Shear and Dilatational Rheology and Interfacial Structure of a Monoclonal Antibody Adsorbed at the Air–Liquid Interface

As amphiphiles, proteins adsorb at 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. To investigate this viscoelastic interface, a new interfacial rheometer is used to probe the 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 ν_2D decrease from 0.9 to 0.4 with increasing compression. X-ray reflectivity (XRR) resolves the out-of-plane structure, and Brewster angle microscopy (BAM) measurements provide in-plane structural information on the film. 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, <i>measured</i> surface coverage is determined. Using the Naïve mode-coupling theory, a cage-localization length <i>r</i>_loc between 2.5 and 4.0 Å is calculated, further confirming the hypothesis that the high interfacial elasticity is due to the 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.