Impact of H-Bonds and Porphyrins on Asphaltene Aggregation As

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The impact of H-bonds and porphyrins on asphaltene aggregation as revealed by molecular dynamics simulations Hugo Santos Silva, Ahmad Al Farra, Germain Vallverdu, Didier Bégué, Brice Bouyssiere, and Isabelle Baraille Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01901 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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The impact of H-bonds and porphyrins on asphaltene aggregation as revealed by molecular dynamics simulations H. Santos Silva,∗,† A. Alfarra,‡,¶ G. Vallverdu,†,¶ D. Bégué,†,¶ B. Bouyssiere,†,¶ and I. Baraille†,¶ † CNRS/Univ Pau & Pays Adour, Institut des Science Analytiques et Physico-Chimie pour l’Environnement et les Materiaux, UMR 5254, 64000, Pau, France. ‡Total Research & Technology, Gonfreville, BP 27, 76700 Harfleur, France ¶Joint Laboratory C2MC: Complex Matrices Molecular Characterization, Total Research & Technology, Gonfreville, BP 27, 76700 Harfleur, France E-mail: [email protected]

Abstract The presence of metalloporphyrins alongside asphaltenes in heavy-fractions of crude oil is a key issue in petroleum exploration and upgrading. These compounds are also expected to display interfacial activity in water/toluene mixtures but the origin of this phenomenon remains uncertain. In this second part, we use molecular dynamics simulations to investigate complex asphaltene mixtures constituted of 10 different molecules, under also multifaceted solvation conditions (toluene/n-heptane/water). We add nickel and vanadium (under the form of vanadyl) porphyrins with occasionally grafted polar lateral chains, in these mixtures. The aggregation behavior and interaction with water molecules (as a model to have insights from the interfacial activity of such molecules)

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are intimately linked to the type of porphyrin and to the molecular properties of the asphaltenes (mainly the presence of polar lateral chains). Vanadium porphyrins, even without polar lateral chains, can form H-bonds that might contribute to their presence within asphaltene nanoaggregates. Moreover, when polar lateral chains are present in asphaltene molecules, the systems display a supramolecular organization with several distinct interactions at the same time. The shapes of these systems do not totally agree with the traditional Yen-Mullins model. In the first part of this work, we finally propose that complex asphaltene systems in complex solvent mixtures seem to have a supramolecular behavior with non-negligent colloidal behavior as well. This should be an indicative that Yen-Mullins and Gray’s models of asphaltene self-assembly are neither conflictual nor antagonists. They are two facets of a scale- and molecular structure-dependent complex mechanism.

Introduction Asphaltene aggregation is a topic of vivid discussion in the heavy-oil community. When uncontrolled, it can clog well-bores, rock pores, and up to the whole refining site 1 . Understanding the origin of this physical-chemical behavior is mandatory in order to better design solutions for the oil industry such as asphaltene inhibitors 2 , demulsifiers 3–5 , crude mixtures charts 6 , etc. However, the access to the asphaltenes molecular structure is a challenge that has been overcome 7–9 only very recently. The work of Schuler et al. 8,9 who identified single asphaltene molecules by atomic force microscopy (AFM) became a landmark in asphaltene science. It corroborates several speculative studies on asphaltene structure published since the early ’90s. More particularly, it allows the Molecular Dynamics community to simulate real structures and to render our results closer to the experimental observations. In our previous works, we used such asphaltene systems to probe the effect of several molecular characteristics of their aggregation process. We have step-wisely investigated the

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influence of the presence of heteroatoms on the conjugated core 10 , the size of the conjugated core 10,11 , the length and polarity of the lateral chains 10,12,13 , their number and their symmetry 13 , the number of asphaltenes in the simulation box, and the sensitivity towards temperature and pressure 13 , etc. In another recent work 14 , we used these molecular structures to access their aggregation properties in presence of different solvent mixtures (toluene, water, n-heptane, toluene/water, toluene/n-heptane, toluene/n-heptane/water). With these particular studies, we started establishing a structure-function link between the asphaltene aggregation properties and their molecular structure. This molecular cartography work also leaded us to analyse that the two models used to explain asphaltene aggregation, i.e., the Yen-Mullins model and Gray’s model. We showed that they are neither conflictual nor antagonists but rather intercorrelated. In this way, the Yen-Mullins model would be a particular case of Gray’s model that describes a specific class of asphaltene molecules with specific molecular properties. Our previous molecular dynamic simulations treated asphaltene simulation boxes constituted of a single type of asphaltene molecule 13 . Even if such configuration is insightful enough, to gain in complexity, it is mandatory to simulate more complex systems, constituted of very different asphaltene molecules. Works reporting mixtures of asphaltenes are rare and we can only cite two examples here: first, in 2016, our group 10 indicated that, for very small systems (5 asphaltene molecules), the aggregation of a mixture with two different asphaltenes is intermediary to the behaviour of each molecule. The second work, recently published by Headen et al. 15 , corroborated our findings confirming both the non obvious reduction in cluster size upon mixing of different asphaltenes, and the non significant change in the distribution of the cluster properties. Another factor involved in asphaltene aggregation process is the presence of metalloporphyrins in the heavy-oil phase 16,17 . The metals present in such molecules (Nickel or Vanadium oxide) represent a challenge for molecular dynamics simulations since it is quite difficult to produce a classical force field for such metallic centers. The example of works

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dealing with such parameter are even more scarce and our group presented one of the first molecular dynamics works dealing with such complex asphaltene-porphyrin systems 11 . In this piece of work, we highlighted that porphyrins without polar lateral chains do not present any inherently strong aggregation behavior towards asphaltene molecules and their presence did not induce any stronger aggregation of asphaltenes. However, we identified strong Hbonds interactions between the vanadyl group and polar or even aromatic hydrogen atoms of the asphaltene molecules. This work concluded that if any other strong aggregation property exists between asphaltenes and porphyrins, this might be due to the presence of polar lateral chain grafted to the porphyrins. Since then, and up to our knowledge, no other paper have addressed this issue, inferring or corroborating these findings. This is quite surprising, since the distribution and the interaction between asphaltenes and porphyrins are central to the “Yen-Mullins vs. Gray” dichotomy. Since early 2010, several experimental studies have tried to track down the porphyrins with respect to asphaltene (nano)aggregates 18,19 . In the Yen-Mullins model, since nanoaggregates are limited in size and have a colloidal behavior 20 , porphyrins would preferentially position themselves on the periphery of the nanoaggregates, probably doing π-stacking interactions with asphaltenes. In Gray’s model, since base-metal interactions are also expected, 16,17 , porphyrins would be distributed all over the “aggregate”, even in the interior of the network formed by the supramolecular interactions. A good probe to advocate against one model or the other is the nuclear magnetic resonance experiments (NMRD) performed by Korb et. al. 21 These authors have proven that, around a paramagnetic relaxation center (the vanadyl group of the vanadium porphyrin), protons from maltenes have a very particular relaxation mechanism and diffusion, i.e., they slowly diffuse in the environments where this porphyrin is found in an anisotropic way 18,19 . In such a way, if porphyrins are indeed found in asphaltene aggregates, and if the Yen-Mullins model applies, the maltene diffusion around the paramagnetic center would not be anisotropic since vanadyl porphyrins would be lying on-top of asphaltene aggregates. To explain such mechanism of diffusion, porphyrins should

