Energy & Fuels 2008, 22, 3989–3993
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Determining Asphaltene Aggregation in Solution from Diffusion Coefficients As Determined by Pulsed-Field Gradient Spin-Echo 1H NMR Hiroyuki Kawashima,*,† Toshimasa Takanohashi,† Masashi Iino,† and Shingo Matsukawa‡ National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, 305-8569, Japan, and Tokyo UniVersity of Marine Science and Technology, Konan, Minato, Tokyo, 108-8477, Japan ReceiVed June 13, 2008. ReVised Manuscript ReceiVed September 10, 2008
Asphaltene aggregation causes several problems in the petroleum industry, and an understanding of the aggregation behavior is needed to solve the problems. In this study, the effects of asphaltene concentration on aggregate size were investigated to elucidate the aggregation mechanism of asphaltene. Diffusion coefficients (D) of three asphaltenes from the vacuum residue (VR) of Khafji, Iranian Light, and Maya crude oils and one resin were determined in deuterated chloroform solution using pulsed-field gradient spin-echo 1H NMR. The pulsed-field gradient spin-echo 1H NMR of asphaltenes and resin showed different diffusion behaviors from well-characterized reference compounds such as polystyrene because of structural and compositional irregularities; that is, they are a complex mixture of molecules of various molecular weights and structures. From the D values, their average hydrodynamic radii were estimated. The concentration dependency (0.1-30 g/L) of the D values and the hydrodynamic radii support the widely accepted stepwise aggregation mechanism (i.e., monomer < small aggregates < medium-size aggregates < large aggregates (precipitate)). At low concentrations (0.1 and 1 g/L), the D values corresponding to the range of small to medium aggregates were observed, and at higher concentrations (10 and 30 g/L), only medium aggregates were detected. Similar D values were obtained for the three asphaltenes, although Maya asphaltene gave slightly lower D values. For the resin, higher D values (lower hydrodynamic radii) were obtained than the asphaltenes.
Introduction Asphaltene is a complex mixture of molecules of various molecular weights and structures, including condensed-ring aromatic compounds with alkyl groups, heteroatoms, and metals. Their aggregation causes major problems in the petroleum industry, such as the deactivation of catalysts, coking and plugging in refineries and transportation lines, and the formation of sludge in products. Although an understanding of the aggregation behaviors of asphaltenes is needed to solve these problems, they are not completely understood. For example, as asphaltenes readily associate to form aggregates, the determination of molecular weights and molecular sizes of asphaltenes has been very difficult. Several analytical methods, such as XRD,1,2 13C NMR,3,4 thermal analysis,5,6 small-angle neutron scattering (SANS),7,8 small-angle X-ray scattering (SAXS),2,9 * To whom correspondence should be addressed. E-mail: h.kawashima@ aist.go.jp. Fax: 81-29-861-8408. † National Institute of Advanced Industrial Science and Technology. ‡ Tokyo University of Marine Science and Technology. (1) Siddiqui, M. N.; Ali, M. F.; Shirokoff, J. Fuel 2002, 81, 51–58. (2) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118–1125. (3) Sato, S.; Takanohashi, T.; Tanaka, T. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2001, 46, 353–354. (4) Carauta, P. R.; Seidl, P. R; Chrisman, E. C. A. N.; Correia, J. C. G.; Menechini, P. de O.; Silva, D. M.; Leal, K. Z.; de Menezes, S. M. C.; de Souza, W. F.; Teixeira, M. A. G. Energy Fuels 2005, 19, 1245–1251. (5) Zhang, Y.; Takanohashi, T.; Sato, S.; Kondo, T.; Saito, I. Energy Fuels 2003, 17, 101–106. (6) Zhang, Y.; Takanohashi, T.; Shishido, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2005, 19, 1023–1028.
