Multiscale Investigation of Oxidative Aging in Biomodified Asphalt

Article pubs.acs.org/JPCC

Multiscale Investigation of Oxidative Aging in Biomodified Asphalt Binder Masoumeh Mousavi,† Farideh Pahlavan,† Daniel Oldham,‡ Shahrzad Hosseinnezhad,‡ and Ellie H. Fini*,‡ †

Center for Innovation in Materials, Methods and Management and ‡Department of Civil and Environmental Engineering North Carolina A&T State University 1601 East Market Street, Greensboro, North Carolina 27405, United States ABSTRACT: Oxidative aging causes major changes in asphalt binder’s physiochemical and rheological properties, giving rise to pavement distress and failure. Asphalt aging occurs due to two main processes: a loss of volatile components and a reduction of the malthene phase and oxidation of certain functional groups in asphalt, increasing the concentration of asphalt’s polar components. Performing SARA (saturate, aromatic, resin, and asphaltene) analysis and FTIR (Fourier transform infrared) spectroscopy on the crude oil shows that the ratio of polar components to nonpolar ones is higher in oxidized asphalt compared to virgin asphalt. However, when a biomodifier is introduced to virgin asphalt, the rate of carbonyl (as a polar functional) formation is reduced, which may indicate a delayed oxidation due to the presence of biomodifier molecules in the asphalt. To understand the fundamental origin of oxidative aging at the molecular level, this paper provides a comprehensive computational chemistry analysis conducted in conjunction with laboratory experiments. On the basis of the results of our analysis, the enhanced performance of biomodified asphalt binder stems from dual-protection mechanisms of biobinder components that defer asphalt aging: the less reactive molecular species found in biobinder that show little propensity toward oxidation and consequently are less affected by the new polar functionalities and the highly reactive components (such as α-tocopherol) that are the primary targets for oxidative attacks, acting as sacrificing elements to save key components of asphalt materials (such as asphaltenes) from oxidative agents. Polarizability calculations show that biobinder constituents are considerably less polarizable than asphalt molecules. Lower polarizability of biobinder indicates the lower tendency of these chemical species toward new polar functionalities arising from the presence of oxidative agents. In contrast, the high polarizability obtained for asphaltene molecules suggests that they are easily affected by the oxidative agents. Therefore, the presence of α-tocopherol in biobinder acting as a sacrificing element could delay asphaltene oxidation as evidenced by the lower carbonyl formation in asphalt samples containing biobinder.

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INTRODUCTION Asphalt binder is one of the most commonly used construction materials over the past decade.1 Not surprisingly, significant research efforts have been devoted to the development of new binders and additives to prolong the life of asphalt pavement while simultaneously enhancing its mechanical properties, lowering bitumen viscosity, and reducing carbon emissions associated with its application.2−4 A limit for asphalt is that its procurement is limited to the petroleum cycle. In this regard, finding a nonpetroleum source for asphalt binder that also improves the properties of this material is of paramount importance. Because of its cost and performance, finding such a replacement is no easy feat. Success has come in the form of bio-oils, which are derived from nonpetroleum renewable resources like wood pallets, corn stover, miscanthus, and animal waste.5 In the case of animal waste, Fini et al. recently pioneered a process for producing a biomodifier for asphalt, socalled biobinder (BB), from swine manure with enhanced performance for applications in asphalt pavement.6 The addition of BB to asphalt has shown improvement in lowtemperature cracking in studies performed using various © XXXX American Chemical Society

mechanical tests: the asphalt binder cracking device (ABCD), bending beam rheometer (BBR), direct tension tester (DTT), and acoustic emission (AE) tests at the binder level and the indirect tensile (IDT) and disk-shaped compact tension DC(T) tests at the mixture level.7 The addition of this waste-derived BB to reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) showed significant improvement in workability, creep compliance, and fracture energy.8,9 RAP and RAS are oxidized versions of asphalt in which asphaltene molecules are more clustered and agglomerated. The improvement from the presence of BB can be attributed to enhanced dispersion of asphaltene molecules and delayed glass transition, which in turn increase the flexibility of chains and segmental motions in the asphalt molecules of RAP.10 Such improvements in the presence of BB can further cause oxidized asphalt to become more flexible at low temperatures compared to unmodified asphalt.8 Received: May 18, 2016 Revised: July 13, 2016

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DOI: 10.1021/acs.jpcc.6b05004 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

asphalt to give rise to more polar components. This paper mainly focuses on the latter factor. Conceptual DFT provides formal definitions for chemical concepts within a DFT approach. As explained in an extensive and thorough review by Geerlings et al.,21 the descriptors defined within this theoretical framework are able to explain qualitatively and quantitatively the reactivity parameters such as global hardness22,23 and the Fukui function24 for any molecular species. Here, these conceptual DFT descriptors are applied to evaluate and compare the chemical reactivity of nonmodified asphalt and biobinder models. Within this multiscale study, our theoretical findings at the micro level corroborate the experiment data obtained from rheology-based aging descriptors and Fourier transform infrared (FT-IR) spectroscopy measurements. Experimental analysis concerning the chemical structure of asphalt and BB molecules, as well as their aging behavior, is used to characterize the newly developed BMB, which shows an enhanced performance compared to nonmodified asphalt binder.

