Molecular Recognition in Aggregates Formed by Asphaltene and

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Energy & Fuels 1999, 13, 278-286

Molecular Recognition in Aggregates Formed by Asphaltene and Resin Molecules from the Athabasca Oil Sand Juan Murgich* and Jose´ A. Abanero Centro de Quı´mica, Instituto Venezolano de Investigaciones Cientı´ficas, IVIC, Apartado 21827, Caracas 1020A, Venezuela

Otto P. Strausz Department of Chemistry, University of Alberta, Edmonton, Canada T6G 2G2 Received October 16, 1998. Revised Manuscript Received January 11, 1999

The conformation of lowest energy of an asphaltene molecule of the Athabasca sand oil was calculated through molecular mechanics. The molecule has a complex globular shape with small internal cavities. This shape resulted mostly from the existence of polymethylene bridges connecting the aromatic regions. Molecular aggregates formed with the asphaltene and with nine resins from the same oil, and with n-octane and toluene, were also studied. The resins showed higher affinities for the asphaltene than toluene and n-octane and also exhibited a noticeable selectivity for some of the external sites of the asphaltene. This selectivity based on the molecular recognition of the site depends on the fit between the resins and the site of the asphaltene. The selectivity explains why resins of one oil may not solubilize asphaltenes from other crudes. An analysis of the changes in the enthalpic and entropic contributions to the free energy showed that both contributions should be considered when the stability of the asphaltene and resin molecular aggregates is examined.

Introduction Crude oil is a complex fluid where molecular aggregates of different sizes and compositions are found.1 This fluid has been classified as a colloidal dispersion of a solid (asphaltenes) in a nonaqueous solvent.1,2 The micelles in the oil are formed by asphaltene molecules and are assumed to be peptized by a fraction called resins.1,2 As in all complex molecular liquids, the noncovalent interactions, responsible for the molecular recognition between the components of crude oil, determine the structure and the lifetime of these aggregates.3,4 The determination of the molecular structure of the components of crude oil is complicated by the selforganization of the heavier parts and the almost continuous distribution of the molecular properties. This last point makes the interpretation of the spectroscopic data in terms of the molecular structure a very complex task for crude oils. The isolation of the different fractions is of paramount importance, and the use of sophisticated methods of determining the molecular structure is mandatory to unravel the chemical components of the crude oil.1,2 Many of these methods were used in the (1) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991. (2) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: New York, 1978. (3) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic: New York, 1991. (4) Evans, F. D.; Wennerstro¨m The Colloidal Domain; VCH Publishers: New York, 1994.

study of the heavy fraction of oil obtained from the Athabasca sands.5 In particular, the acetone fraction of the asphaltenes was studied in great detail and a hypothetical two-dimensional structure was proposed.5 Besides the information about this particular asphaltene molecule, a careful and systematic analysis of the resins present in the Athabasca bitumen was also available.6-10 Then, not only the molecular structure of a major fraction of the asphaltene but also that of several of their most abundant resins is known for this oil sand. For very few oils is this type of detailed molecular structural information available for both fractions.1,2 Therefore, the knowledge derived from these experimental studies provides an unique opportunity of studying: (a) the three-dimensional structure of the proposed asphaltene molecule and (b) the formation of the micelles present in this bitumen from the point of view of molecular recognition. In this work, a slightly modified structure (Figure 1) from that proposed for the asphaltene molecule from the Athabasca sand oil5 was used as the starting point of a (5) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 13551362. (6) Payzant, J. D.; Hogg, A. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1984, 1, 183-202 and references therein. (7) Cyr, T. D.; Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1986, 9, 139-143 and references therein. (8) Payzant, J. D.; Montgomery, D. S.; Strausz, O. P. Org. Geochem. 1986, 9, 357-369 and references therein. (9) Frankman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, O. P. AOSTRA J. Res. 1987, 3, 131-138 and references therein. (10) Frankman, Z.; Ignasiak, T. M.; Lown, E. M.; Strausz, O. P. Energy Fuels 1990, 4, 263-270 and references therein.

