Interactions of Polyaromatic Compounds. Part 2. Flocculation Probed

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Interactions of Polyaromatic Compounds. Part 2: Flocculation Probed by Dynamic Light Scattering and Molecular Dynamics Simulation Xi Wang, Rongya Zhang, Lan Liu, Peiqi Qiao, Sébastien Charles Simon, Johan Sjoblom, Zhenghe Xu, and Bin Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01511 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Interactions of Polyaromatic Compounds. Part 2: Flocculation Probed by Dynamic Light Scattering and Molecular Dynamics Simulation Xi Wang, 1 Rongya Zhang, 1, 2 Lan Liu, 1 Peiqi Qiao, 1 Sébastien Simon3, Johan Sjöblom, 3 Zhenghe Xu1, 4*, Bin Jiang 2 1

Department of Chemical and Material Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 1H9

2

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. 3

Ugelstad Laboratory, Norwegian University of Science and Technology, 7491 Trondheim, Norway 4

Department of Materials Science and Engineering, Southern University of Science and Technology, 1088 Xueyuan Blvd., Shenzhen, P. R. China

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Abstract In part I of this series of papers, the results of ESI-MS and MD simulation revealed a close relation between nanoaggregation of polyaromatic (PA) compounds and their chemical structures. In this paper, we present the results on investigating the flocculation of fractionated asphaltenes and synthesized PA molecules by dynamic light scattering (DLS). Together, these two papers complement one-another and draws a full picture of asphaltene aggregation.

Three asphaltene fractions were obtained based on their different adsorption characteristics onto calcium carbonate. The DLS results suggest that the irreversibly-adsorbed asphaltenes (Irr-Ads) containing the highest number of polar groups are the fraction of asphaltenes responsible for the observed flocculation in whole asphaltenes. To better understand the aggregation behavior of asphaltenes, flocculation of three synthesized PA compounds: N-(1-hexylhepyl)-N’-(5carboxylicpentyl)-perylene-3, 4, 9, 10-tetracarboxilicbisimide (C5Pe); N-(1-undecyldodecyl)-N’(5-carboxylicpentyl)-perylene-3,4,9,10-tetracarboxylbisimide

(C5PeC11);

and

N,N’-bis(1-

undecyldodecyl)perylene-3,4,9,10-tetracarboxylbisimide (BisAC11) were further studied using DLS. The observed flocculation corresponds well with the results of studying nanoaggregation using electron spray ionization mass spectroscopy (ESI-MS). The flocculation of PA compounds was found enhanced with increasing heptane content in the solvent. Among the three synthesized PA compounds studied, C5PeC11 showed similar flocculation kinetics to the Irr-Ads asphaltenes. Experiments using mixed PA compounds showed reduced flocculation of C5PeC11 in the presence of C5Pe under otherwise identical solution conditions. The presence of polar groups in PA molecules was proven to be critical in accelerating the flocculation of PA compounds beyond nano-scales. The results from molecular dynamics (MD) simulations showed that π-π stacking between polyaromatic cores, hydrogen bonding between polar groups and tail-tail interactions among aliphatic chains are all contributing to the observed flocculation of PA compounds. 2 ACS Paragon Plus Environment

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Introduction Asphaltenes in bitumen and crude oil are defined as a solubility class, which are insoluble in nalkane (typically hexane and heptane) and soluble in aromatic solvents, e.g., toluene and xylene.1–4 With their strong tendency of self-association, asphaltenes can precipitate and deposit on solid surfaces causing one of the major concerns that beset the petroleum industry.5,6 Under unfavorable conditions, e.g., sharp changes in pressure, temperature, solution composition and/or mechanical shear, asphaltenes precipitate out as black solids from the bulk liquid oil. Depositions of asphaltene precipitants lead to clog of wells, reservoirs and fluid transport lines, increasing maintenance costs and economic losses. Therefore, systematic studies on asphaltene aggregation and flocculation can help mitigating its adverse effects on oil production. The traditional view of asphaltene aggregation is a two-step process. Individual asphaltene molecules first self-associate to form nanoaggregates, followed by the further association (flocculation) of these primary nanoaggregates to form larger clusters.7 The nanoaggregation and flocculation are two separate yet closely related processes. In the first paper of this series, we focused on the nanoaggregation processes of three synthetic polyaromatic compounds C5Pe, C5PeC11 and BisAC11, and identified π−π stacking between polyaromatic cores being the major driving force for nanoaggregation. However, the size and structure of asphaltene aggregates formed beyond nano scales remain unknown. Furthermore, in the existing open literature, reports on the relationship between asphaltene physicochemical properties and their aggregation mechanisms remain inconsistent due to the high heterogeneity and unknown structures of asphaltene molecules.

Photo correlation spectroscopy, also known as dynamic light scattering (DLS), has been used to study the aggregation and phase behavior of asphaltenes over the past few decades. Anisimov et al. used DLS to study the flocculation processes of asphaltenes in hydrocarbon solutions.8 By 3 ACS Paragon Plus Environment

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studying the effects of resins on the aggregation of asphaltenes, they showed that resins can serve as inhibitors for asphaltene aggregation.9 More recently, development of DLS instrument has allowed more accurate measurements of absolute particle sizes in the range from 0.001 to 5 m suspended in liquids.10 Optical rearrangement in the spectrometer also made it possible to measure light scattering in opaque systems of strong light absorption.11 These previous studies demonstrated that DLS is a well-suited technique to study the aggregation of asphaltenes and synthetic PA compounds. Since asphaltenes are defined as a solubility class of molecules, they contain organic petroleum compounds of diverse molecular weights and chemical structures. Elemental analysis showed that asphaltenes are mostly composed of hydrogen and carbon at a H:C molar ratio around 1~1.2.12 Other heteroatoms, such as nitrogen, oxygen and sulfur, and trace metals such as vanadium and nickel are also present in the structure of asphaltene molecules.12,13 The accepted molecular weight of asphaltenes measured by mass spectrometry14,15 and molecular diffusion methods16,17 covers a wide range from 300 to 1400 g/mol with an average value of 750 g/mol. Molecular-weight distributions (MWDs) of asphaltenes measured by laser desorption/ionization (LDI) mass spectrometry display a broader MWD with an average of ~680 amu.18–22 Other techniques such as 23,24

NMR and UV-visible spectroscopy were also used to study the aromatic moieties in asphaltenes.

