Intermolecular Interactions of Isolated Bio-Oil Compounds and Their

Research Article pubs.acs.org/journal/ascecg

Intermolecular Interactions of Isolated Bio-Oil Compounds and Their Effect on Bitumen Interfaces Albert M. Hung,† Masoumeh Mousavi,† Farideh Pahlavan,† and Ellie H. Fini*,‡ †

Innovation Center for Materials, Methods and Management, Division of Research and Economic Development, North Carolina A&T State University, 1601 E. Market Street, Greensboro, North Carolina 27411, United States ‡ Department of Civil, Architectural and Environmental Engineering, North Carolina A&T State University, 1601 E. Market Street, Greensboro, North Carolina 27411, United States S Supporting Information *

ABSTRACT: Bio-oils derived from low-value or waste biomass have been shown to be useful as low-cost, sustainable additives to improve the adhesion or moisture resistance of bituminous asphalt binder. The chemical profile of bio-oil is complex, and identifying the most active compounds in the mixture and understanding how they work to modify bitumen properties is essential for synthesizing bio-based additives with desired characteristics and performance. Hexadecanamide and hexadecanoic acid, two surfactants prevalent in both animal- and plant-derived bio-oils, are notable candidates as active compounds responsible for altering the colloidal or interfacial properties of bitumen. In this study, pure forms of these surfactants were mixed into bitumen to examine their effects on binder composition and morphology. AFM and FTIR data show that the amide strongly separated from the mixture, much of it crystallizing at the exposed surface. The DFT-based studies indicate that selfassembly of amide molecules is energetically preferred to adsorption of amides to asphaltenes or wax crystals in bitumen. The acid seemed to mix in well but induced the growth of a unique morphology at the glass interface. However, in combination, the amide and acid improve each other’s solubility without strongly interacting with each other. Other molecules in bio-oil may similarly work to improve the solubility of the total mixture and make it an effective surface-modifying additive. KEYWORDS: bitumen, bio-oil, asphaltene, hexadecanamide, phase separation, density functional theory (DFT)

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INTRODUCTION Chemical additives have long been used to improve the performance of asphalt pavement. More recently, the demands to reduce manufacturing cost and ensure environmental sustainability of asphalt pavement construction have spurred interest in the development of new materials and processes such as recycled asphalt pavement (RAP) and warm-mix asphalt (WMA). These technologies rely even more heavily on additives for specific purposes such as adhesion promotion or softening of asphalt binder. Bio-oils derived from other industrial or agricultural waste streams are attractive candidates as low-cost modifiers that would also improve waste management practices and generate new commercial opportunities.1−5 For example, bio-oils derived from swine manure have been shown to improve the rheological and mechanical properties of RAP binder.5−7 However, the chemical profile of bio-oils is complex and highly source dependent.6−8 Identifying the key active compounds and their characteristic behaviors could enable manufacturers to tailor bio-oil production to yield the optimum chemistry. A major component of bio-oil derived from swine manure is hexadecanamide, a saturated hydrocarbon terminated with a primary amide.7,8 Hexadecanoic acid is a similar hydrocarbon compound terminated with a carboxylic acid that is widely prevalent in plant-based bio-oils as well as animal-based oils.8 © 2017 American Chemical Society

However, it is unclear exactly how these compounds contribute to the efficacy of bio-oil in modifying the properties of asphalt binder5−7 and what molecular interactions are responsible for their activity. Computational modeling suggests that hexadecanamide can disrupt asphaltene stacking, affecting the rheological and mechanical properties of the binder.5,9 Both H-bonding and dispersion interactions are important in the self-assembly of hexadecanamide,10−12 but their relative impacts may differ in the context of bitumen or bio-oil. Bio-oil is typically a viscous liquid or soft solid at room temperature, whereas pure hexadecanamide and hexadecanoic acid are both waxy solids with melting points above 60 °C. Recent results show that the introduction of bio-oil enhanced bitumen’s moisture resistance,13 which is important because water-induced stripping of binder from aggregate is a significant source of failure in asphalt pavement.14−17 Hexadecanamide and hexadecanoic acid are both surfactants, compounds with both polar and nonpolar chemical groups, and thus they may be responsible for improving the adhesion between binder and aggregate in water. Commercially available amine-based compounds are commonly used as antistripping agents for Received: May 10, 2017 Revised: June 30, 2017 Published: July 28, 2017 7920

DOI: 10.1021/acssuschemeng.7b01462 ACS Sustainable Chem. Eng. 2017, 5, 7920−7931

Research Article

ACS Sustainable Chemistry & Engineering asphalt binder.18−21 However, amine-based agents may not perform well under certain conditions depending on binder and aggregate chemistry,18 possibly due to the tendency of amine groups to ionize at higher pH. Hexadecanoic acid is a powerful surfactant often found in common household soaps as a salt, but it may be disfavored as an antistripping agent for ionizing too readily and being too water-soluble.22,23 Amide does have an NH2 group, but it behaves differently than a primary amine due to electronic resonance with the neighboring CO group and does not ionize under most conditions. Thus, amides are nonionic, less sensitive to pH, and less chemically reactive than amines or acids. We hypothesize that both the amide and acid compounds found in bio-oil could play an important role in modifying the chemistry and morphology of asphalt binder interfaces with aggregate, air, and water. In this study, we compare the effects of selected amide- and acid-terminated surfactant modifiers on the surface morphology of bitumen and its interaction with borosilicate glass slides as a model oxide surface. The objective herein is to reveal fundamental aspects of how and why these chemistries alter asphalt binder at the molecular and microstructural levels, which is important for understanding and optimizing bio-oil formulations.

