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 c_s, 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.