Phase Behavior of Pickering Emulsions Stabilized by Graphene Oxide

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Phase Behavior of Pickering Emulsions Stabilized by Graphene Oxide Sheets and Resins Xi Chen, Xinmin Song, Jia Huang, Chaodong Wu, Desheng Ma, Maozhang Tian, Hang Jiang, and Pei Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02672 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Phase Behavior of Pickering Emulsions Stabilized by Graphene Oxide Sheets and Resins Xi Chen, *,†,‡ Xinmin Song,‡ Jia Huang,‡ Chaodong Wu,† Desheng Ma,‡ Maozhang Tian,‡ Hang Jiang,‡ and Pei Huang*,§ †

School of Earth and Space Sciences, Peking University, Beijing 100871, China

‡

State Key Laboratory of Enhanced Oil Recovery, PetroChina Research Institute of

Petroleum Exploration & Development, Beijing100083, China §

College of Aerospace Engineering, Chongqing University, Chongqing 400044, China

To whom correspondence should be addressed. E-mail: [email protected] E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Graphene Oxide is preferable to form stable water in oil (W/O) emulsions with crude oil, owing to its exceptional structure, including 1 nm in thickness, several micrometer in diameter and -COOH, -OH, C-O, C-O-C, C=O groups on the surface. The properties of the as-prepared emulsions are strongly depended on the GO concentrations (CGO) and volume fraction of water to oil. At volume ratio of 1:1, the GO dispersions and crude oil can be miscible into stable W/O emulsions accompanying with largely increased viscosity even when the GO concentration reduces to 0.0001%. Notably, when the concentration of GO ranging from 0.005% to 0.01%, viscosity of W/O emulsions increases to several hundred mPa·s with the increased shear time, which is ascribed to the coalescence of the emulsions under shear. The volume fraction of water in the mixtures (Fw) also affects the phase behavior of the emulsions. At CGO = 0.0001%, 0.001% and 0.01%, GO dispersions and crude oil are miscible into one phase completely at Fw < 0.7. In the range between 0.1 and 0.7, viscosity of emulsion increases as increasing volume ratio of water to oil. More interestingly, high internal phase emulsions can be obtained in the range of 0.0005% ~ 0.008% at Fw = 0.75, in which case viscosity of the emulsions reaches the maximum, i.e., nearly two hundred times higher than that of crude oil, due to the high internal phase volume fractions. These results show that the GO dispersions is favorable to form stable W/O emulsions with high internal phase volume fraction, and has a great potential in improving oil displacement efficiency of chemical flooding.


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1. Introduction Emulsions stabilized by solid particles are usually termed as Pickering or Ramsden emulsions,1 in which solid particles of colloidal size are adsorbed at the interfaces of immiscible fluids such as oil and water, and forms into monolayer/multilayer film to prevent coalescence of emulsions drops.2-5 These Pickering emulsions have attracted continuous attention and show potential in the fields of food, cosmetics, and pharmaceutical, especially, in those areas that hazardous organic surfactants are unwelcome.6-8 Since the particles are irreversibly anchored, unlike surfactant molecules which adsorb and desorb within very short time scales under the effect of thermal fluctuations, they prevent the dispersed phase in the oil (or water) droplets from colliding or deforming,9,10 and thus improve oil recovery in oil field exploration.11-13 The association of oil, water, and particles also allows a large set of materials to be obtained in deep metastable states, such as colloidal glasses and gels, exhibiting robustness to subsequent changes in thermodynamic conditions.14,15 Although great endeavors have been devoted to explicate phase behavior of Pickering emulsions2-10, more systematic research is still needed. To be adsorbed at the interface, particles should be wetted by both of the immiscible liquids.16 Previous studies suggest that emulsion type is mainly determined by the relative wettability in the immiscible liquids, expressed in terms of the contact angle θ.17 Solid particles with optimum contact angles (θ) between 70 and 86° and between 94 and 110° have been employed to stabilize oil-in-water and water-in-oil emulsions, respectively, by sterically hinder the coalescence of droplets


