times are shown in Fig. 3b. Initially the emulsion consists of a monomodal population of oil droplets stabilised by nanoparticles. Within 30 s of mixing in the presence of surfactant, the (volume weighted) drop size distribution becomes bimodal, with drop size populations centred at 4 lm and 30 lm. As the mixing time increases, the population of larger drops shifts to smaller sizes and the relative proportion of the population of smaller drops increases. The relative proportions of the two populations are consistent with the drop size distributions in emulsions stabilised by surfactant alone after homogenising for about 6 min. The drop size distribution shown for a mixing time of 360 s is comparable to the drop size distributions of emulsion stabilized by surfactant alone. The changes to the structure of the nanoparticle-stabilised emulsions caused by mixing with surfactant were visualised by confocal fluorescence microscopy (CFM) and cryo-scanning electron microscopy (cryo-SEM). A fluorescent dye, Rhodamine B, which binds to silica, was used to identify the location of the particles in the emulsions. Imaging of emulsions stabilized by particles revealed that the particles are located at the drop surfaces, as shown in Fig. 4a. After mixing with high concentrations of surfactant, the particles are located in the aqueous phase of the emulsions, as shown in Fig. 4b. Fig. 4c shows an image of an emulsion stabilized by surfactant alone. In the absence of particles, the water soluble dye stains the continuous phase of the emulsions. Electron microscopy images revealed the interfacial structure in the emulsions. Fig. 5a shows that flocs of particles are visible at the interface around the drops in emulsions stabilised by particles alone. The oil–water interface is coated with small aggregates of particles in dense, close-packed arrangements. Energy Dispersive X-ray (EDX) spectra of the interfacial particle layer show peaks centred at 0.5 keV and 1.7 keV, corresponding to oxygen (Ka) and silicon (Ka) respectively. This indicates that the elemental composition of the particulate material is silicon and oxygen, as expected

Fig. 4. Examples of confocal fluorescence microscope images of 8 vol.% dodecanein-water (0.1 M NaCl) emulsions stabilized by 0.54 wt.% particles at SDS concentrations in the aqueous phase of (a) 0 M and (b) 0.1 M. Also shown is (c) an emulsion stabilized by 0.1 M SDS alone. A water soluble fluorescent dye, Rhodamine B, which binds to silica particles was added to all emulsions. The fluorescent emission is colored red in these images. The scale bar corresponds to 50 lm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

for silica. A peak at 2 keV due to platinum (Ma) is also observed since platinum was sputtered on the emulsion surfaces to minimise charging by the electron beam during imaging. At intermediate surfactant concentrations, the attached particle layer at the drop surfaces is ‘‘patchy”, as shown in Fig. 5b with the extent of coverage varying between drops. Fig. 5c shows that at high surfactant concentrations, only a white interphase between the frozen oil drops and aqueous phase is observed. There is no evidence of a particle layer on the drop surface, although it is possible that a layer of

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Fig. 5. Examples of freeze fracture SEM images of 8 vol.% dodecane-in-water (0.1 M NaCl) emulsions stabilised by 0.54 wt.% silica particles at SDS concentrations in the aqueous phase of (a) 0 M, (b) 10 4 M and (c) 0.1 M. The scale bars correspond to 2 lm in (a) and (b) and 1 lm in (c).

nanoparticles (not aggregates) at the interface may not be resolved by electron microscopy. Particulate material is observed in cryo-SEM images of the continuous aqueous phase exposed in the frozen emulsion samples (Fig. 5). The emulsions are binary mixtures of nanoparticle-stabilised drops and excess particles. Assuming that the average radius of the nanoparticles attached to the drop surfaces (R) is 15 nm, it can be estimated that there are about six times as many particles available as required to coat the drops with a single layer. Fig. 5a shows that small aggregates of excess particles exist in the aqueous phase of the particle-stabilised emulsions. Small, stringy aggregates of particles are observed in the aqueous phase of emulsions at intermediate surfactant concentrations (Fig. 5b). Comparison of Fig. 5a and c reveals that the aggregates of excess particles located in aqueous phase become slightly larger in size as the surfactant concentration in the emulsions increases. The freezing process may have contributed to some of the features observed. Ice crystal formation in particle systems tends to cause artefacts [36]. Observations from CFM and cryo-SEM are, however, consistent with the release of particles from the emulsions. These findings demonstrate that mixing nanoparticle-stabilised emulsions with surfactant causes particle displacement from the oil–water interface. Particle detachment is apparently not linked to surfactant adsorption altering the particle hydrophobicity. At low salt concentrations, displaced particles are released from the emulsions at surfactant concentrations below the cmc. There is little adsorption of the anionic surfactant onto the negatively charged

