Molecular Dynamics Simulation: The Behavior of Asphaltene in Crude

Subscriber access provided by BALL STATE UNIV

Article

Molecular Dynamics Simulation: The Behavior of Asphaltene in Crude Oil and at the Oil/Water Interface Fengfeng Gao, Zhen Xu, Guokui Liu, and Shiling Yuan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5020428 • Publication Date (Web): 17 Nov 2014 Downloaded from http://pubs.acs.org on November 19, 2014

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

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

Page 1 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 1a. C5 Pe. 250x78mm (300 x 300 DPI)

ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 1b. Anionic C5 Pe. 250x80mm (300 x 300 DPI)


Page 2 of 34

Page 3 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 2. The snapshots of the initial configurations of system A1 (a), system A2 (b). The final structures of the C5 Pe (c) and anionic C5 Pe (d) simulation in crude oil. In (c) and (d), the crude oil molecules are omitted for clarity. Color scheme: C, gray; H, white; N, blue; O, red; the C5 Pe and anionic C5 Pe molecules are displayed in the stick model. 428x499mm (72 x 72 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 3a. The normalized radial distribution functions of C5 Pe (a) and the according stacking models. 297x209mm (150 x 150 DPI)


Page 4 of 34

Page 5 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 3b. The normalized radial distribution functions of anionic C5 Pe (b) and the according stacking models. 297x209mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 4. The initial structures of system B1 (a) and system B2 (c); the final structures of system B1 (b) and system B2 (d).Time dependant density profiles of C5 Pe (e) and anionic C5 Pe (f) at the crude oil/water interface. For details refer to Figure 2. 448x605mm (72 x 72 DPI)


Page 6 of 34

Page 7 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 5a. The total energy of the system changing as the simulation proceeds for B2. 289x202mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 5b. Normalized RDFs of anionic C5 Pe (b). 289x202mm (150 x 150 DPI)


Page 8 of 34

Page 9 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 6. Left: Snapshots of the configurations of anionic C5 Pe molecules adsorbed to the crude oil/water interface at different simulation times; Right: the corresponding configurations of anionic C5 Pe molecules at the interface. Four molecules are highlighted to follow their movement through the four snapshots: molecule 1 (yellow), 2 (blue), 3 (pink) and 4 (green). 709x880mm (72 x 72 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 7. The snapshots of molecule 3 from Fig. 6a drawn to the interface during the simulation. (a) 0 ns. (b) 1 ns. (c) 2 ns. H-bonded water molecules are highlighted. 605x201mm (74 x 74 DPI)


Page 10 of 34

Page 11 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 8a. Scheme of the angles of two anionic C5 Pe molecules, angles betweenthe interfacial molecule and the crude oil/water interface. 254x190mm (96 x 96 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

figure 8b. Distance and angles in (b) denote the properties of the group 1 (molecules 1 and 2) 289x202mm (150 x 150 DPI)


Page 12 of 34

Page 13 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

figure 8c. Distance and angles in (c) denote the properties of the group 2 (molecules 3 and 4). 289x202mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 34

Molecular Dynamics Simulation:

The Behavior of Asphaltene in Crude Oil and at the Oil/Water Interface

†

‡

†

Fengfeng Gao , Zhen Xu* , Guokui Liu , Shiling Yuan*

†

† Key laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China

‡ Shandong Provincial Key Laboratory of Fine Chemicals, Qilu University of Technology, Jinan 250353, China


1

Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

ABSTRACT:

Carboxyl asphaltene is commonly discussed in the petroleum industry. In most conditions, electroneutral carboxyl asphaltene molecules can be deprotonated to become carboxylate asphaltenes. Both in crude oil and at oil/water interface, the characteristics of anionic carboxylate asphaltenes are different than those of the carboxyl asphaltenes. In this paper, molecular dynamics (MD) simulations are utilized to study the structural features of different asphaltene molecules, namely C5 Pe and anionic C5 Pe, at the molecular level. In crude oil, the electroneutral C5 Pe molecules prefer to form a steady face-to-face stacking, while the anionic C5 Pe molecules are inclined to form face-to-face stacking and T-shaped II stacking because of the repulsion of the anionic headgroups. Anionic C5 Pe has a distinct affinity to the oil/water interface during the simulation, while the C5 Pe molecules persist in the crude oil domain. A three-stage model of anionic C5 Pe molecules adsorbed at the oil/water interface is finally developed.