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over long periods.17 Under such conditions, particles should be able to sterically hinder the coalescence of droplets over long periods of time, which has been experimentally confirmed in a number of recent studies. Particle-stabilized emulsions can be prepared by tailoring the wettability of colloidal particles in the liquid media, so that their adsorption at the liquid-liquid interface is favored. Different approaches have been used to tailor the particle wettability within this range, such as surface adsorption of long-chain amphiphilic molecules (surfactants), surface chemical treatments (e.g., silanization of silica particles) and etc.18-21 Sugita22 and Jiang23 discussed the effect of contact angle between particle and water/oil phase interface on emulsion and finally hybridized with cationic surfactants through electrostatic adsorption to adjust contact angle so that make emulsion stable. Silica nanoparticles are usually hydrophilic and the contact angle is less than 90o.24,25 When surfactants absorb on the particle surface, the contact angle will turn to 90o, and then the stability of Pickering emulsion is enhanced. Akartuna26 reported preparation of stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. Octane-in-water emulsions containing alumina particles and carboxylic acids as amphiphiles kept stable for long periods. Short carboxylic acids with two ~ five carbons were shown to be appropriate surface modifiers, by adsorbed onto the particle surface through electrostatic interactions, leaving the hydrophobic moiety exposed to the aqueous medium. These versatile approaches facilitate the preparation of stable emulsions in a number of applications, including material manufacturing, food, cosmetics, and pharmaceutical products. And


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the purpose of tailoring the wettability of the particles is naturally adjusting hydrophile lipophile balance property, just like surfactant. Graphene Oxide (GO) produced by graphite sheets exhibits certain hydrophilic property due to the ionizable -COOH groups and phenol hydroxyl groups on the basalplane.27,28 According to oxidation reaction, amounts of the unsaturated C=C bonds oxidized into ionizable -COOH groups and phenol hydroxyl groups on the basalplane, grafting GO sheets certain hydrophilic property.29-31 Accordingly, the partly oxidized graphite sheets could be wetted by both water and oil, and is distinguished as an emulsifier candidate.32-34 Kim35 found that GO makes highly stable Pickering emulsions with organic solvents, amphiphilicity of which can be tuned by changing pH as it shuttles between water and the oil-water interface. McCoy36 also found that oil-in-water emulsions stabilized by graphene oxide (GO) can be flocculated by the adjustment of pH. At highly acidic pH, fully reversible flocculation of emulsion droplets can be achieved, whereas the flocculation is irreversible at high pH. It is announced that interfacial charge of the GO and oil−water interface is the overriding drive for the exceptional stability of acidic GO emulsions, by directly measuring the interaction forces between two emulsion droplets coated in GO using the atomic force microscope. He37 studied the effects of the type of oil, the sonication time, the GO concentration, the oil/water ratio, and the pH value on the phase behavior of GO stabilized emulsions. It is found that intermediate oil/water ratios and low pH values will make high stable emulsions, and droplet size decreased with sonication time and with GO concentration.


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Water-in-oil-in-water (W/O/W) multiple emulsion droplets were also observed at low GO concentrations, low pH values, high oil/water ratios, high salt concentrations, or moderately reduced GO in the benzyl chloride. Even if the effects of GO wettability and charge on emulsification performance have been explored in above studies,32-37 elaborate depictions of the phase behavior and emulsion property are still advisable. Especially for complex systems, emulsion types are not only determined by GO wettability, but also affected by the interactions with other amphiphilic compositions, i.e. surfactants, polar organic compounds, which may interact with GO sheets and polish up their emulsion properties. Crude oil is a complex mixture with amphiphilic resin and asphaltene (short in resin) dispersed in low polar organic components. Since GO and resins are different types of amphiphilic compositions in structure, size, and hydrophile-lipophile balance value, emulsion behaviors are seriously affected by interactions between GO and resins. It is expected that the emulsion of crude oil may behave as a complex fluid with various rheological properties, for its more vicious than conventional organic solvent. In this work, the synergic effect between GO and resins on emulsification of crude oil has been evaluated. Volume ratio of water to oil and GO concentrations were proved to affect the phase behavior of the emulsions strongly. Stability and viscosity of the emulsions are greatly enhanced by addition of GO. High internal phase emulsions (HIPEs) can also be obtained in concentration ranges of 0.0005% ~ 0.008% at volume ratio of 2.5/7.5. These investigations may provide a new insight to emulsify the crude oil and construct HIPEs. The W/O emulsions remain stable in high water cut


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reservoir and have great potential to block the water channeling and enhance oil displacement efficiency in water flooding or chemical flooding.

