Combined Molecular Dynamics and Quantum Mechanics Study of Oil

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Combined Molecular Dynamics and Quantum Mechanics Study of Oil Droplet Adsorption on Different Self-Assembly Monolayers in Aqueous Solution Jie Zhong,†,‡ Xiao Wang,†,‡ Jianping Du,†,‡ Lei Wang,†,‡ Youguo Yan,†,‡ and Jun Zhang*,†,‡ †

College of Science and ‡Key Laboratory of New Energy Physics & Materials Science in Universities of Shandong, China University of Petroleum, 266580 Qingdao, Shandong, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, the dodecane is selected as the oil phase, and the adsorption behavior of an oil droplet on six selfassembled monolayer (SAM) surfaces in aqueous solution is investigated by a combined molecular dynamics and quantum mechanics method. First, the adsorption configuration of an oil droplet on these six surfaces is investigated, which indicates that the oil droplets spread on SAMs of −CH3, −OCH3, and −COOCH3 and detach from SAMs of −NH2, −OH, and −COOH. After that, the interactions of oil−SAM and water−SAM are calculated to rationalize the driving force controlling the conformational change of the oil droplet on various SAMs. Researched results suggest that the conformational change of the oil droplet is mainly driven by the interaction between water and SAMs. Finally, the mechanism of oil spreading or detachment behavior is discussed from the aspect of water property near the SAM, and calculated results show that the adsorption capability and dynamic property of interfacial water have a profound effect on the adsorption behavior of the oil droplet. The microscopic adsorption behavior of the oil droplet on SAMs discussed here is helpful to understand many phenomena in scientific and industrial processes better.

1. INTRODUCTION The adsorption of an oil droplet on solid surfaces is crucial for many applications with porous materials involved in multiphase phenomena, such as oil recovery,1−4 geological carbon sequestration,5−8 nonaqueous phase liquid (NAPL) contaminant fate and transport,9,10 and detergency.11−13 The adsorption behavior of an oil droplet is generally probed by the contact angle, θow, which is related to interfacial tension based on Young’s equation.14 σsw = σso + σow cos θow

determining the oil adsorption, and it has been confirmed by some experimental works.22,23 S. A. Morton22 investigated the shape of oil droplets on metal surfaces to which an electrical potential is applied, and they found that the electric potential of the metal surface had a significant influence on the droplet shape. A. W. Rowe23 studied the contact angle of oil droplets on the metal surface in various surfactant solutions. They found that the surfactants could form a different self-assembled monolayer (SAM) on the metal surface, which induced a distinct difference among the contact angles of oil droplets. So we can find that the adsorption behavior of oil droplets on the solid surface is closely related to surface property, and it is necessary to investigate how surface property affects the oil adsorption. Recently, a self-assembled monolayer (SAM) has been developed into a powerful, simple, and highly feasible means of changing the surface property.24−31 Investigating the oil droplet adsorption on different SAMs can reflect how the surface property affects the adsorption behavior of oil droplets and gives us insight into the mechanism of oil spreading or detachment behavior. Due to the substantial increase in computational power over the past few years, molecular simulations have been widely used

(1)

Herein, σ is the interfacial tension, and the subscripts w, o, and s denote the water, oil, and solid, respectively. According to Young’s equation, we can observe that the oil−water contact angle is determined by the three-phase interactions, which are solid− water, solid−oil, and oil−water interactions. In the preliminary research, many efforts have been devoted to the influence of oil−water interactions on the adsorption behavior of an oil droplet.15−21 Qian Liu17 studied the mechanism of oil detachment from the silica surface. It has been observed that the decreasing oil−water interfacial tension could promote the detachment of oil from the surface. A similar conclusion was also approved by V.L. Kolev,19 who found that the interfacial tension between water and oil had profound effects on the adsorption behavior of an oil droplet. Besides the interaction between fluids, the solid−fluid interactions are also crucial in © XXXX American Chemical Society

Received: January 30, 2013 Revised: May 21, 2013

A

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Figure 1. Preparation of initial configuration of an oil droplet adsorbed on the SAM surfaces in aqueous solution.

