Morphology of Aggregated Asphaltene Structural Models - Energy

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Morphology of Aggregated Asphaltene Structural Models J. H. Pacheco-Sańchez,* F. A Ä lvarez-Ramı´rez, and J. M. Martıńez-Magadań Programa de Ingenierıá Molecular, Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Me´ xico D.F. 07730, Me´ xico Received April 12, 2004. Revised Manuscript Received July 8, 2004

Aggregated asphaltene structural models have been generated through a molecular simulation geometry optimization process, using periodic boundary conditions. This methodology has been validated by first applying it to a pure aromatic system. Initially, a random distribution of 35 molecules was chosen and a geometry optimization process was performed, allowing the cell dimensions to vary without restrictions. The structure factor (S(k)) of an optimized final cell was obtained and compared with experimental results, and the agreement between theoretical and experimental S(k) profiles was satisfactory. This methodology was next used in the analysis of the morphology of 32 asphaltene model molecules and their aromatic cores; asphaltene model molecules were taken from literature. It is remarkable that face-to-face stacking of asphaltene aggregates was observed, as well as π-offset and T-shaped stacking geometries. Finally, the effect of aliphatic chains on the aggregates was also analyzed.

1. Introduction Aggregated asphaltene structural models would be useful in developing further understanding of important problems in the petroleum industry, such as the aggregation, flocculation, and precipitation of asphaltenes. An operative definition of asphaltenes is that they are insoluble in low-molecular-weight n-alkanes but soluble in aromatic solvents. It is well-known that asphaltenes self-associate in stacked aggregates, which form clusters.1,2 However, the morphology of these clusters is not yet clearly known. The most common morphology considered until now is the face-to-face type. In the literature, these interactions are usually only referenced as aromatic-aromatic interactions, without going into the fundamental molecular details. Some efforts in determining the morphologies of asphaltene-aggregated systems were performed by Yen et al.,3 Dickie and Yen,2 Wiehe and Liang,4 and Evdokimov et al.5,6 The morphology of these clusters is amorphous in nature. Therefore, a possible way to characterize them geometrically is either through the radial distribution function (RDF) or through the structure factor S(k); this is also called isotropic scattering. The RDF is defined as the number of atoms lying at distances between r and r + dr from the center of an arbitrary origin atom,7 * Author to whom correspondence should be addressed. E-mail address: [email protected]. (1) Sheu, E. Y. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995. (2) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847-1852. (3) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587-1594. (4) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201210. (5) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 135-143. (6) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 143-152. (7) ) Elliott, S. R. Physics of Amorphous Materials; Longman Scientific and Technical: New York, 1990; Chapter 3.

whereas the RDF is related to the observed structure factor S(k) by a Fourier transformation.7-12 In a pioneering work, Yen et al.3 performed X-ray diffraction (XRD) measurements for asphaltenes in powdered solid samples coming from Kuwait visbreaker tar oil. They have compared their experimental S(k) profile for the aromatic component of their asphaltenes to that computed for a blend of five polynuclear aromatic compounds of known structures.13 The S(k) profile and the peak positions agree quite reasonably with that obtained by Diamond.13 Based on these results, they have proposed a model constituted by aromatic sheets that are associated in a stacked cluster for an asphaltene solid phase. A very similar morphology that has been extended to aggregates of asphaltene particles was experimentally obtained by Dickie and Yen.2 Asphaltenes appear as unitary stacking sheets that are composed of a highly condensed polynuclear system of aromatic rings bearing alkyl side chains. They proposed that asphaltene association occurs via a stacking of 3-6 unitary sheets through π-π interactions. They define these entities as unitary cells or particles, indicating that the associations of such particles form micelles. The morphology of polyaromatic compounds and asphaltene aggregates have been studied theoretically as induced aggregation in a vacuum,14,15 as spontaneous (8) McQuarrie, D. A. Statistical Mechanics; Harper and Row: New York, 1973. (9) Hansen, J. P.; McDonald, I. R. Theory of Simple Liquids; Academic Press: New York, 1986. (10) Tildesley, M. P.; Allen, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, U.K., 1987. (11) Lee, L. L. Molecular Thermodynamics of Nonideal Fluids; Butterworth: Boston, 1988. (12) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH Publishers: New York, 1994. (13) Diamond, R. Acta Crystallogr. 1957, 10, 359-364. (14) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. J. Phys. Chem. 1995, 99, 10430-10432. (15) Rogel, E. Colloids Surf. 1995, 104, 85-93.

