Influence of pH and Salt Concentration on Pickering Emulsions

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts. Thriveni G Anjali, and Madivala G Basavaraj Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02913 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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Influence of pH and Salt Concentration on Pickering Emulsions Stabilized by Colloidal Peanuts. Thriveni G. Anjali and Madivala G. Basavaraj* Polymer Engineering and Colloid Science (PECS) Laboratory, Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai - 600 036, India *Email: [email protected] Keywords: Particle stabilized emulsions, shape anisotropic colloids, interfacial adsorption, image charge repulsion, particle wettability

ABSTRACT

Solid stabilized emulsions commonly known as Pickering emulsions offer unique benefits such as superior stability and controlled permeability compared to conventional surfactant stabilized emulsions. In this article, the effect of pH, electrolyte and particle concentration, homogenization speed and volume fraction of oil on the formation, stability and the microstructure of emulsion droplets stabilized by micron size peanut shaped hematite particles are investigated. The influence of surface charge of particles

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on emulsification is studied by varying the pH of the dispersing medium, the addition of an electrolyte or a combination of both. Stable O/W emulsions are formed only when the aqueous dispersions at intermediate pH between 4 and 11 and decane (2:1 volume ratio) are vigorously mixed. However, emulsions are not formed when the particles are highly charged i.e., at pH 2 and 12. The presence of monovalent salt or high-speed homogenization assists the emulsion formation at pH 3 whereas their combination helps in emulsification at pH 2. However, neither the addition of an electrolyte nor the high-speed homogenization or their combination facilitates the formation of emulsions at pH 12. We show that the image charge repulsion and the surface charge induced wettability change can explain the influence of both pH and salt concentrations on the formation of Pickering emulsions. While oil-in-water emulsions typically cream due to the density difference, microscopy observations revealed the presence of a large number of small particle-covered oil droplets in the sediment of the emulsified samples. These drops are observed to be entrapped in dense particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets and thus influences their size and surface coverage. The possibility of tailoring the stability, droplet size as well as the surface coverage discussed in this article can play a crucial role in situations that demand controlled release of active components.

Introduction 2 ACS Paragon Plus Environment

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Emulsions are metastable dispersions of two immiscible liquids that consist of drops of one liquid in a continuous medium of another liquid. Typically most emulsion formulations use a surface active species that aid in emulsification, examples includes surfactants, polymers or solid particles. The adsorption of such species on the surface of drops prevents the macroscopic phase separation of the two immiscible liquids. Depending on the dispersed and the continuous phase, emulsions are of either oil in water (O/W) or water in oil (W/O) type. The spontaneous attachment of colloidal particles to fluid-fluid interfaces encountered in emulsions was first reported by Ramsden1 in 1904 and the technological applicability of particles as emulsion stabilizers was later pointed out by Pickering in 1907.2 The colloidal particles can either solely act as effective emulsion stabilizers3-5 or can be added to reduce the amount of surfactants required.6 Solid stabilized emulsions also called as “Pickering” emulsions have broad applications in a number of fields such as – food, pharmaceutical, cosmetic, paint, petroleum and mineral processing industries.7-11 Pickering emulsions can also be used as a template for the preparation of porous materials and hollow capsules that are commonly known as colloidosomes.12-15 Compared to conventional surfactant stabilized emulsions, Pickering emulsions exhibit enhanced stability, low toxicity (when biocompatible and edible particles are used) and tunable permeability.16-18 In recent years, solid stabilized emulsions with stimuli-responsive behavior have received considerable attention. The controlled destabilization of emulsions can be achieved by


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using external triggers such as pH, magnetic field, temperature, ionic strength, light intensity, CO2 concentration etc. or their combinations.18-24 The use of any one of these external triggers offers the possibility to impart surface activity to particles, thereby transforming them into effective emulsifiers or facilitates their detachment from the interface on demand. The pH responsive Pickering emulsions are one of the simplest and the most widely used stimuli responsive systems.23, 25-29 The remarkable stability of Pickering emulsions is due to the irreversible adsorption of micron sized colloidal particles at fluid-fluid interfaces with adsorption energies in the order of several thousands of kBT.16 Though the surface activity of colloidal particles is recognized over hundred years ago,1-2 extensive fundamental investigations on various aspects of particle stabilized emulsions have progressed only in the last few decades.3, 30Analogous

to hydrophile-lipophile balance (HLB) in surfactant systems, particle

wettability, characterized by the three phase contact angle 𝜃 is the key parameter that dictates the formation, type and the stability of solid stabilized emulsions.16 Particles adsorbed to the interface at contact angles 𝜃 < 90° form O/W emulsions and at contact angles 𝜃 > 90° stabilize W/O emulsions.31-32 In general, when particles are used for emulsification, the more wetting liquid becomes the continuous phase and other, the dispersed phase.32 For effective emulsion stabilization, particles that are partially wetted by both the fluids are desirable i.e., 20° < 𝜃 < 160°. The other parameters that influence the formation and stability of Pickering emulsions are particle size, shape, particle


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concentration, dispersion pH, electrolyte concentration and volume fractions of the immiscible liquids.3, 18, 33-34 In recent years, non-spherical particles from different sources have been shown to be highly effective in the formation of emulsions and foams that exhibit extraordinary stability.35-39 The viscoelastic nature of non-spherical particle networks formed due to shape induced capillary interactions imparts exceptional stability to emulsions. While there are several studies on the stabilization of emulsions using anisotropic particles of different shapes,6,

37-38, 40-50,

studies on the combination of shape and role of other

factors such as the influence of particle surface charge are limited.25, 47 The possibility to synthesize hematite particles of different sizes and shapes and the ability to tune their surface charge density by dispersing in aqueous phase at different pH make them an excellent model system to explore the role of both the particle shape and surface charge on the emulsion formation and stability. The influence of particle surface charge on the interfacial adsorption of hematite ellipsoids and the buckling instability in the resulting particle laden interface has recently been studied.47 The interfacial adsorption of highly charged particles is hindered by the presence of an image charge in the oil phase51, which has the same sign and nearly the same magnitude as that of the real charge. Similar to electrical double layer interactions, the charge-image interactions can also be screened by the addition of electrolytes to enable the adsorption of particles to the interface and can lead to the formation stable emulsions.3, 31, 51


