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Molecular Dynamics Simulations of the Oil-Detachment from the Hydroxylated Silica Surface: Effects of Surfactants, Electrostatic Interactions and Water Flows on the Water Molecular Channel Formation Jian Tang, Zhou Qu, Jianhui Luo, Lanyan He, Pingmei Wang, Ping Zhang, Xianqiong Tang, Yong Pei, Bin Ding, Baoliang Peng, and Yunqing Huang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09716 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Molecular Dynamics Simulations of the Oil-Detachment from the Hydroxylated Silica Surface: Effects of Surfactants, Electrostatic Interactions and Water Flows on the Water Molecular Channel Formation Jian Tanga, Zhou Que, Jianhui Luoc,d, Lanyan Hee, Pingmei Wangc,d, Ping Zhangb, Xianqiong Tang*b, Yong Pei*e, Bin Dingc,d, Baoliang Peng*c,d, Yunqing Huanga a

Hunan Key Laboratory for Computation and Simulation in Science and Engineering, Institute for Computational and Applied Mathematics, Xiangtan University, Xiangtan, 411105, P. R. China b Department of Civil Engineering and Mechanics, Xiangtan University, Xiangtan 411105, P.R. China c Research Institute of Petroleum Exploration & Development (RIPED) d Key Laboratory of Nano Chemistry（KLNC），PetroChina, Haidian District, Beijing, 100083, P. R. China e Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Hunan Province 411105, P. R. China

Abstract The detachment process of an oil molecular layer situated above a horizontal substrate was often described by a three-stage process. In this mechanism, the penetration and diffusion of water molecules between the oil phase and the substrate was proposed to be a crucial step to aid in removal of oil layer/drops from substrate. In this work, the detachment process of a two-dimensional alkane molecule layer from a silica surface in aqueous surfactant solutions is studied by means of molecular dynamics (MD) simulations. By tuning the polarity of model silica surfaces, as well as considering the different types of surfactant molecules and the water flow effects, more details about the formation of water molecular channel and the expansion processes are elucidated. It is found that for both ionic and non-ionic type surfactant solutions, the perturbation of surfactant molecules on the two-dimensional oil molecule layer facilitates the injection and diffusion of water molecules between the oil layer and silica substrate. However, the water channel formation and expansion speed is strongly affected by the substrate polarity and properties of surfactant molecules. Firstly, only for the silica surface with relative stronger polarity, the formation of water molecular channel is observed. Secondly, the expansion speed of the water molecular channel upon the ionic surfactant (dodecyl trimethyl ammonium bromide, DTAB and sodium dodecyl benzene sulfonate, SDBS) flooding is more rapidly than the non-ionic surfactant system (octylphenol polyoxyethylene(10) ether, OP-10). Thirdly, the water flow speed may also affect the injection and diffusion of water molecules. These simulation results indicate that the water molecular channel formation process is affected by multiple factors. The synergistic effects of perturbation of surfactant molecules and the electrostatic interactions between silica substrate and water molecules are two key factors aiding in the injection and diffusion of water molecules and helpful for the oil detachment from silica substrate.

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I. INTRODUCTION The understanding of the oil detachment mechanism from the reservoir rocks is of great importance in the oil recovery industry. With the rapid development of the oil recovery techniques, several enhanced oil recovery (EOR) techniques such as gas injection, chemical injection, microbial injection, or thermal recovery have been proposed during the past few decades.1-8 The recovery enhancement of such oilfields brought new challenge and opportunity in the process of sustainable development on energy strategy. Among various EOR techniques, the chemical flooding techniques including surfactant flooding, polymer flooding and supercritical carbon dioxide etc. are promising for the EOR process, especially for reservoirs where the traditional thermal methods are not feasible.9 Numerous oilfield tests proved that chemical flooding techniques can greatly improve oil recovery.10 For instance, it was demonstrated that the dilute anionic and non-ionic surfactant solutions can give oil recovery as high as 60% from oil-wet carbonate cores.11-12 To date, numerous studies focused on the bulk oil recovery properties upon the surfactant flooding. The phase behavior,13 wettability,14 interfacial tension,15-19 water mobility,17 and foaming performance20 of oil recovery using surfactant flooding have been studied by a variety of experimental techniques. It was found that the surfactant molecules can significantly lower the interfacial tension of oil-water interface and alter the wetting properties of solid surface.14,

