Evaluation of Asphaltene Adsorption Free Energy at the Oil-Water Interface: Effect of Oil Solvents

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.