Alteration of Ï Electron Distribution to Induce Deagglomeration in

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Alteration of # Electron Distribution to Induce Deagglomeration in Oxidized Polar Aromatics and Asphaltenes in an Aged Asphalt Binder Farideh Pahlavan, Albert M Hung, Mehdi Zadshir, Shahrzad Hosseinnezhad, and Ellie H. Fini ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00364 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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ACS Sustainable Chemistry & Engineering

Alteration of π Electron Distribution to Induce Deagglomeration in Oxidized Polar Aromatics and Asphaltenes in an Aged Asphalt Binder Farideh Pahlavan,a Albert M. Hung,a Mehdi Zadshir,b Shahrzad Hosseinnezhad,c Ellie H. Finib* * Corresponding author. Tel.: 336-285-3676; fax: 336-334-7126; e-mail: [email protected] (E. H. Fini). a. Innovation Center for Materials, Methods and Management, Division of Research and Economic Development, North Carolina A&T State University, 1601 E. Market St., Greensboro, NC 27411, USA b. Department of Civil, Architectural and Environmental Engineering, North Carolina A&T State University, 1601 E. Market St., Greensboro, NC 27411, USA c. College of Science and Technology, North Carolina A&T State University, 1601 E. Market St., Greensboro, NC 27411, USA

ABSTRACT This paper features electronic evaluation explaining molecular association and dissociation during oxidative aging and rejuvenation process in polyaromatic hydrocarbons. Specifically, we employ both computational modeling and laboratory experiments to show how the presence of external stimuli such as Bio-Rejuvenator can alter π-electron distribution to further induce deagglomeration. To better link findings to real-world materials and application, the study is done on the oxidized asphaltene and resin molecules; in addition, the disturbance of electron distribution is done using a Bio-Rejuvenator derived from animal waste. As petroleum asphalt binder oxidizes, the association forces between polar aromatics are strengthened owing to the introduction of polar chemical functionalities to their molecular structure. This is evidenced by X-ray diffraction (XRD) measurements showing that a new peak at high d-spacing appears after oxidation that is consistent with graphene-oxide-like (GO-like) structures. The GO-like structures become amorphous after the addition of BR, and the related peak in the XRD spectra vanishes. Density functional theory calculations show a destructive effect of BR on the π-π interaction between polyaromatics in the aggregation. The resulting π-electron disruption is induced by the polar head of BioRejuvenator with electron-withdrawing nature and its hydrocarbon tail with the ability of CH-π stacking with oxidized asphalt molecule. This, in turn, indicates that the Bio-Rejuvenator can act to restore oxidized asphalt binder by disassembling asphaltene agglomerates and disrupting resinous GO-like

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structures through three stages mechanism: partial penetration into small pores around agglomerates, partial segregation of agglomerates, and finally dispersion of smaller agglomerates. Keywords: Asphaltene, aged asphalt binder, oxidative aging, Bio-Rejuvenator, rejuvenation mechanism, density functional theory (DFT), X-ray diffraction (XRD).

INTRODUCTION Polar aromatics and asphaltenes in petroleum asphalt binder are subject to irreversible oxidative aging by reaction with atmospheric oxygen. Oxidation can proceed by either physicochemical or chemical mechanism. Through physicochemical aging, asphalt can be enriched in heavy polyaromatic compounds, possibly due to the evaporation of light volatile components and the consequent reduction in the maltene to asphaltene phase ratio. Chemical aging of asphalt fractions is mostly thought to proceed by exposure to oxidation reactions that create oxygen-containing functional groups in the asphalt structures; this causes the asphalt fractions to evolve irreversibly into more polar components, consequently intensifying intermolecular aggregation.1 The four generic asphalt fractions, which include saturates, naphthene aromatics, polar aromatics (resins), and asphaltenes (referred to as SARA in the order of increasing polarity),2 each show different behaviors toward oxidative aging because of their differences in polarity and chemical reactivity. Saturates are relatively resistant to reactions with oxygen, and their content is mostly unaffected by oxidation.3-4 In contrast, oxidative aging leads to losses in the naphthene and polar aromatics content, accompanied by an increase in the asphaltenes.5-6 Association forces and molecular aggregation are responsible for the physical properties of asphalt. Oxidative aging over time is a crucial factor that changes the rheological properties of asphalt through the alteration of its chemical composition, leading to undesirable properties and performance failure.7-8 Therefore, in-depth understanding of underlying mechanisms controlling aggregation phenomenon at the molecular level before and after oxidative aging as well as their influence on the


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chemo-mechanical properties of asphalt is critical to developing effective strategies for restoring the features of virgin asphalt leading to so-called rejuvenation. Rejuvenators are asphalt additives designed to restore the original adhesive and mechanical characteristics of oxidized asphalt binder to facilitate reuse and recycling of aged pavements while extending the service life of asphalt in the field. True rejuvenation of asphalt binder seems to be simple in principle: first, compensate for the loss of volatiles and light bitumen components imposed by oxidation910

and return the asphaltene-maltene ratio to the balance of fractions seen in unaged asphalt;9, 11-12 second,

restore the initial level of peptizing and colloidal structure found in unaged asphalt.13 The role of rejuvenators in the treatment of oxidized asphalt and pavement maintenance has been studied extensively from two viewpoints: rheological and morphological. Rheological studies focus on improvement of the mechanical properties and performance of aged bituminous binder by adding rejuvenators.14-18 Morphological studies evaluate the effectiveness of rejuvenating agents in terms of restoring performance properties and then correlate the rheological results with the microstructural morphology changes observed after the rejuvenation process.9, 19-22 Popular techniques for characterizing the microstructure of asphalt binders include differential scanning calorimetry (DSC) for detecting thermodynamic phase transitions and atomic force microscopy (AFM) for direct imaging of both surface and bulk microstructure.23-24 However, the lateral resolution of AFM is roughly 10 nm, which is too large to examine some structures like asphaltene aggregates. Beyond AFM and DSC, transmission electron microscopy (TEM) has been used to image the molecular organization of extracted asphaltenes,25-26 and neutron scattering was employed to examine wax separation in binders,27 however these tools often have their own limitations such as sample preparation requirements or accessibility. Bio-modifiers are a new generation of additives derived from biomass which have recently received attention as promising asphalt rejuvenators due to the compatibility of their rheological and chemical properties with those of asphalt binder.28 Bio-modifiers derived from non-petroleum renewable resources can have either animal28-30 or vegetal origin.31-33 Plant-based residues show significant



