Molecular Dynamics Characterizations of the Supercritical CO2

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Molecular Dynamics Characterizations of Supercritical CO2-Mediated Hexane-Brine Interface Lingling Zhao, Lu Tao, and Shangchao Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie505048c • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Industrial & Engineering Chemistry Research

Molecular Dynamics Characterizations of Supercritical CO2-Mediated Hexane-Brine Interface Lingling Zhao1,*, Lu Tao1, and Shangchao Lin2,*

1

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of

Energy & Environment, Southeast University, Nanjing, Jiangsu 210096, China 2

Department of Mechanical Engineering, Materials Science & Engineering Program, FAMU-FSU

College of Engineering, Florida State University, Tallahassee, FL 32310, United States *Corresponding authors: Lingling Zhao, phone: (86) 138 5168 0995, e-mail: [email protected] Shangchao Lin, phone: (850) 645 0138, e-mail: [email protected]

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ABSTRACT In the carbon dioxide (CO2)-enhanced oil recovery (EOR) process and the subsequent geological CO2 sequestration, a ternary system consisting of CO2, crude oil and brine exists in the reservoir due to the common practice of injecting CO2 together with brine. In this paper, we carried out molecular dynamics simulations to study the interfacial properties of the ternary CO 2, hexane and 1.52 mol/L sodium chloride (NaCl) solution system under 330 K and 20 MPa with different CO2 compositions at the supercritical state, which are very important for the efficiency of the EOR and CO2 sequestration processes. We observed that CO2 mixes well with hexane and a clear interface separates the CO2-hexane mixture with the NaCl solution. The interfacial roughness increases with the CO2 composition, indicating deeper molecular penetrations and shorter capillary wave lengths, which leads to the reduced interfacial tension. Interestingly, the surface excess of CO2 reaches maximum at a CO2 molar fraction of 62.5% (or a weight fraction of 46%), which implies the amphiphilic feature of CO2, acting like surfactants, towards the hexane-brine interface. The orientational preferences of CO2, hexane and water molecules at the interface are more random at higher CO2 compositions, as a result of the increased absolute amount of CO2 and the absence of hexane at the interface.

KEYWORDS: CO2-enhanced oil recovery, CO2 Sequestration, molecular dynamics, interfacial properties, CO2 adsorption


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1. Introduction Enhanced oil recovery (EOR) by CO2 flooding is an important process to extract energy resources and protect the environment. It not only enhances the oil recovery efficiency, but also facilitates the geological CO2 storage to reduce greenhouse gas emission.1 To ensure both the efficiency and economy of the process, CO2 is typically injected after water flooding where a significant amount of oil is left behind in the reservoir due to capillarity.2, 3 Water and CO2 are also often injected alternately to improve the mobility between the injected fluid and petroleum crude oil.4, 5 In practice, the injected water is normally in the form of brine to reduce cost. Therefore, a fluid system composed of brine, CO2 and crude oil usually co-exists in the reservoir. Moreover, in geological carbon sequestration, CO2 is proposed to be injected in a supercritical state and stored in depleted oil and gas reservoirs where a great amount of brine exists after water flooding,4 which also leads to the formation of the ternary brine, CO2 and crude oil system. It has been found that CO2-EOR and successful CO2 sequestration are largely controlled by the interfacial characteristics among the injected CO2, the reservoir crude oil, the brine and the rock.6-9 The interfacial tension (IFT) under high pressure directly relates to the capillary pressure, the maximum storage height of CO2, and the flow behaviors in porous media.9-11 If the reservoir pressure is higher than the minimum miscibility pressure (MMP), CO2 and oil become completely miscible12 and they contact with brine together at the interface. When CO2 achieves complete miscibility with oil, their IFT drops to zero.13 Thus, in the ternary CO2, oil, and brine system the IFT should be measured between the CO2-oil mixture and brine. Experiments showed that this IFT decreases when increasing the CO2 molar fraction in the mixture.14 CO2 solubility in either phase of the crude oil or the reservoir brine is a key factor in reducing IFT.15 3 ACS Paragon Plus Environment


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However, the role of CO2 at the ternary CO2, oil and brine interface and its influence under its supercritical condition at molecular details have not been reported due to the complexity of the system. Therefore, it is of fundamental and practical importance to study the molecular interactions in the ternary CO2, oil and brine system and identify key factors in order to improve the CO2-EOR and carbon sequestration process, including reducing the capillary forces, increasing the sweep efficiency and storage capacity, and improving the oil mobility, etc. A few molecular dynamics (MD) simulation studies have been conducted to help investigate interfacial properties at the CO2-fluid interface and oil-fluid interface. da Rocha et al.16 simulated the supercritical CO2-water interface and concluded that the interface is sharp at the molecular level with capillary wave-like corrugations, which are characterized as interfacial roughness by Culcer et al.17. Many studies have proved a negative correlation between IFT and the interfacial width.16, 18-22 Nielsen et al.23 simulated CO2-water IFT under various environmental conditions and discussed the proper choice of models, while Muller and Mejia24 also observed the IFT variation with pressure and temperature and mapped out the phase diagram of the same system. In the MD study of the CO2-brine interface by Li et al.25, the surface excess of CO2 is found to be a positive term, which confirms that CO2 molecules prefer to accumulate at the interface. Panagiotopoulos et al.26 studied the thermodynamic and transport properties of the NaCl solution. Zhao et al. investigated the atomic force contributions on IFT of CO2-water and CO2-brine interface and observed that water and CO2 molecules have special molecular orientational tendencies at the interface.27 At the hexane-water interface, differences between the interfacial and bulk liquids were observed, such as the hydrogen-bonding characteristics of water.28 Special molecular orientational order patterns for hexane molecules were found in the miscible 4 ACS Paragon Plus Environment

