Asphaltene Aggregation Behavior in Bromobenzene Determined By

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Asphaltene Aggregation Behavior in Bromobenzene Determined By Small-angle X‑ray Scattering Masato Morimoto,*,† Hiroshi Imamura,‡ Satoshi Shibuta,‡ Takeshi Morita,‡ Keiko Nishikawa,‡ Hideki Yamamoto,§ Ryuzo Tanaka,∥ and Toshimasa Takanohashi† †

Research Institute of Energy Frontier, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan ‡ Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan § Department of Chemical, Energy and Environmental Engineering Faculty of Environmental and Urban Engineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan ∥ Advanced Technology and Research Institute, Japan Petroleum Energy Center, 1-4-10 Ohnodai, Midori-ku, Chiba-shi, Chiba 267-0056, Japan S Supporting Information *

ABSTRACT: Small-angle X-ray scattering (SAXS) analyses of an asphaltene (a heptane-insoluble fraction in Canadian oil sand bitumen (CaAs)) at various concentrations in bromobenzene (BB) were performed at a synchrotron facility. BB is the first trial medium in which the aggregation behavior of asphaltenes has been elucidated, and is considered to be one of the “best” pure solvents for CaAs when determining the Hansen solubility parameters (HSP). Although the aggregation behavior of the CaAs in toluene (TL) and toluene−pentane mixed solvent (TL-PT10, containing 10% pentane on a volume basis) was confirmed to be similar to that reported in previous SAXS studies, the behavior in BB was markedly different. The results indicated that aggregates with a soft boundary of ∼30−60 Å in the radius of gyration (Rg), which were observable in TL and TL-PT10, disappeared in BB and larger aggregates with a clear boundary appeared simultaneously. This phenomenon supported a colloidal aggregation model, with HSP analyses suggesting that BB dispersed the colloid surface fraction at the molecular level and isolated the colloid core fraction, which led to the formation of a rigid aggregation of the core fraction. The HSP analyses enabled us to evaluate the aggregation behavior quantitatively, and the results obtained by SAXS were consistent with those obtained by Rayleigh scattering that we reported previously. parameters (HSP), δd, δp, and δh, which correspond to the dispersive, polar, and hydrogen-bonding forces in the cohesive energy density, respectively.22 Theoretically, any compound has a unique set of HSP values, and two compounds having similar values typically show a good affinity for each other. When the HSP difference between solute and solvent, Δδ, is small, Δδ is given by

1. INTRODUCTION Asphaltene is an undesirable component of petroleum, which causes fouling and catalyst deactivation in petroleum recovery and conversion processes.1,2 Asphaltene is generally defined as a heptane-insoluble fraction, and consists of a complex mixture of many polycyclic aromatic hydrocarbons, including not only carbon and hydrogen but also heteroatoms, such as sulfur, nitrogen, oxygen, nickel, and vanadium. Because asphaltene exists as aggregates in petroleum and organic solvents, it is essential to understand its aggregation behavior to utilize petroleum and heavy crude effectively. Small-angle X-ray/neutron scattering (SAXS/SANS) has been used for the analysis of asphaltene aggregates for a long time, and provides structural information for the aggregates in different solvents, such as mineral oil,3 crude oil,4−7 cisdecahydronaphthalene,3 benzene,8−10 decalin,8,11 tetrahydrofuran (THF),9,12 pyridine,9 quinoline,11 1-methylnaphethalene (1MN),11,12 toluene (TL), 12−20 and TL−heptane mixed solvent.7,21 Most studies that have measured the aggregate size have reported a radius of about 20−200 Å at room temperature, depending on the type of solvent and the concentration. In general, asphaltene is prone to form smaller aggregates at lower concentrations in a good solvent. Considering the influence of the solvent on aggregation, the solvent power can be evaluated using the Hansen solubility © 2015 American Chemical Society

Δδ 2 = 4(δd,solvent − δd,solute)2 + (δp,solvent − δp,solute)2 + (δ h,solvent − δ h,solute)2

(1)