I. INTRODUCTION Asphaltenes, consisting of polyaromatic rings and various proportions of aliphatic chain lengths [1, 2] are the heaviest fractions of petroleum. Since asphaltene molecules can reduce the interfacial tension, they are always taken as the “natural surfactants” in improved oil recovery (IOR) [3, 4]. In the petroleum industry, asphaltene molecules having carboxyl headgroups have been widely discussed [5, 6]. Commonly, asphaltene molecules can form aggregate structures in the bulk phase of crude oil due to the stacking of polyaromatic rings [7, 8]. The aggregation causes many problems during production and transportation; for instance, low solubility [9], high viscosity [10], precipitation and deposition [11]. The aggregation of asphaltene can also induce serious problems in oil recovery and transport, such as reservoirs plugging and production pipelines fouled, etc [12]. Therefore, in IOR, it is necessary to understand the mechanism of the aggregation clearly. In the past, different aggregation mechanisms have been proposed. Cameron et al. hypothesized that parts of asphaltene molecules associated to form micellar structures through H-bonds [13]. Islam proposed that the aggregation was caused by the charge transfer between molecules [14]. Besides being present in the bulk phase, asphaltene molecules are generally considered an important fraction


2

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

in stabilizing the oil/water interface [15]. However, in numerous conditions, carboxyl asphaltene transforms from the nondissociated state into the hydrophilic carboxylate asphaltene easily. The carboxylate functional groups have strong influence on the properties of the asphaltene molecules. Due to the anionic charged headgroups, the carboxylate asphaltenes are preferentially adsorbed at positive surfaces, such as montmorillonite [16, 17]. In addition, the headgroups also have important effects on the behavior of the asphaltene molecules at the oil/water interface. Takamura et al. found that when carboxyl asphaltene molecules were deprotonated, the negatively charged carboxylate groups adsorbed at the water/oil interface [6]. Poteau et al. concluded that the charged asphaltene molecules were more easily to accumulate at oil/water interface too [18]. In the oil field, it is of prime importance to understand the behavior of asphaltene molecules at the oil/water interface [12, 19]. Many experimental technologies are used to investigate the interfacial phenomena, for example, surface tension measurements [13], interfacial tension measurements [20], vapor pressure osmometry [21] and small-angle neutron scattering [22]. However, it is still a challenge to understand the aggregation of asphaltene by experimental methods since this behavior cannot be explained through the common colloidal interaction models and the mesoscale aggregation theories [2]. Due to the increased computational power over recent years, computer simulations have proven to be valuable tools to study the behavior of asphaltene molecules at the molecular level. Molecular dynamics (MD) simulation, based on empirical force fields, is an efficient and reliable method to study the motions of molecular architectures [23, 24]. MD also allows us to extract information about dynamic and structural properties at a microscopic level which is not easy to get though experiments. In previous research the shape and structure of asphaltene molecules in aqueous solutions [25-27] or organic media [28-30] have been wellestablished using MD methods. However, there is still a lack of a fundamental understanding of the properties of asphaltene molecules at the oil/water interface. In this work, two types of asphaltene molecules are discussed. One is the carboxyl asphaltene, and the other has an anionic carboxylate group on the end of its chain. MD simulations are performed to investigate the interfacial phenomena and the dynamic behavior at the molecular level. The present study is divided into two parts: The first is the behavior of the two kinds of asphaltene molecules in crude oil, focusing on the difference


3

Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

of their aggregation configuration; the second is about the asphaltene aggregations at the water/oil interface. The simulations provide considerable insights into the behaviors of asphaltene molecules in crude oil and at oil/water interfaces.

II. SIMULATION METHOD All MD simulations were performed in the GROMACS 4.0.5 software package [31] and GROMOS 45a3 force field was used [32, 33]. The force filed is widely used in exploring the dynamics of polyaromatic molecules [34, 35].

1. Molecular Models N-(1-hexylheptyl)-N’-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxilicbisimide (C5 Pe) (Figure 1a) is an prototypical asphaltene model reported in several studies [35, 36]. In order to investigate the influence of charged groups on C5 Pe, the terminal carboxylic headgroup was deprotonated to be anionic (Figure 1b), which was named anionic C5 Pe.

(a) C5 Pe

(b) Anionic C5 Pe Figure 1. Two asphaltene models studied in the simulations

The crude oil model was based on that proposed by Miranda [37, 38]: alkanes [72 hexane, 66 heptane, 78


4

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 34

octane, 90 nonane, 48 cyclohexane, and 78 cycloheptane molecules] and aromatics [78 toluene and 30 benzene molecules]. The coordinate and topology files of the crude oil and C5 Pe molecules were generated by the PRODRG program [39]. All aromatic regions and double bonds were modeled by sp2 hybridized carbons [35], and aliphatic chains were adopted as united atom structures [40]. The anionic asphaltene molecules, described by van der Waals and Coulomb terms [37], were neutralized by sodium ions. The simple point charge (SPC) model was adopted for the water molecules [41].