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hydrocarbons/fluid interface.29, 30 Patel et al.31 developed a refined polarizable force field for hexane, which described the interfacial properties of the hexane-water system more accurately. To the best of our knowledge, no MD studies have been carried out to investigate the molecular interfacial structure of the ternary oil-CO2-water system. In this work, hexane is used as a simply representation of crude oil in the ternary system and will not affect the qualitative trends and the main conclusions in this work. The ternary CO2, hexane and NaCl solution (1.52 mol/L) system at different CO2 compositions (i.e., molar fractions, XCO2) under 330 K and 20 MPa (a typical reservoir condition with CO2 in the supercritical state) are studied using MD simulation, to investigate the detailed molecular influence of CO2 on the interfacial characteristics. The interfacial properties, including the IFT, width of the Gibbs dividing interface (GDS), surface excess (adsorption) of CO2 and hexane molecules, and molecular orientations of CO2, hexane and water molecules are computed under different ХCO2 values to analyze IFT variation and better characterize the behavior of each substance, especially CO2. With fundamental understandings of the influence of each component on the interfacial properties, we can better explain the experimental IFT values and understand the molecular behaviors occurred during CO2 injection. 2. Computational Methods 2.1. Molecular Potentials and Models Nonbonded interactions between molecules and bonded interactions inside molecules are both considered here. Nonbonded interactions consist of van der Waals (vdW) attractions and electrostatic interactions. The Lennard-Jones (LJ) potential was used to model the vdW attractions and steric repulsions between two atoms, i and j, as follows: 5 ACS Paragon Plus Environment


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12

𝜎

𝑉𝑖𝑗𝐿𝐽 = 4𝜀𝑖𝑗 [( 𝑟 𝑖𝑗 ) 𝑖𝑗

𝜎

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6

− ( 𝑟 𝑖𝑗 ) ] 𝑖𝑗

(1)

where 𝜀𝑖𝑗 = (𝜀𝑖𝑖 𝜀𝑗𝑗 )1/2, 𝜎𝑖𝑗 = (𝜎𝑖𝑖 𝜎𝑗𝑗 )1/2, and 𝑟𝑖𝑗 is the distance between i and j. According to the typical range of cutoff values reported in the literature,27, 32-34 we chose 0.9 nm for the cutoff distance for calculating vdW and electrostatic interactions. The electrostatic interaction between two atoms, i and j, with partial charges qi and qj, was modeled using the Coulomb’s law: 𝑉𝑖𝑗𝐶 =

1

𝑞𝑖 𝑞𝑗

4𝜋𝜀0 𝑟𝑖𝑗

(2)

where 𝜀0 is the vacuum permittivity. Long-range electrostatic interactions were treated using the particle mesh Ewald (PME) summation method with a Fourier spacing of 0.12 nm.35 The flexible, three-center (F3C) model36 was chosen for water molecules, since the bond flexibility is critical in predicting both the bulk properties and the surface tension simulation of water.37 CO2 molecules were modeled using the flexible EPM2 model.38 The flexible F3C and EPM2 models have been successfully utilized together before to simulate the CO2-water and CO2-brine interfaces, and the observed IFTs are close to experimental values.27 Kvamme et al.39 also verified the reliability of combining the above two models, and used them in their studies on the adsorption of water and CO2 on hematite. The force-field parameters of hexane molecules were obtained from the OPLS-AA force field.40 Na+ and Cl- were modeled using the parameters reported by Chandrasekhar et al.41 2.2 Simulation Details and Parameters All MD calculations were carried out using the GROMACS 4.5 software package.42 The cross section of the computational domain is 4 nm by 4 nm, as shown in Figure 1 (a). The middle


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part of the simulation box is filled with 2080 water molecules, which is large enough to render bulk water properties.27, 37 The interface is created such that the normal to the interface is along the z-axis. 50 Na+ and 50 Cl- ions are included in the system to mimics the brine with a salt molarity of 1.52 mol/L. 241 hexane molecules were placed on one side of the water phase, while CO2 molecules, whose amount depends on the CO2 molar fraction with respect to the hexane phase, were placed on the other side. Note that the real XCO2 values used in the simulation results are slightly smaller than the initial settings, because some CO2 molecules are dissolved in the NaCl solution. Since periodic boundary conditions are applied in all directions, hexane and CO2 can form a miscible phase and the resulting system could consist of only two phases, if the applied pressure is above the MMP: the mixed CO2-hexane phase and the brine phase. All simulations were performed under the NPzT ensemble. A constant temperature of 330 K was maintained using the velocity-rescaled Berendsen thermostat43, and a constant pressure of 20 MPa was coupled to a semi-isotropic Berendsen barostat44. The Nose-Hoover thermostat45, 46 and the Parrinello-Rahman barostat47,

