Water Interface: A New

Subscriber access provided by GAZI UNIV

Article

Assembly of graphene oxide at crude oil/water interface: a new approach to efficient demulsification Shenwen Fang, Ting Chen, Rong Wang, Yan Xiong, Bin Chen, and Ming Duan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00195 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 56

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

1

Energy & Fuels

Figure Captions:

2

Figure 1. TEM morphology of the GO nanosheet

3

Figure 2. Demulsification process driven by GO (a)variation of the process with the

4

increase of temperature(25 oC, 30 oC, 40 oC, 50 oC, 60 oC, 65 oC); (b) variation of the

5

process with the increase of the dosage(0ppm, 50ppm, 75ppm, 100ppm, 150ppm,

6

200ppm).

7

Figure 3. Confocal microscopy images of oil droplets in the emulsion before and after

8

treated by GO. (Confocal microscopy images of oil droplets in the emulsion: (a) the

9

oil droplets with diameters smaller than 10 μm uniformly distributed in the untreated

10

oily wastewater; (b) the oil droplets flocculate together after introducing the GO

11

nanosheet; (c) a selected region of the flocculating constituent for a better observation;

12

(d) the fluorescence images of the selected flocculate constituents showing a strong

13

blue fluorescence emission).

14

Figure 4. (a) The interfacial tension (IFT) as a function of temperature (with the GO

15

dosage fixed on 200mg/L); (b) the interfacial dilational modulus (E) as a function of

16

temperature (with the GO dosage fixed on 200mg/L); (c) the effect of GO dosage on

17

the IFT and E (with the temperature fixed on 65 oC); (d) Zeta potential measurements

18

as a function of temperature (with the GO dosage fixed on 200mg/L).

19

Figure 5. The wrinkling and buckling process of a pendant drop of aqueous GO

20

solution in the oil phase. 1

ACS Paragon Plus Environment

Energy & Fuels

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

1

Figure 6. Results obtained from the single-droplet experiments: (a) Effect of

2

temperature on the coalescence process for the untreated emulsion, and (b) the

3

emulsion after treated by GO (with the GO dosage fixed at 200mg/L); (c) Effect of

4

GO dosage on the coalescence process (with the temperature fixed at 65 oC); (d) the

5

drainage time for the emulsion before and after treated by GO.

6

Figure 7. Different stages of the coalescence process. (a) Droplet-to-planar model

7

with 1mM KCl solution as the water phase; (b) 1mM KCl solution containing

8

200mg/L GO as the water phase.

9

Figure 8. Schematic illustration of coalescence process of the oil droplet. (a)a released

10

oil drop in the water phase, and a protective interfacial film was formed; (b)the GO

11

nanosheets diffused to the interface and penetrate the initial interfacial film; (c)the

12

flexible nature of the GO nanosheets enabled them to orient parallel to the interface

13

and self-assembled to form a new “GO film”; (d)when the oil drop was released to the

14

interface, the upper GO nanosheets tend to stack with the GO wrapped outside the oil

15

drop, thus the oil drop was allowed to contact with the bulk oil; (e)the oil drop started

16

to merge into the bulk oil and the external “GO” film became wrinkled, like a deflated

17

balloon; (f)the oil drop completely merged into the bulk oil and the external “GO”

18

film was finally assemble at the upper interface.

19

Figure 9. The changing process of a released crude oil drop contacting with oil/water

20

interface (and the “GO film” was observed within 0.08s).

21

Figure 10. Optical (a) and corresponding fluorescence images (b) of the assembled

2


Page 2 of 56

Page 3 of 56

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

Energy & Fuels

1

“GO film” at the interface.

2

Figure 11. The changing process of a released oil drop contacting with oil/water

3

interface. (a) a drop of saturates ruptured within 3.64s;(b) a drop of aromatics

4

ruptured within 3.88s;(c) the “GO film” produced immediately once the drop of resins

5

contacting the interface;(d) the “GO film” produced immediately once the drop of

6

asphaltenes contacting the interface.

7

Figure 12. Three-dimensional diagram of the molecular structure of GO. The grey

8

spheres represent carbon atoms, white spheres represent hydrogen atoms, and red

9

spheres represent oxygen atoms. (For interpretation of the references to colour in this

10

figure legend, the reader is referred to the web version of this article.)

11

Figure 13. Molecular dynamics simulations of the assembly and orientation behaviors

12

of GO nanosheets. The oil phase was represented in gray, and the water molecules

13

were hidden.

14 15


16

3


Energy & Fuels

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 4 of 56

（b）

（a） 1 2

Figure 2. Demulsification process driven by GO (a)variation of the process with

3

the increase of temperature(25 oC, 30 oC, 40 oC, 50 oC, 60 oC, 65 oC); (b) variation of

4

the process with the increase of the dosage(0ppm, 50ppm, 75ppm, 100ppm, 150ppm,

5

200ppm).

4


Page 5 of 56

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

Energy & Fuels

(a)

(b)

(c)

(d)

1 2

Figure 3. Confocal microscopy images of oil droplets in the emulsion before and

3

after treated by GO. (Confocal microscopy images of oil droplets in the emulsion: (a)

4

the oil droplets with diameters smaller than 10 μm uniformly distributed in the

5

untreated oily wastewater; (b) the oil droplets flocculate together after introducing the

6

GO nanosheet; (c) a selected region of the flocculating constituent for a better

7

observation; (d) the fluorescence images of the selected flocculate constituents

8

showing a strong blue fluorescence emission).

5


Energy & Fuels

20

30

25

(b)

19

Blank GO

GO Blank

18

Modulus E(mN/m)

Interfacial tension(mN/m)

(a)

20

15

17 16 15 14 13 12

10

11

5 25

30

35

40

45

50

55

60

65

10 25

70

30

35

40

45

50

55

60

65

70

o

o

Temperature( C)

Temperature( C)

10

-15

15

(c)

(d)

9

14

Oily wastewater GO

13

7 12 6 11

5

-25

-30

10

4 3

Zeta potential(mV)

-20 8

Modulus E(mN/m)


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 6 of 56

0

50

100

150

200

-35 25

9

30

35

40

45

50

55

60

65

o

Temperature( C)

GO concentration(mg/L)

1


2


3


4


5


6


70

Page 7 of 56

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

Energy & Fuels

Suck in

Squeeze out

1 2


3

solution in the oil phase.