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AFM, ATR-FTIR, and Contact Angle Measurement. Atomic force microscopy (AFM) was performed with a large-stage 5600LS AFM (Keysight Technologies) in tapping mode with TAP-300 silicon cantilever tips (Budget Sensors, 40 N/m nominal spring constant, 300 kHz nominal frequency). All samples were imaged in at least two different locations and with at least one 20 μm scan to get representative images. Images were processed and analyzed with Gwyddion open-source software. Measurements of the contact angle between bitumen and glass were performed using a Ramé-Hart Model 260 standard goniometer with DROPimage Advanced v2.6 software. After AFM imaging and contact angle measurement, Fourier transform infrared spectroscopy in attenuated total reflectance mode (ATRFTIR) was performed on a Cary 670 FTIR (Agilent Technologies) using a diamond ATR crystal and 45° reflection angle. The bitumen droplets on glass were simply inverted to touch the top surface (the air interface) to the diamond ATR crystal. To qualitatively assess bitumen adhesion to glass under aqueous conditions, bitumen droplets on glass were annealed in water at 80 °C for 2 h following a procedure similar to that reported elsewhere.25 Five milliliters of DI water in a 20 mL glass vial was preheated to 80 °C in a temperature-controlled water bath for uniform heating. The temperature in the vial was monitored with a thermocouple inserted through the cap. A bitumen sample on glass was placed in the vial, and the vial was held in the water bath for the requisite time and temperature. To cool the sample, the entire vial was removed from the heated bath and set in a room-temperature water bath for 5 min to cool before the bitumen sample was removed, dried with a nitrogen gun to remove visible water (10−20 s), and immediately characterized by AFM and contact angle measurements. A study of morphological evolution over time was not within the scope of this study, but features observed by AFM and contact angle measurements appeared stable under ambient conditions for longer than an hour. Although the contact angles developed after 2 h in hot water may not necessarily be equilibrium values, they can still be qualitative measures of adhesion, where higher angles indicate greater dewetting or stripping. The test is similar to the boil test (ASTM D3625) in which stripping of bitumen from aggregate after 10 min exposure to boiling water is empirically measured by visual inspection.26 Preparation of Fractured Samples. Access to interior surfaces and glass interfaces of the binder mixtures was obtained by brittle fracture of thin film samples using a procedure similar to that reported elsewhere.27 AFM and FTIR of interior fracture surfaces reveal morphology and composition of the bulk bitumen compared to that of the bitumen−air interface. Characterization of the glass interface would be similarly useful to understand bitumen wetting behavior and identify any segregation of surfactant compounds to that interface that could result in modification of wetting. To begin, 15−20 g of bitumen was deposited between two 1″-square cleaned glass slides and annealed for 30 min at 150 °C. The bitumen spread out into a thin film 10−100 μm thick sandwiched between the glass slides with no extra compression necessary. To assist the next step, a strip of thin Teflon tape could be placed between the slides along one edge. A razor blade was wedged a little bit between the glass slides and used to lever apart the two glass pieces with enough speed and force to crack the bitumen film (caution: risk of broken glass). Glossy surfaces and sudden separation of the sandwich indicated brittle fracture. For especially ductile samples, the chance of brittle fracture could be improved by first chilling the sample in a freezer at −4 °C for 20−30 min and then fracturing the sample under a flowing stream of dry N2 gas to prevent moisture condensation on the sample which could produce anomalous features. This method yielded in-plane crosssection surfaces parallel to the glass surface, but crack propagation was uncontrollable. By chance, the film might separate almost completely from the glass and enable characterization of the glass interface (see Supporting Information, Figure S1). To produce cross-section surfaces perpendicular to the glass interface, long-stem glass Pasteur pipets were first cleaned as described above. A molten bitumen sample was sucked up the capillary tube tip of a pipet that could then be scribed with a diamond-tipped pen and broken off. A heat gun was typically required to warm the pipet while

MATERIALS AND METHODS

Materials. The bitumen used in this study was PG 64-22, a binder grade commonly used in the United States, acquired from Associated Asphalt Inc. (http://associatedasphalt.com). The fraction of crystalline wax was too small to be detected by differential scanning calorimetry (DSC). The chemical properties of this binder are provided in Table 1

Table 1. SARA Fractions and Elemental Analysis of the PG 64-22 Binder SARA fractions saturates naphthene aromatics polar aromatics asphaltene

elemental analysis 9.3 44 39.9 6.8

C H N O

81.6 10.8 0.77 6.83

and also reported elsewhere.24 Hexadecanamide (>95%, mp 103−107 °C) was obtained from Fisher Scientific. Hexadecanoic acid (≥99%, mp 61−63 °C) was obtained from Sigma-Aldrich. For brevity, hexadecanamide and hexadecanoic acid are referred to in this paper as amide and acid, respectively. Glass slides, pipets, and solvents for cleaning were obtained from Fisher Scientific. Doped bitumen samples were made by adding 0.3 or 1% w/w of additive relative to the starting weight of binder and mixing by hand for at least 3 min at about 120 °C. For doping with both amide and acid, a 50:50 w/w mixture of the two additives was melted together at 120 °C in a glass vial and quenched to room temperature in a water bath to prevent the formation of large crystals upon solidification. The solid mixture was added to bitumen to reach concentrations of 0.3 or 1% of each additive (0.6 or 2% total additive). Sample Preparation. Glass slides were cut into 0.5″ or 1″ squares and precleaned immediately before sample preparation by sequential ultrasonication in acetone, isopropanol, 30% ammonium hydroxide, then deionized (DI) water for 10 min each to obtain a hydrophilic surface. The slides were dried under a stream of N2 gas followed by at least 30 min baking at 120 °C in a convection oven. Bitumen mixtures were heated to 120 °C and briefly mixed again before 15−20 mg of the mixture was deposited onto a glass slide. All samples were annealed at 150 °C for 30 min and then allowed to cool to room temperature and equilibrate for at least 1 day under ambient laboratory conditions (21 °C, 50% humidity) before any further treatment or characterization. 7921


