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B: Biomaterials, Surfactants, and Membranes

Experimental and Theoretical Study on Supramolecular Ionic Liquids (ILs)-Asphaltenes Complexes Interactions and Their Effects on Flow Properties of Heavy Crude Oils Raiza Ojaliju Hernandez-Bravo, Alma Delia Miranda, JoséManuel Martínez-Magadán, and Jose Manuel Dominguez J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01061 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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The Journal of Physical Chemistry

Experimental and Theoretical Study on Supramolecular Ionic Liquids (ILs)-Asphaltenes Complexes Interactions and Their Effects on Flow Properties of Heavy Crude Oils R. Hernández-Bravo1,*, A.D. Miranda1, J-M. Martínez-Magadán1 and J.M. Domínguez1,* 1

Instituto Mexicano del Petróleo. Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, Gustavo A. Madero, México City, C.P.07730. México.

Abstract A combined study for understanding the molecular interactions of asphaltenes with molecular species such as ionic liquids (ILs) comprised experimental measurements and computational numerical simulation calculations, using Density-Functional Theory (DFT) with dispersion corrections, molecular dynamics (MD) calculations and experimental rheological characterization of the heavy crude oils (HCOs), before and after doping with ILs, respectively. The main results show that ILs influence the asphaltenic dimmers association by forming supramolecular complexes that modify the properties of crude oils such as viscosity and interfacial tension. The (ILs)-cation and Asphaltene-π ligands molecular interactions seem dominating the interactions between ionic liquids and asphaltenes, where ILs’ high aromaticity index induces a strong interaction with the aromatic hard core of asphaltenes.

1. Introduction The asphaltenes are a complex part of petroleum crude oils that keep in suspension under specific thermodynamic conditions. Asphaltenes have no specific molecular structure but are a solubility class that is present in higher proportion in the heavy crude oils (HCOs), according to the oil formation characteristics and its origin. The modification of initial thermodynamic conditions of petroleum fluids may cause precipitation of asphaltenes with deleterious effects of industrial installations along the production chain1-5, i.e., plugging of pipelines, valves fouling in oil fields, refractory effects for processing in refineries, etc., which provoke large operation cuts and enormous economic losses, a lower recovery factor and lower benefit/production ratios overall

6-8

. From a molecular viewpoint asphaltenes are complex

polyaromatic hydrocarbons (PAHs) with about five to ten fused rings having lateral alkyl type 1 ACS Paragon Plus Environment

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chains attached to the molecule outskirts, have high aromaticity, high polarity and a marked tendency to form asphaltene-asphaltene supramolecular aggregates by a mechanism of π-π orbitals type association8. Another solubility class of molecules associated to asphaltenes are resins, which are aromatic type molecules with a molecular weight smaller than asphaltenes, having only 3 to 6 fused rings and short lateral chains. Resins have a similar tendency to associate with each other and to form stable composite aggregates with asphaltenes, i.e., the resin type molecules tend to stabilize the asphaltenic aggregates in the oil suspension, hence delaying their flocculation onset9. These properties are of industrial interest and are a subject of research10 in view of obtaining potential stabilizers. Other molecules have proved useful as viscosity and interfacial tension (IFT) modifiers for the HCOs11. One example of the former is cocoamidopropylhydroxysultaine with sodium dodecyl alpha-olefin sulfonate and dodecyl hydroxyl sulfonate4; the second case is illustrated by an ionic liquid (IL) like 1-hexyl-3-methyl imidazolium chloride; the general properties of ILs include a low melting point and a vapor pressure much lower than common organic solvents. The design and synthesis of ILs encompass a large variety of combinations of aromatic and acid-base moieties that act as functionalities for specific applications12,13. Therefore, it is worth to explore their interactions with asphaltenes for studying their aggregation state and possible modifications of the HCOs’ properties, i.e., 1-propylboronic acid, 3-alkylimidazolium bromides and 1-propenyl-3alkylimidazolium bromide show some solubility properties in a nonpolar environment12,13. These Ionic liquids (ILs) may present specific interactions with other molecules such as hydrogen bonding, comparable to properties observed in different environments, though ion– counterion interactions may compete with ion–solute interactions, for reducing their effective strength. Other cation–π interactions are important for the ILs; however, the nature of the

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electrostatic response of ILs is still under study. Similarly, a number of theoretical studies addressed these questions, but still it is not clear, for example, the influence of dipole moment of an ion that is dependent on the coordinate scheme chosen, making it an ill-defined physical quantity14. In order to explore the interactions present in the aggregation of asphaltenes Gray et al15 proposed an alternate paradigm based on supramolecular assembly of molecules. This model takes in account the nature of the molecular interactions that dominate the supramolecular assembly of asphaltenes, including the interactions with porphyrins present in those structures, which contain nickel. Recently, Mardani et al16 used the same model of asphaltenes for investigating the interactions of mono and multi-functional acids with asphaltene for designing new dispersant agents. They conclude that the efficiency of acids relates with the asphaltene-acid interaction degree. However, Dechaine et al17 studied that some components like vanadium present in porphyrins may have implications in the mechanisms of molecular association of asphaltenes, which is of interest for elucidating the molecular interactions between asphaltenes and metalloporphyrins, hence the inclusion of metalloporphyrins within the asphaltenes. Previous work by Chávez-Miyauchi et al18 reported that initial asphaltene content in oils is important for screening chemical agents as potential viscosity reducers; also, Firoozabadi et al.19 reported a significant viscosity reduction of oil when using ILs such as dodecylpyridinium chloride (35%), which seemed more effective than ILs containing imidazolium or thiazolium head groups. In addition, Nezhad et al.20 studied some mechanisms involved in the aggregation derived from interactions asphaltene-ILs, where acidic moieties seemed to enhance the interaction with asphaltenes, hence limiting aggregation. Rashid et al21 used