differences in composition and physiochemical characterization compared to bio-residues extracted from animal manure processing due to the difference in chemical composition of the bio-mass feedstock. The main difference is the presence of carbohydrates in wood-based bio-mass leading to higher amounts of oxygen-containing compounds such as ether and alcohols in wood-based bio-residue. Consequently, wood-based bio-residues can be more susceptible to oxidative aging compared to the high-nitrogencontent bio-residues from animal waste, like swine manure.34 Based on our findings, bio-modifiers from animal wastes are less vulnerable to oxidative agents35 and can be promising candidates for use as additives in asphalt.28, 30 This study combines computational modeling and microstructural characterization to understand the mechanism of rejuvenation of aged asphalt at the molecular and electronic level. X-ray diffraction (XRD) is used to characterize the molecular conformation in aged asphalt before and after the introduction of a rejuvenator made from swine manure. This technique helps to determine the degree of asphalt fragment crystallinity as a measure of the extent of rejuvenation. Based on our macro-level understanding of the modified system, we try to explain the XRD observations at a molecular scale through molecular dynamics (MD) simulation and quantum mechanical studies using density functional theory (DFT). To do so, we focus on the electronic perturbation of the oxidized asphalt aggregates induced by the presence of a Bio-Rejuvenator (BR). The changes in the electronic structures through rejuvenation are reflected in energy parameters and d-spacing results for the molecular agglomerates. These two properties can be further used to inform the paradigm for design and engineering of highly effective bio-rejuvenators based on the material by design concept in which the fundamental properties at the molecular level are correlated with macro-level properties and performance. METHODS AND MATERIAL DFT-Based Quantum Mechanical Calculations. Quantum mechanical calculations in this study employed a density functional theory framework using a dispersion corrected DFT (DFT-D) approach, implemented in the DMol3 module36-37 of the Accelrys Materials Studio program package, version 6.0.


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The functional we used here is based on the Perdew-Burke-Ernzerhof (PBE)38 formulation of generalized gradient approximation (GGA). Grimme’s dispersion correction39 was included in our computations, (PBE-D), as a treatment for the long-range behavior of the density functional in such polyaromatic stacking systems. “Fine grid” was selected for the matrix numerical integrations of the exchange-correlation functional. The convergence criterion of the energy gradient, maximum force, and maximum displacement, respectively, were 1.0 × 10 hartree, 2.0 × 10 hartree Å-1, and 5.0 × 10Å. The basis set selected has double numerical quality with polarization function for hydrogen atoms (DNP). Binding energy (Ebind) was calculated to estimate the thermodynamic stability of oxidized molecular aggregations (asphaltene-asphaltene and resin-resin) before and after the bio-rejuvenation process. Ebind is the energy difference between the aggregate and its constituents in their lowest energy state,

= − ∑

(1)

Energy values for interacting systems were corrected for basis set superposition error (BSSE) using the counterpoise (CP) method.40 To detect and characterize non-covalent interactions (NCI) between asphaltene and resinous molecules in their agglomerates, we visualized reduced density gradient (RDG) surfaces between the two fragments in their interacting complex. The foundation of this technique, introduced by Johnson et al.,41 is tracking the changes in RDG at low-density regions. The RDG (s) values are measured based on density, ρ, and its first derivative, ∇.

=

|∇(| $ ) !" # &% ( &%

(2)

One of the advantages of the NCI approach, especially for weak interactions, is its insensitivity to the computational method.41 This feature led us to use the Gaussian calculations near our Dmol3 level of computations to generate electron densities appropriate to visualize RDGs. Accordingly, we employed the structures optimized at the PBE-D/DNP level of Dmol3 to reproduce electron density by an almost



equivalent Gaussian42 method, PBEPBE/6-31G*. RDG isosurfaces were visualized by the Visual Molecular Dynamics (VMD) program.43 Molecular Dynamics Simulation The molecular structures were built via molecular builder in Materials Design module of Medea® and Large-scale Atomic and Molecular Massively Parallel Software (LAMMPS) was used to perform the simulations.44 The force field, PCFF+, which is an all-atom force field and has shown to work well with the organic molecules was used for the simulations.45 The systems were minimized using conjugate gradient method and ran at 408.15 K (135˚C) via two consecutive NVT and NPT ensembles for 5 and 15 ns, respectively. Although the selected temperature is above the boiling point of heptane, the authors intentionally chose it as to not only make the solvent dilute enough to reduce its effect on aggregation of asphaltenes, but also since this is the mixing temperature of the binder with aggregates at asphalt plants at which the first stage of aging (short-term aging) happens. The time step was chosen to be 1 fs for all the stages and a Nose-Hoover thermostat and barostat was selected to control the temperature and pressure in each system. X-Ray Diffraction The Panalytical X'Pert Pro was used to capture XRD patterns of binder samples. The instrument uses a 1.8kW sealed X-ray tube source with Cu target and a vertical circle θ:θ goniometer with 24cm radius. The instrument is configured in Bragg-Brentano geometry with an X'Celerator position-sensitive detector and Open Eulerian Cradle sample stage. The inter-planar spacing (d) is calculated by Bragg’s Law: nλ=2d sin θ, where θ is the diffraction angle, n is the order of reflection, and λ is the wavelength of the X-ray radiation (Cu Kα = 1.54 Å). Materials The neat asphalt binder selected for this study is a Superpave PG 64-22 (Sharpe Brothers, Greensboro, NC), a binder grade commonly used across the U.S. The binder was produced from the Philips 66 Wood

River refinery in Roxana, IL that receives primarily Canadian and U.S. crude oils. The rejuvenator 6 ACS Paragon Plus Environment

