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Surface and Interface Characterization of Asphaltenic Fractions Obtained with Different Alkanes: A study by Atomic Force Microscopy and Pendant Drop Tensiometry Iago Oliveira, Larissa Gomes, Elton Franceschi, Gustavo Rodrigues Borges, Juliana Faccin De Conto, Flavio Cortinas Albuquerque, and Claudio Dariva Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02784 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018
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Surface and Interface Characterization of Asphaltenic Fractions Obtained with Different Alkanes: A study by Atomic Force Microscopy and Pendant Drop Tensiometry Iago Oliveira1, Larissa Gomes1, Elton Franceschi1, Gustavo Borges1, Juliana F. de Conto1, Flávio Cortinas Albuquerque2, Claudio Dariva1*
1 - NUESC/ITP, Núcleo de Estudos em Sistemas Coloidais, PEP/PBI/UNIT, Universidade Tiradentes, Av. Murilo Dantas, 300, Aracaju SE 49032-490, Brasil. 2 - PETROBRAS/CENPES/Gerência de Química, Av. Horacio de Macedo, 950, Cidade Universitária, Rio de Janeiro, 21941-915 Rio de Janeiro (RJ), Brazil. * Corresponding author. E-mail address:
[email protected] Abstract
It is recognized that asphaltenes have a tendency to aggregation and precipitation. Even at low concentrations they can adsorb at interfaces/surfaces where each type of interaction plays an important role in different stages of oil production. In order to evaluate the behavior of asphaltenes, it is necessary to conduct studies that allow the understanding of their chemical and physical structure, as well as to assess how they behave interfacially and superficially. By using the Pendant Drop Tensiometry and Atomic Force Microscopy techniques, the present work aims to characterize the interfacial behavior and the surface structure of two asphaltenic fractions obtained through the precipitation using n-heptane and propane as
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flocculants. Asphaltenic fractions were characterized by Fourier transform infrared spectroscopy, elementary analysis, gel permeation chromatography and differential scanning calorimetry, to obtain their physicochemical characteristics. Fractions were deposited in glass substrates at different concentrations and the contact angle between water and the substrate was determined, identifying how the different asphaltenic fraction influence the wettability of the surface, even at low concentrations. When precipitation was conducted using propane, the asphaltenic fraction showed distinct structural characteristics that obtained by precipitation with heptane. Atomic force microscopy suggested the formation of different surface arrangements between the fractions, caused by the higher presence of resins in the fractions precipitated by propane. The pendant drop tensiometry evidenced that the asphaltenic fraction insoluble in heptane showed greater affinity to migration to the interface, producing films more resistant to the deformation, than the asphaltene fraction insoluble in propane. Keywords: Asphaltenes, atomic force microscopy, wettability, alkanes precipitation, interfacial tension.
1. Introduction Responsible for several problems in the petroleum production and refining processes, asphaltenes are not a homogeneous fraction, but are composed by polydisperse molecules in molecular weight, structure and functionality. Due to its chemical and structural complexity, the understanding of its properties is not a simple task1. Asphaltenes are formed by aromatic and naphthenic nuclei, aliphatic chains and heteroatoms, such as oxygen, nitrogen or sulfur, presenting the highest polarity and molecular weight among petroleum components. Usually asphaltenes are defined as the fraction insoluble in paraffinic hydrocarbons such as nheptane, and soluble in aromatics, such as toluene2–5. The yield and the properties of the asphaltenic fractions depend strongly on the precipitation method and the precipitant used6. The most used precipitation methods are based on the use of solvents n-pentane and n-heptane7. As observed by Luo et al.8, heptane precipitated asphaltenes have a higher molecular weight and aromaticity compared to asphaltenes precipitated in lighter alkanes such as pentane. Thus, the chemical and structural characterization of asphaltenes and their sub-fractions can help the understanding of its properties and behavior. Even at low concentrations, asphaltenes tend to aggregate and precipitate, which causes losses in the oil industry processes. Furthermore, asphaltenes can be adsorbed on liquid-air and liquid-liquid interfaces and liquid-solid surfaces, where each type of interface/surface represents an important detrimental role in different stages of petroleum production9. Such interactions involve asphaltenes aggregation and are
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partially responsible for the formation and stabilization of emulsions, as well as pore obstruction in the reservoir rock and transport ducts, causing losses in the oil production process9. Asphaltene precipitation can occur due different factors including changes in pressure, temperature, chemical composition of crude oils, acid stimulation, and mixing of oil with diluents, other oil components and gases such as CO23,10. Thus, asphaltene precipitation can occur both during the primary oil recovery phase and during advanced oil recovery operations, such as CO2 or natural gas injections. In addition to CO2, propane is often used in solvent-based heavy oil recovery processes, such as the vapor extraction process (VAPEX), where the vaporized solvent is injected with the purpose of reducing the viscosity of