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Molecular Insights into Glass Transition in Condensed Core Asphaltenes Pradeep Venkataraman, Kyriacos Zygourakis, Walter G Chapman, Scott L. Wellington, and Michael Shammai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02322 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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Radial Distribution Functions showing effect of chain cleavage on π-π stacking in Zajac asphaltene model 64x45mm (96 x 96 DPI)

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Molecular Insights into Glass Transition in Condensed Core Asphaltenes Pradeep Venkataraman1†, Kyriakos Zygourakis2, Walter G. Chapman2, Scott L.Wellington2, Michael Shammai1 1. Baker Hughes Oilfield Operations Inc. 2929 Allen Parkway, Suite 2100 Houston, TX 77019 2. Department of Chemical and Biomolecular Engineering – MS 362 Rice University 6100 Main Street Houston, TX 77005

Submission to Energy and Fuels, XXXX XXX.

†Corresponding author. Phone: 504-460-9252. E-mail: [email protected]


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ABSTRACT Glass transition in a condensed core asphaltene model was investigated using molecular dynamics simulations performed in isobaric-isothermal ensemble. Glass transition temperature obtained from the discontinuities in the slope of specific volume versus temperature plots was in close agreement with experimental results reported in the literature. These discontinuities also correspond to those in isothermal compressibility versus temperature plots. In this paper we separate the contributions of aliphatic and aromatic regions of the asphaltene molecule to the glass transition behavior. We demonstrate that the aliphatic chains contribute significantly to volumetric changes and impose restrictions to the molecular orientations. Glass transition is accompanied by breaking of π-π stacking of asphaltene molecule. Therefore the size of the fused aromatic region in the condensed core determines the strength of intermolecular interactions and the glass transition temperature Tg.

Keywords: Asphaltenes, Molecular Dynamics, Glass Transition, Group Contribution


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1. INTRODUCTION Asphaltenes are complex high molecular weight organic molecules present in crude oils and are generally insoluble in alkanes but soluble in aromatic solvents such as toluene and naphthalene derivatives.1-2 Studies suggest that asphaltenes may exist in the form of nanoaggregates in solvents such as toluene.3 Determining the structure of asphaltenes is a scientific challenge and has been a subject of research for many years now. Various models including Yen-Mullins model,4 pendant-core model5 and Sirota model6 were suggested to describe the asphaltene structure and behavior. Although there is heterogeneity in the structures of asphaltene from various sources of crude, these molecules primarily contain aliphatic and aromatic regions with varying degrees of hydrogen and carbon content. Asphaltene structure and content play a critical role in determining the viscosity of crude oil. Understanding the physical behavior and structure of asphaltenes is important for trouble free and economical production, transport and refinery processing of hydrocarbons.7 Early interest in thermal behavior of asphalt and asphaltenic compounds was due to their application in construction of roads, which required blending them with other binding materials. Asphaltenes are known to demonstrate glass-like behavior and therefore glass transition in asphalt and blends of asphalt with binders are of specific interest.6, 8-11. Glass transition is a phenomenon exhibited by amorphous and semi-crystalline material in which a material transitions from a hard and brittle state to a rubbery or liquid like state.12-13 The phenomenon of glass transition is associated with thermodynamic signatures such as abrupt changes in measured thermodynamic quantities like the specific heat, thermal expansion coefficient and the isothermal compressibility in the proximity of the glass transition temperature (usually denoted by Tg).14 Another interpretation of glass transition is described in terms of the viscous behavior of supercooled liquids.13 Glass transition in macromolecules is associated with molecular relaxation and is related to structural factors.15 Understanding quantitatively the various contributions to the glass transition, especially for asphaltenes, continues to be an area of scientific research. Various interpretations were used to model the high viscosity of asphaltenes and asphaltene containing systems.16 Hirose et al showed that the Tg of asphalt increases with increase in asphaltene content.17 Khong et al observed that the composition of a given asphalt fraction controls its Tg and not the molecular weight.18 Asphalt fractions with higher paraffin