In

view of the complex and inaccessible structures of individual asphaltene molecules, fractionation based on the overall physicochemical properties pertaining to asphaltenes have been used.25–27 Based on

polarity, Fogler et al. divided asphaltenes into different sub-fractions. Dielectric constant and dipole moment measurements showed that the more polar asphaltene fraction has a higher tendency to form aggregates at low concentrations even in toluene. Solubility and flocculation experiments confirmed that asphaltene fractions with higher polarity are more likely to form


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deposits in the field and therefore more difficult to remediate.27, 28 Yang et al. developed a new procedure for subfractionation of asphaltenes based on their oil-water interfacial activity. The most interfacially active asphaltene fraction (IAA) was extracted from emulsified water droplets in asphaltene solutions. Measurements using ESI-MS, elemental analysis, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy showed that IAA molecules typically have higher molecular weights and higher concentration of oxygen and sulfoxide groups. The sulfoxide groups were shown to be responsible for the observed aggregation of IAA molecules in the bulk oil phase and adsorption at oil/water interfaces.29,30 Recently, Sjöblom et al. reported a new asphaltene fractionation method based on their adsorption onto calcium carbonate.31 FT-IR analysis revealed that the sub-fractions differ in the amount of carbonyl, carboxylic acid and functional groups containing heteroatoms. The fraction with the highest concentration of carbonyl and/or carboxylic acid was found to exhibit the maximum adsorption and tend form visco-elastic layers on stainless steel. However, the relation between the aggregation and structural properties of different asphaltene fractions remains to be investigated. An alternative strategy to investigate asphaltene aggregation is to study synthetic PA compounds of well-defined structures that possess similar properties to real asphaltenes. By simplifying the complex asphaltenes into known structures, studying the interactions between different functional groups to understand their roles in molecular aggregation/flocculation becomes possible. More importantly, the use of PA compounds also permits the implementation of molecular dynamics simulation (MD) to provide insights on the interactions between different PA moieties. In light of previous literature, MD simulations prevail over other experimental techniques in providing detailed interaction mechanisms involved in nanoaggregation and 5 ACS Paragon Plus Environment

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interfacial behaviors of model compounds.32–36 In this work, the aggregation of a single and mixtures of PA compounds, C5Pe, C5PeC11 and BisAC11 (Fig.1) in organic solvents was investigated. Together with MD simulation, DLS studies provide for the first time the link between nanoaggregation and flocculation of PA compounds. Polydispersity effects were probed by studying binary model compound systems. The results shed more lights on the role of interactions between key functional groups in controlling the aggregation of real asphaltenes.

Figure 1. Structures of (a) C5Pe, (b) C5PeC11, and (c) BisAC11.

Materials and Methods Samples and Sample Preparation Three PA compounds, C5Pe (686.8 Da), C5PeC11 (827.1 Da) and BisAC11 (1035.6 Da) were synthesized according to procedures described in the previous papers.37,38 All chemicals were purchased from Fisher Scientific (Ottawa Canada) and used as received. Toluene (HPLC grade, 99.5%)


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and heptane (HPLC grade 99.5%) were used as solvents in all the experiments reported here. Asphaltenes were first extracted from crude oil by n-hexane precipitation before being further separated into three subfractions based on their adsorption characteristics on calcium carbonate. The three separated asphaltene sub-fractions were labeled as irreversible adsorbed (Irr-Ads), adsorbed (Ads) and non-adsorbed (Non31

Ads). Details on the fractionation procedure and sub-fraction yields can be found elsewhere.

DLS

experiments were performed using sub-fractionated asphaltenes. The stock solutions of asphaltene subfractions and PA compounds were prepared by dissolving a known mass of materials in toluene to yield a final concentration of 0.1 ~ 1 g/L. The stock solutions were sonicated and filtered using 0.2 m PTFE needle filters each time prior to their use in DLS measurements. Dynamic Light Scattering Experiments In heptane/toluene solutions, the aggregate size distribution of the fractionated asphaltenes and synthesized PA compounds was monitored as a function of solution life time with DLS.39,40 Due to its ability to provide ensemble average estimate of particle size in suspensions over a wide accessible size range, DLS has been proven to be an effective tool for monitoring asphaltene aggregations.9,41,42 In this study, a commercial laser light scattering instrument (ALV 5022) equipped with a cylindrical He-Ne laser (model 1145p-3083; output power 22mW) in combination with an ALV SP-86 digital correlator was used for determining the size of various particles in a non-aqueous media. DLS was based on the theory of Brownian motion of colloids. Under ambient conditions, small colloidal particles undergo a random walk in a suspension medium. When a beam of coherent monochromatic radiation is impinged on the particles, each particle becomes a secondary scattering source. Due to the movement of particles concerned, the scattered waves fluctuate 7 ACS Paragon Plus Environment

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randomly in time. Small particles travel fast, resulting in a fast decorrelation of the scattered intensity trace. For DLS, the primarily measured property is the time-dependent correlation function of light scattering intensity fluctuations. For a suspension of monodisperse particles, the time-dependent autocorrelation function () of the scattered light is a function of the lag time . For a single exponential decay, the time-dependent autocorrelation function () can be represented by () = [1 + exp −

]

(1)

where is the baseline correlation level which is proportional to the total light scattering intensity, is the lag time between measurements, and is the characteristic time of diffusion at a given length scale.9 The value is a coefficient which is usually smaller than unity.9 The diffusion coefficient of the moving particles is then deduced from the characteristic time of diffusion , using

= 2!" #

(2) $%&

)

In Eq. 2, " is the scattering vector given by " = ' sin #, where * is the refractive index of (

dispersing medium, +, is the wavelength of the laser beam, and - is the detection angle. Equation 2 provides the basis to obtain diffusion properties of the system. The mean hydrodynamic radius of the particles can then be calculated using the Stokes-Einstein equation given by !=

./

0%12 3

(3)

where 4 is the Boltzmann constant, 5 is the absolute temperature, 67 is the hydrodynamic radius of the spherical particle, and 8 is the viscosity of the fluid. This equation relates the diffusion rate of a particle in a suspension to temperature, particle size, and the fluid viscosity. The measured 8 ACS Paragon Plus Environment

