Synthesis and Stability of Water-in-Oil Emulsion Using Partially

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Synthesis and stability of water-in-oil emulsion using partially reduced graphene oxide as a tailored surfactant Tanesh D. Gamot, Arup Ranjan Bhattacharyya, Tam Sridhar, Fiona Beach, Rico F. Tabor, and Mainak Majumder Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02320 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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Synthesis and stability of water-in-oil emulsion using partially reduced graphene oxide as a tailored surfactant Tanesh D. Gamot†‡ǁ, Arup Ranjan Bhattacharyya‡*, Tam Sridhar§, Fiona Beach¶, Rico F. Tabor┴ Mainak Majumder*ǁ †IITB-Monash Research Academy, Indian Institute of Technology Bombay, Main Gate Road, Powai, Mumbai 400076, India ‡Department of Metallurgical engineering and Materials Science, Indian Institute of Technology Bombay, Main Gate Road, Powai, Mumbai 400076, India ǁNanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia §Department of Chemical Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia ┴School of Chemistry, Monash University, Wellington Road, Clayton, VIC 3800, Australia ¶Orica Mining Services, George Booth Drive, NSW 2327, Australia *E-mail: [email protected], [email protected]

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ABSTRACT Graphene oxide (GO) is widely known as an amphiphile having hydrophilic oxygen functionality and unoxidized graphitic patches as the hydrophobic domains. Exploiting this amphiphilicity, GO serves as a surfactant to stabilize oil-water interfaces. While there are numerous reports on GO as a surfactant, most of these reports concern oil-in-water (O/W) emulsions, and there are very few on the formation of water-in-oil (W/O) emulsions. We prepared W/O emulsions using partially reduced graphene oxide (prGO) as a surfactant. The partial reduction introduces a subtle hydrophilic-lipophilic balance (HLB), which favors the formation of the W/O emulsion. The morphological features and rheological characteristics of the W/O emulsion with 75:25 water-to-oil ratio were investigated and analyzed in detail. The W/O emulsion was found to have polydispersity with wide range of droplet sizes varying between 2 µm to 500 µm. Using confocal microscopy, the role of parameters such as extent of reduction, continuous phase volume fraction and the concentration of GO on the stability, microstructure and variation of droplet size distribution of the W/O emulsion were carefully monitored. With prGO concentration as large as 0.05% (w/w), highly concentrated emulsion will form, and are stable up to 20 days from formation; destabilization occurred from sedimentation and subsequent coalescence as the partially reduced GO was limited by its dispersion ability in the oil-phase studied here. Understanding the mechanisms behind the transient stability will enable the development of novel emulsion compositions containing GO as a multifunctional additive.

KEYWORDS Graphene oxide, wetting, water-in-oil, emulsion, stability, cross-over, storage modulus, loss modulus, destabilization, settling, coalescence.

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INTRODUCTION An emulsion is a type of a colloid which forms when two immiscible liquids are mixed and the dispersed liquid exists as droplets in the continuous liquid of different composition, stabilized by a surfactant or an emulsifier1,2. As well as simple classifications based on which is the continuous and dispersed phases (i.e. oil-in-water vs water-in-oil), emulsions can be further classified based on volume fraction of internal phase: dilute, concentrated and highly concentrated. Highly concentrated emulsions are of particular interest as the high volume fraction of the dispersed phase changes the overall properties of the emulsion. For example, an emulsion with dispersed phase volume fraction >0.74 (above the critical close packing fraction for spheres) is generally highly viscous with deformed polyhedral (polygonal) droplets3. An emulsifier is a surfactant that adsorbs to the surface of emulsion droplets; it reduces the interfacial tension by forming a protective coating around the droplets during emulsification. This prevents the rapid phase separation of emulsion droplets, ultimately protecting them from aggregation and coalescence1. An emulsifier may determine the type of emulsion formed based on the oriented-wedge theory: when an emulsifying agent is preferentially wetted by one of the phases, more of the agent can be accommodated at the interface; if that interface is convex towards that phase, then that phase forms the continuous phase. This theory applies to both solids and surfactants as emulsifying agents, while, in case of fine, solid particles at the liquid/liquid interface, the liquid which preferentially wets the solid particles will tend to form the continuous phase. In case of surfactant molecules, this thumb rule is referred to as Bancroft’s rule and can be restated as: the liquid in which the surfactant is most soluble becomes the continuous phase4. Pickering emulsions are solid-stabilized emulsions where solid particles minimize the interfacial energies of two immiscible liquids by adsorbing at the interface. Depending on the particle