10.1021/ef980228w CCC: $18.00 © 1999 American Chemical Society Published on Web 02/20/1999

Molecular Recognition in Aggregates

Energy & Fuels, Vol. 13, No. 2, 1999 279

Figure 1. Two-dimensional drawing of the proposed asphaltene molecule from Athabasca. The numbers indicate the different adsorption sites used in this work.

molecular mechanics calculation in order to find its three-dimensional conformation of lowest energy. The determination of this conformation allowed the study of the molecular aggregates formed with the resins and their stability. The stability of the micelles of asphaltenes and resins is quite significant because severe production, transportation, and distillation problems are linked to their separation from the oil.1,2 In order to obtain information about these aggregates, a molecular mechanics study of the molecular recognition pattern acting on asphaltene and resins from the Athabasca oil sands was undertaken. Molecular Models The application of molecular mechanics to the study of the asphaltene and resin molecules seems, at first sight, quite direct.11 As already pointed out, information about the structure of the asphaltene and resin molecules is scarce.1,2 For many years, a large number of condensed benzene rings were assumed to form the aromatic system of asphaltenes.1,2 Recently, careful chemical and thermal decomposition studies performed (11) Frenkel, D.; Smit, B. Understanding Molecular Simulation; Academic: New York, 1996.

in acetone-extracted asphaltene from the Athabasca bitumen has led to a quite different molecular structure for this fraction.5 Instead of a highly condensed aromatic system with a large number of rings, a set of much smaller aromatic “islands” linked by polymethylene bridges, many containing S atoms, was obtained for the asphaltene fraction studied.5 Using the same procedure employed for the Athabasca sand oil, similar results were found in several other types of crude oils, including that from Boscan. Thermal and chemical degradation methods were employed for the molecular structural study of Athabasca and other asphaltenes. The latter included methods of oxidation (ruthenium ion catalyzed, RICO), reduction (with nickel boride), and hydrolytic (with OHand BBr3). RICO permits the selective oxidation of aromatic carbon to CO2 while leaving saturated ones essentially unaffected.5 The alkyl-substituted benzene rings are also oxidized except at the site of alkyl attachment, which is converted to a carboxylic group bonded to the alkyl chain, forming an alkanoic acid. When the chain is between two aromatic carbons, a dicarboxylic acid results after the oxidation. RICO then permits the quantitative determination of alkyl side chains attached to and bridges between aromatic car-

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bons in asphaltenes as well as their size distribution as a function of length and number of C atoms in them. Benzenepolycarboxylic acids are produced by RICO from condensed aromatic structures that may be present in asphaltenes and resins. The type of benzenepolycarboxylic acid produced gives direct information about the condensation mode present in the aromatic parts of the asphaltene molecule. 5 Heteroaromatics such as alkyl-substituted benzo-, dibenzo-, and higher condensed thiophenes are also oxidized. In these compounds, the sulfur-attached ring is also destroyed and, additionally, SO2 (or SO3) is generated as a result of RICO. The very low yield of the penta- and hexabenzenepolycarboxylic acids showed that pericyclic aromatic moieties are few and far between in the average molecule of the Athabasca asphaltene.5 RICO also produced an oxidized, nondistillable residue containing a very low aromatic content.5 This material was essentially formed by the oxidation of the naphthenic-aliphatic core of the asphaltene molecule. To this core structure, originally attached were the aromatic rings which together made up the asphaltene molecule.5 It was also found that RICO transformed saturated sulfides to saturated sulfones thus giving direct proof of the existence of sulfide links in the Athabasca asphaltene structure.5 Nickel boride reduction showed that the asphaltene molecule core is made of segments connected by sulfide linkages and hydrolytic degradation revealed the presence of significant amounts n-alkanoic/n-alkanol ester and ether side chains.5 Additional data were obtained from detailed studies of the mild thermal decomposition of the Athabasca asphaltene. The main result obtained with this technique was that the n-alkyl substitution in the structural elements in this asphaltene is highly specific.5 The information obtained from all these sources led to the formulation of the hypothetical two-dimensional molecular structure for the acetone extracted Athabasca asphaltene shown in Figure 1. This is a slight modification of the first model proposed in ref 5 and has a formula of C412H509S17O9N7, with an H/C ratio of 1.23 and a molecular weight of 6239 g/mol. As in the construction of any large molecule using limited experimental data, uncertainties are always present in the final structure. It is unavoidable to make a certain number of arbitrary choices during its construction.1 So, the resulting model is somewhat biased toward what is thought to be more characteristic of the higher molecular weight complement of the polydispersed asphaltene macromolecular mixture. The different types of resins were first isolated by means of preparative scale fractionation of the Athabasca bitumen.6-10 The resulting fractions were analyzed by infrared and mass spectrometry using different types of techniques.6-10 Most of the fractions containing the resins were subjected to different derivatization reactions in order to facilitate the determination of their molecular structure. Each of the resins employed in this work was the most abundant type found in the corresponding fraction of the Athabasca sand oil.6-10