We successfully determined the HSP of a type of asphaltene, a heptane-insoluble fraction in Canadian oil sand bitumen (CaAs). The HSPs, δd, δp, and δh, were 19.1, 4.2, and 4.4 MPa0.5, respectively.23 Table 1 lists the HSP values of organic solvents and the Δδ against CaAs. The solvents used in previous SAXS/SANS studies have Δδ values ranging from 3.4 (quinoline) to 6.8 MPa0.5 (cis-decahydronaphthalene). The Δδ for TL and maltene are almost identical at ∼4 MPa0.5, with the result that asphaltene aggregation behavior in crude oil is similar to that in TL.5 Through HSP analyses, bromobenzene Received: July 1, 2015 Revised: August 20, 2015 Published: August 21, 2015 5737

DOI: 10.1021/acs.energyfuels.5b01491 Energy Fuels 2015, 29, 5737−5743

Article

Energy & Fuels Table 1. HSPs of Asphaltene and Solventsa

2.3. SAXS Data Treatment. The absorption correction of each raw SAXS intensity, I(s), was conducted using the X-ray transmittance of each solvent, and absolutized using a scattering of water as a standard for each experimental setup. The Supporting Information provides the details of the data treatment. The absolute intensity, I(s)abs, was standardized using the electron density difference between asphaltene and each solvent (Δρ), to compare all the SAXS profiles. The plot of I(s)abs Δρ−2 against s was the SAXS profile analyzed in this study. The analytical results of CaAs were used for the data treatment. The elemental compositions were: C, 81.3; H, 7.2; N, 1.3; S, 8.1; O, 1.5; Ni, 0.037; V, 0.100; and others, 0.46 wt %. The number-average molecular weight was 775 g/mol, determined by our method29 using a gel permeation chromatograph (GPC: Gulliver series, JASCO, Tokyo, Japan) with a column (Mixed-D: exclusive limit of 4 × 105 amu, Varian Inc., Palo Alto, CA, U.S.A.) and a refractive index detector (RI-104, Shodex, Tokyo Japan). The weight density of CaAs, ρw, was taken as 1.17 g/cm3 from our previous report,11 because X-ray absorption measurements for CaAs give consistent values. As a result, the electron density of CaAs was estimated to be 1.06 × 10−5 Å−2, and the Δρ2 for TL, TL-PT10, and BB were 6.94 × 10−12, 7.93 × 10−12, and 2.91 × 10−12 Å−4, respectively.

HSP/MPa0.5 CaAs BB quinoline 1-MN maltene TL TL-PT10 pyridine benzene THF decalin (cis) cis-decahydronaphthalene heptane PT

δd

δp

δh

Δδ to CaAs

19.1 19.2 20.5 19.7 17.7 18.0 17.7 19.0 18.4 16.8 18.0 17.6 15.3 14.5

4.2 5.5 5.6 0.8 5.8 1.4 1.3 8.8 0.0 5.7 0.0 0.0 0.0 0.0

4.4 4.1 5.7 4.7 2.5 2.0 1.8 5.9 2.0 8.0 0.0 0.0 0.0 0.0

1.3 3.4 3.6 3.7 4.3 4.8 4.8 5.0 6.0 6.5 6.8 9.7 11.0

a

CaAs: heptane-insoluble asphaltene in Canadian oil sand bitumen. TL: toluene. BB: bromobenzene. PT: pentane. TL-PT10: TL-PT mixed solvents containing 10% PT by volume. 1-MN: 1-methylnaphthalene. Maltene: heptane-soluble fraction in Venezuelan bitumen.25

3. RESULTS AND DISCUSSION Figure 1 shows all of the SAXS profiles of CaAs in BB, TL, and TL-PT10. The profiles of CaAs in BB are very different from