(CH2)11OCH3, ester-terminated (CH2)11COOCH3, amineterminated (CH2)11NH2, alcohol-terminated (CH2)11OH, and carboxyl-terminated (CH2)11COOH. After that, a supercell is built to ensure that the SAM surface is large enough for accommodating the oil droplet to wet. Each SAM consists of 90 alkyl chains, arranged in a well-ordered, defect-free array of 10 × 9 chains with a surface occupied area of 20 Å2 per chain. The SAM surface is built with the dimensions of x = 44.19 Å, y = 42.52 Å, and z ≈ 27 Å. Oil Droplet in Aqueous Solution. In our work, the dodecane (C12H26) is selected as a typical component of the oil droplet, and a hemispheric oil droplet composed of 22 dodecane molecules is constructed with radius of 14 Å. A detailed explanation of such construction can be found in the Supporting Information. Then, the oil droplet is put into the aqueous solution which contains 1905 water molecules. The dimension of the solution box is x = 44.19 Å, y = 42.52 Å, and z = 40.42 Å. Next, the solution box is placed on the surface of SAMs to produce the initial model (see Figure 1). The condensed-phase-optimized molecular potentials for atomistic simulation studies (COMPASS) force field is adopted in all the simulations.43 The total potential can be expressed as

to study the SAMs, which could investigate the dynamical, energetic, and structural behaviors in many scientific and industrial processes,32−40 such as wetting, micropatterning, lubrication, corrosion, medicine, and biocompatibility. For example, Zhen Xu33 investigated the microscopic wetting behavior of different SAMs. Rahul Godawat39 studied the hydration of different SAMs from hydrophobic (−CF3, −CH3) to hydrophilic (−OH, −CONH2). These researches illustrated that the molecular simulation can be adopted to investigate the property of fluids on SAMs scientifically. In this work, our objective is to shed light on the spreading or detachment mechanism of oil on different SAMs. For this purpose, a combined theoretical approach (molecular dynamics (MD) and quantum mechanics (QM)) is adopted. First, the adsorption behavior and structural transition of the oil droplet on six SAMs in aqueous solution is investigated. Second, the interactions of oil−SAM and water−SAM are calculated to rationalize the driving forces controlling the conformational change of oil droplet on various SAMs. Finally, the water property near the SAM surface is investigated to illustrate the mechanism of oil spreading or detachment behavior on SAMs. The microscopic adsorption behavior of an oil droplet on SAMs discussed here is helpful to understand many phenomena in scientific and industrial processes better.

Epot =

∑ [k 2(b − b0)2 +k 3(b − b0)3 + k4(b − b0)4 ] b

2. THEORETICAL MODELS AND METHODS 2.1. Quantum Chemical Calculation. In this paper, the quantum chemical calculation is conducted to investigate the polarity of SAMs and hydrogen bonds formed between SAMs and aqueous solution. The quantum chemical calculation is carried out by the DMol3 module in Materials Studio software of Accelrys Inc.41 All electron calculations are accomplished by the GGA/PBE exchange-correlation function and a double numerical plus polarization (DNP) basis set.42 Self-consistent field calculations are done with a convergence criterion of 10−6 Hartree on the total energy, and the cutoff of the atomic basis set is 0.37 nm. 2.2. Molecular Dynamics Simulation. Molecular dynamics simulations are carried out with Discover and Amorphous Cell modules in Materials Studio of Accelrys Inc. The built model is composed of a SAM surface and an oil droplet in aqueous solution. SAM Surface. The initial silica lattice is derived from the structural database of Material Studio. A repeat unit of silica with the depth of 1.41 nm is cleaved along the (001) crystallographic orientation. Then, the surface is modified by six different SAMs, including methyl-terminated (CH2)11CH3, ether-terminated

