Molecular Dynamic Simulations for Determining Change in

ARTICLE pubs.acs.org/EF

Molecular Dynamic Simulations for Determining Change in Thermodynamic Properties of Asphaltene and Resin Because of Aging Rafiqul A. Tarefder* and Iffat Arisa Department of Civil Engineering, University of New Mexico, MSC01 1070, Albuquerque, New Mexico 87131, United States ABSTRACT: In this study, a molecular dynamic (MD) simulation is employed to determine the thermodynamic properties of asphalt binder components, namely, asphaltene and resin, before and after oxidative aging. For oxidative aging of asphaltenes, the percentage of oxygen considered in MD simulations is 0.1, 1, 12, 23, and 46.5% of asphaltenes. For oxidative aging of resins, the percentage of oxygen used in MD simulations is 5, 15, and 25% of resins. Using few oxygen, asphaltene, and resin molecules as input, MD simulations are run on a system at a fixed number of molecules and pressure to predict internal energy, structure, and density as function of the temperature. Simulation outputs are analyzed to determine density, glass-transition temperature, and potential and kinetic energies of the system. Results show that density has an inverse relationship with the temperature for both asphaltene and resin systems. At high temperatures, asphaltene and resin molecules gain high thermal energy that makes the molecules mobile and capable of breaking molecular association. The density decreases with the increasing temperature because the free volume expands, with or without the association of molecules. The percentage of oxygen affects the glass-transition temperature. At low oxidation levels (20%), the value of the glass-transition temperature of asphaltene decreases. In resin, no definite relationship between the glass-transition temperature and the oxidation level could be established through the approach used in this study. Finally, changes in potential and kinetic energies of asphaltene and resin because of oxidative aging are discussed.

1. INTRODUCTION Aging is a phenomenon in which asphalt molecules undergo chemical and physical changes or modification because of the interaction with oxygen. These changes or modifications have considerable effects on asphalt binder and asphalt concrete properties, such as cohesive and adhesive bond strengths and stiffness, viscoelastic properties, fracture properties, healing, and/ or recovery during rest periods.1 To this day, a number of methods have been developed and used to predict the aging behavior of asphalt binders.2
4 In the 1980s, several rheological tests were developed to characterize the physical properties of aged/unaged asphalt binders.5 In the 1990s, bulk rheological tests using a dynamic shear rheometer, a bending beam rheometer, and the direct tension test were developed to measure asphalt binder deformation and shear properties that were related to field performance.2,3 From these test results, quantities such as aging index, defined by the ratio of viscosity or penetration of aged and unaged binder, are determined to define the aging behavior. While these quantities and test methods are commendable approaches, they do not address the changes in thermodynamic properties because of aging. These tests did not develop an understanding of phenomena, such as phase separation or molecular movement, as well as the components of an asphalt molecule.6,7 There is a need for understanding the thermophysical behavior of individual components of an asphalt molecule because of aging, which is performed in the study. The asphalt binder is composed of organic compounds: asphaltenes (polar), maltenes (nonpolar), and resins (in between). Asphaltenes have nonvolatile and polar components that are insoluble in n-alkanes.8,9 Asphaltenes are kept dispersed by resins and are glued by maltenes.10 Both asphaltene and resin r 2011 American Chemical Society

molecules contain a significant number of aromatic rings, branched alkyls, cycloalkyls, and polar groups that include heteroatoms, such as sulfur, nitrogen, and oxygen atoms. These heteroatoms (nitrogen, oxygen, and sulfur) give asphalt its unique physical and chemical properties; they can replace carbon atoms in an asphalt molecule and have the ability to form associations with other molecules using hydrogen bonding. Heteroatoms make the molecule polar. This in turn allows for such molecules to react easily with other molecules.11 Asphalt aging usually dictates the amount of heteroatoms present in an asphalt molecule.12 As a result, asphaltene and resin are more likely to be oxidized. Recent studies have been shown that there may be a considerable amount of oxidative aging in asphaltene.13,14 Previous studies have shown that the resin component of asphalt is more likely to be oxidized than the asphaltene component.15,16 Maltene is not reactive with oxygen. Only limited studies have been conducted on oxidative aging of asphalt.10 To date, no studies on the aging of individual components of asphalt have been performed. In this study, for the first time, the aging behavior of individual components of asphalt, such as asphaltene and resin, is performed using molecular dynamic (MD) simulation. Studies that include the thermodynamic behavior of molecules are suitable for MD simulation.