It should be noted that resistance to failure is often a measure of performance for structural materials. As an organic-based material, asphalt is prone to chemical oxidation through reaction with atmospheric oxygen, which leads to hardening of the asphalt and negatively affects its mechanical properties and performance. This hardening leads to reduced stressreleasing capabilities of asphalt, giving rise to crack initiation due to environmental and mechanical loadings. This process of oxidative hardening remains an active area of research11−15 wherein the generation of oxygen-containing polar chemical functionalities is believed to be a controlling mechanism affecting asphalt rheology. Hydrogen bonding, van der Waals forces, Coulombic interactions, and even aromatization can arise from these new polar functionalities. In the case of aromatization, reactive compounds such as polycyclic hydrocarbons react with oxygen to produce hydroperoxides, which then react with asphalt sulfides to form sulfoxides that are accompanied by the aromatization of hydroaromatics. Ring aromatization makes hydroaromatics more planar, which in turn facilitates molecular agglomeration. These new forces cause agglomeration among molecules, which in turn leads to hardening. Further studies showed that an increase of polar content, specifically in asphaltene components, correlates with stiffening of asphalt as well as decreasing elasticity.16 It has been documented that as a result of oxidative aging the ratio of polar to nonpolar components increases, as evidenced through a comparison of saturate, aromatic, resin, and asphaltene (SARA) components and a comparison of the extent of carbonyl groups in aged and virgin asphalt.11 Although it is extremely difficult to account for a complete array of chemical reactions occuring during oxidation, it is possible to evaluate the chemical reactivity of the molecules that compose current models of petroleum asphalt and biomodifiers. Such evaluation can help develop guidelines for evaluating and selecting promising additives to enhance the long-term performance of asphalt. To do so, molecular modeling and computational chemistry are useful tools to predict not only the properties that result from molecular interactions and intrinsic chemical structure but also the reactivity and chemical processes involved in the performance of asphalt. Despite the importance of a molecular-level understanding of asphalt’s macroscopic properties, the complexity and massiveness of asphalt structural units along with limited available computational power have reduced theoretical approaches mainly to the classical molecular mechanics/dynamics (MM/ MD) studies (which are found to be lacking a proper description of electronic effects) and very few preliminary quantum-based studies at the DFT (density functional theory) level.17−20 Nevertheless, it is critical to evaluate the chemical reactivity of the molecular species that are expected to be influenced by the oxidative agents. Therefore, adopting an approach capable of simulating electronic properties is instrumental. To this end, this paper employs rigorous quantum mechanical calculations, through a high quantum level of DFT approach and conceptual DFT reactivity descriptors, in addition to experiment techniques to investigate oxidative aging of biomodified binder (BMB) compared to nonmodified asphalt binder. It should be noted that asphalt aging occurs due to two main processes: a loss of volatile components and reduction of the maltenes phase and oxidation of certain functional groups in

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EXPERIMENTAL CHARACTERIZATION In order to study the susceptibility of BMB to oxidative aging, laboratory experiments were conducted to evaluate the physiochemical changes that take place in BMB during aging. To do so, rheological properties along with the chemical structure characterization of BMB were compared with those of nonmodified binder. Modified samples were prepared by blending BMB with virgin asphalt binder at 135 °C and 750 rpm for 30 min, following the approach developed in prior work.6 To simulate the mechanical aging effect, samples were conditioned using the rolling thin-film oven (RTFO) following the ASTM D2872 specification. In this standard, specimens are placed in an RTFO at a 163 °C environment for 85 min while undergoing oxidation by a constant airflow of 4000 mL/min. This procedure is widely accepted in the asphalt community for simulating short-term aging that occurs in the field. The elevated temperatures, combined with the airflow, age the material by driving off a substantial amount of the sample’s volatiles, while oxygen creates covalent bonds with asphalt molecules and creates more polar species. The exact mechanism of this oxidation, and whether it is a radical reaction or another chemical process, is still unknown. However, the absence of a complete and robust explanation for the kinetics and mechanism of oxidation does not hamper the evaluation and characterization of the reactivity properties of the asphalt chemical components. In addition to the modeling approach, to examine the extent of change in rheological properties, the rotational viscometer (RV), dynamic shear rheometer, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy were utilized to evaluate each sample before and after aging. Rotational Viscometer. The viscosity measurements were performed using a Brookfield rotational viscometer DVIII-Ultra following the ASTM D4402 standard. In this test, a shear force was applied to a sample in order to determine its viscosity. Samples were prepared by pouring 10.5 g of sample into an aluminum chamber and preheating it for 30 min in order to ensure that thermal equilibrium was reached. Readings were taken at 135 °C and a rotational speed of 20 rpm. This procedure was performed on each sample before and after B


