Competitive Adsorption of Naphthenic Acids and ... - ACS Publications

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Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Toluene−Water Interface Robel B. Teklebrhan,† Cuiying Jian,‡ Phillip Choi,† Zhenghe Xu,*,†,# and Johan Sjöblom§ †

Department of Chemical and Materials Engineering and ‡Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 1H9 # Institute of Nuclear and New Energy Resources, Tsinghua University, Beijing 100084, China § Department of Chemical Engineering, Ugelstad Laboratory, Norwegian University of Science and Technology (NTNU), Trondheim NO 7491, Norway S Supporting Information *

ABSTRACT: The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene−water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the toluene−water interface. At low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the toluene−water interface. At high NA concentrations, the adsorption of PA molecules (∼5.75−17.25 wt %) was greatly hindered by the presence of small NA molecules (∼1.6−4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at toluene−water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications. functionality to a variety of other basic groups.17−21 These functional groups together with the aromatic core of PA molecules interact through various types of intermolecular interactions, including hydrogen bonding, π−π stacking, dipole−dipole, CH···π, OH···π, NH···π, steric interactions, and so forth.1−6 Over the years, a wide range of interfacial phenomena of PA molecules has been investigated, including amine-modified xylenol orange,22 violanthrone-78 (VO-78),22 perylene bisimides (PBIs),23−25 and asphaltene-like model compounds.26−29 The findings from these early studies suggest that the interfacial activity of these molecules depends more on their polar terminal groups than on the structure of their PA ring regions. Upon adsorption of PA molecules at the oil−water interface, the polar groups are incorporated into the aqueous phase, whereas the hydrocarbon part of the PA molecules resides in the oil phase.26−29 The PA molecules adsorbed at the oil−water interface are known to stabilize emulsions, causing

1. INTRODUCTION Detection and identification of early-stage molecular adsorption of interfacially active molecules from the bulk oil phase and the subsequent association at the oil−water interface are of great importance in industries such as pharmaceuticals, food and cosmetics, petroleum, and water treatment plants.1−9 Although the adsorption of these interfacially active molecules is usually considered as desirable for industries such as pharmaceuticals, cosmetics, and food processing, for the most part of petroleum industry and water treatment plants the emulsions formed by these interfacially active molecules can have deleterious effects.6−14 Thus, it is highly desirable to understand the adsorption mechanism and interfacial behavior of these interfacially active molecules to help design appropriate coemulsifiers or chemical demulsifiers. Polyaromatic (PA) molecules found in petroleum industry are characterized as interfacially active hydrocarbon molecules. These molecules feature a large fused PA core that has five to seven aromatic rings with a wide range of functional groups attached to its aliphatic tails.8−12,15−19 These groups range from carboxylic, thiophenic, sulfide, hydroxy, pyridinic, and pyrrolic © 2016 American Chemical Society

Received: August 5, 2016 Revised: November 19, 2016 Published: November 21, 2016 12901

DOI: 10.1021/acs.jpcb.6b07938 J. Phys. Chem. B 2016, 120, 12901−12910

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The Journal of Physical Chemistry B

Figure 1. (a) PBI-based PA molecule (C5Pe); (b) three types of NA molecules; (c) five π−π stacked aggregates of C5Pe molecules (pentamer, colored black); and (d) five π−π stacked aggregated of C5Pe molecules (colored black) bounded to NA-2 molecules (colored green) or NA-solvated PA nanoaggregates. The two C5Pe pentamers (c and d) are identical, except that one has NA-2 molecules bounded to the exterior of the five π−π stacked aggregate of C5Pe molecules. These stacked aggregates are obtained after 50 ns simulation time in bulk toluene.

structure−property relationships, yet not detectable at a pico to nano second time scale by current experimental techniques.

