Insights into the Oil Adsorption and Cyclodextrin Extraction Process on

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Insights Into the Oil Adsorption and Cyclodextrin Extraction Process on Rough Silica Surface by Molecular Dynamics Simulation Xinzhe Zhu, Daoyi Chen, Yan Zhang, and Guozhong Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10511 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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The Journal of Physical Chemistry

Insights into the Oil Adsorption and Cyclodextrin Extraction Process on Rough Silica Surface by Molecular Dynamics Simulation

Xinzhe Zhu†,‡, Daoyi Chen†,‡, Yan Zhang†,‡, Guozhong Wu*,†,‡

†

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

‡

School of Environment, Tsinghua University, Beijing 100084, China

*Corresponding Author: Phone: +86 0755 2603 0544 Fax: +86 0755 2603 0544 Email: [email protected]

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Abstract Molecular dynamics simulation was performed to investigate the adsorption and aqueous extraction of oil contaminants on silica surface with various roughness. The oil dispersion and immersion were characterized by molecular configuration, adsorption energy and contact angle, while the oil detachment in water and cyclodextrin solution was evaluated by the overall extractability, extractability from individual groove and free energy analysis. Results demonstrated that the main resistance for oil release from the relatively shallow grooves was the strong intermolecular interactions among the oil molecules orderly stacked inside the grooves. It highlighted the role of cyclodextrin in breaking through the energy barrier for pulling out one oil molecule from the tightly stacked oil structure, which resulted in the fast release of the remaining oil. Previous studies attributed the oil extraction to the sequestration of oil into the hydrophobic cavity of cyclodextrin, while our results demonstrated that such inclusion process was critical in initially destroying the stable structure of the oil compacted inside the grooves but contributed little afterwards. To the best of our knowledge, this was the first molecular-level study on the cyclodextrin-aided oil detachment from mineral surface, which is off added knowledge to the oil clean-up mechanisms during aqueous extraction.

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1．．INTRODUCTION Oil spillage due to natural and anthropogenic cause induces long-term soil contamination, which is of growing interest due to the high toxicological impact of petroleum and the pressure on soils for food security and growing urbanization. The resulted soils and the associated risks demand the development of realistic appraisal framework and appropriate remedial strategies, which are heavily dependent on a comprehensive appreciation of the physicochemical behavior of oil compounds in the soil and their bioavailability to microorganisms.1, 2 Of the complicated oil fate and transport processes, the adsorption of oil to the soil minerals is especially important for its persistence in the environment. Therefore, there have been a number of experimental works on the adsorption kinetics and thermodynamics of oil on soil minerals,3-5 which suggested that the oil adsorption depended on the mineral properties (e.g. chemical composition and particle size), oil properties (e.g. polarity, components, viscosity) and environmental conditions (e.g. temperature, ageing time, PH, salinity). The rapid development of computational techniques such as molecular dynamics (MD) simulation offers essential assistance to gain insights to the environmental behavior of hydrocarbons if proper precautions are taken in the model construction and data analysis.6 We have a long-standing research interest in the molecular interactions between oil and soil mineral. Previous applications of the MD simulation included but not limited to its use for characterizing the distribution of oil fractions on the silica surface,7 multi-phase partitioning of oil fractions on the soil organic matters amended silica surface,8 3



diffusion of polycyclic aromatic hydrocarbons inside the silica pores,9 and self-aggregation and co-aggregation of oil asphaltenes with humic acids in the clay pores.10, 11 However, scientific insights into the oil fate and transport on the rough mineral remain limited as smooth surfaces were often used in previous simulations. On the smooth surface, the wetting property of solid surface depends on the liquid-solid interaction strength,12 while the contact angle of liquid on a smooth surface can be described by Young’s equation.13 However, the surface hydrophobicity, wetting mode and adsorption capacity will be changed when the surface become rough.14, 15 Recent studies reported the dependence of the wetting behavior of rough surfaces on the temperature, reentrant geometry, surface chemistry and droplet size, where the water droplet was often used as fluid.14-18 Although a few simulations also investigated the equilibrium structure of hydrophobic fluid,19 information is still inadequate on the molecular-level dispersion and immersion dynamics of petroleum hydrocarbons on the rough minerals. Another key process for the oil clean-up and bioavailability assessment is its detachment from the soil in aqueous medium. For example, cyclodextrins are non-toxic and biodegradable aqueous solvents for hydrocarbon recovery from contaminated soils, which also offer an alternative method for fast evaluation of hydrocarbon bioavailability due to the linear relationship observed between extractability and biodegradability.20, 21 The oil release from the smooth surface of soil minerals in water medium (e.g. pure water and surfactant solution) has been well documented, which highlighted the key role of water channel formation for the oil 4