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surface chemistry, the emulsion can be oil-in-water (O/W) or water-in-oil (W/O). This is sometimes characterized (usually for molecular surfactants) by the hydrophilic to lipophilic (or hydrophobic) balance, abbreviated as HLB5. Historically, W/O emulsions are an important class of emulsion6. For example, W/O emulsions are the basic ingredient in emulsion explosives, which are widely used in mining and construction7. More recently, W/O emulsions have been researched in the areas of corrosionresistant coatings8 for their hydrophobic characteristics, paints for water-soluble colors, petroleum products9 from crude-oil, food colloids10 for oil-soluble fatty acids, and many more11,12. Given the water dispersible components in a W/O emulsion tend to form spherical and hollow spherical droplets, they has also been extensively researched as a route for templated synthesis of polymer nanocomposites13, porous microstructures14 and electrodes for the batteries15 and supercapacitors16. Graphene oxide (GO), the oxygen-derivatized graphene, and its counterpart reduced graphene oxide (rGO) are being widely researched currently as novel materials. These materials have found applications in a broad swathe of applications such as in transistors17, membranes18, electrodes18, coatings19 to biomaterials and bionics20,21. Despite having poorer inherent properties than pristine graphene, rGO is an economic and efficient material for electrical22, electronics23 and electrochemical devices24 and technologies. From the perspective of interfacial science, facile tuning of oxygen functionality from GO to rGO offers tuned hydrophilicity and surface energies leading to novel and exciting interfacial properties25. A growing research area is related to the tuning of fluid properties by tailoring GO and rGO at the interface of liquids to produce multifunctional soft materials such as electro-rheological fluids26, or fluids with enhanced thermal conductivity27.

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Amphiphilic GO has been studied as a surfactant since the last few years28. A variety of reports have studied different parameters of GO29,30 such as pH31, oil volume fraction, and salt concentration32,33. Most of these reports have focused on the stabilization of an oil-in-water (O/W) emulsion30, as GO is more hydrophilic than graphene and disperses well in water34. It is therefore unsurprising that GO will stabilize oil droplets in a continuous water phase, making an O/W emulsion28. A fair amount of research has looked in detail to understand the microstructure35, rheology36 and electrochemical properties16 of O/W emulsions stabilized by GO. Surprisingly, there are very few reports on the preparation of water-in-oil (W/O) emulsions stabilized by GO15,13. Guo et al.15 first reported the formation of hollow GO via a W/O emulsion route through a mechanism believed to occur by the flocculation of GO sheets at basic pH; However, there was little detail on the emulsion stability or the lack thereof, as the main goal was to utilize these emulsions as a template for the preparation of hollow spheres for Li-ion batteries. The formation of W/O emulsions by GO was further explored by Zheng and coworkers13. They formulated W/O emulsions by modifying GO with a surfactant cetyltrimethylammonium bromide (CTAB) to prepare a high internal phase emulsion (HIPE). Though the HIPE W/O emulsion was stable for more than one week, such emulsions could only be obtained at higher concentration of GO and CTAB. Also, during the modification, GO flocculated and stacked, limiting the adsorption of CTAB on GO. In this paper, we focus on the synthesis and evaluation of the properties of W/O emulsions stabilized while using partially reduced GO (prGO). The role of various parameters such as prGO concentration, extent of reduction of GO, and oil phase volume fraction on the preparation and properties of the emulsions are investigated in detail. The properties of W/O emulsions made

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from prGO and the relatively well known O/W emulsions made from GO are also compared and contrasted. The W/O emulsion remains stable up to 20 days, and the cause of the metastability is carefully analyzed and ascertained using microscopic observation and electrical conductivity measurements.

EXPERIMENTAL Materials and methods The graphite flakes (+100 mesh size) were procured from Sigma-Aldrich (99.95% purity). Methylated canola oil (density = 0.913 kg/L at 25 °C, kinematic viscosity = 5 cSt at 40 °C) was supplied from Orica mining services Pty. Ltd., Australia. Deionized water was used to prepare the emulsions. Synthesis of GO GO was synthesized using Hummers’ method37,38. In this method, 2.0 g of graphite flakes (Sigma-Aldrich 99.95%) and 1.0 g of the salt sodium nitrate NaNO3 (Merck 98.5%) were mixed with 46 mL of concentrated sulfuric acid H2SO4 (Merck 98%) in a 500 mL beaker and stirred on ice bath for 15 min. The temperature of the ice bath was maintained at 0 oC. Then, 6.0 g of KMnO4 (Merck 98.5%) was added, maintaining the reaction temperature at 20 oC with continuous stirring, which was continued for 2 h at 35 oC. The mixture turned into black gel type slurry eventually during stirring. Exactly 100 mL of DI water (18.2 MΩ-cm) was slowly added leading to a large exotherm whereby the temperature increased to 98 oC. The reaction temperature was kept at 98 oC for 30 min. Next, the bath was removed and the mixture was allowed to cool to room temperature. After cooling, around 12 mL of H2O2 (Merck 30% purified) was added until the color of the mixture changed to golden yellow and more DI water

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was added. The mixture was centrifuged at 4000 rpm for 2 h and supernatants were discarded. The residual material was washed 3-4 times with 10% HCl to remove the metal ions, and finally with DI water till it attained a pH value around 5. The dispersion was filtered using Whatman filter paper and the solid was dried in vacuum for 4 h at 50 ºC and finally a brown colored GO powder was produced. Thermal reduction of as-synthesized GO As-synthesized GO was dispersed using a probe sonicator for 5 min and centrifuged at 12000 rpm for 15 min. The supernatants were discarded. The GO dispersion was filtered in a vacuum filter using a cellulose acetate filter paper. The filtrate along with the filter paper was placed in a petri-dish containing commercial grade acetone. Acetone dissolves the filter paper and the GO filtrate in the form of a paper was separated. This GO paper was placed on a Teflon® sheet in a petri dish, and heated in a vacuum oven at 300 °C for 24 h. This annealed GO paper was termed as partially reduced GO (prGO). For emulsion preparation, this GO paper was used24,39.

