Experimental and Theoretical Study on Supramolecular Ionic Liquid

Article pubs.acs.org/JPCB

Cite This: J. Phys. Chem. B 2018, 122, 4325−4335

Experimental and Theoretical Study on Supramolecular Ionic Liquid (IL)−Asphaltene Complex Interactions and Their Effects on the Flow Properties of Heavy Crude Oils R. Hernández-Bravo,* A. D. Miranda, J.-M. Martínez-Magadán, and J. M. Domínguez* Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, Gustavo A. Madero, México City, C.P. 07730, México S Supporting Information *

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 dimer association by forming supramolecular complexes that modify the properties of crude oils such as viscosity and interfacial tension. The IL−cation and asphaltene−π ligand molecular interactions seem to dominate 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 chain,1−5 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 5−10 fused rings having lateral alkyl type chains attached to the molecule outskirts and have high aromaticity, high polarity, and a marked tendency to form asphaltene− asphaltene supramolecular aggregates by a mechanism of π−π orbital type association.8 Another solubility class of molecules associated with asphaltenes is resins, which are aromatic type molecules with a molecular weight smaller than asphaltenes, having only three to six 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 onset.9 These properties are of industrial interest and are a subject of research10 in view of obtaining potential stabilizers. Other © 2018 American Chemical Society

molecules have proved useful as viscosity and interfacial tension (IFT) modifiers for the HCOs.11 One example of the former is cocoamidopropylhydroxysultaine with sodium dodecyl alphaolefin sulfonate and dodecyl hydroxyl sulfonate;4 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 applications.12,13 Therefore, it is worth exploring 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 1propenyl-3-alkylimidazolium bromide show some solubility properties in a nonpolar environment.12,13 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 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 the dipole moment of an ion that is Received: January 30, 2018 Revised: March 27, 2018 Published: March 27, 2018 4325

DOI: 10.1021/acs.jpcb.8b01061 J. Phys. Chem. B 2018, 122, 4325−4335

Article

The Journal of Physical Chemistry B

asphaltenes and found that the effectiveness of some amphiphiles relies on its ability to form stabilizing layers around the asphaltenic molecules, which correlates with the polarity of the headgroup and the 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 the 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−IL− asphaltene complex formation. The molecular dynamics allowed exploring over a period of time the dynamical evolution of a system composed of asphaltene and IL 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 asphaltene−IL interactions as well as to the flow behavior of the HCO complex fluids.31

dependent on the coordinate scheme chosen, making it an illdefined physical quantity.14 In order to explore the interactions present in the aggregation of asphaltenes, Gray et al.15 proposed an alternate paradigm based on supramolecular assembly of molecules. This model takes into 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 al.16 used the same model of asphaltenes for investigating the interactions of mono- and multifunctional 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 al.17 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 al.18 reported that the 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 al.21 used 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 denitrification and removal of pyrrol-based compounds from liquid fuels. In addition, Pons et al.25,26 studied the formation of supramolecular 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 al.27 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 noninteracting hard sphere model within the temperature range 298−373 K and included the role of solid maltenes. In addition, Rogel et al.28 studied the effect of inhibiting asphaltene 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, Hu et al.29 studied the effect of structural features of ionic liquids and other amphiphiles for controlling the precipitation of

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All synthesis reagents were analytical grade and used without further purification. Methylimidazole, bromotetradecane, pyridine, quinoline, benzymidazole, sodium hydride, tetradecyltrimethylammonium bromide, and imidazole were from 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 13C NMR analysis allowed characterizing the molecular structure, using DMSO-d6 as a solvent and TMS as an 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 saturate, aromatic, resin, and asphaltene (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 fractions, the ASTM-D-2007-91 method (i.e., in nC7) applied. Table 1 show the initial properties of the heavy crude oil (HCO). 2.2. Rheological Measurements. Knowing the flow behavior of the original and modified HCOs required determining the apparent viscosity by means of a stresscontrolled 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 10 g samples of Maya heavy crude oil; then, doping followed using 1 mL of IL solution in n-heptane, for obtaining a final 4326