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studied was derived from a bio-oil produced by hydrothermal liquefaction of swine manure as described elsewhere.28 The neat binder was aged in the laboratory by a standard method combining aging in a Rolling Thin Film Oven (RTFO) and a Pressure Aging Vessel (PAV). RTFO aging was performed following ASTM D2872, in which samples were exposed to oxidation by placing asphalt binder samples in glass bottles in a rotating carriage in an oven at 163°C and 4 L/min airflow for 85 minutes to simulate conditions at an asphalt-mixing plant. The sample was then exposed to two cycles of PAV aging according to ASTM D6521. In the standard procedure, binder samples in portions of approximately 50 g each are subjected to an air pressure of 2.10 MPa at 100°C for 20 h. To create the artificial RAP, the samples were aged for 40 h, which is referred to as “2PAV”. Samples were then degassed in a vacuum oven at 170°C for 30 minutes at 15 kPa. To prepare rejuvenated samples, 2×PAV was blended with 510% bio-rejuvenator (by weight of binder) at 135°C.

RESULTS AND DISCUSSION XRD of Oxidized and Rejuvenated Binder. Figure 1 shows the XRD spectra of unaged binder and oxidized binder with different concentrations of added BR. They can be used to gather information about crystallinity and the extent of molecular agglomeration during asphalt oxidation and rejuvenation. The intensity and location of the peaks vary with changes in arrangement of the asphalt molecules, especially the aggregates of sheet-like polyaromatics which likely give the highest electron density contrast in the material. The degree of molecular packing can be used as a measure of the rejuvenation and could be directly linked to asphalt’s molecular polydispersity and rheological properties. As shown in Figure 1, the asphaltene peak for unaged binder was observed at 2θ = 21°, which corresponds to a d-spacing of 0.42 nm, consistent with previous diffraction measurements.46-47 After aging, another peak appeared at 0.91 nm (9.7°). A similar peak around 8 – 11° appears in the XRD pattern of graphene oxide (GO).48-49 In that case, the larger spacing results from distortion of graphene π-π stacking by the addition of hydroxyl and carboxyl groups. However, some reports have suggested GOs have a graphitic structure with a clear



stacking order.50-51 The exact chemical identity of the 9.7° structure in aged binder is unknown, but

the observation of a similar d-spacing in GO suggests that the two material systems may share aspects in common. One of the popular approaches for exfoliation of graphitic like structures into few- or monolayered aggregations is using surfactants which interact noncovalently with polyaromatic sheets.52-54 Similarly, the addition of BR to the aged binder appears to marginally increase the asphaltene peak while decreasing the intensity of the GO-like peak at 9.7° until it disappeared entirely at 10% BR dosage. BR is not expected to chemically reduce the aged binder but instead to alter the strength of stacking forces between sheet-like polyaromatic fragments. The presence of a strongly bonded GO-like molecular structure could be related to age-related hardening of asphalt binder, and dissolution of this structure by BR could be the basis of the rejuvenation effect. Figure 2 shows the XRD pattern of oxidized binder at three different temperatures of 25°C, -12°C, and -22°C. The peak at 9.7° of aged binder did not show much change at reduced temperatures , although there was a weak change in the shape of the 21° shoulder, shifting slightly towards smaller d-spacing upon cooling from 25°C to -12°C.

Figure 1. XRD results for the unaged binder, and 2PAV with 0%, 5%, and 10% BR.


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Figure 2. XRD results for aged binder measured at 25°C, -12°C, and -22°C. The XRD peaks are very broad which is likely due to the small size of aggregates and the degree of disorder in the material. From the peak widths, the minimum theoretical aggregate sizes are calculated to be 0.9 nm for the 21° peak and 2 nm for the 9.7° peak (see Supporting Information Figure S1 and Table S1) which corresponds to a minimum of 2 – 3 stacked molecules. The real aggregates are expected to be larger than 1 nm, but probably not by much as the small aggregate size is believed to be the more important factor contributing to peak broadening in the XRD data. Computational Inspection of the Molecular Behavior in the Presence of Rejuvenators. To further study the latter two phenomena, MD simulation and DFT were utilized to model a system of aged asphalt molecules in presence of the BR. Modeling approaches are able to provide insight into the fundamental mechanisms underlying the rejuvenation process. Therefore, we first present here molecular models of involved components based on quantum mechanical theory and DFT. Molecular Models of Fragments Bio-Rejuvenator Components The molecular structures of the main components of the BR were characterized and introduced in our previous studies.30 Due to the high percentage of nitrogen in animal wastes,28 amide functional groups (NH2) are among the dominant groups in the present BR. Also, other significant elements in this BR are



alcohols and oxygen-rich compounds such as long-chain fatty acids containing a polar carboxylic group (COOH).28 Accordingly, the BR molecules considered in our study belong to the categories of linear amides and carboxylic acids, hexadecanamide and hexadecanoic acid, as shown in Figure 3. “Bio-Rejuvenator Molecular Models” C15H31C(=O)NH2 C15H31C(=O)OH

Figure 3. Molecular structures of selected BR components. Both of the above molecular structures of the BR in Figure 3 were optimized at dispersion-corrected DFT, PBE-D, using the DNP basis set, in preparation to study their interactions with oxidized asphaltene molecules. Oxidized Asphaltene and Non-Asphaltene Fragments One of the outstanding structural features of graphene is the carbon sheet with the aromatic hexagonal pattern. Through oxidation of graphene to polar GO, the basal surface of the graphene layer is populated with hydroxyl and epoxide functional groups, while the edge parts are decorated with carbonyl and carboxylic acid groups.55-56 The chemical composition of GO-like colloidal structures in our aged binder is not specified precisely, and it could be a mixture of oxidized SARA fractions. Most importantly, aromaticity and polarity are two characteristics common to oxidized fractions of asphalt and GO. Since asphaltene peaks are located at a higher angle of 2θ, we can favor the possibility that the GO-like colloids in the aged asphalt binder include mostly resinous structures. Thus, our GO-like model can be a polyaromatic hydrocarbon containing an aromatic core surrounded by some polar side chains. This expectation is verified to a certain extent by our results extracted from DFT calculations57 indicating that asphaltenes and polar aromatics (resins) have the maximum polarizability and minimum resistance against deformation of the electronic cloud. Therefore, asphaltenes and polar aromatics have higher chemical reactivity toward oxidative agents compared to the other fragments in asphalt binder and are consequently more susceptible to aging.