Preparation of W/O emulsion using prGO GO paper was dispersed in 7.5 mL DI water with a concentration of 1 mg/mL by ultrasonication for 30 min. The pH of GO dispersion was maintained at 6. This dispersion was heated to 65 °C on a water bath. 2.5 mL of the methylated canola oil was taken in a 100 mL beaker and heated to 90 °C on a hot plate. Hot canola oil was stirred at 700 rpm using a high shear mixer and the GO dispersion was added to it slowly. The addition was performed in such a way that the entire 7.5 mL of GO was fully added within 1 min. The stirring was continued for an additional 2 min. Following this, the shearing speed was increased to 1400 rpm and the

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mixture was stirred for a further 2 min. At the end of the stirring, a yellowish white paste-like emulsion was obtained. Characterization The as-synthesized GO and partially reduced powder was dispersed in the DI water and ultrasonicated for 30 min to achieve uniform dispersion. For Raman spectroscopic analysis, XPS and FTIR (on KBr pellet) analysis, the dispersion was drop casted on a glass slide, heated at 50 °C in vacuum for 4 h and was used for the analysis. Raman spectroscopic analysis Raman spectroscopic analysis was performed using a Raman spectrometer HR 800 microRaman (HORIBA Jobin Yovon, France) on as prepared samples. The scanning range was from 1000 to 1800 cm-1 with incident laser excitation wavelength of 514 nm. Fourier transform infrared spectroscopic analysis (FTIR) FTIR investigations were carried out on 3000 Hyperion Microscope with Vertex 80 FTIR System (Bruker, Germany). The samples were prepared by depositing the dispersion on KBr pellets and drying the pellets in vacuum. X-ray photoelectron spectroscopic analysis (XPS) The XPS analysis was performed using Twin anode (MgKα/ZrLα) 300 W and microfocused monochromatic concentric hemispherical analyzer (CHA). The drop casted samples of both graphene oxide and reduced graphene oxide were used to obtain the raw data which was further deconvoluted to fit different peaks corresponding to different functional groups.

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AC electrical conductivity measurements AC electrical conductivity of the emulsion samples and their components was investigated using a broadband dielectric spectrometer (Novocontrol technology, Germany) with an Alpha A analyzer at room temperature. The measurements were carried out in a frequency range 10-2 to 108 Hz in a home-made cell. The cell was constructed using two copper plates as electrodes, a rubber ring as separator and a hollow circular Teflon plate for support. The samples were placed between two electrodes separated by the rubber ring such that there is short circuit between the two plates. The measurements were repeated three times for consistency. Cryo-scanning electron microscopic observation The droplet fracture morphology was investigated using a FEG-SEM (JSM-7600F, JEOL, Japan) and cryo preparation system (PP3000T, Japan). The cryo preparation system features variable temperature conduction cooled specimen stage (-185 oC to 50 oC) and gas-cooled nitrogen cold stage assembly (-192 oC to 50 oC). About 2-3 drops of the emulsion sample were placed onto a copper crucible and frozen using liquid nitrogen. The frozen sample was introduced into the SEM chamber and fractured using an attached knife in the chamber. Finally, the fractured sample was transferred to the cooled specimen stage to observe the microstructure. Transmission electron microscopic observation in cryo-mode prGO encapsulation on the water droplets was investigated using a HRTEM (JEM 2100 ultra HRTEM, and a cryo mode facility with cryo specimen holder). The sample was prepared in cryo mode. For this, a drop of emulsion sample was cast on a holey carbon grid and was plungefrozen using cryo plunger (Gatan Inc. USA). Frozen-hydrated specimens were transferred to the TEM using a cryo transfer unit under liquid nitrogen. The frozen samples were imaged using a FEI Vitrobot equipped with a LaB6 filament operating at 200 kV.

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Fluorescence imaging The fluorescence imaging was carried out using Olympus IX 81 (combined with FV-500) confocal laser scanning microscope (Zeiss, Germany) using the emulsion having prGO, mildly functionalized with fluorescein isothiocyanate (FITC). FITC was loaded onto prGO by sonication of a FITC solution (0.05 wt%, 10 mL) in DI water with prGO dispersion (0.5 mg/mL, 10 mL) followed by overnight stirring in the dark for 12 h. Unreacted FITC was removed by centrifugation at 6000 rpm for 2 h. The obtained FITC functionalized prGO was further used for W/O emulsion preparation. The sample preparation was performed using a similar approach as for the confocal microscopic analysis. The images were taken in the fluorescence mode by setting the absorbance ~519 nm wavelength corresponding to the excitation wavelength of the FITC. Confocal laser scanning microscopic analysis Confocal micrographs were obtained using Olympus IX 81 (combined with FV-500) confocal laser scanning microscope at magnification of 100x. A drop of the emulsion was placed on a glass slide and immediately covered with a cover slip to get a thin layer of emulsion between the glass surfaces. A drop of type-F immersion oil (n=1.518 at 23 °C) was applied on the lens to improve the resolution. The diameter of individual droplets in the emulsion samples were measured using the software ImageJ 1.47v (National Institute of Health, USA). The diameters of at least 100 droplets from each system were measured and the data were numerically processed to obtain droplet-size distribution. Polarized optical microscopic analysis Polarized light micrographs were obtained using a Leica Abrio imaging system from CRI Inc. The samples for imaging were prepared by placing a minute droplet of the emulsion on a glass

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slide and covering with a cover slip. A little pressure was applied to the cover slip to squeeze the sample for uniform distribution of the sample and to reduce the sample thickness in order to allow the light to transmit through the opaque sample. Before imaging the sample, a background was taken.

