Adsorption of a Polyaromatic Compound on Silica Surfaces from

Article pubs.acs.org/JPCC

Adsorption of a Polyaromatic Compound on Silica Surfaces from Organic Solvents Studied by Molecular Dynamics Simulation and AFM Imaging Yong Xiong,† Tiantian Cao,† Qian Chen,† Zhen Li,† Yue Yang,† Shengming Xu,*,† Shiling Yuan,§ Johan Sjöblom,∥ and Zhenghe Xu*,†,‡ †

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada § Key Laboratory of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China ∥ Department of Chemical Engineering, Ugelstad Laboratory, Norwegian University of Science and Technology (NTNU), Trondheim NO 7491, Norway ‡

S Supporting Information *

ABSTRACT: Molecular dynamics simulation was used to investigate the adsorption of a polyaromatic compound (C5Pe) on silica surfaces from organic solvents. Heptane and toluene were used as oil phase to probe the effect of solvent properties on C5Pe adsorption. The results showed that C5Pe molecules tend to adsorb rapidly on silica surface in heptane and assemble to form long strip shaped aggregates, while in toluene C5Pe prefers to form aggregates which remain mostly in bulk oil phase. The van der Waals interactions were found to provide the largest contribution for driving the adsorption of C5Pe from heptane solutions due to the protonated state of C5Pe molecules. The calculated lower system free energy of C5Pe adsorption from heptane than from toluene corresponded well with the observed stronger adsorption of C5Pe from heptane than from toluene. AFM imaging confirmed the observed trend of C5Pe adsorption on silica from heptane and toluene.

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INTRODUCTION

poorly understood, which greatly hindered the progress in resolving the problems associated with asphaltenes. To understand at molecular level the problems associated with asphaltenes in petroleum production, use of model compounds in research has become an effective approach as it allows developing links between molecular structure and interfacial properties by both experimental approaches and molecular modeling. The most commonly considered asphaltene model compounds along with measured properties and relevant references are summarized in Table 1. More details of asphaltene model compounds were reviewed by Sjöblom et al.4 and Greenfield et al.5 Because of the critical and practical importance, great efforts have been devoted to investigating the driving forces of asphaltene aggregation and adsorption in a wide range of systems. Murgich designed experiments to determine the forces of asphaltene aggregation.16 Their results showed that π−π stacking interaction between conjugated aromatic sheets of asphaltene molecules, H-bonding, polar induction force, and

The adsorption and aggregation of polyaromatic compounds from their organic solutions on inorganic solid surfaces plays a significant role in many important systems. In the petroleum industry, for example, the adsorption of polyaromatic asphaltenes on rock surfaces reduces wettability of host rocks, which increases difficulties in oil release from reservoir rocks and causes plug of production wells and transportation pipelines.1 Understanding the aggregation/adsorption mechanisms of polyaromatic molecules on inorganic solid surfaces in organic solutions provides a scientific basis for improving oil recovery and avoiding the plug of oil transport pipelines. The problematic asphaltenes defined as a solubility class of petroleum molecules are the heaviest components of crude oils, consisting of fused polyaromatic rings with alkyl linkages, and branches, and tails of variable lengths. They are soluble in aromatic solvents such as toluene and insoluble in aliphatic solvents such as heptane and hexane.2 Until now the exact molecular structures of asphaltenes are largely unknown, with only two recognized structure models of asphaltene molecules: archipelago model and continental model.3 Despite recognized challenges of asphaltenes in petroleum production and intensive research, the exact structures of asphaltenes remain © 2017 American Chemical Society

Received: November 22, 2016 Revised: February 3, 2017 Published: February 6, 2017 5020

DOI: 10.1021/acs.jpcc.6b11763 J. Phys. Chem. C 2017, 121, 5020−5028

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The Journal of Physical Chemistry C

model asphaltenes and resins on neutral kaolinite surface and analyzed the contributions of each interaction forces.28 The interaction of kaolinite surface with resins was shown to be stronger than with asphaltenes. Boek et al. determined for the first time the potential of mean force (PMF) of asphaltene molecules on a calcite surface by the constraint force method and obtained the reasonable value of adsorption free energy in vacuum.29 Lu et al. probed the wettability of different clay mineral surfaces by investigating the migration of hydrocarbons.30 Their results revealed that decane molecules tend to aggregate and transport faster than water between the clay layers. In this work, we study the adsorption process and driving force distribution of a known polyaromatic compound, C5Pe, from organic solutions onto a silica surface by the molecular dynamics (MD) simulation method. The effect of solvent on C5Pe adsorption from toluene or heptane solution on silica surfaces was investigated using both the MD simulation method and atomic force microscope (AFM) imaging.

