Coarse-Grained Molecular Simulations to Investigate Asphaltenes at

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Coarse-Grained Molecular Simulations to Investigate Asphaltenes at the Oil-Water Interface Yosadara Ruiz-Morales, and Oliver C. Mullins Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502766v • Publication Date (Web): 19 Feb 2015 Downloaded from http://pubs.acs.org on February 27, 2015

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Coarse-Grained Molecular Simulations to Investigate Asphaltenes at the Oil-Water Interface Yosadara Ruiz-Morales* Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Mexico City 07730, Mexico Oliver C. Mullins, Schlumberger-Doll Research, Cambridge, MA 02139, USA

ABSTRACT. In the present work we investigate, by means of theoretical simulation, the preferred orientation of a model asphaltene molecule at the oil-water interface (monomer). The coarse-grained model molecules at the mesoscale level, using dissipative particle dynamics (DPD), are adopted. The central polycyclic aromatic hydrocarbon (PAH) core and the peripheral alkanes in the asphaltene are considered. The asphaltene model construction by coarse grain mapping is proposed and analyzed, as well as the effect of using different solubility parameters in the construction of the potential interaction of the beads in the coarse grained asphaltene model. Also, the effect of surface coverage for a structure where steric effects dominate is presented as well as the effect of asphaltene coarse-grain nanoaggregates at the oil-water region. Finally, the orientation at the oil-water interface of an asphaltene with peripheral oxygen moieties is studied. Toluene is used as a model of oil. Three different orientations of the asphaltene model are used as starting configurations: horizontal to the oil-water interface, perpendicular to the oil-water interface, and tilted 45° with respect to the oil-water interface. In all cases it is found that the asphaltene molecule stays at the oil-water interface with the preferred orientation, where the aromatic region lays in the plane of the oil-water interface while the aliphatic chains are perpendicular to the oil-water interface and in the oil region. This molecular orientation remains in the case of higher asphaltene surface coverage. Due to jamming and steric repulsion some of the asphaltenes migrate to the bulk and some remain at the oil-water interface. Asphaltene molecules in the bulk oil are found to interact by π-π stacking interaction of the aromatic cores. For the case of the nanoaggregate, where the aromatic core is surrounded by alkyl chains it is observed that the aggregate migrates to the bulk of the oil region, thus these nanoaggregates do not load onto the oil-water interface. For the case of the coarse grain asphaltene with peripheral oxygen moieties it is found that the oxygen moieties orient in plane at the oil-water interface while the PAH orients out of plane and into the toluene region. These many findings are consistent with extensive experimental results as discussed. The combination of coarse grain and DPD dynamics, while maintaining the

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major structural characteristics in the coarse grain mapping of asphaltene species, represents a powerful tool that will help to answer questions about oil-water emulsions.

1. INTRODUCTION The formation of stable emulsions with crude oil and water is of considerable scientific and economic interest. One particular area that continues to receive much attention is the rapid and efficient separation of water and oil in the produced fluids from oil reservoirs. With all production methods, the creation of problematic emulsions occurs particularly for asphaltene-rich oils.1-6 Emulsified water, carries dissolved salts which cause severe corrosion problems for downstream refinery equipment and transporting pipelines.7,8 Also, emulsion droplets can increase the viscosity, water in oil emulsions can increase pumping costs.9,10 In the production of crude oil, water is also produced. Moreover, the so-called water cut (WC), i.e., the volume of water divided by the total volume of fluids produced, increases over the lifetime of an oil-well. Therefore, mature oil-wells present WC as high as 90%.11 The separation of oil and water is hindered by stable emulsions, and even in acquisition of subsurface samples of crude oil, stable emulsions can be rather detrimental. Understanding these emulsions is key to developing treatment strategies from first principles. Emulsions are thermodynamically unstable but kinetically stable. Indigenous components in the crude oil can contribute to the stability of emulsions, including waxes,11 solid particles,13-16 such as Pickering emulsions,17 and asphaltenes.1,18-24 Naphthenic acids have also shown to impact the stability of emulsions, depending on concentration and pH,1,25,26 by forming naphthenates, when in contact with alkaline solutions. Heavy oils, which are naturally rich in asphaltenes, have a significant tendency to form stable emulsions.1,18-24 This indicates that asphaltenes are likely key agents in stabilizing oil-water emulsions. Crude oils and water tend to form invert emulsions, that is, water-in-oil emulsions. This indicates that the interfacial agents are predominantly in the oil phase. In such a case, coalescence of oil drops in water can occur unimpeded, while coalescence of water drops in oil is impeded by the surfactant layer in the oil at each oil-water interface; thereby forming invert emulsions.27 Many studies have characterized aspects of increased emulsion stability due to asphaltenes.1,18,19,23,28-38 For oils of lower resin and higher asphaltene fraction, stable emulsions form due to preferential partitioning of asphaltenes to the interface.1,18,19,23,31,39 Czarnecki et al40,41 have studied how asphaltenes act as emulsion stabilizers above or around the onset of asphaltene precipitation. High resolution mass spectrometry of interfacial material of bitumen-water emulsions has shown a variety of molecular classes