serious problems in petroleum industry, such as refinery catalyst poisoning, scaling, and corrosion of pipelines.8,15,23−25 Over the recent years, through experimental methods, the adsorption and partitioning of PA molecules between the bulk oil phase and the oil−water interface in the presence of other interfacially active molecules have been studied extensively.5,6,11,14,19 For example, recent experimental studies11,14,19 showed a decrease in emulsion stability by the addition of resins (small-size aromatic or cycloaliphatic polar molecules) to the oil having a constant PA (e.g., asphaltene) concentration. The observed reduction in the interfacial activity of PA molecules was attributed to the solvation of PA aggregates by resins in the bulk. The results from these studies revealed that the intermolecular interaction between resins and PA molecules and the competitive adsorption dynamics of these interfacially active molecules to the oil−water interface determine the overall stability of the emulsion. The competitive adsorption dynamics of interfacially active PA molecules and naphthenic acids (NAs) to the oil−water interface was also studied in detail.5,6,23,30,31 The results suggest that the presence of NA molecules can destabilize, to some degree, the stability of the PA-mediated water-in-oil emulsions by reducing the rigidity of the oil−water interface.23,30,31 Tremendous progress5,23,30,31 has been made on understanding the macroscopic behavior of these mixed interfacially active molecules; however, the molecular mechanism of the competing adsorption remains unexplored. In the present work, we investigated the influence of NAs on the adsorption and partition of PA molecules at the toluene− water interface using molecular dynamics (MD) simulation. MD is a powerful theoretical tool to simulate complex systems of mixed interfacially active molecules. MD simulation enables us to explore molecular interactions and track rapid molecular processes, which are important for understanding the molecular

2. SIMULATION METHODS 2.1. Molecular Models and Simulation Setup. PBI and cycloaliphatic-based molecules were used to represent the interfacially active PA and NA molecules, respectively. Their chemical structures are shown in Figures 1a and 1b. The PA molecule (Figure 1a) chosen for this study is N-(1hexylheptyl)-N′-(5-carboxylicpentyl)-perylene-3,4,9,10-tetracar-boxilicbisimide (C5Pe), which has a chemical formula of C47H48N2O6 and molecular weight of 689 (g/mol). It is important to note that C5Pe molecules were selected in this study as a representative of PA molecules because of their strong potency of hydrogen bond formation that led to their nanoaggregation in toluene. Since NA molecules are anticipated to interact with PA molecules via hydrogen bonds,34−37 the feature of carboxylic acid group in C5Pe allows us to probe the effect of NA−C5Pe association on C5Pe adsorption at the toluene−water interface. NA molecules of three different sizes (Figure 1b) are used to mimic a diversity of natural NAs in oil. The general chemical formula of NA molecules is CnH2n+Z O2, where n represents the number of carbon atoms, and Z represents a homologous series. As shown in Figure 1b, NA-2 having two rings belongs to Z = −4 family, whereas NA-4 having four rings belongs to Z = −8 family.31−34 The initial structures of PA and NA molecules are obtained using Chem 3D ultra 10.0 software. It is of great importance to point out that, to be consistent with our previous published paper,34 the same names for NA molecules are used in this study (i.e., NA-2, NA-3, and NA-4). In this atomistic simulation study, 10 different systems were simulated. The first three of these 10 simulations were performed to investigate molecular interactions of C5Pe 12902


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Figure 2. Density profiles of C5Pe (red line) and NA (blue line) molecules averaged over the last 5 ns of simulation time. For C5Pe only (a), NA-2 mixed with C5Pe (b), NA-3 mixed with C5Pe (c), and NA-4 mixed with C5Pe (d) at the toluene−water interface.

one base simulation was conducted for the above system but without any NA molecules. To probe adsorption of “C5Pe nanoaggregates” and “NAsolvated C5Pe nanoaggregates” from the bulk to the oil−water interface, six additional simulations were performed starting from a “co-aggregated” state of C5Pe and NA molecules in the bulk toluene phase. To do so, we first extracted a pentamer of five π−π stacked C5Pe molecules bound to NA-2 molecules (Figure 1d) from a separate simulation of monomer C5Pe and NA-2 molecules in the pure toluene phase (Supporting InformationS2). The main objective here was to investigate the role of NA molecules in controlling the adsorption of C5Pe nanoaggregates from the bulk toluene to the oil−water interface in comparison to nonsolvated C5Pe molecules. For this purpose, we extracted the nanoaggregated states of C5Pe molecules in bulk toluene from our previous simulations to study the migration dynamics of C5Pe nanoaggregates from the bulk toluene region to the interface. The results in Sections 3.1 and 3.2 indicate that all types of NA molecules studied compete with individual PA molecules and their nanoaggregates to migrate to the toluene−water interface. Despite the strongest interactions with C5Pe nanoaggregates in bulk toluene without toluene−water interface, NA-2 appears to exhibit the weakest interaction with asphaltene nanoaggregates in bulk when there

