Microcalorimetry as a New Technique for Experimental Study of

Nov 27, 2013 - PETROBRAS/CENPES, Av. Horácio Macedo, 950, Cidade Universitária, 21941915, Rio de Janeiro, Brazil. ABSTRACT: Prediction of the potent...
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Microcalorimetry as a New Technique for Experimental Study of Solubility Parameters of Crude Oil and Asphaltenes Janaina I. S. Aguiar,*,† Maria S. E. Garreto,† Gaspar González,‡ Elizabete F. Lucas,† and Claudia R. E. Mansur*,† †

Federal University of Rio de Janeiro, Institute of Macromolecules, Laboratory of Macromolecules and Colloids for Petroleum Industry, Av. Horácio Macedo, 2030, Cidade Universitária, 21941598, Rio de Janeiro, RJ, Brazil ‡ PETROBRAS/CENPES, Av. Horácio Macedo, 950, Cidade Universitária, 21941915, Rio de Janeiro, Brazil ABSTRACT: Prediction of the potential for precipitation of asphaltenes is very important to the oil and gas industry. This precipitation can occur at various stages of oil production, during extraction from the rocky formation until refining. It is related to the variation of factors such as the crude oil composition and pressure. Incompatibility of oils coming from different streams can also cause undesired deposition of asphaltenes. Models to predict the stability of asphaltenes typically consider solubility parameters of the oil and its fractions, but as yet, no relatively simple experimental procedure has been presented. In this study, a relatively simple experimental procedure was developed relying on microcalorimetry and ultraviolet−visible spectroscopy (UV− vis) to determine these solubility parameters of two samples of crude oil and five of asphaltenes (four extracted from crude oils and one an asphalt residue). Similar results were obtained between these techniques, demonstrating the potential of using them for this type of analysis. Furthermore, the influence of the solvent in determining the solubility parameter of an asphaltene was studied.

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of crude oil.15−21 For this purpose, thermodynamic models have been created to describe the aggregative behavior of the asphaltene fractions, in an attempt to forecast the starting point for flocculation and/or formation of deposits during the extraction, transport, and processing of crude oil.8,19−23 Petroleum can be viewed as a multicomponent system, composed of asphaltene molecules dispersed in a mixture of various other components. From this perspective, the precipitation of asphaltenes in petroleum, related to their solubility, is also governed by the ratio between the solubility parameter of the asphaltene fraction and that of the rest of the system in which the asphaltenes are dispersed.9,10 However, it is difficult to measure the solubility parameter of asphaltenes due to their great molecular complexity and high molar mass. Because this parameter is directly determined by measuring the vaporization temperature, which is very high for asphaltenes due to their high molar mass, the molecules can be degraded before reaching the vaporization temperature, as also occurs with polymers.24−27 In the case of polymers, various experimental techniques are already used.28−30 These techniques are based on the principle known as “like dissolves like”;31 i.e., polymers will dissolve in solvents whose solubility parameters are not too different from their own.31 The solubility parameter values of polymers can be expressed as a single point or as a range of solubility parameter values. The microcalorimetry is a suitable technique to measure small quantities of heat. This term arose due to the high sensitivity of the developed sensors that can detect amounts of

INTRODUCTION Asphaltenes are by definition the fractions of petroleum not soluble in light hydrocarbons (e.g., pentane and heptane) but soluble in benzene and toluene.1 The equilibrium of phases of asphaltenes in crude oil is very complex and is not yet fully understood, despite research over the past 50 years, due to the problems caused mainly by asphaltenes during petroleum production and processing. The aggregative nature and stability of asphaltenes have been found to be related to their interfacial and colloidal properties2−4 as well as the solubility parameter (δ) of the medium in which these fractions are dispersed and/ or dissolved, mainly in studies with model solvents.5−10 The oil phase behavior is complex because of the many types of molecules present and because petroleum presents some properties of colloidal dispersions and other of solutions.11 The stability of asphaltenes in crude oil is not yet understood well enough to be able to establish more efficient methods to minimize the effects of the formation of deposits. Measurement of the solubility parameter of petroleum has been indicated as a way to determine the stability of asphaltenes. At present, the most common experimental method to calculate this solubility parameter is by the addition of an asphaltene flocculant in the oil sample until their precipitation, determined by microscopy, and then applying an equation that correlates the volumetric quantities in the tests with the solubility parameter of the flocculant.12−14 With the addition of large volumes of a nonsolvent to the system containing asphaltenes, the mass fraction of the precipitated asphaltenes increases due to the increase in the difference of the solubility parameter between the asphaltenes and the solvent.15 In recent years, the solubility parameter has been widely applied in studies focused on the petroleum industry to correlate and predict the stability of the components © 2013 American Chemical Society