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content had lower glass transition temperatures. Small Angle Neutron Scattering (SANS) experiments showed that asphaltene-containing systems have high viscosity in the proximity of glass transition temperature.6 It was proposed that the reduction of viscosity with dilution is no more than a plasticization effect.8 The very high viscosities associated with the glass transition can have implications to the apparent structures in both single-phase and phase-separated systems. Experimental studies using differential scanning calorimetry suggest that asphaltenes from Mayan crude have Tg in the temperature range of 120-130 °C.19 Gurkaynak et al determined glass transition temperatures of Turkish asphaltenes to be in the range 200-220 oC.20 The authors report that asphaltenes with lower hydrogen to carbon (H:C) ratio had higher glass transition temperatures. Using rheological and DSC measurements Shaw et al found that Mayan asphaltenes exhibit broad endothermic transition with peak around 140 oC.9 Molecular Dynamics (MD) simulations can be a powerful tool in studying the physicochemical behavior of asphaltene molecules.21 These simulations employ empirical force field parameters obtained through quantum mechanical calculations and integrate Newton’s equations of motion for atoms.22-23 The macroscopic properties can then be predicted through application of statistical mechanics on ensemble averages. Greenfield compiled a review of computational approaches to modeling and predicting asphaltene structure and phase behavior.21 Molecular simulations of asphaltenes have largely focused on their interaction with solvent molecules and their aggregation behavior in various media.24-30 Pacheco-Sánchez et al determined molecular orientations and structure factors for various asphaltene models using molecular simulations.31 Their work demonstrates that in addition to face-to-face stacking, the asphaltenes molecules also acquire π-offset and T-shaped stacking geometries (terminology described by Leach32). Zhang and Greenfield studied intermolecular orientations in asphaltenes in model asphalt mixtures.33 They suggested the presence of aliphatic chains have significant influence in intermolecular orientations. Atomistic simulations of model asphaltene molecules from Arabian crude suggested a glass transition of 573 oC.34 However, these simulations were carried out for 75 ps and had considerable scatter in the specific volume data. Zhang and Greenfield used the Green–Kubo and Einstein methods to calculate the viscosity of model asphalt systems using molecular simulations.35 More recently Yao et al calculated the viscosity of three component mixtures containing model asphaltene molecules using four different


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methods.36 These investigators showed that the Müller-Plathe method37 in non-equilibrium simulation are closer to the laboratory determined data. However, the effect of molecular structure of asphaltenes on their glass transition behavior is still not very well understood. In an effort to answer some of the still open questions, this work will focus on understanding the relative contributions of aliphatic and aromatic regions on glass transition in condensed core asphaltenes. Of the several model structures for asphaltenes proposed over the years,38-42 we chose as our model compound the molecular structure suggested by Zajac et al38 for asphaltenes from Mayan crude oil (Figure 1a). Our choice of model molecule was dictated by the presence of distinct aromatic and aliphatic regions. The Zajac model has nine aromatic rings and two alicylic rings in the fused aromatic region (FAR). It also has a single aliphatic chain of variable length containing an isopropyl group. Nitrogen and sulfur atoms are present as heteroatoms in pyridinic and thiophenic forms respectively. A hexadecyl aliphatic chain with an isopropyl group was used in this study. The chemical properties of the model are given in Table 1. By varying the structure of the aliphatic and aromatic groups of our model molecule, we aimed to establish a molecular basis for describing the structure-property relationship for glass transition in an asphaltenic system. To this end, MD simulations of the Zajac model of asphaltene were first performed with the complete molecular description and also in the absence of the aliphatic chain. Studies by Pacheco-Sánchez et al indicated that the attraction between the aromatic cores is enhanced in the absence of the aliphatic chains, resulting in lower free volumes and increase in densities of model asphaltene.31 However, the implication of the structure to the macroscopic properties were not investigated. We performed additional simulations with model molecules of variable chain length and varying number of aromatic rings to study the effect of aliphatic chain length and the size of FAR on glass transition in asphaltenes. Thermodynamic signatures such as discontinuities in the slope of specific volume vs. temperature plots and in isothermal compressibility vs. temperature plots are used as indicators for determining the glass transition temperature. The diversity in the structure of petroleum asphaltenes and their polydispersity presents a challenge in realistic molecular descriptions of these systems. Relating macroscale thermal and mechanical properties obtained using experiments to molecular interactions is therefore difficult.