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are present including heteroatom containing compounds, in particular oxygen, and compounds of high aromaticity.40,41 Recently, Rane et al42,43 and Pauchard et al44 have performed pendant drop experiments. In contrast to some Langmuir film and Langmuir-Blodgett film experiments, in pendant drop experiments, the solvent is retained so there is no forcing of all asphaltenes to the interface. These pendant drop experiments have shown that oil-water interfacial tension obeys a universal curve dependent on asphaltene coverage but independent of film aging, viscosity; nor the presence or absence of asphaltene nanoaggregates. This curve was fit with the Langmuir equation yielding a molecular contact area of a PAH with 6.2 fused aromatic rings, in close agreement with other determinations of fused ring number in asphaltenes.4547 This is quite close to the molecular architecture of asphaltene monomers. These pendant drop experiments are in accord with the Yen-Mullins model as clarified by the authors.4244,46,47 Universal curves in the pendant drop experiments were obtained for the elastic and loss moduli of the interface versus relative asphaltene coverage. The existence of three universal curves one of which is described with the Langmuir equation validates the interpretation in these experiments. The interpretation of the surface tension data presumes the orientation of the asphaltene PAH being in-plane and the alkanes out-of-plane. Exactly this molecular orientation has been found in sum frequency generation (SFG) experiments of asphaltene Langmuir-Blodgett films.48 In addition, the SFG experiments showed that PAH compounds with peripheral oxygen moieties orient with the oxygen in-plane and the PAH out-of-plane, opposite to the asphaltene molecular orientation. Moreover, the pendant drop experiments showed the lack of interfacial activity of the asphaltene nanoaggregate which is expected given its hydrophobic peripheral alkanes. Nanoaggregates were originally predicted to have small aggregation numbers with a central PAH core and peripheral alkanes due to the predominant ‘island’ molecular architecture.45,49 This nanoaggregate structure has been confirmed in combined small angle neutron scattering (SANS) and small angle x-ray scattering (SAXS) experiments.50,51 These pendant drop and SFG experiments42-44,48 relating asphaltene molecular structure, nanoaggregate structure, and orientation to interfacial properties offer the opportunity to perform molecular modeling. Theoretical molecular dynamics simulations studies of the interface had been carried out to study asphaltenes,52-55 asphalts and asphaltenes in vacuum,56-59 and asphaltene dimers in different solvents.60 Molecular dynamics simulation techniques have been used to study the nanoaggregation of one resin and two asphaltene structures by Headen et al.61 As pointed out by these authors the length scales are too large and time scales are too long to fully study asphaltene nanoaggregation by atomistic molecular simulation but valuable information is obtained at small-scale simulations. Mikami et al62 carried out a study of the of asphaltenes at the oil-water interface using molecular dynamics. They found that the plane of the PAH of island-type asphaltenes is


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attracted by the interfacial water molecules to form stable parallel structures, while the asphaltene aggregate adopts the vertical and parallel orientations at the oil-water interface. However, these molecular dynamics62 ran for 4 ns only due to limitations to reach larger time scales with molecular dynamics. For larger systems, the coarse-grained model molecules at the mesoscale level, using dissipative particle dynamics (DPD), provide a promising methodology. Mesoscale modeling allows for key chemical interaction to be represented without requiring excessive book keeping at the atomic level. DPD is a relatively new method proposed to study the hydrodynamic behavior of complex fluids.63-65 This method is based on the dynamics of soft particles interacting by conservative, dissipative, and random forces. Bead-spring-type particle models, where particles represent a group of atoms or a liquid volume, are used in the DPD calculations. DPD is a widely used methodology at the mesoscale and can process much larger spatial and temporal scales than molecular dynamics (MD) can. DPD applications have been done in simulations of lipids,66 block copolymers,67 vesicle formation,68 surfactants,69 carbon nanotubes,70 Very recently, DPD is being used in the area of oil chemistry. Zhang, et al71 used DPD to study the aggregate behavior of asphaltenes in heavy crude oil. Rekvig et al70 carried out a DPD investigation of surfactant efficiency to form emulsions in an oil/water/surfactant system. Alvarez et al72 used DPD to study the oil/water emulsions in the presence of a functionalized copolymer. Li73 et al used DPD simulations to study wettability alternation induced by the adsorption of surfactants on a solid surface, wetting hysteresis phenomena, and the process of oil-drop detachment from a solid surface. These results showed that the mesoscopic DPD model can be successfully used to assess the performance of a surfactant-aided oil-recovery fluid. Chen et al have studied the water-oil displacement in a capillary with external force using the many-body dissipative particle dynamics (MDPD) method.74,75 In the present work we investigate, by means of theoretical simulation, the preferred orientation of a model asphaltene molecule and related structures at the oil-water interface. The central PAH and the peripheral alkanes are considered as well as changes accompanying oxygen substituents. The effect of higher surface coverage by asphaltenes is discussed. A model asphaltene nanoaggregate is also investigated, and the behaviors of asphaltene molecules in bulk are treated. For the case of PAHs little work has been done with DPD involving fused aromatic rings due to its complex topology and proved to be a challenge for the coarse-graining process. Therefore, in the present work the coarsegraining process of PAHs is also presented and discussed. Results are compared with extensive experimental work discussed above.

2. METHODOLOGY.


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2.1. DPD Method. Dissipative particle dynamics (DPD) is a coarse-grained simulation technique introduced by Koelman and Hoogerbrugge65 to simulate the Navier-Stokes hydrodynamics of complex fluid systems over long length and time scales. Small regions of fluid and complex material are modeled with one or several soft beads. Bead-spring-type particle models are used in the DPD calculations. Dynamic evolution of the beads is governed by Newton’s second law. Three types of forces describe the interactions between pairs of beads: (1) a harmonic conservative interaction (2) a dissipative force representing viscous drag between moving beads (i.e., fluid elements) and (3) a random force to maintain energy input into the system in opposition to dissipation. Interaction parameters between particles are typically mapped from a well-defined theory, such as Flory-Huggins. DPD is limited to short-ranged interactions and also limited to the NVT ensemble (i.e., the number of particles N, the volume V, and the temperature T of the system are kept constant). Full details for the DPD methodology are provided elsewhere.64,65,76 Here we just provide generalities of the method to explain the parameters used in our calculations. The DPD model is constructed with N particles in a continuum domain of volume V. These particles, or beads bonded by springs, represent a cluster or group of atoms or molecules. During a dissipative particle dynamics the system is updated in discrete time steps ∆t. In DPD the time evolution of the particles of mass is governed by Newton's equations of motion (Eq. 1) which have been set out in detail by Moeendarbary et al76 and by Groot and Warren.64 = , = (1)

Where , , and are the position, velocity and momentum vectors, respectively, of particle i, and is the total interparticle force exerted on particle i by other particles j. The total force, on a given particle or bead i is:

= ( + + ) =

(2)

Where is the mass of the particle; is the interactions exerted on particle i by other particles and can be written in terms of conservative, dissipative, and random forces


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6 ( , , , respectively). Each part of the total force, , is pairwise additive. For simplicity the mass of all the particles is set equal to 1, so the force acting on a particle equals its acceleration. The sum runs over all other particles within a certain cutoff radius, , which is the length scale parameter of the system and set equal to 1. All forces between particles i and j vanish beyond .

is a purely repulsive conservative force: =

1 − <

0 ≥

$ (3)

Where is the interaction parameter between particle i and j or conservative force parameter. It is a maximum repulsion between particle i and particle j. is the relative distance between the beads i and j, and the inter-bead unit vector, pointing from particle i to particle j, is defined as: &

= |&'(| , & = & − & , = |& | (4) '(

The DPD technique is based on soft sphere interactions, therefore the repulsion parameter is chosen while taking into account the compressibility of the system. Defining the repulsion parameter as a soft repulsive potential facilitates accessibility of much larger length and time scales, ant it is one of the most important aspects of DPD simulations. In the following section it is explained how the repulsion parameter was chosen for the calculations in this work. is a dissipative or frictional force, which represents the effects of viscosity and slows down the particles motion with respect to each other; and a random force .

