ARTICLE pubs.acs.org/EF
Characterization of Physically and Chemically Separated Athabasca Asphaltenes Using Small-Angle X-ray Scattering Jesus Leonardo Amundaraín Hurtado,† Martin Chodakowski, Bingwen Long,‡ and John M. Shaw*,‡ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada ABSTRACT: Athabasca asphaltenes were characterized using small-angle X-ray scattering (SAXS). Two methods were used to separate asphaltenes from the Athabasca bitumen: namely, chemical separation by precipitation with n-pentane and physical separation by nanofiltration using a zirconia membrane with a 20 nm average pore size. The permeate and chemically separated samples were diluted in 1-methylnaphtalene and n-dodecane prior to SAXS measurements. The temperature and asphaltene concentration ranges were 50310 °C and 110.4 wt %, respectively. Model-independent analysis of SAXS data provided the radius of gyration and the scattering coefficients. Model-dependent fits provided size distributions for asphaltenes assuming that they are dense and spherical. Model-independent analysis for physically and chemically separated asphaltenes showed significant differences in nominal size and structure, and the temperature dependence of structural properties. The results challenge the merits of using chemically separated asphaltene properties as a basis for asphaltene property prediction in hydrocarbon resources. While the residuals for model-dependent fits are small, the results are inconsistent with the structural parameters obtained from modelindependent analysis.
’ INTRODUCTION Crude oils comprise a distribution of molecules with diverse chemical structures and molecular weights.1 Asphaltenes, a selfassembling fraction of crude oil, are responsible for numerous complications during production, refining, and transport.16 The properties and phase stability of crude oil depends, to a great extent, on the behavior of asphaltenes.2,7 Asphaltenes, remain poorly defined. Numerous operational definitions coexist. The absence of clarity on their properties is a major stumbling block, as it leads to divergent perspectives on the relevant physics and chemistry associated with them,8,9 and their impact on the properties of crude oils.10 A growing body of literature targeting the definition and the behavior of asphaltenes, from a molecular, thermodynamic, reaction, surface science, and aggregation perspective is beginning to emerge.11,12 However, the nominal mean molecular structure, the mean molar mass, and the methods for their determination continue to be debated in the literature.1322 In many industrial applications, the characteristics of petroleum liquids may be driven by asphaltene aggregate as opposed to molecular properties because aggregation occurs at ppm level concentrations.23,24 Here too, there is limited agreement on the size and structure of these aggregates, and credible nonparticulate interpretations of SAXS and rheology data continue to be reported.25,26 The literature follows two principle directions. The majority of reports concern the properties of chemically separated asphaltenes at low concentration in pure solvents. Few reports address the properties of chemically separated asphaltenes at high concentration or the behavior of asphaltenes as they exist in situ, that is, in their native hydrocarbon resources. Modelindependent and model-dependent measures of size and structure remain poorly discriminated as well, although progress on asphaltene structure determination with various r 2011 American Chemical Society
experimental and computational techniques has been reviewed critically.11,27,28 Model-dependent and model-independent measurements yield nominal sizes of asphaltene aggregates in diluents as high as 17 μm using particle size analyzers, confocal microscopy, and confocal laser-scanning and fluorescence microscopy,2931 while small-angle X-ray and small-angle neutron scattering (SAXS and SANS) yield aggregate sizes of 26 nm.3,4,23,32 With the joint use of SAXS and USAXS (ultrasmall-angle X-ray scattering), aggregates as large as 1 um have been reported. For Athabasca asphaltenes in toluene, Xu et al.33 obtained bimodal distributions for asphaltene aggregates in toluene, with the first peak at 3.3 nm and the second one at 14 and 9 nm for 5% and 15% asphaltene dispersions, respectively, on the