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We have attempted to present a method for separating contributions from different chemical moieties in glass transition behavior. A group contribution based method for describing thermophysical behavior of asphaltenic system allows for more accurate modeling of these complex systems using Equation of State approaches.43-45 We demonstrate that while the magnitude of changes in specific volume versus temperature plots is dictated by the size of aliphatic part, the size of the FAR is critical to glass transition temperature. A broader outcome of the paper is development of methods based on thermodynamic signatures such as in calorimetric studies to obtain structural information about asphaltenic systems 2. SIMULATION METHODOLOGY Zajac model and other model molecules were constructed using Materials and Processes Simulation software suite (Version 3.4, Scienomics, Paris, France, 2014).46 Figures 1(b) and Figure 4(b) show the molecular models of the Zajac asphaltene molecule and the modified Zajac molecule without the aliphatic chain. Molecular geometries were optimized using the Gaussian program47 with Hartree-Fock level of theory and 6-31G* basis set. Partial charges on atoms were calculated using the RESP charge derivatization method.48 To start a simulation, 100 asphaltene molecules were packed in a box of volume 1000 nm3 using the Amorphous Builder program provided in MAPS®. Generalized Amber Force Field (GAFF)49 parameters were used to describe the interactions between the atoms in all simulations. Molecular Dynamics (MD) simulations were performed using GROMACS 4.0.7 software package50 in the temperature range 25 oC – 300 oC. The choice of the upper limit of temperature is due to experimental results indicating possible thermal cracking of asphaltenes at temperatures beyond 300 oC.51 The trajectories were visualized using MAPS® and VMD52 programs. Lorentz-Berthelot combination rules53-54 were applied to the dispersive interactions and long-range electrostatic interactions were evaluated using the particle mesh Ewald method55. Cut-off distance of 10 Å was used for both Lennard-Jones interactions and electrostatic interactions. Energy minimization was carried out using the steepest descent algorithm with a tolerance of 100 kJ mol-1 nm-1for derivative of energy. Isochoric-isothermal (NVT) simulations with periodic boundary conditions were performed for 200 ps to relax the molecular structure and equilibrate system temperature. The equations of motion were integrated with a time step of 2 fs. Bonds involving hydrogen atoms were constrained using the LINCS algorithm56. The


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molecules were subjected to one cycle of simulated annealing beginning at 25 oC and heated up incrementally to 200 oC every 50 ps before lowering them to the temperatures of interest. This was done to ensure that the asphaltene molecules are able to access all molecular conformations. The system was equilibrated for pressure at 1 bar and temperatures of interest using periodic isotropic isobaric-isothermal (NPT) ensemble for 2 ns. Following equilibration at the specified temperature and pressure, 15 ns long production runs were carried out at each temperature point. Simulation data was sampled every 100th time step. Nosé-Hoover thermostat57-58 and ParrinelloRahman barostat59 were respectively used to control the temperature and pressure in the simulations. 3. RESULTS AND DISCUSSIONS A snapshot of the simulation box containing Zajac asphaltene molecules after 15 ns of production run at 1 bar and 25 oC is shown in Figure 1(c). Asphaltenes display short range stacking containing two to three asphaltene molecules in various orientations. The density of the box was 1.12 g/cm3 at 25 oC. This is slightly greater than those observed by Pacheco-Sánchez et al. for Zajac asphaltene.31 It should be noted that Pacheco-Sánchez et al. used a model with a longer aliphatic chain in their simulations. The T-shaped and offset π-stacked structures were the most commonly observed orientations. Pacheco-Sánchez et al. and Greenfield et al. have discussed the aggregation behavior and intermolecular orientations of asphaltenes in detail.31, 33 Our observations are consistent with results from Pacheco-Sánchez et al. We discuss the molecular orientations in our simulations using radial distribution functions between sulfur atom pairs in later sections. Specific volumes of Zajac model asphaltene as obtained from MD simulations are plotted as a function of temperature at 1 bar in Figure 2. The circles represent average specific volumes calculated using trajectories obtained from 15 ns NPT simulations. These volumes are results from independent simulations at each temperature obtained after simulated annealing, equilibration and long production runs. There are two distinct regions in the plot represented by linear fits to data points. The specific volume exhibits linear dependence with temperature with a slope discontinuity in the region 175-200 oC. This discontinuity in thermal expansion coefficient is a thermodynamic signature of glass transition.14 Discontinuities in temperature vs. other calculated properties such as isothermal compressibility and molecular diffusivity are also


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observed in the same temperature region (Supporting Information S-1, S-2). These results are in close agreement with experimental data for Mayan asphaltenes.9, 19 The structural transitions were also observed through inter-molecular radial distribution functions (RDF) between thiophenic sulfur atoms at three different temperatures. Figure 3 shows the RDF between sulfur pairs at three temperatures 25 oC (below Tg), 175 oC (near Tg) and 275 o

C (above Tg). The RDF at 25 oC (solid line) shows distinct peaks at 3.8, 4.7 and 7.7 Å