gregates.11,12 This method makes use of analytical functions to represent bond stretching, bending, and torsional as well as nonbonded (electrostatic interactions, dispersion attraction, and exchange repulsion) energies of molecules.11,12 The procedure is such that an initial configuration is specified and the interatomic distances and bond angles are adjusted, using an iterative computational method, until the minimum energy configuration is obtained.11 The algorithm used in this work was part of the InsightII and Discover set of programs.13 Molecular mechanics can only guarantee to locate the nearest local minimum of the energy surface to the starting point of the calculation.11 In order to obtain the most stable conformation for the molecules and aggregates, it is necessary to use procedures that include molecular dynamics.11 In this way, the system may surmount energy barriers that lead to more stable molecular (or aggregate) conformations. The search for the lowest energy conformation of such a large asphaltene molecule as that proposed for the Athabasca sand oil is a formidable task. Such a large molecule can be built in many different ways from the starting blocks. Clearly, one has to choose one of the ways in order to obtain the most suitable conformation. In this work, we started with the blocks that contained the aromatic or saturated rings (Figure 1). Each of these fragments were first subjected to a minimization process of several hundred steps. The main purpose of this minimization was to relieve any residual strain left from their construction. The molecule was then assembled by adding the different fractions shown in Figure 1. In each step, the resulting molecule was subjected to a short minimization process to relieve any construction strain. After the asphaltene molecule was built from these fragments, it was subjected to an initial minimization of 4000 steps. In order to better explore the conformational space,10,11 dynamical runs of 15 ps of duration each were performed at the following temperatures: 300, 370, and 450 K and again at 300 K. The evolution of the total energy in a typical run is shown in Figure 2 a. Finally, a minimization process was applied until the maximum energy derivative was less than 0.1 kcal mol-1 Å-1. The resulting complex globular conformation of the proposed asphaltene molecule is shown in Figure 3. The molecular mechanical calculation was performed for the aggregates formed by the asphaltene with resins (Figures 4-8), toluene, and n-octane. The Amorphous Cell module13 was used to set a cell containing the asphaltene with periodic boundary conditions. The resin molecules of a specified type were introduced over the different sites and the dimensions of the cell were adjusted until a density of 1.0 was obtained. Again a short initial minimization of 500 steps was performed to release any unwanted strain produced in the construction stage. Later, a full minimization was performed followed by dynamical runs of 25 ps each at 400, 500, and 300 K. The evolution of the energy for a typical run extended up to 25 ps is shown in Figure 2b. The resulting aggregate was minimized using the conjugate

Molecular Mechanics Molecular mechanics provides information about the most stable conformation of molecules and their ag-

(12) Catlow, C. R. A. In Computer Modelling of Fluids, Polymers and Solids; Catlow, C. R. A., Parker, S. C., Allen, M. P., Eds.; NATO ASI Series Vol. 293; Kluwer Academic: Dordrecht, 1990. (13) Molecular Similations Inc., San Diego, CA.



Figure 2. (a) Time evolution of the energy of the system for the 15 ps dynamical run. (b) Time evolution of the energy of the system for the 25 ps dynamical run. Figure 4. Two- and three-dimensional drawings of resin molecules 1 and 2 from the Athabasca sand oil.

interactions. This field has been extensively tested against both experimental and theoretical results obtained in a large number of different organic molecules.13 The conformation of minimal energy of the asphaltene and resin molecules either isolated or forming aggregates was determined using a summation cutoff distance6 of 14 Å. This value provides a reasonable compromise between accuracy and computing time.12 Theory

Figure 3. Three-dimensional structure of the asphaltene molecule from the Athabasca sand oil.

gradient procedure. The energy of these aggregates was analyzed by means of the Docking module of the InsightII program.13 The results obtained for the different resins and for toluene and n-octane are shown in Figure 9. The enthalpy of interaction is directly the intermolecular energy as calculated by programs such as Discover.11 The CFF91 interatomic force field13 has been used in this work to describe the intra- and intermolecular

