Mapping the Degree of Asphaltene Aggregation, Determined Using

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Mapping the degree of asphaltene aggregation, determined using Rayleigh scattering measurements and Hansen solubility parameters Masato Morimoto Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502053c • Publication Date (Web): 26 Dec 2014 Downloaded from http://pubs.acs.org on December 30, 2014

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Mapping the degree of asphaltene aggregation, determined using Rayleigh scattering measurements and Hansen solubility parameters

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Energy & Fuels ef-2014-02053c.R1 Article 07-Dec-2014 Morimoto, Masato; National Institute of Advanced Industrial Science and Technology, Energy Technology Research Institute Sato, Takashi; kansai University, Department of Chemical, Energy and Environmental Engineering Faculty of Environmental and Urban Engineering Araki, Sadao; Kansai University, Chemical, Energy and Environmental Engineering Tanaka, Ryuzo; Japan Petroleum Energy Center, Yamamoto, Hideki; Kansai Univ., Faculty of Engr., Dept. of Chem. Engr. Sato, Shinya; AIST, Energy Technology Research Institute Takanohashi, Toshimasa; National Institute of Advanced Industrial Science and Technology, Energy Technology Research Institute

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Mapping the degree of asphaltene aggregation, determined using Rayleigh scattering measurements and Hansen solubility parameters

Masato Morimoto1*, Takashi Sato2, Sadao Araki2, Ryuzo Tanaka3, Hideki Yamamoto2, Shinya Sato1, and Toshimasa Takanohashi1

1

Advanced Fuel Group, Energy Technology Research Institute, National Institute of

Advanced Industrial Science and Technology, 16-1, Onogawa, Tsukuba 305-8569, Japan 2

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 3

Petroleomics Laboratory, Advanced Technology and Research Institute, Japan Petroleum

Energy Center, 40-10, Ohnodai 1-Chome, Midori-ku, Chiba 267-0056, Japan

*

Corresponding author.

E-mail address: [email protected] (M. Morimoto).

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Abstract The degree of asphaltene aggregation (DAgg) over a wide range of concentrations in various solvents under ambient conditions was examined using a quantitative index derived from Rayleigh scattering measurements by UV-vis spectrometry and the Hansen solubility parameter (HSP). The source of asphaltene was Canadian oil sand bitumen. The target concentration was ≤ 10% and that of solvent was organic solvents having ∆δ ≤ 5.5 MPa0.5 of the HSP distance between each solvent and the asphaltene, such as toluene (TL), bromobenzene (BB), and toluene-pentane (TL-PT), toluene-bromobenzene (TL-BB), and toluene-quinoline (TL-QL) mixed solvents. Through these measurements, the effects of concentration and solvent on DAgg were examined quantitatively, which enabled the mapping of DAgg. The DAgg map obtained facilitates estimation of the DAgg under desired conditions, understanding of asphaltene aggregation behavior, and creation of a reasonable aggregation model.


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1. Introduction Asphaltene is an unfavorable component in petroleum that causes fouling. Asphaltene is defined as a heptane-insoluble fraction, generally, and consists of a complex mixture of many polycyclic aromatic hydrocarbons including not only carbon and hydrogen but also hetero atoms, such as sulfur, nitrogen, oxygen, nickel, and vanadium.1 Asphaltene exists as complex aggregates in petroleum and organic solvents.1-8 Thus, it is important to clarify the influence of asphaltene aggregation in asphaltene problems. For that purpose, quantification of the degree of asphaltene aggregation (DAgg) at different concentrations in various solvents is essential. There is a need for methods to evaluate the DAgg and the affinity between asphaltene and solvents (solvent power) quantitatively. One possible method for the quantification of DAgg is small-angle X-ray/neutron scattering (SAXS/SANS) because it can be performed at a wide range of asphaltene concentration in various solvents, which give information about size and shape of scatterer.2-5, 7, 8

However, the techniques require special experimental skills and a synchrotron facility, so

is not always readily available. Recently, Dechaine et al. applied ultraviolet-visible (UV-vis) spectroscopy to asphaltene aggregation analyses. 9-11 They found that asphaltene aggregates dispersed in toluene showed no absorption in the visible range (wavelength (λ) > 550 nm) and Rayleigh scattering behavior, which correlated with the size of the aggregates. Thus, UV-


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vis measurements may be a promising method to quantify DAgg under various conditions because a large number of data points can be obtained easily and rapidly. The Hildebrand solubility parameter (δ MPa0.5) is a conventional and popular concept to quantify the solvent power to asphaltene, defined as the square root of cohesive energy density.12,