7


Energy & Fuels

0.13

(a)

3.5

1.6

0.12

(b)

50

0.11

3.0

1.4

45

0.10 40

2.0

1.0

35

0.08

k

1.2

t1/2(s)

2.5

k

t1/2(s)

0.09

0.07

30

0.06 25 0.05

1.5

0.8

20

0.04

25

30

35

40

45

50

55

60

65

70

25

30

35

45

50

55

60

65

70

Temperature( C)

Temperature( C) 2.0 1.8

40

o

o

3.0

(c)

50

(d)

Blank GO

2.5

1.6

1.2

30

k

1.0 0.8

20

0.6 10

0.4

Drainage time(s)

40

1.4

t1/2(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

Page 8 of 56

2.0 1.5 1.0 0.5

0.2 0

0.0 0

50

100

150

0.0 25

200

30

35

40

45

50

55

60

65

o

Temperature( C)


1


2


3


4


5


8


70

Page 9 of 56

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

Energy & Fuels

(a)

0s

3.36s

Bulk oil

Rupture

Roll Oil Blank (b) 0s Bulk oil

0.08s Film

Wrinkle Oil GO

1


2


3


9


Energy & Fuels

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

(c)

(b)

(a)

(e)

(d) Water phase

Page 10 of 56

Bulk oil

(f)

Oil droplet

Interfacial film

GO

1 2

Figure 8. Schematic illustration of coalescence process of the oil droplet. (a)a

3

released oil drop in the water phase, and a protective interfacial film was formed;

4

(b)the GO nanosheets diffused to the interface and penetrate the initial interfacial film;

5

(c)the flexible nature of the GO nanosheets enabled them to orient parallel to the

6

interface and self-assembled to form a new “GO film”; (d)when the oil drop was

7

released to the interface, the upper GO nanosheets tend to stack with the GO wrapped

8

outside the oil drop, thus the oil drop was allowed to contact with the bulk oil; (e)the

9

oil drop started to merge into the bulk oil and the external “GO” film became

10

wrinkled, like a deflated balloon; (f)the oil drop completely merged into the bulk oil

11

and the external “GO” film was finally assemble at the upper interface.

10


Page 11 of 56

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

Energy & Fuels

1 2


3


11


Energy & Fuels

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

1

Page 12 of 56

=

(a)

(b)

2 3


4


12


Page 13 of 56

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

Energy & Fuels

(a)

(b)

(c)

(d)

1 2


3


4


5


6


7 8 9 10 11

13


Energy & Fuels

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

1 2


3


4


5


6

14


Page 14 of 56

Page 15 of 56

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

1

Energy & Fuels

(b)

(a)

2


3


4

were hidden.

5

15


Energy & Fuels

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

1

Assembly of graphene oxide at crude oil/water interface: a new approach to

2

efficient demulsification

3

Shenwen Fang a,b, Ting Chena, Rong Wanga, Yan Xiong a,b, Bin Chena, Ming Duana,b,*

4 a

5

School of Chemistry and Chemical Engineering, Southwest Petroleum

6 7

University, Chengdu, Sichuan 610500, China. b

Oil & Gas Applied Chemistry Key Laboratory of Sichuan Province, Chengdu,

8

9

Sichuan 610500, China.

Abstract

10

Graphene oxide (GO) has gathered widespread interest within the scientific

11

community due to its unique properties. In the present work, the interfacial behavior

12

of the grapheme oxide (GO) nanosheet at the crude oil/water interface was explored

13

to investigate its demulsification mechanism for crude oil/water emulsions. The

14

interfacial rheology properties and the interfacial tension were systematically

15

discussed. The results revealed that GO was able to decrease the interfacial tension of

16

the emulsion to a large extent, implying that the GO nanosheet was interfacially active.

17

Unexpectedly, the dilational modulus monotonically increased with increasing the GO

18

dosage. In addition, the coalescence kinetics and the interfacial assembly behaviors of

19

GO were investigated. It was observed that the oil droplet became wrinkled once 1


Page 16 of 56

Page 17 of 56

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

Energy & Fuels

1

contacting with the crude oil/water interface, and a thin film was finally left at the

2

interface. Therefore, the GO nanosheet was thought to be able to diffuse to the

3

oil/water interface and self-assembled to jam into a new solid thin “GO film”, leading

4

to the increase of the determined dilational modulus of the interface. The morphology

5

of the film was revealed by a confocal fluorescence microscope, and a wrinkled and

6

continuous morphology was observed, implying that the GO nanosheet aligned

7

parallel to the oil/ water interface. The findings in the present study are crucial for

8

fully understanding the demulsification mechanism of GO and might provide a facile

9

way to prepare large area graphene oxide thin films.

10 11

Keywords: Graphene oxide; interfacial; assembly; demulsification; film

12

Introduction

13

Crude oil is generally produced commingled with water in the form of emulsions,

14

causing a series of operational problems during oil production and large quantities of

15

oily wastewater1. Oil pollution resulted from the directly discharge of these oily

16

wastewater has become one of the most urgent environmental problems that afflicting

17

people all over the world, which is supposed to grow worse in the coming decades

18

with the increasingly consumption of petroleum products2, 3. Thus, the treatment of

19

the oily wastewater, especially those produced from polymer flooding containing

20

residual polymer, remains to be a worldwide challenge. In fact, various approaches

2


Energy & Fuels

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

1

have been used in research for the treatment of oily wastewater, including the gravity

2

separation, air flotation4, coagulation5, absorption6, 7, and membrane filtration8-10.

3

However, most of the adopted technologies suffer from the limitations of complex

4

operation process, energy-cost, low efficiency and time-consuming. Therefore, the

5

development of novel efficient approaches to the treatment of oil/water emulsions is

6

highly desired.

7

Graphene oxide (GO)11 – a giant, flexible planar thin sheet -- has gathered

8

widespread interest within the scientific community due to its unique properties12-14. It

9

is commonly accepted that abundant oxygen containing functional groups such as

10

hydroxyl, carboxyl and epoxy groups are introduced on the surface of GO during

11

oxidation of the graphite15-17, which enables GO to be a good amphiphilic surfactant

12

with a hydrophobic carbon basal plane and these hydrophilic functional groups

13

decorating the periphery.11, 18-23. These attributes of GO make it a promising candidate

14

for the demulsification of oil/water emulsions. Most recently, Sili Ren et al.24 first

15

reported that GO can be used as an environmentally friendly and highly efficient

16

demulsifier to quickly separate the oil from an O/W emulsion within just a few

17

minutes, and a possible mechanism of the demulsification process was proposed.

18

Unfortunately, a direct evidence of the mechanism of the demulsification process has

19

not been obtained.

20

A fundamental understanding of the demulsification mechanism of GO is

21

necessary for the further development of GO as an efficient solution to demulsify

3


Page 18 of 56

Page 19 of 56

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

Energy & Fuels

1

crude oil emulsions. It is now well-established that these crude oil emulsions are

2

mostly stabilized by natural surfactants, such as asphaltenes and resins, which adsorb

3

irreversibly at the interfaces and tend to inhibit the inter drop film drainage by

4

building a viscoelastic, physically cross-linked film, thereby preventing the drop from

5

coalescence25-28. Hence, demulsifiers are required to be able to diffuse to the oil/water

6

interface and interact with the initial surfactant molecules to cause destabilization of

7

the emulsions29-31. Typically, the interaction occurred in the interface during the

8

demulsification process mainly involves three major processes: (1) the demulsifiers

9

exhibit stronger interfacial activity than the initial surfactant and penetrate the

10

interfacial film32, 33; (2) the demulsifiers disrupt or soften the interfacial film34; (3) the

11

demulsifiers enhance film drainage by suppressing the tension gradient35. Therefore, a

12

systematically discussion of the interfacial behaviors of demulsifiers at the crude

13

oil/water interface is the key to fully understanding its demulsifiaction mechanism.

