Microscopic Mechanisms for the Dynamic Wetting of a Heavy Oil

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Microscopic Mechanisms for the Dynamic Wetting of a Heavy Oil Mixture on a Rough Silica Surface Qi Meng,†,‡ Daoyi Chen,†,‡ and Guozhong Wu*,†,‡ †

Division of Ocean Science and Technology, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China School of Environment, Tsinghua University, Beijing 100084, China

‡

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S Supporting Information *

ABSTRACT: Molecular dynamics (MD) simulations and optical contact angle measurements were performed to shed light on the microscopic mechanisms for the dynamic wetting of heavy oil on rough silica surfaces. The surface wetting process was characterized by interaction energy, wettability transition, density profile, and penetration of each oil fraction, while competitive adsorption between oil and water droplets on rough surfaces was also investigated. Results highlighted the role of asphaltenes on the distribution and penetration of resins and aromatics, which might become either a driving force or mass transfer resistance for the wettability transition on rough surfaces. The competitive adsorption process was divided into three steps including water bridge formation, water penetration, and oil penetration. Water penetration promoted oil penetration by providing a channel for asphaltene invading. The overall tendency of simulation results agreed with experimental observations. To our knowledge, this was the first attempt to gain atomistic insights into the heavy oil adsorption on rough mineral surfaces. Future works should focus on the interactions between heavy fractions of crude oil and a rough mineral surface, while oil mixture models involving asphaltenes should be encouraged in MD simulations for better interpreting the adsorption of oil contaminants in soils. wetting transition rate of an oily fluid on a rough surface was calculated using the forward flux sampling method by Savoy and Escobedo.11 The effects of surface topography on the driving forces for the conformational change of hexadecane on alumina surfaces were reported by Xie et al.12 Most recently, we also modeled the dispersion, immersion, and extraction of dodecane on silica surfaces with varying roughness.13 There are still numerous issues to address before accurately describing the dynamic wetting and adsorption of oil on rough minerals. For example, the heavy and polar fractions of crude oil (e.g., asphaltenes and resins) were not taken into account in the above studies. Isolation of light fractions from the host oil phase may lead to deviant results by ignoring their interactions with heavy fractions. It has been evidenced that the heavy fractions like asphaltenes have properties analogous to surfactants.14,15 Asphaltenes also have profound effects on altering the wettability of solid surfaces, because their adsorption on hydrophilic surfaces is irreversible and the asphaltenecontaminated solids remain hydrophobic even after washing with good solvents such as toluene.16 On an ideal smooth quartz surface, a conceptual oil distribution model was proposed that the aromatics and saturates transported randomly into and out of the complex consisting of asphaltenes surrounded by resins.17

1. INTRODUCTION Oil adsorption and wetting on a natural mineral surface are critical for the fate and transport of petroleum contaminants in the soil environment. The macroscopic kinetics and thermodynamics of oil adsorption on soil minerals have been extensively studied during past decades. There is growing interest in the microscopic mechanisms for the dynamic wetting of oil on rough surfaces, because even a macroscopically flat surface has microscale roughness and the surface structure determines the dynamic wetting.1 Although essential progress has been made in understanding the wetting of smooth surfaces, information on the dynamic spreading and penetration of oil on rough surfaces remains limited. Generally, liquids can exhibit the Cassie state by suspending on the microstructures or Wenzel state by penetrating into the microstructures of a rough surface.2,3 Previous studies demonstrated that these two states might coexist or transit under external factors such as gravity, pressure, surface roughness, electric field, bouncing of droplets, evaporation of droplets, and vibration of droplets.4 For example, the water droplet in either the Wenzel or Cassie state depended on the height, spacing, and shape of the pillars.5−8 The water droplet was often used in previous studies, while only a few scattered studies were focused on the oil wetting on a rough surface. For example, the spreading of silicone oil on glass surfaces and silicon substrates with different degrees of roughness was tested by Cazabat and Stuart9 and Yuan and Zhao,10 respectively. The © XXXX American Chemical Society