corresponding to three different orientations of the FARs in the short ranged stacking of the asphaltene molecules. The first two peaks show the T-shaped orientations of the asphaltene molecules. These molecular orientations are shown in Figure 7(a) and are discussed in detail in section 3.1. At 175 oC, a temperature close to Tg, we observe distinct changes such as broadening of peaks in the sulfur-sulfur RDF (dotted line). A single broad peak replaces the first two peaks at 3.8 and 4.7 angstroms. This is a direct consequence of molecules overcoming steric barriers imposed by the aliphatic chain to arrange in more random orientations. At temperatures beyond the glass transition at 275 oC (circles), note that the RDF represents a liquid structure with the primary peak corresponding to nearest neighboring sulfur atom in a T-orientation of asphaltene molecules. The offset π-stacking represented by the peak at 7.7 angstroms is no longer present at temperatures above Tg. Observations with RDF between the pyridinic nitrogen pairs (Supporting Information S-3) are similar to those obtained with the thiophenic sulfur pairs. 3.1 Effect of the aliphatic chain on molecular orientations To investigate the role of the aliphatic chain on the structure of asphaltene, MD simulations of the modified Zajac molecule without the aliphatic chain were carried out at 1 bar and temperatures of interest as described earlier. The chemical structure of the modified Zajac molecule is shown in Figure 4(a-b). Figure 4(c) shows a snapshot of the simulation box containing 100 model molecules after 15 ns NPT simulation at 25 oC. Visual observations indicate increased short range stacking of Zajac asphaltene cores. The density of the Zajac asphaltene core molecules at 25 oC was 1.28 g/cm3. This is in agreement with the values reported by Pacheco-Sánchez et al.31 As discussed by Pacheco-Sánchez et al, the aliphatic chains hinder the stacking of aromatic cores and considerably increase the free volume in the asphaltene aggregates. Consequently, asphaltenes with shorter aliphatic chains have larger extent of πstacked structures than one with longer aliphatic groups.


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Figure 5 shows the specific volume versus temperature plot for the modified Zajac asphaltene molecule. The circles represent average specific volumes calculated from trajectories obtained from 15 ns NPT simulations. As in the previous case, these volumes are results from independent simulations at each temperature obtained after simulated annealing, equilibration and long production runs. In contrast to the results of Figure 2 for the Zajac asphaltene model, we note that there are no distinct regions in the specific volume vs. temperature plot of Figure 5. We observe that there is a modest increase in the specific volume at higher temperatures. But the magnitude of the change in specific volume is very small compared to the case where aliphatic chains are present (Figure 2). We speculate that this small increase in the volume is due to weakening of π- π bonding between the stacks. As noted earlier, the contribution to specific volume change is primarily due to the aliphatic chains that are absent in this case. We also hypothesize that in the absence of aliphatic chains, restrictions to molecular orientations are limited. Without aliphatic chains, the modified Zajac molecule can assume conformations at lower temperatures otherwise restricted by aliphatic chains in the unmodified molecule. To corroborate this hypothesis we studied the RDFs between thiophenic sulfur atom pairs for the Zajac model and the modified Zajac model at 25 oC. Again, the thiophenic sulfur atom was used as a marker to study the molecular orientations in the two models. Due to its proximity to the aliphatic chain in the molecular structure (Figure 1(a)), the RDF between sulfur atom pairs is a good indicator of the degrees of freedom related to molecular orientations. Figure 6 shows a comparison between RDFs of sulfur atom pairs at 25 oC for the Zajac asphaltene model (solid line) and for the modified Zajac model without aliphatic chain (dashed line). RDF between sulfur atom pairs for the Zajac model shows three distinct peaks as 3.8, 4.7 and 7.7 angstroms corresponding to three different orientations of asphaltene molecules. Figure 7(a) is a visual representation of the molecular orientations corresponding to these peaks. The first peak at 3.8 Å in RDF represents a T-shaped orientation between pair of molecules. The second peak at 4.7 Å corresponds to a T-shaped orientation between asphaltene molecule pairs when one of the molecules is in a stacked conformation. Given the relative heights of the peaks, these orientations are most abundant in the Zajac asphaltene at 25 oC. The third peak at 7.7 Å corresponds to the distance between sulfur atoms of asphaltene molecules participating in a