=

=

−*+ ∙ <

(5)

-+ . Δ 01/3 <

(6)

0 ≥

0 ≥

+ , + are r-dependent weight functions that ensure that and vanish when becomes greater than . * is the friction factor or the amplitude of the dissipative force, -


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defines the fluctuation amplitude of the random force or noise parameter. . in Eq. (6) is a randomly fluctuating variable with Gaussian statistics white noise term with . = . , zero mean 〈. ()〉 = 0, and unit variance. There is an independent random function for each pair of particles. Δ is the time step of the Newtonian Eqs. (1) and (2). To obey the fluctuation-dissipation theorem:63 1 − < + = 6+ = 0 ≥

(7)

And the system temperature will follow from the relation between the amplitudes * and of equations 5 and 6:63 ;
= −?@ (A − )D BC

(9)

where ?@ is the spring constant, A is the length of the spring involved or the equilibrium bond length.77

2.2. Coarse-Grained Model Molecules. Dissipative particle dynamics (DPD) simulations were carried out in the NVT ensemble using the Mesocite DPD module as implemented in the Materials Studio package.78 Simulations are carried out for an asphaltene model compound (monomer), which is composed of an ovalene-PAH core and three aliphatic chains (hexane), at the oil-water interface (see section 2.2.1 for model construction). Toluene is used as model of oil, and this approximation is possible due to the fact that the solubility parameters of oil and of


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toluene are similar (see Table 1). Three different orientations of the asphaltene model are used as starting point: horizontal to the oil-water interface, perpendicular to the oil-water interface, and tilted 45° with respect to the oil-water interface. Table1. Experimental Hansen Solubility Parameters (J/cm3)1/2 System Hansen Solubility Molar Volume 3 1/2 Parameter (J/cm ) (cm3/mol)c Oil 18.2d Toluene 18.2a 106.8 Water 47.9b 18.0 b Benzene 18.6 89.13 Hexane 14.9b 131.6 c Methanol 29.6 40.7 Asphaltene 20.4d 669e a Ref. 79. bRef. 80. cRef. 81. dRef. 82. e Parameter calculated indirectly, see text in section 3.2. for explanation. In the DPD methodology, in general, a bead corresponds to EF water molecules. The number EF (degree of coarse graining) can be viewed as a real-space renormalization factor. The mass (in mass units), length (in Å), and time (in ps) scales can be defined in terms of the coarse graining as:66 = EF GHIJK

(10)

L = 3.107(OEF )1/P

(11)

T/P

Q = 14.1 ± 0.1EF

(12)

The degree of coarse graining used in here is EF = 3; i.e., one DPD water bead represents three water molecules. According to Table 1, one water molecule has a volume of 30 Å3 following: 1ZLF[ /F@X

U$V@WJ GHIJK F@XJLYXJ = \.A3P ×1A/F@X = 3 × 1003P ^P = 30ÅP

(13)

therefore, the volume of one water bead, with EF = 3 is 90 Å3. We assigned the reduced density O = 3 beads per LP , defined as the number of beads in a cubic volume of radius L , and accordingly (Eq. 11) L = 6.46 Å. The volume of each bead, 90 Å3, in the simulation is maintained constant. In DPD all beads have the same volume, size, and mass. The mass scale used is 54 amu.


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2.2.1. Model construction and coarse grain mapping. Obtaining the coarse grained model of the molecules involved in the calculations is an essential and limiting step in the DPD simulation. Each bead in the coarse grain models represents a group of atoms or a liquid volume. Because the volume of each bead in the coarse grain models is the same; the definition of the beads, as a group of atoms, has to be done in such a way that the difference in volumes, between groups of atoms, is not too high. Also, in the case of the coarse grain model of the fused aromatic region (FAR), in the polycyclic aromatic hydrocarbon, the topology must be conserved as well as the rigidity due to the fact that the structure of asphaltenes is a crucial component of their behavior, and of their interfacial activity. One key molecular attribute of the asphaltenes is the size distribution of their polycyclic aromatic hydrocarbon (PAH) cores. Comparison of measured spin singlet−singlet absorption and emission transitions, as well as for the triplet manifold (S-T and T-T), with exhaustive molecular orbital (MO) calculations indicates that asphaltene PAHs have a population centroid of pericondensed ∼7 fused aromatic rings (7FAR), as most probable, with the bulk of the population width of pericondensed 4FAR to 10FAR83-87 along with population extending to 15 FAR.84 Analysis of PAHs with heteroatoms showed small differences in electronic structure from nonheteroatom-containing PAHs.87 Indeed, as expected the large FAR PAHs are needed to account for the longest wavelength electronic absorption.84,87 These structural characteristics of asphaltenes are enclosed in the widely validated and accepted Yen-Mullins model,46,47 which specifies the predominant molecular and colloidal structure of asphaltenes in crude oils and laboratory solvents. Therefore, we have chosen ovalene as the PAH in the aromatic core for the asphaltene model used in the calculations here presented. Ovalene is a pericondensed PAH which contains 10 fused aromatic benzenoid rings or hexagons (10FAR). The coarse graining of the asphaltene structure used in here has to reproduce the observed experimental behavior of π-staking, aggregation, and molecular orientation at the oil-water interface. Zhang et al71 in a DPD study of the aggregation behavior of asphaltenes in heavy crude oil, and in a challenging attempt to coarse grain the PAH structure, used the rigid sheet of the hexa-particle ring to coarse grain the fused aromatic rings in asphaltenes. In Zhang’s et al71 paper, the pericondensation of the beads is maintained in some models; however, these authors use PAHs with 10FAR, 13FAR, 16FAR and 19FAR. The coarse grained model for 10FAR that Zhang et al71 used does not show pericondensation, while the other coarse grained models present certain degree of pericondensation. Von Lilienfeld and Andrienko88 computed the potential energy surfaces of the interaction of pairs of PAHs with 1FAR to 13FAR, using density functional theory (DFT), and from the results these authors obtained parameters for a coarse grain potential. These authors88


