Mesoscale Dissipative Particle Dynamics to Investigate Oil Asphaltenes and Sodium Naphthenates at the Oil−Water Interface

Abstract

The effect of the structure of oil asphaltenes and sodium naphthenates at the oil–water interface on the reduction of the oil–water interfacial tension and the formation of stable emulsions is investigated by dissipative particle dynamics (DPD). The coarse-grained model molecules at the mesoscale level are adopted instead of using atomistic models. DPD can process a much larger spatial and temporal scale system than molecular dynamics (MD) can. Two types of island asphaltene model structures are used in the calculations: one with O atoms and the other one without O atoms. A small sodium naphthenate (benzoic acid sodium salt) is included in the simulations. The interfacial tension (IFT) is evaluated at different concentrations of sodium naphthenate while maintaining the asphaltene concentration fixed to conclude on the stability of the emulsions formed. The IFT results obtained for both asphaltene models are compared to reach conclusions regarding the relationships between asphaltene structure, surface activity and coverage, orientation at the oil–water interface, and stability of the emulsion formed by the asphaltenes and sodium naphthenates. Initially, when there is no sodium napthenates in the systems but only asphaltenes, the IFT decreases from 45 mN/m to 40 mN/m, because of the presence of asphaltene molecules at the oil−water interface. With the addition of sodium naphthenates to the systems, it is found that the asphaltene with no O atoms in the structure is less oil−water interface active and it is required to add a concentration of 0.363 M of sodium naphthenate to lower the IFT from 40 mN/m to 35 mN/m. The reduction of the IFT is mainly due to the formation of a uniform film of sodium naphthenates that covers the oil−water interface and displaces most of the asphaltenes with no heteroatoms to the oil region. This would explain why only <2 wt % of the total content of asphaltenes in some oils is found in the material extracted from the oil−water interface. For the asphaltene with O atoms in the structure, only a concentration of 0.033 M of sodium naphthenate is required to lower the IFT from 40 mN/m to 35 mN/m, i.e., a 10 times less sodium naphthenate concentration is required to lower the interfacial tension. The asphaltene with O atoms in the structure is more oil−water interface active and forms stable emulsions with sodium naphthenates by creating a uniform film at the oil−water interface with the sodium naphthenates. Once the film is formed, the asphaltenes with oxygen in the structure do not desorb from the interface, in agreement with experimental observations. A detailed knowledge of the chemical structure of interfacial active asphaltenes and other interfacial materials such as naphthenates can help in the design of a new generation of demulsifiers to resolve water-in-oil emulsions problems in the oil industry.

Structure–Dynamic Function Relations of Asphaltenes

“…Related results have been obtained previously with a dissipative particle dynamics study of two different island asphaltene structures (one more interfacial active than the other) and naphthenic acids, where the interfacial tension is calculated at different concentrations of naphthenic acid, while keeping constant the concentration of asphaltene constant. Displacement of less interfacially active components with higher concentrations by more interfacially active components with lower concentrations is obtained …”

“…Displacement of less interfacially active components with higher concentrations by more interfacially active components with lower concentrations is obtained. 166…”

Virtual Experiments on Real Asphaltenes: Predicting Properties Using Quantum Chemical Simulations of Structures from Non-contact Atomic Force Microscopy

“…It has been argued that only a subfraction of petroleum asphaltenes are responsible for emulsion stabilization. , High-resolution mass spectrometry of material adsorbed to emulsified water droplets identified heteroatom-containing compounds and compounds of high aromaticity. , Interfacial material was found to constitute < 2% of the total asphaltene present in oil and contain an abundance of acidic oxygen and sulfur. − Molecular dynamics simulations of uncharged asphaltene model compounds predict that the model compounds accumulate at heptane/water or toluene/water interfaces, with the aromatic regions aligned parallel to the interface. For oil–water interfacial tension dependence upon asphaltene interfacial coverage, a single fitted parameter is consistent with flat-on adsorption of monomeric asphaltene structures consisting of aromatic cores with an average of six fused rings. − Experiments on acidic perylene diimide and violanthrone-78 model compounds, find the aromatic region lies perpendicular to air/water interfaces. , Sodium naphthenates were predicted to displace non-heteroatom-containing asphaltenes from the oil–water interface, providing a possible explanation for the small fraction of asphaltenes found in interfacial material …”

“…Figure illustrates V s of three representative asphaltenes: two steam-cracked tar asphaltenes and one petroleum asphaltene. All three asphaltenes have a modest charge in the aromatic region and a small charge on the aliphatic chains. , All three asphaltenes have the largest V s near heteroatoms, indicating the importance of heteroatoms in aggregation . Steam-cracked tar asphaltene SCT8 has a region of negative electrostatic potential as a result of its peripheral keto group, consistent with its relatively small log P ow .…”

Outlook on the Role of Natural Surfactants on Emulsion Stability and Gas Hydrate Antiagglomeration in Crude Oil Systems

“…Recently, Ruiz-Morales and Alvarez-Ramírez use dissipative particle dynamics (DPD) to study joint effects of asphaltenes and a carboxylic salt (benzoic acid sodium salt) on the reduction of the oil–water interfacial tension and the formation of stable emulsions. Findings indicate that asphaltenes with oxygen (O) in their structure becomes more interfacial active than those with no oxygen and forms a uniform interfacial film with the sodium naphthenate capable of stabilizing emulsions.…”