Adsorption of Crude Oil on Surfaces Using Quartz Crystal Microbalance with Dissipation (QCM-D) under Flow Conditions†

Adsorption of crude oil on surfaces is successfully measured with a quartz crystal microbalance with dissipation (QCM-D) under flow conditions. The amounts and thicknesses of adsorbed films are determined with good accuracy using liquid loading corrections. Measurements are performed in solvents where the degree of asphaltene stability is high (toluene) and poor (n-alkanes and heptane/toluene mixtures). In toluene, Langmuir-type adsorption is recorded with saturation film thicknesses of 3−4 nm and limited desorption after rinsing. The size of adsorbing species is also determined at early times where adsorption is diffusion-controlled up to 3 wt % crude oil (or 835 ppm asphaltenes) in toluene. The primary asphaltene species that adsorb on the crystal surface are molecules with a diameter of 0.5−1.6 nm at 139−278 ppm asphaltenes and nanoaggregates with a diameter of 2.6−5.6 nm at 835 ppm asphaltenes in toluene. In n-alkanes (nC7, nC10, and nC12), saturation plateaus are not observed within the experimental time scale. Film thicknesses recorded after 3.5 h are all higher than those in toluene and increase with increasing n-alkane carbon number, mainly because of increasing polarity of aggregates. Atomic force microscopy (AFM) analyses reveal that the size of adsorbed aggregates decreases with n-alkane carbon number. Aging effects show that, with time, the adsorbed films become more rigid in toluene and more viscoelastic in n-alkanes. In heptane/toluene mixtures, a significant increase in the dissipation factor is observed close to the flocculation threshold. Adsorption on various surfaces from toluene shows a high affinity of asphaltenes to hydrophilic surfaces. On the other hand, asphaltenes are almost amorphous in n-alkanes. X-ray photoelectron spectroscopy (XPS) analyses of the adsorbed films confirm these observations.