(BB) is considered to be one of the “best” pure solvents, with a small Δδ of 1.3 MPa0.5. Our recent study, using a visible light scattering method, suggested that the degree of asphaltene aggregation decreases substantially in BB compared to that in TL.24 There is a strong demand to investigate the aggregation/ dispersion behavior of asphaltenes in such a good solvent using the SAXS/SANS technique, because it has not been performed previously. This paper presents the first result of SAXS analyses of asphaltene in BB. First, the SAXS analytical results using TL and TL−pentane mixed solvents are shown for consistency with previous SAXS studies of asphaltene aggregation. Finally, the validities of some conventional aggregation models are discussed using the asphaltene aggregation behavior in BB.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The asphaltene and the sample preparation method are as in our previous study.24 CaAs was dispersed in BB, TL, and TL−pentane mixed solvents containing 10% pentane by volume (TL-PT10) at concentrations of 20, 500, 1000, 10 000, and 100 000 mg/L. The solutions were shaken by hand, ultrasonicated for 5 min, and then allowed to stand for a couple of days. Then, after a second 5 min ultrasonication, SAXS measurements were performed within 60 min. 2.2. SAXS Experiments. The SAXS measurements were conducted at beamline BL-6A in the Photon Factory (PF) at the High Energy Acceleration Research Organization (KEK), Tsukuba, which produces an 8.3 keV monochromatic X-ray beam, with a wavelength (λ) of 1.5 Å. The beam spot at the sample position was 1.0 (width) × 0.5 mm (height). An in situ beam monitoring apparatus26 was constructed for the SAXS beamlines, and the absorption factor of the sample, which exponentially affects the precision of absorption correction, was measured using the apparatus simultaneously during the SAXS measurements. The sample cell used for BB was made of titanium with single-crystal diamond windows and had a sample pass length of 0.2 mm.27 The cell used for TL and TL-PT10 was made of stainless steel with single-crystal diamond windows and had a sample pass length of 1.75 mm.28 A two-dimensional detector (Pilatus 300 K, Dectris Ltd., Baden, Switzerland) was located at a distance of 2170 mm from the cell. The exposure time was 10 min for the 20 mg/L sample or 5 min for greater than or equal to the 500 mg/L sample. The range of the observable scattering parameter (scalar), s = 4πsin(θ/ λ), where 2θ is the scattering angle, was 0.005−0.2 Å−1.

Figure 1. SAXS profiles of CaAs in BB, TL, and TL-PT10.

those in TL, TL-PT, and any other solvents used in previous studies. We discuss the results using TL and TL-PT10 first, and then clarify the differences with results using BB. 3.1. Asphaltene Aggregation Behavior in TL and TLPT10. Figures 2 and 3 show the SAXS profiles of CaAs in TL and TL-PT10, respectively, with trends similar to those shown in previous reports.12,18,19,30 To elucidate asphaltene aggregation from these SAXS results, a general method, namely the Zimm approximation,31 was applied to give the radius of gyration (Rg) of aggregates. The Rg is extracted from the SAXS profile by curve-fitting using the following equation for sRg < 319 5738


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variation against the volume fraction of asphaltene in solution, Φ, which is calculated using ρw. The Rg increased with the increase in the asphaltene concentration at Φ ≤ ∼0.01 (10 000 mg/L) and decreased at Φ = ∼0.1 (100 000 mg/L), irrespective of the solvent. These results, namely, the range of Rg, the smaller Rg in a good solvent, and the Rg variation against the concentration, were consistent with the results of a recent study, in which Hoepfner et al. reported values for two different asphaltenes in TL, THF, and 1-MN by SAXS.12 The I(0)est Δρ−2 can also be used to estimate the molecular weight of aggregate, Mw, using the relationship30 M w = I(0)Δρ−2

NAρw Φ

(3)

where NA is the Avogadro constant. Table 2 and Figure 5 show the relationship between Rg and Mw, and indicates a power law relationship, with the exception of the 20 mg/L sample M w ∝ R g1.7

The exceptions at the lowest concentration may be derived from the error due to low signal-to-noise ratios. The fractal dimension of 1.7 was the same as in a previous study.12 All results shown above indicate that the asphaltene in this study displayed the conventional aggregation behavior in TL and TLPT10. 3.2. Asphaltene Aggregation Behavior in BB. Figure 6 shows the SAXS profiles of CaAs in BB. The trend of the profiles in BB is very different from the others. As shown in Figure 5, the relationship between Rg and Mw for BB does not obey the 1.7th power relationship of eq 4, which is valid for various organic solvents (TL, TL-PT10, 1-MN, and THF). This inconsistency indicates that the asphaltene aggregation behavior in BB is different from that in the other solvents, while the Rg at 100 000 mg/L in BB is almost identical to that in TL and TLPT10, as shown in Figure 4. The specific features for BB are as follows: linear relationships with a slope of −4 in log I(s)Δρ−2 vs log s at s < ∼0.02 Å, irrespective of concentration, and no relationship obeying the Zimm approximation at s > ∼0.02 Å, except for the 100 000 mg/L sample. The fourth power law is known as Porod’s law32,33

Figure 2. SAXS profiles of CaAs in TL.