13

Two components having similar solubility parameters typically show good

affinity for each other, although there are exceptions. The Hansen solubility parameter (HSP) is defined as δ = (δd2 + δp2 + δh2)0.5, where δd, δp, and δh are parameters corresponding to dispersive, polar, and hydrogen-bonding forces in the cohesive energy, respectively.13 Redelius applied HSP to the solubility of asphaltene in various organic solvents, and confirmed that it was more appropriate than Hildebrand’s solubility parameter.14, 15 Thus, HSP is considered a better method of quantifying solvent power. Here, we used HSP for solvent power quantification and Rayleigh scattering analysis as a potential method of determining DAgg in various solvents to clarify the effects of solvent and concentration. The objectives were to examine the validity of our proposed index, DAgg, and to draw a distribution map of DAgg over wide ranges of concentration and solvent power.


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2. Experimental 2.1 Sample preparation and UV-Vis measurement We used heptane-insoluble asphaltene recovered from Canadian oil sand bitumen (CaAs, C: 81.3, H: 7.2, N: 1.3, S: 8.1, O: 1.5, Ni: 0.037, V: 0.100, other: 0.46 wt%). CaAs was dissolved in toluene (TL), toluene-pentane mixed solvents containing 5, 10, 15, or 20% pentane by volume (TL-PT5,

TL-PT10, TL-PT15,

and

TL-PT20,

respectively),

bromobenzene (BB), bromobenzene-toluene mixed solvents containing 20 or 40% bromobenzene by volume (TL-BB20 and TL-BB40, respectively), or toluene-quinoline mixed solvent containing 50% quinoline by volume (TL-QL50) at concentrations of 10 to 105 mg/L. The solutions were ultrasonicated for 5 min and allowed to stand overnight. Then, after a second 5-min ultrasonication, absorbance (A) was measured in the λ range of 330 to 1000 nm at room temperature, using a multi-channel photodetector (MCPD-3000, Otsuka Electronics Co., Ltd.) with optical cells having path lengths of 10, 0.5, and 0.1 mm (for concentrations of < 103, 103-104, and 104-105 mg/L, respectively). The exposure time was 0.1 s and the cumulated scan number was 64 times.

2.2 HSP of asphaltene and solvents Table 1 lists the HSP values of CaAs and the solvents used. We reported the HSP of CaAs in our previous paper.16 The HSP values of pure solvents were from the database and 5 ACS Paragon Plus Environment

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those of mixed solvents were calculated by the mixing rule on a volume basis.13 The solvent power corresponds to ∆δ, which is the HSP difference between CaAs and each solvent, calculated using Eq. (1): ∆δ = (4 (δd-CaAs − δd-Solvent)2 + (δp-CaAs – δp-Solvent)2 + (δh-CaAs – δh-Solvent)2)0.5

(1)

A smaller ∆δ indicates a higher solvent power to asphaltene, because the closer HSP means a higher affinity between asphaltene and the solvent. Thus, BB and PT were selected as the ‘best’ pure solvent and a poor solvent for CaAs, respectively. The solvent power of TL was increased by mixing with BB and decreased by mixing with PT. The solvent having the highest solvent power to CaAs was TL-QL50, the ∆δ of which was as low as 0.9. The range of ∆δ in this study was from 0.9 to 5.5 MPa0.5. We used ∆δ as the index for solvent power quantification.

2.3 Degree of aggregation (DAgg) derived from Rayleigh scattering theory According to Rayleigh scattering theory17, A is proportional to λ-4 when the diameter of scatterer is much smaller than the λ. For spherical scatterer, the following equation can be derived (see Appendix), including our proposed factor, DAgg:  (n n )2 − 1  − 4 zC  p s 2  λ ,  (n p ns ) + 2  2

A = D Agg

−1

(2)

where z is the light path length, C is the mass concentration, and np and ns are the refractive indices of the scatterer and the solvent, respectively. The scatterer in the system corresponds 6 ACS Paragon Plus Environment

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to the asphaltene molecule and aggregate, the refractive index of which are 1.7.11, 18, as the scattering intensity of molecule is much lower than that of aggregate. Because the relationship between A and λ-4 on asphaltene solutions was linear, as shown below, the DAgg was derived from the proportional factor (slope, S = A/λ-4):

 (n p ns )2 − 1  = S zC   . 2  (n p ns ) + 2  2

DAgg

−1

(3)

The DAgg is an index about scatterer only, excluding the influence of conditions (light path length, concentration, and solvent (refractive index)) on Rayleigh scattering behavior (S). Although asphaltene molecules and aggregates may not be spherical and have various size and shapes, we considered that DAgg in different solutions could be evaluated quantitatively using Eq. (3).