basis of SAXS measurements. Rastegari et al.29 employed a particle size analyzer to investigate the particle growth in Athabasca heptane asphaltenes in heptane/toluene mixtures. Individual asphaltene aggregates were found to be approximately 0.52 μm in diameter. These aggregates tended to flocculate, up to 400 μm in size, with increasing asphaltene concentration and heptane content or decreasing mixing rate. The structural attributes of individual asphaltene aggregates remain equally uncertain. Different classifications of asphaltene aggregates have been proposed,34 and aggregates have been modeled and described as spheres, prolate ellipsoids, and oblate cylinders, which are dense, or fractal physical structures, all on the basis of experimental measurements.3,29,32,3538 From SAXS and SANS measurements in the power-law region, scattering coefficients (also referred to as power-law slopes), though diversely interpretable, as shown in Table 1, provide information on the Received: June 16, 2011 Revised: September 17, 2011 Published: September 19, 2011 5100
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Table 1. Mass-Fractal Power-Law Scattering56,62 fractal structures sharp interface
scattering coefficient
surface fractal
3< p > > #> > > pffiffiffi = 7 q2 Rg2 < erf 6 7 þ Bi exp 7 > 5 3 > q > > > > ; : "
ð3Þ
where G = N(ΔF)2V2 is the exponential prefactor, and B is a constant prefactor specific to the type of power-law scattering. Only one structural level was required to fit the scattering data in this work. The prefactors were regressed along with the radii of gyration and the scattering coefficients. For the example shown in Figure 3, a radius of gyration of Rg = 56 Å and a scattering coefficient of P = 2.1 were found. The quality of the fit, exemplified by the small values of the standardized residuals, is typical. Size Distribution Estimation. The size distribution is obtained from a summation over a discrete sized histogram of 5104
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Table 4. Repeatability of Unified Fitsa for 20 wt % AB Permeate in n-Dodecane radius of gyration (Å)
a
scattering coefficient
temp (°C)
1
2
3
std. dev. (Å)
1
2
3
std. dev.
70
62.4
63.4
62.1
0.7
1.96
1.94
1.98
0.03
90
60.5
61.2
60.6
0.4
1.96
1.94
1.96
0.01
110
58.9
56.4
57.8
1.3
1.95
2.04
1.97
0.04
130
60.6
60.9
60.9
0.2
1.90
1.91
1.91
0.003
150
60.3
58.3
59.5
1.0
1.90
1.95
1.91
0.03
170
57.7
59.1
59.4
0.9
1.97
1.90
1.94
0.04
190
59.0
59.3
59.1
0.1
1.94
1.94
1.94
0.001
210 230
60.2 57.0
59.9 59.3
59.2 58.3
0.5 1.2
1.91 2.02
1.93 1.94
1.93 1.99
0.01 0.04
250
57.9
58.1
57.9
0.1
2.03
1.99
2.03
0.02
270
59.8
59.4
57.5
1.2
2.04
2.06
2.08
0.02
290
60.8
61.8
63.1
1.1
2.08
2.03
1.98
0.05
310
62.2
61.3
60.8
0.7
2.16
2.15
2.15
0.01
eq 3.
spherical particles: IðqÞ ¼
∑k jΔFkj2 SðqÞ ∑i, k jFk ðq, Di, kÞj2VkðDi, kÞ fkðDi, kÞΔDi, k ð4Þ
where the subscript i represents the bins in the size distribution and ΔDi is the width of bin i. Subscript k denotes different scatterer populations; each population has its own binning index i,k. D is the dimension of the particle (radius for spheres). This equation allows the calculation of scattering intensity from particles in multiple populations of scatterers. The volume fraction distribution for scatterers, f(D), was chosen in this model because of its greater numerical stability.51 Of the four size distributions available in the Irena software tool for particle size distribution (Gauss (normal), log-normal, Lifshitz SlyozovWagner (LSW), and power-law), better fits were encountered for Gauss (normal), and log-normal size distributions, and log-normal distributions were assumed for particle size distribution modeling. Natural binning for the size distributions was performed. This method results in a nonlinear step size and uses two input parameters: the number of bins, Nb, set at 100 and a minimal fractional volume set at 0.01. The center of the first bin (smallest dimension) is found as the value for which the cumulative volume fraction is equal to 0.01, and the center of the last bin (largest dimension) is the value for which the cumulative function equals 0.99. The impact of interparticle interference was neglected. The form factor was assumed to be that of a spheroid or another shape, and the structure factor corresponded to the infinite dilution limit, S(q) = 1.