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ACS Sustainable Chemistry & Engineering

Figure 1. AFM height images of the surface of (a) undoped bitumen and bitumen doped with (b) 1% amide, (c) 1% acid, or (d) both 1% amide and 1% acid. Inset in panel b is an amplitude image of the indicated area. on dimensionless RDG (s) quantity, measured from the electron |∇ ρ| 1 density and its first derivative, s = 2 1/3 4/3 , and the sign of the

the bitumen was being drawn up, and excess bitumen on the outside of the tube was wiped off with laboratory tissues wet with acetone or toluene. The capillary tube samples were annealed at 150 °C as described above and then scribed and snapped in half immediately before AFM imaging (see Supporting Information, Figure S1b). Computational Methods. Molecular level calculations were performed through a dispersion-corrected density functional theory (DFT-D) approach using the DMol3 module28,29 of the Accelrys Materials Studio program package, version 6.0. We employed the Perdew−Burke−Ernzerhof (PBE)30 formulation of generalized gradient approximation (GGA) as the density functional. Grimme’s dispersion correction31 is combined with the density functional (PBED) to provide a complete representation of the long-range electron correlation in this kind of system involving dispersive forces.32 Fine-grid mesh points were specified for the matrix numerical integrations. The geometry optimization was performed to reach the tolerance levels of 1.0 × 10−5 hartree, 2.0 × 10−3 hartree Å−1, and 5.0 × 10−3 Å for energy, maximum force, and displacement convergence, respectively. The basis set assigned for all atoms has all-electron triplenumerical quality with polarization function, TNP. The corrections of basis set superposition error (BSSE) were taken into account while studying intermolecular interactions by means of the counterpoise method.33,34 Binding energy (Ebind) as a thermodynamic stability indicator was evaluated for all associations in this study. Ebind is the energy difference between the complex and its fragments in their lowest energy structure:

2(3π )

ρ

second eigenvalue (λ2) of the electron density Hessian matrix, (λ2)ρ. The 3D RDG surfaces demonstrated in this study correspond to s= 0.5 au and are colored on a BGR (blue-green-red) scale according to the values of (λ2)ρ, ranging from −0.05 to 0.05 au red-, blue-, and greenfilled 3D-gradient-isosurfaces indicate the repulsive, attractive, and weak van der Waals interaction, respectively. Deeper color is associated with higher local electron density and stronger interaction.

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RESULTS Surface Morphology of Modified Binder Annealed in Air. Figure 1 shows AFM images of the bitumen−air surface of undoped and amide- or acid-doped bitumen. The surface of the 1% amide-doped sample was entirely coated with lamellar crystals of excess amide, whereas the acid-doped sample looked no different from the control, displaying characteristic wrinkled “bee” structures that are believed to be patches of native crystalline wax that had separated out at the surface.24,27,36,37 Even at 3% acid, no apparent disruption of the bee structures was observed (data not shown). With a mixture containing 1% amide and 1% acid, there was still significant crystallization of additive at the surface, but large bee structures were still present. Despite being doped with a greater total amount of additives, the binder samples doped with the amide/acid mixture exhibited distinctly less separation of excess additive at the air interface than did the samples doped only with pure amide.

E bind = Ecomplex − (Efrag1 + Efrag2) To visualize noncovalent interactions, 3D reduced density gradient (RDG) plots were employed.35 RDG isosurfaces are visualized based 7922


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Figure 2. AFM height images of bitumen mixed with (a) 0.3% amide or (b) both 0.3% amide and 0.3% acid. Images were taken 6 days after annealing and are of areas in the center of the bitumen drops.

Figure 3. AFM images of cross sections of (a, e) undoped bitumen and bitumen doped with (b, f) 1% amide, (c, g) 1% acid, or (d, h) both 1% amide and 1% acid. Top row (a−d) are height images of cross sections of the bulk bitumen parallel to the glass surface. Bottom row (e−h) are phase images of the glass-bitumen interface.

open areas between the bees first before eventually overgrowing them after 6 days (Figure 2a). The crystal growth was centered in the middle of the bitumen droplet and covered roughly half of the total surface area. In contrast, the mixture containing 0.3% amide and 0.3% acid showed no change in surface morphology after 6 days. Qualitatively, the bee structures of the dual-doped sample appeared larger than those of the undoped bitumen, but this has yet to be quantified.

