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Using atomic force microscopy to detect asphaltene colloidal particles in crude oils Lia Beraldo da Silveira Balestrin, Mateus Borba Cardoso, and Watson Loh Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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Using atomic force microscopy to detect asphaltene colloidal particles in crude oils

Lia Beraldo da Silveira Balestrina, Mateus Borba Cardosob, Watson Loha a

Institute of Chemistry – University of Campinas (UNICAMP) - Campinas, SP, Brazil. b

LNLS (Brazilian Synchrotron Laboratory) and LNNano (Brazilian Nanotechnology Laboratory) – Campinas, SP, Brazil * corresponding author: [email protected]

ABSTRACT

Asphaltene precipitation is a key problem in petroleum industry and has been the focus of many studies on their aggregates present in crude oils and on the effects of additives to inhibit their formation and/or deposition. However, most of these studies were performed using model systems such as asphaltene solutions in organic solvents, turning the comparison with real systems more difficult. Herein we combine different modes (height and phase mode) of atomic force microscopy to identify colloidal particles associated with asphaltene aggregates present in crude oils. Following this methodology, a mica plate is inserted into oil and washed with toluene

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to remove excess oil. In addition, nanoparticles with dimensions ranging from a few to hundreds of nanometers were observed.

Overall, more particles are observed when flocculants such as heptane are added, whereas their size decreases when a good solvent for asphaltenes (toluene) is added. Similar colloidal particles are also observed repeating this methodology with asphaltene solutions in toluene, confirming that these somewhat reproduce the asphaltene association observed in crude oils. Addition of inhibitor such as DBSA led to observation of more and smaller nanoparticles.

The present experimental approach not only confirms the existence of asphaltene colloidal particles in crude oils, but also provides an accessible methodology to directly assess how these particles are affected by changes in oil composition or inhibitors.

INTRODUCTION Asphaltenes are components of the heavy polar fraction of petroleum and are associated with a series of flow-restriction problems caused by their aggregation and deposition from crude oils. They are a mixture of chemical compounds that contain an aromatic core with pendant alkyl groups1 and, due to their chemical nature, they are soluble in aromatic solvents such as toluene, and insoluble in alkanes.1,2,3 For laboratory purposes, asphaltenes are isolated from crude oils following precipitation with n-alkanes (typically pentane and heptane), occurring as a brown to black solid of amorphous nature. Normally, the longer the alkyl chain of the flocculant, the smaller the amount of asphaltenes removed from crude oils. Asphaltenes are now considered to display an average molar mass around 750 Da.1


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It is assumed that asphaltene molecules can associate to form different types of aggregates, as the result of several types of intermolecular interactions, such as π-π stacking,2,4 but also acidbase Brønsted interaction,2,5 hydrogen bonding,2,6 polar interactions,6 metal coordination,2 interactions between cycloalkyl and alkyl groups forming hydrophobic pockets2 and interactions with resins (another fraction of less polar molecules present in the crude).7 This association is proposed to continue hierarchically according to what is now known as the Yen-Mullins model3 up to formation of larger particles and their deposition. Evidences supporting this general model come from studies using a variety of experimental techniques. The structure of the aggregates has been investigated by X-ray diffraction, revealing the semi-crystalline nature of the asphaltene aggregate proposed as an evidence for an ordered aromatic core (via π-π stacking).3,8 A series of results from SAXS (Small Angle X-Ray Scattering) or SANS (Small-angle Neutron Scattering) measurements revealed colloidal aggregates with dimensions around a few nm,9-11 but whose size can increase up to three times when dispersed in organic solvents.10,12 Studies on asphaltene solutions in organic solvents such as toluene also suggested aggregates with a porous structure, of fractal nature,2 also described to occur in crude oil measurements.11 Moreover, there are various other oil components that selfassemble leading to a complex structure, which until today has not been fully elucidated.2,10 Recent studies using optical microscopy and centrifugation showed that there is not a specific onset concentration below which asphaltenes are stable, but their precipitation can take a long time (days to years) depending on particle concentration. Therefore, even with a small concentration of a flocculant may create nanoparticles, whose growth can be observed with time.13,14


