Liquid-Liquid Transition in Ionic Liquids

The properties of liquids have been linked to the existence of the liquid-liquid transition (LLT), a first-order thermodynamic transition from one liquid phase to another in a single- component liquid. LLT is fundamental to the understanding of the liquid state and has been theorized to manifest from a two-state feature of local order in the liquid. LLT has been reported in a variety of liquids with computer simulations comprising the bulk of the evidence. Experimental evidence for LLT remains controversial because it frequently manifests in the supercooled state, obscured by crystallization. In this dissertation, evidence is presented revealing LLT in ionic liquids (ILs) based on the trihexyltetradecylphosphonium (P666,14) cation. These studies consist of ILs containing the P666,14 cation with a series of anions: borohydride (BH4), propionate (Prop), nonafluoropentanoate (NFP), nonafluorobutanesulfonate (NFBS), bis(trifluoromethylsulfonyl)imide (TFSI), and camphor sulfonate (CS). These ILs are glass-formers, allowing unambiguous interpretation of the results in the supercooled state. The BH4, Prop, NFP, NFBS, and TFSI samples display evidence of LLT, while the CS sample shows a continuous transformation without LLT, suggesting the anions significantly influence the phase behavior of P666,14-based ILs. A suite of complimentary methods was employed to investigate structure, dynamics, thermodynamics, and macroscopic properties spanning a wide temperature range. Using wide-angle X-ray scattering (WAXS), the local structures and ion coordination numbers were deduced, showing abrupt changes at the LLT. Via broadband dielectric spectroscopy (BDS), an increase in the static dielectric permittivity, εs, was observed at the LLT, arising from local ion reorganization. A dynamic crossover was revealed at the LLT with an increase in fragility. Differential scanning calorimetry (DSC) revealed a peak in heat capacity, Cp(T), corresponding to LLT. Comparison of the dynamic crossover with configurational entropy, Sc(T), revealed the two are governed by the same underlying two-state feature. Experimental access to the evolution of local order, dynamics, thermodynamics, and macroscopic properties across the LLT opens up unprecedented pathways to understanding the liquid state. It is argued that a two-state description of liquids is generally applicable, generating new directions for the design of neoteric liquids with bespoke properties for a host of critical applications.

Portions of this document were previously published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).