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found that a computationally more efficient coarse grain representation is the one in which each benzene is presented as an interacting point located at its center of mass, thus reducing the number of degrees of freedom. Von Lilienfeld and Andrienko88 did not found significant difference in the interaction profiles upon use of the united atom or the benzenebead representation. Therefore, we have coarse grained the fused aromatic ring (FAR) region of our model asphaltene, which is composed of an ovalene core as follows: each fused benzenoid ring is represented by a bead. Ovalene is composed by ten fused aromatic hexagonal rings (10FAR); therefore, ten beads compose the aromatic region. The position of each bead is at the geometric center of each hexagon in ovalene. The smallest pericondensation of the PAH in asphaltenes is two internal carbon atoms, or Y-carbons (internal carbons in the vertex of what looks like a Y letter).83,85 In general the number of Y-carbons, present in asphaltene structures, are even and starting with two.83,85 Ovalene has ten internal Y-carbons, shown in bold in Figure 1a. These internal Y-carbons compose two internal hexagons that, in the coarse grain model, are represented by two internal beads, or Y-beads because in the atomic structure they conformed the Y-carbons, which is an even number of internal beads. All the overall shape of the FAR region is maintained in the coarse grain model as shown in Figure 1a. We notice that Zhang et al71 obtained the results that best agrees with the π-stacking of asphaltenes when in their coarse grain model there are two internal beads, while for an odd number of internal beads the aggregation obtained do not resembles the experimental observations. Therefore we found that in the coarse grain mapping of the PAH of asphaltenes the number of internal Y-beads has to be an even number. The volume of one hexane molecule is 218 Å3, the volume of one molecule of toluene is 177 Å3, and the volume of one benzene ring is 148 Å3 (according with Table 1 and Eq. 13). As discussed above, a bead corresponds to EF water molecules, and the volume of one water bead in this work is 90 Å3. Thus, toluene is represented by two beads, hexane (which is the alkyl chain in the asphaltene structure) is represented by 3 beads, and the ovalene PAH core is represented by 10 beads (representing 32 carbon atoms) where each bead represents a benzenoid ring. Although the volume of a benzene ring is 1.6 times the volume of a water bead we use one bead for each benzenoid as approximation. Therefore, each simulated system is comprised by five interacting beads, representing water, benzene in toluene, methyl in toluene, aliphatic chain, as part of the asphaltene structure, and the aromatic region in the asphaltene, see Figure 1. The connected beads are connected via harmonic bonds (Eq. 9). We assigned the equilibrium bond length A = 3Å, and a spring constant of 300 Kcal/mol/Å2 in order to maintain the rigidity of the aromatic region in ovalene.


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In the literature it is reported the use of another mesoscopic method to simulate the deposition of colloidal asphaltene in capillary flow by Boek et al.89 These authors have used a particulate simulation technique called stochastic rotation dynamics, where the fluid particles are represented by N point-like (ideal gas) particles. The coarse-grained description of the asphaltene aggregates was performed using an effective interaction potential with an attractive well, the depth of which can be tuned. However, in this theoretical technique there is no 3D structure only 2D.

H bead

(a)

B bead

H H

O

H O H

W bead

H O

(b)

H

T2 bead T1 bead

(c)

Figure 1. Schematic representation of: (a) Atomistic asphaltene type-molecule and its coarse grained bead representation. The pink color beads represent two C atoms with its hydrogens in the aliphatic chain. The aqua-blue color bead represents a benzenoid ring in the ovalene aromatic core. The ovalene is composed by 10 fused aromatic hexagonal rings (10FAR), therefore 10 beads compose the aromatic region. Ovalene has ten internal Ycarbons, (internal carbons in the vertex of what looks like a Y letter) shown in bold in the atomistic asphaltene, see text for explanation. (b) Atomistic and coarse grain bead representation of water. The DPD water bead, in dark blue, represents three molecules of water. (c) Atomistic and coarse grain bead representation of toluene. The aromatic ring of


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toluene is represented by a green bead and the methyl group of toluene is represented by the light-pink bead.

2.3. DPD parameters used in the simulations. The simulations are intended to reproduce ambient conditions. Because DPD is a coarse-grained method, all simulations runs were conducted at the reduced temperature (energy scale) ?8 9 = 1, which is the unit of energy in the reduced units system. The default is 0.59191 kcal/mol (thermal energy scale in physical units), and is chosen so that a reduced temperature of 1 corresponds to 298 K. In simulation we set L = 1, and all DPD particles have the same mass = 1. The random and dissipative parameters were set to - = 3 and * = 4.5, respectively. The time step ∆ = 0.005Q was used to integrate the equation of motion (Eqs. 1 and 2). The simulation time scale Q is 3.02 ps expressed in the natural unit of time: Q = L d⁄?8 9

(14)

The positions and velocities of the particles are solved using a modified velocity-Verlet algorithm proposed by Groot and Warren.64 The DPD method in general has been shown to produce a correct (N,V,T) ensemble, if the fluctuation-dissipation relation is satisfied.63,64 With the modified velocity-Verlet algorithm used the temperature does not depends on the step size. The steps size dependence is negligible64 up to time step ∆ = 0.06. As explained in section 2.1. in the DPD framework the intermolecular pair interactions are represented as a sum of a dissipative force, a random force, and a conservative force. Only the latter depends on the type of atoms considered in each pair and is modeled as a soft repulsion that requires only one parameter, , for each pair to fully describe it, Eq. 3. The bead-bead interaction parameters were determined, following Groot and Warren, by:64 = + 3.27f

(15)