10.1021/ef049911a CCC: $27.50 © 2004 American Chemical Society Published on Web 08/19/2004

Aggregated Asphaltene Structural Models

aggregation in a vacuum,16 and in the presence of solvents15,17,18 by classical molecular dynamics (MD).19 An important result is that the structure of aggregates could not be formed by considering only stacking through π-π interactions. Two additional orientations, such as those mentioned by Hunter and Saunders20 and Leach,21 for aromatic-aromatic interactions seem to be necessary. These authors summarized the results of their investigations as follows: main orientations are face-to-face geometry (π-π interactions), edge-on or T-shaped geometry (π-σ interaction), and offset π-stacked geometry (σ-σ interactions). Therefore, it was considered that these orientations can lead to different forms of the asphaltene aggregates.2,16,22 These findings agree with the results reported by Sheu,17 based on molecular simulations for 64 asphaltene molecules, the structures of which range in size from 3 aromatic rings to 11 aromatic rings, for simulations conducted in toluene. He also found that asphaltene aggregates are formed not only through face-to-face stacking but also through other types of asphaltene clustering; such clustering is much looser and rather irregular in appearance. Generally, it is widely accepted that every designed asphaltene molecule includes an aromatic component, its own quantity of heteroatoms (such as N, S, and O), and one or more aliphatic chains linked to the aromatic region.23-26 Some asphaltene molecular models that have the latter description have been proposed in the literature. Four asphaltene structure model molecules were selected from the following references: Groenzin and Mullins,27 Speight and Moschopedis,28,29 Zajac et al.,23 and Murgich et al.18 These molecules are represented in Figures 1 and 2. The corresponding asphaltenes of these references were extracted from Californian, Venezuelan, Mayan, and Venezuelan crude oils, respectively. Some physicochemical properties of these molecules are given in Table 1. It is well-known that petroleum fluids are comprised of asphaltene polydisperse systems. One of the main limits on the asphaltene theoretical analysis lies in the diversity of structures in which asphaltenes can exist. The difficulty associated with the construction of molecular models of asphalt(16) Pacheco-Sańchez, J. H.; Zaragoza, P. I.; Martıńez-Magadań, J. M. Energy Fuels 2003, 17, 1346-1355. (17) Sheu, E. Y. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter IV. (18) Murgich, J.; Rodrı´guez, J.; Aray, Y. Energy Fuels 1996, 10, 6876. (19) Cerius2 software, Molecular Simulations, Inc. (MSI) (now Accelrys), San Diego, CA. (20) Hunter, C. A.; Saunders, J. K. M. J. Am. Chem. Soc. 1990, 112, 2008-2010. (21) Leach, A. R. Molecular Modeling; Addison-Wesley Longman, Ltd.: Singapore, 1996. (22) Takanohashi, T.; Sato, S.; Tanaka, R. Pet. Sci. Technol. 2003, 21, 491-505. (23) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scanning Microsc. 1994, 8, 463-470. (24) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999; Chapter X. (25) Cimino, R.; Correra, S.; Del Bianco, A. In Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Chapter III. (26) Pfeiffer, J. Ph.; Saal, R. N. J. Presented at the Sixteenth Colloid Symposium, Stanford University, Palo Alto, CA, July 6-8, 1939; pp 139-165. (27) Groenzin, H. G.; Mullins, O. C. Energy Fuels 2000, 14, 677684. (28) Speight, J. G.; Moschopedis, S. E. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1979, 24, 910-923. (29) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1991.

