Project: Probe Rotation |
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In this project, the main concern is the understanding of the effects which determine the rotational behavior of a guest (solute) molecule embedded in a host (solvent) supercooled liquid [105, 119]. Using time-resolved optical triplet state depolarization, the rotational correlation function of a probe is determined by depolarization, while solvation dynamics assess the environmental dynamics. Analysing the dynamics in terms of KWW decays leads to τrot, βrot for the rotation and τsol, βsol for the solvation data. We find that rotation slows down with increasing relative probe size, and that a transition occurs from dispersive to more exponential rotation around a ratio of τrot/τsol = 20. This approach to more hydrodynamic rotation is interpreted in terms of time-averaging over the dynamical heterogeneity. |
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Scaled stretching exponent (βg
- βh)/(1 - βh)
versus correlation time ratio τg/τh
for a number of guest/host combinations. The results associate exponential orientation
correlation functions with slow probe rotation if βg
and τg are gauged relative to the host counterparts.
The ordinate scale is zero for the guest being as dispersive as the host and unity in the
case of exponential guest dynamics. Yellow diamonds are based on dielectric results, green
circles reflect optical rotation/solvation results after shifting on the abscissa scale.
The relative position of guest rotation relative to the exchange time scale is made
responsible for this transition, as indicated by the insets. [137, 140] |
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Using high-resolution dielectric equipment, we are also capable of observing guest and host dynamics simultaneously in a dielectric relaxation measurement [122, 133, 140]. An clearcut case is dibutylether in 3-methylpentane, where an additional solute peak appears at a peak frequency position which is a factor of 9 below that of the bulk liquid, a situation that is sufficient to lead to exponential dynamics of the probe. In contrast to the optical experiment, the relaxation times of the guest (τg) and of the host (τh) are determined on the basis of the same correlation function. |
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Dielectric loss ε'' for dibutylether (DBE) at a level of 1 wt%
in 3-methylpentane (3MP). The higher amplitude/lower frequency peak originates from DBE and is
of Debye type. The other peak originates from the host 3MP and shows significant dispersion.
The situation is unchanged across the entire frequency range shown. [140] |
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Reference numbers refer to the list of publications |
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Experimental techniques:
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Selected projects:
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