Dodecylbenzenesulfonic Acid as a Bitumen Modifier - American

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Dodecylbenzene sulfonic acid (DBSA) as a bitumen modifier: a novel approach to enhance bitumen’s rheological properties Francisco J Ortega, Francisco Javier Navarro, and Moises Garcia-Morales Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00419 • Publication Date (Web): 30 Mar 2017 Downloaded from http://pubs.acs.org on April 2, 2017

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Dodecylbenzene sulfonic acid (DBSA) as a bitumen modifier: a novel approach to enhance bitumen’s rheological properties

Francisco J. Ortega, Francisco J. Navarro*, Moisés García-Morales Departamento de Ingeniería Química, Centro de Investigación en Tecnología de Productos y Procesos Químicos (Pro2TecS), Campus de ‘El Carmen’, Universidad de Huelva, 21071, Huelva (Spain)

*

Author to whom correspondence should be addressed:

E-mail address: [email protected] Tel.: +34 959218205 Fax: +34 959219983

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Abstract Bitumen deterioration has been traditionally overcome by its modification with polymers, sometimes producing unstable blends due to their mutual lack of affinity. A new and different approach, based on the use of dodecylbenzene sulfonic acid (DBSA), is herein reported. DSBA is a surfactant that contains a strong acid group capable of interacting with asphaltene molecules, commonly used as a dispersing agent for asphaltenes in crude oils. However, the contrary effect can also be obtained, as long as a critical concentration is not surpassed. In this work, the influence that additive concentration and processing temperature exert on the thermo-mechanical behavior of DBSA-modified bitumen has been assessed by means of rheological, thermal and thermogravimetric analysis. Within the concentration range used (0.4 to 3 wt.%), the attachment of DBSA to asphaltene molecules promoted the association of asphaltene molecules/aggregates into larger clusters in bitumen, with 3 wt.% DBSA noticeably improving bitumen’s rheological properties. Keywords: bitumen, amphiphile, DBSA, asphaltene, ion-pairing interaction


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1. Introduction Bitumen corresponds to the heaviest fraction obtained from crude oil distillation, resulting in a complex material with aliphatic, aromatic and naphthenic hydrocarbons in its composition, among other molecules containing some heteroatoms (e.g. sulphur, nitrogen and oxygen).1 Based on its solubility in n-alkanes, two major fractions can be separated from bitumen: asphaltenes (which is the most polar fraction) and maltenes. In addition, the maltenic fraction can be further sub-divided into saturates (straight and branch-chain aliphatic hydrocarbons), aromatics (naphtene aromatics) and resins (polar aromatics), according to their solubility and polar properties1,2. Structurally, it is widely assumed that these four families of compounds (traditionally known as SARAs) are associated forming a colloidal suspension of asphaltene aggregates within the maltenic medium, stabilized to a certain extent by resins. Therefore, the degree of association and dispersion of these aggregates plays the most important role in bitumen’s microstructure and mechanical behavior. Chemically, asphaltenes are average-sized polycyclic aromatic hydrocarbon ring systems with peripheral alkane substituents. These molecules tend to associate to form nanoaggregates of approximately six unities, which can further merge into clusters, with estimated aggregation values of around eight2. Asphaltene aggregation is attributed to intermolecular attractive interactions arising from charges on the rings, due to the presence of dissociated ions, along with acid and basic functional groups, containing heteroatoms such as nitrogen, sulphur and oxygen3. Nitrogen is mainly contained within the rings, either 3 ACS Paragon Plus Environment

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in pyrrolic structures (acidic) or in pyridinic structures (basic). Sulphur is present in thiophenic structures; whilst oxygen appears in hydroxyl, carbonyl and carboxylic groups2. Thereby, the dissociation of charged ions must cause opposite surface charges on asphaltenes. Basic functional groups (e.g. pyridinic groups) should generate negative charges in non-polar medium; whereas hydroxyl groups in carboxylic acids, alcohols and phenols, as well as mercapto groups (associated to mercaptans) and organic sulfides would produce positive charges3. The mentioned asphaltenic aggregates may also include a fraction of resins, either linked with the asphaltene molecules, which self-associate into an aggregate core, or occluded within its structure 2. Accordingly, the content of asphaltenic and resin fractions in bitumen, as well as their aggregation and dispersion state, will determine the microstructure and, therefore, the thermomechanical and rheological behavior of bitumen. As for the uses of bitumen, it is primarily employed to provide waterproofing and binding properties in engineering applications, especially in road construction. However, during its in-service life, bitumen deteriorates leading to rutting problems (permanent deformation), thermal cracking or fatigue cracking, and eventually, to the failure of the paved surface. In order to increase its resistance against these problems and durability, it is usually modified with a wide range of materials, mainly polymers, also seeking to improve some of its properties, and so, its performance. Polymers normally used for the modification of bitumen can be classified into plastomers (e.g. polyethylene (PE), polypropylene (PP), ethylene-vinyl acetate (EVA), ethylene–butyl acrylate (EBA)); and thermoplastic elastomers (e.g. styrene-butadiene-styrene (SBS)).4