This effect was most noticeable for samples doped with 0.3% amide or 0.3% of both additives, as shown in Figure 2. Immediately after dry annealing at 150 °C, both samples showed bee structures that looked no different from those of undoped bitumen (data not shown). After 3 days of storage under ambient conditions, the sample doped with 0.3% pure amide showed lamellar crystals beginning to grow at the surface (see Supporting Information, Figure S2), nucleating in the 7923


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ACS Sustainable Chemistry & Engineering Morphology of Interior and Glass Interfaces. Figures 3a−d show AFM height images of cross sections of the bitumen interior parallel to the glass surface produced by brittle fracture of bitumen samples. The fractured surface of the undoped bitumen showed line features that were previously identified as cross sections of lamellar inclusions of native wax in the bitumen27 with no noticeable change in morphology at the glass surface. The 1% amide-doped sample showed an increase in both number and size of inclusions (Figure 3b), often appearing clustered together, and most of which are probably crystallized amide that had separated from the mixture. The acid-doped samples also appeared to show more inclusions in some images, but this varied significantly depending on how close the parallel fracture surface was to the glass interface. Figures 3e−h show phase images of perpendicular fracture surfaces that reveal the morphology of the bitumen−glass interface. Larger area scans of the same regions shown in Figures 3e−h are also provided in the Supporting Information, Figure S3. The morphology of the undoped (Figure 3e) and 1% amide-doped samples (Figure 3f) at the glass interface did not appear significantly different from the morphology deeper into the bulk. However, the glass interface of the 1% acid-doped bitumen sample was coated with a layer about 2.5−3 μm thick that appeared to be a brush of lamellar crystals oriented perpendicular to the surface (Figures 3g and S3c). The composition of this brush is currently unknown; preliminary FTIR data failed to confirm an initial hypothesis that the brush was crystallized acid. Cross sections of bitumen doped with a mixture of amide and acid show the same features that appear in the cross sections of samples doped with only pure amide or pure acid with no readily apparent interaction between the characteristic features (Figure 3h). However, the thickness of the brush at the glass surface is certainly thinner (∼2 μm) in the dual-doped sample than in the acid-only sample (Figure S3d). Thus, mixing in both additives moderates the extreme phase separation effects of doping with the pure additives alone; the acid reduces the separation of amide at the air interface, and the amide reduces the acid-induced structuring at the glass interface. Contact Angle Measurements. Table 2 lists measured contact angles of droplets of bitumen mixtures on clean glass

higher contact angle (37°). After water annealing, the undoped sample dewet from the glass to a contact angle of 104°, higher than any of the doped samples and suggesting that the undoped bitumen is most susceptible to stripping. The doped samples also beaded up, but the 1% amide-doped sample beaded up significantly less than any of the other samples, including the dual-doped sample. Surfactants generally act to reduce interface energies and affect the highest-energy interfaces the most. In air, the surfactant may bleed out from the edge of the drop and coat the glass, lowering the glass−air interface energy locally in the area around the bitumen drop. This effect is analogous to what is observed for droplets of water and propylene glycol on glass; the propylene glycol causes the liquid to bead up where pure water would otherwise spread out.40 This is a possible cause of the higher θdry observed for the doped samples. Doping of bitumen with 1% amide appeared to significantly improve its resistance to water stripping. In water, the amide could lower θwet by passivating either the bitumen−water or bitumen−glass interface. Interestingly, the acid not only appeared ineffective in preventing dewetting but negated any effect of the amide in the dual-doped bitumen sample. For contact angles close to 90°, the drop shape depends less on the bitumen−water interface energy and more on the bitumen− glass interface energy. Thus, in acid-doped bitumen, the glass interface energy and subsequent wetting/dewetting behavior is likely dominated by the same compounds that produce the brush layer observed in Figure 3. FTIR Analysis. Infrared absorption spectra of the doped bitumen samples are shown in Figure 4. They are normalized to the peak at 1455 cm−1, which is a combination of the CH2 scissoring and CH3 antisymmetric deformation peaks.41 FTIR spectra of the pure additives are provided in the Supporting Information, Figure S5. ATR-FTIR is moderately sensitive to surface composition because the infrared penetration depth is at most a few micrometers into the sample.42 The air interface of 1% amide-doped bitumen shows large NH2 and CO stretching peaks (3184, 3354, and 1659 cm−1) that are dramatically reduced in the spectra of an interior surface, confirming that the lamellar crystals observed by AFM are large amounts of solid amide. The position of the NH2 stretching peaks, shifted down in frequency from where they would be in dilute solution,41 also indicates that the amide groups are Hbonded together. The CH2 stretching peaks are enhanced in the spectra of the air interface of amide-doped samples (see the Supporting Information, Figure S6), indicating that the original bitumen contains a lower CH2/CH3 ratio than the amide. This suggests that, on average, the aliphatic chains in bitumen are shorter, more branched/substituted, or less saturated than in amide. Carboxyl groups are also known to H-bond to each other, forming very stable dimers even in the melt phase. FTIR spectra of the acid-doped binder samples show the CO stretching vibration at the dimer frequency (1700 cm−1) instead of shifting closer to 1750−1800 cm−1, which would correspond to the monomer.41 Spectra of the dual-doped bitumen are consistent with a lack of any specific molecular interaction between the amide and acid. The carboxyl group also lacks the extra hydrogen with which amides can form additional bonds between amide dimers. Thus, acid dimers are more weakly bonded to each other and disperse more uniformly into the bitumen. The FTIR detects little difference in the amount of acid at the air interface versus the interior. However, preliminary FTIR of bitumen separated from very near the

Table 2. Contact Angles (±SD) of Bitumen Droplets on Glass before and after Water Annealing 1 wt % additive additive undoped amide acid amide + acid

dry 10 22 27 37

± ± ± ±

wet 2 2 2 3

104 65 87 96

± ± ± ±

3 4 7 2

before (θdry) and after (θwet) annealing under water at 80 °C for 2 h. Pictures of the bitumen droplets are also shown in the Supporting Information, Figure S4. Upon annealing at 150 °C, the bitumen droplets spread out into thin films over the glass, but all of the doped samples did not spread out as much as the control. Contact angles in air for the undoped and amide-doped bitumen are roughly consistent with other studies,38,39 although low contact angle numbers are prone to significant errors due to both measurement limitations and kinetic effects such as contact line pinning and viscous flow. However, the sample doped with both 1% amide and 1% acid showed a significantly 7924