14

In the present study, we focused on the exploration of the interfacial behaviors of

15

GO at the crude oil/water interface to systematically investigate its demulsification

16

mechanism for crude oil/water emulsions. The grapheme oxide was discovered to be

17

able to assemble at the crude/oil interface and jammed into a solid thin film. In

18

addition, the interfacial rheology properties, the interfacial tension, and the assembly

19

kinetics of GO at the crude oil/water interface were systematically studied.

4


Energy & Fuels

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

1

Experimental section

2

Materials

3

GO suspension (1.0 wt%) was purchased from Aladdin Chemistry Co., Ltd. China.

4

The oily wastewater produced from polymer flooding (with a polymer content of 50

5

mg/L) and the crude oil sample were obtained from an offshore oilfield in China. The

6

SARA fraction (saturates, aromatics, resins, and asphaltenes) of the crude oil was

7

analyzed by a classical chromatography separation method. All the chemical reagents

8

were used as received without further purification.

9

Demulsification test

10

To evaluate the demulsification performance, the GO suspension was added into 25

11

mL of the oily wastewater (oil-in-water emulsion) contained in a colorimeter tube.

12

Then, the colorimeter tube was shaken for 200 times to ensure that the GO and the

13

emulsion can be uniformly mixed. For the blank test, the GO suspension was replaced

14

by the same volume of deionized water. The mixture was then placed in a

15

thermostated water bath and gravity settled for oil/water separation observation.

16

Specifically, the effect of the dosage of GO and the temperature on the

17

demulsification performance were systematically investigated. To have a better

18

understanding of the demulsification process, a Nikon C2 confocal microscope

19

(Nikon, Tokyo, Japan) was applied to observe the change of the oil droplets during

20

demulsification. The Zeta potential measurements were conducted on a Brookhaven 5


Page 20 of 56

Page 21 of 56

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

Energy & Fuels

1

Zeta PALS (Brookhaven Corp., USA) at desired temperatures. The GO suspension

2

was uniformly dispersed in 1 mM KCl solution under ultrasonic treatment. The

3

measurement was repeated for 10 times to ensure the accuracy of the results. The

4

interfacial tension (IFT) was measured with a spinning drop tensiometer (Model

5

TX500, CNG USA CO.)

6

Interfacial rheology measurement

7

The rheological measurements were performed on a drop shape analyzer (Model

8

DSA30, (Krüss GmbH, Hamburg, Germany) equipped with an oscillating pendant

9

drop tensiometer. The dehydrated crude oil was diluted by kerosene to 0.25 wt% and

10

was employed as the oil phase, and the 1 mM KCl solution containing GO with

11

desired concentrations was used as the water phase. The water phase was filled in a

12

glass capillary which was rinsed by petroleum ether, absolute ethanol and deionized

13

water alternatively. The capillary was then attached to the instrument and was then

14

lowered into a transparent thermostated cell containing the oil phase. After thermal

15

equilibrium of the desired temperature was achieved, a drop of the GO solution was

16

produced and maintained vertically at the end of the capillary tip (with an inner

17

diameter of 0.514mm). In the oscillation test, the dilational modulus (E) was

18

measured by performing sinusoidal variation of the drop area at a desired amplitude

19

and frequency (0.1 Hz). To retain permanent visual control, images of the drop are

20

continuously recorded on a video monitor in real-time. A Drop Shape Analysis

21

program was applied for the picture analysis, and the drop profile is processed 6


Energy & Fuels

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

1

according to the fundamental Laplace equation 36

2

Generally, the dilational rheology reveals the interfacial resistance to changes in

3

the interface area and is described by the interfacial dilational modulus (E), which is

4

defined as Equation (1):

E =

5

∂γ ∂ ln (A A0 )

6

where γ is the interfacial tension, A the expanded film area, and A0 the initial

7

film area. In the oscillation system, the dilational rheology gives a measurement of the

8

change in interfacial energy with the change in interface area. The dilational modulus

9

(E) is a complex quantity including both a real and an imaginary component and is

10

defined as follows:

11

E = E ′ + iE ′′

12

where the elastic modulus E ′ is the real part, and the loss modulus E ′′ is the

13

imaginary part. The total modulus E reveals the variation of the energy of the

14

system with a change in the interfacial area. The elastic modulus can be regarded as

15

the elastic energy stored in the interface, and the loss modulus, as the loss energy. E ′

16

and E ′′ may also be expressed in terms of the total modulus E and the phase angle

17

θ and can be written as: E ′ = E cos θ

18 19

and

20

E ′′ = E sin θ = ωηd

21

where ω is the oscillation frequency and ηd is the viscosity of the dilational 7


Page 22 of 56

Page 23 of 56

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

Energy & Fuels

1

interface.

2

Coalescence kinetics evaluation

3

To further understand the demulsification process, a single-droplet protocol37

4

was used to investigate the rupture rate constant of the oil/water interfacial film. The

5

coalescence kinetics of these oil droplets was evaluated by using a droplet-to-planar

6

coalescence model. The oil droplets were released from a droplet extrusion device and

7

coalesced at the oil/water interface. The coalescence cell was connected to a

8

temperature controller to maintain constant temperature, and a microscope was

9

equipped to observe the coalescence and rupture process of the oil droplets. The

10

drainage time was finally calculated by a computer.

11

Specifically, the water phase (1 mM KCl solution containing GO with desired

12

concentrations) was filled in the coalescence cell, and the oil phase (0.25 wt% diluted

13

crude oil) was then added on top of the water phase to form an oil/water interface. An

14

oil droplet was squeezed out of a curved needle by a droplet extrusion device and

15

entered into the water phase. When the oil droplet reached the oil/water interface,

16

timing was triggered. The process was repeated for 30 droplets under each set of

17

conditions.

18

Assembly of GO at the oil/water interface

19

GO, a giant flexible sheet, was supposed to conform to a curved interface

20

geometry and effectively cover the oil/water interface38. To figure out the packing 8


Energy & Fuels

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

1

process of GO at the oil/water interface, the interfacial assembly of GO at the

2

interface and its kinetics were investigated. The water phase (GO in 1 mM KCl

3

solution) was filled in a transparent thermostated cell, and the oil phase (0.25 wt%

4

diluted crude oil) was added on top of the aqueous phase to form an oil/water

5

interface. An oil drop was squeezed out from a curved needle fixed upside down in

6

the cell and maintained at the end of the tip in the aqueous phase. Then the oil droplet

7

diffused to the interface and, upon contacting the interface, a wrinkled film formed.

8

The whole diffusion procedure was recorded by a video monitor in real-time for a

9

visual observation of the assemble process. The experiment was repeated for a

10

number of droplets until large-area GO film were assembled at the interface. The film

11

was then retrieved with a glass slide for morphology characterization.