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stacked conformation. As shown in Figure 7(a), the aliphatic chains impose steric restrictions on molecules allowing them to arrange in an offset π-stacking orientation. In the case of the modified Zajac asphaltene molecule, the RDF between sulfur atom pairs is significantly different with a higher first peak at 3.7 Å followed by a diminished second feature at 4.7 Å and a third prominent peak at 7.0 Å. Figure 7(b) is the visual representation of the molecular orientations corresponding to these features in the RDF. In a significant contrast to the Zajac asphaltene model, the first peak at 3.7 Å corresponds to sulfur atoms stacked on top of each other in face-to-face conformation. This orientation is in fact the most probable conformation for the modified Zajac asphaltene molecule at 25 oC. The presence of the aliphatic chains in the Zajac molecule does not favor the orientation of asphaltene molecules where sulfur atoms are stacked in a face-to-face conformation. The peak at 4.7 Å corresponds to T-shaped orientation between asphaltene molecule pairs when one of the molecules is in a stacked conformation. This orientation is similar to the T-shaped orientation found in Zajac asphaltene molecule. It is noteworthy that this peak is significantly diminished in the case of the modified Zajac molecule. This suggests that the relative probability of the molecules existing in Torientation is lower in the case of the modified Zajac molecule. Consequently, it can be construed that in the absence of aliphatic chain, the asphaltene cores tend to have increased πstacking and a higher density. The third peak at 7.0 Å corresponds to the offset π-stacking between the asphaltene molecules. The shift of the peak from 7.7 Å to 7.0 Å is another indication of closer packing between the aromatic cores. Based on the features in the RDFs it can be deduced that the modified Zajac asphaltene molecules show closer packing due to increased πstacking. Also, there is no restriction on sulfur atom to arrange in a face-to-face conformation due to the removal of aliphatic chains. RDF between sulfur atom pairs at temperatures 275 oC shows broadening of the peaks at higher temperature (Supporting Information S-5). When compared with the RDF at 25 oC, peaks corresponding to the three orientations can still be seen in the RDF at 275 oC. As can be seen, Tshaped orientation becomes relatively more favorable. These observations suggest that although the molecules are more labile at elevated temperature, there is no significant change in the molecular orientations even at very high temperature. This explains the absence of distinct regions in the specific volume versus temperature plots of Figure 5 for the modified Zajac


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asphaltene model. These observations also underscore the contribution of aliphatic moieties to the “free volume” present in the asphaltene aggregates and their influence on the thermoelastic behavior of asphaltenes. This result suggests that such thermodynamic signatures obtained, for example, from experimental determination of glass transition in asphaltenes may be useful in making deductions about their structural features. To further quantify the role of the aliphatic and aromatic regions of asphaltenes, we performed simulations with variable sizes of FAR and lengths of aliphatic chain. The results are discussed in the following sections. 3.2 Effect of the aliphatic chain length and number of chains on glass transition To study the effect of aliphatic chain length on the thermoelastic behavior of the asphaltene, we performed MD simulations with model molecules consisting of 10 aromatic rings in FAR and one aliphatic chain of variable length. Figure 8(a) shows the chemical structure of the model molecule with 10 aromatic rings and an aliphatic chain containing 8 carbon atoms. Simulations with molecules having the same FAR and aliphatic chains with 12 and 16 carbon atoms were also carried out at 1 bar and at temperatures between 25 and 300 oC. The specific volume versus temperature behavior of these model molecules is shown in Figure 8 (b). The three sets of data points represent average specific volumes calculated from trajectories from 15 ns NPT simulations respectively for model molecules with 8, 12 and 16 carbon atoms in aliphatic chain. As in the previous cases, each data point is obtained from independent simulations at the specified temperature. We observe that irrespective of the length of the aliphatic chain, the discontinuities in the slope of the specific volume vs. temperature curves occur in the same temperature range around 175 oC. These discontinuities are highlighted using a dashed line in Figure 8(b). As the length of the aliphatic chain increases, the change in specific volume with temperature becomes more prominent. This is again consistent with the conclusions reached by Stoyanov and co-workers using ab initio calculations.60 These authors suggested that the interaction between molecules with the same number of aromatic rings is stronger for the ones with the largest number of aliphatic groups. Our results suggest that while the length of the aliphatic chains determines the volume change upon glass transition it does not significantly impact the temperature of glass transition.


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Figure 8(c) presents estimates of the incremental increases in specific volume as methylene groups are added to the aliphatic chain of a molecule with a 10 ring aromatic core. We obtained one set of estimates by subtracting the specific volumes computed for chains with 16 and 12 carbons at every temperature and dividing by four (open circles in Figure 8c). To obtain the second set of estimates, we subtracted the specific volumes computed for chains with 12 and 8 carbons at every temperature and divided again by four (open triangles in Figure 8c). The significant overlap between both sets of data points indicate that the changes in specific volume between the molecules over the temperature range correspond primarily to the volume change due to addition of four methylene groups to the chain. The line represents temperature correlation for partial molar volume of –CH2 group in alkanes61-62. This result concurs with the observation that the change in the specific volume versus temperature plots is proportional to the size of the aliphatic chain. Consequently, one can separate contributions to volume changes due to the aliphatic and aromatic regions of the asphaltene molecules. This forms the basis for developing a group contribution based approach for molecular description of asphaltenic compounds. This approach also alludes to early models of petroleum components such as a pendant-core model discussed by Wiehe and Liang.5 More recently such group contribution approaches were implemented in molecular equation of state such as SAFT to describe asphaltene behavior.45 To study the effect of number of aliphatic chains, we performed MD simulations with model molecules consisting of 10 aromatic rings in FAR and two aliphatic chains containing 8 carbon each. These simulations were also carried out at 1 bar and at temperatures between 25 and 300 o

