Pressure-induced liquid-liquid transition in a family of ionic materials

Abstract

Liquid−liquid transition (LLT) between two disordered phases of single-component material remains one of the most intriguing physical phenomena. Here, we report a first-order LLT in a series of ionic liquids containing trihexyl(tetradecyl)phosphonium cation [P666,14]+ and anions of different sizes and shapes, providing an insight into the structure-property relationships governing LLT. In addition to calorimetric proof of LLT, we report that ion dynamics exhibit anomalous behavior during the LLT, i.e., the conductivity relaxation times (τσ) are dramatically elongated, and their distribution becomes broader. This peculiar behavior is induced by isobaric cooling and isothermal compression, with the τσ(TLL,PLL) constant for a given system. The latter observation proves that LLT, in analogy to liquid-glass transition, has an isochronal character. Finally, the magnitude of discontinuity in a specific volume at LLT was estimated using the Clausius-Clapeyron equation.

Pretransitional of behavior of electrooptic Kerr effect in liquid thymol

“…( 3 ). It allows posing the hypothesis for the liquid–liquid (L–L) transition [ 38 – 40 ] in liquid thymol at . L–L transition is a challenging phenomenon for which the direct experimental evidence is minimal [ 38 – 40 and refs therein].…”

“…It allows posing the hypothesis for the liquid–liquid (L–L) transition [ 38 – 40 ] in liquid thymol at . L–L transition is a challenging phenomenon for which the direct experimental evidence is minimal [ 38 – 40 and refs therein]. The vast majority of results is related to indirect indications of L–L transitions hidden in hardly accessible experimental domains.…”

“…The vast majority of results is related to indirect indications of L–L transitions hidden in hardly accessible experimental domains. Only recently, L–L was evidenced in a supercooled liquid just above the glass transition [ 40 ]. In the given report, the possible L–L transitions strongly manifests in only slightly supercooled thymol, in the easily accessible domain ~ 10 K above the freezing temperature ( ).…”

Effect of bulky anions on the liquid-liquid phase transition in phosphonium ionic liquids: Ambient and high-pressure dielectric studies

“…Namely, isothermal compression triggers the LLT, with the τ σ at LLT being independent of temperature and pressure conditions. The time scale of ion dynamics at LLT was found anion-specific, i.e., dependent on anion size, geometry, conformational flexibility, and the strength of interionic interactions 15 . Furthermore, it was postulated that large anions most likely complicate the ordering of the alkyl chains in the non-polar domains and consequently prevent the LLT 15 .…”

“…Such a well-defined physical picture of ion dynamics of P-RTILs has been upended by recent studies of trihexyl(tetradecyl)phosphonium [P 666,14 ] + based ionic liquids. In particular, it has been shown that at a certain temperature, dc-conductivity and conductivity relaxation times τ σ = ε 0 ε s σ dc −1 , reveal a substantial departure from VFT law accompanied by an abrupt increase in apparent activation energy 15 , 16 (note that during the liquid-glass transition, an apparent activation energy decreases 17 ). This peculiar behavior, reported for five different [P 666,14 ] + -based ILs, has been recognized as the dielectric signature of the first-order liquid–liquid phase transition (LLT).…”