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RESULTS AND DISCUSSION State of oxidation of graphite and the partial reduction of graphene oxide (b)

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Figure 1. (a) Shows deconvoluted C1s XPS spectrum of pristine GO. The spectrum was fitted to different peak intensities corresponding to sp2 carbon and carboxyl functional group (-COOH) and the peak intensity values are consistent with literature. (b) Shows deconvoluted C1s spectrum of prGO. The reduction in the intensities of carbonyl functional groups can be attributed to the partial reduction of the pristine GO. (c) Shows deconvoluted C1s spectrum of rGO. The reduced peak intensities correspond to complete reduction of the pristine GO. (d) Shows FTIR spectra which re-confirms the partial and complete reduction via reduced peak intensities at ~1620 cm-1. (e) Shows the Raman spectra of graphite, GO, prGO and rGO. Partial reduction induces rupturing of GO sheets, generates disorder and broad G-band along with a shift in lower frequencies due to dominance of sp2 carbon. Complete reduction induces more defects as depicted from the higher relative intensity of the D-band

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In figure 1 (a), (b) and (c); the XPS spectra were fitted to different peaks corresponding to sp2 carbon (C=C) and carbonyl (C=O) functional groups. The carbon to oxygen ratio (C/O) of GO, prGO and rGO was found to be 2.3, 4.5 and 8 respectively. The ratio was determined using atomic percentage of carbon and oxygen40. The atomic percentage of carbon and oxygen was calculated using the integrated intensities in C1s spectra and the sensitivity factor of carbon (FC = 1) and oxygen (FO = 2.93). It can be observed that there is a decrease in the intensity of the peak corresponding carbonyl functional group in case of prGO and rGO. This indicated that partial reduction has removed some of the carbonyl groups along with the hydroxyl groups (as can be determined from FTIR) giving more sp2 carbon in the vicinity of interaction. Further, complete reduction has been demonstrated by the larger reduction in relative peak intensities. The reduction in carboxyl and carbonyl groups was confirmed by the FTIR spectroscopic analysis as shown in figure 1 (d). The thermal dissociation of oxygen groups is clearly indicated in the reduced transmitted intensity of C-O groups, which corresponds to –COOH and –COOR groups. Also, the thermal reduction of GO will remove some of the O-H and –O– bonds on the basal plane. This reduction will expose more aromatic islands at the basal plane indicated by the C=C bonds at the basal plane, which can be confirmed from the C=C stretching at ~1634 cm-1. Some hydrophilic groups at the edges are present as indicated by C-O stretching at 1344 cm-1. In figure 1 (e), the G band shifts to lower frequency from 1593 cm-1 to 1581 cm-1. In reality, the complete reduction is exhibited in the form of a higher intensity D band, whereby the intensity of the D band increases as compared to the G band41,24. This is because defect density increases with the formation of amorphous graphene and less sp2 carbon exposed due rupture and breakage. Herein, the partial reduction may not only expose the sp2 carbon but also there is

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breaking and rupturing of GO sheets which lead to an increase in more amorphous region and hence the higher intensity D band is noticed along with the G band shift. Stabilization of oil-water phases using graphene oxide (oil-in-water emulsion) and partially reduced graphene oxide as surfactant (water-in-oil emulsion)

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GO Oil (43.9o)

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GO Water (25.3o)

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prGO Oil (24.0o)

prGO Water (115.3o)

Figure 2. Contact angle of GO and prGO with canola oil and water. (a) GO and oil (b) GO and water (c) prGO and oil and (d) prGO and water Figure 2 shows the contact angle of GO and prGO films for the solvents utilized to prepare the emulsions. It is observed that GO makes contact angles of 43.9°and 25.3° with oil and water respectively as indicated in figure 2(a) and 2(b). On partial reduction, the contact angle with oil

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decreases to 24.0° and with water increases to 115.3° as indicated in figure 2(c) and 2(d) for prGO, which is consistent with our XPS and FTIR results 25,26. (a)

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Figure 3. Photographs of the (a) GO dispersion in DI water, (b) prGO dispersion in DI water and the (c) W/O emulsion

Figure 3 shows photographs of the (a) GO dispersion in DI water, (b) prGO dispersion in DI water and the (c) W/O emulsion. The beakers containing dispersions and emulsion are 4 cm in diameter. Since the GO is only partially reduced, it retains some dispersibility in water, while still being hydrophobic. The W/O emulsion is slightly yellowish in color owing to the yellow color of the continuous oil phase. The partial reduction of GO increases the hydrophobic character by the removal of hydrophilic oxygenated functional groups viz. C-O, C=O, C-O-H, etc.27-29. This decreases the HLB value for GO and corresponds to the HLB range suitable for the formation of a W/O emulsion. As a result, oil wets the prGO while water does not. Accordingly, by the Bancroft rule3, oil becomes the continuous phase and water becomes the dispersed phase with prGO at the interface. The observations from figure 2, figure 3 and figure S1 (supporting information) also confirm that the prGO forms a stable suspension in water. This can be attributed to the prGO retaining sufficient hydrophilic functionality as to make a dispersion in water.

