Solvent Effect on the Morphology of the Bee – Structure Observed by Atomic Force Microscopy on Bitumen Sample

Programa de Engenharia Metalúrgica e de Materiais – PEMM, Departamento de Engenharia Metalúrgica e de Materiais – DMM, Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa de Engenharia – COPPE,Universidade Federal do Rio de Janeiro – UFRJ, Ilha do Fundão, Rio de Janeiro, RJ, Brazil Centro de Pesquisas da Petrobras, Centro de Pesquisas e Desenvolvimento Leopoldo Américo Miguez de Mello – CENPES, Ilha do Fundão, Rio de Janeiro, RJ, Brazil


Introduction
Bitumen is obtained by the distillation of petroleum refineries, when presents appropriate consistency is called a paving asphalt binder or asphaltic cement (CAP).
Bitumen is a complex mixture of hydrocarbons that presents impermeability, viscoelastic, thermoplastic and non reactive behavior and adhesion to aggregates.Ot is basically constituted of carbon, hydrogen, oxygen, sulfur atoms together with some metals (mainly nickel and vanadium).CAP has a melting point of 440 K and presents a viscoelastic behavior at room temperature.
Bitumen is composed of principles fractions: asphaltenes and maltenes, Ot is considered by some authors as a colloidal dispersion where the asphaltenes (components of highest molecular weight), are dispersed in a medium constituted by the remaining components, the maltenes, divided into three generic groups of different molecular weight and aromaticity: resins, aromatics and saturates, Figure 1 [1][2][3] .The dissolution of bitumen in a solvent has a large effect on its structure 4 .Toluene is a solvent able to dissolve the asphaltene fraction.The maltene phase of bitumen, that is present in larger quantity (70%), remains when the asphaltenes are removed and constitute the fraction of asphalt which is soluble in n-alkane solvent such as n-heptane.The relationship between the composition of asphaltene and maltene has an important effect on the viscoelastic properties and, consequently, the performance of asphalt layers paving the roads.
The micellar model of asphalt binder, states that it may be composed of "primary aggregates" of asphaltenes, undergoing flocculation, in an ordered structure with the asphaltene colloid at the core of the micelle surrounded by a shell of surfactant like resins, shielded by the aromatics and saturates [5][6][7][8][9] .
On order to further detail the structures, atomic force microscopy (AFM) was used for the observation of a heat-cast bitumen film, thus preserving the solid state morphology 10,11 .Masson at al, 2005 modeled asphalt as a multiphase bitumen consisted of a nanostructure called "bee" and dispersed phases on the matrix referred as the catana or catanic phase 4 .On the literature there are a lot of discussion about the composition of the bees.Ot was formelly attributed to asphaltenes 12 Recent work from the same author related also the bee to the presence of wax in the binder 13 .Jäger et al and Moraes et al agreed that the bees are composed of a soft and hard phase that may be composed of asphaltenes and any other malthenic phase 10,14 .On this respect, the extent of bee structure was found to correlate with the asphaltene and maltene fractions contents in bitumen, which would lead to a direct relationship between the amount of "bees" and asphaltenes.
Asphaltene molecules may be dissolved in a petroleum fluid, or may flocculate out of the solution, due to high paraffin content of the oil, forming random aggregates.Asphaltenes micelles have an average diameter around 4-16 nm and is possible aggregated to form structures of greater size 15 .
Recently, some researches studied the thermal variation of the bee morphology and based on their observation 14 .Jäger et al. 10 related the bees are composed of both a soft and hard phase.The predominant phase observed on the surface of the sample is dependent of the thermal history both when considering the bees as well as for the matrix.Bee flocculation was followed in situ and it was observed a modification of the bee pattern when submitting the sample to a thermal cycle.
Even now no correlation was undoubtedly found between the AFM morphology and the composition based on asphaltenes, polar aromatics, naphthene aromatics and saturates.Asphaltene molecules may be dissolved in a petroleum fluid, or may flocculate out of the solution, due to high paraffin content of the oil, forming random aggregates.Solution behavior of associated molecules can explain many features too often uncritically attributed to colloid behavior.The authors defined that a phase separation induced by dissolution with different solvent.This way, the asphaltenerich phase was dispersed on matrix phase, called maltene.This dispersion was observed in this work.Asphaltenes are composed of aromatic there are a lot of discussion about the the real shape of asphaltenes.Aromatic rings form an aromatic core around of 5-8 rings that containing naphthenic and alkyl substitution.This structure can be observed by Mullins 7 , that study suggested a few kind of asphaltene moleculas.
On this sense, the main goal of our work it to determine the possible constitution of the bitumen fractions observed by AFM and relate them to the so called bee nanostructure in order to try to gather further information on the bee composition.

Experimental
Asphalt cement penetration grade 30/45 were supplied by REDUC and samples were prepared by spin coating as described before 10 .These samples will be for now on called CAP films.CAP samples were heated at about 440K and then spin casted on glass.Figure 2a, shows the scheme of sample preparation by spin coating where drops of the heated asphalt were deposited on a heated substrate (glass plate) during rotating motion.Evaporation of the more volatile solvents may occur during spreading.Films produced this way presented a thickness of about 1 mm.
The sample was then cooled to room temperature and humidity and stored for 24 h before analysis.
Atomic force microscopy (AFM) analysis was carried out in a JPK OA microscope (Berlin -Germany) in tapping mode using a NSC 14 (Ti-Pt) tip.The microscope was equipped with a heating stage as described elsewhere 14 .
To study the influence of different solvents on the morphology of the 'bees', the surface of CAP film was spark attacked with 200 µL with toluene to dissolve the asphaltenic fraction, Figure 2b.The same procedure was used to obtain n-heptane films where the maltenic fraction was dissolved.For AFM analysis, these films have been dried to room temperature and were then left at 50% relative humidity for 24 hours.All solvents were spectroscopic grade from VETEC and toluene was UV/HPLC and n-Heptane 95% UV/HPLC.CAP30/45 films were spike attacked with 200µL of toluene and n-heptane in the spin coating.This way, a surface dissolution was induced, without leaving dissolved material on the surface.

