As amphiphiles, proteins adsorb to hydrophobic air-liquid interfaces, often forming a viscoelastic film. This surface viscoelasticity is of interest in many fields of science and technology, such as foamability and coalescence in food and emulsion science as well as stability in biopharmaceutical formulations. For example, the long-term stability of monoclonal antibody (mAb) formulations correlates with surface aggregation and interfacial shear elasticity (J. Phys. Chem. B 2023). To investigate this viscoelastic interface, a new interfacial rheometer is used to probe dilatational and shear rheology of an adsorbed mAb interfacial film as a function of coverage, including “jammed” interfacial states that have not been explored in previous studies. This interfacial film is primarily elastic (solid-like) and the calculated 2D Poisson ratios decrease from 0.9 – 0.4 with increasing compression. X-ray reflectivity (XRR) and Brewster Angle Microscopy (BAM) measurements resolve the out-of-plane and in-plane structure of the film respectively. BAM images confirm the homogeneity of the film upon compression while XRR reveals a thin, high protein concentration region at the air interface and enables accurate determination of the true surface excess. Thus, the scaling of interfacial rheology with the actual measured surface coverage is determined. Using naïve Mode-coupling theory a cage-localization length between 2.5 - 4.0 Å is calculated, further confirming the hypothesis that the high interfacial elasticity is due to beta-sheet structure forming from localized, partial unfolding of mAb at the interface. These results provide a fundamental understanding of the structure – rheological property of the adsorbed mAb interfacial layer with scientific importance and technological application.
