Chapter 1
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Asphaltenes, What Art Thou? Asphaltenes and the Boduszynski Continuum Michael E. Moir* Chevron Energy Technology Company, Richmond, California 94801, United States *E-mail:
[email protected].
Throughout his career Mietek Boduszynski challenged the prevailing ideas about the nature of asphaltenes, a solubility fraction obtained from petroleum by the addition of alkane solvents. He demonstrated that the long-held notion of asphaltenes as molecules of high molecular weight was incorrect. In addition, he reasoned that the molecules contained in this fraction were simply part of the continuum of molecules in petroleum. In this chapter, the origin of the term asphaltene, the history of the science of asphaltenes, and how the current state of knowledge aligns with the model of petroleum proposed by Boduszynski will be examined.
Introduction In 1994, Klaus Altgelt and Mieczyslaw (Mietek) Boduszynski published an astonishing book (1), “Composition and Analysis of Heavy Petroleum Fractions,” as relevant today and it was revolutionary at the time. Preceding this book was more than a decade of work by Mietek that challenged conventional ideas of the nature of heavy hydrocarbons and called into question much of the previously published literature. Mietek’s body of work inspired a generation of petroleum chemists to understand the nature of heavy hydrocarbons, and in particular, the mystery of asphaltenes. One particular conference publication, “Asphaltenes, where are you?” caused a considerable stir in 1980 and provided the inspiration for the title of this chapter (2). The purpose of this chapter is to hopefully provide clarity around what asphaltenes are, and what they are not.
© 2018 American Chemical Society Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
What are Asphaltenes? Today, we think of asphaltenes as a fraction of crude oil that is isolated by precipitation by the addition of a poor solvent such as heptane. Properties such as heteroatom content, high molecular weight, and polarity are often seen to be defining characteristics of asphaltenes. There is considerable confusion in the literature about the definition of asphaltenes because of the numerous methods used to produce the fraction. Originally, asphaltene was produced from bitumen by extraction of a petroleum residuum by a sequence of solvents of increasing strength, not by precipitation. To various degrees, all of the experimental conditions matter in the preparation of asphaltenes; temperature, volume of solvent, the solvent used, the time allowed for the precipitation to occur but to name a few (3). In addition, the source of the asphaltene fraction is important. The geological source of crude oil, whether or not the asphaltenes are isolated from a petroleum residuum, an intermediate refinery stream, or even from coal or coal liquids will influence the detailed chemical make-up of the asphaltene fraction (4). From a modern perspective it is obvious that asphaltenes from one source should be different from asphaltenes from another. However, this was not always seen to be true and it may surprise some readers to learn that asphaltene was once thought to be a pure substance. The phrase “pure asphaltenes” is still seen in current literature. This erroneous belief is the source of much of the confusion that we confront even today. In their book (1), Mietek and Klaus write: “Our reluctance to use the term “asphaltenes” (as well as “maltenes” and some other ones) does not mean that we are denying the existence of this refractory portion of crude oil. We recognize that the insolubles … are a fraction which has the most adverse affect on crude oil refining … Why not call the precipitates “insolubles” rather than “asphaltenes”?” Why not indeed? The answer to Mietek’s plea requires a journey into the history of petroleum.