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The Journal of Physical Chemistry C aging (four replicates were tested for each scenario). The measured viscosities of each sample were then used to determine the viscosity aging index (VAI), which is defined as the change in viscosity resulting from the aging process, normalized by the unaged viscosity value (eq 1). VAI =

aged viscosity value − unaged viscosity value unaged viscosity value

(1)

Dynamic Shear Rheometer. The complex shear modulus (G*) and phase angle (δ) of the material were determined by utilizing a Bohlin Gemini 2 rheometer. G* is defined as the measure of the material’s resistance to deformation when being repeatedly sheared, and δ is the time lag between the applied shear stress and the resulting shear strain. The test was performed on both aged and unaged materials in accordance with the ASTM D7175 standard. The data were collected at 64 °C and 10 rads/s. The resulting data were then used to determine the rheological aging index (RAI) of each material; the definition of the RAI accounts for aging changes in both G* and δ (eq 2). RAI =

* GAged e(δaged − δunaged) * G Unaged

Figure 1. Aging indices calculated using RV, DSR, and FTIR results.

Table 1. Aging Indices Calculated Using RV, DSR, and FTIR Results bio-binder (%)

VAI

RAI

CAI

0 5 10

0.5 0.404 0.258

0.0912 n/a 0.0133

0.141 n/a 0.078

5% BB case. This was further evidenced in the RAI, which is based on the variation in complex modulus and phase angle due to aging. It was found that an 85% reduction in the RAI aging index could be realized when 10% BB was introduced to the base binder, compared to the nonmodified case. Furthermore, chemical analysis utilizing FT-IR was found to be in agreement with the mechanical test results. It was found that the increase in carbonyl and sulfoxide functional groups after aging was 44% less in the modified case (10% BB) compared to the nonmodified scenario. That increased aging may be due to excessive volatilization in the BB when tested at similar high temperatures as asphalt binder. In contrast to volatilization, oxidative aging, which is the focus of the present work, stems from the creation of oxygen−carbon bonds that promote further polar molecular interactions and agglomeration. Computational Details. In the theoretical section, molecular-level calculations were performed through a density functional approach embedded in the Gaussian 09 package.26 Becke’s three-parameter hybrid exchange functional and the Lee−Yang−Parr’ correlation functional (B3LYP)27,28 were used as the functional. In this section, we deal with characterization of the parameters, such as electron affinity, dipole moment, and polarizability, which have special sensitivity to the size and efficiency of the basis functions employed. Therefore, identifying the highly efficient, as well as computationally affordable, basis function is of particular importance. In this respect, the necessity and importance of diffuse functions in describing a system has been the subject of many studies. What should be noted is that in almost all these studies, single atoms that represent point charge, or at best, very small molecules have been examined; such calculations are impractical for large molecules such as PAHs or asphaltenes. Computational difficulties due to the sizable number of atoms in target molecules become more intractable for anions and corresponding parameters like electron-affinity. While a common belief about anions is that the delocalized nature of electron density in anions can be properly treated by adding diffuse functions, some studies on PAH anions indicate that omission of diffuse functions has a negligible effect when computing some parameters like geometry and total energy.29 The less noticeable effect of diffuse functions in PAH anions is attributed to the ability of hyper-conjugation in these systems,

(2)

Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. To determine the chemical changes resulting from aging, a spectrum of each sample was obtained using a Shimzadzu v. 1.30 ATR-FTIR in transition mode with a single-reflection zinc selenide prism. The use of FT-IR for studying the oxidative effect in asphalt binders has been well established.25 The wave numbers in the experiment ranged from 500 to 4000 cm−1. The spectrum was obtained by placing a thin layer of the sample against a preheated internal reflection element and recording the total internal reflectance of infrared light in a nonabsorbing prism (two replicates were tested for each scenario). The contact of absorbing substrates with the prism surface attenuates the internally reflected light while providing the infrared absorption spectrum. This corresponds to a recorded spectrum of the light that passed through the surface layer of the material under study. Furthermore, the changes in the amounts of carbonyl and sulfoxide functional groups were tracked between aged and unaged samples. Equations 3 and 4 were used to determine the carbonyl and sulfoxide indices. The sum of both indices was used to determine the chemical aging index (CAI). ICO = area of the carbonyl bond area 1700 cm−1 area of the spectral bonds between 2000 and 600 cm−1 (3)

ISO = area of the sulfoxide bond area 1030 cm−1 area of the spectral bonds between 2000 and 600cm−1 (4)

Oxidative Aging of Biomodified Binder. Figure 1 and Table 1 show the aging indices for the RV, DSR, and FTIR results. The VAI, determined from the RV results, shows that the introduction of 5% BB leads to a 20% reduction in the aging index compared to the nonmodified binder (0% BB), while 10% BB leads to an additional 36% decrease compared to the C


