Pressure and Temperature Dependence of Contact Angles for CO2

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Pressure and temperature dependence of contact angles for CO2/ water/silica systems predicted by molecular dynamics simulations cong chen, Bo Dong, Ning Zhang, Weizhong Li, and Yongchen Song Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00171 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 8, 2016

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Pressure and temperature dependence of contact angles for CO2/water/silica systems predicted by molecular dynamics simulations Cong Chen1*, Bo Dong1, Ning Zhang2, Weizhong Li1, Yongchen Song1 1

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of

Education, Dalian University of Technology, Dalian 116024, P. R. China. 2

School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin

124221, P. R. China Keywords: Molecular dynamics simulation, contact angle, wettability, CO2 sequestration

Abstract: Wettability controls the capillary behavior of injected CO2 including capillary entry pressure, relative permeability and residual fluid saturation and it is one of the most active topics in geologic carbon sequestration (GCS). However, the large uncertainty of water contact angle (CA) data as well as its pressure, temperature and salinity dependences in literature limits our understanding on wettability. Molecular dynamics (MD) simulations have been performed to investigate the pressure and temperature (P & T) dependence of water contact angles on silica surface. Three typical molecular surface models for silica namely Q2, crystalline Q3 and amorphous Q3 were selected and simulations were conducted at wide pressure (2.8-32.6 MPa)

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and temperature (318-383K) conditions. The results show that P & T dependences of water contact angles on silica surfaces are controlled by surface functional groups. These findings provide new information to help better understand wettability alteration under different GCS conditions.

1 Introduction

Injection of CO2 into subsurface geologic formations is considered as one of the most promising options to limit greenhouse gas emissions and mitigate its impacts on global climate change.1-2 During geologic carbon sequestration (GCS), the movement of injected CO2 and the reservoir brine must be understood to determine the final fate of CO2 and to evaluate the efficiency and safety of sequestration.3 The movement of injected CO2 and reservoir brine is strongly dependent on wettability of CO2/brine/mineral systems.4-6 The water contact angle (CA) is a main factor to evaluate wettability and it is related with interfacial tensions through the following equation:7 cos θ =

(1)

where γ , γ and γ are interfacial tensions between mineral-water (brine), mineral-CO2 and water (brine)-CO2, respectively. However, water contact angle cannot be calculated directly from the three interfacial tensions as only γ can be measured by experiments.8-9 Water contact angle controls the capillary behavior of injected CO2 including capillary entry pressure, relative permeability and residual fluid saturation and it is one of the most active topics in studying GCS.4-7 However, there is a great ambiguity for water contact angle data in literature as well as its pressure, temperature and salinity dependences.7,10-13 The pressure and temperature (P & T)


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dependences of water contact angles for the CO2/brine/silica system found by several authors have been summarized in Table 1. Table 1 Pressure and temperature dependence of water contact angles for the CO2/brine/silica system studied by experiments. Authors

P (MPa)

CA vs P

0-7

increase

10-20.4

independent

1-10

increase

0-9

increase

12-27

independent

0-7

nearly constant

0.1-7

slightly increase

7-10

increase

10-25

independent

Dickson11 Chiquet14 Sutjiadi-Sia15 Espinoza16

Jung17

0-40 Farokhpoor18

T (K)

CA vs T

Solution

Surface

296

DI water

glass

293

0.01-1M NaCl

quartz

313

water

glass

298

DI water, ~3.4M NaCl

glass

318

0-5M NaCl

glass

309, 339

0, 0.2, 0.8M NaCl

quartz

water

quartz

CA vs P: first increase, then decrease and finally constant CA vs T: increase 3.4-11.7

308-333

Saraji19

CA vs P: water advancing, increase; water receding, slightly increase (333K) or constant (308 and 318K) CA vs T: water advancing, increase; water receding, constant in subcritical zone, increase in supercritical zone

Iglauer20

0-25

increase

296

DI water

quartz

Saraji21

13.8-27.6

slightly increase

323-373

0.2-5M NaCl

quartz

CA vs T: has a tendency to decrease Al-Yaseri22

0.1-20

increase

296-343

increase

DI water

quartz

Sarmadivaleh23

0.5-20

increase

296-343

increase

DI water

quartz

318-348

constant

0-3M NaCl, CaCl2

quartz

7-18 Chen24

CA vs P: slightly increase in 7-10 MPa, constant after 10 MPa


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Table 2 Pressure and temperature dependence of water contact angles for the CO2/brine/silica system studied by the molecular dynamics simulation method. Only studies with at least four data points for each condition are summarized. Authors

P (MPa)

CA vs P

T (K)

LIU25

7.3,9.4,10.9,18.8 a

increase

318.15

Javanbakht26

3.4,6.2,9,11.7

CA vs T

Solution

Surface b

308, 333

water decrease

water

1.6 OH/nm2 c

Q2-50%

CA vs P: Increase during sub- to super-critical transition (6-9) a

The pressures were collected from NIST using temperature and CO2 densities in the reference.

b

Silanol number density. The surface is composed of bare Si atoms and a few protonated silanol functional groups. c

A Q2 surface with 50% of nonbridged oxygen atoms at the surface were protonated.