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Molecular Structure of Polar Aromatic Monomer and Dimer. To further study the formation of GOlike structures upon oxidation, we examine electronic perturbation of the polar aromatic fragments via DFT. The polar aromatic structure shown in Figure 4-I is used to represent molecular structures whose oxidized version might be found in the GO-like associations. This molecule was introduced by Dmitriev et al. as a molecular species present in petroleum,58 which consists one big aromatic block, containing aromatic and naphthene rings and hydrocarbon chains. These characteristics make this resinous structure more closely resemble the asphaltene motif. To decrease the steric energy reported for this molecule and obtain a more stable isomer of the above structure, the cyclopentane was relocated to a less restricted situation to decrease the internal energy of the fragment by reducing pentane effect while keeping the chemical formula unchanged.59 In Figure 4-I, the pentane ring undergoing displacement is shown in red and the new location is indicated by a blue dashed arrow. The modified isomer with more planarity and stability compared to the parent structure is demonstrated in Figure 4-II, referred to as modified polar aromatic (PA) in this paper. Figure 4-III shows the oxidized structure of our polar aromatic model. It has been documented that the percentage of polar functional groups increases during oxidation.

60

The design of the oxidized

molecules is of key importance to properly investigate the intermolecular interactions and their plausible alteration in presence of rejuvenator. In this study, the three common types of functional groups: sulfoxide, carbonyl, and carboxyl groups, are considered in developing a model of the oxidized PA. The benzylic carbons (carbons immediately attached to the aromatic rings) are the active sites of the aromatic compounds. The oxidation process usually takes place through the detachment of hydrogen and radical substitution at the benzylic position.61-62 Sulfoxide is another oxidation product that is considered in this section. The benzylic carbon and reactive sulfide sites in the polyaromatic model, which are more susceptible to oxidation, are indicated in Figure 4-II with red arrows, and the final oxidized structure is presented in Figure 4-III.



I.

II.

Polar Aromatic (PA)58

Modified PA

III.

Oxidized Modified PA Figure 4. Polar aromatic molecular structures: I-original, II-modified, and III-oxidized modified polar aromatic. The PA dimerization was modeled by identifying the most stable conformation of PA molecules. To do so, we determined a proper structure for the oxidized dimer while considering three crucial features: (1) the effective π-stacking interactions, (2) the least spatial hindrance between peripheral chains overhanging from the aromatic core, and (3) the most hydrogen bonding interactions between polar functional groups. The three likely arrangements with the greatest tendency for intermolecular interactions are shown in Table 1. The most probable configuration was determined using DFT-D calculations performed using DMol3, at the PBE-D/DNP level of theory. The thermodynamic stability of the dimer systems and the strength of inter-fragment interactions are represented in the form of binding energy; the more negative Ebind value, the more stable the interacting system. Based on the energy information, the lowest-energy configuration is oxidized dimer II.


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Table 1. Structures of three different conformers of the polar aromatic dimer. (Ebind, kcal/mol)

I: Ebind = -31.4

II: Ebind = -37.8

III: Ebind = -34.1

Molecular Structure of Asphaltene Monomer and Dimer. The characteristic high molecular weight, aromaticity, and polarity of asphaltenes make them important components in determining rheological and mechanical properties such as viscosity and hardening.63-65 Asphaltene fragments in the asphalt matrix have mostly “continental” or “archipelago” conformers.66-67 The main difference between these two conformers is the design of their aromatic core. Continental asphaltenes have an integrated polyaromatic core, while archipelago asphaltenes have several isolated cores interconnected by aliphatic chains. Our preferred conformation in this study is the “Island” construction, a medium-sized continental asphaltene that is the predominant structure in asphalt media68-69. The asphaltene-pyrrole molecular structure selected for this research has the Yen-Mullins island motif

68, 70

modified by Greenfield-Li71 and Fini et al.,72

(Figure 5). During oxidation, more polar functional groups appear in the carbonic skeleton of the asphalt fragments, so, the percentage of heteroatoms in the asphalt matrix increases. Asphaltenes are not exceptions in this respect, and ketones, as a major product of oxidation, are formed in high concentrations in asphaltenes.61 Alkyl side chains in the asphaltene structures are the part most vulnerable to irreversible ketonization, due to the presence of benzylic carbons as the active sites for the formation. Despite the type of functional groups, their distribution is a vital factor affecting the intermolecular forces and the extent of asphaltene association. The mildly oxidized asphaltene used here is our previously modeled molecule (Figure 5) .26


“Virgin Asphaltene Model”

Top-view

Mildly Oxidized Asphaltene Dimer:

Side-view

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

“Oxidized Asphaltene Model”


Figure 5.Molecular structures of mildly oxidized asphaltene monomer and dimer. (taken from our previous work).26 Oxidized asphaltenes with high polarization and charge separation arising from polar functional groups surrounding the aromatic core are eligible candidates to form π-π and π-σ interactions in the parallel and near-parallel orientations.22 The predominance of parallel geometries of asphaltenes in asphalt binder73-75 is the basis of our strategy to find a proper orientation for the asphaltene dimer.26 Thus, we employed the oxidized asphaltene dimer in the parallel conformer from our previous work26 with “mirror twisted hexagonal” nomenclature. The selected conformer has a stacking order with a mirror plane and a rotation angle between two polycyclic aromatic cores. This structure is the thermodynamically stable conformer, Ebind= -48.8 kcal/mol, in which the two nitrogens have the greatest distance from each other (Figure 5).