A Brief History of Asphaltenes In his 1936 book, “Bitumen and Petroleum in Antiquity”, R. J. Forbes describes humankind’s long interaction with petroleum, a story that spans at least 5000 years (5). Known to the ancients from natural seeps, and lacking chemical means of classification, the materials oozing or flowing from the seeps were named in the language of the area they were found. This is evident in Pliny the Elder’s first-century multi-volume “Historiae Naturalis (6)” and Dioscorides’ “Materia Medica (7)”, at the time of their publication up-to-date compilations of scientific knowledge, folklore, and mythology. Pliny used words that are recognizable to us, asphalt(os), bitumen, maltha, pitch (pix), naphtha, and tar. Unfortunately for our comprehension, Pliny’s usage of these terms was imprecise with asphaltos being roughly equivalent to bitumen; naphtha, maltha, and tar were used interchangeably; and pitch was described as arising from either plant 4 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
or mineral sources (8). Materials found at natural seeps were named for what they looked like. In a very real sense, if a substance looked like pitch and had similar important properties, such as the ability to seal cracks in the hull of a boat, it was pitch, if it looked like tar, it was tar. “Historiae Naturalis” and “Materia Medica” were the main sources of western knowledge for the subsequent 1500 years. It is not surprising that confusion about the true nature of the natural seeps flourished. Derived from the Greek word asphaltos, the word asphaltene was coined by the esteemed French agricultural chemist Jean Baptiste Boussingault. He lived on a farm in Pechelbronn and he had worked in the nearby oil mines as a young man. In his 1837 paper (9), “Memoir Upon the Composition of Bitumens”, Boussingault describes how he distilled bitumen from Pechelbronn at about 250°C to obtain a distillate and a residue that he named petrolene and asphaltene respectively. He concluded from limited observations of other samples that all bitumens consisted of these two minerals, that is, substances having a definite composition. Boussingault’s asphaltene was insoluble in alcohol; soluble in ether, fatty oils, and turpentine and had a hydrogen to carbon ratio (H/C) of 1.6. We would call this material atmospheric residuum, and it is not the asphaltenes as we would identify them today. The Belgian-American chemist E.J. de Smedt is credited with the invention of modern paving material. In 1870, he laid the first sheet of modern pavement on a street in Newark, New Jersey, a mere 5000 years after bitumen was first used as paving material in Mesopotamia (5). In any event, de Smedt patented the mixture of sand and Trinidad asphalt and a means of application to create a smooth and dust-free road surface (10). As the District of Columbia’s Inspector of Asphalts and Cements, de Smedt was extremely influential for the next 30 years and espoused the idea that bitumen consisted of the mineral substances petrolene, retene, and asphaltene. At the time, this idea was not without its detractors. One critic, Stephen Farnum Peckham strongly objected to the use of ill-defined terms. Peckham, a University of Minnesota professor, Southern California oil explorer, and analytical chemist, determined that “retene” was in fact, an experimental artifact (11). He argued that “petrolene” and “asphaltene” were not substances and therefore the terms should not be used. His reasoning was that the elemental composition of petrolene and asphaltene varied widely by source and method of analysis. In Peckham’s words (11): “It is the height of folly to attempt to compare things that are different or that are stated differently.” Based on extraction in different solvents, asphaltene clearly contained multiple constituents. Rather than continued use of the ill-defined terms “asphaltene” and “petrolene”, Peckham preferred more descriptive terminology: “… I have sought to discard them and to report the percentage soluble in petroleum ether and other solvents as “the petroleum ether soluble,” not as petrolene, etc.” 5 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
This is precisely what Mietek and Klaus wrote nearly a century later. This glimmer of hope for more precision in terminology was quickly extinguished by the paving establishment. Clifford Richardson, de Smedt’s successor as the Inspector of Asphalts and Cements, sought to clarify the misconceptions about “asphaltene’ and “petrolene”. In an early paper (12), Richardson actually avoided the use of the terms “asphaltene” instead using the phrase “portion insoluble in naphtha”. However, in Richardson’s exceptional work first published in 1905 the matter was settled (13). In “The Modern Asphalt Pavement” Richardson added an “s” to “asphaltene” and “petrolene” to convey the idea that asphaltenes and petrolenes were classes of compounds, not specific minerals or hydrocarbons. Richardson defined petrolenes to be the volatiles collected at 325°F, malthenes as the solubles collected in either 88° or 62° Baumé naphtha, asphaltenes as the fraction soluble in carbon tetrachloride, and carbenes as the fraction not soluble in carbon tetrachloride but soluble in carbon disulfide. In Richardson’s experimental scheme, asphaltenes typically had an H/C of 1.1, typical