490
Energy & Fuels 2002, 16, 490-496
Probing Order in Asphaltenes and Aromatic Ring Systems by HRTEM Atul Sharma,† Henning Groenzin,‡ Akira Tomita,† and Oliver C. Mullins*,‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan, and Schlumberger-Doll Research, Ridgefield, Connecticut 06877 Received October 2, 2001. Revised Manuscript Received December 19, 2001
Recent findings regarding the controversy surrounding asphaltene molecular weight coupled with increasing understanding of their molecular structure has enabled the understanding of asphaltene properties. It has been shown previously that larger ring systems require more alkane substituents to maintain a balance between ring stacking propensity vs steric repulsion. Here, stacking and its disruption in asphaltenes and aromatic ring systems are explored using highresolution transmission electron microscopy (HRTEM). It is shown that molecularly disparate asphaltenes exhibit stacking invariants. Solubility data herein suggests these stacking invariants naturally follow from the solubility classification of asphaltenes.
I. Introduction “If you want to understand function, study structure,” exhorts Francis Crick.1 For systems such as asphaltenes that are defined by an operational solubility classification (e.g., soluble in toluene, insoluble in n-heptane2-5) as opposed to a standard chemical structural definition, Crick’s exhortation is doubly important. For 20 years, this sage counsel was not applicable to asphaltene science due to the order of magnitude controversy over asphaltene molecular weight. The recent findings regarding this controversy6,7 confirming early mass spectroscopy studies,8 increasingly supported by a wide variety of techniques,9-12 has enabled Crick’s dictate to be followed at long last in asphaltene science. Petroleum asphaltenes have relatively low molecular weights (∼750 amu) and coal asphaltenes are even smaller (∼500 amu).6,7 This knowledge coupled with bulk molecular structural information has led to tightly constrained proposed asphaltene structures, thereby allowing relations between structure and function to be established. * Corresponding author. † Tohoku University. ‡ Schlumberger-Doll Research. (1) Crick, F. Physics Today March 2001, p 19. (2) Bitumens, Asphalts, and Tar Sands; Chilingarian, G. V., Yen, T. F., Eds.; Elsevier Scientific Publishing Co.: New York, 1978. (3) Chemistry of Asphaltenes; Bunger, J. W., Li, N. C., Eds.; American Chemical Society: Washington, DC, 1984. (4) Asphaltenes: Fundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Publishing Co.: New York, 1995. (5) Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Publishing Co.: New York, 1998. (6) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237. (7) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (8) Boduszynski, M. M. Energy Fuels 1988, 2, 597; Boduszynski, M. M. Ch. 2, ref 2. (9) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290. (10) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972. (11) Sheu, E. Y. Energy Fuels, submitted. (12) Sheu, E. Y.; De Tar, M. M.; Storm, D. A. Fuel 1991, 70, 1151
13C NMR shows that petroleum asphaltenes have approximately 40% to 50% of their carbon as aromatic.5,10 Some coal asphaltenes differ significantly here in that ∼85% of their carbon can be aromatic.10 IR studies show that >90% of the hydrogen is substituted on aliphatic groups.2-5 There is a moderate dependence of specific asphaltene chemical properties on the petroleum source material and on the exact procedure used for asphaltene extraction. There is a significant difference between coal and petroleum asphaltenes that has been very useful in ferreting out structure-function relations. XANES (X-ray absorption near edge structure) studies on sulfur13,14 show that the sulfur is present in sulfide and thiophene groups with increasing thiophene accompanying greater maturation. Alkyl sulfoxide can also be present;15 with such a strong dipole moment, the size of the aromatic ring systems are reduced to maintain solubility.7 That is, the alkyl sulfoxide and the fused ring system present two binding sites in the molecule (bidentate). The large dipole moment of the sulfoxide group requires a reduced fused ring system to keep the total binding energy small enough to maintain solubility. Nitrogen XANES studies show that asphaltene nitrogen is all aromatic with pyrrolic nitrogen dominating.16 Carbon X-ray Raman spectroscopy has been performed on asphaltenes.17,18 New preliminary results show that asphaltene rings systems tend to be pericondensed18 and that this sym-
(13) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 318. (14) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939. (15) Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P. Fuel 1992, 71, 53. (16) Mitra-Kirtley, S.; Mullins, O. C.; van Elp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252. (17) Bergmann, U.; Mullins, O. C.; Cramer, S. P. Anal. Chem. 2000, 72, 2609. (18) Bergmann, U.; Groenzin, H.; Mullins, O. C.; Glatzer, P.; Fetzer, J.; Cramer, S. P. Chem. Phys. Lett., submitted.