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be inserted within a network formed by asphaltenes and forming bonds through H-bonds and acid-base (Coulomb) interactions, as it is suggested in Gray’s model 16,17 . Our recent studies have shown that vanadium porphyrins are indeed able to interact with apolar hydrogens via the radical delocalized over the VO group 11 . This attractive potential confirms in part the results obtained by NMRD 21 . Even with such experimental and theoretical data, the topic merits to be further investigated, addressing more complex asphaltene systems. In this way, the objective of this paper is to fill the lack of description of complex asphaltene mixtures including metalloporphyrins. We have performed molecular dynamics simulations of a complex mixtures of asphaltene molecules (50 molecules within the simulation box, 10 different molecules, 5 molecules of each type) with vanadium and nickel porphyrins (5 molecules of each) that can have or not polar lateral chains grafted to the tetrapyrolic macrocycle. Willing to better understand the origin of the behavior induced by the polar lateral chain’s ends, we also aimed to study polar groups other than the traditional carboxylic acid group (−C(−O)OH). In this way, we have chosen, ad hoc, to study both phosphonic acid (−P(−O)(OH)2 ) or sulfonic acid (−S(−O)2 OH) derivatives as well. We are aware that it is improbable that these groups could be found in crude oil since they would have undergone hydrolysis during petroleum genesis and maturation. However, having the possibility to screen the H-bond formation effect against the structure of the polar group is of high importance to our bottom-up strategy. Even if, in the present work, these groups were inserted as asphaltene’s lateral chain’s ends, one could also think of them as additives to be added to crude oil for specific purposes. Particularly, it was already demonstrated that these groups are wellknown for their interactions with metallic centers as chelants 22–24 . Given that interfacial activity in water-in-oil systems depends on the polar groups 25 , modeling polar groups other than carboxylic acids, such as phosphonic and sulfonic acid derivatives, is a required step to understand the mechanisms behind H-bond networks formation and asphaltene stabilization

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thereafter. Globally, this work is divided to answer specific questions as follows: 1. Heterogeneous asphaltene systems in different solvents: does the mixture of different asphaltene molecules change the conclusions already obtained on the aggregation behavior of one type of molecule? 2. Lateral chain’s polar groups other than −C(−O)OH: has the change of the carboxylic acid group by a phosphonic or sulfonic acid has any impact on the formation of H-bonds between asphaltenes, asphaltene and water, and asphaltene and porphyrin molecules? Is it also the case for heterogeneous systems? 3. Presence of porhyrins and their lateral chains: have porphyrins any effect on the asphaltene-asphaltene interactions? How is the asphaltene-porphyrin interaction impacted by the presence of lateral chains? What about the porphyrin-water interactions?

Methodology In this work, we consider two types of simulation boxes, which asphaltene content can be described as follows: 1. homogeneous systems: constituted of a single type of asphaltene molecule, with a total of 50 asphaltene molecules within the box. For the sake of these studies, this molecule was chosen to be the AAH molecule (see below) which polar group was varied among C(−O)OH, P(−O)(OH)2 , or S(−O)2 OH (carboxylic acid, phosphonic acid, and sulfonic acid, respectively). 2. heterogeneous systems: constituted of a mixture of 50 asphaltene molecules containing 10 different molecules, 5 molecules of each type (see below.)

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The molecular systems are based on the so-called CA22 (Figure 1.b) molecule identified by Schuler et al., 8 to which we have grafted two lateral chains on the opposite sides of the molecule. Then, the length of these chains was set to be n-C6 H13 or n-C16 H33 , and the number of fused rings on the conjugated core was set to 7 or 11, as seen in Figure 1.a-b. The so-called PA3 molecule type described by the above authors was also studied with two different lateral chains. These molecules are presented in Table 1 alongside with the labels used. Other characteristics of these molecules are presented in Table S.2 of the ESI material. Finally, the experimental determination of asphaltene molecular structure by AFM is not free of bias, i.e., selecting only planar, aromatic-rich structures. As our study is based on these observed systems, we acknowledge that this a priori choice of molecular structures can have some influence on the results herein presented.

(a)

(b)

(c)

Figure 1: Molecular structure of the studied molecules with increasing conjugated core size and lateral chain length. (a) represents molecules of the type AAX (derived from CA22), having 25 π-conjugated carbon atoms; (b) ADX molecules, 35 π-conjugated carbon atoms. Finally, (c) the PA3-type molecule, having 32 π-conjugated carbon atoms.

Table 1: Definition of all the 10 asphaltene molecules considered in this work and the labels system. NC Stands for Number of Conjugated Carbon Atoms NC

n-C6 H13

n-C16 H33

25 35 32

AAC ADC A13

AAF ADF A14

n-C6 H13 / n-C5 H10 COOH AAH ADH -

n-C16 H33 / n-C15 H30 COOH AAI ADI -

We have also studied the effect of nickel and vanadium porphyrins. Two different cases 7


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were envisaged: porphyrins having no polar lateral chains (but only very short and apolar ones) and porphyrins having a n-C5 H10 COOH chain. Their molecular structures are depicted in Figure 2. The simulation boxes contained the same number of porphyrin molecules (5 nickel porphyrins and 5 vanadyl porphyrins), but the ratio X : Y between porphyrins having or not a lateral chain was varied, X and Y being the number of porphyrins of the first and second types, respectively. The following ratios were studied: 0:5, 1:4, 3:2, and 5:0.

