Understanding the mechanism of LCST phase separation of mixed ionic liquids in water by MD simulations

Abstract

Recently, it has been found experimentally that two different amino acid ionic liquids (ILs) can be mixed to show unique lowest critical solution temperature (LCST) phase separation in water. However, little is known about the mechanism of phase separation in these IL/water mixtures at the molecular level. In this work, five kinds of amino acid ILs were chosen to study the mechanism of LCST-type phase separation by molecular dynamics (MD) simulations. Toward this end, a series of all-atom MD simulations were carried out on the ternary mixtures consisting of two different ILs and water at different temperatures. The various interaction energies and radial distribution functions (RDFs) were calculated and analyzed for these mixed systems. It was found that for amino acid ILs, the –NH₂ or –COOH group of one anion could have a hydrogen bonding interaction with the –COO⁻ group of another anion. With the increase of temperature, this kind of hydrogen bonding interaction between anions was strengthened and then the anion–H₂O electrostatic interaction was weakened, which led to the LCST-type phase separation of the mixed ILs in water. In addition, a series of MD simulations for [P₆₆₆₈]₁[Lys]_n[Asp]_1−n/H₂O systems were also performed to study the effect of the mixing ratio of ILs on phase separation. It was also noted that the experimental critical composition corresponding to the lowest critical solution temperature was well predicted from the total electrostatic interaction energies as a function of mole fraction of [P₆₆₆₈][Lys] in these systems. The conclusions drawn from this study may provide new insight into the LCST-type phase behavior of ILs in water, and motivate further studies on practical applications.