Where f is the Flory-Huggins parameter between bead i and j and is the repulsion parameter between beads of the same type. To satisfy the compressibility of water the mapping of three water molecules per bead led to = 78.66 The Flory-Huggins parameters, f , where calculated using the formula: h

'( f = i (j − j )3

(16)

Where k is the average molar volume of the species, R is the gas constant, T is the absolute temperature (298 K), and j and j are the solubility parameters for systems i and


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j. The molar volumes and solubility parameters used are presented in Table 1. The calculated values, in reduced DPD units, conform to a symmetric matrix presented in Table 2. Table 2. Conservative force parameter, , for the interacting beads shown in Figure 1. The different beads are shown in Figure 1. H T1 B T2 W H 78 T1 80 78 B 78 78 78 T2 80 78 78 78 W 192 155 142 155 78 For all the results presented in this section the conservative parameters shown in Table 2 are used. All the calculations are performed in a cubic simulation box with size 50 × 50 × 50 LP , with periodic boundary conditions applied in all directions. The enclosed system is composed of 50% water and 50% of toluene. Initially the asphaltene coarse grain model molecule is placed at the oil-water interface region with a high degree of molecular alignment at the interface. Three different orientations were the starting point of the calculations: parallel to the oil-water interface, perpendicular to the oil-water interface, and tilted 45° with respect to the surface normal. Each system is initially geometry optimized using the mesocite module embedded in the Materials Studio package.78 The mesocite force field DPD manager is used together with the conservative parameters listed in Table 2 to calculate the DPD forcefield for the calculations. After the corresponding system is geometry optimized the dissipative particle dynamics (DPD) simulations were carried out. The total number of beads was 1,382. A total of 1,000,000 DPD simulation steps were carried out. Each system is simulated as long as 15,078 ps. Once in 2000 steps, the configurations were saved for analysis. All properties can be expressed in reduced units derived from L , , y ?8 9. The scale used in the Mesocite78 DPD units: Length scale: 6.46 Å, mass scale; 54 amu, energy scale: 0.59191 Kcal/mol, time scale: 3.0158 ps.

3. Results and Discussion 3.1. Molecular Orientation at the oil-water interface


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In Figure 2 the results for the DPD dynamics are shown for the system whose configuration starts with the coarse grain asphaltene model placed horizontally to the normal at the oilwater interface (Figure 2a). The structure shown in Figure 2b is of lowest energy and was obtained at 162,544 steps (2,451 ps). It is observed that the aromatic PAH region stays inplane at the oil-water interface, while the aliphatic legs go out of the oil-water interface and into the toluene region. During the DPD simulation the asphaltene comes off the oil-water interface but returns to it. Most of the time of the simulation the asphaltene stays at the oilwater interface oriented with its PAH core parallel to the oil-water interface, in-plane, while the aliphatic chains are oriented out-of-plane.

Figure 2. (a) Initial configuration for the PAH in-plane in the oil-water interface, (b) Low energy frame obtained at around 162,544 steps (2,451 ps), In Figure 3 the results of the DPD dynamics using as starting conformation the asphaltene molecule placed perpendicular to the oil-water interface are shown. Two of the lowest energy frames are shown, one obtained at 242,058 steps (3,650 ps), Figure 3b, and the other obtained at around 520,923 steps (7,855 ps). For this simulation it is observed that even though the initial configuration of the asphaltene model molecule was perpendicular to the oil-water interface the lowest energy configurations show the asphaltene molecule at the oil-water interface with the aromatic region horizontal to the surface and the aliphatic chains out-of-plane and into the toluene region.


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Figure 3. (a) Initial configuration for the perpendicular to the oil-water interface conformation. (b) Low energy frame obtained at around 242,058 steps (3,650 ps). (c) Low energy frame obtained at around 520,923 steps (7,855 ps). In Figure 4 the lowest energy frames obtained during the DPD simulation for an initial conformation where the asphaltene model is placed at an angle of 45° with respect the oilwater interface. As it can be seen in Figure 4 spite of the initial configuration, the asphaltene model stays at the oil-water interface with its PAH horizontal and on plane to the oil-water interface and with the aliphatic chains out-of-plane and into the toluene region.


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Figure 4. (a) Initial conformation for the configuration tilted 45° with respect the oil-water interface. (b) Low energy frame obtained at around 344,519 steps (5,195 ps). (c) Low energy frame obtained at around 638,504 steps (9,628 ps). For all the initial configurations, presented in Figure 2, Figure 3, and Figure 4, it is observed that during the DPD dynamics the asphaltene molecule stays most of the time at the oil-water interface. Eventually it goes to the toluene region; however, returns in few steps to the oil-water region. Also, the fact that the asphaltene molecule stays at the oilwater interface with its PAH in-plane to the oil-water interface, and its aliphatic chains out of the plane towards the toluene region it is not due to the initial configuration as it is shown here. The asphaltene aromatic region has affinity for the oil-water interface and it orients with its PAH region parallel to the oil-water interface. This same configuration has been found experimentally by direct determination by Sum Frequency Generation (SFG) of asphaltene molecular orientation in Langmuir-Blodgett films of asphaltenes where it has been shown that asphaltene PAHs are in-plane while the alkanes are out-of-plane.48 In addition, this molecular orientation is consistent with the pendant drop interfacial studies of Rane et al42,43 and Pauchard et al.44 Therefore, we are proving that the coarse grain mapping and model construction of the asphaltene molecule proposed in here is adequate and also


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we are proving that the coarse grain DPD methodology is a powerful method, fast and less expensive to simulate aspects of asphaltene science and petroleum science. To our knowledge, this is the first molecular orientation coarse grain modeling to match the extensive results from the SFG experiments48 and the pendant drop experiments.42-44

3.2. Parameter sensitivity in the calculations. In this section the sensitivity of the conservative force parameters used is tested as well as its effect on the simulations. Results from the DPD simulations for the three initial conformations of the asphaltene molecule (horizontal, perpendicular and tilted 45° with respect to the oil-water interface) are presented but instead of using the solubility parameter of benzene to define each bead in the aromatic region of the asphaltene model, the solubility parameter of asphaltene is used. In Table 3 the conservative force parameter obtained, using the data presented in Table 1 and Eqs. 15 and 16, are presented. Table 3. Conservative force parameters, , for the interacting beads shown in Figure 1 but considering in the model construction of the asphaltene aromatic core (bead B) the asphaltene solubility parameter presented in Table 1. H T1 B T2 W H 78 T1 80 78 B 95 81 78 T2 80 78 81 78 W 192 155 445 155 78 The molar volume of asphaltenes, needed to calculate the conservative force parameter for beads B in Table 3, was calculated indirectly. Yarranton and Masliyah90 found that the solubility parameter can be expressed as: j = (lO)1/3

