Molecular Orientation in Model Asphalts Using ... - ACS Publications

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Energy & Fuels 2007, 21, 1102-1111

Molecular Orientation in Model Asphalts Using Molecular Simulation Liqun Zhang and Michael L. Greenfield* Department of Chemical Engineering, UniVersity of Rhode Island, Kingston, Rhode Island 02881 ReceiVed September 2, 2006. ReVised Manuscript ReceiVed December 4, 2006

Molecular simulations were used to analyze orientations of molecules within model asphalt mixtures. After choosing typical compounds to represent resin, maltene, and asphaltene components, molecular orientations and structures in two ternary asphalt mixtures were studied. The following conclusions are obtained from analyzing simulation results: (1) For nearest asphaltene molecules, orientations between neighboring molecules are affected by molecule structure and temperature. At high temperature, asphaltene molecules with long alkane branches prefer to pack almost parallel; at low temperatures, they prefer to pack almost perpendicular; at intermediate temperatures, they have a peak at around 40°. Highly aromatic asphaltene molecules prefer to stay almost parallel to each other at low temperatures and almost perpendicular at high temperatures. (2) Average orientation between neighboring molecules depends on distance between molecule centers of mass. Naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene pack randomly over large center of mass distances, while at short distances, they pack almost parallel. (3) Deviations from planarity for the pure compounds naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene are in the range of 0-20°, as are deviations for asphaltene molecules in ternary mixtures. The deviation increases for rings that include heteroatoms. (4) Different initial conditions indicate two alternatives for the presence and position of the first peak in the center of mass radial distribution function, while small distance intermolecular orientations remain almost the same.

Introduction Asphalts are mixtures composed of many kinds of organic compounds, with some estimates suggesting approximately 105106 different molecule types.1 Asphalts mainly come from crude oil distillation and are widely used on road pavement.2 On the basis of its solubility in different solvents, an asphalt can be separated into three parts: asphaltene, resin, and maltene. Here, asphaltene is the most viscous and most polar part and maltene is the least viscous and most nonpolar part, while resin is in between. This work is the first publication from a larger project in which molecular simulation is used to develop molecular-scale mixtures that are capable of representing the chemical, physical, and mechanical properties of asphalt. The overall project objective is to create a computational platform for describing the structure-function relationships in asphalts and modified asphalts at the molecular level. We hypothesize that increased understanding of molecular asphalt modification mechanisms and microstructures can provide guidance for further improved asphalts for use in road construction. This paper addresses the packing among asphaltene, resin, and maltene molecules relative to each other in asphalt, motivated by how different arrangements between neighboring large molecules can affect chemical, physical, and mechanical properties. The role of molecular simulation is to supply detailed nanoscale packing information, within the limits posed by a computer model of asphalt. * Author to whom correspondence should be addressed. (1) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201210. (2) Roberts, F. L.; Kandhal, P. S.; Brown, E. R.; Lee, D.; Kennedy, T. W. Hot Mix Asphalt Materials, Mixture Design, and Construction, 2nd ed.; National Asphalt Pavement Association: Lanham, MD, 1996; Chapter 2.

Several ideas concerning intermolecular structure of component molecules of asphalts and their small molecule analogs are found in the literature. On the basis of liquid theories, Lowden and Chandler3 predicted that benzene nearest neighbors pack in a T configuration due to repulsive interactions. Narten4 confirmed Chandler’s prediction with experimental evidence, using X-ray diffraction data for liquid benzene at 25 °C to analyze scattering factors for C-H groups rather than for C and H atoms. Evans5 used the standard Monte Carlo method together with a six-center Lennard-Jones potential to calculate the structures of liquid benzene and obtained the same results. He found that influence of temperature on the structures was small in the temperature range he studied (298-328 K). Asphaltenes have many aromatic rings and are known to aggregate in hydrocarbon solvents.6 Simulations predict that asphaltenes aggregate in vacuum and in hydrocarbon solvents.7 Brandt et al.8 used thermodynamic modeling to predict that asphaltenes stack in very good solvents, driven by an entropic excluded-volume effect for flat objects. They also found that the maximum number of aggregated asphaltene molecules was five. Pacheco-Sańchez et al. found that the asphaltene aggregation number decreased with increasing temperature9 and that aggregate orientation mostly depends on the crude oil characteristics.10 (3) Lowden, J. L.; Chandler, D. J. Chem. Phys. 1974, 61, 5228-5241. (4) Narten, A. H. J. Chem. Phys. 1977, 67, 2102-2108. (5) Evans, D. J.; Watts, R. O. Mol. Phys. 1976, 32, 93-100. (6) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1999. (7) Rogel, E. Colloid Surf. A 1995, 104, 85-93. (8) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. J. Phys. Chem. 1995, 99, 10430-10432. (9) Pacheo-Sańchez, J. H.; Zaragoza, I. P.; Martıńez-Magadań, J. M. Energy Fuels 2003, 17, 1346-1355.

