Asphaltenes Adsorption onto Metal Oxide Nanoparticles: A Critical Evaluation of Measurement Techniques

The adsorption of asphaltenes onto nanoparticles (NPs) has received a lot of attention in recent years. However, the effect of the measurement technique on the adsorption isotherms has never been addressed. In this paper, the adsorption of <i>n</i>-heptane-precipitated asphaltenes, C7-asphaltenes, from toluene model solutions onto three metal oxide NPs, namely Fe₂O₃, Fe₃O₄, and Al₂O₃, was studied. Asphaltenes uptake calculated from UV–vis spectroscopy at three different wavelengths were compared with thermogravimetric analysis (TGA) results. Although the adsorption trends followed Langmuir isotherms, instrument as well as wavelength-dependent coefficients were obtained. We believe TGA results are more reliable, provided complete oxidation and account of mass loss due to NPs is attained. UV–vis measurements may be impacted by the chemical structure of the asphaltenes sub fractions as well as their state of association. Al₂O₃ showed the highest adsorption capacity of 385 ± 5 mg/g, followed by Fe₃O₄ and Fe₂O₃. However, based on mg/m², Fe₂O₃ displayed the highest adsorption capacity. TGA analysis revealed that the NPs promoted the oxidation of adsorbed asphaltenes in a reverse order to their adsorption capacity, <i>q</i>_max (mg/g) (Al₂O₃ > Fe₂O₃ ≈ Fe₃O₄). This trend is in line with our previous observation of mass-dependent thermo-oxidative profile and surface exposure role, rather than a catalyst role, of the NPs. Lastly, the C7-asphaltenes from this study were characterized, and their structural parameters were compared to 45 asphaltenes from the literature. The size and structural parameters of the asphaltenes clusters are in good agreement with the literature values