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After polymer modification, a kinetically stable, although thermodynamically unstable system is obtained, with a polymer dispersed phase partially swollen by the light components of bitumen5, exhibiting properties that are mainly determined by the characteristics of bitumen and polymer, their contents and the manufacturing process applied. Nonetheless, despite the advantages that polymer addition has shown to confer on bitumen, this type of modification presents some drawbacks including, among others: high cost, low ageing resistance, and polymer degradation. Yet, the major problem of polymer-modified bitumen is its poor storage stability, which arises as a consequence of incompatibilities between the polymers and bitumen, due to differences in properties like density, molecular weight, polarity and solubility4. In this paper, a new and different approach for bitumen modification is proposed. The target is to modify the colloidal structure of bitumen by promoting the interactions between the asphaltene aggregates. Within a certain range of low concentrations, amphiphilic surfactants are known to favor this process, what might derive in enhanced in-service behavior of bitumen. Amphiphilic surfactants are commonly applied as dispersing agents for asphaltenes in crude oils3,6–10, so as to prevent their precipitation in pipelines, which may result in clogging problems. Examples of these compounds, widely used by the petrochemical industry, are alkyl derivatives of phenol, sulfonic acid and resorcinol, and among them, it is noteworthy the use of dodecylbenzene sulfonic acid (DBSA). Beyond a critical concentration8,9,11, the surfactant molecules, which progressively attach to the available sites on asphaltene molecules/aggregates, occupy the virtual totality of sites and sterically hinder the attractive interactions with other asphaltene aggregates. Then, 5 ACS Paragon Plus Environment

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multilayered coverages on asphaltene aggregates provide steric stability, preventing them from associating into larger structures8,9,11. However, this effect strongly depends on both the additive concentration and the chemical structure of asphaltenes3. Thus, before the critical concentration is achieved, the opposite behavior has also been reported. Thus, the surfactant’s polar head group interacts with asphaltene molecules in crude oils so that the colloidal stability of the dispersion is diminished, a situation which eventually promotes the association of aggregates into larger clusters6–8,12. Based on that premise, we present a comprehensive study on the effect provoked by the addition of the amphiphilic surfactant dodecylbenzene sulfonic acid (DBSA) on bitumen’s behavior and microstructure, a new approach which has never been reported so far. With this aim, the influence that the additive concentration and processing temperature exert on bitumen’s thermomechanical behavior was assessed by means of rheological, thermal and thermogravimetric analysis. 2. Experimental 2.1. Materials Bitumen was used as the base material for the formulations, characterised by a penetration grade of 13/22 (1/10-mm), a softening point of 68 ºC, and a composition, in terms of fractions of saturates, aromatics, resins and asphaltenes (“SARAs” fractions), included in Fig. 1. The following additives were used as modifying agents:


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•

4-Dodecylbenzenesulfonic acid (“DBSA”, hereafter), supplied by Sigma-Aldrich®,

contains a mixture of isomers with a purity greater than 95 % and less than 2 % sulphuric acid, corresponds to a yellow-to-brown liquid, soluble in water, characterized by an average molecular weight of 326.49, along with a density of 1.06 g/mL at 20 ºC, and a boiling/decomposition point of 204.5 ºC.

•

Sodium dodecylbenzenesulfonate (“DBSS”, hereafter”), which is the sodium salt of

DBSA, supplied by Sigma Aldrich with a purity of 95 %, also contains isomers of various chain lengths, with a melting point higher than 300 ºC. Both DBSA and DBSS are commonly used as anionic surfactants, given their amphiphilic properties, provided by a molecular structure comprising a polar head and a non-polar long alkyl chain. 2.2. Sample Processing Samples were processed in glass containers (70.6 mm diameter and 85 mm height), using a low-shear mixer composed of a 50 mm four-bladed impeller, coupled to the blending device “IKA RW20”, with stirring speeds of 800-1000 rpm, and heated by immersion in a recirculating oil digital bath. The modification of bitumen was carried out by melt blending. Thus, the corresponding amount of additive (0.4 – 3 wt.%) was poured into neat bitumen, previously heated to the selected temperature (130, 150 or 170 ºC), and then blended at low-shear (800 – 1000 rpm) for 1 h, while maintaining temperature conditions.