10.1021/ef060449z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007

Molecular Orientation in Model Asphalts

de Leeuw et al. found that for systems including polar and nonpolar molecules, polar molecules decrease their orientational ordering at increased concentration, in the absence of induced dipoles.11,12 So for a system having not only asphaltene, but also resin and saturate, the size and lifetime of aggregates depends on a delicate balance among the different intermolecular forces, including the polarity of aromatic compounds in the system and the polarity of saturate.13 Some explicit analyses of structure within asphaltene aggregates have been reported. Yen et al.14 found that the interlayer distance between aggregated asphaltene molecules is 3.5-3.8 Å and the distance for short-range bonding is 0.5-20 Å, using X-ray diffraction to investigate the structure of petroleum asphaltenes. Several studies analyzed the arrangements of amorphous asphaltene aggregates using the radial distribution function (RDF). Pacheco-Sańchez et al.10 used molecular dynamics to calculate the RDF for four kinds of asphaltene models and compared their results with experimental data. After analyzing the aggregation of asphaltene molecules under vacuum at different temperatures, they observed three kinds of aggregate orientations: face-to-face geometry, edge-on or T-shaped geometry, and offset π-stacked geometry; these are consistent with geometries suggested previously for porphyrins.15 PachecoSańchez et al. cited a claim by Sheu16 that asphaltene aggregates not packing face-to-face are much looser and rather irregular in appearance. When using model average asphaltene molecules, the offset π-stacked was the most common orientation between aggregates.10 Alvarez-Ramirez et al.17 found the preferred configuration for asphaltene and resin self- and cross-interactions is a face-to-face orientation using density functional theory, for isolated molecules at zero temperature (i.e., energy minimum). This paper focuses on describing orientation among asphaltene and naphthene aromatic molecules in two different asphaltlike ternary mixtures at five different temperatures. The deviations from planarity (intramolecular orientation), radial distribution function (RDF), and angles between aromatic compounds (intermolecular orientation) are calculated via molecular dynamics simulations. Orientations for four smaller aromatic compounds are also computed and analyzed as a reference case. In addition to analyzing average geometries between asphaltene molecules, work described here investigates packing among molecules that constitute asphalt. Simulation Methods Asphalt components can be classified based on solubility and chromatography into groups such as asphaltene, resin, and maltene, with further subdivisions into groups such as polar and naphthene aromatics.2 For this work, we represent three groups in the model asphalt mixture in the molecular simulations using one molecule each. Wiehe et al.1 have suggested that there are more than 105 (10) Pacheo-Sańchez, J. H.; A Ä lvarez-Ramı´rez, F.; Martıńez-Magadań, J. M. Energy Fuels 2004, 18, 1676-1686. (11) de Leeuw, S. W.; Smit, B.; Williams, C. P. J. Chem. Phys. 1990, 93, 2704-2714. (12) Mooij, G. C. A.; de Leeuw, S. W.; Smit, B.; Williams, C. P. J. Chem. Phys. 1992, 97, 5113-5120. (13) Murgich, J.; Rodrı´guez, J.; Aray, Y. Energy Fuels 1996, 10, 6876. (14) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33, 1587-1594. (15) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1999, 112, 5525-5534. (16) Sheu, E. Y. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter IV. (17) Alvarez-Ramirez, F.; Ramirez-Jaramillo, E.; Ruiz-Morales, Y. Energy Fuels 2005, 20, 195-204.

Energy & Fuels, Vol. 21, No. 2, 2007 1103

Figure 1. Asphaltene1 molecular structure.21 Small letters indicate the label of atoms used to calculate orientations.

Figure 2. Asphaltene2 molecular structure.22 Small letters indicate the label of atoms used to calculate orientations.