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detachment.9, 22-25 However, little has been discussed on the mechanisms for the oil release from the grooves of rough surface in water or cylodextrin solutions. It is known that cyclodextrins can form inclusion complex with organic molecules due to their hydrophobic cavity, but our recent study demonstrated that only 4% of the selected hydrocarbon molecules were indeed entrapped inside the cavity of β-cyclodextrin when they were used for extraction at the oil-water interface.26 It suggested that molecular inclusion process might not be the only mechanism for cyclodextrin extraction especially when cyclodextrin aggregation occurred at high concentration. It remains unclear about how and to what extent the cyclodextrins break through the barriers for the release of the residual oil sequestrated inside the reentrants of rough surfaces. To bridge the perceived gaps in our understanding of the oil adsorption and detachment from rough soil minerals in water and cyclodextrin solution, MD simulations were performed on silica surface with various roughness. Dodecane was selected as representative oil compound, because (i) alkane is a main component of commercial low viscosity oil products such as gasoline and diesel oil,27 which are more likely to absorb and deposit on skin than aromatic hydrocarbons due to the greater affinity of alkanes for stratum corneum and the volatility of aromatic.28, 29 (ii) single alkane is widely used to model the oil in previous simulations,24, 30-35 because the force field parameters for alkane molecules are readily available. Although mixture oil models including asphaltenes and resins were also reported in recent literatures, it remains challenging to accurately define the force field parameters for 5



the asphaltenes which are a complex mixture of compounds. A general way is to directly use the analogue structures in the force field library to parameterize the asphaltenes,36 but it still lacks a specific force field for asphaltenes. Specific objectives of this study were to (i) characterize the dynamics of oil dispersion and immersion on the mineral surface with different geometry, (ii) distinguish the main resistance and driving force for the oil detachment from the rough surface during aqueous extraction, (iii) identify the contribution and limitation of cyclodextrin to the extraction of the sequestrated oil.

2. SIMULATION METHODS 2.1. Modeling and simulation A quartz (1 0 0) surface was created using a unit cell (a = b = 0.4913 nm, c = 0.5405 nm, a = b = 90°, c = 120°).

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It was replicated to create a larger surface (12 × 6

supercell), which was then converted to a 3D periodic cell (5.9 nm × 6.5 nm × 2.5 nm). To model quartz surface with various roughness, rectangular grooves were periodically created by manually removing the undesired atoms and protonating the resulted bare oxygen atoms (Fig. S1 in the supporting information). Basically, the surface roughness was defined by the deviations in the direction of the normal vector of a real surface from an absolutely smooth surface. Accordingly, the surface roughness (R) was quantified by the ratio of the surface area of a rough surface to that of a smooth surface as follows: R=

+ + 2ℎ 2ℎ =1+ + + 6


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where the w, h, s, and l represented the width, height, step width and length of the grooves of a unit surface structure, respectively. Energy minimization was performed after fixing the internal silica and oxygen atoms and keeping the surface hydroxyl hydrogen atoms flexible. Dodecane was selected as a representative oil compound, while β-cyclodextrins (βCD) dissolved in water was used as aqueous solvent. The bond and non-bond parameters for the SiO2 were taken from the ClayFF force field.37 The CHARMM36 force field, which is compatible with ClayFF,11, 38 was used to model the inter- and intra-molecular interactions among the oil. The simple point charge (SPC) model was adopted for all the water molecules. The force filed parameters are listed in Table S1. MD simulations were carried out using Gromacs software (version 5.1.1).39 The oil adsorption on silica surface was investigated by randomly placing 120 dodecane molecules on the top of silica surface in the simulation box. In the direction perpendicular to the oil layer, a vacuum layer of 25 nm thickness was added to eliminate direct interactions between oil molecules and the top of the slab. The system was initialized by minimizing the energy to 1000 kJ mol-1 nm-1 with the steepest descent approach. MD simulations were performed at NVT ensemble (constant number of atoms, volume and temperature) for 20 ns with a time step of 1 fs. The resulted oil-wetted silica surface was used for the oil detachment simulations by adding pure water, 10 βCD molecules in water or 26 βCD molecules in water, respectively, in the simulation box. The number of water molecules was about 6000 in all simulations. After energy minimization, MD simulation was carried out for 200 ns 7



at NVT ensemble. In all simulations, periodic boundary conditions were applied in all directions and the temperature (300 K) was controlled using the V-rescale thermostat method.40 The cutoff distance was fixed at 1.5 nm for the short range van der waals and electrostatic pair wise calculations, while the long range electrostatics was dealt with the particle mesh Ewald summation.41 The trajectories were saved and output at 0.1 ns interval for data analysis.

