Effect of oil hydrophobicity on adsorption and rheology of protein adsorption layers at oil-water interfaces

Abstract

The rheological behavior of protein layers at water-oil interfaces is critical to understand and predict emulsion formation and stability. Effects of ionic strength, pH, temperature and several pre-treatments are well-known. However, the oil phase has been regarded as exchangeable and its role in the protein adsorption mechanisms has been widely ignored. By measuring adsorption kinetic, dilatational and interfacial shear moduli, the influence of systematically selected oil interfaces of high purity on the formation and properties of beta-lactoglobulin layers was evaluated. A delayed protein solution injection enabled the measurement of the viscoelastic response of protein layer formation at an uncontaminated water-oil interface. It was found that depending on molecular size, flexibility, hydrophobicity, and polarizability of the oils, globular proteins adsorb distinctively. Stronger interactions of polar oils with the hydrophilic exterior of the native beta-lactoglobulin lead to a decelerated protein unfolding. This results in lower surface pressures and slower formation of viscoelastic networks. In addition, polar oils interact stronger with the protein network by hydrophilic bonding and thereby act as softening agents. The found effects of hydrophobic subphases on the adsorbed protein layers provide knowledge, which promotes higher reproducibility in rheological studies and precise tailoring of interfacial films for an enhanced formation and stability of emulsions.