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silica nanoparticles except above the critical micelle concentration [33]. There is no detectable difference between the surface tension of particle dispersions containing surfactant and surfactant solutions at the same concentration [33]. Typically the particles are flocculated in aqueous dispersions. The particles redisperse, however, at high surfactant concentrations [33]. This is due to the surfactant molecules adsorbing in conformations which exposed the anionic head group to the aqueous solution [37], effectively increasing the surface charge on the particles. The particles displaced from the emulsions remain flocculated and sediment out (rather than being dispersed into the aqueous phase as is typical for particles bearing adsorbed surfactant layers). So particle detachment likely occurs due to surfactant adsorption at the oil– water interface. Thermodynamic analysis of particle partitioning between water and a planar dodecane–water interface indicates that an isolated spherical silica nanoparticle is trapped in a deep energy well, given [3] by pR2cow(1 cos h)2 (4000 times the typical Brownian thermal energy for a particle–oil–water contact angle, h, of 60°). For a trapped nanoparticle, a detachment energy at least equivalent to this value is required to remove the nanoparticle from the interface. With surfactant adsorption, the dodecane–water interfacial tension (cow) decreases from 51 mN m 1 [33] to about 10 mN m 1 at the cmc. Although particles can stabilize interfaces with low interfacial tensions, we propose that the reduction in interfacial tension due to surfactant adsorption favours competitive adsorption of the surfactant during rehomogenisation. The dramatic decrease in the interfacial tension associated with surfactant adsorption, reduces the particle detachment energy from 4469 to 859 kT. Complete interfacial displacement is possible at the cmc, since there are sufficient surfactant molecules available to stabilize the drops. Importantly, although nanoparticle displacement by surfactant molecules is thermodynamically favoured, the process is not straightforward. Two minutes of mixing is required after dilution in a high surfactant concentration to displace the particles and fragment the drops to sizes comparable to those of drops stabilized by surfactant alone (Fig. 3a). Comparison of the drops suggests a significant population of satellite drops (4 lm in diameter) form during mixing (Fig. 1b and c). This is confirmed by the drop size distributions (Fig. 3b). In surfactant-stabilised systems, the formation of a population of satellite drops is known as tip streaming. deBruijn [38] showed that accumulation of surfactant at the tips of drops results in the drops developing a sigmoidal shape. A stream of tiny drops ruptures off the tips of the deformed drops under simple shear flow. Typically this only occurs where the ratio of the viscosities of the dispersed and continuous phases is low (<0.1) [39]. Here the ratio of the viscosities of dispersed and continuous phases (dodecane and water, respectively) is close to unity. We did find that satellite drops form even where drop fragmentation is achieved simply by forcing nanoparticle-stabilised emulsions diluted in concentrated SDS solutions under pressure through narrow channels (sub-millimetre-sized needles). A population of satellite drops forms after a single pass through the channel. The number of satellite drops increases with the number of passes. Perhaps the formation of satellite drops is due to the development of surfactant tension gradients as surfactant molecules adsorb on the drop surfaces. These results provide clues to how surfactants and particles can be mixed in emulsions to take advantage of the different interfacial properties of those emulsifiers. For example, nanoparticle-stabilised emulsions typically have high interfacial tensions and hence relatively large drops, as attached particles do not dramatically alter the interfacial tension. Surfactant adsorption reduces the interfacial tension. Importantly, emulsions remain stable at interfacial