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It was only recently that direct studies for the visualization of asphaltene molecules and their aggregates were published. One of the most recent deals with STM (Scanning Tunneling Microscope) measurements that allowed direct visualization of individual asphaltene molecules.15 Atomic force and electron microscopy techniques have also been used for analysis of asphaltenes aggregates or films.16-27 Results obtained with an apparatus for surface force measurements using AFM (Force Atomic Microscope) shows repulsive interactions between two surfaces covered with asphaltene in a good solvent (such as toluene), ascribed to steric repulsion as explained by de Gennes model for polymers. On the other hand, in a bad solvent such as heptane, adhesion observed between both surfaces is the result of attractions via van der Waals forces.16 One of the first studies on asphaltenes adsorbed onto surfaces was published by Toulhoat et al. in 1994.17 The adsorption process was made on mica from asphaltene solutions of different sources. Elongated objects of about 2 nm x 30 nm were observed while larger objects with lengths in the micron range and thicknesses of 10-20 nm were also reported. These display a finger-like design described as a fractal object proposed to be the result of destabilization of smaller objects and their subsequent aggregation by a heterogeneous nucleation process.17 Goual and colleagues also studied films of asphaltenes using a quartz microbalance. These studies showed a decrease in the size of the aggregates adsorbed with increasing carbon chain length of the alkane employed as flocculant.19 Furthermore, this group also employed transmission electron microscopy to investigate asphaltene aggregates deposited from model solutions and the effect of dodecylbenzenesulfonic acid, DBSA, as dispersant, reporting aggregates with dimensions in the range of 0.9-3.7 nm, which became more elongated upon addition of DBSA.20


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However, most of these studies involve analysis of model systems or the deposition of asphaltenes from oil samples in which a flocculant was employed. The use of both approaches is arguable for not ensuring to represent the real asphaltene situation in crude oils. Few studies were focused on obtaining results directly from crude oils.21,22 The methodology reported in the literature for these studies describes the surface exposure to the oil followed by removing its excess by placing the substrate immersed in toluene for a determined time. These studies show the possibility of identifying asphaltene aggregates by AFM topography mode. The present study was conducted to extend these previous ones using AFM in its different modes (topography and phase) to detect and monitor asphaltene aggregates that were adsorbed onto mica from crude oil and did not desorb when immersed in toluene. The AFM technique is adequate to analyze non-electrically conducting objects in the range of few to hundreds of nanometers. These measurements were conducted in crude oils with different asphaltene contents and focusing on the effects of good and bad solvents for asphaltenes, and of an asphaltene inhibitor (DBSA).

EXPERIMENTAL SECTION Materials Two Brazilian crude oils with different asphaltene contents (OFA and OFB) were employed for this study. These oils were provided by Petrobras and a summary of their composition data is listed in Table 1. The asphaltene content determined as the amount insoluble in n-pentane (asphaltene C5I) and n-heptane (asphaltene C7I) for both oils was determined by a gravimetric method, adapted from IP143/84.28 Additionally, water and the light fraction of crude oil A were removed as described in literature29 in order to avoid emulsion formation. Finally, asphaltene


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extraction to produce maltene was performed using n-pentane and following IP143/8428. After that, the pentane was removed with a rotary evaporator.

Method In order to prepare samples for AFM images, one droplet of crude oil was deposited on freshly cleaved mica. This system was horizontally (oil upside) immersed into 6 mL of toluene for 30 minutes in order to remove the excess of oil. Tests were performed with longer immersion times (from 30 to 60 min) with no significant changes in the pattern and size of the objects detected by AFM. After, the samples were dried in air for at least two days in the vertical position to drain excess liquid. The washing effect was evaluated by changing toluene for heptol (70 wt.% of toluene and 30 wt.% of n-heptane). The immersion time was also 30 min and the next steps were the same as described above. The effect of a bad solvent was studied using crude oil A, into which heptane was added until its concentration was 70% of the onset concentration (previously determined by optical microscopy as 3 mL/g). Samples with crude oil were also prepared upon addition of 1 wt.% of dodecylbenzenesulfonic acid (DBSA). For all the cases, DBSA or heptane were added to hot oil (at 55 °C) under mechanical stirring for 3 min and left for 15 min at high temperature, followed by another round of mixing. This mixture was left to equilibrate for one month before AFM measurements that used DBSA, and during four days for experiments with heptane. Additional analyses were performed using model systems. For that, asphaltene solutions were prepared dissolving the C5I fraction obtained from crude oil A into toluene at the same concentration initially found in this crude oil (18.7 wt %). Next, to repeat the same procedure used for crude oil, mica was immersed in this solution for 30 min. Its surface was dried for one


