Solvation dynamics and electric field relaxation in an imidazolium-PF 6 ionic liquid: From room temperature to the glass transition

Naoki Ito, Ranko Richert

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76 Citations (Scopus)

Abstract

Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of η/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature Tg. The dielectric modulus relaxation follows this trend only for temperatures T > 1.2Tg and departs significantly from η/T in the 1.1Tg > T > Tg range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2Tg.

Original languageEnglish (US)
Pages (from-to)5016-5022
Number of pages7
JournalJournal of Physical Chemistry B
Volume111
Issue number18
DOIs
StatePublished - May 10 2007

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Ionic Liquids
Solvation
Ionic liquids
solvation
Glass transition
Electric fields
electric fields
glass
room temperature
liquids
glass transition temperature
quinoxalines
Quinoxalines
Phosphorescence
Temperature
Structural relaxation
translational motion
phosphorescence
decoupling
Relaxation time

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

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title = "Solvation dynamics and electric field relaxation in an imidazolium-PF 6 ionic liquid: From room temperature to the glass transition",
abstract = "Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of η/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature Tg. The dielectric modulus relaxation follows this trend only for temperatures T > 1.2Tg and departs significantly from η/T in the 1.1Tg > T > Tg range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2Tg.",
author = "Naoki Ito and Ranko Richert",
year = "2007",
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AU - Ito, Naoki

AU - Richert, Ranko

PY - 2007/5/10

Y1 - 2007/5/10

N2 - Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of η/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature Tg. The dielectric modulus relaxation follows this trend only for temperatures T > 1.2Tg and departs significantly from η/T in the 1.1Tg > T > Tg range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2Tg.

AB - Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition temperature. The solvation dynamics results are compared with the rotational motion of the probe and with the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of η/T for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition temperature Tg. The dielectric modulus relaxation follows this trend only for temperatures T > 1.2Tg and departs significantly from η/T in the 1.1Tg > T > Tg range. This deviation is reminiscent of the enhanced translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids. Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this effect to the viscous regime with T < 1.2Tg.

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