molecules with different NA molecules (e.g., interaction of C5Pe with NA-2). In these three simulations, 18 C5Pe molecules were evenly placed in two layers in a regular lattice configuration within a 10 nm edge cubic box of toluene. In this configuration, each of the two layers contained 9 C5Pe molecules, approximately 7.5 nm away from each other. Then, nine single type of NA molecules shown in Figure 1b (i.e., NA2, NA-3 or NA-4) were added in between these two layers of C5Pe molecules, resulting in a system containing two layers of C5Pe and one layer of NA molecules immersed in ∼3199 toluene molecules. A 10 nm × 10 nm × 5 nm (z axis) simulation box containing ∼14 153 water molecules was placed on the top of the aforementioned cubic simulation box to create an oil−water interface (Supporting InformationS1). The mass concentrations, in toluene, for these systems are approximately 2.3 wt % for C5Pe molecules and 0.4 wt % for NA molecules. The systems constructed as such were used to capture the early-stage competitive adsorption of individual molecular species to the toluene−water interface, thereby revealing molecular association dynamics of NA and C5Pe molecules at the toluene−water interface during this stage. The number of toluene and water molecules used in each simulation system is shown in Supporting InformationS1. As a control, 12903


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The Journal of Physical Chemistry B is an oil−water interface. The observed “conflicts” of NA-2 interactions with asphaltene nanoaggregates in the presence and absence of toluene−water interface result from quick migration of NA-2 to the toluene−water interface because of its strong interfacial activity compounded by its smaller sizes and hence higher diffusivity. Following this hypothesis, three independent simulation systems containing 9, 18, and 27 of the co-aggregated pentamers bound to NA-2 were arranged in a cubic box of dimension 10 nm × 10 nm × 10 nm. As a comparison, a reference simulation was also conducted by removing all of the NA-2 molecules from C5Pe aggregated pentamers while keeping all other aspects of the stacked C5Pe molecules the same (Figure 1c). After setting the initial configurations for C5Pe and NA molecules, all of the simulation boxes were filled with toluene molecules as the bulk organic solvent. The simulation box was then extended in the z direction by 5 nm and randomly filled with water molecules. The initial configurations for these systems and the number of toluene and water molecules used in the system are also shown in Supporting InformationS2. 2.2. Simulation Details. In this study, the topologies for C5Pe, NA, and toluene molecules were developed using the force field in GROMOS96 with the 53a6 parameter set and are adopted from our previously published papers.34−37 The applicability of this united atom force field to PA molecules (e.g., C5Pe molecules), NA molecules, and toluene molecules was extensively tested and validated in our previous papers with the experimental results and literature data.34−37 The extraction of the topology files, parameterization, simulation setup, and the validity of the simulation time were also tested and validated in our previous studies.34−37 For the water molecules, the extended simple point charge (SPC/E) model in GROMACS is used in our simulation as this model adds the polarization correction to the potential energy function and reasonably reproduces the properties of water at ambient temperatures.38−41 All of the simulations were carried out using the GROMACS 4.0.5 software package.39−41 Each system was first energy minimized using the steepest descent method. The cutoff for both the short-range electrostatic and van der Waals interactions was set to 1.2 nm, and the long-range electrostatic interaction was handled using the particle-mesh Ewald method.40 For each system, full dynamic simulations were performed for 30 ns in an NPT ensemble at 298 K temperature and 1 bar pressure using canonical coupling schemes of the Nosé−Hoover thermostat and Parrinello−Rahman barostat.42−44 Throughout the simulation time, we used a time step of 2 femtosecond (fs), the SETTLE algorithm to constrain all bonds for water molecules, the LINCS algorithm to constrain all bonds for the other organic molecules, a pressure coupling constant of τp = 3 ps, and a temperature coupling constant of τT = 0.3 ps.39−41

