Atomistic investigation on the detachment of oil molecules from defective alumina surface

Atomistic investigation on the detachment of oil molecules from defective alumina surface

Applied Surface Science 426 (2017) 504–513 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

10MB Sizes 1 Downloads 77 Views

Applied Surface Science 426 (2017) 504–513

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full Length Article

Atomistic investigation on the detachment of oil molecules from defective alumina surface W.K. Xie, Y.Z. Sun, H.T. Liu ∗ Center for Precision Engineering, Harbin Institute of Technology, P.O. Box 413, Harbin, Heilongjiang, 150001, China

a r t i c l e

i n f o

Article history: Received 28 February 2017 Received in revised form 22 June 2017 Accepted 18 July 2017 Available online 23 July 2017 Keywords: Oil molecules Surface defect Detachment Molecular dynamics simulation Water channel

a b s t r a c t The mechanism of oil detachment from defective alumina surface in aqueous solution was investigated via atomistic molecular dynamics (MD) simulations. Special attention was focused on the effect of surface defect on the oil detachment. Our simulation results suggest that compared with perfect Al2 O3 surface, defective substrate surface provides much more sites for the adsorption of oil molecules, thus it has higher oil adsorption energy. However, higher oil-solid adsorption energy does not mean that oil contaminants are much more difficult to be detached. It is found that surface defect could induce the spontaneous imbibition of water molecules, effectively promoting the detachment of oil molecules. Thus, compared with perfect alumina surface, the detachment of oil molecules from defective alumina surface tends to be much easier. Moreover, surface defect could lead to the oil residues inside surface defect. In water solution, the entire detachment process of oil molecules on defective surface consists of following stages, including the early detachment of oil molecules inside surface defect induced by capillary-driven spontaneous imbibition of water molecules, the following conformational change of oil molecules on topmost surface and the final migration of detached oil molecules from solid surface. These findings may help to sufficiently enrich the removal mechanism of oil molecules adhered onto defective solid surface. © 2017 Published by Elsevier B.V.

1. Introduction Detachment mechanism of oil molecules from solid surface is of significance in many application fields, such as detergency [1], enhanced oil recovery(EOR) [2], high-power laser facility [3,4]. Deeply understanding the microscopic detachment process of oil molecules occurring at the oil/water/solid interface is often key in further improving these important industrial processes and providing fundamental physical insights into detachment mechanism of oil molecules in aqueous solution. An important feature of solid substrate surface should be further considered. That is, the machined solid surfaces obtained by various kinds of machining methods, such as precision milling [5], laser machining [6], and polishing, et al. are rarely perfect and defect-free. It has been extensively observed in many investigations that surface defect has evident influence on the adsorption behavior of various molecules on different solid surfaces [7–12]. For instance, by using QCM technology, Wu et al. experimentally found that surface roughness greatly affected the total adsorption amount and the adsorption kinetics of CTAB molecules [7]. From the theory

∗ Corresponding author. E-mail address: [email protected] (H.T. Liu). http://dx.doi.org/10.1016/j.apsusc.2017.07.163 0169-4332/© 2017 Published by Elsevier B.V.

and simulation point of view, Maria et al. investigated the effect of point and line defects on the structure and aggregation kinetics of SDS and dodecane molecules on a graphite surface [8,9]. It was demonstrated that vacancies on substrate surface could interface with the aggregation formation of SDS and dodecane molecules, while line defects are capable of localizing and orienting the aggregates of surfactant molecules. Wu and Song et al. studied the effect of various surface structures, including reduced groove, pit, and step et al. on the adsorption behavior and dynamics of RGD tripeptides onto the rutile TiO2 (110) surface in aqueous solution [10,11]. They noted that all of these surface defects could provide more sites for the adsorption of RGD molecules. Claudio Melis et al. found that the adhesion of poly(3-hexylthiophene) on nanostructure titanium surface is easily affected by the local morphology and surface curvature of substrate surface [12]. All of these researches clearly indicate that taking the effect of surface defects on solid surface into account is of significance for deeply revealing the interfacial interactions at aqueous/solid interface. Thus, the presence of surface defect on solid surface is believed to have great effect on the detachment of oil molecules. Better understanding the effect of surface defect on the detachment of oil molecules is important for many practical applications. Moreover, most of surface defects on the processed surface are micro-scale and even nano-scale. Owing to the restriction of experimental

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

505

Fig. 1. Surface topographies of defective alumina surface using vdw spheres (a) and quick surface styles (b).