In contrast to simple liquids, many complex molecular fluids contain aggregates that have a significant lifetime (up to 10-1 s or more for macromolecules and their aggregates).3,4 In solutions of a pure organic surfactant (amphiphile) in water, these aggregates can have a lifetime up to several seconds. These micelles are formed and dissolved continuously, and their constituents are exchanged3,4 within microsecond to tenths of a second. These values reflect the forces that exist inside these aqueous solutions for just one kind of amphiphile. When several types of closely related amphiphiles are present in the solution, the resulting phase diagram displays an astonishing complexity above the critical concentrations. The richness of these diagrams for just only two slightly different types of amphiphiles is an indication that in crude oil, one can expect an enormous variety of aggregates of diverse shapes and properties due to its intrinsic molecular complexity. The variety of aggregates formed by simple surfactants arises from slight differences in their molecular shape (geometrical factors).3,4 In this way, spherical, cylindrical, tubular, and disklike aggregates, bilayers, and vesicles are formed. An analysis of the chemical components of crude oil/ bitumen shows that the molecular shapes range from a


Figure 5. Two- and three-dimensional drawings of resin molecules 3 and 4 from the Athabasca sand oil.

rather simple cylindrical one for the n-paraffins to very complex for resins and asphaltenes as seen in Figures 3-8. Clearly, the number and the shapes of the many different types of molecular aggregates found in petroleum and bitumens can be expected to be quite large and complex. As a result of this intrinsic complexity, the lifetimes of the molecular aggregates in oil are expected to be extended over a much wider time range than that found for the pure surfactants in water. This distribution of lifetimes is reflected in properties such as the viscosity and the diffusivity of the asphaltene and resins and should play a role in the resulting wettability of the rocks forming the oil reservoir. Thermodynamics of Micelle Formation The formation and the stability of molecular aggregates such as the micelles present in crude oil and bitumen will be determined by the changes in the total free energy ∆G ) ∆H - T∆S of the system.15 We have to analyze each of the different contributions to ∆G to gain insight on the aggregation process occurring between resins and asphaltene molecules. Unfortunately, (14) Vila-Romeu, N.; Taddei, G. Colloids Surf. 1997, 129-139, 399403. (15) Searle, M. S.; Willamas, D. H. J. Am. Chem. Soc. 1992, 114, 10690-10697. Willamas, D. H.; Cox, J. P. L.; Doig, A. J.; Gardner, M.; Gerhard, U.; Kaye, P. T.; Lal, A. R.; Nicholls, I. A.; Salter, C. J.; Mitchell, R. C. J. Am. Chem. Soc. 1991, 113, 7020-7030.

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the calculation of the entropic contributions is still an open problem even for small molecules in solution within simple liquids15 so only estimates will be made here for the aggregates of asphaltene with other molecules. The binding of a molecule A with molecule B always has an unfavorable entropic term arising from the reduction in the number of translational and rotational degrees of freedom15 from 12 to 6. This reduction in entropy was estimated to generate a change in free energy of around 10 kcal/mol for small molecules binding to large receptors. Nevertheless, the molecules in the aggregates are not rigidly attached so residual low-frequency intermolecular vibrations are always present after adsorption.15 These vibrations produce an entropic contribution that partially compensates the changes due to the reduction in the degrees of freedom. The reduction will be connected to the intensity of the interactions between the molecules.15 A strong interaction between them implies a deeper potential well than in weaker interactions. Upon aggregation, the decrease in entropy is much larger in the deep well than in the shallow one. For bimolecular interactions, there is a compensation between the enthalpic and entropic parts arising from these differences.15 In the case of the formation of typical asphaltene-resin micelles, the interactions are, in general, weak and involve mainly the van der Waals interaction plus some H bonding. One may then expect a rather shallow potential well upon



Figure 8. Two- and three-dimensional drawings of resin molecule 9 from the Athabasca sand oil.


adsorption with closely spaced levels capable of generating a sizable entropic contribution. This vibrational source compensates a significant part of the entropy loss brought about by the reduction of translational and rotational degrees of freedom upon adsorption.15 The partial desolvation of the contact area between the molecules forming the aggregate also generates an additional source of compensation of the entropy loss already mentioned. This desolvation generates a gain in the translational and rotational degrees of freedom of the molecules of the solvent involved. These molecules have rather restricted motions when solvating the asphaltene and gain degrees of freedom as they are released into the bulk of the liquid, thus increasing the entropy of the system. As in the case of drug-receptor interaction,15 the gain in entropy in this case is also smaller than expected because the solvent molecules are not rigidly attached and residual vibrations exists during solvation. This contribution is connected to the transference of a number of solvent molecules to the bulk phase with the accompanying increase in the translational and rotational entropy ∆Gt+r. The contribution will depend on the number of solvent molecules that are desorbed, consequently, it is proportional to the area of direct contact between the molecules forming the aggregate. The final geometry of the aggregate will determine the extension of this area. The area of contact is relatively large for the components of the Athabasca