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In the present study, the influence of the dispersion pH, the electrolyte concentration, particle concentration, homogenization speed and volume fraction of oil on the physical and microstructural aspects of Pickering emulsions stabilized by monodispersed micron sized peanut shaped hematite particles are investigated. The use of micron-sized particles facilitates the direct visualization of particles and the microstructural changes of droplets in the emulsified samples. The effect of surface charge is investigated by dispersing the hematite particles in aqueous solutions of different pH range from 2 to 12. At highly acidic and basic pH, the surface charge on the particles is high and so is the magnitude of the image charge and therefore no stable emulsions are formed under these conditions. However, when particles are weakly charged, they stabilize O/W emulsions. Further, the addition of an electrolyte to screen the repulsive image charge interactions is observed to facilitate the emulsion formation at pH between 3 and 11. The peanut shaped hematite particles stabilize O/W emulsions in the pH range from 4 to 11 in the absence of sodium chloride (NaCl). But in the case of pH 12, neither the high electrolyte concentration nor the high speed homogenization (20,000 rpm) could facilitate the formation of stable emulsions, whereas their combination helps the emulsion formation at pH 2. However, the presence of an electrolyte alone can assist the stabilization of O/W emulsions at pH 3. We show that the image charge repulsion and the modification of the wettability of particles with change in pH can explain the influence of pH and salt concentrations on the formation of Pickering emulsions. At pH


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12, particles being completely hydrophilic, most of them remain in the aqueous phase and therefore no emulsions are formed even when the image charge repulsion is completely screened by the addition of an electrolyte. Along with the presence of oil-inwater emulsion droplets that cream due to the density difference, an unusual presence of a large number of small particle stabilized oil drops in the sediment is observed. The microscopy observations reveal that these particle-covered oil droplets are entrapped in dense particle networks. This leads to a considerable reduction in the number of particles available for the stabilization of floating emulsion droplets such that their size, surface coverage and stability are significantly affected. EXPERIMENTAL SECTION Materials Iron (III) chloride hexahydrate (FeCl3.6H2O) from Sigma Aldrich, sodium hydroxide (NaOH) and sodium sulphate (Na2SO4) from Merck, Germany were used as reagents for the synthesis of hematite particles. n-Decane (99%, AR grade, Spectrochem Mumbai, India) without further purification was used as the oil phase in all emulsification experiments. Nitric acid (HNO3- 65%, Merck, Germany) and NaOH were used to adjust the pH of aqueous dispersions. Polydimethylsiloxane (PDMS) elastomer (Sylgard-184, Dow Corning) was employed to investigate the effect of pH on the equilibrium position of particles at interfaces. The deionised water from a Milli-Q system (Millipore) of 18.2 MΩcm resistivity was used in all experiments.


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Synthesis of peanut shaped particles: Peanut shaped hematite particles were synthesized particles were prepared by a gel-sol based synthesis procedure.52-53 In a typical synthesis, 90 ml of 5.4 M NaOH solution was added to 100 ml of 2 M FeCl3 solution under vigorous magnetic stirring in about 5 minutes, followed by the addition of 10 ml of 0.6 M Na2SO4 solution. Thus obtained condensed ferric hydroxide gel in a clean 250 ml Pyrex bottle was tightly closed and aged in a preheated oven at 100 °C for 8 days. After aging, the sample was cooled to room temperature and the hematite particles in the resulting suspension were separated by repeated centrifugation and washing with MilliQ water. Finally the peanut shaped particles in the sediment were redispersed in MilliQ water and stored. The particles were characterized using highresolution scanning electron microscope (HR-SEM, Hitachi S-4800, Japan) and transmission electron microscope (TEM, Tecnai 12, Philips, operating at 200 kV). The size analysis of particles was carried out using ImageJ.54 Emulsion preparation All emulsions were prepared by considering aqueous hematite particle dispersions and decane in 2:1 volume ratio. The particle concentration in the aqueous phases was varied from 0.25 to 7 wt % and the pH of the aqueous phase from 2 to 12. All aqueous particle dispersions were sonicated for 10 min prior to the emulsification process. Emulsions were obtained through 2 min of vigorous manual mixing. The emulsification experiments were also carried out in the presence of a monovalent electrolyte at


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different pH. To this end, sodium chloride (NaCl) at concentrations range from 50 mM to 2 M was used. To study the effect of oil to water volume ratio, the volume fraction of decane used for emulsifications were varied from 0.1 to 0.9 at 1 wt % fixed particle concentration. The effect of speed of mixing on the emulsion formation was investigated by homogenizing (IKA T25 Digital ULTRA Turax) 1 wt % aqueous dispersion (at pH 6.5) and decane (2:1 volume ratio) at speeds range from 2500 rpm to 20,000 rpm for a fixed duration of 3 min. The influence of pH on emulsification was studied by varying the pH of the dispersion from 2 to 12 at a fixed speed of 20,000 rpm. The possibility of exploiting the combined effect of electrolyte (50 mM to 2 M) and high energy input was also investigated at pH 2, 3 and 12. After the emulsification process, all the emulsified samples were allowed to equilibrate for 24 hours and further analyzed using an optical microscope. Emulsion characterization The emulsion type was identified to be of oil-in-water (O/W) type from the creaming of particle stabilized droplets, and was further confirmed by the drop test. When emulsion droplets were added to each of the pure liquid phases (water and decane), the particle-covered droplets disperse and float on water and confirmed it as the continuous phase. The emulsion phase was further characterized by optical microscopy (Inverted Microscope, DMI3000B, Leica Microsystems, Germany). The self-assembly of particles at the drop surface, surface coverage of particles on their surface and the percolating


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particle networks that connect the droplets were imaged at using 5X, 10X, 20X, 63X and 100X OE objectives. Modified gel trapping technique The equilibrium position of particles as a function of the aqueous dispersion pH was investigated by using the modified ‘gel trapping technique’ (GTT).28,

