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Energy & Fuels 1996, 10, 68-76
Articles Molecular Recognition and Molecular Mechanics of Micelles of Some Model Asphaltenes and Resins Juan Murgich,*,† Jesu´s Rodrı´guez M., and Yosslen Aray Centro de Quı´mica, IVIC, Apartado 21827, Caracas 1020A, Venezuela Received June 15, 1995. Revised Manuscript Received August 18, 1995X
Molecular mechanical calculations of resins and a highly aromatic model asphaltene extracted from the spectroscopic data of a 510 °C residue of a Venezuelan crude showed that the driving interaction in the micelle formation is the attraction between their aromatic planes. The molecular recognition process is completed by the interactions produced by the alkyl and cycloalkyl groups present in these molecules. The complex three-dimensional shape of the alkyl parts of this model asphaltene limits the growth of its micelles through steric interference. The shape of these asphaltene molecules allows the aggregation of only those resins that fit its aromatic regions and show the lowest steric interference with its alkyl groups. This type of molecular recognition may explain why only some resins are able to solubilize specific types of asphaltenes through the formation of stable aggregates. It was found also that the steric interference of the alkyl groups may limit the number of available sites for H bonding and other directional interactions in this type of model asphaltenes.
Introduction Crude oil is a complex molecular liquid where micelles and/or molecular aggregates of different sizes and composition are found.1 It has been classified as a sol, a colloidal dispersion of a solid in a nonaqueous solvent formed by the rest of the crude oil.1-3 These micelles are formed by a heavy fraction called asphaltenes and are known to be peptized by an intermediate fraction called resin.1,2 As in all complex liquids, the noncovalent interactions responsible for the molecular recognition between the components, determine the microstructure of these aggregates.4 The asphaltene and resin molecules contain a significant number of aromatic rings, branched alkyls, and cycloalkyls plus some polar groups that include S, N, and O atoms.1 The asphaltene and resin fractions are then the polar parts of a crude oil and contain a large fraction of metals1,2 such as V and Ni. A crude oil can be described also as a solution of polar components (asphaltenes and resins) in a nonpolar solvent formed by the remaining hydrocarbon fractions. This type of solution generates phase diagrams with self-assembling processes that range from the formation of ordered clusters of the polar molecules †
Email:
[email protected]. Abstract published in Advance ACS Abstracts, October 1, 1995. (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) Storm, D. A.; Sheu, E. Y. In Asphaltene and Asphalts, Developments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 1994. (4) Israelachvili, J. Intermolecular and Surface Forces, 2nd. ed.; Academic: New York, 1991.
up to a separation of phases.5 The behavior of these solutions depends on a delicate balance between the different intermolecular forces and on the polarizability of the solvent molecules. This balance and the solvent polarizability determine the size and lifetime of the clusters and, consequently, are related to the stability of the solution.5 The stability of the micelles of asphaltenes and resins is very important as severe production, transportation, and distillation problems are linked to their precipitation from the oil.1,2 In order to obtain information about these aggregates, a molecular mechanics study of the molecular recognition pattern acting on model asphaltene and resins extracted from residues of some Venezuelan crude oil was undertaken. Molecular Models In recent years, many papers dealing with molecular conformation studies and the energetics of molecular aggregates have been published.6 The vast majority of them have been focused on biological macromolecules, with special emphasis on the study of the drug-receptor interaction and similar processes.6 Important information has been obtained using molecular mechanic and dynamic calculations in these6 and other systems.7 In particular, the use of atom-atom potentials has allowed extensive studies of the macromolecular conformations
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(5) de Leeuw, S. W.; Smit, B.; Williams, C. P. J. Chem. Phys. 1990, 93, 2704-2714. Mooij, G. C. A.; de Leeuw, S. W.; Smit, B.; Willams, C. P. J. Chem. Phys. 1992, 97, 5113-5120. (6) Brooks III, C. L.; Karplus, M.; Pettitt, B. M. Proteins: a Theoretical Perspective of Dynamics, Structure and Thermodynamics; John Wiley: New York, 1988. (7) 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.