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Table 2. Effect of Four Extended Basis Sets on Reactivity Parameters of Asphaltene−Phenol (As2) and a Polar Aromatic Molecule (PA3): Adiabatic Ionization Potential (IP), Adiabatic Electron Affinity (EA), Hardness (η), and Chemical Potential (μ) As2

PA3

basis set

IP (eV)

EA (eV)

η (eV)

μ (eV)

basis set

IP (eV)

EA (eV)

η (eV)

μ (eV)

6-311G* 6-311+G* 6-311G** 6-311+G**

6.36 6.40 6.36 6.40

0.32 0.42 0.33 0.44

3.02 2.99 3.02 2.98

−3.34 −3.41 −3.34 −3.42

6-311G* 6-311+G* 6-311G** 6-311+G**

7.12 7.17 7.13 7.18

0.46 0.61 0.48 0.63

3.33 3.28 3.33 3.28

−3.79 −3.89 −3.80 −3.90

at which extra charge is dispersed more effectively compared to that in anions of atoms, small molecules, or n-alkyls. In an effort to find an adequate and affordable basis set for the target molecules in the present study, we have examined the effect of four extended basis sets (with and without diffuse basis functions) on some of the chemical concepts, as reported in Table 2. Accordingly, two aromatic structures were chosen for this study; one asphaltene molecule with aliphatic side chains (asphaltene−phenol, As2) and one polar aromatic molecule without aliphatic side chains (benzo-bis-benzothiophene, PA3), which are structurally shown in Figure 2. As reported in Table 2, IPs do not show any improvement when diffuse functions are added. In the case of EAs, despite some degrees of underestimation while excluding diffuse functions, no significant differences are observed in computed values for reactivity parameters of hardness and electron affinity. A similar trend is observed when polarization functions become smaller. Accordingly, B3LYP/6-311G* was chosen as an appropriate level for our calculations. To better understand the chemical reactivity of the constituent molecules of biobinder and asphalt models, chemical hardness (η) and polarizability (α) will be used as conceptual DFT descriptors for global reactivity. Chemical hardness corresponds to the response of the energy, E, to changes in the number of electrons, N, at a fixed external 1 potential η = 2 (δ 2E /δN 2)v(r).22 This parameter indeed characterizes the resistance of the system to variation in electron distribution and intramolecular charge transfer. We use eq 5 for estimating the global chemical hardness, η, in terms of IP and EA. (This equation was introduced by Parr and Pearson. The division by 2 was omitted in their original definition.30) η = (IP − EA)/2

(5)

Here, IP and EA refer to the ionization potential and electron affinity of the system, respectively. In this study, IP and EA are calculated adiabatically, based on the Born−Oppenheimer/ adiabatic approximation at which the electronic eigenstates (ground as well as excited states) are parametrically linked to the nuclear coordinates. Adiabatic IP and EA are evaluated by taking energy differences between neutral and charged species, where all the neutral and charged species are relaxed and at their geometric minimum energy. Polarizability (α) is the other chemical descriptor that is calculated here at B3LYP/6-311G* level to evaluate the reactivity of molecules in biobinder and asphalt models. The α quantity is computed in terms of its x, y, z components: ⟨α⟩ =

1 [αxx + αyy + αzz] 3

Figure 2. Main components of an asphalt model, based on the latest Yen−Mullins-modified model16,40 proposed by Greenfield,41 containing: asphaltene molecules (As1−As3), naphthene aromatics (NA1− NA2), polar aromatics (PA1−PA5), and saturates (Sa1−Sa2). Note that As1 and As2 have been updated based on Clar’s sextet theory.43 All structures are optimized at B3LYP/6-311G*.

static potential and nucleophilic-electrophilic Fukui functions, known as the local reactivity factors, were computed for the systems under review. Quantitative analysis of electrostatic potential, V (r), in our case, study of biobinder and asphalt molecules, can help explain the local reactivity behavior of the species. This property is created by the nuclei and electrons of the molecule at point r in surrounding space31

(7)

V (r ) =

In addition to computing the hardness and polarizability, known as the global reactivity factors, the molecular electro-

∑ A

D

ZA − |RA − r |

∫ |rρ′ (−r′)r| dr′

(8)


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The Journal of Physical Chemistry C where ZA is the charge on nucleus A located at RA and ρ(r) is the electronic density of the molecule. The sign of V(r), positive or negative, at a given region is dependent upon the dominant contribution of the nuclei or the electrons. V(r) is a local property that can be widely used to predict the nucleophilic and electrophilic attack sites. Accordingly, the electronic potential extrema computed on the surface of a molecule are categorized as maxima (positive values) and minima (negative values). The larger negative (or positive) region of electrostatic potential surrounding a molecule indicates the site favored for an electrophilic (or nucleophilic) reaction. The Fukui function is another concept in conceptual density functional theory to predict local reactivity. This concept describes how the electron density changes in response to an increase or a decrease in the number of electrons. In the present study, we use the dual descriptor of the Fukui function that shows both nucleophilic and electrophilic attack sites in the three-dimensional representation of the function.32 The derivative of the dual descriptor, Δf(r), is evaluated by finite difference approximation; density variations are based on electron subtraction or addition (N ± 1)