3. Results and Discussion 3.1. Rayleigh scattering behavior Figures 1(a), (b), and (c) show examples of the relationship between A and λ-4, obtained using CaAs solutions in BB, TL, and TL-PT20, respectively. For all experiments, A was almost perfectly proportional to λ-4 for λ = 700-1000 nm, regardless of solvent and concentration, indicating that Rayleigh scattering alone, and no absorption, occurred. Thus, each DAgg value could be calculated from the slope in that λ region using Eq. (3), and the


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results were normalized to DAgg in TL at 105 mg/L to assess the effects of concentration and solvent.

3.2. Effect of concentration on DAgg Figure 2 shows the relative DAgg at various concentrations in each solvent. DAgg values were determined for a wide range of concentrations. The range of DAgg corresponded to the general understanding of asphaltene aggregation: DAgg increased with increasing concentration, regardless of solvent. This method enabled a quantitative evaluation of asphaltene aggregation state. For example, at 105 mg/L, the asphaltene aggregation states in TL-PT20 and BB had the relative DAgg values of ~1.4 and ~0.46, suggesting 40% higher and 54% lower than that in TL, respectively. The influence of concentration on DAgg was smaller in ‘better’ solvents; the ranges of relative DAgg values were 0.26-0.46 in TL-QL50, 0.28-0.45 in BB, 0.52-0.75 in TL-BB40, 0.59-0.90 in TL-BB20, 0.70-1.00 in TL, 0.70-1.11 in TL-PT5, 0.70-1.15 in TL-PT10, 0.81-1.30 in TL-PT15, and 0.84-1.32 in TL-PT20. At < 1000 mg/L in TL, TL-PT5, and TL-PT10, the concentration showed little effect on DAgg; relative DAgg values at < 40 mg/L were almost the same at 0.70, those at 100-2000 mg/L were almost equal at 0.73. However, those at > 2000 mg/L increased linearly with increasing common logarithm of concentration (log C). At relative DAgg values > 0.8, DAgg values increased markedly with increasing concentration for all TL-PT mixed solvents. These 8 ACS Paragon Plus Environment

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results were consistent with the Yen-Mullins model,19 which proposed a three-stage aggregation

depending

on

concentration:

asphaltene

molecules

at

< ~50

mg/L,

nanoaggregates at ~200-2000 mg/L, and clusters at > ~2000 mg/L. Thus, it was confirmed that our proposed methodology was valid, based on these previous studies. Moreover, the results obtained using the ‘better’ solvents than TL indicated a limitation of the Yen-Mullins model; if asphaltene existed as molecules at the lower concentrations in TL (< 40 mg/L) as Mullins modeled, we cannot explain why the DAgg values in BB and TL-QL50 were considerably lower than those at low concentrations in TL. This was consistent with recent results obtained using SAXS,8 indicating that asphaltene nanoaggregates were observed at concentrations as low as 15 mg/L in TL.

3.3 Effect of solvent on DAgg Figure 3 shows the relationship between relative DAgg and ∆δ at concentrations of about 20, 103, 104, and 105 mg/L. DAgg was determined for a wide range of solutions. The range of DAgg corresponded to the general understanding of asphaltene aggregation: the DAgg decreased with increasing solvent power (decreasing ∆δ), regardless of concentration. As a result, the decrease in relative DAgg values was small at ∆δ < 1.3 MPa0.5, suggesting that the relative DAgg for CaAs could show a minimum of ~0.25 in the best solvent having the smallest ∆δ. 9 ACS Paragon Plus Environment

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The influence of ∆δ on DAgg was smaller at lower concentrations. In the case of 20 mg/L, DAgg increased from 0.26 to 0.84, with the increase in ∆δ from 0.9 to 5.5, for a difference of 0.58. Similarly, the differences were 0.69, 0.79, and 0.86 at concentrations of 103, 104, and 105 mg/L, respectively. At ∆δ = 4.3-5.2 at ≤ 103 mg/L, the DAgg values were largely unaffected by ∆δ, which was ~0.70 for 20 mg/L and ~0.75 for 103 mg/L. However, at higher concentrations, DAgg increased with increasing ∆δ. This behavior was consistent with Yen-Mullins model,19 which described that the aggregation state was “insensitive to solvent type” at < ~2000 mg/L but “size-sensitive to solvent type” at higher concentrations. Thus, it was confirmed again that our proposed method was valid, based on previous research for asphaltene aggregation analyses performed in TL and TL-heptane mixed solvents.