55

These

experiments also revealed the effect of dispersion pH on the number of particles trapped at the interface for a given spreading volume. In this method, the air-water interface was created by using aqueous dispersions at different pH (2 to 12). The particles dispersed in corresponding pH solutions were spread at the interface. All experimental conditions except pH were maintained the same. The air above the particle laden interface was replaced with a thin layer of PDMS elastomer mixed with curing agent in the 10:1 mass ratio. The mixture was poured carefully through the sides of the container such that it spread uniformly above the aqueous phase with minimal disturbance to the particle monolayer. The samples were kept undisturbed for the next 48 hour at room temperature for the elastomer to cure and the soldified PDMS was peeled off, washed and dried for further characterization. The equlibrium position of particles at different dispersion pH was direcly visualized using HR-SEM at different observation angles. Results and discussion Characterization of hematite particles


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The electron microscopy images of hematite particles, their size distribution and the zeta potential as a function of dispersion pH are presented in Figure 1. The average length and the average lobe diameter of peanut shaped particles measured from SEM and TEM images respectively are 1.89±0.04 μm and 0.73±0.03μm. The surface charge of these particles can be tuned by dispersing them in aqueous solutions of different pH. The electrophoretic dynamic light scattering (SZ-100 Nanopartica, Horiba, Japan) technique is used to measure the zeta potential of hematite particles dispersed in aqueous solutions containing 10 mM NaCl concentration at different pH. The particles are positively charged at acidic pH and negatively charged at basic pH with an isoelectric point close to pH 7.6 Effect of aqueous dispersion pH on the formation of emulsions The hematite particles are partially hydrophilic53 and are reported to stabilize O/W emulsions.29,

48

The influence of pH on the formation and stability of emulsions is

studied by dispersing peanut shaped particles in aqueous solutions of different pH range from 2 to 12. The digital photograph of vials about 1 week after the emulsification is shown in Figure 2. The particles are observed to form stable emulsion droplets at pH range from 4 to 11, relatively large droplets of limited stability at pH 3 and no emulsions at pH 2 and 12. The yellow color of the aqueous phase of the emulsified sample at pH 2 is due to the initial dissolution of hematite particles in HNO3. The intensity of the yellow color did not change with time indicating that the dissolution occurs only initially. By optical microscopy, it is confirmed that the particle size and shape are not affected by this initial dissolution. 11 ACS Paragon Plus Environment

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Interestingly, the walls of the glass vials above the emulsion phase appears red because of the film-climbing effect56-58 i.e., formation of a particle laden film along the walls of glass vials due to the coalescence of unstable particle stabilized drops. While the film climbing is also observed at pH 2, due to the coalescence of particle stabilized emulsions that completely destabilized soon after the formation, no such effect is observed at pH 12. This indicates that at pH 12, the particles are not surface active. When the particles are highly charged i.e., at pH 2 and 12, their adsorption to the interface is hindered or suppressed by the repulsion experienced due to an image charge51 located in the oil phase whose magnitude is proportional to the surface charge of the particle. The higher the surface charge density, the greater the strength of image charge repulsion. We have estimated the overall interaction experienced by a peanut shaped particle as it approaches an oil-water interface (supporting information S1). The overall interaction is the sum of the DLVO interaction between the particle and the interface (sum of van der Waals and electrical double layer interactions) and that between the particle and the image charge, Uoverall = UvdW+UEDL+ Uimage. In these calculations, we treat peanut shaped particles as low aspect ratio ellipsoids and the particles are assumed to be oriented with their long-axis parallel to the interface. The zeta potential values employed in interaction calculations are listed in the supporting information (Table S1 in). The overall interactions calculated at pH 2, 6.5 and 12 are shown in Figure S1. At pH 6.5, the overall interactions, Uoverall is attractive indicating that the particle adsorption to the interafce is favoured, which is inline with the formation of particle stabilzed emulsions at this 12 ACS Paragon Plus Environment

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pH. Eventhough the high surface charge of particles at both pH 2 and pH 12 are comparable, the repulsion experienced by particles dispersed in pH 12 sufficiently high to prevent its adsorption to the interface. The positively charged particles dispersed in pH 2 experience repulsion from the image charge, but its interaction with the negatively charged interface is attractive.47,

51, 59

. The energy supplied during the process of

emulsification can overcome this repulsive energy barrier and promote the adsorption of particles to interfaces . Therefore, particle stabilized emulsions are expected to form during initial stages of emulsification. However, the concentration of particles at interface is insufficient to arrest the droplet colescence, which will be discussed later. The coalescence of partially covered drops eventually leads to film climbing effect. But in the case of pH 12, the particle charge-image charge and particle charge-interface charge interactions are both repulsive and the adsorption of particles to the interface is prevented. 47 Even the high speed homogenization is insufficient to overcome the very high energy barrier that the particles experience at pH 12 and therefore not even the film climbing is observed. The influence of pH on the arrangement of particles around the droplet surface is further characterized by the optical microscopy technique. Microstructure of particle stabilized emulsion droplets The optical microscopy images of the emulsion droplets and the interfacial assembly of particles on the drop surface at different pH are shown in Figure 3 and Figure S2 (Supporting information). As shown in Figure 3, the droplet surface is covered by a dense layer of particles with interlocked long particle chains. The surface coverage is decreased marginally in the case of emulsions stabilized by particles of high surface


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charges i.e., on either side of pH 6.5. In these emulsions, the area occupied by particles on the drop surface is greater than 90% and the close packing of particles imparts a steric barrier for the coalescence of drops.60 The microstructural arrangement of the peanut shaped particles on the drop surface shown in Figure 3 (d), (e) and (f) reveals that the particles are oriented with their major axis parallel to the interface and majority of the particles assemble in the side-by-side manner along with some particles arranged in the tip-to-tip configuration. The emulsion droplets formed at different pH (except pH 8) are observed to be stable for more than a year proves their long term stability due to the irreversible adsorption of the particles to the interface. The partial hydrophilic nature, the high detachment energy due to the micron size and the shape induced capillary attraction assisted interfacial assembly, impart high stability to the emulsion droplets. The digital photographs of glass vials containing the particle stabilized emulsions at different pH, taken 10 month after their preparation are given in Figure S3. The complete destabilization of emulsions formed in the case of pH 3 indicates the low stability of emulsions when particles are highly charged. In the case of pH 8, which is close to the isoelectric point, the fraction of emulsion phase reduced significantly after 10 month of storage and the final emulsion is observed to contain few drops as shown in Figure S3. This can be attributed to the low surface coverage of particles on the drop surface (Figure S2c) due to near neutral wetting of particles by both the fluids at this pH.