Figure 9. Energy of interaction for nine different sites of the asphaltene molecule from Athabasca with nine resins plus toluene and n-octane.

oil sand and a considerable number of solvent molecules must be desorbed to allow the direct contact between the asphaltene and resin molecules. There is also an additional entropic contribution to ∆G arising from the restrictions on the internal rotors of the molecule or micelle generated by the aggregation15,16 ∆grot. The contribution to the free energy per rotor was estimated to be around 1 kcal/mol, although smaller values have been used because these rotations are generally replaced by hindered ones rather than being totally frozen.15 The proposed model asphaltene and resins contain some relatively short paraffinic branches5 that may have their rotational freedom impaired upon adsorption. Nevertheless, it is not expected that these restrictions will be much larger than those found in the isolated molecule within the oil. Then, all these rotational contributions to ∆G seem to be negligible and can be disregarded in the present case.


Murgich et al.

The comparison of the molecules before and after aggregation shows that the process of micelle formation generates some changes in their conformations that produce an unfavorable enthalpic contribution, ∆Hconf, to the free energy.15 This term represents the strain energy in the aggregate associated with the introduction of bond bending, rotation around bond, and dihedral angles, etc., produced by the aggregation process.15 Most of the terms mentioned above are unfavorable to aggregation so, in order that a lasting binding may exist, additional favorable factors must be present in the process. One of them is the sum of the changes in the energy of interaction between the charges present in the system, Σ∆Gi. This contribution contains the differences in the Coulomb interaction between the molecules when they are free in solution and when aggregated. Asphaltenes and resins contain some heteroatoms such as N, O, and S that have significant atomic charges. However, the number of such atoms is small5 (e6%) so it can be disregarded in systems such as those studied here. Nevertheless, this contribution to ∆G may become quite important if the participating molecules carry net electrical charges. The second enthalpic term favorable toward aggregation arises from the changes in the van der Waals interaction ∆GvdW resulting from the substitution of a portion of the solvent molecules by the atoms of the other molecule forming the aggregate. Also an additional term arises from changes in intramolecular van der Waals interaction due to variations in the packing of the molecules forming the aggregate.15 If the conformational changes upon aggregate formation are not very significant then this last contribution can be neglected. In general, the binding of the asphaltene and resins in the micelles will be favored if the resulting ∆G is such that15

∆G ) ∆Gt+r + ∆grot + ∆Hconf + Σ∆Gi + ∆GvdW < 0 (1a) In the present case, we have that

∆G ≈ ∆Gt+r + ∆Hconf + ∆GvdW < 0

(1b)

Contributions from all the terms of this last equation are present in the aggregation process of asphaltenes and resins. In some cases, the enthalpic contribution (∆Hconf + ∆GvdW) may become dominant while, in others, the adsorption will be governed by the entropic one (∆Gt+r) or, more generally, by a combination of both contributions. The aggregation of asphaltenes and other heavy fractions of the crude oil has been interpreted by considering enthalpic contributions arising from only H bonds, or the Coulomb interaction or even intermolecular charge transfer. The van der Waals, most of the enthalpic, and all the entropic contributions mentioned above were ignored in many of these analysis.17 One has to recall that the aggregation of asphaltene and resin molecules takes place in a condensed phase. There, the entropic changes may provide a significant contribution or even be the determinant factor as found in the adsorption of many drug agonists,16 polymers, and (16) Gilson, M. K.; Given, J. A.; Bush, B. L.; McCammon, J. A. Biophys. J. 1997, 72, 1047-1069. (17) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: Amsterdam, 1979.