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among the main components of amide-doped bitumen (amide, asphaltene, and wax) using DFT-based computational quantum mechanical modeling. To this end, asphaltene flakes, wax branches, and layered structures of surface-migrated amides are simulated at atomistic levels and individually optimized to their equilibrium states, and then their interactions are examined and compared. Asphaltene−Amide Interactions. Despite an extensive controversy over the contribution and predominance of different conformers proposed for the asphaltene molecular structure, there is a consensus on the presence of a variety of asphaltene molecular structures in the bitumen matrix varying from “continental” to “archipelago”.43,44 A continental is an asphaltene structural motif with a single large core of highly condensed poly aromatic rings surrounded by dangling peripheral chains. Unlike continentals, archipelagoes do not have a uniform aromatic core. They consist of many separated small aromatic units bridged by short aliphatic chains. “Islandtype” conformers are medium-sized structures, between continental and archipelago, composed of a comparatively small and uniform aromatic core of 5−7 fused rings with shorter aliphatic chains and an average molecular weight of ∼750 g/mol.45,46 The asphaltene molecular structure selected for the present study is a Yen−Mullins45,47 island-type molecular model modified48,49 to reduce the high internal energy of the model by rearrangement of some aliphatic side chains and some rings in the aromatic zone. Figure 5 shows the molecular structures

Figure 4. (a) ATR-FTIR spectra of undoped and 1% amide- or aciddoped bitumen taken at the air interface. (b) FTIR spectra of a dualdoped binder taken at different interfaces, and (c) AFM height image of the modified binder surface near where the glass interface FTIR spectra were taken.

Figure 5. Asphaltene-pyrrole (upper) and hexadecanamide (lower) molecular structures.

glass interface also showed no discernible difference in acid content (Figure 4b), suggesting that the brush layer observed by AFM may not necessarily be composed of acid, as initially thought. The increased area of the broad absorption around 3600−3100 cm−1 for the spectra of the glass interface might be due to an enrichment in a variety of OH or NH groups, but additional data is required before firm conclusions can be drawn about the composition at the glass interface. DFT-Based Computational Modeling. The formation of lamellar crystals of amide on the surface of the amide-doped bitumen sample is one of the interesting aspects of this study. Therefore, we studied the assembly of these lamellar crystalline structures in the context of prominent molecular interactions

of hexadecanamide and an island asphaltene that includes one pyrrolic nitrogen atom. It is worth noting that amide functional groups are planar because of electron delocalization within the O−C−N array (charge transfer between the nN and π*CO orbitals). The energetic and geometrical structure of the proposed asphaltene monomer and hexadecanamide molecule were first refined by full optimization at the PBE-D/TNP level (TNP numerical basis set is comparable with 6-311G**). In the next step, optimized structures were used to construct adsorption complexes of amide−asphaltene, as shown in Table 3. 7925


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Table 3. BBSE-Corrected Binding Energies (kcal/mol) for the Four Most Likely Arrangements of Asphaltene−Amide Absorption Complexesa

a

All structures were optimized at the PBE-D/TNP level.

Figure 6. (I) Unit cell of wax crystal employed in the present study. (II) Central chain of each unit cell is mutually perpendicular to other surrounding chains. (III) BSSE-corrected binding energy between an amide molecule and chains of wax crystal. All structures were optimized at the PBE-D/TNP level.

energy of −27.8 kcal/mol and shows the molecular plane of the amide functional group to be almost parallel to the asphaltene aromatic rings. Conformers in which the molecular plane of hexadecanamide is perpendicular to the asphaltene aromatic domain are least stable. This behavior could be indicative of the predominant π−π interactions in this specific orientation. Localization of the active frontier orbitals (HOMOs and LUMOs, the highest and lowest molecular orbitals) on the amide functional group might reinforce the assumption that the main contribution of amide−asphaltene interaction is coming from the amide head and the asphaltene aromatic core, but our findings do not support this assumption. Calculations show that the binding energy between asphaltene and acetamide (a truncated amide with the aliphatic tail removed) is −10.6 kcal/ mol, which is 17.2 kcal/mol less stable than the asphaltene− hexadecanamide complex. This means that about 62% of the thermodynamic stability of a hexadecanamide−asphaltene complex arises from the interaction of the amide aliphatic tail and the asphaltene aromatic core. This comparison highlights the prominent role of CH-π multiple interactions in stabilizing the amide−asphaltene complex.

The interaction of hexadecanamide and a different monomer of asphaltene (asphaltene−thiophene, which includes a thiophene ring in its aromatic zone) was investigated and described recently,9 so it is not necessary to discuss the fine details of this interaction. Nevertheless, a brief look at the main features of this interaction reminds us that to achieve the most effective interactions, all reasonable and possible orientations between two components should be considered. To sketch the structural models with the best orientations possible, four key factors are of paramount importance: (1) the general orientation of the polar groups of amide toward the asphaltene aromatic core, particularly toward the heteroatom of the aromatic zone; (2) the planarity of the amide functional group, which may provide a suitable overlap between π-molecular orbitals of amide and aromatic rings of asphaltene; (3) the contribution of CH-π interactions between the amide aliphatic tail and the asphaltene aromatic zone; and (4) the steric hindrance between asphaltene aliphatic side chains and the amide aliphatic tail.9 The lowest energy conformers of asphaltene−amide and their corresponding BSSE-corrected binding energies are shown in Table 3. The most stable conformer has a binding 7926


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Figure 7. (a) Schematic illustration of molecular arrangement of hexadecanamide with p2 (rotational) symmetry. This structure is designed based on the molecular arrangement of decanamide fitted with X-ray and neutron diffraction patterns taken from ref 12. (b) H-bonding interactions between two arrays of amide chains.