methods, it remains to be a challenge to directly observe the effect of surface defect, particularly those nanoscale defects, on the dynamic detachment process of oil molecules from defective surface. Computational simulations, including molecular dynamics (MD), Monte calro (MC) and dissipative particle dynamics (DPD) simulations, have become powerful tool to analyze the microscopic dynamics of oil molecules adsorbed to solid surface in aqueous solution environment has been the subject of many studies [13–15]. For instance, in Wang and co-authors’ MD study on the oil detachment from solid surfaces immersed in the charged nanoparticle suspensions, it was found that the removal efficiency of oil molecules could be evidently enhanced by using nanofluids of charged nanoparticles [16]. And Zhang et al. studied the effect of substrate surface wettability on the detachment of oil droplet in water solution [17]. For these reasons, we report the MD study of the dynamic conformational change process of oil molecules on defective ␣-Al2 O3 surface in the aqueous solution and the effects of closed-end surface defect are mainly focused. To solve these problems, the present paper is organized in the following sections: The Section 2 briefly introduces the simulation models and protocols used in this study. In Section 3, the main findings and results are presented and discussed. And our conclusions are summarized in section IV. 2. Simulation details 2.1. Simulation model In this work, aluminum oxide surface was selected as the substrate surface. As is known, the oxide layer on aluminum alloy surface has evident effect the interfacial properties of aluminum surface [18,19]. To obtain the precision processed aluminum surface with high surface cleanliness, we investigate the microscopic removal detachment of those oil molecules adsorbed to defective Al2 O3 surface. Al2 O3 substrate surface was simulated by using the (0001) crystallographic face of ␣-Al2 O3 (space group). Many stud-

ies have been conducted to analyze the interfacial issues occurring at ␣-Al2 O3 (0001) surface, such as interfacial water, adsorption of phenolic compounds [20,21]. In the present simulations, the ␣-Al2 O3 (0001) substrate surface was placed in x-y plane and its xy-size is 5.7701 × 5.7108 nm2 . Meanwhile, the defective ␣-Al2 O3 (0001) surface was constructed via building surface defects on perfect solid surface. The rectangular pit was adopted to represent surface defect. The surface topography of defective alumina surface is shown in Fig. 1. As for oil molecules, similar to our previous work [22], they are simulated by hexadecane molecules (C16). To clearly elucidate the effect of surface defect on detachment process of oil molecules in the water solution, two groups of simulation systems consisting of oil/solid and oil/water/solid systems were established. Group I was designed to indicate the reproducibility of MD simulation results. A defective Al2 O3 surface with a rectangular pit defect of the size 20.2 × 24.2 × 6.485 Å3 was established by removing 140 Al atoms and 210O atoms from perfect Al-terminated Al2 O3 (0001) surface. Then the defective Al2 O3 surfaces contaminated by 40, 60 and 80 C16 molecules were respectively prepared, according to Liu’s method [23]. Further, the oil/water/solid systems, which were applied to visualize the microscopic details in the detachment process of oil molecules, were obtained by adding 5508 water molecules over those contaminated defective surfaces and they are labelled to C16-40-groove2, C16-60-groove2 and C16-80-groove2, respectively. The construction procedure of simulation systems in Group I was also adopted to prepare other systems, which were involved in our present work. Group II was used to analyze the effect of geometry parameters, such as defect depth and cross-sectional area, on the spontaneous conformational dynamics of oil molecules. More details of the established oil/water/␣-Al2 O3 systems are listed in Table 1. 2.2. Simulation protocol In simulations, CHARMM [24] force field was adopted to adequately model the oil molecules and the CLAY [25] force field was applied to describe the interactions involving the Al2 O3 atoms. Sim-

Table 1 Model parameters of oil/water/␣-Al2 O3 systems.

Group I

Group II

ID

Dimension/A´˚ 3

Groove/A´˚ 3

Oil

Water

C16-40-groove2 C16-60-groove2 C16-80-groove2 C16-80-groove1 C16-80-groove3 C16-80-groove4 C16-80-groove5

57.687 × 57.108 × 26.50

24.2 × 20.17 × 12.970

40 60 80

5504

57.687 × 57.108 × 31.993 57.687 × 57.108 × 51.480

24.2 × 20.17 × 6.4850 24.2 × 20.17 × 19.455 24.2 × 20.17 × 25.940 24.2 × 20.17 × 32.425

506

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

Fig. 2. Conformation snapshots of contaminated defective ␣-Al2 O3 surfaces at different coverages of oil molecules: (a) coverage1, (b) coverage2, (3) coverage3. Oil molecules and ␣-Al2 O3 surface are shown by CPK mode. Color schemes: substrate oxygen (Os ), red; substrate aluminum (Als ), green; carbon, cyan; hydrogen, white. Part of substrate atoms are omitted for clarity.