2. Experimental Section 2.1 Materials Graphite flake with a purity of 99.8% was purchased from Alfa Aesar. The Graphene Oxide nanosheet was prepared as particle emulsifier by oxidation of graphite flake. The crude oil (Table 1) used in this paper was got from Daqing Oilfield. The inorganic reagents used to oxidize Graphite, i.e. HCl, H2SO4, KMnO4, and NaNO3 are chemically pure agents, purchased from Sinopharm Chemical Reagent Co., Ltd. The water used to prepare GO dispersions was from Milli-Q. Table 1 The Properties of Daqing Crude Oil at 45 oC Density (kg/m3)

Viscosity (mPa·s)

Acid number (mg KOH/g oil)

Saturate (%)

Aromatic (%)

Resin (%)

Asphaltens (%)

Wax (%)

0.86

12.7

0.1

60.76

14.75

8.98

0.68

24.11

2.2 Preparation of Graphene Oxide Nanosheets Modified Hummer’s method was used to synthesize GO in the lab. Graphite (3.0 g) and NaNO3 (1.5 g) were mixed together in a flask, and then 100 ml H2SO4 (95%) was added and stirred constantly in an ice bath. Afterwards, potassium permanganate (8.0 g) was added to the suspension in small amounts each time to avoid overheating. The mixture was kept at room temperature for two hours to allow the color of the suspension to turn light brown. Once the suspension turned to light brown, 90 ml of distilled water was added and the temperature would rise to 90 degree Celsius. Soon


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after, the color would turn to yellow. The diluted suspension was stirred for 12 hours at 98 degrees Celsius while adding 30 ml of 30% H2O2 to the mixture. For purification, 5% of HCl was mixed with the mixture and rinsed, then ammonia water was used to separate graphene oxide from each other, followed by purified water. Later, the suspension was centrifuged for 6 minutes at 4000 rpm and then, filtrated and dried in a vacuum. Graphene oxide was obtained finally in form of black powders. Scanning electron microscopy (SEM) observations were carried out using a Phenom ProX operated at accelerating voltage of 15 kV. Atomic force microscopy (AFM) characterization was carried out on a Multimode scanning-probe microscope (Veeco). Graphene oxides were spin-coated onto a freshly cleaved sheet of mica surface. After 45 min of drying in ambient conditions, the images were taken. Tapping-mode experiments using supersharp tips (AppNano ACL-SS) (2 nm) allowed for the intricate characterization of all samples. Fourier transform infrared (FTIR) spectra were recorded using a Varian 3100 FT-IR spectrometer equipped with 2 cm-1 resolution and an accumulation of 24 scans. X-ray photoelectron spectroscopy (XPS) spectra were obtained with ESCALAB220i-XL Photoelectron Spectrometer (VG Scientific). A Gaussian curve fitting program was used to treat the C1s signal and the following binding energies. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated by using adsorption data in P/P0 = 0.05 ~ 0.3 (six points collected). 2.3 Preparation of Pickering Emulsion GO and crude oil were used to prepare emulsions by RW20 overhear stirrers (IKA,


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Germany). Before stirring, GO dispersion and crude oil are preheated in thermostat at 45oC. Then they were added in a beaker and stirred for 2 minute at stirring rates of 20000 rpm. Finally, the mixtures were placed in the thermostat at 45 °C to investigate the phase behavior of emulsions. Volume ratio of separated water was used to evaluate the stability of the emulsions. Q is the ratio of separated water from emulsion; Vs is the volume of separated water from emulsion; and Vt is the total volume of water. = ⁄ 2.4 Viscosity Measurement ThermoHaake RS300 rheometer (Haake, Germany) was used to test the viscosity at constant rate of 7.34 1/s and 45 degree. The plate and rotor used in this experiment are plate MP60 Steel 18/8 and C60/1 Ti (D = 60 mm, 1°), individually. The measurements were performed 10 minutes after the 1 ml emulsion was placed on between the plate and rotor to make sure the emulsions are heated thoroughly. 2.5 Microstructure Characterization of Emulsion Leica DM 3000 LED (Leica, Germany) was used to observe the morphology of emulsion droplets. Droplets of various dispersant-oil mixtures were sandwiched together with seawater between glass slides and the resulting oil-water interfaces imaged via light microscopy. Digital images and video of this interface were captured using digital camera, at magnifications ranging from 100 to 400.