(a)

(b)

(c)

(d)

Figure 2: Molecular structure of the porphyrin molecules herein studied: (a) Nickel porphyrin having no polar lateral chain; (b) Vanadyl porphyrin having no polar lateral chain; (c) Nickel porphyrin having a polar lateral chain; and (d) Vanadyl porphyrin having a polar lateral chain. All the systems were solvated with a toluene/n-heptane/water mixture, except when otherwise stated. Water molecules topologies were created using the SPC/E 26 model. The concentration of water was equal to 10 g/L (320 water molecules for a total of 6417 toluene 8


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molecules). n-Heptane topology was built similarly to the asphaltene ones. Its concentration was set to 154,8 g/L (900 molecules of n-heptane for a total of 6417 toluene molecules). The so-formed solutions have an asphaltene concentration around ∼ 5 wt.%. Independently of the system, the simulation boxes contained 50 asphaltene molecules and 6417 toluene molecules. Whenever they were requested to be present, the numbers of water, n-heptane and porphyrins molecules were set to 320, 900, and 10 (5 nickel and 5 vanadyl), respectively. Full details on how these different proportions were obtained can be found in ref. 27 . The computational details behind the Classical Molecular Dynamics (CMD) simulations are fully described in the ESI. Briefly, the simulations consist of using the GROMOS96 force field with the 53a6 parameter set 28 within the Gromacs 5.0 29 package. This UnitedAtom (UA) force field allows to increase the size of the systems being treated without compromising accuracy. Moreover, bond vibrations have been constrained since they occur at time scales that are orders of magnitude faster than the physical-chemical process of interest. More details on the force field parameters chosen in this study can be found in ref. 10 . Last but not least, the molecular interactions herein depicted can suffer from some degree of inaccuracy due to this choice and adaptation of the existing force field, even if some extensive experimental validation exist that corroborate the quality of the results obtained by this specific force field 30,31 . Notwithstanding these evidences, the reader must keep in mind that absolute energy values must not be regarded as such and one should focus on the trends one can draw from the results hereafter presented.

Results and Discussion Heterogeneous systems in mixtures of solvents As already described in the previous section, the simulation boxes contained 50 asphaltene molecules (5 molecules for each of the 10 different molecule types) with solvents that could be: a) pure toluene; b) toluene/n-heptane; c) toluene/water; d) toluene/n-heptane/water. 9


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The objective is to determine if such a complex mixture of asphaltenes behaves in a similar manner as isolated molecules or simple mixtures (limited to two different molecules). The most common way to describe the structure of a liquid system is using a radial distribution function (RDF). Briefly, these are probability functions that determine how the liquid is structured. They are calculated by counting the number of molecules found in each increment of the radial coordinate centered in a reference for each time step of the dynamics. In our case, the reference is a surface around the reference molecule rather than individual atomic coordinates. In order to eliminate intramolecular peaks, only distances superior to 2 Å are considered. If one iterates this process for all the molecules of interest in the system, one can obtain a histogram-like curve that indicates how asphaltenes (or any other solute) are arranged at short and long ranges. In this way, Figure 3 presents these RDFs calculated between asphaltene molecules (as a whole) for each solvation system and also the CN/N (%) ratio normalized using the case with pure toluene as the sole solvent as reference. Asphaltene-Asphaltene

Asphaltene-Asphaltene

1.00 Normalized RDF g(r) (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Toluene/n-heptane/water

0.75

Toluene/water

0.50

Toluene/n-heptane

0.25 0.00

Pure toluene Toluene/n-heptane Toluene/water Toluene/n-heptane/water 5

10

15

20 25 30 35 Radial distance (Å)

40

45

50

Pure toluene 55

70

80

90 100 Normalized CN/N (%)

110

120

Figure 3: (left) RDFs and (right) CN/N (%) ratio calculated between asphaltene molecules in different solvation systems. The CN/N (%) ratio, discussed in details in ref. 27 , is a good indicator of the asphaltenes macroaggregation. It is based on taking the ratio between the integral of the RDF function up to the first minimum (0.6 nm) after the first π-stacking peak (0.38 nm) and the integral of the whole RDF. This ratio estimates the proportion of the molecules that have a π-stacking aggregation pattern among the first neighbors compared to all of the other molecules present 10


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in the system; i.e., this ratio is an index that compares the proportion of the number of asphaltene molecules within the nanoaggregates to the number of nanoaggregates in the simulation box. This measure can be thought of as an index of local, quasi-crystalline order that can be found around the asphaltene molecules with a high aggregation potential. The RDF structure of peaks demonstrate that the intra-nanoaggregate π-stacking is little sensitive to the different solvent mixtures. Nevertheless, at long range, the organization of the system (inter-nanoaggregates) is more dependent on the nature of the solvent: the nanoaggregates are further apart in toluene than in the other solvents. This first observation is also corroborated by the analysis of the interaction energy (Coulomb and van der Waals parts). This energy is calculated for each atom i interacting with another atom j within a sphere of radius equal to the cut-off value, i.e., 12 Å. This range is better-suited to describe the interaction among asphaltenes within the same nanoaggregate. Figure 4 presents these energy values as well as their dispersion taking into account the whole dynamics. The asphaltene-asphaltene average interaction energies are equal to 42.5, 44.2, 45.5 et 45.7 kcal/mol, in toluene, toluene/n-heptane, toluene/water or toluene/n-heptane/water, respectively. These results are in agreement with the values reported in ref. 27 , indicating that mixtures of asphaltene molecules have intermediate behavior between the extremes of the constituting asphaltenes 10,15 . Asphaltene-Asphaltene

Asphaltene-Asphaltene

1.0


Pure toluene Toluene/n-heptane Toluene/water Toluene/n-heptane/water

0.8

Toluene/water C

FDP (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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vdW

Toluene/n-heptane

0.6 0.4 0.2

Pure toluene 0

5

0.0

10 15 20 25 30 35 40 Interaction energy per asphaltene molecule (kcal/mol)

45

50

60

55 50 45 40 35 30 Interaction energy per asphaltene molecule (kcal/mol)

25

Figure 4: Asphaltene-asphaltene interaction energies divided by the number of molecules for each different system: (left) Coulomb and van der Waals contribution and (right) evaluation of the variation of this energy throughout the simulation. 11