48

were also utilized to perform the simulation (results not

shown), and the calculated properties are very close to those using the Berendsen method here. The Verlet (Leap-Frog) algorithm49, 50 was used to integrate the equations of motion with a time step of 1 fs. The binary CO2-NaCl solution (ХCO2 = 1) and hexane-NaCl solution (ХCO2 = 0) systems under the same environmental conditions were also simulated using the same method above. All systems reached dynamic equilibrium during 30 ns of simulations. 2.3 Interfacial Property Calculations All data analyses were carried out using the data collected from 25 to 30 ns. The IFT was 7 ACS Paragon Plus Environment


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determined by the difference between the normal and lateral components of the pressure tensor.51, 52

Since there are two interfaces along z direction due to the periodic boundary condition, the

interfacial tension, γ, can be expressed as follows:53 =

𝐿 ∫ 2 0 1

1

[

− 2(

)]

(2)

where Lz is the box length in the z direction and Pxx, Pyy, Pzz are the three z-dependent diagonal components of the pressure tensor along x, y, z directions, respectively. The Gibbs dividing surface (GDS) is defined along the z axis (normal to the interface) where the excess of water molecules at the CO2-hexane side is equal to the deficiency of water molecules at the water side, both with respect to the bulk water densities in the above two phases far away from interface.54 The interfacial width is commonly characterized by the distance between the two surfaces at the 10% and 90% of the bulk water density along the z axis,37 as shown in Figure 2. The surface excess of species i with respect to the bulk represents the difference between the total amount of the species i in the system and in the bulk phase25 and is a good measure of the tendency of species i to adsorb onto the GDS. In the simulated system here, the surface excess of species i, Гi, is calculated by: 𝑖

where

𝑖

=

𝑖

𝐶𝑖

𝐶𝑖

(3)

is the total amount of species i, A and B denote the mixed CO2-hexane phase and

the NaCl solution phase, respectively, VA and VB are the volumes of the two phases,

𝑖

and

𝑖

are the concentration of species i in its bulk phase, and S is the area of the GDS. The orientational probability distribution P(θ) is used to investigate the molecular orientation of each substance at the interface. Figure 3 shows the definitions of θ for each substance. The vector normal to the interface is defined as the normal vector pointing towards the NaCl solution 8 ACS Paragon Plus Environment

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phase. θO-H is defined as the angle between the water O-H bond vector and the normal vector. In a similar way, the angle between the CO2 C=O bond vector and the normal vector defines θC=O. According to the previous literature, the n-hexane orientation can be represented by the longest principal axis of the molecule’s ellipsoid of inertia.30 Because hexane is not a long chain molecule, we use the longest principal axis of the hexane molecule’s carbon chain to represent the orientation here. θHexane is therefore defined as the angle between the longest principal axis of the hexane molecule’s carbon chain and the normal vector. Note that all the orientational probability distributions, P(θ), reported here are corrected by a weighting factor of 1/sinθ in order to remove the bias resulting from the variations in the solid angle.55, 56 3. Results and Discussion 3.1 Validation of the Models The equilibrated simulation snapshot of the ternary CO2, hexane and NaCl solution system at ХCO2 = 72% is shown in Figure 1 (b). The MMP for heptane and CO2 system at 330 K was experimentally determined to be about 9 MPa.57 The heavier the paraffin is, the higher the corresponding MMP.57 Therefore, the pressure of 20 MPa used here is above the MMP for hexane and CO2. Hence, the CO2 phase mixed completely with the hexane phase, which leads to a two phase system with only one interface: the mixed CO2-hexane phase and the brine phase. The IFT values at this interface were calculated from the simulations and are consistent with experimental values14, 58 (see Figure 4) under various ХCO2 values. The brine concentration in our simulations (1.52 mol/L) is higher than that in the experiment (2695 ppm), but it shouldn’t affect the validation of the simulated IFT due to the relationship between IFT and brine salinity59, 60 (see the analysis in the caption of Figure 4). The IFT difference between simulations and experiments 9 ACS Paragon Plus Environment


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becomes larger at higher XCO2, due to the error caused by the combination of EPM2 and water models at high pressures, which was observed in simulations of the CO2-water23 and CO2-brine25 systems. In addition, the simulated bulk CO2 (in the CO2-NaCl system) and hexane (in the hexane-NaCl system) densities are 800.5 ± 6.3 and 652.8 ± 4.9 kg/m3, respectively, close to the experimental density values of 743 kg/m3 for pure CO2 phase (at 333 K and 20 MPa)

62

61

and 647 kg/m3 for the pure hexane

. The above comparisons verified the reliability of the

computational models used here. 3.2 Interfacial GDS Width The IFTs and interfacial GDS widths at various ХCO2 values are shown in Figure 4. It is known that the IFT decreases when increasing ХCO2 in experiments and our simulation results show the same trend that a clear negative correlation exists between the IFT and the interfacial width. This is consistent with the conclusion that sharp interfaces lead to high IFT values in the literature16. The interfacial width is related to molecular penetrations and capillary waves due to thermal fluctuations at the interface, also known as the interfacial roughness17,

63

. To observe the

molecular penetrations and capillary waves, instantaneous configurations of the molecules at the NaCl solution side at ХCO2 = 0, 30.7, 72.0 and 100% were obtained by the QuickSurf method in Visual Molecular Dynamics (VMD)64 (see Figure 6). By comparing between the above several cases, we can see that the surface roughness increases with ХCO2, which verifies that deeper molecular penetrations and shorter capillary wave lengths can be achieved when more CO2 molecules participate in the miscible phase with hexane. It has been found that for partially miscible systems with lower IFT values, such as 10 ACS Paragon Plus Environment