Received: June 6, 2013 Revised: November 27, 2013 Published: November 27, 2013 409

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Table 1. Characteristics of Oil A and Oil B10 petroleum samples

density (g/ml)

° API

saturates (wt %)

aromatics (wt %)

resins (wt %)

asphaltenes (wt %)

A B

0.934 0.974

19.4 13.2

40.2 35.7

33.3 24.6

23.4 32.4

3.1 7.3

heat flow in the range of microwatts. This technique is applied in various fields, such as biological,32 chemistry,33 and petroleum.34 More specifically, on the study of asphaltenes, this technique was used to evaluate the interaction of these fractions with other compounds, such as nonylphenol and toluene, the influence of the water on the aggregate, and other studies.35−38 The aim of this study was to determine the solubility parameter of asphaltenes and crude oils using a principle similar to that for determining the solubility parameter of polymers, but applying the microcalorimetry technique. The results obtained were in agreement with those determined by ultraviolet spectroscopy, demonstrating the potential of microcalorimetry to study solubility parameters.

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Determination of the Solubility Parameter Using Ultraviolet−Visible Spectroscopy (UV−vis). Mixtures of n-hexane/ toluene and toluene/ethanol were prepared in different proportions to obtain solubility parameters from 14.9 MPa1/2 (pure n-hexane) to 26.2 MPa1/2 (pure ethanol) and used for all asphaltenes and oil samples. For asphaltenes C7-AR, mixtures of toluene/dimethyl sulfoxide were also used, covering the solubility parameter range from 18.2 to 26.7 MPa1/2. The solubility parameter values of the solvent mixtures were calculated from the weighted average of the solubility parameters of the pure solvents in relation to the volumetric fraction of each solvent in the mixture.45 This same calculation has been used by authors in previous studies.9 These media from one point to another on average by 0.17 MPa1/2 in the range of the n-hexane/toluene mixture and 0.4 MPa1/2 in the range of the toluene/ethanol mixture. Portions of each sample (1.0 g of the oil samples and 0.010 g in the case of the asphaltenes) and the solvent (5 mL) were placed in individual test tubes. The tubes were closed and placed in a thermostatic shaker at a temperature of 26 °C and an agitation of 150 cycles per minute for 2 h. Then the samples were centrifuged for 30 min at 1500 rpm for complete sedimentation of the precipitated particles. The absorption intensity of the supernatant was measured in a Varian Cary 50 UV−vis spectrophotometer operated at 850 nm, with a 2 mm optical path. The wavelength of 850 nm was selected according to the results obtained in a previous study,46 which showed that, at longer or shorter wavelengths, the absorbance values fell below the instrument’s scale or suffered deviation from the Lambert−Beer law.47 To confirm that the technique is effective to determine the range of the solubility parameter of macromolecules, a sample of poly(methyl methacrylate) (PMMA) was analyzed using the methodology employed in this article excepting by the mass: 0.100 g was weighed into each tube. All the analyses were performed in triplicate, and the respective standard deviations were below 0.05. The results were plotted in graphs of absorption intensity of supernatants as a function of the solvent’s solubility parameter. Determination of the Solubility Parameter Using Microcalorimetry. For this method, a Setaram μDSC III instrument was used, equipped with two mixture cells (Figure 1), one for reference, which remained empty in all the tests, and the other for the sample of interest. Each cell has an upper and lower compartment, which are interconnected during the analysis. Before the microcalorimetry analyses, two standard procedures were carried out: drying of the instrument and verification of the baseline. These procedures were repeated at intervals of at most 15 days.