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Watch glass

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W/O emulsion Water drop

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Figure 4. Top: Continuous phase test of the synthesized emulsion viz. (a) Dilution test and the (b) AC electrical conductivity. Bottom: Confocal images (c) GO enabled O/W emulsion, (d) prGO enabled W/O emulsion and (e) Oil-water biphasic mixture with rGO As shown in figure 4 (a), the formation of W/O emulsion was verified by the dilution test34-39. When oil is the continuous phase, addition of oil will dilute the W/O emulsion and if water is added it will not mix with the emulsion as demonstrated in figure 4(a). The frequency dependent AC electrical conductivity measurements in figure 4(b), are in agreement with the dilution test. It can be seen that O/W emulsions follow the similar variation in electrical conductivity as the conductivity of water42 but with slightly lower value. The lower value of electrical conductivity can be accounted for the presence of oil droplets in the emulsion42. Also, the O/W emulsion is

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observed to be electrically conducting at lower frequencies as well as the water does. The ambiguity in the electrical conductivity of water at lower frequencies could be accounted for the resistance caused by polarization of electrodes. However, the electrical conductivity of W/O emulsion is found to be behave in a similar fashion as the conductivity of oil, showing much lower conduction (i.e. behavior as an insulator) at lower frequencies. Further, the higher conductivity of W/O emulsion than that of pristine oil can be accounted for by the presence of water droplets encapsulated by prGO. These tests confirm the formation of the W/O emulsion using prGO. A comparison of the microstructure of O/W emulsions synthesized using GO, W/O emulsions synthesized using prGO and oil-water biphasic mixture with rGO in it are shown in Figure 4 (c), (d) and (e). Figure (d) shows a high internal phase W/O synthesized using prGO. The microstructure shows polygonal droplets with large polydispersity and wider droplet size distribution than those formed using GO. In the case of rGO, the loss in hydrophilicity prohibits emulsion formation, instead leading to separate oil and water phases. The rGO remains undispersed at the interface of the biphasic mixture. While the appropriate HLB in prGO enables formation of a W/O emulsion with critical close packing fraction of the droplets, the droplet size distribution in this emulsion is intrinsically linked to the technique for formulating it. Herein, the synthesis technique has been adapted from a standard emulsion preparation method43 and optimized it to obtain stable emulsions for our composition. The stirring speed was maintained at 700 rpm during formation step and does not exceed 1400 rpm during refining step as over-speeding leads to breaking of emulsion droplets. Although changes to such practice will likely change the droplet size distribution and subsequently the rheological properties.

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Characteristics of the water-in-oil emulsion synthesized using prGO: Rheology and microscopy

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Figure 5. Rheological properties of W/O emulsion. The emulsion was analyzed using (a) amplitude sweep measurements, (b) frequency sweep oscillatory measurements and (c) steadystate shear flow measurements (viscosity curves)

Figure 5 presents the rheological characteristics of the W/O emulsion synthesized using prGO in comparison to that of an O/W emulsion synthesized using GO. The oil-to-water ratio in O/W emulsion is 75:25 while the water-to-oil ratio in W/O emulsion is 75:25. The GO concentration in both of the emulsions is 0.01% (w/v). Figure 5(a) demonstrates the typical evolution of the storage modulus (G') and loss modulus (G") of the emulsion with respect to the increase in strain amplitude at a constant frequency of 1 Hz44. The rheological measurements were performed in the linear viscoelastic (LVE) regime. The measurements were repeated three times at different gap value between the parallel plates in order to eradicate the effect of wall sleep. There are two characteristic points highlighted in Figure 5(a): point 1 corresponds to the limit of linearity of storage modulus, G' and point 2 corresponds to the maximum loss modulus Gmax". For an O/W emulsion, the strain amplitude, γ at point 1 is close to 0.1% and at point 2 is around 5%. Both G' as well as G" are independent of the strain in an amplitude domain up to about γ

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=1%. Moreover, G' is independent of strain in a rather wide amplitude domain up to γ =0.6%. Further, at higher values of the deformation both the moduli are no longer linear and the emulsion exhibit the viscosity dominant behavior. This is generally attributed to the breakdown of the inter-droplet structure. For W/O emulsion, the strain amplitude, γ at point 1 is less than 0.01% and at point 2 is close to 0.01%. Both G' as well as G" are independent of the strain in an amplitude domain up to about γ =0.5%. Additionally, G' is independent of strain for the amplitude domain only up to γ =0.5% starting from 0.01%. The deformation occurs at a lower value of strain amplitude as compared to that for an O/W emulsion. The elastic-to-viscous transition (cross-over, γ*) for W/O emulsion takes place at lower value of γ* = 0.01 as compared to that of O/W emulsion with γ* = 6. The W/O emulsion possess coarse morphology with low surface-area-to-volume ratio of the droplets as compared to that of the O/W emulsion. The models by Princen and Mason44, 45 have theoretically predicted the dependence of elastic modulus G, as G d-1. Malkin et al.46 have experimentally demonstrated the elastic moduli to be G d-2. This analysis to some extent explains why elastic modulus G of the O/W emulsion is higher than W/O emulsion. Additionally, the amplitude sweep shows that O/W emulsion shows higher elastic and solid-like behaviour as compared to that of W/O emulsion which shows liquid-like behaviour even in the LVE range indicated the destruction the emulsion structure on application of stress. Figure 5(b) shows the oscillatory shear measurements at the linear viscoelastic domain for an O/W (GO-stabilised) as well as W/O (prGO-stabilised) emulsion. For an O/W emulsion, the elastic modulus is almost constant in a frequency range covering a few orders of magnitude i.e. up to angular frequency 10 Hz. In the high frequency region, the elastic modulus increases with