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day at room temperature and then the sample was horizontally immersed into 6 mL of toluene for 30 min. Finally, the sample was dried in air vertically for at least two days to drain excess liquid. The surfaces were analyzed by AFM in a Park Systems NX-10 microscope. Topographical and phase images were obtained with a PPP-NCHR (330 kHz; 4.2 N m-1) probe. The resolution in zaxis is 0.02 nm. Multiple locations (at least four, at two different zoom scales) were imaged for all of the samples at 25 °C and humidity was kept below 10 %. To perform a statistical treatment, the height of all particles was measured using the software Gwyddion 2.45. The number of particles was determined using always the same sample area of 20 x 20 µm². Next, histograms were plotted choosing the bin width to conform best a log-normal distribution and the center of each interval (bin center) represents the size of the particles.

RESULTS Before presenting the results we need to comment on the terms nanoparticles, particles or colloids that will be used. They all refer to asphaltene colloidal aggregates detected by AFM, while the term precipitate refers to asphaltene deposits and hence, much larger than these particles. Figure 1 shows an AFM image of a surface of mica on which crude oil A was deposited, followed by a subsequent washing in toluene. These images were obtained in two modes: height and phase contrast. Figures 1a and 1c show the sample topography, where the brighter points represent the higher regions. These images show a height profile showing the deposition of a material on mica (lower region), referenced in the image. To differentiate between oily residues and asphaltene aggregates, one has to consider both the height (Figures 1a and 1c) and phase (Figures 1b and 1d) information. In Figure 1d, there are three regions with distinct mechanical


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properties (distinguished in the image). Region 1 can be identified as the mica bare surface that could be covered by asphaltene or other components of the oil, as seen in the topography map. Mica is a harder material and the phase image shows lighter tones for this region. As a consequence, region 3 (the darker one) can be understood as a material softer than mica, such as an oily residue arising from remaining of the crude oil. Besides, region 2 displays similar hardness to region 1, as seen in Figure S1 of Supplementary Material, thus being identified as a solid present in the oil, in this case ascribed to asphaltene particles. Furthermore, it is possible to distinguish in this image that these larger objects are formed by smaller aggregates of around (4 ± 1) nm in height. Consequently, this set of results indicates that the AFM technique is efficient to identify the presence of asphaltene particles already present in crude oil and, possibly, how they change with respect to addition of other components. Figure 2 shows AFM results, height and phase contrast maps, for samples of oil B. Similar to what has been observed for oil A, it is possible to identify that the brighter points in the topography image are correlated with harder structures (lighter tones) in the phase map, associated with the mica surface. These points are surrounded by soft material identified as oily residue. Also, the phase contrast image shows larger objects observed on the height map, and it is possible to see that they are composed by smaller aggregates grouped together (phase image). The analysis of AFM results obtained for these two crude oils reveals the presence of aggregates of smaller nanoparticles that display hardness similar to mica. We propose that these particles are composed by asphaltene aggregates. Figure 3(a) shows an image obtained in maltene prepared from crude oil A, with a clear decrease in the number of particles when compared with crude oil A, Figure 3(b). The remaining particles appearing in Figure 3(a) may be ascribed to other oil fractions. Moreover, Figure 3(c)


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shows nanometric particles in systems prepared with C5I asphaltene from oil A prepared in toluene at a concentration of 18.7 wt.% (more images are available in Figures S2, S3 and S4 of the Supplementary Material). One interesting observation is that the structures found from asphaltene solutions are, on average, larger than those observed in the parent crude oil. This may be a consequence of the influence of the maltene fraction, which was not present in the model organic solutions. This comparison among maltene, crude oil and asphaltene solution endorses the idea of asphaltene aggregates. Additionally, mica plates exposed to crude oil and washed with toluene according to our standard procedure were left immersed in toluene, a good solvent for asphaltenes, for two days. Figure S5 (Supplementary Material) shows that the particle size decreases indicating that the particles are partially solubilized by the solvent as expected for asphaltene aggregates. Following this attribution, a more thorough analysis on the size distribution of these aggregates was performed under different conditions. For that, their dimensions will be estimated from height measurements of particles displaying hardness similar to mica. Figure 4 shows the height distribution of the structures identified as asphaltenes in a series of images of samples of oil A taken along the entire extent of the mica plate (some height maps are available in Figure S6 of Supplementary Material). The curve in red represents a lognormal fit for the population distribution, which was found to represent well the size distributions derived from this approach (the fit parameters are available in Table S1 of the Supplementary Material). This distribution shows the presence of structures ranging from a few to hundreds of nanometers, with the distribution median equal to 16 nm. The same analysis was performed in samples of crude oil B. Figure 5 shows the height distribution of the structures identified as asphaltenes in a series of images of samples of oil B taken along the entire extent of the mica


