Rheology of Globular Protein Solutions: Apparent Yield Stress and Interfacial Viscoelasticity of Bovine Serum Albumin Solutions

摘要

Surface-active molecules like low molecular weight surfactants, proteins and polymeric surfactants that adsorb to liquid/fluid interfaces, effectively reduce the interfacial tension, and provide the interface with dynamic properties like elasticity and viscosity . The dynamic interfacial properties or the shear rheology of the adsorbed interfacial layers are important for various technological applications like foam and emulsion formation and stabilization in food processing, cosmetics and pharmaceutical industry . Globular proteins like serum albumins influence the dynamics, phase behavior and transport of biomolecules and drugs in the mammalian body . Interestingly, when globular protein solutions are tested on a torsional rheometer, the solutions exhibit a yield-like behavior for a relatively wide range of concentrations (0.03-10 wt%) . with the apparent viscosity dropping by several orders of magnitude as the imposed shear rate is raised from 0.01 to 100 s-1 . It is argued that the presence of a yield stress is a consequence of strong long-range repulsive forces that are present even at concentrations as low as 0.03 wt % (or 3 mg/ml), and the concentration of added salts is reported to have little or no effect on the observed response . This has been attributed in the past to the presence of long-ranging weak colloidal forces that are stronger than electrostatic attractions and which lead to an ordered "bulk colloidal structure" in the solutions , though no structure is detected in scattering studies. In this study, we use Bovine Serum Albumin as a model globular protein and we demonstrate that this apparent bulk yield stress and shear thinning arises from the presence of a viscoelastic layer formed due to protein adsorption at the air-water interface. We provide direct interfacial rheometric measurements to study the 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.