2. OBJECTIVES AND SCOPE The objectives of this study are to (1) evaluate thermodynamic properties of asphaltene and resin before and after oxidative aging (in particular, determine the glass-transition temperature of Received: September 23, 2010 Revised: April 5, 2011 Published: April 11, 2011 2211

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Figure 1. Schematic asphalt components.

asphaltene and resin for both unaged and aged states from the density
temperature relation using MD simulations) and (2) examine the effects of the amount of oxygen and temperature on the density, glass-transition temperature, and potential and kinetic energies. The structures of asphaltene and resin used in this study are taken from the existing literature. Several variations of these structures are available in the literature. They are sometimes very complex. Only simplified structures of asphaltene and resin are carefully selected, and they suffice the purpose of this study.

3. SOME PRELIMINARIES 3.1. Asphaltene and Resin Chemistry. Figure 1 shows a schematic of asphalt components. The asphaltene fraction is responsible for viscosity and colloidal behavior of the asphalt.15 Asphaltene is surrounded by resin, which is connected by maltene. Resin is highly polar, solid or semi-solid at ambient temperature, but fluid when heated. The resin fraction disperses (or peptizes) asphaltenes throughout maltene to provide a homogeneous liquid and imparts ductility to the asphalt.15,16 The difference between asphaltene and resin is in the amount of the elements. The heteroatoms, such as oxygen, nitrogen, and sulfur are higher in asphaltene than those in resin. In addition, asphaltene has more condensed aromatic polycyclic rings than those in resin. The resin fraction contains more polar molecules than the asphaltene fraction.17 Resin is soluble in n-pentane, whereas asphaltene is insoluble in n-pentane. The carbon/ hydrogen ratio in asphaltene is bigger than that in resin and saturate. Although aging is most likely to occur in the outer core, which is resin, oxygen also diffuses through the outer core and causes aging in asphaltene.18 3.2. Aging in Asphalt. There are two types of aging in asphalt. One is physical aging because of the temperature, and the other is oxidative aging caused by air or oxygen. Physical aging is a reversible phenomenon that takes place as a consequence of cooling or quenching amorphous materials from melt temperatures to below the glass-transition temperature.19 Oxidative aging is irreversible hardening of asphalts. Asphalt composition changes through bond formation and breakage when asphalt is exposed to oxygen. King and Corbett first showed that the saturate fraction is relatively inert to the reaction with oxygen.20 They also showed that the naphthene aromatic fractions are slightly reactive and the polar aromatic fraction is highly reactive with oxygen. The asphaltene fraction shows intermediate reactivity.20 Petersen et al. measured the amount of ketone that was formed from oxidation of Wilmington (Wilmington, CA) asphalt fractions and

Figure 2. Definition of the glass-transition temperature.

based on the ketone amount.21 They ranked the relative reactivity of the saturate, naphthene aromatic, polar aromatic, and asphaltene fractions with oxygen as 1:7:32:40, respectively. Both asphaltene and resin have heteroatoms that are strongly associated with polar functional groups. Therefore, they have strong interaction and/or association forces and are highly reactive with highly electronegative oxygen. Asphaltene and resin contain highly condensed (multi-ring) aromatic ring systems and chemical functional groups of oxygen and nitrogen atoms. The functional groups make them highly polar. The molecules of asphaltenes also interact by close packing of their planar aromatic ring systems.22 The association, agglomeration, and interaction of asphaltene and resin mainly depend upon the temperature and the amount of oxygen. Previous studies have shown that, at high temperatures, 87% of the total oxidation occurred in the polar aromatic fractions (i.e., polar asphaltenes and resins) of the asphalt.21 Asphaltene is less likely to oxidize by air at ambient temperatures. At high temperatures (around 160 °C), the molecular mobility increases. Because of high molecular mobility, resin and asphaltene molecules become unassociated and susceptible to oxidation. 3.3. Glass-Transition Temperature. The glass-transition temperature (Tg) is a temperature below which a material shows glassy (elastic) behavior and above which shows viscous behavior. In materials science, it is defined as the temperature at which there is a change in the slope of the specific volume
temperature curve. It is possible that asphaltene and resin may depict the glasstransition temperature.23 Obviously, asphalt is very sensitive to the temperature. Asphalt materials change from a fluid-like lowstiffness material to a solid-like glassy material because of temperature variation. The concept of the glass-transition temperature is schematically shown in Figure 2. The glass-transition temperature is a point temperature at which the rate of change of specific volume with respect to temperature undergoes a discontinuity. Below the glasstransition temperature, material is in a glassy state. Above the temperature, materials show rubbery behavior. The glass-transition temperature is important to understand the hardening behavior, which generally appears below Tg. Hardening is caused by slow 2212

