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Stabilization of Oil-in-Water Emulsions with Non-Interfacially-Adsorbed Particles Brandy Kinkead Pilapil, Heidi Jahandideh, Steven L. Bryant, and Milana Trifkovic Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00873 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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Figure 1. Optical photographs of dodecane-in-water emulsions studied in this work immediately after preparation (a-e) and after 1 month (f-j). Labels “1 mM”, “B” and “H” here and in succeeding figures indicate the presence of surfactant, bare-silica and HMDS-coated silica nanoparticles. Components in each sample are: (a,f) bare-silica nanoparticles (NPs) (10 mg, 0.33 wt%); (b,g) HMDS-silica NPs (10 mg, 0.33 wt%); (c,h) 1mM surfactant; (d,i) 1mM surfactant with bare-silica NPs (10 mg, 0.33 wt%); and (e,j) 1mM surfactant with HMDS-silica NPs (10 mg, 0.33 wt%). The pink coloration of the solutions is due to the dye incorporated into the silica-NPs. (k) Plot of emulsion droplet size before and after the 1 month settling period. In the case of the 1mM surfactant sample, droplet size after 1 month of settling is not shown because of the essentially complete coalescence of the emulsion. 84x101mm (300 x 300 DPI)

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Figure 2. Three dimensional reconstructed LSCM images of dodecane-in-water emulsions prepared with (a) 1mM surfactant with bare-silica NPs (10 mg, 0.33 wt%) or (b) 1mM surfactant with HMDS-silica NPs (10 mg, 0.33 wt%). These images demonstrate that bare-silica NPs are distributed uniformly through the aqueous phase (a), while HMDS-silica NPs preferentially accumulate at the dodecane-water interface (b). The oil phase is labelled green, the nanoparticles are labelled red, and the water phase is unlabelled (transparent). Note that in (a), the small transparent wedge in the right corner of the image is due to the sample container, such that this region is outside of the glass cell holding the emulsion, and not part of the water phase. 84x71mm (300 x 300 DPI)


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Figure 3. Cryo-scanning electron microscopy images for dodecane-in-water emulsions stabilized with 1mM surfactant and either (a) bare-silica NPs (10 mg, 0.33 wt%) or (b) HMDS-silica NPs (10 mg, 0.33 wt%). Bare-silica NPs are found to be distributed in the aqueous phase (a), while HMDS-silica NPs reside preferentially at the dodecane-water interface (at the relict interface formed when oil droplets were torn from the surface during cryo-fracturing). Note that partial sublimation of (a) was required in order to visualize a significant amount of bare-silica NPs, resulting in the observed roughening of the water phase. (c) Energy dispersion X-ray spectroscopy (EDX) map for silicon, and (d) overlay of silicon EDX map onto the corresponding SEM image for a randomly selected water region of the sample containing bare-silica NPs. Red arrows indicate grain boundaries within the ice that formed despite the use of a nitrogen slush in trying to form completely amorphous ice. Silica NPs can be seen to accumulate at these boundaries. 84x84mm (300 x 300 DPI)


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Figure 4. High-magnification dynamic LSCM images taken at the oil-water interface for the emulsions shown in Figure 2, confirming that bare-silica NPs have no affinity for the interface (a), while the HMDS-silica NPs are generally pinned to the interface (b). The oil phase is labelled green, the NPs are labelled red, and the water phase is unlabelled (black). Trajectories of three randomly chosen particles (based on the length of their trackable trajectories) from videos from which these frames are taken (see SI Appendix) are overlaid on the images and plotted versus distant to the interface (c). This trajectory analysis shows that bare-silica nanoparticles diffuse randomly throughout the water phase, while HMDS-silica NPs generally have confined motion about the interface. 84x63mm (300 x 300 DPI)


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Figure 5. Schematic plot summarizing the relationship between surfactant and NP type/concentration on the stability and morphology of the resulting emulsion. Emulsions containing both surfactants and NPs may be stabilized without the need for synergy (green region) or via synergistic interactions (white region), whereby both the NPs and surfactants are necessary in order for a stable emulsion to form. Moreover, synergy may arise via NP interactions with the oil-water interface that are either Pickering (yellow circles) or nonPickering (blue circles) in nature. The dotted vertical line which separates the synergistic region from the non-synergistic region along the NP concentration signifies that not all NPs may impart stability to an emulsion without synergistic interactions. Previous work reported by Binks et al. (ref. 16) has also been included. 84x101mm (300 x 300 DPI)