of the asphaltenes as we know them today. In 1909 Peckham published a competing work that was essentially a compilation of his earlier publications and presentations (14). Although of historical importance, “Solid Bitumens” did not convince the scientific community to abandon the terms asphaltenes, etc. The penetration of these terms into the common language was just too much to overcome. However, Richardson did recognize that with these terms come potential problems (13). As Peckham also pointed out, naphtha composition can have an effect on extraction results: “For the purpose of determining the percentage of bitumen soluble in naphtha distillates, 88° and 62° B, are used. It is extremely important that these naphthas should be of the exact degree specified, since differences in density will make an appreciable difference in the amount of bitumen extracted. The distillate should be that obtained from a paraffine petroleum.” Furthermore, Richardson recognized that the definition of asphaltenes is purely operational: “The bitumen of Trinidad asphalt which is insoluble in 88° naphtha is of the class known as asphaltenes, according to our purely arbitrary classification; the relative proportion of the two forms of bitumen, asphaltenes and malthenes, being dependent upon the nature of the solvent used, so that any information derived from a determination of the percentages of the two classes of hydrocarbons will be purely relative as compared with other bitumens which have been examined by exactly the same methods and with the same solvents.” Lastly, Richardson warns the reader not to confuse classification with chemical structure: 6 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
“With these facts in view the analyses of the native bitumens which are to be presented will be of interest for the purpose of comparing the characteristics of the various materials, but the chemical data must not be looked upon as being absolute in any case.” The errors made by Boussingault and de Smedt were the same; they asserted that asphaltene was a single substance by glossing over obvious differences between asphaltene isolated from including different sources. Like the ancients before them, they focused on the similarity of a few properties and arrived at the faulty conclusion that asphaltene was a single substance. Any chemistry student today would recognize that Peckham’s counter-argument derives from the concept of a chemical substance first articulated by Joseph Proust in the late 18th century in his Law of Definite Proportions (15). Today we take it as axiomatic that: Substances that have the same properties are the same substance, and its corollary, Substances that have different properties are different substances. The problem for Boussingault and de Smedt was that all properties of two materials need to be the same for them to be identical, not just selected ones. Richardson recognized the problem, but he had a less formal view of substance. His properties, the amount of asphaltenes, petrolenes, etc. while having no chemical significance worked well enough for understanding paving. From this point of view, asphaltenes and their solubility fraction cousins do not have vague definitions at all, but the definitions may not be as useful as we would like. The important point here is that only issues pertaining to solubility can truly be addressed by solubility fractions. There can be no necessary link to chemical behavior or structure. Despite this problem, and the warning given by Richardson in 1905, the search for the chemical structure and universal properties of asphaltenes continued throughout the 20th century. After Richardson, the prevailing view of asphaltenes and the heavy ends of petroleum was that they were high molecular weight molecules and polymeric in nature. This idea was propagated principally by Abraham with the view that petroleum buried in the earth gradually transformed with the passage of time and at elevated pressure and temperature to higher molecular weight, more aromatic compounds (16). The trend to lower H/C ratios in the non-distillable fraction of petroleum was well understood. Therefore, asphaltenes were believed to be the polymeric end product of this geological transformation. It was Nellensteyn who first suggested an alternative explanation (17). He discovered the colloidal nature of petroleum and that it might be the association of individual molecules that mimicked the behavior of polymeric materials. In 1938 Nellensteyn showed the first X-Ray diffraction evidence that asphaltenes precipitated from bitumen had a graphitic character. This finding suggested a structure of the asphaltene micelles where the aromatic cores are stacked like sheets of graphite. However, it was not clear whether the graphitic structure came about from the association of small 7 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
molecules or from an intramolecular association of a larger molecule. The view of asphaltenes as polymeric persisted for another 40 years. It should now be clear the phrase “pure asphaltene” is an oxymoron. Numerous authors have noticed that upon further solvent treatment, asphaltenes continue to yield solubles. Figure 1 shows an example of this where asphaltenes in a vacuum residuum yield additional maltenes upon solvent treatment (18).