2. Initial Simulation Configuration In this work, the simulation systems were divided into two parts. The initial configuration of asphaltene molecules in crude oil was defined as system A; and then water was added on one side of the results for system A to form system B. The simulation details are summarized in Table 1. Table 1. Details of the Different Systems of Simulations System System A1 System A2 System B1 System B2

Asphaltene molecules NAsphaltene NNa+ NWater Final box size (nm3) System A: asphaltene molecules in crude oil C5 Pe 24 5.2×5.2×5.2 Anionic C5 Pe 24 24 5.2×5.2×5.2 System B: aggregated asphaltene at crude water/oil interface C5 Pe 24 4615 5.2×5.2×10.4 Anionic C5 Pe 24 24 4698 5.2×5.2×10.4 a

The initial simulation box size was set at 9×9×9 nm3.

In the simulation of system A, 24 asphaltene molecules and crude oil molecules were randomly put into a cubic box of 9×9×9 nm3 designed as the literature [2, 35]. Then NPT ensemble was carried out to obtain a reasonable density, and the final dimensions of simulation boxes were displayed in Table 1. The simulation system A1 (Figure 2a) was used to study the aggregation of the C5 Pe in crude oil, while system A2 (Figure 2b) was performed to investigate the effect of anionic terminal groups on aggregation. For system A2, the system was made electroneutral by adding 24 Na+ ions, where each Na+ ion corresponds to one COO-. System B was explored to study the aggregation structures at the crude oil/water interface. The initial configurations of system B were taken from the equilibrated structure of system A (C5 Pe molecules in crude oil). The boxes of system A were expanded in the Z direction, and water molecules were added to the empty


5

Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

volume to create the crude oil/water interface. The two new constructed simulation systems were named B1 and B2 (Figure 4a and 4c), respectively.

3. Details of Molecular Dynamics All the initial configurations were minimized by the steepest descent and conjugate gradient methods [31]. During the energy minimization, the cutoff of Coulomb and van der Waals interactions was 1.2 nm. When the maximum force of the system was converged to a threshold of less than 1000 kJ·mol−1·nm−1, the system was considered to be stabilized [35].The simulations were performed in the NPT ensembles at 298 K and 0.1 MPa, which caused the systems to have appropriate densities and box dimensions. Finally, 200 ns NVT simulations were carried out at 298K. In the NVT ensembles, the periodic boundary condition was applied in all directions. The Berendsen thermostat was used as the temperature coupling algorithm, and bond lengths were constrained by the LINCS algorithm [42]. The particle mesh Ewald (PME) method was adopted to compute the electrostatic interactions [43]. The Maxwell-Boltzmann distribution was employed to set the initial atomic velocities of the systems [31]. The trajectories were integrated by leapfrog Verlet algorithm [35]. The dynamic properties of the simulation systems were analyzed by the built-in analytical tools in GROMACS.

III. RESULTS AND DISCUSSION 1. Asphaltene Molecular Aggregation in Crude Oil In this section, the aggregation behaviors of asphaltene models, anionic and neutral C5 Pe, in crude oil are discussed. The aggregate configurations of C5 Pe (Figure 2c) and anionic C5 Pe (Figure 2d) are obviously different in crude oil. The former is more compact. The difference is likely caused by the H-bonds among the headgroups of the C5 Pe molecules. In the aggregation of C5 Pe, the electronegative oxygen atom in the carboxyl groups attracts the electropositive hydrogen atoms to form the H-bonds, which are highlighted in Figure 2c. The H-bonds cause small stacks to form and lead to aggregation. However, due to the deprotonation, H-bonds can’t be formed between the anionic C5 Pe molecules, which lead to a loose aggregation configuration in the crude oil.


6

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

Figure 2. The snapshots of the initial configurations of system A1 (a), system A2 (b). The final structures of the C5 Pe (c) and anionic C5 Pe (d) simulation in crude oil. In (c) and (d), the crude oil molecules are omitted for clarity. Color scheme: C, gray; H, white; N, blue; O, red; the C5 Pe and anionic C5 Pe molecules are displayed in the stick model.