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TOC Graphic Demonstrated here is the formation of non-interfacially-adsorbed particle stabilized oil-in-water emulsions, generated through synergistic interaction between surfactants and nanoparticles that do not adsorb to the oil-water interface. 82x44mm (300 x 300 DPI)


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Stabilization of Oil-in-Water Emulsions with NonInterfacially-Adsorbed Particles Brandy K. Pilapil, Heidi Jahandideh, Steven L. Bryant* and Milana Trifkovic* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr. NW, Calgary, AB, T2N 1N4 Keywords: Emulsion, Pickering, Nanoparticle-Surfactant Synergy, Silica Nanoparticles, Laser Scanning Confocal Microscopy, Cryo-scanning electron microscopy


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Abstract: Classical (surfactant-stabilized) and Pickering (particle-stabilized) type emulsions have been widely studied to elucidate the mechanisms by which emulsion stabilization is achieved. In Pickering emulsions, a key defining factor is that the stabilizing particles reside at the liquidliquid interface providing a mechanical barrier to droplet coalescence. This interfacial adsorption is achieved through the use of nanoparticles that are partially wet by both liquid phases, often through covalent surface modification of or surfactant adsorption to the nanoparticle surfaces. Herein, we demonstrate particle induced stabilization of an oil in water emulsion with fully water wet nanoparticles (no interfacial adsorption) via synergistic interaction with low concentrations of surfactants. Laser scanning confocal microscopy analysis allows for unique and vital insights into the properties of these emulsions via both three-dimensional imaging and real-time monitoring of particle dynamics at the oil-water interface. Investigation of these “non-Pickering” particle stabilized emulsions suggests that the non-adsorbed particles impart stability to the emulsion primarily via entropic forces imparted by the accumulation of silica nanoparticles in the coherent phase between dispersed oil droplets.


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INTRODUCTION Since the discovery of particle stabilized emulsions by Pickering and Ramsden some 100 years ago,1, 2 much work has explored the mechanism of this stabilization and the applications of these soft materials.3 In “Pickering” emulsions, the particle stabilizers are partially wet by both liquids and the type of emulsion formed depends on which phase preferentially wets the particles.4 The interfacially localized particles provide a mechanical barrier to droplet coalescence, interfering with the ability of dispersed liquid droplets to coalesce.5 In addition, the particles may impart electrostatic repulsion between droplets, further enhancing emulsion stability.6 In the case of classical emulsions, amphiphilic molecules lower the interfacial tension between the two immiscible liquids and provide steric and/or electrostatic hindrance to droplet coalescence.7 This behavior is generally analogous to that of Pickering-type particle-stabilized emulsions,8 such that the stabilizing agent preferentially sits at the liquid-liquid interface and the type of emulsion formed similarly depends on the level of miscibility of the surfactant molecule in the two immiscible liquids (the hydrophilic-lipophilic balance).9 Recently, more research has explored the synergy that can exist between particles and surfactants or co-stabilizers in the formation and breaking of emulsions (although earlier studies had previously explored the concept to a lesser extent).10 In cases where the particles are attracted to the interfaces, emulsions may be either stabilized or destabilized by the interactions between the particles and surfactants.11-15 For example, Whitby et al. reported on the ability of anionic surfactant to destabilize Pickering emulsions prepared with hexadecylsilane modified silica when added at concentrations greater than the critical micelle concentration (CMC).13 In cases where synergistic stabilization of the emulsions exist, the effect may be attributed to surfactant-induced flocculation of the particles leading to greater steric hindrance to coalescence, or enhanced


