An ab initio molecular dynamics analysis of lignin as a potential antioxidant for hydrocarbons

Journal of Molecular Graphics and Modelling 62 (2015) 325–341

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An ab initio molecular dynamics analysis of lignin as a potential antioxidant for hydrocarbons Tongyan Pan ∗ , Cheng Cheng Department of Civil, Architectural and Environmental Engineering, Illinois Institute of Technology, 3201 S. Dearborn St. Alumni Memorial Hall 215, Chicago, IL 60616, United States

a r t i c l e

i n f o

Article history: Received 16 July 2015 Received in revised form 2 October 2015 Accepted 26 October 2015 Available online 31 October 2015 Keywords: Lignin Hydrocarbon oxidation Ab initio molecular dynamics

a b s t r a c t Lignins are complex phenolic polymers with limited industrial uses. To identify new applications of lignins, this study aims to evaluate the conifer alcohol lignin as a potential antioxidant for hydrocarbons, using the petroleum asphalt as an example. Using the ab initio molecular dynamics (AIMD) method, the evaluation is accomplished by tracking the generation of critical species in a lignin-asphalt mixture under a simulated oxidative condition. The generation of new species was detected using nuclear magnetic resonance and four analytical methods including density of states analysis, highest occupied molecular orbital and lowest unoccupied molecular orbital analyses, bonding and energy level analysis, and electrostatic potential energy analysis. Results of the analyses show that the chemical radicals of carbon, nitrogen and sulfur generated in the oxidation process could enhance the agglomeration and/or decomposition tendency of asphalt. The effectiveness of lignins as an antioxidant depends on their chemical compositions. Lignins with a HOMO–LUMO gap larger than the HOMO–LUMO gap of the hydrocarbon system to be protected, such as the conifer alcohol lignin to protect petroleum asphalt as was studied in this work, do not demonstrate beneficial anti-oxidation capacity. Lignins, however, may be effective oxidants for hydrocarbon systems with a larger HOMO–LUMO gap. In addition, lignins may contain more polar sites than the hydrocarbons to be protected; thus the lignins’ hydrophobicity and compatibility with the host hydrocarbons need to be well evaluated. The developed AIMD model provides a useful tool for developing antioxidants for generic hydrocarbons. © 2015 Elsevier Inc. All rights reserved.

1. Introduction Hydrocarbons originated from living organisms on earth are the raw materials for many industrial sectors. To name a few as examples, hydrocarbons have been used to synthesize or make various plastics, textiles, and rubber commonly used in people’s daily lives. Hydrocarbons however are prone to degradation in air through a series of exothermic reactions with oxygen. Such oxidative reactions could produce thousands of new species by means of rapid combustion or slow aging at temperature ranging from some hundred to a few thousand K [1–3]. To prevent or minimize the long-term chronic oxidative aging, lignins have been used as antioxidants in different hydrocarbon-based materials such as food [4], paper pulp [5], or in generic hydrocarbons as a radical scavenger [4,6]. Being a waste product from pulp and paper industry, lignins are complex phenolic polymers with limited industrial use

∗ Corresponding author. Fax: +1 312 567 3519. E-mail addresses: [email protected] (T. Pan), [email protected] (C. Cheng). http://dx.doi.org/10.1016/j.jmgm.2015.10.013 1093-3263/© 2015 Elsevier Inc. All rights reserved.

and have often been treated as a fuel material in the process of pulping [6]. New and more sustainable applications of the massive lignins produced annually would produce significant economic and environmental benefits. Research efforts to date have focused on utilizing the antioxidation capacity of lignins, leading to two categories of applications of lignins. The first category of applications entails direct blending large amounts of underivatized lignins in the hydrocarbons to be protected [4,7], and the second category uses relatively small amounts of derivatized lignins that have demonstrated capability to prevent or minimize oxidation [8,9]. The first category of applications of lignins could reduce the desirable mechanical properties of the matrix materials being protected [7]. Regarding the second category of applications, lignins are polar polymers with one or two hydroxyl groups per monomer and thus can only dissolve in fairly polar matrix materials. Such low solubility of lignins could limit their contact with the radicals responsible for oxidation and thus limit the effectiveness of lignins as an antioxidant. As such, studies of lignins as an antioxidant have been limited

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to polar hydrocarbons and composite material systems that contain sufficient polar components [9]. Petroleum asphalt is a representative hydrocarbon system with widespread industrial applications, such as for making roof shingles and road pavements. To date, in the United States more than 70% of commercial and residential buildings are cover with shingles made of asphalt mixtures [10] and over 93% of road miles are paved with asphalt concrete [11]. Petroleum asphalt is a balanced system between polar radicals and nonpolar ingredients, composed of saturates, aromatics, resins and asphaltenes based on their polarizability and polarity different solvents [12–14]. The saturate part of asphalt are nonpolar linear, branched and/or cyclic saturated hydrocarbons. The aromatic component of asphalt is more polar than saturates owing to the aromatic rings contained. The resins and asphaltenes of asphalt both are apparent polar radicals, and the resins are soluble with heptane or pentane while asphaltenes are insoluble in heptane. Asphalt mixtures and concrete used for buildings and roads have a service life of 5–10 years under the typical outdoor service conditions due to the chronic oxidative aging of asphalt binder. Such a service life of materials is significantly shorter than the structural life expectances of the buildings and roads which is typically 50 years [11]. Thus, asphalt as a hydrocarbon system needs to be protected against oxidative aging. Numerous chemical agents have been proposed as potential antioxidants for hydrocarbon systems, such as the dibutyldithiocarbamates and naphthenic oil for general organic polymers and styrene-butadiene-styrene and styrene-b-butadiene for petroleum asphalt in specific [15,16]. Considering the scavenging capability of lignins on the radicals of a hydrocarbon system, this study aims to explore the potential application of lignins as an antioxidant for asphalt binder. This study takes the approach of numerical analysis towards this goal to avoid the expensive and laborious experimental endeavors. Moreover, since the study of the effectiveness of antioxidants for hydrocarbons is constantly challenged by the complexities of organic chemistry and the slow process of oxidative aging at service temperature, a clear understanding of the anti-aging mechanisms of lignins has not been adequately addressed experimentally although many antioxidants have been proposed for hydrocarbons. The numerical approach can hopefully circumvent these challenges by accurately tracking new species generated in oxidation. 2. Ab initio molecular dynamics Generic atomistic modeling approaches enable the calculation for molecular geometries, structures, reactivities, spectra, and other properties. There are five major types of atomistic model-

