Evaluation of Adsorption Characteristics between an Asphaltene Molecule and Silica Through Free Energy Calculation by Molecular Dynamics Simulation

Abstract

Asphaltene adsorption on silica surface plays an important role in wettability alternation in an oil reservoir and the durability of asphalt pavement. In this study, we investigated the relationship between asphaltene structure and adsorption characteristics via potential of mean force (PMF) by molecular dynamics simulation. We prepared six types of asphaltene molecules, and studied the adsorption Gibbs free energy on two different silica surfaces (e.g., hydrophilic and hydrophobic silica). In total, twelve PMFs between a single asphaltene molecule and silica were calculated. On both hydrophilic and hydrophobic silica surfaces, the calculated adsorption Gibbs free energy of asphaltene with a hydroxyl terminal was larger than the similar types of asphaltene without hydroxyl terminals because of hydrogen bonding between a hydroxyl terminal and silica surfaces. The adsorption Gibbs free energy of asphaltene with larger aromaticity was also large, while those of asphaltene with less aromaticity and no alkyl chain were small. Moreover, for all asphaltene molecules, the calculated adsorption Gibbs free energy is significantly larger for hydrophobic surface than hydrophilic one (by 8-54 kJ/mol). The underlying mechanism can be ascribed to the polarity difference of those two surface structures. This study shows a quantitative evaluation of the adsorption characteristics in different types of asphaltene and mineral surfaces.