Shear rheology of hydrophobin adsorption layers at oil/water interfaces and data interpretation in terms of a viscoelastic thixotropic model

Abstract

Here, we investigate the surface shear rheology of class II HFBII hydrophobin layers at the oil/water interface. Experiments in two different dynamic regimes, at a fixed rate of strain and oscillations, have been carried out with a rotational rheometer. The rheological data obtained in both regimes comply with the same viscoelastic thixotropic model, which is used to determine the surface shear elasticity and viscosity, E_sh and η_sh. Their values for HFBII at oil/water interfaces are somewhat lower than those at the air/water interface. Moreover, E_sh and η_sh depend on the nature of oil, being smaller for hexadecane in comparison with soybean-oil. It is remarkable that E_sh is independent of the rate of strain in the whole investigated range of shear rates. For oil/water interfaces, E_sh and η_sh determined for HFBII layers are considerably greater than for other proteins, like lysozyme and β-casein. It is confirmed that the hydrophobin forms the most rigid surface layers among all investigated proteins not only for the air/water, but also for the oil/water interface. The wide applicability of the used viscoelastic thixotropic model is confirmed by analyzing data for adsorption layers at oil/water interfaces from lysozyme and β-casein – both native and cross-linked by enzyme, as well as for films from asphaltene. This model turns out to be a versatile tool for determining the surface shear elasticity and viscosity, E_sh and η_sh, from experimental data for the surface storage and loss moduli, G′ and G′′.