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water/2-heptanone systems19, broader distributions of the interface are observed, and that for immiscible systems with higher IFT, such as water/isooctane18 and water/nitrobenzene21 systems, sharper distributions exist at the interface, which proved the negative correlation relationship between IFT and the interfacial roughness. Because of the small and simple molecule structure, CO2 can form partially miscible with water, while hexane is immiscible with water for its complicated structure with multi C-H interactions. CO2 seems to be less hydrophobic in comparison with hexane. As ХCO2 increases, more CO2 molecules may come to the interface to play a key role and form strong interactions with water, and leads to the deeper molecular penetrations and stronger capillary waves, which can be featured by interfacial width. This phenomenon indicates some influence to IFT. At a higher ХCO2, there are more CO2 molecules at interface to penetrate into water, leading to a higher interfacial width, and the closer interactions between water and CO2, rather than hexane, make a negative contribution to the IFT. 3.3 Surface Excess and Adsorption The CO2 and hexane surface excess curves are shown in Figure 6. They exhibit a parabolic distribution with XCO2. It is worth mentioning that ГCO2 and (‒ ГHexane) exhibit peaks at ХCO2 = 62.5%. There exists a clear negative correlation between the surface excesses of CO2 and hexane. The density profile may show the surface excess phenomenon more directly, so CO2 and hexane density distributions in Figures 7 are combined with Figure 6 to conduct the investigation. The special surface excess phenomenon would be analyzed from the influence of both CO2 and hexane on system. For ХCO2 < 62.5%, there is an enhanced accumulation of CO2 at the interface as XCO2 increases because of the hydrophobicity of CO2 is less than hexane, while hexane molecules are driven away from interface. At ХCO2 = 62.5%, we noticed that in the entire system, 11 ACS Paragon Plus Environment


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including both interface and bulk, the weight fraction of CO2 is 46%. The weight fraction is considered here to judge whether CO2 or hexane has a larger proportion in the miscible phase and have an effect on the surface excess of CO2 and hexane. Under this condition, CO2 starts to play a more important role in the miscible phase than hexane. For ХCO2 > 62.5%, the surface excess of CO2 reaches saturation like a surfactant laying between water and hexane. It further decreases as XCO2 increases because of two possible reasons: (i) the entropic effect, for which hexane cannot be completed separated away from water, and (ii) the adhesion between CO2 and hexane, which requires more hexane at the interface to accompany CO2 and ultimately reduces the CO2 adsorption. 3.4 Molecular Orientation Distribution The simulated orientational probability distributions at the two different sides of the GDS are similar for CO2 and hexane, respectively, and therefore, P(θC=O) and P(θHexane) within the entire GDS are shown in Figure 8 (a) and (b). P(θC=O) and P(θHexane) are random (with a constant value) at all angles for CO2 and hexane in the bulk phase. Water molecular orientation at the water (or NaCl solution) side of the GDS is quite different from that at the CO2-hexane side, so P(θO-H) is calculated at the two sides of the GDS respectively in Figures 8 (c) and (d). P(θO-H) exhibits a peak at ~ 90°at the water side, and two peaks at ~ 70°and ~ 170°at the CO2-hexane side. This is consistent with the findings that the water molecules at the water side of the GDS tend to lie parallel to the interface, while those on the non-aqueous side prefer to orient themselves with an O-H bond pointing towards the non-aqueous phase.27, 30, 65 A broad peak near 90°in P(θC=O) is observed within the GDS, which agrees with the fact that CO2 molecules tend to lie parallel to the interface at the CO2-water interface.27 P(θHexane) shows a wide peak near 120°, suggesting that


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hexane molecules tend to orient at an angle of ~ 60°to the interfacial normal vector, consistent with the work by Nicolas et al.30 In Figure 8 (a), the peak at 90°in P(θC=O) of the ternary CO2, hexane, and NaCl solution system becomes lower when increasing ХCO2. The special C=O bond orientational preference maximizes the interactions between CO2 and water when only a single layer of CO2 can contact with water directly.66 This finding is consistent with the previous report67 that CO2 molecules may form certain interfacial structural compounds with water molecules. When the amount of CO2 increases, more CO2 molecules can be shielded from water by the first CO2 layer that contacts with water directly, and their interactions with water becomes weaker. So the tendency for CO2 molecules to lie parallel to the interface becomes less as ХCO2 increases. CO2 molecules not only accumulate at the interface, but also show a strong molecular orientation tendency, which implies that CO2 is actually amphiphilic (or less hydrophobic) and behaves like a surfactant at the hexane-water interface. In Figure 8 (b), hexane molecules tend to orient with an angle of 60°to the interfacial normal vector. At ХCO2 = 0 and 30.7%, the orientational tendency is more pronounced than that at higher values of ХCO2 = 54.2% and 72.0%. As ХCO2 increases, CO2 starts to show a dominant role at the GDS, while hexane molecules move away from the interface to bulk phase. The remaining hexane molecules at GDS have less contact with water because of the active interactions between CO2 and water molecules. This leads to the lower molecular orientational tendency of hexane when increasing ХCO2. As shown in Figure 9 (c) and (d), whether at the CO2-hexane side or the NaCl solution side, the peak in P(θO‒H) decreases when increasing ХCO2. From the two extreme conditions of ХCO2 =



