Impact of ionic liquid-type imidazolium surfactant additions on dynamic properties of β-casein adsorption layer

In this article, by means of interfacial dilational rheology and interfacial pressure methods, we have investigated the interfacial properties of adsorbed films at water/decane interface, formed by β-casein and a series of ionic liquid-type imidazolium surfactant [Cnmim]Br (n = 12, 14, 16). The results obtained demonstrate that the addition of ionic liquid-type imidazolium surfactant [Cnmim]Br strongly influenced the dynamic interfacial properties of β-casein solutions due to the β-casein/[Cnmim]Br complex formation. The β-casein/[Cnmim]Br complex has been formed by different charged ions of protein and surfactant, so there is competitive adsorption between the complex and surfactant. In the beginning, the surfactant molecules can adsorb to the interfacial freely according to their higher diffusion coefficient, and the β-casein/[Cnmim]Br complex was not influenced as well. As the interfacial pressure increasing, the interfacial concentration of the complex increases gradually and interactively. At higher surfactant concentrations, the complexes were formed owing to the hydrophobic interactions between the component and the complexes structure have changed. Consequently, the adsorption layer structure and the dynamic elasticity properties also changed. Because of the hydrophobic interactions, the complex became relatively hydrophilic and unstable, the ratio of the free surfactant molecules to the complexes also increased. Moreover, due to the stronger attraction with aromatic rings through π–π interaction resulting from the existence of imidazolium head group, the elasticity of β-casein/[Cnmim]Br system are much higher than that of the traditional surfactant, indicating that the adsorption layer formed by β-casein/[Cnmim]Br are much stronger.