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pellet): 3061, 3026, 2920,1735, 1178, 727 cm−1. HRMS calcd C19H34BrN 356.38, found 355.19. 2.3.3. 2-Tetradecylisoquinolin-2-ium Bromide (IL3). The first step was placing quinoline (1 mmol) and 1-bromotetradecane (1 mmol) 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, 727 cm−1. HRMS calcd C23H36BrN 406.44, found 405.20. 2.3.4. 1-Tetradecyl-3-[3-(1-methyl-1H-imidazol-3-ium-3yl) Propyl]-1H-imidazol-3-ium Dibromide (IL4). 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 followed; hence, stirring the mixture for 7 h at 80 °C followed, always under a 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 was applied; hence, it was 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 90% yield of N-tetradecylimidazol (compound 1) resulted. Afterward, adding 1,3-dibromopropane (1 mmol) and acetonitrile (10 mL) applied for synthesizing N-tetradecylimidazole (1 mmol). Heating the mixture at 80 °C and stirring for 4 days in a nitrogen atmosphere followed. Removal of acetonitrile under reduced pressure at 90° for 6 h was applied. Washing the white solid with ethyl acetate several times allowed removing all of the unreacted 1,3-dibromopropane and N-tetradecylimidazole. Hence, removal of the solvent in a vacuum at 80 °C followed. Compound 2 was a white powder, with yield 92%. Afterward, adding 1-methylimidazole (1 mmol) to the synthesized product 1-bromopropyl-3-tetradecylimidazolium bromide (1 mmol) followed. Heating the mixture at 80 °C and stirring for 4 days in a nitrogen atmosphere allowed a yellow solid to be obtained; hence, ethyl acetate allowed washing for removal of all of the unreacted compounds, and then, removal of the solvent in a vacuum at 80° followed. Compound 3 was obtained as a yellow powder, 95% yield. 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). 13C 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 837 cm−1. HRMS calcd C24H44Br2N4 548.44, found 548.15.

Table 1. Properties of Heavy Crude Oil properties of crude oil

Maya

API gravity density (25°), g/mL sulfur, wt % carbon, wt % asphaltenes in nC7, wt % saturates in nC7, wt % aromatics in nC7, wt % polar in nC7, wt %

21−22° 0.9199 3.4−3.8% 10.57 11.2 3.6 67.5 12.7

concentration of 1000 ppm 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 homogeneous mix. Each test was performed with loads of 1 mL, and this procedure applied for all of the experimental series, followed by resting for 12 h at 298.15 K, and preconditioning the samples by dehydration and rest at 40 °C for 2 h before determining their rheological behavior. 2.3. Synthesis of Ionic Liquids. The ionic liquids (ILs) studied in this work (IL-1, IL-2, IL-3, IL-4, and IL-5) were synthesized by following previous reports in the literature,32−36 and their structural features were characterized by 13C, 1H NMR, MS, and FTIR. All of the ILs were dried under a vacuum for at least 48 h at temperatures between 313 and 353 K to remove organic solvents and water by the Karl−Fischer titration method or coulometry (Brinkmann Metrohm 756 KF Coulometer). The ionic liquids used thereafter in the tests contained less than 10 ppm of water, while the purity of the ionic liquids was more than 98%. 2.3.1. 3-Methyl-1-tetradecyl-1H-imidazol-3-ium Bromide (IL1). The first step was placing N-methylimidazole (1 mmol) and 1-bromotetradecane (1 mmol) 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 a 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 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, 837 cm−1. HRMS calcd C18H35BrN2 359.39, found 358.29. 2.3.2. 1-Tetradecylpyridin-1-ium Bromide (IL2). The first step was placing pyridine (1 mmol) and 1-bromotetradecane (1 mmol) 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 a 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, DMSOd6) 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 4327


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The Journal of Physical Chemistry B 2.3.5. 3-Methyl-1-tetradecyl-1H-benzimidazol-3-ium Bromide (IL5). The first step was placing 1-methylbenzimidazole (1 mmol) and 1-bromotetradecane (1 mmol) into a dry roundbottomed 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 was applied down to room temperature. Hence, acetonitrile as solvent allowed washing the product, and 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, 29.0−29.2, 29.5, 32.4, 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, 727 cm−1. HRMS calcd C22H37BrN2 409.45, found 408.21.