© 1996 American Chemical Society
Some Model Asphaltenes and Resins
and the nonbonded interaction between drugs and receptor site models.6 Recently, several interesting studies dealing with the conformation and the resulting densities were performed in model macromolecules of certain coals.8 The application of molecular mechanics to the study of the asphaltene and resin molecules seems, at first sight, quite direct. Unfortunately, the development of this type of calculation has been based on the accurate knowledge of the atomic composition and, sometimes, also of the three-dimensional structure of the molecules.6 In the case of crude oil, the information about the composition and structure of the molecules is still incomplete.1,2 Nevertheless, spectroscopic information exists for selected fractions of asphaltenes and resins of different crudes and its residues.1,3 For many years, a large number of benzene rings were assumed to form the aromatic system of asphaltenes. Recently, chemical and thermal decomposition studies performed on some Canadian crude oils suggest a different structure.9 Instead of a single condensed aromatic system with a large number of rings, a set of smaller aromatic “islands” linked by bridges was postulated.9 The asphaltene and resin fractions of the 510 °C residue of a Venezuelan crude oil were analyzed by means of different techniques such as 1H NMR, IR, vapor pressure osmometry in CH2Cl2 at 40 °C plus elemental analysis.10 Also, a study of the distribution of functional groups was performed for the asphaltene fraction.11 The 1H NMR results were analyzed in order to obtain the average molecular weights12 and structural features. The average structure of the resins was similar to others already reported.1,2 The average structure obtained from the spectroscopic data and atomic ratios of the asphaltene fraction of the residue had a large aromatic plane with the heteroatoms and alkyl branches in the periphery. This result may be questioned on the grounds that there are indications about molecules of asphaltenes with more “open” structures.9 Nevertheless, there is no apriori reason to assume that the asphaltenes found in a 510 °C residue must have a structure equal to that found in some crude oils. The thermal decomposition that occurs at this temperature may have well increased the aromaticity of the asphaltene molecules of the residue.1 This process of condensation has been assumed to be responsible for coke formation in residues subjected to temperatures1 above 350 °C so it is not unlikely that the aromaticity of the average asphaltene molecule found for the 510 °C residue may differ noticeably from that detected in untreated crude oils. In order to obtain trends in the behavior of an asphaltene with a large aromatic core and resins, molecular models of these compounds extracted from (8) Takanohashi, T.; Iino, M.; Nakamura, K. Energy Fuels 1994, 8, 395-398. Murata, S.; Nomura, M.; Nakamura, K.; Kumagai, H.; Sanada, Y. Energy Fuels 1994, 7, 469-472. (9) Strausz, O. P.; Mojelsky, T. W.; Lowm, E. M. Fuel 1992, 71, 1355-1362. (10) Carbognani, L. Technical Report INT-02552,92, INTEVEP S.A., Los Teques, Edo. Miranda, Venezuela, 1992. (11) Green, J. A.; Green, J. B.; Grigsby, R. D.; Pearson, C. D.; Reynolds, J. W.; Shay, J. Y.; Sturm, G. P.; Thomson, J. S.; Vogh, J. W.; Vrana, R. P.; Yu, S. K. T.; Diehl, B. H.; Grizzie, P. L.; Hirsch, D. E.; Hornung, K. W.; Tang, S. Y.; Carbognani, L.; Hazos, M.; Sa´nchez, V. Topical Report NIPER-452, NTIS Nos. DE90000200 and DE90000201; NTIS: Springfield, VA, 1989; Vol. 1 and 2. (12) Leon, V. Fuel 1987, 66, 1445-1446.
Energy & Fuels, Vol. 10, No. 1, 1996 69
the available spectroscopic data obtained from a 510 °C residue of a Venezuelan crude were used.10,11 The inherent ambiguity in the structures of these “average” molecules1 led us to emphasize here that the results obtained should be considered with caution. General trends rather than the details arising from some particular aspect of the model molecules will, then, be considered. Methodology The equilibrium conformation of large molecules such as the asphaltenes is difficult to predict from quantum mechanical calculations.6,7 As in biological macromolecules,6 molecular mechanics can provide information about the most stable conformation of model asphaltenes and resins and its aggregates. This method makes use of analytical functions to represent bond stretching, bending, and torsional and nonbonded (electrostatic interactions, dispersion attraction, and exchange repulsion) energies of molecules and atoms.6,7 These functions and their derivatives can be evaluated even for molecules containing several thousand atoms.6,7 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.6 The algorithm used in this work was part of the INSIGHTII and DISCOVER 2.20 set of programs.12 In the energy minimization process, the steepest descent method was employed in the first 100 steps of the procedure and, then, switched to the conjugate gradient method.6,7 The CFF91 interatomic force field13 has been used in this work to describe the intra- and intermolecular interactions. This field has been extensively tested against both experimental and theoretical results obtained in many different organic molecules.13 The conformation of minimal energy of the model asphaltene and resin molecules either isolated or forming aggregates was determined using a summation cutoff distance6,14 of 15 Å. This value provides a reasonable compromise between accuracy and computing time.13,14 Molecular mechanics can only guarantee to find the local minimum of the energy surface which is nearest to the starting point of the calculation. In order to obtain the most stable conformation for macromoleclues, it is necessary to use procedures that include molecular dynamics.13,14 In this way, the system may surmount energy barriers that lead to more stable molecular conformations.15 This is a very efficient way of searching an important part of the conformational space and is used extensively in the analysis of macromolecules of biological importance.14 The molecular mechanical calculation was performed for monomers, dimers of resin, and asphaltene; dimers, trimers, and tetramers of asphaltenes plus a model of a micelle formed with two asphaltene and two resin molecules. At the start of a run, a minimization was first done in order to relax any initial strain that may be left from the construction of the molecule or aggregate. Once the (13) Hagler, A. T.; Osguthorpe, D. J.; Dauber-Osguthorpe, P.; Hemple, J. C. Science 1985, 227, 1309-1315. (14) Va´squez, M.; Ne´methy, G.; Scheraga, H. A. Chem. Rev. 1994, 94, 2138-2239. (15) Biosym Technologies Inc., San Diego, CA.
