Structural Behavior of Isolated Asphaltene Molecules at the OilâWater

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Structural Behaviour of Isolated Asphaltene Molecules at the Oil-Water Interface Meena B Singh, Nakul Rampal, and Ateeque Malani Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01648 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Structural Behaviour of Isolated Asphaltene Molecules at the Oil-Water Interface

Meena B. Singh1, Nakul Rampal1,2, Ateeque Malani*,1 1

Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai, 400076 India. 2

Department of Chemical and Biomolecular Engineering, University of California, Berkeley 94720, United States

*To whom correspondence should be addressed. Email : [email protected] Ph : +91-22-2576 7205 Fax : +91-22-2576 6895

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ABSTRACT Asphaltenes are the heaviest component of crude oil causing the formation of a stable oilwater emulsion. Even though asphaltenes are known to behave as an emulsifying agent for emulsion formation their arrangement at oil-water interface is poorly understood. We investigated the effect of asphaltene structure (island type vs archipelago type) and heteroatom type (Oxygen-O, Nitrogen-N, and Sulphur-S) on their structural behaviour in the oil-water system. Out of six asphaltenes studied here, only three asphaltenes remain at the oil-water interface while others are soluble in the oil phase. Molecular orientation of asphaltene at the interface, position, and angle of asphaltene with the interface has also been determined. We observed that N-based island type asphaltene is parallel while O-based island type asphaltene and N-based archipelago type are perpendicular to the interface. These asphaltene molecules are anchored at the interface by the heteroatom. The S-based asphaltenes (both island and archipelago type) and O-based archipelago type asphaltenes are soluble in oil phase due to their inability to form a hydrogen bond with water and steric crowding near the heteroatom. This study will help in understanding the role of asphaltenes in oil-water emulsion formation based on its structure and how to avoid it.

Keywords: Asphaltenes, Oil-water interface, Molecular dynamics (MD) simulation, Molecular orientation, Interfacial phenomena.


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1. INTRODUCTION Asphaltenes are defined as the higher molecular weight fraction of crude oil, which are mostly polar, dense and surface active.1–5 They are found along with other aromatic hydrocarbons, resins, and saturated hydrocarbons.6,7 There is no specific structure of asphaltene and therefore they are also defined based on their solubility.8–10 Components of crude oil soluble in toluene and insoluble in n-heptane are called as asphaltenes.11,12 The concentration of asphaltene in the crude generally varies from 0.02-0.1 g/l depending on the geographical source of crude oil.6,8 Asphaltenes basically consist of Carbon - C, Hydrogen - H, the heteroatom (Oxygen - O, Nitrogen - N, Sulphur S) and trace amounts of heavy metals such as Vanadium and Nickel.13 The C:H ratio of asphaltenes varies depending on the source of crude oil and average value is around 1:1.2.9 The heteroatoms are present as thiophene, sulfoxide, sulfidic (S - heteroatom); pyridine, pyrrolic, quinolone (N heteroatom); and hydroxyl, carboxyl, carbonyl (O - heteroatom) functional groups.1,14,15 Asphaltenes also contain alkyl linkages, aliphatic side chains and many times contain paraffin rings.16 In literature, there are various views regarding the exact structure of asphaltenes. Many researchers claim that asphaltenes have multiple aromatic rings linked together by alkyl chains, also known as archipelago model.1,6,11,14,17–20 Others proposed that it has a single aromatic core made of multiple aromatic rings fused together and alkyl chains are attached to the periphery of this core like a pendant, known as island model.1,11,14,17,21 Few studies claim that island model of asphaltenes is more precise and valid as compared to other models.11 For the current study, both archipelago and island model of asphaltenes are considered. Water-in-oil emulsion is one of the major concerns for the petroleum industry.7,13–15,17,22–28 The size of droplets and their stability in the emulsion depends mainly on surface tension, gravity, and, chemical components of crude oil and water.25 Asphaltenes are one of the major components responsible for the formation and stabilization of oil-water emulsion by acting as an emulsifying agent.7,15,23,14,22,29,30 Asphaltenes aggregate at the interface of oil-water and form a physical barrier