Figure 1. Asphaltenes extracted from vacuum residuum with n-heptane (solid line) yield additional maltenes after additional solvent treatment (dashed line). Essentially, asphaltenes become maltenes. In this experiment where solvent power is changed gradually, maltenes appear as the peak on the left, asphaltenes in the peak on the right. Adapted with permission from reference (18). Copyright 2015 American Chemical Society.
Some authors have interpreted this as meaning that the original deposit or asphaltene was “contaminated” by maltenes. This interpretation presupposes that every asphaltene molecule is identical, and the mixture that is asphaltenes is like a sodium chloride deposit contaminated by calcium chloride that can be purified by recrystallization. The interpretation shows a misunderstanding of what asphaltenes are. For any complex mixture of molecules like asphaltenes, there will always be molecules of differing solubility, the more soluble species being, in general, less aromatic than the bulk material. Further treatment with solvent will inevitably produce additional soluble species. The exact mechanism of occlusion of the soluble species is immaterial. In fact, the observation of continued extraction of soluble species is consistent with the very complex mixture that is asphaltenes. There are no “pure asphaltenes”. Continued solvent treatment of asphaltenes is just creating a different and more aromatic mixture. 8 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
The Modern Era In 1961, Teh Fu Yen, Erdman, and Pollack published a pivotal paper on the aromaticity and crystallite parameters of asphaltenes isolated from a visbreaker tar (19). From the X-Ray diffraction (XRD) data, Yen and his colleagues deduced that asphaltenes consisted of stacked structures where 4 to 6 aromatic cores are separated from each other by approximately 3.6 Å. Each core was found to be between 8.5 and 15 Å in diameter. The Yen model of asphaltenes, as it became to be known, and its structural parameters are shown in Figure 2. Like Nellensteyn before him, Yen could not tell if the structures were created from inter- or intramolecular interactions or a combination of both.
Figure 2. The Yen model of asphaltenes circa 1961. Key structural parameters of asphaltene nanoaggregates are identified. Most nanoaggregates are formed from “island” type molecules. Reproduced with permission from reference (19). Copyright 1961 American Chemical Society.
By 1967 Yen had extended his model to include descriptions of a hierarchy of asphaltene particle sizes as it became clear that there were larger assemblies of asphaltene molecules present than could be explained by the earlier XRD data (19). The Yen model of 1967 was also based on measurements that suggested that asphaltene contained molecules having molecular weights ranging up to tens of thousands of Daltons (20). This was explained by intramolecular associations of aromatic cores on the same molecule (Figure 3). This so-called archipelago model of asphaltene interaction plagued asphaltene science for the next 30 years. 9 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 3. The Yen model of asphaltenes circa 1967. Here, the “archipelago” type structure is more prominent. Yen identified a number of structural features in the model: A - crystallite, B - chain bundle, C - particle, D - micelle, E - weak link, F – gap, K - petroporphyrin. Reproduced with permission from reference (20). Copyright 1967 American Chemical Society.
In 1980, Boduszynski, MacKay and Latham gave the presentation, “Asphaltenes, Where Are You?” to what must have been a shocked audience of asphalt science professionals (2). Mietek presented unequivocal evidence that the asphaltenes were not polymeric, in fact, the molecular weight of the asphaltenes tended to be lower than the more soluble fractions in the same sample of asphalt, in this case a vacuum residue. In addition, the molecules present in the asphaltenes existed in a continuum that spanned a wide range of molecular weights, from about 400 to 1900 Daltons. The molecules forming the asphaltenes did not have molecular weights of 5000 to 50000 as reported earlier, these molecules were relatively small. The high molecular weights reported were artifacts of poorly conducted vapor phase osmometry (VPO) measurements which were known to give erroneously high molecular weights due to molecular aggregation. Gone then, finally, was the notion that asphaltenes were a single substance of high molecular weight. 10 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Later, Mullins sought to refine the Yen model by sorting out the archipelago – island controversy (21, 22). By a combination of spectroscopic and mass spectrometry measurements, Mullins reasoned that the archipelago molecules are simply too unstable to survive geological time and that the number of aromatic rings in the central aromatic core was around seven. Although this is a significant insight, the Yen-Mullins model for asphaltenes requires some additional refinement. As shown in Figure 4, the sizes of the nanoaggregates and the nanoaggregate clusters are likely reasonable and are borne out by measurements.