ing approaches that have been developed and used for modeling materials, which are listed as follows in the order of their development time. (1) Molecular mechanics based on a ball-and-springs type of analogy of motion of molecules, (2)Ab initio methods based on theoretical solutions of Schrödinger equation without fitting to experiment, (3) Semiempirical methods based on approximate solutions of Schrödinger equation with appeal to fitting to experiment using parameterization, (4) Density functional theory (DFT) method that solves Schrödinger equation by means of electron density instead of wavefunctions as used in ab initio methods, (5) Molecular dynamics methods that simulate motion atoms of a molecular system based on classical Newtonian or Hamiltonian mechanics while meeting the thermodynamic equilibrium at the same time [17]. The method of molecular mechanics can be used to determine the geometries, structures and energies of large hydrocarbon molecules such as proteins and nucleic acids. Molecular mechanics however does not give information on electronic structures and therefore cannot simulate new bonding or de-bonding that is not pre-defined in the interatomic potentials [18]. The method of molecular dynamics based on classical Newtonian or Hamiltonian mechanics in general does not give electronic information either and therefore does not simulate chemical reactions that are not predefined. Ab initio and DFT methods enable generating new species by solving the Schrödinger equation; however, these two methods do not allow computing big systems that exceed a few hundred atoms [19]. The semiempirical methods, which are much faster than ab initio and DFT methods, can be applied to much larger molecular system. The semiempirical methods however involve more approximations in solving the Schrödinger equation. The very complicated integrals that must be calculated in the ab initio method are not evaluated but fitted to the experimental values [17]. This plugging of experimental values into a mathematical procedure to get the best calculated values is known as parameterization. The semiempirical methods therefore are the mixing of theory and experiment based on the Schrödinger equation but parameterized with experimental values. As such, a semiempirical method cannot give good answers for molecules or molecular systems for which the method has not been parameterized. Semiempirical calculations in general are about 100 times slower than molecular mechanics, but can be 1000 times faster than the DFT or ab initio calculations [20]. Aiming at predicting the unknown species and particularly the radicals that are responsible for the oxidative aging of petroleum asphalt and those that are responsible for the anti-oxidation capacity of lignins, this study adopts the ab initio molecular dynamics (AIMD) method for numerical simulation. The hybrid ab initio and

Fig. 1. Asphalt-lignin-oxygen system before (left) and during (right) oxidation simulation.

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molecular dynamics method has the combined advantages of predicting new radicals that might be critical in promoting or resisting asphalt aging and simulating the thermodynamics of an asphaltlignin system in the typical service conditions, e.g., the surface of roofs or highway roads. One key component of the AIMD methods is the inter-nucleus forces needed to determine the atomic trajectories of the molecular system being simulated. Such inter-nucleus forces in AIMD can be obtained by the ab initio or DFT calculations per Eq. (1), where Ri is the nuclear positions, i is the number of nucleus, and ˚(R) is the potential energy function. ˚(R) in Eq. (1) can be determined as Eq. (2), in which He (ri , R)is the multi-body, electronic Hamiltonian, ri is the electronic coordinates, and E is the total energy of the system. Fi = −∇ Ri ˚(R)

(1)

 (R) = ␸0 |He (ri , R) |␸0  + Eii (R)

(2)

The numerical determination of the multi-body Hamiltonian He (ri , R) constitutes a eigenvalue problem of multiple dimensions as formulated in Eq. (3). The equation is also known as the timeindependent, multi-body, electronic Schrödinger Equation, where the eigenfunctions 0 (ri ) and eigenvalues 0 (R), respectively. He (ri , R)ϕ0 (ri ) = 0 (R)ϕ0 (ri )

(3)

Solving Eq. (3) is an astounding task due to the 3N degrees of freedom of the system (N being the number of electrons of the system). In practices, the time-independent, multi-body, electronic Schrödinger Equation is often solved by the DFT method (or by another ab initio method). DFT is based on the one-to-one correspondence between the ground state electronic density 0 (r) and the external potential E(r), which is known as the Hohenberg–Kohn theorem [19,20]. Since the electronic density (r) of a system only depends on the three degrees of freedom of electrons, not the complex multidimensional wavefunction as used in other ab initio methods, DFT significantly reduces the computation cost and has been the most widespread first-principles method used in electronic structure calculations. In DFT, the ground state energy E0 DFT , the nondegenerate ground state wavefunction 0 (ri ), and the multi-body Hamiltonian He (ri ) in Eq. (2) and (3) are all functionals of the ground state electronic density 0 (r). The exact ground state energy E0 DFT of the system is the global minimum value of the functional EDFT , and the density which minimizes this functional is the exact ground state density0 (r). The ground state energy E0 DFT thus can be evaluated by iterating He [0 (r)]within a self-consistent field per Eq. (4), where (r) is electronic density at any other states of the system [19]. E DFT [0 (r)] = ϕ0 [0 (r)]|He [0 (r)]|ϕ0 [0 (r)] ≤ ϕ[(r)]|He [(r)]|ϕ[(r)] = E DFT [(r)]

(4)

Eq. (4) implies the functional connection between the electron density to the system’s energy, however does not specify the mathematic form, or provide any methods to determine such as a mathematic form, of the functional. Theoretically, any approximate functionals that could give a satisfactory result could be used. In reality, many such approximation functionals have been developed and used in research and development practices. The commonly used approximation functionals include the Local Density Approximation (LDA), the Generalized Gradient Approximation (GGA), as well as some the transformed GGA such as the Meta-GGA functionals, and the Hybrid functionals based on both LDA and GGA [21].