the oil11. The dissolution of a light hydrocarbon in a viscous crude oil is an effective method to significantly reduce its viscosity and thus greatly facilitate its long-distance transport through ducts8. In the literature, there are few data available that focus on the interfacial and surface behavior of precipitated asphalt fractions with a light hydrocarbon such as propane. Another asphaltene property to be considered is the wettability, usually defined as the tendency of a fluid to spread over or adhere to a solid surface in the presence of other immiscible fluid. It plays an important role in the oil extraction process, which may facilitate or hinder the oil flow on different surfaces. The asphaltenes deposition is considered one of the main causes of reversibility of the wettability inside an oil reservoir. This is strongly related to the interfacial interactions of these compounds with the different surfaces found in the oil production stages12. To better understand the problems caused by asphaltenes in the oil industry, it is necessary to characterize the chemical and physical structure of these compounds and conduct studies to understand their behavior when in interfaces or in surfaces. Despite the many studies carried out in this area, there is a lack of information on the asphaltenes characteristics, which makes difficult to define their molecular structure and aggregation mechanisms. Atomic Force Microscopy (AFM) technique is a promising tool to understand the phenomena related to the surface behavior, since it allows a visual analysis of structures formed by the molecules in different conditions13. Balestrin et al.14 used different methods of AFM technique to identify colloidal particles associated with asphaltenes aggregates present in crude oils using a methodology where a mica substrate was inserted into the oil and into a toluene solution of asphaltene. Similar colloidal particles were observed in both methodologies, confirming that the solutions of asphaltenes and toluene reproduce to a certain extent the association of asphaltenes in crude oils. These measurements were performed in oils with different asphaltenes contents using also an asphatene precipitation inhibitor. Raj et al.15 investigated the structure of asphaltene molecules deposited on substrates of variable polarity to chemically mimic minerals from reservoir rocks using the AFM technique. Force-distance spectroscopy was performed on asphaltene
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nanoaggregates to reveal the key role of surface properties such as polarity or hydrophilic/hydrophobic nature, directing size, shape, spatial distribution, as well as adhesion and mechanical properties of asphaltene nanoparticles adsorbed on the substrates. Castillo et al.16 used the AFM technique to observe the interaction forces between asphaltenes and the surface of macro and nano iron particles. At low concentration of asphaltenes, no aggregations were observed in solution, with a clear difference in adsorption process on nano surfaces when compared to macro surfaces. To investigate the role of asphaltenes at the interface, the pendant drop tensiometry technique has been used to determine the properties of interfacial films e.g total modulus of elasticity, elastic and viscous modules, and film compressibility.17–19 Alves et al.19 observed the influence of the aqueous phase salinity on the interfacial properties of Brazilian crude/brine solutions. Pendant drop technique was used to carry out rheological dilatation studies, giving information about the adsorption and desorption process of the tensoactive fractions present in the oil, and how these fractions behaved in the presence of different concentrations of salt at the aqueous phase. Results indicated that the values of the total interfacial elasticity and its components, viscous and elastic modulus were increased when increasing the salt content of brine solutions, suggesting that the salt induces formation of a more rigid interfacial film. In addition, the presence of salt led to a higher interfacial activity of the surfactants, producing higher interfacial elasticity and compressibility. Lashkarbolooki and Ayatollahi20 investigated the effect of asphaltenes and resins on the interfacial tension between a crude oil and a sulfate solution. Results demonstrate that asphaltene molecules are more interfacially active compared to resin molecules. Authors attributed this fact to the high affinity of the asphaltene to be disposed at the interface between the crude oil and the sulfate solution. Morais et al.21 used Pendant Drop Tensiometry to describe the rheological interfacial properties of asphaltenes extracted from two Brazilian oils in heptol (heptane and toluene)/water systems using different ratios of organic solvents and asphaltene concentrations. The results showed that in systems containing a good solvent (toluene), the attraction between solvent and asphaltene molecules was stronger due to the high solubility, thus reducing the interfacial activity of asphaltene molecules. In this scenario, the aim of the present work is to characterize the interfacial behavior and the surface structure of two distinct asphaltenic fractions obtained by precipitation with n-heptane and propane as flocculating agents. The techniques of Pendant Drop Tensiometry and Atomic Force Microscopy were used to determine the effects of the concentration of asphaltenic fractions on the formation of interfacial/superficial films and to study how the structural and resistance behavior of these films are related to the asphaltic fraction characteristics that are obtained from distinct type of flocculating agents.