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Moreover, surfactant molecules can also disturb

adsorption layers of oil molecules formed on the solid surface,23-24 which is helpful for the occurrence of oil molecules detachment. Kolev et al. reported governing factors for spontaneous detachment of oil drops from solid substrates.25 Thoreau et al. analyzed the roles of substrate material properties and surfactants playing in the removal process of oil droplet.26 Morton et al. proposed a thermodynamic model for the prediction of contact angles of oil droplets on solid surfaces in SDS solution.27 Gupta and Mohanty studied the effect of salinity, surfactant concentration, electrolyte concentration and temperature factors on the wettability alteration of carbonate reservoirs.28 Li et al. investigated the process of oil detachment from silica surface modified by carboxy groups in aqueous surfactant (cetyltrimethylammonium bromide, CTAB) solution.29 Many other studies30-37 are also helpful to understand the nature of efficiency of chemical flooding in oil recovery. ACS Paragon Plus Environment

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However, However, despite these studies have provided great insights into the macroscopic dynamics of oil recovery process, due to the different properties of reservoir rocks and the complicated components of crude oil, the understanding of molecular mechanism of oil detachment upon chemical flooding is still a challenging research subject. Therefore, studies on the nanoscale dynamics of surfactant flooding process are of fundamental importance for designing the surfactant molecules with high efficiency of oil recovery. Molecular dynamics (MD) simulation is a powerful technique to probe the microscopic details of chemical processes. Over the past decades, MD approaches have been applied to describe the molecular scale properties of oil/water, oil/water/surfactant interfacial systems.38-39 On the basis of MD simulations, recently a three-stage mechanism was proposed to describe the oil detachment from silica surface upon surfactant flooding.40 The theoretical simulations proposed that the oil detachment from the silica surface involved three stages. In the first stage, upon the addition of surfactant in the aqueous phase, the water molecular channel perpendicular to the silica surface is first formed, in this stage the driving force is the interaction between the water molecule and the substrate. In stage two, the nearest oil layer from the silica surface is destroyed due to the water diffusion on the silica surface through the Columbic interaction between water molecules and the hydroxyl group of the silica. In stage three, the buoyancy force facilitates detachment of the oil drop from the substrate. After these three stages, the substrate will be cleaned and the oil layer becomes oil droplets. In this three-stage oil detachment mechanism, the formation of water molecular channel is a crucial step. Through formation of water molecular channel, water molecules can penetrate into the oil-water interface by diffusing and form a gel layer at the water/silica substrate interface. The propagation of water molecules on the silica substrate then significantly accelerated the removal of the oil molecules from the solid substrate. Experimental evidences of the water molecular channel formation were reported by Chen et al.41 Using nanoscale characterization techniques including quartz crystal microbalance with dissipation, atomic force microscopy, ellipsometry, and dynamic light scattering, Chen et al.41 suspected that the pores and nanochannels within the asphalt film could guide



the formation of water molecular channels, which facilitated the removal of asphalt from solid substrate. However, although the formation of water molecular channels was confirmed by both theoretical simulations and experiment characterizations, several details still need to be further investigated. The first issue is that whether the water molecular channel formation is a popular process of surfactant flooding or not? As the previous theoretical studies40 only studied one kind of surfactant, e.g. cationic type surfactant CTAB, it is therefore of importance to survey the effect of other types surfactant molecules such as the anionic and non-ionic surfactants. Secondly, whether the intermolecular interaction between surfactant and oil molecules is the exclusive factor that drives the formation of water molecular channels? What is the effect of the electrostatic interaction between water molecules and polar substrates on the formation of water molecular channels? Thirdly, in the actual oil recovery process, the water phase flows relative to the attached oil phase. Then, what is the effect of aqueous phase flow on the water molecular channel formation? In order to understand these questions, we have performed a series of parallel MD simulations using a rectangular basic box containing 130 oil molecules (dodecane), 20 surfactant molecules and 5450 water molecules. In these simulations, three factors have been taken into considerations. Firstly, different types of surfactants, including cationic dodecyl trimethyl ammonium bromide (DTAB), anionic sodium dodecyl benzene sulfonate (SDBS) and non-ionic octylphenol polyoxyethylene(10) ether (OP-10), are considered. Secondly, by changing the atomic charges of Si and O atoms of the silica substrate based on different empirical force field parameters, the effect of substrate polarity on the water molecular channel formation is explored. Finally, a non-equilibrium MD simulation is carried out to elucidate the effect of aqueous phase flow on the formation and expansion speed of water molecular channel. Through these MD simulations, we confirmed that the water molecular channel formation is a common step for all three types of surfactant flooding, which induces the oil molecule detachment from the silica surface. The surfactant molecules can perturb the packing structure of adsorbed oil molecules, leading to some ‘cracks’ on the top oil layer. After then, water molecules may diffuse into these ‘cracks’. The electrostatic interaction