3.4 Mapping of DAgg to concentration and solvent power Figure 4 shows the three-dimensional plot of relative DAgg versus ∆δ versus log C, on the vertical, horizontal, and depth axes, respectively. Each data point in the figure corresponds to that in Figures 2 and 3. The plane was drawn by fitting with the following cubic function: f (p, q) = K0 + K1 p + K2 q + K3 p2 + K4 pq + K5 q2 + K6 p3 + K7 p2q + K8 pq2 + K9 q3


(4)

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where f, p, and q are DAgg, ∆δ, and log C, respectively, and K0-K9 are the variables for fitting. As a result, all the data fitted reasonably well with Eq. (4) using the variables shown in Table 2. Figure 5 is the contour plot of relative DAgg on concentration and ∆δ, drawn using the fitted equations above, which was the top view of the three-dimensional plot in Figure 4. Figures 4 and 5 clarified the continuous aggregation-relaxation behavior of asphaltene quantitatively. Thus, we obtained a DAgg map over wide ranges of concentration and solvent power. This map facilitates not only estimation of the DAgg under given conditions but also understanding and modeling of the complex aggregation behavior of complex mixtures of asphaltene molecules.

4. Conclusions Our proposed method using Rayleigh scattering and HSP provided quantification of the effects of solvents and concentration on asphaltene aggregation at concentrations of ≤ 10% and ∆δ of ≤ 5.5 MPa0.5, revealing continuous aggregation/relaxation behavior. DAgg variation was consistent with previous reports regarding asphaltene aggregation on concentration and solvent effects, indicating the validity of this methodology. The DAgg map obtained is helpful for determining DAgg at desired concentrations in various solvents, and to understand asphaltene aggregation behavior. 11 ACS Paragon Plus Environment

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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. Prof. Murray Gray provided the asphaltene and valuable suggestions regarding Rayleigh scattering measurements.


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References (1) Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics, Springer Science+Business Media LLC, New York, 2007. (2) Barré, L.; Jestin, J.; Morisset, A.; Palermo, T.; Simon, S. Oil & Gas Science and Technology 2009, 64, 617-628. (3) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127-134. (4) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118-1125. (5) Eyssautier, J. l.; Levitz, P.; Espinat, D.; Jestin, J.; Gummel, J. r. m.; Grillo, I.; Barré, L. c. The Journal of Physical Chemistry B 2011, 115, 6827-6837. (6) Gray, M. R.; Tykwinski, R. R.; Stryker, J. M.; Tan, X. Energy Fuels 2011, 25, 3125-3134. (7) Hoepfner, M. P.; Vilas Bôas Fávero, C.; Haji-Akbari, N.; Fogler, H. S. Langmuir 2013, 29, 8799-8808. (8) Hoepfner, M. P.; Fogler, H. S. Langmuir 2013, 29, 15423-15432. (9) Dechaine, G. P., PhD Thesis, University of Alberta, 2010. (10)

Dechaine, G. P.; Gray, M. R. Energy Fuels 2011, 25, 509-523.

(11)

Derakhshesh, M.; Gray, M. R.; Dechaine, G. P. Energy Fuels 2013, 27, 680-693.


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(12)

Yen, T. F.; Chilingarian, G.V. asphaltenes and asphalts, 1; Elsevier science:

Amsterdam, 1994. (13)

Hansen, C. M. Hansen solubility parameters: a user's handbook. 2nd ed.; CRC: Boca

Raton, 2007. (14)

Redelius, P. Energy Fuels 2004, 18, 1087-1092.

(15)

Redelius, P. Hansen Solubility Parameters of Asphalt, Bitumen, and Crude Oils; In

Ref. 6; pp.151-175, 2007. (16)

Sato, T.; Araki, S.; Morimoto, M.; Tanaka, R.; Yamamoto, H. Energy Fuels 2014, 28,

891-897. (17)

van de Hulst, H. C. Light Scattering by Small Particles, Dover Publications Inc.; New

York, 1981. (18)

Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Von Drasek, S.; Wang, J. X.; Gil, B. S. Pet. Sci.

Technol. 1998, 16, 251-285. (19)

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, 39864003.


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Appendix The Beer-Lambert law states that the transmittance (T) varies exponentially with the number of excitation sites in the light pass length (z),

T = I I0 = exp(− zNGQext ) ,

(A1)

where I0 and I are the intensity of the incident and the transmitted radiation, respectively, N is the number density of particles, and G is the geometric cross-section of the particles. Qext is the extinction coefficient, which is equal to the scattering coefficient (Qscat) in a case with no absorption:

Qext = Qscat .