10.1021/ef010240f CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002
Probing Order in Asphaltenes and Aromatic Ring Systems
metric arrangement of aromatic ring systems is consistent with formation of the most stable ring systems.19 The resulting type of petroleum asphaltene structures consistent with this mountain of data is that asphaltenes are shaped “like your hand”. There is a central core (palm) consisting of a fused aromatic system with alicyclic substitution with peripheral alkane constituents (fingers). With this simple structure, freshman chemistry principles have been shown to be consistent with defining asphaltene solubility, thus asphaltene definition.10 Fused ring systems have a propensity to stack via van der Waals interaction (and dipolar interactions, etc.) thereby decreasing solubility. Alkane peripheral substituents increase steric interaction thereby increasing solubility. Comparison of coal and petroleum asphaltenes illustrates how these simple principles apply. Petroleum asphaltenes with their substantial alkane substituents possess large ring systems, while (some) coal asphaltenes, with their relative lack of alkanes, possess only small ring systems.10 That is, large ring systems need alkanes to disrupt stacking to maintain solubility in toluene; small ring systems can only have a few alkane substituents or else they would be soluble in n-heptane. Since the coal source material often contains a smaller fraction of alkanes compared to oil source material, the asphaltene fraction of the corresponding coal will have only small fused ring systems. Of course, the whole coal has large fused ring systems that are not soluble in toluene, thus not asphaltene. Previous TEM direct imaging of asphaltenes and other carbonaceous materials supports the view that stacking disruption is an important issue for asphaltenes.20-24 Extensive analysis of asphaltenes by HRTEM has shown that long range order is largely lacking in asphaltenes.20,21 The aromatic ring systems are readily imaged and are found to associate in stacks comprised typically of only 2 to 3 fused ring systems. The size of these systems is slightly larger than 1 nm in width and roughly 1 nm in stack height.20,21 This size range for the aromatic component of petroleum asphaltenes is consistent with the overall molecular size as determined by fluorescence depolarization.6,7 The spacing of the ring systems is determined to be slightly larger than the graphitic spacing of 3.35 Å. If certain carbonaceous materials are subjected to elevated temperatures, graphitization occurs. Because carbon is refractory, temperatures in excess of 1500 °C are required.20 Graphitization can be monitored using HRTEM to see the growth of long range order. HRTEM has been shown to be a powerful tool to explore carbonaceous materials particularly with regard to ordering.20-24 Here, we test these simple ideas about stacking and stacking disruption of various samples using HRTEM. This method provides direct imaging of the ordering of molecules in samples providing a stringent test for (19) Ruiz-Morales, Y. J. Phys. Chem. A, submitted. (20) In Chemistry and Physics of Carbon; Oberlin, A., Thrower, P. A., Eds.; Marcel Dekker: New York, 1989; Vol. 22, p 1. (21) Oberlin, A.; Bonnamy, S.; Rouxhet, P. G. In Chemistry and Physics of Carbon; Thrower, P. A., Radovic, L. R., Eds.; Marcel Dekker: New York, 1999; Vol. 26, p 1. (22) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 2001, 80, 1467. (23) Sharma, A.; Kyotani, T.; Tomita, A. Carbon 2000, 38, 1977. (24) Sharma, A.; Kyotani, T.; Tomita, A. Energy Fuels 2000, 14, 1219.