chemically distinct asphaltene molecules. Dozens of model asphaltene structures and “average” asphalt molecules have been recommended7,13,18-26 in the literature. Computation limitations prevent trying all those proposed structures, so here, we chose two alternate proposed compounds21,22 to represent asphaltene molecules. We label these as asphaltene121 and asphaltene222 and illustrate their molecule structures in Figures 1 and 2. Their sizes are consistent with the lower molecular weights found for many (but not all) asphaltene compounds after long dissociation times in dilute solution.27 The asphaltene1 molecule is composed of many fused aromatic rings with short branches (methyl to propyl), while asphaltene2 has longer aliphatic chains surrounding a core of fewer aromatic rings. They represent two typical structures and enable comparing the effects of short vs long pendent alkyl substituents. 1,7-Dimethylnaphthalene and linear n-C22 were chosen as the resin and maltene parts, respectively, at concentrations similar to those of saturates and naphthene aromatics represented in the literature.19,28,29 Thus, we have two ternary mixtures to represent asphalts. The overall composition of the mixture is guided by the measurements of Storm and co-workers.29 In the asphaltene1 mixture, 5 asphaltene1, 27 1,7-dimethylnaphthalene, and 41 n-C22 molecules were chosen. The asphaltene2 mixture contained 5 asphaltene2, 30 1,7-dimethylnaphthalene, and 45 n-C22 molecules. Both mixtures thus are 21% asphaltene and 59-60% saturate; for detailed (18) Jennings, P. W.; Pribanic, J. A.; Desando, M. A.; Raub, M. F.; Stewart, F.; Hoberg, J.; Moats, R.; Smith, J. A.; Mendes, T. M.; McGrane, M.; Fanconi, B.; VanderHart, D. L.; Manders, W. F. Binder Characterization and EValuation by Nuclear Magnetic Resonance Spectroscopy; SHRP-A335, Strategic Highway Research Program: Washington, DC, 1993. (19) Kowalewski, I.; Vandenbroucke, M.; Huc, A. Y.; Taylor, M. J.; Faulon, J. L. Energy Fuels 1996, 10, 97-107. (20) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 278-286. (21) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287-296. (22) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677-684. (23) Rogel, E.; Carbognani, L. Energy Fuels 2003, 17, 378-386. (24) Gray, M. R. Energy Fuels 2003, 17, 1566-1569. (25) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377-1384. (26) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227-1234. (27) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809822. (28) Storm, D. A.; DeCanio, S. J. Fuel 1990, 69, 735-738. (29) Storm, D. A.; Edwards, J. C. Energy Fuels 1994, 8, 561-566.

1104 Energy & Fuels, Vol. 21, No. 2, 2007 information about the systems, see refs 30 and 31. In order to interpret the orientation results in asphaltene mixtures, we also simulated small pure aromatic compounds: naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene. In these pure systems, 343 molecules were always used. An explicit atom representation of each molecule was used in the simulations. Parameters were taken from the OPLS-aa force field.32,33 Sulfur parameters were chosen based on similarities to furan and pyrrole. Each aromatic ring is maintained close to planar using Amber-style improper torsion angles. Two common molecular simulation approaches were applied: Monte Carlo (MC) and molecular dynamics (MD), using the programs Towhee (versions 3.12.x and 4.6.x)34,35 and Lammps (version 2001).36,37 The Towhee program was modified to enable continuation of the random number generator sequence when simulations were continued. “Diagnostics” were added to both programs to enable run-time and postprocessing calculations of the molecular orientations described here. To confirm that those programs and force field could lead to sufficiently accurate simulation results compared to experimental data, temperature-density simulations were done on individual compounds that have similar aromatic structure as the components in asphalt. Those tests are described in detail in refs 30 and 31. The temperature-density results provide us with confidence that the OPLS-aa force field leads to reasonable results for the temperaturedependent density of such aromatics. For original ternary mixture simulations, MC was first used to run 90 000 and 99 000 cycles for the asphaltene1- and asphaltene2based mixtures, respectively. Then MD was used for at least 1500 ps of equilibration and then for another 3000 ps of sampling to obtain atom position results for calculating orientation. Independent intermolecular arrangements, based on a cubic lattice and different random number seeds, were used to initiate the simulations at each temperature. Periodic boundary conditions were used in all simulations. The time step equaled 0.5 fs. The Nose-Hoover method38 was used to maintain constant temperature and pressure. The cutoff distance in Monte Carlo for interaction was 10 Å; in molecular dynamics, the Charmm spline brought the potential smoothly to zero between 10 and 10.10 Å. Molecular Orientation. One goal of the simulations was to learn how different molecules in asphalt pack relative to each other. Intermolecular orientation and intramolecular orientation provide such kinds of measures. Intermolecular orientation refers to angles between planar regions of different molecules. Intramolecular orientation refers to angles between planes within a single molecule. In the calculation, we confine the inter- and intramolecular orientation results to be in the range of 0-90°. Each orientation is calculated from the dot product of normal vectors defined by a portion of the molecule. In-plane vectors are obtained from the positions of two atoms. Those vectors are defined as shown in Figure 3, using 1,7-dimethylnaphthalene as an example. Each aromatic compound contains more than one aromatic ring, so intramolecular orientation from two rings can be calculated. From the positions of carbon atoms a, b, c, and d, we obtained vector ab B (30) Zhang, L.; Greenfield, M. L. submitted to Energy Fuels. (31) Greenfield, M. L.; Zhang, L. DeVeloping Model Asphalt Systems using Molecular Simulation; University of Rhode Island Transportation Center: Kingston, RI, 2007; to be available on-line via http:// www.uritc.uri.edu. (32) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225-11236. (33) McDonald, N. A.; Jorgensen, W. L. J. Phys. Chem. B. 1998, 102, 8049-8059. (34) Martin, M. G. MCCCS Towhee home page, Sandia National Laboratories. http://towhee.sourceforge.net (accessed Aug 1, 2006). (35) Martin, M. G.; Thompson, A. P. Fluid Phase Equilib. 2004, 217, 105-110. (36) Plimpton, S. J. LAMMPS Molecular Dynamics Simulator home page, Sandia National Laboratories. http://www.cs.sandia.gov/∼sjplimp/ lammps.html (accessed Aug 1, 2006). (37) Plimpton, S. J. J. Comput. Phys. 1995, 117, 1-19. (38) Frenkel, D.; Smit, B. Understanding Molecular Simulation, 2nd ed.; Academic Press: New York, 1996.