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3. AIMD model development This study aims to evaluate the effects and capacity of lignins as an anti-oxidant for hydrocarbons using the petroleum asphalt as an example. Lignins are the macromolecular component of the cell walls of nearly all land plant species on earth, with an overall abundance only after cellulose and hemi-cellulose. Lignins constitute around 25% by dry weight of woods and less than 20% in grasses and crops. Dependent on the plant species, lignins could consist of one, two, or three types of phenylpropane monomers, including (1) coniferyl alcohol monomer that exists in all plant species and is the dominant monomer in softwoods like conifers, (2) syringyl alcohol monomer that composes up to 40% weight of hardwood species, and (3) coumaryl alcohol monomer that occurs in grasses and crops. These three types of monomers could form different possible bonding patterns, resulting in complex and distinct structures and geometries of lignin molecules. Although playing an important role in defending plants from natural degradation, lignins are byproduct materials in industries using woods as a raw material, such as the paper-making or pulping industry. Today, the sources of lignin include mainly (1) sulfite pulping, a source of papermaking grade bleached pulp in which the sulfite process is used, (2) kraft pulping, a process that separates lignin from cellulose by using alkali with a sodium sulfide catalyst, and (3) cellulosic ethanol as a by-product from cellulosic ethanol industry [4–9]. Petroleum asphalt is a mixture of polar hydrocarbon species dispersed in less polar or neutral hydrocarbon matrix, forming a three-dimensional compatible polymeric networks. The polar species in asphalts are more sensitive to oxidative aging under the service conditions than the matrix materials. Oxidative aging could enhance polar content to cause asphalt agglomeration, increased viscosity and elasticity, and reduced flexibility of asphalt binder [12–14]. According to the classical Corbett Method, petroleum asphalt can be divided in three representative radicals [22], including (1) asphaltenes, (2) aromatics, (3) resins, and (4) saturates. Zhang and Greenfield proposed an average asphalt molecule based on molecular analysis [23], which has been adopted in many numerical analyses. Table 1 shows the asphalt components and lignin molecule (coniferyl alcohol) used to build the asphalt-lignin model. This model will be exposed to oxygen molecules to evaluate lignin as a potential antioxidant for asphalt as hydrocarbon system. The asphalt molecules in Table 1 have been used for studying oxidation of asphalt not modified by lignins [24]. Fig. 1 shows the asphalt-lignin-oxygen system before and after oxidation. The model includes one lignin molecule (C59 H72 O21 ), one average asphalt molecule (C82 H101 N3 O2 S2 ), one resin (C12 H12 ), one saturate: C16 H34 , and one saturate: C5 H12 surrounded by 50 oxygen molecules. An NVT ensemble was built in a 50 × 50 × 50 angstrom3 domain to simulate the air-asphalt interface at the surface of a lignin-modified asphalt shingle (see Fig. 1). The simulation was executed at an intermediate environmental temperature of 28 ◦ C. As expected, the systematic energy and temperature of the model both well converged before the specified 1000 computational steps were finished. The convergences of temperature and energy demonstrated that the AIMD model was properly built. Based on the chemical-bond vibrations of radicals, an infrared (IR) analysis was ran in this study on asphalt molecules to identify the radical species generated in the oxidation process. The radical species of carbon, nitrogen and sulfur generated during asphalt oxidation, as indicted by the characteristic absorption frequency bands are given in Table 2. The mechanism of each absorption band is also provided in the table. The weak absorption occurred at 3500–3700 cm−1 can be attributed to the OH stretch of phenolic groups (3550–3650 cm−1 ) and N H stretch of pyrrolic group (3600–3700 cm−1 ). The hydroxyl groups give a strong absorption band at the 1050–1150 cm−1 and a weak

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Table 1 Lignin and asphalt molecules for evaluating lignin as a potential antioxidant. Items

Lignin (C59 H72 O21 )

Average asphalt (C82 H101 N3 O2 S2 )

Resin (C12 H12 )

Saturate (C5 H12 , C16 H34 )

Molecular structure

absorption band at 3620–3640 cm−1 . The area of strong absorption bands at 1050–1150 cm−1 greatly increases after oxidation, implying the formation of hydroxyl groups. The spectrum of oxidized asphalt shows two strong absorption peaks at wavenumbers of 1375 cm−1 and 1109 cm−1 relative to asphalt before oxidation, as caused by the asymmetric (1420–1300 cm−1 ) and symmetric (1200–1000 cm−1 ) stretch of the S O bonds, indicating that the oxidation of the thioether (C S C) structures of the asphalt molecule. The N H stretch of amine causes IR absorption at the wavenumber of 3500–3300 cm−1 , and the H N H scissoring causes an absorption peak at the wavenumber of 1765 cm−1 . 4. Evaluation of lignin as an antioxidant During the simulation process, it was observed that the asphalt molecules got oxidized first, at one carbon atom on a branch acyclic alkane followed by another carbon atom on an aromatic ring structure. Under the same oxidative condition, oxidization occurred to the C OH group on one side chain of the lignin molecule after the alkyl carbon atom of the asphalt molecule became oxidized.

The sulfur and nitrogen atoms of the average asphalt molecule also got oxidized and form oxidants after the oxidation of the lignin molecules. The nitrogen oxidants seemed to be more unstable than the sulfur oxidants as caused by the aromatic structure next to the nitrogen atoms. These observation indicates that the lignin may not be able to get oxidized prior to the oxidation of the asphalt molecule to lower the probability of asphalt oxidation. Also, the lignin molecule did not show any radical-scavenging behavior to reduce the extent of asphalt oxidation. A series of analytical-chemical analyses were conducted to explain the phenomena observed from the asphalt-lignin-oxygen system. The evaluation of lignin as a potential antioxidant of asphalt is accomplished by tracking the generation process of critical radicals occurred to the lignin and asphalt molecules. This task involves the use of one spectrum method, i.e., the nuclear magnetic resonance (NMR), and the analytical methods of density of states (DOS) analysis, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) analyses, bonding and energy level analysis, and electrostatic potential energy analysis. The results of these analyses are detailed in the following sections.