Energy & Fuels, Vol. 16, No. 2, 2002 491
establishing the importance of stacking in asphaltene identity. HRTEM coal and petroleum asphaltene images have been generated and analyzed. Consistency of these results with findings from previous work utilizing various techniques lends credence to the analyses presented here. The asphaltene images are compared with images of several model compounds including both alkyl substituted and unsubstituted aromatics. Analysis of the effects of alkane substitution on long range order provides direct support for the importance of fused ring stacking in asphaltene identity. II. Experimental Section Samples. The petroleum asphaltene samples Ven20 from a Venezuelan crude oil (API gravity ) 10), UG8 from a Kuwaiti crude oil (API gravity ) 26), and BG5 from a Kuwaiti crude oil (API gravity ) 29) were prepared as described previously.6 These are n-heptane asphaltenes prepared by addition of 40 cm3 of n-heptane to 1 g of crude oil. After 24 h, the solution was filtered and the precipitate was washed with hot nheptane. These samples were all individually redissolved in toluene and reprecipitated; no effect of this extra purification step on any of our data was observed. The Arabian Medium Heavy asphaltene vacuum resid was obtained from Dr. Eric Sheu and was separated as an nheptane asphaltene. The procedure was described previously.4 The bituminous coal sample was Tanito Harum (TH) from Indonesia. The TH coal asphaltene was obtained from Professor M. Iino as an n-hexane asphaltene from the coal liquifaction product from Tanito Harum coal. The liquification residue was first extracted with pryidine. The pyridine-soluble fraction was isolated and dissolved in toluene and the n-hexane asphaltenes were obtained.5 The model compounds are naphtho[2,3-a]pyrene, coronene, N,N′-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide, which is a solar dye, and perylenetetracarboxylic acid dianhydride. These compounds were obtained from Aldrich and used without purification. HRTEM Methods. HRTEM fringe imaging requires thin samples that partly transmit the electron beam. For poorly ordered structures, the thickness of the sample is the most likely cause of errors as it is very difficult to eliminate the superimposition of lattice fringes.20 In the present study, the samples were hand-ground to fine powder in ethanol and sprayed over a copper microgrid for TEM observation. TEM observation was performed with a 200 kV transmission electron microscope (JEOL, JEM-2010) and several pictures were taken for each sample from different spots to get a general view. The transmission electron microscope was equipped with an anti-contamination trap, a computerized imaging system, and EDS (energy dispersive spectroscopy) for elemental analysis. For disordered structures or small crystallite sizes, spherical aberration influences on TEM lattice images must be considered. The phase transfer function was calculated for λ ) 0.00251 nm (electron beam wavelength) and CS ) 0.5 mm (spherical aberration coefficient) which are the conditions for the present observations. The transfer function is used to obtain the defocus position at which smoothness in contrast was guaranteed for 0.3 nm and higher basal spacing. In all cases, submicron size particles were first examined at moderate magnification to locate the wedge-shaped particles that are optically thin at the edge. The diffraction pattern was taken and elemental analysis was performed. A number of such regions were then imaged at high magnification (×500 k). Structural Parameters. The TEM images were then subjected to image analysis for semiquantitative information such as interlayer spacing d, stacking distribution, and layer size distribution. The methodology has already been described elsewhere;25 we will describe it here briefly. The digitized TEM
492
Energy & Fuels, Vol. 16, No. 2, 2002
Sharma et al.
Figure 2. HRTEM and skeletonized image of TH coal asphaltene.
Figure 1. HRTEM and skeletonized images of asphaltenes (a, b) BG5, (c, d) Ven20, (e, f) UG8, and (g, h) Arab resid. images were converted to skeletonized images that showed a network of layers connected to each other by Y or T shape links. The Y and T shape links were mathematically removed to separate the layers. The resultant image is subjected to a layer identification program to obtain the characteristics of layers which are the inputs to the image analysis algorithm. We developed the image analysis algorithm in FORTRAN 77 for this purpose. Briefly, the algorithm makes use of four parameters: aspect ratio, parallelism, overlap view parameter, and the interlayer spacing to evaluate the structural parameters. The validity of the algorithm was established by using a rather simple image; a good agreement was obtained between computed parameters and those counted manually.