Figure 2 shows the asphalt model used in the present study. This is likely the most complete and robust model proposed so far, including SARA (saturates, aromatics, resins and asphaltenes) fractions of asphalt binder. Components of this model comprise different heteroatoms such as S, N, and O, as well as a variety of chemical functionalities that are commonly included in asphalt binders, such as groups of carbonyl, hydroxyl, thiophene, pyrrolic, etc. An island-type structure of asphaltenes (a unified and medium-sized aromatic center containing 5−7 aromatic rings surrounded by some aliphatic chains)16,39 in this model has been designed based on the latest Yen−Mullins-modified model16,40 proposed by Greenfield.41 Greenfield’s modification was the rearrangement of some aliphatic chains of the Yen-Mullins model to reduce the pentane effect42 and its corresponding high internal energy. Two asphaltene molecules (As1 and As2 in Figure 2) in this model have been replaced by more stable isomers recently proposed by Fini and her group.43 These isomers have been designed based on a rearrangement of some rings in the aromatic core to meet the requirement of Clar’s sextet theory; this rearrangement leads to an increase in the number of Clar π-sextets in the aromatic zone and a decrease in the pentane effect, resulting in isomers with more stability. Biobinder is produced from swine manure through thermochemical conversion at high temperature (T = 305 °C) and pressure (P = 10.3 MPa) in a batch isothermal reactor, reaching 70% efficiency (based on dry mass) for the conversion. The chemical characterization of the material, according to our previous studies with GC-MS and NMR,6 shows the presence of the following in BB: hexadecanamide, hexadecanoic acid, tetradecanal o-methyloxime, 2-tridecanone o-methyloxime, nbutyl octadecanamide, octadecanoic acid, cholest-7-ene, cholest-3-ene, cholest-4-ene, and vitamin E. Other components present would be in very small amounts. Considering the fully characterized nature of this specific biomodifier derived from swine manure, when compared to other commercial modifiers, as well as the fact that it is being synthesized directly by the authors, we consider the main components of this biomodifier as the main constituents of the model employed in the present study. The structure of the molecules composing this model of biobinder is shown in Figure 3. In this figure, BB1 and BB2 are linear amides. The amide linkage is a common and very important functional group; it is especially stable due to the resonance stabilization from the interaction of the lone pair of electrons in the amino group and the adjacent carbonyl group. BB3, BB4, and BB5 are steroid forms by a skeleton of cyclopenta[a]phenanthrene with different double bond locations. BB6 and BB7 are linear carboxylic acids, some of the most widely occurring functional groups, and very polar structures. BB8 and BB9 are linear methyl oximes, and BB10 is α-tocopherol, the most biologically active form of vitamin E. In the present work, these BB molecules are compared to asphalt molecules in such a way that we can evaluate the effect of BB when it is mixed with petroleum binder to form BMB. Chemical Reactivity. Polarizability. It is documented that the nanoscale mechanisms underlying asphalt hardening stem from new polar functionalities formed due to oxidation, leading to an increase in intermolecular interactions. The aromatic core of asphaltenes, with the highest portion of polarization and charge separation (arising from heteroatoms), is considered a primary site for intermolecular interactions.16 Dipole−dipole

Fukui function for nucleophilic attack: f + (r ) = ρN + 1(r ) − ρN (r )

(9)

Fukui function for electrophilic attack: f − (r ) = ρN (r ) − ρN − 1(r )

⎛ δ 2ρ(r ) ⎞ Δf (r ) = ⎜ ⎟ = f + (r ) − f − (r ) ⎝ δN 2 ⎠v(r) = ρN + 1(r ) − 2ρN (r ) + ρN − 1(r )

(10)

where ρN+1, ρN, and ρN−1 represent the electron density of three states (N + 1, N, N − 1) at the same nuclear coordinates because v(r) is defined as constant in the dual derivative. The dual descriptor representation reveals both types of reactive sites simultaneously: the favorable region for a nucleophilic attack with Δf > 0 and the favorable region for an electrophilic attack with Δf < 0. Asphalt and Biobinder Models. To gain more insight into the oxidation process and the subsequent asphalt hardening, the first step is to identify the “chemical composition” and “molecular structure” of asphalts. The physicochemical and mechanical properties of bituminous materials used in paving asphalts are strongly affected by these two factors. However, the complexity of the material’s origin at the bottom of crude oil distillation, which contains millions of small and large organic molecules, has always been a great obstacle to presenting a unified and explicit definition for molecular structures and their microstructural arrangements of asphalt components. Hence, the real composition of the mixture remains unclear. Among different proposed models in the literature, those from Greenfield33 and Mullins16,34 are the most representative ones.35−38 In spite of this uncertainty, analyzing the microstructure of asphalt components and their molecular interactions is crucial to understanding the linkage between chemical and mechanical properties, which will supply guidance for modifying asphalt compositions at the molecular level to enable development of superior asphalts for pavement applications. E


