Molecular Dynamics Simulation of the Oil Detachment Processwithin Silica NanoporesHui Yan† and Shiling Yuan*,‡

†School of Pharmacy, Liaocheng University, Liaocheng 252059, China‡Key Laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China

*S Supporting Information

ABSTRACT: We investigated the effect of surfactants on the oil displacement process inside a nanoscale silica pore usingmolecular dynamics simulations. First, an oil cylinder was built inside a silica pore to mimic residual oil in the porosity of thereservoir rock after water flooding. In the simulations, we focused on a layering organization of oil molecules adsorbed onto thepore surface, and then a series of equilibrium MD simulations were run to obtain the organization structures of the oil drop in thepresence or absence of surfactant molecules. These simulated results showed that the surfactant could disturb the layeringorganization of the oil drop, since the hydrophobic chains of surfactant molecules could penetrate into the oil phase. And aroundthe polar head of the surfactant, water molecules could form water channels between the oil phase and solid surface, which is vitalto the displacement process. Finally, we used steered molecular dynamics (SMD) method to mimic the displacement andmigration process of an oil drop inside the pore. From SMD calculations, detailed information about the process was obtained,and the free energy of the process was calculated using the WHAM method. Through analysis of the free energy, wedemonstrated the mechanism of surfactants aiding in the oil recovery at a molecular level. Our study provided information on theoil displacement within a nanoscale pore at the molecular level, which is expected to provide useful information for enhanced oilrecovery (EOR) experiments.

1. INTRODUCTIONAfter water flooding, trapped residual oil is distributed in thepores of the reservoir rock in the shape of static oil droplets dueto capillary action, which makes the oil recovery process moredifficult.1 In recent years, enhanced oil recovery (EOR)techniques have been widely used to increase crude oilproduction. Many EOR techniques are applied in the oilrecovery process, such as chemical injection and gas injection.2

In these techniques, the surfactant is widely used in chemicalinjection, which makes it feasible to recover the oil. It is knownthat the surfactant molecules can be adsorbed onto the oil−fluid interface, which could reduce the oil/water interfacialtension (IFT) and cause wettability alteration on the fluid/solidinterface.3,4

The wettability-altering behavior of the solid/fluid system is amajor factor in the EOR process, which can be controlled bysurfactants due to their surface activity. Many experimentalstudies were performed to understand the mechanism ofsurfactants on residual oil by micro models of oil displacementover the past few decades.5−11 In these experiments, a single

cylindrical capillary is usually employed as the micro model toconstruct a simplified system for multiphase flow in porousmedia, which is convenient to mimic the micro situations in theoil recovery process.Tiberg et al.8 used an oil-filled capillary tube to study the

imbibition behavior of a surfactant solution. They found that itwas difficult for the surfactant solution to invade into thecapillary tube even in the case of high surfactant concen-trations.8 On the basis of their observations, Hammond andUnsal9 modified the model for forced imbibitions. Their resultsshowed that under additional forced conditions the meniscuswould move faster. Later, they reported a further investigationwith diffusion of surfactant solutions in front of the mobile oil/water meniscus.10 They found that the adsorption of surfactantmolecules onto the capillary surface is significant for the fluidflow in the capillary tubes. Recently, Wasan et al.11 investigated

Received: October 8, 2015Revised: January 16, 2016Published: January 20, 2016

Article

pubs.acs.org/JPCC

© 2016 American Chemical Society 2667 DOI: 10.1021/acs.jpcc.5b09841J. Phys. Chem. C 2016, 120, 2667−2674

the displacement of hexadecane by sodium dodecyl sulfate(SDS) micellar solution using glass capillaries. They found thatthe cylindrical droplets were finally detached from the capillarysurface, forming spherical drops within the capillary. Based ontheir observations, a theoretical model to assess the dewettingvelocity for the immiscible liquid inside a cylindrical tube wasdeveloped.These experimental investigations help us to understand the