’ RESULTS AND DISCUSSION Measurement Quality. At a synchrotron, the speed of individual measurements is short—less than 1 s. This permits multiple exposures, at the same temperature, composition, and beam alignment. The repeatability of the emissions measurements is illustrated in Figure 4 for 20 wt % AB permeate in n-dodecane at 110 °C. All cases evaluated in this work are comparable. The repeatability of model-independent radii of gyration and scattering coefficient values derived from emissions data using the unified
Guinier-exponential/power-law approach is illustrated in Table 4 for 20 wt % AB permeate in n-dodecane. These results are typical for the large number of SAXS emissions evaluated. The 95% confidence limits for the radii of gyration values are less than 5 Å, while the 95% confidence limits for scattering coefficients values are less than 0.1. Model-dependent number mean radii are presented, and the particle size distribution widths were benchmarked using monodispersed silica nanospheres in water. Nominal mean sizes but not size distribution data were available. Figure 5 exemplifies the particle size distribution results obtained. For a 1 wt % silica nanosphere sample, nominal radius 5 nm, dispersed in water, a mean radius of 4.73 nm based on volume distribution was obtained. Because no size distributions were available, a comparison of the polydispersity obtained is not feasible. However, nanoparticle samples are expected to present some degree of polydispersity, and the number means are smaller than volume means. A parity plot for the average sizes obtained for the silica nanosphere samples, for more than 200 measurements, versus the nominal size of the silica spheres is presented in Figure 6. The SAXS mean particle size estimates are within 10% of the nominal size of the silica particles in all cases. Thus, the means and standard deviations of the model-dependent size distribution parameters— mean and width of the distributions—for asphaltenes are welldefined, as exemplified in Table 5. Model-Independent Analysis. Even a cursory examination of the emissions data reveals significant differences among the samples, as illustrated in Figure 7 for chemically separated AVR asphaltenes and diluted permeate with 4 wt % asphaltenes in ndodecane and 1-methylnaphthalene at 70 °C. Because the permeate comprises 10.4 wt % asphaltenes, the sample composition is 4 wt % asphaltenes, 36 wt % maltenes, and 60 wt % diluent. For the AVR asphaltene case, the mixture comprises 4 wt % asphaltenes and 96 wt % diluent. The diluted permeate samples possess a low q plateau followed by power-law scattering, permitting the extraction of size and structure parameters. The AVR asphaltene + diluent samples exhibit only power-law scattering in the low to middle q range, and only a structure parameter can be extracted from these data in a model-independent manner. This latter scattering pattern can be attributed to significant differences in the electron density between the 5105
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Figure 5. Computed silica particle size distribution for 1 wt % silica nanospheres (nominal radius 5 nm): (a) model fit and (b) calculated number and volume size distributions.
Figure 6. Parity plot for computed (this work) and nominal (supplied) number means for silica particles.
electron-rich and highly aromatic, asphaltene aggregates and the diluents. The upturn in the low q region is consistent with the
presence of large particles. This scattering pattern has been observed previously in small angle scattering studies of asphaltene in oil or solvents.11,39,64,65 The size of these large aggregates cannot be determined in the absence of a Guinier plateau. However, from the qmin value in Figure 7, these particles should be larger than 250 A. The absence of a low q upturn for the AB permeate samples indicates that nanofiltration successfully prevents large aggregates from entering the permeate at a nominal pore size of 200 A. Further, large aggregates do not appear to reform over time, as the nanofiltration experiments and SAXS measurements were performed weeks apart. Figure 8a shows the impact of temperature on the SAXS emissions for 4.3 wt % AVR asphaltenes in n-dodecane. The scattering profiles show a two level structure in the solvent; both nanoscale and microscale aggregates appear to be present. As the temperature increases, the power-law slope decrease and the location of the Guinier plateau shifts to higher q values, suggesting that large compact aggregates present in dodecane at low temperatures break into smaller, looser structures at high temperatures. By contrast, Figure 8b, SAXS emission profiles of 4 wt % AVR asphaltenes in 1-methylnaphthalene are almost 5106
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Table 5. Repeatability of Model-Dependent Particle Size Distribution Calculationsa for 80 wt % AB Permeate in n-Dodecane distribution mean (Å)
a
temp (°C)
1
2
70
40.3
40.2
90
39.1
40.3
110
33.3
130
half height full width (Å) 3
std. dev. (Å)
1
2
3
std. dev. (Å)
39
0.7
8.7
9
8.8
0.2
39.9
0.6
9
8.6
9
0.2
37.6
38.6
2.8
9.6
8.6
8.7
0.6
41.6
40.8
41.4
0.4
9.5
150
40.9
41.1
40.9
0.1
9.2
170
40.2
40.3
41.6
0.8
190
41.1
41.9
41.9
0.5
210 230
39.8 41.7
40.7 41.6
41.3 41.2
250
39.2
40.3
270
40.3
41.6
290
38.8
310
42.9
11
11.5
1.0
9.4
8.8
0.3
9.3
8.3
12.5
2.2
9.8
11.1
11
0.7
0.8 0.3
8.1 8.2
10.4 8.5
10.5 8.2
1.4 0.2
41.4
1.1
11.5
12.1
13.5
1.0
41.2
0.7
8.5
10.9
9.8
1.2
38.4
39.9
0.8
11.3
10.1
11.4
0.7
41.4
41.6
0.8
9.3
9.3
9.1
0.1
eq 4.