2.2. Free energy calculation The umbrella sampling method 42 was applied to calculate the potential of mean force (PMF) for the escape of one oil molecule from the parallel compacted oil molecules inside the grooves. The initial structures of parallel stacking oil molecules inside the grooves were obtained from the results of adsorption simulation. Water molecules were filled randomly at the top of the simulation box. For each system, energy minimization was performed with steepest descent approach, followed by 1 ns NVT equilibration at 300 K. A series of configurations were generated by pulling the COM of one randomly selected oil molecule along the Z-direction at the rate of 2.5 nm ns-1 with the force constant of 10000 kJ mol-1 nm-2. Subsequently, approximately 30 windows were picked out with a distance of 0.1 nm and each was used for the 10 ns MD simulations. For each simulation, the biased umbrella potential was used to keep the molecules within the window. To gain insights into the oil sequestration process by cyclodextrin, the GROMACS tool “gmx genrestr” was used to restrain the targeted oil molecule in the x- and y- directions with a force constant of 1000 kJ mol-1 nm-2,

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which allowed the oil passing through the cavity of cyclodextrin. Meanwhile, the other oil molecules remaining inside the grooves were limited at the three-dimension with the same force constant. The weighted histogram analysis method43 was employed to transform the biased distribution to unbiased distribution based on the MD results and obtain the PMF curves along the reaction coordinates. The PMF curves demonstrated the energy changes during the oil transport from inside the groove to the water. The above procedures were repeated by randomly selecting another target oil molecule and the mean value of the free energy was obtained by averaging the two calculations. The influence of βCD on the PMF was investigated by repeating the above procedures except that a βCD molecule was located right above the selected target oil molecule during the pulling process. The βCD molecule was restrained with a force constant of 1000 kJ mol-1 nm-2 in all directions.

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3. RESULTS AND DISCUSSION 3.1 Oil adsorption on the rough silica surface Fig. 1 shows the equilibrium configurations of oil adsorption on the silica surfaces with different roughness. When the silica surface was smooth (R0 = 1), three layers of oil formed on the hydrophilic silica surface as evidenced by the three clear peaks in the density profile of oil molecules (Fig. S2), which was consistent with previous study.24 The low-surface tension of oil made the smooth surface fully wetted, which changed the surface from hydrophilic to hydrophobic.19 On the rough silica surface (R = 2.02 ~ 3.57), some oil molecules spontaneously moved into the grooves leaving the remaining irregularly distributed above the surface. To gain insights into the dynamics of oil immersion and dispersion on the rough silica surface, the number of oil entering the grooves and the spreading area of oil above the grooves were monitored during the first 5 ns (Fig. 2). Irrespective of the surface roughness, a sharp decline was observed in the spreading area during the first 100 ps, suggesting the rapid accumulation of the scattered oil molecules on the silica surface which was mainly driven by the van der Waals interactions among oil molecules. Snapshots of an example is shown in Fig. S3. Although the number of oil entering the grooves was very limited during such a short time, oil immersion was obviously observed in the following 2 ns (Fig. 2). It was mainly attributed to the attraction of the silica internal wall with the CH2 groups of the dodecane molecules.24, 25 During this stage, the oil immersion was accompanied by the fast oil dispersion on the silica with relatively shallow grooves (Figs. 2b and 2d). However, oil dispersion was not 10


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observed on the silica with relatively deep grooves until 55 - 70% of the oil entering the grooves (Figs. 2c, 2e, 2f). Similar phenomenon was also observed by Prabhu et al.,44 which indicated that the oil spreading rates on the rough surface decreased with the grooves depth. The oil spreading stopped when about 75% of the oil was trapped inside the grooves and further oil immersion did not change the oil spreading area. Particularly, only slight changes were observed in the spreading area on the surface with very wide groove throughout simulation (Fig. 2c). Although the oil immersion and spreading dynamics varied with the surface roughness, the oil wetting preferred to be in a Wenzel mode in all simulations (Fig. 1). Such wetting mode considered a fluid in contact with an extended flat surface with larger interfacial area than that given by a planar projection of the rough substrate.12 Results demonstrated that a higher surface roughness did not change the above wetting mode and the wetting-to-dewetting transition previously reported14, 45 was not observed in this study. As suggested by Pandey and Roy,14 the wettability was determined by the competition between surface-liquid interactions and liquid-liquid interactions. The latter had to overcome the former for the wettability transition. In this study, the attraction of silica surface to oil was one-order of magnitude larger than the interactions between the oil inside and outside the grooves independent on the surface roughness (Fig. 3), making it favorable for the oil remaining inside the groove. Previous study indicated that the contact angle of oil droplet increased with the surface roughness.46 In the present study, such tendency was also obviously observed on the silica surface with the same width (Fig. 1), which was attributed to the 11



decreased interactions between silica and the oil outside the grooves (Fig. 3c). Our results further demonstrated a larger contact angle on the silica with wider grooves providing the similar roughness if we compared the R2 and R3 surfaces. Results also indicated that the oil-oil and oil-silica interactions were dominated by the van der Waals energy other than the Coulomb electrostatic energy, because the former accounted for up to 89% of the total non-bonding interaction irrespective of the surface roughness (Fig. 3). Additionally, the total oil adsorption strength on the silica increased with the surface roughness (Fig. 3a). For instance, the interaction of the total oil with the rough surface (R5) was more than 3-fold larger than with the smooth surface. It implied an increased difficulty for the extraction of the oil in a deep groove which was elaborated in the following sections.