III. Results and Discussion Figures 1a-h and 2 show the TEM and skeletonized images of four petroleum asphaltenes and a coal asphaltene, respectively. In these and subsequent images,
the light regions correspond to electron transmission and the dark to electron scattering. As has been shown previously,20 the aromatic ring systems are readily imaged and are seen as dark lines while the alkanes are not readily observed. These micrographs exhibit some very local order and long range disorder and are in agreement with extensive previously published work.20,21 These images illustrate that the ring systems often occur with two or three stacking together. This observation has been repeatedly observed in carbonaceous materials, the small stacks have been referred to as basic structural units.20,21 To examine the effects of alkanes on aromatic ring stacking, we collected HRTEM images of several model compounds consisting of aromatic ring systems. Their structures are shown in Figure 3. Figures 4 and 5 show the images obtained for two unsubstituted aromatic compounds, Figure 4 shows the HRTEM image for naphtho[2,3-a]pyrene. Long range order and aromatic ring stacking are quite evident. Clearly, overall curvature is quite evident for this sample. HRTEM requires samples that are very thin in order to obtain little distortion of the electron beam, typically a few nanometers. Consequently, sample edge effects may be important. In Figure 4, the curvature might be associated with edge effects and not present in bulk crystals. Nevertheless, the important feature for our purposes is the occurrence of long range order in this sample. (25) Sharma, A.; Kyotani, T.; Tomita, A. Fuel 1999, 78, 1203.
Probing Order in Asphaltenes and Aromatic Ring Systems
Energy & Fuels, Vol. 16, No. 2, 2002 493
Figure 3. Chemical structures of compounds shown in Figures 4-8.
Figure 5 shows another aromatic compound, coronene. Long range order is also evident in this sample. Stacks can be observed in many parts of this image. In addition to these stacks, some ordered fringes are also visible toward edges that are quite different from graphitic or turbostratic carbon structure. To elucidate the nature of these ordered fringes, we made XRD measurement of coronene as shown in Figure 6. Several peaks at different angles can be seen. On the basis of the d spacing calculations, the peaks at about 28-29° the in XRD pattern correspond to the ordered fringes that are visible toward edges of the HRTEM image in Figure 5. The XRD pattern also shows peaks at 24-26° corresponding to 0.37-0.34 nm d spacing. The stacks observed in Figure 5 therefore identified to these peaks. However, these peaks in XRD (Figure 6) are sharp suggesting a high degree of ordering that is not apparent in the HRTEM image. This difference between TEM and XRD is not unrealistic considering the fact that TEM examines a tiny path length and can acquire distortions due to this fact. That is, the small stacking energy for a small molecule like coronene could easily be overcome by edge effects, charging, etc. This is more difficult to do in the bulk. The graphitic carbon would be more resistant to such problems due to the larger sheet area, thus higher binding energies. Previous work implies that the presence of alkanes can disrupt stacking in aromatic ring systems.10 To test this, we use two similar compounds; one has long alkane chains, the other does not. Figures 7 and 8 show the HRTEM images of perylenetetracarboxylic dianhydride and N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide, respectively. From the perspective of ordering, the primary difference between these two compounds
Figure 4. HRTEM and skeletonized image of naphtho[2,3a]pyrene.
is the presence or absence of the two n-C13 carbon chains (cf. Figure 3). The image of the aromatic system without the alkane chains (Figure 7) exhibits greater long range order. For the alkylated molecule, the ordering is disrupted. This result corroborates previous work utilizing fluorescence depolarization (FD), 13C NMR, and IR results indicating that alkane substitutents disrupt stacking.10 Melting point data of alkylbenzenes, alkyl naphthalenes, and alkyl anthracenes also exhibit the effect of disruption caused by alkane substituents.10 The HRTEM results are a direct observation of this alkaneinduced disruption. Solubility classification defines asphaltenes and provides another approach to understanding the effect of alkane substituents. The alkylated dye compound (solar dye) is >100 times more soluble in toluene than the unsubstituted compound (the dianhydride). Figure 9 shows the absorption spectrum for two toluene solutions of these two dyes (cf. Figure 3). The unsubstituted compound is not detectable in the toluene solution, whereas the alkylated compound produces strong absorption. The absorption coefficients of the two compounds are comparable as shown by diffuse reflection spectroscopy. This is exactly the point we are attempting to make about asphaltenes. If the intermolecular interaction is too strong, then the molecule will not dissolve (in toluene). By definition, asphaltenes are soluble in toluene. We view that the solubility classification dictates the stacking behavior, 2 to 3 molecules per stack. These constraints dictate a class of molecular structures
494
Energy & Fuels, Vol. 16, No. 2, 2002
Figure 5. HRTEM and skeletonized image of coronene.