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Figure 1. Depiction of two asphaltene molecule structures: one designed by Groenzin and Mullins27 from Californian crude oil, and the other was designed by Speight and Moschopedis28 for Venezuelan crude oil.

Figure 2. Depiction of two asphaltene molecule structures: one designed by Zajac et al.23 from Mayan crude oil, and the other exhibited by Murgich et al.18 for Venezuelan crude oil.

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Pacheco-Sa´ nchez et al.

Table 1. Physicochemical Properties of Each of the Asphaltene Molecules Used in This Work value property

Mullins

Speight

Zajac

Murgich

formula molecular weight (amu) molecular volume (Å3) elemental analysis (%) C H N O S number of fused aromatic rings CA CS HA HS

H98C72S1 995.64 794.6

H79C80N2O1S2 1149.67 892.7

H63C57N1S1 794.2 637.3

H159C138N3O2S2 1955.95 1574.6

86.86 9.92

83.58 7.01 2.44 1.39 5.58 14 47 33 9 70

86.2 8 1.76

84.74 8.19 2.15 1.64 3.28 24 70 68 19 140

3.22 7 30 42 11 87

enes is well-recognized, and the difficulty is even more severe for a polydisperse system of a specific crude oil. However, a reasonable starting guess is that the amorphous solid asphaltene phase is mainly comprised of an average size molecule, as evidenced from experimental work, which varies for each type of crude oil. Zajac et al.23 proposed three asphaltene model molecules for Mayan crude oil; they are built by rearrangement of some aromatic rings or some aliphatic chains for the asphaltene structures they proposed. As a consequence, a polydisperse system can be constructed by a set of monomer asphaltene molecular models in a vacuum, which, as a first approach, should be a good model to simulate asphaltene aggregates, as well as to investigate how they agglomerate in an amorphous solid phase. Self-association of covalent asphaltene model molecules was recently simulated through molecular simulations by Pacheco et al.16,30 The stacked asphaltene molecules were observed in the form of dimers, trimers, tetramers, and pentamers. The morphologies of those aggregates were proposed as a stacking of asphaltenes not only assembled face to face but also in T-shaped and offset orientations, in agreement with the results of Hunter and Saunders20 and Leach.21 Furthermore, using the same Groenzin and Mullins model utilized in this work, the interaction energy between two asphaltene models was calculated by minimizing the energy at different distances, which, indeed, provided the energy as a function of distance. Similar morphologies can be observed in other systems,12 where it is also possible to observe face-to-face (FF), edge-to-face (EF), and edgeto-edge (EE) clay-particle associations. In this work, a method to generate stable asphaltene aggregates is presented; this methodology uses a periodic cell ensemble of 32 asphaltene molecules. To find a stable structure, an optimization process was performed, using a force-field method. Our objective is mainly (i) to find morphologies of asphaltene aggregates that properly describe the experimentally reported S(k) profile, (ii) to analyze the geometries through which the aggregates are formed, and (iii) to study the effect of the aliphatic chains, on the aggregated structure, through the evolution of the structure factor. To this end, a comparison between predicted and experimental structure factors is examined for asphaltene aggregation (30) Pacheco-Sańchez, J. H.; Alvarez-Ramı´rez, F.; Martıńez-Magadań, J. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2003, 48, 7173.