+

∑ [k 2(θ − θ0)2 +k 3(θ − θ0)3 + k4(θ − θ0)4 ]

+

∑ [k1(1 − cos φ) + k 2(1 − cos 2φ) + k 3(1 − cos 3φ)]

+

∑ k 2χ 2

θ

φ

χ

+

+

∑ k(b − b0)(b′ − b0′) b,b′

∑ k(b − b0)(θ − θ0) + ∑ (b − b0)[k1 cos φ b,θ

b,φ

+ k 2 cos 2φ + k 3 cos 3φ] +

∑ (θ − θ0)[k1 cos φ

+ k 2 cos 2φ + k 3 cos 3φ] +

∑ k(θ − θ0)(θ′ − θ0′)

θ ,φ

b,θ

+

∑ θ ,θ ,φ

k(θ − θ0)(θ′ − θ0′)cos φ + Eele + EvdW (2)

The nonbond interactions are represented by electrostatic potential Eele = ∑i>j[(qiqj)/rij] and van der Waals potential EvdW = ∑ELJ{2[(r0ij)/(rij)]9 − 3[(r0ij)/(rij)]}. The canonical ensemble NVT is performed at 298 K for each system using the velocity Verlet algorithm,44 and the integration B

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Figure 2. Matrix of snapshots from SAMs (a) −CH3, −OCH3, and −COOCH3 and (b) −NH2, −OH, and −COOH simulations as a function of changing surface (along the y-axis) and the increase of simulation time (along the x-axis). The blue lines are contact lines, and the atoms are colored as follows: O, red; C, gray; N, blue; Si, orange; and H, white.

fractional free volume is calculated to illustrate the interior structure of the oil droplet on different SAMs. Snapshots from the simulations of six SAMs in contact with the oil droplet in aqueous solution are summarized in Figure 2. It gives an intuitive vision of the adsorption configuration of the oil droplet on the SAMs with different terminal groups (−CH3, −OCH3, −COOCH3, −NH2, −OH, −COOH). It can be seen that none of the water and dodecane molecules penetrate into the compact SAMs due to strong steric hindrance between alkyl chains in SAMs, and it is consistent with many simulation researches.46−48 Furthermore, it can be observed that the adsorption behavior of oil droplets on six SAMs has a significant difference. At the initial configuration, the contact line is 28 Å for all systems. After simulation, the contact line increases for SAMs of −CH3, −OCH3, and −COOCH3, and it indicates that the oil droplet spreads on these surfaces during the simulation. For SAMs of −CH3 and −OCH3, the contact lines both extend to 44.19 Å (the width of the simulation box). However, compared to the SAM of −OCH3, the contact line of the −CH3 surface extends more rapidly. The increase of contact line for the −COOCH3 surface is not obvious, and it reaches to 31.09 Å at the end of the simulation. On the contrary, for SAMs of −NH2, −OH, and −COOH, the oil droplet

step is set as 1 fs. The temperature is controlled by the Andersen thermostat.45 The van der Waals interactions are calculated by the atom-based method, and the cutoff distance is 12.5 Å. The electrostatic interactions are calculated by the Ewald method, which is rather costly but accurate to the long-rang interactions. Due to the strong steric hindrance between chains in compact SAMs, the thermal motion of SAMs can be neglected for the time scale in which the oil droplet spreads or detaches on SAM surfaces. Therefore, the atom positions of SAMs are constrained during the simulation. At the top of every model, a 65 Å thick vacuum slab is added to avoid interactions between solvent molecules and the periodic images of silica molecules due to the periodic boundary conditions. Finally, 1 ns simulations are conducted to relax the system fully, and the last 100 ps of the trajectory is used for analysis.