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That is, the particle position may be in the neighborhood of the region where the emulsions are unstable.28 Influence of dispersion pH on the interfacial adsorption and the wettability of particles In addition to the charge effects discussed so far, the variations in the pH of aqueous dispersions can also influence the particle wettability, which in turn can affect the ability of particles to stabilize emulsions.23-24 To this end, the influence of dispersion pH on the wettability of particles and their interfacial adsorption is investigated via the modified gel trapping technique.28, 55 The SEM images of particles trapped in PDMS are given in Figure 4. It is evident that under highly acidic and basic conditions, relatively very few particles adsorb at the interface (Figure 4a and 4c), compared to pH 6.5 (Figure 4b) irrespective of identical experimental conditions. The interfacial adsorption of hematite particles dispersed in pH 6.5 is evident in the electron microscopy images and it agrees well with previous

investigation

of

similar

phenomena

studied

by

dynamic

surface

tension

measurements.47 Thus the adsorption of particles to the interface is significantly affected by the repulsive interactions when the particles are highly charged and therefore do not participate in emulsion stabilization. The images in Figure 4d, e and f show that the interfacial position of particles is also affected by the dispersion pH. In these images, the portion of the particle that is immersed in the aqueous phase is visible. The particles become more hydrophilic with an increase in the particle surface charge, i.e., at pH 2 and 12 the particle area exposed to the water phase (Figure 4


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d and f) increases compared to that at pH 6.5 (Figure 4e). Though the wettability studies are conducted at air-water interface, similar trend can be expected in the case of oil-water interface. We now proceed to investigate the dense sediment, a feature commonly observed at the bottom of all the vials following emulsification. Particle-covered oil droplets in the sediments It is well accepted that the excess particles that do not contribute to emulsification are expected to settle to the bottom and this phenomena has been observed earlier.45,

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This is true in spite of the fact that particles used for emulsification are of uniform surface chemistry. When the sediments in the emulsified samples are observed under optical microscopy, to our surprise, the presence of small particle-covered droplets is observed. The drop test confirmed that the sediment consists of oil drops. Figure 5a and b respectively show the optical microscopy images of the droplets in the sediment formed in emulsions prepared by manual mixing and homogenization. Similar to floating particle stabilized emulsions, these drops in the sediment are observed to exist even after 12 month of preparation. No such tiny particle-covered droplets are observed in the sediment formed in samples that do not emulsify i.e., at pH 2, 3 and 12. This indicates that the formation of drops in the sediment is also dictated by the particle surface charge. The presence of particle-covered droplets and the particle networks is the principal reason for the decrease in the surface coverage of the floating droplets. These droplets, both in the emulsion phase and in the sediment, are typically surrounded by


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networks of particles. The drop are stabilized both due to monolayer of particles at the oil-water interface and also due to layer (s) of particles attached to the monolayer as shown in Figure 5c. The images captured demonstrate the presence of more than one layer of particle assemblies on the surface of droplets, with the top layer extending to surrounding particle networks that connect different droplets. The reason for the formation of particle network around the droplets is probably due to magnetic dipolar interactions. The hematite particles are known to assemble in water in response to earth’s magnetic field.61,62 The number of tiny particle-covered droplets in the sediment is observed to increase with an increase in the particle concentration, the electrolyte concentration and the homogenization speed. Effect of high speed homogenization The effect of mixing speed on the formation and stability of particle stabilized emulsions is investigated by homogenizing the oil and the particle dispersed aqueous phases at varying speeds from 2500 rpm to 20000 rpm for 3 min. The aqueous hematite dispersions containing 1 wt % of particles at pH 6.5 and decane in 2:1 volume ratio are used in the preparation of emulsions. The high speed homogenization imparts kinetic energy to the particles and is expected to promote the adsorption of particles at the oilwater interface. A homogenization speed less than 10,000 rpm is observed to be not sufficient for producing stable emulsions. At lower homogenization speeds, the emulsions formed are observed to sparsely covered with particles than the corresponding samples prepared by vigorous manual shaking. The digital photographs of the emulsified samples are given in Figure S4 (Supporting Information) 17 ACS Paragon Plus Environment

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We further investigated the application of high speed homogenization on the emulsification of dispersion with 1 wt % particles at different pH from 2 to 12 and decane in 2:1 volume ratio. The emulsification is carried out at 20,000 rpm for a duration of 3 min. The emulsions are observed to form in the pH range from 3 to 11 as shown in the supporting information (Figure S5). The particle dispersions at either pH 2 or pH 12 are found to be unsuitable for emulsion stabilization due to the high surface charge of particles. Whereas at pH 3, the particles form emulsions at 20,000 rpm, however, exhibit limited stability up to few days after the preparation. The short term stability of the particles is due to the low surface coverage resulting from the poor interfacial adsorption of particles. As shown in Figure 6, the surface coverage of droplets formed by high speed homogenization at pH 4 to 11 is observed to be significantly less compared to the corresponding emulsions prepared by manual shaking. The low surface coverage of droplets affects the long term stability of emulsions prepared at different pH as shown in Figure S5 (supporting information). The homogenized sample at pH 6.5 alone is stable for more than ten month, whereas the emulsions prepared by manual shaking at several intermediate pH exhibit long term stability. Plausible reasons for the limited stability of emulsions prepared by homogenization than that formed by manual mixing are: 1) the high speed homogenization produces much smaller drops and therefore the total interfacial area created during emulsification is large. For the same surface coverage, a higher particle concentration is required for stabilization of interfaces. 2) the microscopy observation of the sediment shows the presence of larger number of smaller particle stabilized drops in the 18 ACS Paragon Plus Environment

