Molecular Dynamics Simulation: The Behavior of Asphaltene inCrude Oil and at the Oil/Water InterfaceFengfeng Gao, Zhen Xu,*, Guokui Liu, and Shiling Yuan*,

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, ChinaShandong Provincial Key Laboratory of Fine Chemicals, Qilu University of Technology, Jinan 250353, China

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

I. INTRODUCTION

Asphaltenes, consisting of polyaromatic rings and variousproportions of aliphatic chain lengths,1,2 are the heaviestfractions of petroleum. Since asphaltene molecules can reducethe interfacial tension, they are always taken as the naturalsurfactants in improved oil recovery (IOR).3,4

In the petroleum industry, asphaltene molecules havingcarboxyl headgroups have been widely discussed.5,6 Commonly,asphaltene molecules can form aggregate structures in the bulkphase of crude oil due to the stacking of polyaromatic rings.7,8

The aggregation causes many problems during production andtransportation; for instance, low solubility,9 high viscosity,10

and precipitation and deposition.11 The aggregation ofasphaltene can also induce serious problems in oil recoveryand transport, such as reservoir plugging and productionpipelines fouled, and so forth.12 Therefore, in IOR, it isnecessary to understand the mechanism of the aggregationclearly. In the past, different aggregation mechanisms have beenproposed. Cameron et al. hypothesized that parts of asphaltenemolecules associated with form micellar structures through H-bonds.13 Islam proposed that the aggregation was caused by thecharge transfer between molecules.14

Besides being present in the bulk phase, asphaltenemolecules are generally considered an important fraction instabilizing the oil/water interface.15 However, in numerousconditions, carboxyl asphaltene transforms from the non-dissociated state into the hydrophilic carboxylate asphalteneeasily. The carboxylate functional groups have a stronginfluence on the properties of the asphaltene molecules. Dueto the anionic charged headgroups, the carboxylate asphaltenesare preferentially adsorbed at positive surfaces, such asmontmorillonite.16,17 In addition, the headgroups also haveimportant effects on the behavior of the asphaltene moleculesat the oil/water interface. Takamura et al. found that whencarboxyl asphaltene molecules were deprotonated, thenegatively charged carboxylate groups adsorbed at the water/

oil interface.6 Poteau et al. concluded that the chargedasphaltene molecules were more easily to accumulate at oil/water interface too.18

In the oil field, it is of prime importance to understand thebehavior of asphaltene molecules at the oil/water interface.12,19

Many experimental technologies are used to investigate theinterfacial phenomena, for example, surface tension measure-ments,13 interfacial tension measurements,20 vapor pressureosmometry,21 and small-angle neutron scattering.22 However, itis still a challenge to understand the aggregation of asphalteneby experimental methods since this behavior cannot beexplained through the common colloidal interaction modelsand the mesoscale aggregation theories.2

Because of the increased computational power over recentyears, computer simulations have proven to be valuable tools tostudy the behavior of asphaltene molecules at the molecularlevel. Molecular dynamics (MD) simulation, based on empiricalforce fields, is an efficient and reliable method to study themotions of molecular architectures.23,24 MD also allows us toextract information about dynamic and structural properties at amicroscopic level which is not easy to get though experiments.In previous research the shape and structure of asphaltenemolecules in aqueous solutions2527 or organic media2830

have been well-established using MD methods. However, thereis still a lack of a fundamental understanding of the propertiesof 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 anioniccarboxylate group on the end of its chain. MD simulations areperformed to investigate the interfacial phenomena and thedynamic behavior at the molecular level. The present study isdivided into two parts: The first is the behavior of the two kinds

Received: July 27, 2014Revised: November 17, 2014Published: November 17, 2014

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of asphaltene molecules in crude oil, focusing on the differenceof their aggregation configuration; the second is about theasphaltene aggregations at the water/oil interface. Thesimulations provide considerable insights into the behaviorsof asphaltene molecules in crude oil and at oil/water interfaces.