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Table 3. DFT-Based Polarizability Values for Molecular Species of Virgin Asphalt and Biobinder

Figure 3. Biobinder model based on the experimental characterization in our previous work.6 All structures are optimized at B3LYP/6311G*.

interactions between polar groups of asphaltene−asphaltene and also other components like asphaltene−resin are among the key factors promoting the formation of asphaltene nanoaggregates. Therefore, understanding the electric susceptibilities of the system is of fundamental importance for examining the underlying mechanism for asphalt hardening and oxidative aging. Dipole moment and “polarizability” are among the polar properties that can highly affect asphalt affinity for oxidation. In a general sense, polarizability is the response of the electron density toward an external electric field, which may be due to the proximity of other ions or dipoles. This physical property is associated with the formation of instantaneous dipoles that are oriented in the applied field. Polarizability can be shown as α = μ/E, indicating that it is a function of the induced dipole moment (μ) and the intensity of the field (E) that induces it. Such deformability of the electron cloud is an indicator of chemical softness. Indeed, polarizability is a key factor in the description of chemical reactivity of the system under consideration. In terms of the present study, lower polarizability is indicative of lower propensity of the chemical species to interact with other species in the environment and to form molecular aggregates. In the specific case of asphalt oxidation, it helps to understand how the intermolecular interactions are affected by the appearance of the abovementioned new polar functionalities. Table 3 shows polarizability (α) values for the molecules in the biobinder and asphalt models. These values have been plotted in Figure 4 to gain a better insight into the trends governing the interaction mechanisms. As clearly shown, BB molecules are considerably less polarizable than asphalt molecules. In terms of the hardening process, it means that BB molecules will be less affected by the presence of new polar functionalities.

Figure 4. Comparing the polarizability trend for molecular species of virgin asphalt and biobinder (values are taken from Table 3 and arranged from maximum to minimum).

One of the noticeable points in Table 3 is the high polarizability of the asphaltene molecules (As1, As2, and As3), which supports the presumed mechanism for asphalt hardening. Indeed, the high polarizability of asphaltenes makes them more vulnerable to any polar molecules in the environment. Based on this evaluation, it is plausible that the presence of BB in the asphalt mixture would make the blended material more resistant to forming aggregates after oxidation, with a consequent reduction in asphalt hardening. This is in agreement with the experiment results from both rheology and the viscosity aging index. Global Hardness. The correlation between maximum hardness and minimum polarizability in Pearson’s original approach emphasizes the concept of chemical hardness as a parameter measuring the resistance to deformation of charge distribution or resistance to change in the number of electrons. Additionally, the definition of chemical hardness based on IP and EA, η = (IP − EA)/2, is a compromise between the resistance of the system to lose its electron as well as the propensity of the system to acquire an electron. High hardness F


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In fact, it has been shown that vitamin E could be used in asphalt materials as an antioxidant agent.44 The results of these DFT-based reactivity calculations also show that the asphaltene molecules As1, As2, and As3 always present the lower values of chemical hardness; that means they are more likely to be altered upon oxidation as an overall reactivity description. However, as we will see in the following section, certain positions within the molecules do react against oxidant agents. This point is of fundamental importance, since asphaltenes are known to be the key components of asphalt, responsible for most of the mechanical properties. Therefore, any strategy toward protecting asphaltenes from oxidation will improve the performance of asphalt in adverse environmental conditions. As evidenced by SARA analysis, some of the resins and aromatics are converted to asphaltenes under oxidation.45−47 More precisely, the carbonyl group is the most common functional group that forms due to aging, which can be understood from local reactivity calculations discussed below. Local Reactivity: Fukui Function and Molecular Electrostatic Potential (MEP). In this section, we employ quantitative analysis of electrostatic potential (ESP) on the van der Waals (vdW) surface of fragments and a dual Fukui function calculation as a local description to characterize the reactivity toward oxidative agents. These approaches not only suggest the possible locations for eventual new functional groups arising from the oxidation reaction but also provide information about the effect of the presence of BB molecules in the asphalt mixture. In this respect, the maximal points of electrostatic potential on the vdW surface, representing the large negative and positive ESP values, are set as electrophilic and nucleophilic sites. Through this work, vdW surfaces denote the isosurface of ρ = 0.001 au (electrons/bohr3) and are colored on a BWR (blue− white−red) scale; the red- and blue-filled surfaces correspond to electron-depleted and electron-accumulated regions, respectively. Accordingly, the left columns of Figures 6 and 7 show the color-filled molecular surface map and its extrema, plotted via the Multiwfn and VMD programs, for some selected fragments of the asphalt and biobinder models, respectively. The right columns of Figures 6 and 7 are assigned to the isosurfaces of Fukui dual descriptors (FDD). Fukui descriptors are employed here as a complementary indicator to evaluate local reactivity of biobinder and asphalt binder toward oxidative agents. The plots of FDD in this paper are computed based on spin density of two states (N + 1, N − 1), using the Multiwfn program. The red and blue isosurfaces (+0.00126 and −0.00126 au) represent the favorable sites for a nucleophilic or electrophilic attack, respectively. The numbers labeled on isosurfaces are global maxima, corresponding to the most favorable sites for nucleophilic attacks. Since oxidative agents like OH− mostly contribute to the nucleophilic reaction, we label here the global maximum points, the candidates which interact with a nucleophile. In Figure 6, special attention should be given to the asphaltene molecules, since asphaltene molecules control many of the physiochemical properties of asphalt. The larger ESP values at the global maximum point for asphaltene (labeled for As1 in Figure 6, Vmax = 40.7 kcal/mol) along with high global hardness makes asphaltenes better candidates to interact with oxidative agents compared to other asphalt components: polar aromatics (PA), naphthene aromatics (NA), and saturates (Sa). As an example, we can refer to the hardest fragment in the