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Furthermore, in order to evaluate the specific effects of processing conditions on the resulting properties, blank samples of neat bitumen without any modifier were processed according to the sequence above. For the sake of brevity, only selected blank samples that underwent significant ageing are presented. 2.3. Characterization Steady-state viscous flow tests were performed at 60 ºC in stress-controlled mode, using a controlled-stress Physica MCR-301 rheometer (Anton Paar, Austria). Temperature sweep tests under oscillatory shear, at a frequency of 10 rad/s, were conducted using the same rheometer. A continuous heating ramp was applied at 1 K/min, from 30 ºC up to the maximum possible temperature at which reliable results were achieved (approximately 110 ºC). Previously, strain sweeps were carried out to ensure a linear viscoelastic response within the entire temperature interval applied. Smooth plate-and-plate geometry, with 25 mm diameter and a gap size of 1 mm, was used for the rheological tests. Before testing, in every case, samples were allowed to rest for at least 20 minutes. Modulated differential scanning calorimetry tests (MDSC) were conducted using a TA Q100 (TA Instruments, USA) on 5 – 10 mg of sample placed in hermetic, aluminum pans under nitrogen atmosphere. In order to ensure the same thermal history, prior to testing, samples were heated up to 150 ºC for 30 minutes, and annealed at room temperature for 24 h. Then, the following testing procedure was applied, starting by quenching to a temperature of -70 ºC, which was hold for 5 minutes, followed by a heating ramp up to 150


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ºC at a rate of 5 K/min in modulated mode, with period and amplitude values of 60 s and 0.5 ºC, respectively. Bitumen composition, based on the so-called “SARAs” fractions, was obtained by thin layer chromatography coupled to a flame ionization detector (TLC/FID), by means of an Iatroscan MK-6 analyser (Iatron Corporation Inc., Japan). The elution procedure consisted on the sequential use of hexane, toluene and dichloromethane/methanol (95/5, in volume), following the method outlined elsewhere13. Thermogravimetric analysis tests (TGA) were conducted using a TA Q-50 (TA Instruments, USA), under nitrogen atmosphere, on 5 – 10 mg of sample placed in aluminum pans, which were first heated up to 50 ºC and equilibrated at that temperature for 5 min., and subsequently subjected to a continuous heating ramp at 10 K/min up to 600 ºC. In order to ensure the accuracy of the results obtained, every sample was examined at least three times, with the resulting values corresponding to the averaged data collected in each set of tests. Asphaltene fraction from neat and 3 wt.% DBSA modified bitumen was obtained by precipitation and centrifugation, according to the following procedure. Bitumen was first dissolved in n-heptane (1 g/20 mL) in an Erlenmeyer flask and heated at reflux for 1 h. Afterwards, the solution was left to cool down to room temperature under ambient conditions and then centrifuged at 3000 rpm for 5 min. The supernatant was removed, and the sediment (asphaltenes) washed by resuspension and centrifugation again until the supernatant was clear enough. Finally, asphaltenes were dried and placed into a glass flask.


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3.

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Results and Discussion

3.1.

Rheological properties of modified bitumen: effect of additive concentration

The effects of DBSA on the steady viscous flow properties at 60ºC of modified bitumen were first assessed in relation to the concentration of this additive, within the range from 0.4 to 3 wt.%. Measurements for neat bitumen and its corresponding blank, resulting from neat bitumen with no additive, processed following the same sequence as the modified binders, are also included for comparison. As depicted in Fig. 2, the profiles for all the systems are described by a wide Newtonian plateau, followed by a decay of the viscosity, which corresponds to a shear-thinning zone, whose onset is established by the critical shear rate value. Blank shows that stirring for 1 h at 150ºC leads to a subtle increase in viscosity, and therefore negligible “primary ageing”, due to oxidation processes during processing14,15. The addition of DBSA noticeably enhances the Newtonian viscosity of the systems in the entire range of shear rates studied, with increasingly greater values at higher DBSA concentrations, seemingly exerting a viscosity building effect. Furthermore, the critical shear rates are slightly shifted towards lower values, pointing at the development of a more complex microstructure, which arises as a consequence of the modification. The modifying effect of the additive, at 60ºC, regardless of oxidation effects, was quantified defining a modification index (M.I.), given by the following expression: . . =

, ,

(1)

,

where , and , are the zero-shear-rate limiting viscosity of the modified bitumen and its corresponding blank sample, respectively. As displayed in Fig. 3, an approximately