The stacking models of the asphaltene molecules are expressed by the normalized radial distribution function (RDF), g(r)/g(r)max peak, where g(r) is the distribution of asphaltene molecules away from a reference


7

Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

asphaltene surfactant molecule, and g(r)max peak is the maximum peak value of g(r) [35]. Figure 3 depicts the normalized RDFs of systems A over the last 100 ns of the simulation. Evidently, the curves of the RDFs are different for system A1 and system A2. Figure 3a shows that system A1 has one sharp peak (ca. 0.4 nm) and a small peak (ca. 0.75 nm), while the system A2 (Figure 3b) has an additional broad peak (ca. 1.23 nm). The different peaks correspond to different stacking models, which are displayed in Figure 3. The sharp peak at about 0.4 nm corresponds to the face-to-face stacking, which is formed by the stacking of polyaromatic systems. The other peaks are due to the T-shaped (edge-to-face) stacking models, which are broadened due to the numerous orientations possible. In this work, the peak at 0.75 nm is defined as the T-shaped I stacking model and the peak at 1.23 nm correspond to the T-shaped II stacking model. The appearance of the T-shaped II stacking in system A2 is mainly caused by the negatively charged headgroup, which increases the distance between the headgroups to reduce their repulsion. In other words, the T-shaped II stacking is another reason to cause the anionic C5 Pe to aggregate more loosely than the denser C5 Pe aggregations.

(a)

(b)

Figure 3. The normalized radial distribution functions of C5 Pe (a) and anionic C5 Pe (b) and the according stacking models. The face-to-face stacking (left) and the T-shaped stacking (right) of system A1 are shown in (a); the face-to-face stacking (left), T-shaped I stacking (middle) and T-shaped II stacking (right) of system A2 are shown in (b). The stacking models are randomly taken from the trajectories and the color scheme refers to Figure 2.


8

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

The ratio of the peaks in the RDF curves expresses which stacking is dominant in the final configuration. In system A1, the ratio of the face-to-face stacking and the T-shaped I stacking is about 4:1, illustrating the faceto-face staking is the primary stacking model of C5 Pe in the crude oil. In the plot for system A2, there are three peaks according to the face-to-face, T-shaped I and T-shaped II stacking models, and their ratio is about 3:1:3, respectively. The data shows that anionic C5 Pe molecules mainly form the face-to-face and T-shaped II stackings in crude oil. To further validate the result of Figure 3, the possible stacking configurations of asphaltene aggregations shown in dynamics process are calculated using quantum chemical methods. All the energies of the stackings in system A are explored using PM6, a semi-empirical method in the Gaussian09 suite of programs [44]. For the C5 Pe molecules, the energies of the face-to-face stacking and the T-shaped I stacking are -1285.18 kJ/mol and -830 kJ/mol, indicating the C5 Pe molecules prefer to the face-to-face stacking comparing with the Tshaped I stacking. For system A2, the energies of the face-to-face stacking, T-shaped I stacking, T-shaped II stacking are -1079.08, -958.31, -1042.32 kJ/mol, respectively. Compared to the energies of the stacking structures, the energy of the T-shaped I in system A2 is higher than those of other stackings. Therefore, the face-to-face stacking and T-shaped II stacking are more suitable for the anionic C5 Pe molecules. From the energy, we conclude that the face-to-face stacking in system A1 is dominant, while the face-to-face and Tshaped II stacking are the primary structures of anionic C5 Pe molecules in crude oil. The conclusion obtained from quantum calculations are consistent with the RDFs results shown in Figure 3.

2. The Behavior of the Asphaltene Molecules at the Crude Oil/ Water Interface We investigate the behavior of the two aggregations of anionic and neutral C5 Pe molecules at the oil/water interface with the systems B1 and B2 (vide supra). The density profiles (Figure 4e and Figure 4f) are used to detect the adsorption process of the anionic and neutral C5 Pe molecules. The width of the crude oil/ water interface is defined as the length where the water density is 10 to 90% of its bulk value [45]. For system B1 and system B2, the interfacial width of the oil/water interface is about 1 nm.


9

Page 23 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 4. The initial structures of system B1 (a) and system B2 (c); the final structures of system B1 (b) and system B2 (d).Time dependant density profiles of C5 Pe (e) and anionic C5 Pe (f) at the crude oil/water interface. For details refer to Figure 2.

For the two systems, at the beginning of the simulation, the anionic C5 Pe molecules are distributed in the


10

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

oil phase (Figure 4a and 4c). During the simulation process, the density profile of the C5 Pe molecules (Figure 4e) has almost no changes, suggesting the C5 Pe molecules persist in the crude oil all the time (Figure 4b). The reason can be attributed to the aromatic molecules in the crude oil, which hinder the asphaltene molecules to be adsorbed at oil/water interface [46]. However, in Figure 4f, the peak closest to the interface increases with time evolution indicating that the anionic C5 Pe molecules are adsorbed to the interface gradually (Figure 4d). The results provide further evidence that the headgroups have strong influence to the behavior of asphaltene molecules, which is in agreement with the experimental phenomenon [18].