Figure 6. XRD profile of coronene.
that scale. Large aromatic ring systems necessitate alkyl substitution; small ring systems necessitate small alkane chains (or it would dissolve in n-heptane). Table 1 presents the structural parameters for asphaltenes obtained using image analysis; stacking numbers, average layer size and d spacing. Table 2 presents the same structural parameters for the several model compounds shown in Figure 3. A striking feature about the comparison of Tables 1 and 2 is the similarity of the respective structural features. For instance, the lengths of the fringes for the asphaltenes and model compounds are about 1 nm, which is the approximate lateral size of coronene. HRTEM fringes image the aromatic portion of the molecule. These results support previous work that the aromatic ring sizes in petroleum
Sharma et al.
Figure 7. HRTEM and skeletonized image of perylenetetracarboxylic dianhydride.
asphaltenes consist of ∼7 fused rings (of course with variation). The coal asphaltene is a bit smaller at 0.7 nm, thus somewhat smaller aromatics. The average number of layers per stack for the three petroleum asphaltenes and the TH coal asphaltene is nearly the same, ∼2.4. The number of molecules per stack in the models varies, the model with long alkanes exhibits the least number per stack, 2.1, while the naphthopyrene exhibits the most, 3.7. The models with no alkane side chains compared to the asphaltenes generally exhibit a smaller fraction of single molecules not in a stack and generally exhibit larger stacks. In addition, the model with alkane side chains exhibits the least order of the models and almost no stacks of more than two molecules. It is probable the alkane side chains disrupt long range order in this model and in asphaltenes. The spacing for both the models and for the asphaltenes is ∼3.7 Å and is somewhat larger than for graphite (3.35 Å), perhaps due to some relaxation effect in HRTEM. These results support previous conclusions10 that asphaltene structures are determined by the interplay of stacking vs steric hindrance. Here, we see for disparate asphaltenes of significantly different molecular size and alkane content, the stacking parameters (number of molecules in stack, layer spacing) are nearly constant. That is, the invariant defining characteristic of the asphaltenes is not molecular size or aromatic to saturate fraction. The invariant of asphaltenes is the balanced
Probing Order in Asphaltenes and Aromatic Ring Systems
Energy & Fuels, Vol. 16, No. 2, 2002 495 Table 1. Structural Parameters of Asphaltenesa % of total layers identified in image stacking no.
BG5
Ven20
UG8
AR
1(single) 2 3 4 5 6
81.9 14 2.1 1.9
82.3 14.4 2.5 0.8
70.5 20.1 4.4 3.5 1.5
72.5 16 6.9 4.6
86.3 9.3 3.1 1.3
TH coal
avg. #/stack length d-space
2.3 1.1 0.37
2.2 0.97 0.363
2.5 1 0.38
2.6 1 0.37
2.4 0.7 0.367
a Length is the length in nm of the molecule (fringe). d 002 is the fringe spacing in nm in a stack.
Table 2. Structural Parameters of Model Compoundsa % of total layers identified in image stacking no. naphthopyrene coronene dianhydride solar dye 1(single) 2 3 4 5 6 7 8 9
50 13.5 13.5 11.5 4.8 3.8 0 0 2.9
69.2 21.7 5.6 2.5 1
75.6 17.7 3.2 2.5 1.1
72.4 24.6 3
avg #/stack length d-space
3.7 1.2 0.36
2.4 0.9 0.37
2.5 0.9 0.37
2.1 0.8 0.37
a Length is the length in nm of the molecule (fringe). d 002 is the fringe spacing in nm in a stack.
Figure 8. HRTEM and skeletonized image of N,N′-ditridecyl3,4,9,10-perylenetetracarboxylic diimide.
Figure 9. The optical absorption spectrum of saturated solutions of the two “perylene” dyes shown in Figure 3. The alkylated dye is quite soluble, the unsubstituted dye is not detectable in toluene.
