Insights into asphaltene aggregation in the Na-montmorillonite interlayer

• Pore water was most critical for asphaltene (ASP) aggregation and diffusion in MMT.
• Salinity had little impact on ASP aggregation but obviously affected ASP mobility.
• ASP aggregation decreased by 32% when the pore size decreased from 5 to 2.5 nm.
• ASP aggregate was sequestrated in the hollow cone-shaped cavity of HS in organoclay.
• Co-aggregation with HS decreased ASP mobility by one-order of magnitude in MMT pore.

This study aimed to provide insights into the diffusion and aggregation of asphaltenes in the Na-montmorillonite (MMT) interlayer with different water saturation, salinity, interlayer space and humic substances. The molecular configuration, density profile, diffusion coefficient and aggregation intensity were determined by molecular dynamic simulation, while the 3D topography and particle size of the aggregates were characterized by atomic force microscopy. Results indicated that the diffusivity of asphaltenes was up to 5-fold higher in the MMT interlayer filled with fresh water than with saline water (salinity: 35‰). However, salinity had little impact on the asphaltene aggregation. This study also showed a marked decrease in the mobility of asphaltenes with decrease in the pore water content and the interlayer space of MMT. This was more pronounced in the organo-MMT where the humic substances were present. The co-aggregation process resulted in the sequestration of asphaltenes in the hollow cone-shaped cavity of humic substances in the MMT interlayer, which decreased the asphaltene diffusion by up to one-order of magnitude and increased the asphaltene aggregation by about 33%. These findings have important ramifications for evaluating the fate and transport of heavy fractions of the residual oil in the contaminated soils.