2.1 The Three-Stage Model of Anionic C5 Pe Adsorbed at Crude Oil/Water Interface In this section, we focus on the dynamic process of anionic C5 Pe molecules adsorbed at the oil/water interface. The total energy of system B2 (Figure 5a) varies as the simulation progresses suggesting that the adsorption process can be divided into approximately three stages. The density profile and detailed snapshots of the adsorption process are shown in Figure 4f and Figure 6. In stage I, the critical event is a few anionic C5 Pe molecules adsorbed at the oil/water interface from the crude oil phase (Figure 6a to Figure 6b). Initially, most of the anionic C5 Pe molecules are distributed in crude oil and only a few of the anionic C5 Pe molecules are close to the interface. Then, some water molecules enter into the crude oil phase by the influence of Hbonds, which are formed between the hydrogen atoms in water and the negatively charged oxygen atoms in the anionic C5 Pe molecules. Consequently，those water molecules pull the anionic C5 Pe molecules to the interface. Examination of the density profile (Figure 4f) reveals a trend of the anionic C5 Pe molecules moving towards the interface. After about 60 ns, the density peak close to the oil/water interface increases. This data means some anionic C5 Pe molecules are already being attracted to the interface (Figure 6b). In this paper, the anionic C5 Pe molecules at the interface are defined as interfacial molecules, which are marked in red in figure 6b for clarity.


11

Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

(b)

Figure 5. The total energy of the system changing as the simulation proceeds for B2 (a) and normalized RDFs of anionic C5 Pe (b).

In the second stage, more anionic C5 Pe molecules are continually attracted to the interface (Figure 6b to 6c) and their stacking models transform from T-shaped stacking to the face-to-face stacking. In this process, the total energy of the system decreases quickly compared to stageⅠand stage Ⅲ (Figure 5a). In Figure 4f, the peaks close to the oil/water interface clearly increase from 60 to 120 ns, denoting more anionic C5 Pe molecules are adsorbed at the interface during this stage. In the adsorption process, the molecules already at the oil/water interface play the vital roles. They attract other anionic C5 Pe molecules to the oil/water interface through non-covalent interactions. The stacking pattern is obtained from the normalized RDFs (Figure 5b). At 60 ns, the ratio of the three stacking models is about 3:1:2, while it changes to 3:1:1 at 120 ns for stacking of face-to-face, T-shaped I and T-shaped II, respectively. The reduction of the peak at about 1.23 nm indicates the T-shaped II stacking in system B2 is vanishing. The two large peaks displayed in Figure 5b represent the faceto-face stacking and the T-shaped I stacking. Due to the dominating peak at 0.4 nm, we assume that majority of the anionic C5 Pe molecules are participating in face-to-face stacking. The reason for changes in the stacking models is ascribed to the hydrated headgroups, which are formed after the headgroups are exposed to the water phase, evidently reducing the repulsion of the headgroups (Figure 6c).


12

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Figure 6. Left: Snapshots of the configurations of anionic C5 Pe molecules adsorbed to the crude oil/water interface at different simulation times; Right: the corresponding configurations of anionic C5 Pe molecules at the interface. Four molecules are highlighted to follow their movement through the four snapshots: molecule 1 (yellow), 2 (blue), 3 (pink) and 4 (green).

Stage III reflects the anionic C5 Pe molecules in a well-organized and ordered arrangement at the interface


13

Page 27 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(Figure 6c to Figure 6d). In this process, a slight decrease of the energy profile suggests that a small adjustment of the system occurs (Figure 5a). The decrease of energy is illustrated by the normalized RDFs in Figure 5b, where the peak of the T-shaped II stacking vanishing companies with the energy decrease. The observed adsorption process implies that the molecules already exiting at the interface are very important for the formation of the anionic C5 Pe slab, and that the hydration of the headgroup causes the stacking model to change to T-shaped II. From the top view in figure 6d, we can observe that the anionic C5 Pe molecules form side-on arrangement at oil/water interface. Their molecular orientation agrees well with the experimental data reported using the sum frequency generation (SFG) vibrational spectroscopy [47]. This three-stage model based on the total energy of the system adequately explains the process of anionic C5 Pe aggregation from the crude oil to the oil/water interface.