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designer solvents based on ILs to prevent the precipitation of asphaltenes, by combining 34 anions and 3 cations for obtaining 102 combinations that were investigated by means of models of asphaltene molecules and COSMO-RS calculations. Those results showed that smaller cations with a steric shielding on the large anion caused improvement of the HCO’s flow. In addition, Ogunlaja et al.22 demonstrated that ILs based on imidazolium may have properties for dispersing asphaltenes within HCOs. From the theoretical viewpoint, molecular calculations showed that ILs having a small HOMO-LUMO energy gap would contribute to high polarizability and reactivity of the ILs vis-a-vis asphaltenes. Hizaddin et al.23,24 used quantum chemical calculations to demonstrate that ILs based on aromatic rings and either [ETSO4] or [Ac] anions were active in the extractive de-nitrification and removal of pyrrolbased compounds from liquid fuels. In addition, Pons et al.

25,26

studied the formation of

supra-molecular complexes showing reduction of oil viscosity and wettability alteration of reservoir rocks, for enhanced oil recovery (EOR). These authors proposed that ion-dipole forces act in the interaction of cationic surfactants with different aromatic molecules. Hassan et al27. studied rheological properties of Athabasca bitumen and Maya crude oil by nanofiltration techniques at 473 K and showed a rheological behavior that is consistent with Newtonian behavior; this study proposed a non-interacting hard sphere model within the temperature range 298-373 K and included the role of solid maltenes. In addition, Rogel28 et al. studied the effect of inhibiting asphaltenes aggregation using a molecular thermodynamic approach and found that some inhibitors attached to active sites of asphaltenic aggregates become part of the crown surrounding the polyaromatic core. In addition, Feng-Hu, Y et al29 studied the effect of structural features of ionic liquids and other amphiphiles for controlling the precipitation of asphaltenes and found that effectiveness of some amphiphiles relies on its


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ability to form stabilizing layers around the asphaltenic molecules, which correlates with polarity of the head group and length of the ILs’ alkyl tail. Recently, Hernández-Bravo et al.30 studied the solubility behavior of ILs within asphaltenes using quantum-chemical density functional theory in combination with COSMO-RS, showing that structure and size of ILs’ cations have an influence on their ability for dispersing asphaltenes through VDW, π-π, cation-π and hydrophobic interactions. Therefore, the present work focused on the hypothesis that ILs can reduce the viscosity of heavy crude oils (HCOs) by means of weakening the asphaltene-asphaltene link; for this, we used a combination of both theoretical and experimental studies. These results should provide a better understanding about the mechanisms involved in the interactions and could lead to development of specific molecules for improving the flow in HCOs. In this work, molecular dynamics (MD) and DFT methods allowed investigating the asphaltene-ILsasphaltenes complex formation. The molecular dynamics allowed exploring over a period of time the dynamical evolution of a system composed of asphaltenes and IL’s models, while DFT calculations allowed analyzing the different interactions involved in the supramolecular complex formation. The validation study by means of rheological experiments using HCO, HCO’s asphaltenes and several ILs allowed comparing the viscosity variations and interfacial tension modifications with respect to theoretical results, thus leading to a better understanding of the asphaltenes – ILs interactions as well as to the flow behavior of the HCOs complex fluids31.

2. Experimental



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Materials and Methods. All synthesis reagents were analytical grade and used without further purification. Methylimidazole, Bromotetradecane, Pyridine, Quinoline, Benzymidazole, Sodium hydride, Tetradecyltrimethylammonium bromide, Imidazole, were Sigma Aldrich (≥99%). The compositional and structural characterization involved some techniques like FTIR in a Nicolet 8700 FTIR Thermo Scientific instrument. In addition, 1H and

13

C NMR

analysis allowed characterizing the molecular structure, using DMSO-d6 as a solvent and TMS as internal standard in a Varian 300 MHz Oxford spectrometer. A heavy crude oil (HCO) from the Gulf of Mexico coastline was chosen for these studies and their main properties were characterized, by means of SARA, Total acid Number (TAN), Total Basic Number (TBN), Viscosity (µ), interfacial tension(IFT), Gravity (°API) and water content. For determining the content of asphaltenes and other SARA’s fractions, the ASTM-D-2007-91 method (i.e. in nC7) applied. Table 1 show the initial properties of the heavy crude oil (HCO).