3.2 Oil extraction from the rough silica surface 3.2.1 Pure water Results demonstrated that the oil detachment from the smooth silica surface consisted of several steps including the water penetration under electrostatic interaction, formation of water channel under H-bonding interaction, formation of a surface gel layer through water diffusion, and strip of oil from solid surface (Fig. S4). This was in good agreement with the conceptual model proposed by Zhang et al.,25 which had been well elaborated previously and therefore was not detailed in the present study. On the rough silica surfaces, the oil above the grooves simultaneously assembled to form a droplet as soon as the water was added, while the oil inside the grooves were 12


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gradually released with different degrees. To clarify the dependence of oil release rate upon the surface roughness, we characterized the oil extractability by the number of oil molecules with half of the atoms moving out from the silica grooves. It demonstrated that the oil release from the silica with shallow grooves (R1 = 2.02) was very fast and the overall extractability reached 100% before 58 ns (Fig. 4a). As expected, the extractability decreased with the increase of surface roughness (Fig. 4). On the silica with largest roughness (R5 = 3.57), more than 55% oil remained inside the groove without being extracted by pure water. In order to gain insights into the driving forces for the oil release, we further labeled the grooves in each simulation according to their relative positions to the above oil droplet and quantified the corresponding oil extractability in each groove. For example, the two grooves above which there was an oil droplet were defined as G1 and G2, while the remaining grooves were defined as G3 and G4, respectively (Fig. 5a). It showed that the oil extractability from the grooves covered by the oil droplet was higher than from those without oil droplet when the groove depth ranged from 1.3 to 1.7 nm (i.e. R2, R3, R4 surfaces). For instance, the percentage of oil released from G1 and G2 was about 3-fold of that from G3 and G4 on the R4 surface (Fig. 5c). This finding suggested that the oil release in these cases was mainly contributed by the van der Waals interactions from the above oil droplet. However, this contribution became insignificant when the groove depth increased to 2.14 nm (i.e. R5 surface) as a result of the strong binding energy between the silica groove and the inside oil (Fig. 3b). It implied that the variance in the oil extractability from different grooves in this case 13



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was associated with some other mechanisms. To find out the potential attributable factors, the oil configurations on the rough surfaces were reexamined. An interesting finding from the R3 surface was that the oil droplet was separated from the silica surface and migrated to the water phase after almost all the oil molecules was released from G1 and G2, which was pulled back onto the

silica surface

but located above the neighbor groove such as G4 (Fig. S5). Similar phenomenon was found on the R2 surface except that the oil droplet directly shifted from G1 and G2 to G3 without entering the water phase (Fig. S6). In both cases, the remaining oil appeared to be blocked inside the groove without being successfully pulled out by the above oil droplet according to the mechanism aforementioned. A closer examination of the molecular configurations demonstrated that the residual oil molecules were all parallel compacted inside the groove (Fig. S7). Such ordered structure was stable rending them difficult to release. This implied that the oil distribution pattern inside the groove was another factor influencing the extractability. To quantify such effects, the number of oil clusters was defined according to the stacking directions of oil molecules inside the grooves. Methods for the oil clusters counting are elaborated in Fig. S8. The oil molecules in one cluster distributed across those in the neighbor cluster, while all the oil molecules in the same cluster were parallel to each other. A larger number of oil clusters indicated a less ordered distribution of oil molecules inside the grooves. For better interpreting the association of oil release with the oil distribution pattern, selected snapshots of the oil and water configurations inside the 14


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grooves of the R4 surface are shown in Fig. 6. According to our definition, the number of oil clusters in the four grooves was 2, 4, 4, 2, respectively. It clearly showed that the water started to penetrate into the grooves through the cross section between the horizontal and vertical stacking sheets (i.e. the interspace between two oil clusters) where the combination among oil molecules should be weaker than among those orderly stacked in one direction. During the initial stage of this process, the water penetration through the interfaces between the sequestrated oil and the internal wall of silica groove was not observed on the surface with narrow grooves (Fig. 6), but was found on that with wide grooves (Fig. S9). Moreover, the oil release from the G1 groove was obviously slower and less than from the G2 groove due to the smaller number of oil clusters in the former. The same trend was observed for the G3 and G4 above both of which the oil droplet was absent (Fig. 6). Overall results suggested that the oil extractability from the grooves was simultaneously influenced by the oil distribution pattern and the presence of oil droplet above the groove. For example, although the oil inside the G1 groove of R2 surface was also distributed in very good order (Fig. 5b), the corresponding extractability (Fig. 5c) was very high due to the strong driving force provided by the oil droplet above it (Fig. 5a).