(13)

Where O is the density and l is the linear fit parameter for the enthalpy of vaporization vs. molar mass, and j is the solubility parameter for asphaltenes presented in Table 1. The value used for A was that of PAHs as proposed by Yarranton and Masliyah90 and equal to 398 J/g. We used Eq. 13 to calculate the density of asphaltenes and obtained a value of 1.046 g/cm3. We used this density together with an average molar mass of asphaltenes of 700 g/mol,46,47 to calculate a molar volume of 669 cm3/mol for asphaltenes. This value is shown in Table 1. The information in Table 1 is used to construct the matrix presented in


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Table 3. In Figure 5 the snapshots of the starting configuration, before optimization, left side of Figure 5, together with the lowest energy configurations obtained in the DPD simulations are presented for each orientations of the asphaltene model with respect to the oil-water interface: horizontal, parallel, and tilted; respectively.

Figure 5. On the left side there are presented the initial conformations of the asphaltene at the oil-water interface: (a) horizontal, (b) perpendicular, (c) tilted 45°. For the DPD


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dynamics in figures (a)-(c) the asphaltene solubility parameter is used to construct the conservative force matrix shown in Table 3. On the right the lowest energy configurations are shown. The lowest energies are obtained at around: (a) 11,653 ps (772,822 steps), (b) 7,161 ps (474,899 steps), (c) 6,969 ps (462,166 steps). As it can be seen in Figures 5 when the experimental solubility parameter for asphaltenes is used in the calculation of the conservative force parameter, , for the case of the beads composing the aromatic region of the asphaltene, light blue beads in Figure 1a, it is found that, regardless the initial starting configuration, in the DPD simulation, of the asphaltene at the oil-water interface: horizontal, perpendicular or tilted, in all cases the asphaltene molecule migrates to the oil region and stays there most of the time of the simulation. This is because the use of the asphaltene solubility parameter for each bead in the aromatic region of the coarse grain model is akin to defining the aromatic region as composed by individual asphaltenes that are aggregated or bonded together. Such system would move to the oil region as it is experimentally observed with asphaltene aggregates. Therefore, the sensitivity of the conservative force parameter in the DPD simulation agrees with the experimental observations. However, to study the orientation of the asphaltene at the oilwater interface it is necessary, as shown here, to define the aromatic region of the coarse grain model of asphaltene as fused benzenoid rings, one benzenoid ring per bead.

3.3. Aggregation in bulk In this section the effect of surface coverage at the oil-water interface by asphaltene and concentration is studied. In Figure 6 two views of the initial horizontal configuration of five asphaltene molecules at the oil-water interface are presented. The conservative force parameter, , used for the interacting beads are those presented in Table 2, and correspond to the use of the benzene solubility parameter for each bead in the definition of the aromatic region of the asphaltene molecules.


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Figure 6. Two views of the initial horizontal configuration for a calculation at higher concentration of asphaltene. In Figure 7 the lowest energy frame configuration, obtained at 972,000 steps (15,348 ps), is presented. Both views corresponded to the same snapshot but in the right box the toluene molecules are not shown for clarity. As it can be seen the asphaltene molecules tend to stay at the oil-water interface with the aromatic region horizontal to the normal and the aliphatic chains out of the plane. Some of the asphaltene molecules migrate to the oil region, and π-π stacking interaction of the aromatic cores is observed in the bulk of the oil region.

Figure 7. Low energy frames obtained at around 972,000 steps (15,348 ps) for the calculation at higher concentration of asphaltene. In the right view the toluene coarse grain molecules are not shown for clarity.


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Rane et al42,43 and Pauchard et al44 have measured the oil−water interfacial tension by the pendant drop technique for a range of oil-phase asphaltene concentrations and viscosities. The interfacial tension was found to be related to the relative surface coverage during droplet expansion. These authors found that cross-linking did not occur at the interface and that only asphaltene monomers were adsorbed. At longer times and higher interfacial coverage the data indicate that when coverage reaches 35−40%, the adsorption rates slow down considerably, and at higher coverages the surface pressure is only dependent on asphaltene monomer concentration. The analysis of the long-term adsorption kinetics by means of the previously identified Langmuir EoS by the same authors42-44 confirmed an expected transition to a regime governed by steric hindrance of adsorption with a maximum coverage or jamming limit of 85%, and thus a fairly large degree of order within the asphaltenes monolayer. Too high jamming at the oil-water interface leads to asphaltene desorption, as pointed out by Pauchard et al;44 however, the simulation presented in Figure 7 corresponds to a surface coverage significantly lower than jamming. Due to the small size of the calculation box, we consider that some of the asphaltene monomers migrate to the bulk of the oil, where they aggregate, mainly due to steric hindrance at the oil-water interface. As pointed out by Pauchard et al44, the hindrance factor increases severely above 30% coverage leading to a much slower adsorption rate.44 In the case of soluble asphaltenes (i.e. in toluene like in the presented asphaltenes) this means that the proportion of asphaltenes desorbing to the organic phase will increase severely as initial surface coverage increases. This experimental result is in accordance with our calculation. Therefore, we conclude that our methodology is correct and close to the observables. When the solubility parameter for asphaltenes is used in the DPD calculation of the conservative force parameter, , for the initial system shown in Figure 6, for the case of beads composing the aromatic region of the asphaltene in the calculation (Table 3), instead of using the solubility parameter of the benzene for each bead in the asphaltene aromatic core, it is found that the asphaltene molecules migrate to the toluene bulk and stay there for the entire simulation, see Figure 8. The asphaltene molecules tend to aggregate, and interact face to face. The upper view and lower view in Figure 8 is the same but in the upper view the toluene molecules are not shown for clarity. The use of the asphaltene solubility parameter for each bead in the aromatic region of the coarse grain model is equivalent to defining the aromatic region of the asphaltene as composed by individual asphaltenes that are aggregated or bonded together; the aggregation of aggregates of asphaltenes (cluster) tends to occur in the oil bulk in accordance with the Yen-Mullins model.46,47 Therefore, what actually is being simulated in Figure 8 is the initiation of an aggregate of aggregates, e.i. a cluster, according to the Yen-Mullins model.


