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The Role of Supercritical/Dense CO2 Gas in Altering Aqueous/ Oil Interfacial Properties: A Molecular Dynamics Study Sohaib Mohammed, and G. Ali Mansoori Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03863 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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The Role of Supercritical/Dense CO2 Gas in Altering Aqueous/Oil Interfacial Properties: A Molecular Dynamics Study Sohaib Mohammed a, * and G.Ali Mansoori b a
Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA. b
Department of Bio and Chemical Engineering, University of Illinois at Chicago, (M/C 063) Chicago, IL 60607-7052, USA. *Corresponding author.
Abstract One of the main mechanisms contributing to enhanced oil recovery (EOR) processes using compressed (supercritical) carbon dioxide (sc-CO2) is alterations in the oil-water interfacial properties. However, it has been a challenge to experimentally investigate such effects. In our investigation presented here, we performed molecular dynamics (MD) simulation to explore these changes. We studied the role of sc-CO2 in changing the interfacial and transport properties of systems composed of water and pure hydrocarbons, namely hexane, octane, benzene, and xylene. The simulations were performed at 100bar and 350K. It was observed that sc-CO2 accumulates at the interface which leads to a reduction in the interfacial tension (IFT) of wateroil systems. Our further analysis of such accumulation showed that the ratio of sc-CO2 density at the interface to sc-CO2 bulk density decreases as the sc-CO2 mole fraction increases. This interesting behavior owed to the difference in the interaction between CO2-water and CO2hydrocarbon which diverges as CO2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO2 forms a film between the two phases which displaces oil molecules away from the interface. This film was stabilized by H-bonds between water and CO2. We also found that as CO2 content increase, the interfacial width increases which contribute negatively to the IFT. Furthermore, it was found that as sc-CO2 mole fraction increase, the hydrocarbons diffusion coefficients increase. The diffusivity response to CO2 addition was determined by the molecular weight and the polarity of the hydrocarbon. 1. Introduction It is now a proven fact that carbon dioxide (CO2) is the major cause of global warming due to its presence in the atmosphere in a much larger amount than it used to be, before the industrial revolution. CO2 emission has increased significantly during recent decades 1. One of the practical solutions that may help to reduce the accumulation of CO2 in the atmosphere is its storage in underground reservoirs and the employment of the gas in enhanced oil recovery (EOR) processes
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that provide the advantage of geological sequestration as well as petroleum production. EORs due to CO2 injection for oil production has been used since the 1970s. In EOR, tertiary oil recovery, using CO2 injection, it is applied in shallow reservoirs of light and medium light oils at low temperature. CO2 has a relatively low minimum miscibility pressure (MMP) with a broad range of crude oils. Thus, it is considered a unique displacing agent in EOR. EOR processes, including those using CO2 flooding, are typically applied after secondary recovery (i.e., water flooding) to extract the oil left in the reservoirs. In CO2-induced EOR practice, CO2 and water are injected alternately to enhance the mobility between the injected fluid and the oil in the reservoir, so a mixture of water, CO2 and oil usually exist in the reservoir 2. One of the main mechanisms contributing to CO2-induced EOR is the change in the interfacial properties 3. At a pressure above its MMP, CO2 is in its supercritical conditions, and it may become miscible with oil, causing swelling of the oil phase, as well as changing its interfacial properties with the other phases present, i.e., water phase and reservoir wall surfaces. It is challenging to experimentally measure the effect of sc-CO2 on the interfacial behavior of (CO2/oil) phase, water phase, and reservoir wall surfaces due to the complexity of the problem. There are limited studies, both theoretical and experimental, that investigated the ternary system of CO2-hydrocarbons-water. We found two experimental and two simulation studies in the literature that studied the ternary systems with CO2 present as a