Understanding interactions between immunoassay excipient proteins and surfactants at air–aqueous interface

•Interfacial shear rheology of excipient proteins was studied using Du Noüy ring.•Globular BSA formed stronger, and disordered caseinate formed weaker networks.•Concentration dependent displacement of proteins by nonionic surfactants was observed.•Surface tension characterization complimented the shear rheology observations, yet provided distinct information about protein–surfactant interactions. Air–aqueous interfacial properties of four excipient proteins commonly used in immunoassay reagent formulations were studied with shear rheology and surface characterization methods. A Du Noüy ring geometry was utilized to quantify the elastic (G′) and viscous (G″) shear moduli of protein interfacial networks and to probe the effect of several nonionic surfactants at various concentrations. Time sweep protocols of buffered protein solutions yielded G′ in the range of 16mN/m for bovine serum albumin (BSA), 6mN/m for bovine gamma globulin (BGG), 7mN/m for Mouse IgG, and 0.9mN/m for sodium caseinate. G′s were higher than G″s for a given protein. Effect of nonionic surfactants on G′ of a protein was concentration dependent and the magnitude of protein displacement from the interface varied with Tween 20>Triton X-100>Triton X-405, with the exception of Mouse IgG. Degree of displacement of BSA from the interface by Tween 20 was approximately 66-fold greater than that of BGG whose displacement by Tween 20 was approximately 7-fold greater than that of Mouse IgG. Degree of displacement by Triton X-100 was comparable in case of studied proteins. Surface tension characterization suggests that the interfacial interactions between proteins and surfactants are driven not only by their surface activity but also by the network formation abilities of the proteins. Data presented here demonstrates a potential application of interfacial studies to sensitively identify discriminatory interactions between proteins and surfactants in immunoassay solutions.