Figure 8. BSSE-corrected binding energies (kcal/mol) for two possible pathways of amide dimerization, head-to-head and side-by-side. Both structures have been optimized at the PBE-D/TNP level.

Wax−Amide Interactions. Paraffin and microcrystalline waxes are two distinct types of waxy derivatives of crude oil. Paraffin waxes (n-alkanes) are regular, large clusters of unbranched hydrocarbons. In contrast, higher-melting microcrystalline waxes are small and irregular crystals of saturated alkanes in addition to a significant portion of branched and cyclic saturated hydrocarbons.50,51 The presence of cyclic compounds in microcrystalline waxes results in a H/C ratio lower than that of paraffin waxes.51 The projected wax crystal structure here is a layered structure of n-alkane wax, which is observed in both microcrystalline and paraffin waxes. On the basis of FTIR results, microcrystalline and paraffin waxes contain 55 and 63−78% linear chain methylene, respectively.51 In the present study, a methylene chain of C17H36 was used for designing the unit cell of chainfolded polyethylene as n-alkane wax. As shown in Figure 6, our typical wax model is a simple orthorhombic packing of uniform rows of C17H36 chains that was introduced by Bunn in 193952 based on X-ray diffraction studies and reexamined for different alkane chains by other groups in the following years.53,54 Except for the central chain, this packing is a regular array of CH2 chains aligned in parallel rows. The central chain is mutually perpendicular to other chains. The optimized unit cell dimensions (without imposing any constraint on the system) are a = 4.89 Å, b = 7.25 Å and γ

= 89.3°, which are in reasonable agreement with the results predicted by Smith55 (a = 4.97 Å, b = 7.48 Å). Note that c dimension corresponds to the length of the hydrocarbon chain. To simulate the wax−amide interaction, the amide molecule was located on top of the wax crystal model. To maximize coverage between the two components, the amide was arranged parallel to the wax chains. Due to the very weak interaction between the deeper layers of the wax crystal and the amide molecule, only two upper surface layers (containing three alkane chains) were taken into account. In a separate series of DFT calculations, this two-layer surface was optimized in its equilibrium state, and BSSE correction was added to the calculations. The BSSE-corrected binding energy between the amide and wax is −28.7 kcal/mol. Amide−Amide Interactions. Surface migration of amide molecules and the formation of layered microstructures on the surfaces of water, polymer, and graphite have recently been the focus of several studies. Scanning tunneling microscopy (STM) was used to characterize two-dimensional, ordered arrays of self-assembled, H-bonded amide molecules.56−59 Surface monolayers/multilayers of alkyl amide have also been studied by differential scanning calorimetry (DSC)60 and X-ray and neutron diffraction studies.12,61,62 Figure 7a illustrates a representative molecular structure for crystalline layers of hexadecanamide (C16) that has been 7927


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Figure 9. Noncovalent interaction analysis through RDG plots for two possible pathways of amide dimerization: head-to-head and side-by-side. RDG plots correspond to s = 0.5 au and a BGR (blue-green-red) color scale of −0.05 < ρ < 0.05 au.

Figure 10. Successive chain packing through two different pathways: (a) packing of head-to-head H-bonded dimers and (b) packing of side-by-side H-bonded dimers.

stability of the hexadecanamide dimers in a side-by-side arrangement is due to the effective van der Waals interactions between its aliphatic chains, which compensates for the shortage of H-bond interactions in this configuration compared to the head-to-head configuration. To gain a deeper understanding of the key role of van der Waals forces (in addition to the H-bond interactions) in closepacking of amide chains, we compared reduced density gradient (RDG) plots34 for the two aforementioned arrangements. Visual analysis of RDG plots for head-to-head association exhibits just deep blue pills, indicating strong H-bond interaction in the amide functional group zone. With side-byside association, in addition to the light blue pills that represent weak H-bond interactions, an extensive greenish surface is observed, reflecting the region of attractive dispersive forces in the aliphatic chains’ domain. This extensive green surface is indeed indicative of the multiple interactions involving the multicentric electron density in the region of aliphatic chains when two amide molecules are placed in a parallel position. It is worth noting that weak H-bonding interaction in sideby-side arrangement, shown with light blue pills in Figure 9, is due to the slight deviation of amide groups from their ideal side-by-side arrangement in crystalline structure. In a more optimum position, amide functional groups may find their proper orientation at which H-bonding interactions become more significant than what has been shown by light blue color in side-by-side RDG plots.

designed on the basis of decanamide (C10) molecular arrays fitted to X-ray and neutron diffraction patterns, as proposed by Bhinde et al.12 A similar layout was reported for solid crystalline structures in multi/monolayer coverage of carboxylic acid on a graphite surface.63 As shown in Figure 7b, to form this highly extended H-bonded chain, dimers are formed first; then, depending on the carbon chain length, they bond together and adopt p2 (with two amide molecules per unit cell) or pgg (with four amide molecules per unit cell) symmetry groups.12,63 As shown in Figure 8, two possible configurations are predicted for dimerization of two amide monomers: head-tohead (centrosymmetric amide pair), and side-by-side. Head-tohead dimerization is associated with the formation of two Hbonds, eventually leading to the formation of extensive Hbonded chains of amide molecules. In side-by-side interactions, two amide chains are arranged in a parallel position to form one H-bond. The corresponding interaction energy released with head-to-head amide dimerization is −16.8 kcal/mol, which is only 2.2 kcal/mol more stable than the side-by-side arrangement. To estimate the H-bond interaction energy between the two dimerization pathways, we calculated BSSE-corrected binding energy (Ebind) for dimerization of two acetamide monomers. As expected, in the absence of aliphatic chains, Ebind for head-tohead dimerization is almost twice the corresponding energy of side-by-side dimerization (−16.6 compared to −7.5 kcal/mol). This simple comparison indicates that almost 49% of the 7928