2.2 The Formation of the Interfacial Molecules The formation of interfacial molecules parallel to the oil/water interface is vital to catalyze the anionic C5 Pe molecular aggregation. The interfacial molecules are formed by the H-bond interaction between the oxygen atoms of the anionic C5 Pe molecules and the protons of water molecules. This is the key step for the anionic C5 Pe molecules accumulating at crude oil/water interface [38, 48].

(a)

(b)

(c)

Figure 7. The snapshots of molecule 3 from Fig. 6a drawn to the interface during the simulation. (a) 0 ns. (b) 1 ns. (c) 2 ns. H-bonded water molecules are highlighted.

Molecule 3 (marked in Figure 6a) is taken as an example to investigate the formation process of the


14

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

interfacial molecules. The snapshots of the anionic C5 Pe molecule over time are shown in Figure 7. Initially, some water molecules come into the crude oil and then connect with molecule 3 (Figure 7a). The electropositive hydrogen atoms of the water molecules attract the negative oxygen atoms of the anionic C5 Pe molecules to form new H-bonds. Then, the anionic C5 Pe molecule is dragged to the interface due to a hydrophilic interaction (Figure 7b). At the same time, the aromatic plane is forced to parallel to the interface under the function of the numerous H-bonds (Figure 7c). This behavior is energetically favorable for the anionic C5 Pe molecule to reside at the interface. Due to the numerous other H-bonding interactions, more anionic C5 Pe molecules are extracted from the crude oil and are adsorbed at the interface gradually (Figure 6b). To study the behavior of the interfacial molecules at the oil/water interface, the distances and angles of two anionic C5 Pe molecules are discussed. The angle between two anionic C5 Pe molecular planes is defined as angle α, while the angle of one anionic C5 Pe molecular plane to the oil/water interface is angle β (Figure 8a). Molecule 1 (orange) and molecule 2 (blue), defined in Figure 6a, are taken as group 1; meanwhile, molecule 3 (pink) and molecule 4 (green) are taken as group 2. The data for group 1 is shown in Figure 8b, and that of the group 2 is displayed in Figure 8c. Though both of them can form face-to-face stacking, the trends of their angles relative to the interface are different. For group 1, molecule 2 acts as the interfacial molecule (Figure 6a) and the cosine of angle β is close to 1 at the beginning. As molecule 1 closing to molecule 2, the cosine of angle β is perturbed. After about 12 ns, molecule 1 approaches molecule 2 and their distance reduces from 3.0 nm to 0.6 nm as a result of the noncovalent interactions. Hence, molecule 2 is forced to leave the crude oil/water interface due to the hydrophobic interaction of the polyaromatic rings. In the next 8 ns (12 ns to 20 ns), the cosine of angle β trends toward 1, suggesting the molecule 2 is adsorbed to the interface again due to the H-bonds between the oxygen in anionic C5 Pe and water molecules. However, during this period, the distance between molecule 1 and molecule 2 doesn’t reduce to the minimum 0.4 nm, which indicates the face-to-face stacking has not been formed between the two anionic C5 Pe molecules. After about 20 ns, the cosα is close to 1 and their distance approaches 0.4 nm, implicating that the face-to-face stacking is formed between the two molecules.


15

Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

(b)

(c)

Figure 8. (a) Scheme of the angles of two anionic C5 Pe molecules, angles betweenthe interfacial molecule and the crude oil/water interface. Distance and angles in (b) and (c) denote the properties of the group 1 (molecules 1 and 2) and group 2 (molecules 3 and 4).

The properties of group 2 (molecule 3 and molecule 4) are shown in Figure 8c. The cosine of the angle β increases to 1 in the first 2 ns of the simulation. The data is in agreement with Figure 7, where the molecular plane becomes parallel to the oil/water interface at 2 ns due to the H-bond interaction with water molecules. From 2 to 8 ns, both the angle α and the distance between molecule 3 and molecule 4 show little variation, similar to the 12-20 ns regime of group 1 (Figure 8b). After 8 ns, the distance is minimized and the cosine of angle α approximately equals to 1, indicating the face-to-face stacking formed. However, in this period, the cosine of angle β (Figure 8c) contains fluctuation in comparison to Figure 8b. This fluctuation is the result of the hydrophobic interaction. That is the delicate balance of the repulsive hydrophobic interaction and the


16

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

attractive hydrogen bonding between the anionic C5 Pe molecules and water. After the formation of the faceto-face stacking, it presents different tendencies for the two groups at the oil/water interface. Group 1 stay at the interface steady and group 2 is more prone to oscillate at the interface. The reason can be ascribed to the aggregation number, the large aggregation number can defense the oscillation.