The large uncertainties of contact angle and its pressure and temperature dependences may be caused by several factors including surface contamination and chemical reactions during experiments.10,20 In both cases, the functional groups on silica surfaces will be changed and the variation of water contact angles with surface functional groups may cover the nature of water contact angle dependences. During molecular dynamics (MD) simulations, functional groups on silica surfaces can be fixed to eliminate any contact angle variation due to functional groups changing. The MD simulations of P & T dependences for CO2/brine/silica system are summarized in Table 2. Different surface models seem to get different P & T dependences.25-26 Typical molecular models for silica surfaces (Q2, Q3, Q4, et al.) have been used to predict water contact angles and the number density and space distribution of silanol groups has been proven to play a very important role in silica wettability.27 However, the P & T dependences of water contact angles on these typical molecular models for silica surfaces are not studied. In the present study, several silica surfaces with different functional groups were selected and the pressure and


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temperature dependences of water contact angles were predicted by performing a lot of molecular dynamics simulations at temperatures from 318 to 383K and pressures from 2.8 to 32.6 MPa. 2 Methods 2.1 Boxes The half-cylindrical water droplets method was used to predict water contact angles. Water droplets with cylindrical diameters around 3.5-4 nm were placed on silica surfaces under CO2 atmosphere. The geometries of initial simulation boxes are similar as those used in previous studies.24,27-28 Three silica surfaces were selected as Q2, Q3 and amorphous Q3 (Figure 1). NaCl and CaCl2 were chosen to study the effect of ions type. The parameters for simulation boxes are summarized in Table 3. c is the molality of ions. P is the system pressure (MPa). Lx, Ly and Lz are the dimensions in x, y and z directions (nm), respectively. Nw, NCO2 and Nions are the numbers of water molecules, CO2 molecules and ions, respectively. It should be noted that the system pressure P was measured by CO2 density in bulk area (far from silica surfaces and the water droplet, >2nm) based on the pressure-density relationship from NIST (http://www.nist.gov/).

amorphous Q3

Q2

Q3

Figure 1 Structures of selected model surfaces.


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Table 3 Parameters for simulation boxes. Surface

c

T/K

P range/MPa

Q2

0M

318

3-23.7

0M

333

3.6-16.5

Lx,Ly,Lz

Nw range

NCO2 range

Nions

2085

823-6571

0

2085

823-6571

0

2085

823-6571

0

2085

823-6571

0

Lx: 20.95-21.20 0M

363

4.4-22.1 Ly: 3.47-3.72

0M

383

5-32.6 Lz: 11.66-11.67

Q3

3M NaCl

318

3.5-23.5

1883-2085

823-6571

75-111

1M CaCl2

318

3.1-23.5

1980-2085

823-6571

35-37

0M

318

3.3-22.6

2085

793-6240

0

Lx: 20.06-20.23 Ly: 3.51-3.77

Amorphous Q3

0M

383

5.4-32.4

Lz: 12.99-13.39

2085

793-6240

0

0M

318

2.8-26.2

Lx: 20.18-20.5

2387-2568

990-7178

0

2387-2568

990-7178

0

Ly: 4.27-4.59 0M

383

4.8-23.2

Lz: 12.83-12.89

2.2 Molecular dynamics simulation details A force field for silica and water was selected to compute interactions between silica and water molecules.29 A flexible CO2 model was chosen.30 Ions interaction energies were calculated using parameters from Beglov et al.31 The combination of these force fields has been applied in former studies and good results have been obtained.24,27 Other details of the force field used in the simulations are summarized in section S1 of the Supporting Information. Softwares NAMD32 and VMD33 were used to perform molecular dynamics simulations and analyze data using inhouse codes, respectively. Standard techniques such as periodic boundary conditions, neighborhood lists, switching function, Particle mesh Ewald (PME) method34 and integration technique r-RESPA35 were applied. Atoms in silica except for atoms in hydroxyl groups were fixed to their initial positions using SHAKE algorithm.36 NVT ensemble was applied and temperature was maintained to target value using a Langevin dynamics method37 with a damping


6

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coefficient of 5/ps. The positions of atoms in silica were fixed except for atoms in hydroxyl groups. The parameters for these simulation techniques were same as those used in former studies.24,27 The time step was 1 fs and all boxes were simulated for 15 ns where the 12-ns run is for equilibrium and the rest 3-ns run is for contact angle measurement.