MATERIALS AND METHODS

A sample of asphalt residue, called AR, and two crude oil samples, called Oil A and Oil B were supplied by Petrobras (Brazil). The contents of saturates, aromatics, resins, and asphaltenes (SARA) of each oil were determined in the PETROBRAS, by chromatographic analysis (ASTM D5186)39 and method IP-143/8440 (Table 1).10 The ́ solvents supplied by Vetec Quimica Fina (Brazil) were used, and their purities and solubility parameters are shown in Table 2. Poly(methyl methacrylate) was supplied by LMCP/IMA/UFRJ, Rio de Janeiro, with ⟨Mn⟩ = 277 039 g/mol and ⟨Mw⟩ = 891 071 g/mol.

Table 2. Hansen Solubility Parameters of Solvents41 Hansen solubility parameters (MPa1/2) solvents

purity

δ

δD

δP

δH

cyclohexane dimethyl sulfoxide ethanol n-heptane n-hexane n-pentane toluene

99.0 99.9 95.0 99.0 99.0 99.0 99.5

16.8 26.7 26.5 15.3 14.9 14.5 18.2

16.8 18.4 15.8 15.3 14.9 14.5 18.0

0.0 16.4 8.8 0.0 0.0 0.0 1.4

0.2 10.2 19.4 0.0 0.0 0.0 2.0

Extraction of Asphaltenes from the Crude Oils. The asphaltene fractions were extracted from each of the crude oil samples, called C5A and C7A (extracted from Oil A with n-pentane and nheptane, respectively) and C5B and C7B (extracted from Oil B with npentane and n-heptane, respectively). Only one fraction was extracted from the asphalt residue, denoted C7AR, employing n-heptane. The asphaltenes were separated by precipitation induced by the addition of the excess flocculant (n-alkane) in a Soxhlet extractor, based on the IP 143 method.42 The reason asphaltene/flocculant used was 30 g/1 L, and the reflux was done until the reflux solvent became clear. Then toluene was used to extract the asphaltenes contained in the precipitate using the Soxhlet extractor until the reflux solvent was clear. Finally, the asphaltenes in solution were recovered after evaporation of the toluene in an IKA RV05 basic rotary evaporator. Characterization of the Asphaltenes. The asphaltene samples C7A, C7B, and C7AR were characterized regarding content of carbon, hydrogen, nitrogen, and oxygen (CHN) with a Thermo Finnigan EA Flash 1112 Series analyzer, based on the ASTM D5291-02 method.43 The oxygen content was determined indirectly by subtracting from 100% the percentages of C, H, N, and sulfur, the last of which was measured in a LECO SC 632 sulfur analyzer coupled to an SC 632 automatic sample loader, based on the ASTM D1552-03 method.44