Received: June 18, 2018 Revised: September 1, 2018 Published: October 1, 2018 A

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Figure 1. Initial configuration of the oil droplet (a) without or (b) with a water droplet on the rough silica surfaces. The surface structure was varied by changing the spacing and depth of the grooves, as shown in panel c. For example, D1H2 represents that the groove is 0.843 nm deep and the spacing between each two grooves is 0.806 nm (red, asphaltenes; green, resins; blue, aromatics; gray, saturates; pink, water).

nonbond parameters for the silica surface are listed in Table S2, which were obtained from the ClayFF force field.20 The oil mixture was modeled by mixing 21 saturates (C20H42), 18 aromatics (C46H50S),21 12 resins (C49H78S),22 and 6 asphaltenes (C54H65NO2S).22 The mass ratio of the SARA (saturate, aromatic, resin, and asphaltene) fractions was about 15:30:35:20, which was designed according to the ratio in the real heavy oil sample in our previous experiments.23,24 Threedimensional structures of these molecules were created using the Material Studio software (Figure S2). The coordinate files were exported and input into the automated server PRODRG.25 The atom types, atom charges, charge groups, bonds, angles, and dihedrals in the resulted topology files were manually adjusted according to the analogue structures in the CHARMM36 force field library. These molecules were then randomly located in a simulation box (8.0 nm × 8.0 nm × 8.0 nm), which spontaneously formed an oil droplet (diameter: ∼4.5 nm) after running a MD simulation (10 ns) at 300 K under 3D periodic boundary conditions. 2.2. Molecular Dynamics Simulation. MD simulations were performed using the Gromacs software (version 5.0.5).26 The oil adsorption process was investigated by placing the oil droplet on the top of the silica surface (Figure 1a). To investigate the competitive adsorption of water and oil, a water droplet was also located aside (Figure 1b). The systems were initialized by minimizing the energy to 1000 kJ mol−1 nm−1 with the steepest descent approach. MD simulations were performed at the NVT ensemble (constant number of atoms, volume, and temperature) for 20 ns with a time step of 1 fs. Temperature (300 K) was controlled using the V-rescale thermostat method.27 The cutoff distance was fixed at 1.5 nm for the short-range van der Waals and electrostatic pairwise calculations, while the long-range electrostatics was dealt with the particle mesh Ewald summation.28 The periodic boundary condition was applied throughout the simulations. Trajectories were saved, which wereoutput at 0.1 ns and 0.01 ns intervals, respectively, for the system with oil droplet and the system with both oil and water droplets.

However, it remains unclear how different oil fractions distribute on rough surfaces and how the intermolecular interactions influence the spreading and penetration dynamics of each fraction. Another issue of particular interest is the competitive adsorption between a water and oil mixture on rough surfaces. On the oil-wetted surface, water is able to penetrate under electrostatic interaction, form a water channel under H-bonding interaction, form a surface gel layer through water diffusion, and finally result in the oil strip from a solid surface.18 On the waterwetted surface, a two-step adsorption process was proposed that the polar oil components preferentially absorbed on the mineral surface by displacing the occupied water molecules, which then induced the adsorption of apolar components.19 Future works are required to verify the competitive adsorption mechanisms on rough surfaces in the presence of heavy fractions of crude oil. To address the above issues, we performed molecular dynamics (MD) simulations and optical contact angle measurements to gain insights into the heavy oil wetting process on silica surfaces with varying roughness. Competitive adsorption was also investigated by quantifying the adsorption energy, wettability transition, density profile, and penetration dynamics of both oil droplet and water droplet on the rough silica surface.

2. METHODOLOGY 2.1. Molecular Model. The quartz (1 0 0) surface was produced using a unit cell (a = b = 0.4913 nm, c = 0.5405 nm, a = b = 90°, c = 120°). It was replicated to form a larger surface and then converted to a 3D cyclic cell. The dangling oxygen atoms on the quartz surface were saturated with hydrogen atoms. To build rough quartz surfaces, rectangular grooves were periodically created by manually removing the undesired atoms and protonating the exposed oxygen atoms. A total of 15 rough surfaces with different spacing and depth were generated (Figure 1). More details of surface construction are available in the Supporting Information (Figure S1). Dimensions of the simulation boxes are shown in Table S1. The bond and B