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2. Experimental Section 2.1. Extraction of asphaltenic fractions The asphaltenic fractions investigated in this work were extracted from a Brazilian crude oil with an amount of asphaltenes of 3.1%(wt%) and ºAPI of 23.3. To evaluate the behavior of different oil fractions, two oil fractionation methodologies were used: one to recover the heptane insoluble fraction, and other to obtain the propane insoluble fraction. To obtain the heptane insoluble fractions, a methodology was adapted from ASTM 656022 in which 10 g of oil was diluted in 400 ml of n-heptane (Sigma Aldrich, ≥99%) in a flask protected from light. The solution was stirred for 4 h with magnetic stirrer and then kept without agitation for more 24 h. After this, the solution was filtered using cellulose filter paper Whatman n°42. The precipitate was oven dried for 30 min at 110 °C. Different from the norm the asphaltenes purification step was not carried out in order to obtain a fraction with the presence not only of asphaltenes but of other compounds that could influence the interfacial and superficial behavior of the studied oil. This procedure was repeated until a sufficient amount of asphaltenic fraction was obtained for the conduction of all characterization experiments. This fraction was named AH. The asphaltic fraction insoluble in propane was obtained by an adaptation of the method above. 30 g of the crude oil was diluted in 200 ml of propane (White Martins S.A., ≥99%) in a high-pressure cell. Figure 1 illustrates a schematic view of the experimental setup used in this study. Briefly, the system is composed by a high-pressure variable-volume view cell with maximum volume around 300mL, coupled to a syringe pump (ISCO 500DX) to fed the propane into the cell and to control the system pressure and volume. The procedure consisted in adding an amount of propane into the high-pressure cell (around 200mL), stirring the solution for 4 h with a magnetic stirrer, and then kept the solution to rest for 24 h. After this process the supernatant was drained, and all the procedure repeated until the observation of a clear supernatant phase that typically was found after 3 or 4 cycles. The obtained precipitate was then collected, and this fraction was named AP. Figure 1 2.2. Chemical characterization of asphaltenic fractions Fourier Transform Infrared Spectroscopy (FTIR) Fourier Transform Infrared Spectroscopy (FTIR) technique was used to identify the functional groups present in the asphaltenic fractions employing an attenuated total reflectance (ATR) spectrometer (PERKIN ELMER, Frontier FTIR). Spectra of the fractions sample were conducted with resolution of 4cm−1 and 256 scans in a range of 4000–400 cm−1.
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Elemental Content (CHN) The elemental content analysis was performed on a Truspec 630-200-200 equipment operated with Helium (99.995%) and Oxygen (99.99%), with an oven temperature of 1075 °C and post-combustion temperature of 850 °C. The equipment was calibrated with EDTA standard (41.0% C, 5.5% H and 9.5% N) using a mass range of 10-200 mg. The mass of sample used was approximately 2 mg. The percentages of carbon and hydrogen are given by infrared absorption and nitrogen by thermal conductivity.