scaled version of the density profiles is shown in the inset of each graph to improve the visibility of the profile. The interface is located at the point where toluene and water lines intersect, or the dotted line in the inset of the figure. The density distribution is averaged over the last 5 ns of the simulation time. The description and procedure to calculate the density profiles of the simulated systems were reported previously.34−37 The snapshots in the inset were taken at the end of the simulation time. Demonstration of the system achieving dynamic equilibration by averaging radial distribution function (RDF) over the last 5 ns of 30 ns simulation is available in Supporting InformationS3. It is worth noting that the number of hydrogen bonds seems not to fully converge within the simulation time used. For the purpose of this study, however, showing only the converging in the extent of hydrogen bonding should not change our general conclusions on the adsorption dynamics of NA and PA molecules at the toluene-water interface. As shown by the results in Figure 2, the partition of C5Pe molecules at the toluene−water interface is affected by the presence of NA molecules. In the absence of NA molecules, for instance, C5Pe molecules prefer to concentrate near the toluene−water interface (inset in Figure 2a). This strong affinity of C5Pe molecules for the interface was confirmed by the presence of two relatively large peaks near the two toluene−water interfaces in the density profile shown in Figure 2a. The presence of a comparable third peak in the middle of the density profile reveals the moderate affinity of C5Pe molecules to aggregate in the bulk toluene phase. The results suggest that in the absence of NA molecules, C5Pe molecules distribute slightly more at the two interfaces than in the bulk toluene phase. In contrast, there exists a large and broad peak of C5Pe molecules in the bulk toluene phase with two peaks near the toluene−water interface that diminish significantly when NA-4 molecules are added to the system (Figure 2d), indicating a significant reduction in the interfacial affinity of C5Pe molecules in the presence of NA-4 molecules (inset in Figure 2d). The strong self-association of C5Pe molecules, as shown in the snapshot inset panel of Figure 2d, appears to be responsible for the observed weak interfacial activity of C5Pe molecules. A further inspection of the density profile for C5Pe molecules with NA-3 reveals two overlapping peaks of C5Pe molecules in the bulk toluene phase, whereas two moderate C5Pe peaks near the toluene−water interface remain when NA3 molecules are present (Figure 2c). This finding indicates the less significant effect of NA-3 molecules than that of the larger ring NA-4 molecules on the interfacial activity of C5Pe molecules, most likely as a result of weaker interactions of NA-3 than that of NA-4 molecules with C5Pe molecules, which is reflected in the snapshot panel of Figure 2c. For NA-2 molecules (Figure 2b), the density profile of C5Pe molecules is similar to the baseline case of C5Pe without NA molecules, indicating the weakest interaction of NA-2 molecules with C5Pe molecules as shown in the inset panel of Figure 2b, with most NA molecules being unbound to C5Pe molecules. For NA molecules studied here, small NA molecules (NA-2) exhibit a weak affinity and higher tendency to concentrate at the toluene−water interface in comparison to large NA molecules (NA-3 and NA-4). These observations suggest that the partition of NA molecules and their effect on partition of C5Pe molecules at the toluene−water interface depend greatly on the structures of NA molecules.