IV. CONCLUSIONS In this work, the effects of the terminal groups on asphaltene molecules have been studied by MD simulations. The simulated results indicate the presence of anionic terminal groups on the aliphatic chains can dramatically influence the neutral C5 Pe behavior both in the oil phase and at the oil/water interface. In crude oil, the carboxyl C5 Pe favors the face-to-face stacking, while the carboxylate C5 Pe molecules prefer the faceto-face stacking in addition to the T-shaped II stacking because of the repulsive interaction of the headgroups. What’s more, the terminal groups also affect the properties of the C5 Pe molecules to adsorb at the crude oil/water interface. The anionic carboxylate C5 Pe molecules can transform from an aggregate to a stable slab at the interface, while the carboxyl C5 Pe aggregations persist only in the bulk crude oil during the whole simulation. The adsorption process of anionic C5 Pe molecules can be explained by the three-stage model: First, a few asphaltene molecules are brought to the interface through the H-bond interaction between water and the oxygen atoms of anionic C5 Pe. Then, more anionic C5 Pe molecules in the crude oil are drawn to the interface via non-covalent interactions. Ultimately, an asphaltene slab is formed at the interface including the face-to-face stacking.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]


17

Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

ACKNOWLEDGMENT We gratefully appreciate the financial support from NSFC Project (No. 21173128), Key NSF Project of Shandong province (No. ZR2011BZ0003 and No. ZR2012BM004) and the HESTP Project of Shandong Province (J13LD01). The authors thank Dr. Bradley D. Rose, King Abdullah University of Science and Technology, for helpful discussions and manuscript editing. We are thankful for support by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

REFERENCES [1] Speight, J. G. The Chemistry and Technology of Petroleum (electronic resource), 4th ed.; CRC Press/Taylor and Francis: Boca Raton, FL 33487-2742, 2007. [2] Kuznicki, T.; Masliyah, J. H.; Bhattacharjee, S. Energy & Fuels 2008, 22, 2379-2389. [3] Zhang, L. Y.; Lopetinsky, R.; Xu, Z. H.; Masliyah, J. H. Energy & Fuels 2005, 19, 1330-1336. [4] Cadena-Nava, R. D.; Cosultchi, A.; Ruiz-Garcia, J. Energy & Fuels 2007, 21, 2129-2137. [5] Silva, I.; Borges, B.; Blanco, R.; Rondón, M.; Salager, J. L.; Pereira, J. C. Energy & Fuels, 2014, 28, 3587-3593. [6] Takamura, K.; Chow, R. S. Colloids Surf. 1985, 15, 35-48. [7] McLean, J. D.; Kilpatrick, P. K. J. Colloid Interface Sci. 1997, 196, 23-34. [8] Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy & Fuels 2007, 21, 2785-2794. [9] Speight, J. G. Oil & Gas Sci. Technol. 2004, 59, 467-477. [10] Pierre, C.; Barré, L.; Pina, A.; Moan, M. Oil & Gas Sci. Technol. 2004, 59, 489-501. [11] Spiecker, P. M.; Gawrys, K. L.; Trail, C. B.; Kilpatrick, P. K. Colloids Surf., A 2003, 220, 9-27. [12] Branco, V. A. M.; Mansoori, G. A.; Xavier, L. C. De A.; Park, S. J.; Manafi H. J. Pet. Sci. Eng. 2001, 32, 217-230. [13] Cameron, J. R.; Briggs, D. E. Colloids Surf. 1982, 4, 285-303. [14] Islam, M. R. Asphaltenes, Fundamentals and Applications. Sheu, E. Y.; Mullins, O. C. eds.