I (s ) ∝ s − 4

I(s)Δρ

−1 ⎛ s 2R g 2 ⎞ ⎟ = I(0)est Δρ ⎜1 + 3 ⎟⎠ ⎝

sR g > 1

(5)

which is valid when the scattering system has a phase-separated structure with a well-defined smooth surface (i.e., a large difference in electron density between the scatterer and the environment), such as polymer−water,34 silica−water,35 glass− water,36 and asphaltene−air37 systems, no matter what the type of scatterer type, for example, single particle, densely packed system, and nonparticulate structures. Because the scattering behavior under Porod’s law is influenced by only the surface structure and not the magnitude in scale or the mutual arrangement of the scatterer, it is impossible to obtain information about the shape of the scatterer. On the other hand, the aggregate obeying the Zimm approximation is considered to have a soft boundary against the solvent, because the approximation was conducted for a dilute solution of chain molecules with a moderate degree of polymerization.31 Therefore, the results indicate that the aggregates with a soft boundary that are observable in TL disappear in BB, and simultaneously the asphaltene in BB forms aggregates with a clear boundary to the solvent. In Figure 6, the upper limit of s for the fourth power law, ∼ 0.02 Å, suggests that the Rg of the

Figure 3. SAXS profiles of CaAs in TL-PT10.

−2

(4)

−2 ⎜

(2)

−2

In eq 2, I(0)est Δρ is the intensity at zero angle. The solid lines in Figures 2 and 3 are the fitted curves of eq 2 with the determined variables listed in Table 2. The analyzed s range was 0.02−0.05 Å−1, corresponding to Rg < 60 Å. The values of Rg in TL and TL-PT10 were 34−55 and 47−60 Å, respectively; the values of Rg in TL were smaller than that those in TL-PT10, irrespective of the concentration. Figure 4 shows the Rg 5739


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Energy & Fuels Table 2. Mw and Rg Determined Using the Zimm Approximation Solvent TL

TL-PT10

BB

Conc./mg L−1 18.8 498.2 1004.7 9762.6 97 484.1 17.3 487.7 998.7 9720.7 97 420.6 19.4 490.0 985.4 10 124.9 100 447.1

Φ 1.61 4.26 8.58 8.28 7.69 1.48 4.17 8.53 8.24 7.69 1.66 4.19 8.42 8.58 7.91

× × × × × × × × × × × × × × ×

−5

10 10−4 10−4 10−3 10−2 10−5 10−4 10−4 10−3 10−2 10−5 10−4 10−4 10−3 10−2

I(0)estΔρ−2 /Å3

Rg/Å

Mw/g mol−1

× × × × × × × × × ×

0

34.0 46.8 48.4 54.5 44.1 46.8 52.9 55.0 60.1 47.6

5.54 5.86 6.69 8.38 5.53 4.00 7.37 7.87 9.73 6.40

8.17 × 102

50.6

7.23 × 103

1.26 3.54 8.15 9.84 6.04 8.40 4.36 9.52 1.14 6.98

10 101 101 102 103 10−1 101 101 103 103

× × × × × × × × × ×

104 104 104 104 104 104 104 104 104 104

Figure 4. Rg variation against concentration.

Figure 5. Relationship between Rg and Mw.

Figure 6. SAXS profiles of CaAs in BB.

scatterer is > ∼50 Å due to the judgment rule of sRg > 1. This indicates that the asphaltene forms larger aggregates in BB than in TL, despite being a superior solvent. Some of the asphaltene molecules exist as aggregates, even at concentrations as low as 20 mg/L in the good solvent. Considering the fact that BB has higher affinity for asphaltene than TL, judging from the HSP analysis described above, some of the asphaltene molecules may completely dissolve in BB and the residual molecules can form another type of aggregate. This mechanism is consistent with the result in which the fourth power law was found in the SAXS profiles, not only for BB but also for TL in the 20 mg/L sample, as shown in Figure 2, because such a highly diluted condition may enable some molecules to dissolve even in TL. In contrast, even in BB, aggregates with a soft boundary appear at higher concentrations in the SAXS profile in the 100 000 mg/L