Rheological measurements. Knowing the flow behavior of the original and modified HCOs required determining the apparent viscosity by means of a stress-controlled rheometer Haake Mars III Thermo Scientific, at atmospheric pressure and 25°C; the shear rates scale comprised the interval between 0 and 200 s-1, within a parallel plate geometry. In addition, n-heptane as received allowed preparing diluted binary mixtures of Maya crude oil and ionic liquids. Weighing each constituent in a vial allowed preparing 10g samples of Maya heavy crude oil, then doping followed using 1 ml of ILs solution in n-heptane, for obtaining a final concentration of 1,000ppm of IL in the HCO. A magnetic stirrer placed inside the vial allowed mixing at room temperature for at least 2 h for obtaining a homogenous mix. Each test performed with loads of 1 ml and this procedure applied for all the experimental series, followed by resting for 12 h at


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298.15 K, and pre-conditioning the samples by dehydration and rest at 40 oC during 2 h before determining their rheological behavior.

Table 1. Properties of heavy Crude Oil Properties Crude Oil

Maya

API gravity Density (25°), gr/ml Sulphur, wt % Carbon, wt % Asphaltenes in nC7, wt % Saturates in nC7, wt %

21-22° 0.9199 3.4-3.8% 10.57 11.2 3.6

Aromatics in nC7, wt %

67.5

Polar in nC7, wt %

12.7

Synthesis of Ionic Liquids: The ionic liquids (ILs) studied in this work (LI-1, LI-2, LI-3, LI-4, and LI-5) were synthesized by following previous reports in the literature32-36 and their structural features were characterized by 13C, 1H NMR, MS and FTIR. All the ILs were dried under vacuum for at least 48 h at temperatures between 313 K and 353 K to remove organic solvents and water by Karl–Fischer titration method or Coulometry (Brinkmann Metrohm 756KF Coulometer). The ionic liquids used thereafter in the tests contained less than 10-ppm water, while the purity of the ionic liquids was more than 98%.

3-methyl-1-tetradecyl-1H-imidazol-3-ium bromide (LI1) The first step was placing N-methylimidazole (1mmol) and 1-Bromotetradecane (1mmol) into a dry round-bottomed flask equipped with a reflux cooler, a magnetic agitator and a glass thermometer. The heating of the mixture under stirring proceeded at temperatures between 80 and 85°C, under nitrogen atmosphere. After 36 h of reaction, the cooling of the mix down to room temperature followed. Then, ethyl acetate as solvent allowed washing the samples, hence removing the solvents followed under reduced pressure, at 70° for 6 h. The product is 7 ACS Paragon Plus Environment


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a white powder, Mp 54.5 °. 1H NMR (300 MHz, DMSO-d6) δ (ppm) 0.88 (t, 3H), 1.15–1.4 (m, 22H), 1.86 (q,2H), 4.07 (m, 3H), 4.18 (s,2H), 6.94 (s, 1H), 7.17 (s, 1H), 7.91 (s, 1H). 13C NMR (300 MHz, DMSO-d6) δ (ppm) 14.0, 22.6, 27.1, 29.0, 29.2, 29.4, 32.4,30.3, 36.2, 50.1, 122.0, 123.7, 137.9. FTIR (400–4000 cm-1, KBr pellet): 3116, 1776, 1063, 837cm-1; HRMS calcd C18H35BrN2 359.39, found 358.29.

1-tetradecylpyridin-1-ium bromide (LI2) The first step was placing Pyridine (1mmol) and 1-Bromotetradecane (1mmol) into a dry round-bottomed flask equipped with a reflux cooler, a magnetic agitator and a glass thermometer; heating the mixture under stirring followed and the temperature range was between 80 and 85°C under nitrogen atmosphere. After 36 h of reaction, the mixture cooling down to room temperature followed. Then, ethyl acetate as a solvent allowed washing the sample. Drying the product at a reduced pressure and 70°C for 6 h applied for removing the residual solvent. The product was a white powder. 1H NMR (300 MHz, DMSO-d6) δ(ppm) 0.86 (m, 3H), 1.15-1.41 (m, 22H), 1.87 (tt, 2H), 4.48 (t, 2H), 7.85 (m, 2H), 8.56 (t, 1H), 8.80 (m, 2H). 13C NMR (300 MHz) δ(ppm) 14.2, 22.5, 25.8, 29.0, 29.4, 31.2, 32,4, 61.3, 128.3, 145.2, 146.2. FTIR (400–4000 cm-1, KBr pellet): 3061, 3026, 2920,1735, 1178,727 cm-1. HRMS calcd C19H34BrN 356.38, found 355.19. 2-tetradecylisoquinolin-2-ium bromide (LI3) The first step was placing Quinoline (1mmol) and 1-Bromotetradecane (1mmol) into a dry round-bottomed flask equipped with a reflux cooler, a magnetic stirring and a glass thermometer. Heating the mixture under stirring and keeping the temperature between 80 and 85°C followed. After 36 h of reaction, the cooling of the mixture down to room temperature followed. Hence, acetonitrile as a solvent allowed washing the product and drying followed under a reduced pressure for removing the residual solvent and water. The product was a violet powder. 1H NMR (300 MHz, DMSO-d6) δ(ppm) 0.864 (m, 3H), 1.23 (tt, 2H), 1.26-1.37 (m, 20 H), 1.915 (m, 2 H), 4.45 (d, 2 H), 7.51 (m, 1H), 7.81 (m, 1H), 8.04 (m, 1H), 8.26(m, 1H), 9.35 (m, 2H), 9.71 (s, 1 H). 13C NMR (300 MHz) δ(ppm) 14.2, 22.6, 25.7, 29.0, 29.4, 31.5 32.4, 61.4, 121.8, 126.6-126.9, 128.4, 132.9, 142.8, 144.2. FTIR (400–4000 cm-1, KBr pellet): 3061, 3026, 2920, 1736, 1178, 727cm-1; HRMS calcd C23H36BrN 406.44, found 405.20