18

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

Plenum Press, New York, London, 1995, 191-218. [15] Yang, X. L.; Czarnecki, J. Energy & Fuels 2005, 19, 2455-2459. [16] Jada, A.; Debih, H.; Khodja, M. J. Pet. Sci. Eng. 2006, 52, 305-316. [17] Zhou, Z. A.; Xu, Z.; Masliyah, J. H.; Czarnecki, J. Colloids Surf., A 1999, 148, 199-211. [18] Poteau, S.; Argillier, J. F.; Langevin, D.; Pincet, F.; Perez, E. Energy & Fuels 2005, 19, 1337-1341. [19] Moore, F. G.; Richmond, G. L. Acc. Chem. Res. 2008, 41, 739-748. [20] Mohamed, R. S.; Ramos, A. C. S.; Loh, W. Energy & Fuels 1999, 13, 323-327. [21] Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. [22] Gawrys, K. L.; Blankenship, G. A.; Kilpatrick, P. K. Langmuir 2006, 22, 4487-4497. [23] Zhang, L. Q.; Greenfield, M. L. J. Chem. Phys. 2007, 127, 194502. [24] Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy & Fuels 1999, 13, 278-286. [25] Baram, J.; Shirman, E.; Ben-Shitrit, N.; Ustinov, A.; Weissman, H.; Pinkas, I.; Wolf, S. G.; Rybtchinski, B. J. Am. Chem. Soc. 2008, 130, 14966-14967. [26] Wang, K. R.; Guo, D. S.; Jiang, B. P.; Sun, Z. H.; Liu, Y. J. Phys. Chem. B 2010, 114, 101-106. [27] Zhang, X.; Rehm, S.; Safont-Sempere, M. M.; Würthner, F. Nature Chem. 2009, 1, 623-629. [28] Khoshandam, A.; Alamdari, A. Energy & Fuels 2010, 24, 1917-1924. [29] Wang, S. Q.; Liu, J. J.; Zhang, L. Y.; Xu, Z. H.; Masliyah, J. Energy & Fuels 2009, 23, 862-869. [30] Murray. R. G.; Tykwinski, R. R.; Stryker, J. M.; Tan, X. L. Energy & Fuels 2011, 25, 3125-3134. [31] van der Spoel, D.; Lindahl, E.; Hess, B.; Kutzner, C.; van Buuren, A. R.; Apol, E.; Meulenhoff, P. J.; Tieleman, D. P.; Sijbers, A.; Feenstra, K. A.; van Drunen R; Berendsen, H. J. C. Groningen University: Groningen, The Netherlands, 2005. [32] Oostenbrink, C.; Villa, A.; Mark, A. E.; van Gunsteren, W. F. J. Comput. Chem. 2004, 25, 1656-1676. [33] Schuler, L. D.; Daura, X.; van Gunsteren, W. F. J. Comput. Chem. 2001, 22, 1205-1218. [34] Kuznicki, T.; Masliyah, J. H.; Bhattacharjee, S. Energy & Fuels 2009, 23, 5027-5035. [35] Teklebrhan, R. B.; Ge, L. L.; Bhattacharjee, S.; Xu, Z. H.; Sjöblom, J. J. Phys. Chem. B 2012, 116, 5907-5918. [36] Wang, J.; Opedal, N. van der T.; Lu, Q. Y.; Xu, Z. H.; Zeng, H. B.; Sjöblom, J. Energy & Fuels 2012, 26,


19

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2591-2599. [37] de Lara, L. S.; Michelon, M. F.; Miranda. C. R. J. Phys. Chem. B 2012, 116, 14667-14676. [38] Kunieda, M.; Nakaoka, K.; Liang, Y. F.; Miranda, C. R.; Ueda, A.; Takahashi, S.; Okabe, H.; Matsuoka, T. J. Am. Chem. Soc. 2010, 132, 18281-18286. [39] Schuttelkopf, A. W.; van Aalten, D. M. F. Acta Crystallogr. Sect. D: Biol. Crystallogr. 2004, 60, 13551363, part 8. [40] Kuznicki, T.; Masliyah, J. H.; Bhattacharjee, S. Langmuir 2007, 23, 1792-1803. [41] Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J. Intermolecular Forces, Reidel, Dordrecht, The Netherlands, 1981, 331-342. [42] Hess, B.; Bekker, H.; Berendsen, H. C. J.; Fraaije, J. G. E. M. J. Comput. Chem. 1997, 18, 1463-1472. [43] Essmann, U.; Perera, L.; Berkowitz, L. M.; Darden, T.; Lee, H.; Pedersen, L. G. J. Chem. Phys. 1995, 103, 8577-8592. [44] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian, Inc., Wallingford CT, 2009. [45] Frost, D. S.; Dai, L. L. Langmuir, 2011, 27, 11339-11346. [46] Teklebrhan, R. B.; Ge, L. L.; Bhattacharjee, S.; Xu, Z. H.; Sjöblom, J. J. Phys. Chem. B 2014, 118, 1040-1051. [47] Andrews, A. B.; McClelland, A.; Korkeila, O.; Demidov, A.; Krummel, A.; Mullins, O. C.; Chen, Z. Langmuir 2011, 27, 6049-6058.


20

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

[48] Mikami, Y.; Liang, Y. F.; Matsuoka, T.; Boek, E. S. Energy & Fuels 2013, 27, 1838-1845.


21

Molecular Dynamics Simulation: The Behavior of Asphaltene in Crude

Recommend Documents