Designing new LCST ionic liquids

Abstract

This research addresses existing gaps in the understanding of the Lower Critical Solution Temperature (LCST) phenomenon in ionic liquid-water mixtures, with particular emphasis on developing a systematic, predictive approach to investigate this thermoresponsive behaviour. Chapter 1 introduces the LCST concept, reviews limitations in current mechanistic understanding, and highlights the profound potential of LCST ILs for energy-efficient processes. Specifically, these ILs offer a sustainable solution as draw fluids in forward osmosis-based water desalination, providing clean water from saline sources with reduced energy consumption.To achieve this, a series of novel ILs incorporating the 5-phenyl tetrazole moiety as the anion precursor, together with mono- and di-cationic ammonium and phosphonium cations, were designed, synthesised, and characterised. Chapter 2 details the synthesis of 51 compounds, developing a unified family of ILs through systematic modification of both cationic and anionic components. The spectroscopic and thermophysical characterisation is provided.The phase behaviour of the synthesized ILs with water is studied in Chapter 4, focusing on how structural modifications influence LCST Tc. The importance of examining the full phase diagram is emphasised, leading to the discovery, detailed in Chapter 5, of a novel anomaly: the formation of unexpected co-crystals, resulting in discontinuous LCST phase diagrams. This chapter presents the first recorded description of these anomalous transitions, which is critical for understanding unique ion-water interactions at low temperatures. Chapter 6 adopts a methodical, multi-scale approach to investigate the various interactions governing the LCST mechanism. Techniques including variable-temperature (VT-NMR), isothermal titration calorimetry (ITC), and neutron scattering are employed to provide comprehensive insight into ion-water associations, thermodynamic aspects, and atomic-level liquid structure. Chapter 7 concludes the research, summarising the definitive findings—including the establishment of a predictive design tool and the quantitative, entropy-driven mechanism and outlines potential directions for future studies. Chapter 8 compiles all essential supporting data.

Thesis is embargoed until 31 December 2027.

Date of Award	Dec 2025
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Tezpur University
Supervisor	John Holbrey (Supervisor), Leila Moura (Supervisor) & Ruli Borah (Supervisor)