4.04 9 29 28 6 57

modeling, using molecular simulations for a set of one type of asphaltene molecule. 2. Methodology The methodology used is based on force-field concepts; it is an analytical function that is mainly composed of two types of terms. The first terms are associated with bond-interaction energies, such as torsion, bending, and stretching. The second terms are associated with nonbonded interactions, such as Coulombic and van der Waals forces.31-34 Because of the forcefield features, this methodology is dependent on the molecular characterization of the crude oil under study. One objective behind MD is to find a stable molecular configuration where that configuration is located in a local minimum of the potential energy surface around the initial configuration. However, as a consequence of the high molecular weight, the asphaltene motion in a MD simulation is known to be very slow. For this reason, finding the local minimum energy requires hundreds of picoseconds. First, a model system constituted by a blend of 35 molecules composed of equal quantities of five different polynuclear aromatic molecules was set up, as proposed by Diamond.13 These 35 molecules were randomly distributed in a simulation cell using the Amorphous Cell program.35 The cell dimensions (a, b, and c) are all equal to 63.25 Å, and the initial density in the cell is equal to 0.1 g/cm3. Second, a complete cell geometry optimization process was conducted, allowing free movement of molecules, including internal lengths and angles, bringing as a consequence, the effect of a comprised cell with a representative density. The final density for the relaxed structure was 1.39 g/cm3. This value is larger than the usual experimental values.36 This behavior can be explained by the lack of aliphatic chains pendant to the aromatic cores. The COMPASS98_02 force field35 was chosen because it was designed for organic and inorganic molecules, and it has been extensively applied to these types of systems with successful results. An internal stress of 1 × 10-4 GPa was selected as the criterion of convergence. Figure 3 shows the same final structure from two different points of view, highlighting different molecules in each one. The relaxed structure displays the presence of stacking, as expected. Stacked polydisperse domain sheets were formed in the cell; they can be seen in Figure 3a and b, where the (31) Sun, H.; Rigby, D. Spectrochim. Acta A 1997, 53, 1301-1323. (32) Rigby, D.; Sun, H.; Eichinger, B. E. Polym. Int. 1997, 44, 311330. (33) Sun, H.; Ren, P.; Fried, J. R. Comput. Theor. Polym. Sci. 1998, 8 (1-2), 229-246. (34) Sun, H. J. Phys. Chem. B 1998, 102, 7338-7364. (35) Force Field-Based Simulations; MSI, Inc.: San Diego, CA, 1997; pp 29, 265-268. (36) Rogacheva, O. V.; Rimaev, R. N.; Gubaidullin, V. Z.; Khakimov, D. K. Colloid J. USSR 1980, 490-493.



Figure 4. Comparison between the structure factor S(k) obtained in our simulation and those S(k) profiles reported by Yen et al.3 asphaltene models, and the results will be compared against at least one of those curves of the S(k) profile for several asphaltenes reported by Yen et al.3

3. Asphaltene Aggregation Structures

Figure 3. Final structure of 35 aromatic molecules obtained after geometry optimization process. Panels a and b show two angles of different stacking structures of the same final relaxed cell; the macrostructure of asphaltic material representing just the aromatic portion is shown in panel b. Panel c shows a planar representation of Figure 3b, which is a typical Dickie and Yen2 model for aromatic components without considering aliphatic chains. domains are themselves constituted by different types of molecules, which give the polydisperse character to the formed aggregate. The generated aggregates are formed of dimer, trimer, and tetramer domains. This description is consistent with the results found by Dickie and Yen.2 One very important detail, in the present study, is that the face-to-face stacking geometrical orientation, as well as the offset π-stacked geometry, was observed. The positions of every molecule and atom can be determined in the model, and then it is possible to get structural properties as the spherically averaged distribution of interparticle vector lengths (the radial distribution function, RDF) and its inverse Fourier transformation (the structural factor, S(k)) within the model. The structure factor was obtained for this model system, and good agreement was observed between the theoretical and experimental S(k) profile (Figure 4). In this graph, we rescaled the abscissa axis (sin θ)/λ of Yen et al.3 by a value of 4π, such that this axis now is denoted as k, where k ) 4π(sin θ)/λ. The dotted curve in Figure 4 represents the experimental curve of the amorphous blend of five aromatic compounds of known structure, according to those results computed by Diamond.13 He computed the intensity of X-rays diffracted from randomly oriented, perfect aromatic molecules of various sizes, using the Debye RDF. The dashed curve in Figure 4 represents approximately the experimental XRD pattern of the aromatic clusters in the asphaltene computed by Yen et al.3 The solid curve in Figure 4 represents our own simulation of the same amorphous blend of five aromatic compounds of known structure. Therefore, the structural model in Figure 3 can be considered to be a good approximation of the real system. Because of these agreements, this methodology will be extrapolated to different structure cases of the