Figure 1. Scheme of mixture cells of the μDSC III Setaram microcalorimeter. 410

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To validate the microcalorimetry method for mixtures of substances by use of mixture cells, the solubilization heat of a sample of KCl48 and the dilution heat of a sample of sucrose, both published in the literature,49 were calculated. To determine the solubilization heat, 10 mg of KCl was placed in the lower compartment and 150 μL water was put in the upper compartment of the mixture cell. During the isothermal test, at 26 °C for 30 min, the KCl sample was dissolved in water by opening the upper compartment. At this moment, the instrument detects the heat involved in the mixture of the samples and that from movement of the piston. Therefore, to determine the heat involved only in the solubilization process of the sample, after the heat flux stabilizes, the upper compartment, which is now empty, is opened again. In this way, the heat involved in dissolving the sample is obtained from the heat generated from the first movement of the piston subtracted from the heat involved in the second piston movement. All the heat values generated during the analyses were calculated by integrating the area under the heat-flux peaks that are formed during the events. The experimental procedure is essentially the same to determine the dilution heat. In this case, 30 μL of a solution of 25% p/v of sucrose was placed in the lower compartment while 300 μL of water was placed in the upper compartment of the mixture cell. To measure the heat involved between the oil/asphaltene samples and the solvent systems, the same procedure described above was used, with the exception of the temperature and isotherm time, which were 25 °C and 25 min, respectively. In the tests with asphaltenes, 1 mg of sample was placed in the lower compartment, and in the tests with the oil samples, this was 32 mg. In both cases, 100 μL of the solvent system was placed in the upper compartment with the aid of a syringe. The solvent systems were the same as those used in the UV− vis technique, except for the analysis of the C7AR sample, in which mixtures of n-heptane/cyclohexane, cyclohexane/toluene, and toluene/ethanol were used to obtain media with solubility parameters ranging from 15.3 MPa1/2 (for pure n-heptane) to 26.2 MPa1/2. These media differed from one point to the other by an average of 0.8 MPa1/2 in the range of the cyclohexane/toluene mixture and 0.4 MPa1/2 in the range of the toluene/ethanol mixture. After performing each test at least three times, the results were plotted on a graph of the median of the interaction enthalpy (J/g) versus the solubility parameter of the solvent system (MPa1/2), with the respective standard deviations of the measurements.

fractions obtained from these two crude oil samples have almost equal aromaticity values. In relation to samples C5A and C5B, it can be inferred that they are less aromatic than samples C7A and C7B, because it has been widely reported in the literature that a larger quantity of asphaltenes with a lower molar mass and polarity precipitate out when the flocculant’s chain is smaller.50−52 Determination of the Solubility Parameter Using Ultraviolet−Visible Spectroscopy (UV−vis). In the spectroscopic analyses, the solubility parameter was measured based on the principle that the higher interaction between solute and solvent, the higher the absorption intensity. To validate the experiments carried out with asphaltenes, the solubility parameter of a polymer sample was determined and compared with data already published in the literature. Figure 2 shows the

Figure 2. Absorption intensity versus solubility parameter of the solvent added to PMMA, at 850 nm.

results of absorption intensity as a function of solubility parameter of the solvent system, for poly(methyl methacrylate) (PMMA). It is clear that the highest absorption intensities were detected for the solvents presenting solubility parameters of 18.6 and 19.0 MPa1/2, which is in good agreement with the values already reported in the literature (18−26 and 17−27 MPa1/2).53 The general principle can be applied to the asphaltenes and oils; that is, the interaction was measured based on the principle that the more miscible an oil sample is (and the more dispersed the asphaltene fractions are), the more opaque the solution will be, a characteristic caused by the dark color of these substances. Therefore, it is reasonable to expect that the greater the interaction of the solvent system is with the oil and/or asphaltenes, the more intense will be the absorption of the resulting solution/dispersion. Figure 3 presents the graph of the absorption intensity as a function of the solubility parameter (δ) of the solvent system for Oil B and asphaltenes C7B and C5B. It can be seen that, as expected, there is a tendency for increased absorption intensity

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RESULTS AND DISCUSSION Characterization of the Asphaltene Samples. The chemical characteristics of the asphaltenes are reported in Table 3. Table 3. Chemical Characteristics of the Asphaltenes Extracted with n-Heptane (C7) samples elements

C7AR (asphalt residue)

C7A (Oil A)

C7B (Oil B)

total carbon (% w/w) total hydrogen (% w/w) oxygen (% w/w) nitrogen (% w/w) sulfur (% w/w) molar ratio (H/C)

83.3 7.4 1.2 0.6 7.5 1.07

86 8.8 2.2 1.8 1.2 1.23

88.1 8.8 1.3 1.1 0.7 1.20

The H/C molar ratio indicates that the C7AR sample is more aromatic than the C7B and C7A samples, which have very similar aromaticity characteristics. This indicates that the concentration of asphaltenes contained in the petroleum is not related to the average aromaticity of the molecules that compose a determined fraction, since Oil B has a higher content of asphaltenes than Oil A does and the asphaltene

Figure 3. Absorption intensity versus solubility parameter of the solvent added to the Oil B, asphaltenes-C7B, and asphaltenes-C5B, at 850 nm. 411