Figure 4. The Yen-Mullins model of asphaltenes. By this time, asphaltenes are known to exist as a mixture of single molecules, nanoaggregates, and clusters. Reproduced with permission from reference (22). Copyright 2012 American Chemical Society. (see color insert)
However, the details of the interactions shown cannot be correct. The nanoaggregate must be more ordered that shown in Figure 3. The interplanar distance between aromatic cores averages about 3.5 Å as dictated by van der Waals forces. There is no such thing as “steric repulsion,” as all intermolecular forces are attractive until the distance between molecules reaches bonding distance, at which point the interaction becomes strongly repulsive. Either the aromatic cores are interacting or they are not. They are either forming a stable structure or they are not. Steric effects come into play to prevent aromatic cores from ever reaching a distance where the interaction is strong enough to form a stable structure. A more refined view of the asphaltene nanoaggregate would have the stacked aromatic cores capped at each end by a molecule that prevents additional growth of the stack by either distortion of the aromatic cores or by steric interference due to alkyl substituents. Indeed, a number of theoretical studies have shown that the stable forms of stacked aromatic cores may include aromatic cores in various configurations having cores perpendicular to stacks (Figure 5) (23). 11 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 5. Theoretical study of coronene stacking as a model of asphaltene aggregation. The most stable nanoaggregate arrangement is not always a vertically arranged stack of aromatic cores. (a)-(c) show the most stable configuration to least stable configuration. Reproduced with permission from reference (23). Copyright 2005 American Chemical Society. (see color insert)
In addition, Rogel published a thermodynamic model for asphaltene nanoaggregration wherein the attraction between asphaltene molecules is driven by the interaction between polyaromatic cores which is limited by, among other factors, steric interference between the aliphatic chains surrounding the aromatic core (24). An interfacial contribution to the interaction arises from the formation of the aggregate-solvent interface that is important in determining aggregation behavior in different solvents. In this view, the number of individual molecules in each nanoaggregate is governed by the geometry of available building blocks (25), the solubility of the resulting nanoaggregate, and the ability of the nanoaggregate to assemble into larger clusters. The depiction of the cluster in Figure 4 is likewise imprecise. As shown, the cluster would simply be unstable. The loosely associated side chains would provide insufficient force to stabilize a large cluster. However, clearly the clusters exist. What is unknown at present is the exact source of the forces involved, certainly to be van der Waals in nature, that maintain the stability of the larger asphaltene clusters. Recently, atomic force microscopy studies on coal asphaltenes, asphaltene model compounds, and petroleum asphaltenes have provided astonishing revelations into the nature of asphaltenes. Schuler and coworkers provide evidence of the tremendous variety of molecules that make up the building blocks of asphaltenes. Figure 6 shows a few of the 200 hundred or so molecules that were imaged (26, 27). 12 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 6. Molecules in asphaltene fractions imaged by Atomic Force Microscopy (AFM). These images show the astonishing variety of molecules present. Reproduced with permission from reference (27). Copyright 2017 American Chemical Society. The images in Figure 6 are spectacular. Without exaggeration, the images show that virtually everything we believed about asphaltenes at the molecular scale is wrong. First, no two molecules are remotely similar in geometry. In addition, we can see that there is no apparent residual signature of their biological origin. It is as if Mother Nature took some atoms in the observed proportions and shook them in a box, the only rule being that the resulting molecules were to exhibit poor solubility in alkane solvents. In retrospect, perhaps that is not too surprising, since it is precisely by solubility that the asphaltene fraction is derived! As already discussed, we should not have expected that solubility could necessarily