third component. Experimentally it was shown the interfacial tension of crude oil/brine, in the presence of CO2, decreased as the pressure and CO2 concentration increased 4-5. To understand the role of CO2 in altering aqueous/oil interfacial properties, Liu et al. 6 used MD simulation on a system of CO2, water, and decane under 313 K and 10 MPa. According to their simulation results, CO2 prefers to accumulate at the water decane interface. Liu’s MD simulation study also showed an increase in the diffusivity of all the participated fluids toward the interface as the concentration of CO2 increases in the system. Zhao et al. 7 studied the CO2-hexane-brine ternary system and found that the interfacial roughness increases as CO2 content increases in the system. They also found that CO2 has an amphiphilic behavior, like surfactants, toward the hexane-brine interface. Due to the importance of this topic and, the insufficient past studied, we intend to perform a detailed investigation on the effect of supercritical/dense CO2 gas on IFT of a variety of aqueous/oil systems. For this purpose, we chose four different pure hydrocarbons (benzene and xylene, nhexane, n-octane) resembling the oil phase. Benzene and xylene are aromatics while n-hexane, noctane are paraffins. Benzene and n-hexane contain six carbon atoms while xylene and octane contain eight. Our simulation operating conditions are 350 K and 100 bar which are above the critical temperature and pressure of CO2, so CO2 in its supercritical state and the pressure is also above MMP. This paper is organized as follows. In section 2, we describe the simulation methodology we have used to extract the data. Also, we included a validation of this methodology. Section 3 contains the results and discussions about the findings in this research and explanations of the observed behaviors. We discussed the density profiles, CO2 accumulation, hydrocarbon
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displacement from the interface, interactions, interfacial tension, and diffusivity. Finally, we list the conclusions in section 4.
2. Simulation methodology 2.1. Simulation details Classical MD simulations reported here were performed using GROMACS 5.1.2 package 8. Both intermolecular and intramolecular interactions were considered in this simulation. The intramolecular interaction potential was calculated for the bond stretching, angle bending, and dihedral potential. The intermolecular interactions accounted for van der Waals (vdW) attraction and steric repulsion, which were modeled by Lennard-Jones (LJ) pair potential energy function, and electrostatic (ES) interactions, which were modeled by Coloumb’s law. A cutoff of 1.4 nm was determined to calculate the vdW interaction. Long-range ES interactions were treated using particle mesh Ewald (PME) summation method 9. The optimized potentials for liquid simulations-all atoms (OPLS-AA) force field 10 was employed to model the hydrocarbons. CO2 molecules were modeled by an EPM2 force field 11. Water molecules were described by simple point charge/extended (SPC/E) 12. The simulation cell for CO2-hydrocarbon-water mixture was initially set as a rectangular box with dimensions Lx = Ly = 4 nm and Lz = 12 nm containing CO2, hydrocarbon, and water. The initial configuration was composed of 400 hydrocarbon molecules, various numbers of CO2 molecules and 3000 water molecules. The various number of CO2 molecules in the box were 0, 100, 267, 600 and 1600 to represent 0, 0.2, 0.4, 0.6 and 0.8 mole fractions (xCO2), respectively. The number of CO2 molecules for each mole fraction was calculated as follows: nCO 2 = xCO 2 nt
(1)
Where nCO 2 is the number of CO2 molecules, xCO 2 is the required CO2 mole fraction, and nt is the summation of the number of hydrocarbon and CO2 molecules. For each simulation case, an energy minimization was performed using the “steepest descent” method to ensure that the maximum force is less than (100 kJ/mol.nm). The simulation box was equilibrated at the required temperature using NVT (canonical ensemble) for 100 ps. Berendsen thermostat 13 was used to control the temperature during the NVT. After the system reached the required temperature, MD simulation was performed for 10 ns under NpnAT ensemble, where pn is the normal component of the pressure tensor and A is the interface area. The area of the interface was kept constant while Z-axis changes using a semiisotrpic coupling. Berendsen thermostat and Parrinello-Rahman barostat 14 were employed to control the temperature and pressure during NpnAT, respectively. Three dimensions periodic boundary conditions were applied. The IFT was calculated using formulation of Gibbs as written regarding pressures,