laser desorption mass spectroscopy,10 and high-resolution transmission electron microscopy,11 have been used to clarify asphaltene aggregate structures. Pulsed-field gradient spin-echo (PGSE) 1H NMR is a widely used method for measuring diffusion coefficients (D) of polymers in solution.12-15 For oil, Desando et al.16 estimated the hydrodynamic radii of Athabasca oil sand asphaltenes from their diffusion coefficients in chloroform. They found that the sizes increased with increasing concentrations, suggesting larger formations of asphaltene aggregates in more concentrated ¨ stlund et al.17 measured diffusion coefficients of solutions. O asphaltene from vacuum residue (VR) of a Venezuelan crude oil in toluene and reported a concentration range of 0.044-5 wt %. The D values decreased with the asphaltene concentration, (7) Liu, Y. C.; Sheu, E. Y.; Chent, S. H.; Storm, D. A. Fuel 1995, 74, 1352–1356. (8) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127–134. (9) Fenistein, D.; Barre, L. Fuel 2001, 80, 283–287. (10) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405–1413. (11) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Energy Fuels 2002, 16, 490–496. (12) Callaghan, P. T.; Pinder, D. N. Macromolecules 1980, 13, 1085– 1092. Callaghan, P. T.; Pinder, D. N. Macromolecules 1981, 14, 1334– 1340. (13) von Meerwall, E. D. J. Magn. Reson. 1982, 50, 409–416. (14) von Meerwall, E. D. AdV. Polym. Sci. 1984, 54, 1–29. (15) Kaerger, J.; Pfeifer, H.; Heink, W. AdV. Magn. Reson. 1988, 12, 1–89. (16) Desando, M. A.; Lahajnar, G.; Ripmeester, J. A.; Zupancic, I. Fuel 1999, 78, 31–45. ¨ stlund, J.-A.; Andersson, S.-I.; Nyden, M. Fuel 2001, 80, 1529– (17) O 1533.
10.1021/ef800455g CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
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Table 1. Properties of the Asphaltenes and the Resin elemental analysis (wt %) asphaltene or resin
C
H
S
N
O
H/Ca
mol wtb
density (g/cm3)
Maya Khafji Iranian Light Maya Resin
82.0 82.2 83.2 83.4
7.5 7.6 6.8 10.4
7.1 7.6 5.9 4.6
1.3 0.9 1.4 0.4
1.2 1.1 1.5 0.5
1.1 1.1 1.0 1.5
4000 4000 2400 720
1.1767 1.1683 1.1669 1.0367
a
Atomic ratio. b From vapor pressure osmometry.
but the distribution width of the D values was invariant with concentration, indicating that the effect of aggregation of the asphaltene was small. Norinaga et al.18 measured diffusion coefficients of Khafji VR asphaltene in pyridine and showed that the D values decreased with increasing asphaltene concentrations, which suggests that aggregation occurred even at low concentrations of 0.1 wt %. PGSE 1H NMR is a useful method to the aggregation behaviors of asphaltenes in solution. In this study, we measured the D of three kinds of asphaltenes and one resin in chloroform by PGSE 1H NMR and estimated the sizes of the aggregates formed. The effects of asphaltene concentration on aggregate size are discussed. Experimental Section Samples. n-Heptane-insoluble asphaltenes from VR (>500 °C) of Khafji, Iranian Light, and Maya crude oils, and a resin from Maya VR were used. Detailed procedures have been described elsewhere.5,6 Table 1 shows the properties of the samples used. The carbon and hydrogen contents were measured using a CHNO-Rapid analyzer (Elementar). The sulfur, nitrogen, and oxygen contents were measured using an AQS-6W sulfur tester (Tanaka Scientific Instruments), an ANTEK7000 (Antek), and a CHN-ORapid (Heraeus) analyzer, respectively. Densities were measured in conformity with Japanese Industrial Standard JIS K 7112, using a DMA45 apparatus (Paar). Molecular weights were measured by vapor pressure osmometry method using an Automatic Molecular Weight Apparatus (Rigosha).2,6,8,10 Polystyrenes (molecular weight (MW) ) 350, 700, 1400, 2000, 2500, and 3000), poly-4-vinylpyridine (PVP, MW ) 1800), poly-2-vinylnapthalene (PVN, MW ) 1800), pyrene, and coronene were used as reference compounds. Deuterated chloroform (>99.95% D) was used as a solvent. PGSE 1H NMR. PGSE 1H NMR spectra were measured using a Brucker DRX-300 NMR spectrometer. Asphaltene concentrations in deuterated chloroform were 30, 10, 1, and 0.1 g/L. Each solution was placed in a flat-bottomed Pyrex glass NMR tube (diameter 10 mm). To avoid convection and vaporization, which can cause measurement errors, a plug that covered the surface of the solution was inserted into the NMR tube. For the PGSE 1H NMR measurements, two gradient pulses with identical magnetic field gradients, G (G/cm), and pulse widths, δ, were applied, separated by a time, ∆. Diffusion of the molecules in an inhomogeneous static field during the two field gradients caused the spin-echo signal to decay. The PGSE 1H NMR measurements determine the displacement of protons in the diffusion time, ∆, between the two gradient pulses, and signal intensities, A, are described by the following equation:
Figure 1. Pulsed-field gradient spin-echo 1H NMR spectra of 10 g/L Maya asphaltene in deuterated chloroform by varying magnetic filed gradient (G/cm). Chemical shift (ppm) was calibrated with respect to tetramethylsilane proton peak at 0 ppm.