mechanism of oil displacement in a capillary, but themicroscopic insights into the detachment process are difficultto be obtained through experimental technologies. Moreover,experimental measurements of thinner capillaries are challeng-ing because of the relatively small size, especially for themeasurements of thinner capillaries of a nanoscale diameter. Amore adequate approach to gain more microscopic insights intothe oil displacement process, which is considered supplementalto experimental observations, is to carry out computationalsimulations.Over the past decades, molecular simulations have become

an adequate approach to investigate the behavior of liquidsconfined in nanoscale geometries.12−16 Simulations ofsurfactants or liquid systems adsorbed on a slab of solidsurface are now relatively common;17−20 however, to ourknowledge, molecular simulation studies on the oil displace-ment process inside capillaries are scarce. Early simulationstudies have focused on the spreading drop dynamics on aporous surface or liquid penetration into a cylindrical poreusing molecular kinetic theory (MKT) dynamics.21−23 Morerecently, dissipative particle dynamics (DPD) simulations havebeen applied to research fluids in nanoscale geometries. Millanand Laradji used the DPD method to understand structural andtransport properties of driven polymer solutions in nanoscalechannels.24 Chen et al. carried out many-body DPD simulationsto study the water−oil displacement process in capillaries underan external force.25 They demonstrated that both strengtheningthe interaction between water and capillaries and weakening theapplied force can displace the entire oil drop from thecapillaries.In this work, we performed a series of simulation studies of

oil displacement within a cylindrical silica pore using the all-atom molecular dynamics (MD) method. Silica was selectedsince it is a major composition of glass capillaries or rockminerals (e.g., quartz sandstone) in many geological environ-ments. Our attention focused on the effect of the surfactant,cetyltrimethylammonium bromide (CTAB), on the adsorptionof oil drops on the pore wall with equilibrium MD simulations.Using the steered MD method, the displacement and migrationprocess of an oil drop under an externally applied force wasinvestigated. We concluded with some observations from thesimulation that may be useful in understanding the mechanismof oil detachment for EOR experiments.

2. SIMULATION METHOD2.1. Model Systems. For crude oil, a model proposed by

Matsuoka et al. was adopted, which consists of eight kinds ofhydrocarbon molecules, including hexane (HEX), heptane(HEP), octane (OCT), nonane (NON), cyclohexane (CHEX),cycloheptane (CHEP), toluene (TOL), and benzene (BEN)molecules.3 A model silica nanopore was obtained according toprevious publications.14,15 First, a cylindrical hole of diameter d= ∼30 Å was carved from an amorphous silica block ofdimensions Lx = Ly = 98.26 Å, Lz = 108.10 Å, by removing allthe atoms lying along the z-axis within the diameter (Figure 1).

The pore was set to be large dimensions to provide sufficientspace for the subsequent pulling simulations to take place.Then, silanol groups were used to saturate the bare Si atoms onthe internal pore walls of the silica block, resulting two kinds ofsilanol groups (Si−(OH)2 and Si−OH), adsorbed on theinternal surface (Figure S1, Supporting Information), with atotal surface density of ∼7.0 −OH/nm2, in accordance withprevious simulation studies.16

In order to get a model of the residual oil inside thenanopore, system I was performed with oil molecules fullyfilling the inside of the cylindrical pore. The oil moleculesincluded the eight hydrocarbon types, and the proportion ofcomponents was in accordance with previous studies (Table1).3 Systems II and III were constructed to investigate the effectof pure water and surfactant solution on the residual oil bytruncating an oil cylinder with a length of ∼30 Å, in which theoil cylinder was selected from the final configuration of system Iafter a 10 ns MD run. We note that the proportion ofcomponents of the oil drop was very similar to that in system I,which suggested that the hydrocarbon species were mixeduniformly along the pore in system I. In constructing the modelof system III, six CTAB molecules were placed at each side ofthe oil cylinder in the pore. After water molecules were addedto fill the rest space of pores besides the oil cylinder in systemsII and III, the box length Lz was increased to 113.10 Å to setthe local water density to the bulk value under ambientconditions (ρbulk = 33.3 nm−3) in the subsequent MDsimulations. In system III, bromine anions were inserted tokeep electrically neutral.