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The Journal of Physical Chemistry C 2.3. Data Analysis. The surface roughness was defined as the deviations in the direction of the normal vector of a real surface from its ideal form (i.e., absolutely smooth surface). Accordingly, the roughness (R) was calculated by the ratio of the surface area of a rough surface to that of a smooth surface. The apparent contact angle was calculated by the radius and height of the oil droplet on the silica surface,29 which was determined by the relative concentration of oil molecules along the X and Z directions (see details in Figure S3). To quantify the penetration capability of each oil fraction on the silica surface, the amount of oil molecules entering the grooves was characterized by counting the number of oil molecules with half of the atoms inside silica grooves.13 The interaction energies were calculated using the standard modules of Gromacs. The self-diffusion coefficient was computed using the Einstein equation.30 2.4. Instrumental Analysis. The contact angles of oil on a silica surface were characterized using the optical contact angle analyzer (KRUSS DSA25). Heavy crude oil was sampled from the Jianghan oilfield of China, while light mineral oil was purchased from Sigma-Aldrich. SiO2 samples were purchased from Aladdin reagent. The SiO2 substrates were prepared according to Lebedeva and Fogden.31 Briefly, an aqueous suspension was prepared by mixing 20 g of SiO2 with 100 g of deionized water under rapid magnetic stirring. After sonication for 10 min, NaOH was added to adjust the pH to 9.8. The suspension was centrifuged at 160g for 3 min, after which the suspended upper phase was decanted to remove the sediment. It was then rapidly stirred and sonicated for 10 min. Suspensions were pipetted as several drops onto a precleaned microscope glass slide and manually spread over the glass surface. The particles were immobilized using a heat gun (∼120 °C) to sweep in one direction over the wet surface. The treated surface was further oven-dried at 70 °C.

silica surface decreased with the spacing between grooves providing the same groove depth (with one exception for the shallowest grooves). However, a clear relationship was not observed between the interaction energy and the depth of silica grooves. Transition of Oil-Wetting Mode. Snapshots of the oil molecules on the rough silica surfaces at the end of simulation are shown in Figure 3. It indicated that the contact angles on the rough surface (65−121°) were larger than that on the smooth surface (62°, Figure S6a), but it did not vary monotonically with the surface roughness. Additionally, it decreased with the spacing between grooves providing the same groove depth and increased with the groove depth providing the same groove spacing. An important finding was the sharp increase in the contact angle when the groove depth increased to 1.7 nm in the relatively narrow grooves (Figure 3d and i). The oil-wetting mode started to transit from Wenzel to Cassie after increasing the roughness of the silica surface (Figure 3e and i). The phenomenon of wetting to dewetting transition due to roughness changes was first reported by Pandey and Roy,5 which modeled the water-wetting process and demonstrated the possibility of hydrophilicity decrease by changing the surface structure. However, the oil-wetting mode transition was not observed in our previous study where only saturates were present on the rough silica surface.13 The above findings suggested that it was the asphaltenes that led to the wettability alternation on the rough surface. To confirm this, we removed the asphaltene molecules during the model construction and ran the simulation following the same procedures. It demonstrated that the oil-wetting mode became the Wenzel state again when the asphaltenes were absent in the oil droplet (Figure 4). Moreover, the oil-wetting mode transition after the roughness changing was not found on the relatively wide grooves where the contact angle ranged from 65 to 70° (Figure 3k−o). Density Profile of Oil Fractions. The density profiles of each oil fraction in each scenario are shown in Figure 5. It clearly demonstrated that saturate was the fraction distributed most closely to the bottom of the silica surface, which was evidenced by the first narrow and sharp peaks at 1.4−3.5 nm. The peaks for resins and asphaltenes appeared very similar, suggesting that these two fractions were most bundled to each other. This finding agreed with experimental observations that resins were tightly coated on the surface of asphaltenes.32,33 These two fractions were located relatively far from the silica surface, while aromatics were located between saturates and the asphaltene− resin complex. It was interesting to compare the above oil distribution pattern with that reported in the literature. Early studies supported that the conventional crude oil was a colloid system with a complex blend of oligomeric hydrocarbons. According to the conceptual model proposed in our previous studies,17,33 the SARA fractions on a sand surface were neither distributed homogeneously in the oil layer in the form of a colloid system nor distributed successively from the inner to outer surface of the sand. This was consistent with the present study. However, previous studies also proposed that the content of asphaltene−resin complexes decreased from the inner to outer of the oil layer on the sand surface,17,33 which was different from the findings in this study (Figure 5). It was attributed to the fact that the ideally smooth sand surface or the sand surface saturated with a water film was previously used, where the dynamic penetration of SARA fractions in the microscopic grooves of rough surfaces was not taken into account. The varied distribution pattern resulted from the different penetration