3. Results and Discussion


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3.1 Structural Characterization of GO Morphologies of GO sheet made by Hummers Method were investigated by SEM and AFM. As shown in Figure 1 and Figure S1, GO sheets GO is about 1 nm in thick and extends to 1 µm or more on a plane. It is thicker than single layer grapheme, attributed to the oxygen functional group which change the surface of GO. The surface area of GO is 61 m2/g according to the BET result (Figure S2).

Figure 1 Morphology of GO sheet obtained by (a) SEM and (b) AFM. FTIR was performed to investigate the structure and functional groups of graphene oxide. As shown in Figure 2a, GO sheets have apparent adsorption peaks in regions of 1000 cm−1 ~ 1400 cm−1, 1500 cm−1 ~ 1700 cm−1, and 2800 cm−1 ~ 3050 cm−1. These peaks are ascribed to epoxy C-O (1250 cm−1), alkoxy C-O (1050 cm−1), sketch in graphene (1600 cm-1) and O-H hydroxyl groups (2900cm-1 and 2990 cm-1), respectively.38,39 To further investigate the bond change after oxidation, XPS was employed to confirm the binding energy of the functional groups. The summarized content of different elements is shown in Figure 2b and Table S1.The content of C element is about 69.0%, which is consistent with the results from EA analysis (Figure


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S3). The O element is about 26.4% and additional N element originated from ammonium hydroxide during production process is observed. The oxygen-containing functional groups was further confirmed by XPS analysis. As shown in Figure 2b, the binding energy at 531.3eV is assigned to O=C-OH which promotes GO sheet to stabilize the emulsions.40

a

b O-H

2900 cm

-1

2990 cm

-1

Sketch in graphene

1600 cm

Raw Data Background C-OH C-O-C C=O O=C-OH

-1

Intenstiy (a.u.)

Hydroxyl groups

Absorption (a.u.)

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C-O -1

1350 cm

-1

1250 cm

-1

1050 cm

3500

3000

2500

2000

1500 -1

Wavenumber (cm )

1000

542

540

538

536

534

532

530

528

526

524

Binding Energy (eV)

Figure 2 (a) Variation of FTIR absorption of GO sheet plotted against wavenumbers. (b) Binding energy of the oxygen-containing functional groups according to XPS spectra. 3.2 GO Stabilized Emulsions at Volume Ratio of Water to Oil 1:1 Emulsions formed by GO and crude oil were prepared by mixing 20 mL oil and 20 mL GO dispersions in a cylinder with fixed volume ratio of water to oil (1:1). Figure 3 shows the visual appearances of the emulsion at different concentrations of GO (CGO). It can be observed that mixtures of crude oil and water apparently is separated into two phases. The bottom phase is white and turbid, indicating crude oil is rarely dissolved in the water phase. The upper phase becomes ~ 25 ml, about 5 ml water are miscible into crude oil to form W/O emulsions. Emulsions formed by the mixtures of crude oil and GO dispersions are obviously different from that without GO. Crude oil


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and GO dispersions are miscible into brown emulsions with slice oil at the top, demonstrating the excellent performance in stability. Meanwhile, there is seldom change in stability after they are placed in the thermostat for several months. Stable emulsions can be obtained with addition of slice GO, even at CGO = 0.0001%. High stability of the emulsions can be attributed to laminated structure of GO, which increases the interaction area with oil and water, and lower the system free energy by reducing the liquid-liquid interfacial area. The hydrophobic conjugated structure together with hydrophilic carboxyl and hydroxyl groups makes GO easier to be adsorbed at the interface of oil and water, enhances interactions with crude oil and water, lower the system free energy by reducing the liquid-liquid interfacial area, and thus, limit the exchange between the emulsion drops as a steric or electrostatic barrier and hider.