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and hence increase the viscosity of crude oil, which increases the pumping and transportation cost of crude oil.11,15 Asphaltenes also deposit at the surfaces of pipes, causing the clogging of pipelines and borewells of crude oil. They also result in fouling, corrosion of equipment and coke formation.2,10,14,25,29,31 Various (experimental and theoretical) studies1,10,18,20–22,24,29–40 report aggregation of asphaltene at oil-water interface and solid surface. Alvarez and co-workers22 studied the influence of crude oil dilution by toluene on the oil-water emulsion stability and observed that critical aggregation concentration of asphaltenes are independent of dilution of crude oil but effects the disruption of asphaltene layer. Sojoblom and co-workers24 found that stability of oil-water emulsion depends on the interfacial pressure rather than oil-water interfacial tension. Stability of emulsion increases with increase in interfacial pressure. Jiang and co-workers29 concluded that reduction in interfacial tension of oil-water interface depends on surface concentration of asphaltene and not on the bulk concentration in oil phase. Cruz and co-workers31 developed a mathematical model for deposition of asphaltene in porous media. They proposed two distinct mechanisms for asphaltene deposition; i) adsorption of asphaltene on pore surface and ii) mechanical trapping of asphaltene molecules on porous medium. From various studies,20–22,24,29–40

it was observed that aggregation

and deposition of asphaltene depends on various factors; such as the concentration of asphaltene, solvents, pH and C:H ratio. Most of the molecular simulation studies1,14,18,21,25,26,39,41 have used Yen42 and modified Yen17 model to construct the model structure of asphaltene. Many of these studies reported that asphaltenes first form nanoaggregates at the oil-water interface and later these nanoaggregates also form clusters by self-association.1,9,18,21,30,43 Headen and co-workers1 studied aggregation of five types of asphaltene molecules (island and archipelago types) and observed that asphaltene forms clusters with aggregation number of 3.6 – 5.6. Further archipelago model of asphaltene forms longer prolate structures, while island model favours oblate shape clusters. Rogel9 investigated four model structures of asphaltenes and concluded that van der Waals interaction between asphaltene


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molecules play a crucial role during aggregation and stabilization. Wang and Ferguson18 studied the aggregation of three types of asphaltenes based on quantitative molecular representations (island types) and concluded that asphaltene aggregation occurs in 3 stages. During the first stage, parallel stacking of ~10 asphaltenes aromatic cores results in the formation of rod-like structure (nanoaggregates), followed by self-assembly of ~6 nanoaggregates in various shapes and finally formation of porous webbed network congregated by the weak association of side alkyl chains of nanoaggregate clusters. Costa and co-workers21 studied the behaviour of hexa-tert-butylhexa-perihexabenzocoronene (HTBHBC) molecules which has a structure similar to model asphaltene and concluded that the tertiary-butyl side chain restricts the dimer formation and further aggregation of HTBHBC due to steric repulsion. Murgich and co-workers43 also claim that three-dimensional shape of side alkyl chains limits the growth of aggregates of asphaltenes due to steric interferences. Gao and co-workers30 compared the aggregation behaviour of neutral and anionic carboxyl asphaltene and concluded that neutral carboxyl asphaltene forms aggregates by parallel face-to-face stacking and stays in oil phase while the anionic carboxyl asphaltene forms aggregates by both parallel and T-shape stacking and adsorb at the oil-water interface. Jian and co-workers29 found that model asphaltene Violanthrone-79 forms aggregates at the interface and the aromatic plane remains perpendicular to the oil-water interface. All the previous studies have focused on aggregation of asphaltene in bulk and at the oilwater interface. However, we believe that, their interfacial properties are mainly governed by their native structural arrangement at oil-water interface which has not been investigated thoroughly. In order to understand their behaviour, we performed molecular simulation studies of six asphaltene molecules having different structure (island and archipelago) and heteroatom in biphasic oil-water system. We found that not all of them remain at oil-water interface. We analyzed the paircorrelation function, orientational distribution and hydrogen bonds to obtain their structural arrangement.