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between the silica substrate and water molecules is found to be a determined factor for water molecular channel formation. By considering the three surface models in this work, we found that on the low polarity model silica surface, the formation of water molecular channel is prohibited. However, on the model silica surface with stronger polarity, formation water molecular channel is very feasible upon surfactant flooding. The molecular structure of surfactants also affects the formation and expansion speed of water molecular channels. Our simulation results indicate that the expansion speed of the water molecular channels upon SDBS and DTAB surfactant flooding is more rapidly than that of OP-10 system. This is largely due to the long molecular chain configuration of the OP-10 molecule. The aqueous phase flow may also affect the formation and expansion speed of the water molecular channel. Our simulation results indicate that the expansion speed of water molecular channel does not in proportion to the water flow speed and the static water layer entails the rapidest expansion speed of water molecular channel. For comparison, the increase of water phase flow speed will suppress the water molecular channel expansion speed to different extents.

II. Simulation Methods and Details 2.1 Force Field Parameter Sets All MD simulations were carried out using LAMMPS42 and all the organic molecules were parameterized using the polymer consistent force field (PCFF)43 and the SPC/E44 model was used for water molecules. The PCFF includes the bonding and the non-bonding potential function. The bonding potential function is composed of bond stretching, angular bending, dihedral angle torsion, out-of-plane interactions and cross terms and the non-bonding potential functional consists of the long-range electrostatic interactions and the short-range van der Waals (vdW) interactions. The PCFF is a widely used all-atom force field and has been validated to be capable of accurately predicting structural and thermodynamics properties for a broad range of organic and inorganic compounds, and the -C-SO3, -N-(CH3)3 and -OC- groups can be also well described by the PCFF.45-53 2.2 Construction of Silica Surface In this work, the (001) silica surface is used to model the silica substrate. The ACS Paragon Plus Environment


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dangling O atoms on the (001) surface are all saturated by the H-atom, as shown in Figure 1. The density of surface hydroxyl group is 9.57 nm-2, which is in the range of crystal chemistry calculations (5.9~18.8 nm-2)54 and constituent to the previous theoretical work5. Dodecane (C12) is used as the model oil molecule. The chemical surfactants include DTAB, SDBS and OP-10, respectively. In order to study the effect of surface polarity on the water-channel formation, we have chosen three atomic charge sets for the Si, O and H atoms of silicon substrate, which are extracted from the COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies)55, CVFF (consistent valence force field)56 and INTERFACE force field57, respectively (as show in Figure 1b-1d). The L-J parameters of Si and O atoms of the silica surface are extracted from the PCFF (combine with charge set A and B) or INTERFACE force field (combine with charge set C).

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Figure 1. (a) Top and side view of the (001) surface of silica used in the simulations. Red balls denote O atoms; Yellow balls denote Si atoms and the gray balls denote H atoms. Three kinds of atomic charge set used for silica surface: (b) charge set A from the COMPASS force field; (c) charge set B from the CVFF force field and (d) charge set C from the INTERFACE force field, which are extracted from the COMPASS, CVFF and INTERFACE force fields, respectively.

2.3 Construction of Simulation Model To prepare the configuration of the simulations, a rectangular box of 58.96×59.57×69.27 Å3 is used, with the z-axis being perpendicular to the silica surface. Periodic boundary conditions are applied in all directions. At the beginning, 130 C12 molecules are randomly placed on the silica surface and the initial molecular