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to the also high dispersion of the types of molecules that compose a petroleum sample. Comparison of the solubility parameter ranges identified for these three samples shows that the range for the crude oil encompasses the entire range of sample C5B, which, in turn, includes the entire range of sample C7B. This is to be expected, since, in this order, one fraction is contained in the other. It is curious to note that the solubility ranges of these three samples start at values near the solubility parameter, while the upper limit significantly differs, especially for the petroleum sample in relation to the two asphaltene fractions. This behavior suggests that the fraction that precipitates at the lower solubility parameters is really the one that contains more polar molecules and higher molar mass, and that this fraction is present in all three samples analyzed. The solubility of the crude oil observed at a higher solubility parameter can be attributed to the interactions of the various molecules that compose the oil but are not present in the extracted asphaltene fractions. Figure 4 shows the graph of the absorption intensity as a function of the solubility parameter for sample C7AR. Note

with rising solubility parameter of the solvent system until a certain point, after which there is a decrease in this intensity as the solubility parameter values increase further. This behavior reflects the stronger interaction of the solute with the solvent that has a solubility parameter equal to or similar to that of the solute. For sample C7B, a fraction with a narrower polarity, the absorption intensity remains constant at higher values before starting to decay. This range of greater interaction with sample C7B was observed at solubility parameters between 17.1 and 19.8 MPa1/2. The absence of a single maximum absorption intensity point indicates the polydispersed character of the sample in relation to the polarity. For sample C5B, the plateau of maximum intensity is not as well-defined as that for sample C7B and the solubility parameter range at which the absorption intensities are highest is between 16.4 and 20.2 MPa1/2. This range is wider than that observed for sample C7B, which can be explained by the fact that sample C5B has a fraction of less polar molecules, which start to dissolve at lower solubility parameter values. Although molecules of high polarity are present in the two fractions,51 the upper limit of the solubility parameter of fraction C5B is slightly greater than that of fraction C7B, indicating that the less polar molecules present in fraction C5B help dissolve the molecules with greater polarity in solvent systems with higher solubility parameter values. The fluctuation of the absorption intensities can be related to the process of dissociation of the asphaltene aggregates. The asphaltene molecules are initially aggregated and precipitated, not causing an increase in the absorption intensity of the solution. As the solubility parameter of the solvent medium increases (up to ∼16.6 MPa1/2), some of these aggregates tend to swell and are solvated by the medium, i.e., become more soluble, although still with relatively large sizes, which leads to a relatively high absorption intensity. As the solubility parameter of the medium increases even more (∼17.3−19.5 MPa1/2), these aggregates tend to dissociate, and consequently decrease in size, causing the reduction observed in the absorption intensity of the asphaltene dispersion. However, at still higher solubility parameter values (∼20.0 MPa1/2), the solubility of these molecules is once again disfavored, so that the system returns to the previous point at which there are aggregates that are still solvated and dispersed in the medium, causing the absorption intensity to increase. In solvent systems with solubility parameters above 20.5 MPa1/2, the medium is not able to keep these aggregates solvated, so precipitation can be observed. This behavior is coherent with other findings in the literature,54 reporting the detection of larger asphaltene aggregates just before the onset of precipitation of the asphaltenes in model solvent systems. The fact that this fluctuation of absorption intensities was not observed for sample C7B suggests that the more aromatic fractions, with a narrower solubility range (because they do not have the other molecules to promote stabilization), aggregate more quickly, possibly because they interact with each other, making it easier for a sheetlike aggregation to occur, as proposed by Stachowiak et al.55 The fluctuation of the absorption intensities as a function of the solubility parameters of the solvent system was similar to that observed for asphaltene fraction C5B, but even more pronounced. The solubility parameter range at which the petroleum was soluble varied from 16.6 to 24.0 MPa1/2. The high dispersion of the absorption intensity values can be related

Figure 4. Absorption intensity versus solubility parameter of the solvent added to the asphaltenes-C7AR, at 850 nm.

