Study On The Phase Behavior Of Lower Critical Solution Temperature Ionic Liquids In Water

The phase behavior of temperature-sensitive ionic liquids（ILs）could be easily achieved by adjusting the temperature.This property has a wide variety of applications,such as dynamic control of dissolution,reaction,extraction,and separation.In general,there are two types of temperature-driven phase separation in ILs–water binary systems:lower critical solution temperature（LCST）phase transition and upper critical solution temperature（UCST）phase separation.Most of the temperature responsive ILs displays UCST-type phase separation in water,while the LCST phase behavior was rarely found in mixtures of ILs with water.Up to date,the LCST-type ILs was only found by trial and error,and the rational design of ILs with LCST phase behavior is still a great challenge.Thus,it is necessary to study the mechanism of LCST phase behavior of ILs in water.In this work,the mechanism of the LCST-type phase separation of phosphonium-based ILs in water was investigated and some new types of mixed ILs that could show LCST phase transformation behavior with water were designed.The main research content included the following aspects.1.First,tetrabutylphosphonium trifluoroacetate[P₄₄₄₄][CF₃COO],tetrabutylphosphonium acetate[P₄₄₄₄][CH₃COO]and tetrabutylphosphonium hexafluorophosphate[P₄₄₄₄][PF₆]were chosen to investigate the mechanism of the LCST-type phase separation of phosphonium ILs in water.By analyzing the interaction energy,the radial distribution function,and spatial distribution function and the number of hydrogen bonds changes of the ILs-water binary system,it was found that the hydrogen bond between anion and water is the main driving force for LCST-type phase behavior.Then,a series of ILs-water binary systems with different water contents were investigated.It was noted that the trend of interaction energy change of per unit volume（△Einter/V）of anion-water for ILs-H₂O system with different water content could well describe the trend of lowest critical solution temperature for ILs-H₂O systems with different composition.2.The volume and interaction energy of ionic liquid combined with different anions and cations were calculated by using quantum chemistry method.Then,the range of was obtained based on the calculated data of the known LCST-type ILs.According to this range of the binding energy density,a series of mixed ionic liquids combinations which could show LCST phase behavior in water were predicted.3.The above-mentioned ternary system was experimentally verified.Ion exchange method was used to synthesize these predicted ILs.Then,the phase diagram of ILs in water was obtained through determining of the phase separation temperature and water content of the mixed ILs-water ternary system.From the above result,it could be verified whether the predicted mixed ILs-water systems could show LCST phase behavior.4.Molecular dynamics simulation was used to study the mechanism of ILs-water ternary system with LCST behavior.By analyzing the trajectory files and interaction energies and radial distribution function of the mixed ILs-water ternary system,it was found that the interaction between[P_4,4,4,14]⁺with water molecules and the interaction of[NO₃]^-with water molecules were decreased with the temperature increased,resulting in the aggregation between[P_4,4,4,14]⁺and[NO₃]^-to form phase separation.