II. SIMULATION METHODAll MD simulations were performed in the GROMACS 4.0.5software package,31 and the GROMOS 45a3 force field wasused.32,33 The force field is widely used in exploring thedynamics of polyaromatic molecules.34,35

1. Molecular Models. N-(1-Hexylheptyl)-N-(5-carboxy-licpentyl)-perylene-3,4,9,10-tetracarboxylicbisimide (C5 Pe)(Figure 1a) is an prototypical asphaltene model reported in

several studies.35,36 In order to investigate the influence ofcharged groups on C5 Pe, the terminal carboxylic headgroupwas deprotonated to be anionic (Figure 1b), which was namedanionic C5 Pe.The crude oil model was based on that proposed by

Miranda:37,38 alkanes (72 hexane, 66 heptane, 78 octane, 90nonane, 48 cyclohexane, and 78 cycloheptane molecules) andaromatics (78 toluene and 30 benzene molecules). Thecoordinate and topology files of the crude oil and C5 Pemolecules were generated by the PRODRG program.39 Allaromatic regions and double bonds were modeled by sp2

hybridized carbons,35 and aliphatic chains were adopted asunited atom structures.40 The anionic asphaltene molecules,described by van der Waals and Coulomb terms,37 wereneutralized by sodium ions. The simple point charge (SPC)model was adopted for the water molecules.41

2. Initial Simulation Configuration. In this work, thesimulation systems were divided into two parts. The initialconfiguration of asphaltene molecules in crude oil was definedas system A, and then water was added on one side of the

results for system A to form system B. The simulation detailsare summarized in Table 1.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 ensemblewas carried out to obtain a reasonable density, and the finaldimensions of simulation boxes were displayed in Table 1. Thesimulation system A1 (Figure 2a) was used to study theaggregation of the C5 Pe in crude oil, while system A2 (Figure2b) was performed to investigate the effect of anionic terminalgroups on aggregation. For system A2, the system was madeelectroneutral by adding 24 Na+ ions, where each Na+ ioncorresponds to one COO.System B was explored to study the aggregation structures at

the crude oil/water interface. The initial configurations ofsystem B were taken from the equilibrated structure of systemA (C5 Pe molecules in crude oil). The boxes of system A wereexpanded in the Z-direction, and water molecules were addedto the empty volume to create the crude oil/water interface.The two new constructed simulation systems were named B1and B2 (Figure 4a and c), respectively.

3. Details of Molecular Dynamics. All of the initialconfigurations were minimized by the steepest descent andconjugate gradient methods.31 During the energy minimization,the cutoff of Coulomb and van der Waals interactions was 1.2nm. When the maximum force of the system was converged toa threshold of less than 1000 kJmol1nm1, the system wasconsidered to be stabilized.35 The simulations were performedin the NPT ensembles at 298 K and 0.1 MPa, which caused thesystems to have appropriate densities and box dimensions.Finally, 200 ns NVT simulations were carried out at 298 K. Inthe NVT ensembles, the periodic boundary condition wasapplied in all directions. The Berendsen thermostat was used asthe temperature coupling algorithm, and bond lengths wereconstrained by the LINCS algorithm.42 The particle-meshEwald (PME) method was adopted to compute the electro-static interactions.43 The MaxwellBoltzmann distribution wasemployed 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 wereanalyzed by the built-in analytical tools in GROMACS.

III. RESULTS AND DISCUSSION

1. Asphaltene Molecular Aggregation in Crude Oil. Inthis section, the aggregation behaviors of asphaltene models,anionic and neutral C5 Pe, in crude oil are discussed. Theaggregate configurations of C5 Pe (Figure 2c) and anionic C5Pe (Figure 2d) are obviously different in crude oil. The formeris more compact. The difference is likely caused by the H-bonds among the headgroups of the C5 Pe molecules. In theaggregation of C5 Pe, the electronegative oxygen atom in the

Figure 1. Two asphaltene models studied in the simulations.

Table 1. Details of the Different Systems of Simulations

system asphaltene molecules NAsphaltene NNa+ NWater final box sizea (nm3)

System A: Asphaltene Molecules in Crude OilSystem A1 C5 Pe 24 5.2 5.2 5.2System A2 anionic C5 Pe 24 24 5.2 5.2 5.2

System B: Aggregated Asphaltene at Crude Water/Oil InterfaceSystem B1 C5 Pe 24 4615 5.2 5.2 10.4System B2 anionic C5 Pe 24 24 4698 5.2 5.2 10.4

aThe initial simulation box size was set at 9 9 9 nm3.

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Figure 2. Snapshots of the initial configurations of system A1 (a) and 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 Peand anionic C5 Pe molecules are displayed in the stick model.

Figure 3. 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-shapedII stacking (right) of system A2 are shown in (b). The stacking models are randomly taken from the trajectories, and the color scheme refers toFigure 2.

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carboxyl groups attracts the electropositive hydrogen atoms toform 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 cannot be formedbetween the anionic C5 Pe molecules, which lead to a looseaggregation configuration in the crude oil.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 ofasphaltene molecules away from a reference asphaltenesurfactant molecule, and g(r)max peak is the maximum peakvalue of g(r).35 Figure 3 depicts the normalized RDFs ofsystems A over the last 100 ns of the simulation. Evidently, thecurves of the RDFs are different for system A1 and system A2.