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homogenized samples (Figure 5b) compared to manually shaken samples (Figure 5a). Therefore, the availability of lesser number particles for the stabilization of floating drops leads to partially covered drops in homogenized samples. Effect of electrolyte concentration The effect of electrolyte concentration on the formation of emulsions is studied by considering aqueous dispersions at different pH range from 2 to 12 which contain 50 mM to 2 M NaCl. A particle concentration of 1 wt % and 2:1 water to oil ratio are maintained in all the experiments. Figure 7 is the state diagram of the results of emulsification process showing the possibility of the formation and the nature of emulsions prepared at different electrolyte concentrations and aqueous dispersion pH. Oil-in-water emulsions are produced at pH range from 3 to 11 for the salt concentrations considered, whereas in pH 2 and 12, no emulsions formed even in the presence of 2M NaCl. Film climbing is observed in the case of pH 2 and in pH 12 at salt concentrations of 0.5 M and above. The occurrence of film climbing points to the favorable interfacial adsorption of particles, but the particle concentration is insufficient to stabilize the emulsion droplets. At pH 12, the hematite particles are reported to be completely hydrophilic owing to their surface hydroxylation and therefore, they prefer to remain in the aqueous phase.63 In the case of pH 3, the presence of salt facilitates the formation of emulsions. At moderate salt concentrations, the electrolyte screens the surface charge of particles and promotes their adsorption to the interface and assists in emulsion stabilization. But at high salt concentrations, the particles in the dispersion 19 ACS Paragon Plus Environment

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flocculate and hence less number of particles is available for the emulsion stabilization and results in the formation of sparsely covered large droplets. This behavior is in accordance with the previous report that the weakly flocculating particles enhance the emulsion stability, whereas highly flocculating particles cause the destabilization of emulsions.64 Microscopy images of emulsion droplets formed and the particle surface coverage at pH 3 and at different electrolyte concentrations are given in Figure 8. In the absence of an electrolyte, emulsion droplets with limited stability are formed, and only small droplets with very low surface coverage (Figure 8a and d) are retained in the emulsified samples. At low salt concentrations, the surface coverage of particles on the drop surface is observed to increase, followed by a significant reduction at a higher salt concentration of 2M NaCl as evident in the microscopy image of the drop surface (Figure 8b, c and e, f). While the particles adsorbed on the drops impart stability to the emulsion due to arrested coalescence at low NaCl concentration, the drops that are sparsely covered at higher NaCl concentration are stabilized through the aggregated network of particles in the bulk. The dark regions in the low magnification microscopy images are the flocculated particle networks that exist in the continuous phase of the emulsified samples. Similar trend is observed at pH range from 4 to 11 as shown in Figure S6-S10 in the supporting information. The concentration of particle-covered droplets in the sediments is observed to increase with an increase in NaCl concentration.


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This leads to a significant reduction in the droplet surface coverage especially at high salt concentrations, yet, the emulsions are observed to exhibit long term stability. Effect of particle concentration Oil-in-water emulsions are prepared by vigorous manual shaking of the aqueous dispersions of varying particle concentrations at pH 6.5 and decane in 2:1 volume ratio. The particle concentration is varied from 0.25 wt % to 7 wt %. The digital photographs of the glass vials shown in Figure 9 display the emulsions formed with an increase in the particle concentration. The droplet size decreases with an increase in the particles concentration up to 2 wt % as expected. With further increase in the particle concentration, an increase in both the droplet size and the polydispersity are observed. Moreover, the absence of excess oil in these cases indicates that the entire volume of oil is transformed into particle-covered droplets and the fraction of emulsion phase formed is found to be almost same at all particle concentrations. The fraction of emulsion phase remains unchanged over months indicates the long term stability of emulsion droplets. . The optical microscopy images in Figures 10, S11 and S12 display the micro-structure of the particle stabilized decane drops and demonstrate an increase in the surface coverage of droplets up to 2 wt% and then a decrease in the surface coverage with an increase in the particle concentration. At high particle concentration (>3 wt %), emulsions formed are characterized by an increase in the droplet size, a decrease in the droplet surface coverage and the presence of particle networks in the continuous phase.


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This fact indicates that the particles prefer to remain in the continuous phase rather than being trapped at the decane-water interface and therefore form particle aggregates that impart additional stability to the droplets. It must be noted that even at very low particle concentration (0.25 wt %), there is significant fraction of particles at the bottom of the vial. The optical microscopy images show the presence of a number of small particlecovered droplets in the sediments formed in emulsified samples and their number density increases with an increase in particle concentration. Effect of volume fraction of oil The volume fraction of oil used for emulsification is varied from 0.1 to 0.9 by keeping the particle concentration and the pH fixed at 1 wt % and 6.5 respectively. Figure S13 shows the emulsions that are formed by the vigorous manual shaking of the particle dispersed aqueous phase and the decane phase. The microstructure of emulsified droplets formed at different oil volume fractions is shown in Figure S14-15 in the supporting information. An increase in both the average size of droplets and the fraction of emulsion phase is observed up to an oil volume fraction of 0.5. As the volume fraction of the oil phase is further increased to 0.6 and 0.7 sparsely covered bigger droplets form due to insufficient number of particles available for emulsification and no emulsion is formed at 0.8 and 0.9 oil volume fractions. Therefore, the emulsions formed by considering water, decane and hematite particles do not exhibit a


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catastrophic phase inversion. The average droplet size decreases and the concentration of excess particles settled increases with decrease in the oil volume fraction. CONCLUSIONS The effect of various parameters such as particle surface charge, concentration, homogenization speed, volume fraction of oil and electrolyte concentration on the formation, stability and microstructure of decane/water emulsions stabilized by peanut shaped hematite particles are investigated. The surface charge of hematite particles tuned by dispersing them in aqueous solutions of different pH is observed to affect the emulsion formation. In the case of aqueous dispersions at pH range from 4 to 11, stable emulsions are formed upon vigorous manual mixing, but no emulsions formed at pH 2 and 12. At pH 3, the emulsions formed exhibit limited stability due to low concentration of particles on the drop surface. The presence of an electrolyte is observed to screen the image charge repulsion and facilitate the emulsion formation at intermediate pH. A combination of high speed homogenization and addition of an electrolyte facilitates the formation of emulsion at pH 2. Even the presence of 2M NaCl or the high speed homogenization (at 20,000 rpm) is not sufficient to facilitate the adsorption of particles to the interface and effect emulsification at pH 12, due to the complete hydrophilic nature of hematite particles at that pH. The optical microscopy images of stable emulsion drops reveal that the surface is covered with interlocked chains of peanut shaped particles. The particles in these chains show long multi-particle side-by-side