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2. SIMULATION METHODOLOGY Crude oil is a complex mixture of hydrocarbons, whose composition depends on its geographical source. The exact molecular-level composition is very difficult to obtain and is hence often reported in terms of elemental atoms (such as C, H, N, O, S, metals) or group contributions (such as saturates, aromatics, resins and asphaltenes).44,45 This makes creating model crude oil extremely difficult. In this scenario, we could use short chain (heptane) or long chain alkane (saturates), as done in previous literature.1,14,29 We believe, the short chain alkane does not represent the viscosity and dynamic behavior of crude oil accurately whereas large chain alkane leads to large computational expense. As a compromise, we have chosen dodecane molecule as model crude oil, which has been used in the literature as well.46 The simulation box consists of a repeated slab of oil and water phase having two oil-water interfaces in a three-dimensional periodic box as shown in Figure 1. The dimensions of both oil and water phase are around 6 X 6 X 6 nm3, which is sufficiently large to avoid any finite size effects. One asphaltene molecule is placed at each oil-water interface. The initial configuration was generated using GROMACS47–50 software. The total interaction energy of the system includes both non-bonded and bonded interaction given as, 𝜎

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𝑈 = ∑𝑖 ∑𝑗>𝑖 4𝜀𝑖𝑗 [( 𝑟 𝑖𝑗 ) 𝑖𝑗

2

𝜎

6

𝑞 𝑞𝑗 0 𝑟𝑖𝑗

− ( 𝑟𝑖𝑗 ) ] + ∑𝑖 ∑𝑗>𝑖 4𝜋𝜖𝑖 𝑖𝑗

0 ) + ∑𝑙 ∑5𝑛=0 𝐶𝑛 (𝑐𝑜𝑠(∅ − 180°)) 𝜃𝑖𝑗𝑘

1

2

1

𝑏 𝜃 + ∑𝑙 2 𝑘𝑖𝑗 (𝑟𝑖𝑗 − 𝑏𝑖𝑗 ) + ∑𝑙 2 𝑘𝑖𝑗𝑘 (𝜃𝑖𝑗𝑘 −

𝑛

where the first two-terms account for non-bonded interactions modelled using Lennard-Jones (LJ) and Coloumbic interaction potential, respectively. rij is the distance between i and j atoms, σij and εij are the LJ interaction parameters evaluated using Lorentz-Berthelot mixing rule (𝜎𝑖𝑗 = (𝜎𝑖𝑖 + 𝜎𝑗𝑗 )⁄2 , 𝜀𝑖𝑗 = √𝜀𝑖𝑖 𝜀𝑗𝑗 , where σii and εii are the size and energy parameters of particle i), qi is the charge on the atom i and 𝜖0 is the vacuum permittivity. The long-range electrostatic interactions were treated using particle mesh Ewald (PME) method.51 The last three terms are ACS Paragon Plus Environment

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𝑏 𝜃 (𝑘𝑖𝑗𝑘 ) is the bond bonded interaction which includes, bond, angle and torsional potential. 𝑘𝑖𝑗 0 (angle) constant, 𝑟𝑖𝑗 (𝜃𝑖𝑗𝑘 ) is the instantaneous bond length (bond angle) and 𝑏𝑖𝑗 (𝜃𝑖𝑗𝑘 ) is the

equilibrium bond length (bond angle). ∅ is the torsional angle and Cn are the Ryckaert-Bellemans (RB) parameters.52 The forcefield parameters for asphaltene molecules in the current work is taken from the work of Li and Greenfield41 which are based on OPLS-AA forcefield.53–57 The dihedral parameters of asphaltene molecules from OPLS-AA forcefield were converted to RB parameters. The dodecane and water molecules were modelled using Siu et al.58 and SPC/E forcefield.59 An open source GROMACS47–50 code is used to perform molecular dynamics (MD) simulation. Total six types of asphaltene molecules, three island types (Asphaltene-phenol, APH, Asphaltene-pyrrole, APY, and Asphaltene-thiophene, ATH) and three archipelago type (Trimethylbenzene-oxane, TMBO, Quinolinohopane, QHP, and Thio-isorenieratane, TIR) as shown in Figure 2 are considered for the current study. Total 6 systems were studied with a single type of asphaltene molecule at the oil-water interface. Each system consists of around 8200 water molecules, 573 dodecane molecules and 2 asphaltene molecules in a box of around 6 X 6 X 13 nm3. Each simulation was started by keeping one asphaltene molecule near each oil-water interface inside the water phase (as shown in Figure 1).