Table 1. Most Commonly Considered Asphaltene Model Compounds

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SIMULATION MODELS AND METHODOLOGY GROMACS 4.5.5 software has been proven to be a powerful tool to simulate surface adsorption process and is used for molecular dynamics simulations in this study. Previous studies6,7 on aggregation of asphaltenes and corresponding model compounds demonstrated suitability of the GROMOS96-53a6 united atom force field to match the simulation and experimental results.31 This force field is therefore adopted in the current simulations. C5Pe Molecule. N-(1-Hexylheptyl)-N′-(5carboxylicpentyl)perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe) (shown in Figure 1) was used as a typical polyaromatic

electrostatic interaction collectively drive asphaltenes to form supermolecular structures. To understand adsorption processes for developing removal strategies of asphaltenes, different adsorption systems were studied as reviewed by Adams.17 The asphaltene adsorption was found to be a multifaceted process and sensitive to many system variables such as source and concentration of asphaltenes, type of solvents, properties of rock surfaces, etc. For example, early study showed monolayer adsorption characteristics under the static conditions in contrast to continuous adsorption in dynamic flow experiment using the QCM-D technique, AFM imaging, and other spectroscopy analysis.18 Sjöblom et al. investigated adsorption of fractionated asphaltenes on a number of different solid surfaces.19 Their results of asphaltene adsorption showed that the surface active sites of H-bonding make a great contribution to asphaltene adsorption. Other research by Zahabi et al. on adsorption of asphaltenes on the silica surface containing different number of Si−OH sites supported the results of Sjöblom et al.20 Nicolas et al. measured the adsorption of asphaltenes on hydrophilic and hydrophobic silica surfaces.21 Their results showed stronger interactions of asphaltenes with hydrophilic silica than with hydrophobic silica, leading to a higher adsorption capacity on hydrophilic silica surfaces. The studies reported by Cohen et al. and Simon et al. revealed an increased degree of asphaltene aggregation on solids with decreasing the quality of solvents, achieved by increasing heptane content in a heptane−toluene mixture.22,23 Although the adsorption of polyaromatic asphaltenes on solid surface has been studied extensively by experimental methods, the critical information on molecular adsorption process, orientation of molecules, binding sites on solid surfaces, binding groups of asphaltenes, the state of molecular aggregates, etc., remains unavailable. Molecular dynamics simulations6,7,9,14,15,24 and dissipative particle dynamics (DPD)25−27 give insight into the dynamics processes of molecular adsorption from both molecular and microscopic views. Using the molecular dynamics simulation method, Murgich et al. calculated the adsorption energy of

Figure 1. Molecular structure of C5Pe used in the current simulations.

compound with similar fused polyaromatic rings to asphaltenes.4 Since adsorption of C5Pe on silica occurs in organic solvents, the carboxylic groups were all protonated in the current simulations.6,7 The geometry and topology files were generated by Automated Topology Builder (ATB) and Repository Version 2.2,32 which are similar to those reported by Teklebrhan et al.6,7 The force field parameters of simulation are shown in Supporting Information Table S1 and Figures S1.1−1.3. It should be noted that OPLS-aa force field could be also used for similar type of simulations as reported by Boek,29,33,34 Lamia,35,36 and Greenfield,5 who produced the simulation results that are comparable with the corresponding experimental observations. It would be interesting to conduct simulations by applying both approaches to the same systems and compare the results as currently being done in our group. To mimic the reservoir rock surface, a 20 × 20 × 1 SiO2 supercell (slab) was built using Material Studio (MS) with O atoms exposed on surface. The oxygen on the surface was further hydrolyzed to form a surface of silanol groups (Si−OH) which were considered to be the active adsorption sites. The force field parameters of ∥ SiOH (where ∥ represents the 5021


