Modeling Solvent Effects on Asphaltene Dimers - Energy & Fuels

Energy & Fuels 2005, 19, 1245-1251

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Modeling Solvent Effects on Asphaltene Dimers† Alexandre N. M. Carauta,‡ Peter R. Seidl,*,§ Erika C. A. N. Chrisman,§ Ju´lio C. G. Correia,# Prı´scila de O. Menechini,§ Daniel M. Silva,‡ Katia Z. Leal,‡ Sonia M. C. de Menezes,⊥ Wladimir F. de Souza,⊥ and Marco A. G. Teixeira⊥ Departamentos de Quı´mica Inorgaˆ nica e Fı´sico Quı´mica, Instituto de Quı´mica, Universidade Federal Fluminense (UFF), Outeiro de S. J. Batista s/n, Nitero´ i RJ, Brazil, Cep: 24020-150, Departamento de Processos Orgaˆ nicos, Escola de Quı´mica, Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro RJ, Brazil, Coordenaç a˜ o de Apoio a Pequena e Me´ dia Empresa do Centro de Tecnologia Mineral (CETEM), Av. do Ipeˆ , 900, Ilha do Funda˜ o, Rio de Janeiro RJ, Brazil, Cep: 21941-590, and CENPES, PETROBRAÄ S, Rio de Janeiro RJ, Brazil Received July 31, 2004. Revised Manuscript Received December 15, 2004

Asphaltene deposition is a well-known problem in the petroleum industry. Nevertheless, there seems to be a lack of information on the processes involved in asphaltene association and its relationship to asphaltene solubility under certain conditions. Molecular mechanics and molecular dynamics have had an important role in the investigation of these phenomena. To better understand the role of solvents in fractionating asphaltenes extracted from vacuum residues and evaluate their tendency to dissociate under different conditions, we modeled the effect of toluene, n-butane, isobutane, and n-heptane on an aggregate formed by two asphaltene molecules that would have a tendency to associate (not average structures commonly used in similar studies). Molecular dynamics simulations were performed on an asphaltene dimer after minimizing the conformation of each molecule and verifying the most stable position for docking. They reveal the extent to which these solvents are able to separate the aggregate at different temperatures after a given period of time. As expected, toluene is the most effective and n-heptane affects the aggregate the least, with n-butane and isobutane falling between these two bounds.

Introduction Asphaltene deposition is a problem that is related to the tendency of certain petroleum constituents to form aggregates.1 It is a very well-known problem that generates a large cost increase in the petroleum industry.2,3 Yet, there is a lack of information concerning the mechanism of asphaltene deposition and its origins.4,5 Asphaltenes are defined by their behavior in the presence of solvents. They correspond to fractions that are dissolved in aromatic solvents while remaining insoluble in aliphatic compounds. A considerable amount of research has been dedicated to the characterization of asphaltenes; however, even using standard separation methods, variations in properties may be observed. In an attempt to characterize representative constituents of this complex chemical entity better, recent work has †

Presented at the 5th International Conference on Petroleum Phase Behavior and Fouling. * Author to whom correspondence should be addressed. E-mail address: [email protected]. ‡ Universidade Federal Fluminense (UFF). § Universidade Federal do Rio de Janeiro (UFRJ). # Coordenac ¸ aõ de Apoio a Pequena e Me´dia Empresa do Centro de Tecnologia Mineral (CETEM). ⊥ CENPES, Petrobra ´ s. (1) Taylor, S. E. Fuel 1992, 71, 1338. (2) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd Edition; Marcel Dekker: New York, 1978. (3) Park, S. J.; Mansoori, G. A. Energy Sources 1988, 10, 109. (4) Rassamdana, H.; Dabir, B.; Nematy, M.; Farhani. M.; Sahim, M. AIChE J. 1996, 42, 10. (5) Hammami, J.; Ferworn, K. A.; Nighswander, J. A. Pet. Sci. Technol. 1998, 16, 227.