Research Article


While this study focused primarily on amide assembly and its impact on bitumen morphology, the development of the brush layer at the bitumen−glass interface upon the addition of acid to the bitumen was unexpected. Preliminary FTIR measurements could not confirm the composition of the brush layer as either acid or any other easily identifiable compound. Whatever its composition, the brush layer appeared to be detrimental to adhesion of bitumen on silica under water exposure. However, just as the acid inhibited amide separation at the air interface, amide also decreased the thickness of the brush layer at the glass interface. Thus, the same bio-oil compounds that disrupt amide assembly could also disrupt this acid-induced assembly and promote the antistripping effect of bio-oil as observed experimentally.13

Thus, ordering of the amide molecules in the crystalline layers on the bitumen surface is controlled not only by the Hbonding interactions of the head functional groups but also by the van der Waals interactions of aliphatic side chains. As stated above, there is some evidence that amide molecules adsorb as dimers,60 and depending on their ordering (head-to-head or side-by-side), successive chain packing can occur through two different pathways. To understand which molecular arrangement is more favored, the binding energy between two adjacent dimers was calculated for two head-to-head and two side-byside dimers. As shown in Figure 10, the binding energy between head-to-head dimers is more than that between side-by-side dimers (−30.8 compared to −26.8 kcal/mol). This result is in agreement with diffraction experiments showing that adsorbed layers of amides grow in a layer-by-layer fashion.12 These quantitative and qualitative analyses highlight the valuable role of noncovalent interactions not only in head functional groups but also in tail alkyl chains in the assembly and stabilization of amide chains packing into solid adsorbed layers at interfaces.

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CONCLUSION Amide- and acid-terminated surfactant modifiers show very different effects on the surface morphology of bitumen and on the interaction of bitumen with a silica surface. The amide compound in pure form did not mix well with the bitumen and would eventually separate out, even at 0.3 wt % of additive. This behavior is attributed to the tendency of amide molecules to form multiple strong H-bonds with other amide molecules, effectively hiding their polar heads and limiting their function as a surfactant. DFT-based calculations confirm that it is energetically more favorable to form extensive chains from self-assembled amide dimers than from adsorption complexes of amide−asphaltene or amide−wax. In contrast, the acid appeared to mix well into the bitumen and did not affect the morphology of the bitumen−air interface; however, the acid severely altered the morphology of the bitumen−glass interface with significant impact on wetting behavior. Solubilizing agents could reduce the amide intermolecular interactions and amide’s affinity for forming extended chains, increasing its effectiveness as a surfactant that can improve the adhesion properties of bitumen. Organization of the amide molecules in crystalline layers is influenced by both van der Waals forces and H-bonding interactions; almost 49% of the stability of the hexadecanamide dimers in a side-by-side arrangement is due to van der Waals interactions between the aliphatic chains. Thus, solubilizing agents do not need to interact specifically with the amide headgroup to be effective. Even a nonoptimized mixture of amide and acid exhibited improved dispersion of both components. Accordingly, the presence of other compounds in bio-oil (such as heterocyclic derivatives) in combination with amides may explain the effectiveness of the bio-oil as an antistripping agent, as reported elsewhere.13

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DISCUSSION The presence of well-formed crystalline solids on the surface of amide-doped bitumen is the product of a hidden intermolecular competition between amide molecules and various components available in the matrix of asphalt binder, eventually resulting in the amide-rich mono/multilayered structures on the surface. A DFT-based comparison was made on the strength of noncovalent interactions between the amide units and constituent units of asphaltene flakes, wax chains, and selfassembled layers of amides. As summarized in Table 4, the Table 4. Comparison of the DFT-Based BSSE-Corrected Binding Energy between the Amide Molecules and the Main Components (Asphaltene, Wax, and Amide) Available in the Matrix of Amide-Doped Bitumen

binding energy (kcal/mol)

asphaltene− amide

wax− amide

amide− amide

−27.8

−28.7

−30.8

interaction of amide molecules with each other is energetically preferred to the interaction of amide with asphaltene or wax. This could justify the presence of self-assembled layers of amide chains on the surface of amide-doped bitumen. The intermolecular interactions of additives are an important consideration because the strength of some of these interactions can inhibit them from fully interacting with oxide surfaces as intended in the case of antistripping agents. Coadditives that improve the solubility of amide and disrupt its tendency to self-assemble may improve its efficacy as a surfactant or antistripping agent,64 and because the amide is neutral rather than basic, its adhesion might be less sensitive to aggregate composition and pH. Bio-oils are complex liquid mixtures, and any or all of those compounds could contribute to amide solubility and the ability of bio-oil to enhance bitumen moisture resistance as observed experimentally elsewhere.13 Our results show that acid can improve the dispersion of amide even in the absence of any specific interaction between the two compounds. Similar amide−acid mixtures exhibit a eutectic phase diagram with a eutectic concentration of about 60% amide.65 This mixture gives the greatest melting point depression and may be closer to the relative ratio of amide/ acid in real bio-oil.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01462. Additional AFM images and FTIR spectra, including Figure S1: (a) example of a sandwiched bitumen film after fracturing to reveal an in-plane cross-section, and (b) bitumen sample in a capillary tube that was fractured to reveal a vertical cross-section and the bitumen−glass interface for AFM imaging; Figure S2: AFM height phase image of 0.3% amide-doped bitumen after 3 days; Figure S3: AFM height images and phase images of cross 7929