Table 2 Carbon, nitrogen and sulfur radicals generated during oxidation of asphalt. IR absorption frequency (cm−1 )

Absorption mechanism

1050–1150 (strong) 3620–3640 (weak)

Broad adsorption bands of hydroxyl groups bonded to the primary, secondary and tertiary carbon atoms

S O

1000–1200 1300–1420

Asymmetric stretch of S O bonds Symmetric stretch of S O bonds

N H

1765 3500–3300

H N H scissoring N H stretch of primary amines

C O

1690–1700

Stretch of C O groups (ketone, carbonyl)

Polycyclic aromatic rings

690–900

A broad absorption band of polycyclic aromatic structures

Phenolic group

3550–3650

Pyrrolic group

3600–3700

Radical Species

OH

Structure

OH stretch of phenolic group

N H stretch of pyrrolic group

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4.1. NMR analysis The NMR analyses were performed to obtain the overall changes in the molecular structures of the lignin and the average asphalt during oxidation. In the C-NMR spectrum of the asphalt molecule before oxidation shown in Fig. 2a, 43% of carbon atoms belong to the aromatic structure. The NMR signals observed in this study are in the range of 91.47–139.77 ppm, which is within the theoretical chemical shift region of 90–150 ppm [14,15]. Aromatic carbon chemical shifts were observed at 139.77 and 133.21 ppm. These shifts are possibly caused by the chemical group (phenol) and the heterocyclic element (N). The chemical shift of the amide-group carbon occurs at 178.48 ppm and the lowest signal observed at 91.47 ppm is caused by an aromatic carbons atom. In the oxidized asphalt molecule shown in Fig. 2b, less aromatic carbon atoms were detected (40%) due to the destruction of aromatic structure during oxidation. The observed signal region is in the range of 87.11–138.32 ppm, and shifts of aromatic carbon were observed at 138.32 ppm, 135.53 ppm and 129.73 ppm. The lowest signal of the aromatic carbon was observed at 87.11 ppm. Comparing to the location of the carbon atom on the amide-group before oxidation, the NMR signal occurs at 199.35 ppm as caused by the structural transform from an amide-group to an aldehyde during oxidation. Chemical shifts of the protons (H nuclei) of hydrocarbon molecules vary greatly with the surrounding electronic environment of the protons [16]. Hydrogen-attached or electronwithdrawing atoms/groups can lower the shielding and move the resonance of attached protons towards a higher frequency. By contrast, electron-donating atoms or groups increase the shielding and move the resonance towards to a lower frequency. The H NMR spectrum of the average asphalt molecule before oxidation is given in Fig. 3a. Theoretical chemical shifts of aromatic protons of organic compounds occur in the range of 6.00–8.00 ppm. The signals of the aromatic protons of the average asphalt molecule before oxidation were observed at 6.24 ppm, 6.65 ppm, 7.61 ppm and 7.94 ppm. The chemical shifts for the two heterocyclic-ring protons are observed at 5.87 ppm and 9.17 ppm, respectively. The H NMR spectra for the oxidized asphalt molecule is given in Fig. 3b. Comparing to Fig. 2b, the signals of the aromatic protons of the oxidized asphalt molecule were observed at 4.60 ppm, 6.28 ppm, 6.65 ppm and 7.12 ppm. The great changes of the aromatic protons were caused by the unstable state of the structures after oxidation. The NMR signals of the two heterocyclic-ring protons shifted to 5.34 ppm and 8.46 ppm, respectively after oxidation. It is also noteworthy that the protons on the aldehyde group, which were generated from the amide group before oxidation, showed a higher chemical shift at 10.45 ppm. The C-NMR spectrum of the lignin molecule is given in Fig. 4a, of which 61% of the carbon atoms belong to the aromatic rings. Comparing to the theoretical chemical shift region of 90–150 ppm for aromatic carbons, the aromatic-carbon signals observed in the lignin are in the range of 113.56–149.23 ppm. The aromatic carbon chemical shifts observed at 113.56 ppm was possibly caused by the penta-heterocyclic ring containing an oxygen atom. According to the C-NMR spectrum, the chemical shift of the ketone and aldehyde carbons occur at 183.12 ppm, 196.43 ppm and 205.71 ppm, respectively. The C-NMR peaks of the lignin molecule at aromatic carbon region are distributed more averagely comparing to the C-NMR spectrum of the asphalt molecule before oxidation. This phenomenon can be explained by the structural difference between two molecules. The aromatic structures of lignin are single phenyl rings at different parts of the molecule, while the asphalt molecule has a polycyclic aromatic structure at the center of the molecule. The separate phenyl rings in lignin may have reduced polarity (electrical attraction) and/or reactivity than that of the polycyclic

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aromatic structure of the asphalt molecule, leading to the later oxidation of the lignin molecule. The H-NMR spectrum for the lignin molecule is given in Fig. 4b. Comparing to the H-NMR spectrum for the asphalt molecule before oxidation shown in Fig. 3a, there are more signals between 5.00 ppm and 10.00 ppm, and the H-NMR peaks between 0.00 ppm and 5.00 ppm are distributed more averagely. The reason is that lignin has more protons attached to aromatic rings, and the populations of H atoms bonded to different structures are similar. The more uniformly distributed OH radicals in lignin may be responsible for the later oxidation of the lignin molecule than the asphalt molecule under the same condition. 4.2. Density of states analysis In quantum mechanics, the density of states D(E) per Eq. (5) determines the number of allowable electron states per unit volume for a range of energy, where L is the Lorentzian function and ei denotes the one-electron energies. Integrating the density of states over an energy range per Eq. (6) will produce a number of states. The number of electrons at each energy level, also known as the total DOS, then can be obtained by multiplying the number of states N with the probability that a state is occupied by an electron [17–19]. The density of states can be used to evaluate the number and characteristics of molecular orbitals for a given molecular system. D(E) =



 i

L(E − ei )

(5)

E

N=

D(E)dE

(6)

E

The density of states can also be determined for each chemical element in a molecular system at a given energy level, which is known as the partial density of states (PDOS). In DFT calculations, molecular orbitals are expressed as density functions. In order to determine the PDOS or the contribution of one chemical element to the total DOS at a given energy level, the expression for the total DOS per Eq. (5) can be transformed to Eq. (7), where the Pi,j is the percentage factor of the element with respect to the total population of the system [20]. If the percentage factor Pi,j of an element j is determined by its density functionωj , the partial density of states can be obtained by Eq. (8), where i = (ri ) is the wavefunction of the chemical element j. Dj (E) = Dj (E) =

 

i

i

Pi,j L(E − ei )

(7)

|ωj |ϕi |2 L(E − ei )

(8)