Using the same methodology as before for aromatic molecules proposed by Yen et al,3 four periodic cells with 32 asphaltene model molecules were built. The cells were built using the four asphaltene molecular models mentioned previously; for simplicity, let us call them Mullins, Speight, Zajac, and Murgich. The minimization of the energy was performed by allowing the relaxation of the cell dimensions to obtain spontaneous selfaggregation of the asphaltene molecules. Asphaltene self-aggregation in crude oil is believed to occur as a spontaneous aggregation, which can be investigated using a geometry optimization process, such as that in this work. An initial density of ∼0.1 g/cm3 was chosen for all the cells. During the optimization process, the equilibrium configuration of the system gradually decreases the cell length, leading to a squeezed cell. The cells that have been built in this way have the following final densities: 0.98 g/cm3 for the Mullins model molecules, 1.04 g/cm3 for the Speight model molecules, 1.02 g/cm3 for the Zajac model molecules, and 0.69 g/cm3 for the Murgich model molecules. 3.1. Mullins Model Case. The final structure of the cell that has been built using the Mullins asphaltene model shows a very small number of stacking formations, compared to that of aromatic molecules. This is attributed to the presence of aliphatic chains, which hinder a close interaction between the aromatic cores of the asphaltenes, as shown in Figure 5a. In particular, both the aliphatic and aromatic regions images were isolated. Figure 5b shows the aliphatic region image, which displays a homogeneous distribution over the cell. The aromatic region image, in Figure 5c, shows two highlighted stacking formations: one in an offset π-stacked geometry, and the other in a faceto-face geometry. The stacking behavior can be explained by the repulsive steric effect of the aliphatic chains, which decreases as the length of these chains diminishes. Asphaltene face-to-face stacking has a moreprobable existence when short aliphatic chains overhang from the aromatic rings.

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Figure 6. Comparison between the asphaltene S(k) profile of our relaxed cell and two aromatic S(k) profiles reported by Yen et al.3

Figure 5. (a) Cell showing the final asphaltene structure of the Mullins model. (b) Isolated aliphatic region of the complete final structure. (c) Isolated aromatic region, where some stacking were highlighted.

For an optimized configuration, the following two parameters of the simulation cell allow us to discuss the diminishing of the stacking. The free-volume percentage in Figure 5a is 34.68%, which means that asphaltene monomers are highly condensed in the cell. The ratio of surface areas between the aromatic region and the aliphatic region is ∼0.41, which means that the aromatic portion is bigger than the aliphatic portion. The small amount of stacking, in this case, is due to the aliphatic chain size and number, with respect to the aromatic core size, which does not allow a close interaction between the aromatic components of the asphaltene. These chains form walls between the asphaltene nearest neighbors, which hinder the direct π-π interaction. The increased separation generated by the presence of the chains is reflected directly in the S(k) profile. This phenomenon can be observed when the S(k) profiles for the aromatic and asphaltene cases are compared. The presence of these chains produces a discrepancy between the aromatic and asphaltene S(k) profiles, which is reflected in the number of peaks and their positions, for k < 2.5 Å-1. It was observed that the first peak (the nearest to zero) for the aromatic case is still present in the asphaltene S(k) profile; however, it is shifted to a k value of 0 in Figure 14. First, the first peak on the aromatic core (AC) of Mullins asphaltenes is the smallest one, whereas the first peak on the AC of Murgich asphaltenes is the largest one, and its second peak is the smallest one among all of the AC asphaltenes. Second, the S(k) profile of Baxterville asphaltene3 includes the major component (38) Relative density (60/60 °F), which was called density by Speight, is the ratio of the mass of a given volume of liquid at 60 °F to the mass of an equal volume of pure water at the same temperature. (39) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999; p 315.