12

Molecular dynamics simulation

13

Molecular dynamics simulations on the assembly and orientation behaviors of

14

GO nanosheets at the oil/water interface were carried out to further understand the

15

mechanism of demulsification process. The simulation box sizes were chosen

16

according to the given system. The time step was set as 1fs, and the total simulation

17

step was set as 1ns. The three-dimensional (3D) structures of molecules of water,

18

toluene, n-heptane, asphaltenes, and grapheme oxide were created and optimized by

19

using the Accelrys Material Studio software package 7.0 (MS). Besides, 3D periodic

20

boundary conditions were applied during the model construction and simulations. In

21

the simulations, the system energy and the temperature between molecular groups 9


Page 24 of 56

Page 25 of 56

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

Energy & Fuels

1

were monitored to indicate the equilibrium of the systems. Graphene oxide nanosheets

2

were randomly set into the water phase. All of the molecular configurations and

3

snapshots were visualized and acquired with visual molecular dynamics.

4

Results and discussion

5

Demulsification performance

6

The morphology of the GO nanosheet was revealed by using TEM, as displayed

7

in Figure 1. It was observed that wrinkles and defects are formed on the surface of the

8

GO nanosheet.

9

The demulsification process of the oily wastewater driven by GO was illustrated

10

in Figure 2. The effect of temperature and the dosage of GO was taken into

11

consideration. It was observed that after introducing the GO suspension, the stability

12

of the emulsion was destroyed immediately. Besides, the oil/water separation process

13

was speeded up by increasing the temperature and the GO dosage.

14

To further understand the demulsification process, the changes of the

15

morphology of the oil droplets in the emulsions before and after treated by GO were

16

observed using a confocal microscope, and the results are displayed in Figure 3. As

17

shown in Figure 3a, the oil droplets with diameters smaller than 10 µm uniformly

18

distributed in the untreated oily wastewater, implying that the wastewater existed in a

19

form of emulsion. In contrast, the oil droplets flocculate together after introducing the

20

GO nanosheet, as presented in Figure 3b, c. Furthermore, the fluorescence images of 10


Energy & Fuels

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

1

the flocculate constituents were recorded using scanning confocal microscopy. As

2

presented in Figure3d, a strong blue fluorescence emission could be observed, which

3

might be attributed to the heterogeneous electronic structure of GO, according to

4

previous reported literatures39, indicating that the flocculated oil droplets might be

5

wrapped by the GO nanosheets.

6

Interfacial rheology

7

The equilibrium interfacial tension and the dilational modulus of the interfacial film

8

were determined as a function of the temperature and the GO dosage. The results are

9

shown in Figure 4. As illustrated in Figure 4a, it was found that the addition of GO

10

decreased the interfacial tension of the emulsion to a large extent, implying that the GO

11

nanosheet had excellent interfacial activity and was capable of diffusing to the oil/water

12

interface easily. The effect of temperature on the interfacial dilational modulus was

13

presented in Figure 4b. It was observed that the interfacial dilational modulus decreased

14

with temperature, indicating a decrease in the interfacial film strength. However, both of

15

the interfacial tension and the dilational modulus monotonically increased with increasing

16

the GO dosage, as presented in Figure 4c. One possible explanation for this result was that

17

the GO nanosheet diffused to the oil/water interface and self-assembled to form a new “GO

18

film”. As a consequence, the GO nanosheet occupied most of the interfacial area and the

19

substantial assembly of the initial natural surfactants in the crude oil was prevented, thus

20

an increase of the GO concentration leading to a higher equilibrium interfacial tension.

21

Furthermore, the newly formed “GO film” was more robust than the initial interfacial film 11


Page 26 of 56

Page 27 of 56

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

Energy & Fuels

1

stabilized by the natural surfactants, thus, the determined dilational modulus increased with

2

increasing the GO concentration. Similar results have been reported for silica nanoparticles

3

in a previous study40, where an increase in oil/water interfacial tension was observed when

4

increasing the amount of silica nanoparticles at the interface because the assemble of the

5

nanoparticles at the oil/water interface would cause depletion in the interface of the initial

6

small-molecule surfactant. The zeta potential measurements for the crude oil/water

7

emulsion (i.e., the oily wastewater) and the GO nanosheet were conducted as a function of

8

temperature. As presented in Figure 4d, the results showed that the zeta potential for both

9

the wastewater and GO became less negative with increasing the temperature, leading to a

10

decrease in the electrostatic repulsion between GO nanosheets and oil droplets. The results

11

indicated that higher demulsification efficiency might be obtained at higher temperatures,

12

which was in good agreement with the experimental phenomena.

13

In addition, an intriguing phenomenon was observed during the interfacial

14

rheology test. The images are presented in Figure 5. As shown in Figure 5, a pendant

15

drop of the aqueous GO solution was maintained in the oil phase (diluted crude oil)

16

for 10-15 minutes. When sinusoidal variation of the drop area (0.1 Hz).was performed,

17

the interfacial area decreased and a wrinkling and buckling process of the GO drop

18

was observed. This observation further confirmed that an elastic film (i.e., the “GO”

19

film) was produced at the interface, which was robust enough to withstand massive

20

shrinkage of the droplet. Thomas P. Russell et al.38 first reported that graphene oxide

21

can be trapped at water/oil interfaces and jammed into a solid thin film using a block

12


Energy & Fuels

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

1

copolymer, due to the interaction occurred between the quaternized block copolymer

2

chains and the peripheral carboxyl groups on the GO surface. In fact, similar

3

phenomenon can be also observed for some amphiphilic spherical nanoparticles

4

according to previous literatures41, 42.

5

Coalescence kinetics

6

To further understand the coalescence kinetics, a single-droplet protocol was

7

applied to analyze the demulsification process. It has been reported that the rupture

8

process of the emulsion droplet consists of two sequential procedures37. Firstly, the

9

bulk oil phase and some demulsified water droplets distributed in the oil phase is

10

drained into the water phase. This step is regarded as the drainage process, where

11

droplets have not yet broken, and the corresponding time is regarded as the initial

12

drainage time. Subsequently, the oil droplets completely merge into the bulk oil phase

13

and the oil/water interface turns to completely flat. According to Cockbain’s theory33

14

the examination curve at the rupture stage can be expressed as follows:

15

ln (N N 0 ) = kt + C

16

where N is the number of droplets which do not coalesce within the drainage

17

time; N 0 is the total number of determined droplets; k is the rupture rate constant;

18

t is the drainage time; C is regression factor.

19

The coalescence speed of the oil droplets can be determined by their half lifetime

20

t1 2 ，namely the time required for half of the droplets to disappear. Thus, the

21

following relationship can be obtained: 13


Page 28 of 56

Page 29 of 56

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

Energy & Fuels

t1 2 =

1

ln 2 + C

k

2

The results obtained from the single-droplet experiments were illustrated in

3

Figure 6. Figure 6a, b displayed the effect of temperature on the coalescence process

4

for the the emulsion before and after treated by GO (200ppm), respectively. The

5

results showed that with increasing the temperature, the value of half lifetime (t1/2)

6

decreased while the rupture rate constant (k) increased, implying that the temperature

7

had a positive impact on the coalescence process. More precisely, an increase of k was

8

indicative that the thinning rate of the interfacial film was increased, thus the

9

interfacial film beecam more unstable (i.e., the film was easier to rupture). A similar

10

conclusion was obtained for the effect of GO dosage, as revealed in Figure 6(c). A

11

possible explanation might be that the increase of the GO concentration made it easier

12

for water to be drained into the water phase in the drainage process, thus the oil

13

droplets aggregated with each other and merged into the bulk oil phase. The drainage

14

time for the emulsion before and after treated by GO was displayed in Figure 6 (d). It

15

was obvious that the additive of GO can accelerate the coalescence process to a large

16

extent.

