Interaction between β-casein micelles and imidazolium-based ionic liquid surfactant

Abstract

The interaction of β-casein micelles (β-CMs) with imidazolium based ionic liquid (IL) surfactant ([C12mim]Br) has been studied using turbidity, isothermal titration microcalorimetry, fluorescence spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. Below c1, the individual [C12mim]Br monomers bind onto the β-CM shell close to the hydrophobic core to form a β-CM–[C12mim]Br (monomer) complex. The hydrophobic tail of [C12mim]Br leads to a significant decrease in the environmental polarity of β-CM. Just over c1, [C12mim]Br molecules aggregate into micelle-like aggregates on the micellar shell. This leads to the collapse of the N-terminal of β-casein and strengthens the hydrophobicity of the protein molecules, resulting in a more compact structure of β-CM. With a continuous increase in[C12mim]Br concentration, β-CMs associate with each other into a network-like structure. Beyond c3, the net positive charges on the complexes, owing to the binding of more cationic surfactant molecules, lead to redissociation of the complexes, corresponding to the formation of the new nano-sized β-CM–[C12mim]Br complexes. All the β-casein molecules are saturated by [C12mim]Br aggregates above cs, and free [C12mim]Br micelle-like aggregates appear in the bulk phase above critical aggregation concentration (cac). From a combination of experimental results and discussion on various interactions in the β-CM–[C12mim]Br system, the hydrophobic interaction between the hydrophobic tail of ILs and the hydrophobic domain of β-CM, the electrostatic attraction between [C12mim]+ and negative charged amino acid residues on β-CM shell, and the hydrogen bonding between [C12mim]+ and carboxylic moiety on β-CM shell are the main forces for [C12mim]Br binding to β-CM. Modifications in the physicochemical properties of β-CM upon the addition of [C12mim]Br will expand and enhance the overall capabilities and applications of β-CM.