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Figure 8. Two low energy snapshots obtained at around 102,527steps (1,546 ps, left side), and at around 888,000 steps (13,401ps right side), for the calculation at higher concentration of asphaltene using for the model of the asphaltene aromatic core (bead B) the experimental asphaltene solubility parameter (see Table 1 and Table 3). In the top views the toluene coarse grain molecules are not shown for clarity. From their analysis of the relationship between interfacial tension, obtained from pendant drop experiments, and surface coverage with a Langmuir equation of state, Rane et al42-43 concluded a flat-on adsorption of monomeric asphaltene structures supporting the hypothesis that nanoaggregates do not adsorb on the interface, and consistent with the asphaltene aggregation behavior in the bulk fluid expected from the Yen−Mullins model.46,47 in accordance also with our findings. 3.4. Asphaltene Nanoaggregate. There are evidently different stages of aggregation in asphaltenes, as proposed in the YenMullins model,46,47 which consists of a hierarchy of aggregation for asphaltene solutions.


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Different hierarchies of aggregation correspond to different energies of interaction. The growth and its termination of nanoaggregates can be understood directly from molecular structural considerations. The polydispersity in a standard asphaltene sample ensures nanoaggregate initiation from the least soluble molecular fraction and nanoaggregate termination from the most soluble fraction, thereby creating a stable (nano)colloidal suspension. The resulting nanoaggregates resemble a “hairy tennis ball” with alkanes surrounding the outside. The predominant asphaltene intermolecular interaction is via polarizability forces as shown by evaluation of the Hansen solubility parameters for asphaltenes.91 We carried out a calculation where we model a nanoaggregate that resembles a “hairy tennis ball” with alkanes surrounding the outside, as shown in Figures 9 and 10.

Figure 9. Top: Different stages of aggregation in asphaltenes, as proposed in the YenMullins model.46,47 Bottom: Cartoon representation of a nanoaggregate that resembles a “hairy ball”. The orange horizontal section represents the piled aromatic regions and the “hairs” represent the alkyl chains in the entire nanoaggregate.


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Figure 10. Two views of the coarse grain representation of an asphaltene nanoaggregate equivalent to the nanoaggregate presented in the cartoon of Figure 9 (bottom section of Figure 9). Initially the hypothetical nanoaggregate is placed at the oil-water interface as shown in Figure 11a. For the DPD dynamics the conservative force parameters used are presented in Table 2. In Figure 11b and 11c two low energy snapshots are shown. It is seen that the nanoaggregate leaves the oil-water interface and stays in the oil region most of the simulation time. This happens because the exterior of the hypothetical nanoaggregate is surrounded by the alkyl chains that have affinity for the oil region.


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Figure 11. (a) Initial configuration of the system containing the molecule with external alkyl chains horizontal to the oil-water contact. Bottom: Morphology of two of the lowest energy frames obtained during the DPD simulation at (b) 198,000 steps (2,986 ps), and at (c) 676,000 steps (10,190 ps).

The study of the of asphaltenes at the oil-water interface using molecular dynamics has been carried out by Mikami et al.62 These authors describe the interfacial structures and the dynamics of the asphaltene from a single asphaltene molecule, the accumulated nanoaggregates, and the thin asphaltene film at the oil−water interface. Mikami, et al62 found that island-type asphaltenes aromatic plane is attracted by the interfacial water molecules to form stable parallel structures, according with our findings, and with observations from pendant drop experiments42-44 and sum frequentcy generation,48 but Mikami et al62 found that the nanoaggregates have a vertical and parallel structures at the oil-water interface and cause molecular oscillations. We found that the nanoaggregate do not stay at the oil-water interface due to the presence of the aliphatic chains which force the aggregate into the oil region. The MD simulations by Mikami et al62 were run for 4 ns. To


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fully study the nanoaggregate behavior at the oil-water interface by atomistic molecular simulation requires a length scales which is too large and time scales which are also too long. We consider that coarse grain DPD is a method that can provide more information of the effect of the nanoaggregate at the oil-water interface because it can process a much larger spatial and temporal scale system than molecular dynamics (MD) can. We ran the dissipative particle dynamics for a total of 15 ns and found that the nanoaggregate migrates to the oil interface.

3.5. Orientation of an asphaltene with peripheral oxygen moieties at the oil-water interface. The orientation of an asphaltene with peripheral oxygen moieties, as well as in one of the aliphatic chains, was also calculated, as shown in Figure 12, where the yellow bead represents methanol. The conservative parameters, , used in the DPD simulation are presented in Table 4, and were obtained from the information provided in Table 1 and using Eqs. 15 and 16. The conservative parameters of the blue beads in the aromatic region of the asphaltene in Figure 12, which correspond to bead B in Table 4, were calculated using the benzene solubility parameter (Table 1).

Figure 12. Asphaltene with peripheral oxygen (methanol) represented by the yellow beads.


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Table 4. Conservative force parameter, , for the interacting beads. The A bead represents methanol shown as yellow bead in Figure 12. The letters for the beads are shown in Figure 1. The letter A corresponds to the yellow bead. A H T1 B T2 W H 78 T1 80 78 B 78 78 78 A 102 92 89 78 92 T2 80 78 78 78 92 W 192 155 142 155 78

The initial configuration of the DPD dynamics is presented in the top part of Figure 13 and some of the lowest energy configurations obtained in the simulation are presented at the bottom of Figure 13. As it can be seen the asphaltene with peripheral oxygen moieties orients at the oil-water interface with the oxygen moieties in plane, including the oxygen in the alkyl chain, and the PAH out of plane and into the oil region, opposite to the orientation of the asphaltene with no oxygen moieties (see section 3.1). Work by Stanford et al40,41 suggest that the interfacially active asphaltenes may contain oxygen-rich functional groups. We have found that the asphaltenes with no oxygen and the asphaltenes with oxygen stay at the oil-water interface but they orient differently. The PAH core of asphaltenes with no oxygen orient in plane at the oil-water interface (see Figures 2, 3, and 4), while the asphaltenes with peripheral oxygen on one side orient with the oxygen atoms in plane at the oil-water interface (see Figure 13). The PAH region has electronic density, distributed in resonant sextets that have both affinity towards toluene and affinity towards water. However, the affinity for the water region of the oxygen atoms is much higher.