Figure 1. Asphaltene model.43

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

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

3. THEORETICAL CALCULATIONS 3.1. 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 the Dmol3 module,37 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 base.40 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 IL−asphaltene interactions. 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 noncovalent interactions.42 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 ΔEij = Eij − (Ei + Ej)

C53H55NS 737 11 1.04

(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. 3.2. 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., 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 the COMPASS force field for the NPT simulations.37 The atomic charges for the electrostatic component of the force field were determined from quantum mechanical 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

(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 improves the local spin density; therefore, GGA is appropriate for calculating the interaction energies of the asphaltene−IL−asphaltene system. The correction to the van der Waals interaction effect keeps 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 six imidazole-based ILs 4328


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The Journal of Physical Chemistry B distribution of conformational states.44 Hence, this study allowed verifying the supramolecular complex formation. With the purpose of visualizing the formation of the supramolecular complex, the molecular dynamics simulation was performed independently of the DFT calculations. Computational calculations in a vacuum environment were performed; because the cohesive energy density is related too, the stability of the asphaltenic aggregate was used.

Table 5. Total Energy for Single Molecules

4. RESULTS AND DISCUSSION 4.1. Modeling Asphaltene−IL 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 (ΔEij) represents the simplest case of two asphaltene molecules linked to each other by means of a π−π type interaction, ΔEij 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,

steps

interactions Asph + ILs → Asph−ILs ILs + Asph−Asph → Asph−ILs−Asph Asph−Asph + Asph−ILs → Asph−ILs−Asph + Asph 2Asph−Asph + 2ILs → 2Asph−ILs−Asph

total energy × 106 (kcal/mol)

Asph IL1 IL2 IL3 IL4 IL5 IL6

−1.571 −2.116 −2.120 −2.212 −3.932 −2.248 −1.996

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)

Table 4. Interaction Mechanism of Asphaltenes with ILs 1 2 3 global

single molecules

molecular species

total energy × 106 (kcal/mol)

ΔE (kcal/mol)

Asph−Asph IL1−IL1 IL2−IL2 IL3−IL3 IL4−IL4 IL5−IL5 IL6−IL6 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

−3.142 −4.205 −4.232 −4.425 −7.865 −4.496 −3.992 −3.673 −3.687 −3.784 −5.503 −3.819 −3.567 −5.245 −5.258 −5.355 −7.075 −5.390 −5.138

−74.899 −19.277 −35.417 −14.388 −112.927 −15.175 −25.746 −12.836a −25.749a −31.349a −24.091a −37.361a −7.262a −2.210b 12.177b −331.561b 20.545b −10.689b 32.971b

a

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

that is, the viscous energy, i.e., Evisc; thus, in the onset of flow, Evisc = −ΔEij, since these are opposite terms with respect to the flow dynamics; i.e., asphaltene aggregation increases 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 = A exp(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 = −ΔEi, as shown in Table 4. The molecular modeling by DFT allowed calculating thermodynamic data of adduct formation between ILs and asphaltenes. In addition, Table 5 reports the energies associated with single asphaltenic molecules and their complex supramolecular aggregates. Tables 6 and 7 show the interaction energies. Hence, according to these calculations, the molecular disaggregation of asphaltenes from the asphaltene−asphaltene dimer model requires about +74.899 kcal/mol, while the energies needed for disaggregating two lateral asphaltenic molecules linked to an ILX type molecule in between are 4.422, 282.981, and 35.054 kcal/mol, respectively, where X = 1, 3, or 5. Tables 6 and 7 show the energies associated with steps 2 and 3 (Table 6) which follow the order EAsph−IL3−Asph < EAsph−Asph. This leads us to conclude that ILs inserted in between two

Table 7. Interaction Energy Values of Step 3 and Global Energy molecular species

ΔEstep3 (kcal/mol)