17

Different stages of the coalescence process for the emulsion before and after

18

treated by GO were illustrated in Figure 7. As shown in Figure 7a, when the oil

19

droplet was released to crude oil/water interface, it rolled at the interface and ruptured

20

within 3.36s. However, when GO was added into the water phase, it was observed

21

that the oil droplet became wrinkled once contacting with the crude oil/water interface,

14


Energy & Fuels

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

1

and a thin film was finally left at the interface within 0.08s (see Figure 7b).

2

The schematic illustration of this process was shown in Figure 8. The strong

3

interfacial activity of GO nanosheet enabled it to be attracted to the oil/water interface.

4

As a giant, flexible planar sheet, GO was expected to penetrate the initial interfacial

5

film and self-assembled at the interface to form a new film with the oil droplet

6

wrapped in it. Once the oil droplet was released to the upper oil/water interface, the

7

“GO film” outside the drop stacked with the “GO film” assembled at the upper

8

interface immediately, thus the wrapped oil drop was drained into the bulk oil phase.

9

As a result, the external “GO film” became wrinkled, just like a balloon that had been

10

deflated, and finally packed at the upper interface.

11

Interfacial assembly of GO

12

To further understand the mechanism of the demulsification process, the

13

interfacial self-assemble behavior of the GO nanosheet at the crude oil/water interface

14

was investigated detailedly. As displayed in Figure 9 is the changing process of a

15

released oil drop when contacting with oil/water interface. It was observed that once

16

contacting with the interface, the oil drop deformed and a transparent film was formed.

17

The observation in Figure 9 further confirmed that the GO nanosheet assembled at the

18

interface and wrapped outside the oil drop to form the “GO film”.

19

A large-area “GO film” was prepared by releasing several oil drops to the

20

interface, and the packed “GO film” was retrieved with a glass slide. The morphology

21

of the “GO film” was characterized by the confocal fluorescence microscope. As 15


Page 30 of 56

Page 31 of 56

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

Energy & Fuels

1

shown in Figure 10, a wrinkled and continuous morphology was observed, implying

2

that the GO nanosheet aligned parallel to the oil/ water interface. In addition, a strong

3

blue fluorescence emission could be readily observed for the assembled “GO film”,

4

which might be attributed to the heterogeneous electronic structure of GO, according

5

to previous reported literatures39.

6

To figure out which components in the crude oil contribute to the adsorption

7

force between the GO nanosheet and the oil drop exactly, the interfacial assembly

8

experiment was repeated for the SARA fraction (i.e., saturates, aromatics, resins, and

9

asphaltenes) of the crude oil. As shown in Figure 11a, b, when the oil drops of

10

saturates and aromatics contacted with the oil/water interface, no sign of the “GO film”

11

was observed. However, the “GO film” was observed for both of the oil drops of

12

resins and asphaltenes. It was well recognized that asphaltenes and resins have high

13

affinity for the water-oil interface and tend to build viscoelastic films at the interface

14

to prevent drop-drop coalescence, hence the stability of water-in-oil emulsions was

15

ensured43-45. In addition, asphaltenes and resins are featured by large, polar,

16

polynuclear molecules which are constitutive of condensed aromatic ring systems and

17

heteroatoms such as sulphur, nitrogen and oxygen46. The GO nanosheet was also

18

known to have the chemical structure of conjugated aromatic rings with huge

19

delocalized π systems47. It has been reported that the π-conjugated system of

20

asphaltenes or resins is capable of interacting with the polarized π orbitals of GO to

21

form strong π−π interactions, and the anti-bonding orbital of asphaltenes or resins

16


Energy & Fuels

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

1

tend to interact with electron-rich functional groups on the GO surface to form n−π

2

interactions24. Therefore, it can be concluded that the formation of the “GO film”

3

might is highly related to the π−π or n−π interactions occurred between the asphatenes

4

or resins and the GO nanosheet.

5

Molecular dynamics simulation

6

Molecular dynamics simulations on the assembly and orientation behaviors of

7

GO nanosheets at the oil/water interface were carried out to further understand the

8

mechanism of demulsification process. In the simulation, the molecular structure of

9

graphene oxide was sketched using the Accelrys Material Studio software package,

10

and the three-dimensional diagram is shown in Figure 12. According to the as shown

11

structural model23,

12

two-dimensional molecular amphiphile, with hydroxyl and epoxy functional groups

13

interspersed on the basal plane, a network of hydrophobic polyaromatic islands of

14

unoxidized benze rings, and the hydrophilic –COOH groups decorating on the edges.

15

Simulation results of the assembly and orientation behaviors of GO at the oil/water

16

interface were displayed in Figure 13. It was found that the GO nanosheets quickly

17

diffused to the oil/water interface from the water phase, due to the amphiphilicity of

18

GO. More importantly, it was observed that the GO nanosheets tended to be

19

perpendicular to the interface at first, thus the initial interfacial film was penetrated by

20

the GO nanosheets, and the oil droplets were allowed to coalesenced. Subsequently,

21

the flexible nature of the GO nanosheets enabled them to orient parallel to the

48, 49

, the graphene oxide can be basically viewed as a

17


Page 32 of 56

Page 33 of 56

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

Energy & Fuels

1

oil/water interface, and finally to be anchored to the interface, thereby the

2

macroscopic “GO film” was produced.

3

Conclusions

4

GO nanosheet was employed as a demulsifier to break up crude oil/water

5

emulsion, and interfacial behavior of the GO nanosheet at the crude oil/water

6

interface was explored to investigate its demulsification mechanism. The interfacial

7

rheology properties and the interfacial tension were systematically discussed. The

8

results revealed that GO was able to decrease the interfacial tension of the emulsion to

9

a large extent, implying that the GO nanosheet was interfacially active. However, the

10

dilational modulus was found to monotonically increased with increasing the GO

11

dosage. In addition, the coalescence kinetics and the interfacial assembly behaviors of

12

GO were investigated. It was observed that the oil droplet became wrinkled once

13

contacting with the crude oil/water interface, and a thin film was finally left at the

14

interface. Therefore, the GO nanosheet was thought to be able to diffuse to the

15

oil/water interface and self-assembled to jam into a robust “GO film”. As a result, the

16

dilational modulus was determined to increase with increasing the GO concentration.