3. RESULTS AND DISCUSSION 3.1. Adsorption of an Oil Droplet on a Rough Silica Surface. Energy of Oil Adsorption. Results showed that the fluctuation of the running average of temperature and potential energy was very slight, indicating that the systems reached equilibrium stage (Figures S4 and S5). Figure 2 shows the nonbonded interaction energy between oil and silica. It indicated that the van der Waals interaction accounted for 53−85% of the binding energy on rough silica surfaces, while less than 46% of the interactions were contributed by the Coulomb electrostatic force. The adsorption strength between the oil and

Figure 2. Interaction energy between silica and oil during the adsorption process. C

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Figure 3. Snapshots of oil droplets on rough silica surfaces with different structures. The oil droplet contains saturates (gray), aromatics (blue), resins (green), and asphaltenes (red). R and θ represent the roughness and oil−silica contact angle, respectively.

increased to 83%. This suggested that the oil penetration was determined by its interactions with both silica and asphaltenes. For saturates, the adsorption strength from the silica surface was more than twice of that from asphaltenes (Figure 7a). Accordingly, saturates had the strongest penetration capability independent of the asphaltenes’ transport (Figure 6). For the aromatics and resins, the adhesion from asphaltenes obviously outperformed that from the silica surface (Figure 7b and c), which resulted in the dependence of their distribution on the asphaltenes. Two distinct examples were the D2H4 and D3H4 silica surfaces. On the D2H4 surface, aromatics and resins were not observed inside the grooves (Figure 6b). The inhibited penetration was attributed to the strong binding from the asphaltenes outside of the grooves (Figure 3i). Results showed that up to 29% aromatics and 21% resins successfully penetrated into the grooves if asphaltenes were absent in the oil droplet (Figure 4). On the contrary, the binding to asphaltenes became a driving force for the penetration of aromatics on the D3H4

capacity of different oil fractions on the unsaturated rough surface, which would be discussed in the following subsection. Penetration of Oil Fractions. Results indicated that more than 65% of the saturate molecules entered the silica grooves with only one exception when the groove depth was 0.4 nm which was too shallow to accommodate more oil molecules (Figure 6). By contrast, the percentages of the other three oil fractions entering the grooves were relatively low. For example, none of the aromatics, resins, or asphaltenes moved into the grooves when the groove spacing was 0.8 nm irrespective of the groove depth (Figure 6a). An interesting finding was that the penetration capability of the aromatics and resins highly relied on the movement of asphaltenes. For instance, the percentage of the former two fractions penetrating into the grooves was less than 30 and 15%, respectively, when no more than 17% asphaltenes entered the grooves. When the percentage of asphaltenes inside the grooves increased to 33%, the corresponding percentage of aromatics D

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Figure 4. Snapshots of oil droplet penetration on the D2H4 silica surface. The oil droplet contains 25 saturates (gray), 21 aromatics (blue), and 14 resins (green). Figure 6. Percentage of oil fractions inside the silica grooves.