Gel Permeation Chromatography (GPC) Molecular weight was determined by Gel Permeation Chromatography (GPC). Analyzes were performed by injecting 20 μl of a 0.5 wt% solution of the sample dissolved in Tetrahydrofuran (THF, Merck), prefiltered through a Teflon filter with a mesh size of 450 nm. The analysis was conducted on a highperformance liquid chromatograph (HPLC, 20A, Shimadzu) equipped with a RID-10A detector at 35°C, a pre-column (PL gel 5 mm MINIMIX-C guard, 50 x 4 mm, Agilent) and a 250 x 4.6 mm column set of two columns in series (PL gel 5 mm Mini MIX-C, Agilent). Molecular weight and molecular weight distributions were calculated against polystyrene standards in a range of 580 and 3,800,000 g.mol-1.
Differential Scanning Calorimetry (DSC) The thermal properties of the fractions were characterized by differential scanning calorimetry (DSC). To perform the analyzes, approximately 10 mg of sample was weighed and placed in a hermetically sealed aluminum crucible. Samples were first heated from 40 °C up to 350 °C at a heating rate of 10 °C min-1 under nitrogen at 50 mL min-1, to eliminate their thermal history. Then samples were cooled to 40 °C at a cooling rate of 10 °C min-1, maintained at this temperature for 1 min and heated again to 350 °C at a heating rate of 10 °C min-1. The thermogravimetric effects are observed on the three heating/cooling ramps.
2.3. Surface characterization The surface films of the different asphaltenic fractions were analyzed by Atomic Force Microscopy (AFM - Shimadzu). To perform the analyzes, toluene solutions of the both AP and AH fractions were prepared at different concentrations (1, 5, 10, 20, 50, 100, 500 and 1000 mg.L-1). The solutions were sonicated for 30 min to ensure complete solubilization of fractions. To induce a scenario with higher precipitation, heptane was added in the solutions of toluene with asphaltenic fractions of 1000 mg.L-1. Solutions of heptol 2:1
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(heptane – toluene) with asphaltenic fractions were prepared to evaluate the influence of n-heptane (paraffinic solvent) on aggregation process of the fractions obtained. 10 mL of solutions of heptol 2:1 with 1000 mg.L-1 of asphaltenic fraction were prepared by adding 3.33 mL of toluene to ensure complete dissolution of the asphaltenic samples. Samples were sonicated for 30 min, then 6.66 mL of heptane was added and it was sonicated again for 30 min. Glass substrates (18x18 mm, WWR) were used in the analyzes following the preparation methodology proposed by Labrador et al.23. The substrates were initially subjected to a four steps cleaning procedure. In the first step substrates were placed in Falcon tubes submerged in tetrahydrofuran (THF - Synth, 99%).In the second step the substrates were placed in tubes with water and soap and in the third step they were placed in tubes with only distilled water. At each stage the tubes with the substrates were sonicated for 5 min and at the end of each step dried with N2 gas. In the last step the substrates were submerged in acetone (Synth, 99%), sonicated for 15 min and dried with N2. After the cleaning procedure the substrates are placed vertically in flasks containing asphaltenic fractions solutions. A substrate was submerged only in pure toluene to evaluate the surface behavior without the influence of the asphaltene fractions. After 24 h the substrates were removed from the solution and placed vertically in a container over a paper filter and insulated in a toluene atmosphere until complete evaporation of the solution. Four substrates were prepared for each experimental condition, two used for the contact angle analysis and two for the AFM analysis, as shown in the sequence. The contact angle analyzes were performed in a pendant drop tensiometer (Teclis Tracker, IT Concept) at 5 random points of each prepared substrate. A 5 μL water drop was dripped onto the substrate and the contact angle between the water and the surface. The topography of the samples was obtained in a microscope (SPM-9700, SHIMADZU) equipped with a 30 μm scanner through dynamic mode and the tips used were Econo-TESP type with a resonance frequency of 300 kHz and a 40 N.m-1 constant of elasticity, provided by Oxford Instruments. The analyzes were performed at 5 random points of each substrate with a scale of 20 x 20 μm, so 10 images were obtained for each experimental condition. Several parameters such as height and roughness distribution can be measured from AFM images based on a specific area. The parameters evaluated in this work were the arithmetic mean of the roughness (Ra) and the maximum height of the analyzed areas (Rz), both provided by the software of the equipment. Results were analyzed by analysis of variance (Tukey test). Differences were considered significant for p < 0.05.
