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Energy & Fuels 2007, 21, 1712-1716
Analyzing Properties of Model Asphalts Using Molecular Simulation Liqun Zhang and Michael L. Greenfield* Department of Chemical Engineering, UniVersity of Rhode Island, Kingston, Rhode Island 02881 ReceiVed December 29, 2006. ReVised Manuscript ReceiVed February 10, 2007
Molecular simulations have been used to estimate the properties of three-component mixtures whose constituents were chosen to represent the chemical families found in paving asphalts. Naphthene aromatics and saturates were represented by 1,7-dimethylnaphthalene and n-C22, respectively. Two different asphaltene model structures were considered. The first has a large aromatic core with a few short side chains; the second contains a moderate size aromatic core with larger branches. Both types have been proposed in the recent literature based on experimental characterizations of asphaltene fractions. Properties calculated from atomistic molecular simulations of the mixtures include density and isothermal compressibility (inverse of bulk modulus). The thermodynamic properties suggest a high-frequency glass transition above 25 °C for both model mixtures. The mixture based on the more aromatic asphaltene shows a more pronounced transition and has a higher bulk modulus. For a polymer-modified model asphalt, the calculations are consistent with increases in the bulk modulus.
Introduction Asphalts are mixtures that come mainly from crude oil distillation1 and are composed of many kinds (some estimate millions) of organic compounds.2 Chemical methods separate an asphalt into multiple parts, such as asphaltene, resin, and maltene. Solubility differences and chromatography (Corbett method) subdivide an asphalt into asphaltene, polar aromatic, naphthene aromatic, and saturate components.1 Asphaltenes are the most viscous and polar components; maltenes are the least viscous and most nonpolar components; and resins are in between the other two components.3 We make the assumption that analyzing the microstructure of asphalt components and understanding molecular interactions among them can supply guidance on how to modify asphalt compositions, which could eventually make asphalt work better as road pavement. Molecular simulation is one way to predict the macroscopic properties that result from specified microscopic molecular interactions and structures. Because asphaltenes are highly polydisperse molecules,3,4 asphalt compositions are not well-defined. It is not easy to separate asphalt into individual components. To do molecular simulations on a complex mixture, such as asphalt, building model asphalt mixtures to provide input for simulation is needed. Prior studies have recommended some average model structures for whole asphalts and also for asphaltene and resin components based on experimental analysis. * To whom correspondence should be addressed. Telephone: 401-8749289. Fax: 401-874-4689. E-mail:
[email protected]. (1) Roberts, F. L.; Kandhal, P. S.; Brown, E. R.; Lee, D.-Y.; Kennedy, T. W. Hot Mix Asphalt Materials, Mixture Design, and Construction, 2nd ed.; National Asphalt Pavement Association: Lanham, MD, 1996; Chapter 2. (2) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201210. (3) Petersen, J. C.; Robertson, R. E.; Branthaver, J. F.; Harnsberger, P. M.; Duvall, J. J.; Kim, S. S.; Anderson, D. A.; Christiansen, D. W.; Bahia. H. U. Binder Characterization and EValuation Volume 1. Report SHRPA-367, Strategic Highway Research Program; National Research Council: Washington, D.C., 1994. (4) Rogel, E. Colloids Surf., A 1995, 104, 85-93.
Several ideas concerning average model structures for whole asphalts are found in the literature. On the basis of experimental results of asphalt composition, molecular-weight distribution, and structure analysis, Jennings et al.5 built average molecular structures of eight core asphalts, which were standardized samples studied in the Strategic Highway Research Program (SHRP). Pauli et al.6 compared the physical properties of those core asphalts to predictions from correlations and recommended an alicyclic sheet molecule to represent asphalt. They found that average molecular structures could be useful in correlating physical properties of real asphalts. Several average molecular structures for asphaltene and resin have been recommended. Murgich et al.7 built average asphaltene and resin molecule structures to analyze molecular recognition and the mechanism in asphaltene aggregation. Artok et al.8 built several model structures for asphaltene based on experimental data, and later, Groenzin and Mullins9 recommended more asphaltene model structures based on fluorescence measurements. Rogel and Carbognani10 constructed some asphaltene average structures based on Venezuelan crude oils from different origins and estimated their density using molecular dynamics. Takanohashi et al.11 used a mixture of three molecules to represent the average model structure of Khafji asphaltene; the structure parameters of the mixture are very close to those of (5) 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; Report SHRP-A-335, Strategic Highway Research Program; National Research Council: Washington, D.C., 1993. (6) Pauli, A. T.; Miknis, F. P.; Beemer, A. G.; Miller, J. J. Prepr. Pap.Am. Chem. Soc., DiV. Pet. Chem. 2005, 50, 255-259. (7) Murgich, J.; Rodrı´guez M., J.; Aray, Y. Energy Fuels 1996, 10, 6876. (8) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287-296. (9) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677-684. (10) Rogel, E.; Carbognani, L. Energy Fuels 2003, 17, 378-386. (11) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135-139.