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67 is the hydrodynamic radius, which may add a negligible correction to the size of bare particles. For systems containing particles that interact with each other, Eq. 3 is still applicable to monitoring the change of apparent particle size, since the characteristic time of aggregation is much longer than the time of measurement. For multimodal systems as in our case, the size distribution of the aggregates is determined using the CONTIN algorithm which has been described in detail elsewhere.43 Samples were prepared by first adding 200 l stock solutions of either asphaltene subfractions or synthetic PA compounds into a clean optical cell. The aggregation was initiated by adding 1800 l heptane/toluene mixture (heptol) of known ratio, i.e., 75-heptol indicates a heptane-toluene mixture containing 75 wt% heptane. The optical cell was manually shaken for 1 min before being placed in the DLS sample holder. All measurements started 2 min after heptol solution was added. Although the measured hydrodynamic radius is highly dependent on concentration in DLS experiments, the sample concentration needs to be optimized first for each type of samples so that the size of aggregates formed during the experiment falls within the measurement limit of DLS and the sedimentation of the aggregates avoided. The optimized final concentration was 0.1 g/L for fractionated asphaltenes and PA compounds C5PeC11 and BisAC11. A concentration of 0.02 g/L was used for C5Pe due to its lower solubility in heptol solutions. For DLS measurements, the optical cell was placed in an index matching vat filled with highpurity, dust-free toluene, which was kept at a constant temperature of 22°C. Scattering experiments were performed at a scattering angle of 75° to minimize backscattering effects from the sample. The data acquisition time was chosen to be 60 s to maximize the precision of the measurement without causing significant changes in the observed hydrodynamic radius of the substances in solution/suspension. To avoid directional motions of the scatters due to 9 ACS Paragon Plus Environment

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sedimentation caused by gravity, the total time allowed for sample aggregation was limited within 2 hr. A lag time of 1 min was allowed between any two consecutive measurements.

Molecular Dynamics (MD) Simulations To provide molecular level insights into the observed flocculation characteristics of asphaltene molecules, molecular dynamics (MD) simulations of four systems containing well-defined PA molecules in 50-heptol were performed for a simulation time up to 80 ns. All the simulations were carried out using the GROMACS 5.1.2 software package and the 53a6 parameter set according to procedures stated in the preceding paper.10,

32–36, 44-46

The Material Studio 8.0

software was used to construct the initial structures of three PA compounds and toluene, which upon optimization were transferred to the Automated Topology Builder (ATB) and Repository server (version 2.2) to generate the necessary molecular topology and GROMACS structure files.49, 50 The default topology obtained from ATB was then modified by manually adjusting the partial charges and charge groups so that they were compatible with the GROMOS96 force field parameter set 53A6. It has been shown in literature that this type of charge adjustment was necessary as the default partial charges can lead to unphysical results. On the contrary, using analogous functional group existing in GROMOS96 has been proven to be a more reliable for probing the dynamics of polyaromatic cores. 3336, 47

Charges for PA molecules and toluene were adopted from existing parameters of analogue

atom groups in dipalmitoylphosphatidylcholine (DPPC), phenylalanine (PHE) and peptide. The topology for toluene and heptane was generated from phenylalanine amino acid fraction and DPPC through the pdb2gmx routine in GROMACS, respectively. Since heptane is a nonpolar molecule, its atomic charge was set to zero.


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The simulation boxes were constructed by first placing a desired number of PA molecules in a cubic box of 12 nm edges. A total of four simulation boxes were constructed as shown in Table 1. The boxes were then solvated with heptane and toluene molecules followed by energy minimization. The parameters used for the energy minimization, NPT equilibration and NPT MD production were described in our previous papers.33-36 After simulations for 80 ns, the structure and dynamic properties of the system were analyzed using the GROMACS built-in analytical tools. The time evolution of the molecular structure of the system was visualized using visual molecular dynamics (VMD).46 Table 1. Composition of PA molecules in toluene-heptane mixture systems used in MD simulations. System

PA molecules

Time (ns)

Number of solvent molecules (Nheptane + Ntoluene)

Final volume (nm×nm×nm)

1

24 C5Pe

80

2678 + 2705

10.40×10.40×10.40

2

24 C5PeC11

80

2670 + 2680

10.41×10.41×10.41

3

24 BisAC11

80

2638 + 2651

10.40×10.40×10.40

4

24 C5Pe + 24 C5PeC11

80

2589 + 2610

10.39×10.39×10.39

Results and Discussion Aggregation of Asphaltene Subfractions Monitored by DLS Aggregation kinetics of the fractionated asphaltenes were first studied under similar solvent conditions. Shown in Fig. 2 are the DLS correlation functions for 0.1 g/L Irr-Ads asphaltenes in various heptol solutions at 30 min after the measurement started. An increase in the amplitude of the correlation function is anticipated due to the growing sizes of scatters with increasing heptane concentrations. This result confirms that heptane is an effective precipitant, which induces aggregation for Irr-Ads asphaltenes. With increasing heptane concentration, the inflection points 11 ACS Paragon Plus Environment

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also increases in magnitude, indicating a decrease in the Brownian diffusion rate due to larger sizes of the aggregates.

Figure 2. DLS correlation functions of 0.1 g/L Irr-Ads asphaltenes in heptol solutions at 30 min solution lifetime.

Upon obtaining the DLS correlation functions, hydrodynamic radii can be plotted as a function of time, as shown in Fig. 3(a). At low heptane concentrations (30-39 wt%), the floc size of IrrAds asphaltenes remained small throughout the entire experiment. Further increase in the heptane concentration (44-50 wt%) led to significant increases in both the rate of aggregation and the size of the flocs. In light of the asphaltene separation process, the addition of heptane is without doubt the most effective method to induce asphaltene flocculation as described in previous publications.9,10 Increasing heptane concentration to 54 wt% caused more rapid aggregation and formation of larger flocs. Unfortunately at the high heptane concentration of 54 wt%, the correlation function became distorted after 1 hr due to precipitation militating


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continuous monitoring. Below the heptane concentration of 60 wt%, aggregation rates for IrrAds asphaltenes always depend on heptane concentration.

Figure 3. (a) Time dependent aggregation of Irr-Ads asphaltenes (0.1 g/L) and (b) Time resolved hydrodynamic radii of Irr-Ads asphaltenes (0.1 g/L) with increasing heptane concentrations.

Figure 3(b) plots the same data in Fig. 3(a) on double logarithmic scales. A linear relationship was observed between the measured hydrodynamic radii and aggregation time, indicating that the size of the aggregates can be approximated by equation 6(9) = :9 ;

(4)

where : is a system-specific constant reflecting the size of aggregates.9 Upon fitting, a value of 0.36±0.02 for the exponent constant < was obtained, which agrees well with the values reported for whole asphaltenes.9,48 The observed power law dependence indicates that the aggregation of Irr-Ads asphaltene follows the diffusion-limited aggregation (DLA) kinetics. The similarities between the current results for Irr-Ads asphaltenes and that reported for whole asphaltenes lead us to conclude that the Irr-Ads asphaltenes are the fraction responsible for the observed massive aggregation in whole asphaltenes. 13 ACS Paragon Plus Environment

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For the rest two fractions, Ads and Non-Ads asphaltenes, corresponding DLS results were summarized in Fig. 4. Compared with Irr-Ads asphaltenes, Ads asphaltenes showed much lower tendency of flocculation in solutions with up to 50 wt% heptane (Fig 4(a)). At lower heptane concentrations, negligible aggregation was observed for Ads asphaltenes. For Non-Ads asphaltenes (Fig 4(b)), no large flocs could be observed in all heptol solutions tested. Even in 71heptol, the measured hydrodynamic radii of Non-Ads asphaltenes remained small at around 6080 nm over the entire measurement period.