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0 and 100%, water molecules show a much weaker orientational tendency when contacting directly against CO2 at the interface, in comparison with that against hexane. Based on the work by Fan et al.65, when water interfaces with a non-aqueous hydrophobic phase, water molecules will orient themselves in order to maximize the number of hydrogen bonds and minimize the partial charges that are exposed to the non-aqueous phase. CO2 is less hydrophobic than hexane, leading to the less repulsive interaction with water at the GDS. 4. Conclusions In this paper, molecular dynamics simulations were performed to study the interfacial properties of the ternary CO2, hexane and NaCl solution system with different CO2 compositions at the supercritical state under 330 K and 20 MPa. CO2 molecules mixed well with hexane molecules, which reduces the ternary system to a two phase system with a clear interface separating the CO2-hexane mixture with the NaCl solution. As the CO2 composition increases, the GDS width also increases while IFT decreases. Further investigation shows that the interfacial roughness increases when increasing the CO2 composition, indicating deeper molecular penetrations of the two phases into each other and shorter capillary wave lengths, which ultimately lead to the reduced IFT. Interestingly, the surface excess of CO2 reaches a peak at a CO2 molar fraction of 62.5% (or a weight fraction of 46%), which implies the surfactant-like feature of CO2 at the hexane-brine interface. The surface excess of hexane exhibits the highest negative value at this CO2 fraction, indicating its strong hydrophobicity. During the CO2 injection process in EOR, the composition of CO2 and oil is changing, and molecular adsorptions at the interface exhibit the interesting nonlinear behavior above. The orientational preferences of CO2, hexane and water molecules within the GDS are more random at higher CO2 compositions, all


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related to the increased absolute amount of CO2 and absence of hexane at the local interfacial region. In the CO2-hexane mixture, CO2 plays the major role and interacts more actively with water, while hexane shows stronger hydrophobicity and is separated from water by CO2, which causes a series of influence on the interfacial properties. The unique interfacial properties observed in the CO2, hexane and brine system could provide better explanations for the experimental IFT values and clearer pictures of the molecular behavior of materials during the CO2 injection process.

ACKNOWLEDGMENT L. Zhao and L. Tao would like to acknowledge funding from the National Natural Science Foundation of China (Grant No. 51106027). S. Lin would like to acknowledge the startup funding from the Energy and Materials Initiative at the Florida State University.



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(27) Zhao, L.; Lin, S.; Mendenhall, J. D.; Yuet, P. K.; Blankschtein, D. Molecular Dynamics Investigation of The Various Atomic Force Contributions to the Interfacial Tension at the Supercritical CO2-Water Interface. J. Phys. Chem. B. 2011, 115, 6076-87. (28) Carpenter, I. L.; Hehre, W. J. A Molecular Dynamics Study of the Hexane/Water interface. J. Phys. Chem. 1989, 94, 531-536. (29) de Lara, L. S.; Michelon, M. F.; Miranda, C. R. Molecular Dynamics Studies of Fluid/Oil Interfaces for Improved Oil Recovery Processes. J. Phys. Chem. B. 2012, 116, 14667-76. (30) Nicolas, J. P.; de Souza, N. R. Molecular Dynamics Study of the n-Hexane-Water Interface: Towards a Better Understanding of the Liquid-Liquid Interfacial Broadening. J. Chem. Phys. 2004, 120, 2464-9. (31) Patel, S. A.; Brooks, C. L. Revisiting the hexane-water interface via molecular dynamics simulations using nonadditive alkane-water potentials. J. Chem. Phys. 2006, 124, 204706. (32) Klauda, J. B.; Wu, X.; Pastor, R. W.; Brooks, B. R. Long-Range Lennard-Jones and Electrostatic Interactions in Interfaces: Application of the Isotropic Periodic Sum Method. J. Phys. Chem. B. 2007, 111, 4393-4400. (33) Lague, P.; Pastor, R. W. Pressure-Based Long-Range Correction for Lennard-Jones Interactions in Molecular Dynamics Simulations: Application to Alkanes and Interfaces. J. Phys. Chem. B. 2004, 2004, 363-368. (34) Venable, R. M.; Luo, Y.; Gawrisch, K.; Roux, B.; Pastor, R. W. Simulations of Anionic Lipid Membranes: Development of Interaction-Specific Ion Parameters and Validation Using NMR Data. J. Phys. Chem. B. 2013, 117, 10183-92. (35) Darden, T.; York, D.; Pedersen, L. Particle Mesh Ewald: An N⋅ log (N) Method for Ewald