10.1021/ef060658j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/17/2007
Analyzing Properties of Model Asphalts
the real asphaltene. Gray and co-workers12,13 recommended new archipelago asphaltene models that can fit observed experimental data better than previous pericondensed models. Siskin et al.14 recommended six average chemical structural models for asphaltenes based on different sources and analyzed their chemical influences on the morphology of coke produced in delayed coking. In the following, one way to build model asphalt mixtures for computer simulation is explained and several physical properties of those model asphalts are analyzed. The objectives of this work are (i) to devise simple mixtures for modeling asphalts on the computer, using compounds that represent different asphalt components, and (ii) to compare the model asphalt properties, predicted using molecular simulations, to those of real asphalts. Intermolecular characterizations of the model asphalt mixtures, such as microstructure and relative orientations, are discussed in another paper.15
Energy & Fuels, Vol. 21, No. 3, 2007 1713
Figure 1. Asphaltene1 molecular structure.8
Simulation Methods Asphalt Composition Models. Analyzing the physical properties of asphalt using molecular simulation necessitates devising model asphalt mixtures of reasonable compositions, with different compounds representing each constituent (asphaltene, resin, and maltene). Several compounds are chosen, rather than just a single “average” molecule as done by Jennings et al.,5 to incorporate the diversity of chemistry and polarity (i.e., solubility parameters)16,17 found in real asphalts. Each component is represented using one molecule type. The molecule choice was aided by measurements of Storm and co-workers.18 They applied nuclear magnetic resonance (NMR) to asphalt fractions separated by the common alkane precipitation method, and through the carbon atom peak positions, they identified the balance among aromatic and alkane carbons in the asphaltene, resin, and saturate fractions. Here, n-docosane (n-C22H46) is chosen as a representative saturate. Storm et al.18 reported 72.2% alkane carbon atoms (per total carbon), subdivided into different types (branches, ends, etc.). These data18 can be interpreted19 as suggesting C16 to C36 average chain lengths. A chain length of 22 is near the middle of this range. Kowalewski et al.20 reported that n-C22 is the normal alkane with the highest concentration in the asphalt that they considered. Its melting point Tm ) 44 °C and boiling point Tb ) 369 °C21 are consistent with this saturate being a waxy component of an overall asphalt.19 1,7-Dimethylnaphthalene is chosen as a representative naphthene aromatic. Its 16.7:83.3 alkane/aromatic ratio differs from the overall 58:42 balance reported for one resin,18 but it does resemble some molecules depicted earlier for asphaltenes.22 The number of aromatic rings and side chains makes it intermediate between saturates and (12) Gray, M. R. Energy Fuels 2003, 17, 1566-1569. (13) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. Energy Fuels 2004, 18, 1377-1384. (14) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227-1234. (15) Zhang, L.; Greenfield, M. L. Energy Fuels 2007, 21, 1101-1111. (16) Redelius, P. G. Fuel 2000, 79, 27-35. (17) Redelius, P. Energy Fuels 2004, 18, 1087-1092. (18) Storm, D. A.; Edwards, J. C.; DeCanio, S. J.; Sheu, E. Y. Energy Fuels 1994, 8, 561-566. (19) Greenfield, M. L.; Zhang, L. Developing Model Asphalt Systems Using Molecular Simulation, Final Report for the RIDOT and URI Transportation Center, 2007. (20) Kowalewski, I.; Vandenbroucke, M.; Huc, A. Y.; Taylor, M. J.; Faulon, J. L. Energy Fuels 1996, 10, 97-107. (21) Linstrom, P. J.; Mallard, W. G. NIST Chemistry WebBook, NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg MD, 2005. http://webbook.nist.gov (accessed 2005). (22) Bunger, J. W.; Li, N. C. Chemistry of Asphaltenes, Advances in Chemistry Series 195; American Chemical Society: Washington, D.C., 1981.