3. RESULTS AND DISCUSSIONS 3.1. Structure of the Oil Droplet on SAM Surfaces. In this section, the structure of the oil droplet on SAMs is investigated from two aspects. First, the spreading and detachment behavior is investigated by contact line, the height of the oil droplet, and the contact angle of the oil droplet. Second, the C

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keeps the spherical shape during the simulation. As the simulation time increases, the contact line shrinks gradually. After simulation, the contact lines of −NH2, −OH, and −COOH terminated SAMs are 22.65, 13.03, and 10.54 Å, respectively. For quantitative description of conformational dynamics on SAMs, the height of the oil droplet, viz., the vertical distance between the center of the oil droplet and the SAM surface, as a function of time is investigated and depicted in Figure 3. From

These parameters in the above equations are depicted in Figure 4. The height h and the radius of droplet contact surface r can be

Figure 4. Geometrical parameters for the calculation of the contact angle θ.

obtained from the corresponding radial density distribution (RDD) along the z-direction. Figure 5 shows the RDD profiles on different

Figure 3. Time dependence of the height of the oil droplet center on SAMs (a) −CH3, −OCH3, −COOCH3; (b) −NH2, −OH, and −COOH surfaces.

Figure 3, it is observed that the height of the oil droplet center decreases gradually for SAMs of −CH 3 , −OCH 3 , and −COOCH3 during the simulation. Especially, the height of the oil droplet on the −CH3 surface decreases to 3 Å, and it is almost the height of monolayer dodecane lying on the surface, which suggests complete wetting of the oil droplet on the −CH3 surface. On the other hand, the height of the oil droplet center increases a lot for SAMs of −NH2, −OH, and −COOH during the simulation. For the −COOH surface, the height of the oil droplet center is 13 Å after simulation, and it is much closer to the droplet radius, which indicates the oil droplet nearly detaches from the −COOH surface. Comparing the height of the oil droplet center on different SAMs, it follows the order of −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH. The contact angle of the oil droplet (θow) is often used to describe the oil adsorption behavior on the surface, and it is also a crucial parameter to characterize the wettability involved in multiphase phenomena. Generally, the wettability is defined as oil wet for oil−water contact angles from 0° to 65°, intermediate wet from 65° to 105°, and water wet from 105° to 180°, respectively. Further, the region from 45° to 65° is considered as weakly oil wet, while the region from 105° to 125° is considered weakly water wet.49 In this article, the method of Fan and Cagin is exploited for the calculation of contact angle (θ).50 In this method, the θ could be calculated by the following equations h R

(3)

h S + 2 2πh

(4)

cos θ = 1 − R=

S = πr 2

Figure 5. Radial density distribution profiles for the oil droplet on SAMs.

SAMs. On the basis of Figure 5, the contact angle of the oil droplet is calculated as shown in Table 1. Table 1. Contact Angle of the Oil Droplet θow on SAM Surfaces SAM

−CH3

−OCH3

−COOCH3

−NH2

−OH

−COOH

h r cos θow θow

6.30 29.68 0.91 23.98

11.94 25.77 0.65 49.72

13.91 21.35 0.40 66.20

23.69 14.36 −0.46 117.64

23.39 11.03 −0.64 129.56

27.17 9.92 −0.76 139.97

From Table 1, it is noticed that the contact angles θow on −CH3 and −OCH3 surfaces are 23.98° and 49.72°, respectively, and these two angles are lower than 65°. According to the definition of surface wettability, it can be concluded that the −CH3 surface is oil wet and the −OCH3 surface is weakly oil wet. As for the −COOCH3 surface, the θow is 66.20° and is slightly larger than 65°, so the −COOCH3 surface can be defined as intermediate wet. As for the −NH2 surface, the θow is 117.64°, and it is in the region from 105 to 125°, which indicates that the −NH2 surface is weakly water wet. As for −OH and −COOH surfaces, the θow are 129.56° and 139.97°, respectively, and both

(5) D

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Table 2. Fractional Free Volume (FFV) of the Oil Droplet on SAM Surfaces SAM