C. Figure 8(d) shows the specific volume versus temperature plots for two model molecules

with 10 aromatic rings in FAR and different numbers of aliphatic chains. There are 8 carbon atoms in each aliphatic chain attached to core. As in the previous cases, each data point is obtained from independent simulations at the specified temperature. We observe that irrespective of the number of the aliphatic chains, the discontinuities in the plots occur in the same temperature range around 175 oC highlighted using a dashed line in Figure 8(d). As with the case of differing lengths of the aliphatic chains, we observe that the number of aliphatic chains does not affect the glass transition temperature of the model asphaltene molecules. It can then be speculated that the structural dependence of glass transition temperature of asphaltenes can be


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attributed to the size of FAR. To this effect, we also performed simulations of model molecules with varying number of rings in the aromatic core. The results from those simulations are discussed in the next section. 3.3 Effect of the size of FAR on glass transition The effect of the size of aromatic core on thermoelastic behavior of asphaltene molecules was investigated through systematic study of homologous model compounds. These model compounds consist of FARs with varying number of aromatic rings each having a dodecyl aliphatic chain. Figure 9 shows the four model compounds used in this study respectively with 4, 6, 8 and 10 aromatic rings in the FAR. MD simulations of the four model molecules were carried out at temperatures of interest as described earlier. The specific volume versus temperature plots for these model molecules is shown in figure 10 (a-d). The circles represent average specific volumes calculated from trajectories from 15 ns NPT simulations and show linear fits. As in the previous cases each data point is obtained from independent simulations at the specified temperature. In all four cases we observe two distinct regions in the specific volume versus temperature plots marked by a discontinuity corresponding to the glass transition temperature. The discontinuity in the curve for 4 rings occurs between 50-75 oC (Figure 10(a)). With increase in the number of rings in the FAR the discontinuities in the plots shift to higher temperatures. Discontinuities observed in temperature vs. isothermal compressibility plots also exhibit similar trends (Figure S-6). These results suggest that the glass transition temperature is closely related to the size of the FAR. Using ab initio calculations, Stoyanov and co-workers showed that compounds with large aromatic cores system have strong aggregation involving π–π stacking.60 It follows from our previous discussion that the glass transition in asphaltenes is accompanied with breaking of π–π stacking between molecules. A bigger core implies stronger interactions between the molecules and therefore greater amount of thermal energy is required to overcome these interactions. 4. CONCLUSIONS Glass transition in condensed core asphaltenes was investigated using isothermal-isobaric molecular dynamics simulations. Glass transition appeared as a discontinuity or change in the


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slope of specific volume vs. temperature plots for the condensed core asphaltene. In the case of the original Zajac asphaltene model, at lower temperatures, two to three molecules were able to stack allowing for short-range structuring. The presence of the aliphatic moiety restricts certain molecular orientations and stacking by offering steric barriers. Glass transition is accompanied with breaking of π–π stacking between molecules and is also associated with molecules overcoming the steric barriers and restrictions in molecular orientations. In the absence of the aliphatic chains the FAR have additional degrees of freedom with respect to molecular orientations even at lower temperatures. As an implication, significant changes in specific volumes were not observed at higher temperatures. This study identified two significant contributions to glass transition phenomena in asphaltenes – 1. The rate at which the specific volume of asphaltenes increases with temperature is largely determined by the length of aliphatic chains. The longer the aliphatic chain, the faster the specific volume increases as the temperature rises. However, the chain length or number of chains does not influence significantly the glass transition temperature. 2. The glass transition temperature of asphaltenic molecules with same length of aliphatic chain is dictated by the size of the FAR. The larger the number of aromatic rings in the FAR, the higher is the temperature of glass transition. In addition to providing insights at the molecular level into the glass transition in asphaltene molecules, this study has broader implications to the study of structure-property relationships of these complex organic molecules. Chemical modification of asphaltenes significantly impacts the thermoelastic behavior of asphaltenes and has implications to flow properties at higher temperatures. This study demonstrates that thermodynamic signatures such as glass transition of asphaltenes yield important details about their molecular structure. This information can be used as basis for developing a group contribution based approach or using a molecular equation of state such as SAFT to describe asphaltene behavior. The role of heteroatoms such as sulfur, nitrogen and oxygen, and multiple aliphatic chains on the glass transition behavior remains to be investigated and will be pursued in future studies.