17

Confocal fluorescence microscope images of the “GO film” showed that a wrinkled

18

and continuous morphology was observed, implying that the GO nanosheet aligned

19

parallel to the oil/ water interface. Furthermore, the interfacial assembly experiment

20

was repeated for the SARA fraction of the crude oil. The results indicated that it was 18


Energy & Fuels

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 34 of 56

1

asphaltenes and resins contributed to the formation of the “GO film” and the

2

demulsification process. The findings in this work are crucial for fully understanding

3

the demulsification mechanism of GO and might provide a facile way for the

4

preparation of large area GO thin films.

5

Acknowledgements

6

The authors gratefully acknowledge the financial support offered by National

7

Natural Science Foundation of China (Project No: 51504201), Program for New

8

Century Excellent Talents in University (NCET-13-0983), and Sichuan Youth Science

9

& Technology Foundation for Innovation Team (2015TD0007). Besides, we

10

acknowledge National Supercomputing Center in Shenzhen for providing the

11

computational resources and materials studio (version 7.0, Forcite module)

12

Reference

13

1.

Fan, Y.; Simon, S. b.; Sjöblom, J., Chemical Destabilization of Crude Oil

14

Emulsions: Effect of Nonionic Surfactants as Emulsion Inhibitors. Energy & Fuels

15

2009, 23, (9), 4575-4583.

16

2.

17

Mayes, A. M., Science and technology for water purification in the coming decades.

18

Nature 2008, 452, (7185), 301-10.

19

3.

Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.;

Chu,

Z.;

Feng,

Y.;

Seeger,

S.,

Oil/water

separation

19


with

selective

Page 35 of 56

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

Energy & Fuels

1

superantiwetting/superwetting surface materials. Angewandte Chemie 2015, 54, (8),

2

2328-38.

3

4.

4

separation of emulsified oil/water mixtures by ultrathin free-standing single-walled

5

carbon nanotube network films. Advanced materials 2013, 25, (17), 2422-7.

6

5.

7

110-125.

8

6.

9

effective continuous absorption and expulsion of oil pollutants from water. ACS

Shi, Z.; Zhang, W.; Zhang, F.; Liu, X.; Wang, D.; Jin, J.; Jiang, L., Ultrafast

Srivastava, S. N., Macromolecular Emulgents in Emulsion Stability. 1975, 9,

Wang, C. F.; Lin, S. J., Robust superhydrophobic/superoleophilic sponge for

10

applied materials & interfaces 2013, 5, (18), 8861-4.

11

7.

12

Breathing

13

Superhydrophilic Sponge. ACS applied materials & interfaces 2015, 7, (40),

14

22264-71.

15

8.

16

membranes for emulsified oil/water separation. NPG Asia Materials 2014, 6, (5),

17

e101.

18

9.

19

Membrane via Binding TiO2(2)Nanoparticles and a PVA Layer onto a Membrane

20

Surface. ACS applied materials & interfaces 2015, 7, (16), 8427-36.

21

10. Chakrabarty, B.; Ghoshal, A. K.; Purkait, M. K., Ultrafiltration of stable

Xu, L.; Chen, Y.; Liu, N.; Zhang, W.; Yang, Y.; Cao, Y.; Lin, X.; Wei, Y.; Feng, L., Demulsification:

A

Three-Dimensional

(3D)

Free-Standing

Zhu, Y.; Wang, D.; Jiang, L.; Jin, J., Recent progress in developing advanced

Qin, A.; Li, X.; Zhao, X.; Liu, D.; He, C., Engineering Highly Hydrophilic PVDF

20


Energy & Fuels

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

1

oil-in-water emulsion by polysulfone membrane. Journal of Membrane Science 2008,

2

325, (1), 427-437.

3

11. Frost, R.; Jonsson, G. E.; Chakarov, D.; Svedhem, S.; Kasemo, B., Graphene

4

oxide and lipid membranes: interactions and nanocomposite structures. Nano letters

5

2012, 12, (7), 3356-62.

6

12. Yan, X.; Chen, J.; Yang, J.; Xue, Q.; Miele, P., Fabrication of free-standing,

7

electrochemically active, and biocompatible graphene oxide-polyaniline and

8

graphene-polyaniline hybrid papers. ACS applied materials & interfaces 2010, 2, (9),

9

2521-9.

10

13. Wang, J.; Chen, Z.; Chen, B., Adsorption of polycyclic aromatic hydrocarbons by

11

graphene and graphene oxide nanosheets. Environmental science & technology 2014,

12

48, (9), 4817-25.

13

14. Ban, S.; Xie, J.; Wang, Y.; Jing, B.; Liu, B.; Zhou, H., Insight into the Nanoscale

14

Mechanism of Rapid H2O Transport within a Graphene Oxide Membrane: Impact of

15

Oxygen Functional Group Clustering. ACS applied materials & interfaces 2016, 8, (1),

16

321-32.

17

15. Larciprete, R.; Fabris, S.; Sun, T.; Lacovig, P.; Baraldi, A.; Lizzit, S., Dual path

18

mechanism in the thermal reduction of graphene oxide. Journal of the American

19

Chemical Society 2011, 133, (43), 17315-21.

20

16. Li, B.; Hou, W.; Sun, J.; Jiang, S.; Xu, L.; Li, G.; Memon, M. A.; Cao, J.; Huang,

21

Y.; Bielawski, C. W.; Geng, J., Tunable Functionalization of Graphene Oxide Sheets

21


Page 36 of 56

Page 37 of 56

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

Energy & Fuels

1

through Surface-Initiated Cationic Polymerization. Macromolecules 2015, 48, (4),

2

994-1001.

3

17. Barroso-Bujans, F.; Fernandez-Alonso, F.; Pomposo, J. A.; Cerveny, S.; Alegría,

4

A.; Colmenero, J., Macromolecular Structure and Vibrational Dynamics of Confined

5

Poly(ethylene oxide): From Subnanometer 2D-Intercalation into Graphite Oxide to

6

Surface Adsorption onto Graphene Sheets. ACS Macro Letters 2012, 1, (5), 550-554.

7

18. Imperiali, L.; Liao, K. H.; Clasen, C.; Fransaer, J.; Macosko, C. W.; Vermant, J.,

8

Interfacial rheology and structure of tiled graphene oxide sheets. Langmuir : the ACS

9

journal of surfaces and colloids 2012, 28, (21), 7990-8000.

10

19. Xu, C.; Xu, Y.; Zhu, J., Photocatalytic antifouling graphene oxide-mediated

11

hierarchical filtration membranes with potential applications on water purification.

12

ACS applied materials & interfaces 2014, 6, (18), 16117-23.

13

20. Chen, X.; Chen, B., Macroscopic and spectroscopic investigations of the

14

adsorption of nitroaromatic compounds on graphene oxide, reduced graphene oxide,

15

and graphene nanosheets. Environmental science & technology 2015, 49, (10),

16

6181-9.

17

21. Hu, K.; Kulkarni, D. D.; Choi, I.; Tsukruk, V. V., Graphene-polymer

18

nanocomposites for structural and functional applications. Progress in Polymer

19

Science 2014, 39, (11), 1934-1972.

20

22. Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S., The chemistry of

21

graphene oxide. Chemical Society reviews 2010, 39, (1), 228-40.