2.4. Interfacial characterization
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To evaluate the influence of the asphaltenic fractions on interfacial film properties, the interfacial tension and the viscoelastic properties of the film were determined using the pendant drop technique in systems involving asphaltenic fractions in toluene with different concentrations (0 to 3000 mg.L-1) and ultrapure water (MiliQ). Toluene solutions of the fractions were sonicated for 30 min in an ultrasonic bath to ensure complete dissolution of the samples. The experiments were carried out on a pendant drop tensiometer (Teclis Tracker, IT Concept).
Static measurements The interfacial tension values are determined as a function of time in the static measurements and these results can be correlated with the adsorption of surfactants at the interface. To determine the curves, a drop with area and volume of 39.6 µm2 and 24.8 µm3, was formed and remained at rest until the end of the experiment. All experiments were performed in duplicate. To perform tests on the pending drop tensiometer, some initial care needs to be taken to ensure the best results and the absence of errors during the tests a previous study to determine area and volume ideas to be performed the experiments was performed before the experiments presented here. Information about this study of initial parameters is attached as Support Information. As one of the input parameters in the pending drop tensiometry technique, the phases density was determined in a digital densimeter (DMA 4500M from Anton Paar) at of 25 ºC.
Dilatational rheological experiments From the dilatational rheological experiments, the values of the total dilatational, elastic and viscous modules can be determined, and they are correlated with the stiffness of the interfacial film. These tests are based on sinusoidal perturbations around the interfacial area, with frequency and amplitude of pre-defined oscillation. In addition to the area and volume of ideas to perform the experiments, other parameters such as amplitude and frequency of oscillation require previous studies to determine the best conditions to perform the experiments. Information on this parameter study is attached in the supporting information. The area and volume of the drop were the same of the static tests and the oscillations were performed with an amplitude of 6% of the drop area and a frequency of 0.02 Hz. The dilatation modules were obtained by forming a drop which remained at rest for 30 min and then the oscillation procedure was performed and repeated every 30 min during the experiment time. Four
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oscillations were conducted in a total time of 2 h for each experiment. Four concentrations of the
solutions asphaltenic fractions / toluene were selected to carry out the tests, being 10, 250, 1000 and 3000 mg.L-1 of the fractions AH and AP. All experiments were performed in duplicate.
3. Results and discussion 3.1. Chemical characterization FTIR spectra of samples shows three main peaks regions corresponding to aliphatic (2960 to 2850 cm-1), aromatic (1500 and 1450 cm-1 and 866 and 600 cm-1) and heteroatoms having a composition consistent with the expected for asphaltenes and resins, according to the literature7,8,23–26 as observed in Figure 2. The two fractions presented similar peaks.
Figure 2
The peaks in the region between 2920 and 2850 cm-1, correspond to the stretching vibrations in the CH2 and CH3 groups, and in the region between 1455 and 1375 cm-1, corresponding to the symmetric and asymmetric axial deformations of the CH3 groups. The peak present at 1602 cm-1 indicates the axial deformation of C=C and C=O of the aromatic ring. The peak absorption at 1030 cm-1 corresponds to heteroatoms such as O, N and S. Peaks between 870 and 810 cm-1 relate to out-of-plane deformation of the CH-bond ring in aromatic rings and the peaks in 746 cm-1 correspond to the vibrations of the four hydrogens adjacent to the aromatic ring25. Comparing the spectra of the AH and AP fractions we can observe more pronounced peaks in 1600 and 746 cm-1 corresponding to the assignments with aromatic nuclei for the AP fraction. In addition, the relative ratio between the peaks in 1600 and 2920 cm-1 and between 1660 and 2850cm-1 were slightly higher for the AH fraction (1600/2920 and 1600/2850 ratios were 0.174 and 0.244 for AH, and 0.159 and 0.243 for AP), indicating a higher number of aromatic groups in the AH fraction27. Table 1 presents the results of the elemental analysis in which the mass percentages of C, H, and N of the samples were determined. Results are similar to those found in the literature for asphaltenes and resins28–30. Carbon was the main element found in the fractions, along with smaller amounts of hydrogen and other heteroatoms (N and O).