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linear relation can be established between the DBSA concentration and the corresponding M.I. values. This result points out the mentioned viscosity building effect of DBSA on bitumen within the concentration range studied, most probably due to asphaltene clustering. Asphaltene flocculates/aggregates are thus assumed to increase both the microstructural complexity, as supported by lower critical shear rate values at greater DBSA concentrations16–18; and the resistance to flow, similarly observed in systems with high aspect ratio particles15,19. In addition, it is also expected that clustering process brings about some entrapment of the surrounding medium within the structure, increasing its effective volume19,20, which becomes manifest as an additional contribution to viscosity and, consequently, to M.I. build-up19,21, as observed in Figs. 2 and 3. DBSA modified binders and neat bitumen were also subjected to temperature sweeps under oscillatory shear within the temperature range from 30 to 110 ºC (Fig. 4), in order to determine their linear viscoelastic (LVE) response, and delve deeper into the corresponding thermomechanical properties and microstructure. As illustrated, all the systems studied show terminal-flow-shaped curves, with a prevailing viscous response in the entire range of temperature considered. The addition of DBSA to neat bitumen leads to an enhancement in both the elastic and viscous moduli, which result greater as concentration increases. The observed increase in moduli with similar viscoelastic patterns suggests the assembly of asphaltene aggregates into individual larger structures. Asphaltene aggregates, when flocculated, are presumably tightly bonded to each other in filamentary structures, giving rise to a slight enhancement in the resistance to shear and thermo-mechanical stability19. Besides, external forces imposed during shear are mainly subjected to viscous dissipation due to the prevailing viscous properties of the surroundings, with little influence of the


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long-range interactions between different large-sized asphaltene flocculates15. Thereby, as the DBSA content increases within the concentration range studied, a greater number of larger aggregates are liable to form, yielding an enhanced viscoelastic response. 3.2.

Rheological properties of modified bitumen: effect of processing temperature

The influence of processing temperature (within the range from 130 to 170 ºC) on the endproperties of the systems was also studied. With this aim, a concentration of 3 wt.% DBSA was chosen so that noticeable differences with respect to neat bitumen were obtained. These systems were subjected to a rheological study consisting on viscous flow tests at 60 ºC and temperature sweeps in oscillatory shear at 10 rad/s. The results were portrayed in Figs. 5 and 6, respectively. As can be observed, processing temperatures equal or below 150ºC do not significantly affect the end-properties of the systems. Little or no variation is distinguishable between the viscosity curves or the LVE moduli of the modified bitumen processed at 130 and 150 ºC (Figs. 5 and 6). Analogously, when the system is processed at 170 ºC, an almost negligible increase is obtained both in viscosity and in LVE moduli, indicating that oxidation processes as a consequence of the high processing temperature have little effect on the rheological properties of bitumen, as demonstrated by the blank sample that results from processing at 170 ºC (Fig. 5). The modification index at 170ºC in Fig. 3 (which quantifies the modification effects regardless of ageing) is nevertheless slightly increased, suggesting despite all, some effect of DBSA interactions on the rheological properties of bitumen. Thus, under temperature conditions applied, oxidation does not seem to play a relevant role, although higher processing temperatures could potentially increase the asphaltenic fraction


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in bitumen, and so, the effective volume of the dispersed asphaltenic phase20–22. Moreover, oxidation would generate more polar groups in the asphaltene molecules, which could further interact with DBSA. Consequently, at high temperatures, both factors can be assumed to potentially lead to an enlarged asphaltenic phase with a greater effective volume fraction, and probably, larger flocculates, which would influence the hydrodynamic properties, bringing about the enhancement of the viscoelastic and viscous properties19. Finally, according to the rheological results obtained, and taking into account the adverse effects that ageing can cause at low in-service temperatures, a processing temperature of 130ºC is enough to enhance high in-service temperature properties. 3.3.

Chemical origin of DBSA modification

As discussed above, the rheological results can be attributed to the development of largesized aggregates or flocculates promoted by DBSA-asphaltene interactions. In order to gain a deeper insight into the role of this additive, bitumen was also modified with 3 wt.% of its sodium salt (DBSS) to compare and analyze the influence of the acidic properties of the amphiphile. Figs. 5 and 6 point out that the use of DBSS barely modifies the rheological response of neat bitumen. Those results only show a minimal rise in viscosity and viscoelastic moduli, respectively, with respect to those of neat bitumen or its blank. Taking into account that DBSA and DBSS only differ in the polar group (sulfonic acid or sodium sulfonate), the reported results reveal that acid-base interactions between DBSA and asphaltenes (not involving the apolar coil) must play a major role in the modification process. Consequently,


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the presence of sulfonic acid groups (─SO3H) is required for the surfactant to effectively interact with asphaltenes. The interaction between DBSA and asphaltenes in crude oils has been extensively studied in the literature3,6–12,23, evidencing that the concentration of DBSA and the chemical and structural properties of asphaltenes are major controlling variables. As shown in Fig. 7, the modification takes place through the protonation of heteroatomic components in the asphaltenes, which result positively charged, whereas proton-donor DBSA molecules become negatively charged ions. Hence, this process leads to an ion pair with a strong ionic bonding, able to promote further electrostatic interactions with other ion pairs in neighboring

molecules/aggregates7,8,12,23.