Equation 1 shows that D is obtained from the slope of plots between γ2G2δ2(∆ - δ/3) and log A. To check the accuracy of D, we measured the D of water molecules in bulk water with dissolved CuSO4 that was added to reduce the T1 of the water protons. A D of 2.0 to 2.2 × 10-9 m2/s obtained, consistent with the reported value of 2.1 × 10-9 m2/s.19
Results and Discussion
where A0 is the intensity at zero gradient, γ is the gyromagnetic ratio, and D is the diffusion coefficient (m2/s). In this study, δ and ∆ were 1 and 10 ms, respectively, and G was varied from 5 to 1144 G/cm. The number of scans was 16-4000, depending on the concentration of the solution. Measurements were taken at 21 °C.
Characteristics of PGSE 1H NMR of Asphaltenes Compared to Reference Compounds. The PGSE 1H NMR spectra of 10 g/L Maya asphaltene solution in CDCl3 are shown as a function of the magnetic field gradient (G) in Figure 1. The peaks at around 1 and 7 ppm were assigned to aliphatic protons and aromatic protons, respectively. The aliphatic peak intensity was over 10 times higher than the aromatic peak intensity, so the signals of the aromatic peaks became very low in the spectra. Plots of log A against γ2G2δ2(∆ - δ/3) for the aliphatic and aromatic proton peaks for this sample are shown in Figure 2. The intensity decay for both the peaks at 1 and 7 ppm was due to the diffusion of the asphaltene molecule and was not due to intramolecular motion of each moiety in the molecule. The slopes in Figure 2 gave the same D values, i.e.,
(18) Norinaga, K.; Wargardalam, V. J.; Takasugi, S.; Iino, M.; Matsukawa, S. Energy Fuels 2001, 15, 1317–1318.
(19) Chemical Society of Japan. Chemistry Handbook; Maruzen: Tokyo, 1984; Vol. 2.
A ) A0 exp[-γ2G2Dδ2(∆ - δ/3)]
(1)
Asphaltene Aggregation by NMR
Figure 2. Semilogarithmic plot of A versus K ()γ2G2δ2(∆ - δ/3)) (106 m2/s) for the aliphatic and aromatic protons of 10 g/L Maya asphaltene in deuterated chloroform.
Figure 3. Diffusion time, ∆ (ms), dependency of diffusion coefficient, D (10-10 m2/s), for Maya asphaltene and polystyrene with MW of 700 and 3000 (10 g/L).
1.3 × 10-10 m2/s. For all the samples, same D values were obtained for the aliphatic and aromatic proton peaks, so, in the following discussion, only results for aliphatic protons are shown. Figure 3 shows the diffusion time (∆) dependency of D for Maya asphaltene and polystyrene with MWs of 700 and 3000 (10 g/L). Polystyrenes showed normal behavior (i.e., no dependency of D on ∆), but the D of Maya asphaltene increased with increasing ∆. Similarly, ∆ dependency was observed for the other asphaltenes. This may have occurred because they are a complex mixture of many different molecules, and as a result, a wide distribution of D values exists. The contribution of large species (asphaltene and aggregates) with a small D decreases with increasing ∆; because of their short spin-spin relaxation times, T2, they decay faster than smaller molecules. This small contribution of small D species increases the average D values
Energy & Fuels, Vol. 22, No. 6, 2008 3991
Figure 4. Semilogarithmic plot of A versus K ()γ2G2δ2(∆ - δ/3)) (106 m2/s) for the aliphatic and aromatic protons of 1 g/L Maya asphaltene in deuterated chloroform.