3.2. Adsorption of Oil and Water Droplets on a Rough Silica Surface. To investigate the adsorption process of an oil droplet in the presence of a water droplet, the silica surfaces with the deepest grooves were selected for simulation (i.e., D1H5, D2H5, and D3H5). The final snapshots are shown in Figure 8a−c, while the adsorption of a water droplet on the smooth silica surface is also shown in Figure S7. Overall, the presence of a water droplet obviously decreased the oil−silica contact angle at the end of the simulation (Figure 8d). Especially on the D1H5 and D2H5 surfaces, the contact angle declined from about 120 to 60°. The transition from the Cassie heterogeneous to the Wenzel homogeneous wetting occurred. Such a tendency was validated by instrumental analysis where the measured contact angle of heavy crude oil on silica surfaces decreased from about 102.5 to 78.6° after the water droplet was added (Figure 9a−c). However, this change was not observed for the light mineral oil; instead, some slight increase was noted for the contact angle (Figure 9g−i). It was inferred that these findings might be attributed to the interactions between water molecules and the polar fractions of oil. For better interpreting the dynamic process, an example is shown in Figure 10, which demonstrated that the adsorption process could be divided into three stages as follows: Fast Formation of a Water Bridge (0−0.05 ns). As moving toward the silica surface, obvious deformation was observed for the water droplet which fast formed a tower-shape bridge with the bottom skirt standing on the top of two pillars (Figure 10a− d). This was a very fast process lasting for only 50 ps, when a small portion of water molecules immersed into the grooves. During this stage, the oil droplet was not deformed, but it rotated toward the water bridge using the pillar as a fulcrum until becoming tangent to the water bridge.

Figure 5. Relative concentration of saturates (black), aromatics (red), resins (blue), and asphaltenes (cyan) along the normal direction of the silica surface.

surface where more asphaltenes entered the silica grooves (Figure 6c). E

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bonds between 10 and 13 ns. A closer examination of the molecular configurations indicated that the top asphaltene dimers moved toward the silica surface along the water bridge, while the bottom asphaltene aggregates consisting of four monomers became bended and inserted into the grooves along the direction of water penetration (Figure 13). Oil Penetration (13.14−20 ns). This process was similar to the cases without a water droplet. Following the penetration of saturates, the asphaltenes entered the grooves and then drove the movement of resins. The difference was that saturates were more tightly combined with the oil droplet when the water droplet was present, although the total amount of saturates penetrating into the grooves was not obviously changed. To confirm this, we calculated the distance between the center of mass of the saturate molecules and that of the remaining oil molecules. It indicated that the corresponding distance was about 9.8% less when the water droplet coexisted with the oil droplet (Figure 12). This process also resulted in a 27−71% decrease in the self-diffusion coefficient of saturates (Figure 8e). It should be noted that the water bridge formed at the first step was attributed to the attractions between the bottom and top silica surfaces due to the periodic boundary conditions (top surface not shown). To confirm this, we repeated the simulation by setting the z-axis dimension of the simulation box (i.e., the distance between the top and bottom silica surfaces) as 20, 27, 35, and 40 nm, respectively. The water bridge was observed in all of these systems, suggesting that such a finding was not an accidental phenomenon. As expected, the water bridge disappeared when the z-axis dimension increased to 100 nm (Figure S8). Results further demonstrated that it did not change the final results after increasing the z-axis dimension. In other words, the oil droplet deformation and penetration process did not depend on whether or not the water bridge was initially formed. To support this, we increased the z-axis dimension from 27 to 100 nm and then compared the corresponding oil configuration (Figure S9), water distribution (Figure S10), diffusion coefficient of saturates (Figure S11), and distance between saturates and other oil components of the oil droplet (Figure S12). The only difference observed between these two cases was that the water penetration time decreased from 13 to 4.3 ns after increasing the z-axis dimension (Figure S9). The above findings demonstrated the priority of adsorption for water droplets on silica surfaces, which was attributed to the 2 or 3 order of magnitude larger nonbonded interaction energy

Figure 7. Oil−silica and oil−asphaltene interaction energy.

Water Penetration (0.05−13.14 ns). The water bridge started to shrink, and all of the water molecules entered grooves along the pillars. Finally, the water molecules were closely adhered to the interior wall of the silica surface, leaving little in the central channels, which was especially true in the grooves with wide spacing (Figure 11b and c). The oil droplet appeared as the Cassie state above between two pillars. During this stage, the most pronounced phenomenon was the changes in the number of hydrogen bonds between asphaltenes and water. As shown in Figure 8f, strong peaks were found in the hydrogen

Figure 8. Snapshots of oil droplet (a−c), contact angle of droplet (d), diffusion coefficient of saturates (e), and number of H-bonds between asphaltenes and water on rough silica surfaces with water droplet (f) (red, asphaltenes; green, resins; blue, aromatics; gray, saturates; pink, water). F

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Figure 9. Snapshots of the oil and water wetting process on silica surfaces obtained by optical contact angle measurement: (a) final state of heavy oil, (b) adding water droplet besides heavy oil, (c) final state of water besides heavy oil, (d) final state of water, (e) adding heavy oil above water, (f) final state of heavy oil above water. Processes g−i are the same as a−c, while processes j−l are the same as d−f, but light mineral oil is used in g−l.