1
γ = Pxx − 2
Pyy + Pzz Lz 2
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(2)
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Where Pii (i = x, y, and z) is the diagonal elements of the pressure tensor and Lz is the length of the simulation box in the Z axis. 2.2. Simulation verifications A series of simulations were conducted to calculate the density of pure hydrocarbons and IFT of water/hydrocarbons at the atmospheric pressure and 298K. The hydrocarbons used in the validation simulations were hexane, octane, benzene, and xylene. The simulations were performed for 10 ns and the data were averaged for the last 5 ns. The governed data were in good agreement with the experimental data as shown in Table 1. Table 1. A comparison between the density of pure hydrocarbons and IFT with water governed from MD simulations with the experimental data at 1bar and 298K. Hydrocarbon Hexane Octane Benzene Xylene
Density (kg/m3) MD Experimental 653 654.78 15 696 698.27 15 876 873.60 15 873 875.53 17
MD 49.8 51.3 34.8 35.2
IFT (mN/m) Experimental 50.5 16 50.7 16 34.4 16 36.3 18
3. Results and discussion 3.1. Constituents Density profiles We measured the density (ߩ) profile for all the constituents in the systems of water-CO2-octane and water-CO2-benzene along Z-axis, which is the axis normal to the interface, for xCO2 of 0.2, as shown in Figure 1. Bulk density (ߩbulk) of water was about 975 ± 23 kg/m3 which is consistent with the experimental data under 350K and 100bar. The simulation results showed that the system of oil-water forms two separate phases. CO2 exhibited low and high solubilities in water and oil phases, respectively. The simulation also showed that there is an accumulation of CO2 at the interface in all systems which is consistent with previous studies that performed on brine-hexane and waterdecane 6-7. However, we observed that the accumulation in the water-n-alkanes is higher than that in the water-aromatics interface. The variation in the amount of CO2 accumulated is attributed to the difference in the interaction of water-aromatics and water-n-alkanes. Water and hydrocarbons are immiscible so the only possible interaction between the two phases occurs at the interfacial region. Aromatics have higher interaction with water than alkanes due to the weak hydrogen bonding between the aromatic ring and the water proton 19. Thus, aromatics have lower IFT with water than alkanes. CO2 are partially miscible with water so it has lower IFT than hydrocarbons. The difference between IFT of water/CO2 and water/hydrocarbons is the driving force for CO2 accumulation. The accumulation in water/alkanes is higher than water/aromatics because the difference in the IFT between water/CO2 and water/alkanes is higher than the difference between water/CO2 and water/aromatics.
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Figure 1. Density profile of the constituents in water-CO2-benzene (left) and water-CO2-octane (right) along with the axis normal to the interface (z-axis). The x-axis represents Z-axis normalized by the length of the system in the z-direction. CO2 mole fractions are equal to 0.2 in these systems.
To shed more light on CO2 accumulation at the interfaces, we analyzed the distribution of CO2 near the interface at each CO2 addition as illustrated in Figure 2. We observed that as xCO2 increases, the relative density, the ratio of CO2 accumulated at the interface to the bulk density (ߩ/ߩbulk), decreases in all systems. The relative density of CO2 was decreased more significantly in case of alkanes than aromatics as xCO2 increased from 0.2 to 0.6. This means that as xCO2 increases, the amount of CO2 dissolved in the oil bulk increases. The decrease in the peak of the ratio as xCO2 increases is due to the increase in the gap between the magnitude of water-CO2 and hydrocarbon-CO2 interactions, thus CO2 dissolves in the phase that has higher interaction with.
Figure 2. Relative CO2 density along with the axis normal to the interface. The X-axis represents Z-axis in the simulation box normalized by the length of the system in that direction. The left
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figure corresponds to the system where CO2 mole fraction is equal to 0.2 while the right one corresponds the systems where CO2 mole fraction is equal to 0.6.