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Table 1. Distribution of Atoms in Asphaltene percent carbon

percent hydrogen

percent sulfur

(86.88% by mass)

(9.92% by mass)

(3.2% by mass)

aromatic (%) alkane (%) aromatic (%) alkane (%) 41.67

58.33

11.11

88.89

Figure 3. Asphaltene molecular structure used in this study.9

isothermal volumetric shrinkage that results from the bias in internal energy because of the lag of molecular adjustment behind the temperature drop.11 Therefore, there are speculations in the asphalt community that the glass-transition temperature of asphalt is responsible for low-temperature transverse cracking.24 Asphalt can have a wide range of values of the glass-transition temperature depending upon its composition. Strategic Highway Research Program (SHRP) core asphalts exhibit a glass-transition temperature between
3.1 and
27.5 °C.25 3.4. Concept of MD Simulations. MD simulations compute the motions of a number of atoms and molecules in a system as a function of time. The basis of MD simulation is Newton’s law of motions and statistical mechanics, where statistical ensemble averages are equal to time averages of a system. MD simulation is a useful tool for predicting system properties by animating forces and allowing insight into molecular motion at an atomic scale.26 In MD simulation, an atom is treated as a point mass, simple force rules describe the interactions between atoms, Newton’s equations are integrated to determine the positions and velocities of atoms, and thermodynamic statistics are extracted from the motion of the atoms. The process of MD simulations generates cumulative errors in numerical integration that are minimized with appropriate selection of algorithms and potential functions known as force field. A MD simulation involves intensive computations. In MD simulations, the equations of motion (i.e., a set of ordinary differential equations) are solved using the finite difference approach.27 The equations are solved on a step-by-step basis. The general idea is that, given the atomic positions and velocities at time, the positions and velocities at a later time can be obtained with sufficient accuracy. The selection of simulation size (n = number of particles) and time step is critical to perform MD simulation within a reasonable amount of time. Fortunately, these are performed in the most commercial software for MD simulations.27,28 In MD simulations, a periodic boundary is usually introduced to consider an infinite, space-filling array of identical copies of the simulation region.29 For periodic boundary conditions, the system or simulation box should be created large enough to represent the bulk material.30

4. METHODOLOGY 4.1. Selection of Model Asphaltene and Resin Structures. 4.1.1. Asphaltene. The asphaltene fraction contains at least

105 different molecules; therefore, the isolation and identification of individual components of asphaltene are impossible.31 As a result of extraordinary complexity, the chemical characterization of asphaltenes often ends up with only the identification of molecular types and structural groups. A previous study by Rogel and Carbognani has shown that asphaltenes are composed of polyaromatic condensed rings, aliphatic chains, and heteroatoms, such as nitrogen, oxygen, sulfur, and various metals.31 Figure 3 shows the asphaltene molecule structure that is employed for MD simulations