Figure 7. SAXS intensity profiles for AVR pentane asphaltenes + diluent and diluted permeate samples containing 4 wt % asphaltenes at 70 °C.
temperature independent for both the nanoscale and microscale aggregates. Such diluent-dependent structural effects can be simulated using quantum mechanics, on the basis of either force fields or potential energies related to interactions between and among hypothetical asphaltene molecules and diluents.67,68 However, solvent-dependent behaviors may be equally well attributable to complex phase transitions occurring in AVR pentane asphaltenes over the same temperature interval.69,70 Figure 8c presents intensity profiles for 10 wt % AB retentate in 1-methylnaphthalene. The asphaltene content in this sample was 5.67 wt %, permitting a meaningful comparison with the 4% AVR asphaltene. The emissions only display power-law scattering, as a result of the large aggregates in the mixture. Scatterers larger than 20 nm are anticipated for retentate samples. The parallel emissions curves in Figure 8c indicate a temperature invariant
scattering coefficient for these asphaltene aggregates with a value of approximately 3.2. This value is much higher than that obtained for bitumen and AVR asphaltenes in the same diluent. Interpretations of this result include that these domains constitute bulk asphaltene rich phases or fully dense aggregates. Apparent Radii of Gyration, Rg. With the SAXS beamline configuration used at APS, the observable size range is limited to radii less than 100 nm. The radii of gyration values fell well below this size limit for permeate samples, as shown in Figure 9. This result is superficially unsurprising, as the permeates passed through a 20 nm membrane. However, the bitumen was nanofiltered, and the individual samples were prepared weeks prior to the SAXS measurements. If asphaltenes possess an equilibrium size distribution within a crude, aggregate regrowth would be expected. While not probed directly, there is no evidence to support significant aggregate regrowth within the nanofiltered bitumen sample. The radii of gyration results for AB permeate in n-dodecane are presented in detail in Figure 9a. Radii of gyration values are composition dependent but are temperature invariant within experimental error. The radii of gyration increase from 18 Å in the permeate to 60 Å at 90 wt % dodecane. In 1-methylnaphthalene, Figure 9b, radii of gyration are also temperature invariant, and at 1525 Å, the values are significantly smaller than the values in n-dodecane. The temperature averaged results for the radii of gyration for AB permeate + diluent mixtures are summarized in Figure 10. It is evident that even though the radii of gyration remain in the size range proposed for elementary asphaltene aggregates, the nature and concentration of the diluents have a clear effect on aggregate size. The 80 wt % AB permeate in 1-methylnaphthalene result appears anomalous but does not disrupt the trend of the data. Aggregates are small in the permeate and are unaffected by dilution with 1-methylnaphthalene. The aggregate radii grow to approximately 60 Å on dilution with n-dodecane. The samples are well mixed, and there is no evidence of larger aggregates in the SAXS measurements or precipitation in the sample tubes. Scattering Coefficients, P. While scattering coefficient values are frequently associated with specific particle shapes and with aggregation models, assigning a particle shape or defining particle 5107
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Figure 8. SAXS emission profiles for (a) 4.3 wt % AVR asphaltenes in n-dodecane; (b) 4 wt % AVR asphaltenes in 1-methylnaphthalene; and (c) 10 wt % AB retentate in 1-methylnaphthalene (5.7 wt % asphaltenes). Temperature is a parameter.