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The Journal of Physical Chemistry C surface) were taken from the work of Warne et al.,37 which have been widely used by Yuan et al.38−40 Configuration and Simulation Details. A box of 10.8 × 9.8 × 11 nm containing 24 C5Pe molecules in a solvent (3000 heptane or 3600 toluene) was first built. Energy minimization was then performed by the steepest descent method, followed by the conjugate gradient method. The system was further simulated for 500 ps in isothermal−isobaric (NPT) ensembles at 300 K and 1 bar pressure using V-rescale thermostat and Parrinello−Rahman pressure coupling algorithm in the Z direction. The SiO2 slabs were then placed on the top and bottom of the simulation box as shown in Figure 2 and Figure

groups of asphaltenes and DBSA in both one-ion pair and twoion pair configurations as well as the free energy of asphaltene aggregation in solvents.35,36 In this paper the PMF profile of C5Pe molecular adsorption on silica surface was investigated in heptane or toluene in the Z (perpendicular to silica surface) direction. A series of windows were created by pulling a C5Pe molecule from the silica surface to 2.6 nm away (shown in Figure S7) and then run 10 ns NVT simulation with harmonic potential for each window. The umbrella force constant is set at 1000 kJ/(mol·nm2) and the pulling rate at 0.01 nm/ps. The other parameters were the same as used in the former NVT simulations. We used the weighted histogram analysis method (WHAM) implemented in GROMACS to calculate the free energy of systems. Materials and Sample Preparation for AFM Imaging. Synthesis and structural characterization of C5Pe molecules were performed at the Ugelstad Laboratory (NTNU, Trondheim, Norway). Toluene and heptane are all of HPLC grade (>99.9%, Aladdin, China). The silica-coated silicon wafers were purchased from Zhongxingbairui Co. Ltd. (Beijing, China). To ensure the silica surface to be hydrophilic as encountered in nature, the wafers were immersed in NaOH solution (about pH = 11) for 30 min, which resulted in a water drop contact angle less than 10° as measured by a drop shape analyzer (DSA100, Krüss, Germany). To conduct the experiment of C5Pe adsorption on silica in toluene solutions, 0.01 g of C5Pe was dissolved in 20 mL of toluene by sonication for 10 min. The treated silica wafers were immersed in the prepared C5Pe-in-toluene solution for 30 min, with the working surface facing down to eliminate possible gravitational deposition of C5Pe aggregates on the silica surface. The silica wafer was then taken out of the solution and blow-dried by pure nitrogen prior to AFM imaging. It is wellknown that C5Pe as a model compound of asphaltenes is insoluble in heptane by definition. The adsorption experiments of C5Pe on silica wafer cannot be conducted directly using C5Pe-in heptane “solutions”. To investigate the effect of solvent type on adsorption of C5Pe on silica surfaces, the silica wafer was first immersed in C5Pe-in-toluene solutions prepared using the procedures described above, followed by gradual addition of heptane to the solution to change the concentration of heptane in toluene, often known as heptol. After reaching a desired heptane concentration with the wafer remained in the resultant solution for 30 min, the wafer was taken out and blowdried by pure nitrogen prior to AFM imaging. It should be noted that at the heptane-to-toluene volume ratio of 5:1 the system would reach C5Pe precipitation state and hence mimic the simulation of C5Pe in the heptane system for comparison.44,45 The C5Pe molecules/aggregates adsorbed on silica wafers in toluene−heptane after blow-dried with pure nitrogen gas were imaged in air at room temperature using an atomic force microscope (AFM, Dimension Icon, Bruker). The images of 2 × 2 μm were obtained using SCANASYST-AIR probe purchased from Bruker and Peak Force Tapping mode scan operated by ScanAsyst software.

Figure 2. Initial configuration of simulation box with the area between two SiO2 slabs being filled with solvent (heptane or toluene) and 24 C5Pe molecules, which were not displayed for clarity. Colors for atoms are white = H, yellow = Si, and red = O.