with increasing ∆. In this study, a short diffusion time (∆) of 10 ms that included the contribution of large molecules was chosen. Figure 4 shows that the plots of log A against γ2G2δ2(∆ δ/3) for the aliphatic and aromatic proton peaks of Maya asphaltene at a lower concentration (1 g/L) did not give a straight line, unlike the case in Figure 2 (10 g/L). For an even more dilute solution (0.1 g/L), the other asphaltenes and resin, the plots also showed curves. However, no such curves were seen with the reference compounds (polystyrene with a MW of 350 to 3000, PVN, PVP, pyrene, and coronene at 1 g/L, or for polystyrene with a MW of 700 to 3000 at 0.1 g/L). Several reasons may exist for the results obtained with the diluted ¨ stlund et al.17 measured PGSE asphaltenes and resin solution. O 1H NMR spectra of asphaltene from Venezuelan oil VR in deuterated toluene at concentrations of 0.044 to 5 wt %, and the curves observed at all concentrations were well fitted to lines calculated by considering a broad distribution of species with different diffusion coefficients. Desando et al.16 measured PGSE 1H NMR spectra of Athabasca oil sand asphaltene in deuterated chloroform and the plots were found to be composed of two very different slopes. They attributed the first decaying component to CHCl3 because of incomplete deuteration (99.8% deuterium content). However, in our study, the curves observed were not likely due to the presence of CHCl3 because further dilution from 1 to 0.1 g/L did not increase the contribution of the first decaying component. In this study, the observed curves can be represented to a first approximation by a two-exponential fit with fast and slow D values. Two-state model (twocomponent model) is widely used as a simple method to consider the D value distribution.12-14 As shown in Figure 4, the experimental points fit well on lines calculated by a twoexponential fit and can be described by the following equation: A ) A0{ff exp[-γ2G2Dfδ2(∆ - δ/3)] + fs exp[-γ2G2Dsδ2(∆ δ/3)]} (2) where ff and fs are fractions for the fast and slow diffusion, respectively, and Df and Ds are the D values for the fast and
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Table 2. Diffusion Coefficient Values (×10-10 m2/s) for the Asphaltenes and the Resin in Deuterated Chloroform
Table 3. D (×10-10 m2/s) Values for Reference Compounds in Deuterated Chloroform
concentration asphaltene or resin
30 g/L
10 g/L
Maya
1.0
1.3
Khafji
1.0
1.6
Iranian Light
1.0
1.6
Maya Resin
2.5
2.7
1
g/La
(%)
2.2 (70) 1.3 (30) 3.4 (87) 1.0 (13) 2.6 (45) 2.0 (55) 4.3 (62) 2.3 (38)
concentration 0.1
g/La
(%)
5.1 (70) 0.9 (30) 5.6 (88) 1.0 (12) 6.5 (50) 1.1 (50) 8.7 (69) 2.1 (31)
a The values in parentheses are the percentages of each component obtained from the echo signal intensity, calculated from the ratio of ff and fs values in eq 2 for each component.
slow diffusion, respectively. Note that the ff and fs are fractions on echo signal intensities and have influence T2 as follows: ff⁄s)Ff⁄s exp(-2τ/T2,f/s)/[Ff exp(-2τ/T2,f) + Fs exp(-2τ/T2,s)] (3) where Ff and Fs are fractions of diffusants. For the other asphaltenes and the resin, the two-exponential fit also showed good agreement with the experimental data. The D values of the two-exponential fit for each sample were determined from a least-squares fitting of the plots using eq 2. It should be emphasized here that we approximated the results by the two-exponential fit. Usually, the plot of log A against γ2G2δ2(∆ - δ/3) deviates from a straight line if there is D value distribution and the exchange between the distribution is not rapid enough to be averaged during the diffusion time (∆). The asphaltenes and the resin at high concentration, which showed the straight lines, probably had narrow D value distribution. The asphaltenes and the resin at low concentration, which did not show the straight lines, probably had the D value distribution and the exchange was not rapid. On the occasion of the twoexponential fit, two components having different D values really exist in certain cases, and a continuous distribution of components with several D values (polydispersity) exists in other cases. In this study, it is thought that two different kinds of groups (bimodal distribution) were probably formed in the asphaltene solutions. But, as an evaluation of the D value distribution was not carried out in the present study, the possibility of one wide distribution (monomodal distribution) of the