Our methodology for calculating the minimum distance between two asphaltenes in the stacking can give us a reasonable prediction if the asphaltene molecule has a compacted aromatic region that is well-designed, which can be done using the methodology developed by Ruiz-Morales.40 This is due to very good results that have been obtained for the distance of binding energy between two asphaltene molecules on the Speight and Murgich cases; however, although the Zajac case is 9% deviated from experimental values, the Mullins case has a deviation of 15%. These deviations clearly indicate the presence of aliphatic chains, which restrict both the closest interaction distance between two asphaltene molecules and its geometry of stacking. The following comment of Ebert,41 that “only 36% of the aromatic carbon was in stacks of two (‘dimers’) and 64% of the aromatic carbon was not in a stack of any kind (‘amorphous’)” suggests agreement with our observations. Another point is related to the number of aromatic rings in the AC of one asphaltene molecule. RuizMorales40 suggests that asphaltenes present a polyaromatic core size of 1-2 aromatic systems with 4-10 fused rings in each one; according to the Mullins experimental work.27 Rogacheva et al.36 reported asphaltenes with 4-10 fused aromatic rings. Rogel42 has drawn polyaromatic rings with 9-14 fused rings to represent the aromatic moieties of asphaltenes. We used four different asphaltene molecules, with 7, 9, 14, and 24 aromatic rings, following previous models in the literature. We believe that the use of molecular simulations to calculate the structure factor S(k) is a good starting point to improve this design. By cutting aliphatic chains of asphaltenes, just ACs were obtained. These ACs were also energetically optimized, and their structure factors S(k) were obtained, as shown in Figure 15. Figures 8c and 14a show that an asphaltene molecule designed by Mullins with seven fused aromatic rings exhibits at least one 5-aromatic-stack system. As we can also see in Figure 15, (40) Ruiz-Morales, Y. J. Phys. Chem. A 2002, 106, 11238-11308. (41) Ebert, L. B. Fuel Sci. Technol. Int. 1995, 13, 941-944. (42) Rogel, E. Langmuir 2004, 20, 1003-1012.



Figure 15. Comparison between structure factors of aromatic cores (AC) calculated by us (continuous line) and those proposed by Yen et al.3 (dotted and dashed lines).

the AC of Speight asphaltenes with 14 fused aromatic rings gave the best agreement between its second S(k) peak and the first S(k) peak of AC compounds; however, the AC of Mullins asphaltenes is deviated from matching. Second, the S(k) peak of the AC of Zajac asphaltenes with nine aromatic fused rings has a very similar behavior to that of the AC of Mullins asphaltenes. In the case of Murgich asphaltene, we found that the AC of 24 fused aromatic rings has a behavior more similar to asphaltenes than to ACs of asphaltenes (see Figure 13). After this analysis, it can be concluded that the Murgich asphaltene case is only meaningful when aliphatic chains are removed. We must stress that minimization of the energy is independent of the temperature, and a complete study of the stability using entropy and thermodynamic potentials is not possible within the present methodology. At this time, we just want to show that we found a very good matching between this theory and experiment stabilizing asphaltenes by molecular simulations of energy optimization. It must be stated that, knowing the interaction potential, the structure factor S(k) can be approximated from the solution of the Ornstein-Zernicke (O-Z) integral equation and an additional closure relation (PY, MSA, RMSA, HNC, RY, etc.).43,44 The methodology of Henderson-Barker-Abraham and of density functional theory (DFT) can be used to obtain the solution of the O-Z equation.9,45,46 However, these methods are de(43) D’Aguanno, B.; Klein, R. J. Chem. Soc., Faraday Trans. 1991, 87 (3), 379-390. (44) Ortega-Rodrı´guez, A.; Cruz, S. A.; Gil-Villegas, A.; GuevaraRodrı´guez, F.; Lira-Galeana, C. Energy Fuels 2003, 17, 1100-1108. (45) von Gru¨nberg H. H.; Klein R. J. Chem. Phys. 1999, 110 (11), 5421-5431.

