The bitumen microstructure: a fluorescent approach
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Five bituminous samples were carefully studied by confocal laser scanning microscopy using 488 nm excitation radiation and observing 500–530 nm of emission. The images revealed the microstructure of bitumen. The influence of the admixture of mineral aggregates concerning the microstructure was tested. For the minerals, no significant influence was found. For understanding the origin of fluorescent signals, the samples were separated into asphaltenes and maltenes and analyzed with fluorescence spectroscopy. Although former works have assumed the origin of fluorescent emissions in bitumen to be found in the asphaltene fraction, the asphaltenes produce little to no emissions, but the maltenes exhibit strong fluorescence in the observed spectral region. For deeper insight, fractionation of the bitumina into the SARA fractions by chromatographic column separation was necessary. The fluorescence spectra of these fractions were analyzed and revealed the aromatics and resin phases to be the only components capable of sufficiently intense fluorescent emission. This is a strong argument for a complex internal microstructure consisting of a mantle of aromatics surrounding an inner core.
KeywordsBitumen Microstructure Photoluminescence spectroscopy Optical microscopy
Since the times of ancient Babylon, bitumen has been used in road engineering and construction [1, 17]. Today it is one of the primary construction materials in road engineering. About 95 % of the world production of bitumen (approximately 100 Mt/year) are used by the pavement industry as asphalt mixtures . The mechanical properties of asphalt mixtures can be assessed through a multi-scale modelling approach [24, 42, 43]. The primary assumption of multi-scale analysis is that the material properties of the composite can be calculated, if the properties of the basic materials are known and the microstructure of the composite is understood. Hence, the microstructure of bitumen plays a crucial role.
As every bitumen is a very complex material and the properties tend to vary dependent on source, production cycle, additives, and additional chemical-physical treatments, bitumen according to European regulations is described simply as a “virtually involatile, adhesive and waterproofing material derived from crude petroleum or present in natural asphalt, which is completely or nearly completely soluble in toluene, and very viscous or nearly solid at room temperature” . Due to its limited economic importance, most research excludes the natural asphalt, because most bitumen used nowadays is obtained as the residue of the vacuum distillation in the refinery process of crude oil .
The complex chemical nature of bitumen complicates many analytic methods to the limit of applicability. A widely accepted analytical approach is to separate bitumen according to similar chemical behavior into a saturated, an aromatic, a resin, and an asphaltene fraction (SARA). The asphaltene fraction is hereby defined as the n-heptane insoluble and toluene soluble part of bitumen, while the soluble fractions are called maltenes. Maltenes can then be separated by column chromatography [4, 9, 27]. To avoid confusion, the SARA nomenclature was preferred to the Corbett and ASTM nomenclatures [4, 9]. Hence, aromatics refer to naphthenic aromatics and resins to polar aromatics in the ASTM-Standard . Although concerns regarding repeatability of the chromatographic separation process have been raised, it is still the most validated and practical separation technique for bitumen . The rheological properties of bitumen can be explained by various modelling approaches . However, contemporary research has all but eliminated single phase approaches, by providing solid evidence that bitumen has indeed a definite microstructure that can be measured and visualized by various techniques. Small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) experiments have proven beyond doubt that ordered structures in bitumen exist [12, 13, 44, 48, 59]. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have revealed a morphology of characteristic and reproducible appearance, which is often described as “bee-like” structures of 1–5 µm [8, 15, 27, 28, 30, 35, 36, 38, 51, 53, 60, 61]. Confocal laser scanning microscopy (CLSM) has uncovered the existence of fluorescent centers in bitumen of 1–10 µm size, indicating high concentrations of fluorescent species in definite volume elements evenly dispersed through the material [5, 33]. Since both structures range in the same order of magnitude, it can reasonably be assumed that they are identical or at least strongly linked to each other. The combination of physico-chemical analysis and rheological data concludes the existence of at least two separate phases in bitumen. The identification of these structures and the investigation of their properties are currently in the focus of the scientific community [27, 60].
