Effect of Anionic Structure on the LCST Phase Behavior of Phosphonium Ionic Liquids in Water

• Published in: Industrial & engineering chemistry research 2018-09, Vol.57 (38), p.12935-12941
• Main Authors: Zhao, Yuling, Tian, Li, Pei, Yuanchao, Wang, Huiyong, Wang, Jianji
• Format: Article
• Language: English
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/acs.iecr.8b02073

To shed light on the mechanism of lowest critical solution temperature (LCST) phase behavior, the phosphonium-based ionic liquids (ILs) with different anions ([CF3COO]−, [CH3COO]−, and [PF6]−) were chosen and studied in this work by molecular dynamics (MD) simulations. In this work, a set of MD simulations on IL/H2O systems were implemented at different temperatures. It was found that, among these systems, H-bonds between [CF3COO]− and water molecules in the [P4444]­[CF3COO]/H2O system were most seriously destroyed with increasing temperature, leading to the aggregation of cations and anions and phase separation. Furthermore, the simulations for [P4444]­[CF3COO]/H2O and [P4444]­[CH3COO]/H2O systems were also performed to reveal the effect of water contents on phase separation. It was shown that each unit volume of anion–water interaction change with temperature (ΔE inter/V) can well describe the change of experimental lowest critical solution temperature in the [P4444]­[CF3COO]–H2O system, which is an important predictive factor for the phase transition behavior. These findings are useful for a deeper understanding of LCST phase behavior of IL/H2O systems and would accelerate further studies on the design and application of new LCST-type ILs.