2.2. Computational Details. GROMACS 4.6.3 softwarepackage26 was employed to carry out all the MD simulations.The all-atom optimized performance for the liquid systems(OPLS-AA) force field27 was adopted for all of the potentialfunction terms to calculate the interatomic interactions. Thesimulation parameters for CTAB and oil molecules used in thisstudy were derived from OPLS-AA force field, which is similarto recent studies on the adsorption of CTAB on carbonnanotubes.28 The partial atomic charges of CTAB and oilmolecules were assigned according to the OPLS-AA force fieldbecause of the force field’s good representation of small organicmolecules. The force field parameters of amorphous SiO2 werereferred to the work of Lorenz,29 which gave good results in thedynamics properties of amorphous SiO2. The particle charge ofthe surface atoms of the pore was derived from the previousreport,30 which gave a good prediction of adsorbed water layerson the silicon oxide surface. Water molecules were described bythe simple point charge/extend (SPC/E) model.31 More

Figure 1. Representation of the silica nanopore model (left panel) anddetails of the pore wall functionalization (right panel). The axis labelon the left panel shows the orientation of the nanopore in thesimulation box relative to the z-axis, and d is the diameter of the silicapore.

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detailed information on the force field parameters could befound in Table S1 of the Supporting Information.Each of the systems was first minimized using the steepest

descent method. Following the minimization, MD simulationsunder canonical ensemble (NVT) were carried out for eachsystem with periodic boundary conditions applied in the x, y,and z directions. During the minimization and the equilibrationMD simulation, position restraints were applied to the silicacrystal except for surface atoms. The V-rescale thermostatalgorithm32 was adopted to keep a constant temperature of 298K. Bond lengths were constrained by the LINCS algorithm.33

The particle mesh Ewald (PME) method34 was used tocompute electrostatics interactions. The Lennard-Jones inter-actions were cut off at 1.4 nm for the nonbonded potential. Foreach system, the simulations were performed for 10 ns with atime step of 1 fs. Trajectories were collected at intervals of 0.1ns and were viewed by VMD 1.9.1.35 The last 5 ns trajectorieswere used for further analysis.To study the displacement process of the residual oil inside

the pore, we performed steered molecular dynamics (SMD)36

(i.e., pulling simulations) using the pull code of the GROMACSpackage.26 Structures derived from the equilibrated MDtrajectories of systems II and III were used to perform thepulling simulations. For system II, the driving force was applied

on the whole oil drop (i.e., the pulled group), while for systemIII it was on the oil drop together with CTAB molecules whichwere solubilized into the oil phase. A point on the central axisof the nanopore was selected as a fixed reference using in thepulling simulations. Figure S2 shows a schematic representationof pulling direction. For each system, the external force woulddraw the pulled group from its original position along the z-axis, using a potential with K = 1000 kJ mol−1 nm−2. Fourpulling rates (0.0025, 0.005, 0.01, and 0.025 nm ps−1) wereused to investigate the influence of pulling rate on the SMDsimulations. The target distance of the pulling simulations wasset to be 4 nm.The potential of mean force (PMF) as a function of the

center of mass (COM) of the oil drop displacement ξ along thepore was calculated through a series of biased simulations withvarious ξ spanning the path of oil drop displacement. The 20starting configurations which correspond to the 20 samplingwindows were derived from trajectories of the pullingsimulation with a window spacing of 0.2 nm, for systemswith and without CTAB. In each window, a short equilibrationMD run (500 ps) has been performed before the samplingsimulations. Then, umbrella sampling37 simulations wereperformed for 5 ns in each window. The details of othermethodology for pulling and sampling simulations are the same