Figure 4. Aggregation of Ads (a) and Non-Ads (b) asphaltenes in heptol of variable heptane concentrations.

To better compare aggregation characteristics of the three asphaltene subfractions, the hydrodynamic radii of these asphaltenes in 44-heptol solution was plotted as a function of time in Fig. 5. It can be clearly seen that Irr-Ads asphaltenes showed the highest tendency to aggregate at this heptane concentration. For Ads- and Non-Ads asphaltenes, no significant increase in measured aggregate sizes was observed. For the Non-Ads asphaltenes, the measured 14 ACS Paragon Plus Environment

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hydrodynamic radius stayed almost constant at ~ 60 nm for the entire two hours, suggesting the lowest aggregation tendency.

Figure 5. Time dependence of the aggregate size for three asphaltene fractions in heptol containing 44 wt% heptane.

The results in Figs. 3, 4 and 5 collectively suggest that among the three asphaltene fractions, the Irr-Ads asphaltenes have the highest tendency to aggregate by diffusion limited aggregation mechanisms. Since the separation of the three asphaltene fractions is quantitative with a recovery of 98-99 wt%, our results confirm that the Irr-Ads asphaltenes are the fraction responsible for the observed flocculation in whole asphaltenes.31 Early studies using FT-IR spectroscopy showed that the primary difference between the three asphaltene subfractions is the higher concentrations of polar groups such as carbonyl and carboxylic acid in the Irr-Ads asphaltenes.31 It is therefore reasonable to conclude that the binding interactions between various polar groups are one of the dominating forces responsible for the flocculation in whole asphaltenes. Previous results also support the same conclusion that the more polar asphaltene fraction is seen increasingly less


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soluble in heptol solutions. As a result, the addition of more polar resin could disrupt the interactions among the polar groups of asphaltenes and hence depress asphaltene aggregation.44 Since asphaltenes are defined as a solubility class, studying fractionated asphaltenes remains insufficient to understand on molecular level their aggregation mechanisms. To better understand how various types of molecular interactions contribute to the aggregation of asphaltenes, we further studied three synthetic PA compounds of well-defined molecular structures. With previous knowledge of nanoaggregation obtained using ESI-MS and MD simulations, the flocculation processes of the synthetic PA compounds were monitored using DLS as discussed below. Combined with MD simulations, such study provides at the molecular level further insights into the real asphaltene aggregation/ flocculation. Flocculation of PA compounds measured by DLS The flocculation of C5Pe, C5PeC11 and BisAC11 was studied using DLS. Similar procedures as in previous measurements on flocculation of asphaltene fractions were used. However the solution concentrations of PA compounds were optimized for each compound in terms of signal to noise ratios of the scattered light. The measured hydrodynamic radius was plotted in Figs. 6 (a), (b) and (c) as a function of time for three PA compounds, C5Pe, C5PeC11 and BisAC11, respectively. As shown in Fig. 6 (a), C5Pe exhibits the highest tendency to aggregate due to its shortest side chains. At a concentration as low as 0.02 g/L in 65-heptol, C5Pe aggregates rapidly to form flocs of sizes between 800 to 1000 nm in less than half an hour. Higher concentrations of C5Pe tend to form sedimentation in 65-heptol, leading to distortions of the resultant correlation functions. For C5PeC11, a higher concentration of 0.1 g/L is needed to initiate flocculation at solvent conditions similar to that of C5Pe. For BisAC11, no obvious flocculation was observed even at very high solute concentrations of 0.1~0.2 g/L. A possible explanation for these 16 ACS Paragon Plus Environment

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observations is that the longer aliphatic chains in C5PeC11 and BisAC11 are more solvated by interacting with the solvent molecules, resulting in higher solubility of C5PeC11 and BisAC11 in heptol solutions. The observed flocculation behavior of the synthetic PA compounds correlates well with the results of previous nanoaggregation study using ESI-MS measurements, where the average nanoaggregation number for a given solution condition follows the trend of C5Pe > C5PeC11 > BisAC11.

Figure 6. Hydrodynamic radius measured by DLS of synthetic PA compounds as a function of time for (a) C5Pe, (b) C5PeC11, and (c) BisAC11 in heptol solutions of varying heptane concentrations. Snapshots of PA molecule aggregation from MD simulation in 50-heptol, taken at the end of 80ns simulation time: (d) 24 C5Pe; (e) 24 C5PeC11; (f) 24 BisAC11. Solvent molecules are removed, polar group associations formed by C5Pe and C5PeC11 are circled in the snapshot for clarity.


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In addition, the aggregation/flocculation of C5Pe and C5PeC11 appears to be sensitive to heptane concentrations in solution. For C5Pe, a higher heptane concentration leads to more flocculation within the first 30 min, with the flocs size at a given measurement time doubled for a 10 wt% heptane concentration increment. For C5PeC11, a 5-6 wt% increase in heptane concentration led to a significant variation in the observed flocculation. In the case of BisAC11, the presence of longer aliphatic chains and the lack of polar groups significantly hindered its ability to flocculate. Even at heptane concentrations as high as 82 wt%, there was no obvious change in the flocculation behavior of BisAC11. Furthermore, a higher concentration of 0.2 g/L BisAC11 in 65-heptol also failed to induce appreciable flocculation. The measured hydrodynamic radius of BisAC11 flocs at all solution conditions remained below 200 nm even after 2 hr of continuous monitoring. These results are consistent with the previous ITC data in support with the observed heptane effects in the nanoaggregation of PA compounds, suggesting that the inherent molecular structures and functional groups play a vital role in controlling the nanoaggregation and flocculation of PA compounds in bulk solutions.48 Table 2. Number of hydrogen bonds averaged over the last 5 ns of 80 ns simulation for 24 C5Pe, 24 C5PeC11 and 24 BisAC11 in 50-heptol. System

NH-bond

24 C5Pe

22.55

24 C5PeC11

22.86

24 BisAC11

0

Besides the presence of a precipitating solvent such as heptane, previous results also show that the aggregation also hinged upon polar group interactions among PA molecules. Comparing