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Utilizing MD Simulation to Evaluate Rejuvenator Effect To investigate the effect of aforementioned BR molecules on the conformational packing of the oxidized asphaltene molecules, a system of 15 oxidized asphaltene-pyrrole molecules was examined in n-heptane via MD simulation. To prepare the system, using amorphous material builder in Medea® considering no coordinate or orientation bias for each hexadecanamide and hexadecanoic acid in separate cells, a 10 wt% ratio of the 15 oxidized asphaltenes was added to the matrix of molecules. Thus, two different cells with the same size of 220×220×220 Å were chosen for the simulations with one having 15 molecules of asphaltene, 6 hexadecanamide, and 378 heptane; and the other with 15 molecules of asphaltene, 6 hexadecanoic acid, and 378 heptane. The average aggregate size, as well as the diffusion coefficient of the BR molecules in the matrix of oxidized asphaltenes, is then calculated. The latter was done using the slope of the mean square displacement (MSD). The results show that the oxidized asphaltene molecules interact with each other forming parallel and semi-parallel stacked structures (Fig. 6-a). However, in the presence of BR molecules, the nano-aggregates of oxidized asphaltenes deagglomerated forming a larger number of reduced-size nano-aggregates (Fig. 6-b). “Snapshot of Simulation Before and After Rejuvenation”

a.

b.

Figure 6. Snapshot of simulation for the system of a) oxidized asphaltenes; b) oxidized asphaltenes plus 10 wt% BR (hexadecanamide). As it can be seen in Table 2, the introduction of hexadecanamide to the matrix of oxidized asphaltene was more effective than hexadecanoic acid in alteration of nano-aggregates. This was evidenced by the



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formation of a greater number of reduced-size nano-aggregates due to the presence of hexadecanamide. However, the hexadecanoic acid led to a higher diffusion coefficient, indicating its enhanced movement capability in the matrix of oxidized asphaltenes (Table 2). Considering the importance of both deagglomeration and diffusion capabilities for effective rejuvenation, it appears that the two molecular species have a complementary role in the rejuvenation of oxidized asphaltenes. The XRD data (the peak at 2θ = 9.7°) further suggest that aging of asphalt introduces GO-like structure here, possibly in the form of colloids of oxidized molecules that coexist with the native asphaltene colloids. It should be noted that the microstructure of asphalt has previously been described as a dispersion of resin-coated asphaltene colloids in a saturate-rich matrix.76 An increased volume fraction of colloids through the aging process could be a fundamental cause of stiffening in aged asphalt. As an analogy, the viscosity of colloidal fluid suspensions also depends strongly on the volume fraction of particles (ɸ), independent of particle size.77-78 Viscosity scales as ɸ2 in dilute colloidal suspensions and decreases as polydispersity in colloid size increases. A similar dependence could underlie the rheological behavior of aged binders and rejuvenated binders. Table 2. The number of asphaltene aggregates, the average size of aggregation and diffusion coefficient of the bio-molecules in the systems in the final configuration. Diffusion Number of Average Size of Molecule Group Coefficient Aggregates Aggregation (cm2.s-1)

Oxidized Asphaltene C66NO3H75

4

3.75

_____

6

2.5

1.65E-05

3

5

3.30E-05

& “Pyrrole”

“Carbonyl” O

Hexadecanamide C15H31C(=O)NH2

N

“Amide”

Hexadecanoic Acid C15H31C(=O)OH “Carboxylic Acid”


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Response Assessment of Oxidized Asphalt Associations in the Presence of Bio-Rejuvenator Using DFT Approach. To examine the GO-like associations in the presence of BR molecules, we simulated an interacting system of a dimer of the oxidized polar aromatic that has been surrounded by BR molecules, as shown in Table 3. To set up the initial structures, BR molecules were located in the positions around the interlayer space having the maximum interaction with the active sites of the oxidized dimer. Table 3. The interaction between the components of BR and oxidized polar aromatic dimer. Shown are binding energy (Ebind, kcal/mol) and binding distance for oxidized dimer (dbind, Å)

Ebind = -14.6 Ebind = -37.8

dbind = 5.57

dbind = 3.33

Ebind = -10.3

dbind = 5.75 Table 3 shows the molecular binding energy (Ebind) and binding distance (dbind) between the oxidized planes before and after addition of the BR molecules. Here, the interlayer distance, dbind, is the vertical



distance between two molecular planes, almost near the center of dimer. Based on the results reported in this table, rejuvenator molecules largely mitigate the effect of polar functional groups that have been intensified by aging. As reported in this table, dbind increased from 3.33 Å to 5.57 and 5.75 Å in two- and three-additive complexes, respectively. Compared to unmodified oxidized dimers, binding energy between the rejuvenated planes has decreased noticeably: 61.4% in the system with two BR molecules, and 72.8 % in the system with three BR molecules. As the figures in this table show, after adding rejuvenators, adjacent monomer planes are no longer parallel and the dimer is opened up, showing a greater separation distance on one side compared to another side. This structural deformation can trigger the intercalation mechanism, in which additive molecules penetrate the empty spaces created between two interacting planes. This intercalation mostly through the CH-π interaction is followed by a disturbance in the π-π interaction between polyaromatics in aggregate and loading an extra spatial hindrance between the planes which facilitate their separation. To examine the asphaltene molecule associations, the interaction of an oxidized asphaltene dimer with two amide-type and two acid- type molecules was investigated (Table 4). The study results show that BR components have diminished the influence of the polarity developed due to oxidation, and have mitigated the molecular agglomeration of fragments oxidized by aging; this was further evidenced by a 69 % decrease in Ebind (from -48.8 to -15.0 kcal/mol), and a significant increase in the interlayer distance (from 3.48 to 7.17 Å) in the oxidized asphaltene dimer in presence of the BR. Table 4. Interaction between the BRs and an oxidized asphaltene dimer. Shown are binding energy (Ebind, kcal/mol) and binding distance for an oxidized dimer (dbind, Å). Oxidized Asphaltene Dimer

Rejuvenated Asphaltene Dimer

Ebind = -48.8 dbind = 3.48

Ebind = -15.0 dbind = 7.17


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Electronic Analysis of Rejuvenated Molecular Aggregates. To summarize the theoretical justification for observed XRD pattern, we divide the graph in Figure 7 into two parts. Part “A” is a series of peaks that are related to the GO-like structures that appeared through oxidative aging. Part “B” is specific to the asphaltenic molecular aggregations. Changes in the appearance of the peaks in parts A and B can be attributed to variations in the scattering intensity of layered structures in the asphalt matrix due to the alteration of electron density of fragments and the number of sheet-like molecular structures assembled in parallel conformation.