that the behavior of this sample is similar to that of C7B, in relation to both the shape of the curve and the solubility parameter range, which is 17.2−19.0 MPa1/2. The only difference is that the upper limit of sample C7B is slightly higher (19.8 MPa1/2) than that of sample C7AR, suggesting that C7B contains molecules with greater polydisperson in relation to solubility. The behavior of Oil A and asphaltene sample C7A (Figure 5) is similar to that of Oil B and asphaltene sample C7B (Figure 3) in terms of the shape of the curves of absorption intensity versus solubility parameter of the solvent system and in terms of the relative breadth of the ranges found for the three samples; i.e., the range of the oil encompasses that of sample C5, which, in turn, includes the range of sample C7. The

Figure 5. Absorption intensity versus solubility parameter of the solvent added to the Oil A, asphaltenes-C7A, and asphaltenes-C5A, at 850 nm. 412

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process itself and the heat from the movement of the mixture cell’s piston, whereas the second peak only refers to the heat involved in the piston movement. Therefore, the heat involved in the process under study is determined by the difference between the two heat values determined for the first and second peaks. To validate the experimental procedure, we first determined the solubilization heat of KCl in water and the dilution heat of sucrose, which can be found in the literature.48,49 The solubilization assay of the KCl presented ΔH1 = 209.9392 J/ g (referring to the first peak) and ΔH2 = −19.8944 J/g (second peak). The solubilization heat of KCl in water can be calculated as ΔHsolub = ΔH1 − ΔH2 = 230 J/g. This value found is very near that cited in the literature (235 J/g).48 This test was performed in triplicate, and the standard deviation of the measurements was 2 J/g and the accuracy was 2%. We applied the same procedure and calculation method to the dilution heat of sucrose and found ΔHdilution = 1.2019 J/g, which is also very near the value described in the literature (1.4620 J/g).49 In this case, the standard deviation was 0.06 J/g and the accuracy was 17.8%. These results validate the proposed method, which was applied to study the samples of crude oil and asphaltenes. The solubilization heat of the asphaltenes and crude oil in the various solvent systems was measured to determine the solubility parameter range of these compounds. In this case, the solubility parameter of the material will be near that of the solvent in which the solute presents the strongest interaction, that is, the highest solubilization heat value. The solubilization heat results were plotted on graphs of the solubilization enthalpy as a function of the solubility parameter of the solvent. Figure 8 presents the graphs obtained for Oil B

solubility parameter ranges found for these samples were the following: Oil A: 15.9−21.7 MPa1/2; fraction C5A: 16.5−21.2 MPa1/2; and fraction C7A: 16.9−20.4 MPa1/2. The asphaltene fractions extracted from Oil A thus have a slightly higher upper solubility parameter limit than the fractions extracted from Oil B. This suggests that the fractions C5A and C7A have slightly more polar molecules than do fractions C5B and C7B, which remain soluble in solvents with a higher solubility parameter. This result is in agreement with the chemical characterization of fractions C7A and C7B presented in Table 3. These fractions have very similar aromaticities, but sample C7A contains marginally higher levels of oxygen, nitrogen, and sulfur than C7B, contributing to the increase of the polarity of sample C7A. To investigate the influence of the solvent presenting a higher solubility parameter (ethanol), which contains a high contribution of the hydrogen- bonding component (19.4 MPa1/2) (Table 2), the same experiment was carried out for asphaltenes-C7AR using dimethyl sulfoxide instead ethanol, since it presents a lower contribution of the hydrogen-bonding component (10.2 MPa1/2) (Table 2). The results are shown in Figure 6, and a very similar range of solubility parameter values

Figure 6. Absorption intensity versus solubility parameter of the solvent (toluene−DMSO) added to the asphaltenes-C7AR, at 850 nm.