structure based solely on a scattering coefficient value constitutes an over interpretation, as diverse shapes32,36,40,71 and structures possess similar values.6,7274 No attempt is made to assign structures to the scattering coefficient values in this work. The scattering coefficients for AVR asphaltenes in n-dodecane are shown in Figure 11. At low concentrations, Figure 11a, the
scattering coefficients drop sharply in the temperature range 120150 °C, indicating that a significant structural change has occurred. From Table 1, the nature of the change is ambiguous. At higher asphaltene concentrations, Figure 11b, the impact of temperature on scattering coefficients is more gradual, but the transition to a lower scattering coefficient plateau is complete 5108
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Figure 9. Radius of gyration for AB permeate in diluents (a) n-dodecane and (b) 1-methylnaphthalene. The wt % permeate is a parameter in both figures: 10% (green square); 20% (blue circle); 40% (black triangle); 80% (red del); and 100% (dark yellow).
by 200 °C. Scattering coefficients for AVR asphaltenes in 1-methylnaphthalene, Figure 12, are temperature and concentration invariant, from 75 to 250 °C. Above 250 °C, thermolysis reactions begin to occur in the samples. Bagheri et al.70 found liquid crystals in precipitated asphaltenes and showed that Athabasca pentane asphaltenes comprise at least two solid phases at room temperature (solid I and solid II). As temperature increases, solid I undergoes an endothermic transition to liquid, which begins at room temperature. Solid II, on the other hand, undergoes an endothermic transition to liquid crystal at 65 °C, followed by an exothermic dissolution that ends at approximately 150 °C. Above 150 °C, asphaltenes are composed by two phases, solid I and liquid. The impact of diluents on this complex phase transition remains a matter of ongoing inquiry. Scattering coefficient values for diluted permeate samples are temperature invariant up to more than 275 °C. This behavior is consistent with the radius of gyration values that are similarly temperature invariant. Composite average values are summarized in Figure 13. The scattering coefficient for AB permeate is ∼1.3. Upon dilution in 1-methylnaphthalene, the scattering coefficient is slightly smaller and composition invariant, suggesting that the structures present are similar to one another. In n-dodecane, the scattering coefficient is a function of dilution and reaches values greater than 2. As aggregates in n-dodecane are also larger than in 1-methylnaphthalene, this result appears to conflict with the suggested hierarchy of Tanaka et al.34 for asphaltene aggregates: core aggregates, formed by ππ stacking of asphaltene molecules and with a size of around 20 Å; medium aggregates, consisting of secondary aggregates formed from interactions with media, with a size range of 50500 Å; and fractal aggregates, comprised of secondary aggregates that result from diffusion-limited cluster aggregation (DLCA) or reactionlimited cluster aggregation (RLCA). According to this hierarchy, small aggregates are more densely packed than larger aggregates and therefore should possess larger scattering coefficients than the less structured but larger aggregates. The apparent conflict can only be resolved if the dimensionality of the aggregates changes; for example, rods (scattering coefficient 1) form clusters of rods (P ∼ 2). The scattering coefficients for diluted permeates in n-dodecane differ significantly at low and at high temperatures vis-a-vis
Figure 10. Temperature independent average aggregate size for AB permeates diluted in n-dodecane and 1-methylnaphthalene (radii of gyration, open symbols; mean radius from the calculated polydispersed sphere model, filled symbols).
the scattering coefficients for AVR asphaltenes in dodecane. The scattering coefficients for the AVR pentane asphaltenes transition from a higher to a lower value as temperature is raised, while the scattering coefficient for the diluted permeate is invariant. Neither the transition nor the start and end states are shared. In 1-methylnaphthalene, the scattering coefficients are temperature and composition invariant. For diluted permeates the value is ∼1.2, while for diluted AVR asphaltenes the value is ∼2.2. Again, the structures present differ significantly. The absence of a temperature dependence for the scattering coefficient is also consistent with a prior finding showing that 1-methylnaphthalene acts as an asphaltene aggregation inhibitor and not as a dispersant.67 It is also important to recognize that only a fraction of the AVR asphaltenes is observed in the SAXS measurements and comparisons among observations must be made with caution. Athabasca retentate (56.7 wt % pentane asphaltenes) diluted to 10 wt % in 1-methylnaphthalene only displayed power-law scattering. This result was expected because large aggregates are 5109
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Figure 11. Scattering coefficients for the AVR asphaltenes in C-12: (a) lower and (b) higher concentration range.