interplay between stacking propensity and steric hindrance to stacking. The fact that asphaltenes routinely lack long range order indicates that the lack of order is not some peculiarity of sample preparation or peculiar to certain
asphaltenes. This is a general finding. Here, we have examined many samples of several asphaltenes and consistently observe that the length scale of the aromatic ring systems is ∼1 nm for the petroleum asphaltenes and is 0.7 nm for the TH coal asphaltene. Smaller ring systems for the coal asphaltene are clearly shown by fluorescence spectroscopy and by fluorescence depolarization (FD) spectroscopy.7 FD obtains the molecular size from the rotational correlation times in solution, a very different technique than HRTEM. For UG8 asphaltene a molecular diameter of 2.1 nm by FD was obtained and for the TH coal asphaltene, 1.1 nm, respectively. The FD is sensitive to the entire molecule, not just the aromatic ring portion, and so gives somewhat larger sizes. In addition, the FD result corresponds to a hydrodynamic diameter. Reasonable agreement between two very different methods especially as to the relative sizes of the petroleum and coal asphaltene is very encouraging. Because HRTEM measures very thin edges, sample preparation or other spurious effects can impact the images. For example, the extent of grinding or mechanical disruption can have a large effect on the data. As one would expect, it is easy to obtain spurious disorder. Nevertheless, all samples we examined that had alkyl substitution exhibited disordered images. Octaethyl porphyrin complexes are an example; these were chosen because they are about the same size as the TH coal asphaltene.7 The TH coal asphaltene possesses very little alkane and always shows disorder, so it is not surprising that the alkyl substituted porphyrins exhibit disorder as well. We found that unsubstituted phthalocyanine did not exhibit order, which is surprising.
496
Energy & Fuels, Vol. 16, No. 2, 2002
Perhaps edge effects dominate. Two alkyl substituted phthalocyanines did not show order as expected. The salient conclusion is that the asphaltene solubility class precludes significant long range order. For petroleum asphaltenes with their significant alkanes, long range order is precluded by the alkanes even for relatively large ring systems, about 7 rings on average.7,19 For coal asphaltenes which have very little alkane substitution, long range order is precluded by consisting of ring systems sufficiently small that intermolecular binding is weak. Because van der Waals interaction of aromatic ring systems scales with the number of rings, coal asphaltenes can only possess small ring systems, about 4 rings on average.7 Of course, both coal and petroleum asphaltenes possess a significant width of the distribution of ring sizes. Nevertheless, the governing principles of the relations between structure and solubility still apply. It is possible that the petroleum asphaltenes with their long alkane chains stack somewhat better in micelles in solution or in crude oil. In the solid the alkane chains will tend to form spheres distorting stacking further, whereas with micellar petroleum asphaltenes in solution or in crude oil, the alkane chains will have some tendency to distend into the continuous hydrocarbon (or toluene) phase, thereby removing some of the steric interaction responsible for stacking disruption. For those coal asphaltenes with very small alkane fractions, this effect might be quite small. The TH coal asphaltene has been shown by 13C NMR to have very small fraction of saturated carbon (15%) whereas the BG5 is ∼60% saturated carbon fraction. The XRD profile shown in Figure 10 confirms this result. For BG5, a large peak is seen at 20° that is known to originate with saturated carbon. The TH coal asphaltene does not show this peak. Agreement on ring system size between these HRTEM measurements and FD measurements10 strengthens all conclusions. IV. Conclusions HRTEM images of asphaltenes and various model compounds corroborate the ideas that simple chemical principles govern the identity of asphaltenes. The solubility classification of asphaltenes mandates certain invariants in the stacking behavior of asphaltene molecules, the average intermolecular spacing and the average number of molecules in the stack. In turn, these
Sharma et al.
Figure 10. X-ray diffraction profile of petroleum asphaltene (BG5) and TH coal asphaltene.
invariants require a balance between intermolecular stacking of aromatic ring systems vs steric disruption induced by alkanes. To achieve these invariants, larger ring systems mandate larger alkane chains; likewise, smaller ring systems mandate smaller alkane chains. The ability to relate simple chemical principles to asphaltene identity is crucially dependent on the solution of the 20 year, order of magnitude controversy over asphaltene molecular weight. Testing FD results that indicate small asphaltene molecular weights with predictions for HRTEM provides a stringent test of these FD results. The HRTEM results are consistent with FD and 13C NMR results, melting point data, and solubility data. All of this consistency relies on the accuracy of the FD results that indicate petroleum asphaltenes are ∼750 amu. The connection between asphaltene structure and function is being made. The complex interaction of asphaltenes with various materials, and the phase behavior of asphaltenes should now be more amenable to investigation. Acknowledgment. We are very grateful to Professor Iino of Tohoku University for supplying us with the TH coal asphaltene sample. We thank Dr. Eric Sheu of Vanton Research Laboratory for supplying us with the vacuum resid asphaltene. EF010240F