sample, while the increase in intensity in the s range of 0.01− 0.1 Å−1 could arise from interparticle interference due to the high concentration. 3.3. Evaluation of Asphaltene Aggregation Model. The results of this study can be used to evaluate the asphaltene aggregation models proposed previously. In the Yen−Mullins model, it is assumed that the asphaltene molecules show a twostep aggregation behavior in organic solvents depending on concentration, namely independent molecules at < ∼50 mg/L, nanoaggregates at ∼200 to ∼2000 mg/L, and clusters of nanoaggregates at ≥ ∼2000 mg/L.38,39 The nanoaggregates correspond to the aggregate having a soft boundary, whose size and shape in TL were estimated by SAXS/SANS.40 They used the term critical nanoaggregate concentration (CNAC) to describe the molecule−nanoaggregate transition. However, the observable scattering intensity from aggregates at 20 mg/L in 5740


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Energy & Fuels TL (Figure 2) and 15 mg/L in TL and THF,12 are inconsistent with the CNAC concept. In the Yen−Mullins model, it is considered that the molecular aggregation behavior to form nanoaggregates is insensitive to solvent type, and as a result the aggregation behavior in BB is outside the concept. These inconsistencies originate from the assumption that asphaltene molecules have only one type of structure, that is, a large fused ring (island type), and the molecules form an aggregate only by the π−π stacking of aromatic rings. Tanaka et al. also proposed an aggregation model based on the π−π stacking of island-type molecules, and they considered that asphaltene molecules form three types of aggregate at the same time: the core aggregate (similar to a nanoaggregate), medium aggregate (an in-line connected core aggregate), and fractal aggregate (secondary aggregate of the core aggregate).41 The abundance ratio of each aggregate may be controlled by the regulation of the thermodynamic equilibrium as Hoepfner et al. described.12 The aggregate with a soft boundary in TL corresponds to the core/medium aggregate, and the aggregate with a clear boundary in BB corresponds to the fractal aggregate. This hierarchy aggregation model cannot explain the asphaltene aggregation behavior in BB, because the fractal aggregate is no longer formed under such conditions as core/ medium aggregates become molecules, and the fractal aggregate is the secondary aggregate of a core aggregate; that is, this model does not allow molecules to form the fractal aggregate in BB without forming the core/medium aggregates. The inconsistencies of the Yen−Mullins and Tanaka models with the results obtained using BB suggests that it is necessary to involve the molecular type distribution to construct a reasonable aggregation model, for example, the supramolecular assembly model proposed by Gray et al.42 or the model described below. Acevedo et al. proposed a colloid model for asphaltene aggregates that consists of A1 and A2 fractions, where A1 and A2 are different molecular-type fractions representing the core and surface of colloid particles, respectively.43 A model was developed based on their findings, in which A1 and A2 can be separated from an asphaltene using cumene and p-nitrophenol, and the solubility of A1 to TL was as low as ∼90 mg/L at room temperature, whereas that of the original asphaltene was >50 000 mg/L,44 and the asphaltene displayed a thermal diffusivity minimum at ∼50 mg/L in TL using thermal lens spectroscopy.45 These results suggested the contribution of A1 (TL-insoluble fraction) to the first step aggregation at low concentrations in TL. The particular aggregation behavior at ∼50 mg/L was also observed in other studies.20,46 Although A1 is prone to flocculate without A2 in TL due to the low solubility, it is able to exit as stable aggregates in the surrounding A2 fraction, even at high concentrations. This A1/A2 colloidal aggregation model seems to give a reasonable explanation of the aggregation behavior in BB; BB disperses A2 (colloid surface) at the molecular level due to a marked affinity with A2 and isolates the A1 fraction (core), which lead to a rigid aggregation of A1. This concept is acceptable because the Δδ between A2 and BB is as small as 0.9 MPa0.5, whereas that between A1 and BB is 4.3 MPa0.5, which is not small enough, as shown in Table 3. It is not surprising that the aggregate comprising only the core fraction has a large and rigid structure with a clear boundary. 3.4. Influence of Δδ on Aggregation Behavior. The aggregation behavior observed in this study can be explained based on the A1/A2 colloidal aggregation model with HSP

Table 3. HSPs of Asphaltene Fractions and Solvents HSP/MPa0.5 CaAs Asa A1a A2a TL BB a

δd

δp

δh

Δδ to TL

Δδ to BB

19.1 19.5 20.9 19.6 18.0 19.2

4.2 4.7 5.6 5.8 1.4 5.5

4.4 4.9 6.8 4.4 2.0 4.1

4.3 5.3 8.6 5.9

1.3 1.3 4.3 0.9 5.2

5.2

Reported by Acevedo et al.43

analyses. Figure 7 shows the relationships between the values of Δδ for each solvent to CaAs (Table 1) and the Rg values of an

Figure 7. Relationships between Δδ and Rg (left axis) or relative Dagg (right axis).