Figure 12. Scattering coefficients for AVR asphaltenes in 1-methylnaphthalene. The wt % of asphaltenes is a parameter.
present in the mixture. More than 30% of pentane asphaltenes in Athabasca bitumen do not pass through a 200 nm membrane during nanofiltration.8,9 The temperature independent scattering coefficient derived from these data is 3.25 ( 0.15, a value significantly larger than that for diluted permeate, where the comparative value is 1.2, or that for diluted asphaltenes, where the comparative value is 2.2. It is approaching the value of 4 obtained for large aggregates, which was detected using a combination of SAXS and USAXS.3 Model-Dependent Aggregate Size Distribution Analysis. AVR pentane asphaltenes present a significant challenge for particle size distribution estimation because the SAXS emission profiles for these samples do not possess low q plateaus, which corresponds to the average structure size according to Guinier’s law (eq 1), and radii of gyration cannot be obtained. However, an approximate size range can be extracted from q-minimum values,3,4 where qmin1 is the maximum particle size observable in experiments. The scatterers present possess particle sizes larger than qmin1. The AVR asphaltenes include scatterers larger than 250 Å in all cases. Particle size distribution calculations were successfully performed for Athasbaca bitumen permeate samples. Several particle shapes such as spheres,24,33,40 thin disks,32,39 oblate cylinders, and prolate ellipsoids35,42 have all been proposed for characterizing asphaltenes and asphaltene fractions. However, small-angle
Figure 13. Impact of the addition of 1-methylnaphthalene and ndodecane on the temperature independent scattering coefficients of AB permeates.
scattering emission profiles tend to be featureless. Significant differences in scatterer shape only induce subtle impacts in the resulting intensity profiles. For the temperature invariant aggregate radii calculations reported in Figure 10, asphaltene aggregates were assumed to be polydispersed hard spheres. For AB permeates diluted in 1-methylnaphthalene, the values of the radii fall in the 1020 Å size range and conform with those obtained from the model-independent radius of gyration calculations. However, the fitted average radii should be larger than the radii of gyration if the scatterers are spherical. For AB permeates diluted in n-dodecane, the computed radii values and trends with composition diverge from those obtained from radii of gyration calculations. The lack of agreement, despite high quality fits, reflects the inappropriateness of assumptions present in the model, such as particle shape, and particle density. Results from radius of gyration, scattering coefficient, and particle size distributions all seem to concur with Stoyanov et al.,67 who suggested 1-methylnaphthalene acts as an aggregation inhibitor and lacks the dispersant qualities of other aromatic materials such as toluene. The effect of n-dodecane is clear. Assumptions made in model-dependent analyses, however, need to be assessed carefully to reconcile results with model-independent analyses. 5110
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’ CONCLUSIONS Chemically separated AVR pentane asphaltenes displayed power-law scattering in the entire q-range studied, indicating the presence of large aggregates that are not observable in the experimental window of the measurements. Values of qmin indicate the presence of aggregates larger than 250 Å in AVR asphaltenes when diluted in either n-dodecane or 1-methylnaphthalene. Only the properties of smaller aggregates are observed directly in these measurements. Changes in scattering coefficient values with temperature suggest that AVR asphaltene aggregates undergo a significant structural transition when diluted in n-dodecane, where larger and more numerous aggregates present at lower temperatures appear to break into lower mass and more loosely structured aggregates at higher temperatures. The temperature of the transition is composition dependent, but it falls in the range 110200 °C. This transition is consistent with calorimetric, rheologic, and cross polarized light microscopy results for neat asphaltenes, which undergo a complex sequence of phase transitions from solid to liquid that include this temperature range, as noted elsewhere.70 Upon dilution in 1-methylnaphthalene, AVR asphaltenes possess a different structure at room temperature than in n-dodecane, and the structure is both composition and temperature invariant. This result may be linked to the dissolution of the liquid crystal phase present