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Figure 1: A representative simulation system containing water (red) and dodecane molecules (cyan) in a periodic box having two oil-water interfaces and one asphaltene molecule at each interface. The geometry of water molecules was maintained using the LINCS algorithm for bond constraints.60 Each system was simulated for 80 ns with a time step of 2 fs. In order to prevent the initial configuration effect on trajectory and equilibrium position of asphaltene molecules, they were restrained for the initial 40 ns of the simulation during which time dodecane and water phase was equilibrated. After 40 ns the restraint was relaxed and the system was analyzed for further 40 ns. The simulations were performed in NPzAT ensemble (N - number of particles, Pz - pressure in z direction, A - lateral area in x-y plane and T – temperature). The temperature and pressure of the system was maintained at 300 K and 1 bar using Nose-Hoover thermostat61,62 and ParrinelloRahman pressure coupling,63,64 with a damping factor of 0.1 and 2.0 ps, respectively. A cut off distance of 1.2 nm was applied for the LJ and short-range part of the Coulombic interaction and neighbours list was updated after every 10 fs. The reciprocal part of long-range electrostatic interactions was evaluated using PME with an accuracy of 10-4. Island Type (a) Asphaltene-phenol (APH)

Archipelago Type (b) Trimethylbenzene-oxane (TMBO)


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(c) Asphaltene-pyrrole (APY)

(d) Quinolinohopane (QHP)

(e) Asphaltene-thiophene (ATH)

(f) Thio-isorenieratane (TIR)

Figure 2: (a,c,e) Island type and (b,d,f) archipelago type asphaltene molecules containing (a,b) Oxygen (c,d) Nitrogen and (e,f) Sulphur heteroatom. Island type asphaltene molecules (left column) are characterized by large aromatic core (containing around 8-10 fused aromatic rings) and aliphatic side chains. The archipelago model (right column) has a lesser fraction of aromatic rings with sizes (in terms of the number of Carbon atoms) similar to alkyl chain part. The aromatic plane was defined by C9-C5-C18 atoms in APH, C30-C2-C18 atoms in APY and C20-C28-C24 atoms in QHP molecules. 3. RESULTS AND DISCUSSION 3.1 Equilibrium position of asphaltene molecule in the biphasic system To examine whether various asphaltene molecules prefer to stay at the interface as reported in literature1,9,18,21,29,32 or remain in the bulk dodecane phase, we monitored the position of all the 6 model asphaltenes (along z-direction) in the biphasic system. As stated earlier, the asphaltene molecules were placed in the water phase near the oil-water interface in the beginning (Figure 1) and their position was monitored by tracking z-coordinates of the heteroatom (Figure 3). The APH, ACS Paragon Plus Environment

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APY and QHP molecules moves from their initial position to the oil-water interface within 3.5 ns (Figure 3a) and remains there for the rest of the time. However the remaining three types of asphaltene molecules i.e ATH, TIR and TMBO move from their initial position, cross the waterdodecane interface and remain in the dodecane phase (Figure 3b). These results were also confirmed by starting with two different initial configurations of system i.e (a) when water phase is separate from dodecane phase in the initial configuration and (b) when water molecules were distributed uniformly in the box containing dodecane phase and vacuum region. The final equilibrium position of the asphaltene molecules (i.e. retention at oil-water interface and dispersion in oil phase for respective molecules) was unaltered by the initial configurations. Though trajectory of ATH molecule exhibit tendency of aggregation, however, in an extended 40 ns simulation neither aggregation nor presence near oil-water interface was observed (see supplementary information (SI)). (a)

(b)


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(c)

(d)

Figure 3: Position of (a) APH and (b) ATH asphaltene molecules tracked by monitoring the zcoordinates of the heteroatom in the oil-water system. (c) and (d) Snapshot of systems illustrating equilibrium positions of APH and ATH molecules respectively. The z-coordinates of z=0 in subfigure a and b correspond to the left oil-water interface (as shown in subfigure c and d) obtained using Gibbs dividing surface methodology (see text for details). In order to obtain the average location of these asphaltene molecules, we evaluated their 𝑁𝑖 density profile in the z-direction as 𝜌𝑖 (𝑧) = 〈∑𝑗=1 𝑀𝑗 𝛿(𝑧 − 𝑧𝑗 )〉, where, Mj is the molecular weight

of atom j, Ni is the total number of atoms of (i =) water/oil/asphaltene molecules, 𝛿(𝑧) = ∆𝑧 2

∆𝑧 2

𝜃(𝑧− )−𝜃(𝑧+ ) ∆𝑉

is a delta function evaluated numerically as combination of Heaviside step function