focused on how these properties are affected by the variables that are used in separation procedures.6,7 In particular, the intermolecular forces that cause the association of asphaltenes remain as one of the lessunderstood aspects that are related to asphaltene precipitation. According to the literature, hydrogen bonding,8-11 van der Waals forces,12 and charge-transfer interactions13,14 are involved in asphaltene association. However, there is controversy about the relative importance of each of these forces in the association process.15,16 Moreover, the associated structure and association dynamics still are not well-understood.16 Welldefined concepts seem to be desirable, to understand the nature of the asphaltene aggregation process better.17 Modeling studies should have an important role in their (6) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W.; Akbarzadah, K. Energy Fuels 2002, 16, 462. (7) Seidl, P. R.; Chrisman, E. C. A. N.; Silva, R. C.; de Menezes, S. M. C. Pet. Sci. Technol. 2004, 22 (7&8), 949. (8) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55, 187. (9) Ignasiak, T. M.; Strausz, O. P. Fuel 1978, 57, 617. (10) Tewari, K. C.; Galya, L. G.; Egan, K. M.; Li, N. C. Fuel 1978, 57, 245. (11) Acevedo, S.; Leon, O.; Rivas, H.; Escobar, G.; Gutierrez, L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1987, 32, 426. (12) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (13) Wright, J. R.; Minesinger, R. R. J. Colloid Interface Sci. 1963, 18, 223. (14) Siffert, B.; Kuczinski, J.; Papirer, E. J. Colloid Interface Sci. 1990, 135, 107. (15) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 68. (16) Schabron, J. F.; Speight, J. G. Pet. Sci. Technol. 1998, 16, 361. (17) Murgich, J. Pet. Sci. Technol. 2002, 20, 1029.

10.1021/ef049809d CCC: $30.25 © 2005 American Chemical Society Published on Web 03/29/2005

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investigation of aggregation processes and in identification of the nonbonded interactions that are involved. Asphaltene aggregates have been modeled under vacuum,18-20 in the presence of solvents19-21 and in the presence of resin-solvent systems,22 by classical molecular dynamics (MD). Aromatic and asphaltene aggregated models have been generated as periodic systems using the compass force field,23 where a system of 32 asphaltene model molecules were studied and a stable aggregate of several stacking sheets inside the cell of aromatic molecules was observed. Simulations on model compounds were performed to investigate the effect of solvents and heating on the stability of aggregates. Variations in the distance between aromatic ring systems were used to probe tendencies for disaggregation.24 The reliability of modeling studies is directly related to the quality of the asphaltene structures that are used to simulate a certain type of behavior. Because asphaltenes are defined by their solubility rather than as specific chemical entities, most simulations are based on average molecular structures. We have developed a different approach, based on a combination of molecular modeling and NMR data, that takes into consideration the asphaltene’s propensity for aggregation.25 Fractionation procedures are normally used to isolate and characterize representative constituents. Elemental composition, approximate molecular weights, and functional groups that can be identified by chemical or spectrometric techniques are then used to construct average asphaltene structures that can be related to certain properties. Molecular weights are an essential component of this approach, because they reflect the number of atoms that correspond to a given structural unit. The difficulties associated with obtaining representative molecular weights of asphaltenes have been extensively discussed in the literature.26 Problems related to the size of representative constituents of a certain fraction may be circumvented by concentrating the analysis on the structures that would be mainly responsible for the formation of aggregates. The combination of certain variables in isolation procedures may lead to the removal of constituents that would hinder aggregation thus favor “cleaner” asphaltenes.7 Structures that are consistent with analytical data may be modeled, revealing the molecular features that favor association. Structural units that are best suited for self-association can then be used to simulate properties related to aggregation. (18) Brandt, H. C. A.; Hendriks, E. M.; Michels, M. A. J.; Visser, F. J. Phys. Chem. 1995, 99, 10430. (19) Rogel, E. Colloids Surf. 1995, 104, 85. (20) Pacheco-Sańchez, J. H.; Zaragoza, P. I.; Martıńez-Madagan, J. M. Energy Fuels 2003, 17, 1346. (21) Sheu, E. Y. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Chapter IV. (22) Rogel, E. Energy Fuels 2000, 14, 566. (23) Pacheco, J. H.; Ramı´rez, F. A.; Martinez, J. M. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 2003, 48 (2), 71. (24) Takanohashi, T.; Sato, S.; Tanaka, R. Structural Relaxation Behaviors of Three Different Asphaltenes Using MD Calculations. In Proceedings of the 2002 International Conference on Heavy Organics Deposition; 7 pp. (CD-ROM.) (25) Seidl, P. R.; Leal, K. Z.; Chrisman, E. C. A. N.; de Menezes, S. M. C.; de Souza, W. F.; Teixeira, M. A. G. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 2003, 48 (3), 145. (26) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. Energy Fuels 2001, 15, 2001.