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The second observation is corroborated by the snapshots after 60 ns of simulation presented in Figure 5. For all systems except pure toluene, the majority of the asphaltene nanoaggregates form clusters. In toluene, several nanoaggregates can clearly be distinguished indicating that the clusterization (macroaggregation) phenomenon is less intense in this solvent. This behavior is in perfect agreement with what is expected and what was already observed in previous studies on single asphaltene systems: the solubility of the nanoaggregates in toluene is higher. When other co-solvents are added, the first signs of macroaggregation/cluster formation can be noted. The addition of water, n-heptane or both to toluene increases the asphaltene-asphaltene interaction energy of around 3 kcal/mol per asphaltene molecules. The RDFs reporting the interactions between the asphaltene and the solvent molecules can be found in the ESI material, Figure S.4. Water molecules are closer to asphaltene molecules in default of n-heptane, as shown by the intensity of the RDF for intermediary values of r. Instead, interactions with n-heptane seem to be independent of the presence of water, probably because these interactions are only possible via the asphaltene’s lateral chains (peak around 5Å), region which remains inaccessible to water molecules. Finally, asphaltene-toluene interactions are not dependent on the presence of other co-solvents, for the range of concentrations herein studied. The asphaltene-asphaltene average interaction energies equal to (see Figure S.15) 72.2, 60.7, 63.2 and 58.5 kcal/mol per asphaltene molecule, in toluene, toluene/n-heptane, toluene/water or toluene/n-heptane/water, respectively. Since the number of toluene molecules was kept unchanged for all the cases, these values indicate that asphaltene molecules also interact up to some extent with other solvents, when they are present. The average asphaltene-water (∼ 0.9 kcal/mol per asphaltene molecule – see Figure S.16) are almost independent of the presence of n-heptane. Vice-versa, the average asphaltene-nheptane interactions (∼ 8.8 kcal/mol per asphaltene molecule) are also independent of the

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presence of water in the simulation boxes. The interaction energies per asphaltene molecule and per solvent molecule (0.010, 0.009 et 0.002 kcal/mol for asphaltene-toluene, asphaltene-n-heptane and asphaltene-water, respectively) indicate that asphaltenes interact more or less equally with toluene and n-heptane and in much lower intensity with water molecules.

Different polar groups at asphaltene’s lateral chain’s end We measure the impact due to the change of polar groups at asphaltene’s chain ends on the aggregation properties. We study: a) homogeneous systems consisting of single molecules of AAH type for which their polar group is changed and two different solvents (toluene and tolune/water) are screened. In this way, we can determine the asphaltene-water interactions in response to the change of the polar group. b) heterogeneous systems consisting of the complex mixture of asphaltenes, previously investigated, for which only the polar groups of AAH molecules are varied. In order to keep the complexity of this study, the solvent is toluene/n-heptane/water.

Homogeneous systems Figure 6 presents the snapshots of the homogeneous systems for the three polar groups in toluene. The same images for the toluene/water systems are given in the ESI, Figure S.29. The presence of polar groups at the lateral chains ends is enough to increase the clusterization effect. Generally speaking, π-stacking interaction create the so-called nanoaggregates and hydrogen bonds can be formed between them, reducing the distance in-between and forcing them to interact even more, forming clusters or even merging different nanoaggregates into a unique one. These clusters take either very condensate (oblate) or more wire-like (prolate) forms. These observations are confirmed by the RDF and CN/N (%) ratios (see ESI, Figure S.5) calculated for the asphaltene-asphaltene interactions. In toluene, the interactions 13


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among nanoaggregates are the lowest for the P(−O)(OH)2 group. Instead, C(−O)OH and S(−O)2 OH seems to be equivalent, with low CN/N (%) values and Gaussian bands for large r values. For P(−O)(OH)2 termination, the clusters (macroaggregates) are less compact, with an increased prolate form. This observation remains valid with water in the simulation box. We can conclude, by extension, that the interaction energies between molecules having P(−O)(OH)2 termination should be lower than for the other types of molecules. In this way, the P(−O)(OH)2 group would be a weaker cluster formation agent than C(−O)OH or S(−O)2 OH (vide infra). The asphaltene-asphaltene interaction energy is a good indicator to complete this analysis for the situation inside nanoaggregates, as it is presented in Figure 7 for toluene solutions (for toluene/water solutions, see ESI Figure S.18.). Molecules with C(−O)OH terminations have the largest interaction energies, regardless of the presence of water in toluene. In this solvent, substituting C(−O)OH by S(−O)2 OH or P(−O)(OH)2 lowers this energy of ∼ 15% (6 kcal/mol) and ∼ 21% (8.4 kcal/mol), respectively. If water is present, these values become ∼ 15% (6 kcal/mol) and ∼ 12% (4.4 kcal/mol), respectively. These findings are corroborated by the analysis of the RDFs – see Figure S.5: the C(−O)OH group induces the highest proportion of hydrogen bonds between asphaltene molecules and is the most efficient inducer of macroaggregation. To get a clearer insight of the underlying H-bond mechanism, we have analysed these interactions during the whole dynamics. The results are reported in the ESI, Figures S.33 and S.34. Both the distance and angle distributions of the asphaltene-asphaltene H-bonds for different polar terminal groups indicate that the H-bonds formed by C(−O)OH are tighter and lye on the same plane as the group itself. This is not the case for P(−O)(OH)2 and S(−O)2 OH, which are sterically hindered even though they have more sites to accept/donate charges in the H-bond formation process. The result of such molecular configuration is the overwhelming capacity of C(−O)OH to form such H-bonds in comparison to P(−O)(OH)2 or S(−O)2 OH. Figure S.6 shows the RDFs calculated for asphaltenes and water. The RDF for inter-

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mediary values of r confirms that the interactions between the C(−O)OH group and water molecules is the most important in perfect agreement with the highest interaction energies with water (almost 0.5 kcal/mol per asphaltene molecule, as can be seen from Figure S.19). Moreover, the analysis of asphaltene-water H-bonds (Figure S.35) confirms that the C(−O)OH forms much more of these bonds with water than the other terminal groups. This set of results advocates towards the fact that in toluene or humid toluene, (asphaltene) molecules having carboxylic acid groups would display the highest potential to induce macroaggregation and probably interfacial activity in emulsions (given the interactions with water molecules) when compared to the polar groups derived from sulfonic or phosphonic acids. Finally, it is interesting to stress that these finding were obtained considering a nondissociated O−H bond of these polar terminations. As CMD force fields do not allow bond dissociation, we keep these polar groups as fixed and consider non-dissociated structures to elucidate their role in the oil phase (polar groups expected not to be ionized). As the systems are rich in apolar solvents, the interfacial properties are determined at limit conditions (no dissociation). It is clear that, if we treated the polar groups in their anionic form, the asphaltene-water interactions would be largely magnified, as shown in the work of Kuznicki et al. 25 .