1-tetradecyl-3-[3-(1-methyl-1H-imidazol-3-ium-3-yl) propyl]-1H-imidazol-3-ium dibromide (LI4) The first step was placing sodium hydride (1 mmol), imidazole (1 mmol) and tetrahydrofurane (10 ml) in a reaction flask. Hence, after stirring for 1 h, 1-bromotetradecane (1 mmol) doping 8 ACS Paragon Plus Environment

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followed; hence, stirring the mixture for 7 h at 80°C followed, always under nitrogen atmosphere. After completing the reaction, removal of tetrahydrofurane followed under reduced pressure at 90°C for 1 h. Adding a large excess of 15 (wt%) NaCl solution and chloroform followed, and then stirring the mixture at room temperature for 1 h applied, hence it kept at room temperature until observing the apparent interface between the organic and water solution. After removing the substrate solution, a viscous white liquid remained, even after washing several times with distilled water. After removal of residual solvent under reduced pressure, a yield 90% of N-tetradecylimidazol (compound 1) resulted. Afterwards, adding 1,3 dibromopropane (1 mmol) and acetonitrile (10 ml) applied for synthesizing Ntetradecylimidazole (1mmol). Heating the mixture at 80°C, and stirring for 4 days in nitrogen atmosphere followed. Removal of Acetonitrile under reduced pressure, at 90° for 6 h applied. Washing the white solid with ethyl acetate several times allowed removing all the unreacted 1,3-dibromopropane and N-tetradecylimidazole. Hence, removal of the solvent in vacuum at 80°C followed. The compound 2 was a white powder, with yield 92%. Afterwards, adding 1methylimidazole (1 mmol) to the synthesized product 1-bromopropyl-3-tetradecylimidazolium bromide (1mmol) followed. Heating the mixture at 80°C, and stirring for 4 days in nitrogen atmosphere, allowed to obtain a yellow solid; hence, ethyl acetate allowed washing for removal of all of the unreacted compounds, then removal of the solvent in vacuum at 80° followed. The compound 3 was obtained as a yellow powder, yield (95%). 1H NMR (300 MHz, DMSO-d6) δ(ppm) 0.84 (t, 3H), 1.232 (m, 2H), 1.25-1.27 (m, 20H), 1.86 (tt, 2H), 2.21 (tt, 2H), 4.079 (s, 7H), 4.26 (t, 2H), 7.20-7.217 (dd, 2H),7.489 (dd, 1H), 7.824 (dd, 1H), 7.93(dd, 2H). 13 C NMR (300 MHz) δ(ppm) 14.3, 22.1, 22.6, 27.1, 29.0-29.2, 29.4, 30.3, 32.4, 36.2, 49.6, 50.1, 53.8, 122.1, 122.3 124.1, 123.7, 136.8, 137.9. FTIR (400–4000 cm-1, KBr pellet): 3134, 3050, 2921, 2850, 1736, 1563 1469 and 837cm-1; HRMS calcd C24H44Br2N4 548.44, found 548.15 3-methyl-1-tetradecyl-1H-benzimidazol-3-ium bromide (LI5) The first step was placing 1-methylbenzimidazole (1mmol) and 1-Bromotetradecane (1mmol) into a dry round-bottomed flask equipped with a reflux cooler, together with a magnetic agitator and a glass thermometer. Heating the mixture under stirring and keeping the temperature between 80 and 85°C followed. After 48 h of reaction, cooling of the mixture applied down to room temperature. Hence, acetonitrile as solvent allowed washing the product, then drying followed at a reduced pressure for removal of the residual solvent. 1H NMR (300 MHz, DMSO-d6) δ(ppm) 0.83 (t, 3H), 1.24 (m, 2H), 1.27 (tt, 4H), 1.31-1.37 (m, 16H), 1.952 (tt, 2H), 4.08 (t, 3H), 4.51 (s, 2H), 7.26 (dd, 1H), 7.36 (m, 1H), 7.49 (m, 1H), 7.61(m, 1H), 8.205 (m, 1H). 13C NMR (300 MHz) δ(ppm) 14.1, 22.6, 23.7, 29.029.2,29.532.4, 37.8, 53.8, 117.3, 124.6, 127.5, 127.8, 140.5, 141.9, 143.3; FTIR (400–4000 cm-1, KBr pellet): 3061, 3026, 2920, 1736, 1178, 727cm-1; HRMS calcd C22H37BrN2 409.45, found 408.21. 9 ACS Paragon Plus Environment