(17.21−19.05 MPa1/2) is observed than that obtained from Figure 4, using ethanol in the solvent mixture (17.20−19.00 MPa1/2), evidencing that, in this case, the large difference between hydrogen-bonding components of these two solvents does not affect the final results of the solubility parameter range. Determination of the Solubility Parameter Using Microcalorimetry. Figure 7 contains two curves, the heat flux (solid line) and temperature (dotted line), both as a function of the measurement time of a typical analysis employing this method. As mentioned in the experimental part, the first peak refers to the heat involved in the mixing

Figure 8. Solubilizing enthalpy (J/g) versus solubility parameter of the solvent added to the asphaltenes-C5B (black square), asphaltenes-C7B (gray circle), and Oil B (black line), at 25 °C and 1 atm. The double arrow shows a range of the solubility parameter of asphaltenes-C5B.

and its asphaltene fractions (C5B and C7B). Just as observed in the UV−vis spectroscopy tests, microcalorimetry allowed the determination of a solubility parameter range for each sample, i.e., an interval of greatest solubilization enthalpy. The samples that showed a range of the solubility parameter started with a low solubilizing enthalpy related to a low solubility parameter. Their enthalpies of solubilization were increasing and remained constant during their range of the solubility parameter and after the enthalpy of solubilization decreases. The double arrow, in Figure 8, shows the range of the solubility parameter of asphaltenes-C5B. The results for all the samples are presented in Table 4, together with the values found by UV−vis spectroscopy, for comparison. It can be seen that, although the results found by both techniques are not exactly the same, they are very close.

Figure 7. Typical result of analysis using the microcalorimeter according to the methodology developed. 413

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i.e., they do not consider their polydispersity, which can cause inaccuracy.59 Andersen and Speight60 already discussed that the ideal thermodynamic models for asphaltene solubility and precipitation should contain a range of solubility parameter, and the microcalorimetry technique was effective and economical for investigating this range of the solubility parameter. Table 5 presents the maximum solubilization enthalpy and absorption intensity values obtained for each sample. These values also agree between the two procedures. Comparison of the asphaltene samples from a single oil type (for example, Oil B) shows that the highest solubilization enthalpy (33.9365 J/g) is associated with the lowest absorption intensity (0.20) and vice versa. This behavior could be related to the fact that the solubilization process is endothermic and a higher solubilization enthalpy reflects a greater difficulty to dissolve the sample. This greater difficulty to dissolve the sample could be associated with the stronger intramolecular interaction in detriment to the solute−solvent interaction, reflected in a lower maximum absorption intensity value. However, by this reasoning, the difference in solubility of each of these fractions should lead to an opposite result than that obtained; i.e., the solubilization enthalpy of the C5 asphaltenes should be lower than that of the C7 asphaltenes because the C5 asphaltenes are easier to dissolve than the C7 ones. In reality, the results obtained reflect the aggregation characteristic of the asphaltene molecules. The C5 asphaltene fraction, due to its greater polydispersion in relation to solubility (broader solubility parameter range), has a smaller percentage by mass of molecules that remain aggregated, leaving a larger amount of material to dissolve, reflected in higher solubilization enthalpy. This behavior also explains the higher absorption intensity value found for the C7 fraction, because this sample has a stronger tendency to form dispersed aggregates, which are responsible for the increase in the absorption intensity. This reasoning is confirmed by comparing the maximum enthalpy values for the C7 asphaltene fractions extracted from different sources (C7AR, C7A, and C7B). The enthalpy value is highest for the sample (C7A from Table 5) with the least tendency to remain aggregated, i.e., the sample with the highest polydisperson in relation to solubility (C7A from Table 4). This behavior is in agreement with the results presented in Table 3, which show that sample C7A, with the highest concentrations of oxygen and nitrogen, is the one with the most polar molecules, with consequent higher polydispersion in relation to solubility. Between samples C7B and C7AR, C7B has the highest polydisperson in relation to solubility because it contains slightly more polar molecules. In this case, only the level of nitrogen present is contributing to the greater polarity, since C7B has an equal oxygen content, lower sulfur content, and lower aromaticity (higher H/C ratio). Although the level of

Table 4. Ranges of the Solubility Parameter of Oil and Asphaltene Fractions Determined by the Techniques of Microcalorimetry and UV−vis solubility parameter, δ (MPa1/2) microcalorimetry

UV−vis

sample

LIa

LSb

LIa

LSb

asphaltenes-C7B asphaltenes-C5B Oil B asphaltenes-C7A asphaltenes-C5A Oil A asphaltenes-C7AR

17.9 17.5 17.5 19.1 17.9 17.9 18.2

22.3 23.0 24.6 23.8 23.8

17.1 16.4 16.6 16.9 16.5 15.9 17.2

19.8 20.2 24.0 20.4 21.0 21.7 19.0

22.5

a

Lower limit of the range of solubility parameter. bUpper limit of the range of solubility parameter.