S2. A similar approach was used by Le et al. and Naruke et al.41,42 After applying the same energy minimization by the steepest descent and conjugate gradient methods to this configuration, the simulation was conducted for 100 ns at 300 K in a canonical (NVT) ensemble with periodic boundary conditions extending in all directions. The temperature of the system was kept constant by weak coupling of a V-rescale thermostat with the temperature coupling constant of τT set at 0.3 ps throughout the simulation while the pressure coupling was set at off. All the bond lengths were constrained by the LINCS algorithm, and the neighbor list cutoff distance was set at 1.2 nm. Nonelectrostatic interactions were described by the L-J potential and calculated with a cutoff distance of 1.4 nm. The particle mesh Ewald (PME) summation method with a fast Fourier transform grid spacing of 0.135 nm and cutoff distance of 1.2 nm was adopted to calculate the electrostatic interactions. A Maxwell distribution was applied to generating the initial velocities of atoms in the system. The trajectories of atoms were calculated using the leapfrog Verlet algorithm and recorded in an interval of 1 ps for further analysis. All the analysis, including system energy, distribution and orientation of molecules, and degree of molecular aggregation, was performed using the built-in analytical tools in GROMACS. The MS and VMD program were used as visualization tools. Umbrella Sampling. Umbrella sampling is one of the most accurate methods to estimate the free energy of systems from the potential mean force (PMF) profiles.43 It should be noted that the sampling in the current study is all by MD simulations. Lamia et al. computed the interaction free energy of ionic head

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RESULTS AND DISCUSSION Equilibrium and Stability of Systems. The root-meansquare displacement (RMSD) of C5Pe in heptane and toluene was first calculated using eq 1 to determine the dynamics of C5Pe adsorption and estimate required simulation time: 5022


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The Journal of Physical Chemistry C RMSD =

1 N

N

∑ ⟨|ri(t ) − ri(0)|⟩2 i=1

(1)

where N is the total number of atoms in C5Pe molecules and ri(0) and ri(t) are the initial position and the position of atom i at time t. Figure 3 shows the RMSD of C5Pe adsorption on

Figure 3. RMSD of C5Pe in heptane (black) and toluene (red) as a function of time.

Figure 4. Number of C5Pe and density of solvents along the Z-axis in (a) heptane and (c) toluene over the last 10 ns of simulation time; right side is the final adsorption configuration at 100 ns in (b) heptane and (d) toluene. The heptane and toluene are not shown for clarity.

silica in heptane and toluene as a function of simulation time. After about 35 ns, there are little changes in RMSD for both systems as indicated by the dashed line. It is interesting to note a higher RMSD value for C5Pe in toluene than in heptane. Considering toluene being a better solvent than heptane for C5Pe, this contrast result is not unexpected. In general, stronger solvation of C5Pe by toluene leads to a weaker adsorption of C5Pe on silica from toluene than from heptane, as confirmed in our MD simulation and AFM imaging experiments to be discussed later. The observed higher fluctuations in RMSD of C5Pe in the toluene system than in the heptane system reflects more mobile nature of C5Pe in toluene than in heptane, most likely as a result of much weaker adsorption of C5Pe on silica in toluene than in heptane that leaves most C5Pe molecules in bulk toluene. Compared with C5Pe adsorbed, C5Pe remaining in toluene would led to a higher mobility and hence a higher RMSD value of C5Pe in toluene than in heptane, as observed. The corresponding interaction energy of adsorption process of C5Pe molecules on the silica surface was calculated, and the results are shown in Figure S3. It is interesting to note that the interaction energies reached constant values with anticipated fluctuations after around 35 ns at which RMSD reached relatively constant values for both heptane and toluene systems. The contributions from van der Waals forces (−6280 and −6730 kJ/mol) were shown to be much higher than that from electrostatic interactions (−69 and −320 kJ/mol) in both heptane and toluene. Since all C5Pe molecules in organic solvents were protonated, it is not surprising to observe the van der Waals interactions as the major driving force for C5Pe adsorption on silica surface. Adsorption Process and Configuration. The number distribution of C5Pe and density profile at silica−solvent interfaces were calculated by averaging the configurations over the last 10 ns in the Z direction. The results in Figure 4 show clear accumulation of C5Pe molecules on silica surface in heptane, in contrast to more diffused distribution in toluene. The partition of oil (heptane and toluene) in the vicinity of