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plate. Additionally topographic maps and fit parameters are available in the Supplementary Material (Figure S7 and Table S1, respectively). In order to assess how these detected nanoparticles respond to changes in oil composition, heptane, an asphaltene flocculant, was added to samples of oil A, at a concentration of 70% of its asphaltene precipitation onset (Figure 6). Comparing these images with the ones obtained without heptane (Figure 7 shows a comparison between these two cases), we observe very similar size distribution for the particles. However they increase in number, but remaining at similar sizes indicating heptane flocculation effect. Moreover, we checked how changing the solvent to wash the mica with crude oil A would affect the size distribution. We observed that washing the mica surface with toluene or heptol (70 wt.% of toluene) did not change the object distribution size (Figure S8 of Supplementary Material), indicating that, within the washing period used for this methodology, the particles are equally affected by these two solvents. We also studied samples in the presence of DBSA, a known asphaltene dispersant. The topographical maps are shown Figure S9 for the oil A and Figure S10 for oil B. The height distribution of the objects is available in Figure 8, while the fit parameter in Table S1 of Supplementary Material. For crude oil A, the size distribution shows that large aggregates persist even with the addition of 1 wt.% DBSA on oil basis, some still in the size range of hundreds of nanometers. However the median of the population was significantly reduced to 9 nm, confirming DBSA dispersing effect. On the other hand, DBSA effect was more prominent over asphaltenes present in crude oil B, as can be seen by the difference in the distribution curves shown in Figure 8b. We believe this


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is related to the different chemical composition of these oils, though this should be further investigated. In this oil, we observed that the addition of DBSA led to a decrease in structures size and an increase in the number of objects per area. Figure 9 shows a comparison between AFM images of the crude oil B before and after the addition of DBSA. In this figure, we note that the size is wider in the system without DBSA. Moreover, it is noted that we did not detect structures with size in the range of hundreds of nanometers with the addition of DBSA. In addition, the median of asphaltene particles decreased to 8.8 nm, while without DBSA the median was 56 nm. It is also important to mention that some elongated objects are present in some regions of the substrate surface (as seen in Figure S10 of Supplementary Material), revealing a tendency of growth in a preference direction. We have also attempted to suppress the washing step by using more dilute asphaltene solutions in toluene. Attempts at the concentration of 1 wt.% still produced a thick layer of adsorbed material on the mica surface that prevented observation of possible particles. By diluting the solutions further, to 0.03 wt.%, (see Figures S11 and 12 in Supplementary Material) we were able to identify colloidal particles whose size distribution and median size are in the same range of those observed in crude oil. Moreover, Table S1 show that increasing the asphaltene concentration causes an increase in the median size of the lognormal distribution. The fact that the AFM images contain particles with a wide size range agrees with the Yen-Mullins stepwise asphaltene aggregation model.3,30,31


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DISCUSSION The results presented above reveal that AFM is capable of detecting colloidal particles after exposing mica plates to crude oils and asphaltene solutions. Their hardness similar to mica as suggested from AFM results, and the finding that they seem to be affected by known good or bad solvents for asphaltenes and an asphaltene inhibitor, led us to ascribe these colloidal particles to asphaltene nanoaggregates. The presence of these nanoaggregates in crude oils, as well as in asphaltene solutions in organic solvents, has been earlier proposed based on evidences from a variety of experimental results such as SAXS (or SANS) data,9-12 and even other microscopy techniques such as TEM (Transmission Electron Microscopy).20 The size distribution derived from the present AFM analyses agrees with the sizes reported from the above studies. We believe the strategy presented here presents two important advantages over the techniques used in the earlier investigations cited above: first, it provides the possibility of a direct investigation using crude oil samples, that could additionally be compared with results obtained in model solutions; second, AFM is a technique that is more easily accessible than some used for these earlier studies as, for instance, SAXS or SANS. Moreover, in earlier studies from our group,11 the scattering pattern showed that the aggregates are composed by at least two levels; the first one, of a few nanometers and a second one, bigger than 100 nm. SAXS was unable to describe the second organization level due to limitations from the experimental setup used.11 The scattering intensity of this level of organization suggests that there are either more particles or more voluminous particles. AFM was able to detect small particles (1-10 nm) that could aggregate, forming particles in the range of 100-1000 nm, in agreement with the earlier SAXS results.