WCCL SCCL

SWCL

Figure 2 An initial simulation box (left top) and its equilibrium configuration (left bottom) as well as the density profile of water (right) 2.3 Contact angle calculation Water contact angles were predicted based on the water density profiles (2D, x-z plane). At first, the water density profiles were calculated from the trajectory generated from the last 3-ns run. Then, the triple phase contact point, water-CO2 contact line (WCCL), silica-water contact line (SWCL) and silica-CO2 contact line (SCCL) were then determined from the density profile. Finally, water contact angles were calculated by measuring the angle formed between WCCL and SWCL. An initial simulation box and its equilibrium configuration as well as the density profile are shown in Figure 2 to better illustrate the contact angle prediction protocol. It should


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be noted that other contact angle prediction methods were also used in literature.27-28 However, the comparison and evaluation of different contact angle prediction protocols are not the scope of the present study. The contact angle prediction method used in this study has been applied in other studies and good results were gained.24,27 2.4 Hydrogen bonds definition The definition of H-bonds is in some way arbitrary and two criteria are usually used namely geometrical and energetic definitions. The geometrical criterion was selected in this study. An interaction X-H…A was identified as a hydrogen bond when (1) the distance between H and A atoms was less than its threshold; (2) the angle XHA was less than its threshold. The two thresholds were selected as 0.3 nm and 130°, as used in other studies.27,38 2.5 Calculation of CO2 surface excess

0.030

o3

Atom number density (N/A )

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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Si CCO2

0.025

position of the outmost silicon atoms

0.020

P

L

0.015

0.010

0.005

0.000 20

30

40

50

60

70

80

90

100

o

Distance normal to surface (A )

Figure 3 An illustration of CO2 surface excess definition based on atom number density as a function of distance normal to surface (3 MPa, 0 M, 318 K, Q2 surface)


8

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To calculate surface excess, a mathematical dividing surface is required and the value of surface excess depends on the location of the mathematical dividing surface. The Gibbs dividing surface of solvent is usually used to calculate solute surface excess. In the case of solid/gas adsorption, the Gibbs surface is equivalent to the adsorbent surface.39-40 So, the position of the outmost silicon atoms is selected as the dividing surface. An illustration of CO2 surface excess definition can be found in Figure 3. For unit area of interface, the surface excess of CO2 can be calculated by:

Γ = N − c V

(2)

where A = L L is the area of silica surface (L and L are dimensions in x and y directions, respectively), N = L L

#$ cdz #%

is the number of CO2 molecules in real system from the

dividing surface (P in Figure 3) to system boundary (L in Figure 3), c is the mole density of CO2 molecules in bulk CO2 phase and V = L L (z' − z( ) is the volume of system. Equation (1) changes to: Γ =

#$ cdz #%

− c (z' − z( )

(3)

The location of system boundary is selected far from the dividing surface and at least 2/3 of the system is within bulk CO2 phase. It should be noted that the position of system boundary does not affect the value of CO2 surface excess unless no bulk CO2 phase is involved in the system. 3 Results and Discussion 3.1 Pressure, density and contact angles of equilibrium simulation boxes


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The equilibrium bulk CO2 densities and final system pressures as well as predicted water contact angles for each simulation box are summarized in Tables 4-6. Table 4 Pressure, density and water contact angles on crystalline and amorphous Q3 surfaces Surface & Concentration

CA(°)

P(MPa)

Density(kg.m-3)

T(K) Av.

Range

Av.

±

Av.