3. RESULTS AND DISCUSSION 3.1. Effect of NA Molecules on the Initial Adsorption Dynamics of C5Pe Molecules to the Oil−Water Interface. The main focus of this section is to investigate the competitive adsorption dynamics of individual C5Pe and NA molecules to the toluene−water interface from the bulk toluene phase. Figure 2 shows the density distribution profiles of the simulated C5Pe and NA molecules for each of the four simulated systems, starting from the well-dispersed phase. The 12904


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Figure 3. RMSD of C5Pe (a) and NA (b) molecules from the toluene−water interface as a function of time. (c) and (d) show the partition ratio (%) of C5Pe and NA molecules from the toluene−water interface as a function of simulation time.

molecules approaching the toluene−water interface is greatly reduced by the presence of NA-3 and NA-4 molecules in the system. However, the RMSD for C5Pe molecules in the presence of NA-2 molecules shows a comparable trend to our reference simulation, suggesting that NA-2 molecules have little influence on the rate of C5Pe molecules migrating to the toluene−water interface at this mass concentration (∼2.3 wt %). Toward the end of the simulation (from 28 to 30 ns), the average separation distances between C5Pe molecules and the toluene−water interface in the presence of NA-3 and NA-4 molecules were 2.34 ± 0.03 and 2.95 ± 0.05 nm, respectively, which are larger than those in the presence of NA-2 (1.76 ± 0.04 nm) and the reference simulation (2.04 ± 0.04 nm). Significant difference also exists in the RMSD for NA molecules shown in Figure 3b. First, the RMSD of NA-2 molecules continues to decrease till the last 5 ns of 30 ns simulation time, corresponding to continuous migration of NA-2 molecules to the interface. In contrast, the RMSD for NA-3 and NA-4 molecules approach a near steady-state value after 15 ns. These different dynamics of NA migration finally lead to the distinct separation distance of NA molecules to the interface. The separation distance of NA-2 molecules, for example, is ∼0.31 ± 0.03 nm, which is much smaller than that of NA-3 (∼2.53 ± 0.04 nm) and NA-4 (∼2.10 ± 0.13 nm) molecules. This finding accounts for the prominent accumulation of NA-2 molecules at the interface shown in the inset of Figure 2b. The differences in adsorption dynamics observed lead to distinct partition of C5Pe and NA molecules between bulk and toluene−water interface (interface-bound molecules), as shown

To explore the dynamics of the adsorption at the toluene−water interface in the current systems, the rootmean-square distance (RMSD) of C5Pe and NA molecules was calculated using the following equation

RMSD =

1 N

i=1

∑ δi2 N

where N is the total number of C5Pe or NA molecules in the system, and δi is the corresponding distance of the ith C5Pe or NA molecules from the toluene−water interface. The distance was evaluated by calculating the minimum (“MIN”) distance between any atom of the central ring of C5Pe or NA molecules and any atom of water molecules at the vicinity of the interface. Therefore, the RMSD is a good indicator of the relative locations of molecules to the interface. Figures 3a and 3b shows the RMSD plots of C5Pe and NA molecules, respectively, from the toluene−water interface as a function of time, in which different systems are represented by different colors. Clearly, there is an initial decrease in all of the RMSD values, corresponding to the migration of C5Pe and NA molecules to the interface. Detailed examination of these profiles reveals considerable differences among the plots for the four systems in each figure. As seen from Figure 3a, the rate of C5Pe molecules approaching the toluene−water interface, indicated by the ongoing drops of the RMSD, varied greatly in different simulation systems, indicating that the type and size of NA molecules present in the system can greatly influence the interfacial activity of C5Pe molecules. Compared with our reference simulation, the rate of accumulation of C5Pe 12905


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Figure 4. Formation of hydrogen bond as a function of time for (a) C5Pe and water molecules, (b) NA and water molecules, (c) C5Pe and NA molecules, (d) among C5Pe molecules, (e) among NA molecules, and (f) type of π−π stackings and their occurrence in each system.