3.2.2 Cyclodextrin solution Fig. 4 showed that the βCD addition was favorable for improving the oil release from the grooves, but the effects of βCD concentration on the total extractability was 15



insignificant. From the surface with shallow grooves (depth: 0.85 nm), the total oil extractability already reached 100% in pure water, while the βCD addition increased the kinetics of oil release by up to one-fold (Fig. 4a). When the groove depth increased to 1.30 nm, the oil extractability increased by 30% in the βCD solution compared with that in the pure water (Fig. 4c). However, such promotion effects decreased with the further increase of groove depth. Particularly, the extractability was only enhanced by 10% after βCD addition when the groove depth increased to 1.7 nm (Fig. 4d), while the promotion effects even disappeared on the surfaces with 2.14 nm depth (Fig. 4e). Results further demonstrated that the promotion effects of βCD addition on the oil extractability increased with the groove width at given depth. For example, the percentage of the βCD-aided release of oil from grooves increased by about 33% when the groove width increased from 0.69 to 1.22 nm, although only very slight difference was noticed in the oil extractability by pure water between surface R4 (Fig. 4d) and R2 (Fig. 4b). Two critical steps were recognized for the enhanced oil release in the presence of βCD solution. One was the fast adsorption of βCD on the oil droplet followed by the formation of hollow bucket-like blocks at the interface between oil droplet and silica surface (Figs. 7a-c). The hydrophilic shell of this structure made it easy to form water channels at the interface from which the water penetrated into the grooves. Another step was the destabilization of the ordered structure of the residual oil inside the grooves by directly pulling out one of those oil molecules into the cyclodextrin cavity (Figs. 7g-k). As aforementioned, the strong intermolecular forces among the parallel 16


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compacted oil molecules inside the grooves accounted for the main resistance for aqueous extraction. Results of PMF analysis demonstrated the presence of a high energy barrier (~111 kJ mol-1) for the escape of one oil molecule from such ordered structure to above the grooves. This was evidenced by the sharp increase in the energy profiles along the reaction coordinates (i.e., from about 0.9 nm to 1.9 nm in Fig. 8). The oil molecules tend to keep their positions inside the grooves (i.e., at position P1) if the external force was too small to counteract the oil adsorption energy with the internal wall of the grooves and with the surrounding oil molecules. It required extra energy to make the oil migrate by overcoming the energy barrier, making it difficult for the pure water to displace the oil inside the grooves. Once such energy barrier was overcome, the oil would spontaneously separate from the groove as suggested by the decrease in the energy profile (i.e. at position P2) and would enter the bulk water without extra energy supply (i.e. at position P3). By contrast, the βCD-mediated extraction was obviously more energetically favorable as the corresponding energy barrier was 39-42 % lower than that in the pure water. It was also found that the energetic advantages resulting from βCD addition was not obvious in terms of the total energy for the transport of oil into the bulk water, because it needed to overcome another energy barrier (~34 kJ mol-1) for the oil to desorb from the βCD cavity before entering the bulk water (Fig. 8). These findings suggested that the oil extraction by directly passing through the βCD cavity was an alternative but not the most priority pathway for the oil removal from rough surface. In fact, only about 10% of the oil was released from the grooves by this way during our simulations. Nevertheless, this study 17



highlighted the contribution of cyclodextrin to the breakthrough of the first energy barrier for destroying the ordered structure of residual oil. Once such tight structure was destroyed, the remaining oil released fast (Figs. 7b-d).

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4. CONCLUSIONS This study demonstrated the different dynamics of oil dispersion and immersion on silica surface with different roughness, but the equilibrium wetting mode was independent on the surface roughness. It also demonstrated that the oil adsorption strength on the silica surface increased with the depth of the grooves, which was more than 3-fold larger than that on the smooth surface. The water-driven release of the oil from the grooves decreased with the groove depth, which was attributed to (i) the decreased binding energy between the oil outside and inside the grooves and (ii) the strong intermolecular interactions among the oil molecules with ordered structure inside the grooves. The latter was recognized as the main resistance for oil detachment in pure water, because water molecules could only penetrate into the grooves through the cross section between horizontal and vertical stacking sheets where the combination among oil molecules was weaker than among those orderly stacked in one direction. The addition of βCD significantly promoted the oil detachment by (i) destroying the ordered structure of the residual oil inside the grooves by pulling out one of those oil molecules into the cyclodextrin cavity, and (ii) forming hollow bucket-like blocks at the interface between silica and the above oil droplet which favored the water penetration into the grooves. The presence of βCD decreased the energy barrier for the release of the entrapped oil from the grooves by 39 - 42 %, but the oil extraction by directly passing through the βCD cavity was not the predominant pathway for the oil removal from rough surface because of the energy barrier for separating the oil-βCD complexes. 19