However,

depending

on

the

additive

concentration, the structural effect provoked by DBSA may be very different, since it can act promoting the aggregation, enhancing the dispersion (inhibiting aggregation), or not even exerting any effect3. Thus, in the case of crude oils and model solvents, at low DBSA concentration, the formed ion pairs show enhanced long-range interaction capabilities, promoting the association of asphaltene molecules/aggregates into larger asphaltene clusters3,8,9,12,23. Nevertheless, as DBSA concentration rises, a greater number of DBSA molecules are attached to the available sites on asphaltenes molecules/aggregates, up to either completion or unreachability of any available site due to steric hindrance by already attached DBSA molecules in the vicinity. This process progressively leads to the full coverage of aggregates, hindering interactions as well as inhibiting aggregation8,9,11. Here, according to the obtained results, DBSA promotes asphaltene clustering/aggregation inside bitumen, leading to the formation of structures with greater effective volumes20,21,23. Moreover, these larger asphaltene aggregates also entrap a higher amount of maltenic 14 ACS Paragon Plus Environment

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continuous medium, hence exhibiting a greater hydrodynamic influence on the surroundings, associated to a higher effective volume19,21, and resulting in the enhancement of the rheological properties observed in Figs. 5 and 6. In this regard, the concentration range applied for DBSA would not be high enough to produce the complete coverage or micellization of individual aggregates, as it would have led to a worsening in the mechanical behavior. According to the literature, the critical concentration for micellization can be estimated from the stoichiometry of the chemical interaction between DBSA and asphaltenes, taking into account the complex nature of this interaction, and bitumen composition. Information either on the stoichiometry or on amphiphile per asphaltene requirements has been given in several works11,12,24, as an average of “active sites” per asphaltene molecule, which corresponds to the average number of amphiphile molecules capable of attaching to each asphaltene molecule. This number is assumed to range from 2.67 to 16.27 mmol of amphiphile per gram of asphaltene, if an average molecular weight of 750 ± 250 g/mol is considered for the asphaltene molecular unity2,7,8,11,12,23–26. Thereby, considering the lower bound of the previous range, along with the asphaltene concentration of the bitumen studied (26.60 wt.%), the minimum requirement to get the micellar stability of all asphaltenes amounts to 71.02 mmol of DBSA (23.19 g) per 100 g. bitumen. However, the concentrations of DBSA used in the present work to modify bitumen (≤ 3 wt.%) are, at least, eight times lower than this requirement. Consequently, DBSA would act promoting the electrostatic interaction among aggregates, yielding larger clusters with increased effective volume (Fig. 8).


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3.4.

Thermal properties

The bituminous systems were also subjected to modulated differential scanning calorimetry (MDSC), a technique that allows separating the overlapping reversing and non-reversing thermal events in the total heat flow signal, hence proving to be a powerful technique when looking into bitumen’s microstructure27,28. The derivative of the reversing heat flow has been frequently used to determine glass transition temperatures (Tg’s), as evidenced by the corresponding peaks in Fig. 9a. In general, three peaks, related to glass transitions defined as Tg1, Tg2 and Tg3 are obtained (Fig. 9b), assumed to arise from each of the different amorphous phases that can be found in bitumen. Accordingly, Tg1 would arise from the maltenic phase, mainly from the low molecular weight compounds; Tg2 would correspond to a maltene-asphaltene interfacial region; and Tg3 would be associated to a phase mostly consisting of asphaltenes5,27,28. As expected, Tg values resulting from the addition of additive sodium salt (DBSS) do not significantly differ from those of neat bitumen. This indicates that DBSS does not affect either the chemical or the thermal properties of any of the amorphous phases in bitumen, which is in good agreement with the results provided by the rheological study. By contrast, the addition of the acid form of the surfactant (DBSA) led to a noticeable shift in Tg1 and Tg2 to lower temperatures, from -23.86 ºC down to -28.32 ºC, and from 10.26 ºC down to 5.36 ºC, respectively. This decrease in both Tg’s might be a consequence of the clustering of the asphaltene aggregates, which is assumed to bring about the occlusion and immobilization of a fraction of surrounding molecules into the asphaltene clusters. Then, this process would provoke some phase segregation that results in the enrichment of both