SiO2 surface (solid−liquid interface) formed unique layered structures. As shown in Figure 4b, all the C5Pe molecules tended to gather on the surface of silica, and two modes of C5Pe adsorption from heptane on silica were observed: single molecule adsorption and C5Pe aggregates adsorption (shown in Figure S4). While in toluene, C5Pe adsorption is minimal even at prolonged simulation time. Most C5Pe molecules prefer to stay in the bulk toluene phase in the form of nanoaggregates (shown in Figure 4d). The results in Figure 4 clearly illustrate the critical role of solvent type in adsorption/ deposition of polyaromatic molecules on silica (rock) surfaces as encountered in petroleum production. The resulting density of 684 and 874 kg/m3 from MD simulation, on the other hand, is in an excellent agreement with the experimentally measured (true) density values of 684 and 868 kg/m3 for bulk heptane and toluene, respectively. The spikes near the silica surface in the density profile is a result of C5Pe molecules adsorbed on silica surfaces, with density profile following the same patterns as C5Pe number distributions for both solvents. As we can see, all the C5Pe molecules adsorbed on silica surface from heptane, but there was almost no adsorption in toluene. For this reason, the interface behavior on the silica− toluene surface was not discussed here. In order to further understand the adsorption process of C5Pe molecules from heptane on silica surfaces, the radial distribution function (RDF) between single C5Pe molecule (center of mass) and silica surface was calculated. The results in Figure 5 show two distinct peaks of distribution at 0.26 and 0.55 nm, indicating most of polyaromatic core of C5Pe molecules on silica surfaces being tiled to the silica surface with a few C5Pe molecules lying at the slant state. It should be noted that C5Pe molecules at the slant state could turn to the tiled state (Figure S5). The distance between the terminating −COOH groups and silica surface was found to be less than 0.26 nm, indicating all the carboxyl groups being bond to the silanol groups of modified silica surfaces through H-bond interactions. 5023


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mainly of π−π stacking and T stacking of C5Pe molecules. For this reason, the details on C5Pe aggregation in heptane will not be discussed in this paper again. To further understand the mechanism of C5Pe adsorption, the distribution of C5Pe clusters either adsorbed or remained in the bulk solvent was calculated from the MD simulation results. In this set of simulations, all 24 C5Pe molecules were initially completely dispersed in the form of monomers in bulk solvents. The cluster number calculated based on the cutoff distance of 0.55 nm (the distance of mass center of two C5Pe) as a function of time is shown in Figure 7. In this calculation, the cluster size was updated whenever a C5Pe molecule moved into 0.55 nm around a cluster, including a single C5Pe molecule as monomer. The number of clusters was calculated over every 10 ns interval. Furthermore, the clusters adsorbed (solid bars) were distinguished from that in the bulk (dashed bars). The results in Figure 7 show that the adsorption reached dynamic equilibrium at about 50 ns in both solvents as shown by the same cluster distributions between 50 and 100 ns simulation time. However, almost all C5Pe molecules were adsorbed on silica surfaces in heptane solutions, in contrast to a much lower fraction of C5Pe molecules adsorbed on silica from toluene. It is interesting to note that compared with the case in heptane, the number of C5Pe monomers in toluene decreased more rapidly due to rapid formation of C5Pe aggregates in toluene as a result of its higher mobility. Also interesting to note is larger size C5Pe aggregates stayed in the bulk toluene than adsorbed on silica surfaces (Figure S4). The observed differences were caused by higher solubility of C5Pe molecules in toluene than in heptane. The low solubility of C5Pe molecules in heptane drives them to adsorb on silica surfaces rather than aggregate in bulk solutions. It appears that the adsorption of C5Pe on silica surface also limits C5Pe aggregation as compared with its counterpart in bulk toluene. The observed trends in toluene and heptane here are in line with the results of RMSD in Figure 3. To gain more insights into the mechanism of C5Pe adsorption on silica surfaces, the formation of hydrogen bonds between C5Pe molecules and silica surface was quantified as a function of time. In our simulation, the pendant alkyl groups were modified by sp2 hybridization of carbon in the united atom force field as such that they cannot form hydrogen bond. The hydrogen bonds of polar termination (−COOH) and perylene core of C5Pe molecules were calculated separately, and the results are shown in Figure 8. The number of hydrogen bonds between C5Pe and silica (total) in heptane increased rapidly with simulation time up to 30 ns after which it reached a steady value. In this system, the polyaromatic ring (perylene) in heptane contributed almost equally to the formation of hydrogen bonds with silica as carboxylic acid groups did. In comparison, the number of corresponding hydrogen bonds in toluene is much smaller. In this case, the contribution from perylene to forming hydrogen bonds with silica surface in toluene is negligible, accounting for weak adsorption observed in Figure 7. The observed weak adsorption in toluene is most likely from hydrogen bonds between carboxylic acid group of C5Pe and silanol groups on silica, with a negligible contribution from hydrogen bonds between silanol groups of silica and polyaromatic rings of C5Pe. This result matches with the observed preferential “upright” conformation of toluene on silica surface, as a result of hydrogen bonding between carboxylic acid group of C5Pe and silanol groups on silica.46 The lower number of hydrogen