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configurations are minimized using the steepest descent method. After the energy minimization, a NVT MD simulation of 10 ns with a time step 1fs is carried out to obtain the equilibrium absorption configuration of the C12 molecules. The temperature and pressure are kept at 300 K and 1 atm, respectively. Figure 2 displays the snapshot of the equilibrium C12 molecular adsorption configuration on the silica surface. After 10 ns simulations, it can be seen that the C12 molecules have migrated to the silica surface rapidly. The analysis of the density profile indicates that three clear peaks located at z = 1 nm, z = 1.5 nm, and z = 1.9 nm, respectively, corresponding to the formation of three C12 molecular layers on the silica surface. This kind of C12 molecular adsorption configuration is in good agreement with the previous theoretical reports.40 As shown in Figure 2, it is found that the C12 layers are parallel to the silica surface. A sharp peak centered at around 1 nm indicates the formation of the first adhesive layer (Figure 2a) on the silica surface. In this molecular layer, the C12 molecules show well-organized structures and extend over the whole silica surface. The decreases of density of the second and third C12 layer indicate there are more ‘vacancies’ in these molecular layers comparing to the first molecular layer. A pre-equilibrated water/surfactant layer containing 5450 water molecules and 20 surfactant molecules (DTAB, SDBS or OP-10, molecular structure diagrams are displayed in Figure 2c) is then added on the top of C12 layer. Briefly, a water layer containing 5450 water molecules (with density of 1 g/cm-3) is first constructed and relaxed by 1 ns NPT MD simulation with the temperature and pressure are kept at 300 K and 1 atm, respectively. After constructing the water layer, 20 surfactant molecules are inserted into the water phase and both the SDBS and DTAB molecules are tilted perpendicularly to the oil/water interface. As for the OP-10 molecule, we have tilted their molecular chain orientations due to long molecular chains. This water/surfactant system is then underwent further relaxation by both steepest descent method and subsequent 2 ns NPT MD simulations with the atomic coordinates of surfactant molecules are fixed. These procedures are aiming to remove the bad contact between the water and surfactant molecules. The produced water/surfactant layer is then added on the top of oil layer. After adding the layer, a 2 ns NVT simulation is first performed with the surfactant molecules are fixed. This procedure is used to relax the interface structure between the



water and oil phase and remove some bad molecular contact. After then, a 40 ns NVT MD simulation is performed with the temperature of system kept at 300 K by the Nosé-Hoover method, in which the coupling constant is 0.1 ps, and the trajectories are collected with an interval of 1 ps in the production period. In all simulations, the long-range Columbic interactions are computed using the particle-particle particle-mesh (PPPM) algorithm58, with a convergence parameter of 1.0×10-4. The cut-off of vdW interactions is set to 12 Å. During the MD simulations, we adopt separate thermostats for the silica substrate and the fluid phase. The force and velocity of the Si and O atoms in the silica substrate are set to 0.

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(c) Figure 2. (a) The top and side views of equilibrium configuration of C12 molecule layer on the silica surface. (b) The density profile of the C12 molecules along the vertical direction. (c) The molecular structure diagrams of DTAB, SDBS and OP-10 molecules.

III. Results and Discussions 3.1 The Interfacial Structure of the Silica/Oil/Water Interface without Inclusion of Surfactant Molecules In order to verify the effect of surfactant molecule on the oil detachment process, we first investigate the interfacial properties of a three-phase system including a layer of pure water molecules, a layer of C12 oil molecules and a silica substrate. As shown in Figure 3, in the initial stage of MD simulations, the oil phase consists of three-layer structure. With the progress of the simulation (4 ns), the layered oil phase remains stable, in which the packing structure of underlying two layers of oil phase changes slightly. For comparison, because of top layer of oil phase contact directly with the water molecules, the configuration change of the top layer C12 molecules is remarkable. However, although the top-layer oil molecules have been disturbed, the water molecules cannot penetrate into the oil layer. From the energy diagram and the snapshot of 4 ns simulations, as displayed in Figure 3a and 3b, it can be seen that the interfacial structure between water and C12 molecules as well as the C12 molecules and silica surface remains stable. These simulation results indicate that in the absence of surfactant molecules, the rock reservoir, oil phase and water phase keep a stable interfacial configuration, the oil phase cannot be peeled from the silica surface.



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(b) Figure 3. (a) The snapshots of molecular configurations of the silica/oil/water interface without inclusion of surfactant molecules. Red balls denote O-atom. Grey lines denote the C12 chains and the silicon atoms are marked by yellow color balls. The H atoms are not displayed for clarity. (b) The evolution of potential energy of the system.

3.2.1 Effect of Surfactant Molecules on the Oil Detachment Process and Water Molecular Channel Formation Figure 4 gives the density profile of C12 and water molecule perpendicular to the oil/water interface and the MD snapshots of the silica/oil/water system upon the surfactant solution flooding. The simulation time lasts 40 ns for each surfactant system. We note that in this section of discussions, the silica surface has a stronger polarity. The atomic charges of silica substrate are assigned from charge set B, as shown in Figure 1c.


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(c) OP-10 system Figure 4. Density profiles of C12 and water molecules perpendicular to the oil-water interface and MD snapshots of the water molecular channel formation upon three different types of surfactant flooding: (a) DTAB, (b) SDBS and (c) OP-10.