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BisAC11 with C5PeC11, the absence of polar functional groups in BisAC11 reduces the binding interactions between molecules in relatively non-polar heptol solutions. Consequently the flocculation of BisAC11 in bulk heptol solutions is hindered. In addition, the presence of longer aliphatic chains in the molecules of BisAC11 also poses difficulties in its aggregation. With higher degrees of freedom, the C-C single bonds increase the steric hindrance of =-= interactions, leading to more difficult bindings between molecules and nanoaggregates of BisAC11. To control the aggregation tendency of PA compounds, the length and number of aliphatic chains as well as polar functional groups in PA compounds must be controlled to achieve suitable solubility in aliphatic/aromatic solvents. It is interesting to note in Fig. 6 (a) that the size of C5Pe flocs increases almost linearly with time during the first ten to fifteen minutes, after which the observed size of aggregates stayed constant in the ensuing measurement period. Higher heptane concentrations in solution produce larger final flocs of C5Pe. This unique property of C5Pe suggests its extremely high tendency to aggregate. In heptol solutions, the small nanoaggregates of C5Pe bind together rapidly to form larger structures until reaching the point where the nanoaggregate concentration no longer suffices to produce favorable flocculation. For C5PeC11, the longer aliphatic chains are more solvated and hence better stabilize the PA molecules, in favor of higher solubility and lower aggregation tendency in heptol. Even though C5PeC11 and C5Pe have identical π−conjugated core structures and polar functional group, a small change in the length of aliphatic chains led to dramatically different flocculation profiles for the two PA compounds. At the same concentration, C5PeC11 contained notably similar size ranges of aggregates to that of Irr-Ads asphaltenes, leading to comparable flocculation profiles. This observation indicates that C5PeC11 exhibits similar aggregation behavior to Irr-Ads asphaltenes and support the use of 19 ACS Paragon Plus Environment

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C5PeC11 as a potential model compound to study real asphaltene molecules. Moreover, compared with C5Pe, the measured hydrodynamic radius of C5PeC11 increased more slowly with time. Instead of reaching a plateau as in the case of C5Pe, the size of C5PeC11 flocs continues to increase even after 2 hr of measurement, which echoes with the behavior of Irr-Ads asphaltene fraction as shown in Fig. 3(a). In both cases, the rate of aggregation depends strongly on precipitant (heptane) concentrations. Considering the previous results obtained with ESI-MS, smaller nanoaggregates of C5PeC11 contribute to smaller building blocks, which limits the growth of hydrodynamic radius under similar conditions, resulting in the observed flocculation characteristics of C5PeC11. This finding confirms the aggregation model proposed by YenMullen that small nanoaggregates form first before they bind together to form larger structures.7

To better illustrate the aggregation difference between the three PA compounds, MD simulation snapshots at 9 = 80 ns are presented in Fig. 6 in concert with DLS results, though more detailed discussion is followed in the ensuing sections. In MD simulation, the box size and the number (24 molecules) of PA molecules are kept the same for all PA molecules. As a result, the influence of PA concentration on its aggregation is eliminated for better comparison. The GROMACS built-in analytical tool, gmx cluster, was used to calculate the number of molecules in the largest aggregates as a measure of the degree of nanoaggregation. For three types of PA molecules, the number of molecules per largest aggregates was found to be 6, 5 and 4 for C5Pe, C5PeC11 and BisAC11, respectively. As can be seen from the snapshots (Figs. 6 (d) and (e)), nanoaggregates of C5PeC11 are far apart from each other than the case of C5Pe, most likely as a result of larger steric hindrance arising from longer hydrocarbon chains of C5PeC11. Such large steric hindrance makes C5PeC11 nanoaggregates harder to bind together to form large clusters. Bigger C5Pe nanoaggregates and shorter distances between the aggregates contribute to the formation of large flocs in C5Pe systems. Although the MD simulation represents only nano-scale aggregation, which is the first step for further aggregation and does not predict the size of flocs measured 20 ACS Paragon Plus Environment

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by DLS, the nanoaggregates are building blocks for the subsequent flocculation. It is therefore not surprising to see the similar trend in the size of flocs measured by DLS for these PA molecules.

As shown in Figs. 6 (d) and (e), C5Pe molecules form a large cluster, which correlates well with the larger hydrodynamic radius detected by DLS. C5PeC11 molecules form more relaxed and smaller aggregates than C5Pe, showing a certain degree of steric hindrance on the π-π stacking interactions. In contrast, BisAC11 in Fig. 6 (f) shows the least inclination towards aggregation. With the presence of four long aliphatic chains, the odds for BisAC11 molecules to form direct π-π stacking between polyaromatic cores are scarce. Collectively, DLS results and MD snapshots present a vivid picture of the aggregation tendencies of PA compounds in heptol solutions.

Figure 7 presents hydrodynamic radius of C5PeC11 flocs as a function of time on a doublelogarithmic scale. Similar to Irr-Ads asphaltenes, the size of C5PeC11 aggregates, after the initial stabilization period, follows a power law relation, showing a linear dependence of time on loglog scale. At different heptane concentrations, aggregate sizes 6(9) can be fitted with the same equation of 6(9) = :9 ; , where < = 0.32 ± 0.03. The similar slope values between C5PeC11 and Irr-Ads asphaltenes in this case suggests at similar solvent qualities and sample concentrations, the aggregation behaviors of the two are comparable which is consistent with previous studies. 42,48 However minor aggregation differences between the model compound and real asphaltenes remain. These differences are largely attributed to a wider range of functional groups in asphaltene molecules, which enhances intermolecular interactions. The result points at an issue that for mixtures such as asphaltenes, the aggregation behavior is also controlled by polydispersity among molecules. Interactions between different molecules affect the measured aggregation profiles. As a result, a single compound such as C5PeC11 is unlikely to reflect the


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complete aggregation behavior of real asphaltenes. This phenomenon was further probed by studying the aggregation behavior of mixed PA compounds to be discussed in the later sections.

Figure 7. Time dependence of the size of C5PeC11 aggregates plotted in double logarithmic scale. The same slope value of 0.32 was obtained for all the heptane concentrations studied.