CH3

OCH3

COOCH3

NH2

OH

COOH

Vo Vf FFV

5577.31 2689.08 32.53%

5658.79 2607.6 31.54%

5788.65 2477.74 29.97%

5864.70 2401.69 29.05%

5878.38 2388.01 28.89%

5918.59 2347.80 28.40%

of them are a water wet surface. Comparing the θow on six surfaces, it follows the sequence of −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH, which agrees well with the experiment and simulation references51,33 and indicates that our simulation results are reasonable and credible. In the above analysis, the oil is treated as a droplet, and the spreading and detachment behavior is investigated by a series of parameters. In the next part, the fractional free volume (FFV) is calculated to investigate the interior structure of the oil droplet on six surfaces. The fractional free volume (FFV) is calculated from the Connolly task simulation as follows52

FFV =

Vfree

Vfree + Voccupy

Table 3. Nonbond Interaction Energies Ebind, van der Waals Potentials Evdw, and Electrostatic Potentials Eelec between the Oil Droplet and SAM Surfaces Ebind/kJ·mol−1

Evdw/kJ·mol−1

Eelec/kJ·mol−1

CH3 OCH3 COOCH3 NH2 OH COOH

50.55 53.36 58.42 67.38 62.32 74.99

50.38 52.95 58.32 67.23 62.72 73.93

0.17 0.41 0.1 0.15 −0.4 1.05

the Ebind between the oil droplet and SAM should not be very large. As a result, the Ebind difference of the oil droplet on six SAMs ought to be small. This inference is verified by comparing the Ebind of different SAM surfaces in Table 3. On the basis of the above discussion, there is no direct correlation between oil−SAM interaction and conformational change of the oil droplet on six SAMs. So, we infer that the adsorption behavior of the oil droplet on the SAM is mainly dominated by the water−SAM interaction. 3.2.2. Interactions between Water and SAMs. The interaction between water and the SAM should be discussed from two aspects: the first one is the nonbond interaction, and the second one is the hydrogen bond formed between water and the SAM surface. (1). Nonbond Interaction Energy. Table 4 shows the nonbond interaction energy between six SAMs and water molecules.

(6)

where Vfree is the sum volume of large cavities in the oil droplet which the probe particle could get in, and Voccupy is the sum volume that atoms occupy together with small cavities which the probe particle could not get in. Generally, the larger the FFV, the more relaxed the oil droplet is. In this work, the selected probe radius is default 1.0 Å in MS software, and the reason for this selection is explained in the Supporting Information. Table 2 shows the calculated FFV of the oil droplet on six SAMs. From Table 2, it can be seen that the FFV of the oil droplet on six surfaces follows the order of −CH3 > −OCH3 > −COOCH3 > −NH2 > −OH > −COOH, which indicates that the FFV decreases with increasing water wettability of the surface. As for an oil wet surface, the oil droplet spreads on it, and the larger FFV indicates that the structure of the oil droplet has more cavities. On the contrary, the oil droplet detaches from the water wet surface and turns to be spherical configuration, and the smaller FFV indicates that the oil droplet is more compact with less interior cavities. 3.2. Driving Forces of Conformational Change. The different configurations of oil droplets on six SAMs could be ascribed to the changing interactions of oil−SAM and water− SAM. Here, the interactions of oil−SAM and water−SAM are calculated to rationalize the driving forces controlling the conformational change of oil droplet. 3.2.1. Interactions between Oil Droplet and SAM. We investigate the nonbond interaction including van der Waals and electrostatic interactions between the oil droplet and SAMs in the absence of solvent. The interaction energy can be calculated by the following equation E bind = Eoil + ESAM − Etotal

SAM

Table 4. Nonbond Interaction Energies Ebind, van der Waals Potentials Evdw, and Electrostatic Potentials Eelec between Water Molecules and SAM Surfaces SAM