ple point-charge (SPC) model was used to describe water molecules [26]. All simulations for the detachment of oil molecules on defective Al2 O3 surface in aqueous solution were carried out by using the LAMMPS free software package [27]. The total potential energy consists of bond stretching, angle bending, torsion, and nonbonded interactions. The nonelectrostatic interactions between system atoms were modeled by using the Lennard-Jones potential, and the van der Waals interactions between different atom species were calculated by the standard geometric mean combination rules. A simulation box with the size of 5.7701 × 5.7108 × 10.65 nm3 was used for the construction process of contaminated defective Al2 O3 surfaces and the following simulations for removal process of oil molecules adsorbed to defective surface in aqueous solution. Each oil/water/solid system was performed by the following strategy: Firstly, the simulation systems were minimized for 10,000 steps via a steepest descent algorithm, which is conducted to eliminate the possible overlapping in the initial configurations. Then a relaxation process for only water molecules were carried out for 50 ps at 300 K to randomize their positions, while oil molecules and defective Al2 O3 surface were fixed to their initial positions. Finally, the restraints on oil molecules were removed and the simulations for detachment of oil molecules were started. The MD simulations of oil/water/solid systems were performed at T = 300 K in canonical ensemble (NVT) by using the Nose-Hoover thermostat with a coupling time constant of 0.1 ps and a timestep of 2 fs. Simultaneously, the SHAKE algorithm in LAMMPS was adopted to constrain all bonds of oil molecules and water molecules connected to H atoms [28]. The cut off radius of Lennard-Jones interactions were set to be 1 nm, and the particle-particle-particle-mesh (PPPM) solver [29] was employed to handle the long-range electrostatic interactions to minimize artifacts resulting from artificially truncating such interaction. Periodic boundary conditions were applied in both of x and y directions, while the reflecting wall was adopted in the z direction. All defective ␣-Al2 O3 surfaces are frozen over the whole simulation time. The MD simulations in the present study consist of three parts: First-stage simulations were performed to gain the contaminated defective ␣-Al2 O3 surfaces. Second-stage simulations were conducted to investigate effect of surface defect on dynamic detachment process of oil molecules with different surface coverages. Third-stage simulations were used to indicate the influences of depth and cross area of surface defect on the removal mechanism of oil molecules.

3. Results and discussion In the following sections, we report MD computational results on the microscopic details for the oil adsorption on defective alumina surface, and the detachment of oil molecules driven by the

combined interactions of defective substrate surface and aqueous solution. 3.1. Analysis for oil adsorption on defective Al2 O3 surface To clearly indicate the effects of closed-end surface defects on the oil detachment in the aqueous solution, the adsorption behavior of oil molecules onto defective alumina surface is first discussed. As shown in Fig. 2, oil molecules on the defective ␣-Al2 O3 surface are absorbed onto not only the topmost substrate surface, but also the inner surfaces of surface defect. By averaging the instantaneous values in the last 400 ps of the constructed process of contaminated Al2 O3 surface, the number density profile of oil carbon atoms as a function of vertical distance z from the top-most surface atom(z = 0) is calculated. Fig. 3(a) and (b) exhibit the effects of oil surface coverages and defect depths on the density of oil molecules, respectively. As shown, above topmost substrate surface (z > 0), the number density profile of carbon atoms of oil molecules in each system respectively Exhibits 1–3 high sharp peaks, which represent adsorption layers of oil molecules formed on ␣-Al2 O3 surface. Similarly, below the topmost substrate surface (z < 0), several sharp high peaks with lower amplitude values are also observed. It implies that oil molecules inside surface defect are also layering distributed. Additionally, it can be clearly found in Fig. 3(a) that amplitude values of sharp peaks tend to be larger with increasing of surface coverage of oil molecules. Therefore, all of oil molecules are adsorbed to defective surface layer by layer. In Fig. 3(b), for oil molecules consisting of 80 C16 molecules, the z-direction number density profiles of oil carbon atoms on defective surfaces at different defect depths were compared. As displayed, number density of carbon atoms at about 2.5, 6.7, and 11.0 Å tends to be lower, with increasing of defect depth, which also means that more oil molecules are adsorbed inside surface defect at a larger defect depth. In order to indicate the effect of surface defect on the adsorption strength of oil molecules on ␣-Al2 O3 surface, according to the following Eq. (1), the oil-Al2 O3 interaction energy (EOS ) of each system is calculated and illustrated in Fig. 4. EOS = Eoil+surface − Eoil − Esurface

(1)

where EOS is the interaction energy between oil residues and Al2 O3 surface, and Eoil+surface refers to the total energy of the molecular system consisting of oil molecules and Al2 O3 surface. Eoil and Esurface are energies of the isolated adsorbed oil molecules and the clean substrate surface, respectively. It can be found that for equal content of oil molecules, defective substrate surface has higher adsorption energy with oil molecules. As described in Li and co-workers’ study, the adsorption energy of normal alkanes on relaxed ␣-Al2 O3 surface is much higher, up to 20%, that of the unrelaxed surface [13]. It is ascribed to more

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

507

Fig. 3. z-direction number density profiles of carbon atoms on contaminated defective ␣-Al2 O3 surfaces: (a) different oil surface coverages, (b) different defect depths.

