Abstract:

Adsorption of proteins and asphaltenes at the oil/water interface is known to produce viscoelastic networks. These interfacial networks slowly evolve in time and result from desorption barriers and conformational changes of the adsorbing molecules. We utilize interfacial shear and dilatational rheology to probe noninvasively proteins and asphaltenes adsorbed at the oil/water interface. We find that the interfacial moduli are more revealing of structural changes within the interfacial region than the surface pressure. Additionally, we perform washout experiments to investigate the contribution of reversible surface-active molecules to the interfacial dilatational moduli.

To gain further understanding of interfacial transport processes and rheology, we compare the interfacial rheological response in dilatational and shear deformations for a globular protein, lysozyme, and a flexible protein, ?-casein. We find that the interfacial dilatational storage modulus is comprised of a static and dynamic response. The static response corresponds to a change in the surface pressure upon interfacial-area change for the irreversibly adsorbed species. The dynamic contribution corresponds to rearrangement and reconfiguration of the protein molecules within the interface, analogous to the shear storage response. However, we find that upon washout, the dynamic response in dilatational deformation disappears for ?-casein in contrast to lysozyme. Therefore, shear and dilatational moduli are comparable only for insoluble monolayers.

Similar to proteins, surface-active crude oil components form interfacial gels. We show that wettability alteration on a mica surface is linked to the growth of the interfacial dilatational moduli at the oil/water boundary. Thus, this work emphasizes the role that these interfacial microstructures have on establishing the mixed-wet state in oil reservoirs. To understand the nature of these crude-oil interfacial films, we measure the interfacial rheological response of a model crude oil. We find that the frequency response of the dilatational moduli fit a combination of diffusion dissipation and in-plane surface relaxation models. We verify the model by washing out the asphaltenes from the organic phase and observing that the diffusion component of the frequency response disappears. The relaxation time of the interface increases significantly after washout suggesting that the reversibly adsorbed species prevent formation of an asphaltene gel phase at the interface.