ΔEglobal (kcal/mol)

Asph−IL1−Asph Asph−IL2−Asph Asph−IL3−Asph Asph−IL4−Asph Asph−IL5−Asph Asph−IL6−Asph

10.624 31.426 79.929 44.635 12.996 39.684

−4.422 17.854 −282.981 41.089 −35.054 65.393

asphaltenic molecules (Figures 2 and 3 and step 3 of Table 6) cause the formation of stable aggregates, as is the case of Asph− IL1−Asph, Asph−IL3−Asph, and Asph−IL5−Asph, which present negative energies down to about −4.422, −282.981, and −35.054 kcal/mol, respectively (i.e., Table 7). Those supramolecular aggregates seem more favorable from the energetic viewpoint, while other aggregates like Asph−IL2− Asph, Asph−IL4−Asph, 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 the 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 4329


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In summary, IL1, IL3, and IL5 are the most probable supramolecular aggregates, while IL4, IL2, and IL6 seem nonfavorable (Tables 6 and 7). In particular, the supramolecular Asph−IL2−Asph complex presents an energy of 17.854 kcal/mol, which explains previous reports by Firoozabadi et al.,19 who reported that ionic liquids containing the pyridinium moiety are not as efficient as viscosity reducers for some HCOs. However, structural features such as head groups and alkyl chain lengths play an important role in the rheological behavior of the aggregates, as reported by Hernández-Bravo et al.30 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, and IL6 being 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 tends 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 the AI from the exact molecular mass of an organic compound. Hence, the AI might measure the C−C double-bond “density”, which in turn involves the contribution of π-bonding by heteroatoms, according to the following relationship:53

Figure 2. Interaction models for IL−IL and Asph−Asph (dimer).

AI =

1 + C − O − S − 0.5H C−O−S−N−P

(2)

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

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

DBE = C −

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−IL−asphaltene 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, interaggregation of IL3 should tend to destabilize the asphaltenic cluster by intercalating between at least one Asph−Asph dimer, which could induce cluster breaking and asphaltene 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 IL−asphaltene and asphaltene dimers to form asphaltenes−ILs−asphaltenes + asphaltenes, a process that is not favored for all of these cases.

H N + +1 2 2

(3)

The AI order is the following: IL3 > IL5 > IL2 > IL4 > IL1 > IL6. However, DBE’s sequence is as 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 Table 8. Calculated Aromacity Index (AI) and Double-BondEquivalent (DBE) Values for the ILs

4330

models

AI

DBE

IL1 IL2 IL3 IL4 IL5 IL6

0.094 0.166 0.273 0.158 0.205 0.000

2.5 3.5 6.5 5.0 5.5 0.0 DOI: 10.1021/acs.jpcb.8b01061 J. Phys. Chem. B 2018, 122, 4325−4335

Article

The Journal of Physical Chemistry B liquids.50 In particular, IL3 has the highest degree of interaction, 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.56−58 Hence, this finding has a great potential for aromatics separation in general, but the concept applies too for the asphaltene 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 interactions.59 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.

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 factor.64 Furthermore, a Mülliken and NBO population analysis of ILs gave results shown in Tables S1, S2, S3, S4, and S5 and Figures S1, S2, S3, S4, and S5 of the Supporting Information. These two analyses 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 six-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 counterion 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 orbital interaction, and van der Waals forces including hydrogen bonding.65,22 Some ILs having anions and a low value of HOMO−LUMO energy gap, such as chloride or bromide, were reported to reduce the viscosity of the HCOs, by their tendency to form soft molecules that lead to better polarizability and reactivity.65 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 is the case for IL3, which has a Lewis acid character in opposition to IL6, which has a quaternary amine group that tends to form “hard molecules” (high HOMO−LUMO gap). By using the HOMO and LUMO energy values EH and EL, respectively, the global chemical reactivity descriptors such as chemical potential (μ) and global hardness (η) can be defined for a molecule as66

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

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 hydrocarbon families, i.e., paraffinic, naphthenic, and aromatic, as reported previously.60 Table 9. Watson Characterization Factor (KW) for Different Hydrocarbon Families homologue series

KW

paraffinic naphthenic aromatic

13.1−13.5 10.5−13.2 9.5−12.5

μ = (E L + E H)/2

(5)

η = (E L − E H)/2

(6)

In a reaction between two molecules A and B, the electronic transfer will occur until their chemical potentials μA and μB equal one another.67 If μB ≈ μA, 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).