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stacks. The emulsions showed stability of more than 12 months due to the high detachment energy of particles from the drop surface, the high surface coverage of droplets and the presence of particle networks that connect the droplets. Remarkably, the emulsified samples also showed the presence of large number of small particle stabilized emulsion droplets that are connected by particle chains in the sediments. The fraction of emulsified droplets in the sediment is observed to be affected by dispersion pH, homogenization speed, electrolyte concentration and particle concentration. The possibility to tune the stability, size and the surface coverage of particle stabilized droplets provide an additional handle for modulating release of encapsulated components from such emulsions.

Conflicts of interest There are no conflicts of interest to declare. ASSOCIATED CONTENT Supporting Information The manuscript is accompanied by supporting document with details of – DLVO interaction calculations, and digital/microscopy images showing the effect of pH, electrolyte concentration, homogenization speed, particle concentration and volume fraction of oil on emulsification. This material is available free of charge via the Internet at http://pubs.ac.org. 24 ACS Paragon Plus Environment

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Acknowledgements We acknowledge Venkateshwar Rao Dugyala for the interaction calculations. The authors acknowledge the Department of Science and Technology, India for funding the project (research grant no:SB/S3/CE/053/2015). The use of scanning electron microscope (SEM) facility at the department of chemical engineering (procured through DST FIST grant) and the transmission electron microscope (TEM) facility at the department of Metallurgical and Materials Engineering are also acknowledged. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors contributed equally.


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REFERENCES 1. Ramsden, W., Separation of Solids in the Surface-Layers of Solutions and 'Suspensions'

(Observations

on

Surface-Membranes,

Bubbles,

Emulsions,

and

Mechanical Coagulation). -- Preliminary Account. Proc. Royal Soc. A 1903, 72, 156-164. 2. Pickering, S. U., CXCVI.-Emulsions. J. Chem. Soc. Trans. 1907, 91, 2001-2021. 3. Aveyard, R.; Binks, B. P.; Clint, J. H., Emulsions stabilised solely by colloidal particles. Adv. Colloid Interface Sci. 2003, 100–102, 503-546. 4. Ge, L.; Friberg, S. E.; Guo, R., Recent Studies of Janus Emulsions Prepared by OneStep Vibrational Mixing. Curr. Opin. Colloid Interface Sci. 2016, 25, 58-66. 5. Binks, B. P., Colloidal Particles at a Range of Fluid–Fluid Interfaces. Langmuir 2017, 33, 6947-6963. 6. Pang, M.; Cairns, A. J.; Liu, Y.; Belmabkhout, Y.; Zeng, H. C.; Eddaoudi, M., Synthesis and Integration of Fe-soc-MOF Cubes into Colloidosomes via a Single-Step EmulsionBased Approach. J. Am. Chem. Soc. 2013, 135, 10234-10237. 7. Dickinson, E., Food emulsions and foams: stabilization by particles. Curr. Opin. Colloid Interface Sci. 2010, 15, 40-49.


Page 27 of 45 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8. Frelichowska, J.; Bolzinger, M.-A.; Pelletier, J.; Valour, J.-P.; Chevalier, Y., Topical delivery of lipophilic drugs from o/w Pickering emulsions. Int. J. Pharm. 2009, 371, 5663. 9. Whitby, C. P.; Wanless, E. J., Controlling Pickering emulsion destabilisation: A route to fabricating new materials by phase inversion. Materials 2016, 9, 626. 10. Binks, B. P.; Horozov, T. S., Colloidal particles at liquid interfaces. Cambridge University Press 2006. 11. Chevalier, Y.; Bolzinger, M.-A., Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloids Surf. A: Physicochem. Eng. Asp. 2013, 439, 23-34. 12. Velev, O.; Nagayama, K., Assembly of latex particles by using emulsion droplets. 3. Reverse (water in oil) system. Langmuir 1997, 13, 1856-1859. 13. Velev, O.; Furusawa, K.; Nagayama, K., Assembly of latex particles by using emulsion droplets as templates. 1. Microstructured hollow spheres. Langmuir 1996, 12, 2374-2384. 14. Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A., Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles. Science 2002, 298, 1006-1009.


Langmuir 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 45

15. Bollhorst, T.; Rezwan, K.; Maas, M., Colloidal capsules: nano- and microcapsules with colloidal particle shells. Chem. Soc. Rev. 2017. 16. Binks, B. P., Particles as surfactants—similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21-41. 17. Berton-Carabin, C. C.; Schroën, K., Pickering emulsions for food applications: background, trends, and challenges. Annu. Rev. Food Sci. Technol. 2015, 6, 263-297. 18. Wu, J.; Ma, G. H., Recent Studies of Pickering Emulsions: Particles Make the Difference. Small 2016, 12, 4633-4648. 19. Ngai, T.; Auweter, H.; Behrens, S. H., Environmental responsiveness of microgel particles and particle-stabilized emulsions. Macromolecules 2006, 39, 8171-8177. 20. Ngai, T.; Behrens, S. H.; Auweter, H., Novel emulsions stabilized by pH and temperature sensitive microgels. Chem. Commun. 2005, 3, 331-333. 21. Amalvy, J.; Unali, G.-F.; Li, Y.; Granger-Bevan, S.; Armes, S.; Binks, B.; Rodrigues, J.; Whitby, C. P., Synthesis of sterically stabilized polystyrene latex particles using cationic block copolymers and macromonomers and their application as stimulus-responsive particulate emulsifiers for oil-in-water emulsions. Langmuir 2004, 20, 4345-4354.


