Intermolecular Interactions of Isolated Bio-Oil Compounds and Their

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Intermolecular Interactions of Isolated Bio-Oil Compounds and Their Effect on Bitumen Interfaces Albert M Hung, Masoumeh Mousavi, Farideh Pahlavan, and Ellie H. Fini ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01462 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Sustainable Chemistry & Engineering

Intermolecular Interactions of Isolated Bio-Oil Compounds and Their Effect on Bitumen Interfaces Albert M. Hung,a Masoumeh Mousavi,a Farideh Pahlavan,a Ellie H. Finib,* * Corresponding author. Tel.: 336-285-3676; fax: 336-334-7126; e-mail: [email protected] (E. H. Fini). a. Innovation Center for Materials, Methods and Management, Division of Research and Economic Development, North Carolina A&T State University, 1601 E. Market St., Greensboro, NC 27411, USA b. Department of Civil, Architectural and Environmental Engineering, North Carolina A&T State University, 1601 E. Market St., Greensboro, NC 27411, USA

ABSTRACT Bio-oils derived from low-value or waste bio-mass 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 self-assembly 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.

Keyword: bitumen, bio-oil, asphaltene, hexadecanamide, phase separation, density functional theory (DFT) 1 ACS Paragon Plus Environment


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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 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.

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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 non-polar 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 anti-stripping agents for 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.

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



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wax was too small to be detected by DSC. The chemical properties of this binder are provided in Table 1 and also reported elsewhere.24 Hexadecanamide (>95%, m. p. 103 – 107°C) was obtained from Fisher Scientific. Hexadecanoic acid (≥99%, m. p. 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.

Table 1 SARA fractions and elemental analysis of the PG 64-22 binder SARA fractions Elemental analysis Saturates 9.3 C 81.6 Naphthene aromatics 44 H 10.8 Polar aromatics 39.9 N 0.77 Asphaltene 6.8 O 6.83

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 pre-cleaned 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 one day under ambient laboratory conditions (21 °C, 50% humidity) before any further treatment or characterization. 4 ACS Paragon Plus Environment

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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 (ATR-FTIR) 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. In order 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 similar procedure as reported elsewhere.25 5 mL 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 de-wetting or stripping. The test is similar to



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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 similar procedure as 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, 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” cross-section 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 electronic supplementary information, ESI Figure S1). In order 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 the bitumen was being drawn up, and excess


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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, then scribed and snapped in half immediately before AFM imaging (see ESI 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 (PBE-D) 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 hartree, 2.0 × 10 hartree Å-1, and 5.0 × 10 Å for energy, maximum force, and displacement convergence, respectively. The basis set assigned for all atoms has all-electron triple-numerical 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:

= − + (1) To visualize non-covalent interactions, 3D reduced density gradient (RDG) plots were employed.35 RDG isosurfaces are visualized based on dimensionless RDG (s) quantity, measured from the

|∇,|

electron density and its first derivative, $ = (%& )'⁄( ,-⁄( , and the sign of the second eigenvalue (. ) of the electron density Hessian matrix, $/01 (. )2. The 3D RDG surfaces demonstrated in this study correspond to s= 0.5 a.u. and are colored on a BGR (blue-green-red) scale according to the values of 7 ACS Paragon Plus Environment


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$/01 (. )2, ranging from -0.05 to 0.05 a.u. Red-, blue-, and green-filled 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.

RESULTS Surface morphology of modified binder annealed in air. Figure 1 shows AFM images of the bitumenair 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 the samples doped only with pure amide. 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 ESI Figure S2), nucleating in the 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


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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.

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 (b) is an amplitude image of the indicated area.



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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. Morphology of interior and glass interfaces. Figures 3a-d show AFM height images of crosssections 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 ESI 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 (Figure 3g and S3c). The composition of this brush is currently unknown; preliminary FTIR data 10 ACS Paragon Plus Environment

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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.

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 crosssections of the bulk bitumen parallel to the glass surface. Bottom row (e – h) are phase images of the glass-bitumen interface.



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Contact angle measurements. Table 2 lists measured contact angles of droplets of bitumen mixtures on clean glass before (θdry) and after (θwet) annealing under water at 80 °C for 2 h. Pictures of the bitumen droplets are also shown in ESI 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 higher contact angle (37°). After water annealing, the undoped sample de-wet 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. Table 2 Contact angles (± s.d.) of bitumen droplets on glass before and after water annealing Additive 1 wt% additive Dry Wet Undoped 10 ± 2 104 ± 3 Amide 22 ± 2 65 ± 4 Acid 27 ± 2 87 ± 7 Amide + Acid 37 ± 3 96 ± 2

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.


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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 de-wetting 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/de-wetting 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 ESI Figure S5. ATR-FTIR is moderately sensitive to surface composition, because the infrared penetration depth is at most a few microns 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 H-bonded together. The CH2 stretching peaks are enhanced in the spectra of the air interface of amide-doped samples (see ESI 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



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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 glass interface also showed no discernable 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.


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

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. So, we studied the assembly of these lamellar crystalline structures in the context of prominent molecular interactions among the main components of amide15 ACS Paragon Plus Environment


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doped bitumen (amide, asphaltene, and wax) using DFT-based computational quantum mechanical modelling. 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, 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 polyaromatic 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. “Island-type” 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 islandtype 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 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 13 and 4 ∗ 678 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.


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The interaction of hexadecanamide and a different monomer of asphaltene (asphaltenethiophene, which includes a thiophene ring in its aromatic zone) has been 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 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



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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.

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

Table 3 BBSE-corrected binding energies (kcal/mol) for the four most likely arrangements of asphalteneamide absorption complexes. All structures have been optimized at the PBE-D/TNP level.

a) Ebind=-27.8

b) Ebind=-23.3

c) Ebind=-22.6

d) Ebind=-21.0


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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 lower H/C ratio 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 chain-folded 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



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state, and BSSE correction was added to the calculations. The BSSE-corrected binding energy between the amide and wax is -28.7 kcal/mol.

Ebind= -28.7 kcal/mol I

II

III

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 have been optimized at the PBE-D/TNP level.

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) has been used to characterize two-dimensional, ordered arrays of self-assembled, Hbonded 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 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 has been 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,


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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

a

b 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 Xray and neutron diffraction patterns; taken from Reference 12. (b) H-bonding interactions between two arrays of amide chains.

As shown in Figure 8, two possible configurations are predicted for dimerization of two amide monomers: head-to-head (centrosymmetric amide pair), and side-by-side. Head-to-head dimerization is associated with the formation of two H-bonds, 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-to-head 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 stability of the hexadecanamide dimers in a side-by-side



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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-tohead configuration.

head-to-head amide pair

side-by-side amide pair

Ebind= -16.8

Ebind=-14.6

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.

To gain a deeper understanding of the key role of van der Waals forces (in addition to the Hbond interactions) in close-packing 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-by-side 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 side-by-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


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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.

Amide head-to-head dimerization

Amide side-by-side dimerization

Figure 9 Non-covalent 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 a.u. and a BGR (bluegreen-red) color scale of -0.05

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