Figure 3a shows that system A1 has one sharp peak (ca. 0.4nm) and a small peak (ca. 0.75 nm), while the system A2(Figure 3b) has an additional broad peak (ca. 1.23 nm). Thedifferent peaks correspond to different stacking models, whichare displayed in Figure 3. The sharp peak at about 0.4 nmcorresponds to the face-to-face stacking, which is formed by thestacking of polyaromatic systems. The other peaks are due tothe T-shaped (edge-to-face) stacking models, which arebroadened due to the numerous orientations possible. In thiswork, the peak at 0.75 nm is defined as the T-shaped I stackingmodel, and the peak at 1.23 nm corresponds to the T-shaped IIstacking model. The appearance of the T-shaped II stacking insystem A2 is mainly caused by the negatively chargedheadgroup, which increases the distance between the head-

Figure 4. Initial structures of system B1 (a) and system B2 (c); the final structures of system B1 (b) and system B2 (d). Time-dependent densityprofiles of C5 Pe (e) and anionic C5 Pe (f) at the crude oil/water interface. For details, refer to Figure 2.

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groups to reduce their repulsion. In other words, the T-shapedII stacking is another reason to cause the anionic C5 Pe toaggregate more loosely than the denser C5 Pe aggregations.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 Istacking is about 4:1, illustrating the face-to-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 show that anionic C5 Pe

molecules mainly form the face-to-face and T-shaped II

stackings in crude oil.

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

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 movementthrough the four snapshots: molecule 1 (yellow), 2 (blue), 3 (pink), and 4 (green).

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To further validate the result of Figure 3, the possiblestacking configurations of asphaltene aggregations shown indynamics process are calculated using quantum chemicalmethods. All the energies of the stackings in system A areexplored using PM6, a semiempirical method in the Gaussian09suite of programs.44 For the C5 Pe molecules, the energies ofthe face-to-face stacking and the T-shaped I stacking are1285.18 kJ/mol and 830 kJ/mol, indicating the C5 Pemolecules prefer to the face-to-face stacking comparing withthe T-shaped I stacking. For system A2, the energies of theface-to-face stacking, T-shaped I stacking, and T-shaped IIstacking are 1079.08, 958.31, and 1042.32 kJ/mol,respectively. Compared to the energies of the stackingstructures, the energy of the T-shaped I in system A2 is higherthan those of other stackings. Therefore, the face-to-facestacking and T-shaped II stacking are more suitable for theanionic C5 Pe molecules. From the energy, we conclude thatthe face-to-face stacking in system A1 is dominant, while theface-to-face and T-shaped II stacking are the primary structuresof anionic C5 Pe molecules in crude oil. The conclusionsobtained from quantum calculations are consistent with theRDFs results shown in Figure 3.2. Behavior of the Asphaltene Molecules at the Crude

Oil/Water Interface. We investigate the behavior of the twoaggregations of anionic and neutral C5 Pe molecules at the oil/water interface with the systems B1 and B2 (vide supra). Thedensity profiles (Figure 4e and f) are used to detect theadsorption process of the anionic and neutral C5 Pe molecules.The width of the crude oil/water interface is defined as thelength where the water density is 1090% of its bulk value.45For system B1 and system B2, the interfacial width of the oil/water interface is about 1 nm.For the two systems, at the beginning of the simulation, the

asphaltene molecules are distributed in the oil phase (Figure 4aand c). During the simulation process, the density profile of theC5 Pe molecules (Figure 4e) has almost no changes, suggestingthe C5 Pe molecules persist in the crude oil all of the time(Figure 4b). The reason can be attributed to the aromaticmolecules in the crude oil, which hinder the asphaltenemolecules to be adsorbed at oil/water interface.46 However, inFigure 4f, the peak closest to the interface increases with timeevolution indicating that the anionic C5 Pe molecules areadsorbed to the interface gradually (Figure 4d). The resultsprovide further evidence that the headgroups have stronginfluence to the behavior of asphaltene molecules, which is inagreement with the experimental phenomenon.18