After decades of research, the ideas of the micelle theory still form the basis of one of the two major models for bitumen microstructure [27, 45]. Asphaltenes are thought to agglomerate to form kind of micelle like structures surrounded by other high-polar material and dispersed through the maltene phase [40, 52]. This model is strongly supported by the distinctive agglomeration behaviour of asphaltenes, which also aggregate in crude oil and most pure solvents . This explains the structural investigations as well as the rheological properties of bitumen as a highly viscous visco-elastic material . Basically, the model assumes the aggregation of asphaltene micelles into larger structures of around 1–20 µm. Typically, these structures are in the range of a microcapsule by IUPAC definitions, which is essentially a core–shell particles with a diameter of around 0.1–100 µm  as opposed to micelles, which are generally considered to be much smaller. However, the use of the term “micelle” for these “super-micelles” can be attributed to the cross-linking between them and the asphaltene micelle model (e.g. in ).
The second model explains the expression of structural features in the µm range on the bitumen surface as the crystallization of waxes. This argumentation is based on several studies, which prove a strong correlation between wax/saturates content and the frequency of occurrence of structural features [27, 33, 51]. Key works in this area have been conducted by Redelius . Additionally, mixed models have developed by combining both asphaltene precipitation and wax crystallization approaches .
CLSM is capable of analyzing highly localized fluorescence emission and allows the detailed investigation of the bitumen scale of asphalt observation . This can be used to efficiently visualize the microstructure of bitumen [5, 18, 21, 33] and structural effects of bitumen modification [14, 29, 47, 54]. Bearsley et al. presented a very detailed comparison of CLSM with other imaging techniques . In general, the fluorescent centers are interpreted as asphaltene micelles. Previous works relied heavily on CLSM images for the identification of the fluorescent phase . However, CLSM can only provide leads and indications, but is not suited to determine fluorescence characteristics of molecules or composites. Fluorescence spectroscopy can be used to study aromatic and conjugated carbohydrates, as the capability of electronic transitions that decay emitting a photon is almost exclusive to these classes of molecules [25, 49]. Fluorescence spectroscopic studies of bitumen and bitumen fractions can provide a solid base for the identification of these structures [16, 56].
Although adhesion between mineral aggregates and bitumen is a major topic in materials research [6, 7, 22, 24, 31, 57], CLSM has not been used yet to investigate bitumen/mineral aggregate mixtures. Comparing five different bitumina by CLSM we were able to monitor the dependence of the microstructure on the phase composition. The focus was on the bitumen scale, when CLSM can be employed on bitumen and filler mixtures, providing information about the changes in microstructure induced by the admixture of the mineral phase.
Five different bitumina and bitumen precursors were provided by OMV AG obtained from vacuum distillation. The samples were classified and named by their needle penetration value. The first is 50/70 bitumen ready for use in asphalt mix production. Two soft bitumina 70/100A and 70/100B, typically used in the production of polymer modified bitumen were investigated. PE1005 is a bitumen precursor and is a vacuum flashed and cracked vacuum residuum. The samples were chosen for their practical relevance mostly. 50/70 is commonly used in road engineering and 70/100A and B are the precursors for SBS modified bitumina often used in high performance roads. All samples were stored in sealed metal cans to avoid ageing. All solvents used in the experiments were obtained from Carl Roth Gmbh+Co. KG and of ROTIPURAN quality (≥99 % p.a.). The aluminum oxide used in the chromatographic column had a grain size of 63–200 µm and pH value of 3.5–4.5.
2.2 Separation and fractionation
This volume was then transferred onto a chromatographic column filled with 450 g of dry aluminum oxide to a column length of 80 cm (±1 cm) and prewetted with 400 mL of n-heptane. The solvent schedule was chosen according to ASTM 4124. However, due to the prewetting of the column, forerunnings of 350 mL were collected. The eluate was collected in 50 mL (±2 mL) glass beakers, sealed airtight and kept at −15 °C in a fridge to slow degradation and oxidation processes. The solvent was removed by evaporation according to EN 1297-3  and the fractions were analyzed as the solid phase.