Table 1. Details of the Simulation Systems

components of oil

system HEX HEP OCT NON CHEX CHEP TOL BEN CTA+ Br− water

I 54 54 60 72 36 60 60 24II 13 12 13 21 7 13 7 6 2431III 13 12 13 21 7 13 7 6 6 6 2312

Figure 2. (a) Number density profiles along the cylindrical radial direction with respect to the z-axis of the pore. (b) Snapshots of the configurationsof oil molecules inside the silica nanopore from the cylindrical axial view. (c) Scheme of orientation angle θ between vectors of the alkane moleculesand a vector that extends from the pore axis to the pore wall through the COM of a given alkane molecule. (d) Probability distribution of theorientation angle for alkane molecules at each distance from the pore axis.

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as the equilibrated MD simulations. The weighted histogramanalysis method (WHAM)38 was used to analyze the resultsfrom the sampling simulations.

3. RESULTS AND DISCUSSION3.1. Structural Features of Crude Oil in the Silica

Nanopore. To confirm the equilibrium of system I, thepotential energy with time evolution was checked (Figure S3).It was found that the energy remained steady during the last 5ns, suggesting that the system was well equilibrated. Moreover,considering the system is complex, we performed one moresimulations for system I with different initial configuration toguarantee equilibration. The structural properties of oilmolecules inside the nanopore were investigated by exploringthe average number density profiles of relevant species in thepore:

∑ρπ

δ= ⟨ − ⟩ααr

rLr R( )

12

( )Z i

i(1)

where Riα denotes the distance between the position of the

molecule of species α and the pore axis, and Lz denotes thelength of the oil cylinder. The relevant species including carbonatoms of oil molecules and oxygen atoms of the silanol groupswere computed to represent the density distribution of oilmolecules and the silanol groups on the nanopore wall. Figure2a shows the density profiles along the cylindrical radialdirection with respect to the central axis of the pore (i.e., z-axisof the box) for system I.Three evident peaks can be observed from the density

profiles, which exhibited a layering organization of oil moleculesinside the nanopore. The width of a peak in the density profilewas about 0.5 nm, which is approximately equal to a diameterof one n-alkane molecule, suggesting that monolayers hadformed. Figure 2b shows a view of the layering structure. Fromthe figure, three distinct layers, which correspond to the threepeaks shown in the density profile, can be observed. The threepeaks have different intensities, which suggests that the threelayers formed with different organization aggregations. Theresults from the repeated simulations for system I can be foundin Figure S4, which also confirmed the final equilibration andreproducibility of system I. A detailed view of the three layersinside the pore is shown in Figure S5. We observed that the oilmolecules in the outer layer covered the pore surface, resultingin well-organized aggregates, while in the inner layers, there aremore vacancies compared with that in the outer layer. This isprobably due to the different molecular orientation for oilmolecules with respect to the axis of the pore.The orientation of oil molecules inside the pore was

calculated, which is described by the angle θ between vectorsof oil molecules and a vector that extends from the pore axis tothe pore wall through the COM of a given molecule. Theorientation was considered for three kinds of the oil molecules,i.e., n-alkanes, cycloalkanes, and aromatic hydrocarbons. Themolecular vector of the n-alkane molecules was defined byconnecting the two terminal carbon atoms, and a schematicillustrating the orientation angle of n-alkane kind moleculesbetween the two identified vectors is shown in Figure 2c. Notethat when the value of θ is 90°, the corresponding molecularvector is parallel to the pore wall. If the value is 0° or 180°, themolecular vector is vertical to the pore wall. The orientationprobability distribution for n-alkane molecules is illustrated inFigure 2d. The distribution of cycloalkanes and aromatic

hydrocarbons is provided in Figure S6. It can be seen that then-alkane and aromatic molecules have a preference for orientingparallel to the surface of the pore wall at a distance of ∼1.5 nmfrom the pore axis. This means that these species in the outerlayer tend to lie on the pore wall. In the inner layers, the oilmolecules tend to lie at various angles to the pore surface.There is no preferential orientation of cycloalkanes inside thepore. This is because the structure of these molecules is neitherlinear nor planar. When they adsorbed onto the pore wall, partof the carbon atoms of the cycloalkanes anchored onto thepore. The fully covered and well-ordered layer of oil moleculesadsorbed on the pore wall is considered to be crucial for theprocess of oil detachment from the silica surface.