Carauta et al.

The alternative procedure for the design of asphaltene structures is based on molecular modeling, combined with 1H and 13C NMR and the elemental analysis of “clean” asphaltenes. Rather than average structures, it is specifically directed toward the types of molecular species in the asphaltene (i.e., toluene-soluble) fraction that would have a tendency to aggregate. The correct order of solubility and its temperature dependence was reflected by these simulations. Dissolution of asphaltenes in organic solvents involves very complex processes.27 It can be simplified considerably by assuming an asphaltene dimer to be a model for the aggregrate that it has a tendency to form. Dissociation into monomers can be monitored by the distance between aromatic ring systems, under given conditions. Our simulations correctly predict this tendency in different solvents. However, an increase in temperature will not promote dissociation of the dimer. A striking similarity with a model proposed for the type of asphaltene that would have a tendency to form aggregates from a different heavy crude oil was observed.28 Experimental Section Samples of asphaltenes from vacuum residues processed in Brazilian refineries were extracted using the IP-143 method; these samples have been labeled A, B, and C. Variables were adjusted within the limits of specifications for this method, to obtain the “cleanest” asphaltene possible under those conditions.7 The IP-143 method follows standard procedures for the separation of asphaltenes from crude oils and vacuum residues, and the details will not be repeated here. Statistical methods were applied to the planning of experiments, to probe the influence of different variables on the amounts of asphaltenes that are extracted and their respective chemical properties.7 Thus, modifications were introduced at the time the sample was refluxed or stored in the absence of light, so that asphaltenes A and B would be the cleanest possible. For the purpose of comparison, another methodsreferenced as HEPT and based on the exhaustive extraction of samples with heptane in a Soxhletswas also applied to asphaltene B. NMR experiments were performed on a Varian model INOVA300 spectrometer. Hydrogen spectra were run at 300 MHz on 5% (weight/volume) samples dissolved in a 1:1 mixture of deuteriochloroform and tetrachloroethylene at ambient temperature, using 4.9-µs (45°) pulses and 128 transients. 13C spectra were run at 75.4 MHz on a solution of ∼100 mg of sample dissolved in 1 mL of a solution of 0.05 M of chromium acetilacetonate in deuteriochloroform as a relaxation reagent and tetramethylsilane as an internal reference. The acquisition used 90° pulses, 10 s interval between pulses, 5000 transients, and the decoupler in the gated mode to avoid nuclear Overhauser effect (NOE).

Computer Simulations The Accelrys Discover Program, operated on a 2.4 GHz personal computer (PC), was used for molecular mechanics (MM) and molecular dynamics (MD) studies. The asphaltene molecule was initially minimized by a consistent valence force field (CVFF), using 5000 interactions and the conjugate gradient method. MD was conducted on this molecule (300 ps) at 300 K, and (27) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135. (28) Acevedo, S.; Escobar, O.; Echevarria, L.; Gutie´rrez, L. B.; Meńdez, B. Energy Fuels 2004, 18, 305.