Zhang and Greenfield

Figure 3. Molecular structure of 1,7-dimethylnaphthalene, showing how the vectors in a molecule and the orientation of a ring are defined. Table 1. Atom Labels Used in Inter- and Intramolecular Orientation Calculations

and vector dc B. From the cross product of those two nonparallel vectors, we obtained the normal vector 1, which is the normal vector of ring a-b-c-d and is used to represent the ring; similarly, we can calculate vector 2. Vectors bf B and gc b lead to vector 3. For all small aromatic compounds, vector 3 is used to represent the orientation of the molecule. The angle between two different vectors 3 is the intermolecular orientation, and the angle between vector 1 and vector 2 is the intramolecular orientation. Atoms listed in Table 1 and labeled in Figures 1 and 2 are used to calculate inter- and intramolecular orientations for asphaltene1 and asphaltene2. Radial Distribution Function. The radial distribution function (RDF) g(r) gives the probability of the occurrence of a particle a at a distance r from particle b, compared to the average number density. Peaks in g(r) vs r plots can be associated with specific neighbors and can describe the microscale properties of a system. Molecular Orientation Probability Density. The angle between aromatic rings ranges from 0-90°. We divided this range into bins of width 0.05°. For each kind of molecular orientation, the probability density equals the ratio of the number of molecules having their orientation in each bin to the total possible number of orientations, divided by the bin width. The integral over all angles is thus normalized to unity. For distances, a bin width of 0.1 Å is used, if not indicated otherwise. For a mixture, we can calculate the intermolecular orientation between similar and different molecules and the intramolecular orientation within each kind of molecule. This leads to multiple corresponding orientation probability densities. A totally random orientation probability density follows the function sin(θ). If the orientation density curve is very close to sin(θ), then there are few correlations in orientation. Orientation was calculated in a limited range up to a center of mass separation distance d. To choose this distance, the RDF was analyzed based on the center of mass and the nearest neighbor peak was identified. We use d0 to represent the end position of the first peak. When orientation sampling was taken, d equaled 15 Å if not stated otherwise. Check on Sampling by Repeating Simulations. For asphaltene-solvent systems, Rogel7 found that simulations using different initial conditions and thus different temporal evolution of the system could lead to different extents of aggregate formation from asphaltene monomers. Different initial conditions could also lead to different interaction forces between molecules, which changed the extent of aggregate formation. These indicate the risk of insufficient sampling in simulations of asphaltenes. In our system, we mixed asphaltene molecules with naphthene aromatic and saturate. To check the sampling, we used different initial positions and velocities to rerun several cases: the asphaltene1 mixture at 298.15 and 358.15 K and the asphaltene2 mixture at 443.15 K. The label B represents repeated simulations to distinguish them from the initial results, labeled simulation A.



Figure 4. Intramolecular orientation probability density: (a) four pure compounds (s) naphthalene at 358.15 K, (- -) 1-methylnaphthalene at 343 K, (-‚) 2-methylnaphthalene at 313 K, (‚‚‚) 1,7-dimethylnaphthalene at 298.15 K; (b) naphthalene at two different temperatures; (c) asphaltene1 at five different temperatures; (d) 1,7-dimethylnaphthalene at five different temperatures in the asphaltene1 mixture; (e) asphaltene2 molecules at five different temperatures; (f) 1,7-dimethylnaphthalene in the asphaltene2 mixture at (s) 443.15, (- -) 358.15, (‚-) 298.15, (- - ‚) 268.15, (‚‚‚) 238.15 K.

Results Intramolecular Orientation. Results for intramolecular orientations are shown in Figure 4a for pure compounds naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene. These compounds are smaller and less polar than resin in asphalt and differ in the number and location of methyl groups attached to the aromatic rings. Results at two different temperatures for naphthalene are shown in Figure 4b, with one temperature below the melting point (Tmelt ) 353.39 K) and another above Tmelt. At a higher temperature, the naphthalene intramolecular orientation probability density distribution becomes wider and lower because the vibration movement of atoms becomes more intense and the angle between two fused rings spans a wider range. Over this temperature range for three other compounds, we can see they have very similar intramolecular orientation densities to naphthalene. For two fused rings, an intramolecular angle of zero minimizes improper torsion contribution to the potential energy. Our simulation results suggest that molecules balance away from this lowest energy state. Thus, the OPLS-aa force field leads to a prediction that small aromatic compounds deviate from planarity by angles of 0-20°.