Figure 2. Asphaltene2 molecular structure.9
asphaltenes. Its size is relatively small compared to a reported overlap in resin and asphaltene molecular-weight distribution.9 Two proposed asphaltene structures were chosen from examples in the literature and are shown in Figures 1 and 2. The first contains a moderate-size aromatic core with small branches. It is taken from NMR studies by Artok et al.;8 they proposed a collection of sample molecules that in total represent statistics of molecular asphaltenes. The second, taken from discussions by Groenzin and Mullins9 of their fluorescence depolarization studies, contains a somewhat smaller aromatic core and much longer alkane side branches. It is also one molecule from a collection of proposed asphaltenes. These models were chosen because they represent different proposed styles for the kinds of bonding patterns present in asphaltenes. Rogel and Carbognani chose asphaltenes with much larger pericondensed aromatic rings (8-20 fused rings) for their simulations.10 Two model asphalt mixtures were created, using these asphaltene, saturate, and naphthene aromatic molecules. The overall mixture composition was guided by the measurements by Storm et al.18 An asphaltene mass fraction of 21% was selected, similar to the 22 wt % that they reported for Ratawi vacuum residue. The concentrations of n-C22 and dimethylnaphthalene were chosen based on the alkane/aromatic carbon ratio (72.2:27.8 ≈ 8:3) reported by Storm et al.18 for the oil components, thus treating dimethylnaphthalene within both the oil and resin categories. This led to mixtures with 59 wt % n-C22 and 20 wt % 1,7-dimethylnaphthalene. If more molecules were desired for each subsection of the asphalt, an alternative would have been to use the overall oil/resin/asphaltene balance reported for this mixture23 (71.0/7.5/ 21.5) in conjunction with the alkane and aromatic carbon speciations for resin and oil. The resulting compositions and corresponding molecular characterizations are listed in Tables 1 and 2. To see how a polymer modifies asphalt mixture physical properties, a single polystyrene chain of molecular weight 5223.6 g/mol (50 repeat units and 2 hydrogen ends) was added to the asphaltene2-based mixture. The resulting system had 5440 atoms and 18 mass % polymer. Simulation Methods and Conditions. In simulation systems, an explicit atom representation for each molecule was used. This is computationally more expensive than representing many atoms (23) Storm, D. A.; DeCanio, S. J.; Detar, M. M.; Nero, V. P. Fuel 1990, 69, 735-738.
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Table 1. Overall Composition of Asphaltene 1 and Asphaltene 2 Mixtures number of molecules
mass fraction (%)
mixture
asphaltene
1,7-dimethylnaphthalene
n-C22
asphaltene
1,7-dimethylnaphthalene
n-C22
1 2
5 5
27 30
41 45
20.7 21.1
19.7 19.8
59.6 59.1
Table 2. Distribution of Atom Types in the Asphalt-like Mixtures mass percent by atom
carbon (versus total C)
hydrogen (versus total H)
mixture
C
H
S
H/C ratio
percent aromatic
percent alkane
percent aromatic
percent alkane
1 2
86.85 86.87
11.65 12.45
1.50 0.68
1.60 1.71
33.0 26.3
67.0 73.7
9.4 8.0
90.6 92.0
as a single site (united atom model)24 but can lead to more accurate estimates of intermolecular packing. The OPLS-aa (all-atom optimized parameters for liquid simulations) force field25 supplied required parameters for each atom. Two common molecular simulation approaches were applied: Monte Carlo (MC) and molecular dynamics (MD), using the publicly available simulation programs Towhee26,27 and Lammps.28,29 To analyze how the properties of asphaltene1- and asphaltene2based ternary mixtures vary with temperature, molecular simulations were conducted for both systems at 1 atm pressure and five different temperatures: 238.15, 268.15, 298.15, 358.15, and 443.15 K (-35, -5, 25, 85, and 170 °C). Those temperatures span a range from cold winter conditions to exceeding the “hot-mix” temperatures reached during paving operations. Preliminary simulations for pure compounds were conducted at additional temperatures. For polymermodified model asphalt, physical properties at T ) 25 °C were analyzed. A combination of MC and MD were used to estimate properties of the ternary mixtures and the polymer-modified asphalt mixture. MC was used to initiate each ternary system at five different temperatures, with a different random number seed chosen to ensure independent simulations. The large size of semirigid aromatic groups and expected slow molecular reorientations compared to small molecule liquids made it difficult to obtain the global molecular rearrangements necessary for equilibration. To reach a better equilibrated and lower energy state, MD was used to continue running the simulations for at least 1500 ps. Sampling was then performed for both ternary mixture systems at the five different temperatures.