The total DOS and PDOS spectra are used herein to demonstrate the molecular orbital compositions of the lignin and asphalt systems, based on which the relative easiness of oxidation of the two systems can be compared. The total DOS and PDOS of different species in lignin vs. asphalt before oxidation are shown in Fig. 5a and b. At the energy levels of −5.0–−6.0 eV, a dramatic energy gap can be observed between the HOMO and LUMO energy levels in both systems. The spectra of occupied molecular orbitals (OMO) are located on the left side of the gap, while the spectra of unoccupied molecular orbitals (UMO) are on the right side of the gap. In the total DOS spectrum of lignin shown in Fig. 5a, approximately 98% of the occupied orbital electron states lie within the interval of −28.4975 eV–−6.0409 eV, while 98% of the unoccupied orbital electron states lie within the interval of −3.9729 eV–89.7123 eV. In the total DOS spectrum of the asphalt molecule before oxidation shown in Fig. 5b, 98% of the occupied orbital electron states lie within the interval of −27.7847 eV–−5.1647 eV, and 98% of

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Fig. 2. (a) C-NMR spectrum of asphalt molecule before oxidation (b) C-NMR spectrum of asphalt molecule after oxidation.

the unoccupied orbital electron states lie within the interval of −3.6001 eV–79.2483 eV. The differences between Fig. 6a and b tell that the total DOS of the lignin molecule distributes more averagely in the unoccupied molecule orbital (UMO) region as compared to the total DOS of the asphalt molecule before oxidation. At the high energy level (about 20–60 eV) of the UMO region, the apparently higher total

DOS of the lignin system shows that more atoms are located at the high energy level than the asphalt system. Among all the species of the lignin system, the aromatic carbon atoms contribute about 50% to the total DOS between 20 eV and 60 eV; while the contribution given by aromatic carbon to the asphalt system is in the region of 0–20 eV. The different total DOS is caused by the structural difference between the two molecules. In lignin molecule, the aromatic

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Fig. 3. (a) H-NMR spectrum of asphalt molecule before oxidation (b) H-NMR spectra of asphalt molecule after oxidation.

structures are single phenyl rings connected by aliphatic and ester groups, the UMO energy of each phenyl ring is rather different. In contrast, the asphalt molecule has a polycyclic aromatic structure located at the center of the molecule, of which the energy distribution is narrower due to the conjugation effects [32]. The spectra of lignin and asphalt before oxidation indicate that the nuclei of the aromatic structure of lignin has greater affinity towards electrons, or that lignin cannot be oxidized as easily as the asphalt molecule selected in this study.

4.3. HOMO and LUMO analyses The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of a molecular system has been commonly used to evaluate the system’s chemical reactivity [33]. When chemical reaction occurs in a molecular system, electrons transfer across the HOMO–LUMO gap at the cost of an energy change [34]. The magnitude of the energy gap between the HOMO and LUMO orbitals can be used to predict the chemical reactivity

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Fig. 4. (a) C-NMR spectrum of the lignin molecule (b) H-NMR spectrum of the lignin molecule.

of different molecular systems [35]. In general, a molecular system with a larger HOMO–LUMO gap is less reactive than a system with a smaller gap. Based on HOMO and LUMO, the hardness ( ) defined as one half of the HOMO–LUMO energy gap, the ionization energy (IE) defined as the HOMO orbital energies, i.e., IE = − D HOMO , and the electron affinity (EA) defined as the LUMO orbital energies, i.e., EA = − D LUMO are energy indicators for characterizing the relative oxidation reactivity of different materials [36–40]. These indicators of reactivities

are used in this study to compare the oxidation of the selected lignin and asphalt molecules. The computation of the HOMO and LUMO of the lignin and asphalt molecules was conducted using the Density Functional Theory through the qB3LYP/6-311++G(d,p) method [41]. The molecules were optimized in the ground state in electronic-structure calculations using the Geometry Optimization Functional built in DFT. The energies of the orbitals C O C, CH2 OH, CH OH, phenyl OH, and phenyl groups were calculated individually.

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Fig. 5. (a) Total DOS and PDOS of species of lignin molecular system (b) Total DOS and PDOS of species of asphalt molecular system before oxidation.

Fig. 6. Overlapped HOMO (a) and Overlapped LUMO (b) of asphalt before and after oxidation.

The HOMO region of an electronic system tends to donate electrons to get oxidized, and the LUMO region is more likely to accept electrons to get reduced. Fig. 6a shows the HOMO of the asphalt molecule before oxidation overlapped with the HOMO of the asphalt molecule after oxidation. Fig. 6b shows the LUMO of the asphalt molecule before oxidation overlapped with the LUMO of the asphalt molecule after oxidation. In Fig. 6a, the HOMO locates at the C S C radical on one branch chain. The central sulfur atom

shares electrons with the two neighbor carbon atoms via bonding and the spare electrons form two occupied orbitals after orbital hybridization. In Fig. 6b, the LUMO is located separately on the aromatic rings. After oxidation, both the HOMO and LUMO regions of the asphalt molecule become less distinct, demonstrating a system that is more unstable. Fig. 7a and b present the HOMO and HOMO-1 of the lignin molecule respectively, and Fig. 8a and b present the LUMO and

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Fig. 7. Illustration of HOMO energy (a) and HOMO-1 energy (b) of lignin.

Fig. 8. Illustration of LUMO energy (a) and LUMO + 1 energy (b) of lignin.

Table 3 HOMO and LUMO based analyses of lignin and asphalt before oxidation. Energies (eV)