Figure 3 Appearance of emulsions formed by GO and crude oil after 3 hours with different GO concentrations: (a) without GO, (b) 0.0001%, (c) 0.0005%, (d) 0.001%, (e) 0.005%, and (f) 0.01%. To investigate the effect of GO concentration on rheological properties of the emulsions, viscosity of the crude oil and emulsions as a function of time at shear rate of 7.34 s-1 was measured. As shown in Figure 4, when crude oil was made into


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emulsions with water, viscosity of continuous phase becomes 33.4 mPa·s, about three times of that of the crude oil (12.7 mPa·s). The introduction of GO dramatically increases the viscosity of emulsion, compared to that of crude oil and emulsions without GO. For instance, at CGO below 0.001%, viscosity of emulsions merely changes with prolonged time and the increased concentration of GO, keeping in the range from 104.5 mPa·s to 111.2 mPa·s, about ten times of that of crude oil. Figure 5a is the morphology of the emulsions before shearing at CGO = 0.001 wt%. The spherical emulsion droplets are sized in the range of 1 ~ 10 µm. External phase of the emulsion are visually deeper than inner in contrast, suggesting the formation of W/O emulsions. Figure 5b is the image of the emulsion after shearing, similar to that before shearing. When CGO is higher than 0.005%, curves of viscosity to shear time is different with that at low concentrations. The increase of shear time will lead to striking viscosity increase, to 252.1 mPa·s after 20 minutes while the initial viscosity is nearly equal. Viscosity increase is attributed to the formation of droplet clusters by strong coupling between particles at semidilute or concentrated dispersions.41-43 It is difficult for particles to flow around in clusters, and the resistance to flow gradually increases during shearing, leading to the increase of viscosity. In this work, droplet number may be in the range of “semidilute or concentrated dispersions” at volume ratio of 1:1. Droplets form into bubble-clusters according to interactions between interfacial films, which may be responsible for the viscosity phenomenon. Since the emulsion is colloidal stabilization system, some of droplets in clusters will fuse into larger ones during shearing. Figure 5c and 5d are images of emulsion before and after


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shear. Size increase can be observed after shearing, which may be another reason for the increase of viscosity. The enlarged drops are steady, resulting in the viscosity of emulsions keeping at high level during shearing. 1000

Viscosity (mPa.s)

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crude without GO CGO = 0.0001% CGO = 0.0005%

CGO = 0.001% CGO = 0.005% CGO = 0.01%

100

10 0

200

400

600

800

1000

1200

Time (s)

Figure 4 Viscosity of emulsions changes with time at shear rate of 7.34 s-1.

Figure 5 Morphologies of W/O emulsion made of GO and crude oil obtained by optical microscope at (a) CGO = 0.001% before shearing, (b) CGO = 0.001% after shearing, (c) CGO = 0.01% before shearing, and (d) CGO = 0.01% after shearing.


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These results indicate that the addition of slice GO makes oil and water miscible into steady W/O emulsions accompanying with dramatic increase of the viscosity. The interactions between resins and GO, together with the large specific area may be responsible for the formation of emulsion. Meanwhile, the interfacially adsorbed particles lower the system free energy by reducing the liquid-liquid interfacial area, and therefore enhance the stability of emulsion in comparison to resins-stabilized emulsions. 3.3 Effect of Volume Fraction of Water on GO Stabilized Emulsions Since GO and resin have different amphiphilic molecules, phase behavior of GO stabilized emulsions with crude oil is greatly dependent on the ratio of GO to resin at the interface. The effect of volume fraction of water in the mixtures (short in Fw) on emulsion behavior was investigated at CGO = 0.0001%, 0.001% and 0.01%. Appearances of the emulsions at 0.01% are shown in Figure S4. The calculated volume fraction of water separated from the emulsions at different Fw are shown in Figure 6. It can be observed that, at low volume fraction of water, GO suspensions and crude oil are almost miscible into one phase at volume fraction below 0.7. These emulsions stand for a long time, and water can hardly be separated from the miscible phase. Figure S5 shows that emulsion droplets are W/O emulsions sized 1 ~ 10 µm. Interestingly, although the internal phase reaches 70% in the emulsions, the emulsion is still stable and behaves as a complete miscible system. At high volume fraction, from 0.75 to 0.9, dramatically increased volume of water separated from emulsions was detected as increasing the water volume fraction. However, concentration of GO


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affects volume of water slightly. For emulsions at CGO = 0.0001%, 0.001% and 0.01%, volume fraction of separated water versus Fw shows the similar trend. Volume of separated water changes slightly as increasing GO concentration. Volume fraction of seperated water (%)