CAP film morphology
Typical topographic images and phase contrast images of CAP30/45 asphalt binder samples are presented in Figure 3. On the topographic image (Figure 3a), the bees or the catanic phase could be observed 2,6 .Dn the phase contrast image (Figure 3b), a clear contrast can be observed on the matrix in the micrometer range, where the bees were completely surrounded by a softer phase.A line profile was performed on one typical bee and it is clearly observed that the bees presented protusions and depressions like the catanic phase (Figure 3c).Phase contrast line profile in the same indicated that there may be more than one phase on the bee composition as the phase shift oscillated between softer (lighter) phases and more rigid phases, that appear darker in the phase contrast image when compared to the  matrix.Protrusions presented typical heights in the 10-30 nm range and the overall phase contrast accounts for more than 60 deg for one bee, confirming the presence of more than one phase.On Figure 3d, 3D mode of typical bee are observed, where the bee have the same height profile independently of its size and direction.

Heptane film morphology
By AFM it was possible to observed the morphological modifications of the CAP film presented in Figure 3 after being spiked with n-heptane.The film was not observed at the same spot, but all CAP films were tested for the presence of bees before spiking.Typical images obtained for these films, that will be called, from now on heptane films, are presented in Figure 4. Ot can be observed that the film surface is monotonous, presenting only protusions.Also, a phase contrast can be observed, although not as clearly as the one presented for the unmodified binder.Phase contrast for the overall image accounts for less than 10 deg which can be due only to topographic effects.
Ot is possible to observe on AFM images that protrusions whose height is between 3 -6 nm and width of 200-500 nm are similar to the one observed for the bees.Jäger et al. 10 , proposed the bees are composed of both a soft and a hard phase.Of the hard phase is due to asphaltene, this phase will not be dissolved in the heptanes film.This way these protrutions are probably constituted of the asphaltenes because heptane is able to dissolve only the maltene fractions of bitumen This way, it can be said that the protrusions of the bees are related to the more rigid, asphaltene phase.This result is in agreement with the measurents presented by Jager et al. 10 that performed force-distance curves in different positions of the bees and related the protrusions to the bee harded phase.

Toluene film morphology
CAP30/45 films were also spiked with toluene.Figure 5 presents the typical images observed for the Toluene Films.By AFM it was impossible to observe either protusions or depressions from the bees and the surface roughness is in the nanometric range.Also, no phase contrast was observed within the role image.From that, it can be concluded that toluene completely dissolved the bee as it can dissolve all fractions of the bitumen.Ot must also be pointed out that the   spike induced only a surface dissolution of the film and that the dissolved material was spread by spin coating, leaving only the undissolved material behind.

Conclusions
The asphalt is composed of principle fractions: asphaltene, maltene, resins.The way these fractions are dispersed had been studied by several researchers.The aim of our study was to evaluate precisely these structures.For this, first we started with the morphology observed by AFM.Structures at the nanometer scale (bee) appeared in the scan and could be understood by the line profile, as being composed of successive protusions and depressions.The main conclusion of our study was to assess what was the fraction of the ligant that composed the protusions of the bee.Knowing the basic fractions are sustained by intermolecular interaction forces, it would be possible to associate a composition for these structures.As the AFM is a surface analysis technique we were seeking to determine the composition of these regions observed .Through surface attacks it was argued that the bulge in the bee is very likely to be composed entirely of asphaltenes since it was possible to observe only protusions in the surface attacked by heptanee (solvent not able to solve asphaltenes) in AFM analysis.But when asphalt surface undergoes a bout of n-heptane (solvent able to solve the maltenes), it is still possible to see the lumps, but not being organized as proposes the micellar system.The n-heptane was a solvent that broked stability of colloidal system when was spiked on asphalt surface because this solvent was able to break intermolecular interactions, but the protusions still could be observed.By the other side, when the surface was spiked with toluene, no features are left in the surface as the solvent is able to wash out all ligant components.
The CAP 30/45 sample surface as observed by AFM presents three different aspects: the CAP films presented the bee structure and it can be observed that bees are mostly always composed of a sequence of hills and valleys.Film surface, after mild dissolution on heptanes, presents only hills and, after dissolution on toluene films it was impossible to observed either hills or valleys.
Ot was possible, though to conclude that the surface of the analyzed bitumen sample is constituted of different fractions, and the effect of solvent on the morphology of the bee on this structure is observed by the action of specific solvents that dissolve each of the fractions constituents and that led us to conclude that bees protusions would be entirely composed of asphaltenes dispersed in a maltenic matrix.
AFM images shows that the bee structure possible originates from a super complex structure containing asphaltene, which is insoluble in n-heptane and resists attacked surface.

Figure 2 .
Figure 2. a) Production of CAP film using spin-coating; b) A schematic diagram of the dissolution of the surface material.