An inverted confocal fluorescence microscope ECLIPSE TE2000 (Nikon Corporation, Tokyo, Japan) was used in this study. The microscopic setup offers both a transmission and a CLSM array. For the transmission mode, a T-DH 100 W Illumination Pillar (Koehler Type) is employed. An Argon-ion laser is used as source of excitation radiation for the fluorescence scanning mode. Samples were studied using an excitation wavelength of 488 nm, the most intense frequency of emission provided by the Argon-ion laser, and emission sided, a band pass filter 515/30 was employed, setting the window of observation to 500–530 nm of fluorescent emission. The standard software Nikkon EZ-C1 was used to control and operate the TE2000 setup. Image post-processing for optimal contrast and conversion to black and white was performed with Corel PaintShop Photo X3 version 18.104.22.168.
For sample preparation, the bitumen was heated to about 150–200 °C as necessary for melting the sample. A small drop of bitumen was placed on an ultrathin glass slide. Subsequently, another glass slide was placed onto the bitumen drop. An additional heating period of 15 min at 150 °C for the samples was implemented, allowing the bitumen film to spread and become very thin. These thin films were then examined by CLSM. The mineral aggregates/bitumen mixtures were prepared in a similar fashion. A small drop of bitumen was allowed to spread on an ultrathin glass slide and then the mineral aggregates were added on top. Then the second glass slide was put onto the mixture. In order to introduce shear forces into the material, the still hot glass slides were slid against each other. Then the sample was prepared following the same procedure employed on the pure bitumen samples.
2.4 Fluorescence spectroscopy
For fluorescence spectroscopy, an Edinburgh Instruments FSP920 photoluminescence spectroscopy setup was employed. The setup is equipped with a XE900 Xenon Arc Lamp, which provides high intensity radiation on a broad spectrum. The setup employs double Czerny-Turner monochromators (type TMS300) at both excitation and emission arms, guaranteeing a very narrow spectral bandwidth selection. The detector is a S900 single-photon photomultiplier (type R928). The spectrometer was used to conduct both excitation and emission measurements. For sample preparation, the bitumina were heated to 150 °C and a drop of bitumen was applied to a standard microscopic slide. This slide was stored at elevated temperature (80 °C) for 5 min to allow the drop to spread and increase its surface. Afterwards the samples were cooled to room temperature and subjected to fluorescence spectroscopy. To rule out any oxidative influence on the sample surface, one set of samples was prepared under a continuous N2 flow, limiting the oxygen uptake of the sample. For bitumen fractions, this treatment was not necessary, because the maltene phase and its components are viscous liquids at room temperature and can be applied directly at room temperature to the glass slide. The asphaltenes were taken up with toluene and then dripped slowly on the warm glass surface (80 °C) to allow the formation of a thin film. Two different modes of measurement were employed, excitation scans (variable excitation, fixed detection wavelength) and emission scans (fixed excitation, variable detection wavelength). For the excitation scans, the same detection wavelength as used at the microscope was chosen, 525 nm, and a spectral range of 200–500 nm excitation wavelength was observed. It is important to insert a 340 nm cutoff filter emission sided to avoid second diffraction order radiation of the Rayleigh line at 207.5 nm on the detector. For the emission spectra, two wavelengths have been studied carefully. First an excitation wavelength of 488 nm was selected, because this is the most intense excitation wavelength available in the CLSM setup. For detection, a spectral range of 500–750 nm (detection limit) was chosen. To check for high energy fluorescence transitions, we employed a wavelength of 280 nm and a detection range of 300–540 nm without filter, and 480–750 nm with an emission sided 340 nm cutoff filter. The overlap was used to append the spectra. All spectra were measured with the following setup parameters: step width 1 nm, scan slit set for Δλ value of ±1 nm, dwell time 1 s, with 5 repeats. Additionally, emission mappings of selected bitumen samples were performed for a range of 280–530 nm excitation. An emission window of 390–640 nm was observed corrected by an offset value of 20 nm to avoid the respective Rayleigh lines. Also, a 340 nm cutoff filter was employed. The mappings were performed over 18 h. The software used to record the spectra was the Edinburgh Instruments F900 Version 6.87 (Build 1). The spectra were processed employing OriginPro 8.6.0G by OriginLab Corporation.
3.1.1 Bitumen samples
3.1.2 Bitumen and mineral aggregate mixtures
The measurement of mixtures of bitumen and filler was at the center of our interest. The latter, is essentially finely powdered mineral aggregate. We have assumed an interaction between the most polar molecules of bitumen, which are asphaltenes and resins, with the polar surface of mineral aggregates.