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3. Theoretical Calculations DFT calculations: The interaction energy between asphaltenes and ILs was calculated from molecular models that were constructed and optimized for obtaining the molecular geometries of minimal energy using Dmol3 module37, with the parameters 2.0 × 10−5 Ha in energy, 0.004 Ha/Å and 0.004 Ha/Å. These are the thresholds for energy, force strength and displacement convergences, respectively. The Generalized Gradient Approximation (GGA-PBE)38 functional applied for exchange-correlation energy, while Tkatchenko-Scheffler correction methods (TS)39 applied for dispersion energy, unrestricted spins, automatic multiplicity, zero electric charge, effective core potential treatment for internal electrons and DND base40. Turbomole software41 using the Kohn-Sham Hamiltonian and the gradient-corrected Perdew-Becke-Ernzerhof (PBE)38 allowed calculating the Van der Waals surfaces. The analysis of surface mapping allowed revealing the nature of ILs-asphaltenes interaction. This van der Waals surface not only conveys a variety of information including polarity, electronegativity and the nature of the bonds, but it is widely used for studying and predicting non-covalent interactions42. The calculation of interaction energy of the interaction pairs obeyed the expression linking the difference of total energies between monomeric and dimer type species according to: ∆ = − ( + )

(1)

Where Ei is the total energy of the reactive m (i.e., the single molecule of Ils) and Eij is the total energy of the product (i.e., the interacting reactive i and j).

DFT level calculations

allowed studying the nature of exclusive electronic interactions involved in the formation of the molecular complex. GGA is good for studying systems with a slowly varying charge density, because it improve the local spin density; therefore, GGA is appropriate for calculating the 10 ACS Paragon Plus Environment

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interaction energies of the asphaltene-ILs-Asphaltene system. The correction to the van der Waals interaction effect keeping molecules attached to the asphaltene surface. Figure 140 and Table 2, illustrate the molecular properties and asphaltene structure model, respectively. The molecular formula for asphaltenes derives from the construction model, which is the basis for molecular simulation (Table 3). The calculations (14) focused on a total of 6 imidazole-based ILs (IL1-IL6) having a different cationic head, a fixed alkyl chain length and a bromide anion, which allowed comparing with the experimental viscosity data before and after adding ILs to HCOs.

Figure. 1. Asphaltene Model43

Table 2. Molecular Characteristics of the average structure of the asphaltene model

mol formula mol wt (g mol−1) no. of fused rings H/C

C53H55NS 737 11 1.04

Table 3. Structures and nomenclature of Ionic Liquids (ILs)

Models

Molecular Formula

Molecular Weight (g/mol) 11

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IL1: C18H35BrN2

359.39

IL2: C19H34BrN

356.39

IL3: C23H36BrN

406.44

IL4:C23H42Br2N4

534.41

IL5: C23H39BrN2

423.47

IL6: C17H38BrN

335.22

Molecular Dynamics The Amorphous Builder system allowed building a cubic periodic unit cell containing 10 and 1 molecules for asphaltenes and ionic liquids, respectively, at a low density, i.e., of 0.5. The running molecular dynamics was realized at constant pressure and temperature, using a cell under periodic boundary conditions, i.e., an isothermal–isobaric ensemble (NPT simulation), with the Forcite module from Materials Studio software, with COMPASS force field for the NPT simulations37. The atomic charges for the electrostatic component of the force field were determined from quantum mechanical


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calculations of isolated molecular models, using DMOL3 programs37 and fitting the molecular electrostatic potential, ESP. The amorphous Builder allowed creating an initial random and low-density sample using a suitable Monte Carlo procedure, for achieving a right distribution of conformational states44. Hence, this study allowed verifying the supramolecular complex formation. With the purpose of visualizing the formation of the supramolecular complex the molecular dynamics simulation performed independently of the DFT calculations. Computational calculations in a vacuum environment were performed; because the cohesive energy density is related to, the stability of the asphaltenic aggregate was used.

4.

Results and Discussion

4.1.

Modeling Asphaltene-ILs Interactions

For the present case the Yen-Mullins’ model7,8 was applied, which proposes a hierarchical aggregation structure from single asphaltene molecules (i.e., “island” type model) to lumps of asphaltenic clusters that form under certain thermodynamic conditions, leading to macroscopical particles that precipitate from the liquid phase hydrocarbons. Thus, if the interaction energy (∆Eij) represents the simplest case of two asphaltene molecules linked to each other by means of a π-π type interaction, thus ∆Eij can be defined as the energy needed to bring those molecules together, like a cohesive energy. On the other hand, viscosity is from a phenomenological viewpoint the resistance of a fluid to flow or the resistance of momentum transference that is the friction forces act between single molecules and supramolecular aggregates, as represented in Table 4. Hence, an activation energy barrier must be overcome, that is the viscous energy, i.e., Evisc, thus in the onset of flow Evisc= -∆Eij , since these are opposite terms with respect to the flow dynamics, i.e., asphaltenes aggregation increases 13 ACS Paragon Plus Environment


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viscosity, while disaggregation enhances fluidity by means of a viscosity decrease. Previous works reported that a viscosity pattern for a number of HCOs can be represented by an Arrhenius type model, i.e., η=Aexp(Evis/kT)45-50, where η is the viscosity, A is the collision or frequency factor, Evisc is the activation energy for the viscous flow, k is the Boltzmann constant and T is the absolute temperature. Thus, a primary model of a supramolecular aggregate involves two asphaltenic out-shell molecules and one IL molecule at the middle, in which case the energy needed to disaggregate this ensemble is represented by Evisc= -∆Ei, as shown in Table 4.