3.2. Hydrocarbon displacement away from the interface by CO2 As shown in Figure 3-a, we observed that as xCO2 increases in the system, it displaces the hydrocarbon away from the interface. We measured ߩ of the hydrocarbon within the interface for all the hydrocarbons. The interface boundaries were determined where the hydrocarbon ߩ in the water phase is approximately equal to the water ߩ in the hydrocarbon phase. It characterized by Gibbs dividing surface (GDS). ߩ of hydrocarbon increases in the direction from water phase toward hydrocarbon phase at all xCO2. ߩ of the hydrocarbons is higher in the absence of CO2 and decreases with the increase of xCO2. The conclusion that CO2 displaces the hydrocarbons away from the interface is supported by the water density profile within the interface which is not affected by the addition of CO2 as shown in Figure 3-b.
a
c
b
d
Figure 3-a. The density of (a) hexane, (b) octane, (c) benzene and (d) xylene in the interface at 0, 0.2, 0.4, 0.6 and 0.8 CO2 mole fraction. the X-axis represents the interface region, which is determined by Gibbs Dividing Surface, normalized by the width of the interface (GDS* = GDS/interface width).
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a
b
d
c
Figure 3-b. Water density within the interface region at various CO2 mole fractions for systems composed of water and (a) hexane, (b) octane, (c) benzene and (d) xylene. 3.3. Intermolecular Interactions We also calculated the interaction between water-hydrocarbons, water-CO2 and hydrocarbonsCO2 as a function of xCO2, as shown in Figure 4. Both electrostatic and vdW interactions were calculated. The intermolecular interaction between the constituents at the interface is responsible for the variation of the interfacial properties. Intermolecular interactions between water and the hydrocarbons were decreased in both vdW and electrostatic as xCO2 increased. This decrease was due to the displacement of hydrocarbons away from the interface. Both electrostatic and vdW participated in Water-CO2 intermolecular interactions, however, electrostatic interactions were higher than vdW. Water-CO2 interaction occurs between the accumulated CO2 molecules in the interfacial region and the penetrated CO2 molecules into the water phase with water molecules. The intermolecular interactions between CO2 and hydrocarbons came mainly from vdW, however, an observation regarding electrostatic interaction could be noticed. It is that the
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electrostatic interaction between CO2-aromatics is higher than CO2-alkanes due to the aromatics polarity. In case of vdW, CO2-hydrocarbons increased significantly in all systems. In term of total intermolecular interaction, it was noticed that as the xCO2 increases, the total interaction of water-CO2 and hydrocarbon-CO2 increase while the interaction of waterhydrocarbon decrease. In all systems, the interaction of water and hydrocarbon with CO2 increased significantly, and the hydrocarbon-CO2 interaction is stronger than water-CO2 interaction. Another observation could be shown here, the interaction between aromatics-CO2 was stronger than the interaction of alkanes-CO2. The higher interaction between aromatics and CO2 over alkanes-CO2 may be attributed to the polarity of the aromatic ring. Calculating the interaction energies between the constituents can describe the mechanism of the reduction of IFT using sc-CO2. If the difference between the interaction energies of waterCO2 and hydrocarbon-CO2 is small, then CO2 accumulates at the interface to balance the interaction energies. As the difference between the interaction energy between water-CO2 and hydrocarbon-CO2 increases, CO2 prefers to be dissolved in the phase of which the interaction is stronger. This conclusion is supported by the relation between ߩ/ߩbulk and xCO2 in the system (see Figure 2). When the interaction between hydrocarbon-CO2 increases and become higher than the interaction between water-CO2, more CO2 dissolves in the hydrocarbon phase than the amount accumulated at the interface. Moreover, the interaction between aromatics-CO2 is higher than the interaction between alkanes-CO2, thus we noticed lower peaks in water-CO2-aromatics systems than in Water-CO2-alkanes systems.
a
b
c
d
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Figure 4-a. The electrostatic interaction energies between water-CO2, water-hydrocarbon and hydrocarbon-CO2 for (a) water-CO2-hexane, (b) water-CO2-octane, (c) water-CO2-benzene and (d) water-CO2-xylene as a function of CO2 mole fraction.
a
b
c
d
Figure 4-b. vdW interaction energies between water-CO2, water-hydrocarbon and hydrocarbonCO2 for (a) water-CO2-hexane, (b) water-CO2-octane, (c) water-CO2-benzene and (d) waterCO2-xylene as a function of CO2 mole fraction.