Heterogeneous systems The snapshots in Figure 8 present the final configuration of the heterogeneous systems in toluene/n-heptane/water solvation conditions. The RDFs and CN/N (%) ratios are reported in the ESI, Figure S.7. The systems with AAH molecules having P(−O)(OH)2 terminations form tighter macroaggregates, i.e. there is a reduced number of isolated nanoaggregates within the simulation box. The asphaltene-asphaltene interaction energies (ESI Figure S.20) are very similar for the three cases. Indeed, replacing the C(−O)OH by P(−O)(OH)2 induces a reduction of

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less than 1 kcal/mol (∼ 0.4). If replacing it by S(−O)2 OH, this reduction is equal to ∼ 1.6 kcal/mol. Despite the differences observed for homogeneous systems, heterogeneous ones are less sensible to the character of the polar group at lateral chain ends. Changing carboxylic acids by phosphonic acids induces a reduction of less than 1 % and 3 % if the replacer is the sulfonic acid group. Finally, the asphaltene-water interactions follow the order: S(−O)2 OH (∼ 1.3 kcal/mol) > C(−O)OH (∼ 1.0 kcal/mol) & P(−O)(OH)2 (∼ 1.0 kcal/mol).

Influence of metallic porphyrins on the aggregation of asphaltenes In order to measure the impact of metallo-porphyrins on asphaltene aggregation, we use the following strategy: a) we add 5 nickel porphyrins and 5 vanadyl porphyrins to the simulation boxes constituted uniquely of 50 AAH molecules, with the different polar C(−O)OH, P(−O)(OH)2 , or S(−O)2 OH terminations. The solvent is pure toluene. These porphyrins have only methyl or ethyl substituents as lateral chains. The aim of such study is to detect if there is any preferential interaction between any of the three polar groups and the metallic centers of the porphyrins 32 . Such systems are called hereafter homogeneous. b) we add 5 nickel porphyrins and 5 vanadyl porphyrins to the system constituted of a complex mixture of 50 asphaltene molecules among which AAH molecules with the C(−O)OH termination, in a solvent mixture constituted of toluene/n-heptane/water. The impact of the presence of the lateral chains on porphyrins is studied by grafting hexanoic acid chains on one of the peripheric carbon atoms of each molecule. The ratio between porphyrins without polar lateral chains and those with polar lateral chains is varied, keeping the total number of porphyrins constant. Such systems are called hereafter heterogeneous.

Homogeneous systems Figure 9 presents the snapshots of the molecular configurations after 60 ns of dynamics. As already mentioned, the presence of polar groups at chain ends induces the formation of more or less compact clusters of nanoaggregates. 16


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Energy & Fuels

In this case, the homogeneous system with S(−O)2 OH terminations on AAH molecules exhibits tighter macroaggreates what is also retrieved in the RDF and CN/N (%) ratio analysis (ESI Figure S.8). The asphaltene-asphaltene interaction energies (ESI Figure S.22) show that molecules with C(−O)OH groups interact the most strongly within nanoaggregates, as shown in the previous cases. Replacing this group by S(−O)2 OH or P(−O)(OH)2 lowers the energy of 7 % (∼ 2.6 kcal/mol) and 6 % (2.3 kcal/mol), respectively. To summarize, asphaltene-asphaltene interactions in presence of porphyrins without lateral chains follow the same trends as those observed for the cases where porphyrins are not present, even if the relative energies are slightly lowered. Concerning the porphyrin-asphaltene interactions, taking a closer look on the snapshots of Figure 9, one can note that nickel porphyrins do not seem to interact with asphaltene molecules, despite the presence of polar groups. On the contrary, vanadyl porphyrins are found in closer proximity to asphaltene molecules and nanoaggregates. This is due to the formation of H bonds between the vanadyl and the polar groups of asphaltene molecules. In other words, these results agree with the conclusions we did previously using density functional theory: the vanadyl group has a high affinity with polar hydrogens and this energy can assume values up to 50 kcal/mol 11 . The asphaltene-porphyrins RDFs reported in Figure 10 (CN/N (%) ratios can be found in Figure S.9) confirm that nickel porphyrins do not significantly interact with asphaltene molecules, regardless on the presence of polar lateral chains. However, the low intensity peak at around 4 Å indicates that π-stacking can exist between these porphyrins and asphaltene molecules. On the contrary, vanadyl porphyrins exhibit a significantly higher probability of interaction with asphaltene molecules. The C(−O)OH termination has the highest affinity towards vanadyl porphyrins, followed by P(−O)(OH)2 and S(−O)2 OH groups. These results are confirmed by the asphaltene-porphyrins interaction energies reported in Figure 11. The mean asphaltene-nickel porphyrin interaction energy equals to ∼ 4, ∼ 3.2 and ∼ 2.8

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kcal/mol when the polar group is S(−O)2 OH, P(−O)(OH)2 or C(−O)OH, respectively. This order is reversed for asphaltene-vanadyl porphyrin interactions and their values are inflated: they are now equal to ∼ 15, ∼ 6.5 and ∼ 2.5 kcal/mol for C(−O)OH, P(−O)(OH)2 and S(−O)2 OH, respectively. Vanadyl porphyrins have an increased tendency to interact with C(−O)OH groups and this interaction could play a role on the asphaltene-porphyrins self-aggregation phenomena. Nickel porphyrins, instead, have much lower interactions with asphaltene molecules and this seems to be fairly independent on the nature of the polar termination of asphaltene molecules. Finally, it is important to highlight that these porphyrin have only very short apolar lateral chains (< 2 carbon atoms). These observations could be changed if the porphyrin molecules also have lateral chains what is going to be studied in the next section.

Heterogeneous systems Figure 12 presents the snapshots of heterogeneous systems at the end of the simulations. The RDFs, CN/N (%) ratios and interaction energies between asphaltene molecules can be found in the ESI, Figures S.10 and S.24. These data show that asphaltene-asphaltene interactions are not significantly impacted by the presence of porphyrins regardless of the fact that they can have polar groups. Compared to the case where no porphyrin is present (first section of this paper), the interaction energies are lowered of ∼ 2-3 kcal/mol. These results imply that the presence of porphyrins do not induce any improvement in the aggregation state of these asphaltenes. This does not exclude the fact that asphaltene can interact with porphyrins, but this interaction do not induce any change in the asphaltene-asphaltene interaction. In other words, one could say that porphyrins do not seem to be an aggregation inducing agent. Concerning these asphaltene-porphyrins interactions, it is worth recalling that the total number of porphyrins is kept constant. So, the changes are introduced by the variation of the ratio between the porphyrins with and without polar lateral chains. The RDFs reported