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stabilization due to lowering of the interfacial tension between the oil and water phases without a significant loss of particles from the interface.14 In other cases, particles with no affinity for the oil-water interface may be modified through the use of a surfactant or co-stabilizer to induce changes in the particle wettability. Binks et al. have demonstrated interactions between negatively charged silica and positively charged surfactant, and vice versa, as a route to stable Pickering emulsions.16, 17 Interaction between surfactants and hydrophilic nanoparticles (no affinity for the oil-water interface) has indeed been exploited by a number of researchers for its ability to enhance emulsion stability, 16, 18-22 and induce emulsion phase inversion.10, 23, 24 In this work, the combination of bare (fully water wet) or surface modified (partially wet by both the oil and water phases) silica nanoparticles (NPs) with surfactant, at concentrations where neither component can form a stable emulsion alone, is shown to form stable emulsions via synergistic effects. The modified silica NPs (hexamethyldisilazane (HMDS) modified silica NPs) are found to impart stability to the emulsions via adsorption to the oil-water interface alongside the co-stabilizing surfactant. In contrast, the bare-silica NPs impart stability to the emulsion while remaining entirely in the coherent water phase. This mechanism differs significantly from a synergistically formed Pickering-type particle stabilized emulsions, where particles demonstrate a large adhesion energy with the oil-water interface and cannot leave it spontaneously.5, 25 Herein, we describe the formation of “non-Pickering” particle stabilized emulsions, compare this form of emulsion stabilization to its Pickering and surfactant-only counterparts and suggest a mechanism by which this emulsion stabilization is achieved. The use of laser scanning confocal microscopy (LSCM) with suitable image resolution allowed for direct viewing of both the nanoparticles and oil droplets within the emulsion. This imaging capability provided valuable insights into the emulsion stabilization mechanism through three dimensional


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(3D) imaging of the emulsions and 2D real-time imaging of particle dynamics at the oil-water interfaces. In addition, we propose the concept that particle stabilization of emulsions may be imparted over a spectrum of interfacial adsorption; whereby the primary mechanism by which the stabilization is achieved varies depending on the level of NP adsorption to the liquid-liquid interface. MATERIALS AND METHODS Fluorescently labelled silica NPs were synthesized by a modified Stöber method.26 Prior to use, all glassware was cleaned by treatment with Aqua Regia (1:3 Nitric Acid to Hydrochloric Acid), followed by treatment with Piranha solution (7:2 Sulfuric Acid to 30% Hydrogen Peroxide). In brief, 44 mg (3-aminopropyl)triethoxysilane (99%, Sigma Aldrich), 5 ml Ethanol (200 proof) and 28 mg rhodamine B isothiocyanate (mixed isomers, Sigma Aldrich) were combined and allowed to stir smoothly overnight. Additional ethanol (176 ml), 7.7 ml ammonium hydroxide solution (ACS reagent, 28.0-30.0%, Sigma Aldrich), 4.6 ml deionized water, and 7.7 ml tetraethyl orthosilicate (reagent grade, 98%, Sigma Aldrich) were then added to the previous mixture and allowed to stir for at least 8 hours. The resulting cloudy pink solution was centrifuged for 2 hours at 4500 rpm, decanted from the resulting supernatant and redispersed in deionized water. Centrifuging and redispersion was repeated two more times with ethanol to ensure removal of excess reagents. The resulting powder was freeze dried to remove residual solvent and this dried powder used in the preparation of subsequent samples. Note that freeze drying was essential in enabling the particles to redisperse into water. Drying at ambient conditions resulted in irreversible aggregation of the particles. Transmission electron microscopy (TEM) analysis was performed using a Tecnai F20 FEG-TEM operated at 200 kV by drop casting the particles from an ethanol solution onto a C-Formvar 200