±

3.3

3-3.6

17

3

65.11

7.41

3.9

3.6-4.2

18

3

78.28

7.90

5

4.6-5.3

21

4

107.53

10.70

9.2

8.8-9.5

29

6

368.61

52.72

11.6

11.4-11.9

24

4

640.34

9.93

14.6

14-15.4

24

3

735.08

14.56

22.6

21.5-23.8

24

4

838.31

10.68

Q3 318 0M

5.4

5-5.8

4.00

1

84.81

6.96

6.4

6-6.8

16

5

102.96

8.00

8.1

7.7-8.5

14

4

136.28

8.30

18.6

17.7-19.7

18

4

403.28

27.51

32.4

30.7-34.2

17

4

652.34

20.67

2.8

2.5-3.1

25

3

52.97

5.69

3.5

3.2-3.9

21

6

69.68

7.72

4.6

4.2-5

27

2

98.23

10.61

9.1

8.8-9.4

38

4

356.53

42.18

12.4

12-12.9

33

3

676.88

16.92

13.6

13.3-13.9

35

5

711.87

8.29

14.9

14.2-15.8

35

4

741.70

15.26

4.8

4.6-5.0

17

4

73.60

3.56

5.9

5.4-6.3

16

2

92.86

8.83

8.1

7.3-8.8

21

4

135.48

14.63

11.7

11.4-12

27

5

217.53

8.08

14.5

13.6-15.5

27

4

291.45

25.67

20.7

20.3-21.0

29

4

453.79

8.43

23.2

22.7-23.9

25

2

511.39

12.70

Q3 383 0M

Amorphous Q3 318 0M

Amorphous Q3 383 0M


10

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Table 5 Pressure, density and contact angles of pure water on Q2 surface CA(°)

P(MPa) Concentration(M)

0

0

0

T(K)

318

333

Av.

Fluctuation

Av.

±

Av.

3.0

2.8-3.1

19

5

57.40

2.31

3.8

3.5-4.1

18

2

76.26

8.51

5.0

4.9-5.1

21

3

108.00

3.18

9.0

8.9-9.2

32

3

340.63

20.05

9.5

9.4-9.6

33

2

413.91

17.87

11.7

11.3-12.3

32

4

647.51

22.71

14.5

13.6-15.5

33

4

732.46

20.05

16.5

15.8-17.2

32

5

768.28

10.47

23.7

23.2-24

42

3

847.12

3.76

3.6

3.1-4

18

3

65.25

9.59

4.3

3.8-4.8

21

3

80.97

11.81

5.5

5-5.9

21

4

110.50

11.80

13.7

13.2-14.1

23

4

542.91

24.76

16.5

16.0-17.0

31

6

651.60

14.34

4.4

4.1-4.8

17

3

73.34

7.08

±

5.7

5.3-6.1

16

4

97.55

8.07

7.6

7.0-8.2

20

3

139.10

13.96

21.4-22.9

24

3

582.89

15.05

5

4.8-5.2

12

2

77.17

3.46

6.1

5.7-6.5

16

1

97.23

7.31

8.6

7.8-9.4

16

3

147.63

16.65

26.4

25.5-27.3

23

2

568.11

14.68

32.6

31.3-34

32

7

654.03

15.04

363 22.1

0

Density(kg.m-3)

383


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Table 6 Pressure, density and contact angles of brine on Q2 surface CA(°)

P(MPa) Concentration(M)

Density(kg.m-3)

T(K) Av.

Fluctuation

Av.

±

Av.

3.5

±

3.1-3.7

32

6

68.07

9.02

4.5

4.3-4.8

24

4

95.15

7.90

5.5

5.2-5.9

34

3

125.08

11.16

9.4

9.3-9.6

44

2

403.58

26.80

22.6-24.5

50

3

845.97

7.87

3.1

2.8-3.3

26

3

58.40

6.06

3.8

3.5-4.1

34

2

75.60

7.36

4.6

4.2-5.1

36

6

98.73

13.57

9.6

9.5-9.8

38

3

437.82

27.60

22.2-24.9

44

3

845.92

11.33

3M 318 NaCl 23.5

1M 318 CaCl2 23.5

3.2 P&T dependences of CA on Q2 surface Water contact angles as a function of pressure on Q2 surface at 318K for pure water and brine solutions are illustrated in Figures 4 and 5, respectively. Water contact angles show an increasing trend as pressure increases for pure water and brine solutions. For pure water, when pressure increases from 3 to 23.7 MPa, water contact angle increases by one time (from 19° to 42°). For 3M NaCl solutions, water contact angle increases from 32° to 50° as pressure increases from 3.5 to 23.5 MPa. Water contact angle increases 18° when pressure changes from 3.1 to 23.5 for 1M CaCl2 solutions. Water contact angles as a function of pressure at temperatures from 318K to 383K are summarized in Figure 6. As temperature increases, water contact angle decreases. Former studies on Q2 surface were performed at different temperature and pressure.41-42 At 300K, water contact angles were calculated to be 29.9° (±1°) at 18.9 MPa and 31.9° (±5.2°) at 17.3 MPa.42 At 275K, the calculated water contact angles at 19.1 MPa and 20.67 MPa were


12

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52.4° (±5.7°) and 39° (±7°), respectively.42 The P dependence cannot be predicted with only two data points at each temperature. Water contact angle increases as temperature decreases which agrees well with the T dependence found in this study. Much smaller contact angles (~20°) were obtained at 296K when pressure ranges from 7.8 to 28.8 MPa.41 However, they applied rigid potential models for water and CO2 molecules. When brine is present, water contact angle increases. The dependence of water contact angles with brine salinity is consistent with those found by other authors.17,22 However, the effects of Na+ and Ca2+ are different even with the same ionic strength.