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Some observations such as the fact that the nanoparticles detected with crude oils are smaller than those observed with asphaltene solutions confirm the importance of assessing these nanoparticles directly in crude oils. This observation suggests that the presence of the maltenes plays an important role in the stabilization of the particles, reducing the size of the observed structures. On the other hand, we foresee a series of questions about possible artifacts derived from the procedure taken to prepare the samples for AFM analyses. As we described above, one of the key requirements for proper AFM analysis is the removal of oil excess after exposure to crude oils. According to our methodology, this is performed by washing the mica plate with toluene (or toluene/heptane mixtures) for a certain interval (up to 60 min, as we tested). It is expected that this procedure may remove, not only oil excess, but also other asphaltene particles. In fact, by extending this washing period to many hours, we have noticed a significant decrease in the size of nanoparticles detected by AFM (as can be seen in Figure S5). Washing with a bad solvent such as heptane was not sufficient to produce samples that could be analyzed by AFM as a thick layer of adsorbed material remained on the mica plates. Image analyses always require careful consideration of statistics. For that, we have always confirmed results by analyzing replicates of each sample, prepared independently. In addition, a series of images were taken from each specimen, in order to produce well over one hundred particles to produce the size distributions and sizes derived from this methodology. Another source of concern of this methodology is related to sampling of these colloidal particles. According to this approach, AFM can only analyze particles that remain attached to the mica surfaces. One point that should be raised in this discussion is that, if this methodology is to be used to investigate oils for their instability with respect to asphaltene deposition, the fraction


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of particles that are expected to adsorb should be more representative of those which should be related to flow restriction problem, including interaction with solid surfaces such as those of rocks in the reservoir or pipelines. In the same line of thought, one can ask what could be the role of the mica surface in nucleating formation of particles that, otherwise, may not exist in the oil sample or asphaltene solution. We believe that this should be tested by employing other surfaces than mica, and this is planned as a continuation of the present study. However, once more, if surfaces may act as nucleating agents for asphaltene deposition, it is important that this is detected because this is likely to occur in real systems during oil exploration or transportation. In a related review, Adams32 shows that asphaltene adsorption onto surfaces of different nature: iron, mica and glass, for example. His analyses indicate that the nature of the surface did not play a major role in asphaltene adsorption.32 Bearing in mind the points discussed above, the present methodology provides an experimental approach capable of producing, at least complementary, evidences related to colloidal particles formed by asphaltenes. AFM results can be presented as size distributions and particle counts, both constituting relevant information for the evaluation of asphaltene aggregation/deposition processes. Our results confirm that size distribution and particle counts are both sensitive to changes in composition of the oil or asphaltene solutions with results that agree with those commonly reported in related studies. It is interesting that the size distributions for asphaltene particles detected with both crude oils did not differ significantly, although their asphaltene contents were different (18.7 vs. 3.6 wt.%). Moreover, the particles detected with model systems such as toluene solution of asphaltenes displayed very similar size distributions in diluted solutions, as shown in Figures S4 and S12, although at higher concentration larger particles are observed. As mentioned above, prolonged


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exposure of oil treated mica plates to toluene produced a decrease in the size of detected particles, in agreement with the expected removal of these asphaltene particles by a good solvent. On the other hand, addition of a bad solvent such as heptane, even in concentrations significantly below its asphaltene precipitation onset determined by optical microscopy, produced a significant increase in the number of detected particles, though they displayed similar size distributions. The observation of these aggregates below the optical microscopy onset agrees with the earlier proposition of a continuous aggregation process rather than a critical one.13,14 The action of DBSA as asphaltene inhibitor is already well-established, and associated with its amphiphilic character and the presence of an acidic group (SO3H), ascribed as responsible for their stabilizing effect.5 In present investigation, AFM analyses reveal that addition of DBSA led to a reduction on the size of the particles present in the oil. However, as can be seen in Figure 8, the action of the DBSA differs for the two studied oils. This may result from the fact that asphaltene polarity, and hence their interaction with DBSA polar group, is different depending on the oil source. Goual et al.33 previously reported that the precipitation of asphaltenes by the addition of a flocculant could increase or decrease due to an addition of DBSA. In the present study, DBSA was more effective over asphaltene deposition from oil B, in which there was an increase in the number of particles detected, but with decrease in their size. This could be ascribed to either disruption of larger asphaltene particles or prevention of their formation due to interaction with DBSA. The lack of observation of particles with dimensions around hundreds of nanometers, and the observation of elongated structures upon addition of DBSA, the latter similar to what has been reported by Goual from TEM images,20 may suggest that DBSA effect on the aggregation of smaller particles may be dominant.