SUPPORTING INFORMATION The unit structure of rough silica surface (Figure S1), Force field parameters (Table S1), Density profile of oil on the smooth silica surface (Figure S2), Snapshots of oil adsorption on the rough silica surface (Figure S3), Snapshots of the oil detachment process on the smooth silica surface in water (Figure S4), the movement of oil droplet above silica surface in R3 (Figure S5) and R2 (Figure S6), Molecular configuration of residue oil molecules inside the groove of silica surface (Figure S7), The definition of numbers of oil clusters (Figure S8), Water penetration in R2 (Figure S9).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Telephone/Fax: +86-0755-26030544 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This study was financially supported by the Shenzhen Peacock Plan Research Grant (KQJSCX20170330151956264), the Fundamental Research Project of Shenzhen, China (JCYJ20160513103756736), the Economy, Trade and Information Commission of Shenzhen Municipality (HYCYPT20140507010002), and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. 20


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Handbooks,

Springer-Verlag GmbH Berlin Heidelberg 2016, pp. 165-180. (7) Wu, G.; He, L.; Chen, D. Sorption and distribution of asphaltene, resin, aromatic and saturate fractions of heavy crude oil on quartz surface: molecular dynamic simulation. Chemosphere 2013, 92, 1465-71. (8) Wu, G.; Zhu, X.; Ji, H.; Chen, D. Molecular modeling of interactions between heavy crude oil and the soil organic matter coated quartz surface. Chemosphere 2015, 119, 242-249. (9) Sui, H.; Li, L.; Zhu, X.; Chen, D.; Wu, G. Modeling the adsorption of PAH mixture in silica nanopores by molecular dynamic simulation combined with machine learning. Chemosphere 2016, 144, 1950-1959. (10) Zhu, X.; Chen, D.; Wu, G. Molecular dynamic simulation of asphaltene co-aggregation with humic acid during oil spill. Chemosphere 2015, 138, 412-421. (11) Zhu, X.; Chen, D.; Wu, G. Insights into asphaltene aggregation in the Na-montmorillonite interlayer. Chemosphere 2016, 160, 62-70. (12) Kumar, V.; Errington, J. Impact of small-scale geometric roughness on wetting behavior. Langmuir 2013, 29, 11815-11820. (13) Young, T. An essay on the cohesion of fluids. Philos. Trans. R. Soc. London 1805, 95, 65-87. (14) Pandey, P.; Roy, S. Is it possible to change wettability of hydrophilic surface by changing its roughness. J. Phys. Chem. Lett. 2013, 4, 3692-3697. (15) Wu, C.; Kuo, L.; Lin, S.; Fang, T.; Hsieh, S. Effects of temperature, size of water droplets, and surface roughness on nanowetting properties investigated using molecular dynamics simulation. Comp. Mater. Sci. 2012, 53, 25-30. (16) Kanungo, M.; Mettu, S.; Law, K.; Daniel, S. Effect of roughness geometry on wetting and dewetting of rough PDMS surfaces. Langmuir 2014, 30, 7358-7368. (17) Acharya, H.; Mozdzierz, N. J.; Keblinski, P.; Garde, S. How chemistry, nanoscale roughness, and the direction of heat flow affect thermal conductance of solid–water interfaces. Ind. Eng. Chem. Res. 2011, 51, 1767-1773. (18) Katasho, Y.; Liang, Y.; Murata, S.; Fukunaka, Y.; Matsuoka, T.; Takahashi, S. Mechanisms for 21



