In this study, we investigated the stability of asphaltene adsorption structures at the oil–water interface, focusing on the role of heteroatoms, by molecular dynamics simulations. We employed an oil (1:1 mixture of heptane and toluene, by volume)–water system and used 13 types of asphaltene molecules. Two sets of asphaltene models with the alkyl side chain at different locations were considered. For each set, six models were employed, which have essentially the same structures but with different heteroatoms (such as nitrogen, oxygen, and sulfur) on the aromatic ring (i.e., heteroaromatic ring). Besides 12 models, an additional asphaltene molecule with a carboxyl group at the end of the alkyl side chain was included. We evaluated the asphaltene adsorption Gibbs free energy at the oil–water interface using potential of mean force calculations. It is found that the basic pyridine-type nitrogen-containing asphaltene presents the highest adsorption Gibbs free energy among six asphaltene molecules for both sets. The heteroatom of the asphaltene molecule forms a hydrogen bond with the water molecules so that it can stabilize asphaltene adsorption at the oil–water interface. The strength of the hydrogen bond depends on the negative charge of the heteroatom, with the basic pyridine-type nitrogen being the highest, and the highest adsorption Gibbs free energy. Furthermore, it is found that the acidic pyrrole-type nitrogen-containing asphaltene has the most significant weak hydrogen bonding between the heteroaromatic ring and water molecules due to the charge of the carbon atom in that ring being higher than others. The thiophene-type sulfur-containing asphaltene has the most significant van der Waals interaction; the adsorption Gibbs free energy shows a significant value for both sets. The carboxyl asphaltene molecule has the highest affinity to the oil–water interface among 13 models because it has two heteroatoms. The detailed understanding of the asphaltene adsorption behavior presented in this study would be useful to solve the stability issue of oil–water emulsions in crude oil production.

We investigated asphaltene adsorption behaviors at the oil–water interface, focusing on the effect of oil solvents, by molecular dynamics simulations. Heptane, toluene, and their mixtures with ratios of heptane to toluene of 25:75, 50:50, and 75:25 by volume (namely, heptol25, heptol50, and heptol75) were used as the oil models. Two asphaltene models with essentially the same structure were employed: one contains a basic pyridine-type nitrogen heteroatom; another contains no heteroatoms. The asphaltene adsorption Gibbs free energy at the oil–water interface was evaluated by potential of mean force (PMF) calculations using the umbrella sampling method. The results show that the oil solvent not only affects the adsorption Gibbs free energy but also the adsorption structure as revealed by PMF minimum numbers, locations, and asphaltene orientation angles. For nitrogen-containing asphaltene, the adsorption Gibbs free energy increases linearly with the heptane volume fraction. Noteworthily, the adsorption Gibbs free energy value of this nitrogen-containing asphaltene is high enough to adsorb at the toluene–water interface. It implies that solely adding solvent (such as toluene) may not be enough for solving the emulsion problems induced by interface-active asphaltenes. For the asphaltene without heteroatoms, the asphaltene prefers to be solvated in the oil phase, and there is no well-defined adsorption state when the heptane volume fraction is less than half. For heptol75–water and heptane–water interface systems, the asphaltene adsorption minima can be detected. This clearly indicates that the oil solvent can influence the surface activity of asphaltenes. The study highlights the importance of subtle balances of different noncovalent interactions between asphaltenes, water, and oil components in the oil solvent. The detailed understanding of the asphaltene adsorption behaviors presented in this study will be helpful to solve oil–water emulsion problems and understand the effect of water cut to asphaltene deposition in crude oil production.