From Figure 4, the density profile of water and MD trajectories indicate that for all three types of silica/water-surfactant/oil/ systems, the water molecular channel formation is a common step. In the initial stage, as shown in Figure 4a – 4c, at the beginning of the simulation, the thickness of the C12 layers is about 2 nm and no water molecule enter into the C12 layer. However, in the presence of surfactant interactions, the hydrophilic silica surface which is covered by C12 molecules becomes thermodynamically unstable. Surfactant molecules such as SDBS, DTAB and OP-10 diffuse rapidly from the aqueous ACS Paragon Plus Environment


phase to the water-C12 interface. For ionic surfactant molecules, it is found that in a very short time period, e.g. about 0.5 ns, almost all surfactant molecules have penetrated into the oil phase. The hydrophobic carbon tails of the surfactant molecules approach the water-C12 interface and gradually insert into the oil phase. In this process, the hydrophobic interaction is the major driving force. As shown in Figure 4a – 4c, we also noticed that a few surfactant molecules can aggregate to form micelle or premicelle in the aqueous solution. For comparison, the diffusion of non-ionic surfactant (OP-10) to the water-C12 interface takes more time. Because of the relative long molecular chain, the OP-10 molecules tend to aggregate into micelle or premicelle firstly, and then gradually approach the water-C12 interface. From the present simulations, at about ~5 ns, almost all OP-10 molecules have diffused to the water-C12 interface. One interesting phenomenon is found that the OP-10 molecules tend to form an adhesive layer between the water and C12 phases and the alkyl tails of OP-10 do not fully penetrate into the C12 layer. For all three types of the surfactant flooding, the water molecular channel formation is a rapid process. As shown in Figure 4a - 4c, during the surfactant molecules approaching the C12 layer, the long alkyl chains of surfactant molecules significantly disturb the aggregates of alkane molecules, especially the alkane molecules in the upmost layer which contacts directly with the water phase. The less ordered molecules in the upmost C12 layers are ripped by the alkyl tails of surfactant molecules, resulting in some ‘cracks’, which can be considered as the sign of water molecular channel formation. Upon the continuous perturbation by alkyl chains of surfactant molecules, some water molecules have entered into the next alkane layers caused by the long-range electrostatic interactions between the silica surface and the water molecules. These water molecules destruct the ordered layers of alkane molecules and finally reach the hydrophilic silica surface. The analyses of water molecular channel formation processes in three surfactant systems give important information about the initial stage of oil detachment processes. Our results indicate that the water molecular channel formation is a general step for all three kinds of surfactant solution flooding, but the formation speed of water molecular channel and the diffusion rate of water molecules on the silica surface are much different.


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In Figure 5, we have monitored the amount of water molecules in the water molecular channel at different simulation time. As shown in Figure 5b, at the initial stage of water molecular channel formation (within ~2 ps), three systems have similar amount of water molecules in the channel. In this time period, the formation speed of water molecular channel in the non-ionic OP-10 system is slightly more rapidly than other two systems. Moreover, the MD snapshots shown in Figure 4c indicate that in the OP-10 system two water molecule channels are formed, which suggests stronger perturbations of OP-10 surfactant molecules on the C12 layer. Once the water channel has formed, the H-bonding interactions are proposed to be a major driving force for the detachment of the C12 layers from the solid surface, we denote this time period as the expansion stage of water molecular channel. Liu et al.40 proposed that the types of H-bonding include the ones between the water molecules and the hydroxyl group on the silica surface, as well as those among the water molecules in the water channel. These H-bonding interactions were helpful for the adsorption and propagation of water molecules on the silica surface. In the present simulations, we found that the expansion speeds of water molecular channel in three kinds of surfactant solution systems were different. From Figure 5, the water channel expansion speed shows an order of anionic SDBS system > cationic DTAB system > non-ionic OP-10 system. These results indicate that the interactions between water molecules and surfactants may also affect the water molecule diffusion rate in the oil phase and on the silica surface. From the MD trajectories shown in Figure 4a - 4b, for the ionic surfactants such as SDBS and DTAB, because of their rapid diffusion rate, some surfactant molecules may diffuse into the water channels rapidly and wrap the C12 molecules. The polarized head groups in these ionic surfactant molecules may also bond certain number of water molecules through the H-bonding interactions, which facilitate more water molecules propagating into the water molecular channel. On the other hand, the surfactant molecules in the water molecular channels also employ lateral interactions on the attached C12 layer, which accelerate the shrink of C12 layer and lead to more water molecules adsorb on the silica surface. For comparison, the water molecular channel expansion speed in the OP-10