For 1,7-dimethylnaphthalene, asphaltene1, and asphaltene2 in ternary mixtures, the adjacent rings have a small folding angle, as shown in Figure 4c-f. At 443.15 K, 1,7-dimethylnaphthalene has the widest intramolecular orientation probability density, which is consistent with results in a pure system, and its range remains at 0-20° as shown in Figure 4d. For asphaltenes, two types of ring choices are shown in Figure 4. Ring a-b-c-d chosen for calculating intramolecular orientation in asphaltene1 includes a sulfur heteroatom, which has a stronger interaction on the second ring than a carbon atom does. Its intramolecular orientation always has a distribution of 0-30°, which is bigger compared to 1,7-dimethylnaphthalene and asphaltene2 in mixtures, as shown in Figure 4. Thus, the rings are distorted and have a larger deviation from planarity. For asphaltene2, the sulfur heteroatom is far away from both rings chosen, so the angle between rings is similar to that in 1,7dimethylnaphthalene, as shown in Figure 4e. These tendencies are related to the presence or absence of sulfur in the aromatic ring. When rings not including sulfur atoms are chosen for asphaltene1, the resulting intramolecular orientation distribution is similar to that of 1,7-dimethylnaphthalene and asphaltene2 (figure not shown). Choosing a sulfur-containing ring for

1106 Energy & Fuels, Vol. 21, No. 2, 2007


Figure 5. Intermolecular orientation probability density changes with intermolecular distance d for naphthalene at 358.15 K. The inset is the g(r) based on center of mass for naphthalene at 358.15 K; circles are the positions of d chosen in the main graph.

Figure 6. Intermolecular orientation probability density of 1,7-dimethylnaphthalene in asphaltene1-based ternary mixtures at five different temperatures: (a) d ) 5.0; (b) d ) 6.0; (c) d ) 15; (d) d ) 15 Å in an asphaltene2-based ternary mixture; (s) T ) 443.15; (- -) T ) 358.15; (‚-) T ) 298.15; (- - ‚) T ) 268.15; (‚‚‚) T ) 238.15 K; (×-×) sin(θ).

asphaltene2 leads to a distribution as broad as that for asphaltene1 (not shown). Intramolecular orientation for both kinds of asphaltene molecules is less sensitive to temperature than for 1,7-dimethylnaphthalene. We hypothesize this is because big molecules like asphaltene have many aromatic rings and stronger interactions within the molecules, so temperature changes influence them less. We conclude that for small aromatic compounds, both neat and in asphaltlike mixtures, the angle between two fused rings is 0-20°. For rings containing heteroatoms, the intramolecular orientation has a wider range. Intermolecular Orientation. In a liquid, each molecule directly affects the orientation of its neighbors over a limited distance range. Indirect effects persist beyond nearest neighbors but eventually decay to zero. The smaller the center of mass separation distance d considered, the more the orientation results focus on the nearest neighbors. The maximum influence of the

value of d on the intermolecular orientation probability density is shown in Figure 5. For naphthalene at 358.15 K, molecules at a very close distance (d ) 4.1 Å) prefer to pack parallel. At increased separations, the intermolecular orientation distribution becomes closer to random. Small Molecules. The intermolecular orientation probability density for naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene follows the sin(θ) distribution when intermolecular distances bigger than half of the box length (15-20 Å) are used, indicating random orientation. When using small d, for example 4.1 Å for naphthalene, we see a peak at around 10°. All those small compounds have similar intermolecular orientation probability density relationships with intermolecular distance to that for naphthalene shown in Figure 5. For molecules having multiple aromatic rings and their accompanying π-π interactions, nearest neighbors stay close to being parallel to each other. Small aromatic compounds



Figure 7. Radial distribution function for distances between asphaltene1 molecules in the asphaltene1 mixture at five different temperatures. The inset compares results from two independent simulations at 358.15 K.

Figure 8. Intermolecular orientation at d0 for asphaltene1 in a ternary mixture. The inset compares the intermolecular orientations at d ) 15 Å for asphaltene1 molecules in two independent simulations at T ) 358.15 K.