There is only a slight shift of the solubility parameter (lower and upper limits) to higher values. Therefore, the explanations for the differences between the solubility parameter ranges found for each sample are essentially the same as those discussed for the results obtained by UV−vis spectroscopy. Furthermore, the range of solubility parameters determined by microcalorimetry was higher than the range determined by UV−vis. This could be related to the higher sensitivity of the technique of microcalorimetry that is capable of detecting the interactions between the asphaltenes and the solvents that the UV−vis technique cannot detect. The standard deviations of all the analyses were very low. The standard deviation of the asphaltenes-C5B, asphaltenesC7B, and Oil B, by microcalorimetry, can be seen in Figure 8; however, they are small and the standard deviation cannot be seen at all points. The average standard deviation for all points made by microcalorimetry was 1.2. The values found in this work are concordant with the values of onset of precipitation of asphaltenes obtained in previous studies,10,13,14,56 because the values previously found are near the lower limit of the range of solubility parameter. Moreover, in another study,57 the range of the solubility parameter of asphaltenes was determined and it comprised between 15.5 and 22.3 MPa1/2, and these values are in agreement with those obtained in this work. Note that, although some previous works attribute a single value of the solubility parameter for the asphaltenes and the oils,10,13,14,18 asphaltenes constitute a family of molecules having different molar masses, structures, and contents of heteroatoms,50 and therefore, it is expected to have a range of solubility parameter values, as previously proposed.57,58 It is important to note that models currently used to predict the precipitation do not consider a range of solubility parameter;

Table 5. Maximum Solubilization Enthalpy and Absorption Intensity Values Obtained for Each Sample sample

maximum enthalpy (J/g)

maximum absorption intensiy

asphaltenes-C7AR asphaltenes-C7A asphaltenes-C5A Oil A asphaltenes-C7B asphaltenes-C5B Oil B

16.4232 23.3513 29.1449 5.8793 21.4124 33.9365 6.3823

0.29 1.32 0.51 2.55 0.40 0.31 0.79

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sulfur in C7AR is nearly 11 times greater than that in C7B while the nitrogen content is only 2-fold that in C7B, nitrogen has a greater contribution to the polarity than sulfur, according to the Pauling electronegativity scale (oxygen: 3.5; nitrogen: 3.0; sulfur: 2.5).61 Therefore, it can be said that sample C7AR has a greater tendency to form aggregates because it has lower polydispersion in relation to solubility and also has higher aromaticity (Table 3), facilitating the formation of sheetlike aggregates.55

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CONCLUSIONS By means of absorption spectroscopy in the ultraviolet−visible (UV−vis) region and microcalorimetry (μDSC), it was possible to determine the solubility parameter range of different samples of asphaltenes and crude oils. The results obtained by both techniques are not exactly the same, but they were very close, with only a small shift in the solubility parameters (lower and upper limits) to higher values when using the μDSC technique. The solubility parameter ranges were also coherent with certain punctual values of the solubility parameter determined previously.10,13,14,56 The results obtained by UV−vis and μDSC were also concordant with respect to the solubilization enthalpy and absorption intensity values, as expected for a system with a tendency to form aggregates, as is the case of asphaltene fractions. From a technical standpoint, UV−vis and μDSC can be considered equally effective, although μDSC has the advantage of requiring a quantity of sample 10 times lower than that needed for analysis by UV−vis. From the results obtained, it can be suggested that microcalorimetry is more sensitive than UV−vis spectrometry and is able to detect a broader solubility parameter range.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank CNPq, CAPES, FAPERJ, ANP, and PETROBRAS for the financial support.

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