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80

CGO = 0.0001% CGO = 0.001% CGO = 0.01%

60

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

Fw

Figure 6 The calculated volume fractions of separated water varies with Fw at CGO = 0.0001%, 0.001% and 0.01%. Viscosity of the emulsions at different volume ratio of water to oil at CGO = 0.0001%, 0.001% and 0.01% are shown in Figure 7a ~ 7c, and the summarized results of viscosity variation against volume ratio at shearing time of 10 minutes are shown in Figure 7d. It can be observed that viscosity of the emulsions increases with volume fraction of water in the range of 0.1 to 0.7. Similar trends can be obtained at CGO = 0.0001%, 0.001% and 0.01% in this Fw range. The maximum of the viscosity is in the range between 0.7 and 0.8, and viscosity slightly declines with shear time for emulsions in this Fw range. Taking CGO = 0.001% and Fw = 0.7 as an example, viscosity of the emulsions reach 1805 mPa·s at the beginning of shear. It decreases rapidly at initially, then slowly a few minutes later, and reaches 928 mPa·s after 10 minutes of shearing. Although there is a sharp decrease, the viscosity is still much


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higher than that in the range between 0.1 and 0.7, obviously. The GO concentration also affects emulsion viscosity at volume fraction above 0.7. At Fw = 0.7, viscosity of emulsions increases with concentration. While for emulsions at Fw = 0.8 and 0.9, viscosities of 0.001% are higher than that of 0.01% and 0.0001%. It is in accordance with results of volume fractions of separated water, which reduces down to the lowest at CGO = 0.001%. For emulsions in the range between 0.1 and 0.9, volumes of separated water are equal, but viscosities increases with Fw, so the volume of separated water is not the determinants. The volume fractions of internal phase (water) increases with Fw, in the similar manner with viscosity to Fw. Accordingly, volume fractions of internal phase may be the key factor on the emulsion viscosity. 10000

a

Viscosity (mPa.s)

CGO = 0.0001%

FW = 0.1 FW = 0.2 FW = 0.3 FW = 0.4

1000

FW = 0.5 FW = 0.6 FW = 0.7 FW = 0.8 FW = 0.9

100

Viscosity (mPa.s)

10000

b

FW = 0.5 FW = 0.6 FW = 0.7 FW = 0.8 FW = 0.9

FW = 0.1 FW = 0.2 FW = 0.3 FW = 0.4

CGO = 0.001%

1000

100

10

10

10000

200

400

c CGO = 0.01%

600 Time (s)

800

FW = 0.1 FW = 0.2 FW = 0.3 FW = 0.4

1000

1000

FW = 0.5 FW = 0.6 FW = 0.7 FW = 0.8 FW = 0.9

100

10 0

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800

1000

0

1200

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600 Time (s)

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d 1000 Viscosity (mPa.s)

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Viscosity (mPa.s)

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CGO = 0.0001% CGO = 0.001% CGO = 0.01%

100

10 0.0

0.2

0.4

0.6

0.8

1.0

Fw

Figure 7 Viscosity of emulsions formed by GO dispersions and crude oil at 7.34 1/s at CGO = (a) 0.0001%, (b) 0.001%, (c) 0.01% and (d) the summarized results of viscosity variation versus Fw at certain shearing time of 10 minutes


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It can be summarized that emulsion formed by GO suspension and crude oil is greatly dependent on volume ration of water to oil as shown in Figure 8. At low volume ratio of water to oil, i.e. Fw = 0.1, oil is the dominate phase and the interface are dominated by resins which are hydrophobic. So droplets with high curvature are formed in emulsions. As Fw increases, more GO sheets are absorbed onto the interface. Resins and GO sheets are assembled into a mixed interfacial film via weak interactions. Since GO is more hydrophilic in comparison with resin, the absorbed GO sheets weakens the hydrophobic properties of the mixed interfacial film, leading to the decrease of interfacial curvature and formation of droplets with larger radius at medium volume ratio of water to oil, i.e. Fw = 0.5. It should be noted that, despite the appearances of the W/O emulsions are similar when Fw is below 0.7, volume fraction of internal phase are completely different and increase gradually with increasing Fw. This may be responsible for the rise of the emulsion viscosity in this range. At higher volume fraction of water, more GO sheets are absorbed onto the interface, the interfacial films are stronger and emulsions become more stable. GO suspensions and crude oil are miscible into a phase completely even when internal (water) phase reach 75% at 0.001%. Hence, GO sheets may have great potential in forming high internal phase emulsion. The phase behavior of emulsions with higher volume fraction of internal phase will be discussed in the following section.