provides confidence that simulations based on the OPLS-aa force field can lead to reasonable results for the temperature-dependent density of other kinds of aromatic compounds, such as asphaltenes, resins, and their mixtures. Evidence of agreement for a mixture of simple molecules 1-methylnaphthalene, methylcyclohexane, and heptane is shown in Figures 5 and 6.19 These figures show the effects of independently varying temperature and composition, while the other settings are held fixed. Figure 5 shows that density decreases with increasing temperature, which is consistent with that measured experimentally.33 Both the density itself and
Figure 3. Comparison of the specific density and temperature of naphthalene between simulation and experiment.30,31
Results Calculations were performed to estimate several different kinds of properties of the model asphalts. Thermodynamic quantities, such as density, thermal expansion coefficient, and isothermal compressibility, are useful for interpreting glass transition behavior. Density. To know if this simulation approach and force field could yield accurate simulation results compared to experimental data, simulations were first performed on individual compounds that have similar aromatic structures as components in asphalt. Results in Figures 3 and 4 from simulations on naphthalene and 1-methylnaphthalene show that the simulation results are very close to literature data.30-32 This good agreement between predictions and experiment for temperature-dependent density (24) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Oxford University Press: New York, 1987. (25) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives. J. J. Am. Chem. Soc. 1996, 118, 11225-11236. (26) Martin, M. G. MCCCS Towhee home page, Sandia National Laboratories. http://towhee.sourceforge.net/ (accessed Aug 1, 2006). (27) Martin, M. G.; Thompson, A. P. Fluid Phase Equilib. 2004, 217, 105-110. (28) Plimpton, S. J. LAMMPS Molecular Dynamics Simulator home page, Sandia National Laboratories. http://www.cs.sandia.gov/∼sjplimp/ lammps.html (accessed Aug 1, 2006). (29) Plimpton, S. J. J. Comput. Phys. 1995, 117, 1-19.
Figure 4. Comparison of the specific density and temperature of 1-methylnaphthalene between simulation and experiment.30-32
Figure 5. Density and temperature relationship for a ternary mixture of heptane/methylcyclohexane/1-methylnaphthalene with mole fractions of 0.125/0.125/0.750.33
Analyzing Properties of Model Asphalts
Figure 6. Mixture density for the heptane and 1-methylnaphthalene composition with a specified mole fraction of methylcyclohexane, at T ) 30 °C.33
Energy & Fuels, Vol. 21, No. 3, 2007 1715
Figure 8. Thermal expansion coefficient of the model asphalt mixture based on asphaltene2 in the presence of a polystyrene modifier, compared to unmodified model asphalt mixtures.
differential scanning calorimetry experiments occur over time scales longer than the short time scales (1 ns ≡ 10-9 s) readily accessible in MD. A much higher temperature is required for relaxation on MD time scales (i.e., time-temperature superposition), raising Tg. Mixtures based on the more aromatic asphaltene1 show a more pronounced change in the slope and thus a stronger glass transition than in asphaltene-2-based mixtures. Thermal Expansion Coefficient. The thermal expansion coefficient is related to the temperature dependence of the density and is calculated as24 Figure 7. Predicted densities at five different temperatures for unmodified and polymer-modified model asphalt mixtures.