with those on perfect alumina surface, the oil molecules adsorbed to defective alumina surface tends to be much easier to be detached. Besides, one can note that parts of oil molecules are still remained inside surface defect of each system over the whole simulation, clearly indicating that surface defect could induce the residues of oil molecules. It is consistent with the finding in Chen’s DPD study [31] on the displacement of oil molecules in open-ended nano-pore. Thus, in the aqueous solution, the oil molecules on the defective Al2 O3 surface could be divided into two parts: the oil molecules located above topmost surface of substrate and those inside surface defect, which are labelled as OIL A and OIL B in the following analysis, respectively. From above analysis, surface defect has evidently affected the oil detachment from two major aspects. Firstly, surface defect could effectively promoted the detachment of oil molecules onto the topmost surface of the defective alumina surface. Secondly, surface defect could lead to the residual of oil molecules.

Fig. 4. Evolution profile of oil-Al2 O3 interaction energy versus defect depth.

adsorption sites for normal alkanes that relaxed Al2 O3 surface provides. Similar phenomenon was also observed in Zhang’s work on the effect of surface defect on peptide adsorption [30]. Therefore, it can be inferred that, compared with perfect Al2 O3 surface, defective surface could also possess more sites for the adsorption of oil molecules, thereby enhancing the adsorption strength between oil molecules and Al2 O3 surface. The objective of the present study is to clarify the effect of surface defect on the oil detachment, whether the higher adsorption energy means that oil molecules are much more difficult to be thoroughly detached from solid surface? In this work, it is interestingly observed that those oil molecules adhered to defective alumina surface tend to be easily detached, despite of higher adsorption energy. 3.2. Analysis for the final configurations of oil molecules From the MD trajectory files, it can be found that evident change of oil adsorption conformation occurs in each system, once the contaminated alumina surface is submerged into aqueous solution. Fig. 5 shows the final configurations of oil molecules on the defective alumina surface of each system. It can be observed that most of oil molecules have been detached from defective surface and one oil droplet has emerged. Differently, the oil molecules on perfect alumina surface, tends to be remained, owing to the strong interactions between oil molecules and hydrated alumina surface, which has be deeply discussed in our pioneering study. Thus, compared

3.3. Spontaneous detachment of oil molecules from defective alumina surface Fig. 6 gives the time evolution of the adsorption conformation of oil molecules in C16-80-groove2 system. It can be clearly observed that most of oil molecules onto defective alumina surface have been gradually detached by roll-up process, during the MD simulation. At about 15 ns, the oil molecules adsorbed to topmost surface have evolved into one oil droplet, as shown in Fig. 6(f). The entire process of the conformational change of oil molecules onto defective alumina surface is approximately consistent with that occurring on perfect alumina surface. Differently, parts of oil molecules have been adsorbed inside surface defect. Moreover, the adsorption conformation of oil molecules inside surface defect approximately remains unchanged since 1 ns. It indicates that the oil molecules inside surface defect are preferentially detached at the early stage. To quantitatively illustrate the detachment of oil molecules onto topmost surface and those inside surface defect, the z-direction component of the center of mass (z COM) of OIL A and OIL B in each system are respectively calculated. As illustrated in Fig. 7(a), the z COMs of OIL As tend to rapidly increase since 0 ns, implying that the adsorption conformation of oil molecules starts to drastically change, once the contaminated substrate surface is immersed into the aqueous solution. Moreover, the z COM of the OIL A in each system has largely increased by about 20 Å, clearly indicating the detachment process of oil molecules. As for OIL B, it can be seen in Fig. 7(b) that all of the z COMs of OIL Bs in different systems approximately remain unchanged, except for slight change at the

508

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

Fig. 5. Snapshots for final adsorption conformation of oil molecules on defective Al2 O3 surface: (a) C16-40-goove2, (b) C16-60-groove2, (c) C16-80-groove2. Oil molecules and Al2 O3 surface are shown by CPK mode while water molecules are shown by line style. Color schemes: substrate oxygen (Os ), red; substrate aluminum (Als ), green; carbon, cyan; hydrogen, white; water oxygen (Ow ), red; water hydrogen(Hw ), white. Periodic boundary conditions were omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Snapshots of adsorption conformations of oil molecules in C16-80-groove2 system: (a) 1 ns, (b) 3 ns, (c) 5 ns, (d) 7 ns, (e) 9 ns and (f) 15 ns. C16 and Al2 O3 atoms are represented by VDW mode. Color details are identical to those in Fig. 5.

beginning of simulations, illustrating that those oil molecules are always immobile over the whole simulations. Fig. 8 gives the evolution of minimum z-coordinate (z min ) values of OIL A and OIL B as a function of time during the 15 ns simulation and the first 500 ps, respectively. From above analysis, it can be known that the OIL A consists of oil molecules on topmost surface and parts of oil molecule inside surface defect. Thus the z min of the OIL A in each system is negative at the early stage of simulation. After simulation, the (z min ) of OIL A rapidly increase to a positive value, which means that parts of oil molecules inside surface defect

are preferentially detached and become members of OIL A. As for OIL B, the z min of each system always maintains at about 12 Å over the whole simulations, further confirming that (OIL B)s in different systems have been stably adsorbed inside surface defect.

