(713f) Asphaltenes Adsorption at Water/Oil Interface, a Classical Surfactant Approach

Time consuming gelation of asphaltenes covered interfaces has persistently been called to explain both slow evolution of interfacial tension and crude oil emulsion stability, even if the latter can be obtained after short mixing times. To resolve this inconsistency (and some others), asphaltenes adsorption has been re-investigated from scratch. Irrespective to adsorption time, asphaltenes appear to behave as surfactants insofar they follow an equation of state which can be approximated by a Langmuir model. The associated molecular area corresponds to the average asphaltenes polyaromatic core parallel to the water surface, a conformation in line with previously published Sum Frequency Generation spectroscopy experiments and with recent quantum based Self Consistent Charge Density Functional Tight Binding simulations. The coupling of the Langmuir model with a distribution in adsorption coefficient among the asphaltenes solubility class further enables matching quantitatively both dynamic interfacial tension and dilatational rheology measurements with diffusional models. The slow evolution of interfacial properties between water and asphaltenes solution then appears to be governed by the diffusion of a tiny sub-fraction of extreme surface activity. The Langmuir adsorption model is however a gross approximation insofar it assumes surfactants to be smaller than adsorption sites (here water molecules). This can be improved with a Lattice Gas model of hexagons covering 3 triangular adsorption sites (a choice dictated the size and shape of asphaltenes aromatic cores). This model not only matches the experimental equation of state extremely well but it also predicts a fluid to solid transition at the very surface pressure value for which a contracted pendant droplet loses its Laplacian shape. Around phase transition the lattice gas model also exhibits some dynamic frustration that would explain the observed Soft Glass (shear) rheology features of asphaltenes laden interfaces. In turn, the reported onset of emulsion stability at a critical mass coverage can be interpreted in terms of jamming of the interface upon partial coalescence. This would resolve the discrepancy between fast emulsion stabilization and slow evolution of interfacial properties in quiescent experimental setups. Finally the observed transition to multilayer adsorption at high bulk asphaltenes concentration seems to be fairly well captured by a BET model using the same molecular area as the previously mentioned Langmuir and Lattice gas models. The corresponding ratio in adsorption energies between first and subsequent layers is fairly large. This would be consistent with a first monolayer adsorption through strong pi interactions with water and additional layers adsorption through weak van der Waals interactions with alkyl dangling chains.