its temperature derivative (which leads to the thermal expansion coefficient) show good agreement. Figure 6 shows the density change as a function of chemical composition at a fixed temperature (T ) 30 °C). With more cyclic alkane methylcyclohexane and less aromatic 1-methylnaphthalene, the density decreases. The effect of changing the ratio among the three compounds affects the density, and the extent of this change is calculated accurately by the simulation. The density results for model asphalt ternary mixtures at five different temperatures are shown in Figure 7. Circles and squares indicate simulation results for the asphaltene1- and asphaltene2based mixtures. The calculations were performed using MD, because we found that it provided better equilibration than MC for these systems. A change in the slope in Figure 7 suggests the presence of a glass transition. From these density versus temperature results, the glass transition temperature is between 25 and 85 °C for both asphaltene1 and asphaltene2-based model mixtures. This is higher than the ranges below 0 °C reported for SHRP asphalts,34,35 because several relaxations observed in those (30) Egloff, G. Physical Constants of Hydrocarbons; Reinhold Publishing Corporation (American Chemical Society Monograph Series): New York, 1947. (31) Rossini, F. D.; Pitzer, K. S.; Arnett, R. L.; Braun, R. M.; Pimentel, G. C. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds; Carnegie Press: Pittsburgh, PA, 1953. (32) Baylaucq, A.; Boned, C.; Dauge´, P.; Lagourette, B. Int. J. Thermophys. 1997, 18, 3-23. (33) Baylaucq, A.; Dauge´, P.; Boned, C. Int. J. Thermophys. 1997, 18, 1089-1107. (34) Turner, T. P.; Branthaver, J. F. In Asphalt Science and Technology; Usmani, A. M., Ed.; Marcel Dekker: New York, 1997; pp 59-101. (35) Robertson, R. E.; Branthaver, J. F.; Harnsberger, P. M.; Petersen, J. C.; Dorrence, S. M.; McKay, J. F.; Turner, T. F.; Pauli, A. T.; Huang, S-C.; Huh, J.-D.; Tauer, J. E.; Thomas, K. P.; Netzel, D. A.; Miknis, F. P.; Williams, T.; Duvall, J. J.; Barbour, F. A.; Wright, C. Fundamental Properties of Asphalts and Modified Asphalts, Volume 1: InterpretiVe Report, Publication number FHWA-RD-99-212; FHWA: Washington, D.C., 2001.
R≡
1 ∂V 1 〈δVδ(E + PV)〉 ) ) V ∂T P k T2V B 1 (〈VE〉 - 〈V〉〈E〉 + P(〈V2〉 -〈V〉2)) (1) kBT2V
( )
Here, V is the system volume, T is the absolute temperature, and the pressure P remains constant. Results for the calculated thermal expansion coefficients are shown in Figure 8. The temperature dependence is relatively smooth, with the exception of -40 and 85 °C points for mixtures 1 and 2, respectively. The temperature-dependent increase for mixture 1 is completed by 85 °C, which is consistent with the change in the slope of the density. The results for mixture 2 suggest a glass transition closer to 0 °C, lower than observed based on the density. Isothermal Compressibility. Isothermal compressibility βT and bulk modulus K are defined and calculated by24
βT )
1 1 (〈V2〉 - 〈V〉2) ) -V-1(∂V/∂P)T ) K 〈V〉kBT
(2)
Here, the absolute temperature T remains constant. Calculating isothermal compressibility at five different temperatures for asphaltene1 and asphaltene2 mixtures leads to the results shown in Figure 9. At lower temperatures, asphalts are rigid and not easily compressed; therefore, the slope of the βT versus T curve is low. At higher temperatures, they are more easily compressed; therefore, the slope of the βT versus T curve is higher. The glass transition temperature is the transition point where the isothermal compressibility changes more suddenly. From Figure 9, the glass transition temperature can be estimated to occur within the range of 25-85 °C, which is consistent with the density results. Mixtures based on the more aromatic asphaltene1 have a lower compressibility and thus a higher bulk modulus (Figure 10). Polymer-Modified Model Asphalt. A probable initial chain configuration for a polystyrene chain was found using a
1716 Energy & Fuels, Vol. 21, No. 3, 2007
Figure 9. Isothermal compressibility of unmodified and polymermodified model asphalt mixtures.