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5. ACKNOWLEDGEMENTS The authors acknowledge the support and resources from The Center for Research Computing at Rice University and at BHI. The authors would like to thank Dr. Manjusha Verma (Baker Hughes), Dr. Sivaram Pradhan (Baker Hughes), Dr. Ciceron Ayala (Rice University), Dr. Bruce Brinson (Rice University), Dr. Ed Billups (Rice University) and Dr. Michael Wong (Rice University) for their comments and useful discussions. The authors would like to dedicate this article to the loving memory of Scott Wellington, who sadly passed away before the publication of this article. SUPPORTING INFORMATION Isothermal compressibility versus temperature curve for Zajac model (S-1), Diffusivity versus temperature curve for Zajac model (S-2), RDF between the aromatic nitrogen atom pairs of Zajac asphaltenes at different temperatures (S-3), comparison between nitrogen RDF of Zajac asphaltene and modified Zajac asphaltene (S-4), RDF between the thiophenic sulfur atom pairs of modified Zajac asphaltenes at different temperatures (S-5) and Isothermal compressibility versus temperature curves for PAH homologues (S-6) are provided as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. 6. REFERENCES 1.

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LIST OF FIGURES Figure 1 (a) Chemical structure of the Zajac asphaltene model used in this study. The core contains 8 aromatic fused rings and two alicyclic rings. The aliphatic part contains a C16 chain with an isopropyl group. (b) Ball and stick representation of the molecule used in molecular dynamics simulations. (c) Snapshot of the simulation box containing 100 model molecules at 25 o

C after 15 ns of NPT simulation.

Figure 2 Specific volume of the Zajac asphaltene molecule as a function of temperature. Circles represent average specific volume obtained from 15 ns NPT simulations at each temperature. Lines show linear fit to the simulation data. Discontinuity in the temperature range 175-200 oC indicates glass transition. Figure 3 Radial distribution function (RDF) between sulfur atom pairs at three different temperatures (25 oC, 175 oC and 275 oC). Sulfur atom is used as a marker for aromatic core structuring. The RDFs show a transition from a structured system at lower temperatures to a liquid like system at temperatures beyond Tg. Figure 4 (a) Chemical structure of the modified Zajac asphaltene model after removing the aliphatic chain (b) Ball and stick representation of the molecule used in molecular dynamics simulations. (c) Snapshot of the simulation box containing 100 model molecules at 25 oC after 15 ns of NPT simulation. Figure 5 Specific volume of the modified Zajac asphaltene molecule as a function of temperature. Circles represent average specific volume obtained from 15 ns NPT simulations at each temperature. Figure 6 Radial distribution function (RDF) between sulfur atom pairs from the Zajac (solid line) model and the modified Zajac (dashed line) model at 25 oC. The molecular orientations are significantly different in the absence of the aliphatic chains. Figure 7 (a) Molecular orientations corresponding to major peaks of sulfur-sulfur RDF of the Zajac asphaltene model. Distance between the sulfur atoms (represented by yellow spheres) of two asphaltene molecules is shown using dashed lines. (b) Molecular orientations corresponding to major peaks of sulfur-sulfur RDF of the modified Zajac asphaltene model without the aliphatic


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chain. Distance between the sulfur atoms (represented by yellow spheres) of two asphaltene molecules is shown using dashed lines. Figure 8 (a) Model compound with 8-carbon aliphatic chain and an aromatic core containing 10 rings. Model compounds with 8, 12 and 16 carbon aliphatic chains were used to study the effect of chain length. (b) Specific volume versus temperature plots of the model molecules at 1 bar. The number of carbon atoms in the aliphatic chain attached to core is shown on the side. Data points represent average specific volume obtained from 15 ns NPT simulations at each temperature. Dashed line shows the discontinuities in the slope of plot occur at the same temperature irrespective of the length of the aliphatic chain. (c) Difference in the specific volume per methylene group added to the model molecules shown as a function of temperature. The triangles represent the difference in specific volume per methylene group for models with 16 carbon and 12 carbon chain. The circles represent the difference in specific volume per methylene group for model molecules with 12 carbon and 8 carbon aliphatic chain. The line represents temperature correlation for partial molar volume of –CH2 group in alkanes. (d) Specific volume versus temperature plots of two model molecules with same aromatic core size and different numbers of aliphatic chains. There are 8 carbon atoms in each aliphatic chain attached to core. Data points represents average specific volume obtained from 15 ns NPT simulations at each temperature. Dashed line shows the discontinuities in the slope of plot occur at the same temperature irrespective of the number of the aliphatic chains. Figure 9 Homologous model compounds with 12-carbon aliphatic chain and incremental number of aromatic rings in the fused aromatic region Figure 10 Specific Volume versus Temperature curve for the homologous molecules containing 12 carbon chain and (a) four (b) six (c) eight and (d) ten aromatic fused rings. The data points represent results from simulations and the solid line represents a linear fit. The discontinuities representing Tg occur at higher temperatures as the size of fused aromatic region increase.