The correlation factor was determined using the equation developed by Perry and White61 KW = −0.0239(Wt polars) − 0.0109 H C + 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 miscibility.62,30 For comparison 4331


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determined host zones provides a quantitative determination of Lewis acidity strength on the outermost atoms. Thus, in this work, the areas of low potential (red) characterize the abundance of electrons (Figure 6), while the

Table 10. Chemical Potential and Hardness for the Ionic Liquids and Asphaltenea

a

models

EH

EL

EL − EH

μ

η

IL1 IL2 IL3 IL4 IL5 IL6 asphaltene

−85.849 −103.351 −109.174 −97.327 −93.555 −110.832 −109.214

−43.982 −53.100 −52.742 −38.774 −53.269 0.634 −71.045

41.867 50.251 56.432 58.553 40.286 111.466 38.169

−64.915 −78.225 −80.958 −68.050 −73.412 −55.099 −90.130

20.934 25.125 28.216 29.276 20.143 55.733 19.085

Energies are in kcal/mol.

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 involves isolated molecules prior to the reaction; then, their insights for what could occur can be different from the calculations over interacting molecules. Additionally, molecular dynamics (MD) simulation allowed clarifying the possible mechanism of asphaltene dispersion at the molecular scale by interacting with ILs. Figure 5 shows the MD simulation results of asphaltene aggregates within a cluster before and after contacting ILs, with

Figure 6. Top view of the van der Waals surface mapping defining the outermost border of the ionic liquids.

areas of high potential (blue) correspond to the relative absence of electrons. Due to the high Br− electronegativity with respect to nitrogen, Br− ions have an electron density higher than nitrogen atoms. Thus, the intermediary potential energy influences the interaction; i.e., the non-red or blue regions imply that the electronegativity difference is not very large. In a molecule with a large electronegativity difference, the charge is rather concentrated, which means significant differences in electron density at different regions of the molecule. Large electronegativity differences look as if there were regions almost entirely red or blue. Large regions of intermediary potential (yellow and green) and the smaller ones are key indicators of mild electronegativity differences. The cationic ILs have their positive charge localized mostly on their ring. Thus, the cationic ILs’ electric fields should induce delocalized electrons on asphaltenic molecules, thus making the cation−π structures more stable. This analysis about the van der Waals surface shows that the active sites are mainly concentrated in the IL’s cationic head, a poor electron site, thus making the interaction with species that are rich in electrons, i.e., the aromatic core of asphaltenes (π-system), a very probable interaction. Molecular simulations should verify these interactions and compare them with experimental results for viscosity variations of the crude oils, thus validating the calculations therein. Analysis of the van der Waals surface reveals a strong attractive interaction between the components of the binary asphaltene−IL3 complex (Figure 7) as compared with the components of the asphaltene−IL6 complex (Figure 8), since the polar head of IL3 is less hidden than the one of IL6. This is a consequence of the affinity between the two aromatic rings of IL3 with asphaltene (in fact, by definition, an asphaltene is soluble in toluene, which is aromatic), whereas IL6 does not have rings. 4.2. Study of Rheological Behavior of Heavy Crude Oils (HCOs). This experimental study focused on assessing the influence of the series of ILs, IL1−IL6, on the viscosity of the

Figure 5. NPT molecular dynamics simulation: (A) asphaltene− asphaltene aggregates in the absence of ILs; (B) supramolecular Asph−IL−Asph complex formed after IL addition.