Figure 8. Relationships between Δδ and Mw (left axis) or relative Dagg (right axis).

aggregate with a soft boundary (Table 2). Figure 8 shows the relationships between the Δδ and Mw (Table 2). Both Rg and Mw decreased with a decrease of Δδ, except for the Rg at 100 000 mg/L. These results indicate that the size of an aggregate with a soft boundary and the number of molecules in the aggregate decrease with the decrease in Δδ; that is, the number of A2 molecules on the colloid surface decreases, whereas the number in solution increases with the increase in solubility of A2, which decreases aggregate size. The change in the size of the aggregate can be evaluated quantitatively using HSP analyses. Figures 7 and 8 show the relative degree of aggregation (Dagg), which were determined by Rayleigh scattering measurements using a UV−vis spectrometer in our previous study.24 The Dagg was derived as a relative index to Dagg at 100 000 mg/ L in TL. The values of Dagg decreased with decreasing Δδ and reached a plateau at Δδ ≤ ∼1.3, irrespective of concentration. These relationships suggest that the gradual variations in Dagg at 5741


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Energy & Fuels Δδ of 1.3−5.0 reflect a change in the size of an aggregate mainly due to changes in the amount of A2 on the surface of the aggregate, and the plateau at Δδ ≤ 1.3 corresponds to the formation of an A1 aggregate with a clear boundary, which does not change even in more effective solvents than BB. Consequently, SAXS and Rayleigh scattering measurements with HSP analyses provide similar results using the A1/A2 colloidal aggregation model.