with only one ring, like benzene, prefer to stay perpendicular to each other3-5 because that kind of packing helps minimize repulsive interactions between molecules. Ternary Model Asphalt Mixtures. In order to see how molecular orientation in the system varies with temperature, five different temperatures (238.15, 268.15, 298.15, 358.15, 443.15 K) were chosen to simulate both ternary mixtures. The RDF based on the molecule center of mass reflects the distribution of molecules in the system. Its result can help us decide the end position of the first peak (d0), which is used to calculate the close distance orientation probability density. Because only 10 asphaltene-asphaltene pairs were available, some RDF and orientation curves are not very smooth. In ternary mixtures, 1,7-dimethylnaphthalene behaved almost the same as in the pure compound. Using an intermolecular distance of 5 Å for 1,7-dimethylnaphthalene molecules reveals that closely neighboring molecules pack closer to parallel than would be predicted by the random distribution, as shown in Figure 6a. It packed more randomly when the center of mass separation distance is 6.0 Å, as shown in Figure 6b. When using d ) 15 Å, the higher the temperature, the closer its curve is to random, as shown in Figure 6c. This is consistent with the random packing (i.e, g(r) ) 1) expected, based on Figure 5. In asphaltene2 ternary mixtures, 1,7-dimethylnaphthalene has its intermolecular orientation probability density very close to random when using d ) 15 Å, as shown in Figure 6d; these results are almost the same as those in asphaltene1-based ternary mixtures, shown in 6c. Figure 7 shows the RDF among asphaltene1 molecules at five different temperatures. We can see that the end positions of the first peak (d0, listed in Table 2) changed with temperature.

Table 2. Temperature and First Peak End Position d0 for Asphaltene1 and Asphaltene2 asphaltene1, d0 (Å) temp (K)

simulation A

238.15 268.15 298.15 358.15 443.15

10.9 7.1 8.1 5.8 4.3

simulation B

12.8 6.2

asphaltene2, d0 (Å) simulation A

simulation B

9.6 12.3 8.6 8.6 9.3

8.3

At 443.15 K, the first peak position is near 3.5 Å, which is consistent with Yen’s model for interlayer distance in aggregated asphaltene (3.5-3.8 Å).14 The simulations thus suggest asphaltene aggregation at 443.15 K. But, the first peak is not well distinguished from its remaining peaks, as shown in Figure 7, so asphaltene1 aggregate exists but not exclusively. At 358.15 K, its first peak position is at 5.0 Å, which is a little bigger than Yen’s result. The large diameter of the asphaltene1 molecule, 10-20 Å,21 and its high concentration density at 5 Å in g(r), which shows a clear gap between the first and second peaks, suggests that an aggregate exists. At all other temperatures, their first peak positions are even larger than Yen’s results and their first peaks are not as well distinguished as in the T ) 358.15 K case. Simulation results thus suggest asphaltene1 as being more dispersed than aggregated at these temperatures. Figure 8 illustrates the intermolecular orientation probability density of asphaltene1-based mixtures using the temperaturedependent d0 values. At low temperatures, asphaltene1 molecules stayed almost parallel to each other; almost all neighboring pairs are at angles less than 30°. Near-parallel orientation can correspond to a face-to-face geometry, as shown in Figure


Figure 9. Parallel packing between asphaltene1 molecules.

Figure 10. Offset π-stacked packing between asphaltene1 molecules.

Figure 11. T-shaped packing between asphaltene1 molecules.

9, or offset π-stacking, as shown in Figure 10. At 358.15 K, the orientation probability density for asphaltene1 has a broad peak around 40°. At 443.15 K, the probability density is low before 50° and then increases. Neighboring molecules maintain larger intermolecular orientation than at low temperatures and stay nearer to being T-shaped than parallel for the closest molecules. An example of the T-shaped geometry is visualized in Figure 11. The RDF after long-time evolution for asphaltene2-based mixtures is shown in Figure 12. The end positions of the first peak in the RDF are listed in Table 2. We can see that at all five temperatures the first peak positions are much further than the 3.5-3.8 Å nearest layer spacing in Yen’s model. This suggests that asphaltene2 molecules have not formed aggregates. Considering the molecule structure of asphaltene2, it has longer alkyl branches that may prevent the smaller layer separation accessible in the asphaltene1 case. The intermolecular orientation probability density for asphaltene2 based on distances d0 varies with temperature, as shown in Figure 13. At 443.15 K, asphaltene2 molecules stay nearly parallel to each other in simulation A. At 298.15 and 358.15 K, the closest pairs have a peak in probability density around 40° in the long simulation evolution. At 268.15 and 238.15 K, their tendency is closer to a T-shaped orientation. Asphaltene and Naphthene Aromatic. In both kinds of ternary mixtures, we analyzed intermolecular orientation between