enhanced hydrophobicity by atomic-scale roughness. Sci. Rep. 2015, 5. (19) Savoy, E.; Escobedo, F. Molecular simulations of wetting of a rough surface by an oily fluid: Effect of topology, chemistry, and droplet size on wetting transition rates. Langmuir 2012, 28, 3412-3419. (20) Lal, V.; Peng, C.; Ng, J. A review of non-exhaustive chemical and bioavailability methods for the assessment of polycyclic aromatic hydrocarbons in soil. Environ. Technol. Innov. 2015, 4, 159-167. (21) Landy, D.; Mallard, I.; Ponchel, A.; Monflier, E.; Fourmentin, S. Remediation technologies using cyclodextrins: an overview. Environ. Chem. Lett. 2012, 10, 225-237. (22) Chen, J.; Si, H.; Chen, W. Molecular dynamics study of oil detachment from an amorphous silica surface in water medium. Appl. Surf. Sci. 2015, 353, 670-678. (23) Yuan, S.; Wang, S.; Wang, X.; Guo, M.; Wang, Y.; Wang, D. Molecular dynamics simulation of oil detachment from calcite surface in aqueous surfactant solution. Comput. Theor. Chem. 2016, 1092, 82-89. (24) Liu, Q.; Yuan, S.; Yan, H.; Zhao, X. Mechanism of oil detachment from a silica surface in aqueous surfactant solutions: molecular dynamics simulations. J. Phys. Chem. B 2012, 116, 2867-75. (25) Zhang, P.; Xu, Z.; Liu, Q.; Yuan, S. Mechanism of oil detachment from hybrid hydrophobic and hydrophilic surface in aqueous solution. J. Chem. Phys. 2014, 140, 164702. (26) Zhu, X.; Wu, G.; Chen, D. Molecular dynamics simulation of cyclodextrin aggregation and extraction of Anthracene from non-aqueous liquid phase. J. Hazard. Mater. 2016, 320, 169-175. (27) Rotaru, A.; Cojocaru, C.; Cretescu, I.; Pinteala, M.; Timpu, D.; Sacarescu, L.; Harabagiu, V. Performances of clay aerogel polymer composites for oil spill sorption: experimental design and modeling. Sep. Purif. Technol. 2014, 133, 260-275. (28) McDougal, J.; Pollard, D.; Weisman, W.; Garrett, C.; Miller, T. Assessment of skin absorption and penetration of JP-8 jet fuel and its components. Toxicol. Sci. 2000, 55, 247-255. (29) Baynes, R.; Brooks, J.; Riviere, J. Membrane transport of naphthalene and dodecane in jet fuel mixtures. Toxicol. Ind. Health. 2000, 16, 225-238. (30) Li, X.; Xue, Q.; Zhu, L.; Jin, Y.; Wu, T.; Guo, Q.; Zheng, H.; Lu, S. How to select an optimal surfactant molecule to speed up the oil-detachment from solid surface: A computational simulation. Chem. Eng. Sci. 2016, 147, 47-53. (31) Zhong, J.; Wang, X.; Du, J.; Wang, L.; Yan, Y.; Zhang, J. Combined molecular dynamics and quantum mechanics study of oil droplet adsorption on different self-assembly monolayers in aqueous solution. J. Phys. Chem. C 2013, 117, 12510-12519. (32) Xie, W.; Sun, Y.; Liu, H.; Fu, H.; Liang, Y. Conformational change of oil contaminants adhered onto crystalline alpha-alumina surface in aqueous solution. Appl. Surf. Sci. 2016, 360, 184-191. (33) Ginzburg, V. V.; Chang, K.; Jog, P. K.; Argenton, A. B.; Rakesh, L. Modeling the interfacial tension in oil-water-nonionic surfactant mixtures using dissipative particle dynamics and self-consistent field theory. J. Phys. Chem. B 2011, 115, 4654-4661. (34) Bresme, F.; Chacon, E.; Tarazona, P.; Tay, K. Intrinsic structure of hydrophobic surfaces: the oil-water interface. Phys. Rev. Lett. 2008, 101, 056102. (35) Wang, H.; Wang, X.; Jin, X.; Cao, D. Molecular dynamics simulation of diffusion of shale oils in montmorillonite. J. Phys. Chem. C 2016, 120, 8986-8991. (36) Jian, C.; Tang, T. One-dimensional self-assembly of poly-aromatic compounds revealed by molecular dynamics simulations. J. Phys. Chem. B 2014, 118, 12772-12780. (37) Cygan, R.; Liang, J.; Kalinichev, A. Molecular models of hydroxide, oxyhydroxide, and clay 22


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phases and the development of a general force field. J. Phys. Chem. B 2004, 108, 1255-1266. (38) Wright, L.; Walsh, T. First-principles molecular dynamics simulations of NH4(+) and CH3COO(-) adsorption at the aqueous quartz interface. J. Chem. Phys. 2012, 137, 224702. (39) Van Der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A.; Berendsen, H. GROMACS: fast, flexible, and free. J. Comput. Chem. 2005, 26, 1701-1718. (40) Bussi, G.; Donadio, D.; Parrinello, M. Canonical sampling through velocity rescaling. J. Chem. Phys. 2007, 126, 014101. (41) Darden, T.; York, D.; Pedersen, L. Particle mesh Ewald: An N⋅ log (N) method for Ewald sums in large systems. J. Chem. Phys. 1993, 98, 10089-10092. (42) Torrie, G.; Valleau, J. Nonphysical sampling distributions in Monte Carlo free-energy estimation: Umbrella sampling. J. Comput. Phys. 1977, 23, 187-199. (43) Hub, J.; De Groot, B.; Van Der Spoel, D. G_wham: A free weighted histogram analysis implementation including robust error and autocorrelation estimates. J. Chem. Theory Comp. 2010, 6, 3713-3720. (44) Prabhu, K.; Fernades, P.; Kumar, G. Effect of substrate surface roughness on wetting behaviour of vegetable oils. Mater. Design. 2009, 30, 297-305. (45) Lundgren, M.; Allan, N.; Cosgrove, T. Modeling of wetting: a study of nanowetting at rough and heterogeneous surfaces. Langmuir 2007, 23, 1187-1194. (46) Hsieh, C.; Chen, J.; Kuo, R.; Lin, T.; Wu, C. Influence of surface roughness on water- and oil-repellent surfaces coated with nanoparticles. Appl. Surf. Sci. 2005, 240, 318-326.