Θ(z), ∆𝑉 = 𝐴∆𝑧 is the bin volume, A is the cross-sectional area in the x and y-direction, Δz is the bin-thickness in z-direction and angular bracket indicates time-average. In the biphasic NPzAT simulations, the water and dodecane phase recover their respective bulk densities of 1000 and 750 kg/m3 in the central region. The densities are also used to find the location of oil-water interface (h) ℎ

𝑏

using Gibbs dividing surface methodology, ∫𝑎 (𝜌𝑟 − 𝜌(𝑧))𝑑𝑧 = ∫ℎ (𝜌(𝑧) − 𝜌𝑙 )𝑑𝑧 where 𝜌𝑟 , 𝜌𝑙 and 𝜌(𝑧) are the density of water/oil in rich phase, lean phase and variation in z-direction (Figure 4 a,b).


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(a)

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(b) Water

Water

Dodecane

Dodecane

APH

(c)

ATH

(d)

Figure 4: Density profile of water (green), dodecane (red) and asphaltene molecule (black) in a biphasic system containing (a) APH and (b) ATH asphaltene molecule. (c) Density distribution of APH (black), APY (red) and QHP molecule (violet) which are observed at oil-water interface (d) Density distribution of ATH (black), TIR (red) and TMBO molecule (violet) which are found in the bulk dodecane phase of the biphasic system. The reference z=0 in all subfigure corresponds to the left oil-water interface (as shown in Figure 1 and 3) obtained using Gibbs dividing surface methodology. The density profiles of APH, APY and QHP molecules (Figure 4c) exhibit strong peaks located at each oil-water interface in the biphasic system. While the density profile of other three types of asphaltene molecules i.e ATH, TIR and TMBO, exhibits peak inside dodecane phase indicating their preference to stay in the oil-phase (Figure 4d). The broad distribution of asphaltenes in the ACS Paragon Plus Environment

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density profiles of ATH, TIR and TMBO indicate that they remain soluble in dodecane phase. We emphasize that the current study is performed at infinite dilution, and the observed behaviour may differ at higher concentrations due to interface and bulk partitioning, and, cooperative effect among similar and different asphaltenes. A closer analysis of the density profiles of APH, APY and QHP, shows that the larger portion of density distribution at both the interfaces is inside the dodecane phase indicating that major part of the asphaltene molecules lies inside the oil phase and only small part is in the water phase (Figure 4). 3.2 Orientational analysis of asphaltene molecule in the biphasic system To investigate the interaction of asphaltene at the dodecane-water interface, the orientation of various asphaltene molecules with respect to the interface were studied (Figure 5). Asphaltene structure has two major parts: aromatic core and side alkyl chains. The orientation of aromatic core of different asphaltenes with respect to interface normal (z-axis) is studied by calculating the orientational distribution of the angle, θAZ, formed between plane of aromatic core and z-axis (i.e. 𝜃𝐴𝑍 = 𝑐𝑜𝑠(𝑒̂𝐴 ⋅ 𝑒̂𝑧 )−1, where 𝑒̂𝐴 and 𝑒̂𝑧 are the unit vectors of aromatic plane and z-axis). The aromatic plane of APH molecule is slightly tilted with an angle of θAZ = 81 ± 5° indicating that APH molecule is almost perpendicular to the dodecane-water interface (Figure 5a). Whereas, the orientational distribution of APY molecule exhibits two peaks around θAZ = 150° and a shoulder at around 80°. This analysis indicates that APY molecule remains almost parallel to the oil-water interface (Figure 5b). The θAZ distribution of the QHP molecule is broad and distributed as compared to other two APH and APY asphaltene molecules. We believe that this is due to the smaller aromatic core of QHP molecule due to its archipelago type configuration as compared to APH and APY island type molecules which have a larger aromatic core (Figure 2b). The QHP molecule (Figure 5c) also exhibits a prominent peak at θAZ = 105 ± 20° (i.e. perpendicular orientation with the oil-water interface), while the distribution suggests that other configurations are also possible. For the results reported here, we considered aromatic core formed by three atoms which lie at the end of the aromatic core, (mentioned in Figure 2). The validity of these results was ACS Paragon Plus Environment