1000

50

subcritical

supercritical

45 800

3

CO2 density (kg/m )

40

) (

o

35

CA

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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600

30 25

400

CO2 density

20 200 15 10

0 0

6

12

18

24

30

P (MPa)

Figure 4 Water contact angles as a function of pressure for pure water on Q2 surface at 318K. CO2 densities as a function of pressure at 318K are also drawn for comparison. The data at 10.8 MPa (surrounded by a red circle) was collected from former studies.24


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Energy & Fuels

60

3M NaCl 1M CaCl2 50

0M

o

CA ( )

40

30

20

10 0

5

10

15

20

25

30

P (MPa)

Figure 5 Water contact angles as a function of pressure for NaCl and CaCl2 solutions on Q2 surface at 318K. The results for pure water are also shown for comparison. The data at 10.8 MPa (surrounded by a red circle) was collected from former studies.24

50

383K 318K 333K 363K

40

o

CA ( )

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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30

20

10 0

7

14

21

28

35

P (MPa)

Figure 6 Water contact angles as a function of pressure for pure water on Q2 surface at different temperatures.


14

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60

3

Q 3 amorphous Q

50

o

CA ( )

40

30

20

10 0

4

8

12

16

20

24

28

P (MPa)

Figure 7 Water contact angles as a function of pressure for pure water on crystalline and amorphous Q3 surfaces at 318K. The data at 10.5 MPa (surrounded by a red circle) was collected from former studies.24

60 3

Q 383K 3 Q 318K

50

3

amorphous Q 318K 3 amorphous Q 383K

40

o

CA ( )

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Figure 8 Water contact angles as a function of pressure for pure water on crystalline and amorphous Q3 surfaces at different temperatures.


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3.3 P&T dependences of CA on Q3 and amorphous Q3 surfaces Figure 7 illustrates water contact angles on crystalline and amorphous Q3 surfaces as a function of pressure at 318K. On crystalline Q3 surface, water contact angle first increases, then decreases and finally keeps constant with pressure. There is a peak at pressures between 8 and 12 MPa. A similar P dependence was found on amorphous Q3 surface. At all pressures studied, water contact angles on amorphous Q3 surface are larger than that on crystalline Q3 surface which agrees well with the trend found in literature by molecular dynamics simulations and experiments. The final water contact angles on crystalline and amorphous Q3 surfaces are ~24° and ~35°, respectively. The P dependences of water contact angles on crystalline and amorphous Q3 surfaces were also predicted at 383K and the results are shown in Figure 8. On crystalline Q3 surface, water contact angle decreases as temperature increases. At 383K, the final water contact angle is ~18°. However, the P dependence of water contact angles on amorphous Q3 surface at 383K is quite different. At 383K, on amorphous Q3 surface, water contact angle increases with pressure. At high pressures (larger than 14.5 MPa), water contact angle at 383K is larger than the value at 318K. This implies that on amorphous Q3 surface, when T increases, water contact angle decreases at low pressures and increases at high pressures. 3.4 P & T dependence on GCS conditions Based on the results obtained by molecular dynamics simulations, the P & T dependences on silica surfaces with different molecular models are different. Under GCS conditions, the molecular structure on silica surfaces is rather complicated and may be composed of several surface models (Q2, Q3, Q4, deprotonation, protonation).43-44 As a result, the P & T dependences of water contact angles under GCS conditions should be a combination of individual P&T dependence. In experimental studies, although the molecular structures on silica surfaces used