(A2)

The absorbance (A) is defined as minus the common logarithm of T, A = − log 10 ( I I 0 ) = −

ln ( I I 0 ) zNG = Q scat . 2 . 303 2 . 303

(A3)

When the particle size is much smaller than the wavelength (λ) and its shape is spherical, then Qscat is derived as follows: 2

Qscat

8  m2 −1   , = x 4  2 3  m + 2 

(A4)

where m is the relative refractive index of particle (np) to medium (ns) and x is the size parameter: m = n p / n s and x=

2π r

λ

,

where r is the radius of the particle. 15 ACS Paragon Plus Environment

(A5) (A6)

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Then, A can be described using Eq. (A3)-(A6):

zNG  8 4  m 2 − 1    x  A= 2.303  3  m 2 + 2  

2

  

2 4 2   zN A C 2 8  2πr   m − 1    πr   =   2   2.303 M w  3  λ   m + 2  

2

 m 2 − 1  −4 128π 5 N A r 6  λ , = zC  6.909 M w  m 2 + 2 

(A7)

where NA is the Avogadro constant, C is the weight concentration of the particle, and Mw is the molecular weight of the particle. Here, the relationship N = NAC/Mw was used. Eq. (A7) indicates that A is proportional to λ-4; this is termed Rayleigh scattering.


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Table captions: Table 1. Hansen solubility parameters of asphaltene and solvents. Table 2. Variables for Eq. (4), determined by experimental data fitting.

Figure captions: Figure 1. Relationship between absorbance and wavelength obtained at the indicated concentrations in (a) BB, (b) TL, and (c) TL-PT20. Figure 2. Effect of concentration on relative DAgg in various solvents. Figure 3. Effect of solvent on relative DAgg in various solvents. Figure 4. Effects of solvent and concentration on relative DAgg in various solvents. Vertical axis: relative DAgg, horizontal axis: ∆δ, depth axis: log C. Figure 5. Contour plot of relative DAgg.


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Table 1. Hansen solubility parameters of asphaltene and solvents. Solubility parameter / MPa0.5 δd

δp

δh

δ

∆δ

CaAs

19.1

4.2

4.4

20.1

0.0

TL-QL50

19.3

3.5

3.9

19.9

0.9

BB

19.2

5.5

4.1

20.4

1.3

TL-BB40

18.5

3.0

2.8

18.9

2.3

TL-BB20

18.2

2.2

2.4

18.5

3.3

TL

18.0

1.4

2.0

18.2

4.3

TL-PT5

17.8

1.3

1.9

18.0

4.6

TL-PT10

17.7

1.3

1.8

17.8

4.9

TL-PT15

17.5

1.2

1.7

17.6

5.2

TL-PT20 17.3 1.1 1.6 17.4 5.5 CaAs: heptane-insoluble asphaltene in Canadian oil sand bitumen; TL: toluene; BB: bromobenzene; PT: pentane; TL-BB20 and TL-BB40: TL-BB mixed solvents containing 20 and 40% BB by volume, respectively; TL-PT5, TL-PT10, TL-PT15, and TL-PT20: TL-PT mixed solvents containing 5, 10, 15, and 20% PT by volume, respectively.


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Table 2. Variables for Eq. (4), determined by experimental data fitting. ∆δ / MPa0.5

K0

K1

K2

K3

K4

K5

K6

K7

K8

K9

≤1.3

2.11×10-1

-6.51×10-2

6.65×10-2

4.03×10-3

3.66×10-2

-2.63×10-2

3.00×10-2

-1.05×10-2

-4.35×10-3

4.59×10-3

1.3-5.5

-4.18×10-1

6.89×10-1

9.05×10-2

-1.69×10-1

-2.78×10-2

-2.84×10-2

1.56×10-2

1.76×10-3

5.75×10-3

3.75×10-3

19

ACS Paragon Plus Environment

Energy & Fuels

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Figure 1. Relationship between absorbance and wavelength obtained at the indicated concentrations in (a) BB, (b) TL, and (c) TL-PT20. 20 ACS Paragon Plus Environment

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Energy & Fuels

Figure 2. Effect of concentration on relative DAgg in various solvents.


Energy & Fuels

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Figure 3. Effect of solvent on relative DAgg in various solvents.


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Energy & Fuels

Figure 4. Effects of solvent and concentration on relative DAgg in various solvents. Vertical axis: relative DAgg, horizontal axis: ∆δ, depth axis: log C.


Energy & Fuels

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Figure 5. Contour plot of relative DAgg.


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Mapping the Degree of Asphaltene Aggregation, Determined Using

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