2.1. Three-Stage Model of Anionic C5 Pe Adsorbed at theCrude Oil/Water Interface. In this section, we focus on thedynamic process of anionic C5 Pe molecules adsorbed at theoil/water interface. The total energy of system B2 (Figure 5a)varies as the simulation progresses, suggesting that theadsorption process can be divided into approximately threestages. The density profile and detailed snapshots of theadsorption process are shown in Figure 4f and Figure 6. Instage I, the critical event is a few anionic C5 Pe moleculesadsorbed at the oil/water interface from the crude oil phase(Figure 6ab). Initially, most of the anionic C5 Pe moleculesare distributed in crude oil, and only a few of the anionic C5 Pemolecules are close to the interface. Then, some watermolecules enter into the crude oil phase by the influence ofH-bonds, which are formed between the hydrogen atoms inwater and the negatively charged oxygen atoms in the anionicC5 Pe molecules. Consequently, those water molecules pull the

anionic C5 Pe molecules to the interface. Examination of thedensity profile (Figure 4f) reveals a trend of the anionic C5 Pemolecules moving toward the interface. After about 60 ns, thedensity peak close to the oil/water interface increases. Thesedata mean some anionic C5 Pe molecules are already beingattracted to the interface (Figure 6b). In this paper, the anionicC5 Pe molecules at the interface are defined as interfacialmolecules, which are marked in red in Figure 6b for clarity.In the second stage, more anionic C5 Pe molecules are

continually attracted to the interface (Figure 6bc) and theirstacking models transform from T-shaped stacking to the face-to-face stacking. In this process, the total energy of the systemdecreases quickly compared to stage I and stage III (Figure 5a).In Figure 4f, the peaks close to the oil/water interface clearlyincrease from 60 to 120 ns, denoting more anionic C5 Pemolecules are adsorbed at the interface during this stage. In theadsorption process, the molecules already at the oil/waterinterface play the vital roles. They attract other anionic C5 Pemolecules to the oil/water interface through noncovalentinteractions. The stacking pattern is obtained from thenormalized RDFs (Figure 5b). At 60 ns, the ratio of thethree stacking models is about 3:1:2, while it changes to 3:1:1 at120 ns for stacking of face-to-face, T-shaped I, and T-shaped II,respectively. The reduction of the peak at about 1.23 nmindicates that the T-shaped II stacking in system B2 isvanishing. The two large peaks displayed in Figure 5b representthe face-to-face stacking and the T-shaped I stacking. Due tothe dominating peak at 0.4 nm, we assume that majority of theanionic C5 Pe molecules are participating in face-to-facestacking. The reason for changes in the stacking models isascribed to the hydrated headgroups, which are formed afterthe headgroups are exposed to the water phase, evidentlyreducing the repulsion of the headgroups (Figure 6c).Stage III reflects the anionic C5 Pe molecules in a well-

organized and ordered arrangement at the interface (Figure6cd). In this process, a slight decrease of the energy profilesuggests that a small adjustment of the system occurs (Figure5a). The decrease of energy is illustrated by the normalizedRDFs in Figure 5b, where the peak of the T-shaped II stackingvanishing accompanies the energy decrease. The observedadsorption process implies that the molecules already exiting atthe interface are very important for the formation of the anionicC5 Pe slab and that the hydration of the headgroup causes thestacking model to change to T-shaped II. From the top view inFigure 6d, we can observe that the anionic C5 Pe moleculesform side-on arrangement at oil/water interface. Theirmolecular orientation agrees well with the experimental datareported using the sum frequency generation (SFG) vibrationalspectroscopy.47 This three-stage model based on the totalenergy of the system adequately explains the process of anionicC5 Pe aggregation from the crude oil to the oil/water interface.

2.2. Formation of the Interfacial Molecules. The formationof interfacial molecules parallel to the oil/water interface is vitalto catalyze the anionic C5 Pe molecular aggregation. Theinterfacial molecules are formed by the H-bond interactionbetween the oxygen atoms of the anionic C5 Pe molecules andthe protons of water molecules. This is the key step for theanionic C5 Pe molecules accumulating at crude oil/waterinterface.38,48

Molecule 3 (marked in Figure 6a) is taken as an example toinvestigate the formation process of the interfacial molecules.The snapshots of the anionic C5 Pe molecule over time areshown in Figure 7. Initially, some water molecules come into

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the crude oil and then connect with molecule 3 (Figure 7a).The electropositive hydrogen atoms of the water moleculesattract the negative oxygen atoms of the anionic C5 Pemolecules to form new H-bonds. Then, the anionic C5 Pemolecule is dragged to the interface due to a hydrophilicinteraction (Figure 7b). At the same time, the aromatic plane isforced to parallel to the interface under the function of thenumerous H-bonds (Figure 7c). This behavior is energeticallyfavorable for the anionic C5 Pe molecule to reside at theinterface. Due to the numerous other H-bonding interactions,more anionic C5 Pe molecules are extracted from the crude oiland 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 Pemolecules are discussed. The angle between two anionic C5 Pemolecular planes is defined as angle , while the angle of one

anionic C5 Pe molecular plane to the oil/water interface isangle (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 are shown in Figure 8b, and that of thegroup 2 is displayed in Figure 8c. Though both of them canform face-to-face stacking, the trends of their angles relative tothe interface are different.For group 1, molecule 2 acts as the interfacial molecule