3.4. The adsorption of water molecules onto defective alumina surface From the MD trajectory files, It can be observed that once the contaminated defective alumina surface was submerged into

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

Fig. 7. Time evolution of z COM and z

min

509

of oil molecules of each systems: (a) OIL A, (b) OIL B.

Fig. 8. Time evolution of (z min)s of oil molecules:(a)0–15 ns, (b) 0–500 ps.

Table 2 The water-alumina interaction energy(Ews ) and its componts (Evdw, Ecoul ) of each system at various oil surface coverages. Water-alumina (kcal/mol)

C16-40-groove2

C16-60-groove2

C16-80-groove2

Ecoul Evdw Ews

−4539.88 −1659.88 −6199.76

−2353.046 −277.024 −2630.07

−1515.43 −39.9378 −1555.37

aqueous solution, water molecules were immediately imbibed into surface defect, further inducing the obvious change of the adsorption conformation of oil molecules onto defective alumina surface. According to previous work on the water-driven detachment of oil molecules on perfect alumina surface [22,32], the adsorption conformation of oil molecules does not start to change drastically until the formation of water channel after a relatively long relaxation process. The difference should be ascribed to the capillary-driven spontaneous imbibition of water molecules [33]. Herein, surface defect plays the role of capillary structure. It has been extensively recognized that the dominant driving force for the diffusion of water molecules from aqueous solution to alumina surface is the nonbonded interactions between water solution and alumina surface. Thus, the capillary action of surface defect could be clarified from the perspective of the water-solid interactions. Table 2 gives the interaction energy between water solution and defective alumina surface (Ews ) and its VDW and electrostatic

components (Evdw , Ecoul ) in each system. As shown, on the defective alumina surface, there’re strong interactions between water solution and defective alumina surface, thus water molecules can be easily adsorbed to the defective alumina surface after simulation. Meanwhile, the value of the Ecoul of each system is much larger than that of Evdw , despite of different oil surface coverages. This clearly indicates that the electrostatic interactions between water solution and defective alumina surface dominate the early diffusion of water molecules from aqueous solution to defective alumina surface. Moreover, it can be noted that the values of Ews , Evdw , and Ecoul tends to decrease, with the oil surface coverage increasing. Thus, it can be expected that on the defective alumina surface, the nonbonded interactions will become too weak to drive the diffusion of water molecules, with oil surface coverage increasing. That is to say, the capillary action of surface defect could be ignored when the oil surface coverage is large enough. From above analysis, it can be concluded that on the defective alumina surface, the early imbibition of water molecules into surface defect is driven by surface defect’s capillary action, mainly consisting of the long-range electrostatic component of the water-alumina interactions. To clearly indicate the adsorption process of water molecules onto whole defective alumina surface, Fig. 9 gives the time evolution of the values of Ews , Evdw , and Ecoul of C16-80-groove2 system. It can be seen that Ews , Evdw , and Ecoul rapidly decrease at the early stage of the simulation (0–300 ps), which is the consequence of the rapid imbibition of water molecules inside surface defect. Then

510

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513 Table 3 Interaction energy between oil residues and alumina surface (Eos) and its components of each system at various oil surface coverages.

Fig. 9. Time evolution of Ews , Evdw , and Ecoul of C16-80-groove2 system.

water molecules are continually adsorbed to the topmost surface of defective alumina surface. During this stage, we note that the Ecoul approximately remains unchanged, while the Ecoul tends to descend with decreasing slope. It clearly indicating that VDW interactions dominate the adsorption process of water molecules onto topmost surface. Thus, on the defective alumina surface, the main driving force for water molecules undergoes the transformation from electrostatic interaction to VDW interactions. Fig. 10 illustrates the adsorption process of water molecules inside surface defect of C16-80-groove2 system. At 100 ps, large amount of water molecules have been absorbed into surface defect by two water channels (displayed in Fig. 10(a)). In many studies, the formation of water channel has been observed [17,22,23], but only one water channel was observed. For instance, in the Liu and co-workers’ study of oil detachment on the perfect SiO2 surface, only one water channel was discovered [23]. We also previously found that the formation of one water channel on perfect alumina surface, when the contaminated alumina surface is immersed in aqueous solution. The case of two water channels formed on contaminated substrate surface is first observed. Thus, it can be inferred that surface defect favors the formation of water channel, which is helpful for the oil detachment. At 300 ps, the adsorption process of water molecules inside surface defect has approximately finished and that onto topmost surface is in progress (see Fig. 10(c)). 3.5. Detachment of oil molecules inside surface defect On the basis of above analysis, it can be known that the detachment of oil molecule inside surface defect mainly occurs at the early

