10 research outputs found

    Molecular Dynamics Simulations Reveal Inhomogeneity-Enhanced Stacking of Violanthrone-78-Based Polyaromatic Compounds in <i>n</i>ā€‘Heptaneā€“Toluene Mixtures

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    We elucidated the effect of inhomogeneity in solutes on the aggregation of our representative polyaromatic (PA) compounds through a series of molecular dynamics simulations. Two kinds of solutes, a single type of PA compounds and a mixture of four types of PA compounds, were simulated in toluene, <i>n</i>-heptane, and heptol (mixture of toluene and <i>n</i>-heptane). The geometries of the resultant aggregates were quantified using gyradius ratios. Our results revealed that in toluene, while a single type of PA compound can only form short-cylinder-like aggregates, by having a solute mixture, parallel stacking of PA cores is enhanced, leading to the formation of one-dimensional (1D) rod-like structure. The enhanced stacking is caused by collective arrangement of the PA molecules; i.e., PA compounds of different types appear in an alternating manner in the aggregate. In addition, while the aggregated geometries of a single type of PA compounds were found to be affected by the composition of the solvents, the existence of the 1D structure formed by mixture seems to be insensitive to the solvents. On the other hand, the longest range of stacking is achieved by having a small amount of toluene (ā€œgoodā€ solvent) in <i>n</i>-heptane (ā€œbadā€ solvent)

    Molecular Dynamics Investigation on the Aggregation of Violanthrone78-Based Model Asphaltenes in Toluene

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    In order to investigate the aggregation mechanisms of asphaltenes in toluene, a series of molecular dynamics simulations were performed on Violanthrone78-based model asphaltenes with different aliphatic/aromatic ratios. Our simulation results show that the attraction between poly-aromatic cores is the main driving force for asphaltene aggregation in toluene, and that the extent of aggregation is independent of the aliphatic/aromatic ratios. On the other hand, analysis of the aggregated structures indicates that long side chains do hinder the formation of large direct parallel stacking structures. In contrast with water as a solvent, toluene exhibits attractive interactions with both the aliphatic and aromatic regions of the asphaltenes, hence reducing the size and stability of the asphaltene aggregates. Our findings help to elucidate, at a molecular level, the different solubility behaviors of asphaltenes in toluene and in water

    Probing the Effect of Side-Chain Length on the Aggregation of a Model Asphaltene Using Molecular Dynamics Simulations

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    A series of molecular dynamics simulations were performed to investigate the effect of aliphatic side-chain length on the aggregation behavior of a model asphaltene in water. We found that the extent of aggregation has a nonmonotonic relationship with the side-chain length. Asphaltene molecules with very short or very long side chains can form dense aggregates, whereas those with intermediate chain lengths cannot. Through analysis of the kinetics and driving forces of the aggregation, the role of the side chains in affecting the aggregation behavior was elucidated. Long side chains hinder the formation of parallel stacking structures of the polyaromatic cores while also favoring aggregation through hydrophobic association. The simulation results reported here can be used to propose appropriate means to reduce the extent of aggregation of asphaltene in the presence of water

    A Molecular Dynamics Study of the Effect of Asphaltenes on Toluene/Water Interfacial Tension: Surfactant or Solute?

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    A series of molecular dynamics simulations were performed to investigate the effects of model asphaltenes on the toluene/water interfacial tension (IFT) under high temperature and pressure conditions. In the absence of model asphaltenes, the toluene/water IFT monotonically decreases with increasing temperature, whereas, with the presence of model asphaltenes, especially at high concentrations, such monotonic dependence no longer holds. Furthermore, in contrast with the decreasing trend of IFT with increasing model asphaltene concentration at low temperature (300 K), increasing concentration at high temperature (473 K) leads to increasing IFT. This relation can even be nonmonotonic at moderate temperatures (373 and 423 K). Through detailed analysis on the distribution of model asphaltenes with respect to the interface, such complex behaviors are found to result from the delicate balance between miscibility of toluene/water phases, solubility of model asphaltenes, and hydrogen bonds formed between water and model asphaltenes. By increasing the temperature, the solubility of model asphaltenes in toluene is enhanced, leading to their transition from being a surfactant to being a solute. The effect of pressure was found to be very limited under all model asphaltene concentrations. Our results here present, for the first time, a complete picture of the coupled effect of (high) temperature and asphaltene concentration on IFT, and the methodology employed can be extended to many other two-phase or multiphase systems in the presence of interface-active chemicals

    Probing the Adsorption of Polycyclic Aromatic Compounds onto Water Droplets Using Molecular Dynamics Simulations

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    A series of molecular dynamics (MD) simulations were performed to probe the adsorption behaviors of polycyclic aromatic compounds (PACs) from <i>n</i>-heptane and toluene onto water droplets. In <i>n</i>-heptane, the simulations revealed distinct adsorbed structures of PAC molecules on the water droplets with different sizes. In the system with a small water droplet (radius 1.86 nm), the adsorbed PAC aggregate renders a straight one-dimensional (1D) structure; contrarily, in the presence of a large water droplet (radius 3.10 nm), a bent 1D structure of nonzero curvature is formed. Such size effect is a result from the delicate balance between the deformation energy required for adsorption and the available attractions between the PAC and water molecules. While the adsorbed structures are sensitive to the size of the water droplet at relatively low PAC concentration in <i>n</i>-heptane, the size effect in toluene is only prominent when the concentration of PAC molecules is sufficiently high