Page 29 of 45 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

22. Amalvy, J.; Armes, S.; Binks, B.; Rodrigues, J.; Unali, G., Use of sterically-stabilised polystyrene latex particles as a pH-responsive particulate emulsifier to prepare surfactant-free oil-in-water emulsions. Chem. Commun. 2003, 15, 1826-1827. 23. Tang, J.; Quinlan, P. J.; Tam, K. C., Stimuli-responsive Pickering emulsions: recent advances and potential applications. Soft Matter 2015, 11, 3512-3529. 24. Fujii, S.; Nakamura, Y., Stimuli-Responsive Bubbles and Foams Stabilized with Solid Particles. Langmuir 2017, 33, 7365-7379 . 25. Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R.; Huang, J., Graphene oxide sheets at interfaces. J. Am. Chem. Soc. 2010, 132, 8180-8186. 26. Liu, H.; Wang, C.; Zou, S.; Wei, Z.; Tong, Z., Simple, reversible emulsion system switched by pH on the basis of chitosan without any hydrophobic modification. Langmuir 2012, 28, 11017-11024. 27. Li, J.; Stöver, H. D., Doubly pH-responsive pickering emulsion. Langmuir 2008, 24, 13237-13240. 28. Destribats, M.; Gineste, S.; Laurichesse, E.; Tanner, H.; Leal-Calderon, F.; Héroguez, V.; Schmitt, V., Pickering emulsions: what are the main parameters determining the emulsion type and interfacial properties? Langmuir 2014, 30, 9313-9326.


Langmuir 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 45

29. Anjali, T. G.; Basavaraj, M. G., General destabilization mechanism of pH-responsive Pickering emulsions. Phys. Chem. Chem. Phys. 2017, 19, 30790-30797. 30. Xiao, J.; Li, Y.; Huang, Q., Recent advances on food-grade particles stabilized Pickering emulsions: fabrication, characterization and research trends. Trends Food Sci. Technol. 2016, 55, 48-60. 31. Briggs, T., Emulsions with finely divided solids. J. Ind. Eng. Chem. 1921, 13, 10081010. 32. Finkle, P.; Draper, H. D.; Hildebrand, J. H., The theory of emulsification1. J. Am. Chem. Soc. 1923, 45, 2780-2788. 33. 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. 34. Velankar, S. S., A non-equilibrium state diagram for liquid/fluid/particle mixtures. Soft Matter 2015, 11, 8393-8403. 35. Basavaraj, M.; Fuller, G.; Fransaer, J.; Vermant, J., Packing, flipping, and buckling transitions in compressed monolayers of ellipsoidal latex particles. Langmuir 2006, 22, 6605-6612. 36. Botto, L.; Lewandowski, E. P.; Cavallaro, M.; Stebe, K. J., Capillary interactions between anisotropic particles. Soft Matter 2012, 8, 9957-9971.


Page 31 of 45 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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37. Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D., Foam Superstabilization by Polymer Microrods. Langmuir 2004, 20, 10371-10374. 38. Lou, F.; Ye, L.; Kong, M.; Yang, Q.; Li, G.; Huang, Y., Pickering emulsions stabilized by shape-controlled silica microrods. RSC Adv. 2016, 6, 24195-24202. 39. Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G., Shape anisotropic colloids: synthesis, packing behavior, evaporation driven assembly, and their application in emulsion stabilization. Soft Matter 2013, 9, 6711-6725. 40. Noble, P. F.; Cayre, O. J.; Alargova, R. G.; Velev, O. D.; Paunov, V. N., Fabrication of “Hairy” Colloidosomes with Shells of Polymeric Microrods. J. Am. Chem. Soc. 2004, 126, 8092-8093. 41. Yang, F.; Liu, S.; Xu, J.; Lan, Q.; Wei, F.; Sun, D., Pickering emulsions stabilized solely by layered double hydroxides particles: The effect of salt on emulsion formation and stability. J. Colloid Interface Sci. 2006, 302, 159-169. 42. Yang, F.; Niu, Q.; Lan, Q.; Sun, D., Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles. J. Colloid Interface Sci. 2007, 306, 285-295. 43. Ashby, N.; Binks, B., Pickering emulsions stabilised by Laponite clay particles. Phys. Chem. Chem. Phys. 2000, 2, 5640-5646.


Langmuir 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 45

44. Cervin, N. T.; Johansson, E.; Larsson, P. A.; Wågberg, L., Strong, Water-Durable, and Wet-Resilient Cellulose Nanofibril-Stabilized Foams from Oven Drying. ACS Appl. Mater. Interfaces 2016, 8, 11682-11689. 45. Madivala, B.; Vandebril, S.; Fransaer, J.; Vermant, J., Exploiting particle shape in solid stabilized emulsions. Soft Matter 2009, 5, 1717-1727. 46. Daware, S. V.; Basavaraj, M. G., Emulsions stabilized by silica rods via arrested demixing. Langmuir 2015, 31, 6649-6654. 47. Dugyala, V. R.; Anjali, T. G.; Upendar, S.; Mani, E.; Basavaraj, M. G., Nano ellipsoids at the fluid–fluid interface: effect of surface charge on adsorption, buckling and emulsification. Faraday Discuss. 2016, 186, 419-434. 48. de Folter, J. W.; Hutter, E. M.; Castillo, S. I.; Klop, K. E.; Philipse, A. P.; Kegel, W. K., Particle shape anisotropy in Pickering emulsions: cubes and peanuts. Langmuir 2013, 30, 955-964. 49. Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I., New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals. Langmuir 2011, 27, 7471-7479. 50. Inam, M.; Jones, J. R.; Pérez-Madrigal, M. M.; Arno, M. C.; Dove, A. P.; O’Reilly, R. K., Controlling the Size of Two-Dimensional Polymer Platelets for Water-in-Water Emulsifiers. ACS Cent. Sci. 2018, 4, 63-70.