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confirmed by calculating the orientational distribution by defining the aromatic plane using a combination of a different set of atoms. All those orientational distributions (see SI) yield similar conclusions. Orientational, density and snapshot analysis of asphaltene molecules at the interface indicates that asphaltene molecules either remain perpendicular or parallel to the interface with the major part of the molecule immersed in the oil phase and anchored to the oil-water interface via the heteroatom (Figure 4 and 5). We also investigated the orientations of side alkyl chains of asphaltene molecules with respect to the aromatic core of the molecule (θAC). APH molecules have total 5 alkyl chains containing Carbon atoms ranging from 2 to 5. Alkyl chains orient themselves in the major orientation parallel to the plane of the aromatic core by making an angle of around θAC = 72 ± 6° with the plane normal of APH aromatic core (Figure 5d). APY molecules have 6 alkyl chains attached to the aromatic core of molecule and number of Carbon atom varies from minimum 2 to maximum 9 in alkyl chains. These alkyl chains are present in mainly two conformations, forming the angle of θAC = 72 ± 10° and 115 ± 7°, with the cumulative probability of 0.3 and 0.7, respectively. This analysis indicates that approximately 2 chains remain on one side of the aromatic plane and 4 chains on the other side of aromatic plane (Figure 5e). The QHP molecule has two major side alkyl chains connected to the aromatic core, one consisting of 24 Carbon atoms and other 5 Carbon atoms. Alkyl chains form two angles, θAC = 66 ± 7° and 112 ± 5° indicating that each alkyl chain orients itself on each side of the aromatic plane with approximately equal probability (Figure 5f). We did not find any direct correlation between the asphaltene structure (island type vs archipelago type) and their structure at the interface. Both parallel and perpendicular orientations were observed for island type models. The archipelago type models have a broad orientational distribution due to the smaller fraction of rigid aromatic portion.


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θAZ

θAC

Snapshot

(a)

(d)

(g)

(b)

(e)

(h)

APH

APY


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(c)

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(i)

(f)

QHP

Figure 5: Orientational distribution evaluated between (a-c) aromatic plane normal and oil-water interface normal (z-axis), and, (d-f) aromatic plane normal and side alkyl chains. (g-i) Representative snapshots illustrating the structure of asphaltene molecules at the oil-water interface. Red-Oxygen, White-Hydrogen, Blue-Nitrogen, Cyan-Carbon atoms of asphaltene molecules, thin red lines-water molecule and thin cyan lines-dodecane molecules.


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3.3 Role of heteroatoms and hydrogen bonding Even though all the six types of asphaltene molecules have aromatic cores with alkyl side chains and one heteroatom in their structures, they behave differently at the oil-water interface. This we believe is due to the difference in hydrophilicity of the heteroatom and structural arrangement. The partial charge of Oxygen heteroatom of APH, Nitrogen of APY and Nitrogen of QHP is -0.585, -0.339 and -0.697e, respectively. Due to higher negative charge on the heteroatom of these asphaltenes, the heteroatom facilitates the formation of a hydrogen bond (HB) with the water molecules and hence they remain at the interfaces. The ATH and TIR molecules have Sulphur heteroatoms, which have lower partial charges of -0.086 and -0.098, respectively, which are negligible as compared with the partial negative charges on heteroatoms of APH, APY and QHP molecules. Therefore, ATH and TIR do not stay at the oil-water interface due to lesser electrostatic interactions or HB formation. To confirm this hypothesis, we calculated the total number of HB formed between water and asphaltene molecules at the interface. The HB is defined here by the geometric criteria.65,66 The distances between heteroatom (Ma)

of asphaltene molecule, and, Oxygen (Ow) and Hydrogen (Hw) atom of the water molecule (i.e.

rMa-Ow and rMa-Hw) should be within 0.35 and 0.25 nm which corresponds to the first minima location of the corresponding radial distribution function (RDF).65,66 When the heteroatom of asphaltene molecule has Hydrogen atom (Ha) covalently bonded, as in the case of APY and APH molecules, they also form HB with the Oxygen atom of water molecules. For these HBs as well, the two distance criteria used are; rMa-Ow ≤ 0.35 nm and rHa-Ow ≤ 0.25 nm.65,66 We also estimated the HB energy from independent MD simulation in NVT ensemble containing a) single asphaltene molecule and b) asphaltene-water molecule pair in vacuum in a cubic periodic box of 4 nm. The force-field, timestep and thermostat used are same as biphasic simulations. The HB energy is defined as the difference between the total energy of asphaltene molecule with hydrogen-bonded water molecule and isolated asphaltene molecule (i.e. E(HB) = E(Asp-Water) – E(Asp)). Since water molecule is modelled as a rigid molecule, hence the energy of an isolated water molecule in ACS Paragon Plus Environment