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Table 1 The H/C ratio can be used to determine the aromatic character of the fractions, where the high H/C ratio indicates the presence of linear long chain aliphatic compounds, and a low H/C ratio suggests the presence of large polynuclear aromatic systems31,32. Propane insoluble fractions (AP) present a higher H/C ratio compared to the heptane insoluble fractions (AH), suggesting a lower aromaticity in this fraction and the presence of longer aliphatic chains in the sample24. The results obtained in this work corroborate with those observed by Yasar et al.33 where asphaltenes with higher aromaticity have more fused polynuclear aromatic rings (such as the AH fraction) and the less fragmented asphaltenes have less condensed aromatic rings with longer side chains in their molecular structure (such as the AP fraction). The results are also in agreement with that observed by Luo et al.8, where asphaltenic fractions precipitated with propane showed longer carbon side chains, which entails in their lower aromaticity. Such behavior may be associated with higher resins content in the sample having a higher number of aliphatic side chains attached to heteroatoms that generally have a highly polar comportment.34 The differences in the elemental analysis indicate that there is a difference in size and structure of asphaltenic fractions. To better understand the nature of this difference, the molecular weight of the AH and AP fractions was determined by GPC. Number average (Mn) molecular weight and Dispersity (Ɖ) obtained were 1676 g.mol-1 and 3.1 for AH fraction, and 1204 g.mol-1 and 1.9 for AP fraction. Fractions insoluble in heptane (AH) had a higher molecular weight than the fractions insoluble in propane (AP), confirming the structural differences between the fractions. The results corroborate with Luo et al.8 where precipitated asphaltenic fractions with propane presented a lower molecular weight due to the higher presence of resins in the fraction. Figure 3 show the thermograms of samples AH and AP, respectively. In the analysis of the AH fractions it is possible to highlight the presence of an endothermic peak around 100 °C, being the only peak present in the two heating cycles, that can be related to the melting point of the sample. In the analysis of the AP fractions, a very less pronounced endothermic peak around 100 °C it is observed in the two heating cycles, also an endothermic peak close to 320 ºC is most evident, being present only in the first heating cycle, and may be related to the volatilization of organic compounds in the sample.
Figure 3
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The fact of the melting peak at 100 ° C in AP sample shows a less pronounced behavior may be related to the greater amount of maltenes (especially resins) co-precipitated with AP asphaltenes. Kopsch35 stated that asphaltenes which are precipitated with the same solvent have relatively uniform heat of transition independently of the origin of the crude oil. In this work, from the DSC characterization of the asphaltenes samples originated from the same oil, but precipitated with different solvents, different thermal properties were observed in each fraction and these properties can be related to the differences structural of the samples.
3.2.
Surface characterization
Films of AH and AP fractions were formed on the glass substrates at different concentrations. Table 2 shows the contact angle values for each concentration. The contact angle of the clean glass substrate was 23.5 ± 0.3° and after 24 h submerged in pure toluene was 48.6 ± 1.6°. As observed by Zhang et al.31 the contact angle of a drop of water on a surface containing a monolayer of asphaltenes is approximately 90°, regardless of the origin of the asphaltenes. The substrates submerged in solutions with lower concentrations had lower contact angle values because the layer of asphalt fractions on the surface was thinner, with fewer clusters and larger openings in the films. For concentrations above 100 mg.L-1 the contact angle values do not differ greatly between the fractions due to the presence of the monolayer in the substrates. The AH fraction presented higher contact angle values than the AP fractions, mainly in lower concentrations such 1 mg.L-1 where there was a difference of up to 23º of the AP fraction for the AH fraction. Such behavior may be associated with the presence of resins in the AP fractions. Ese et al.34 observed that the addition of small amounts of resins may disturb the arrangement of asphaltenes films, giving a small opening of the structure, but the properties of the film are still dominated by the rigid structure of the asphaltene fraction. These apertures in the film allow greater exposure of the glass substrate surface causing a reduction in the contact angle. Nevertheless, it is possible to say that there is a high capacity of modification in the wettability of surfaces by the fractions AH and AP, even at low concentrations. Tables 3 and 4 presents the analyzes of the variance of contact angles average.