Lignin

Asphalt before oxidation

D HOMO D HOMO -1 D LUMO D LUMO+1 D gap = − (D HOMO –D LUMO ) = 1/2 D gap IE = − D HOMO EA= −D LUMO

−6.0409 −6.2042 −3.9729 −3.7905 2.0680 1.0340 6.0409 3.9729

−5.1647 −5.2219 −3.6001 −3.2137 1.5646 0.7823 5.1647 3.6001

LUMO + 1, respectively. In Fig. 7a, the HOMO locates in the region that contains two alcohol groups and a phenyl group. The hydroxyls of the alcohol groups are polarized chemical groups with strong reducibility [42]. Thus the hydroxyl groups have the occupied molecular orbitals with highest energy. In Fig. 7b, the HOMO-1s lie on the similar region of the HOMOs. The LUMO and LUMO + 1s are observed at the central part of the lignin molecule in Fig. 8a and b, which indicates that it is easier for the central part of lignin to accept the excited electron from the HOMO. To further compare the relative easiness of the oxidation of the selected lignin and asphalt molecules under the same condition, the HOMO energies (D HOMO ), LUMO energies (D LUMO ), hardness ( ), ionization energy (IE), and electron affinity (EA) of the lignin molecular system were computed and listed in Table 3, side by side with such energy indicators of the asphalt molecule before oxidation. To show a broad picture of the electronic structure, four energy levels were determined for the lignin and asphalt molecular systems, including the HOMO, HOMO-1 energies and LUMO, LUMO + 1 energies. The HOMO–LUMO gap is 2.0680 eV for lignin before oxidation and 1.5646 eV and asphalt before oxidation, respectively. In agreement to the DOS analysis, this difference further implies that the selected asphalt is chemically more reactive than the lignin during oxidation. For the simulated condition, the HOMO energy of the oxygen molecules was calculated to be −12.0697 eV and the LUMO energy of the oxygen molecules is −0.4480 eV. According to Table 3, the HOMO energy of lignin (−6.0409 eV) is lower than the HOMO

energy of asphalt before oxidation (−5.1647 eV). Thus, the energy gap between the HOMO of asphalt before oxidation and the LUMO of oxygen molecule (LUMOO2 − HOMOAsphalt ) is smaller than the gap between the HOMO of lignin and the LUMO of oxygen molecule (LUMOO2 − HOMOLignin ). Equally speaking, the electrons at HOMO state of the asphalt molecule needs less excitation energy than those of the lignin molecule to reach the LUMO of the oxygen molecules. Therefore, it is easier for the relevant chemical groups of the asphalt molecule at HOMO state, i.e., C S C group, to get oxidized than the groups of lignin at HOMO state (the alcohol groups). According to Table 3, the IE of the lignin molecule (6.0409 eV) is higher than the asphalt molecule before oxidation (5.1647 eV). This difference also indicates that the lignin needs slightly more energy to lose the first electron (to get oxidized), and thus is harder to get oxidized than the asphalt molecule. The asphalt-lignin-oxygen system possesses two electron transition paths for the first excitation energy of the oxidation reaction. The first excitation energy of oxidation represents the lowest energy required for an electron to jump to the next energy level [43]. One electron transition path is the transition from HOMO to LUMO and the other path is the transition from HOMO-1 to HOMO. As for the asphalt molecule before oxidation, the electron transition from HOMO to LUMO needs significantly more energy than the transition from HOMO-1 to HOMO. Thus, the first electronic excited state for the asphalt before oxidation is due to the HOMO1 to HOMO transition, and the value is 0.0572 eV. For the lignin molecule, the first excitation energy is also due to the HOMO-1 to HOMO transition, which is 0.1633 eV. Comparing the value of first excitation energies of that two molecules, the first excitation energy of the asphalt molecule is smaller, which means that the asphalt before oxidation needs less energy to reach the excitation level. 4.4. Bonding and energy levels analysis The orbital compositions of the asphalt and lignin molecules were further examined to illustrate the chemical changes during oxidation. With respect to the energy levels of the HOMO and LUMO shown in Figs. 9 and 10a and b, and the detailed orbital composition

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Table 4 HOMO and LUMO orbital composition of asphalt molecule before oxidation. HOMO Atom type S H H C C C C H H C S

LUMO Orbital type 1P:y 1S 1S 1P:z 1P:y 1P:z 1P:y 1S 1S 1P:y 1P:x

Percent (%) composition 71.68 4.45 4.08 1.83 1.57 1.44 1.21 1.17 1.10 1.07 1.03

Fragment orbital sfo 709 sfo 847 sfo 845 sfo 607 sfo 597 sfo 599 sfo 605 sfo 859 sfo 853 sfo 581 sfo 707

Energy level = −5.164703 eV

Atom type Orbital type C 1P:z C 1P:y C 1P:z C 1P:x C 1P:x C 1P:y C 1P:x C 1P:z C 1P:z C 1P:y C 1P:y C 1P:y 1P:y C 1P:y C 1P:z C 1P:x C 1P:y C 1P:y C C 1P:x 1P:x C C 1P:y 1P:y C 1P:z C 1P:x C 1P:y C C 1P:y 1P:x C C 1P:x 1P:x C 1P:y C C 1P:y 1P:y C 1P:x C 1P:x C Energy level = −3.213747 eV

Percent (%) composition 7.70 5.69 4.69 4.66 3.54 3.21 3.07 2.64 2.33 2.20 2.19 2.06 1.97 1.94 1.75 1.67 1.67 1.63 1.57 1.51 1.48 1.45 1.43 1.41 1.33 1.33 1.30 1.26 1.25 1.13 1.11 1.08 1.07 1.03

Fragment orbital sfo 351 sfo 125 sfo 175 sfo 155 sfo 331 sfo 37 sfo 171 sfo 335 sfo 159 sfo 117 sfo 317 sfo 301 sfo 21 sfo 325 sfo 119 sfo 347 sfo 173 sfo 181 sfo 123 sfo 315 sfo 333 sfo 285 sfo 23 sfo 19 sfo 197 sfo 29 sfo 179 sfo 323 sfo 203 sfo 61 sfo 69 sfo 205 sfo 91 sfo 115

Table 5 HOMO and LUMO orbital composition of asphalt molecule after oxidation. HOMO

LUMO

Atom type

Orbital type

Percentage composition (%)

Fragment orbital

Atom type

O S O O C C C C O S H S S O C C C C C C C N

1P:z 1P:z 1P:x 1P:z 1P:y 1P:y 1P:y 1P:y 1P:y 1P:x 1S 1P:y 1S 1P:x 1P:z 1P:y 1P:z 1P:z 1P:y 1P:z 1P:z 1P:y

7.95 6.62 6.33 6.00 5.26 4.94 4.86 4.67 4.28 4.04 3.89 3.55 2.81 2.76 1.60 1.43 1.30 1.30 1.27 1.11 1.02 1.00

sfo 253 sfo 981 sfo 241 sfo 245 sfo 435 sfo 451 sfo 467 sfo 251 sfo 243 sfo 977 sfo 145 sfo 979 sfo 975 sfo 249 sfo 325 sfo 635 sfo 613 sfo 469 sfo 491 sfo 493 sfo 357 sfo 947