22


Energy & Fuels

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

1

23. Pei, S.; Cheng, H.-M., The reduction of graphene oxide. Carbon 2012, 50, (9),

2

3210-3228.

3

24. Liu, J.; Li, X.; Jia, W.; Li, Z.; Zhao, Y.; Ren, S., Demulsification of Crude

4

Oil-in-Water Emulsions Driven by Graphene Oxide Nanosheets. Energy & Fuels

5

2015, 29, (7), 4644-4653.

6

25. Razi, M.; Rahimpour, M. R.; Jahanmiri, A.; Azad, F., Effect of a Different

7

Formulation of Demulsifiers on the Efficiency of Chemical Demulsification of Heavy

8

Crude Oil. Journal of Chemical & Engineering Data 2011, 56, (6), 2936-2945.

9

26. Pensini, E.; Harbottle, D.; Yang, F.; Tchoukov, P.; Li, Z.; Kailey, I.; Behles, J.;

10

Masliyah, J.; Xu, Z., Demulsification Mechanism of Asphaltene-Stabilized

11

Water-in-Oil Emulsions by a Polymeric Ethylene Oxide–Propylene Oxide Demulsifier.

12

Energy & Fuels 2014, 28, (11), 6760-6771.

13

27. Souza, V. B.; Mansur, C. R. E., Oil/Water Nanoemulsions: Activity at the Water–

14

Oil Interface and Evaluation on Asphaltene Aggregates. Energy & Fuels 2015, 29,

15

(12), 7855-7865.

16

28. Creighton, M. A.; Ohata, Y.; Miyawaki, J.; Bose, A.; Hurt, R. H.,

17

Two-dimensional materials as emulsion stabilizers: interfacial thermodynamics and

18

molecular barrier properties. Langmuir : the ACS journal of surfaces and colloids

19

2014, 30, (13), 3687-96.

20

29. Sjöblom, J.; Aske, N.; Harald Auflem, I.; Brandal, Ø.; Erik Havre, T.; Sæther, Ø.;

21

Westvik, A.; Eng Johnsen, E.; Kallevik, H., Our current understanding of

23


Page 38 of 56

Page 39 of 56

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

Energy & Fuels

1

water-in-crude oil emulsions. Advances in Colloid and Interface Science 2003,

2

100-102, 399-473.

3

30. Hou, J.; Feng, X.; Masliyah, J.; Xu, Z., Understanding Interfacial Behavior of

4

Ethylcellulose at the Water–Diluted Bitumen Interface. Energy & Fuels 2012, 26, (3),

5

1740-1745.

6

31. Fan, Y.; Simon, S.; Sjöblom, J., Interfacial shear rheology of asphaltenes at oil–

7

water interface and its relation to emulsion stability: Influence of concentration,

8

solvent

9

Physicochemical and Engineering Aspects 2010, 366, (1-3), 120-128.

aromaticity

and

nonionic

surfactant.

Colloids

and

Surfaces

A:

10

32. Wang, Y.; Zhang, L.; Sun, T.; Zhao, S.; Yu, J., A study of interfacial dilational

11

properties of two different structure demulsifiers at oil–water interfaces. Journal of

12

colloid and interface science 2004, 270, (1), 163-170.

13

33. Wang, J.; Li, C.-Q.; An, N.; Yang, Y., Synthesis and Demulsification of Two

14

Lower Generation Hyperbranched Polyether Surfactants. Separation Science and

15

Technology 2012, 47, (10), 1583-1589.

16

34. Varadaraj, R.; Brons, C., Molecular Origins of Crude Oil Interfacial Activity. Part

17

4: Oil–Water Interface Elasticity and Crude Oil Asphaltene Films. Energy & Fuels

18

2012, 26, (12), 7164-7169.

19

35. Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K., Effects of

20

petroleum resins on asphaltene aggregation and water-in-oil emulsion formation.

21

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003, 220, (1-3),

24


Energy & Fuels

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

1

9-27.

2

36. Zhang, Y.; Fang, S.; Tao, T.; Xiong, Y.; Duan, M., Influence of Asphaltene

3

Concentration on the Interfacial Properties of Two Typical Demulsifiers. Journal of

4

Dispersion Science and Technology 2015.

5

37. Wang, J.; Hu, F.-L.; Li, C.-Q.; Li, J.; Yang, Y., Synthesis of dendritic polyether

6

surfactants for demulsification. Separation and Purification Technology 2010, 73, (3),

7

349-354.

8

38. Sun, Z.; Feng, T.; Russell, T. P., Assembly of graphene oxide at water/oil

9

interfaces: tessellated nanotiles. Langmuir : the ACS journal of surfaces and colloids

10

2013, 29, (44), 13407-13.

11

39. Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M., Graphene oxide as a chemically

12

tunable platform for optical applications. Nature chemistry 2010, 2, (12), 1015-24.

13

40. Francesca Ravera; Eva Santini; Giuseppe Loglio; Michele Ferrari; Liggieri, L.,

14

Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air

15

surface layers. The Journal of Physical Chemistry B 2006, 110, (39), 19543-19551.

16

41. Zhang, Q.; Bai, R. X.; Guo, T.; Meng, T., Switchable Pickering Emulsions

17

Stabilized by Awakened TiO2 Nanoparticle Emulsifiers Using UV/Dark Actuation.

18

ACS applied materials & interfaces 2015, 7, (33), 18240-6.

19

42. Whitby, C. P.; Fornasiero, D.; Ralston, J.; Liggieri, L.; Ravera, F., Properties of

20

Fatty Amine–Silica Nanoparticle Interfacial Layers at the Hexane–Water Interface.

21

The Journal of Physical Chemistry C 2012, 116, (4), 3050-3058.

25


Page 40 of 56

Page 41 of 56

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

Energy & Fuels

1

43. Georgieva, D.; Schmitt, V.; Leal-Calderon, F.; Langevin, D., On the possible role

2

of surface elasticity in emulsion stability. Langmuir : the ACS journal of surfaces and

3

colloids 2009, 25, (10), 5565-73.

4

44. Spiecker, P. M.; Kilpatrick, P. K., Interfacial Rheology of Petroleum Asphaltenes

5

at the Oil−Water Interface. Langmuir : the ACS journal of surfaces and colloids 2004,

6

20, (10), 4022-4032.

7

45. Hannisdal, A.; Orr, R.; Sjöblom, J., Viscoelastic Properties of Crude Oil

8

Components at Oil ‐ Water Interfaces. 2: Comparison of 30 Oils. Journal of

9

Dispersion Science and Technology 2007, 28, (3), 361-369.

10

46. Hannisdal, A.; Orr, R.; Sjöblom, J., Viscoelastic Properties of Crude Oil

11

Components at Oil ‐ Water Interfaces. 1. The Effect of Dilution. Journal of

12

Dispersion Science and Technology 2007, 28, (1), 81-93.

13

47. Wu, D.; Zhang, F.; Liang, H.; Feng, X., Nanocomposites and macroscopic

14

materials: assembly of chemically modified graphene sheets. Chemical Society

15

reviews 2012, 41, (18), 6160-77.