3.2. Structural Features of Residual Oil in the SilicaNanopore. The structural properties of residual oil in thepresence of displacing fluid, i.e., pure water (system II) andCTAB solution (system III), were investigated by analyzing thecorresponding equilibrium MD trajectories. The equilibrationof these systems was determined by monitoring the thickness ofthe oil cylinder with time evolution. The plots are shown inFigure S7. It can be seen that the thickness achieved a stableequilibrium after the first several picoseconds of simulation,suggesting that these systems were well equilibrated.Figure 3 shows the structures at the beginning and the end of

the simulation. It can be found that the oil drop did not migrate

after the MD simulations. CTAB molecules were found to besolubilized into the oil phase from the water phase, with theirhydrophobic chains penetrating the oil phase while hydrophilicheadgroups stayed at the oil/water interface. To guaranteeequilibration and reproducibility, two more simulations fromdifferent initial configurations were performed. Configurationsfrom these simulations are presented in the SupportingInformation (Figure S8). It can be noted that the sameobservations were obtained from these results. Note that inpractice the in situ EOR is normally carried out in the region ofthe ground under high pressures. To ensure reasonability of thesubsequent pulling simulations, we also performed simulationsusing NPT ensemble on the silica slab under high pressures.Figure S9 shows a configuration of the NPT MD run at a

Figure 3. Configurations at the beginning (left) and the end (right) ofthe simulations for systems II and III. Color scheme: oil molecules,cyan; CTA+, violet; and Br−, gray.

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pressure of 100 atm. We can observe that CTAB molecules canalso enter into different layers of the oil aggregations with theirpolar headgroups toward the water phase.The location of the oil drop inside the pore was characterized

by the average number density distributions of componentsalong the axial direction of the silica pore (Figure S10). Thelocal density of water inside the pore in each system was foundto be equal to the bulk value under ambient conditions, ρbulk =33.3 nm−3, and the oil molecules were found to be located inthe same region which extended from ∼2.0 to ∼5.0 nm in thetwo systems. Since the simulation models of these systems wereall derived from system I, the unchanged location regionreflects the immobile oil drop in the silica pore. For the systemswith CTAB, the distribution of the oil molecules expanded dueto the surfactant molecules entering into the oil phase, whichresulted in an increase of the thickness of the oil cylinder.The structures of the oil cylinder in the two systems were

also checked by the number density profile of oil moleculesalong the cylindrical radial direction, as shown in Figure S11. Itcan be seen that the layering organization still existed in theresidual oil drops. However, the intensity of the three peaks wassignificantly reduced, especially for systems with CTAB. Thisindicates that the layering structures in the residual oil drophave been changed, after the displacing fluid filled in the pore.For the system without CTAB, the distributions of watermolecules were observed between the oil droplet and the poresurface. Figure 4a shows that water molecules moved into the

vicinity of the oil aggregates and formed a molecular channel.Interestingly, the water channel formed on the surface with Si−(OH)2 groups. The water channel formed along with themobility of oil molecules, which left a gap for water moleculesto enter. We considered that the formation of the channel wasattributed to the competitive adsorption of water molecules ondifferent densities of the hydroxyl groups on the pore surface.The competitive adsorption was investigated by calculating thecorresponding binding energies using the density functionaltheory (DFT). Figure S12 shows that the interaction betweenSi−(OH)2 groups and water molecules was stronger than thatbetween Si−OH groups and water molecules.