molecules. The number of hydrogen bonds is calculated using a geometric criterion of donor−acceptor distance (rDA ≤ 3.5 Å) and acceptor−donor−hydrogen angle (θADH ≤ 30°).40 Figure 4 summarizes the number of hydrogen bonds formed from initially well-dispersed individual C5Pe and NA molecules as a function of simulation time. In the presence of large NA molecules, such as NA-4, less hydrogen bond is formed between C5Pe and water molecules at the interface, as shown in Figure 4a. For small NA molecules, such as NA-2, C5Pe molecules form the highest number of hydrogen bonds with water. The similarity between Figures 4a and 3a indicates that the hydrogen bond is a dominant driving force for partition of C5Pe molecules at the interface. Similarly, a larger number of hydrogen bonds are formed between NA-2 and water molecules than between the other two NAs and water molecules, as shown in Figure 4b, which is consistent with previous observation in Figure 3d that smaller NA molecules prefer to migrate rapidly to the interface and adsorb at the interface. It is the stronger interaction of C5Pe molecules with larger NA molecules that limited the hydrogen bonding of C5Pe with water molecules. Furthermore, a recent experimental study on similar molecules using the electrospray ionization mass spectrometry method revealed a significant hindrance in self-association of C5Pe molecules in the bulk, when larger NA molecules are present, because of stronger steric hindrance of larger NA molecules.45 To probe the effect of NA molecules on the π−π stacking of C5Pe molecules in the presence of water phase (or oil−water system), the types of π−π stacking formed and their occurrence were calculated using the same method reported previously.34,46−48 Here, the types of stacking are quantified by the number of C5Pe molecules in each stacking structure. For instance, three π-stacked molecules indicate that there are three

in Figures 3c and 3d. To quantify this partition, a partition ratio is defined as the ratio of the number of interface-bound molecules (NI) to the total number of C5Pe molecules in the system (NT), that is, PI/T = NI/NT. The interface-bound molecules are those molecules found within 1 nm from the oil− water interface on both sides of the simulation box. To calculate the number of molecules near the toluene−water interface, we followed the same methodology described in our previous work.36,37 As seen from Figure 3c, C5Pe molecules mixed with small NA molecules partition more at the interface in comparison to C5Pe molecules mixed with large NA molecules. The partition ratio of C5Pe molecules in the presence of NA-2 molecules, for example, is similar to the partition ratio of C5Pe molecules in the absence of NA molecules. C5Pe molecules mixed with NA2 molecules, however, have a partition ratio of around 61% in comparison to 17% for C5Pe molecules mixed with NA-4 molecules. In contrast, 88.9% of NA-2 molecules partitioned to the interface in comparison to 33.3% of NA-4 molecules, as shown in Figure 3d. These results indicate that for the concentration studied at a given time, smaller NA molecules partition more than larger NA molecules to the interface, occupying quickly limited area at the toluene−water interface and hence leading to less interactions with the C5Pe molecules in the bulk toluene phase. 3.2. Intermolecular Hydrogen Bond Formation Dynamics and π−π Stacking. The results from our previous study revealed the interaction force of the hydrogen bonding responsible for the observed adsorption dynamics of C5Pe molecules at the toluene−water interface in the absence of NA molecules.36 Therefore, using hydrogen bond formation dynamics as a single criterion to compare the dynamic nature of C5Pe and NA molecules in our current work could provide insights into the interaction mechanisms of C5Pe with NA 12906


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Figure 5. Snapshots taken after 100 ns simulation of C5Pe nanoaggregates at the toluene−water interface in the absence (a) and presence (b) of NA-2 molecules that retard the migration of C5Pe nanoaggregates to the toluene−water interface.