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the maltenic region and the interphase in the lightest compounds, hence reducing the corresponding Tg values5,27. Additional information on bitumen’s microstructure can be obtained from the non-reversing component of the heat flow, in which the thermal events, as depicted in Fig. 10a, are related to different phases in bitumen. When analyzing the non-reversing signal, the following thermal events can typically be found: a wide endothermic background, which extends from -60 to 80 ºC, and arises from the ordering of the lightest saturated and aromatic segments upon cooling; two exotherms centered at approximately 0 and 40 ºC, related to the cold crystallization of low and high molecular weight segments, respectively, of saturated and aromatic compounds; and an endotherm, with its minimum located at around 50 ºC, resulting from the disordering of large structures, found in the resin and asphaltene fractions, which slowly diffuse to form a mesophase upon annealing27,28. Fig. 10b gathers the enthalpy values corresponding to the above thermal events for neat bitumen, and for DBSS and DBSA modified bitumen. As illustrated, either of the two additives diminishes the enthalpy of all the thermal events, being particularly noticeable for the third and fourth events. Surprisingly, even though DBSS does not provoke significant variations on the rheological properties and amorphous phases (unvarying glass transition temperatures), this additive would be prone to hinder the ordering processes that cause these thermal effects. The modification obtained of the thermal properties might be interpreted on the basis of physical effects related to the similar molecular size and structure of both DBSS and DBSA, with alike long aliphatic chains. Thus, the presence of both additives may obstruct


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the diffusion (constrained motion) of large-sized molecules, as those related to the third and fourth thermal events, which require longer times to attain structural stability29–31. Consequently, they seem to reduce the rate of ordering of the phases, upon annealing, preventing them from ordering into crystalline phases or mesophases. Eventually, the decrease obtained in the thermal events of the non-reversing heat flow component for both additives evidences lower degrees of molecular ordering in the different bituminous phases, involving a reduction in the crystalline domains, in favour of the amorphous content32. In the case of DBSA, it is further accompanied by a decrease in Tg1 and Tg2, which translates into improved mechanical performance against fracture at low-to-intermediate temperatures30,31,33. 3.5.

Compositional characterization

As hypothesized from the rheological and MDSC results, and in accordance with the literature3,7–9,11,23, DBSA has been assumed to mainly interact with the asphaltenic fraction of bitumen. Thereby, in attempting to elaborate on the bituminous fractions involved, and to what extent they interact with DBSA, thin layer chromatography coupled with a flame ionization detector was used to separate the different bituminous fractions, based on polarity13. Then, their mass fractions were analyzed before and after the addition of DBSA. As illustrated in Fig. 1, the addition of DBSA to bitumen leads to a decrease in the content of all fractions except that of asphaltenes, which rises by approximately 3 wt.%. Since this increase roughly equates to the added amount of DBSA, it indicates that DBSA is retained along with the asphaltenic fraction when bitumen is subjected to a chromatographic separation. In this regard, in the case of acid-base electrostatic interaction between


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asphaltenes and DBSA, the resulting compounds, containing DBSA, would also be eluted with the asphaltenic fraction12. Nonetheless, the retention of DBSA within the asphaltenic fraction does not necessarily mean that DBSA has reacted or interacted with the asphaltene molecules. It could also stem from the high polarity of the DBSA molecules (dipole moment of 4.5 D), which falls within the average range of polarities attributed to the asphaltenes (3.3 – 6.9 D)6,11, and might cause free DBSA (not reacted) to be eluted together with the asphaltenic fraction. It is worthwhile to point out that the shape of the peaks obtained from TLC-FID can also reveal valuable information about chemical changes affecting the polarity of the bituminous fractions33. In this sense, the dispersive effect of DBSA on asphaltene aggregates under certain DBSA and asphaltene concentration ranges, which would eventually split the aggregates or prevent asphaltenes from aggregating, would presumably affect the polarity of the resulting structures. When DBSA interacts with asphaltenes, charges on surface tend to be balanced, reducing asphaltene polarity, while seeing their stability increased in nonpolar medium3. This would hence provide the dispersive effect associated to DBSA, yielding low-polar, small-sized asphaltene aggregates that should reach greater heights on the stationary phase in TLC. Nevertheless, upon DBSA addition, no significant change that could be attributed to that process or alike was observed in the peak of any fraction, suggesting either not interaction between DBSA and asphaltenes or only partial coverage of the active sites in asphaltene molecules/aggregates. In both circumstances, polar properties would be retained, and in case of interaction, the formation of asphaltene-DBSA ion pairs would promote the aggregation instead of the dispersion of asphaltenes3,8,9,12,23, supporting the information drawn from the rheological results.