Ebind/kJ·mol−1

Evdw/kJ·mol−1

Eelec/kJ·mol−1

CH3 OCH3 COOCH3 NH2 OH COOH

23.61 76.6 88.75 198.82 313.27 333.33

22.52 31.6 31.79 3.46 −35.89 −21.78

1.09 45 56.96 195.37 349.16 355.11

Every individual alkyl chain with different terminated group (−CH3, −OCH3, −COOCH3, −NH2, −OH, and −COOH) is optimized by quantum chemical calculation, and the polarity of each terminated group is calculated as shown in Figure 6. As for the oil wet surface (SAM of −CH3), it is terminated with the nonpolar group −CH3, which could induce the weak polarity of the SAM surface. The weak polarity will result in small electrostatic potential between the surface and water. Correspondingly, from Table 4, it can be seen that the Eele even does not reach ten percent of the Evdw. As for the weakly water wet surface (SAM of −NH2), the polarity of the terminated group is larger than that of −CH3, so it can be found that the Eelec increases a lot and that the small Evdw can be neglected compared to Eelec. As for water wet surfaces of −OH and −COOH, the polarity of terminated groups is strong, and it can be observed that the Eelec is very large. It is worth noting that the Evdw is

(7)

where Ebind is the nonbond interaction energy; Eoil and ESAM represent the energy of the oil droplet and SAM, respectively; and Etotal refers to the energy of the whole system containing the oil droplet and SAMs. Similarly, Evdw and Eele contributed from van der Waals and electrostatic interactions are also calculated. These calculated interaction energies are shown in Table 3. First, as for each SAM, it can be found that the Ebind is approximately equal to the Evdw, and the Eele is very small, which implicates that the interaction between the oil droplet and SAM is mainly contributed by van der Waals potential. Second, because van der Waals potential is one type of weak interaction, E

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Figure 6. Dipole moment of six different terminated groups and the equilibrium distance of specific atoms between water molecules and SAM surfaces. For atom color codes see the caption of Figure 2.

negative for those two surfaces. This interesting phenomenon could be ascribed to strong electrostatic attraction between water and −OH and −COOH surfaces. Due to the strong interaction, the distance between water and the surface would become too close. As a result, the Evdw will become negative according to the Lennard-Jones potential. As for surfaces of −OCH3 and −COOCH3, the polarity of the terminated group is larger, but their Eelec is smaller than that of −NH2, −OH, and −COOH surfaces. This abnormal phenomenon is caused by the steric hindrance from the methyl group in −OCH3 and −COOCH3 groups. The large polarity of −OCH3 and −COOCH3 groups is mainly originated from O atoms; however, due to the steric hindrance from the methyl group, the water molecules are prevented from reaching the right position to interact with O in −OCH3 and −COOCH3 groups, so the Eelec is small for these two surfaces. As a result, the Eelec follows the order of −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH. Meanwhile, it can be seen that the van der Waals interaction is slightly influenced by the polarity of surfaces. Compared with the Eelec, the change of Evdw is much smaller on six SAMs. Therefore, the order of Ebind is consistent with that of Eelec. (2). Hydrogen Bond Interactions. The hydrogen bond can be formed between water molecules and SAMs. Here, quantum chemical calculation is employed to investigate the hydrogen bond between the water molecule and a single chain of each SAM. Figure 6 shows the equilibrium distance of specific atoms between water and a single chain of SAM. It is noticed that, as for the terminated group −CH3, the distance between H (−CH3) and O (H2O) is 3.595 Å, which indicates that there is no hydrogen bond formed between water molecules and terminated group. As for the terminated groups −NH2 and −OH, it can be found that one hydrogen bond forms between the water molecule and the terminated group, respectively, as the distances are 1.939 Å for N(−NH2)···H(H2O) and 1.921 Å for O(−OH)···H(H2O), and the comparison of hydrogen bonds distances indicates that the strength of the O−H type hydrogen bond formed between O(−OH) and H(H2O) is slightly stronger than the N−H type hydrogen bond formed between N(−NH2) and H(H2O). As for the terminated group −COOH, it would form two hydrogen bonds with water molecules. One hydrogen bond (distance 1.823 Å) forms between the carbonylic O(−COOH) and H(H2O), and the other hydrogen bond

Figure 7. Density distribution of water molecules on the −COOH surface at different simulation time.