the former showing the Asph−Asph dimer formation and the second one showing the formation of a supramolecular Asph− IL−Asph complex. The density of the cluster before adding the IL is 1.131 g/cm3, while the supramolecular complex showed a density of 1.104 g/cm3 (data from NPT). These results indicate that the supramolecular complex can form inside a cluster and for the case involving IL3 the supramolecular complex is more stable than a simple Asph−Asph dimer, in agreement with results of Table 6, where (ΔE)Asph−Asph = −74.899 kcal/mol and (ΔE)Asph−IL3−Asph = −331.561 kcal/mol, which could give rise to disaggregation of the complex from the asphaltenic cluster. 4.1.1. van der Waals Surface. The van der Waals surface of a molecule is a model in rough terms, showing where a molecular surface might reside for the molecule based on the hard cutoffs of van der Waals radii for individual atoms, and it represents a surface prone to interacting with other molecules.68 Hence, the mapping of a surface allows identifying where the host sites in which nucleophiles (most positive zone) and electrophiles (most negative zone) should bind. Additionally, the (local) maximum value of electrostatic potential at the 4332


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give an assessment of the viscosity trends, for comparing with energetic parameters of those interactions; these should provide data for establishing a correlation based upon the energy needed to overcome the molecular cohesion for keeping the supramolecular aggregates together.

5. CONCLUSIONS In the search of a correlation between theoretical cohesive and viscosity energies as a function of experimental viscosity data for a series of HCOs and distinct ILs, the DFT methods allowed calculating the interaction energy of the systems asphaltene−asphaltene and asphaltenes−ILs−asphaltenes. The experimental determination of the viscosity of HCOs as a function of shear rates, with and without ILs, allowed determining the viscosity variations caused by the molecular interactions of the ILs with those HCOs. These calculations were based on interaction energies of the ionic liquids with asphaltenes models, and the reactivity order was the following: IL3 > IL5 > IL1 > IL2 > IL4 > IL6, with IL3 being the most effective one for reducing the viscosity of the HCOs. These results might have an 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 the lowest interaction energies between ILs and asphaltenes. This mechanism seems dominated by the (IL)−cation and (asphaltene)−π ligands with a high aromaticity index, which promotes a strong interaction with the aromatic hard core of asphaltenes.

Figure 7. Top view of the van der Waals surface mapping of asphaltene−IL3.

Figure 8. Top view of the van der Waals surface mapping of asphaltene−IL6.

HCOs, with the purpose of understanding the molecular interactions and modifications of thermodynamic properties of the complex fluids.69 It required the experimental evaluation of viscosity before and after treating Maya type crude oil with ILs.43 The SARA analysis (saturate, aromatic, resin, and asphaltene) indicated a content of 11.2 wt % asphaltenes, i.e., the insoluble fraction in n-heptane. The experiments used a HCO blank sample and then a mix of HCO with n-heptane for comparing their rheological properties with respect to HCOs doped with ILs (i.e., Figure 9 and Figure S6 in the Supporting

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.8b01061. Mülliken and NBO population analysis results obtained for IL3, IL6, and asphaltene models (Tables S1−S5 and Figures S1−S5) and rheological behavior (viscosity) (Figure S6) (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

R. Hernández-Bravo: 0000-0002-6608-8135

Figure 9. Rheological behavior (viscosity) at 25 °C of Maya heavy crude oil with ILs.

Notes

The authors declare no competing financial interest.

■

Information, for illustrating the rheological behavior of those fluids at 25 °C, as a function of shear rates, from about 5 to 200 s−1). One observes a typical Newtonian profile for all of the cases with an overall viscosity decrease after introducing ILs, except for IL6. The maximum viscosity reduction occurs for HCOs doped with IL3, more pronounced than the effect of nheptane in the Maya crude alone (i.e, Maya-C7). HCO’s viscosity results from the sum of inner interactions at the molecular level, i.e., asphaltene−asphaltene, asphaltene−resin− asphaltene, and asphaltene−naphtenic acid interactions. Hence, calculating the interaction energy between those cases should

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, the authors want to acknowledge Dr. Raúl Oviedo-Roa from IMP for help in the data analysis and fruitful discussions. 4333


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Experimental and Theoretical Study on Supramolecular Ionic Liquid

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