Modeling Solvent Effects on Asphaltene Dimers

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Table 1. Elemental Analysis of Asphaltenesa Composition (mol %) H N S O

asphaltene

C

A B

87.3 87.2

a

7.7 8.0

2.2 2.2

1.05 1.08

1.75 1.52

amount of asphaltene (mol %) 8.7 9.2

Reproducibility: ( 0.3%.

selected structures were further minimized by the same method. This force field has been successfully used to describe the aggregation behavior of asphaltenes.15,22 Manual docking was used to probe the best spatial approach between two asphaltene structures. The dimer, which corresponds to the most stable combination of monomers, was optimized by MD; the five lowest-energy conformers were further minimized under the same conditions. The interaction between the asphaltene dimer and 80 solvent molecules was simulated using periodic boundary conditions by the Accelrys Amorphous Cell Program. MD (100 ps) was conducted for each solvent at 323 K. The temperature effect was simulated by repeating this procedure for toluene at 573 K and for n-butane and isobutane at 400 K. Results Asphaltenes A, B, and C have been used consistently in our studies on fractionation and characterization of asphaltenes.25 Asphaltenes A and B are from fields that are close geographically, whereas asphaltene C is an imported heavy crude with a high sulfur content. For the purpose of exemplifying the method, the structures generated for asphaltenes A and B are compared. Tables 1 and 2 clearly indicate that their asphaltene contents and elemental analyses are quite similar, although NMR data indicates that asphaltene A has a higher aromatic carbon content than asphaltene B. The data in Tables 1 and 2 indicate that the approximate molecular formula for asphaltene A is C69H73N2O and that for asphaltene B is C53H59NO. Using these molecular formulas and the percentage of aromatic carbon obtained by NMR data shown in Table 2, the number of aromatic rings is determined. The analysis of NMR data according to distinct 13C and 1H chemical-shift regions29 and still further assigned by the use of model compounds, chemical transformations and DEPT experiments.33 For asphaltene B, for example, the possible structures involved eight aromatic rings with two different arrangements. From the aromaticity factor (fa), the number of aliphatic carbons in this unit can be estimated and its hydrogen deficiency calculated from elemental analysis for a hydrocarbon adjusted for its heteroatom content and for the number and position of naphthyl rings.25 Molecular modeling studies on the association of asphaltenes was used as a basis for selecting structures (29) Storm, D. A.; Edwards, J. C.; DeCanio, S. J.; Sheu, E. Y. Energy Fuels 1994, 8, 561. (30) Silva, D. M.; Dias, J. F.; Silva, R. C.; Cunha, J. B. M.; Seidl, P. R. Unpublished work. (31) de Menezes, S. M. C., Petrobras CT QM 027/2002, mimeo, Rio de Janeiro. (32) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (33) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287.

Figure 1. Types of carbons and hydrogens.

that would have a tendency to associate more strongly from among those that reflect the analytical data.30 They indicate that the arrangement of aromatic rings, their common bonds with naphthyl segments and the position of alkyl substituents are the most important factors in calculating their relative energies. The number of aromatic rings does not exert such a strong influence; however, their arrangement is very important. Having established the number of saturated and unsaturated rings, NMR data are used to adjust the structures and estimate the size and position of alkyl chains. The length of the carbon chains is estimated and associated with the number of aromatic carbons connected to hydrogen and the substitution of the ring. The different types of carbons and hydrogens are represented in Figure 1. Representative structures established in this way are given in Figures 2 and 3. These structures are used as a starting point for MM/MD studies on association and solvation. Further refinement of these structures can be made using 13C NMR data on the relative proportion of relevant molecular segments; however, the “best” arrangements of these group will not vary, to a large extent. Discussion Models for asphaltene structures are usually based on 1H and 13C chemical shifts, elemental composition, and approximate molecular weights.26 Despite the recent advances in mass spectrometry, the reliability of methods for average molecular weight determinations has been the subject of much controversy. For three vacuum residues analyzed in other projects,31 gel permeation chromatography (GPC) gave approximately twice the value obtained from cryoscopic measurements and one-fourth of that obtained from vapor-pressure osmometry (VPO) and two reversals in order were observed. On the other hand, a considerable advance in the interpretation of 1H and 13C spectra, according to integration domains. has resulted from the introduction of multiplet-selective NMR techniques, which provide quantitative subspectra, according to the degree of substitution on carbon.32 Further refinements were provided by DEPT experiments on 13C spectra, which led to the tentative assignment of the sharp peaks in the aliphatic region33 and the estimation of the proportion of quaternary aromatic carbons that are shared by

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Figure 2. Representative structures of asphaltene A.