have implied anything about chemical structure. Mother Nature can only satisfy the solubility requirement in one way, the molecules must be aromatic. As known from Peckham’s day, the molecules exhibit an aromatic core. But after the work of Schuler and coworkers (27), in no way can we depict the stacking of the aromatic rings as a simple cylinder. Instead, the stacks will be as irregular as the molecules from which they are produced. These molecules bear no resemblance to polymers. Consequently, the language of polymer science and the analogy to the structure of polymers is largely inappropriate. Asphaltenes are frequently described as “polydisperse”. The idea of polydispersity comes from polymer science and thus conjures up a mental image of a polymer of widely varying and high molecular weight. This image is incorrect in two respects. 13 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
First, the molecular weights of the molecules of the asphaltene fraction are low. Secondly, asphaltenes contain no repeating units, or if there is any repetition, it must be purely by chance. The stacking of aromatic rings is a trick played upon us by Nature to make us think that asphaltenes have a definable structure. Figure 7 shows how we have been tricked. We observe the 3.5 Å spacing of aromatic cores which implies some order in a single dimension. However, rotating the stacked assembly through 90° reveals that there is no order in the other two dimensions. There may be some average dimension that could be inferred from X-ray scattering, but the average is meaningless. Asphaltenes are amorphous. The notions of average molecular weight and average molecular structure are largely if not completely useless.
Figure 7. While we observe order in one-dimension, that of the stacked aromatic cores, there is no order in the other two. Asphaltenes are largely amorphous. (see color insert) 14 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
We have been inculcated with the canonical view that asphaltenes are: a) very high molecular weight, b) must contain heteroatoms, c) are “polar,” d) are hydrogen deficient, and e) contain a flat aromatic core consisting of six-membered rings. It was clear from Mietek’s work in 1980 that a) is not true, but the work by Schuler and coworkers allows us to test the rest of this canonical view. Although commonly used, the term “polar” is not well defined in chemistry, and its use results in much confusion. Here, the terms “polar” and “polarity” are taken to be related to the presence of a permanent dipole moment. Figures 8 through 11 show molecules imaged by Schuler and coworkers produced by HyperChem (HyperChem Professional 8.0, Hypercube, Inc., Gainesville, FL) (27). Molecules were built and optimized using an annealing procedure as the starting point for the calculation of molecule properties. The Austin Model 1 semi-empirical method was used to calculate dipole moments. This molecule represents everything that our canonical view of asphaltenes would lead us to believe. It contains heteroatoms, has a large dipole moment, is very hydrogen deficient, and consists of a flat aromatic core containing six-membered aromatic rings and one five-membered ring.
Figure 8. One molecule found by AFM. This molecule is exactly as the canonical model of asphaltenes would lead us to predict. It is flat, hydrogen deficient, contains heteroatoms, and has a permanent dipole moment. (see color insert)
On the other hand, the asphaltene molecule in Figure 9 does not contain heteroatoms, and is not flat. The central five-membered ring distorts the molecule so that it is more cup-shaped. Although it is hydrogen-deficient, it has a calculated dipole moment of 0.78 D, about that of toluene. Toluene would never be categorized as an asphaltene, and the “asphaltene-ness” of the molecule in Figure 8 is not due to its dipole moment or “polarity.” 15 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 9. This molecule identified by AFM does not abide by the canonical view of asphaltenes. The molecule is not flat, contains no heteroatoms, does not have a large permanent dipole moment. It is hydrogen deficient. (see color insert)
The asphaltene molecule in Figure 10 contains nearly 300 atoms. It contains no heteroatoms, has a very large calculated dipole moment and it decidedly not flat. This molecule consists of 5, 6, 7, 8, and 11 membered rings which results in a highly distorted structure.