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Page 18 of 26

the vacuum is zero (i.e. E(water) = 0). The HB energies obtained from these simulations are given in Table 2. We found that APH molecule forms around 2.22 HBs per molecule, while APY and QHP molecules form respectively 1.39 and 1.19 HBs per molecule. Though APY and QHP molecule contain Nitrogen as the heteroatom, however Hydrogen atoms are also present as -NH functional in APY molecule. This Hydrogen atom of –NH functional increases the number of HB per molecule compared to QHP. Since ATH and TIR have lower heteroatom partial charges, and they do not stay at the oil-water interface, hence the HB analysis was not performed for them. Higher values of HB energy (25-96 kJ/mol) indicate the formation of very strong HB between asphaltene (APH, APY and QHP) and water molecules at the interface. We believe that this strong HB is the driving force for retaining asphaltene molecules at the interface. Table No. 1: Average number of hydrogen bonds (HBs) formed by heteroatom of asphaltene molecules and corresponding HB energy Sr.

Average No. of Hydrogen

Hydrogen bond energy

bonds per molecule

(KJ/mol)

Asphaltene Molecule No. 1

Asphaltene phenol (APH)

2.22

53.734

2

Asphaltene pyrrole (APY)

1.39

96.509

3

Quinolinohophane (QHP)

1.18

25.231

In case of the TMBO molecule, even though the partial charge on Oxygen heteroatom is 0.450e, which is more than the partial charge on heteroatom of APY molecule, it is not observed at oil-water interface and remain soluble inside the dodecane phase. We believe that the reason behind this behaviour of TMBO is due to steric crowding near the heteroatom. The molecule has archipelago type structure, with the single aromatic ring with 5 substitutions and ether Oxygen linkage is attached to this aromatic ring. Since this Oxygen atom is connected to another alkyl chain with methyl (CH3) substitution, chances of steric crowding near Oxygen is more when compared ACS Paragon Plus Environment

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Energy & Fuels

with the APH, APY and QHP molecules, where heteroatom is connected to the aromatic core or is the direct part of the aromatic core. In these cases, rigidity of aromatic core plays a crucial role and heteroatom can easily form HB with water. To confirm this hypothesis we evaluated RDF and coordination number (CN) for the heteroatom of TMBO molecules with respect to 4 methyl groups (identified as A to D in Figure 2f). The RDF between two atomic site α and β is defined as,67 𝑔𝛼𝛽 (𝑟) = 𝑁𝑥

1

𝑁

𝛼 𝑥𝛽

𝑁

𝛽 𝛼 ∑𝑗=1 ⟨∑𝑖=1 𝛿(𝑟 − 𝑟𝑖𝑗 )⟩ , 𝜌

where xi and Ni are the mole fractions and number of atoms of type i, respectively, N is the total number of atoms, and ρ is the overall number density. The cumulative coordination number profile is obtained by integrating RDF over spherical volume as, 𝑟

𝑛𝛼𝛽 (𝑟) = 𝜌𝛽 ∫0 𝑔𝛼𝛽 (𝑟) 4 𝜋𝑟 2 𝑑𝑟

,

where 𝜌𝛽 denotes the number density of atom type β. From the Figure 2f, it is evident that methyl groups A and B are very close to Oxygen heteroatom of TMBO having RDF peak located at 0.285 and 0.245 nm respectively (Figure 6a,b). We also calculated RDF between Oxygen and terminal C and D methyl group to ascertain any folding of molecules. We observe the RDF first peak is located at 1.3 nm indicating their presence far away from Oxygen atom of TMBO molecule (Figure 6c,d). Presence of two bulky methyl groups very close to Oxygen of TMBO confirms the hypothesis of steric crowding near heteroatom and its inability to form HB with water molecules and hence remain at the oil-water interface. (a)

(b)


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(d)

(c)

Figure 6: (a – d) Radial distribution function (black, left axis) and cumulative coordination number profile (red, right axis) evaluated between Oxygen atom of TMBO molecule and intramolecular methyl (CH3) group A – D respectively. The first peak of RDF in a and b subfigure in the range of 0.2-0.3 nm indicates crowding, whereas the absence of peak around this location in c and d subfigure indicates the absence of folding in TMBO molecules.