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mesh TEM grid (Ted Pella). TEM analysis reveals that individual silica particles are roughly spherical and have a mean diameter of 70 ± 20 nm (Figure S1). These particles, though, are commonly aggregated in solution or sometimes fused to one another into agglomerates, such that DLS analysis reveals a larger particle mean diameter of 160 ± 60 nm. In addition, the TEM grid is found to be free of impurities (as indicated by the lack of residues found on the C-Formvar support), suggesting that the cleaning procedure utilized here is sufficient. Surface modification of the prepared silica NPs with hexamethyldisilazane (HMDS, Sigma Aldrich, Reagent Grade ≥99%) was used in order to alter the wettability of the NPs. In brief, 0.4 grams of the bare-silica nanoparticles were dissolved in 100 ml of toluene (Sigma Aldrich, anhydrous, 99.8%) via sonication in a 250 ml round bottom flask equipped with a reflux condenser. To this dispersion, 0.5 mL of reagent (10 vol% HMDS dissolved in toluene) was added. The solution was then heated to 110°C and held at this temperature for approximately three hours. Once cooled, the solids were separated by centrifugation (15 minutes at 4500 rpm), the supernatant removed and the solids redispersed in toluene or acetone. Centrifugation and redispersion was repeated two more times to ensure removal of unreacted HMDS. The samples were finally freeze dried for a duration of three hours in order to obtain the final solid products. Oil in water emulsions were prepared with dodecane (Sigma Aldrich, anhydrous > 99%), and deionized water (18 MΩ). All emulsions were prepared with equal parts oil and water (3 ml of each phase). A zwitterionic surfactant, 3-(N,N-Dimethylmyristylammonio)propanesulfonate (≥ 99%, Sigma Aldrich), was chosen for this study. The critical micelle concentration (CMC) of the surfactant is ~ 0.1-0.4 mM at 25°C.27 Solutions of surfactant and/or particles were prepared in the water phase with defined concentrations in a 7 ml glass scintillation vial prior to addition of the dodecane oil phase. Emulsification was achieved through sonication for 30 seconds (Fisher


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Scientific M Series Ultrasonic Bath), followed by vortexing for 30 seconds (Fisher Scientific Analog Vortex Mixer). Stability of the emulsions was assessed via both settling under gravity and centrifugation (Eppendorf Model 5810R swinging bucket centrifuge). Samples were allowed to cream before centrifuging with a 1 minute ramp up/down and 5 minute hold at 200 rcf. Dynamic light scattering (DLS) and zeta potential analyses were carried out using a Malvern Zetasizer Nano ZSP instrument. Each reported zeta potential value was an average value from a minimum of 3 x 30 runs. Emulsions were diluted to approximately 20 to 1 water phase to oil phase for zeta potential analysis. Laser scanning confocal microscopy (LSCM) imaging was performed using a Leica SP8 inverted confocal microscope, equipped with an 8 kHz resonant scanner and operated using LASX software. Dodecane was tagged with perylene dye (0.01 mg per ml, sublimed grade, ≥99.5%, Sigma Aldrich), such that both the oil phase and the particle additives were visible under fluorescence microscopy. Stability was assessed by visual inspection of the emulsions before and after a predefined settling time, as detailed in the results section of the main manuscript. Samples without perylene dye in the oil phase were prepared as well, and no differences were observed in their stability and emulsification behavior (in comparison to samples prepared with perylene). The emulsions were imaged directly in the vials that they were prepared through the use of a periscope arm attachment on the confocal microscope (LSM Tech) for droplet size determination. Measurement of emulsion droplet size from these images was done using ImageJ software, with a minimum of 100 droplet measurements per sample. Three-dimensional imaging and high-magnification 2D dynamic imaging was performed on emulsions immediately after loading of the emulsion into 100 µl capillary tubes sealed at the ends with parafilm. The 2D time stacks (videos) reported were acquired at a frame rate of 50


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frames per second. These images were median filtered to reduce noise using the LASX software. ImageJ analysis of the 2D time stacks was performed using a trajectory analysis module developed by Sbalzarini and Koumoutsakos.28 Only trajectories that could be mapped for at least 10 frames were considered for the plots seen in Figure 3 and the specific trajectories chosen were randomly selected from this list (e.g., one trajectory from near the beginning, middle and end of the acquired time stack). Reconstruction and image processing of the z stacks to create 3D images was achieved using Avizo software (Version 9.0.1, CMC microsystems). For 3D reconstructions, a median filter was applied to reduce noise and the images thresholded to generate binary images. The binary images were then volume (particle channel) or surface (oil channel) rendered for 3D viewing. Quantification of the NP concentration (area of nanoparticle fluorescence, %) from LSCM images of NP containing samples was achieved via ImageJ analysis of single z planes. The “interfacial region” of these images was evaluated as the water phase region with ~2 µm of the oil-water interface (as defined by the perimeter of fluorescence from the perylene oil dye), while regions greater than ~2 µm from the interface are defined as “bulk”. The 2 µm distance was chosen because ImageJ analysis of the LSCM images used in this quantification measured the average particle diameter to be ~ 2 µm. This is considerably larger than what the diameter of the NPs appears to be in the 2D dynamic images because these images were acquired at a relatively slow rate (~ 15 frames per second), such that NP movement contributes significantly to what is captured in the images. Accordingly, the “interfacial region” in the dynamic 2D images is defined smaller (~ 1 µm). Cryo-SEM analysis was performed using a FEI Quanta FEG 250 SEM with attached Gatan Alto 2500 cryo stage and corresponding xTm version 4.1.12.2162 software. A small volume of freshly mixed emulsion sample was loaded into a rivet and quickly frozen in a nitrogen slush