Figure 4. Resin molecular structure.12 in this study.9 It contains a small aromatic core and long alkane side branches. This structure was proposed by Groenzin and Mullins based on fluorescence depolarization data.9 This model asphaltene is chosen in this study because it includes most of the bonding patterns (such as C
C, C
S, C
H, CdC, etc.) present in an ideal asphaltene. Also, it has a sulfur atom along with a carbon
hydrogen chain and ring structure. Ring structures have significant polarity, although the aliphatic chain is not polar. Only the sulfur heteroatom is present in the model asphaltene structure. However, oxygen or nitrogen may be present in compositions reported for SHRP core asphalts.32 The molecular formula of this model can be written as C72H99S. Its average molecular weight is 997. Distributions of the atom types of this asphaltene are calculated and presented in Table 1. The mass of an asphaltene molecule is predominately due to the mass of the carbon atom. Hydrogen contributes about 10% mass of an asphaltene molecule. The ratio of carbon/hydrogen, C/H, is 8.73. There are two types of C and H in asphaltene. They are aromatic and alkane types. Most of the hydrogen atoms are alkane-type. Alkane carbon is slightly more than the aromatic carbon. 4.1.2. Resin. Two types of resin molecules, as shown in Figure 4, are considered in the simulation. The source of this resin structure is Venezuelan crude oil.12 The first structure of a resin molecule (resin structure 1) shown in Figure 4a has a heteroatom sulfur, aliphatic chains (both straight and branched), and few ring structures, including saturated and unsaturated rings. Its molecular formula can be written as C49H78S1, and its molecular weight is calculated to be 698 Da. The second structure of a resin molecule (resin structure 2) used in this study is shown in Figure 4b. It contains some heteroatoms, such as sulfur, oxygen, and nitrogen, with alkanes and aromatic rings. Its molecular formula can be written as C59H85S1N1O1, and its molecular weight is 855 Da. From Table 2, it can be seen that carbon has the highest percentage of mass among all of the components in both resin molecules. The percentage mass of hydrogen is around 10% in both cases. In resin structure 1, the percentage mass of heteroatom (only sulfur) is less than 5%. However, in resin structure 2, the percentage mass of heteroatoms (combining sulfur, nitrogen, and oxygen) is 7.25%. Carbon/ hydrogen ratios in two structures of the resin molecule are less than that in an asphaltene molecule. Similar to asphaltene, these resin structures 2213

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Table 2. Distribution of Atoms in Resin

resin 1

resin 2

percent carbon

percent hydrogen

percent sulfur

percent nitrogen

percent oxygen

(84.24% by mass)

(11.17% by mass)

(4.58% by mass)

(0%)

(0%)

percent sulfur (3.74% by mass)

percent nitrogen (1.64% by mass)

percent oxygen (1.87% by mass)

aromatic (%)

alkane (%)

aromatic (%)

alkane (%)

40.82

59.18

20.51

79.49

percent carbon (82.81% by mass)

percent hydrogen (9.91% by mass)

aromatic (%)

alkane (%)

aromatic (%)

alkane (%)

42.37

52.54

21.18

78.82

Table 3. Simulation Matrix for Asphaltene and Resin components asphaltene (25 molecules, 4300 atoms) resin 1 (3 molecules, 384 atoms, 128 atoms per molecule) resin 2 (3 molecules, 441 atoms, 147 atoms per molecule)

oxygen atoms

pressure (atm)