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are unknown, the P & T dependences found are somehow similar with the trends obtained by MD simulations. For amorphous Q3 surface, MD simulations predict a trend at 318K: water contact angle first increases (2.8-10.5 MPa), then decreases (10.5-12.4 MPa) and finally keeps constant (>12.4 MPa) with pressure. Several experimental studies on glass surfaces (amorphous) obtained similar trends (at temperatures no more than 318K): 0-7 MPa (increase) and 10-20.4 (constant),11 0-9 MPa (increase) and 12-27 (constant),15 0-10 MPa (increase) and 10-25 MPa (constant).17 However, during these studies, no peak has been found. The reasons may be lack of data within 9.1 and 12.4 MPa11,15 or surface functional group change during step pressurizing.17 For Q2 and crystalline Q3 surfaces, by MD simulations, a peak has been found when pressure is within 7.3811 MPa. Within this range, CO2 density changes sharply with pressure and more data points are required to capture the peak. In fact, most of the experimental work was not performed at pressures within this range19-23 or was conducted at limited pressures.14 However, an experimental study did find the peak at pressures between 7 and 10 MPa.18 If MD simulation results at pressures within peak range are not included, water contact angle shows an increase trend on Q2 surface and first increases, then keeps constant on crystalline Q3 surface. Interestingly, most of the experimental results agree well with these trends: (1) increases with pressure, 1-10 MPa,14 3.4-11.7 MPa,19 0-25 MPa,20 13.8-27.6 MPa,21 0.1-20 MPa,22 0.5-20 MPa;23 (2) first increases and then doesn’t change with pressure, 7-10 (increases) and 12-18 MPa (constant) ,24 0-7.5 MPa (increases) and >10MPa (constant).18 MD simulations predicted lower water contact angles at higher temperatures on all surfaces studied. MD simulations on other surface models also predicted lower water contact angles at higher temperatures.26 However, the T dependence of water contact angles by experimental


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methods is rather different: increases,18-19,22-23 keeps constant24 and decreases.21 The uncertainty of T dependence may be caused by surface functional groups change as the temperature affects rehydroxylation and dissolution processes.45 3.5 Discussion on the effects of surface functional groups The mean number of hydrogen bonds per silanol group nHB(SiOH) for different model surfaces as a function of pressure were calculated and illustrated in Figure 9. Surface excess of CO2 as a function of pressure at 318 K on different model surfaces is illustrated in Figure 10.

2.4 2

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Figure 9 Mean numbers of hydrogen bonds per hydroxyl group at 318K


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Figure 10 CO2 surface excess as a function of pressure at 318K The following discussions are based on the same pressure and temperature condition. (1) For Q2 surfaces, when ions are present the mean number of hydrogen bonds per silanol group decreases which implies that the interaction between Q2 surface and water is weakened. The number of water molecules which diffuse into CO2 atmosphere is rather small especially in nanosize simulation boxes used in MD simulation. As a result, the interaction between Q2 surface and CO2 atmosphere can be assumed to keep constant. It has been found that the interfacial tension between water-CO2 increased with increasing water salinity.46 From equation (1), decreasingγ , increasing γ and nearly constant γ will definitely lead to an increase of water contact angles. (2) For amorphous and crystalline Q3 surfaces, the mean number of hydrogen bonds per silanol group on amorphous Q3 surface is much smaller than the value on crystalline Q3 surface. The interaction between amorphous Q3 surface and water should be weaker than that between crystalline Q3 surface and water in the context of same silanol densities of amorphous and crystalline Q3 surfaces. The surface excess of CO2 molecules on amorphous Q3 surface is larger


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than that of crystalline Q3 surface which shows that the interaction between amorphous Q3 surface and CO2 is stronger than the interaction between crystalline Q3 surface and CO2. As a result, with the same γ (same P, T, same water salinity), smallerγ and largerγ , water contact angles on amorphous Q3 surface are larger than the predicted values on crystalline Q3 surface. Analysis of pressure and temperature dependence of water contact angles is challenging as two of the three interfacial tensions which govern water contact angles cannot be directly calculated.7 The hydrogen bonds between mineral surface and water may reflect the interaction of the two phases. Surface excess of CO2 may be used to analyze the interaction between mineral and CO2. However, no direct quantitative relationship of γ - nHB(SiOH) and γ -Γ can be constructed. When P & T changes, γ varies and the trend for water contact angles cannot be investigated without data of γ andγ . Much more work is required in future study to develop methods to measure interfacial tensions between mineral-water and mineral-CO2. 4 Conclusions Molecular dynamics simulations have been performed to estimate water contact angles for CO2/brine/silica system. Three different molecular surface models were applied and the simulations were conducted at pressures from 2.8 to 32.6 MPa and at temperatures from 318K to 383K. MD simulation results show that the P & T dependences on silica surfaces with different molecular models are rather different: (1) on Q2 surface, water contact angles show an increasing trend as pressure increases for pure water and brine solutions. As temperature increases, water contact angle decreases; (2) on crystalline and amorphous Q3 surface, water contact angle first increases, then decreases and finally keeps constant with pressure. When temperature is