surfactant system is obviously slower than the ionic surfactant systems. As shown in Figure 4c, the MD snapshots indicate that after the water molecular channel formation, because of larger molecule size, the OP-10 molecules cannot diffuse into the water channel. The OP-10 molecules keep the adhesive adsorption configuration on the top of C12 layer. As the result, the diffusion speed of water molecules on the silica surface in the OP-10 system is much slower than that in DTAB and SDBS systems. The snapshots of the water pores formed in three systems at the simulation time of 40 ns are displayed in Figure 6. It can be seen that within the same simulation time, the size of water pore in the SDBS and the DTAB systems is much larger than that in OP-10 system. The radius of the water pores in SDBS, DTAB and OP-10 system is rs= ~22 Å, rd= ~19 Å and ro= ~16 Å, respectively. Among three surfactant systems, the formation speed and the radius of water molecular channel upon anionic SDBS solution flooding is the fastest and largest in all three types of surfactant systems. 400

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(b)

Figure 5. A comparison of the number of water molecules in the water molecular channels at different simulation times (a: 0~25ns; b: 0~2ns) and in different surfactant solution systems. In case of how to distinguish the water molecules that are part of the channels, we have defined a threshold distance perpendicular to the silica surface. The threshold distance value is set as 14.8 Å, corresponding to the thickness of bottom two adsorbed oil molecular layer (c.f. density profile of the C12 molecules displayed in Figure 2b).


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SDBS

DTAB

OP-10

Figure 6. Snapshots of C12 configurations (grey lines) at the 40 ns (t = 40 ns) and comparison of size of water pores in three surfactant solution systems. The blank areas denote the water pores. R is the radius of the water pores.

In order to obtain a deeper understanding of the interactions between oil molecules and the surfactant molecules, we have compared the interaction energies between C12 and surfactant molecules. The interaction energies ∆E5, 59 between surfactant molecules and oil molecules are calculated from eq. (1), where E(C12+Surfactants) is the potential energy of C12 and surfactant molecules (without inclusion of water and silica surface), the E(C12) is the potential energy of C12 molecules and the E(Surfactants) is the potential energy of the surfactant molecules. ∆E = E(C12+Surfactants) – E(C12) – E(Surfactants)

eq. (1)

As shown in Figure 7, it is found that the interaction energies between the C12 and surfactant molecules show an order of ∆EOP-10 > ∆EDTAB ≈ ∆ESDBS. Of interesting, this interaction energy order is opposite to the observed expansion speed of water molecular channel. Here, we think the order of interaction energy is in agreement with the MD snapshots displayed in Figure 4. Firstly, because of the stronger interactions between OP-10 and C12 layer, the initial water channel formation speed in the OP-10 system is the fastest. However, caused by the strong van der Walls interactions, the OP-10 molecules tend to form parallel adhesive layer onto the C12 layer (Figure 4c). Such parallel adhesive layer is not propitious for further shrink of oil molecule layer. For the ionic SDBS and DTAB systems, they have relatively smaller interaction energies with the C12 layer. The amphiphilic property and relatively smaller size of these surfactants also entail them more



rapidly diffusion rate in the water molecular channel. Therefore, the water molecular channel expansion speed in these ionic surfactant systems is much faster than that of OP-10 system. -100

Interaction Energy/(kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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DTAB SDBS OP-10

-200 -300 -400 -500 -600 -700 -800 0

5

10

15

20

25

30

35

Time/ns

Figure 7. Interaction energies between the C12 layer and surfactant molecules.

3.2.2 Effect of Substrate Polarity on the Water Molecular Channel Formation In this session of discussions, we have investigated the effect of substrate polarity on the water molecule channel formation. It was pointed out that the columbic interaction is another factor driving the water molecular channel formation.40 Compared to the vdW interactions, the columbic interaction belongs to the long-range interactions, which begins in the range of several nanometers. In present model system, the thickness of the oil phase is around 2 nm, which is in the range of the columbic interaction. In order to investigate the effect of substrate polarity on the water molecular channel formation and oil detachment process, additional set of surface atom charges is considered. As discussed above, under the atomic charge sets A, the electrostatic interactions between the water molecules and the silica substrate might be a major driving force for the water molecular channel formation. Here, we have considered another set of Si and O atomic charge extracted from the COMPASS force field (charge set A displayed in Figure 1b). When the atomic charge set is switched to charge set A, the polarity of silica substrate is decreased. In the same way as discussed above, we have carried out a 40 ns MD simulation for each system and monitored their molecular configuration changes. As shown in Figure S1-S3, under this atomic charge set, a strike difference from the stronger polarity rock