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Sums in Large Systems. J. Chem. Phys. 1993, 98, 10089-10092. (36) Levitt, M.; Hirshberg, M.; Sharon, R.; Laidig, K. E.; Daggett, V. Calibration and Testing of a Water Model for Simulation of the Molecular Dynamics of Proteins and Nucleic Acids in Solution. J. Phys. Chem. B. 1997, 101, 5051-5061. (37) Yuet, P. K.; Blankschtein, D. Molecular Dynamics Simulation Study of Water Surfaces: Comparison of Flexible Water Models. J. Phys. Chem. B. 2010, 114, 13786-13795. (38) Nieto-Draghi, C.; de Bruin, T.; Perez-Pellitero, J.; Bonet Avalos, J.; Mackie, A. D. Thermodynamic and Transport Properties of Carbon Dioxide from Molecular Simulation. J. Chem. Phys. 2007, 126, 064509. (39) Kvamme, B.; Kuznetsova, T.; Kivelae, P. H. Adsorption of Water and Carbon Dioxide on Hematite and Consequences for Possible Hydrate Formation. Phys. Chem. Chem. Phys. 2012, 14, 4410-24. (40) Kaminski, G. A.; Friesner, R. A.; Tirado-Rives, J.; Jorgensen, W. L. Evaluation and Reparametrization of the OPLS-AA Force Field for Proteins via Comparison with Accurate Quantum Chemical Calculations on Peptides. J. Phys. Chem. B. 2001, 105, 6474-6487. (41) Chandrasekhar, J.; Spellmeyer, D. C.; Jorgensen, W. L. Energy Component Analysis for Dilute Aqueous Solutions of Lithium (1+), Sodium (1+), Fluoride (1-), and Chloride (1-) Ions. J. Am. Chem. Soc. 1984, 106, 903-910. (42) Pronk, S.; Pall, S.; Schulz, R.; Larsson, P.; Bjelkmar, P.; Apostolov, R.; Shirts, M. R.; Smith, J. C.; Kasson, P. M.; van der Spoel, D.; Hess, B.; Lindahl, E. GROMACS 4.5: A High-Throughput and Highly Parallel Open Source Molecular Simulation Toolkit. Bioinformatics. 2013, 29, 845-54.


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(43) Bussi, G.; Donadio, D.; Parrinello, M. Canonical Sampling Through Velocity Rescaling. J. Chem. Phys. 2007, 126, 014101. (44) Berendsen, H. J.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.; Haak, J. Molecular Dynamics with Coupling to an External Bath. J. Chem. Phys. 1984, 81, 3684-3690. (45)Nosé, S. A molecular dynamics method for simulations in the canonical ensemble. Mol. Phys. 1984, 52, 255-268. (46) Hoover, W. G. Canonical dynamics: equilibrium phase-space distributions. Phys. Rev. A. 1985, 31, 1695. (47) Parrinello, M.; Rahman, A. Polymorphic transitions in single crystals: A new molecular dynamics method. J. Appl. Phys. 1981, 52, 7182-7190. (48) Nosé, S.; Klein, M. Constant pressure molecular dynamics for molecular systems. Mol. Phys. 1983, 50, 1055-1076. (49) Verlet, L. Computer" Experiments" on Classical Fluids. I. Thermodynamical Properties of Lennard-Jones Molecules. Phys. Rev. 1967, 159, 98. (50) Hockney, R.; Goel, S.; Eastwood, J. Quiet High-Resolution Computer Models of a Plasma. J. Comput. Phys. 1974, 14, 148-158. (51) Harris, J. G. Liquid-Vapor Interfaces of Alkane Oligomers: Structure and Thermodynamics from Molecular Dynamics Simulations of Chemically Realistic Models. J. Phys. Chem. 1992, 96, 5077-5086. (52) Alejandre, J.; Tildesley, D. J.; Chapela, G. A. Molecular Dynamics Simulation of the Orthobaric Densities and Surface Tension of Water. J. Chem. Phys. 1995, 102, 4574-4583. (53) Senapati, S.; Berkowitz, M. L. Computer Simulation Study of the Interface Width of the



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Liquid/Liquid Interface. Phys. Rev. Lett. 2001, 87, 176101. (54) Leach, A. R. Molecular Modelling: Principles and Applications, 2nd ed. Prentice Hall: Harlow, England, 2001. (55) Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. Effect of Temperature on the Structure and Phase Behavior of Water Confined by Hydrophobic, Hydrophilic, and Heterogeneous Surfaces. J. Phys. Chem. B. 2009, 113, 13723-13734. (56) Lee, C. Y.; McCammon, J. A.; Rossky, P. J. The Structure of Liquid Water at an Extended Hydrophobic Surface. J. Chem. Phys. 1984, 80, 4448-4455. (57) Zolghadr, A.; Escrochi, M.; Ayatollahi, S. Temperature and Composition Effect on CO2 Miscibility by Interfacial Tension Measurement. J. Chem. Eng. Data. 2013, 58, 1168-1175. (58) B.Y Cai; J.T Yang; Guo, T. M. Interfacial Tension of Hydrocarbon + Water/Brine Systems under high pressure. J. Chem. Eng. Data. 1996, 41, 493-496. (59) Chalbaud, C.; Robin, M.; Lombard, J. M.; Martin, F.; Egermann, P.; Bertin, H. Interfacial Tension Measurements and Wettability Evaluation for Geological CO2 Storage. Adv. Water. Resour. 2009, 32, 98-109. (60)Duchateau, C.; Broseta, D. A Simple Method for Determining Brine–Gas Interfacial Tensions. Adv. Water. Resour. 2012, 42, 30-36. (61) Lemmon, E. W.; Huber, M. L.; McLinden, M. O. NIST Reference Fluid Thermodynamic and Transport Properties–REFPROP, 2002. (62) Kumagai, A.; Tomida, D.; Yokoyama, C. Measurements of the Liquid Viscosities of Mixtures of n-Butane, n-Hexane, and n-Octane with Squalane to 30 MPa. Int. J. Thermophys. 2006, 27, 376-393.