temperatures

outputs

5, 50, 500,

from
55 to 85 °C,

1000, 4000

a total of 15

density, potential energy,

temperatures

and kinetic energy

1

15, 45, 75

have two types of carbon and hydrogen atoms: aromatic and alkane. In resins, aromatic hydrogen is much less than alkane hydrogen. However, aromatic carbon is slightly less than the alkane carbon in both resin structures shown in Figure 4. In this study, both of these resin structures are used in a single system for running MD simulations. Both of these structures are very common but have different hateroatoms, and therefore, they are considered together in a single MD simulation in this study. 4.2. Simulation Matrix. MD simulation matrix and input parameters are presented in Table 3. The simulation matrix includes four input parameters: number of asphaltene or resin molecules, pressure, temperature, and oxygen atoms. A total of 25 molecules of asphaltene are considered for MD simulation, with each molecule having 172 atoms. A total of 4300 atoms are used in MD simulations of asphaltene aging. Simulations are run at a constant pressure of 1 atm and at 15 different temperatures varying 10 °C. The values of pressure are not varied in MD simulation because the molar volume of asphaltene does not vary with pressure. As mentioned previously that the glass-transition temperature in asphalt may occur below zero temperature, therefore, negative temperatures are considered in this study. In service pavements, the temperature may reach up to 50
70 °C depending upon the geographic location. Therefore, the positive temperatures are considered in MD simulations. Several oxidation levels or number of oxygen atoms are considered and calculated on the basis of the percentages of asphaltene or resin molecules of a simulation system. These are 0.1% oxidation level (5 oxygen atoms), 1% oxidation level (50 oxygen atoms), 12% oxidation level (500 oxygen atoms), 23% oxidation level (1000 oxygen atoms), and 46.5% oxidation level (2000 oxygen atoms). Simulation outputs are density, potential energy, and kinetic energy. For resin, three molecules of each type are taken, Resin structure 1 molecule has 128 atoms, and the total is 384 atoms. Resin structure 2 has 147 atoms, and the total is 441 atoms. Both of these resin structures are mixed in an equal amount, and the final simulation is run using 825 atoms. A constant pressure (1 atm) is maintained throughout the simulation because resin is solid or semi-solid, whose volume does not vary significantly with pressure. A total of 15 temperatures are considered for simulation of aging in resins. Different levels or amounts of oxygen atoms (as a percentage of the molecular weight) are used. The oxygen levels considered are 5% (15 oxygen atoms), 15% (45 oxygen atoms), and 25% (75 oxygen atoms). The MS simulation outputs are density, potential energy, and kinetic energy, similar to asphaltene. Ideally, three molecules would be sufficient to calculate the potential energy defined by eq 1. Therefore, selection of only three resin molecules for MD simulation suffices the purpose of this study. However, as for

varying 10 °C

asphaltene molecules, 25 molecules are arbitrarily selected for MD simulation. MD simulation results do not vary with the number of molecules.30 For both resin and asphaltene, the number of oxygen atoms is calculated to match the prescribed levels of oxidation (0.1, 1, 12, 23%, etc). In MD simulations, different levels of oxidation are used to simulate the progressive long-term oxidations that occur in the field asphalt pavements. For the purpose of comparison, it can be said, if 0.1% oxidation occurs during the 2 year life of a pavement, probably 1% oxidation occurs at the 20th year of the pavement life. Therefore, MD simulation results are just snapshots of the aged condition of the entire life of a pavement. 4.3. MD Simulation. MD simulation is employed here using commercially available simulation software called Culgi.30 An all-atom model, instead of a united atom model, is selected, although it is computationally more expensive; however, it can estimate intermolecular packing accurately.27 Dreiding force field is used for molecular interactions or intermolecular potential. Because dreiding force field uses parameters that are defined by simple rules. Dreiding force field is suitable to represent structures of molecules for which little to no experimental data are available. Dreiding force field is useful in predicting structures involving new combinations of elements and easily extendable to new atoms.30 To reach an equilibrium or lower energy state, MD simulations were run for 106 cycles. Through trial and error, a time step of 0.1 fs was found to be suitable for energy minimization quickly and stability of simulation runs in this study. To minimize energy, relatively soft short-ranged interaction potentials in a constant NVT ensemble were used in MD simulations. The potential energy can be expressed by eq 130 1 1 E ¼ kr ðr
r0 Þ2 þ Cθ ðcos θ
cos θ0 Þ2 þ V0 ½1
cos nðφ
φ0 Þ 2 2 3 2 !12 !6 σij σij 5 1 qi qj 4 þ
ð1Þ þ Kψ ð1
cos ψÞ þ 4εij rij rij 4πε0 rij where E is the potential energy. The first term in the equation, kr(r
r0)2 models the interaction between pairs of bonded atoms. r is a position vector and deviates from the reference position vector, r0. kr is the stretching constant of the bond. The second term is the angle bending potential in cosine form for θ ¼ 6 180°, where Cθ = Kθ/((sin θ0)2). Kθ is a force constant, and θ0 is a reference value of angle bending. For θ = 180°, the angle bending potential becomes Kθ(1 þ cos θ). The third term is a torsional potential that depends upon bond rotation. The value of V0 indicates the relative barriers to rotation. n is the multiplicity, whose value gives the number of minimum points in the function as the bond is rotated through 360°, and φ is 2214

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The cutoff distance for both asphaltene and resin in MD for interaction is 7.5 Å.