3.4. Interfacial tension We calculated the IFT for all the simulated systems as a function of xCO2, as shown in Figure 5. The IFT decreased as xCO2 increased in the system. The IFT of water-hexane-CO2 system decreased from 50.2 to about 30.7 mN/m as xCO2 rose from 0 to 0.8, respectively. In case of octane, the IFT decreased from about 52.8 to 30.9 mN/m as xCO2 increased from 0 to 0.8. While for aromatics, IFT decreased from 38.6 to 27 and from 36.6 to 23.8 for the case of benzene and xylene, respectively, as a xCO2 increase from 0 to 0.8. It is obvious that the effect of CO2 on IFT of water-alkanes is higher than the effect on water-aromatics. This trend attributed to different factors such as variation of the CO2 accumulation at the interface, the displacement of the
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hydrocarbons away from the interface, hydrogen bonds between water and CO2, and interfacial width.
Figure 5. IFT of water-CO2-hydrocarbon as a function of CO2 mole fraction (xCO2).
3.5. Hydrogen bonds Hydrogen bonds (H-bonds) usually form between hydrogen, which is an electropositive atom, and oxygen and nitrogen, which are strongly electronegative atoms that have at least one lone pair of electrons 20. In addition to the interaction between the components at the interface that leads to a variation in the IFT when CO2 added, H-bonds between CO2 and water were also contributed to this change. We found that the number of H-bonds between water and CO2 increased as xCO2 increases as shown in Table 2. The formation of H-bonds between water and CO2, enhances the interaction between them and as a result, stabilizing CO2 film at the interface. It also reduced the interaction between water molecules by decreasing H-bonds between water molecules which eventually lead to a high interfacial roughness. Thus, the formation of H-bonds between water and CO2 has a vital role in the drop of the IFT. Table 2. Number of hydrogen bonds between water and CO2 at the different system and CO2 mole fractions. xCO2 Hydrocarbon Hexane Octane Benzene Xylene
0.2
0.4
0.6
0.8
19 18 19 17
36 36 34 33
47 52 50 56
61 61 58 77
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3.6. The interfacial width The interfacial width was also calculated against CO2 mole fraction, as shown in Table 3. It was determined by GDS where the hydrocarbons molecules in the water phase were approximately equal to the water molecules in the hydrocarbons phase. The relation between the interfacial width and xCO2 suggests that the addition of CO2 to the water-oil system dilutes the interface. CO2 has less hydrophobicity than hydrocarbons which describes the attraction of CO2 toward the interface. In other words, CO2 can be partially miscible in water due to the strong diffusibility of CO2 regarded as a polar molecule with active and strong bond dipoles 21. On the other hand, hydrocarbon molecules that have several C-H interactions which make it strong hydrophobic and immiscible with water. The increase in the interfacial width contributes negatively to the IFT. Table 3. The interfacial width (nm) of the water-CO2-hydrocarbon system as a function of xCO2. xCO2 Hydrocarbon Hexane Octane Benzene Xylene
0.0
0.2
0.4
0.6
0.8
0.37 0.34 0.44 0.46
0.44 0.44 0.48 0.53
0.5 0.47 0.53 0.58
0.58 0.57 0.59 0.63
0.67 0.65 0.63 0.81
The dilution of the interface and displacing the hydrocarbon away toward the oil bulk suggests that CO2 forms a film between water and hydrocarbons phases. The thickness of this film enhances as xCO2 increases in the system as shown in Figure 6. This film bridges the two phases and as a result, affects the interfacial properties of the system. The film is stabilized by Hbonds with water phase in the interfacial region.
a
b
c
Figure 6. A snapshot for interface at CO2 mole fraction of (a) 0.0, (b) 0.4 and (c) 0.8. The color codes are red (oxygen), white (hydrogen) and cyan (carbon). CO2 are hidden in the snapshots for clarity. VMD22 package was used for imaging.