Figure 4b shows that the CTAB surfactant molecules enteredinto different layers of the oil aggregations, under the dispersiveinteraction of the chain with the oil molecules. In the case ofCTAB adsorbed onto the silica surface, and water moleculesaggregated around the polar head of CTAB, water channelswould form between the oil phase and silica surface. With theperturbation the alkyl chains of the surfactant into the oil layer,the arrangement of the oil molecules changed. Moreover, thethickness of the oil cylinder increased more than in the absenceof surfactants (Figure S7) due to the solubilization ofsurfactants into the droplet. Thus, we can conclude that thesurfactant molecules are more efficient than pure water inchanging the microstructure of the oil drop adsorbed on thepore wall. Although both the structures in the two systems withand without CTAB changed, the oil drop was still undissociatedfrom the silica surface. The molecular orientation of n-alkanemolecules in the two systems was also investigated bycalculating the corresponding orientation angle with respectto the pore surface, as shown in Figure S13. It can be seen thatthe linear alkane molecules that adsorbed on the pore wall stilltended to lie on the pore wall. To further mimic the oil dropdetach from the silica surface, pulling simulations were furtherperformed.

3.3. Dynamical Process of Pulling Simulations. In SMDsimulations, an external force was applied on the oil drop tosimulate the driving force from the displacing fluid in theprocess of oil recovery. By pulling on the COM of the oil drop,the oil molecules were forced to move along the silica pore. Toillustrate the movement of the oil drop inside the pore, thedistance between the COMs of the oil drop and the immobilereference with time evolution was calculated (Figure S14). Itcan be seen that the oil drop was pulled along the pore forabout 3.5 nm. To verify the reproducibility of thesenonequilibrium MD simulations, the pulling simulations ofeach system were performed for three times. The profiles of theexternal force against the reaction coordinate of the repeatedSMD simulations are shown in Figure S15. From the figure, wefound that the variation trend of the force for the parallelsimulations was very similar, which confirmed the reproduci-bility of the SMD method.The curves of the external force were also plotted with time

evolution of the SMD simulations, as shown in Figure S16. Wefound that the force rapidly increases during the first ∼300 ps.On the basis of the variation of the COM distance (FigureS14), we found that the movement of the oil drop is muchslower during this period than that in the subsequent period.Take system II as an example, a series of configurations withtime evolution in the SMD simulations are shown in Figure 5.It can be seen that the oil molecules in the inner layers werefound to be first pulled by the external force.After reaching the maximum at ∼300 ps, the force remained

relatively constant. By checking the SMD trajectories, we foundthat from ∼300 ps the oil molecules in the outer layer werepulled and detached from the pore surface. To confirm theobserved phenomenon, the number of carbon atoms in the outlayer (i.e., the carbon atoms located beyond a distance of 1.3nm from the central axis of the pore) was plotted with respectto the reaction coordinate, as shown in Figure S17. It can befound that the number of carbon atoms began to decrease at1.5 nm (i.e., ∼300 ps), which means that the oil moleculesbegan to be detached from the silica surface. At that time, theforce needed to overcome the adsorption action between oilmolecules and the surface. With the oil molecules adsorbed on

Figure 4. Detailed views of the partial structures for systems II (a) andIII (b).

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the pore wall gradually decreasing, less force was required topull the oil drop and the force was reduced (at ∼600 ps). Untilthe entire oil drop has been pulled along the pore for about 3.5nm (at the end of the SMD simulation), the oil drop wasconsidered to be detached from the silica pore. At the moment,a few oil molecules were detained on the pore wall. For thesystems with CTAB, similar observations were obtained fromSMD simulations (Figures S14−S18).To check the reproducibility of the observations from the

pulling simulations, we performed one more simulation forsystem II by truncating the oil cylinder in a different position ofthe silica pore from system I. Some results including the viewsof SMD simulations and the curves of the applied force andcarbon atoms in the out layer can be found in Figure S19.These results showed good agreement with those derived fromsystem II.The detachment processes of the oil droplet under the other