“XRD Pattern”

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


“GO-like Structuring”

“Asphaltene Aggregation”

Figure 7. Schematic representation of the oxidized agglomerates. Correlation between the molecular-sheet arrangements and the position of XRD peaks. (Aromatic sheets targeted by the XRD beam are filled in blue.)



The molecular mechanism responsible for the restorative effect of the Bio-Rejuvenator on oxidized asphalt can be described in three steps: partial penetration of Bio-Rejuvenators, partial segregation of aggregates, followed by dispersion. This three-step mechanism leads to the rejuvenator fragments sliding into the intermolecular gap between the two interacting asphaltenes in an aggregate, owing to their CH-π interaction with the aromatic zone of one of the ending asphaltenes. In addition to the disturbance in the π-π stacking between asphaltenes, this is followed by imposing an extra spatial hindrance between the molecular sheets, facilitating their separation. This, in turn, can lead to breaking oxidized agglomerates to smaller assemblies (Figure 8). Therefore, the XRD peak (part B in Figure 7) for the asphaltene aggregates includes a pronounced peak after rejuvenation. This is in line with observations from MD simulation indicating the formation of a larger number of reduced-size nano-aggregates in presence of biorejuvenator.

Figure 8. Schematic representation of BR action on the oxidized asphaltene agglomerates. There seems to be the same scenario for the resinous polar aromatic fragments in asphalt binder, though to a different degree. The latter may be attributed to polar aromatic having lower aromaticity and a smaller conjugated zone compared to the asphaltene molecules. The intermolecular interaction in an oxidized asphaltene dimer (Ebind = -48.8 kcal/mol) is stronger than that of an oxidized resin dimer (Ebind = -37.8 kcal/mol). Therefore, it is expected that presence of the BR alters the molecular packing of the latter group more easily.


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This observed phenomenon is controlled by parameters influencing the π-electron distribution and strength of aromatic-aromatic interactions. π-electron distribution over aromatic system creates a quadrupole with partial negative charges on both sides of the aromatic sheets and partial positive charges on σ-framework squeezed between the two π-clouds,79-80 Figure 10. To minimize the electrostatic repulsion between two such quadrupoles, parallel stacking is mainly observed in a form of parallel displaced or turned to the edge-to-face orientation, in which the π-σ attraction is maximized. π-electron delocalization in the aromatic molecules makes π-clouds susceptible toward electronwithdrawing or -donating substitutions, and thought to be responsible for π-π interactions. Accordingly, based on Hunter-Sunders rules,79 electron-withdrawing substituents polarize the π-electron density away from the aromatic core decreasing unfavorable π-π repulsion. The relative electron deficiency of the central area set the stage for the electrostatic attraction with the electron-rich part of the other aromatic, leading to stabilizing the face-center pairing.81 Such kind of π-stacking can be considered as aromatic donor-acceptor interaction. Electron-withdrawing carbonyl groups around the aromatic core of the oxidized asphaltenes contract the π-electron density and reduce π- π repulsion, Figure 10. In this study, the asphaltenes are paired in mirror fashion so that the electron-deficient part of one asphaltene interact attractively with the electron-rich region in the other one, Figure 9. Therefore, alternating electron-deficient and electron-rich aromatic stacking create stronger aggregates of oxidized asphaltenes. “Orientation of Asphaltenes in the Oxidized Dimer”- top-view



Figure 9. Regions with maximum electron accumulation for each monomer in an oxidized asphaltene dimer. For clarity, the two electron sets are different-colored dots. BR attachment to the polar sites of oxidized hetero-polyaromatic (asphaltene and resinous molecules) disturbs the balance between electron distributions of two fragments in the dimer. The polar head of BR can intensify the electron-withdrawal power of carbonyl groups and subsequently make a disturbance in the alternating electron-deficient and -rich aromatic interactions in the π-stacking, Figure 10. In the following section, the analysis of electron density over the molecular surfaces of the corresponding monomers, electrostatic potential (ESP) analysis, confirm the variation in the distribution of π-cloud in the presence of electron-withdrawing agents. “Effect of Electron-Withdrawing Substituent”

Virgin Dimer

Oxidized Dimer

Rejuvenated Oxidized Dimer

Figure 10. Schematic representation of oxidation and rejuvenation effect on the alteration of π-electron distribution and electrostatic interactions in the oxidized hetero-polyaromatic.


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In addition, CH-π stacking interactions between donor CH sites of the additive molecule and acceptor π clouds of polyaromatic has a destructive effect on the π-π interaction between fragments in aggregation.82 Accordingly, the Bio-Rejuvenator hydrocarbon tail can be also considered as a π-stack-disrupting part that provides an apolar surface of interaction, consisting of CH groups, for the aromatic core of the asphaltene or polar aromatic monomer. Therefore, BRs with the aliphatic tail act as breakers of the polyaromatic aggregation and prevent their re-aggregation by locking each aromatic terminus of the molecular aggregation. A substitution of π-π stacking by CH-π BR-polar aromatic interactions interferes with the self-assembly of polyaromatics. The interaction of BR aliphatic chain with the aromatic region of the asphaltene and its role in the disruption of π-π interactions between asphaltene stacks were studied through a peer electronic analysis in our previous works.26, 83 To show the charge redistribution over the aromatic zone from oxidation and rejuvenation, Figure 11 compares ESP surfaces of the asphaltene fragment in the virgin form with its counterparts after oxidation and rejuvenation. In the oxidized asphaltene (Figure 11-II), there is clearly polarity (separation between positively and negatively charged surfaces) through the separation of electrostatic potential into extremely red and blue regions. This is attributed to the presence of electron-withdrawing segments, carbonyl groups, around the aromatic zone. This illustrates how electron withdrawing can incrementally strengthen stacking interactions by the polarization of aromatic sheets with large quadrupole moments.