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increased, lower water contact angles are predicted; (3) despite different P & T dependence, a local peak was found on each surface model at pressures between 8 and 12 MPa. The mean number of hydrogen bonds per silanol group and surface excess of CO2 have been calculated to analyze the different behaviors of water contact angles on different model surfaces. At the same P & T condition, the dependence of water contact angles on water salinity as well as the effect of crystalline and amorphous Q3 surfaces on water contact angles is successfully explained by hydrogen bonds and CO2 surface excess data. However, the P & T dependence of water contact angles cannot be analyzed as missing of direct measurement of interfacial tensions between mineral-water and mineral-CO2. Although the molecular structures on silica surfaces used in experimental studies are unknown, the P & T dependences found in experiments are somehow similar with the trends obtained in MD simulations. The different P & T dependence of water contact angles on mineral surface functional groups found in this study cannot be easily explored by experiments as the composition of mineral surface functional groups varies during experiments. These results are useful to better understand the wettability alteration under different GCS conditions. Further investigations should focus on roughness, charge and functional groups changing on surfaces controlled by chemical reaction among CO2, brine and minerals. AUTHOR INFORMATION Corresponding Author *

Cong Chen, email: [email protected], tel: 86-411-84708774, website: nanoccs.dlut.edu.cn

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the following projects: project 51206016 funded by National Natural Science Foundation of China (NSFC), the Doctoral Startup Funds of Liaoning Province (20121021) and the Fundamental Research Funds for the Central Universities (DUT14LAB13). ACKNOWLEDGMENT The authors thank Computing Center of Department of Energy and Power Engineering in Dalian University of Technology for providing parallel computing environment. Supporting Information Available Tab. S1 Atom partial charges and LJ energy parameters; Tab. S2 Interaction parameters for bond stretching and angle bending. This material is available free of charge via the Internet at http://pubs.acs.org. ABBREVIATIONS GCS, geologic carbon sequestration; MD, molecular dynamics; PME, Particle mesh Ewald; CA, contact angle; P, pressure; T, temperature. REFERENCES 1. Altman, S. J.; Aminzadeh, B.; Balhoff, M. T.; Bennett, P. C.; Bryant, S. L.; Cardenas, M. B.; Chaudhary, K.; Cygan, R. T.; Deng, W.; Dewers, T.; DiCarlo, D. A.; Eichhubl, P.; Hesse, M. A.; Huh, C.; Matteo, E. N.; Mehmani, Y.; Tenney, C. M.; Yoon, H. J. Phys. Chem. C 2014, 118 (28), 15103-15113. 2. Michael, K.; Golab, A.; Shulakova, V.; Ennis-King, J.; Allinson, G.; Sharma, S.; Aiken, T. Int. J. Greenh. Gas Control 2010, 4 (4), 659-667. 3. Song, J.; Zhang, D. X. Environ. Sci. Technol. 2013, 47 (1), 9-22.