molecules concurrently stacking together through π−π stacking. The results summarized in Figure 4f for such π−π stacking types show the presence of large numbers of C5Pe monomers in all systems studied here, suggesting a significant reduction in π−π stacking of C5Pe molecules as C5Pe and NA molecules migrate to the interface from the systems of the same C5Pe and NA concentrations. This finding is in contrast to the prevalence of π−π stacking of C5Pe molecules observed in bulk toluene without water, as reported in our previous work.34 3.3. Adsorption of NA-Solvated C5Pe Nanoaggregates from the Bulk Toluene Phase to the Toluene− Water Interface. It is widely known that PA molecules form nanoaggregates in toluene even at extremely low concentrations.8−13 Therefore, for any given system, both individual PA molecules and their nanoaggregates can coexist. The snapshot in Figure 2, for example, reveals the formation of C5Pe nanoaggregates as well as the presence of C5Pe monomers in the bulk toluene. These nanoaggregates either grow into large aggregates or migrate slowly to the interface with time. Thus we explore in this section the adsorption dynamics of these nanoaggregates to the toluene−water interface. For this purpose, six additional simulations were designed. These simulations were performed on the structures depicted in Figures 1c and 1d, which were extracted from our bulk toluene simulation.34 It is important to note that because of strong interactions between the carboxylic acid functional groups (−COOH) of NA-2 and C5Pe molecules in the system, C5Pe nanoaggregates formed in the bulk toluene phase are usually surrounded by NA-2 molecules in the system. Therefore, the inclusion of NA-2 molecules, as shown in Figure 1d, allows us to explore the effect of NA-2 molecules on the migration of C5Pe nanoaggregates to the oil−water interface. The adsorption dynamics and 3D structural arrangements of nine NA-2-solvated C5Pe nanoaggregates were first simulated. The system achieving dynamic equilibrium toward the end of 100 ns simulation is clearly shown in Supporting Information S4 by overlapping of RDFs averaged over 10 ns toward the end of 100 ns simulation time. Again, although the number of

hydrogen bonds has not fully converged yet, general conclusions regarding interactions of NAs with C5Pe at the toluene−water interface remain valid and would not change by considering only the converging trend. To account for the concentration effect on the adsorption dynamics, two additional simulations were conducted with 18 and 27 NA-solvated C5Pe nanoaggregates (Supporting InformationS2). These three systems have C5Pe mass concentrations of 5.75, 11.5, and 17.25% and NA mass concentrations of 1.60, 3.20, and 4.80%. To examine the effect of NA “solvation” on the adsorption of C5Pe nanoaggregates at the toluene−water interface, three control simulations were performed by removing the NA molecules from the C5Pe nanoaggregates. Figure 5 shows the final structure formed in the system of nine C5Pe nanoaggregates without and with NA-2 molecules solvated in the system. After 100 ns simulation, the results revealed that a large portion of the C5Pe molecules in the absence of NA-2 migrated to the toluene−water interface (Figure 5a). In contrast, majority of the C5Pe molecules remained in the bulk toluene phase and only a small fraction of the C5Pe molecules migrated to the toluene−water interface when C5Pe nanoaggregates were solvated by NA-2 molecules (Figure 5b). These contrast behaviors confirm that the solvation of C5Pe nanoaggregates in the bulk toluene phase by NA-2 molecules hinders the adsorption of C5Pe molecules to the toluene−water interface. At a higher NA-2 concentration, NA-2 can occupy the active sites of C5Pe nanoaggregates, thus making C5Pe nanoaggregates less interfacially active. As discussed in Section 3.2, the main driving force in the adsorption of C5Pe molecules to the toluene−water interface is the strong hydrogen bonding interactions between the carboxylic acid functional group of C5Pe molecules and the water molecules. To further examine the impact of NA-2 interaction with C5Pe nanoaggregates on the hydrogen bond formation of C5Pe molecules with water molecules, the time evolution of hydrogen bonding between C5Pe and water molecules at different C5Pe concentrations (i.e., 9, 18, and 27 C5Pe nanoaggregates) was calculated. A geometric criterion, described in Section 3.2, was used to calculate the number of 12907


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Figure 6. Number of hydrogen bonds formed between (a) C5Pe nanoaggregates and water molecules without NA; (b) C5Pe nanoaggregates and water molecules with NA-2; (c) C5Pe nanoaggregates and NA-2 molecules; and (d) NA-2 and water molecules.

suggest the hindrance of NA molecules to C5Pe adsorption at the toluene−water interface, thereby diminishing the ability of C5Pe molecules to stabilize water-in-oil emulsions. The NA molecules are therefore anticipated to reduce the stability of water-in-oil emulsions. The results obtained from this work provide some molecular insights into the findings of an earlier experimental report by Gao et al.30 that the presence of NA molecules in bitumen destabilized moderately the water-in-oil emulsion by reducing the rigidity of diluted asphaltene−water interface due to the resultant weaker adsorption of asphaltene molecules at the oil−water interface.