MD Simulations for PA Compounds To get a better picture of how intermolecular interactions affect the flocculation of PA compounds, MD calculations were carried out in simulated 50-heptol solutions. The definition for core groups and tails of PA molecules used for analysis in the discussions is given in Supporting Information S1. Radial distribution functions (RDFs) for the center of mass (COM) used as the demonstrations of achieving dynamic equilibrium are available in the Supporting information S2. Figure 8 shows the radial distribution function (RDFs) for the COM separation of PA cores away from a reference PA molecule, g(r). These RDFs are averaged over the last 5 ns of the total 80 ns simulation time. For all three individual PA molecules in 50-heptol, three peaks are procured at 22 ACS Paragon Plus Environment

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the core-to-core distance of 0.37, 0.74 and 1.1 nm. Irrespective of the structure of the PA molecules, the first and the most prominent peak is located at around 0.37 nm in all cases, This distance agrees with the minimum separation of ∼0.35 nm between adjacent parallel PA cores, indicating the formation of strong π-π stacking between PA molecules.49 Interestingly, such stacking distance is independent of the solvent properties as the same value was also observed in methanol-toluene mixtures. The magnitude (g(r)) of the first peak in the RDF plot follows the order of C5Pe > C5PeC11 > BisAC11 (Fig. 8). The progressive reduction in peak height indicates a decreasing number of direct π-π stacking from C5Pe to C5PeC11 to BisAC11. More π-π stacking pairs give rise to higher tendency of molecular aggregation, as confirmed by DLS results with a flocculation trend of C5Pe > C5PeC11 > BisAC11. The reduction in π-π stacking is most likely caused by increased steric hindrance and reduced polarity of the PA compounds, leading to increased interference with π-π interaction. It is interesting to note that the spacing distance between the first and second peak 0.37 nm is nearly identical to the spacing of 0.36 nm between the second and third peak. This proves the existence of an extended π-π stacking of the molecules to the second and third layer, demonstrating π-π stacking being the main driving force for aggregation of these PA compounds in heptol solutions. The observed trend in MD simulation is in support with the results of DLS measurement and provides molecular level insights on bulk flocculation behaviour. The results from previous studies clearly suggested that in addition to the inherent structures of PA molecules, their solubility in solvent also firmly controls the behaviour of molecular aggregation.50–52 In order to identify the role of solubility of PA molecules in determining the observed aggregation trend in our systems, solvent-accessible surface area (SASA) of individual PA molecules was calculated based on MD simulation results. SASA is the surface area of a


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molecule that is accessible to a given solvent. Higher SASA values indicate more solvated states of the PA molecules in favour of less aggregation. Figure 9 (a) shows the SASA value following the order of BisAC11 > C5PeC11 > C5Pe, indicating increasing aggregation tendency from BisAC11 to C5PeC11 and then C5Pe, as observed in our DLS experiments.

Figure 8. Radial distribution function g(r) for COM separation distance between aromatic cores averaged over the last 5 ns of 80 ns simulation time.


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Figure 9. (a) Solvent-accessible surface area and (b) Number of tail-tail contacts between the nonpolar segments of 24 C5Pe, 24 C5PeC11 and 24 BisAC11 PA molecules in 50-heptol.

To further understand the correlation between nanoaggregation and the differences in molecular structures, tail-tail contacts between aliphatic chains on adjacent PA cores were studied through calculation of the minimum distances between the tails. The minimum distance of 0.5 nm between tails is set within which the tails are considered as being ‘in-contact’. The calculated number of tail-tail contacts for pure PA molecules follows the trend of BisAC11 > C5PeC11 > C5Pe (Fig. 9 (b)). This is in accordance with previous reports on MD simulation which showed that the tail-tail contacts of PA compounds can reduce the π-π stacking due to the steric hindrance associated with the branched side chains.36 The presence of longer aliphatic chains on C5PeC11 and BisAC11 allows them to have higher number of side chain contacts, introducing larger steric hindrance to the π-π stacking and contributing to the reduced extent of aggregation. Another factor that significantly reduces BisAC11 aggregation is the absence of the polar group from BisAC11 molecules. As shown in Figs. 6 (d) and (e), C5Pe and C5PeC11 can associate through the interactions of polar groups. To better illustrate this point, the number of hydrogen bonding between the polar carboxyl groups of PA molecules were calculated for each system. Geometric criteria of donor-acceptor distance BC ≤ 0.35 nm and ∠acceptor-donor-hydrogen DCE ≤ 30° were used for calculating the number of hydrogen bonds.53 As shown in Table 2, the number of hydrogen bonds in C5Pe and C5PeC11 systems are similar, while no hydrogen bonds are found in BisAC11 system due to the lack of hydrogen donors on BisAC11. Thus, BisAC11 exhibits the lowest ability to aggregate through hydrogen bond formation, accounting for the negligible flocculation of BisAC11 in bulk heptol solutions. This result support the conclusion to


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the hilt that hydrogen bond formation as a form of polar group interaction between PA molecules is one of the factors that intensify aggregation in C5Pe and C5PeC11 system. Polydispersity effects of PA compounds studied using DLS In order to understand how different molecules interact to affect the overall aggregation in bulk solutions that would better mimic the aggregation behavior of real asphaltenes, the flocculation of C5Pe, C5PeC11 and BisAC11 binary mixtures were studied using DLS. After dissolving C5Pe with either C5PeC11 or BisAC11 in heptol solutions, the hydrodynamic radius is measured as a function of time using the same procedure as described in previous experiments. Figure 10 (a) shows hydrodynamic radius 67 for C5Pe, C5PeC11 and their binary mixtures of various ratios in 65-heptol solutions. As indicated earlier, 0.02 g/L C5Pe or 0.1 g/L C5PeC11 could aggregate by itself up to 1000 nm without sedimentation being observed. After mixing these two compounds together at the same concentrations as in the individual component systems, the measured hydrodynamic radius decreased significantly despite an overall high concentration of the PA molecules in the mixture. At 2 hr of measurement time, the maximum size of aggregates measured by DLS remains below 200 nm. In addition, C5PeC11 showed dispersing effect upon C5Pe, where the increasing of C5PeC11 concentration from 0.06 g/L to 0.1 g/L led to a progressive reduction in the hydrodynamic radius of the resultant aggregates. The aggregation rate was found to be much slower for the binary systems of 0.02 g/L C5Pe and various amount of C5PeC11 than for pure C5Pe or C5PeC11 single compound systems. This mutual depression behavior of PA molecular aggregation corresponds well with the experimental results of ESI-MS study reported earlier. The combination of different PA compounds depressed nanoaggregation of corresponding single component systems as shown by decreasing the 26 ACS Paragon Plus Environment

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average nanoaggregation number. Such mutual depression of nanoaggregation limits massive flocculation of individual C5Pe or C5PeC11 molecules in bulk solutions. The observed effect illustrates the importance of intermolecular interactions governed by the structures of PA molecules in controlling nanoaggregation and subsequent flocculation of PA molecules. In C5Pe + C5PeC11 binary systems, increasing the amount of C5PeC11 increases its interactions with C5Pe molecules. The longer aliphatic chains of C5PeC11 exerted higher steric effects and brought C5Pe-C5PeC11 nanoaggregates to a much lower efficiency of flocculation in heptol solutions. The results suggested that for structurally similar compounds with similar polarity, the more soluble compound could help in the solvation of the less soluble one. Embedded in this observation is the key in understanding the interactions between PA molecules. Similar results were confirmed by our MD simulation results that the addition of C5PeC11 decreases nanoaggregation of C5Pe in either methanol-toluene or heptol solutions.