O 1P:z C 1P:y C 1P:y C 1P:y O 1P:x S 1P:z O 1P:z O 1P:y S 1P:x O 1P:y H 1S S 1P:y S 1S O 1P:x C 1P:z C 1P:z C 1P:y C 1P:z C 1P:y C 1P:z C 1P:z C 1P:y 1P:y C 1P:y N 1P:z C 1P:z N 1P:x C 1P:z C 1P:y C 1P:y C Energy level = −4.862720 eV

Energy level = −4.912276 eV

Orbital type

Percentage composition (%)

Fragment orbital

6.99 6.78 6.06 5.88 5.48 5.41 4.84 3.89 3.49 3.28 3.20 2.72 2.57 2.34 1.70 1.60 1.60 1.58 1.54 1.35 1.34 1.26 1.21 1.21 1.20 1.14 1.08 1.07 1.07 1.04

sfo 253 sfo 435 sfo 451 sfo 467 sfo 241 sfo 981 sfo 245 sfo 251 sfo 977 sfo 243 sfo 145 sfo 979 sfo 975 sfo 249 sfo 325 sfo 469 sfo 635 sfo 613 sfo 491 sfo 493 sfo 357 sfo 339 sfo 379 sfo 947 sfo 381 sfo 949 sfo 465 sfo 517 sfo 515 sfo 355

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Fig. 9. (a) HOMO orbital of asphalt molecule before oxidation (b) LUMO orbital of asphalt molecule before oxidation.

of HOMO and LUMO given in Tables 4 and 5, the orbital composition and the types of the chemical bonds and the status of orbital overlapping can be understood. Obvious differences can be observed between the asphalt before oxidation and the asphalt after oxidation shown in Tables 4 and 5 and Figs. 9 and 10. The HOMO and LUMO energy levels of the asphalt molecule after oxidation share many fragment orbitals, while the HOMO and LUMO energy levels of the asphalt molecule before oxidation are quite separate. This phenomenon indicates that the overlapping of orbitals occurs when excitation energy is received in oxidation. This phenomenon well explains the reduced the HOMO and LUMO energy gap after oxidation.

Based on Fig. 10a and b and Table 5, the HOMO and LUMO of the asphalt molecule after oxidation share fragment orbitals including C1P:y(sfo 435), C1P:y(sfo 451), O1P:x(sfo 241), O1P:y(sfo 243), O1P:z(sfo 245), O1P:x(sfo 249), O1P:y(sfo 251) and O1P:z(sfo 253). To examine the bonding mechanism of S and O atoms during asphalt oxidation, the structure of O S O is taken as an example. There were two oxygen atoms that formed bonds with the sulfur atom on the branched chain of the asphalt after oxidation. At the HOMO states, the fragment orbitals O1P:x(sfo 241), O1P:y(sfo 243), O1P:z(sfo 245) belong to one oxygen atom, and the O1P:x(sfo 249), O1P:y(sfo 251) and O1P:z(sfo 253) belong to the other oxygen atom. Four orbitals, i.e., S1S(sfo 975), S1P:x(sfo 977), S1P:y(sfo

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Fig. 10. (a) HOMO orbital of asphalt molecule after oxidation (b) LUMO orbital of asphalt molecule after oxidation.

979), S1P:z(sfo 981) can be observed at the LUMO level. These four orbitals belong to the sulfur atom on the branched chain of asphalt, and are bonded with the two oxygen atoms. The sulfur atom forms one pair of bond and  bond with each oxygen atom, forming a ..

O = S = O structure. This structure is unstable and can be further oxidized under the typical field service conditions. According to Table 6 and Fig. 11a and b, the HOMO and LUMO of lignin are composed of fragment orbitals of different atoms, which is similar to the case of asphalt molecule before oxidation. This difference can be seen by comparing Fig. 11 to Figs. 9 and 10. The HOMO and LUMO each represent a different region of atoms in each molecule. Based on Table 6, the HOMO and LUMO of lignin molecule

is mainly composed of the p orbitals of carbon and oxygen atoms. For the HOMO composition, two orbitals of the oxygen of one alcohol group located at the end of one branch chain have the greatest contribution. The two orbitals O1P:z(sfo 1053) and O1P:x(sfo 1049) contribute to the HOMO by 11.96% and 10.81% respectively, which indicates that this alcohol group has the highest energy level and tends to lose electrons upon excitation. Equally speaking, the alcohol group has greater reducibility to get oxidized. The phenyl ring on the same branch chain as the alcohol group does also contributes to the HOMO energy level. According to Table 6, the p orbitals of the carbon atoms at the HOMO state on the phenyl ring form ␲ bonds around the phenyl ring, creating an electron-abundant region with

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Table 6 HOMO and LUMO orbital composition of lignin molecule. HOMO Atom type O O C C C C C C O C C O C C H H H

LUMO Orbital type 1P:z 1P:x 1P:z 1P:x 1P:z 1P:z 1P:x 1P:x 1P:y 1P:z 1P:z 1P:y 1P:x 1P:x 1S 1S 1S

Percentage composition (%) 11.96 10.81 9.50 8.04 7.05 6.49 5.50 5.19 3.72 3.09 2.71 2.35 2.35 2.31 2.07 2.01 1.65

Fragment Orbital sfo 1053 sfo 1049 sfo 501 sfo 497 sfo 449 sfo 462 sfo 458 sfo 445 sfo 1064 sfo 488 sfo 475 sfo 1077 sfo 484 sfo 471 sfo 853 sfo 863 sfo 858