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(10) Ectors, P.; Zahn, D. Analysis of the molecular interactions governing the polymorphism of benzamide - a guide to syntheses? Phys. Chem. Chem. Phys. 2013, 15, 9219−9222. (11) Turner, J. D.; Lingafelter, E. C. The X-ray crystallography of the n-aliphatic amides. Acta Crystallogr. 1955, 8, 549−550. (12) de Paiva, L. B.; Morales, A. R.; Valenzuela Díaz, F. R. Organoclays: Properties, preparation and applications. Appl. Clay Sci. 2008, 42, 8−24. (13) Oldham, D.; Yaya, A.; Folley, D.; Fini, E. Effects of warm mix additives on asphalt moisture resistance. In Transporation Research Board Conference; National Academy of Science: Washington DC, 2017; pp 17−05541. (14) Caro, S.; Masad, E.; Bhasin, A.; Little, D. N. Moisture susceptibility of asphalt mixtures, Part 1: mechanisms. Int. J. Pavement Eng. 2008, 9, 81−98. (15) Kringos, N.; Scarpas, A.; Copeland, A.; Youtcheff, J. Modelling of combined physical−mechanical moisture-induced damage in asphaltic mixes Part 2: moisture susceptibility parameters. Int. J. Pavement Eng. 2008, 9, 129−151. (16) Apeagyei, A. K.; Grenfell, J. R. A.; Airey, G. D. Moisture-induced strength degradation of aggregate−asphalt mastic bonds. Road Mater. Pavement Des. 2014, 15, 239−262. (17) Mohd Hasan, M. R.; You, Z.; Porter, D.; Goh, S. W. Laboratory moisture susceptibility evaluation of WMA under possible field conditions. Construction and Building Materials 2015, 101 (Part 1), 57−64. (18) Buddhala, A.; Hossain, Z.; Wasiuddin, N.; Zaman, M.; O’Rear, E. A. Effects of an Amine Anti-Stripping Agent on Moisture Susceptibility of Sasobit and Aspha-Min Mixes by Surface Free Energy Analysis. J. Test. Eval. 2012, 40, 91−99. (19) Cui, S.; Blackman, B. R. K.; Kinloch, A. J.; Taylor, A. C. Durability of asphalt mixtures: Effect of aggregate type and adhesion promoters. Int. J. Adhes. Adhes. 2014, 54, 100−111. (20) Arabani, M.; Hamedi, G. H. Using the surface free energy method to evaluate the effects of liquid antistrip additives on moisture sensitivity in hot mix asphalt. Int. J. Pavement Eng. 2014, 15, 66−78. (21) Xiao, F.; Amirkhanian, S. N. Effects of liquid antistrip additives on rheology and moisture susceptibility of water bearing warm mixtures. Construction and Building Materials 2010, 24, 1649−1655. (22) Lesueur, D.; Petit, J.; Ritter, H.-J. The mechanisms of hydrated lime modification of asphalt mixtures: a state-of-the-art review. Road Mater. Pavement Des. 2013, 14, 1−16. (23) Plancher, H.; Green, E. L.; Petersen, J. C. Reduction of oxidative hardening of asphalts by treatment with hydrated lime − a mechanistic study. Proceedings of the Association of Asphalt Paving Technologists 1976, 45, 1−24. (24) Hung, A. M.; Fini, E. H. AFM study of asphalt binder “bee” structures: Origin, mechanical fracture, topological evolution, and experimental artifacts. RSC Adv. 2015, 5, 96972−96982. (25) Hung, A. M.; Goodwin, A.; Fini, E. H. Effects of water on bitumen surface microstructure at elevated temperature or extended duration. Construction and Building Materials 2017, 135, 682−688. (26) Parker, F., Jr; Wilson, M. S. Evaluation of boiling and stress pedestal tests for assessing stripping potential of Alabama asphalt concrete mixtures. Transportation Research Record 1986, 1096, 90− 100. (27) Pahlavan, F.; Mousavi, M.; Hung, A.; Fini, E. H. Investigating molecular interactions and surface morphology of wax-doped asphaltenes. Phys. Chem. Chem. Phys. 2016, 18, 8840−8854. (28) Musser, B. J.; Kilpatrick, P. K. Molecular Characterization of Wax Isolated from a Variety of Crude Oils. Energy Fuels 1998, 12, 715−725. (29) Dorset, D. L. Crystallography of Real Waxes: Branched Chain Packing in Microcrystalline Petroleum Wax Studied by Electron Diffraction. Energy Fuels 2000, 14, 685−691. (30) Boys, S. F.; Bernardi, F. The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol. Phys. 1970, 19, 553−566.

sections of the bitumen−glass interface for bitumen samples; Figure S4: bitumen droplets on glass before and after annealing in water at 80 °C for 2 h; Figure S5: ATR-FTIR of pure hexadecanamide (“Amide”), pure palmitic acid (“Acid”), and the 50:50 w/w mixture of the two compounds used to dope bitumen samples; Figure S6: plot showing the same spectra presented in main text Figure 4 of undoped and doped bitumens, focusing on the alkane stretching vibrations (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: 336-285-3676; Fax: 336-334-7126; E-mail: efini@ncat. edu. ORCID

Masoumeh Mousavi: 0000-0003-4750-8154 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is sponsored by the National Science Foundation (Awards 1150695 and 1546921). The authors are grateful for the support from NSF and from the State of North Carolina. The contents of this paper reflect the view 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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