Figure 10. This molecule found in the AFM study contains no heteroatoms, many rings that do not contain 6 carbon atoms and has a large calculated permanent dipole moment that arises from its geometry alone. (see color insert) 16 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
The asphaltene molecule in Figure 11 is hydrogen-deficient, contains no heteroatoms and has a dipole moment of nearly zero. The molecules in Figures 8 to 11 are not “polar,” but like all molecules they are polarizable. It is polarizability and geometry that leads to the observed stacking of the aromatic cores. The common characteristic of these molecules is their pronounced hydrogen deficiency.
Figure 11. This molecule is only hydrogen deficient, and yet it is found in an asphaltene fraction. Hydrogen deficiency is the sole defining characteristic of asphaltenes. (see color insert)
Rodgers, Marshall and coworkers were the first to observe by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICRMS) the entire astonishing diversity of molecular formulae in crude oil and asphaltenes (28–33). They showed in their comprehensive series of publications that Mietek was correct in his bold theory that the molecules in crude oil form a continuum which includes the non-distillable fraction forming the “Boduszynski Continuum”. Interestingly, the island-archipelago controversy continues still. Very recently, Rodgers and coworkers have determined from data obtained by FT-ICRMS that both archipelago and island structures exist in asphaltenes fractions (34). This agrees with the visual evidence obtained by Schuler and coworkers of aromatic cores in asphaltene molecules connected by either zero or one-carbon bridges (27). However, this is a very different model than that originally proposed by Yen for very different reasons, and should be described in terms other than “archipelago” to avoid future confusion. Figure 12 shows the species found in an oil field deposit plotted as double bond equivalent versus carbon number. The carbon number range from 30-80 is typical of asphaltenes (35). 17 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 12. FT-ICRMS of an oil-field deposit. The plot of Double Bond Equivalent versus carbon number shows the wide range of species with diverse molecular formula. Adapted with permission from reference (35). Copyright 2016 American Chemical Society.
What remains then of the canonical view of asphaltenes? The molecules making up the asphaltene solubility fraction do not always contain heteroatoms, the aromatic core is not flat and does not consist of just six-membered rings, the molecules are not necessarily “polar,” and they consist of molecules of diverse carbon number. The sole defining characteristic of asphaltenes is their hydrogen deficiency. We must conclude that the lack of solubility in hydrocarbon solvents, the aromatic character, polarizability, and stacking are all consequences of hydrogen deficiency. Likewise, we must deduce that the separation based upon solubility is based on the difference in the strength of the London force that can be exerted by the solvent on the solute asphaltene molecules. Rogel and coworkers found that the density of asphaltenes correlates with hydrogen content (36), consistent with the idea that the forces involved in asphaltene solubility and aggregation are essentially related to polarizability. Dipole moment or “polarity” have little or nothing to do with the solubility behavior of asphaltenes. Recently, Rogel took the molecules from Schuler et al and performed a simulation wherein the molecules were allowed to interact (37). As expected, the molecules associate to form stable, irregular stacked assemblies (Figure 13). 18 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 13. When the molecules found by AFM are allowed to interact in a simulation, irregular stacked assemblies result. Reproduced with permission from the author of reference (37). (see color insert)
The distance between the aromatic cores was found to be 3.5 Å perfectly in line with predictions from interactions due to London forces (38). The molecules stack to minimize the energy of the system. The London force lacks a classical analogy and is thus poorly taught and largely misunderstood. The energy of interaction for even a simple system is difficult to calculate without simplification of the quantum mechanical equations involving several approximations. Equation 1 shows one form of the London interaction that is seen frequently, although it is inexact. The energy of interaction is approximately equal to the sum of each interaction of each pair of atoms in a molecule which in turn is proportional to their polarizabilities, α. Expressions for the interaction of atoms are available and for molecular interactions as well (9).