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systems is that the formation of water molecule channel is not observed in all kinds of surfactant solution flooding. Although the alkyl chains of surfactant molecules may penetrate into the C12 layer, the water molecules cannot inject into the oil phase. It is worth to mention that for the OP-10 system, as shown in Figure S3, during the initial stage of simulations, some water molecules can penetrate into the C12 layer caused by the stronger perturbation of the surfactant molecules. However, due to the weaker substrate polarity, the electrostatic interactions cannot drive water molecules to further diffuse into the oil layer. These parts of simulations indicate that the electrostatic interactions between water molecule and polar substrate have significant impact on the water molecular channel formation. For the silica substrate with stronger polarity, the formation of water molecule channel is favorable. In Figure 8, we further evaluate the columbic interactions between one water molecule and the silica surface with different atomic charge set. For the stronger polarity substrate (charge set B in Figure 1c), the columbic interactions between the water molecule and the silica surface are much larger than the weaker polarity substrate (charge set A in Figure 1b). These results explained why the water molecule channel formation is not observed in the weaker polarity substrate systems. The long range columbic interactions between the substrate and water molecule is one of major driving forces for the water molecular channel formation.

0.0 -0.2

Interaction Energy/(kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.6

Charge set B Charge set A

-0.8 -1.0 -1.2 -1.4 -1.6 -1.8 0

10

20

30

40

Distance/Å


50


Figure 8. Comparison of interaction energies between the water molecule and the polar silica surface with different atomic charge sets.

MD simulations based on the INTERFACE force field parameters were also carried out to validate the effect of substrate polarity (or electrostatic interactions) on the water molecular channel formation. In these simulations, we have switched the force field parameters of silica surface to the INTERFACE force field (charge set C in Figure 1d). The INTERFACE force field parameters had been validated against the heats of immersion of real silica materials. From Figure 1d, the atomic charges of Si and O atoms in the INTERFACE force field are obviously larger than both charge sets A and B. As displayed in Figure S4-S6, because of the stronger substrate polarity, the formation and expansion of water molecular channel in three kinds of surfactant systems are all accelerated. Moreover, within 30 ns MD simulations, it can be clearly seen that the aggregation phenomena of oil phase changed from flat oil layer into spherical oil drop for all systems. These results further confirm that the substrate polarity (or electrostatic interactions) has a significant impact on the water molecular channel formation and expansion. In order to avoid the artifacts of long range interactions, the influence of the thickness of the silica substrate is examined. As the thickness of silica substrate in the present MD simulations is relative small (about 10 Å), the oil, surfactant and water in contact with the oil/surfactant layer may ‘feel’ the interaction with water molecules on the other side of the layer. Here, a simulation model containing a thicker silica substrate (about 20 Å) is also built and the OP-10 system is chosen to investigate the substrate thickness effect. As shown in Figure S7, the MD results indicate the general conclusion such as the water molecular channel formation mechanism is not affected by the substrate thickness. 3.2.3 Effect of Water Phase Flow on the Water Molecule Channel Formation Finally, we discuss the third factor that may affect the water molecule channel formation. In above discussions, a static water phase is considered. It is known that in the oil recovery process, the injected surfactant solution is flowing relative to the oil layer with a certain speed. It is of interesting to explore whether the water molecular channel formation will be accelerated or suppressed by the water flowing effect?


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Herein, a non-equilibrium MD simulation is carried out to study the water flow effect on the oil detachment process. As shown in Figure 9a, a silica/oil/SDBS solution three-phase model is constructed. In order to study the water flow effect, the upper 1 nm water layer is set as the slide layer. During the MD simulations, an external force is added to the water molecules in the slide layer, which leads to the flow speed of this layer is constant 1 m/s, 2 m/s and 3 m/s, respectively. We note that with the flow of the upper water layer, the water molecules included in the below phases obtained a flow speed as well.

350


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


v = 0.0 m/s v = 1.0 m/s v = 2.0 m/s v = 3.0 m/s

300 250 200 150 100 50 0 0

1

2

3

4

5

6

7

Time/ns

(a)

(b)

Figure 9. (a) The initial model with a slide layer (b) A comparison of the number of water molecules in the water molecule channels at different simulation times and in different velocity of slide layer.