(distance 1.722 Å) forms between the hydroxylic H(−COOH) and O(H2O). As for terminated groups of −OCH3 and −COOCH3, the hydrogen bonds can also be formed, as the distances are 1.900 Å for O(−OCH3)···H(H2O) and 1.922 Å for carbonylic O(−COOCH3)···H (H2O). However, in the SAM surface, the steric hindrance originated from the methyl group in −OCH3, and −COOCH3 groups will prevent water molecules reaching the right position to form hydrogen bonds with O atoms (−OCH3 and −COOCH3). On the basis of the above analyses, the hydrogen bond can only form on SAMs of −NH2, −OH, and −COOH, and the strength of the hydrogen bond follows the order of −NH2 < −OH < −COOH. According to the nonbond interaction energy and hydrogen bond strength between water molecules and SAMs, it can be concluded that the interaction strength between water and SAMs follows the order of −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH. This order is consistent with the sequence of θow, implying that the adsorption behavior of oil droplets is dominated by water−SAM interactions rather than oil−SAM interactions. 3.3. Influence of Interfacial Water on Adsorption Behavior. On the basis of the above analyses, the conformational change of the oil droplet is mainly driven by the interaction between water molecules and SAMs. How do these interactions F

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Figure 8. Configurations of oil droplets on the −COOH surface at different simulation time. The light green dashed lines represent the hydrogen bonds. For atom color codes, see the caption of Figure 2.

interaction between water molecules and SAMs since −COOH groups can accept or donate H atoms to form hydrogen bonds. As a result, interfacial water molecules are attracted toward the surface and further induce the migration of water molecules from the bulk solution to the surface, leading to the detachment of the oil droplet from the surface. This phenomenon can also be confirmed by the analysis of the water distribution in x axis and y axis (seen in Supporting Information). Figure 8 shows the detailed information on how the interfacial water molecules repel oil molecules from the surface. From Figure 8, it can be found that at 100 ps of the simulation, the water molecules adsorb at the −COOH surface, and at the vicinity of the contact line, some water molecules form hydrogen bonds with the surface beneath the oil droplet. Due to the strong

affect the adsorption behavior of the oil droplet? Here, we try to explain this problem from the aspect of the interfacial water property. Figure 7 shows the density distribution of water molecules along the z axis on the −COOH surface at different simulation time. The z axis is normal to the SAM surface, and the SAM surface is set as zero point. From Figure 7, it can be found that the density distribution profile has an obvious peak at 2.05 Å in the first 100 ps, which indicates that water molecules form an adsorption layer on the surface at the beginning of the simulation. In the time evolution the density distribution profiles exhibit two typical features: (i) the position of the first sharp peak decreases from 2.05 Å to a final value of 1.78 Å and (ii) the intensity of the peaks gradually increases. It should be ascribed to the strong G