asphaltene and 1,7-dimethylnaphthalene molecules and found that, at small intermolecular distances, they packed in parallel; at long distances, they packed randomly in the system. The relationship between intermolecular distance and intermolecular orientation is shown in Figure 14 and is similar to that of naphthalene (Figure 5). In contrast to naphthalene, its orientation correlations persist to a longer distance. Repeated Simulations. Some cases were repeated to determine if other orientation effects would be observed. The RDF changed compared to the initial case, and the nearest neighbor positions were identified the same way as earlier. Asphaltene1 at 298.15 K. For asphaltene1 at 298.15 K, the first peak end position, d0 ) 12.8 Å, is 50% larger than in the first case. Using the new d0 and bin size 2 Å to calculate the intermolecular orientation probability density for asphaltene1 and 1,7-dimethylnaphthalene led to the results shown in Figure 15a. Although the asphaltene1 molecules are more separated in this case, indicating a lower probability of aggregation, they prefer to reside almost parallel. The intermolecular orientation probability density of 1,7-dimethylnaphthalene molecules remains essentially unchanged, while the RDF (not shown) had slight changes in shape. Asphaltene1 at 358.15 K. The RDF for the asphaltene1 mixture at 358.15 K differed in the second case by shifting its first peak position from 5 Å to around 3.8 Å, as shown in the inset curve in Figure 7. The first peak begins at a position within the range of 3.5-3.8 Å used in Yen’s model.14 Simulation B thus provides further support for asphaltene1 aggregates existing. After using its corresponding d0 ) 6.2 Å, the intermolecular orientation probability density for asphaltene1 molecules stayed almost the same, with peaks shifting from around 40° to around 30°, as shown in Figure 15b. Its intermolecular orientation probability density, using d ) 15 Å, is compared with simulation A in the inset of Figure 8. Packing among 1,7-dimethylnaphthalene molecules stayed close to a random orientation when using the new d0, as shown in Figure 15b. Asphaltene2 at 443.15 K. For asphaltene2 at 443.15 K, the two cases differed significantly in the nature of peaks below 10 Å. Here, the second simulation led to d0 ) 8.3 Å, but at a much smaller concentration, as shown in the inset of Figure 12. The corresponding intermolecular orientation of asphaltene2 is shown in Figure 15c. Asphaltene2 molecules in both cases pack almost parallel at short distances despite the differences in g(r). The intermolecular orientation probability density comparison showed differences between simulation A and B when using d ) 15 Å, as shown in the inset of Figure 13. As in the asphaltene1 case, the packing of 1,7-dimethylnaphthalene molecules did not change. Discussion Comparing intermolecular orientation for neat naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene, all of them prefer to pack randomly. The inferred packing is related to intermolecular distance. Over small enough intermolecular distances, packing among those small aromatic compounds is close to parallel; over larger distances, the packing is more close to random. On the basis of their results, Pacheco-Sańchez et al.10 compared aggregation in simulations of asphaltenes that differed in molecular structure. Asphaltene2 molecules should belong to the Mullins model case, and asphaltene1 should be close to a Murgich model asphaltene (those without aliphatic branches). Using a geometry optimization process in their molecular simulation without explicitly considering the influence of



Figure 12. Radial distribution function of asphaltene2 in a ternary mixture at different temperatures. The inset compares the results from two independent simulations at 443.15 K.

Figure 13. Intermolecular orientation among asphaltene2 molecules in a ternary mixture. The inset compares the results from two independent simulations at 443.15 K and d ) 15 Å.

Figure 14. Intermolecular orientation probability density between different molecule pairs at different d0 values for asphaltene2 and dimethylnaphthalene at 443.15 K. The inset curve is the g(r) at 443.15 K for asphaltene2 and dimethylnaphthalene; the circles are the positions of d used in the main graph.

temperature changing the orientation result, Pacheco-Sańchez et al.10 suggested a possibility of asphaltene aggregation inside systems similar to those studied here. Our RDF results for both kinds of ternary mixtures at five different temperatures showed that only asphaltene1 at 443.15 and 358.15 K had clear asphaltene aggregation. That kind of difference may come from different systems used. They used a vacuum system while our system not only has asphaltene but also naphthene aromatic and saturate, with a total C/H composition chosen based on literature data.29 Because of the delicate balance of interaction force between asphaltene and naphthene aromatic,13 which could

change with temperature and asphaltene molecule structure, there was no aggregate forming in our systems except in the asphaltene1 mixture at 443.15 and 358.15 K. The packing of asphaltene aggregates is not strictly parallel or perpendicular. It is irregular and loose in appearance,16 so its orientation result is not strictly 0° or 90°, even reaching 40° in long simulation evolution as shown in Figure 8. Pacheco-Sańchez et al. also pointed out that, in vacuum or solvents, aggregates should be in an offset π-stacked geometry. Visualization in their paper9 illustrates geometries corresponding to a near parallel intermolecular orientation. In their temperature


Figure 15. Comparison between two independent simulations of intermolecular orientation in (a) asphaltene1 mixtures at 298.15 K; (b) asphaltene1 mixtures at 358.15 K; (c) asphaltene2 mixtures at 443.15 K: (- -) simulation A, asphaltene1 or asphaltene2; (‚‚‚) simulation A, dimethylnaphthalene; (s) simulation B, asphaltene1 or asphaltene2; (‚-) simulation B, dimethylnaphthalene; (×-×) sin(θ).