Figure 10. (a) Hydrodynamic radius as a function of time for 0.02 g/L C5Pe and its mixtures with varying C5PeC11 concentrations in comparison with 0.1 g/L C5PeC11, all in 65-heptol solutions. (b) The hydrodynamic radius distribution for pure C5Pe and its mixture with 0.08 g/L C5PeC11 obtained by CONTIN analysis at 9 = 1800 G after experiment started. 27 ACS Paragon Plus Environment

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Figure 10 (b) shows the measured hydrodynamic radius distribution at 30 min (1800 s) after experiment started. In 65-heptol solutions, the hydrodynamic radius of 0.02 g/L C5Pe is centered around 1000 nm with a 67 breadth of 3165 nm. However by mixing 0.08 g/L C5PeC11 with the same amount of C5Pe, the hydrodynamic radius distribution peak shifted to around 200 nm. The breadth of the size distribution peak for the mixed system decreased to around 919 nm, which is much smaller than that of 3165 nm for the pure C5Pe system. These results suggest a significant effect of molecular polydispersity (binary system in the current case) on decreasing the average size and polydispersity of the aggregates. In addition, for all cases the CONTIN analysis revealed a log-normal distribution of hydrodynamic radius which increases with the accumulation of aggregation time. This finding confirms that the aggregation of perylene bisimide PA compounds remains a multiple-step process, mainly resulting from the three-dimensional structure of aggregates in solution. Despite the reduced nanoaggregation in the binary systems, the concentration of larger aggregates/flocs increases at the cost of reducing the population of smaller nanoaggregates as in the single component system, resulting in a shift of the average hydrodynamic radius.9 By the same token, experiments were carried out with BisAC11 and C5PeC11 binary mixtures to further test mutual dispersion and polydispersity effects.


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Figure 11. (a) Hydrodynamic radius as a function of time for 0.1 g/L C5PeC11, 0.1 g/L BisAC11 and their binary mixtures in 65-heptol. (b) The hydrodynamic radius distribution for 0.1 g/L C5PeC11 and its binary mixture with 0.1 g/L BisAC11 in 65-heptol at 9 = 1800 G obtained with CONTIN analysis.

Though under the same conditions, pure C5PeC11 and BisAC11 share little common in their individual aggregation, the combination of these two compounds lead to a compatible and cohesive behavior. As shown in Fig. 11(a), in 65-heptol solutions, the addition of 0.1 g/L BisAC11 to 0.1 g/L C5PeC11 affords aggregates with hydrodynamic radius ~ 500 nm after 2 hr, which is in between that measured from each of the contributing compound at the same concentration. This is in accordance with previous observations where the long peripheral chains in BisAC11 could tangle up with the side chains of C5PeC11, decreasing the degree of polar group interactions. These results further confirm that adding a compound of higher degrees of steric hindrance and/or higher solubility could depress the flocculation of other PA molecules. Particle size distributions for C5PeC11 in the presence and absence of BisAC11 were plotted in Fig. 11(b). Upon the addition of 0.1 g/L BisAC11 to 0.1 g/L C5PeC11, the hydrodynamic radius 29 ACS Paragon Plus Environment

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of the binary system reduced from 700 nm to around 200 nm with a reduction in the distribution peak breadth from 4000 nm to 1700 nm. This result is consistent with previous results that the addition of increasing concentration of BisAC11 enhances the solubility of C5PeC11 in a mixture of xylene and alkane.54 By comparing Fig. 10(a) with Fig. 11(a), it can be observed that the overall polydispersity effect for the two mixtures: C5Pe+C5PeC11 and C5PeC11+BisAC11 were different. The mixture of C5Pe+C5PeC11 exhibited much smaller aggregation tendency than either of its contributing components at any measurement time. Upon mixing C5Pe and C5PeC11, a mutual depression in the aggregation was observed. On the other hand, C5PeC11+BisAC11 mixture showed an intermediate aggregation behavior. The measured sizes of flocs for the mixture were bigger than that of pure BisAC11 but much smaller than the floc sizes of C5PeC11 at any measurement time. This result further stresses the importance of polar group interactions in the aggregation of PA compounds. For the C5Pe+C5PeC11 binary system, C5PeC11 was in close proximity to C5Pe due to polar group interactions. The long aliphatic chains in C5PeC11 caused steric hindrance and disrupted the flocculation process for the binary system. On the other hand, for the BisAC11+C5PeC11 binary system, due to the lack of polar group interaction, BisAC11 only binds loosely to C5PeC11, which makes BisAC11 a less effective dispersant. BisAC11 is unable to stay closely to C5PeC11 nanoaggregates and the aliphatic chains in BisAC11 can only exert partial effects by sterically disrupting the flocculation of C5PeC11 nanoaggregates. This result corresponds well with observations from early ESI-MS study, where C5Pe-C5PeC11 complexes were common and could be detected in solution, while BisAC11-C5PeC11 complexes were usually absent.55 It can be concluded that for polydisperse systems, the polarity difference between PA compounds 30 ACS Paragon Plus Environment

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is vital in determining the overall aggregation behaviors. For a PA compound known to aggregate in solution, adding a structurally similar PA compound of similar polarity and higher solubility can successfully decrease the nanoaggregation/flocculation of the mixture. Similar result was also seen in real asphaltenes, where the addition of a more soluble and polar compound such as resin could decrease the aggregation/precipitation of asphaltenes. Resins with a bigger dipole moment were found to be more effective in stabilization of asphaltenes in solutions. 56 MD Simulations for Polydisperse System In order to understand the effect of molecular polydispersity on aggregation, the aggregation of C5Pe + C5PeC11 in 50-heptol was studied using MD simulations. The suppression of aggregation between C5Pe molecules by the addition of C5PeC11 is clearly shown in Fig. 12 (a). Due to the polydispersity effect, both C5Pe and C5PeC11 molecules aggregation are reduced. The reduced extent of aggregation is verified by the reduced magnitude of g(r) for C5Pe and C5PeC11 in the binary system compared with the corresponding single component systems as shown in Figs. 13 (a) and (c). The insertion of C5PeC11 between C5Pe molecules disperses C5Pe molecules and in turn increases the solubility of C5Pe.