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increasing frequency. The wide plateau on the frequency dependence of elastic modulus is standard for ideal elastic materials, the elastic modulus of which must be independent of frequency. Dependence of storage modulus on the frequency for O/W and W/O emulsion was demonstrated by Rajinder Pal47. The storage modulus dependence on the frequency O/W of emulsion and W/O emulsion stabilized by GO and prGO respectively follows similar behavior. The linearity of the storage with frequency in case of O/W emulsion indicated the quasi-solid behavior due to the presence of smaller droplets and monodisperse morphology. On the other hand, the non-linearity in case of W/O emulsion can be attributed to the presence of larger droplets. The polydispersity in W/O allows the smaller droplets to go in the voids between large droplet leading to the overall lubricating effect and flow of the emulsion. Figure 5(c) shows the steady-state shear flow measurements in terms of complex viscosity with angular frequency. For both the emulsions, the complex viscosity decreases on going from low angular frequency to high angular frequency regime. This indicates the shear-thinning behavior both the emulsions. However, in an O/W emulsion, there is a decrease in the viscosity is constant for a wide frequency range. This can be attributed to monodisperse droplet morphology which allow droplets to easily slip and roll-over each other on the application of sress. While, in case of the W/O emulsion, complex viscosity increases at low frequency and then increases at high frequency for a wide frequency range. The large droplets in the polydisperse structure in W/O emulsion prevents the movement of droplets at low shear stress. Further, increase in the stress or at high frequency, droplets starts to break and emulsion flows as indicated by decease in complex viscosity. This was thoroughly reviewed by the Svetlana48 on how the droplet size and internal structure influences the complex viscosity.

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(a)

(b)

20 µm

200 nm

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Figure 6. Morphology of the W/O emulsions made from prGO. (a) Polarized microscopy image of an emulsion droplet. The prGO at the interface shows birefringence. (b) Confocal images of the W/O emulsion show the internal droplet strcuture. (c) Cryo-TEM micrograph showing that prGO effectively encapsulates the droplet phase as depicted by the dark-red arrows. (d) Crosssectional cryo-SEM image of fractured droplet. The morphological features of the emulsion droplets were analyzed using various microscopy techniques48, 49 and are shown in Figure 6. It is evident from Figure 6 (a) that prGO positions

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itself at the oil-water interface, as the prGO shows birefringence under the polarized light, noting that neither water nor the oil phase shows any birefringence49. The confocal fluorescence images of the W/O emulsions with prGO are shown in Figure 6 (b) and indicate that some amount of prGO lies inside the droplet water phase asides from being primarily at the interface. The transmission electron micrograph in Figure 6 (c) shows an emulsion droplet with prGO presented at the interface26. Wrinkled and overlapping prGO can be easily seen in the microstructure along the droplet periphery. The blue arrow shows the holes in the droplet created during sampling the emulsion for the measurement. The cryo-SEM image in Figure 6 (d) shows the cross-section of the fractured emulsion droplet. Delaminated and ruptured prGO sheets can be easily seen surrounding the water droplet, confirming the formation of the emulsion. The cryo-TEM technique usually images only smaller droplets since larger droplets flatten on vitrification. Our image is indicative of the presence and orientation of the prGO sheets at the interface.

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Effect of parameters such as prGO concentration and the oil volume fraction on the W/O emulsion

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Figure 7. Top: Confocal images and the droplet size distribution of the W/O emulsion as a function of prGO concentration (w/w %) (a) 0.005, (b) 0.010, (c) 0.05 and (d) 0.1. An increase in prGO concentration stabilizes more water droplets up to a certain concentration, increasing the emulsion volume fraction. Bottom: Confocal images and droplet size distribution of the W/O emulsion as a function of oil volume percentage (v/v %) (e) 40, (f) 30, (g) 20 and (h) 15. Decrease in oil volume percentage changes the distribution from log-normal to Gaussian. Red marks in the confocal images are generated due to contrast of oil with the light source.