three pulling rates (i.e., 0.0025, 0.01, and 0.025 nm ps−1) in theSMD simulations were also investigated. The curves of theexternal force as well as configurations with time evolution ofthe SMD simulations are shown in Figure S20. The force andnumber of carbon atoms in the out layer plotted with thereaction coordinate are also included as shown in Figure S21.We may see that the structural change is following the samesequence under each of the pulling rates. The variation trendsof the external force are also generalized, and the maximum isrelated to the structural change. Thus, the used pulling rates ina certain range do not affect the process of the SMDsimulations. Actually, the rates of the pulling simulations maynot be in accord with the velocities in the real situation. Thebiased MD simulations were used here to achieve a particularphenomenon (i.e., the migration of oil drop along the pore)

that might be difficult to occur using the conventional MDmethods.To further identify the effect of the surfactant on the oil

detachment process from the silica surface, a more detailedanalysis was conducted based on the SMD trajectories. Wefocused on a region of the oil drop where there was a CTABmolecule inserted into the layers of the oil organization. Figure6a shows a configuration of system III at 300 ps of the SMD

simulation. As discussed above, the oil molecules in the outerlayer were pulled in order to be detached from the pore surfaceat about 300 ps. From the figure, we note that water moleculescan aggregate around the headgroup forming a water channel atthe solid−oil interface because the hydrophilic headgroup ofCTAB can strongly interact with water molecules. When the oildrop moved along the pore, the water channel expanded whichfacilitated the detachment of oil molecules from the pore wall(Figure 6b). However, this phenomenon was not observedsystem without CTAB, even when the water channel existed inthe initial structure of the SMD simulation. Figure S22 showsthe configurations of the water channel in the system withoutCTAB. It can be seen that the channel did not expand with thedisplacement of the droplet. Conversely, it became smaller dueto the adsorption of oil molecules onto the pore wall.

3.4. Potential of Mean Force. The potential of mean force(PMF) of the oil detachment process inside the silica pore wasdetermined using the umbrella sampling method. The umbrellasampling method is an useful method to assemble the PMF.39

To perform the umbrella sampling, a series of configurationsare generated by the SMD simulations along a reactioncoordinate ξ between two groups first (e.g., the oil drop and theimmobile reference in this work). A selected increasing COMdistance between the two groups represents the samplingwindows. Then, the position of the pulled group wasmaintained by a biased potential, and independent samplingsimulations were performed in each window. The PMF curveswere plotted with the entire reaction coordinate using theWHAM38 method, which led to the free energy ΔG ofdissociation of the oil drop from the silica surface andmovement along the pore.We have performed sampling simulations from initial

configurations derived from pulling simulations with differentrates (0.01, 0.005, and 0.0025 nm ps−1) for systems with andwithout CTAB. These curves are shown in Figure S23. Asillustrated in these figures, a convergence was achieved bydiscarding the first 2 or 3 ns of the trajectories. Thus, the last 2ns of the simulation time was used for WHAM in all the

Figure 5. Configurations at different periods of the pulling simulationfor system II.

Figure 6. Configuration of the water channel at 300 ps (a) and 400 ps(b) of the SMD simulation.

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sampling simulation systems. The PMF profiles for thedetachment of the oil drop through the pore in systems IIand III under each of the pulling rates are shown in Figure 7.

The histograms of the configurations within the umbrellasampling windows are plotted in Figure S24. The histogramsshow reasonable overlap between windows of the COMspacing, suggesting the entire reaction coordinate was properlysampled.Figure 7 shows clearly that the free energy change is

consistently smaller in systems with CTAB than those withouta surfactant. Although there are certain differences among thePMF curves when using different pulling rates, the overall trendof the energy change is consistent. In the beginning of thereaction coordinate (i.e., the first 0.75 and 1.50 nm for systemII and III, respectively), the energy profiles show a slow growth.This is because that the migration of the oil drop is very slow atthe beginning of the SMD simulation, as discussed above. Inthis stage, we tend to believe that both the adsorptioninteraction of the drop onto the wall and dispersioninteractions among the hydrocarbon molecules make themain contributions to the free energy, which suggests thatthe oil drop tended to keep their positions inside the pore whenthe external force was small, and the force could not counteractthe adsorption energy between the oil drop and the pore wall.Thus, extra energy was required to make the droplet migrate,yielding an increment of ΔG in the two systems. The overallpositive value of ΔG indicates a nonspontaneous process of themovement of the oil drop along the pore.In the subsequent stage, the energy profiles show a faster