Oil-alumina (kcal/mol)

C16-40-groove2

C16-60-groove2

C16-80-groove2

Ecoul Evdw Eos

−64.784 −14.570 −79.354

−22.246 −11.964 −34.210

−88.90 −28.37 −117.27

stage of the MD simulation. Fig. 11 summarizes simulation snapshots for the detachment of oil molecules inside surface defect. For clarity, the oil molecules inside surface defect are represented by yellow VDW spheres. As displayed, with the invasion of water molecules into surface defect, parts of oil molecules are approximately immobile over the whole simulations, while others have been detached from surface defect. To quantitatively describe the dynamic detachment process of oil molecules inside surface defect, the time evolution of the numbers of oil carbon and water oxygen atoms inside surface defect (z < 0) is illustrated in Fig. 12. As shown, at the beginning of the simulation (0–300 ps), the number of water oxygen atoms of each system rapidly increase, while the number of oil carbon atoms tends to decrease. It clearly indicates the dynamic water-driven detachment process of oil molecules inside surface defect. After the drastic change at the early stage (0–300 ps), the numbers of oil carbon and water oxygen atoms inside surface defect of each system finally converge, suggesting that water molecule have approximately finished their adsorption process inside surface defect and parts of oil molecules are still remained. Further, the influence of depth on the number of residual oil molecules inside surface defect (ROC) is investigated. As shown in Fig. 13, the number of oil molecules remained inside surface defect tends to increase with the depth of surface defect increasing, further indicating that surface defect could lead to oil residues. To thoroughly remove the oil residue, it is necessary to determine the driving forces for the residual of oil molecules inside surface defect. Pioneering studies have stated that the residual of oil molecules onto perfect alumina surface is due to the strong interaction between oil molecules and hydrated alumina surface. It can be expected that the residual of oil molecules inside surface defect could be also ascribed to the interactions between the hydrated inner surfaces of surface defect and the remaining oil molecules. According to the Eq. (1), the interaction energy between the remaining oil molecules and defective alumina surface is calculated and summarized in Table 3. As shown, the Eos of each system is still relatively large, indicating that there’re strong interactions between oil residues and alumina surface. From the MD trajectory files, it should be due to the fact that the inner surfaces of surface defect are not fully wetted. As shown

Fig. 10. Consecutive snapshots of the adsorption process of water molecules in surface defect: (a) 100 ps, (b) 200 ps, (c) 300 ps. Water molecules are displayed in VDW mode, and C16 molecules are shown in line mode.

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

511

Fig. 11. Simulation snapshots for detachment process of oil molecules inside surface defect.

Fig. 12. Time evolution of number of particles inside surface defect in various systems.

Fig. 13. Change in the ROC upon varying depths of surface defect.

3.6. Mechanism analysis of detachment of oil molecules from defective surface in Fig. 14, at the end of simulation, the bottom surface of surface defect of each system are not fully wetted by water molecules, and the remaining oil molecules inside surface defect always come into directly contact with the inner surfaces of surface defect. Moreover, with the increasing of defect depth, the area of unwetted inner surfaces of surface defect tends to increase, and much more oil molecules are remained inside surface defect, as illustrated in Fig. 15. On the basis of above analysis, it can be inferred that compared with perfect alumina surface, the spontaneously wetting of inner surface of surface defect is much harder, easily inducing the residue of oil molecules inside surface defect.

To summarize, in the aqueous solution, oil molecules on defective Al2 O3 surface undergo one evident “roll-up” process. It’s discovered that the penetration of water molecules is the key driving force of the oil detachment from defective surface, which is in accordance with that of oil molecules adsorbed to perfect substrate surface in aqueous surfactant solution [23] and pure water solution [32]. The entire water-driven detachment process of oil molecules from defective alumina surface could be clearly divided into three stages: (I) the early detachment of oil molecules inside surface defect induced by capillary-driven spontaneous imbibition of water molecules; (II) the following conformational change of

512

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

Fig. 14. Adsorption conformation of oil residues inside surface defect.