18


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in Figure 13 confirm that asphaltene-nickel porphyrin interactions without lateral chains are improbable. Asphaltenes interact more frequently with such porphyrins when they have lateral chains, probably via H-bonds between the polar groups of both molecules, inducing an increase of the interaction probability via mechanisms, such as π-stacking , for instance, explaining an increase in intensity of the peak at ∼4 Å on the RDF curves. Such increased π-stacking would be due to the fact that with such H-bonds and acid-base interactions, porphyrins are now “forced” to be closer to asphaltene molecules. These results are in line with the experimental studied of Schulze et al. 17 , who demonstrated that acidic nickel porphyrins interact with basic asphaltenes (having a pyridine-like conjugated core) via acidbase interactions and form complexes in a 1:2 proportion. These observations slightly change for vanadyl porphyrins: even without lateral chains, these porphyrins interact with asphaltene molecules, as demonstrated in the previous section and in our first work on the theme 11 . It is clear that π-stacking interaction are also present, given the regular and evenly spaced peaks centered at values multiple of 4 Å. Such interactions are probably triggered by the strong interaction between the vanadyl group and hydrogen atoms of the asphaltene molecules, following a mechanism similar to nickel porphyrins having polar lateral chains. The vanadyl porphyrins with polar lateral chains continue to interact with asphaltene molecules, but now they can form H-bonds via the polar groups as well, and explore more possible interaction sites. These intense interactions between similar vanadium porphyrins and asphaltenes have recently been demonstrated by experimental studied performed by Borisova et al. 32 The asphaltene-porphyrin interaction energies per asphaltene molecule are reported in the ESI material, Figure S.25. These averaged interaction energies between asphaltenes and porphyrins (nickel and vanadyl altogether) equal to 1.3 or 0.63 kcal/mol for porphyrins with or without lateral chains, respectively. The presence of polar lateral chains grafted on porphyrins increases the strength of the asphaltene-porphyrin interaction regardless of the metallic center, even if we now know that vanadyl porphyrins have a more important role

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on these interactions. Figure S.12 and S.22 present the RDFs and the interaction energies between asphaltene and water molecules, respectively. Clearly, such interactions are slightly modified for porphyrins having lateral chains. This is probably due to the fact that the new polar groups introduced in the simulation box can also interact with water, reducing the probability of asphaltene-water interactions. The mean asphaltene-water interaction energy is around 1.1 kcal/mol per asphaltene molecule. Finally, RDFs and energies for porphyrin-water interactions are depicted in Figure S.13 and S.27, respectively. For nickel porphyrins without lateral chains, interaction with water are unlikely, given the absence of peaks at low ranges in the RDFs. In contrast, vanadyl porphyrins interact with water even if they do not have polar lateral chains. As the vanadyl group is very polar, it can form H-bonds with water molecules. When porphyrins have polar lateral chains, regardless of the metallic center, these interactions become more possible, H-bond formation being the main mechanism behind such interactions. Without lateral chains, porphyrin-water interactions vary between 1.2 and 4 kcal/mol (2.8 kcal/mol in avg.) per porphyrin molecules. With lateral chains, these interactions can assume values between 2.6 up to 42 kcal/mol (18.3 kcal/mol in avg.) per porphyrin. The lateral chains on porphyrin molecules induce the formation of H-bonds between porphyrins and both asphaltenes and water. This mechanism could possibly be behind some interfacial activity of porphyrin molecules in water/oil interfaces, as it will be studied in more details in a forthcoming work.

Conclusions In this paper we tried to answer to the following questions using classical molecular dynamics simulations: a) do complex mixtures of asphaltenes containing up to 10 different types of asphaltene molecules behave similarly to homogeneous asphaltene systems? Are

20


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Energy & Fuels

the aggregation properties dependent on the choice of the solvent? b) do polar terminations other than -C(-O)OH induce the same type of behavior concerning the formation of hydrogen bonds? What about interactions with water and with porphyrins? c) do porphyrins with polar lateral −C(−O)OH interact more with asphaltene molecules than the porphyrins without lateral chain? What about porphyrin-water interaction? Do porphyrins induce an increase of the asphaltene-asphaltene interaction strength? These studies were performed in rationally chosen solvent mixtures, that could be pure toluene, toluene/water (1 % v/v), toluene/n-heptane (22,6 % v/v) or toluene/n-heptane/water (22,6 % v/v et 1 % v/v). We have shown that mixtures of asphaltene molecules form nanoaggregates more soluble in toluene, than in toluene/n-heptane, toluene/water or toluene/n-heptane/water. Adding water, n-heptane or both enhances asphaltene-asphaltene interactions. This behavior is consistent with what was previously observed when the simulation boxes contained a single type of asphaltene molecule at a time and with experimental observations. Concerning the different polar groups grafted at lateral chain ends, the carboxylic acid group has the strongest role behind macroaggregation/formation of clusters. Besides, it has also the strongest interaction with water molecules. In heterogeneous media, these differences are less pronounced even though the role of macroaggregation induction and water interaction mediated by the molecules with polar groups can still be identified. Finally, we have also observed that porphyrins in homogeneous simulation boxes do not change the way asphaltenes interact with themselves. However, concerning asphalteneporphyrin interactions, we showed that nickel porphyrins without any polar chains interact only slightly with asphaltene molecules. On the other hand, vanadyl porphyrins, even without polar lateral chains, can interact considerably strongly with asphaltene molecules via the formation of H-bonds. If the porphyrins have polar lateral chains, both nickel and vanadyl porphyrins interact more strongly with both asphaltene and water molecules. As a counter-effect, the formation of H-bonds between porphyrins and asphaltenes enhance the π-stacking interactions. The H-bond formed between these two molecules force them to be

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close together and the probability of π − π interactions between them is increased. Generally speaking, the presence of polar lateral chains grafted on the porphyrin cores induces a twofold increase in their interaction energy with asphaltenes if compared to the case of porphyrins without lateral chain. The interactions with water are also increased with the presence of lateral chains even though vanadyl porphyrins already display such interactions. Porphyrins, mainly vanadyl ones and/or the ones having lateral chains, should display interfacial activity in water/toluene interfaces.

Acknowledgement The authors would like to thank to the University of Pau & Pays Adour, MCIA (Mésocentre de Calcul Intensif Aquitain) and GENCI-CINES (Grant 2017-c2016087698) for providing the computation power needed for this project. Isifor-Carnot Institute and Total Refining & Chemicals are also acknowledged for their financial support to this research project.

References (1) Hsu, C. S.; Robinson, P. R. Springer Handbook of Petroleum Technology; Springer, 2017. (2) Chávez-Miyauchi, T. E.; Zamudio-Rivera, L. S.; Barba-López, V.; BuenrostroGonzalez, E.; Martínez-Magadán, J. M. N-aryl amino-alcohols as stabilizers of asphaltenes. Fuel 2013, 110, 302–309. (3) Adeyanju, O. A.; Oyekunle, L. O. Optimum demulsifier formulations for Nigerian crude oil-water emulsions. Egyptian Journal of Petroleum 2017, (4) Bi, Y.; Li, W.; Liu, C.; Xu, Z.; Jia, X. Dendrimer-Based Demulsifiers for Polymer Flooding Oil-in-Water Emulsions. Energy & Fuels 2017, 31, 5395–5401.