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Figure 8 Possible models of emulsion stabilized by GO and resins. 3.4 High Internal Phase Emulsion (HIPE) Formed by GO HIPEs are normally defined as concentrated emulsions with a minimum internal phase volume fraction of 74.05%. It is reported that nano-particles are favorable candidates in preparing HIPE emulsions at low concentration, compared to conventional surfactant systems. GO is a typically amphiphilic nano-sheet with high surface area, and recognized to be an ideal candidate in stabilizing HIPE emulsions. To evaluate its ability to form HIPE emulsions with crude oil, phase behavior of emulsions formed by GO dispersions and crude oil at Fw = 0.75 was investigated. Figure 9 shows the appearances of the emulsions at GO concentration in the range of 0.0001% ~ 0.05%. Viscosity of the emulsions at shear rate of 7.34 s-1 is shown in Figure 10. It can be observed that in concentration range of 0.0005% ~ 0.008%, GO suspension and crude oil are miscible into a stable W/O completely, with internal phase fraction up to 75%. Increasing or decreasing GO concentration off this range will lead to water separation. Viscosity of


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emulsion also reaches the maximum in this concentration range. Taking 0.005% as an example, viscosity reaches 4848 mPa·s initially, and then decreases slowly down to 1857 mPa·s within ten minutes. Viscosities of emulsions out of this concentration range are lower, especially for emulsions at CGO above 0.01%. Figure 11 is the morphology of the emulsions at CGO = 0.005%. Bubbles are crowded with each other and the emulsions are occupied by internal phase (water). When they are shearing between plates, interactive compression between droplets dominates and massive droplets coalescence occurs. Results of droplets coalescence are oil and water separated from the emulsions and viscosity drop was detected.

Figure 9 Photographs of emulsions formed by GO suspensions and crude oil at Fw = 0.75 and different GO concentrations: (a) 0.0001%, (b) 0.0004%, (c) 0.0005% , (d) 0.001%, (e) 0.005%, (f) 0.008%, (g) 0.01%, (h) 0.03%, and (i) 0.05%. 10000

0.005% 0.008% 0.01% 0.03% 0.05%

0.0001% 0.0004% 0.0005% 0.001%

4000 3000 2000

b phase separation Viscosity (mPa.s)

a

5000

Viscosity (mPa.s)

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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HIPE

phase separation

1000

1000 0

100

0

100

200

300 Time (s)

400

500

600

10

-4

-3

10

10

-2

-1

10

CGO (%)

Figure 10 (a) Variation of emulsion viscosity versus shear time at Fw = 0.75. (b) Viscosity of emulsion plotted against GO concentration at Fw = 0.75.


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Figure 11 The morphology of HIPE emulsions at Fw = 0.75 and CGO = 0.005%. According to the above results, the synthesized GO sheet shows favorable performance in making HIPEs. The appropriate concentration to make HIPEs at Fw = 0.75 is in the range of 0.001% ~ 0.008%. The appropriate amphiphilicity of GO is essential to strongly attach GO to the oil/water interface and fabricates W/O Pickering HIPE. The interaction between GO and resin molecules at the interface is mainly determined by the GO concentration. In a proper concentration range, GO and resin molecules arrange in a certain curve and are assembled at the interface of water and oil, which may promote the formation of stable HIPEs. The HIPEs displays remarkable stability against coalescence, Ostwald ripening, and creaming. No significant change was observed in the appearance and droplet size distribution of the emulsions within a time period of more than half a year after emulsification. 3.5 Discussion on GO Sheets Stabilized Emulsions Firstly, the interactions between GO sheets and amphiphilic components of the crude play a great role in the formation of stabilized W/O emulsions. The difference between crude oil and conventional organic solvent is that crude oil contains lots of


