Dielectric study of probe rotation in viscous liquids

Abstract

Dielectric molecular probes are employed to assess the averaging required for a transition from heterogeneous to more hydrodynamic rotational behaviour in supercooled liquids. The degree of averaging is controlled by the size of the probe molecule relative to the liquid constituents. Larger solutes exhibit longer time constants and more exponential dynamics as a result of effective time averaging over fluctuating environments. With this approach, the time scale of rate exchange is measured across a range of eight decades, in which the characteristic relaxation time varies from 5 s to 50 ns. The sharp transition from dispersive to exponential probe rotation suggests that rate exchange occurs on the time scale of the slowest time constant of structural relaxation. No change in the environmental fluctuation to structural relaxation time ratio is observed across this wide range of frequencies.

Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts

“…96,97 The coupling to the medium viscosity and validity of Stokes-Einstein-Debye (SED) relation for solute rotation is tested next in Fig. 9 where temperature dependent average rotational times ( r τ ) are shown as a function of temperature-reduced viscosity ( T / η ) in a doublelogarithmic plot.…”

The component dynamics of miscible binary mixtures of glass formers: New features

“…In these cases, as reported in our experiments, structural relaxation becomes very broad, with a distribution of relaxation time sometime exceeding that of the pure apolar solvent (see for instance BrEBz in 3S (Fig.4), with FWHM≈2.78, where for pure 3S FWHM≈1.9). Although the latter results cannot be explain by using the concept of heterogeneous dynamics as presented in [30], they are naturally explained by coupling model. The overall scenario is confirmed by studies of rotational motion of probe molecules in various hosts with different viscosity by time-resolved optical spectroscopy [31], for which the agreement with CM predictions has been tested [32].…”

“…Unfortunately in ref. [30] no experiments with solute of smaller size or with lower T g than the solvent were reported. In these cases, as reported in our experiments, structural relaxation becomes very broad, with a distribution of relaxation time sometime exceeding that of the pure apolar solvent (see for instance BrEBz in 3S (Fig.4), with FWHM≈2.78, where for pure 3S FWHM≈1.9).…”

“…This picture is in agreement with CM preditcions. Recently, the probe rotation dynamics of probe polar molecules in viscous liquids has been studied [30] by dielectric spectroscopy as a function of the the size of the probe molecule relative to the liquid constituents. The change of the breadth of the α-relaxation was interpreted in terms of averaging the eniromental fluctuations due to heterogeneous dynamics.…”

On the Decoupling of Relaxation Modes in a Molecular Liquid Caused by Isothermal Introduction of 2 nm Structural Inhomogeneities

“…Probe molecules of dimensions larger than about 2 nm are observed to relax exponentially (implying that any heterogeneities in the mobility on this length scale are being averaged out 3,24 . Accordingly, probe molecules of dimensions approaching that of the host molecule relax with degrees of non-exponentiality that approach that characteristic of the host molecules 25 . This is consistent with the observation of Kawasaki and Tanaka 26 , by MD studies, that a system appears non-ergodic unless sufficient time is allowed for the heterogeneity length scale to be equilibrated, a time scale much longer than that assigned to the !…”