Figure 10. Snapshots of the adsorption process of an oil droplet in the presence of a water droplet on a D2H5 surface (red, asphaltenes; green, resins; blue, aromatics; gray, saturates; pink, water).

between water and silica than that between oil and silica (Table 1). Energy analysis further demonstrated that water adsorption was mainly driven by the Coulomb electrostatic interactions,

while the oil adsorption was dominated by the van der Waals forces. Moreover, the above results highlighted that the water penetration process paved the way for the penetration of G

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Figure 11. Configurations of water molecules (blue) inside the silica grooves (vertical view). Oil molecules are hidden for clarity.

Figure 12. Distance between saturates and other oil components of the oil droplet.

Figure 13. Snapshots of oil droplets on D2H5 silica surfaces in the presence of water droplets. Saturates, aromatics, and resins are hidden to highlight the structure of asphaltenes and water (blue, nitrogens; red, oxygens; yellow, sulfurs; white, hydrogens; cyan, carbons; pink, water).

asphaltenes, which eventually enhanced the penetration of resins and favored the overall wetting process of heavy oil. However, it did not necessarily mean that the adsorption of heavy oil would increase with the water content. For example, our experimental

results showed little difference in the contact angle when the heavy oil was added on the top of the water film which was formed by the spreading of the preloaded water droplet on the silica surfaces (Figure 9d−f). This might be due to the fact that H

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Table 1. Nonbonded Interaction Energies (Etotal), van der Waals Energies (Evdw), and Electrostatic Energies (Ecoul) among Species oil−silica (kJ mol−1) D1H5 D2H5 D3H5

water−silica (kJ mol−1)

Ecoul

Evdw

Etotal

Ecoul

Evdw

Etotal

Ecoul

Evdw

Etotal

−421.5 −233.2 −85.5

−879.4 −495.2 −345.9

−1301.2 −728.5 −431.5

−111056 −131540 −160924

17757.6 21462.8 27970.4

−93298.4 −110077.0 −132954.0

8.2 −57.5 −43.0

−410.1 −329.6 −325.9

−401.8 −387.1 −368.9

ORCID

the microscopic grooves on the surface were already fully occupied by the water prior to oil adsorption.

Daoyi Chen: 0000-0002-6332-1958 Guozhong Wu: 0000-0002-2663-3637

4. CONCLUSIONS Results indicated that the main driving force for the adsorption of heavy oil on a rough silica surface was van der Waals interactions. The adsorption strength decreased with the spacing between grooves but did not have a clear relationship with the depth of grooves. The oil−silica contact angle did not vary monotonically with the overall roughness, but the oil wetting mode transited from Wenzel to Cassie on the surface with large roughness. Results highlighted that the wettability transition was mainly contributed by the asphaltenes, because the penetration capability and density profiles of resins and aromatics highly depended on the asphaltenes. The saturate fraction was an exception, as its interaction strength with the silica surface was more than twice of that with asphaltenes, making it fast entering the grooves after separating from the oil mixture. When water and oil droplets coexisted on the rough silica surface, the water adsorption was dominated by the Coulomb electrostatic interactions while the oil adsorption was mainly contributed by the van der Waals interactions. The presence of water decreased the oil−silica contact angle, which resulted in the oil wettability transition from the Cassie to Wenzel state. This finding was attributed to the fact that the water channel formed during water penetration paved the way for asphaltene penetration, which then promoted the movement of other oil fractions.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Science and Technology Innovation Commission of Shenzhen Municipality (No. KQJSCX20170330151956264), the Economy, Trade and Information Commission of Shenzhen Municipality (No. HYCYPT20140507010002), and the Development and Reform Commission of Shenzhen Municipality (No. DCF-2018-64).