refers to a high ionization energy (IP) and a low electron affinity (EA). Table 4 and Figure 5 show the values and corresponding plot of global hardness for the two models under study; the Table 4. Values of Global Hardness (η), Computed via Adiabatic IP and EA, for Asphalt and Biobinder Models

Figure 5. Comparing hardness (η) of two categories of asphalt and BB models. The values are taken from Table 4 and arranged from maximum to minimum.

hardness values clearly show the higher global hardness of BB molecules compared to that of asphalt molecules, which is interpreted as a lower reactivity of BB components. The lower reactivity of BB under oxidative conditions is a plausible factor underlying the modified performance of biomodified asphalt binder against oxidative aging. There are, however, two different factors against oxidation. The first is the presence of less reactive molecular species, as in the majority of cases for BB. The second is the presence of molecular species that are much more reactive than the key components of the mixture. These molecules would be more likely to undergo oxidation first, saving key components of the material from being oxidized. For instance, this may happen with BB10 (vitamin E), which is a known antioxidant in living organisms, although the mechanism of that kind of oxidative aging is completely different from the case of asphalt materials. G


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Figure 7. ESP color-filled molecular surface map (left column) and isosurfaces of the Fukui dual descriptor (right column) for some representative constituents of the biobinder model. Blue and red regions correspond to the electrophilic and nucleophilic sites, respectively.

In the case of polar aromatics, both descriptors of MEP and Fukui are clearly localized around the functional groups, as evidenced in most polar components of asphalt. Only polar aromatic PA3 does not show such localization, which can be attributed to the highly symmetric geometry of this molecule. In saturates (Sa1 and Sa2), regions covered by red-filled isosurfaces suggest that short aliphatic branches are more susceptible to chemical reactions. These can undergo a nucleophilic attack, according to the Fukui function, as they present a slightly elevated MEP, in red. In the case of the BB model, as represented in Figure 7, the positive-valued dual descriptor (red region) is more localized on the biobinder components carrying functional groups of carboxylic acid (−COOH), amide (−CONRR′), and oxime (−CNO−), compared to other BB species and asphalt components. This correlates to the lower polarizability of these species and the reference to them as “hard” molecules. For steroid molecules (BB3, BB4, and BB5), the higher values of MEP (red area) are near the location of the double bond. This region coincides with the location of the Fukui function for nucleophilic attacks, the red-filled isosurfaces are relatively large in this area, so double bonds are considered the most reactive sites in the steroid molecules. The steroid-type of biobinder molecules present high polarizability; therefore, they may be affected by additional functional groups in the molecules. There is some evidence that some biobinder components are more susceptible to nucleophilic attack than are asphalt fragments; an example is the larger values of electrostatic potential at the global maximum points (Vmax) for some biobinder components compared to those for asphalt components, as shown in Figure 7. This could be attributed to the duality of performance of biobinder components against oxidative agents. On one hand, there are some species with higher values of global hardness compared to asphalt fragments,

Figure 6. ESP color-filled molecular surface map (left column) and isosurfaces of Fukui dual descriptor (right column) for some representative constituents of the asphalt model. Blue and red regions correspond to the electrophilic and nucleophilic sites, respectively. The number on each surface shows the electrostatic potential at the global maxima, corresponding to the most favorable sites for nucleophilic attacks.