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Figure 12. Snapshots of molecular configurations of 24 C5Pe + 24 C5PeC11 in heptol, taken at the end of 80ns simulation time. Each molecule is presented by a different color: C5Pe in green, C5PeC11 in red. Solvent molecules are removed for clarity. (a) 24 C5Pe + 24 C5PeC11 in heptol; (b) Largest aggregates formed by π-π stacking of C5Pe and C5PeC11. Polar group association formed by C5Pe and C5PeC11 is circled in the snapshot for clarity.

The increase in solubility is confirmed by the results shown in Figs. 13 (b) and (d) that the SASA values of 24 C5Pe molecules and 24 C5PeC11 molecules in its binary system are increased compared with that in single component systems. In addition, Figs. 13 (b) and (d) show that the SASA values for C5Pe and C5PeC11 in single component systems decrease rapidly due to aggregation between molecules. However, in the binary systems, the SASA values for both 24 C5Pe and 24 C5PeC11 molecules show less reduction due to the polydispersity effect of the system. These results agree well with the bulk flocculation behaviors of the corresponding systems.


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As shown previously in Figs. 6 (d) and (e), the snapshots indicate that C5Pe and C5PeC11 can interact through polar group association. This is also verified in Fig. 12 (b) that aggregates are formed by polyaromatic molecules of different types through interactions between polar groups. In addition, the formation of hydrogen bonds between C5Pe and C5PeC11 as presented in Table 3 verifies the inherent molecular interactions. This result clearly suggests the role of hydrogen bonds in promoting the insertion of C5PeC11 among C5Pe molecules to disperse C5Pe. On the contrary, with no polar groups available, BisAC11 lacks the ability to form hydrogen bonding with C5PeC11 as shown in Table 2, exhibiting no obvious dispersing effects. Thus, the MD simulation results gives a convincing explanation for the DLS experimental observations and confirms that compare to C5Pe, BisAC11 is less effective in decreasing the aggregation rate and hydrodynamic radius in the binary system with 0.1 g/L C5PeC11. Table 3. Number of hydrogen bonds averaged over the last 5 ns of 80 ns simulation for 24 C5Pe + 24 C5PeC11 in 50-heptol. System

NH-bond

Total

29.44

24 C5Pe - 24 C5Pe

9.72

24 C5Pe - 24 C5PeC11

12.31

24 C5PeC11 - 24 C5PeC11

7.41


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Figure 13. (a) Radial distribution function g(r) for the COM separation distance r(nm) between aromatic cores of 24 C5Pe in 50-heptol for single C5Pe system and 24 C5Pe + 24 C5PeC11 binary system; (b) Solvent-accessible surface area of 24 C5Pe in 50-heptol for single C5Pe system and 24 C5Pe + 24 C5PeC11 binary system; (c) Radial distribution function g (r) for the COM separation distance r (nm) between aromatic cores of 24 C5PeC11 in 50-heptol for single C5PeC11 system and in 24 C5Pe + 24 C5PeC11 binary system; (d) Solvent-accessible surface area of 24 C5PeC11 in 50-heptol for single C5PeC11 system and 24 C5Pe + 24 C5PeC11 binary system.

Conclusions Dynamic light scattering study showed distinctly different aggregation profiles for three asphaltene fractions separated based on adsorption onto CaCO3. The Irr-Ads asphaltene fraction 34 ACS Paragon Plus Environment

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showed the highest aggregation tendency while the Ads and Non-Ads fractions showed significantly less aggregation in heptol solutions. Combined with the results obtained with FT-IR analysis published early, polar group interaction appears to be an important factor in accelerating asphaltene aggreagtion in bulk solution. For synthesized PA compounds, DLS studies showed the flocculation tendency as C5Pe < C5PeC11 < BisAC11, which is opposite to the trend of their solubilities. Increasing heptane concentration leads to higher rate of aggregation, which agrees well with the heptane enhanced nanoaggregation in previous ESI-MS results. MD simulation provided detailed picture for molecular interactions involved in the aggregation of three PA compounds in heptol. The aggregation tendency of PA compounds observed in MD simulation agrees with flocculation results confirming the important role that polar groups play in facilitating nanoaggregation. Furthermore, MD simulation suggests that strong π-π stacking between polyaromatic cores exists within the nanoaggregates formed in solution. Other intermolecular interactions, such as hydrogen bonds as well as tail-tail contacts between aliphatic side chains, also contribute to the self-association of polyaromatic molecules. Longer aliphatic chains with reduced polarity led to the increased solubility of PA molecules hampering their abilities to aggregate. In addition, DLS results showed that among the three PA compounds, C5PeC11 follows a diffusion limited aggregation behavior with a constant exponent of 0.32, which is quite similar to that of the IrrAds asphaltenes fraction. This similarity suggests that C5PeC11 can potentially be used in the future study on the aggregation behavior of asphaltenes. Polydispersity effects were also studied by DLS, where binary mixtures of C5PeC11 + C5Pe and C5PeC11 + BisAC11 showed a decreased flocculation tendency in heptol solutions. This finding suggests the inherent structural properties as well as the presence of polar functional groups of 35 ACS Paragon Plus Environment

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individual components play an important role in determining the overall flocculation behavior of the mixture. These results were also confirmed by MD simulations, where the addition of another compound with less self-association tendency tends to disperse the aggregated molecules and increase the solubility of the system. Together with the previously published results, the results from this study confirm nanoaggregation and flocculation processes being closely related to each other. Factors that drive the nanoaggregation also enhance the flocculation of the nano aggregates. The result further suggests the interactions between PA compounds are of multiple origins. Single type of interactions is not sufficient to explain the complex behaviors of asphaltene aggregation.

Acknowledgement This research was conducted under the auspices of the Natural Sciences and Engineering Research Council (NSERC)−Industrial Research Chair (IRC) Program in Oil Sands Engineering. The partial support from Alberta Innovates Energy and Environmental Solutions, from National Natural Science Foundation of China (Grant 21333005) and from The Joint Industrial Program consortium consisting of Norwegian Research Council and oil industry and chemical vendors is also greatly appreciated. Rongya Zhang acknowledges financial support from Tianjin University and the China Scholarship Council (CSC). The authors also acknowledge computing resources and technical support from the Western Canada Research Grid (WestGrid).

ASSOCIATED CONTENT Supporting Information Definition of the PA core group and tail (Supporting Information SI 1), and demonstration of reaching dynamic equilibrium by RDF (Supporting Information SI 2)


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