    Probing the Effect of Salt on Asphaltene Aggregation in Aqueous Solutions Using Molecular Dynamics Simulations

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    The presence of salts in different processes of oil production has attracted wide attention because of its effects on asphaltene aggregation, stability, interactions of emulsions, etc. In this work, molecular dynamics simulations were employed to study the effect of salts on aggregation of model asphaltenes. Four types of polyaromatic compounds possessing key structural features of continental-type asphaltenes were dispersed into NaCl solutions of different concentrations. These models have the same polyaromatic core but different lengths for the side chains. In the two models with relatively long side chains, the hydrophobic association among side chains is the main driving force for aggregation. The effect of salt on aggregation is therefore closely tied to its influence on the hydrophobic interaction: the salt ions promote the hydrophobic interaction at a low salt concentration while suppressing it at a high salt concentration. For the model with an intermediate side chain length, the hydrophobic interaction between side chains becomes less dominant and the salt has mutual influences on the coreā€“core, chainā€“chain, and coreā€“chain interactions. For the model with the shortest side chains, although the coreā€“core and coreā€“chain interactions are more important, the side chains still play a role in aggregation when the salt is present. Our results provide new insights into the fundamental understanding of the influence of salts on the aggregation and interaction behaviors of polyaromatic compounds in an aqueous environment

    Role of Naphthenic Acids in Controlling Self-Aggregation of a Polyaromatic Compound in Toluene

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    In this work, a series of molecular dynamics simulations were performed to investigate the effect of naphthenic acids (NAs) in early stage self-assembly of polyaromatic (PA) molecules in toluene. By exploiting NA molecules of the same polar functional group but different aliphatic/cycloaliphatic nonpolar tails, it was found that irrespective of the presence of the NA molecules in the system, the dominant mode of Ļ€ā€“Ļ€ stacking is a twisted, offset parallel stacking of a slightly larger overlapping area. Unlike large NA molecules, the presence of small NA molecules enhanced the number of Ļ€ā€“Ļ€ stacked PA molecules by suppressing the hydrogen bonding interactions among the PA molecules. Smaller NA molecules were found to have a higher tendency to associate with PA molecules than larger NA molecules. Moreover, the size and distribution of Ļ€ā€“Ļ€ stacking structures were affected to different degrees by changing the size and structural features of the NA molecules in the system. It was further revealed that the association between NA and PA molecules, mainly through hydrogen bonding, creates a favorable local environment for the overlap of PA cores (i.e., Ļ€ā€“Ļ€ stacking growth) by depressing the hydrogen bonding between PA molecules, which results in the removal of some toluene molecules from the vicinity of the PA molecules

    Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Tolueneā€“Water Interface

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    The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a tolueneā€“water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the tolueneā€“water interface. At low concentrations of PA (āˆ¼2.3 wt %) and NA (āˆ¼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the tolueneā€“water interface. At high NA concentrations, the adsorption of PA molecules (āˆ¼5.75ā€“17.25 wt %) was greatly hindered by the presence of small NA molecules (āˆ¼1.6ā€“4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at tolueneā€“water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications

    Reduction of Water/Oil Interfacial Tension by Model Asphaltenes: The Governing Role of Surface Concentration

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    In this work, pendant drop techniques and molecular dynamics (MD) simulations were employed to investigate the effect of asphaltene concentrations on the interfacial tension (IFT) of the oil/water interface. Here, oil and asphaltene were represented by, respectively, common organic solvents and Violanthrone-79, and two types of concentration, i.e., bulk concentration and surface concentration, were examined. Correlations between the IFTs from experiments and MD simulations revealed that surface concentration, rather than the commonly used bulk concentration, determines the reduction of oil/water IFTs. Through analyzing the hydrogen bonding, the underlying mechanism for the IFT reduction was proposed. Our discussions here not only enable the direct comparison between experiments and MD simulations on the IFTs but also help with future interfacial studies using combined experimental and simulation approaches. The methodologies used in this work can be extended to many other oil/water interfaces in the presence of interfacially active compounds

    Mechanistic Understanding of the Effect of Temperature and Salinity on the Water/Toluene Interfacial Tension

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    In this work, a series of pendant drop measurements and molecular dynamics (MD) simulations were performed to investigate the effects of temperature and salinity on the interfacial tension (IFT) of water/toluene binary systems. Both experimental measurements and theoretical simulations demonstrated that elevating temperature decreased the IFT, while adding salts resulted in an increment of IFT. Furthermore, it was found that the presence of model asphaltene compound could alleviate the effects of temperature and salinity on the IFTs. That is, in the presence of the model asphaltene compound, the decrement effect of elevating temperature as well as the increment effect of adding salts was reduced. Through detailed analysis of the simulated systems, the underlying mechanisms for the effects of temperature and salinity on the IFTs were clarified for cases with and without the presence of the model asphaltene. The results reported here can help to modulate the IFT values of oil/water interfaces in petroleum processing
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