(564b) Adsorption and Interfacial Consolidation of Amphoteric Species at Oil-Water Interfaces

The formation of stable emulsions through the adsorption of polyvalent, complex naturally occurring species is important in a variety of important industrial applications and challenges, including proteins and other macromolecules in food emulsions, and asphaltenes in petroleum and bitumen-based emulsions. The stability of these emulsions is attributed to interfacial films with viscoelastic properties that are known to vary with concentration, solvent quality, and macromolecule chemistry. Here, we explore the impact of aqueous phase chemistry, namely pH and salinity, on the transient interfacial rheological properties of asphaltenic films. With two chemically unique asphaltenes, interfacial shear rheology revealed salt-induced retardation of the interfacial consolidation process that ultimately engenders elasticity to the film. With HO asphaltenes at pH 7, a linear dependence of this retardation on the Debye parameter (κ) indicated that shielding of electrostatic attraction was responsible. Further investigation with dynamic oscillating drop tensiometry at varying pH, to probe dilatational rheological properties as well as dynamic interfacial tension, illustrated that electrostatic interactions, both attractive and repulsive, clearly influence the evolution of the interfacial structure. More specifically, the transient tension and dilatational moduli indicated that several interfacial processes were impacted by the addition of salt, including (i) interfacial activity and the extent of adsorption, (ii) interfacial rearrangement and consolidation, and (iii) interfacial transport and displacement. We believe that these observations are likely to be universal with other macromolecular systems of amphoteric character. We further elaborate on this by considering the globular proteins lysozyme and β-casein, and their adsorption and consolidation at oil-water interfaces.