LIST OF TABLES Table 1 Molecular properties of the Zajac asphaltene molecule


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Total No. Of Carbon Atoms

54

Total No. Of Hydrogen Atoms

57

Hydrogen to Carbon Ratio

1.06

Nitrogen to Carbon Ratio

0.02

Sulfur to Carbon Ratio

0.02

Double Bond Equivalent

26.00

No. Average Molecular Mass (Da)

752.13

% Aromatic Carbon

53.70

%Aromatic Hydrogen

10.53

Table 1 Molecular properties of the Zajac asphaltene molecule


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Figure 1 (a) Chemical structure of the Zajac asphaltene model used in this study. The core contains 8 aromatic fused rings and two alicyclic rings. The aliphatic part contains a C16 chain with an isopropyl group. (b) Ball and stick representation of the molecule used in molecular dynamics simulations. (c) Snapshot of the simulation box containing 100 model molecules at 25 o

C after 15 ns of NPT simulation.


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Figure 2 Specific volume of the Zajac asphaltene molecule as a function of temperature. Circles represent average specific volume obtained from 15 ns NPT simulations at each temperature. Lines show linear fit to the simulation data. Discontinuity in the temperature range 175-200 oC indicates glass transition.


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Figure 3 Radial distribution function (RDF) between sulfur atom pairs at three different temperatures (25 oC, 175 oC and 275 oC). Sulfur atom is used as a marker for aromatic core structuring. The RDFs show a transition from a structured system at lower temperatures to a liquid like system at temperatures beyond Tg.


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Figure 4 (a) Chemical structure of the modified Zajac asphaltene model after removing the aliphatic chain (b) Ball and stick representation of the molecule used in molecular dynamics simulations. (c) Snapshot of the simulation box containing 100 model molecules at 25 oC after 15 ns of NPT simulation.


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Figure 5 Specific volume of the modified Zajac asphaltene molecule as a function of temperature. Circles represent average specific volume obtained from 15 ns NPT simulations at each temperature.


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Figure 6 Radial distribution function (RDF) between sulfur atom pairs at Zajac (solid line) and the modified Zajac (dashed line) model at 25 oC. The molecular orientations are significantly different in the absence of the aliphatic chains.


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Figure 7(a) Molecular orientations corresponding to major peaks of sulfur-sulfur RDF of Zajac asphaltene model. Distance between the sulfur atoms (represented by yellow spheres) of two asphaltene molecules is shown using dashed lines.


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Figure 7(b). Molecular orientations corresponding to major peaks of sulfur-sulfur RDF of the modified Zajac asphaltene model without the aliphatic chain. Distance between the sulfur atoms (represented by yellow spheres) of two asphaltene molecules is shown using dashed lines.


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Figure 8 (a) Model compound with 8-carbon aliphatic chain and an aromatic core containing 10 rings. Model compounds with 8, 12 and 16 carbon aliphatic chains were used to study the effect of chain length

Figure 8(b) Specific volume versus temperature plots of the model molecules. The number of carbon atoms in the aliphatic chain attached to core is shown on the side. Data points represents average specific volume obtained from 15 ns NPT simulations at each temperature. Dashed line shows the discontinuities in the slope of plot occur at the same temperature irrespective of the length of the aliphatic chain.


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Figure 8(c) Difference in the specific volumes per methylene group added to the model molecules shown as a function of temperature. The triangles represent the difference in specific volumes per methylene group for models with 16 carbon and 12 carbon chain. The circles represent the difference in specific volumes per methylene group for model molecules with 12 carbon and 8 carbon aliphatic chain. The line represents temperature correlation for partial molar volume of –CH2 group in alkanes.


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Figure 8 (d). Specific volume versus temperature plots of two model molecules with same aromatic core size and different numbers of aliphatic chains. There are 8 carbon atoms in each aliphatic chain attached to core. Data points represents average specific volume obtained from 15 ns NPT simulations at each temperature. Dashed line shows the discontinuities in the slope of plot occur at the same temperature irrespective of the number of the aliphatic chains.

Figure 9 Homologous model compounds with 12-carbon aliphatic chain and incremental number of aromatic rings in the fused aromatic region


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Figure 10 Specific Volume versus Temperature curve for the homologous molecules containing 12 carbon chain and (a) four (b) six (c) eight and (d) ten aromatic fused rings. The data points represent results from simulations and the solid line represents a linear fit. The discontinuities representing Tg occur at higher temperatures as the size of fused aromatic region increase.


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