(347j) Rheology of Globular Protein Solutions: Apparent Bulk Yield Stress and Interfacial Viscoelasticity of Bovine Serum Albumin Solutions

Globular proteins influence the dynamics, phase behavior and transport of biomolecules and drugs in the mammalian body. These proteins are essential constituents of food, drugs and cosmetics, and their dynamics determine the physical properties and application of these multicomponent materials. In conventional rheological studies conducted using typical geometries on torsional rheometers, solutions of globular proteins are commonly reported to have a solid-like response at concentrations as low as 0.03% by weight. Typical explanations invoke the presence of long-range repulsions that are stronger than electrostatic interactions. In this study, we probe the bulk and the interfacial viscoelasticity of surfactant-free bovine serum albumin (BSA) solutions using a stress-controlled torsional rheometer, augmented by microfluidic rheometry and interfacial rheometric measurements. We demonstrate that the origin of this yield-like behavior, which is manifested as a highly shear-thinning bulk rheological response, lies in the formation of a film of adsorbed protein, formed spontaneously at the solution/gas interface. We provide direct interfacial rheometric measurements to study the concentration-dependent viscoelasticity of the adsorbed protein and we describe a simple, but quantitative, additive model useful for extracting the interfacial viscosity contribution from bulk viscosity measurements over a wide range of shear rates. Finally, we use these techniques to examine the systematic changes in the interfacial and bulk rheology for mixtures of a common non-ionic surfactant, Tween 80, and BSA. The apparent bulk viscosity, interfacial shear viscosity as well as the interfacial viscoelastic moduli are all noticeably suppressed by the presence of even a relatively small amount of surfactant (>10^-3 wt. %). Preferential interfacial adsorption of the mobile surfactant results in complete elimination of the bulk apparent yield stress exhibited by surfactant-free BSA solutions.