Two-Step Adsorption of Endogenous Asphaltenic Surfactants at the Bitumen-Water Interface

We have performed hanging droplet measurements of a straight-run bitumen (SRB) and its corresponding deasphalted oil (DAO) to investigate the adsorption process at the bitumen water interface (pH 2.7, T = 90 degrees C). The analysis of the time and surface pressure dependences of the viscoelastic properties show that two types of endogenous surfactants (called A and B) compete for the same interface in the case of SRB. The A species adsorb first, and their molecular characteristics (size and bulk concentration) can be obtained by fitting the data with a diffusion-limited adsorption model. They form fluid-like interfacial layers. The B species, that diffuse more slowly toward the interface, eventually replace the A species to form highly viscous and hardly compressible interfacial layers. The B species belong to the asphaltene solubility class because they are removed by the n-heptane fractionation. They are not present in DAO, which only contains the A species. We surmise that those are also asphaltenes but of higher aliphaticity and/or lower state of aggregation. The possibility that they are resins cannot, however, be eliminated at this stage.