5. RESULTS AND DISCUSSION 5.1. Interaction of Oxygen with Asphaltene and Resin. 5.1.1. Asphaltene. Figure 6a shows the positions of asphal-

Figure 5. Thermodynamic equilibrium of asphaltene and resin in MD simulation. the torsion angle. The fourth term is the inversion potential for planar configurations in cosine form, where ψ is the inversion angle. The fifth term is the potential for nonbonded van der Waals interactions, where rij denotes the distance between the interacting atoms, εij is the Lennard
Jones well depth, and σij is the size parameter for a pair of atoms i and j. The last term is for the electrostatic interaction, where qi and qj are the point charges and ε0 is the electrical permittivity of media containing those charges. In this study, simulations are run to attain the thermodynamic equilibrium state for a particular temperature and pressure. When the change of potential energy between two successive steps is less than 10
4, simulation is stopped. As shown in Figure 5, the equilibrium state for the temperature and pressure was attained within 500 ps. Berendsen method is used to maintain a constant temperature and pressure.33 According to Berendsen’s approach, the equation of motion is written as30   Dr DUð B r N Þ mi T0 mi B ai ¼
þ
1 Bi τT T Dt DB ri

ð2Þ

where mi is the mass of atom i, Bai is the resulting acceleration of the atom, U(rBN) denotes the potential energy as a function of the positions (rB) of N particles (usually atoms), ∂t is the time step, τT is a damping constant that determines the strength of the coupling to the external bath, and T0 is the reference (target) temperature. In Culgi, τT is referred to as the thermal coupling parameter. Figure 5a shows that thermodynamic equilibrium conditions of asphaltene and resin are reached with progressing time for 1 atm pressure. Figure 5b shows that for 298.15 K (25 °C) temperature.

tene molecules (black and white) and oxygen atoms (red) at 5 °C and 1% oxidation level. At this temperature, density is calculated to be 1024 kg/m3. It can be seen that molecules are densely packed. Indeed, such association of the molecules is responsible for the increase in intermolecular forces and the decrease in the reaction rate with oxygen atoms. Figure 6b shows that, at high temperatures (i.e., 55 °C), the molecules are a bit separated compared to that at low temperatures. Density decreases to 1007 kg/m3 from 1024 kg/m3. At high temperatures, the velocity and kinetic energy of asphaltene molecules increase. Figure 6c shows that a high oxidation level (23%) can dissociate aggregation of asphaltene molecules. From Figure 6d, it can be seen that oxygen atoms are breaking association in the asphaltene structure at a high temperature (55 °C) and high oxidation level (23%). The asphaltene structure is largely unassociated, and the molecular motion increases, creating various paths for oxygen molecules to diffuse through the asphaltene clusters. While moving from high to low temperatures, molecular motions decrease and molecules again form an association, because the energy of the molecules in a condensed or aggregate form is smaller than the sum of the energies of the individual molecules. In such a case, the electronic structure of the aromatic ring is responsible for aggregation and not the localized aliphatic electrons. Also, at low heteroatom content, the asphaltene molecule shows a small degree of charge transfer during such molecular aggregation. In this study, asphaltene contains only a sulfur heteroatom, which has low electronegativity. This makes the asphaltene molecule less reactive during oxidation, and a hydrogen bond does not form. Also, the presence of large aliphatic chains reduces the number of active sites for molecular interactions because they interfere with the molecules approaching the bonding. 5.1.2. Resin. The molecular arrangement of resin during oxidative aging at a low temperature (5 °C) and low oxidation level (5%) is shown in Figure 7a. A low temperature reduces molecular motion; therefore, it is very structured where density changes from 977 kg/m3 at the start of the simulation (at 25 °C) to 998 kg/m3 at the end of the simulation (5 °C). For MD simulation at 55 °C and 5% oxidation, the initial density is calculated to be 977 kg/m3 and the final density at the end of simulation is calculated to be 976 kg/m3. The orientation of molecules at the end of the MD simulation is shown in Figure 7b. Therefore, the effect of the high temperature and effects of interactions with less amounts of high-electronegative oxygen balance each other. Figure 7c shows that, at highly oxidized conditions (50%) and a low temperature, resin molecules are not in a very structured state. Here, the low temperature tries to form an aggregate, but a large amount of oxygen atoms interacts with resins and tries to dissociate. Because resins have various heteroatoms, which are polar in nature, they are more reactive with oxygen and less susceptible to temperature. Figure 7d depicts the positions of resin molecules at a high oxidation level (50%) and high temperature (55 °C). Effects of the temperature on resin molecules are less than oxygen. Probably, this is because resin is a solid, whereas oxygen is a gas. It is possible that a van der Waals attraction force exists among resin molecules, which form 2215