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(63) Rao, D. N.; Lee, J. I. Determination of Gas-Oil Miscibility Conditions by Interfacial Tension Measurements. J. Colloid. Interface Sci. 2003, 262, 474-482. (64) Humphrey, W.; Dalke, A.; Schulten, K., “VMD: Visual Molecular Dynamics,”. J. Mol. Graphics. 1996, pp 33-38. (65) Fan, Y.; Chen, X.; Yang, L.; Cremer, P. S.; Gao, Y. Q. On the structure of water at the aqueous/air interface. J. Phys. Chem. B. 2009, 113, 11672-11679. (66) Perez-Blanco, M. E.; Maginn, E. J. Molecular Dynamics Simulations of CO2 at An Ionic Liquid Interface: Adsorption, Ordering, and Interfacial Crossing. J. Phys. Chem. B. 2010, 114, 11827-11837. (67) Tewes, F.; Boury, F. Thermodynamic and Dynamic Interfacial Properties of Binary Carbon Dioxide-Water Systems. J. Phys. Chem. B. 2004, 108, 2405-2412.



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Figure Captions Figure 1. Simulation snapshots of the ternary CO2, hexane and NaCl solution system at XCO2 = 72.0%. (a) The initial configuration of the simulated system. (b) The equilibrium configuration of the system after 30 ns of simulation. The two solid black lines denote the Gibbs dividing interfaces. It shows that CO2 and hexane are well mixed under 330 K and 20 MPa. Color code for the atoms: green – carbon, white – hydrogen, red – oxygen, blue – sodium, and yellow – chloride. Figure 2. Definition of the Gibbs dividing surface (GDS), including its location and the interfacial width (distance between the surfaces at 10% and 90% of the bulk water density along the z axis). The density profiles of all species present in the simulation is shown at XCO2 = 72.0%. Figure 3. Schematic diagram of the definitions of angular orientations of water, CO2 and hexane. (a) the angle θO-H is defined between the O-H bond of the water molecule and the vector normal to the interface, (b) the angle θC=O is defined between the C=O bond of the CO2 molecule and the vector normal to the interface, and (c) the angle θHexane is defined between the longest principal axis of the hexane molecule’s carbon chain and the vector normal to the interface. The color code is the same as Fig. 1. Figure 4.

Variations of the IFT (simulations and experiments14, 58) and interfacial width as a

function of ХCO2. Symbol codes: square – the simulated IFT values here, circle – the experimental IFT values, dashed line – calculated experimental IFTs considering salinity difference, and up triangle – the interfacial width. A clear negative correlation is observed between the IFT and interfacial width. Note: Experimental IFTs in Ref. 18 were observed in CO2, crude oil and reservoir water system at 330 K and 20 MPa. The salt concentration of reservoir water is very low (2695 ppm), close to pure water. Ref. 19 reported IFT of hexane/NaCl (0.8847 mol/L)


Page 25 of 35

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interface at 323 K and 21.19 MPa. The IFT of CO2, hexane and water system calculated at XCO2 = 62.5% is 38.2 ± 1.0 mN/m, and the one calculated in CO2, hexane and 1.52 mol/L NaCl in this work is 40.4 ± 1.1 mN/m. According to the above result and the research of salinity effect on IFT59, 60, the errors due to salinity differences here are within 2.5 mN/m (as shown by the corrected experimental IFTs in the dashed line), which has little influence on our main conclusions. Figure 5. Instantaneous configuration of the corrugations (interfacial roughness) at the NaCl solution side of the interface under different XCO2. The colored contour plot illustrates atoms at different depths normal to the interface. The blue color represents the locations of hydrogen or oxygen atoms of water molecules close to the bulk NaCl solution phase and far away from the other phase (CO2 + hexane). The red color represents the atoms of water close to the mixed CO2-hexane bulk phase. The transitional color, such as white, represents atoms distributing somewhere in between. Figure 6. Surface excesses of CO2 and hexane (both with respect to the GDS) as a function of XCO2. A clear negative correlation between the surface excesses of CO2 and hexane is observed. A peak CO2 surface excess is obtained at XCO2 = 62.5%. Figure 7.

CO2 and hexane density profiles along the z axis at different XCO2. (a) The CO2

density curves. (b) The hexane density curves. The GDS is located at z = 0 nm. Figure 8. Simulated orientational probability distributions within the GDS width. (a) P(θC=O) for CO2 molecules, (b) P(θHexane) for hexane molecules, (c) P(θO=H) for water molecules at the NaCl solution side, and (d) P(θO–H) for water molecules at the CO2-hexane side. Note that all P(θ) reported here are corrected by a weighting factor of 1/sinθ in order to remove the bias resulting



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from the variations in the solid angle. The P(θ) values are calculated every five degrees. Symbol codes: square – reference state in the bulk CO2 phase, circle – at XCO2 = 0%, up triangle – at XCO2 = 30.7%, pentagon – at XCO2 = 54.2%, down triangle – at XCO2 = 72.0%, and diamond – at XCO2 = 100%.


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Figure 1. Simulation snapshots of the ternary CO2, hexane and NaCl solution system at XCO2 = 72.0%. (a) The initial configuration of the simulated system. (b) The equilibrium configuration of the system after 30 ns of simulation. The two solid black lines denote the Gibbs dividing interfaces. It shows that CO2 and hexane are well mixed under 330 K and 20 MPa. Color code for the atoms: green – carbon, white – hydrogen, red – oxygen, blue – sodium, and yellow – chloride.