macromolecules.17 Consequently, at least estimates of the entropic contribution should be made in any analysis of the formation and stability of aggregates of asphaltene and resins. The van der Waals interaction is always present in matter3 so it must be taken into consideration even for cases where H bonds and charge transfer are important factors for the formation and the stability of the aggregates. Results and Discussion In Figures 1 and 3-8 are shown two-dimensional and space-filling drawings of the model asphaltene and the resins studied in this work. The most stable conformation found in a vacuum for the asphaltene (Figure 3) and resin molecules (Figures 4-8) showed that most have, as expected, a rather complex three-dimensional shape. These conformations did not change appreciably when these molecules were allowed to interact in boxes of solvents such as toluene or n-octane, whose mixtures can represent a model crude oil. The use of only the chemical formula or the twodimensional sketches of organic macromolecules such as the asphaltene from Athabasca can be quite misleading as seen when comparing Figures 1 and 3. In many cases, the number of available active sites in such macromolecules is much less than that expected from two-dimensional drawings. This type of reduction may be quite important in asphaltenes and large resin molecules because several of the sites can be located inside the macromolecule. These sites are, then, totally unavailable for interaction with resins or solvent molecules. This is the case of several segments that are buried within the molecule of the Athabasca asphaltene (Figures 1 and 3). A similar problem occurs for some fragments in the asphaltene molecule from Boscan.18 For some other sites in this type of globular asphaltenes, the intermolecular interactions are only partially limited by steric obstacles produced by neighboring groups. The number of available sites and its selectivity toward the resins and solvent molecules will determine the formation of the micelles that solubilize the asphaltenes in the oil. Some authors have used the concepts developed for the micellization of surfactants having simple molecular shapes and developed “models” of asphaltene micelles of different geometries.19,20 In the case of the simple surfactants, a few geometrical parameters are enough to describe their geometry.3 Polar heads and nonpolar tails can be uniquely defined for the surfactant molecules allowing the use of rather simple equations to describe their behavior in solution.3,4 In the case of large, random, and complex macromolecules such as the asphaltenes, these factors cannot be defined because the polarity is distributed over the whole molecule (Figures 1 and 3) nor can its complex geometry be described with a few simple parameters. Clearly, one has to generalize these concepts first in order to describe the behavior of the micelles formed by resins and asphaltenes by means of equations similar to those used for surfactants in water. It is wise then to recall that the asphaltene and (18) Kowalewski, I.; Vandenbroucke M.; Huc, A. Y.; Taylor, M. J.; Faulon, J. L. Energy Fuels 1996, 10, 97-107. (19) Pacheco-Sanchez, J. H.; Mansoori, G. A. Pet. Sci. Technol. 1998, 16, 377-394. (20) Victorov, A. I.; Firoozabadi, A. AIChE J. 1996, 42, 1753-1764.



resin molecules have complex three-dimensional shapes and that the molecular recognition processes that leads to micelle formation depend on the fit of the resins with the adsorption sites of the asphaltenes. All this shows that the three-dimensional shape reflected in the molecular recognition is paramount in the formation of micelles in crude oil. This site selectivity explains the reason the resins of one crude do not necessarily contribute to the solubility of the asphaltene of another one. If the three-dimensional shape of the resins does not match some of the adsorption sites of the asphaltene molecule, then the aggregate will not have a significant lifetime and, consequently, the resin will not help to its solubilization. Figure 3 shows the overall three-dimensional shape of the proposed molecule of the Athabasca asphaltene. Such a molecule contains a minor axis of around 22 Å and a large one of 28 Å. These values are in accord with the average radius of gyration of 33 Å found for the solvated asphaltene molecule from Athabasca in toluene solutions by small-angle X-ray scattering.21 It is likely that the presence of strongly adsorbed resin or solvent molecules increases the measured average radius of the asphaltene in solution. Additionally, it is known that macromolecules and polymers in a good solvent tend to swell noticeably so it is likely that this effect may also contribute to the measured radius of gyration of the asphaltene from Athabasca. Figure 9 shows a graph of the enthalpies of adsorption of nine resins plus toluene and n-octane at nine different adsorption sites that are available in the asphaltene molecule from Athabasca (Figure 1). These sites were chosen because they contain mostly aromatic parts that are known to strongly bind most of the resins. These molecular parts are rather flat (Figure 3) thus favoring the interaction between atoms of similar regions in other molecules.3,22 The saturated regions found mostly in the bridges between the aromatic parts also attract the resins and the solvent molecules but with lower intensity. These regions have mostly curved parts and therefore only a few favorable close intermolecular contacts are possible when the resin or solvent molecules approach them. The low enthalpy of adsorption in this case cannot compete with the thermal agitation and the aggregate will have a very short lifetime.4 From Figure 9, we see that both toluene and n-octane have a lower enthalpy of adsorption when compared with that found for most of the resins. Most of the resins contain a relatively large number of aromatic C atoms that are available for interaction and that are quite rich in electrons so they interact strongly with similar ones of the asphaltene. The saturated regions have in their external surfaces mostly H atoms with much lower electron densities and consequently produce interactions of lower intensity. The adsorption of the larger molecules on the asphaltene will be also favored by the increase in entropy produced by their partial desolvation. The number of solvent molecules that has to be displaced by the adsorption is larger for most of the resins than for toluene or n-octane thus further favoring this process. Figure 9 also shows that the model solvent