Although each individual interatomic interaction is quite small, indicated as an interaction between atoms a and b in Equation 1, the total London force for a large molecule can be substantial. In the interaction of two large aromatic cores, the most stable geometry is coplanarity. Clearly the force and the energy of interaction depend on the number of atoms in each aromatic core and the asphaltene molecules stack to maximize the points of close contact. Smaller aromatic molecules than those observed in asphaltenes will have insufficient interaction energy to form stable stacks. A somewhat tongue-in-cheek analogy of this interaction is that of 19 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
stacked fridge magnets. The energy of interaction for the individual very weak magnetic dipoles ma and mb in fridge magnets can be approximated as in Equation 2.
Figure 14. A possible depiction of an asphaltene cluster. When the size of the clusters of asphaltene nanoaggregates are measured it is found that they contain thousands of molecules and span a diameter of between 40 and 100 nm. (see color insert) 20 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Fridge magnets behave in a remarkably similar way to the aromatic cores of asphaltenes. The energy of interaction is proportional to the area of the overlapping magnets. A magnet with a small area will have a lower energy of interaction and requires less force to separate than a magnet with a larger area. Earlier work by Rogel indicated that the presence of heteroatoms in the molecules has little impact on the overall stability of the asphaltene nanoaggregates (40). It is the London force that dominates asphaltene-solvent interactions (41), the strength of the force determined by the size of the molecules and area of possible mutual interaction. Although beyond the scope of this chapter, no discussion of asphaltenes is complete without mentioning the large-scale interaction of the asphaltene nanoaggregates. As Yen postulated in the 1960’s, and later confirmed by experiment (42), asphaltene nanoaggregates assemble in large colloidal particles some 40 to 100 nm in diameter containing thousands of individual asphaltene molecules (Figure 14). It is these colloidal particles that are destabilized by the additional of poor solvents like n-heptane to form the asphaltene precipitate. Now that the structure of the asphaltene molecular building blocks has been revealed, the remaining important questions in asphaltene science are in the area of asphaltene aggregation and disaggregation. In particular: What is the role of detailed molecular composition in aggregation and disaggregation? What are the forces that keep these large colloidal particles stable? Can we enhance the stability of the colloidal system to prevent asphaltene deposition? While it had long been postulated that the London force is behind the self-assembly of the larger clusters of nanoaggregates, experimental data exploring this issue is less common. However, there are tantalizing clues in the literature that suggest that just as the interaction of the aromatic cores is inevitable, so is the self-assembly of the larger clusters. In a series of elegant experiments, Carbognani and Rogel showed that asphaltenes can incorporate alkane solvents (43, 44). Not only does this provide a mechanism for cluster formation, it also explains why “non-asphaltenes” are observed in asphaltene deposits. In addition, Stachowiak and coworkers provided direct evidence that the interaction of asphaltene nanoaggregates is energetically favorable with the most likely mechanism being the interaction of the aliphatic side chains (45). Once again it is the London force that drives this behavior. Given the fact that the London force between methylene groups is actually larger than that between two aromatic carbon atoms, we infer that the tendency to form clusters of nanoaggregates is an unavoidable consequence of the formation of the nanoaggregates themselves, the magnitude of the force being dependent on the length and number of side chains on the aromatic cores. Time and some well-designed experiments will tell if this extension of the Yen-Mullins model is correct.
21 Ovalles and Moir; The Boduszynski Continuum: Contributions to the Understanding of the Molecular Composition of Petroleum ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Conclusion Asphaltenes, the fraction of crude oil insoluble in heptane, contain molecules of moderate molecular weight having a hydrogen deficient core. The lack of solubility in hydrocarbon solvents, the aromatic character, polarizability, and stacking are all consequences of hydrogen deficiency. The formation of nanoaggregates or individual asphaltene molecules and the formation of larger clusters are a result of London forces. All of the experimental data obtained so far is consistent with Mietek Boduszynski’s vision of asphaltenes as being not something apart from crude oil, but simply a continuation of the continuum of the diverse molecular ensemble that is petroleum.
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