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Figure 13. Top: Initial configuration of an asphaltene with peripheral oxygen moieties (represented by the yellow bead, bead A in Table 3). Bottom: low energy configurations obtained at around (from left to right): 10,646 ps (706,000 steps), 10,680 ps (708,270 steps), and 10,706 ps (708,271 steps); respectively.

The results shown in Figure 13 are in agreement with the results obtained by sum frequency generation experiments (FG) of PAHs with peripheral oxygen moieties obtained by Andrews et al.48 These authors found that oxygen moieties orient in plan at the oil-water interface while the PAH orients out of plane.

Conclusions. In the present work the study of the orientation of asphaltenes at the oil-water interface is carried out by means of computation theoretical chemistry. The coarse grain dissipative particle dynamics method is used. Obtaining the coarse grained model of the molecules involved in the calculations is an essential and limiting step in the DPD simulation. The


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model construction and coarse grain mapping of the aromatic region of the asphaltene molecule was a main challenge. In this work we propose a definition in which the pericondensation, topology, and rigidity of the aromatic core of the asphaltene is maintained as well as the presence of an even number of Y-beads, which in the atomic molecule come from the internal Y-carbons (internal carbon atoms with a connectivity of three), and which are even in asphaltenes. The coarse graining of the fused aromatic region of the asphaltene is done by considering each fused benzenoid ring as a bead. The position of each bead is at the geometric center of each hexagon in the PAH region. All the topology of the spatial distribution of the benzenoid rings is kept. We found that a bond length A = 3Å, and a spring constant of 300 Kcal/mol/Å2 is adequate in order to maintain the rigidity of the aromatic region. This modeling approach is shown herein to match extensive experimental work on the direct measurement of molecular orientation of asphaltenes and related compounds at the oil-water interface. In addition, this modeling approach is shown to match extensive pendant drop measurements of asphaltene molecules and nanoaggregates as well as related compounds. This excellent match between experiment and theory provides strong validation of the modeling as well as the experiments. Moreover, this augurs well for future applications of DPD modeling for asphaltene molecules and colloidal species. We have found that asphaltenes with no peripheral oxygen moieties orient with the aromatic PAH region in plane and horizontally at the oil-water interface, and with the aliphatic chains out of plane and into the toluene region. The asphaltenes with peripheral oxygen moieties orient with the oxygens in plane at the oil-water interface and the PAH region out of the interface due to the high affinity of oxygen for the water region. In the case of surface coverage by asphaltenes with no oxygen moieties in the PAH region at high concentration, we found that due to jamming of the oil-water interface and steric repulsion some of the asphaltenes stayed horizontally oriented at the oil-water interface, while others migrate to the oil bulk where they tend to stack. All of our findings are according with observations from pendant drop experiments42-44 and sum frequency generation.48 For the case of an asphaltene nanoaggregate we found that it does not stay at the oil-water interface and migrates to the oil region due the presence of many alkyl chains in its exterior which do not have high affinity for the water region. In the literature it is reported combinations of Monte Carlo, ab initio and classical simulations to study the adsorption of small PAH molecules, with no-heteroatoms, on water or ice. 92-94 The aromatic PAH core show flat adsorption,92-94 presumably due to hydrogen bonds with the π−electrons of the PAH.92 These atomic-base simulations results are in agreement with our coarse-grain DPD simulations results on the orientation of the PAH core, with no-heteroatoms, close to the water region. This agreement also reinforces the validation of our proposed coarse-grain mapping of the PAH core and the parameters selection used in the DPD simulations.


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We found that the coarse grain DPD methodology is highly sensitive to the parameters used in the simulations. When the asphaltene solubility parameter and molar volume of asphaltenes is used for the beads of the aromatic PAH region during the construction of the conservative force parameters matrix, , used in the DPD dynamics, it is found that the asphaltenes migrate to the oil bulk region where aggregate. However when the benzene solubility parameter and molar volume of benzene is used for the beads of the aromatic PAH region we obtained the results of the orientation of the asphaltenes at the oil-water interface. It is the first time, to our knowledge, that the coarse grain mapping of asphaltene where all the structural features are kept is presented. The potential of the DPD novel methodology for modeling the oil-water interface is proven and opens the possibility of predicting the physicochemical hydrodynamics in the area of chemical enhanced oil recovery. The implications of this work are very broad beyond a far better understanding of the detailed chemical understanding of asphaltenes. First, this work helps address emulsion stability of crude oils from fundamental principles. This enables a first principles approach to address demulsifiers. Second, EOR (enhanced oil recovery) generally requires disrupting the wettability of the oil-mineral surface. We can use the detailed understanding of the crude oil-water interface to point the direction to analyzing crude oil-mineral interfacial chemistry. In order to understand the complex picture behind water-in-crude oil emulsion stability, a thorough knowledge at both ambient and high pressures should involve the structure and properties of crude oil components, mainly the heavy ones, their association tendencies and accumulation at w/o interfaces, their solubilities and sensitivity towards changes in pressure and temperature, the special features of acidic components and their association structures in water/oil systems, and how these components by themselves or by an intricate interplay form highly elastic interfaces protecting the droplets against coalescence.2 Asphaltenes play an important role in water in crude oil emulsion stabilization. Asphaltenes structure and orientation at the oil-water interface, as shown in here, impacts the stability of W/O emulsions. The identity of the precise asphaltene molecules is under investigaction.1As it is shown in this work the methodology presented here of combining coarse grain-DPD dynamics, maintaining the major structural characteristics in the mapping of the asphaltene structure to the coarse grain molecule represents a powerful tool that will help to answer these questions.

ACKNOWLEDGMENT


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Y.R.-M. acknowledges the support under projects D.60019 and Y.61000 (CONACYTSENER 177007) of the Instituto Mexicano del Petróleo.


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