hydrogen bonds of the systems. A comparison of results in Figures 6a and 6b shows more hydrogen bonds formed between C5Pe and water molecules at the interface in the absence (Figure 6a) than in the presence (Figure 6b) of NA-2 molecules for all simulated C5Pe nanoaggregate concentrations. This finding indicates a diminished formation of hydrogen bonds between C5Pe and water molecules at the interface as a result of the association of C5Pe nanoaggregates with NA-2 molecules in the systems, leading to a weaker adsorption of C5Pe molecules at the toluene−water interface. For comparison, the formation of hydrogen bonds between NA and C5Pe molecules, as well as NA molecules with water molecules, was also calculated to determine the preference of interaction of NA molecules in the system. The results in Figures 6c and 6d show that, despite the migration of NA-2 molecules to the toluene−water interface, they still form a large number of hydrogen bonds with C5Pe molecules in the bulk toluene phase. In fact, a close examination of Figures 6c and 6d show that after 100 ns simulation time the number of hydrogen bonds formed between NA and C5Pe molecules in each system is similar to that formed between NA and water molecules. It appears that the chances of NA-2 molecules to migrate to the interface are comparable with the chances of these molecules to interact with C5Pe molecules in the bulk toluene phase. Although more NA-2 molecules migrate to the interface at high concentration of NA-2 (18 and 27 C5Pe aggregates), there are still plenty of NA-2 molecules to interact with C5Pe molecules, diminishing the hydrogen bonding between C5Pe and water molecules. As a result, more C5Pe molecules remained in the bulk toluene phase to form larger C5Pe aggregates. The preference of C5Pe molecules to adsorb and position at the toluene−water interface is known to stabilize water-in-oil emulsions.23−27 The results of MD simulations presented here

4. CONCLUSIONS The competitive adsorption kinetics to the tolene−water interface between the interfacially active C5Pe and NA molecules was studied in detail using MD simulation. The adsorption of C5Pe molecules at the toluene−water interface was found to be hindered in the presence of NA molecules, depending on the size and structure of NA molecules, and hence intermolecular interactions between C5Pe and NA molecules. C5Pe molecules were found to form more hydrogen bonds with larger NA molecules than molecules in the bulk, resulting in lower adsorption of C5Pe molecules. Furthermore, π−π stacking of C5Pe molecules was greatly reduced in the presence of water phase due to migration of C5Pe and NA molecules to the toluene−water interface. Although small NA molecules at low concentrations prefer to migrate to the toluene−water interface, showing little interaction with C5Pe molecules in the bulk toluene phase, these molecules at high concentrations hinder the adsorption of C5Pe nanoaggregates to the interface by blocking the active sites of C5Pe nanoaggregates, leading to a less stable emulsion by reducing 12908


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interfacial activity of C5Pe nanoaggregates. The results from this study provide valuable molecular insights into developing and enriching the structural databases of crude oil components, which eventually help in optimizing the upstream and downstream process conditions.

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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.jpcb.6b07938. Snapshot of the initial configurations of the simulation setup for the monomer C5Pe and NA molecules, equilibration attainment, and for the NA-solvated C5Pe aggregates at the oil−water interface (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1-780-492-7667. Fax: +1-780-492-2881. ORCID

Zhenghe Xu: 0000-0001-8118-1920 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Canada Research Chairs (CRC) program and the NSERCIndustrial Research Chair in Oil Sands Engineering. We are also grateful to the Oil Sands and Coal Interfacial Engineering Facility (OSCIEF) for assistance and computational resources. Financial support through the Petromaks program (Norwegian Research Council) and JIP 1 (Ugelstad Laboratory) is gratefully acknowledged. We would like to thank Dr. Tang for the constructive comments and suggestions. We would also like to express our special thanks to Dr. Subir Bhattacharjee for his active involvement in the project and insightful discussions on the scientific matter of the subject. Matlab code for the size distribution and the number of π−π contacts are available upon request.

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REFERENCES

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