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tensions where there is insufficient surfactant to stabilize drops alone. Mixing surfactant together with the nanoparticle-stabilised emulsions produces stable emulsions with drop sizes that are tuned by the surfactant concentration (Fig. 1). Varying the surfactant concentration added to the emulsion changes the relative proportions of surfactant and particles at the interface. The incomplete coverage of the drops by particles observed at low surfactant concentrations (Fig. 4b) indicates that changes to the interfacial composition alter the structure of the attached particle layer. Our results suggest this affects the rheological properties of the interface. We also speculate that surfactant displacement of particles from the interface will, for example, affect the rate of release of active ingredients encapsulated in the drops. Another application of surfactant addition could be to trigger the fast release of ingredients from emulsions during stirring. The observation that particles are displaced from the oil–water interface is reminiscent of the displacement of proteins from interfaces by surfactants. A reduction in the interfacial tension of emulsions by 10 mN m 1 is sufficient, for example, to induce displacement of proteins by surfactant in stirred emulsions [40,41]. Protein displacement occurs by a three stage mechanism where surfactant adsorbs in small defects in the protein film and the surfactant domains grow, causing holes in the adsorbed protein layer, until finally the protein network breaks down [41]. Many proteins occur naturally as globular aggregates of nanometre size and the mechanisms by which proteins stabilize interfaces are similar to those by which particles stabilize interfaces. It is speculated that surfactant adsorption occurs in the pores of the close packed particle layer. Perhaps the development of surfactant tension gradients as surfactant molecules adsorb on the drop surfaces enables particle detachment as drops collide during mixing. Once there is sufficient surfactant adsorption at the drop surfaces to enable drop fragmentation, then the faster rate of surfactant diffusion and adsorption at the interface will result in the fresh interface created being stabilized by surfactant molecules. Our results indicate that the deformation, breaking and coalescence of drops bearing mixed surfactant and particle layers requires further investigation. In summary, nanoparticle displacement occurs via surfactant adsorption at the nanoparticle-stabilised interface. These results are different to the observations by Rao et al. [42] that nanoparticles films formed at planar interfaces in the water or oil phases could be redispersed into the bulk liquid phases by adding surfactants. As demonstrated by Subramaniam et al. [43], exposure of individual bubbles stabilised by micrometre-sized particles to surfactant which adsorbs at the particle surfaces enables particle detachment from fluid interfaces, presumably due to changes in particle wettability. In these cases, the liquid phases subsequently phase separated. Binks et al. [44] found that gently stirring solutions of nonionic surfactant with particle-stabilised emulsions increased drop coalescence as the surfactant adsorbed onto the particle surfaces and caused aggregation. The findings presented here highlight, instead, the effects of shear and surfactant adsorption directly at the interface on the interactions between nanoparticles and surfactant at interfaces in bulk emulsions. Careful choice of surfactant enables particles to be recovered from particle-stabilised emulsions without requiring that the emulsion drops (encapsulated drops of contaminants, for example) are destabilized. This is also a route, for example, for detaching particles from drops containing reactive ingredients that modify the particle surfaces once the reaction is complete.

Adsorption of the anionic surfactant at the interface was energetically favourable. The silica nanoparticles were displaced from the interface with the application of shear. Our results suggest that at surfactant concentrations insufficient to stabilise drops alone, the drops obtained are stabilised by a composite layer of nanoparticles and surfactant, as surfactant adsorbs rapidly at the interface created during drop fragmentation. Above the critical micelle concentration, there is complete displacement of the nanoparticles by surfactant. Our observations indicate how attached nanoparticles can be recovered from interfaces in emulsions. They have important applications for the interfacial separation of particles and recycling of particulate encapsulants. Further, these results imply that the interfacial composition and the response of the interface to shear and deformation can be tailored by surfactant addition. Acknowledgments The financial support of the Australian Research Council Linkage Scheme, AMIRA International, and State Governments of South Australia and Victoria, the FABLS network (RN0460002) is gratefully acknowledged. CPW gratefully acknowledges receipt of an Australian Research Council Future Fellowship. The particles were kindly supplied by Evonik Degussa. We thank Dr. L. Waterhouse and Dr P. Self (Adelaide Microscopy, The University of Adelaide) for their help with the confocal fluorescence microscope and cryo-SEM experiments, respectively. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

[31]

4. Conclusions [32]

In this paper we examined displacement of nanoparticles from oil–water interfaces in emulsions in the presence of surfactant.

[33] [34]

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