II. “Oxidized Asphaltene”

“Electrostatic Potential Isosurfaces”

I. “Virgin Asphaltene”

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

III. “Oxidized/BR Complex”


Figure 11. Electron density distribution over electrostatic potential surfaces (ESP). I) virgin asphaltene, II) oxidized asphaltene (ESP in the presence of electron-withdrawing groups, EWG: carbonyl functional groups), III) the complex of oxidized asphaltene and BR in a rejuvenated system, and IV) oxidized polar aromatic. Red shows a region with charge accumulation; blue shows a region with charge depletion. As shown in Figure 11, the concentration of electrons (the red region) in the polar aromatics is not significant compared to that of asphaltenes. Thus, it is expected that the molecular agglomerates generated by polar aromatic fragments have structures that are less layered, and are nearly amorphous. Such molecular arrangements are reflected in the XRD pattern by peaks located at large d-spacing (Part A, Figure 7), resembling the GO-like structures. This may indicate that molecular assembly of polar aromatics is easily disturbed and restored to its amorphous structure when exposed to BR. This was corroborated by the intense peak in part A of the XRD spectra which first converts to a small bump at 5% rejuvenator and then disappears at 10% rejuvenator dosage. Electronic Perturbation Imposed by BR. According to our previous molecular analysis,28, 30 amides and carboxylic acids compounds with their hydrocarbon tail contained in BR have a low spatial hindrance in nature, which allows them to penetrate easily into small pores around the interlayer region. In fact, the rejuvenating behavior of the BR can be ascribed to either their capability to form hydrogen bonds with the heterocycle of the asphaltene and polar aromatic molecules or oxygen-containing functional groups generated by aging, or their capability for electrostatic interactions with positively charged hydrogens in the oxidized fragments. The asphaltene/hexadecanamide interacting complex shown in Figure 11-III exemplifies the effect of BR molecules on the electron distribution of oxidized asphaltene through H-


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bond linkage. As shown in this complex, making a connection between a polar head of BR and a polar functional group of oxidized asphaltene creates a stronger electron-withdrawing center and intensifies electron depletion on the molecular surface. In this figure, we have visualized the ESP color-filled for the interaction of just one isolated molecule of bio-modifier with an asphaltene monomer, while a variety of BR molecules with polar groups and several active sites for H-bonding around oxidized asphaltene are present. This means that more withdrawing forces can be exerted on the oxidized asphalt fragment. Accordingly, the electron accumulation on the aromatic core of conjugated molecules is significantly decreased, leading to the loss of electrostatic quadrupole-quadrupole interactions between asphaltenes. The results of our computational modeling confirm this assertion through a notable reduction in the binding energy and an increase in the interplane distance of aged asphaltene dimers (Table 4). Hirshfeld and Becke Surface Analysis. To further examine the significance of H-bonding between BR and oxidized asphalt component, this type of interaction was studied through a quantum mechanical approach. The molecular structure of BR considered in this paper consists of two main parts with different functions: a polar head and an aliphatic tail. The polar head is more important here because it can perform as a penetrating part to make its way through molecular aggregates and then, as part of a surfactant, disperse smaller agglomerates in the matrix. In this regard, the efficiency of the polar head of BR in the formation of non-covalent interactions (including H-bonding) with the aged fragment should be confirmed. This section focuses on analyzing the Becke surface to reveal H-bonding interactions between BR and oxidized asphalt components in the aged molecular agglomerates. The Becke surface is a kind of inter-monomer surface that is defined based on the concept of Becke weight.84 “Becke Surface Analysis”



Figure 12. Mapped Becke surface for hexadecanamide in rejuvenating resinous complex. Red zones in the maximized frames with top and bottom view indicate regions of high electron density. We employed the Becke surface analysis for a resin dimer/BR interacting system to show the capability of our BR models to form H-bonding networks with oxidized asphalt molecules (Figure 12). In this case, the Becke surface is an open surface, because the bio-molecule is not completely buried in the aged dimer. This visualization detected three red zones between amide functional groups from BR and the resinous structure. In the red local areas, the electron density is high, and there are strong intermolecular interactions (attractive H-bonding) across these regions. The boldest spot on the surface belongs to the strongest H-bonding between *+, = - ⋯ / − 012. Two other light red regions are due to the Hbonding between *+, = - ⋯ /! , − ,-12 and *+0/! ⋯ 0/12. Another pale red spot on the other side of the surface (Figure 12) is related to the interaction between the positively charged hydrogen atom connected to the carbon atom next to the amide functional group of the BR and the π-electron cloud of PA. Another useful technique used here to investigate the nature of H-bonding generated from amide functional groups is a “fingerprint plot”, which is defined based on the framework of Hirshfeld partitioning.85 Two-dimensional 3 ⁄3 fingerprint plots for the H-bonding between hexadecanamide, as


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a Hirshfeld fragment, and the oxidized resinous structure in the aforementioned complex are visualized in Figure 13. I.

II.