components in the samples could not be excluded. Aggregation Behavior of Asphaltenes and Resin. Table 2 shows D values of asphaltenes and those of Maya resin. For the low concentrations (1 and 0.1 g/L), the D values of the fast and the slow components are shown. The percentages of each component obtained from the echo signal intensity, calculated from the ratio of ff and fs values in eq 2, are given in the table. At 0.1 g/L, the signal intensities were weak, and shimming and phase adjustments of the NMR spectra were difficult, so the obtained D values and the percentages may not be as quantitative. Table 2 shows that D values decrease with increasing concentrations. For example, Maya asphaltene gave D values of 3.8, 1.9, 1.3, and 1.0 × 10-10 m2/s at 0.1, 1, 10, and 30 g/L, respectively (D values for 1 and 0.1 g/L are the weighted mean of the two components). Table 3 shows D values of the reference compounds (polystyrenes, poly-4-vinylpyridine, poly-2-vinylnapthalene, pyrene, and coronene). Unlike the asphaltenes and the resin, the D values of the reference compounds did not decrease with concentration. In general, D values of the polymers are constant with concentration change, and decrease with increasing molecular weight, if no aggregation occurs.12,14
references (MW)
10 g/L
1 g/L
polystyrene (350) polystyrene (700) polystyrene (1400) polystyrene (2000) polystyrene (2500) polystyrene (3000) poly-4-vinylpyridine (1800) poly-2-vinylnaphthalene (1800) pyrene (202) coronene (300)
6.2 6.1 4.0 3.6 2.7 2.6 3.6 2.7 15.4 -
6.3 6.0 4.4 3.7 2.9 2.9 3.8 3.1 16.2 11.1
The D value for polystyrene with a MW of 350 was far lower than that of coronene (MW ) 300), probably due to differences in molecular shape, size, and interactions. The concentration dependency of D values for the asphaltenes and the resin can be attributed to self-aggregations. Aggregate sizes increase with increasing concentrations. The smaller D values of the asphaltenes compared to the reference polymers with similar MWs, for example, 1.6 of Iranian light asphaltene (MW ) 2400) and 2.7 of polystyrene (MW ) 2500) at 10 g/L, also indicate that the asphaltene aggregated, resulting in smaller D values. D values of the asphaltenes ((0.9-2.0) × 10-10 m2/ s) at 10 and 30 g/L were similar to those for polystyrene with a MW of 4000 to 10 000.12 The distribution of D values ((0.9-6.5) × 10-10 m2/s, including both the fast and the slow D components) obtained at 0.1 and 1 g/L were similar to those for polystyrene with a MW of 350 to 10 000.12 D values for the resin (MW ) 720), 2.5 × 10-10 and 2.7 × 10-10 m2/s at 10 and 30 g/L, respectively, were similar to those of polystyrene with a MW of 2500 to 3000. These results indicate that the asphaltenes and the resin aggregated under these experimental conditions. Table 2 shows that Maya asphaltene gave smaller D values than the other two asphaltenes at 10 g/L. This was consistent with the findings of Tanaka et al.8 that Maya asphaltene aggregates were the largest in 1-methylnaphthalene and quinoline at 25 °C from SANS measurements, which is related to the high coking tendency of Maya asphaltene. However, Table 2 shows that, in general, no large difference in D values existed among the three asphaltenes, possibly because their aggregation tendencies are not that different. Table 2 also shows that the resin had larger D values (i.e., smaller aggregate sizes) than the asphaltenes. Some studies indicate that the aggregation of asphaltenes in solution proceeds stepwise when the asphaltene concentration increases, or temperature decreases, as shown below.2,20 monomer < small aggregates < medium aggregates < large aggregates (precipitation) Small aggregates are also widely accepted to form through π-π interactions between aromatic rings, and medium aggregates are mainly formed through van der Waals and acid-base interactions between small aggregates; further aggregation of medium aggregates results in precipitation. The results in this study can be explained by this stepwise mechanism. At high asphaltene concentrations, 10 and 30 g/L, all aggregates were medium aggregates, and at low asphaltene concentrations, 0.1 and 1 g/L, small and medium aggregates coexisted, corresponding to the two observed components with different average D values. The slow D values at 0.1 and 1 (20) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barre, L.; Feinstein, D.; Broseta, D. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter V.
