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

The interaction of beta-casein micelles (beta-CMs) with imidazolium based ionic liquid (IL) surfactant ([C(12)mim]Br) has been studied using turbidity, isothermal titration microcalorimetry, fluorescence spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. Below c(1), the individual [C(12)mim]Br monomers bind onto the beta-CM shell close to the hydrophobic core to form a beta-CM-[C(12)mim]Br (monomer) complex. The hydrophobic tail of [C(12)mim]Br leads to a significant decrease in the environmental polarity of beta-CM. Just over c(1), [C(12)mim]Br molecules aggregate into micelle-like aggregates on the micellar shell. This leads to the collapse of the N-terminal of beta-casein and strengthens the hydrophobicity of the protein molecules, resulting in a more compact structure of beta-CM. With a continuous increase in[C(12)mim]Br concentration, beta-CMs associate with each other into a network-like structure. Beyond c(3), 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 beta-CM-[C(12)mim]Br complexes. All the beta-casein molecules are saturated by [C(12)mim]Br aggregates above c(s), and free [C(12)mim]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 beta-CM-[C(12)mim]Br system, the hydrophobic interaction between the hydrophobic tail of ILs and the hydrophobic domain of beta-CM, the electrostatic attraction between [C(12)mim](+) and negative charged amino acid residues on beta-CM shell, and the hydrogen bonding between [C(12)mim](+) and carboxylic moiety on beta-CM shell are the main forces for [C(12)mim]Br binding to beta-CM. Modifications in the physicochemical properties of beta-CM upon the addition of [C(12)mim]Br will expand and enhance the overall capabilities and applications of beta-CM.