Table 4. Interaction mechanism of asphaltenes with ILs Steps Interactions 1 Asph+ILs→ Asph-ILs 2 ILs+Asph− ℎ →Asph-ILs-Asph 3 Asph-Asph+Asph-IL →Asph-ILs-Asph+Asph Global 2Asph-Asph+2IL →2Asph-ILs-Asph

The molecular modeling by DFT allowed calculating thermodynamic data of adducts formation between ILs and asphaltenes. In addition, Table 5 reports the energies associated to single asphaltenic molecules and their complex supramolecular aggregates. The Tables 6 and 7 show the interaction energies. Hence, according to these calculations the molecular disaggregation of asphaltenes from asphaltene-asphaltene dimer model requires about +74.899 kcal/mol, while the energies needed for disaggregating two lateral asphaltenic molecules linked to ILX type molecule in between are 4.422, 282.981 and 35.054 kcal/mol, respectively, where X=1,3,5. Tables 6 and 7 show the energies associated to steps 2 and 3 (Table 6) which follow the order Easph-IL3-asph < Easph-asph. This lead us to conclude that ILs inserted in between two asphaltenic molecules (Figures 2 and 3 and step 3 of Table 6), cause 14 ACS Paragon Plus Environment

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formation of stable aggregates as is the case of Asph-IL1-Asph, Asph-IL3-Asph, Asph-IL5asph, which present negative energies down to about -4.4 22 kcal/mol, -282.981 kcal/mol and 35.054 kcal/mol, respectively (i.e.,Table No. 7). Those supramolecular aggregates seem more favorable from the energetic viewpoint, while other aggregates like Asph-IL2-Asph, Asph-IL4Asph and Asph-IL6-Asph are not, i.e., E= + 17.854, 41.089 and +65.393 kcal/mol, respectively. Those associations could promote intercalation of ILs between layered asphaltenic aggregates, hence weakening π - π bond which limits further the asphaltene-asphaltene association (Figure 3). In addition, Hess's law describes the functionality of the ILs, which allows deducing the enthalpy change in a complex reaction, as the ones illustrated in Table 4, where the sum of partial chemical equations leads to the global reaction equation. In Step 1 the asphaltene-IL aggregate occurs more probably. In step 2 cation-π interactions occur between ILs and asphaltenic dimers, hence forming the supramolecular asphaltene-ILsasphaltene aggregates. Strong interactions of asphaltenes with IL1, IL3 and IL5 occur with higher probability, but only IL3 forms associations with asphaltenes with a probability higher than Asph-Asph associations. Hence, inter-aggregation of IL3 should tend to destabilize the asphaltenic cluster by intercalating between at least one Asph-Asph dimer, which could induce cluster breaking and asphaltenes dispersion, which in turn could contribute to viscosity reduction. Tables 6 and 7 show that this process is more favorable for ILs IL1, IL3 and IL5. In contrast, step 3 involves cation-π interactions between ILs-asphaltene and asphaltenes dimers to form asphaltenes-ILs-asphaltenes + asphaltenes, a process that is not favored for all these cases. In summary, IL1, IL3 and IL5 are the most probable supramolecular aggregates while IL4, IL2 and IL6 seem non-favorable at all (Tables 6 and 7). In particular, the supramolecular Asph-IL2-Asph complex presents an energy of 17.854 kcal/mol, which explains previous



Page 16 of 37

reports by Firoozabadi et al19, who reported that ionic liquids containing the pyridinium moiety are not efficient as viscosity reducers for some HCOs. However, structural features such as head groups and alkyl chains length play an important role on rheological behavior of the aggregates, as reported by Hernández-Bravo et al30. The cationic head of ionic liquids seems to induce stability of supramolecular aggregates with asphaltenes, in the following order: IL3>IL5>IL1>IL2>IL4>IL6, with IL3 being the most efficient one, i.e., Asph-IL3-Asph, while IL6 is the most refractory one. The IL4 dimer presents a very negative potential energy (Table 7) for building an association with other IL4 molecules, that is, IL4 tends to aggregate to other IL4 molecules rather than with asphaltenes. Hernández-Bravo et al.30 reported anion and alkyl chain effects, as well as the ability of the cationic head groups for interacting with asphaltenes, which seems to involve Lewis acid-base interactions or cation-π interactions. Asphaltenes contain π-electrons in their aromatic core (i.e., electronically higher conjugated structures)29,51, that interact with poor electron systems such as the cationic head of ILs, which possess a π system that tend to interact with the asphaltene aromatic core, hence IL3 should correlate with Aromacity Index (AI) and double-bond equivalent (DBE) values. At this respect, Koch and Dittmar52 proposed a procedure for calculating AI from exact molecular mass of an organic compound. Hence, AI might measure C-C double-bond “density”, which in turn involves the contribution of π-bonding by heteroatoms, according to the following relationship53: Table 5. Total energy for single molecules Single molecules Total Energy×106 (kcal/mol) Asph -1.571 IL1 -2.116 IL2 -2.120 IL3 -2.212 IL4 -3.932 IL5 -2.248 16 ACS Paragon Plus Environment

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IL6

-1.996

Figure 2. Interaction models for IL-IL and asph-asph (dimer).