Asphaltene Aggregation by NMR
Energy & Fuels, Vol. 22, No. 6, 2008 3993
g/L, (0.9-2.0) × 10-10 m2/s, were similar to those of medium aggregates at 10 and 30 g/L, (1.0-1.6) × 10-10 m2/s, suggesting that medium aggregates of similar size existed at all concentrations. Many studies have been conducted to determine the concentration at which asphaltenes begin to aggregate when the asphaltene concentration increases. For toluene at room temperature to 35 °C for various asphaltenes, such as Kuwait and Tatarstan (Russia), the concentrations at the onset of aggregation were reported to be 0.01-22.9 g/L by surface tension,21-23 calorimetry,24 viscosity,25 and spectroscopy.26 The reason for this wide range of concentrations is not only due to differences in the kinds of asphaltenes used but may also be due to differences in the sensitivity of the methods used for the determination of the concentration. Thus, suggesting that asphaltene aggregates even at the lowest concentration, 0.1 g/L, is not unreasonable. Table 2 also shows that a similar mechanism may apply to the resin, although no evidence exists that resin aggregation proceeds in a stepwise manner. If asphaltene aggregates are assumed to be spherical, we can calculate their average hydrodynamic radius, rav, from the D values using the Stokes-Einstein equation: D ) kT/(6phsrav)
(4)
where k is the Boltzmann constant, T the temperature (K), and hs is the viscosity of the solvent (0.56 mPas for CDCl3 at 21 °C). Table 4 shows that the rav values for asphaltenes are 2.4-3.8 nm at concentrations of 10 and 30 g/L, and between 3.5 and 4.2 nm (for the slow D value component) and 0.6-0.7 nm (for the fast D value component) at 0.1 g/L. Tanaka et al.8 estimated the size of the aggregates of the same three asphaltenes examined here in 5 wt % solutions in 1-methylnaphthalene and quinoline using SANS. They reported that if a spherical structure was assumed for the shape of asphaltene aggregates, the radius was 3-4 nm at 25 °C, and if an ellipsoidal structure was assumed, the semiaxes would be 6-8 nm and 2-3 nm, respectively, at 25 °C. Their results are in fair agreement with ours in high-concentration (30 g/L) solutions. Tanaka et al.2 also proposed a hierarchy model (similar to the stepwise model, (21) Mohamed, R. S.; Ramos, A. C. S.; Loh, W Energy Fuels 1999, 13, 323–327. (22) Rogel, E.; Leon, O.; Torres, G.; Espidel, J. Fuel 2000, 79, 1389– 1394. (23) Zhang, L.; Yang, G.; Que, G. Fuel 2005, 84, 1023–1026. (24) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38–42. (25) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Fuel 2006, 85, 1465–1472. (26) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Fuel 2004, 83, 1823–1828.
Table 4. Hydrodynamic Radius, rav (nm), for the Asphaltenes and the Resin in Deuterated Chloroform concentration 30 g/L
10 g/L
1 g/La
0.1 g/La
Maya
3.8
2.9
Khafji
3.8
2.4
Iranian Light
3.8
2.4
Maya Resin
1.5
1.4
1.7 2.9 1.1 3.8 1.5 1.9 0.9 1.6
0.7 4.2 0.7 3.8 0.6 3.5 0.4 1.8
asphaltene or resin
a
rav values for slow and fast D value components, respectively.
above) for asphaltene aggregates from XRD and SAXS measurements of solid asphaltenes (i.e., core aggregates (∼2 nm in size) to medium aggregates (3-15 nm), and to fractal aggregates (>100 nm)).2 Our estimated rav values of the asphaltenes suggest that our medium and small aggregates correspond, respectively, to their medium and core aggregates. To further understand the role of interactions, such as π-π interactions between aromatic rings, van der Waals interactions, and acid-base interactions, including hydrogen bonds, in the formation of asphaltene aggregates, the effect of solvents and temperature on the diffusion coefficients of asphaltenes should be examined in future studies. Conclusion Diffusion coefficients, D, of three asphaltenes from Khafji, Iranian Light, and Maya VR and a resin from Maya VR were measured in deuterated chloroform solution by PGSE 1H NMR. Using the Stokes-Einstein equation, the hydrodynamic radius, rav, was calculated from the D values obtained. The dependency of the D values of the asphaltene samples in chloroform solution measured by PGSE 1H NMR and rav on asphaltene concentrations (0.1-30 g/L) indicates that the aggregation of asphaltenes proceeds stepwise, monomer < small aggregates < medium aggregates, as concentration increases. At 1 and 0.1 g/L, all the asphaltenes examined showed the D values corresponding to the range of small to medium aggregates, and at higher concentrations, only medium aggregates were observed. No large difference in D values was observed among the three asphaltenes, although Maya asphaltene gave a slightly smaller D value. The resin gave larger D values than those of the asphaltenes. EF800455G
