Evaluation of Asphaltene Adsorption Free Energy at the Oil-Water Interface: Role of Heteroatoms

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.