1200 1000

Density (kg/m3)

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Water

CO2

Na+

Cl-

Page 28 of 35

C6H14

Width 800

GDS 600 400 200 0 -0.8

-0.4

0.0 z (nm)

0.4

0.8

Figure 2. Definition of the Gibbs dividing surface (GDS), including its location and the interfacial width (distance between the surfaces at 10% and 90% of the bulk water density along the z axis). The density profiles of all species present in the simulation is shown at XCO2 = 72.0%.


Page 29 of 35

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Figure 3. Schematic diagram of the definitions of angular orientations of water, CO2 and hexane. (a) the angle θO-H is defined between the O-H bond of the water molecule and the vector normal to the interface, (b) the angle θC=O is defined between the C=O bond of the CO2 molecule and the vector normal to the interface, and (c) the angle θHexane is defined between the longest principal axis of the hexane molecule’s carbon chain and the vector normal to the interface. The color code is the same as Fig. 1.



0.72

IFT (mN/m)

49 42

0.64

35

0.56

28

0.48

21

0.40 0%

20%

40%

60%

80%

GDS Width (nm)

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100%

XCO2 Figure 4.

Variations of the IFT (simulations and experiments14, 58) and interfacial width as a

function of ХCO2. Symbol codes: square – the simulated IFT values here, circle – the experimental IFT values, dashed line – calculated experimental IFTs considering salinity difference, and up triangle – the interfacial width. A clear negative correlation is observed between the IFT and interfacial width. Note: Experimental IFTs in Ref. 18 were observed in CO2, crude oil and reservoir water system at 330 K and 20 MPa. The salt concentration of reservoir water is very low (2695 ppm), close to pure water. Ref. 19 reported IFT of hexane/NaCl (0.8847 mol/L) interface at 323 K and 21.19 MPa. The IFT of CO2, hexane and water system calculated at XCO2 = 62.5% is 38.2 ± 1.0 mN/m, and the one calculated in CO2, hexane and 1.52 mol/L NaCl in this work is 40.4 ± 1.1 mN/m. According to the above result and the research of salinity effect on IFT59, 60, the errors due to salinity differences here are within 2.5 mN/m (as shown by the corrected experimental IFTs in the dashed line), which has little influence on our main conclusions.


Page 31 of 35

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Figure 5. Instantaneous configuration of the corrugations (interfacial roughness) at the NaCl solution side of the interface under different XCO2. The colored contour plot illustrates atoms at different depths normal to the interface. The blue color represents the locations of hydrogen or oxygen atoms of water molecules close to the bulk NaCl solution phase and far away from the other phase (CO2 + hexane). The red color represents the atoms of water close to the mixed CO2-hexane bulk phase. The transitional color, such as white, represents atoms distributing somewhere in between.


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Surface Excess (×10-6 mol/m2)


0%

20%

40%

60%

80%

100%

40%

60%

80%

100%

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CO2

4 2 0 -1 -2

C6H14 0%

20%

XCO2 Figure 6. Surface excesses of CO2 and hexane (both with respect to the GDS) as a function of XCO2. A clear negative correlation between the surface excesses of CO2 and hexane is observed. A peak CO2 surface excess is obtained at XCO2 = 62.5%.


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800

XCO2

(b)

100% 84.6% 62.5% 30.7% 9.3%

600

GDS

Hexane Density (kg/m3)

(a) CO2 Density (kg/m3)

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400

200

0 -1.5

-1.0

-0.5

0.0

0.5

1.0

800

XCO2

9.3% GDS 30.7% 62.5% 84.6%

600

400

200

0 -1.5

1.5

0

-1.0

z (nm)

Figure 7.

-0.5

0.0

0.5

1.0

1.5

z (nm)

CO2 and hexane density profiles along the z axis at different XCO2. (a) The CO2

density curves. (b) The hexane density curves. The GDS is located at z = 0 nm.



(a)

0.06

(b) 0.08

P (Hexane)

P (C=O)

0.05

0.04

0.03

0.06

0.04

0.02

0.02 0

30

60

90

120

150

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C=O (degree) (d)

(c)

30

60

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120

Hexane (degree)

150

180

150

180

0.056

0.050 0.048

P (O-H)

0.045

P (O-H)

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0.040 0.035

0.040

0.032

0.030 0.024

0

30

60

90

120

150

180

0

30

60

90

120

O-H (degree)

O-H (degree)

Figure 8. Simulated orientational probability distributions within the GDS width. (a) P(θC=O) for CO2 molecules, (b) P(θHexane) for hexane molecules, (c) P(θO=H) for water molecules at the NaCl solution side, and (d) P(θO–H) for water molecules at the CO2-hexane side. Note that all P(θ) reported here are corrected by a weighting factor of 1/sinθ in order to remove the bias resulting from the variations in the solid angle. The P(θ) values are calculated every five degrees. Symbol codes: square – reference state in the bulk CO2 phase, circle – at XCO2 = 0%, up triangle – at XCO2 = 30.7%, pentagon – at XCO2 = 54.2%, down triangle – at XCO2 = 72.0%, and diamond – at XCO2 = 100%.


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Molecular Dynamics Characterizations of the Supercritical CO2

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