Figure 13. Fingerprint plots of a hexadecanamide (BR) and oxidized polar aromatic (PA) dimer interacting complex. In Figure 13, the X and Y axes correspond to di and de, respectively, where di and de are the distances from a point on the surface to the nearest nucleus inside and outside the surface, respectively. In Figure 13-I, there are three spikes at the bottom, almost left, of the plot (at di ~1.00, 1.30, 1.50and de~1.00): the two spikes at the bottom middle and right represent H-bonds between *+, = - ⋯ / − 012 and *+, = - ⋯ /! , − ,-12, respectively; the spike at the bottom left represents *+0/! ⋯ 0/12. Based on the relative values of di and de, the BR molecule behaves as both H-bond acceptor (middle and right spikes, 3 5 3 ) and H-bond donor (the left spike, 3 6 3 ). To check the nature of the spikes, since the di and de for the left spike are similar, we defined just hydrogen atoms of the BR as inside atoms and all atoms in peripheral fragments as outside atoms to visualize the fingerprint plot; consequently, just one of the spikes (the left one) appeared in the plot (Figure 13-II). This means that this spike corresponds to the hydrogen donation from the molecular representative of BR fragment. Non-Covalent Interaction and RDG Analysis. The extension of electrostatic and van der Waals interactions between asphaltene planes and resinous fragments in their aggregations before and after rejuvenation can be well-described by a visual analysis of the electron density and reduced density



gradient (RDG). RDG plots can qualitatively explain how the addition of BR molecules to the aged asphalt affects the stability of agglomerates that have been intensified by oxidation and intermolecular interactions between the oxidized fragments. Accordingly, 3D-gradient isosurfaces of resin and asphaltene dimers in two forms, aged and rejuvenated, are shown in Figure 14. Note that the rejuvenated dimers, Figures 14-b and 14-d, are isolated from their interacting complex by BR molecules. The surfaces reported here correspond to s= 0.5 a.u. and are colored based on a BGR (blue-green-red) color scale. As shown in Figures 14-a and 14-c, the absence of blue and red surfaces in the BGR color scheme indicates that there are no significant H-bonding attractive forces or steric repulsive forces in our oxidized dimer species. Van der Waals forces reveal themselves in the form of extended greenish surfaces between fragments. More importantly, the van der Waals and electrostatic interactions (green RDG isosurfaces) between rejuvenated oxidized asphaltene fragments (Figure 14-b) and between rejuvenated non-asphaltene fragments (Figure 14-d) have almost disappeared. The isosurfaces of RDG in Figure 14-a also confirm the shift of π-electron density toward regions near the electron-withdrawing groups, C=O, which results in stronger π staking, leading to deep green extensive surfaces of van der Waals interaction between oxidized asphaltene monomers. In Figure 14-c for the resinous dimer, the green van der Waal spots are smaller and shaded more with light brown (indicating the low electron density). This comparison qualitatively supports the expected theory that asphaltene fragments are more interconnected in their aggregates, so their molecular associations are not getting amorphous through rejuvenation, but rather convert to smaller agglomerates. Therefore, we can see that the asphaltene peak in the XRD pattern was retrieved, while the corresponding peaks related to the polar aromatics completely disappeared. “Oxidized Asphaltene Dimer”

“Rejuvenated Asphaltene Dimer”


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a.

b.

“Oxidized Polar Aromatic Dimer” c.

“Rejuvenated Polar Aromatic Dimer” d.

Figure 14. Non-Covalent Interaction analysis of the intermolecular interaction between asphaltene monomers, and resin (PA) monomers in their (a, c) oxidized; and (b, d) rejuvenated dimers (top-views). CONCLUSION The association forces between polar aromatics fragments are strengthened due to the introduction of chemical functionalities to their molecular structure. This, in turn, increases the size of molecular agglomerates in petroleum asphalt binder when exposed to oxidative aging. This paper combined computational modeling and microstructural characterization to explain how alteration of π−electron can induce deagglomeration in polar aromatics as a means of restoring virgin asphalt properties. A bio-based rejuvenator was used to induce perturbation of the fused aromatic cores of oxidized asphalt fragments while studying the change of their interaction energy and binding distance.



The analysis of the XRD spectra showed that, in addition to the peak associated with asphaltene stacking, a new peak at a larger d-spacing appeared after oxidative aging which was disappeared after rejuvenation. XRD data suggest that aging introduced a new molecular agglomeration analogous to that of graphene oxide referred to as a GO-like structure. MD simulations and DFT were applied to provide insight into the XRD observations. MD simulations showed that including BR to a matrix of oxidized asphaltene molecules leads to the formation of a larger number of reduced-size nano-aggregates. Selected BR molecule has a destructive effect on the π-π interaction between polyaromatic fragments in the aggregation through its polar head and aliphatic tail. Electron-withdrawing nature of polar head and CH-π stacking between the hydrocarbon tail and polyaromatic core are specified as external stimuli to induce perturbation in the π-cloud. Studying the electronic redistribution of the fused aromatic cores of asphaltene and polar aromatic molecules in presence of BR, it was found that polar aromatic molecules are more susceptible to such perturbation and their stacked structure is easily disturbed by adding BR. Furthermore, ESP maps showed that electron-withdrawing groups generated through oxidation, polarize the charge distribution over the aromatic core of the asphalt fragments in favor of the intermolecular interactions and molecular aggregation. In contrast, BR molecules condense π electrons in a way that decreases interaction between quadrupoles of molecules in aggregates and consequently increases the d-spacing between them. RDG isosurfaces showed that van der Waals and electrostatic interactions between the oxidized asphaltene fragments and between oxidized resinous fragments in the rejuvenated complexes have significantly reduced. The rejuvenation effect of BR molecules can be attributed to the H-bonding and van der Waals interactions of the oxidized fragments with both the polar head and the aliphatic tail of the molecules in BR, respectively. This leads to a three-step rejuvenation: (1) partial penetration of BR into small pores around the interlayer region of the aggregates, (2) partial segregation of aggregates, and (3) dispersion of smaller agglomerates in the asphalt matrix. This three-step mechanism dismantles the asphaltene agglomerates into smaller agglomerates and disorders polar aromatic molecules.


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Supporting Information More information about peak broadening in the XRD data along with additional table and figure relevant to it are presented in the Supporting Information. This file is available free of charge on the ACS publication website. Acknowledgment This research is sponsored by the National Science Foundation Award No: 1546921 and 1150695 and the University Transportation Center: Center for Highway Pavement Preservation. The contents of this paper reflect the view of the authors, who are responsible for the facts and the accuracy of the data presented.

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Graphical Table of Contents

A Bio-based rejuvenator as a sustainable modifier to diffuse into and disperse nano-aggregates containing oxidized- polyaromatic molecules in aged bitumen.


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