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before transfer to the SEM. The samples were either imaged under low vacuum or coated with ~ 10 nm of gold prior to imaging, as specified in the results. Images were acquired with either a secondary electron or backscattered electron detector and an accelerating voltage of 5 or 10 kV. Energy dispersive X-ray spectroscopy (EDX) was performed using a Bruker Quantax 5030 EDX with an energy resolution of 127 eV and the corresponding Bruker Esprit version 2.0 software. Interfacial tension (IFT) between dodecane and the water phases studied was measured using a Future Digital Scientific Corp. SVT20 Spinning Drop Tensiometer. Measurement tubes were filled with the water phase of interest before injecting a single droplet of dodecane (containing 0.01 mg per ml perylene). All water phases were found to have the same density and refractive index regardless of composition of 0.999 mg/ml and 1.3305, respectively. A dodecane density of 0.7487 mg/ml was used in the calculation of IFT and all measurements were acquired at 20°C. Reported values have been calculated from droplet shape analysis by Young-Laplace method. The viscosity of a 0.33wt% NP containing solution was evaluated using an Anton Paar MCR 302 Rheometer equipped with a bob in cup measuring system at room temperature (21°C) using a shear rate ramp test (1 to 10 Hz). The NP containing fluids were Newtonian in nature with a viscosity comparable to that of water (0.95 ± 0.15 mPa·s). RESULTS AND DISCUSSION A series of water phase solutions were prepared containing 0.33 wt% silica NPs (either bare or HMDS functionalized), 1mM surfactant, or a combination of the surfactant and NP components. These solutions were then used to prepare emulsions with equal volumes of dodecane (Figure 1). The samples containing NPs alone at the 0.33wt% concentration could not emulsify the entire oil phase (Figure 1 a,b). The hydrophilic bare-silica NPs (at any concentration) alone could not form an emulsion, while HMDS-silica NPs emulsified the entire oil phase at approximately 5 wt%


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concentration (150 mg in 3ml of water, Figure S2). The stable emulsion prepared with 5 wt% HMDS-silica NPs did not form a typical emulsion with discrete droplets, but rather a type of bicontinuous network or bijel of trapped oil in the water phase (Figure S2c) with NPs at the oilwater interfaces.29, 30 The solutions containing 1 mM surfactant, with or without silica NPs, could initially emulsify the entire oil phase (Figure 1 c,d,e). After one month of settling though, the sample containing only 1mM surfactant had completely coalesced, while the samples containing surfactant and NPs of either type remained stable. Moreover, after the 1 month settling period there were no significant changes to the average emulsion droplet size, as evaluated using LSCM (Figure S3), for either of these samples (Figure 1 k). Note that the use of LSCM to evaluate mean droplet diameter was chosen so that the samples did not need to be disturbed in order for measurements to be taken. The observation that the emulsion droplet sizes are comparable across all samples suggests that the surfactant dictates the formation of the emulsions, although the NPs impart stabilizing effect. Below 0.33 wt% of bare silica nanoparticles this enhanced stabilizing effect was not observed, such that 0.33 wt% represents the minimum concentration for nanoparticle enhanced stabilization using the bare silica NPs in combination with 1 mM surfactant. The same concentration of HMDS-silica NPs was incorporated into the emulsions containing these particles for best comparison. To further evaluate emulsion stability, a centrifugation study was also performed (Figure S4). The results of this study indicated the same trend as that seen from gravity settling, as well as demonstrated that the surfactant exhibited comparable stability to that of the NP containing samples at a concentration of 10 mM for the specific centrifuging conditions examined. Overall, these analyses demonstrate the ability of the silica NPs of both types to enhance the stability of emulsions in synergy with the co-stabilizing surfactants.