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Hence, in order to shed some light on this issue, asphaltenic fractions (insoluble in nheptane) extracted from neat and DBSA-modified bitumen, along with pure DBSA, were subjected to thermogravimetric analysis (TGA). As illustrated in Fig. 11, both asphaltenes and pristine DBSA show a single decomposition process that occurs within different temperature ranges. For DBSA, the main weight loss happens between 200 and 325 ºC, with the maximum of its derivative at approximately 270 ºC, and an almost negligible residue of 3.67 wt.%. By contrast, asphaltenes extracted from neat bitumen start decomposing at much higher temperatures (375 ºC), ending at around 525ºC, with its maximum decomposition rate reached at 460 ºC. In addition, whereas the residue at the highest temperature for DBSA is very low, it amounts to 54.9 wt.% for asphaltenes, typical of polycyclic condensed aromatic hydrocarbon compounds34. On the contrary, TGA of the asphaltenic fraction from DBSA-modified bitumen develops two consecutive weight loss processes. The second process presents a peak in the weight loss derivative that is coincident to that obtained for neat bitumen’s asphaltenes, and consequently, stems from the degradation of asphaltenic structures34. However, the first one, occurring from 280 ºC to 355 ºC, with its maximum decomposition rate at 312 ºC, is located within the temperature range defined by the decomposition processes of DBSA and asphaltenes, hinting that it must likely arise from new compounds not originally present in neat bitumen. Thereby, it is consistent to qualitatively assume its correspondence with a certain fraction of DBSA added to bitumen that might be attached to asphaltene entities through acid-base ion-pairing electrostatic interactions, as commented before. It is also noteworthy that the corresponding peak of free DBSA does not appear in the asphaltenes extracted from modified bitumen, although not clear conclusions about the extent of the 20 ACS Paragon Plus Environment

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interactions between DBSA and asphaltenes can be extracted from it, given that the corresponding signal might be somehow overlapped with that of the asphaltenic fraction from DBSA-modified bitumen. Nevertheless, the results obtained from the TGA technique would demonstrate the attachment of DBSA molecules to those in the asphaltenic fraction, enhancing the rheological and thermomechanical properties of bitumen, as evidenced by the previous results in this work. 4. Conclusions The addition of DBSA to a selected bitumen in concentrations of up to 3 wt.% noticeably enhanced its viscous flow behavior at 60ºC and viscoelastic response in a temperature range from 30 to 110ºC. Glass transition temperatures associated to its lightest fractions were reduced. Accordingly, the modified binder presented an improved thermo-mechanical behavior if compared to its parent bitumen. In general, the magnitude of the improvement increased with the additive concentration, and was almost independent of processing temperature. The obtained outcomes were explained on the basis of both a chemical and a microstructural perspective, as well as their mutual interrelation. Thus, at the molecular level, the modification takes place through the protonation of heteroatomic asphaltene components by the sulfonic acid group of DBSA. This process leads to asphaltene/DBSA ion pairs with a strong bonding interaction, able to promote further electrostatic interactions with other ion pairs. Since the amount of DBSA used per number of active sites in the asphaltene molecules was not high enough to produce a complete coverage or micellization 21 ACS Paragon Plus Environment

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of the individual aggregates, DBSA acted promoting the association of asphaltene molecules. So, at a larger scale, this phenomenon yields greater effective volumes of the asphaltenic fraction, which enhance the rheological and thermal properties of bitumen. These postulates have been confirmed by MDSC, TLC/FID and TGA experiments, which give clear evidences of the attachment of DBSA to asphaltene molecules, accompanied by a further microstructural modification that becomes manifest as a significant enhancement in bitumen’s rheological properties, hence establishing a new and different approach for bitumen modification. Acknowledgements Funding: This work is part of two research projects (CTQ2014-56980-R and TEP-6689) sponsored by a MINECO-FEDER Programme and the “Consejería de Innovación, Ciencia y Empresa, Junta de Andalucía” (“Projects of Excellence” Programme), respectively.