in asphaltenes on exposure to this diluent, as observed with cross polarized light microscopy in toluene. By contrast, more than 60% of scatterers present in Athabasca bitumen passed through a 200 Å filter. Scatterers in nanofiltered and diluted nanofiltered bitumen samples are small, on the basis of the radii of gyration measurements, compared with the filter size and with AVR asphaltenes. They also presented temperature invariant structural parameters, which differed from AVR asphaltenes. On dilution in 1-methylnaphthalene, the radii of gyration were in the range 1520 Å and the scattering coefficients were ∼1.2. On dilution in n-dodecane, the radii of gyration and scattering coefficient values increased with dodecane concentration to ∼60 Å and ∼2, respectively. The values of the scattering coefficients for both AVR pentane asphaltenes and for asphaltenes in nanofiltered and diluted Athabasca bitumen are subject to multiple interpretations and provide little direct information concerning the structure, in the absence of exogenous and independent structural data. For example, the application of a hard sphere model to the asphaltenes in nanofiltered and diluted Athabasca bitumen provides good numerical fits, but the means of the size distributions are inconsistent with the experimental radii of gyration and scattering coefficient values. This study also reinforces the need to extend the q-range for the intensity profiles associated with chemically or physically separated asphaltenes.75 The gulf between the structural properties of physically separated asphaltenes in bitumen and those of chemically separated AVR asphaltenes, with an essentially identical chemical composition, underscores the need to perform research directly on the fluids of industrial interest if the understanding and improvement of industrial processes are primary goals. ’ AUTHOR INFORMATION Corresponding Author
*Corresponding Author E-mail:
[email protected]. Present Address †
now working with Schlumberger, Russia;
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Note ‡
visiting professor
’ ACKNOWLEDGMENT The authors thank Dr. Bei Zhao for preparing nanofiltered bitumen samples and Dr. Soenke Seifert for assistance with SAXS measurements. Funding was provided by the sponsors of the NSERC Industrial Research Chair in Petroleum Thermodynamics: Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta Innovates, KBR Energy and Chemical, Halliburton Energy Services, Imperial Oil Resources, ConocoPhillips Canada Resources Corp., Shell Canada Ltd., Nexen Inc., and Virtual Materials Group (VMG). Total E&P Canada Ltd. is gratefully acknowledged. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38. We also acknowledge support from the Alberta Synchrotron Institute, our host team at APS—BESSRC-CAT 12—and the helpful comments made by the reviewers at Energy & Fuels. ’ NOMENCLATURE B = constant prefactor specific to the type of power-law scattering. D = the dimension of the particle. erf( ) = error function. F(q, D) = form factor model. f(D) = volume distribution function. G = exponential prefactor. I(q) = normalized scattering intensity. N = number density of particles in the scattering volume. NMR = nuclear magnetic resonance. P = scattering coefficient. q = scattering vector, also denoted as Q R = radius of a sphere. Rg = radius of gyration. S(q) = structure factor. V = volume. ΔF = contrast, electron-density difference between the particle and the solvent θ = scattering angle. λ = X-ray wavelength. ’ REFERENCES (1) Chang, C.; Fogler, H. S. Langmuir 1994, 10, 1749–1757. (2) Speight, J. G. Oil Gas Sci. Technol. 2004, 59, 479–488. (3) Savvidis, T. G.; Fenistein, D.; Barre, L.; Behar, E. AIChE J. 2001, 47, 206–211. (4) Gawrys, K. L.; Spiecker, P. M.; Kilpatrick, P. K. Pet. Sci. Technol. 2003, 21, 461–489. (5) Fenistein, D.; Barre, L.; Broseta, D.; Espinat, D.; Livet, A.; Roux, J. N.; Scarsella, M. Langmuir 1998, 14, 1013–1020. (6) Fenistein, D.; Barre, L. Fuel 2001, 80, 283–287. (7) Jestin, J.; Barre, L. J. Dispersion Sci. Technol. 2004, 25, 341–347. (8) Zhao, B.; Becerra, M.; Shaw, J. M. Energy Fuels 2009, 23, 4431–4437. (9) Zhao, B.; Shaw, J. M. Energy Fuels 2007, 21, 2795–2804. (10) Hasan, M. D. A.; Fulem, M.; Bazyleva, A.; Shaw, J. M. Energy Fuels 2009, 23, 5012–5021. (11) Sheu, E. Y. J. Phys.: Condens. Matter 2006, 18, S2485–S2498. (12) Mullins, O. C.; Sheu, E. Y.; Hammami, A.; Marshall, A. G. Asphaltenes, Heavy Oils, and Petroleomics.: Springer: New York, 2007. (13) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377–1384. 5111
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