ranges for an asphaltene2-like model, our results suggest a temperature influence on this local packing among asphaltenes. At lower temperatures, it packed nearly T-shaped; at 298.15 and 358.15 K, it had a peak around 40°; at 443.15 K, it packed almost parallel. Intramolecular orientation shows differences in our systems compared to those of Pacheco-Sańchez et al., which may relate to the differences in aggregation. In our systems, asphaltene2 remained nearly planar; in their system, visualizations show wide deviations from planarity, which allow for locally parallel orientation but lead to highly bent asphaltene molecules. Those results are related to the interactions between aromatic molecules in the system. Pacheco-Sańchez et al.9 suggested that the main orientations between asphaltene aggregates could be face-to-face because of π-π repulsive interactions, could be edge-on or T-shaped because of π-θ attractive interactions, or could be offset π-stacked because of θ-θ attractive interactions. In our system, the orientation between asphaltenes can be influenced by interactions with naphthene aromatics, not just interactions between asphaltenes. Since we only consider nearest-neighbor packing by our choice of d0, most of the interactions still come from asphaltene molecules, with some


coming from aromatic 1,7-dimethylnaphthalene, which leads to intermediate orientations such as peaks at 40°. The asphaltene1 molecule is mainly composed of aromatic rings with heteroatoms and short branches. Because of π-π repulsive interaction between aromatic rings, it prefers to stay parallel to each other at short distance, although long-range packing can deviate from that. At very high-temperature (443.15K), asphaltene1 molecules have much stronger mobility, so nearest neighbors changed into T-shapes to decrease the repulsive force between aromatic rings, as shown in Figure 8. The distance between closest pairs became very short, as shown in Figure 7. At 358.15 K, it has a tendency toward T-shaped packing, compared to that at lower temperatures. So its intermolecular orientation probability density has a peak around 40°. Asphaltene2 molecules have π-π interaction from aromatic rings, which includes asphaltene2-to-asphaltene2 and naphthene aromatic-to-asphaltene2, π-θ interactions between the aromatic ring and aliphatic chains, and θ-θ interaction between aliphatic chains. Under a combination of influences of those three kinds of interactions, the close distance intermolecular orientation probability density for asphaltene2 molecules changed with temperature. Nearest-neighbor molecules preferred to pack almost parallel at high temperatures because of π-π and θ-θ interactions, to pack perpendicular at low temperatures because of π-θ interaction, and to pack at around 40° at intermediate temperatures, as shown in Figure 13. Its longer branches led to a larger separation between neighboring centers of mass, as shown in Figure 12. Steric effects from those side chains likely prevent the close layering inherent in Yen’s model. In analyzing our repeated simulations, we found two possibilities for the center of mass RDF: having a well-distinguished first peak in g(r) or not. In all three cases, our initial and repeat simulations indicate both situations. This is consistent with the differences found by Rogel7 in simulations and suggests that a larger system size is required in order to have all aggregation conditions occur simultaneously in one simulation cell. Despite the difference in distance between neighboring molecule centers of mass in simulations A and B, intermolecular orientations for both kinds of asphaltene molecules did not change significantly with closest separations, which means the interaction force between nearest-neighbor molecules were similar at specific temperatures. This is particularly interesting since the intermolecular orientation probability density distribution is very sensitive to the d0 value chosen, which is different in each set of simulations A and B. Asphaltene1 at 298.15 K and asphaltene2 at 443.15 K still pack almost parallel in both A and B. Asphaltene1 at 358.15 K still has a peak at around 40°. Conclusions From intramolecular orientation calculation in molecular simulations of naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, and 1,7-dimethylnaphthalene, we find that the most probable angle between two fused aromatic rings in a single molecule is 0-20° instead of 0°. In ternary mixtures, 1,7dimethylnaphthalene molecules kept almost the same intramolecular orientation density as in the pure compound. When heteroatoms occur in rings chosen for orientation calculation, a wider intramolecular orientation distribution range than in small aromatic compounds or non-heteroatom-containing rings of asphaltenes is observed. The intramolecular orientation probability density became wider and lower with increasing temperature for small aromatic compounds and also for asphaltene molecules.


1,7-Dimethylnaphthalene, either pure or in a mixture, reaches a randomly distributed state of intermolecular orientation, and its center-of-mass RDF reaches almost the same distribution even from different initial conditions. Asphaltene molecules usually have different center-of-mass RDFs from different initial conditions. At low temperatures, the highly aromatic asphaltene1 molecules prefer to stay almost parallel to each other, while at high temperatures they can form aggregates in a T-shaped orientation. At intermediate temperatures, asphaltene1 aggregate packed at around 40°. For the more branched asphaltene2 molecules, because of the influence of long aliphatic chains surrounding


the aromatic rings in the middle, their intermolecular orientation density results behaved differently from short alkyl branchcontaining asphaltene1. At high temperatures, asphaltene2 molecules prefer to stay almost parallel. At low temperatures, they pack almost T-shaped. At intermediate temperatures, they have a peak intermolecular orientation near 40°. Acknowledgment. This work was supported through grants from the Rhode Island Department of Transportation (Research and Technology Division) and the University of Rhode Island Transportation Center. EF060449Z

Molecular Orientation in Model Asphalts Using ... - ACS Publications

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