Figure 5. RDF of polar −COOH terminal groups and polyaromatic perylene cores in reference to silica surface, averaged over the last 10 ns in heptane.

The orientation of adsorbed C5Pe molecules can be shown by the face-to-face angle distribution between silica surface and polyaromatic planes. The average angle was calculated over the last 10 ns of the simulation time. The angle (θ) value could range from 0° to 90°. When θ ≤ 20°, the polyaromatic plane is referred to as parallel to the silica surface, which is equivalently referred to the distance of 0.26 nm. In the same way, when 50° ≤ θ ≤ 70°, the molecules are considered in the slant state. The result in Figure 6 shows that the angles of adsorbed single C5Pe

Figure 6. Orientation (angle distribution) of plane of C5Pe molecules on silica surfaces in heptane with the angle (θ) being shown in the inset.

molecules are distributed at 0°−20° and 50°−70° in heptane, indicating two types of orientations of C5Pe molecules adsorbed on silica surfaces. However, when all the C5Pe molecules were included to calculate the face-to-face angle, namely single C5Pe molecule adsorption and C5Pe aggregates adsorption, it was found that the aggregation on surface was formed at a higher slant angle rather than parallel to the silica surface. Moreover the C5Pe molecules were mostly aggregated and attached to the silica surface as shown in Figure 4(a and b). The detailed analysis showed that the aggregation configurations were the same as in the bulk oil phase, which was reported in our previous work.6 The assemble structure consists 5024


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Figure 7. Number of each type of C5Pe clusters in (a) heptane and (b) toluene, with solid columns referring to the adsorbed clusters while shaded columns referring to unabsorbed clusters.

Figure 8. Number of hydrogen bonds as a function of time in (a) heptane and (b) toluene.

bonds between polyaromatic rings of C5Pe and silanol groups on silica in toluene than in heptane as shown in Figure 8 is most likely linked with strong interaction of silica in toluene due to hydrogen bonding between aromatic rings of toluene and silanol groups on the silica surface, contributing to weaker C5Pe adsorption on silica in toluene than in heptane. Furthermore, there appears a slight decrease in the number of hydrogen bonds after 50 ns (the time of maximum hydrogen bonds number), indicating some degree of C5Pe desorption from silica surfaces (Figure S6). Free Energy Evaluation by Umbrella Sampling. The PMF profiles of C5Pe molecules in heptane and toluene in Figure 9 show a rapid increase in system free energy when pulling away a C5Pe molecule from silica surface in both toluene and heptane. When a C5Pe molecule is pulled off from the silica surface, the solvent molecules replace the C5Pe molecule. The overall free energy required to pull off a C5Pe molecule from silica surface in heptane is calculated to be 90.3 kJ/mol. The free energy required to pull off a C5Pe molecule from the silica surface in toluene is only 30.4 kJ/mol, representing almost 3 times weaker adsorption of C5Pe molecule on silica surface in toluene than in heptane. The observed higher overall free energy increase of pulling off a C5Pe molecule from silica in heptane than in toluene indicates a stronger binding and hence stronger adsorption of C5Pe on silica in heptane than in toluene. The smaller free energy change of pulling off a C5Pe molecule from and hence weaker binding of C5Pe with silica in toluene than in heptane is attributed to the stronger interaction of silica by toluene through hydrogen bonding between silanol groups on silica and

