Dynamic phase change and local structures in IL-containing mixtures: classical MD simulations and experiments

Abstract

The dynamic phase change between a homogeneous mixture and a liquid–liquid biphase and separation of phases are explored in three-component mixtures composed of 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), and water through a classical simulation method and experiment. Different experimental and theoretical tools, including density measurement, dynamic light scattering study, radial distribution, mean square displacement, an interstice model, and statistical function, are used to describe the structural modifications of the ions as a function of solution concentration. An analysis of the relation between the phase and the state of the component ions indicates that the phase separation pattern is governed by the hydrophilicity/hydrophobicity of anions. We proposed the existence of a critical point, that is, 1 : 3 : 8 for [Bmim][PF₆]/[Bmim][BF₄]/H₂O (mole fraction). Before this critical point, obvious phase separation was seen in the mixtures. The separation phase became homogeneous with the addition of [Bmim][BF₄] after this critical point. However, this homogeneous mixed solution was phase separated again upon the addition of [Bmim][PF₆] or water. The existing nanostructures were present in the [Bmim][PF₆]/[Bmim][BF₄]/H₂O mixtures, and their size abruptly decreased close to the critical point. We provided evidence of the formation of double salt ionic liquids of [Bmim][PF₆]_0.25[BF₄]_0.75·2H₂O and discussed the interactions involved in these systems by examining their physicochemical properties. The ionic phase response of such three-component mixtures could be useful in various applications, especially in the dynamic control of extraction/separation processing.