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system was relaxed, the positions and velocities were assigned in such a way that the velocities were in accord with the set temperature of the system (500 K, Maxwell distribution). A period of equilibrium of several thousand steps (each of 10-15 s) was then established in order to equipartition the potential and kinetic energies. For the monomers, the equilibration period was 400 ps while for the tetramer it was 800 ps with intermediate values for the dimer and trimer. After achieving this equilibration, the system was allowed to further evolve for 100 ps. Systematic minimization of the conformation obtained each 10 ps was performed until the maximum derivative was less than 0.1 kcal/(mol Å). The resulting conformations were analyzed in order to see if new lower energy ones occurred at this particular temperature. In order to better explore the conformational space,6 dynamical runs were performed at 800 K for 50 ps. The high-temperature structures were annealed by subjecting the system to a dynamic calculation at a lower temperature (300 K) for 50 ps. A minimization process was then applied until the maximum derivative was less than 0.001 kcal/(mol Å). The conformations obtained in this way were subjected to two additional runs similar to the previous one. From the set of all these conformations, the one with the lowest energy was chosen as the most stable. Several initial configurations were tried, including ones where the aromatic plane of the approaching molecule was perpendicular to that of the others. The procedure was started by moving the molecules within interacting distance (10 Å) of each other. In some cases where the aromatic planes were parallel to each other, the center of the approaching molecule was also shifted by 15 Å from the target molecule/aggregate. In the case of perpendicular or parallel shifted center approach, the distance between molecules was chosen in such a way that the occurrence of unwanted short intermolecular contacts was prevented.14 In the trimers, the approaching molecule was placed near an already minimized dimer before the minimization process was started. A similar procedure was employed in the tetramer using an already minimized trimer. In all these configurations, it was found that the most stable ones for the aggregates were always those containing stacks of parallel aromatic planes. It was also found that, for the different starting configurations, the final ones were all very close in energy. At most, they showed only a slight difference in the conformation of some of the alkyl branches. Theory Two interacting molecules always attract each other unless they have a large net charge of the same sign and/or some of their interatomic distances are much shorter than the sum of the corresponding van der Waals radii.4 This does not mean that they will always form a stable aggregate when they approach each other. In order to form one, the energy of the aggregate must be lower than the sum of the energies of each molecule when they are separated by an infinite distance.4 Usually, the reference state at infinity is taken to be the molecular conformation of minimum energy.6,7 We may visualize the formation of an aggregate between molecules A and B as a two-step process.6 The first step is the distortion of the molecules involved until they
Murgich et al.