Figure 9. PMF profiles of C5Pe molecule in heptane (black) and toluene (red); the inset is the pulling schematic of umbrella sampling.

aromatic rings of toluene that is absent in heptane. Because of hydrogen bonding characteristics of carboxyl group on C5Pe with aromatic ring of toluene that is absent in heptane, a stronger solvation of C5Pe by toluene than by heptane is anticipated. As a result, additional energy is required to remove solvating toluene molecules from C5Pe in order to accomplish the binding of carboxyl group in C5Pe with silanol group on silica surfaces, which also contributes to lower pulling off energy of C5Pe from silica surface in toluene than in heptane. AFM Imaging of C5Pe Adsorption on Silica. To validate the results from MD simulations, the morphology of C5Pe 5025


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Figure 10. AFM images of C5Pe adsorbed on silica surface: (a) immersed in toluene for 30 min; (b−d) immersed for 30 min in heptane−toluene solution with volume ratio of toluene/heptane = 5:1, 1:1, and 1:5, respectively; (e−g) immersed in an equal volume heptane−toluene mixture solution for 5 s, 5 min, and 15 min, respectively.

showing long striped shape of adsorbed C5Pe molecular aggregates on silica surface from solutions of increasing heptane volume ratio. The calculated adsorption free energy of C5Pe molecules indicated preferred adsorption of C5Pe on silica surface to reduce the system energy in heptane, but it was opposite in toluene.

molecules adsorbed on silica surfaces from toluene and heptane was determined using atomic force microscope (AFM), and the results are shown in Figure 10. As anticipated, silica wafer was flat and featureless with roughness of 0.416 nm after treated with NaOH (3D image is shown in Figure S8). The AFM image of Figure 10a shows a negligible adsorption of C5Pe molecules on silica in toluene after 30 min, which agrees well with the MD simulation results of minimal adsorption of C5Pe in toluene (Figure 7b). When heptane was injected in toluene, clear adsorption of C5Pe on silica in the aggregates of long strip shape is seen (Figure 10b−d). The size and density of the C5Pe aggregates on silica surface increased with increasing volume fraction of heptane as a result of reducing the quality of the solvent. This finding agrees well with the MD simulation results that most C5Pe molecules in heptane adsorbed on silica. AFM images in Figure 10e−g show a rapid adsorption of C5Pe in a 1:1 heptane:toluene mixture. The adsorption of C5Pe is mainly in the form of round nanoaggregates after being immersed in an equal volume heptane−toluene solution for 5 s. With increasing the adsorption time, the more aggregates in long strip shape are seen (Figure 10f,g).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11763. The initial simulation box of system, interaction energy between C5Pe and silica surface, molecular partition, C5Pe adsorption process, and roughness of NaOH treated silica surface, contact angle of before and after treatment (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.X.). *E-mail: [email protected] (S.X.).

CONCLUSION Adsorption of C5Pe molecules on silica surface in toluene and heptane was studied by molecular dynamics simulation. Although there was limited adsorption of C5Pe on protonated silica from toluene, significant adsorption of C5Pe from heptane was found. The van der Waals forces were the main force for driving C5Pe molecules to adsorb from heptane on protonated silica surface. It was the polar terminate (carboxylic acid: −COOH) group that adsorbs on the protonated silica surface first, followed by the interactions of perylene cores with the solid surfaces. The type of solvents showed a great influence on adsorption, mainly due to the difference in the state of molecular solvation. In heptane C5Pe molecules tend to adsorb on silica surface with more hydrogen bonds with solid surface and less self-aggregation. In contrast, C5Pe tends to selfaggregate and even desorbs from silica surface into bulk toluene phase. In situ AFM imaging confirmed the simulation results,

ORCID

Shengming Xu: 0000-0002-6765-9251 Shiling Yuan: 0000-0002-4073-9470 Zhenghe Xu: 0000-0001-8118-1920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The financial support of the project from National Natural Science Foundation of China (Grant 21333005) is greatly appreciated. Partial financial support through the Petromaks program (Norwegian Research Council) and JIP 1 (Ugelstad Laboratory) and NSERC-Industry Research Chair in Oil Sands Engineering is also gratefully acknowledged. 5026


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Adsorption of a Polyaromatic Compound on Silica Surfaces from

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