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4. Bourg, I. C.; Beckingham, L. E.; DePaolo, D. J. Environ. Sci. Technol. 2015, 49 (17), 10265-10284. 5. Hamm, L. M.; Bourg, I. C.; Wallace, A. F.; Rotenberg, B. In Geochemistry of Geologic CO2 Sequestration; DePaolo, D. J.; Cole, D. R.; Navrotsky, A.; Bourg, I. C., Eds.; Mineralogical Soc Amer: Chantilly 2013; Vol. 77, pp 189-228. 6. Tokunaga, T. K.; Wan, J. M. In Geochemistry of Geologic CO2 Sequestration; DePaolo, D. J.; Cole, D. R.; Navrotsky, A.; Bourg, I. C., Eds.; Mineralogical Soc Amer: Chantilly 2013; Vol. 77, pp 481-503. 7. Iglauer, S.; Pentland, C. H.; Busch, A. Water Resources Research 2015, 51 (1), 729-774. 8. Li, X.; Boek, E. S.; Maitland, G. C.; Trusler, J. P. M. In EGU General Assembly, Australia, 2012; Vol. 14, p 11519. 9. Bikkina, P. K.; Shoham, O.; Uppaluri, R. Journal of Chemical & Engineering Data 2011, 56, 3725-3733. 10. Wan, J. M.; Kim, Y.; Tokunaga, T. K. Int. J. Greenh. Gas Control 2014, 31, 128-137. 11. Dickson, J. L.; Gupta, G.; Horozov, T. S.; Binks, B. P.; Johnston, K. P. Langmuir 2006, 22 (5), 2161-2170. 12. Wang, S. B.; Edwards, I. M.; Clarens, A. F. Environ. Sci. Technol. 2013, 47 (1), 234-241. 13. Bikkina, P. K. Int. J. Greenh. Gas Control 2011, 5 (5), 1259-1271. 14. Chiquet, P.; Broseta, D.; Thibeau, S. Geofluids 2007, 7 (2), 112-122. 15. Sutjiadi-Sia, Y.; Jaeger, P.; Eggers, R. The Journal of Supercritical Fluids 2008, 46, 272279. 16. Espinoza, D. N.; Santamarina, J. C. Water Resources Research 2010, 46, W07537. 17. Jung, J.-W.; Wan, J. Energy & Fuels 2012, 26 (9), 6053-6059. 18. Farokhpoor, R.; Bjorkvik, B. J. A.; Lindeberg, E.; Torsaeter, O. Int. J. Greenh. Gas Control 2013, 12, 18-25. 19. Saraji, S.; Goual, L.; Piri, M.; Plancher, H. Langmuir 2013, 29 (23), 6856-6866. 20. Iglauer, S.; Salamah, A.; Sarmadivaleh, M.; Liu, K. Y.; Phan, C. Int. J. Greenh. Gas Control 2014, 22, 325-328. 21. Saraji, S.; Piri, M.; Goual, L. Int. J. Greenh. Gas Control 2014, 28, 147-155. 22. Al-Yaseri, A. Z.; Lebedev, M.; Barifcani, A.; Iglauer, S. The Journal of Chemical Thermodynamics 2016, 93, 416-423. 23. Sarmadivaleh, M.; Al-Yaseri, A. Z.; Iglauer, S. Journal of Colloid and Interface Science 2015, 441, 59-64. 24. Chen, C.; Wan, J.; Li, W.; Song, Y. Int. J. Greenh. Gas Control 2015, 42, 655-665. 25. LIU, S.; YANG, X.; QIN, Y. Chinese Science Bulletion 2010, 55 (21), 2252-2257. 26. Javanbakht, G.; Sedghi, M.; Welch, W.; Goual, L. Langmuir 2015, 31 (21), 5812-5819. 27. Chen, C.; Zhang, N.; Li, W.; Song, Y. Environ. Sci. Technol. 2015, 49 (24), 14680– 14687. 28. Tenney, C. M.; Cygan, R. T. Environ. Sci. Technol. 2014, 48 (3), 2035-2042. 29. Emami, F. S.; Puddu, V.; Berry, R. J.; Varshney, V.; Patwardhan, S. V.; Perry, C. C.; Heinz, H. Chem. Mat. 2014, 26 (8), 2647-2658. 30. Nieto-Draghi, C.; de Bruin, T.; Perez-Pellitero, J.; Avalos, J. B.; Mackie, A. D. Journal of Chemical Physics 2007, 126 (6). 31. Beglov, D.; Roux, B. Journal of Chemical Physics 1994, 100 (12), 9050-9063. 32. Phillips, J. C.; Braun, R.; Wang, W.; Gumbar, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kale, L.; Schulten, K. J Comput Chem 2005, 26, 1781–1802.


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33. Humphrey, W.; Dalke, A.; Schulten, K. Journal of Molecular Graphics & Modelling 1996, 14 (1), 33-38. 34. Darden, T.; York, D.; Pedersen, L. J. Chem. Phys. 1993, 98 (12), 10089. 35. Procacci, P.; Marchi, M. Journal of Chemical Physics 1996, 104, 3003. 36. Ryckaert, J. P. Molecular Physics 1985, 55 (3), 549-556. 37. Brunger, A. T. X-PLOR, 3.1; The Howard Hugher Medical Institute and Department of Molecular Biophysics and Biochemistry: Yale University, 1992. 38. Ozkanlar, A.; Kelley, M. P.; Clark, A. E. Minerals 2014, 4 (1), 118-129. 39. Myers, A. L.; Monson, P. A. Langmuir 2002, 18 (26), 10261-10273. 40. Herrera, L.; Fan, C.; Do, D. D.; Nicholson, D. Adsorption-Journal of the International Adsorption Society 2011, 17 (6), 955-965. 41. Tsuji, S.; Liang, Y. F.; Kunieda, M.; Takahashi, S.; Matsuoka, T. Ghgt-11 2013, 37, 5435-5442. 42. Bagherzadeh, S. A.; Englezos, P.; Alavi, S.; Ripmeester, J. A. J. Phys. Chem. C 2012, 116 (47), 24907-24915. 43. Zhuravlev, L. T. Langmuir 1987, 3 (3), 316-318. 44. Zhuravlev, L. T. Colloid Surf. A-Physicochem. Eng. Asp. 2000, 173 (1-3), 1-38. 45. Fournier, R. O.; Rowe, J. J. American Mineralogist 1977, 62 (9-10), 1052-1056. 46. Bachu, S.; Bennion, D. B. J. Chem. Eng. Data 2009, 54 (3), 765-775.

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The pressure and temperature dependences of water contact angles are strongly affected by silica surface functional groups.


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Pressure and Temperature Dependence of Contact Angles for CO2

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