3.2 Fluorescence spectroscopy
The identity of the fluorescent centers can only be assessed by looking at the fluorescent capabilities of the bitumen fractions. In comparison to the CLSM images, fluorescence spectroscopy yields an integrated spectrum that is not capable of visualizing spatial distributions or point-by-point scanning. However, through separation of bitumen into fractions according to solubility and polarity, the spectral information can provide additional structural information by inference. A closer look on the basic physicochemical properties of bitumen reveals that bitumen contains only three fractions that could theoretically be the origin of the fluorescent signal: a) The asphaltenes, b) the resins, and c) the aromatics are, based on their general chemical description and nature, capable of fluorescence. Saturates can easily be dismissed as a source of fluorescent emission, due to the well-defined chemical nature of the fraction and the lack of conjugated π-electron systems.
3.2.1 Fluorescence mapping of bitumen
The Kasha–Vavilov rule states that the quantum yield of fluorescence transitions is independent of the excitation wavelength, if the provided energy causes the transition into an excited state but does not cause ionization. Although exceptions from this rule exist, generally the rule applies to most aromatic molecules . However, bitumen is not a pure, aromatic substance but rather a colorful mix of various compounds. Although bitumen generally seems to follow this rule, there is an anomaly at wavelengths below 260 nm wavelength of excitation radiation. Generally, fluorescent emission in this spectral region is rare, because the corresponding energy is sufficient to cause dissociation or predissociation in many molecules and the great variety of compounds in bitumen makes the existence of such bonds a certainty .
3.2.2 Emission and excitation scans of bitumen and SARA-fractions
The mapping mode for fluorescence spectroscopy is very limited in terms of signal intensity and spectral resolution, because of the number of spectra and the resulting measurement time. The long measurement times could induce changes in the sample surface of the irradiated area, which is due to the high illumination intensity. Especially for high energy radiation close to the UV range, this might have an important impact, which has been controlled for by performing emission and excitation scans at the exact wavelengths employed in the CLSM setup.
A quenching effect can either be direct by changing the chemical vicinity of the aromatics, or indirect, if the addition of the asphaltenes induces structural changes in the bitumen, which can have an even greater impact on the chemical environment for a chemically similar group of molecules. The formation of aggregated structures of asphaltenes in bitumen is undisputed. These spectra point to a direct structural impact by the asphaltenes on the material, but are not sufficient to identify the fluorescent phase in bitumen.
We have shown that the fluorescent centers that can be visualized by CLSM are not caused originally by asphaltenes, as was assumed before . First fractioning experiments found that the asphaltenes are hardly capable of fluorescence at 488 nm excitation and the 500–530 nm detection window, on the other hand, maltenes produce very strong fluorescence signals. When fractioning the maltene phase, aromatics and resins are the only constituents that show significant capability of fluorescence. Without further considerations, the basic conclusion is that there exists a spatial non-continual distribution of volume elements in bitumen, which exhibit a significantly higher concentration of fluorescence capable molecules of the aromatics fraction. In other words, the existence of structures consisting mainly of aromatics and to a lesser extent of resins was proven beyond doubt.
If the similar size of the structures is taken into account, the fluorescent centers can reasonably be assumed to form the so called “peri-phase” often shown by AFM imaging [36, 51, 53] to surround the “bee-like” catana phase. As there is a sharp phase-contrast between the three phases, the name microcapsule is probably more correct then micelle due to the size of the structures (IUPAC definitions).
The question, whether the inside of the microcapsules is an agglomeration of asphaltenes or wax-crystallization, cannot be answered by CLSM. The wax-theory is mainly based on the observation that the frequency of occurrence of these structures rises with increasing content of wax and saturates respectively [33, 36]. Also, undisputable observations of microstructural phenomena in pure maltenes  were made. Additionally, by using heating and cooling cycles, it was proven that these structures vanish completely at around 90 °C and reemerge when the sample is cooled down , similar to a classical crystallization scenario. This is interpreted as being in the same temperature range as the melting points of waxes commonly found in bitumina .
