Comparative studies on the interaction of [C4mim]Br, and [C8mim]Br with β-casein micelles

The comparative studies on the interactions of β-casein micelles (β-CMs) with imidazolium-based ionic liquids (ILs) containing different hydrophobicity (1-octyl-3-methylimidazolium bromide [C8mim]Br and 1-butyl-3-methylimidazolium bromide [C4mim]Br) have been carried out using turbidity, dynamic light scattering (DLS), transmission electron microscopy (TEM), conductivity, steady-state fluorescence and isothermal titration microcalorimetry (ITC) measurements. The structure and microenvironmental polarity of β-CM can be controlled by the addition of ILs. The controlling effect depends much on the alkyl chain that attached to the imidazolium ring and the polarity of the IL. The partitioning of IL molecules in different locations of β-CM depends on the concentration and chain characteristics of ILs. In addition to the electrostatic attraction, the binding of the cationic [Cnmim]+ headgroup to β-CM should be dominated by the hydrophobic interaction among the alkyl tails of the IL and the hydrophobic domain of β-CM. Also, C2-H of [Cnmim]+ could easily participate in hydrogen bonding interaction with the carboxylic moiety of protein molecules. The complementary results obtained from the multitechnique approaches proved useful in drawing out the mechanism of interaction between amphiphilic protein aggregates and ionic liquid with different chain architecture in different IL concentration regimes, which will in turn facilitate the use of protein–IL systems in foods, pharmaceuticals, biotechnology and related industries.