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The Journal of Physical Chemistry 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

Page 24 of 32

R0

R1

R2

(h: 0, w: 0, s: 0, R: 1.00, θ: 0°)

(h: 0.85, w: 0.69, s: 0.97, R: 2.02, θ: 31°)

(h: 1.70, w: 1.22, s: 0.97, R: 2.55, θ: 67°)

R3

R5

R4

(h: 1.30, w: 0.69, s: 0.97, R: 2.56, θ: 39°) (h: 1.70, w: 0.69, s: 0.97, R: 3.04, θ: 45°)

(h: 2.14, w: 0.69, s: 0.97, R: 3.57, θ: 72°)

Fig. 1 Configurations of oil on silica surface with different roughness (Length unit: nm; θ represents the contact angle of oil on the top surface of silica) 24


45

30

30 15

(a) R0

0 60

(b) R1

0 45

40

30

20

15

(c) R2

0 45

(d) R3

0 60 40

30

20

15 0

2

45

15

Molecular number

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


Spreading area (nm )

Page 25 of 32

(e) R4 0

1

2

3

4

50

(f) R5 1

2

3

4

0 5

t (ns) Fig. 2 Number of oil inside the grooves (red) and spreading area of oil above the grooves (blue) during oil adsorption on silica surface 25



6

6

(a)

4

4

2

2

(b)

3

-1

Interaction energy (-10 kJ 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

Page 26 of 32

0

R0

R1

R2

R3

R4

R5

0

R1

Total oil - Silica

1.6

R2

R3

R4

R5

Inside oil - Silica

1.2

(c)

1.2

(d)

Coulomb electrostatic Van der Waals

0.8

0.8 0.4 0.4 0.0

0.0 R1

R2

R3

R4

R5

R1

Outside oil - Silica

R2

R3

R4

Inside oil - Outside oil

Fig. 3 Interaction energy between silica and oil during adsorption process

26


R5

Page 27 of 32

100 75

%

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


50 25 0

(a) R1 0

50 100 150 200 0

(b) R2 50 100 150 200 0

(c) R3 50 100 150 200 0

(e) R5

(d) R4 50 100 150 200 0

50

100 150 200

t (ns) Fig. 4 Percentage of oil released from the grooves by pure water (black), low concentration βCD (red) and high concentration βCD (blue)

27



Page 28 of 32

Oil droplet (a)

G1

G2

G3

G4

(b)

(c)

Fig. 5 (a) Definition of the serial number of grooves on rough silica surface. The corresponding number of oil cluster inside the grooves and water-driven extractability of oil from each groove are shown in (b) and (c), respectively.

28


Page 29 of 32 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


z

x

G1 G2 G3 G4 z y

(a) 0 ns

(b) 6.6 ns

(c) 14.3 ns

(d) 20.0 ns

(e) 55.2 ns

(f) 200 ns

Fig. 6 Snapshots of water penetration into the silica grooves (R4 = 3.04). The top row shows the molecular configurations at different times from the x-z plane view. The remaining rows show the configurations of oil (green) and water (red) molecules inside each groove at different times from the y-z plane view, where the silica surface and the oil and water outside the grooves are not shown for clarity.

29



(a) 0 ns

(d) 46.2 ns

(g) 2.9 ns

(h) 3.0 ns

Page 30 of 32

(b) 3.3 ns

(c) 45 ns

(e) 98.1 ns

(f) 200 ns

(i) 3.1 ns

(j) 3.2 ns

(k) 3.3 ns

Fig. 7 Snapshots of β-cyclodextrin extraction of oil from silica surface (R2 = 2.55). The dynamic process of one oil molecular passing through the β-cyclodextrin cavity was highlighted in panel g-k. (Red: water, Purple: cyclodextrins, Green: oil)

30


Page 31 of 32

-1

PMF (kJ mol )

120

without βCD with βCD

R3 = 2.56

P1

P1

P2

P2

P3

P3

80

P3 40

0 34

P2 111 64

0

P1 1

2

3

4

Z (nm) 120 -1

PMF (kJ 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



R2 = 2.55

80

P3 40

0 33

P2 93

0

57

P1 1

2

3

4

Z (nm) Fig. 8 Potential of mean force for oil release from silica grooves. Error bars represent the standard deviation of duplicate computation. P1, P2 and P3 represent three typical positions during the transport of oil from inside the groove to the bulk water, with the corresponding molecular configurations shown in the right panels (the silica and water molecules are hidden). The data labeled in the curves represent the free energy (kJ mol-1) for the oil transport between each two positions.

31



TOC Graphic


120

-1

PMF (kJ 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

Page 32 of 32

80 C

40 B

0 A

A

1

57 kJ mol-1 93 kJ mol-1

2 Z (nm)

32


B

33 kJ mol-1 0 kJ mol-1

3

C

4

Insights into the Oil Adsorption and Cyclodextrin Extraction Process on

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