Energy level = −6.040901 eV

high energy that might attract electrophoresis compounds or structures during oxidation [44,45]. According to Table 6, the LUMO of the lignin molecule is composed mainly by the carbon atoms on the phenyl ring at its central part. As a result, this region has the greatest tendency to receive an excited electron. 4.5. Electrostatic potential energy analysis The electrostatic potential energy of a pair of charged entities is dependent on the amounts of the two charged entities and the distance between them. The electrostatic potential map of a molecule is a nucleus-based three-dimensional presentation of the electrostatic potential energy of the molecule. The map represents the charge distribution and electrostatic potential energy calculated from the nuclei and electron clouds. The lignin and asphalt molecules each carry a number of charged entities including electrons and nuclei. The electrostatic potential map enables quick visualization of the electrostatic energy and the charge distribution of the molecules, and thus can be used to study the anti-oxidative properties of the lignin. In the electrostatic potential map of a hydrocarbon molecule, the positively charged nucleus emits a radially distributed electric field. A region with above-average potential energy carries either a stronger positive charge or a weaker negative charge [46]. Given the stable positive nucleus charge in an electrostatic potential map, a region with higher potential energy values implies fewer electrons in this region, and a region with low electrostatic potential indicates an abundance of electrons in this region. The electrostatic potential map thus allows the visualization of the shape, size and density of electrons cloud [47]. In hydrocarbons, the most reactive sites on a molecule often show different levels of electrostatic potential form its surrounding regions [48]. This property is useful in studying the agglomeration and decomposition behavior of asphalt during oxidation. The electrostatic potentials of asphalt and lignin molecules are mapped on an isosurface with an electron density of 0.04 electrons per cubic Bohr radius (e/a0 −3 ). Fig. 12a shows the electrostatic potential map of the asphalt molecule before oxidation, and Fig. 13a shows the electrostatic potential map of the oxidized asphalt molecule. The colors on the electrostatic potential maps denote the value of electrostatic potential, with the red color representing negative potential and blue representing positive potential. From the two

Atom Type Orbital Type C 1P:z C 1P:x C 1P:z C 1P:x C 1P:z O 1P:z C 1P:x O 1P:z C 1P:z O 1P:x C 1P:y C 1P:x C 1P:z C 1P:y O 1P:x C 1P:z C 1P:x 1P:x C 1P:y C O 1P:y 1P:y C Energy level = −3.972947 eV

Percentage composition (%) 9.68 9.15 9.03 5.52 4.85 4.38 4.33 3.98 3.75 3.66 3.56 3.41 3.30 3.27 3.26 3.14 2.81 2.36 2.10 2.09 1.37

Fragment Orbital sfo 332 sfo 276 sfo 280 sfo 328 sfo 254 sfo 988 sfo 250 sfo 1001 sfo 410 sfo 997 sfo 330 sfo 406 sfo 293 sfo 278 sfo 984 sfo 319 sfo 289 sfo 315 sfo 408 sfo 986 sfo 291

figures, the average electrostatic potential of the asphalt molecule before oxidation is 0.085 eV. The sulfur atom on the heterocyclic ring has a potential of 0.7 eV, and the sulfur atom on the branched chain has a potential of −0.01 eV. The oxygen and nitrogen atoms show a potential of about −0.06 eV. The hydrogen nuclei bonded with the charged N and O atoms give higher potential than those bonded with carbon atoms, indicating that the amide and phenol radicals are the reactive sites on the asphalt molecule. The average electrostatic potential of the molecule (Fig. 12b) became doubled after oxidation, with the number of more polarized S O, S OH, and phenol (C6-OH) groups increased. The H atoms bonded to these radicals can be more easily ionized. Fig. 13 shows the two-side views of the three-dimensional electrostatic potential maps of the lignin molecule before oxidation. The colors on the maps indicate the levels of electrostatic potential. Red color represents the negative potential and blue represents the positive potential. From Fig. 13, the average electrostatic potential of the lignin molecule is 0.10 eV. The potential around most of the carbon nucleus is at this state. The potential around hydrogen atoms attached to the phenyl rings is more positive (about 0.236 eV). The carbon atoms on aromatic rings and aliphatic groups have the potential of 0.10 eV approximately, which is slightly higher than that of the asphalt molecule before oxidation (0.07 eV). This phenomenon shows that there are more electrophoresis structures (such as C O) in the lignin molecule than in the asphalt molecule before oxidation. As a result, more ‘red’ regions can be seen in the potential map of lignin, owing to the oxygen atoms. The electrons of aromatic rings and aliphatic groups tend to be attracted by the C O structures, forming relatively lower potential regions [49]. Comparing to the potential energy map of the asphalt molecule before oxidation, there are more regions in the lignin molecule that show great differentiation in potential energy. For example, the oxygen sides of hydroxyl groups have the potential of −0.04 eV, while the hydrogen sides have the potential of 0.50 eV, and these structures are obviously polarized. This phenomenon indicates that on average the lignin molecule is more polarized than the asphalt molecule. If lignin is used as an additive compound to asphalt, some physic properties of asphalt, such as hydrophobicity, can be changed.

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Fig. 11. (a) HOMO orbital of lignin molecule (b) LUMO orbital of lignin molecule.

5. Summary and conclusions This study aims to evaluate the use of lignins as a potential antioxidant for general hydrocarbon systems, using asphalt binder as an example. This objective was fulfilled by building a numerical asphalt-lignin model using the method of ab initio molecular dynamics, and by tracking the generation of new species form the asphalt and lignin molecules under the same oxidative condition. The evaluation of lignin as a potential antioxidant of asphalt involves the use of one spectrum method, i.e., the nuclear magnetic resonance (NMR), and the analytical methods of density of states analysis, highest occupied molecular orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) analyses, bonding

and energy level analysis, and electrostatic potential energy analysis. Based on the research results, it was found that the chemical radicals of carbon, nitrogen and sulfur generated in the oxidation process tend to enhance the agglomeration and/or decomposition of aged asphalt. The effectiveness of lignins as an antioxidant for asphalt depends on its chemical composition. Lignins do not show anti-oxidative capacity in hydrocarbon systems with a smaller HOMO–LUMO gap, such as the conifer alcohol lignin in the asphalt system as is studied in this work. Lignins however can be effective oxidants for hydrocarbon systems with a larger HOMO–LUMO gap. Also, lignins may have more polar and active sites than hydrocarbons; thus the lignins’ hydrophobicity and compatibility with

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Fig. 12. Electrostatic potential map of asphalt molecule before oxidation (a) and after oxidation (b).

Fig. 13. Two-side view of the electrostatic potential map of lignin molecule before oxidation.

the host matrix ought to be evaluated when the lignins are evaluated as a potential antioxidant. The developed ab initio molecular dynamics model is useful for evaluating antioxidants for general hydrocarbon systems.

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