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solution is investigated. These SAMs are terminated with methylterminated (CH2)11CH3, ether-terminated (CH2)11OCH3, esterterminated (CH2)11COOCH3, amine-terminated (CH2)11NH2, alcohol-terminated (CH2 )11 OH, and carboxyl-terminated (CH2)11COOH, respectively. The structure of oil on SAM surfaces is investigated from two aspects. First, the oil is treated as a droplet, and the spreading and detachment behavior is investigated by contact line, the height of the oil droplet, and the contact angle of the oil droplet. Calculated results indicate that the oil droplets spread on SAMs of −CH3, −OCH3, and −COOCH3 and detach from SAMs of −NH2, −OH, and −COOH, respectively. Second, the fractional free volume is calculated to investigate the interior structure of the oil droplet on six SAMs, which suggests that the structure of oil droplets would become more compact when they detach from the SAM. The interactions of oil−SAM and water−SAM are calculated to rationalize the driving forces controlling the conformational change of the oil droplet on the SAMs. First, the interactions between the oil droplet and SAMs are investigated, and no direct correlation between the oil−SAM interaction and conformational change of the oil droplet is observed. Second, the interaction strength between water and the SAM is investigated through nonbond interaction energy and hydrogen bond interactions. The results show that the interaction strength between water and SAMs follows the order of −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH, and this order is consistent with the sequence of contact angle θow, which indicates that the conformational change of the oil droplet is driven by the interaction between water and the SAM surface. Furthermore, the mechanism of oil spreading or detachment behavior is discussed from the aspect of interfacial water property. The results illustrate that the adsorption capability and dynamic property of interfacial water have a profound effect on the adsorption behavior of the oil droplet. As to the surface which has a strong interaction with water, the water molecules would be adsorbed onto the surface continuously and be trapped for a much longer time on SAMs. As a result, this tightly bound interfacial water provides a physical and energy barrier to impede the oil spreading on the surface and make the oil droplet detach from the SAM gradually. Our researches investigate the microscopic adsorption behavior of the oil droplet on different SAMs by a combined theoretical approach (both MD and QM methods), which is helpful to understand many phenomena in scientific and industrial processes better, such as oil recovery, geological carbon sequestration, and detergency, etc.

interaction strength between water molecules and the surface, the adsorbed water molecules would move along the contractive direction of the contact line, and the contact line shrinks gradually. At the end of the simulation, there are a small number of water molecules under the oil droplet, which indicates that the oil droplet nearly detaches from the −COOH surface. On the other hand, the dynamic property of interfacial water can also affect the adsorption behavior of the oil droplet. Here, the diffusion coefficient (D) is calculated to evaluate the dynamic property of interfacial water. It is described by the following equation D=

1 d lim 6 t →∞ dt

n

∑ ⟨|R i(t ) − R i(0)|2 ⟩ i

(8)

where Ri(t) is the position of atom i at time t, and Ri(0) is the initial position. Figure 9 shows the diffusion coefficients of interfacial

Figure 9. Self-diffusion coefficients of interfacial water molecules on SAMs.

water molecules on six SAMs. As for SAMs of −COOH, water molecules are strongly attracted by this surface, and the diffusion coefficient of water molecules is the smallest, which indicates that the water molecules are trapped a much longer time on the surface. As a result, this tightly bound interfacial water would provide a physical and energy barrier to impede the oil spreading on the surface. On the basis of the above analyses, it is observed that the adsorption capability and dynamic property of interfacial water have a profound effect on the adsorption behavior of the oil droplet on the −COOH surface. As for the other five SAMs, the interaction strength between water and surfaces is smaller. Due to the weak interaction, it would be difficult for water molecules in the bulk solution to migrate onto the surface, and also water molecules near the SAM surface would have a larger diffusion coefficient (seen in Figure 9). As a result, the capability of the oil droplet detaching from other SAM surfaces would be smaller. According to the former analyses, the order of the capability of the oil droplet detaching from six SAMs should have the same sequence as that of interaction strength between water and SAMs, −CH3 < −OCH3 < −COOCH3 < −NH2 < −OH < −COOH, which is consistent with the configuration analysis of the oil droplet on different SAM surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The discussion for the construction of an oil droplet, the selection of probe radius in calculating FFV, and the density distribution of water molecules along the x axis and y axis on the −COOH surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-0532-86983418.

4. CONCLUSION In this article, the dodecane is selected as the oil phase, and the adsorption behavior of the oil droplet on six SAMs in aqueous

Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (51034007), CNPC Innovation Foundation (2010D-5006-0204, 2011D-5006-0202), CNPC science and technology development project (2011A-1001), and China University of Petroleum (East China), Graduate Innovation Project funded projects (CX-1254, I4CX06004A).

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