Probing the adsorption behavior and molecular interactions of asphaltene subfractions at Water–Toluene/Heptol interface

Asphaltene-stabilized water-in-oil (W/O) emulsions present significant challenges, contributing to corrosion and fouling in the oil industry. Previous studies have suggested that only a small fraction of asphaltenes is responsible for emulsion stabilization. In this work, whole asphaltenes were fractionated into interfacially active (IAA) and non-active (INAA) subfractions to better understand their interfacial behaviors. Dynamic interfacial tension (IFT), Langmuir–Blodgett trough, atomic force microscopy (AFM) imaging, and AFM drop probe technique were employed to explore the adsorption behavior, interfacial film properties, and interaction forces of IAA and INAA asphaltenes at the water-toluene/heptol interface. The effects of asphaltene subfraction concentrations and solvent aromaticity were systematically evaluated. IAA asphaltenes manifest higher interfacial diffusion coefficients and lower equilibrium IFT at the water-toluene/heptol interface compared to INAA asphaltenes, indicating superior adsorption capabilities. IAA asphaltenes showed decreased interfacial diffusion coefficients at the water-heptol interface compared to toluene, while INAA asphaltenes displayed the opposite trend. Asphaltene subfractions formed protective layers at the water-toluene/heptol interface, sterically inhibiting water droplet coalescence. IAA asphaltenes form more rigid interfacial films than INAA asphaltenes, with the addition of n-heptane further increasing film rigidity. Quantitative force results demonstrated interfacial adhesion between IAA/INAA asphaltenes-adsorbed water droplets. The measured adhesion in toluene increased at low IAA concentrations but decreased at higher concentrations. The addition of n-heptane significantly reduced adhesion between water droplets in the presence of IAA asphaltenes. This work provides quantitative insights into the interfacial behaviors of asphaltene subfractions in both toluene and heptol, with implications for asphaltene-related interfacial phenomena in oil production.