(Figure 6a), and the cosine of angle is close to 1 at thebeginning. As molecule 1 closing to molecule 2, the cosine ofangle is perturbed. After about 12 ns, molecule 1 approachesmolecule 2, and their distance reduces from 3.0 to 0.6 nm as aresult of the noncovalent interactions. Hence, molecule 2 isforced to leave the crude oil/water interface due to thehydrophobic interaction of the polyaromatic rings. In the next 8

Figure 7. Snapshots of molecule 3 from Figure 6a drawn to the interface during the simulation. (a) 0 ns, (b) 1 ns, and (c) 2 ns. H-bonded watermolecules are highlighted.

Figure 8. (a) Scheme of the angles of two anionic C5 Pe molecules, angles between the 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).

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ns (12 to 20 ns), the cosine of angle trends toward 1,suggesting the molecule 2 is adsorbed to the interface again dueto the H-bonds between the oxygen in anionic C5 Pe and watermolecules. However, during this period, the distance betweenmolecule 1 and molecule 2 does not reduce to the minimum0.4 nm, which indicates the face-to-face stacking has not beenformed between the two anionic C5 Pe molecules. After about20 ns, the cos is close to 1, and their distance approaches 0.4nm, implicating that the face-to-face stacking is formed betweenthe two molecules.The properties of group 2 (molecule 3 and molecule 4) are

shown in Figure 8c. The cosine of the angle increases to 1 inthe first 2 ns of the simulation. The data are in agreement withFigure 7, where the molecular plane becomes parallel to theoil/water interface at 2 ns due to the H-bond interaction withwater molecules. From 2 to 8 ns, both the angle and thedistance between molecule 3 and molecule 4 show littlevariation, similar to the 1220 ns regime of group 1 (Figure8b). After 8 ns, the distance is minimized, and the cosine ofangle approximately equals to 1, indicating that the face-to-face stacking formed. However, in this period, the cosine ofangle (Figure 8c) contains fluctuation in comparison toFigure 8b. This fluctuation is the result of the hydrophobicinteraction. That is the delicate balance of the repulsivehydrophobic interaction and the attractive hydrogen bondingbetween the anionic C5 Pe molecules and water. After theformation of the face-to-face stacking, it presents differenttendencies for the two groups at the oil/water interface. Group1 stays at the interface steady, and group 2 is more prone tooscillate at the interface. The reason can be ascribed to theaggregation number; the large aggregation number can defensethe oscillation.

IV. CONCLUSIONS

In this work, the effects of the terminal groups on asphaltenemolecules have been studied by MD simulations. The simulatedresults indicate that the presence of anionic terminal groups onthe aliphatic chains can dramatically influence the neutral C5 Pebehavior both in the oil phase and at the oil/water interface. Incrude oil, the carboxyl C5 Pe favors the face-to-face stacking,while the carboxylate C5 Pe molecules prefer the face-to-facestacking in addition to the T-shaped II stacking because of therepulsive interaction of the headgroups. Whats more, theterminal groups also affect the properties of the C5 Pemolecules to adsorb at the crude oil/water interface. Theanionic carboxylate C5 Pe molecules can transform from anaggregate to a stable slab at the interface, while the carboxyl C5Pe aggregations persist only in the bulk crude oil during thewhole simulation. The adsorption process of anionic C5 Pemolecules can be explained by the three-stage model: First, afew asphaltene molecules are brought to the interface throughthe H-bond interaction between water and the oxygen atoms ofanionic C5 Pe. Then, more anionic C5 Pe molecules in thecrude oil are drawn to the interface via noncovalentinteractions. Ultimately, an asphaltene slab is formed at theinterface including the face-to-face stacking.

AUTHOR INFORMATIONCorresponding Authors*E-mail: xuzhen@qlu.edu.cn.*E-mail: shilingyuan@sdu.edu.cn.

NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSWe gratefully appreciate the financial support from NSFCProject (No. 21173128), Key NSF Project of Shandongprovince (No. ZR2011BZ0003 and No. ZR2012BM004), andthe HESTP Project of Shandong Province (J13LD01). Theauthors thank Dr. Bradley D. Rose, King Abdullah University ofScience and Technology, for helpful discussions and manuscriptediting. We are thankful for support by Program for ScientificResearch Innovation Team in Colleges and Universities ofShandong Province.

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