pendent on Euclidean geometric forms of the molecule, because of the potential they use, as the hard-sphere intermolecular potential of interaction. This is a gross limitation, because the center of mass is translated, which produces deviations from real systems as asphaltenes are. They usually compare their results against MD simulations, Monte Carlo (MC) calculations, and/or experiments.45 Yen et al.3 used a very clever methodology to compare the XRD experimental pattern for aromatic clusters in petroleum asphaltene with the experimental pattern for a blend of five polynuclear aromatic compounds of known structure by computing the Debye RDF implemented by Diamond,13 which is precisely the structure factor S(k). We calculated a very good approximation of the S(k) profile by optimizing the energy of a 35-membered aromatic system, using the same five aromatic compounds mentioned previously in groups with seven-membered rings. Whether the approved SF profile is our guide to validate generated morphologies, the effects of polydispersity on the SF profile are well-known, as shown in some studies developed by D’Aguanno and Klein.43 In that work, the effects of polydispersity in charged colloidal dispersions are exhibited in a manner in which the SF peaks are displaced, modifying its height and width at different levels of polydispersity. Asphaltenes are known to constitute a polydisperse system; however, in the literature, it is usual procedure to characterize crude oil systems by means of an asphaltene average molecule.18,23,27,28 Because of this restriction on the characterization of the entire polydispersity of the asphaltenic system, as a first approach, we have taken (46) Pacheco-Sańchez, J. H.; Rodrı´guez, A. G. Rev. Inst. Mex. Pet. 1993, 25 (2), 55-60.

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a set of average molecules reported in the literature as the starting point in this study. Finally, our results here can help to improve the experimental methodology for designing asphaltene molecules, which can be useful, as many researchers have shown.14-18,22,30,42,47 6. Conclusions One possible way to determine the morphology of asphaltene aggregates is through modeling and simulating such aggregates. This is because (i) our simulated structure factors (S(k)) of asphaltene aggregates agree with those reported by Yen; (ii) the molecular models of asphaltenes are experimentally designed from extracted crude oils; and (iii) experimentally, it is possible to simply guess about the morphology of the aggregates. We have proposed a procedure for generating asphaltene aggregated structures, based on a minimization of the energy of the system in periodic cells. These periodic cells were constructed using different asphaltene models, such as those devised in the research of Mullins, Speight, Zajac, and Murgich. Each cell was built with one type of asphaltene model in an amorphous arrangement to optimize them. The developed methodology is capable of reproducing the position of the experimental S(k) profile. Important differences were observed, with respect to the AC cases. The presence of a shift in the peak located between 1 Å-1 (47) Rogel, E.; Leoń, O. Energy Fuels 2001, 15, 1077-1086.


and 2 Å-1 is associated with the stacking. We observed stacking clusters when aromatic molecules are used. Based on the similarity of the S(k) profiles, we can conclude similarities in the asphaltene agglomeration of the Zajac, Mullins, and Speight cases with Ragusatype asphaltene, and of the Murgich case with Gilsonitetype asphaltene. Generally, the positions of the main S(k) peaks were correctly reproduced, showing a discrepancy on the second peak region. Such a discrepancy can be attributed to the tail presence in the aromatic sheets that inhibits the π-π, π-σ, and σ-σ interactions, bringing, as a consequence, an increase of the distance between these sheets. The good correlation between the experimental and predicted S(k) profiles would imply that the asphaltene aggregate structure is an adequate model for studying the asphaltene aggregation phenomenon. Therefore, based on the present simulations, we conclude that a nearest-neighbor stacking in face-to-face geometry is not the only possible orientation. The observed asphaltene aggregate structure represents just one of the possible ensemble structures that could exist in the system. The final cells are good models as a first approach for representing asphaltene aggregates as well as the way in which they agglomerate in an amorphous solid phase. In addition, we applied our methodology to the different asphaltene models used but without their aliphatic chains, finding a similar stacking behavior for each case. EF049911A

Morphology of Aggregated Asphaltene Structural Models - Energy

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