growth. According to the migration displacement of the dropand the number of carbon atoms in the out layer (Figures S14and S17), we can see that the oil molecules in the outer layer ofthe drop detached from the pore wall under the external forceat the moment. In the systems in the presence of CTAB, waterchannels that formed around the headgroups of the surfactantmolecules expanded on the silica surface which caused awettability alteration of the pore surface and facilitated thedetachment process. It made the force overcome theinteractions between the droplet and the pore wall easily.Thus, less extra energy was required to make the oil dropmigrate in the presence of the surfactant solution. In addition,the increased thickness of the oil cylinder was another factor forthe lower energy in the presence of CTAB, which resulted in aloose pack of oil molecules. We can conclude that, in contrastto water, surfactant solutions may facilitate the process of oildetachment. Therefore, in order to enhance oil-displacementefficiency, it is important to reasonably determine the role of

surfactant molecules on the inversion of wettability, whichhelps to reduce the interfacial tension at the oil−solid interface.

4. CONCLUSIONS

A series of MD simulations were performed to study the effectof a surfactant on the oil detachment process inside a silicananopore. With equilibrium MD simulations the microscopicstructures of crude oil inside the pore were revealed. Theresults showed that there is a layering organization in the crudeoil inside the silica pore, with one well-ordered layer adsorbedon the pore surface. To assess the mechanism of displacingfluid on residual oil, two systems with and without a surfactantwere studied. The MD results showed that with the surfactantsolution the layering structures were evidently disturbed by thesolubilization of surfactant molecules into the oil phase. In thecase of CTAB adsorbed onto the pore wall, water moleculesaround the polar head formed water channels between the oilphase and silica surface, which is vital to the oil detachment.The steered MD method was used to study the displacement

and migration process of the residual oil drop inside the silicapore. Detailed information about the process was achievedfrom SMD simulations. Briefly, oil molecules in the central partof the oil drop moved first under the applied force. At that time,the external force counteracted interactions of oil moleculeswith the pore wall. The structural change of the outer layer onthe pore wall is important to oil detachment. Because ofexpansion of the water channel around the headgroups ofsurfactant molecules, oil molecules in the outer layer are moreeasily pulled away from the surface. The free energy profilesshowed that less energy is required for the fluid to displace theoil droplet along the pore with surfactants present. Theseresults are expected to provide molecular-level insight into themechanism of surfactants aiding in displacing oil drops inenhanced oil recovery.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.5b09841.

Detailed view of the pore surface; force field parameters;illustration of pulling direction; potential energy withtime of system I; layering structures inside the pore;molecular orientation angles; time evolution of thethickness of the oil cylinder; number density profiles;COM distance between the oil drop and the reference;results from repeated simulations for pulling simulation;number of carbon atoms adsorbed on the pore wall;SMD configurations of system III; pulling simulationusing different rates and configurations of the waterchannel in a system without CTAB; PMF curves usingdifferent pulling rates and histograms of the config-urations within the umbrella sampling windows (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail shilingyuan@sdu.edu.cn (S.Y.).

NotesThe authors declare no competing financial interest.

Figure 7. Potential of mean force (PMF) profiles as a function ofCOM distance between the oil drop and reference for the two systemsin this study.

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■ ACKNOWLEDGMENTS

This work was financially supported by the National NaturalScience Foundation of China (21203084). We thank Dr.Edward C. Mignot, Shandong University, for linguistic advice.

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The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.5b09841J. Phys. Chem. C 2016, 120, 2667−2674

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