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In order to probe the mechanism by which these NPs impart the observed enhanced stability, a number of experiments were undertaken. Zeta potential analysis of the bare-silica NPs reveals that the NPs have a near neutral surface charge (-1 ± 6 mV). This finding is not unexpected considering their generally poor stability in water (see Figure 1) and the particle aggregation observed in the DI water solvent, as indicated by the difference between measured DLS and TEM diameters (see SI appendix). We suspect that the near neutral surface charge is the result of the incorporated dye in the silica particles, which would normally (without dye) have a negative surface charge. We note though that measuring zeta potential of these NPs is hindered by their poor stability in solution (rapid settling out from water), and so the uncertainty of these measurements is relatively high. Moreover, the addition of up to 10 mM surfactant to the baresilica NP solutions resulted in no visible change to NP stability, suggesting that the surfactant does not adsorb to the NP surfaces. Correspondingly, the addition of up to 10 mM surfactant to the bare-silica NP solution did not change the measured zeta potential or level of uncertainty in the measurement (1 ± 9 mV in 10 mM surfactant), further suggesting that the surfactant does not adsorb to the NP surfaces. Three dimensional reconstructed LSCM images of the emulsions containing 1 mM surfactant and bare or HMDS-modified silica NPs are shown in Figure 2. The resolution of the LSCM images with the objective used in this study is approximately 300 x 300 x 300 nm voxel size, such that shapes of individual NPs cannot be resolved. However, because the microscope is detecting the fluorescence emitted by the NPs, it is possible to “see” all NPs in the samples, although each individual particle will show up as at least a single voxel. Moreover, the correspondence between fluorescence and NP position is unambiguous, since the fluorescing molecule is incorporated into the silica NPs during synthesis. In samples containing bare-silica,


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the NPs are found to be dispersed throughout the water phase with no specific adsorption to the oil-water interface (Figure 2c). These images confirm that the bare-silica NPs impart stability to the emulsion without adsorbing at the oil-water interface. Rather, the bare-silica NPs impart a stabilizing effect while remaining fully water wet. The images also further suggests that the surfactant does not adsorb at the bare-silica NP surfaces, since surfactant adsorption would drive the NPs to the oil-water interface. Interestingly, despite the tendency of the bare-silica NPs to settle out of solution (as observed in Figures 1 f and i) and their lack of interfacial adsorption, a significant amount of particles is found in the coherent phase surrounding the emulsion droplets (Figure 2a). This observation suggests that there exists either a weak attraction to the interface, and/or a repulsive force or steric hindrance prevents the NPs from moving past the droplet surfaces to the excess water phase below. Meanwhile, HMDS-silica NPs are found to be significantly concentrated about the droplet surfaces (Figure 2c), indicating adsorption of these particles to the oil-water interface. Overall, this image analysis clearly demonstrates the formation of non-Pickering (non-interfacially adsorbed) NP stabilized emulsions with bare-silica NPs, with evident contrast to the Pickering-type NP stabilized emulsions prepared with HMDSsilica NPs. It should be noted here that emulsion stabilization by particles that do not adsorb to the interfaces has been previously mentioned in literature, but never fully studied.16 Binks et al. reported in 2007 that upon addition of 2 wt% Ludox CL (alumina coated silica NPs that are positively charged at low pH) to an emulsion containing 1 mM sodium dodecyl sulfate (SDS, an anionic surfactant), an emulsion formed which incorporated a larger portion of the oil phase and exhibited greater stability than that of the emulsion containing 1 mM SDS alone. They did not observe the adsorption of these particles to the interface though, stating that it “appears that these