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Figure 6. Interaction between asphaltene and oxygen.

a more structured asphaltene molecule with aging time. It can be concluded that resin is less temperature-susceptible than asphaltene. 5.2. Glass-Transition Temperature. 5.2.1. Asphaltene. Figure 8a shows the change in the density of asphaltene at different temperatures without subjecting it to any oxidative agents, such as oxygen. As the temperature increases, the density decreases and a sharp change in the density versus temperature curve is observed at 268.15 K (
5 °C). Basically, tangents are plotted at the straight line portions of the curve. The intersection of the two tangents can be considered as the glass-transition temperature. The glass-transition temperature is found to be 268.15 K (
5 °C). Results for aged asphaltene are found for different oxidation levels. Figure 8b is for 1% oxidation, where the glass-transition temperature is observed at 268.15 K (
5 °C). Figure 8c is for 12% amount of oxidation, where Tg is also 268.15 K (
5 °C). Figure 8d shows the change in density with temperatures for 23% oxidation level, and the glass-transition temperature is 258.15 K (
15 °C). It can be seen that, with a large

increase in the amount of oxidation, the glass-transition temperature of asphaltene is reduced. This is because, at a highly oxidized state, asphaltene molecules are unassociated and react with oxygen and, therefore, Tg changes. At a low oxidation level (20%), the glass-transition temperature of asphaltene decreases, and is below
5 °C. In the case of resin, the glasstransition temperature is established for each specific oxidation level; however, it could not be generalized from this study. (2) For both asphaltene and resin fractions, the density increases with the increase in the oxidation level to a certain extent, reaches a peak value, and then decreases. With aging time, the asphaltene density increases at a low oxidation level and decreases at a high oxidation level. The opposite trend is found in resin. With time, the resin density decreases at low oxidation levels and increases at high oxidation levels. The change in density over time is less in resin than that in asphaltene, which may be an indication that resin is less likely to dissociate and does not have a strong affinity to form aggregates like asphaltene. (3) The density versus time relationship is hyperbolic at low oxidation levels, whereas it is linear at high oxidation levels. (4) Both potential and kinetic energies increase with an increase in the temperature. Energy decreases with an increase in the oxidation level in both asphaltene and resin. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: (505) 277-6083. Fax: (505) 277-1988. E-mail: [email protected].

’ ACKNOWLEDGMENT This project is funded by the National Science Foundation (NSF) through the GOALI Program (NSF Grant 0900778 and the Infrastructure Materials and Structural Mechanics Program). The authors express sincere gratitude toward Shyamal Nath, Chief Development Scientist of Culgi, Inc., and Parveez Anwar, State Asphalt Engineer of the New Mexico Department of Transportation. ’ REFERENCES (1) Knorr, D. B.; Davison, R. R.; Glover, C. J. Effect of various aging techniques on asphalt low-temperature properties. J. Transp. Res. Board 2002, 9–16. (2) Collop, A. C.; Airey, G. D. Effects of pressure and aging in SATS test. Transp. Eng. J. ASCE 2007, 133 (11), 618–624. (3) National Cooperative Highway Research Program (NCHRP). Simulating the effects of hot mix asphalt aging for performance testing and pavement structural design. National Cooperative and Highway Research Report; Transportation Research Board: Washington, D.C., 2007; Research Results Digest 324. (4) Andersen, S. I.; Birdi, K. S. Aggregation of asphaltenes as determined by calorimetry. Colloid Interface Sci. 1991, 142, 497– 502.

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