molecules display some noticeable selectivity for the different adsorption sites of the asphaltene molecule. The adsorption of most of the resins is, in general, more favorable from the point of view of the enthalpy than for the model solvent molecules (Figure 9). The resins display also a higher selectivity for the adsorption sites than the model solvent molecules. This means that in this type of oil, the resins will tend to substitute the solvent molecules adsorbed in most of the sites. This trend will be more noticeable for the specific sites of high affinity for certain resins. Figure 9 shows that the enthalpic contribution favors the formation of micelles with the resin molecules rather than solvation with the other components of the oil. A resolution of energy of interaction in its components showed that the largest contribution to the enthalpy of adsorption arises from the van der Waals force. The intermolecular Coulomb interaction only generated contributions of less than 10% of the total energy. This result again cast doubts about the repeated assumptions made by several authors regarding the overall importance of the polar interactions in the formation of micelles of asphaltenes and resins.23 H bonding may contribute to the enthalpy of adsorption through its formation in different sites of the asphaltene molecule. These sites must be free of steric hindrances in such a way that proper contact is possible between the approaching resin and the accepting group. Otherwise, the H bonding will be weak and contribute little to the change in enthalpy found upon aggregation. As the Athabasca asphaltene structure proposed here contains some carboxylic groups and also some basic N sites, it is expected that H bonding will contribute to the aggregate formation if the sites are available for interaction. Further work is in progress in order to evaluate the importance of H bonding on the formation of these resin and asphaltene aggregates from the Athabasca sand oil. We have to remember that molecular recognition is a dynamic process based on distinct chemical interactions and not only on the passive action of a lock and a key.15 If the molecules of asphaltene and resin have sufficient flexibility, a conformational reorganization or induced fit is possible and an aggregate formed even if small discrepancies exist between the host (lock) and the guest (key) molecules. We have, then, that the flexibility of parts of the participating molecules plays also an important role in the formation of these aggregates. This flexibility and the existence of grooves, internal cavities, and channels in the proposed asphaltene structure suggest that small molecules of the right size and shape such as methane can be trapped inside the macromolecule. It is not difficult to imagine also that some n-paraffins may enter these channels thus becoming entangled with the asphaltene molecule. Further work in this area will clarify its importance in the formation of asphaltene aggregates.

(21) Xu, Y.; Koga, Y.; Strausz, O. P. Fuel 1995, 74, 960-964. (22) Murgich, J.; Rodrı´guez, J.; Aray, Y. Energy Fuels 1996, 10, 6876.

(23) Dubey, S. T.; Waxman, M. H. Paper SPE 18462 presented at the 1989 SPE International Symposium in Oilfield Chemistry, Soc. Pet. Eng., Houston, TX, February 8-10.

Conclusions Molecular mechanics have been used to compute the most stable conformation of a hypothetical native Atha-


basca asphaltene molecule derived from extensive experimental structural studies. In a vacuum, an isolated asphaltene molecule has a three-dimensional globular structure with internal cavities. The conformations of several resin molecules have also been computed and interaction energies have been calculated for solvent and resin molecules at nine different surface sites. The enthalpies of interaction vary between -9 and -39 kcal/ mol. The lowest values obtained are for solvent toluene and n-octane and the highest for polar resins like carboxylic acids. The results are significant because they demonstrate that asphaltene molecules, especially the large ones, are not necessarily two-dimensional flat disks but they have

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the capacity, owing to the presence of long polymethylene bridges, to fold upon themselves into a complex three-dimensional globular conformer with internal structure. Also, the results indicate that the selectivity of the resins for some sites of the asphaltene molecules provides an explanation of the specificity of the resins for its own crude oil and all those containing asphaltenes with similar adsorption sites. Acknowledgment. The authors thank the Natural Sciences and Engineering Research Council of Canada for financial support. EF980228W

Molecular Recognition in Aggregates Formed by Asphaltene and

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