asphalt model, Sa2, with η= 5.13 eV and Vmax= 7.9 kcal/mol, which has the lowest propensity toward oxidative agents. The concentration of blue-filled surfaces around the polycyclic aromatic core of asphaltene molecules, as indicated for As1 in Figure 6, is indicative of a higher concentration of electrons in the aromatic core. These regions coincide with the locations of the Fukui function for both electrophilic and nucleophilic attacks (Figure 6, right column). However, the three-dimensional representation of Fukui is quite small compared to the same value of isosurfaces for other asphalt components. Therefore, considering both the Fukui function and MEP values, we can conclude that these regions of the polycyclic aromatic core are quite unlikely to experience the oxidation. In contrast, we expect that the adjacent hydrogen positions, which are clearly red in the MEP representation, would be attacked by some of the species responsible for the oxidation of asphalt, such as OH−. Naphthene aromatics (e.g., NA2 in Figure 6) reveal a similar behavior to asphaltenes, as their functionality is quite similar, though at a smaller scale. Negative MEP values (blue-filled regions) are mainly localized around the aromatic rings, while lower concentrations of electrons (red-filled regions) in some peripheral hydrogens make them more accessible to oxidation. H


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species responsible for the oxidation of asphalt, such as OH−. This point is of fundamental importance, since molecular species like asphaltenes are known to be the key components of asphalt, responsible for most of the mechanical properties. Despite the wide presence of less-reactive molecular species in BB, which act as a protective shield against oxidative aging, there is some evidence of highly reactive species that are primary targets for oxidative agents. Highly reactive components of BB play the role of sacrificing elements; they would undergo oxidation first, saving key components of the material (such as asphaltene) from being oxidized. BB10 (vitamin E) could be the most representative example for this group; it has recently been used as an antioxidant agent in asphalt materials.44 This duality in the protection mechanism of biobinder components is a plausible contributor to the high performance of BB in asphalt binder.

which show higher resistance to electron transfer and therefore lower reactivity; this happens in the majority of cases for BB, such as with BB2 and BB6. This in turn could suggest that having BB as modifier in an asphalt mixture would be beneficial against oxidative aging, since it would make the material less reactive under oxidative conditions. On the other hand, there are some biobinder molecular species that have low values of hardness and are much more reactive than the key components of the mixture. The importance of these reactive species (such as BB10 with high Vmax, 43.0 kcal/mol, and low hardness, 3.92 eV) lies in their ability to be oxidized first, saving key components like asphaltene from being oxidized.

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CONCLUSIONS Producing biobased materials is a response to the growing demand for nonpetroleum-based materials and moving away from an oil-based economy. The strong dependence of bituminous adhesives on crude oil, the shrinkage of bitumen supplies (from refineries installing coking technologies to produce synthetic fuel from their bitumen supply), and the increasing demand for high-grade asphalt materials have made “bio-based bituminous materials” a significant research topic for the asphalt community. Therefore, this paper focuses on the oxidation and consequent hardening of asphalt and further examines the role of a bioadditive (a biomodifier extracted from swine manure, introduced by Fini’s team)6 to enhance the performance of asphalt against oxidative aging. In a multiscale analysis, macroscopic properties of unmodified and modified asphalt binder are experimentally characterized under oxidation conditions, and then chemical reactivities of the species (asphalt and biobinder molecules) are studied at a molecular level via DFT-based theoretical calculations. The experiments’ results were used to calculate the overall susceptibility of asphalt binder to oxidation via different indices. Viscosity measurements were used to calculate the VAI, which showed that the introduction of 5% BB leads to a 20% reduction in the VAI compared to the unmodified scenario, while 10% BB leads to an additional 36% decrease in the VAI, compared to the 5% BB concentration. In general, it was found that an 85% reduction in the VAI could be realized when 10% BB is introduced to the base binder, compared to the unmodified case. More specifically, the increase in carbonyl and sulfoxide functional groups after aging was 44% less in the BB-modified case compared to unmodified samples. Our theoretical calculations are aimed at the molecular processes and intermolecular interactions underlying asphalt hardening. To do so, we characterized electron density through DFT-based chemical reactivity indices: polarizability and hardness as the global reactivity indices, and electrostatic potential, and Fukui functions as the local reactivity indices. Polarizability calculations clearly show that biobinder constituents are considerably less polarizable than asphalt molecules. Lower polarizability of BB is indicative of lower propensity of these chemical species toward new polar functionalities arising from the presence of oxidative agents. In contrast, the maximum polarizability obtained for asphaltene molecules suggests that this component of virgin asphalt is easily affected by the oxidative agents. Chemical hardness also corroborates our assertion that components of unmodified asphalt are much more easily affected by oxidation. ESP and Fukui functions show that central regions of the polycyclic aromatic core are quite unlikely to experience oxidation, while adjacent hydrogen positions are easily attacked by some of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: efi[email protected]. Tel: 336-285-3676. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is sponsored by the National Science Foundation (Award Nos. 1546921 and 1150695). The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented. This paper does not constitute a standard, specification, or regulation.

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Multiscale Investigation of Oxidative Aging in Biomodified Asphalt

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