IL1-IL1

IL2-IL2

IL3-IL3

IL4-IL4

IL5-IL5

IL6-IL6

Asph-Asph



Page 18 of 37

Figure 3. Formation of supramolecular Asph-IL-Asph type complexes

Asph-IL1-Asph

Asph-IL2-Asph

Asph-IL3-Asph

Asph-IL4-Asph

Asph-IL5-Asph

Asph-IL6-Asph

Table 6. Total energy and interaction energies of ILs and asphaltenes (i.e., “island type model”) including supramolecular Asph-ILX-Asph complexes (X=1,2,3). Molecular Species Asph-Asph IL1-IL1 IL2-IL2 IL3-IL3 IL4-IL4 IL5-IL5 IL6-IL6

Total Energy×106 (kcal/mol) -3.142 -4.205 -4.232 -4.425 -7.865 -4.496 -3.992

∆E (kcal/mol) -74.899 -19.277 -35.417 -14.388 -112.927 -15.175 -25.746 18


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Asph-IL1 Asph-IL2 Asph-IL3 Asph-IL4 Asph-IL5 Asph-IL6 Asph-IL1-Asph Asph-IL2-Asph Asph-IL3-Asph Asph-IL4-Asph Asph-IL5-Asph Asph-IL6-Asph 1 2

-12.8361 -25.7491 -31.3491 -24.0911 -37.3611 -7.2621 -2.2102 12.1772 -331.5612 20.5452 -10.6892 32.9712

-3.673 -3.687 -3.784 -5.503 -3.819 -3.567 -5.245 -5.258 -5.355 -7.075 -5.390 -5.138

Corresponding to step 1 of the interaction mechanism Corresponding to step 2 of the interaction mechanism

;

Table 7. Interaction energy values of step 3 and global energy. Molecular Species ∆Estep3 ∆Eglobal (kcal/mol) (kcal/mol) Asph-IL1-Asph 10.624 -4.422 Asph-IL2-Asph 31.426 17.854 Asph-IL3-Asph 79.929 -282.981 Asph-IL4-Asph 44.635 41.089 Asph-IL5-Asph 12.996 -35.054 Asph-IL6-Asph 39.684 65.393

=

.

(2)

The number of atoms in the molecule allowed determining the AI, for example carbon (C). In addition, the characterization of organics involves the value of DBE, and it is a well-established tool in mass spectrometry studies. DBE can be defined as the summation of the unsaturated bonds within the rings of a molecule, according to the following relationship: 54,55

= − + + 1

(3)

Table.8. Calculated aromacity index (AI) and double-bond equivalent (DBE) values for the ILs Models IL1

AI 0.094

DBE 2.5 19



IL2 IL3 IL4 IL5 IL6

0.166 0.273 0.158 0.205 0.000

Page 20 of 37

3.5 6.5 5.0 5.5 0.0

The AI order is the following: IL3>IL5>IL2>IL4>IL1>IL6, while DBE’s sequence follows: IL3>IL5>IL4>IL2>IL1>IL6 (Table 8). In addition, miscibility decreases with AI and DBE, while the above sequences seem to correlate partially with the interaction degree of asphaltenes with ionic liquids50. In particular, IL3 has the highest degree of interaction, the highest Aromaticity Index and Double Bond-Equivalent values.

Previous works indicated that cations of ILs present a conjugated aromatic core that would lead to specific interactions that may be stronger with aromatic units of asphaltenes’

56,57,58

.

Hence, this finding has a great potential for aromatics separation in general, but the concept applies too for the asphaltenes aggregates.

Theoretical molecular calculations on the interactions of ILs’ structures with benzene rings have revealed there is an attraction dominated by electrostatic and polarization interactions, which may arise from cation-π type interactions59. This is the starting point of a supramolecular architecture as the one shown in Figure 4, which shows the ILs enclosed between two asphaltenic molecules, thus forming an alternate stacking unit together with an IL, for the most stable structure comprising only IL3. Further analysis of the nature of Maya crude oil and the predominant interactions between its asphaltenes and IL3, required determining the Watson-K factor, as shown in Table 9, where the range of K-factor values are displayed for three hydrocarbons families, i.e. paraffinic, naphthenic and aromatic, as reported previously60. 20 ACS Paragon Plus Environment

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Table 9. Watson characterization factor (KW) for different hydrocarbons families Homologue Series KW paraffinic, 13.1-13.5 Naphthenic 10.5-13.2 Aromatic 9.5-12.5 The correlation factor was determined using the equation developed by Perry and White61 !" = −0.0239('( *+,- ) − 0.0109./0 1 + 10.642

(4)

The polar percentage in Maya heavy crude oil was taken from the properties reported in Table 1. The K factor calculated was 10.33, which indicates, according to Table 9, that this oil has an aromatic nature, thus asphaltenes will have a greater preference for associating with ionic liquids, which have an aromatic nature too, hence favoring their miscibility62,30. For comparison purposes, Rogel63 reported previously that a low H/C ratio favored the interaction between asphaltenes and selected solvents, together with high aromaticity and high degree of condensation. In the present work, hence, Figure 4 shows the interaction energy dependence of H/C ratio for ILs, where it is verified that the most favorable interaction energies should correspond to lower H/C ratios, together with a higher aromaticity factor64.