Fig. 15. Adsorption conformation of oil residues inside surface defect of each system:(a) C16-80-groove3, (b) C16-80-groove4, (c)C16-80-groove5.

oil molecules on topmost surface; and (III) the final migration of detached oil molecules from solid surface. In Stage I, owing to the effect of surface defect, water molecules surge into surface defect without any obstacles, once the contaminated alumina surface is immersed into aqueous solution. As a consequence, parts of oil molecules inside surface defect are preferentially detached and the adsorption layers of oil molecules are partly disturbed. Meanwhile, a water channel connecting oil-solid and water-oil interfaces is formed, which is also vital to the gradual detachment from alumina surface. In this stage, the capillary force imposed by surface defect is dominating force for the adsorption of water molecules and detachment of oil molecules inside surface defect. In Stage II, due to the water-solid interactions and hydrogenbond interaction between oil and the alumina surface, large amount of water molecules are adsorbed to the topmost surface of defective alumina surface. Then the water channel is gradually expanded and the oil-solid interface is occupied by water molecules. As a result, oil molecules adhered onto topmost alumina surface roll up into a cylinder oil drop. Meanwhile, the adsorption layers of water

molecules are formed. In this stage, oil molecules slowly mainly complete their conformational change on the defective alumina surface. In Stage III, oil/water/solid system reaches equilibrium, meaning the end of whole entire detachment process of oil molecules. In this stage, under the effect of buoyancy force, oil aggregate is finally detached from solid surface. The microscopic phenomenon in this stage mainly includes the aggregation of detached oil molecules in aqueous solution, and the migration of oil aggregate towards water/air interface. According to above analysis, the influences of surface defect on the mechanism of oil detachment could be identified. Comparing the three-stage processes of oil detachment from defective and perfect alumina surface, it can be observed that the most obvious differences mainly occur in the Stage I. In this stage, surface defect effectively promotes the early formation of water channel, which is the key to induce the early conformational change and even whole detachment of oil molecules. Secondly, surface defect could lead to the residual of oil molecules inside surface defect.

W.K. Xie et al. / Applied Surface Science 426 (2017) 504–513

4. Conclusions In this work, we investigated the whole detachment process of oil molecules from defective alumina surface in aqueous solution via molecular dynamics simulation. The effect of surface defect on the adsorption behavior and water-driven detachment of oil molecules was explored. The MD simulation results indicate that defective substrate surface could provide more adsorption sites for adsorption of oil molecules, thereby possessing higher adsorption energy with oil molecules. However, compared with perfect surface, the detachment of oil molecules on defective surface tends to become much easier. Under the hybrid interactions of water solution and defective surface, the conformational change process of oil molecules exhibits three-stage features. The surface defect on substrate surface effectively accelerates the conformational change process of oil molecules by promoting the formation of water channel. Moreover, surface defect also leads to residual of parts of oil molecules. In addition, with increasing of surface defect depth, more oil molecules are remained in surface defect. Our MD simulations provide microscopic insights into the effects of surface defect on the mechanism detachment of oil molecules, which is believed to deeply understand the surface cleaning process of oil molecules adsorbed to processed surface. Acknowledgement The authors gratefully acknowledged financial supports of National Natural Science Foundation of China (51475108 and 51535003). References [1] Y. Kanasaki, Y. Kobayashi, K. Gotoh, Dynamic analysis of the removal of fatty acid from a pet surface using a quartz crystal microbalance, J. Surfactants Deterg. 19 (2016) 627–636. [2] P.D.I. Fletcher, L.D. Savory, W. Freya, C. Andrew, A.M. Howe, Model study of enhanced oil recovery by flooding with aqueous surfactant solution and comparison with theory, Langmuir 31 (2015) 3076–3085. [3] S.R. Qiu, N.M.A. orton, R.N. Raman, A.M. Rubenchik, C.D. Boley, A. Rigatti, Impact of laser-contaminant interaction on the performance of the protective capping layer of 1 high-reflection mirror coatings, Appl. Optics 54 (2015) 8607–8616. [4] X. Ling, Y. Zhao, J. Shao, Z. Fan, Effect of two organic contamination modes on laser-induced damage of high reflective films in vacuum, Thin Solid Films 519 (2010) 296–300. [5] C. Wang, F. Ding, D. Tang, Modeling and simulation of the high-speed milling of SKD11 (62 HRC) hardened steel based on split hopkinson pressure bar technology, Int. J. Mach. Tool Manu. 108 (2016) 13–26. [6] V. Tangwarodomnukun, P. Likhitangsuwat, O. Tevinpibanphan, Laser ablation of titanium alloy under a thin and flowing water layer, Int. J. Mach. Tool Manu. 89 (2015) 14–28. [7] S. Wu, S. Liu, L. Garfield, R. Tabor, Influence of surface roughness on cetyltrimethylammonium bromide adsorption from aqueous solution, Langmuir 27 (2011) 6091–6098. [8] M. Sammalkorpi, A.Z. Panagiotopoulos, M. Haataja, Surfactant and hydrocarbon aggregates on defective graphite surface: structure and dynamics, J. Phys. Chem. B 112 (2008) 12954–12961. [9] M. Sammalkorpi, A.Z. Panagiotopoulos, M. Haataja, Structure and dynamics of surfactant and hydrocarbon aggregates on graphite: a molecular dynamics simulation study, J. Phys. Chem. B 112 (2008) 2915–2921.