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(5) Zhang, L.; Ying, H.; Yan, S.; Zhan, N.; Guo, Y.; Fang, W. Hyperbranched poly (amido amine) as an effective demulsifier for oil-in-water emulsions of microdroplets. Fuel 2018, 211, 197–205. (6) Hasan, S. W.; Ghannam, M. T.; Esmail, N. Heavy crude oil viscosity reduction and rheology for pipeline transportation. Fuel 2010, 89, 1095–1100. (7) Dutta Majumdar, R.; Gerken, M.; Mikula, R.; Hazendonk, P. Validation of the Yen– Mullins model of Athabasca oil-sands asphaltenes using solution-state 1H NMR relaxation and 2D HSQC spectroscopy. Energy & Fuels 2013, 27, 6528–6537. (8) Schuler, B.; Meyer, G.; Peña, D.; Mullins, O. C.; Gross, L. Unraveling the molecular structures of asphaltenes by atomic force microscopy. J. Am. Chem. Soc. 2015, 137, 9870–9876. (9) Schuler, B. et al. Heavy oil based mixtures of different origins and treatments studied by AFM. Energy & Fuels 2017, 31, 6856–6861. (10) Santos Silva, H.; Sodero, A. C.; Bouyssiere, B.; Carrier, H.; Korb, J.-P.; Alfarra, A.; Vallverdu, G.; Bégué, D.; Baraille, I. Molecular dynamics study of nanoaggregation in asphaltene mixtures: Effects of the N, O, and S heteroatoms. Energy & Fuels 2016, 30, 5656–5664. (11) Santos Silva, H.; Sodero, A. C.; Korb, J.-P.; Alfarra, A.; Giusti, P.; Vallverdu, G.; Bégué, D.; Baraille, I.; Bouyssiere, B. The role of metalloporphyrins on the physicalchemical properties of petroleum fluids. Fuel 2017, 188, 374–381. (12) Santos Silva, H.; Sodero, A. C. R.; Guevara Level, P.; Bouyssiere, B.; Korb, J.-P.; Carrier, H.; Alfarra, A.; Bégué, D.; Baraille, I. Investigation of the effect of sulfur heteroatom on asphaltene aggregation. Energy & Fuels 2016, 30, 4758–4766.

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(13) Santos Silva, H.; Alfarra, A.; Vallverdu, G.; Bégué, D.; Bouyssiere, B.; Baraille, I. Sensitivity of Asphaltene Aggregation toward the Molecular Architecture under Desalting Thermodynamic Conditions. Energy & Fuels 2017, (14) Santos Silva, H.; Alfarra, A.; Vallverdu, G.; Bégué, D.; Bouyssiere, B.; Baraille, I. Asphaltene aggregation studied by molecular dynamics simulations: role of the molecular architeture and solvents on the supramolecular or colloidal behaviour. Petroleum Science submitted, (15) Headen, T.; Boek, E.; Jackson, G.; Totton, T.; Müller, E. Simulation of asphaltene aggregation through molecular dynamics: Insights and limitations. Energy & Fuels 2017, 31, 1108–1125. (16) Gray, M. R.; Tykwinski, R. R.; Stryker, J. M.; Tan, X. Supramolecular assembly model for aggregation of petroleum asphaltenes. Energy & Fuels 2011, 25, 3125–3134. (17) Schulze, M.; Lechner, M. P.; Stryker, J. M.; Tykwinski, R. R. Aggregation of asphaltene model compounds using a porphyrin tethered to a carboxylic acid. Organic & biomolecular chemistry 2015, 13, 6984–6991. (18) Korb, J.-P.; Louis-Joseph, A.; Benamsili, L. Probing structure and dynamics of bulk and confined crude oils by multiscale NMR spectroscopy, diffusometry, and relaxometry. J. Phys. Chem. B 2013, 117, 7002–7014. (19) Vorapalawut, N.; Nicot, B.; Louis-Joseph, A.; Korb, J.-P. Probing Dynamics and Interaction of Maltenes with Asphaltene Aggregates in Crude Oils by Multiscale NMR. Energy & Fuels 2015, 29, 4911–4920. (20) Mousavi, M.; Abdollahi, T.; Pahlavan, F.; Fini, E. H. The influence of asphaltene-resin molecular interactions on the colloidal stability of crude oil. Fuel 2016, 183, 262–271.

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(21) Benamsili, L.; Korb, J.-P.; Hamon, G.; Louis-Joseph, A.; Bouyssiere, B.; Zhou, H.; Bryant, R. G. Multi-dimensional nuclear magnetic resonance characterizations of dynamics and saturations of brine/crude oil/mud filtrate mixtures confined in rocks: The role of asphaltene. Energy & Fuels 2013, 28, 1629–1640. (22) Horwitz**, E. P.; Chiarizia, R.; Diamond, H.; Gatrone, R.; Alexandratos, S.; Trochimczuk, A.; Crick, D. Uptake of metal ions by a new chelating ion-exchange resin. Part 1: Acid dependencies of actinide ions. Solvent Extraction and Ion Exchange 1993, 11, 943–966. (23) Nowack, B. Environmental chemistry of phosphonates. Water research 2003, 37, 2533– 2546. (24) Nowack, B.; VanBriesen, J. M. Chelating agents in the environment; ACS Publications, 2005. (25) Kuznicki, T.; Masliyah, J. H.; Bhattacharjee, S. Molecular dynamics study of model molecules resembling asphaltene-like structures in aqueous organic solvent systems. Energy & Fuels 2008, 22, 2379–2389. (26) Berendsen, H.; Grigera, J.; Straatsma, T. The missing term in effective pair potentials. Journal of Physical Chemistry 1987, 91, 6269–6271. (27) Santos Silva, H.; et al., Asphaltene aggregation studied by molecular dynamics simulations: role of the molecular architeture and solvents on the supramolecular or colloidal behaviour. submitted to Petroleum Science 2018, (28) van Gunsteren, W. F.; Daura, X.; Mark, A. E. Encyclopedia of Computational Chemistry; John Wiley & Sons, Ltd: Chichester, UK, 2002. (29) Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.; Berendsen, H. J. GROMACS: Fast, flexible, and free. J. Comput. Chem. 2005, 26, 1701–1718. 25


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(30) Teklebrhan, R. B.; Ge, L.; Bhattacharjee, S.; Xu, Z.; Sjoblom, J. Probing structure– nanoaggregation relations of polyaromatic surfactants: a molecular dynamics simulation and dynamic light scattering study. The Journal of Physical Chemistry B 2012, 116, 5907–5918. (31) Xiong, Y.; Cao, T.; Chen, Q.; Li, Z.; Yang, Y.; Xu, S.; Yuan, S.; Sjoblom, J.; Xu, Z. Adsorption of a Polyaromatic Compound on Silica Surfaces from Organic Solvents Studied by Molecular Dynamics Simulation and AFM Imaging. The Journal of Physical Chemistry C 2017, 121, 5020–5028. (32) Borisova, Y. Y.; Tazeeva, E. G.; Mironov, N. A.; Borisov, D. N.; Yakubova, S. G.; Abilova, G. R.; Sinyashin, K. O.; Yakubov, M. R. Role of vanadylporphyrins in the flocculation and sedimentation of asphaltenes of heavy oils with high vanadium content. Energy & Fuels 2017, 31, 13382–13391.