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Another interesting observation, that repeated for all of the samples investigated, is that AFM images reveal larger structures that seem to be formed by aggregation of smaller objects. Results with DBSA, discussed above, suggest that their aggregation can be controlled by addition of inhibitors, features that are consistent a stepwise aggregation process. Moreover, increasing the concentration of the asphaltene solutions, the size of the objects also increase. These observations agree with overall picture of the currently accepted Yen-Mullins model3 with regards to nanoaggregate formation, which extends to asphaltene self-assembly into clusters and then larger objects in the micrometer scale.

CONCLUSION The results presented and discussed in this paper support the proposition of this methodology as a valid one for the investigation of colloidal asphaltene particles in crude oils and model systems. The strategy of collection of colloidal particles over a surface that is then analyzed by AFM revealed to produce images that allow unequivocal identification of objects related to asphaltenes, and their analysis in terms of quantity of particles and their size distributions. These parameters were shown to respond to the addition of good or bad asphaltene solvents and inhibitors according to the expected trends reported in similar studies. The general observations derived from this approach agree with those reported from earlier studies on asphaltene colloidal particles obtained from scattering experiments and other microscopy analyses, validating the proposed methodology. We have also discussed possible concerns related to particle sampling and surface nucleating effects that may be associated with the current strategy. In some cases, they would produce information that is relevant because provides an insight on the behavior of less stable asphaltene


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fractions or on how they are affected by surfaces. These are evidences relevant for assessment of possible flow problems related to their deposition during oil exploration or transportation. In summary, this report presents and describes a new experimental strategy that allows direct observation of asphaltene particles in model systems and, most importantly, in crude oils. In addition, the required technique, AFM, is a widely available and versatile methodology capable of providing information on asphaltene deposited quantity and particle size distribution. As such, we believe that this is an important addition to the inventory of techniques currently employed to investigate asphaltene aggregation/deposition and for the test and development of their chemical inhibitors, with relevant impact to the area of flow assurance.

SUPPLEMENTARY MATERIAL This material contains an analysis of the AFM results (images and phase shifts) of oil samples and asphaltene solutions. Fit parameters of lognormal distribution are also available.

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS The authors thank the Brazilian Agencies CNPq and FAPESP for support to this work through a PhD scholarhip to L.B.S.B and senior research grants to W.L. and M.B.C., and for financial support through project FAPESP 2015/25406-5. We also thank the LCS/LNNano/CNPEM for


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access to their AFM equipment and to Dr. Carlos Alberto Rodrigues Costa and Evandro Martin Lanzoni for their important technical support. Finally, we thank CENPES/Petrobras for the oil samples.

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FIGURE CAPTIONS Figure 1. (a), (c): Height and (b), (d): phase maps of a crude oil A sample deposited on a mica surface and subsequently washed in toluene. (c) and (d) were obtained in the black square region indicated in (a) with an increased magnification. Figure 2. (a) Height and (b) phase maps of a crude oil B sample deposited on a mica surface and subsequently washed in toluene. Figure 3. Height maps of (a) maltene-C5 from oil A, (b) crude oil A and (c) C5I asphaltene from oil A dissolved in toluene at a concentration of 18.7 wt.% and subsequently washed in toluene. Figure 4. (a) Histogram of height distribution of the asphaltene aggregates of crude oil A. (b) Zoom of the more populous region. In red, the lognormal fit. Figure 5. Histogram of height distribution of the asphaltene aggregates of crude oil B. In red, the lognormal fit. Figure 6. Height maps of (a) crude oil A sample and (b) this oil with n-heptane deposited on a mica surface and subsequently washed in toluene. For (a) average number of objects per image (20 x 20 μm²) = 110. Median height is (16.4 ± 0.3) nm, obtained by lognormal fit. The others parameters are presented in Table S1. For (b) average number of objects per image (20 x 20 μm²)