Tables 2 to 4
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From the statistical analysis it can be observed that the solvents used for the precipitations can influence the contact angle measurements independent of the concentration. The substrates containing the surface film of AP is relatively more hydrophilic than those with AH. Such behavior may be related to the surface arrangements of the film. Kumar et al.36 observed the influence of the height of interfacial film in the contact angle, where it was found that the higher the film the greater the contact angle. Such behavior was also observed in this work and is discussed later in the atomic force microscopy section. It is observed that the contact angle values increase with increasing concentrations of the asphaltenic fraction in the solution, independent of the solvents used for extraction. As observed in Table 4, a difference between the concentrations is statistically significant, but the results suggest that this influence is less stratified than all the concentration levels evaluated. By adding heptane to the system, a decrease in the contact angle was observed. This can be related to the opening of the film on the surface, caused by the agglomeration of the asphaltenes particles. This phenomenon induces a greater contact of the water droplet with the glass, which has a hydrophilic tendency, showing a behavior similar to the substrates submerged in solutions of toluene/asphaltenic fractions with lower concentrations. The atomic force microscope was used to observe the topography of the AH and AP films deposited on the substrates on a molecular scale, thus revealing the surface layer arrangements of the different asphaltenic fractions. The images were made at five different points of each substrate. The experiments were performed in duplicate for each concentration in order to guarantee a better representation of the existing film. All images were taken on a 20 x 20 μm scale. Figures 4 and 5 show the topographic images obtained by AFM of both films at concentrations of 1, 5, 10, 20, 50, 100, 500 and 1000 mg.L-1. From the AFM images of the AH and AP films it can be suggested that the asphaltenic fractions do not produce a homogeneous layer, but they form aggregates. The same behavior was observed by Balabin et al.37 and Syunyaev et al.38 in different surfaces analyzed, being this behavior related to the tendency of asphaltenes aggregation. In the images of the AH fraction (Figure 4) it can be verified that the fraction layer presents a network formation with larger clusters (lighter points) scattered randomly on the surface, being a standard behavior independent of the concentration. On the other hand, the AP fraction (Figure 5) presented an organization of lower clusters disposed evenly.
Figures 4 and 5
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Energy & Fuels
The presence of peaks and valleys in the substrates containing the asphaltenic fractions results in changes in surface roughness, but only in concentrations above 100 mg.L-1 it is more evidenced. This behavior is observed for both fractions as showed in Figure 6A. The films of the AH fractions showed a higher average height than the ones from the AP fractions, as observed in Figure 6B.
Figures 6
As observed by Soorghali et al.2 the presence of resins changes the way asphaltenes affect the surface, generally leaving the film more homogeneous and less rough. As observed in the characterization, the AP fractions may present a greater amount of co-precipitated resins that are responsible for the variations between the topographies of AH and AP. Compared to AH films, aggregates of AP films increased in number, but decreased in height, these results corroborate with those found by Ese et al.34, which evaluated the effect of resin addition on asphaltenes films and observed that in small to moderate amounts, the resins give a more polydispersive distribution of the material in the films. Thus, the results suggest that the AH fractions have stronger interactions asphaltenes-asphaltenes, which leads to the formation of larger aggregates and changes in the maximum height of the asphaltenic fractions. For the AP fraction, a more homogeneous surface is observed with smaller aggregates and consequently a smaller film height. These aspects can be attributed to the fact that the asphaltene-asphaltene interaction is weaker than the asphaltene-surface interaction. In order to better discuss the influence of the types of asphaltenic fractions on the roughness and maximum height of the surface films, regardless of concentration, a variance analysis was performed (Table 3). From the results it can be observed that the AH fractions form films with an average height 59% higher than the AP fractions, but the averages of roughness do not differ statistically by Tukey's test (p