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Figure 7 illustrates the effect of parameters such as concentration of prGO and oil volume percentage on the total volume fraction of the emulsion. The water-to-oil ratio is kept constant at 75:25. In Figure 7 (a), (b), (c) and (d) confocal microstructure and corresponding droplet size distribution has been analyzed with respect to the increase in the prGO concentration. At low concentration, fewer prGO sheets take part in minimizing the system free energy and stabilizing water droplets, and consequently, the amount of stabilized water will be less and the volume fraction of the stable emulsion will be lower. The droplets are spherical since more free volume is available, and their size distribution is best described by a log-normal distribution with most droplets concentrated in the size range of 2-25 microns as shown in Figure 7(a). The statistical values for the distribution curves at varying prGO concentration are listed in Table S1 (supporting information), where ‘ c’ represents the mean value and ‘σ’ represents the standard deviation for the peak of the log-normal fit. The distribution function assumes ‘ c’ at 9.4 microns with the ‘σ’ 0.53 which corresponds to smaller droplets concentrated in a narrow range. With an increase in prGO concentration, more and more prGO goes to the interface in order to minimize free energy by stabilizing more water droplets. The droplet shape will now change from spherical to polygonal since less volume is available for interstitial oil spaces. The size distribution extends to large droplets, again with a log-normal distribution. The mean shifts to higher droplet size with an increase in standard deviation. The droplet size varies from 2 to 40 microns as shown in Figure 7(b) and (c). At high enough concentration as shown in Figure 7(d), a highly concentrated emulsion will form. All of the water phase will be utilized in stabilization of the emulsion, and the emulsion will achieve its maximum volume fraction. The prGO can be seen in the emulsion. The droplets are polygonal and the emulsion structure is compact. Since

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very little free volume is available, the smaller droplets will tend to collapse to form bigger droplets. This will affect the size distribution. The size distribution is again log-normal with more large droplets in the emulsion. The distribution function decreases with mean shifts to higher droplet size and large standard deviation from the mean. In conclusion, with an increase in prGO concentration, the stable emulsion volume fraction will increase with commensurate increase in polydispersity until ultimately highly concentrated emulsions form. When the prGO concentration is larger than ~0.05%, the emulsion volume fraction will not increase, but rather the emulsion structure changes with the predominance of larger droplets. Figure 7 (e), (f), (g), and (h) shows the confocal images and droplet size distribution with respect to the variation in the oil volume fraction; here, the prGO concentration is kept constant at 0.05%. As shown in Figure 7(e), at oil volume percentage of 40%, the prGO stabilized water droplets assume the minimum energy and a stable spherical shape given the accessible free volume of the oil phase. Hence, the distribution is log-normal with standard deviation 0.53 from the mean 9.2 microns (table S2 in supporting information). This corresponds to the droplet size in the range of 2-25 microns. With the decrease in oil volume percentage as in Figure 7(f) and (g), the accessible free volume decreases, and the emulsion droplets minimize their surface energy by assuming strained polygonal shapes. Furthermore, a decrease in oil volume percentage as in Figure 7(h), results in incomplete stabilization, and droplets collapse to form larger droplets. At this stage, emulsion formation is difficult. The size distribution widens and droplets falls in the range of 5-30 microns. The probability distribution function assumes much lower value than any oil volume percentage emulsion discussed before, with large standard deviation from the mean. We note that any further decrease in the oil volume percentage will not give a

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stable emulsion. In conclusion, a decrease in the oil volume percentage increases polydispersity and the emulsion structure breaks completely at oil volume percentages less than 20%. De-stabilization studies and coalescence dynamics of the W/O emulsion

(b)

(c)

(a)

(d)

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Figure 8 Photographs of W/O stabilized by prGO taken on (a) 6th day, (b) 12th day and (c) 18th day after formulation. The emulsion destabilizes, forming separate phases. Confocal images of the W/O emulsion synthesized using 0.01% prGO, taken on (d) 6th day, (e) 12th day and (f) 18th day after formulation. The droplets coalesce to form bigger droplets and ultimately the two phases separate It was observed that the synthesized W/O emulsion destabilizes completely in about 20 days from the day of its production. The destabilization process is analyzed based on the emulsion stable volume fraction as depicted in the Figure 8(a), (b) and (c). Additionally, the confocal images were taken with time to observe the droplets’ structure within the emulsion. The emulsion structure of (a), (b) and (c) is shown in confocal images (d), (e) and (f) respectively that are taken every 6th day from the day of emulsion formation.

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A W/O emulsion destabilizes mainly by physicochemical mechanisms which are flocculation, coalescence, Ostwald ripening and phase inversion. From Figure 8, it can be seen that during the destabilization, reverse emulsion (O/W in this case) does not form indicating phase inversion could not be a mechanism. In flocculation, the droplets aggregate (without merging) to form clusters. The confocal images of destabilizing emulsion taken on 3rd, 4th and 5th day in Figure S2 (supporting information) show that no droplet aggregates were observed during the destabilization. This observation indicates that flocculation is also not a mechanism for destabilization. In other two processes, coalescence as well as Ostwald ripening, smaller droplets merge or dissolve to give larger droplets. In coalescence, the droplets collapse due to interstitial film thinning to form bigger droplets, while in Ostwald ripening, bigger droplets grow at the expense of smaller droplets. From this one can say that Figure 8 indicates that possibly either or both of these mechanisms could be a probable reason for the destabilization of the W/O emulsion46.