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51. Wang, H.; Singh, V.; Behrens, S. H., Image charge effects on the formation of pickering emulsions. J. Phys. Chem. Lett. 2012, 3, 2986-2990. 52. Sugimoto, T.; Khan, M. M.; Muramatsu, A., Preparation of monodisperse peanuttype α-Fe2O3 particles from condensed ferric hydroxide gel. Colloids Surf. A: Physicochem. Eng. Asp. 1993, 70, 167-169. 53. Anjali, T. G.; Basavaraj, M. G., Contact angle and detachment energy of shape anisotropic particles at fluid-fluid interfaces. J. Colloid Interface Sci. 2016, 478, 63-71. 54. Schneider, C. A.; Rasband, W. S.; Eliceiri, K. W., NIH Image to ImageJ: 25 years of image analysis. Nat. Methods 2012, 9, 671-675. 55. Paunov, V. N., Novel Method for Determining the Three-Phase Contact Angle of Colloid Particles Adsorbed at Air−Water and Oil−Water Interfaces. Langmuir 2003, 19, 7970-7976. 56. Cheng, H.-L.; Velankar, S. S., Film climbing of particle-laden interfaces. Colloids Surf. A: Physicochem. Eng. Asp. 2008, 315, 275-284. 57. Mayya, K.; Sastry, M., A new technique for the spontaneous growth of colloidal nanoparticle superlattices. Langmuir 1999, 15, 1902-1904.


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58. Binks, B.; Clint, J.; Fletcher, P.; Lees, T.; Taylor, P., Growth of gold nanoparticle films driven by the coalescence of particle-stabilized emulsion drops. Langmuir 2006, 22, 4100-4103. 59. Marinova, K.; Alargova, R.; Denkov, N.; Velev, O.; Petsev, D.; Ivanov, I.; Borwankar, R., Charging of oil-water interfaces due to spontaneous adsorption of hydroxyl ions. Langmuir 1996, 12, 2045-2051. 60. Pawar, A. B.; Caggioni, M.; Ergun, R.; Hartel, R. W.; Spicer, P. T., Arrested coalescence in Pickering emulsions. Soft Matter 2011, 7, 7710-7716. 61. Ozaki, M.; Suzuki, H.; Takahashi, K.; Matijević, E., Reversible ordered agglomeration of hematite particles due to weak magnetic interactions. J. Colloid Interface Sci. 1986, 113, 76-80. 62. Anjali, T. G.; Basavaraj, M. G., Shape-Induced Deformation, Capillary Bridging, and Self-Assembly of Cuboids at the Fluid–Fluid Interface. Langmuir 2017, 33, 791-801. 63. Shrimali, K.; Jin, J.; Hassas, B. V.; Wang, X.; Miller, J. D., The surface state of hematite and its wetting characteristics. J. Colloid Interface Sci. 2016, 477, 16-24. 64. P. Binks, B.; O. Lumsdon, S., Stability of oil-in-water emulsions stabilised by silica particles. Phys. Chem. Chem. Phys. 1999, 1, 3007-3016.


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Figure 1. Peanut shaped hematite particle characterization: (a) Scanning electron microscopy image, (b) particle size (length) distribution, (c) Transmission electron microscopy image, AB is the major axis and CD is the lobe diameter and (d) the Zetapotential measurements at 10 mM NaCl concentration, displaying the particle surface charge variation as a function of the aqueous dispersion pH. Scale bar: 1 μm.


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Figure 2. Digital photographs of the vials displaying the emulsion formation at different aqueous dispersion pH from 2 to 12: Emulsions were prepared by vigorous manual shaking of an aqueous phase containing 1 wt. % of particles and decane in 2:1 volume ratio. The photograph was taken one week after the emulsification.


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Figure 3. Optical microscopy images of particle stabilized emulsions formed at different aqueous dispersion pH: The emulsions were prepared by vigorous manual shaking of aqueous dispersions of 1 wt. % of particles and decane in 2:1 volume ratio. (a-c) images acquired at 5X magnification and (d-f) images captured with an oil immersion 100X objective display the arrangement of particle on the drop surface. Scale bar: 200 μm (ac) and 10 μm (d-e).


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Figure 4. The SEM images show the hematite particles trapped in PDMS obtained after the modified gel trapping technique. The projected particle volume is the portion of the particle immersed in the aqueous phase. Figures (a-c) are the top view of the particle monolayer which illustrate that the fraction of particles adsorbed at the interface is affected by the aqueous dispersion pH. The micrographs in Figure (d-e) are the side view, which elucidate a clear change in the equilibrium position of the particles adsorbed at the interface with change in the aqueous dispersion pH. Scale bar: (a-c) 10 µm and (d-f) 1µm.


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Figure 5. Optical microscopy images of the sediments in emulsified samples (pH 6.5 and 1 wt % particle concentration). (a) Particle stabilized droplets connected to particle networks in emulsions prepared by manual shaking, (b) particle stabilized droplets connected to particle networks in emulsions prepared by high speed homogenization and c) particle network constituting the top layer of the droplets. Scale bar: 10 μm.


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Figure 6. Optical microscopy images of the microstructure of emulsion drops formed via high speed homogenization: The emulsions were prepared by mixing aqueous phase containing 1 wt% of particles and decane in 2:1 volume ratio at different dispersion pH through high speed homogenization (20,000 rpm for 3 minutes): The images were acquired by 100X OE objective. Scale bar: 10 μm.


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Figure 7. State diagram representing the possibility and the nature of emulsion formation at different aqueous dispersion pH and electrolyte concentration.


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Figure 8 Optical microscopy images of the particle stabilized emulsion droplets formed by high speed homogenization at pH 3 and at different electrolyte concentration: (a-c) images acquired at 5X magnification and (d-f) at 100X OE objectives. Scale bar: 200 μm (a-c) and 10 μm (d-e).


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Figure 9. Digital images of the glass vials displaying the effect of particle concentration on the formation of particle stabilized emulsions. All the emulsions are formed by the vigorous manual shaking of aqueous dispersion at pH 6.5 and oil (2:1 volume ratio) and the particle concentration is varied from 0.25 wt % to 7 wt %.


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Figure 10 Optical microscopy images of the particle stabilized emulsions formed by vigorous manual shaking of aqueous dispersions of different particle concentrations and decane in 2:1 volume ratio. (a-c) micrographs acquired at 5X magnification and (d-f) by using 100X OE objective. Scale bar: 200 μm (a-c) and 10 μm (d-e).


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Table of Contents (Graphical Abstract)


Influence of pH and Salt Concentration on Pickering Emulsions

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