(4) Headen, T. F.; Boek, E. S.; Stellbrink, J.; Scheven, U. M. Langmuir 2009, 25, 422−428. (5) Eyssautier, J.; Espinat, D.; Gummel, J.; Levitz, P.; Becerra, M.; Shaw, J.; Barré, L. Energy Fuels 2012, 26, 2680−2687. (6) Eyssautier, J.; Hénaut, I.; Levitz, P.; Espinat, D.; Barré, L. Energy Fuels 2012, 26, 2696−2704. (7) Hoepfner, M. P.; Vilas Bôas FáVero, C.; Haji-Akbari, N.; Fogler, H. S. Langmuir 2013, 29, 8799−8808. (8) Pollack, S. S.; Yen, T. F. Anal. Chem. 1970, 42, 623−629. (9) Ravey, J. C.; Ducouret, G.; Espinat, D. Fuel 1988, 67, 1560− 1567. (10) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988, 67, 245−250. (11) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127−134. (12) Hoepfner, M. P.; Fogler, H. S. Langmuir 2013, 29, 15423− 15432. (13) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399−410. (14) Storm, D. A.; Sheu, E. Y.; Detar, M. M. Fuel 1993, 72, 977−981. (15) Sheu, E. Y.; Storm, D. A.; Detar, M. M. J. Non-Cryst. Solids 1991, 131, 341−347. (16) Xu, Y.; Koga, Y.; Strausz, O. P. Fuel 1995, 74, 960−964. (17) Roux, J.-N.; Broseta, D.; Demé, B. Langmuir 2001, 17, 5085− 5092. (18) Espinat, D.; Fenistein, D.; Barre, L.; Frot, D.; Briolant, Y. Energy Fuels 2004, 18, 1243−1249. (19) Barré, L.; Jestin, J.; Morisset, A.; Palermo, T.; Simon, S. Oil Gas Sci. Technol. 2009, 64, 617−628. (20) Morimoto, M.; Morita, T.; Takanohashi, T.; Nishikawa, K. J. Jpn. Pet. Inst. 2013, 56, 58−59. (21) Fenistein, D.; Barré, L.; Broseta, D.; Espinat, D.; Livet, A.; Roux, J.-N.; Scarsella, M. Langmuir 1998, 14, 1013−1020. (22) Hansen, C. M. Hansen solubility parameters: a user’s handbook, 2nd ed.; CRC Press: Boca Raton, FL, 2007. (23) Sato, T.; Araki, S.; Morimoto, M.; Tanaka, R.; Yamamoto, H. Energy Fuels 2014, 28, 891−897. (24) Morimoto, M.; Sato, T.; Araki, S.; Tanaka, R.; Yamamoto, H.; Sato, S.; Takanohashi, T. Energy Fuels 2015, 29, 2808−2812. (25) Redelius, P. Energy Fuels 2004, 18, 1087−1092. (26) Morita, T.; Tanaka, Y.; Ito, K.; Takahashi, Y.; Nishikawa, K. J. Appl. Crystallogr. 2007, 40, 791−795. (27) Morita, T.; Ushio, M.; Kanoh, K.; Tanaka, E.; Nishikawa, K. Jpn. J. Appl. Phys. 2012, 51, 076703. (28) Nitta, A.; Morita, T.; Saita, S.; Kohno, Y.; Ohno, H.; Nishikawa, K. Chem. Phys. Lett. 2015, 628, 108−112. (29) Sato, S.; Takanohashi, T.; Tanaka, R. Energy Fuels 2005, 19, 1991−1994. (30) Barre, L.; Simon, S.; Palermo, T. Langmuir 2008, 24, 3709− 3717. (31) Zimm, B. H. J. Chem. Phys. 1948, 16, 1093−1099. (32) Porod, G. Colloid Polym. Sci. 1951, 124, 83−114. (33) Porod, G. General theory. In Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982. (34) Takenaka, M.; Iwase, N.; Nishitsuji, S.; Ito, K. Polym. J. 2007, 39, 1112−1116. (35) Schaefer, D. W.; Rieker, T.; Agamalian, M.; Lin, J. S.; Fischer, D.; Sukumaran, S.; Chen, C. Y.; Beaucage, G.; Herd, C.; Ivie, J. J. Appl. Crystallogr. 2000, 33, 587−591. (36) Lyonnard, S.; Spalla, O.; Testard, F.; F. Né, O. T. What is measured in a Small Angle X-ray Scattering (SAXS). http://iramis.cea. fr/scm/lions/techniques/saxs/ (accessed April 4, 2015). (37) Savvidis, T. G.; Fenistein, D.; Barré, L.; Béhar, E. AIChE J. 2001, 47, 206−211. (38) Mullins, O. C. Energy Fuels 2010, 24, 2179−2207. (39) Mullins, O. C.; Sabbah, H.; Eyssautier, J.; Pomerantz, A. E.; Barré, L.; Andrews, A. B.; Ruiz-Morales, Y.; Mostowfi, F.; McFarlane, R.; Goual, L.; Lepkowicz, R.; Cooper, T.; Orbulescu, J.; Leblanc, R. M.; Edwards, J.; Zare, R. N. Energy Fuels 2012, 26, 3986−4003.

4. CONCLUSION The SAXS analyses of CaAs in BB revealed a particular aggregation behavior of asphaltene compared to that in conventional test solvents, such as TL. Instead of the aggregates with a soft boundary of ∼30−60 Å in Rg, which were frequently observed in TL, larger aggregates with a clear boundary were present. Some of the asphaltene molecules existed as aggregates even at concentrations as low as 20 mg/L in the BB. This phenomenon can be explained by the colloidal aggregation model proposed by Acevedo et al. in association with HSP analyses. The model suggests that BB disperses the colloid surface fraction at a molecular level due to the great affinity with the medium and isolates the colloid core fraction, which leads to the formation of a rigid aggregation of the core fraction. The HSP analyses enabled us to evaluate the aggregation behavior quantitatively, with the results obtained by SAXS being consistent with those obtained by Rayleigh scattering in our previous study. The new findings of this study will help us to understand the complex phenomena involved in the aggregation behavior of asphaltene molecules.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01491. Scattering intensity of solute obtained experimentally, absolute intensity correction, correction of electron density difference between solute and solvent, estimation of electron density of asphaltene. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Japan Petroleum Energy Center (JPEC) as a technological development project entrusted by Ministry of Economy, Trade, and Industry. We thank PF at KEK for giving us the opportunity to perform the SAXS experiments under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2013G208).

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