16

48. Kim, J.; Cote, J. L.; Kim, F.;Yuan, W.; Shull, R. K.; Huang, J.; Graphene oxide

17

sheets at interfaces. Journal of the American Chemical Society 2010, 132, (23),

18

8180-8186.

19

49. Gilje, S.; Han, S.; Wang, M.; Wang, L. K.; Kaner, B. R.; A chemical route to

20

graphene for device applications. Nano letters 2007, 7, (11), 3394-3398.

21

26


Energy & Fuels

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

1

Figure Captions:

2


3

Figure 2. Demulsification process driven by GO (a)variation of the process with the

4

increase of temperature(25 oC, 30 oC, 40 oC, 50 oC, 60 oC, 65 oC); (b) variation of the

5

process with the increase of the dosage(0ppm, 50ppm, 75ppm, 100ppm, 150ppm,

6

200ppm).

7

Figure 3. Confocal microscopy images of oil droplets in the emulsion before and after

8

treated by GO. (Confocal microscopy images of oil droplets in the emulsion: (a) the

9

oil droplets with diameters smaller than 10 µm uniformly distributed in the untreated

10

oily wastewater; (b) the oil droplets flocculate together after introducing the GO

11

nanosheet; (c) a selected region of the flocculating constituent for a better observation;

12

(d) the fluorescence images of the selected flocculate constituents showing a strong

13

blue fluorescence emission).

14


15


16


17


18


19


20


27


Page 42 of 56

Page 43 of 56

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

Energy & Fuels

1


2


3


4


5


6


7


8


9

Figure 8. Schematic illustration of coalescence process of the oil droplet. (a)a released

10

oil drop in the water phase, and a protective interfacial film was formed; (b)the GO

11

nanosheets diffused to the interface and penetrate the initial interfacial film; (c)the

12

flexible nature of the GO nanosheets enabled them to orient parallel to the interface

13

and self-assembled to form a new “GO film”; (d)when the oil drop was released to the

14

interface, the upper GO nanosheets tend to stack with the GO wrapped outside the oil

15

drop, thus the oil drop was allowed to contact with the bulk oil; (e)the oil drop started

16

to merge into the bulk oil and the external “GO” film became wrinkled, like a deflated

17

balloon; (f)the oil drop completely merged into the bulk oil and the external “GO”

18

film was finally assemble at the upper interface.

19


20


21


28


Energy & Fuels

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

1


2


3


4


5


6


7


8


9


10


11


12


13

were hidden.

14 15


16

29


Page 44 of 56

Page 45 of 56

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

Energy & Fuels

（b））

（a）） 1 2

Figure 2. Demulsification process driven by GO (a)variation of the process with

3

the increase of temperature(25 oC, 30 oC, 40 oC, 50 oC, 60 oC, 65 oC); (b) variation of

4

the process with the increase of the dosage(0ppm, 50ppm, 75ppm, 100ppm, 150ppm,

5

200ppm).

30


Energy & Fuels

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

(a)

(b)

(c)

(d)

1 2

Figure 3. Confocal microscopy images of oil droplets in the emulsion before and

3

after treated by GO. (Confocal microscopy images of oil droplets in the emulsion: (a)

4

the oil droplets with diameters smaller than 10 µm uniformly distributed in the

5

untreated oily wastewater; (b) the oil droplets flocculate together after introducing the

6

GO nanosheet; (c) a selected region of the flocculating constituent for a better

7

observation; (d) the fluorescence images of the selected flocculate constituents

8

showing a strong blue fluorescence emission).

31


Page 46 of 56

Page 47 of 56

30

20

25

(b)

19

Blank GO

GO Blank

18

Modulus E(mN/m)


(a)

20

15

17 16 15 14 13 12

10

11

5 25

30

35

40

45

50

55

60

65

10 25

70

30

35

40

50

55

60

65

70

60

65

70

Temperature( C)

Temperature( C)

10

-15

15

(c)

(d)

9

14

Oily wastewater GO

7 12 6 11

5

Zeta potential(mV)

13

Modulus E(mN/m)

-20

8

-25

-30

10

4 3

45

o

o


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

Energy & Fuels

0

50

100

150

200

-35 25

9

30

35

40

45

50

55 o

Temperature( C)


1


2


3


4


5


32


Energy & Fuels

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

Suck in

Page 48 of 56

Squeeze out

1 2


3


33


Page 49 of 56

0.13

3.5

(a)

(b)

0.12

1.6

50

0.11

3.0

1.4

45

0.10 40

2.0

1.0

35

0.08

k

1.2

k

2.5

t1/2(s)

t1/2(s)

0.09

0.07

30

0.06 25 0.05

1.5

0.8

20

0.04

25

30

35

40

45

50

55

60

65

70

25

30

35

45

50

55

60

65

70

65

70

o

3.0

2.0 1.8

40

Temperature( C)

o

Temperature( C)

(c)

(d)

50

Blank GO

2.5

1.6 1.2

30

1.0 0.8

20

0.6 10

0.4

k Drainage time(s)

40

1.4

t1/2(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

Energy & Fuels

2.0 1.5 1.0 0.5

0.2 0

0.0 0

50

100

150

0.0 25

200

30

35

40

45

50

55

60

o

Temperature( C)


1


2


3


4


5


34


Energy & Fuels

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

(a)

0s

Page 50 of 56

3.36s

Bulk oil

Rupture

Roll Oil Blank (b) 0s Bulk oil

0.08s Film

Wrinkle Oil GO

1


2


3


35


Page 51 of 56

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

Energy & Fuels

(e)

(d) Water phase

(c)

(b)

(a)

Bulk oil

(f)

Oil droplet

Interfacial film

GO

1 2

Figure 8. Schematic illustration of coalescence process of the oil droplet. (a)a

3

released oil drop in the water phase, and a protective interfacial film was formed;

4

(b)the GO nanosheets diffused to the interface and penetrate the initial interfacial film;

5

(c)the flexible nature of the GO nanosheets enabled them to orient parallel to the

6

interface and self-assembled to form a new “GO film”; (d)when the oil drop was

7

released to the interface, the upper GO nanosheets tend to stack with the GO wrapped

8

outside the oil drop, thus the oil drop was allowed to contact with the bulk oil; (e)the

9

oil drop started to merge into the bulk oil and the external “GO” film became

10

wrinkled, like a deflated balloon; (f)the oil drop completely merged into the bulk oil

11

and the external “GO” film was finally assemble at the upper interface.

36


Energy & Fuels

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

1 2


3


37


Page 52 of 56

Page 53 of 56

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

Energy & Fuels

1

=

(a)

(b)

2 3


4


38


Energy & Fuels

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

(a)

(b)

(c)

(d)

1 2


3


4


5


6


7 8 9 10 11

39


Page 54 of 56

Page 55 of 56

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

Energy & Fuels

1 2


3


4


5


6

40


Energy & Fuels

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

1

Page 56 of 56

(b)

(a)

2


3


4

were hidden.

41


Water Interface: A New

Recommend Documents