We found that the flow velocity of the slide layer affects the formation and expansion speed of the water molecular channel. First of all, for the four velocities of slide layer, their common feature is that when the simulation time is less than 6 ns, the number of water molecules in the water channel increases rapidly. This phenomenon means that the water channel has a fast expansion speed at the initial stage. Secondly, we analyzed the effect of the slide layer velocity one by one, as shown in Figure S8– S10. When the slide layer velocity is 1 m/s, the water channel has the fastest expansion speed. When the slide layer speed increase to 2 and 3 m/s, the expansion speed of water channel decrease. After more than 6 ns, the corresponding curve in Figure 9 becomes ‘flat’, the expansion of the water molecular channel becomes slower. According to Bernoulli’s law, when the slide



layer is moving, the pressure in the lower water phase becomes smaller. Compared with the data in Figure 9 and Figure 5, if the velocity of the slide layer is zero, the water molecular channel has the fastest expansion speed.

Conclusion Using the all-atomic force field molecular dynamic simulations, the oil detachment process from a hydrophilic silica surface is investigated. Our simulation results confirm that the water molecular channel formation is one of the possible mechanisms for understanding the oil detachment process upon the surfactant flooding. However, the water molecular channel formation and expansion is affected by several factors including the surfactant properties, the electrostatic interactions between substrate and water molecules, and the flow speed of surfactant solutions. Our simulation results indicate that for both ionic and non-ionic type surfactant solutions, the perturbation of surfactant molecules on the two-dimensional oil molecule layer facilitate the injection and diffusion of water molecules between the oil layer and silica substrate. However, only for the rock surface with relative stronger polarity, the formation of water channel is observed. For different types of surfactants, the DTAB and SDBS solutions lead to more rapidly formation and expansion of water channel than the non-ionic OP-10 surfactant. The electrostatic interactions play a vital role in the water molecular channel formation. For the weaker polarity substrate, because of the weaker electrostatic interactions between substrate and water molecules, the peneration of water molecules into oil/substrate layer is not observed even in the presence of perturbation of surfactants. The flow speed of surfactant solution also affects the formation and expansion speed of water channel. The increase of surfactant solution flow speed suppresses the water molecular channel expansion speed to different extents. Finally, we note that in the present simulations, a defect free and fully hydroxylated silica surface was used to mimic the rock substrate. We emphasized that this simplified model may be quite different from the real geological environment. In the latter case, the surface properties of silica surface are rather complicated and the present simulation results are only validated on the ideal defect free and fully hydroxylated silica surface. Further studies are certainly necessary to clarify the


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water molecular channel formation mechanism using more realistic surface models.

Supporting Information The trajectory snapshots of the different polarity systems (charge set A and C), the MD snapshots of models with different velocity of side layer (v = 1m/s, 2m/s and 3m/s) and the force field parameters (PCFF and INTERFACE) are given. These materials are available free of charge via the internet at http://pubs.acs.org.

Corresponding Authors [email protected] (Y. P.); [email protected] (X. T.); [email protected] (B. P.)

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by PetroChina Scientific Research and Technology Development Project (2014A-1001) and the National Natural Science Foundation of China (21373176, 21422305, 21773201). References (1) Moncayo-Riascos, I.; de León, J.; Hoyos, B. A. Molecular dynamics methodology for the evaluation of the chemical alteration of wettability with organosilanes. Energ. Fuel. 2016, 30 (5), 3605-3614. (2) Zhang, L.; Lu, X.; Liu, X.; Yang, K.; Zhou, H. Surface wettability of basal surfaces of clay minerals: Insights from molecular dynamics simulation. Energ. Fuel. 2016, 30 (1), 149-160. (3) Wang, P.; Li, Z.; Ma, Y.; Sun, X.; Liu, Z.; Zhang, J. The coarse-grained model for a water/oil/solid system: Based on the correlation of water/air and water/oil contact angles. RSC Adv. 2015, 5 (63), 51135-51141. (4) Wang, S.; Li, Z.; Liu, B.; Zhang, X.; Yang, Q. Molecular mechanisms for surfactant-aided oil removal from a solid surface. Appl. Surf. Sci. 2015, 359, 98-105. (5) Yan, Y.; Li, C.; Dong, Z.; Fang, T.; Sun, B.; Zhang, J. Enhanced oil recovery mechanism of CO2 water-alternating-gas injection in silica nanochannel. Fuel 2017, 190, 253-259. (6) Sun, J.; Zhang, H.; Hu, M.; Meng, X.; Yuan, S. Molecular dynamics study on oil migration inside silica nanopore. Chem. Phys. Lett. 2017, 678, 186-191. (7) Wang, S.; Li, Z.; Liu, B.; Zhang, X.; Yang, Q. Molecular mechanisms for surfactant-aided oil removal from a solid surface. Appl. Surf. Sci. 2015, 359, 98-105.



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