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REFERENCES

(1) Wang, J.; Do-Quang, M.; Cannon, J. J.; Yue, F.; Suzuki, Y.; Amberg, G.; Shiomi, J. Surface Structure Determines Dynamic Wetting. Sci. Rep. 2015, 5, 8474. (2) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988−994. (3) Cassie, A.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551. (4) Bormashenko, E. Progress in Understanding Wetting Transitions on Rough Surfaces. Adv. Colloid Interface Sci. 2015, 222, 92−103. (5) Pandey, P. R.; Roy, S. Is It Possible to Change Wettability of Hydrophilic Surface by Changing Its Roughness? J. Phys. Chem. Lett. 2013, 4, 3692−3697. (6) Niu, D.; Tang, G. H. Static and Dynamic Behavior of Water Droplet on Solid Surfaces with Pillar-Type Nanostructures from Molecular Dynamics Simulation. Int. J. Heat Mass Transfer 2014, 79, 647−654. (7) Chen, J.; Chen, W.; Xie, Y.; Wang, Z.; Qin, J. Wettability Behavior of Water Droplet on Organic-Polluted Fused Quartz Surfaces of PillarType Nanostructures Applying Molecular Dynamics Simulation. Appl. Surf. Sci. 2017, 396, 1058−1066. (8) Chowdhury, S. S.; Pandey, P. R.; Kumar, R.; Roy, S. Effect of Shape of Protrusions and Roughness on the Hydrophilicity of a Surface. Chem. Phys. Lett. 2017, 685, 34−39. (9) Cazabat, A.; Stuart, M. Dynamics of Wetting: Effects of Surface Roughness. J. Phys. Chem. 1986, 90, 5845−5849. (10) Yuan, Q.; Zhao, Y.-P. Multiscale Dynamic Wetting of a Droplet on a Lyophilic Pillar-Arrayed Surface. J. Fluid Mech. 2013, 716, 171− 188. (11) Savoy, E. S.; Escobedo, F. A. Molecular Simulations of Wetting of a Rough Surface by an Oily Fluid: Effect of Topology, Chemistry, and Droplet Size on Wetting Transition Rates. Langmuir 2012, 28, 3412−9. (12) Xie, W. K.; Sun, Y. Z.; Liu, H. T. Atomistic Investigation on the Detachment of Oil Molecules from Defective Alumina Surface. Appl. Surf. Sci. 2017, 426, 504−513. (13) Zhu, X.; Chen, D.; Zhang, Y.; Wu, G. Insights into the Oil Adsorption and Cyclodextrin Extraction Process on Rough Silica Surface by Molecular Dynamics Simulation. J. Phys. Chem. C 2018, 122, 2997−3005. (14) Jian, C.; Liu, Q.; Zeng, H.; Tang, T. A Molecular Dynamics Study of the Effect of Asphaltenes on Toluene/Water Interfacial Tension: Surfactant or Solute? Energy Fuels 2018, 32, 3225−3231. (15) Jian, C.; Poopari, M. R.; Liu, Q.; Zerpa, N.; Zeng, H.; Tang, T. Reduction of Water/Oil Interfacial Tension by Model Asphaltenes:

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05784.

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oil−water (kJ mol−1)

Box size for the 15 simulations (Table S1); force field parameters (Table S2); schematic for the rough surface model construction (Figure S1); three-dimensional structure of the model oil fractions (Figure S2); schematic for the contact angle calculation (Figure S3); energy and temperature profile during molecular dynamic simulations (Figures S4 and S5); configuration of the oil droplet on a smooth silica surface (Figure S6); water distribution on smooth silica surfaces (Figure S7); snapshots of the oil and water adsorption process on the D2H5 surface (z-axis dimension = 100 nm) (Figure S8); oil configuration, water distribution, diffusion coefficient, and distance between saturates and other oil fractions on the D2H5 surface (z-axis dimension = 100 nm) (Figures S9−S12) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 0755 2603 0544. Fax: +86 0755 2603 0544. Email: [email protected]. I

DOI: 10.1021/acs.jpcc.8b05784 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b05784 J. Phys. Chem. C XXXX, XXX, XXX−XXX