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amphiphilic resin and asphaltene which can act with GO by molecular interactions and tailor the wettability of GO. When GO dispersions and crude oil are mixed under stirring, both GO nano-sheets and resins arrange at the interface with a certain curvature, decided by interactions between GO and resins directly. Therefore, the GO concentrations and volume ratio of GO to oil affect the amphiphilic property of the molecules at the interface, which decides the arrangement of the molecules at the interface and affects the phase property of the emulsions in turn. Some special emulsions, such as HIPEs, forms at proper mole ratios of GO and resins. Secondly, GO sheets act as a steric barrier at the interface and prevent the exchange between droplets. Since GO sheets has especially large specific surface area, they are prone to be absorbed onto the oil/water surface. It has reported that the energy required to remove a colloidal particle from an oil-water interface can be amounted to thousands of kT,44,45 suggesting an irreversible adsorption of particles at the oil-water interface. Particles irreversibly adsorbed at the interface can impede the rupture of the thin film between neighbor droplets and halt the expected shrinkage of droplets due to Ostwald ripening, that is, the emulsions are ultra-stable. Thirdly, the aromatic components in the crude oil affect the stability of the emulsion. He37 and Thickett46 reported that aromatic solvents exhibit better stabilizing ability than nonaromatic solvents. Benzene rings from aromatic solvents has stronger π-π interaction with GO than nonaromatic reagents. The oil of stronger polarity may form W/O emulsion stabilized by GO, on the contrary, that of weaker polarity oil formed O/W emulsion. To summarize, lots of factors affect the type and stability of emulsions, including interactions between GO


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sheets and amphiphilic resins, GO structures, components of crude oil. There are a variety of factors affecting the viscosity of the oil-water emulsion, relating to the volume concentration of dispersed phase, shearing force, viscosity of continues phase and dispersed phase, different stabilizer or emulsifier and the emulsion size.47,48 Sibree49 attempted to study the relationship between viscosity of the emulsion and volume of inner phase, and deduced the formula using in the oil industry. The equation was applied to a wide range of volume of inner phase and close to the application of oil industry.

=

1 1 − (ℎ∅)/

µ = viscosity of emulsion µ0 = viscosity of solvent or continuous phase Φ = volume fraction of inner phase h = volume factor In this paper, the effect of GO concentration and volume ratio on viscosity of GO dispersion/crude oil emulsions was investigated. Considering those formula deduced by former researchers, the volume of internal phase and viscosity of continues phase play crucial roles on emulsion viscosity. Summarized curves of emulsion viscosity plotted against volume fraction of water phase at CGO = 0.0001%, 0.001% and 0.01% are shown in Figure 12. Viscosity of emulsion rises as increasing volume fraction of water phase at a certain concentration, inclined to exponential variation. GO concentration and bubble size affect the emulsion viscosity slightly. Since emulsion is a thermodynamically unstable system, viscosity also should be affected by the preparation procedure or other


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factors, and, more investigations should be carried out to make deep insight into the emulsion viscosity.

Viscosity (mPa.s)

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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1400

0.0001%

1200

0.001% 0.01%

1000 800 600 400 200 0 5

15

25

35

45

55

65

75

Volume fraction of internal phase (%)

Figure 12 The proportion of water phase volume in emulsion changes with viscosity at different concentration of GO. 4 Conclusion In this paper, stable W/O emulsions with different internal phase proportion were prepared by mixing GO dispersions and crude oil. Phase behaviors of the emulsion are greatly dependent on the GO concentration and volume ratio of water and oil. At volume ratio of 1:1, the introduction of slice GO makes crude oil and water to form miscible stable W/O emulsions completely, accompanying by a sharp increase of viscosity, even though GO concentration is only 0.0001%. Viscosity of the W/O emulsions is over 100 mPa·s, about ten times than that of the oil, while it is only 33.4 mPa·s in maximum without GO. Increase of water volume fraction from 0.1 to 0.7 induces an increase of viscosity, in exponential variation with volume ratio of internal phase. Moreover, this current approach is proved to be convenient in the preparation of high inner phase emulsions. At Fw = 0.75, HIPE with high viscosity can be


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obtained in the range of 0.0005% ~ 0.008%. Viscosity of the miscible phase is strongly dependent on the proportion of the inner phase with approximately exponential correlations and is slightly affected by GO concentration. The present study provides an excellent system of GO dispersion in making stable emulsions at low dosage due to its laminated structure and the strong interactions with hydrophobic resin and asphaltenes at the oil/water interface. It also suggests that GO is an excellent candidate in constructing high viscosity W/O emulsions in high water cut period, which has great potential in improving oil displacement.

Acknowledgement The authors thank financial supports from the China National Petroleum Corporation Major Project (101016306001001b34) of China.

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Phase Behavior of Pickering Emulsions Stabilized by Graphene Oxide

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