reach the conformation found in the aggregate while they are still at infinite distance of each other. In this process, the molecule A will gain net amount of energy ∆EA so that iso ∆EA ) Eagg A - EA
(1)
is the internal energy of the distorted where Eagg A molecule A and Eiso A is the energy of the stable molecular conformation at infinite separation. A similar equation holds for molecule B. In the second step, the distorted molecules are allowed to approach each other and to interact. This process generates a negative contribution to the energy Eint that may or may not be able to overcome the increase in energy produced by the deformation process. The molecular aggregate is stable only if the association energy Ecomp
Ecomp ) Eint + ∆EA + ∆EB
(2)
is negative (that is, if |Eint| > ∆EA + ∆EB). Therefore, if large molecular deformations are required in order to make a reasonable number of attractive contacts, the resulting aggregate will not be stable because the value of Eint will be too small. Results and Discussion Figures 1 and 2 show two-dimensional and spacefilling drawings of some of the model resins and asphaltene studied in this work. The stable conformation found in the vacuum for the model resin (see Figure 1b) and asphaltene (see Figure 2b) molecules showed that they have a complex three-dimensional shape. This conformation did not change appreciably when these molecules were allowed to interact in boxes of solvents such as toluene, cyclohexane, or n-octane. The use of only the chemical formula or the twodimensional sketches of organic macromolecules may be misleading when directional intermolecular interactions have to be studied. H bonds have been assumed to produce an important contribution to the energy of interaction between asphaltenes, resins, and some solvents.1,16 If one looks at the two-dimensional representation of these molecules (Figures 1a and 2a), one may conclude that the contribution of such bonds has to be proportional to the number of H donor and acceptor sites present in the molecules. However, the fact that these large molecules have a complex threedimensional structure (see Figures 1b and 2b) shows that not all the sites may be necessarily available for H bonding. The same is true for other types of directional interactions such as charge transfer, etc. Therefore, the chemical composition shown in two-dimensional drawings provides only an upper limit of the possible number of intermolecular H bonds and other directional interactions. In many cases, the number of active sites in complex macromolecules is less than what is expected from the two-dimensional drawings.17 This type of reduction in the number of sites may be quite important in asphaltenes due to the presence on its periphery of (16) Sieffert, B.; Kuczinski, J.; Papirer, E. J. Colloid Interface Sci. 1990, 135, 107-117. (17) Morawetz, H. Macromolecules in Solution, 2nd. ed.; R. E. Krieger: Malabar, FL, 1975.
Some Model Asphaltenes and Resins
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Figure 2. (a, top) Two-dimensional drawing of model asphaltene molecule. (b, bottom) Space-filling drawing of the model asphaltene molecule. Notice the difference in conformation between the aromatic plane and the alkyl groups.
Figure 1. (a, top) Two-dimensional drawing of model resins. The upper structure corresponds to resin Rc while the lower one to Ra. (b, bottom) Space-filling drawing of the Rc resin molecule. In all the drawings of this type, the radius of the atomic spheres corresponds to some of the known van der Waals radii.
the alkyl branches that could interfere with the molecules approaching the bonding or reaction site. The possible reduction in the number of active sites has an important consequence in processes where specific intermolecular interactions play a determining role. As an example of this, one may analyze its effects, e.g., on the design and selection of additives used to prevent asphaltene deposition. Even if the chemical composition of these additives may be the correct one, unless the three-dimensional shape of both the additive and the target compound fit adequately, little or no effect will be obtained from its use. Therefore, for a successful design, one has to consider the chemical composition and the three-dimensional conformation of the additives that generate a favorable molecular rec-
ognition process with the asphaltenes. Nevertheless, 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. If the molecules of asphaltene and resin have sufficient flexibility,6 a conformational reorganization or induced fit is possible and an aggregate formed even if some apparent discrepancy exists between the host (lock) and the guest (key) molecules. We have, then, that the flexibility of parts of the participating molecules (i.e., alkyl groups in asphaltenes, resins, and dispersants) also plays an important role in the design of additives for the prevention of the solid deposition in the oil production. Intermolecular Interactions and Asphaltene Micelles The precise mechanism of the association between asphaltenes and resins has not been well established yet,18 but H bonding and charge transfer have been cited as responsible for it.1 The forces between two molecules may be divided into4 (a) the electrostatic forces existing between their net charges, permanent dipoles, quadrupoles, etc., (b) the polarization forces that arise from the dipole moments induced by the electric fields of neighboring molecular charges and permanent dipoles, and (c) the forces that are quantum mechanical in origin such as the dispersion, exchange, and charge transfer interactions. (18) Storm, D. A.; Sheu, E. Y.; DeTar, M. M.; Barresi, R. J. Energy Fuels 1994, 8, 567-569 and papers cited therein.
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There is experimental and theoretical evidence that the van der Waals4 (mainly the dispersion part) and the electrostatic interactions plus the desolvation process are the main factors in the molecular recognition in solutions of most organic compounds.19 Unless strong donors and acceptor molecules are involved, the induction (polarization) and charge transfer effects play only a relatively minor role in the molecular recognition process and in the stability of the aggregates.19 The number of atoms that may be part of the strong donor and acceptor groups in asphaltenes and resins is a small fraction (