Table No. 2: The interfacial tension of dodecane-water biphasic system with and without asphaltene molecules present at the oil-water interface Asphaltene molecules

Interfacial Tension, mN/m

Without asphaltene

55.40 ± 0.7 [ 52.3468, 57.2369 ]

APH

52.29 ± 0.3

APY

52.94 ± 0.7

QHP

53.14 ± 0.4

ATH

53.94 ± 0.8

TIR

55.37 ± 0.7

TMBO

55.49 ± 1.2 Number in square bracket indicates literature values.


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Energy & Fuels

3.4 Oil-water interfacial tension We also calculated the oil-water interfacial tension to ascertain the role of each individual (𝑃𝑥𝑥 +𝑃𝑦𝑦 )

asphaltene molecule. From the simulation, the interfacial tension was obtained as 𝛾 =

−𝑃𝑧𝑧

2

2𝐴

,

where 𝑃𝑥𝑥 , 𝑃𝑦𝑦 and 𝑃𝑧𝑧 are the average pressure in x, y and z-direction respectively. The obtained interfacial tensions for all the systems with and without asphaltene molecules are given in Table 3. For the pure oil-water system (without asphaltenes) we obtained an interfacial tension of 55.40 ± 0.7 mN/m at 300 K, which is in agreement with the values reported in literature. Zeppieri and coworkers68 have reported the interfacial tension of 52.34 mN/m from experimental studies, while Xiao and coworkers69 reported value of 57.23 mN/m from molecular simulations using similar potential parameters. When APH, APY and QHP molecules are present at the interfaces, there is a decrease in the interfacial tension as compared to that of a pure dodecane-water system; since these asphaltene molecules are adsorbed at the interface of dodecane and water. In case of APH molecule, we observed a maximum decrease of around 6% from the pure oil-water system, while both APY and QHP showed a similar decrease of approximately 4% as compared to that of a pure oil-water system. The reason, we believe, is due to the formation of more number of HB between APH and water molecule, as compared to APY and QHP molecules (Table 2). The decrease in interfacial tension in the presence of asphaltene molecules at the dodecane-water interface is not very significant because only a single asphaltene molecule is present at the interface. Interfacial tension for systems with TIR, TMBO and ATH molecules is similar to the interfacial tension of a pure oilwater system with the deviation in the range of 0.05 – 2.6%.

4. SUMMARY AND CONCLUSION Formation of the crude oil-water emulsion is one of the major techno-commercial problems for oil and gas industry. These emulsions are often stabilized by asphaltene molecules. In this manuscript, we have investigated the structural behaviour of six model asphaltene molecules having


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Page 22 of 26

different structure (island and archipelago) and heteroatoms (Sulphur, Nitrogen and Oxygen atom) in the oil-water system using molecular dynamics simulations. We found that only three asphaltene molecules; Asphaltene-pyrrole (APY), Asphaltene-phenol (APH) and Quinolinohopane (QHP) remain at the oil-water interface while the remaining three asphaltene molecules; Asphaltenethiophene (ATH), Thio-isorenieratane (TIR) and Trimethylbenzene-oxane (TMBO) are soluble in dodecane oil phase. Asphaltene molecules at the oil-water interface are stabilized by the strong hydrogen bonds (HBs) formed between water molecules and heteroatoms having average HB energy of around 50 kJ/mol. The asphaltene molecules are oriented at the oil-water interface in such a way that the interaction between the heteroatom and water phase is maximized. Overall orientations of asphaltene molecules are either perpendicular to the interface or parallel to the interface. Even at infinite dilution, presence of single asphaltene molecule at the interface leads to decrease in interfacial tension as compared to that of a pure dodecane-water system. This confirms the theory of decrease in interfacial tension by the presence of asphaltene molecules in the crude oil system. The ATH and TIR asphaltene molecules which remain soluble in oil phase have sulphur atom as the heteroatom, which has lower hydrophilicity due to which, it does not form strong interactions with the water molecules. In case of TMBO molecule, the crowding of methyl group near the oxygen heteroatom results in steric repulsion, which reduces the interaction with water phase, and hence the TMBO molecule does not stay at the oil-water interface. This detailed understanding of the behaviour of asphaltene molecules would be useful to circumvent the stability issue of oil-water emulsions in crude oil exploration. REFERENCES (1)

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Structural Behavior of Isolated Asphaltene Molecules at the OilâWater

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