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3.7. Hydrocarbon Diffusivity The self-diffusion coefficient of each hydrocarbon was determined using mean square displacement (MSD) according to Einstein relation as follows: D=
1 d n 2 lim ∑i Ri (t ) − Ri (0) 6 t →∞ dt
(3)
For all the hydrocarbons, we found that as xCO2 increases the diffusion coefficient increases, as shown in Figure 7. The effect of CO2 addition to the diffusivity behavior can be explained by comparing the diffusivities of components of the same type (i.e. alkane with alkane and aromatic with aromatic) as well as alkane with aromatic based on the carbon number. The difference in alkanes diffusivities was due to the difference in the molecular weight. The heavier molecule (octane) was affected less significantly than the lighter one (hexane). In the aromatics, there are two determinants for the effect of CO2 on the diffusivity: the molecular weight and the polarity. Xylene (C8) was affected less than benzene (C6) because it is heavier and more polar. The polarizability of benzene is 10.3 Å3 while xylene has a value of 14.9 Å3 23. For the same carbon number, the effect of CO2 on the diffusivity of the aromatic compound was less significant than on alkane, although it has less molecular weight. This behavior could be attributed the difference in the polarity, whereas the aromatics are more polar than alkanes. The increase of the hydrocarbon diffusion leads to an increase in the mobility of oil and thus gives an advantage in EOR.
Figure 7. Self-diffusion coefficient of hydrocarbons as a function of CO2 mole fraction (xCO2).
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4. Conclusions We conducted a series of MD simulations and present a detailed study on the effect of supercritical/dense CO2 gas (sc-CO2) on interfacial and transport properties of a variety of aqueous/oil systems. We investigated the effect of sc-CO2 on systems composed of water with four different pure hydrocarbons (benzene, xylene, n-hexane and n-octane) resembling the oil phase. The main finding of this research is as follows: I.
The results showed that CO2 accumulates at the interface for all systems. This accumulation was attributed to the difference in IFT between water/CO2 and water/hydrocarbons. The amount of accumulated CO2 depends on the magnitude of the difference between water/CO2 and water/hydrocarbon IFT thus the accumulation is higher in case of alkanes than aromatics. We also found that the relative density, ratio of ߩ at the interface to ߩbulk, decreases as xCO2 increases.
II.
We proposed a mechanism for why CO2 dissolved more in the hydrocarbon phase as xCO2 increases in the system. We ascribed this behavior to the difference in the intermolecular interactions between CO2-water and CO2-hydrocarbon. CO2-hydrocarbon interaction becomes much larger than CO2-water at high xCO2, thus more CO2 is dissolved in the phase that has higher interaction energy with it.
III.
The addition of CO2 leads to a reduction in the interfacial tension for all the studied systems. The reduction in the interfacial tension is a result of CO2 accumulation at the interface, increase in the interface width, the formation of H-bonds between CO2 and water, and the increase in the interfacial roughness.
IV.
Further analysis showed that CO2 forms a film at the interface and as the thickness of that film increases, it displaces the hydrocarbons away from the interface. The displacement is more significant in the case of n-alkanes than aromatics. CO2 formed hydrogen bonds with water which led to a reduction in water-water interaction and resulting in stabilizing the CO2 film. The addition of CO2 in the system dilutes the interface and increases its thickness.
V.
It has also found that the addition of CO2 increases the diffusion coefficient for the hydrocarbons which is an advantage in the EOR processes. There were two factors that affect the diffusivity due to the addition of CO2. The first one is the molecular weight, whereas the lighter molecules diffusivity increased more significantly than the heavier ones. The other factor is the polarizability, whereas the more polar molecules affected less significantly than the less polar molecules.
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Author Information Corresponding Author Email for Sohaib Mohammed:
[email protected], Tel: +1 708 427 6250
Notes The authors declare no competing financial interest.
Acknowledgements: This research is supported, in part, by Higher Committee for Education Development in Iraq (HCED)/Office of the Prime Minister.
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