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3000 1st Day 6th Day 12th Day 18th Day

2500 Population (%)

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2000 1500 1000 500 0

0

20 40 60 Average droplet diameter (µ µm)

80

Figure 9. Decay in normalized droplet size distribution of the W/O emulsion with time. The broader droplet size distribution shows that the coalescence is the prevailing mechanism in the destabilization of prGO stabilized W/O emulsion The assessment of the second destabilization mechanism after sedimentation was undertaken by examining the time-dependent droplet size distributions obtained from the confocal images with aging, as shown in Figure 9 and shows that the average droplet size distribution shifts towards larger droplets along with a decrease in the average population of the droplets. The distribution is wider with size range varying from less than 2 µm to 100 µm. The distribution curve fit shows that the curve shifts from log-normal to Gaussian. The mean value increases with the increase in the standard deviation, while the probability density function decreases drastically

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with time. It is well established that the droplet size distribution is indicative of the destabilization mechanism that occur49. Ostwald ripening shows a narrower distribution, while coalescence shows a wider distribution. Given the wider droplet distribution of the destabilizing W/O emulsions, coalescence can be considered as the prevailing mechanism of secondary destabilization over Ostwald ripening. This can be attributed to the surface layer thinning, allowing droplets to coalesce to form larger droplets.

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Day 1 Day 6 Day 12 Day 18

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σ AC (S/cm)

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Time (hours) Figure 10. AC electrical conductivity of the prGO stabilized W/O emulsion with time. There is no change in the conductivity in the initial days of the synthesis. From day 6 the prGO stabilized water droplets starts settling with evolution of oil phase.

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In prGO sheets, large sp2 domains are available for interaction with adjacent sheets. Previous studies have explored the interaction between GO sheets in an emulsion when two droplets approach each other31. When the droplets are in close proximity after sedimentation, the π- π interaction between the sheets prevail over the wetting with the oil droplets. This could result in stacking of the prGO sheets, removing them from sufficiently covering the water droplet surfaces and resulting in coalescence. The stability of the prGO stabilized W/O emulsions against coalescence and phase separation was also assessed by AC electrical conductivity. As shown in Figure 10, the emulsion is quite stable during the first few hours (24 hrs for day 1 in Figure 10) from the formulation – as shown by constant σAC in the Figure 10. After the 6th day from its production, sedimentation of the emulsion and separation of the oil phase was observed in prGO stabilized emulsions as indicated by the decrease in the σAC from the insulating oil phase (day 6 in Figure 10). From day 12, an increase in the σAC was observed, this is attributed to the increase in phase separated water, and suggests that the water droplets are destabilized and break either by coalescence or Ostwald ripening. This destabilization accelerates, and the phases completely separate by the 20th day from the emulsion formation, at which point, a rise in σAC is observed, with a larger volume of water separating from the emulsion. In coalescence, the change of droplet size with respect to time follows, ……………………………………………………………………. (1)

where, R is the mean radius and ω is frequency of coalescence50 Using this relation, the emulsion life-time (the time for the emulsion to be completely destabilized) can be deduced by:

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………………………………………………………………………….... (2)

Following from the above equations, the time of the emulsion to fully resolve was calculated. The frequency of coalescence ω was calculated by a linear fit to the sauter mean diameter curve (see supplemental figure S4). The value of

e

was found to be 18 days, which is close to the

experimental phase resolution time. This is in agreement with our analysis by confocal imaging and droplet size distribution analysis, as well as direct observation.

Conclusion This work focuses on the stability of W/O emulsions prepared using partially reduced graphene oxide (GO). With partial reduction, more hydrophobic domains on the GO sheets are exposed to the hydrocarbon oil phase, which changes the hydrophilic-to-lipophilic balance (HLB) and ultimately the wettability. This simple strategy is employed to synthesize a W/O emulsion instead of an O/W emulsion. The stability of such emulsions was monitored with changes in parameters including the extent of reduction, concentration of GO, and the continuous phase volume fraction. The emulsions are stable for ~20 days, and the instability mechanism was explored using time dependent electrical conductivity measurements and droplet size distribution analysis via confocal imaging, suggesting that sedimentation followed by coalescence were the governing mechanisms. The emulsion preparation method may have implications in development of synthetic protocols for hollow structures of rGO and other templating applications. For applications involving specifically as-synthesized W/O emulsions, the longer term stability needs further improvement and this can be guided by our understanding of the underlying mechanisms.

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ASSOCIATED CONTENT Supporting Information. Solubility of prGO in water Mean and standard deviation calculations from droplet size distribution curve Destabilization analysis using time-dependent confocal microscopy and emulsion photographs Variation of Sauter mean diameter with time AUTHOR INFORMATION Corresponding Author *E-mail - [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources ORICA Mining Services, NSW, Australia has supported the research by funding for consumables and equipment purchase. ACKNOWLEDGMENT The author would like to acknowledge SAIF and CRNTS facility at IIT Bombay for providing characterization facilities viz. Raman spectroscopy and HR-TEM. Also, the author would like to acknowledge Central Facilities at IIT Bombay for carrying out various characterizations viz. XPS, Broadband dielectric spectroscopy, confocal microscopy, cryo-SEM, cryo-TEM and Rheology. The work is supported financially by ORICA Mining Services, NSW, Australia.

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ABBREVIATIONS GO Graphene Oxide, prGO partially reduced graphene oxide, W/O water-in-oil, O/W oil-inwater. REFERENCES (1)

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For Table of Contents Use Only Hydrophobic domains

Oil

Water

Water

Hydrophilic groups

Oil

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