The asphaltene agglomeration theory is mainly based on the observation of asphaltene precipitation in various solvents and also crude oil . The capability for self-assembly of polyaromatic structures is an often observed phenomenon. Several studies found self-ordering behavior for such molecules and it has been proven that this effect, which is caused by π-π interactions, can also be found in asphaltenes [2, 12, 13, 19, 20, 55, 59, 62]. For most bitumina studied in literature, the natural wax fraction is below 2 wt% as measured by standard distillation methods , while the typical asphaltene content is around 10 wt%.
Detailed studies of waxes in bitumen have shown that these are largely dominated by n-alkanes and iso-alkanes with carbon numbers of C15-C57 , and melting points of around 40–65 °C. Furthermore, a recently concluded round-robin test focused the glass transition temperature reliability via DSC testing and found that the addition of 3 wt% wax had no significant influence on the glass transition temperature (T g) of bitumen , limiting the calculation of wax content by DSC. The same study found that the melting temperature of the natural waxes in bitumen is around 25 °C. These findings make it rather difficult to correlate the vanishing of the structures at 90 °C with the melting point of waxes. Additionally, AFM was successfully employed to visualize these structures for bitumen with very low saturates contents (3.8 %) . The polarity and hence the solubility parameters of a solvent system may change to a great extent, if apolar material is added. This can cause aggregation and phase separation processes of polar constituents that were soluble in the original material. It is thus also possible to interpret this effect as the formation of further aggregates of polar material in an increasingly apolar matrix. From a micromechanical point of view, the addition of around 5 % of asphaltenes has a tremendously high impact on the rheological and mechanical behavior of maltenes, indicating a huge impact on the microstructure by the addition of the asphaltenes .
The addition of mineral aggregates to bitumen has little to no impact on the bitumen microstructure as visualized by CLSM. This indicates a lack of physico-chemical interaction between the two phases, which in turn could indicate no chemical and only little physical interaction between the mantle phase and the minerals. Investigations of this interaction have shown that it is mostly based on dispersive interaction . This suits our model of an apolar matrix quite well. If the asphaltenes as the most polar molecules in bitumen were to interact with the minerals, the aromatic shell would have to be located on the outside of the mineral particle (intermediate halo observation, see Fig. 3, middle). However, this transition state vanishes after a short relaxation time.
CLSM has been used as a highly effective imaging technique on bitumen. The technique allows the confirmation of the asphaltene agglomeration theory in the form of a core–shell particle like arrangement. For the SBS modified bitumen, the microstructure is perfectly visible. The fluorescent phase was also shown to play no detectable role in the interaction between bitumen and mineral aggregates. This indicates a general lack of interaction on a structural level. In total, this limits the interaction to the molecules in the matrix, indicating that adhesion between minerals and bitumen is mostly based on mechanical phenomenon and perhaps to a lesser extent on van der Waals forces .
CLSM pictures similar to ours have been interpreted by many researchers as asphaltene micelles, attributing the fluorescent signals to asphaltenes themselves. Our results of the fractionation experiments, however, indicate that in fact the capability of the asphaltenes for fluorescence at 488 nm excitation is insignificant. Likewise, the emission signal strength at 525 nm detection wavelength is very low. Furthermore, fractionation according to ASTM 4124 and subsequent fluorescence analysis showed clearly that the aromatics fraction is the source of the fluorescent emissions. When combining this structural information with the micelle theory, the fluorescent signals still originate from these asphaltene micelles. However, it has to be pointed out that the signals originate from the stabilizing mantle of declining polarity around the asphaltene agglomerates and not from the inner core of the asphaltenes themselves.
These results have been successfully employed in further investigations regarding microstructure and microstructural changes at the ageing of bitumen . The mantle surrounding the asphaltene agglomerate seems to be the critical part. Overall, these effects play a critical role that has been underrated in former studies of bitumen and asphalt ageing. CLSM and fluorescence spectroscopy can be regarded as solid methods for such investigations.
We express our gratitude towards our governmental funding partners, the Austrian Research Promotion Agency (FFG) in the project “OEKOPHALT – Chemical and physical fundamentals of bitumen ageing for ecological asphalt recycling”. Furthermore, we thank our cooperation and funding partners from asphalt industry, namely: Pittel+Brausewetter GmbH, Swietelsky Baugesellschaft m.b.H., Nievelt Labor GmbH, and the OMV AG for their continued support.
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