513

[10] D.P. Song, M.J. Chen, Y.C. Liang, Adsorption of tripeptide RGD on rutile TiO2 nanotopography surface in aqueous solution, Acta Biomater. 6 (2010) 684–694. [11] M.J. Chen, C.Y. Wu, D.P. Song, RGD tripeptide onto perfect and grooved rutile surfaces in aqueous solution: adsorption behaviors and dynamics, Phys. Chem. Chem. Phys. 12 (2010) 406–415. [12] C. Melis, A. Mattoni, L. Colombo, Atomistic investigation of poly(3-hexylthiophene) adhesion on nanostructured Titania, J. Phys. Chem. C 114 (2010) 3401–3406. [13] C. Li, P. Choi, Molecular dynamics study of the adsorption behavior of normal alkanes on a relaxed ␣-Al2 O3 (0001) surface, J. Phys. Chem. C 111 (2007) 1747–1753. [14] P. De Sainte Claire, K.C. Hass, W.F. Schneider, Simulations of hydrocarbon adsorption and subsequent water penetration on an aluminum oxide Surface, J. Chem. Phys. 106 (1997) 7331–7342. [15] C. Yeh, J.L. Lenhart, B.C. Rinderspacher, Molecular dynamics simulations of adsorption of catechol and related phenolic compounds to alumina surfaces, J. Phys. Chem. C 119 (2015) 7721–7731. [16] F.C. Wang, H.A. Wu, Enhanced oil droplet detachment from solid surfaces in charged nanoparticle suspensions, Soft Matter 33 (2013) 7974–7980. [17] P. Zhang, Z. Xu, Q. Liu, Mechanism of oil detachment from hybrid hydrophobic and hydrophilic surface in aqueous solution, J. Chem. Phys. 140 (2014) 164702. [18] C. Li, P. Choi, Molecular dynamics study of the adsorption behavior of normal alkanes on a relaxed ␣-Al2O3 (0001) surface, J. Phys. Chem. C 111 (2007) 1747–1753. [19] P. De Sainte Claire, K.C. Hass, W.F. Schneider, Simulations of hydrocarbon adsorption and subsequent water penetration on an aluminum oxide surface, J. Chem. Phys. 106 (1997) 7331–7342. [20] D. Argyris, T. Ho, D.R. Cole, Molecular dynamics studies of interfacial water at the alumina surface, J. Phys. Chem. C 115 (2011) 2038–2046. [21] I.C. Yeh, J.L. Lenhart, B.C. Rinderspacher, Molecular dynamics simulations of adsorption of catechol and related phenolic compounds to alumina surfaces, J. Phys. Chem. C 119 (2015) 7721–7731. [22] W.K. Xie, Y.Z. Sun, H.T. Liu, Conformational change of oil contaminants adhered onto crystalline alpha-alumina surface in aqueous solution, Appl. Sur. Sci. 360 (2016) 184–191. [23] Q. Liu, S. Yuan, H. Yan, Mechanism of oil detachment from a silica surface in aqueous surfactant solutions: molecular dynamics simulations, J. Phys. Chem. B 116 (2012) 2867–2875. [24] R.W. Pastor, A.D. Mackerell, Development of the Charmm force field for lipids, J. Phys. Chem. Lett. 2 (2011) 1526–1532. [25] R.T. Cygan, J.J. Liang, A.G. Kalinichev, Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field, J. Phys. Chem. B 108 (2004) 1255–1266. [26] H.J.C. Berendsen, J.R. Grigera, T.P. Straatsma, The missing term in effective pair potentials, J. Phys. Chem. 91 (1987) 6269–6271. [27] S. Plimpton, Fast parallel algorithms for short-range molecular dynamics, J. Comput. Phys. 117 (1995) 1–19. [28] H.C. Andersen, RATTLE: a Velocity version of the SHAKE algorithm for molecular dynamics calculations, J. Comput. Phys. 52 (1983) 24–34. [29] E.L. Pollock, J. Glosli, Comments on P3M, FMM, and the Ewald method for large periodic Coulombic Systems, Comput. Phys. Commun. 95 (1996) 93–110. [30] H. Zhang, X. Lu, Y. Leng, Effects of aqueous environment and surface defects on Arg-Gly-Asp peptide adsorption on titanium oxide surfaces investigated by molecular dynamics simulation, J. Biomed. Mater. Res. A 96 (2011) 466–476. [31] C. Chen, L. Zhuang, X. Li, A many-body dissipative particle dynamics study of forced water–oil displacement in capillary, Langmuir 28 (2011) 1330–1336. [32] W.K. Xie, H.T. Liu, Y.Z. Sun, Conformation evolution of oil contaminants onto aluminum oxide surface in aqueous solution: the effect of surface coverage, Appl. Sur. Sci 392 (2017) 747–759. [33] M.R. Stukan, P. Ligneul, J.P. Crawshaw, Spontaneous imbibition in nanopores of different roughness and wettability, Langmuir 26 (2010) 13342–13352.