Page 22 of 37

Figure 4. Interaction energies as a function of the H/C ratio for ionic liquids.

Furthermore, a Mülliken and NBO population analysis of ILs gave results showed in Tables S1, S2, S3, S4 and S5 and figures 1,2,3,4 and 5 of Supporting Information. These two analyzes show similar calculations, thus the use of a methodology or another is completely valid for predicting the phenomenon presented with ionic liquids and asphaltenes. Hence, one observes the 6-membered ring of IL3 with an electron deficiency, which in turn could be a preferential site for interaction with the electron-rich resonant attractor aromatic cores of asphaltenes. In contrast, IL6 has a high electron density in every atom, which is a result of the inductive effect of the alkyl chain and the counter-ion electron donation, which makes it a very stable chemical species.

Previous works proposed that the interaction between ILs and asphaltenes could depend on orbital interactions like HOMO - LUMO, π-π orbitals interaction and Van der Waals forces including hydrogen bonding65,22. Some ILs having anions and a low value of HOMO-LUMO energy gap, such as chloride or bromide, were reported to reduce viscosity of the HCOs, by their tendency to form soft molecules that lead to better polarizability and reactivity65. Though IL3 and IL6 have the same anion, their energetic gaps are completely different, i.e., IL3´s gap is 56.432 kcal/mol while IL6´s gap is 111.466 kcal/mol. Thus, the viscosity reduction effects of these ILs should depend not only on the anion but also on the cationic head contribution to polarizability, as it is the case for IL3, which has a Lewis acid character in opposition to IL6, which has a quaternary amine group that tend to form “hard molecules” (high HOMO-LUMO gap).


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By using the HOMO and LUMO energy values 4 and 5 , respectively, the global chemical reactivity descriptors such as chemical potential (6) and global hardness (7) can be defined for a molecule as66:

6 = (5 + 4 )⁄2

(5)

7 = (5 − 4 )⁄2

(6)

In a reaction between two molecules A and B, the electronic transfer will occur until their chemical potentials 69 and 6: equal one another67. If 6: ≈ 69 , it is expected there is a poor chemical reaction. On the contrary, chemical reaction (i.e., breaking and formation of bonds) is promoted when the chemical potentials are somewhat different. From Table 10, it can be seen that IL3 is the ionic liquid whose chemical potential (-80.958 kcal/mol) is the closest to the asphaltene one (-90.130 kcal/mol), whereas the chemical potential of IL6 (-55.099 kcal/mol) is the farthest away. The implication is that IL3 can diffuse through the asphaltene environment more easily than IL6, then separating efficiently the asphaltene molecules. The hardness is a measure of the resistance to change the electronic energy by changing the number of electrons. Table 10 reveals IL6 has the greatest resistance to interact with asphaltene, explaining the low interacting energy in the supramolecular complex (-7.262 kcal/mol versus, for example, -31.349 kcal/mol of the IL3:Asph complex, Table 6). Although IL3 doesn´t have the lowest hardness (Table 10), the lowest difference in its chemical potentials relative to the one of the asphaltene determines the major feasibility to be miscible with asphaltene. It is worthy to mention that the concept of chemical potential and hardness involve isolated molecules prior to the reaction; then, their insights for what could occur can be different from the calculations over interacting molecules.



Page 24 of 37

Table 10. Chemical potential and hardness for the ionic liquids and asphaltene. Energies are in kcal/mol. Models IL1 IL2 IL3 IL4 IL5 IL6 Asphaltene

IL1>IL2>IL4>IL6, with IL3 being the most effective one for reducing the viscosity of the HCOs. These results might have important practical relevance for enhanced oil recovery (EOR) and flow assurance applications. From a basic viewpoint, this study demonstrated the coincidence of viscosity modifications of the HCOs with the probability of molecular interactions, in terms of the minimum energy criterion, with lowest interaction energies



Page 30 of 37

between ILs and asphaltenes. This mechanism seems dominated by the (LI’s)-cation and (Asphaltene)-π ligands with a high aromaticity index, which promotes a strong interaction with the aromatic hard core of asphaltenes.

AUTHOR INFORMATION Corresponding Author e-mail: [email protected] (R. H-B), [email protected] (J-M.D).

ASSOCIATED CONTENT Supporting Information Mülliken and NBO Population analysis results obtained for IL3, IL6 and asphaltenes models (Tables S1.S2, S3, S4 and S5) and (Figures S1,S2,S3,S4 and S5). Rheological behavior (viscosity) (Figure S6).

Acknowledgments: The authors are grateful to the Mexican Institute of Petroleum (IMP) for providing facilities and publication permission, through Project Y.61006, as well as to Conacyt-Sener Hydrocarbons’ Program (Project 177007 “Matricial Oil Recovery and Improvement of Extra Heavy and Heavy Crudes Density (API) by means of in situ hydroprocessing” for financial support. In addition, authors want to acknowledge Dr. Raúl Oviedo-Roa from IMP for help in the data analysis and fruitful discussions.

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Figure 10. TOC Graphics


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