Zhang and Greenfield
ics, and saturate was targeted toward the detailed data of Storm et al.18 Thus, it differs from the wide-ranging compositions reported37 for SHRP core asphalts, which vary from 4 to 23% asphaltene. One conclusion is that it is not surprising that the model asphalts have physical properties different from real asphalts. SHRP core asphalts exhibit glass transition temperatures that range from -3.1 to -27.5 °C,3 while our model asphalt systems have glass transition temperatures above or around 25 °C. The short time scales accessible in molecular simulation can also lead in part to these discrepancies. Equilibrium responses that require microsecond time scales to occur would appear glassy over the nanosecond time scales simulated. This increases the predicted glass transition temperature, corresponding to the Tg found using a very fast quenching rate. Conclusions
Figure 10. Bulk modulus of the model asphalt mixture based on asphaltene2 in the presence of a polystyrene modifier, compared to unmodified model asphalt mixtures.
rotational isomeric state approach.36 This single polymer chain was surrounded by asphaltene2, 1,7-dimethylnaphthalene, and n-C22 molecules. Then, equilibration and sampling using MD were applied, leading to the density, thermal expansion coefficient, isothermal compressibility, and bulk modulus results shown by diamonds in Figures 7-10. In comparison to the unmodified model asphalt mixtures, the polymer-modified sample has a lower compressibility and thus a higher bulk modulus. Additional studies are underway to determine if the temperature dependence of properties is affected by the polymer modifier. The thermal expansion coefficient result suggests that the polymer affects the density dependence, likely because of a shift in the glass transition. Comparisons of the temperature dependence are ongoing. Discussion The composition of the model asphalts differs in some significant ways from real asphalts. The small number of molecule types is one clear difference; Wiehe et al.2 estimate that 105-106 molecule types are present in residues and asphalts. 1,7-Dimethylnaphthalene lacks specific polar functionalities, creating a polarity gap relative to asphaltenes that is usually occupied by polar aromatics. The overall and relative heteroatom concentrations do not match values reported in the literature,18,20,23 because only sulfur heteroatoms were present and not nitrogen or oxygen. The balance among asphaltene, aromat(36) Yoon, D. Y.; Sundararajan, P. R.; Flory, P. J. Macromolecules 1976, 8, 776-783.
These calculations constitute one of the first attempts to conduct fully atomistic molecular simulations on a multicomponent mixture whose chemical composition is chosen to be reflective of compounds found in real asphalts. Different molecules were chosen to reflect saturate, naphthene aromatic, and asphaltene components. The ultimate model mixtures displayed properties in simulations that were qualitatively similar to those of real asphalts but not quantitatively the same. Initial calculations focused on small molecules and simple mixtures to assess the accuracy of the methods and force field. It was found that the OPLS-aa force field provides sufficient accuracy for predicting the temperature-dependent density of naphthalene and 1-methylnaphthalene (aromatic compounds). For a simple ternary mixture of heptane/methylcyclohexane/1methylnaphthalene, simulation results of density versus temperature and density versus composition are consistently close to experimental data. Because asphalt mixtures, which contain asphaltenes, resins, and saturates, belong to the same general mixture category as the simple ternary mixture, it is reasonable to believe that present simulation methods can provide correct temperature-dependent density results. Simulations were conducted next for model asphalt-like ternary mixtures based on two different asphaltene molecules proposed in the literature. The temperature-dependent density and isothermal compressibility of both model asphalts show a change of the slope beyond 25 °C. These slope changes suggest the presence of a high-frequency glass transition somewhere between 25 and 85 °C. Polymer modification, a single polystyrene chain added to the asphaltene2-based model asphalt, led to noticeable changes in physical properties. The density increased and the thermal expansion coefficient shrank, suggesting a smaller tendency for density changes over a range of temperatures. The isothermal compressibility decreased, and the bulk modulus (its inverse) increased, indicating stronger mechanical properties in the presence of polymer modification. 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. EF060658J