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= 335. Median height is (15.4 ± 0.2) nm, obtained by lognormal fit. The others parameters are presented in Table S1. Figure 7. Size distribution of crude oil A (black curve) and this oil with n-heptane at 70 % of onset (red curve). Figure 8. Size distribution of (a) crude oil A and (b) crude oil B (black curves) and its respective oils with 1 wt.% of DBSA (red curves). Figure 9. Height maps of (a) crude oil B and (b) this oil with 1 wt.% of DBSA. For (a) average number of objects per image (20 x 20 µm²) = 70. Median height is (56 ± 1) nm, obtained by lognormal fit. The others parameters are presented in Table S1. For (b) average number of objects per image (20 x 20 µm²) = 470. Median height is 8.8 nm, obtained by lognormal fit. The others parameters are presented in Table S1.

TABLE CAPTION Table 1. Properties of the two crude oils studied. °API and their saturated, aromatic, resins and asphaltene contents were determined by the SARA method. Asphaltene C5I and C7I represent the asphaltene fraction insoluble in n-pentane and n-heptane, respectively, determined by IP143/84.


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FIGURES

Figure 1


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Figure 2

Figure 3

Figure 4


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Figure 5

Figure 6


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Figure 7

Figure 8


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Figure 9 TABLE Table 1 Oil

OFA

OFB

°API

11.0

27.9

Saturated (wt.%)

39.5

53

Aromatics (wt.%)

27.5

24

Resins (wt.%)

21.9

21

Asphaltenes (wt.%)

11.1

2.2

Asphaltenes C5I

18.7

3.6

12.0

Not

(wt.%) / IP143/84 Asphaltenes C7I (wt.%) / IP143/84

determined


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TABLE OF CONTENTS


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Figure 1. (a), (c): Height and (b), (d): phase maps of a crude oil A sample deposited on a mica surface and subsequently washed in toluene. (c) and (d) were obtained in the black square region indicated in (a) with an increased magnification. 90x80mm (300 x 300 DPI)


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Figure 2: (a) Height and (b) phase maps of a crude oil B sample deposited on a mica surface and subsequently washed in toluene. 377x161mm (300 x 300 DPI)


Energy & Fuels

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Figure 3. Height maps of (a) maltene-C5 from oil A, (b) crude oil A and (c) C5I asphaltene from oil A dissolved in toluene at a concentration of 18.7 wt.% and subsequently washed in toluene. 182x55mm (150 x 150 DPI)


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Figure 4. (a) Histogram of height distribution of the asphaltene aggregates of crude oil A. (b) Zoom of the more populous region. In red, the lognormal fit. 110x77mm (300 x 300 DPI)


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Figure 5. Histogram of height distribution of the asphaltene aggregates of crude oil B. In red, the lognormal fit. 121x84mm (300 x 300 DPI)


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Figure 6. Height maps of (a) crude oil A sample and (b) this oil with n-heptane deposited on a mica surface and subsequently washed in toluene. For (a) average number of objects per image (20 x 20 µm²) = 110. Median height is (16.4 ± 0.3) nm, obtained by lognormal fit. The others parameters are presented in Table S1. For (b) average number of objects per image (20 x 20 µm²) = 335. Median height is (15.4 ± 0.2) nm, obtained by lognormal fit. The others parameters are presented in Table S7. 203x85mm (300 x 300 DPI)


Energy & Fuels

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Figure 7. Size distribution of crude oil A (black curve) and this oil with n-heptane at 70 % of onset (red curve). 143x99mm (120 x 120 DPI)


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Figure 8. Size distribution of (a) crude oil A and (b) crude oil B (black curves) and its respective oils with 1 wt.% of DBSA (red curves). 201x84mm (300 x 300 DPI)


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Figure 9. Height maps of (a) crude oil B and (b) this oil with 1 wt.% of DBSA. For (a) average number of objects per image (20 x 20 µm²) = 70. Median height is (56 ± 1) nm, obtained by lognormal fit. The others parameters are presented in Table S1. For (b) average number of objects per image (20 x 20 µm²) = 470. Median height is 8.8 nm, obtained by lognormal fit. The others parameters are presented in Table S1. 166x73mm (150 x 150 DPI)


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421x413mm (120 x 120 DPI)


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