Figure 3. Representative structures of asphaltene B. Table 2. NMR Molecular Parameters molecular parameter

asphaltene A

asphaltene B

8.6 13.5 34.4 56.5 43.5

12.9 16.8 22.5 52.2 47.8

18.6 54 18 90.6 9.4

18.5 54.3 15.8 88.7 11.3

aromatic carbon (%) aromatic carbon connected to alkyl or heteroatom, Cal aromatic carbon connected to hydrogen, Cah aromatic carbon in ring junction, Caj total total aliphatic carbon (%) saturated hydrogen (%) R-hydrogen, ΗR β-hydrogen, Ηβ γ-hydrogen, Ηγ total total aromatic hydrogen (%)

three aromatic rings.34 An example of the sensitivity of NMR techniques is given in Table 3. Here, characteristic regions for alkyl carbons25 were carefully integrated. The difference between samples extracted using different procedures clearly is as great as that between two different samples. Asphaltene properties have been modeled from an “average” structure or a combination of structures that fit the analytical data. The reliability of simulations that are based on certain structure(s) will only be as good as the extent that the models that are used reflect the desired property. The distinction between “toluenesoluble asphaltenes” and “toluene-insoluble asphaltenes” that has been recently highlighted28 may provide a basis for distinguishing between constituents that (34) Sato, S.; Takanohashi, T.; Tanaka, R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2001, 46 (2), 353.

Table 3.

13C

NMR Relative Integrated Area of Aliphatic Chemical-Shift Region Integrated Area

region (ppm)

Sample A, IP-143 method

Sample B, IP-143 method

Sample B, HEPT method

37 32.5 31 29 26.5 22 19.5 14

22.7 18.5 17.3 100.0 27.9 21.3 19.2 18.4

19.5 13.2 16.4 100.0 24.3 21.7 18.8 16.9

29.2 17.9 28.2 100.0 25.6 21.8 22.7 17.5

would have a tendency to self-associate and those that would associate with an aggregate that had already been formed. The toluene-soluble constituents generally should correspond to those with a tendency for aggregation.



Figure 4. Chemical structures of (A) the lowest-energy monomer (E ) 349.97 kcal/mol) and (B) the lowest-energy energy dimer (E ) 619.69 kcal/mol).

Figure 5. Variation of the distance between aromatic ring systems for selected solvents at 323 K.

Because these structures are representative of the asphaltene constituents that have a tendency to aggregate, they will probably reflect properties associated with aggregation in way that is more realistic than a structure that represents an average of the molecular features that are present in that type of asphaltene. This observation is confirmed by MM/MD studies on the solubility of asphaltene B. Its relative solubility in toluene and selected aliphatic compounds is wellreproduced by simulations of the behavior of its dimer in the presence these solvents. Dissociation of the aggregated structure by heating and/or soaking in solvent is not well-understood. Modeling studies on structures generated from analytical data on asphaltenes extracted from Khafji vacuum residues may help clarify some aspects of this question. MD simulations based on an average of three model structures in the absence of solvents showed that aggregated forms correspond to the most stable arrangement of these constituents. Changes in the distances between aggregated structures during heating were monitored up to temperatures in which decomposition reactions may occur; yet, stacking interactions between aromatic ring systems may lead to coke formation, rather than dissociation.27 Models for asphaltene B (Figure 4A) have several features in common with average model structures of Khafji asphaltenes. Segments that have a tendency to associate are composed of a minimum of four pericondensed aromatic rings that share several bonds with their saturated counterparts and are connected to side chains that are far apart from each other (heteroatoms