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Pure toluene

Toluene/water

Toluene/n-heptane


Figure 5: Snapshots after 60 ns of simulation for the four solvent systems. The asphaltene phase is a mixture of 10 different asphaltene molecules as previously indicated. Toluene molecules are not present for the sake of clarity. n-Heptane molecules are presented as blue lines.

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P(−O)(OH)2

S(−O)2 OH

Pure toluene

C(−O)OH

Figure 6: Snapshots after 60 ns of simulation of AAH systems with different polar groups attached to the lateral chain in pure toluene. Toluene molecules are not represented for the sake of clarity. AAH-AAH, Pure toluene

AAH-AAH, Pure toluene C

vdW

C( = O)OH P( = O)(OH)2 S( = O)2OH

1.0

S( = O)2OH FDP (arb. un.)

0.8 P( = O)(OH)2

0

0.4 0.2

C( = O)OH 5

0.6


0.0

40

55


15

Figure 7: Asphaltene-asphaltene interaction energies divided by the number of molecules for each system: (left) Coulomb and van der Waals contribution and (right) variation of this energy throughout the simulation. C(−O)OH

P(−O)(OH)2

S(−O)2 OH

Toluene/nheptane/water

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 8: Snapshots after 60 ns of simulation of the asphaltene mixture with different polar lateral groups attached to AAH molecules in a toluene/n-heptane/water solvent mixture. Toluene molecules are not represented for the sake of clarity. 28


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P(−O)(OH)2

C(−O)OH

Pure toluene

S(−O)2 OH

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Figure 9: Snapshots after 60 ns of simulation of AAH-based systems with different polar lateral groups attached to it in the presence of porphyrins in toluene.. Toluene molecules are not present for the sake of clarity. Molecules in green represent vanadyl and in gray nickel porphyrins. 29


Energy & Fuels

AAH-PorNi (PorNi/PorVO), Pure toluene

C( = O)OH P( = O)(OH)2 S( = O)2OH

50 RDF g(r) (arb. un.)

40 30 20 10 0

AAH-PorVO (PorNi/PorVO), Pure toluene

60

C( = O)OH P( = O)(OH)2 S( = O)2OH

50 RDF g(r) (arb. un.)

40 30 20 10

5

10

15


40

45

0

50

5

10

15


40

45

50

Figure 10: Asphaltene-porphyrins RDFs calculated for different lateral polar groups.

PorNi-AAH (PorNi/PORVO), Pure toluene

PorNi-AAH (PorNi/PORVO), Pure toluene C( = O)OH P( = O)(OH)2 S( = O)2OH

1.0

S( = O)2OH FDP (arb. un.)

0.8 P( = O)(OH)2

C 0

0.6 0.4 0.2

C( = O)OH vdW

1 2 3 Interaction energy per porphyrine molecule (kcal/mol)

0.0

4

10

PorVO-AAH (PorNi/PORVO), Pure toluene C

8 6 4 2 0 Interaction energy per porphyrine molecule (kcal/mol)

2

PorVO-AAH (PorNi/PORVO), Pure toluene vdW

C( = O)OH P( = O)(OH)2 S( = O)2OH

1.0

S( = O)2OH

0.8 FDP (arb. un.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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P( = O)(OH)2

2

0.4 0.2

C( = O)OH 0

0.6

4 6 8 10 12 14 Interaction energy per porphyrine molecule (kcal/mol)

16

0.0

30

25 20 15 10 5 0 Interaction energy per porphyrine molecule (kcal/mol)

Figure 11: Porphyrine-water interaction energies per porphyrin molecules.

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5

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Energy & Fuels

No C(−O)OH

1 C(−O)OH

3 C(−O)OH

5 C(−O)OH

Figure 12: Snapshots after 60 ns of simulation for complex systems constituted of a mixture of 10 different asphaltene molecules, 3 solvents and Nickel and Vanadium porphyrins.Toluene molecules are not represented for the sake of clarity and n-heptane molecules are drawn as blue lines. Molecules in green represent vanadyl and in gray nickel porphyrins without lateral chain. In ochre and magenta, respectively, vanadyl and nickel porphyrins with polar lateral chains. “No”, 1, 3, 5 C(−O)OH refer to the quantity of porphyrines of each type (nickel or vanadyl) having polar chains for which the chain end is this polar group.

31


Energy & Fuels

Asphaltene-PorNi (PorNi/PorVO), Toluene/n-heptane/water Normalized RDF g(r) (arb. un.)

40 30 20 10 0

0:5 polar/apolar 1:4 polar/apolar 3:2 polar/apolar 5

10

15

20

25 30 35 Radial distance (Å)

40

45

50

1 0:5 polar/apolar 1:4 polar/apolar 3:2 polar/apolar 5

10

15

20


40

45

4 3 2 1:4 polar/apolar 3:2 polar/apolar 5:0 polar/apolar

1 5

50

55

10

15

20


40

45

50

55

Asphaltene-PorVO(COOH) (PorNi/PorVO), Toluene/n-heptane/water

3

2

0

5

0

55

Asphaltene-PorVO (PorNi/PorVO), Toluene/n-heptane/water

3

Asphaltene-PorNi(COOH) (PorNi/PorVO), Toluene/n-heptane/water

6

Normalized RDF g(r) (arb. un.)


50


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Page 32 of 32

2

1

0

1:4 polar/apolar 3:2 polar/apolar 5:0 polar/apolar 5

10

15

20


40

45

50

55

Figure 13: Asphaltene-porphyrines RDF calculated for with (right) or (left) without polar lateral chains.

32


Impact of H-Bonds and Porphyrins on Asphaltene Aggregation As

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