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Figure captions Fig. 1. Bitumen SARAs fraction for neat bitumen and bitumen with 3 wt.% DBSA. Values shown as mean (standard deviation). Fig. 2. Viscous flow curves, at 60 ºC, for neat bitumen and bitumen with 0.4 – 3 wt.% DBSA. The corresponding blank is also included. Fig. 3. Modification indexes, at 60 ºC, for bitumen with 0.4 – 3 wt.% DBSA (processed at 150 ºC); and for bitumen with 3 wt.% DBSA, processed at 130, 150 and 170 ºC. Fig. 4. Temperature sweeps in oscillatory shear, at 10 rad/s, for neat bitumen and bitumen with 0.4 – 3 wt.% DBSA. Fig. 5. Viscous flow curves, at 60 ºC, for neat bitumen, bitumen with 3 wt.% DBSS, and bitumen with 3 wt.% DBSA (processed at 130, 150 and 170 ºC). Their corresponding blanks are also included. Fig. 6. Temperature sweeps in oscillatory shear, at 10 rad/s, for neat bitumen, bitumen with 3 wt.% DBSS, and bitumen with 3 wt.% DBSA (processed at 130, 150 and 170 ºC). Fig. 7. Attachment of a molecule of DBSA to an active site at the asphaltene molecular structure, through acid-base interaction, where X corresponds to a heteroatom such as N, O or S. Fig. 8. Development of the acid-base ion-pairing electrostatic interaction and association between different aggregates of DBSA-Asphaltenes. Fig. 9. (a) Characteristic pattern of the derivative of the heat capacity versus temperature for the systems studied, displaying the peaks corresponding to the different Tg’s found. (b) Tg values for neat bitumen, bitumen with 3 wt.% DBSS and bitumen with 3 wt.% DBSA.


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Fig. 10. (a) Typical pattern and thermal events of the non-reversing heat flow for the bituminous systems studied. (b) Enthalpy values of the non-reversing heat flow thermal events for neat bitumen, bitumen with 3 wt.% DBSA and bitumen with 3 wt.% DBSS. Fig. 11. Thermogravimetric analysis data of pure DBSA and asphaltenes separated from either neat bitumen or bitumen with 3 wt.% DBSA.


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Graphic Abstract 351x200mm (96 x 96 DPI)

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Fig. 1. Bitumen SARAs fraction for neat bitumen and bitumen with 3 wt.% DBSA. Values shown as mean (standard deviation). Fig. 1 224x170mm (96 x 96 DPI)


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Fig. 2. Viscous flow curves, at 60 ºC, for neat bitumen and bitumen with 0.4 – 3 wt.% DBSA. The corresponding blank is also included. Fig. 2 286x201mm (150 x 150 DPI)


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Fig. 3. Modification indexes, at 60 ºC, for bitumen with 0.4 – 3 wt.% DBSA (processed at 150 ºC); and for bitumen with 3 wt.% DBSA, processed at 130, 150 and 170 ºC. Fig. 3 286x201mm (150 x 150 DPI)


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Fig. 4. Temperature sweeps in oscillatory shear, at 10 rad/s, for neat bitumen and bitumen with 0.4 – 3 wt.% DBSA. Fig. 4 286x201mm (150 x 150 DPI)


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Fig. 5. Viscous flow curves, at 60 ºC, for neat bitumen, bitumen with 3 wt.% DBSS, and bitumen with 3 wt.% DBSA (processed at 130, 150 and 170 ºC). Their corresponding blanks are also included. Fig. 5 286x201mm (150 x 150 DPI)


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Fig. 6. Temperature sweeps in oscillatory shear, at 10 rad/s, for neat bitumen, bitumen with 3 wt.% DBSS, and bitumen with 3 wt.% DBSA (processed at 130, 150 and 170 ºC). Fig. 6 286x201mm (150 x 150 DPI)


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Fig. 7. Attachment of a molecule of DBSA to an active site at the asphaltene molecular structure, through acid-base interaction, where X corresponds to a heteroatom such as N, O or S. Fig. 7 129x120mm (96 x 96 DPI)


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Fig. 8. Development of the acid-base ion-pairing electrostatic interaction and association between different aggregates of DBSA-Asphaltenes. Fig. 8 250x89mm (96 x 96 DPI)


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Fig. 9. (a) Characteristic pattern of the derivative of the heat capacity versus temperature for the systems studied, displaying the peaks corresponding to the different Tg’s found. (b) Tg values for neat bitumen, bitumen with 3 wt.% DBSS and bitumen with 3 wt.% DBSA. Fig. 9 197x288mm (150 x 150 DPI)


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Fig. 10. (a) Typical pattern and thermal events of the non-reversing heat flow for the bituminous systems studied. (b) Enthalpy values of the non-reversing heat flow thermal events for neat bitumen, bitumen with 3 wt.% DBSA and bitumen with 3 wt.% DBSS. Fig. 10 197x288mm (150 x 150 DPI)


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Fig. 11. Thermogravimetric analysis data of pure DBSA and asphaltenes separated from either neat bitumen or bitumen with 3 wt.% DBSA. Fig. 11 286x201mm (150 x 150 DPI)


Dodecylbenzenesulfonic Acid as a Bitumen Modifier - American

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