were not considered, because modeling studies indicate that strong association occurs even if no nitrogen, oxygen, or sulfur is present). On the other hand, it is noteworthy that the distance between rings is shorter for asphaltene B than for the Khafji asphaltenes, because the corresponding structure represents the constituents that would have a tendency to aggregate, rather than an average of all those that are present. These features are the result of modeling studies on structures that are compatible with the analytical data. As shown in Figure 4A, the condensed aromatic system is almost planar, being slightly distorted by the fused naphthenic ring systems and saturated side chains. Among the several conformations obtained by MD methods, the most stable are those in which the aliphatic side chains are on the face opposite to that which approches the other monomer. Our modeling studies indicate that the most-stable dimers are formed when the fused aromatic ring systems are arranged so that they share the largest possible common surface. Naphthyl rings and alkyl side chains introduce steric repulsions between aromatic ring systems, so the most favorable arrangement corresponds to that for which interactions are minimized. In another study, using different asphaltene structures and molecular mechanics/quantum mechanics (MM/QM) methods, we obtained similar results for the dimer conformation.35 In the absence of solvents, the most-stable dimer is lower in energy than its two monomers, by 80.25 kcal/ mol. The (35) Carauta, A. N. M.; Seidl, P. R.; Correia, J. C. G.; Silva, D. M. J. Mol. Struct. (THEOCHEM) 2004, accepted for publication.

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Figure 6. Variation of the distance between aromatic ring systems for n-butane at different temperatures.

Figure 7. Variation of the distance between aromatic ring systems for isobutane at different temperatures.

most favorable distance between the quasi-planar ring systems is 3.844 Å, which is in good agreement with other modeling studies27 and experimental results. Figure 5 illustrates the variation in the distance between the two monomers of the aggregate during the simulation time for the four solvents that were investigated. The two extremes for solubility are wellcharacterized. In n-heptane, the distance between monomers hardly changes, reflecting the almost-negligible effect of the solvent on the aggregate, or its insolubility. On the other hand, in toluene, this distance increases to >5.00 Å, indicating that the aggregate will have a tendency to dissociate and dissolve. The behaviors in n-butane or isobutane fall between these two extremes and are quite similar up to 75 ps, after which point isobutane seems to be more effective. In these simulations, the first 20 ps correspond to the thermalization process; however, this time period is useful for interpretation purposes. During this period, the system relaxes and the conformation assumed by the asphaltenes reflects their interactions with the solvent. After that, this distance hardly varies for n-heptane, whereas, for the other solvents, it increases to ∼4.50 Å.

The effect of temperature on the dissociation process is given in Figures 6-8. An increase in temperature does not seem to increase the distance between monomers; this observation is verified for the MD simulations of Khafji vacuum residues in the absence of solvents.27 Conclusions Asphaltenes extracted from vacuum residues provide an attractive probe for association studies.36 The absence of groups that can contribute to polar types of interactions17 limits the factors that are involved. Our results indicate that simulations that are based on an asphaltene dimer as a model for an aggregate reveal the importance of superposition between aromatic ring systems, which is dependent on the relative position of naphthalene rings and aliphatic chains that are present in the asphaltene. They correctly predict its solubility behavior close to normal temperatures and the decrease in its solubility at higher temperatures. This model is also consistent with those which are proposed to account (36) Seidl, P. R.; Chrisman, E. C. A. N.; Carvalho, C. C. V.; Leal, K. Z.; de Menezes, S. M. C. J. Dispersion Sci. Technol. 2004, 25, 1.



Figure 8. Variation of the distance between aromatic ring systems for toluene at different temperatures.

for the solubility of different types of structures in p-nitrophenol.28 Fractions that are soluble in toluene would correspond to asphaltene constituents that would not have a tendency to aggregate with each other, so they were not included in the simulation. However, the types of structures proposed for its constituents in the work by Acevedo et al.28 should compete with toluene for an asphaltene or its aggregate. Therefore, the sensitivity of these aggregates to factors such as temperature, mixing time, solute-to-solvent ratios, sonication, etc.7 should be reflected by structural factors, such

as aromatic rings and saturated chains,28,36 that would affect the equilibria among these species. Acknowledgment. This research was supported by the CTPetro Program (FINEP and CNPq) and by Petrobra´s. P.R.S. has a research fellowship from the CNPq. We thank Prof. S. Sato for meaningful discussions and access to his analytical data. EF049809D

Modeling Solvent Effects on Asphaltene Dimers - Energy & Fuels

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