Combined effects of ultrasound and [bmim][cl] on heavy oil-solid separation: experiments and molecular dynamics...

Efficient recovery of heavy oil from oil-bearing sands remains constrained by strong oil-solid adhesion, unfavorable interfacial conditions, and slow mass transfer in conventional solvent extraction. In this work, a solvent-ionic liquid-ultrasound intensified route was developed using [Bmim][Cl] as the key additive and toluene/p-xylene/m-xylene/o-xylene as representative solvents, with particular attention to the solvent-dependent interaction between ultrasound and [Bmim][Cl]. In the toluene system, the heavy-oil recovery increased from 90.6 wt% for single-solvent extraction to 92.5 wt% after the addition of [Bmim][Cl], corresponding to an increase of 1.9 wt%. The further introduction of ultrasound increased the recovery to 95.2 wt%, representing an additional increase of 2.7 wt% and an overall increase of 4.6 wt% relative to single-toluene extraction. Factorial interaction analysis demonstrated statistically significant synergy between ultrasound and [Bmim][Cl] in the toluene system (ΔR_syn = 2.10%, 95% CI: 0.38-3.82, p = 0.023) and a borderline statistically significant positive interaction in the p-xylene system (ΔR_syn = 2.00%, 95% CI: 0.01-3.99, p = 0.049), whereas the interaction estimates for the m-xylene and o-xylene systems were positive but not statistically significant (p = 0.198 and 0.158, respectively). The combined treatment also modified interfacial behavior: AFM measurements at the hydrophobized SiO₂ model oil-solid interface showed that the interaction force decreased as the [Bmim][Cl] concentration increased, while interfacial tension decreased with increasing [Bmim][Cl] concentration under both sonicated and non-sonicated conditions. Because this model does not reproduce the mineralogical composition, surface morphology and roughness, or wetting heterogeneity of natural oil sands, the AFM force trend is used for controlled comparison and is not interpreted as a direct quantitative adhesion measurement on natural oil-sand surfaces. Wettability was improved in parallel, with the minimum water contact angle reduced to 24.7° under the combined treatment, consistent with a more water-wet post-treatment surface. Analysis of the properties and composition of the recovered oil showed that the highest C/H ratio of the recovered oil obtained under the combined toluene treatment was 9.08, which was slightly higher than that obtained in the single-toluene extraction system (8.73). Meanwhile, the SARA analysis showed that the resin and asphaltene mass fractions increased slightly from 27.96 and 21.93 wt% in the single-toluene extraction system to 28.56 and 22.39 wt% under the combined toluene treatment, respectively, with similar trends observed in the xylene systems. Given the small magnitude of these differences, this study uses the changes in the C/H ratio and SARA composition as auxiliary evidence for the trend of heavy-component release. Kinetic analysis indicated extraction acceleration, for instance, reducing the toluene equilibration time from approximately 20 min to approximately 10 min after [Bmim][Cl] addition. Molecular dynamics simulations further provide qualitative molecular-scale support for interpreting the experimental trends. In the simplified SARA representative molecular model, [Bmim][Cl] exhibits more pronounced local association with resins and asphaltenes; these short-range correlations do not by themselves establish specific interaction energies. The MD results are mainly used to compare the differences in local associations between different SARA representative molecules and [Bmim][Cl] and to serve as qualitative molecular-scale evidence for explaining the experimental trends, rather than as quantitative simulations of real asphaltene aggregation/disaggregation behavior. Within the simplified model, the polycyclic and polar features of the asphaltene-like molecule were associated with more pronounced local contact with [Bmim][Cl], suggesting a possible influence on the surrounding microenvironment. Overall, the results are consistent with ultrasound and [Bmim][Cl] playing complementary roles in reducing interfacial restrictions and accelerating mass transfer, realizing faster and more efficient heavy-oil release and providing a practical foundation for designing intensified, potentially lower-impact extraction processes.