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particles approach the drop interface but are not adsorbed to it” based on cryo-SEM analysis.16 In addition, the zeta potential of the NPs in this surfactant solution was not significantly altered in comparison to the NPs in a solution of water alone. Similarly, their particles did not form emulsions without the use of surfactant due to a strong preference for the water phase, suggesting that their system exhibited similar properties to those observed here. Cryo-SEM has also be used here to confirm the differences observed for HMDS and bare-silica NPs seen under LSCM (Figure 3). Initial observation of the bare-silica NP containing sample showed very few silica NPs (Figure 3a), with only one region found where multiple silica NPs were observed on the surface (Figure 3 c,d). EDS analysis of this region was performed to evaluate if NPs were detectable below the surface (Figure 3c), since the EDS interaction volume is approximately 300 nm in diameter and 600 nm in depth (as estimated by the software for the beam conditions utilized) such that particles not visible under SEM imaging could be “visible” via EDS mapping. This analysis found that significant signal (red background between brighter red spots from surface particles, Figure 3 c,d) was coming from below the surface, and so sublimation was performed in order to image the buried silica NPs in the sample. Note that the EDS interaction volume is comparable to the resolution of the LSCM, and, correspondingly, the image shown in Figure 3c looks quite similar to the image in Figure 3a. After sublimation, baresilica NPs (both individual and large aggregates) are clearly seen to concentrate in the bulk water phase, away from the oil-water interface (Figure 3a). The accumulation of HMDS-modified silica NPs at the oil-water interface is readily evident in Figure 3b, with the NPs residing primarily at the relict interfaces (oil droplets were dislodged from the sample surface during cryo-fracturing). As expected in cryo-SEM images of emulsions, evidence of particle motion upon freezing/sublimation can be seen, e.g., as the accumulation of NPs along frozen water


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crystal grain boundaries (Figure 3b). This particle motion is not ideal, but is essentially unavoidable. For this reason, we suggest that the LSCM results are indeed a better indicator of particle location within the prepared emulsions, despite the lower resolution of LSCM images. Regardless, it is clear that a mechanism other than affinity for the interface underlies the stabilizing influence of the bare-silica NPs. Investigating real-time dynamics of the NPs at the oil-water interface in emulsions with bare and HMDS-modified silica NPs further highlights the differences in interfacial localization of the NPs. The images shown in Figure 4 are snapshots from high-frame rate 2D LSCM videos acquired of the samples (See SI Appendix for video files). In samples containing bare-silica NPs (Figure 4a) particles are seen to exhibit Brownian motion in the water phase with only random collisions with the oil-water interface. HMDS-modified NPs, in contrast, are seen to adsorb to the oil-water interface (Figure 4b); the Brownian motion of the pinned particles is restricted to motion along the oil-water interface while excess particles exhibit Brownian motion in the water phase comparable to that of the bare-silica NPs. The presence of the excess particles also suggests that a critical interfacial concentration has been reached, such that there is no room for further particle adsorption to the interface and so the oil-water interface does not attract the excess NPs. Differences in the motion of the NPs can also be realized by tracking their motion over time (Figure 4c), demonstrating that the non-adsorbed NPs exhibit greater fluctuation in position in accordance with their unrestricted dispersion in the coherent phase. The drastic differences in observed motion about the interface for the two different silica NPs emphasizes the variances between the traditional Pickering-type particle enhanced stability achieved with HMDS-silica NPs, and the non-Pickering type particle enhanced stability achieved with baresilica NPs.


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Langmuir

A large number of factors may influence the stability of an oil in water emulsion, including interfacial tension between the droplet and coherent phases, electrostatic effects, mechanical barriers at the droplet interfaces, depletion forces and viscosity of the phases.5, 31-34 A series of experiments, in addition to the experiments previously discussed, and an evaluation of previous literature was sought in order to better understand the phenomena behind enhanced stability of the oil-in-water emulsions with the non-adsorbed (to the oil-water interface) NPs. Firstly, we confirmed that the viscosity of the NP containing solutions is not increased in comparison to pure deionized water (see experimental section for details). Therefore, viscosity differences do not impact our system. In 2008, Dai et al. reported on the ability of unmodified hydrophilic silica NPs (negative surface charge) to decrease interfacial tension of an anionic surfactant.35 To probe whether a similar effect occurs here, interfacial tension (IFT) between dodecane and the series of studied water phases were measured (Figure S5). The water phases containing both 1mM of surfactant and either 0.33 wt% of bare-silica NPs or HMDS-silica NPs showed only a very small reduction in IFT (

Stabilization of Oil-in-Water Emulsions with ... - ACS Publications

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