Nature of the Debye-Process in Monohydroxy Alcohols: 5-Methyl-2-Hexanol Investigated by Depolarized Light Scattering and Dielectric Spectroscopy

Abstract

The slow Debye-like relaxation in the dielectric spectra of monohydroxy alcohols is a matter of long-standing debate. In the present Letter, we probe reorientational dynamics of 5-methyl-2-hexanol with dielectric spectroscopy and depolarized dynamic light scattering (DDLS) in the supercooled regime. While in a previous study of a primary alcohol no indication of the Debye peak in the DDLS spectra was found, we now for the first time report clear evidence of a Debye contribution in a monoalcohol in DDLS. A quantitative comparison between the dielectric and DDLS manifestation of the Debye peak reveals that while the dielectric Debye process represents fluctuations in the end-to-end vector dipole moment of the transient chains, its occurrence in DDLS shows a more local signature and is related to residual correlations that occur due to a slight anisotropy of the α relaxation caused by the chain formation.

A microscopic look at the Johari-Goldstein relaxation in a hydrogen-bonded glass-former

“…As already stressed before, the TDI measurements detect at each T and q only one relaxation process, and accounts for the total strength of the α and β JG processes. A clear change in the T -dependence of can be observed for all q values around T g = 154 K, as expected when the glass transition occurs, whereas no discontinuity is observed at T αβ = 181 K. This is a well-know result from DS and depolarized dynamic light scattering measurements, that is when the β JG relaxation separates from the structural one no discontinuity in the total strength is detected 34,35 . It is interesting to notice that also at 37 nm −1 , where the β JG relaxation is the dominant relaxation channel, a clear change in the temperature dependence of the contrast is observed at T g , thus highlighting the sensitivity of the β JG relaxation to the glass-transition, as already shown for the dielectric strength of the β JG process at T g 1 .…”

“…The T -dependence of the β JG -relaxation was instead modeled using the Arrhenius equation: the reduced activation energy found here ( E / k B = 3.3(1) × 10 3 K) agrees with the one reported in ref. 34 . The obtained curves (dash-dotted lines) were then scaled onto the relaxation time data measured by TDI in order to identify unambiguously the α and β JG processes (solid lines in Fig.…”

“…It has to be underlined that the Debye relaxation 28,34 , much slower than the α one, was detected only by DS and in Fig. 2 only its characteristic timescale is reported for the sake of clarity.…”

“…2 only its characteristic timescale is reported for the sake of clarity. Indeed, the Debye process, despite being the dominant one in DS data, is related to fluctuations of the end-to-end dipole moment of transient supramolecular structures and has therefore little or no signature in the spectra of other experimental techniques such as depolarized light-scattering 34,35 , mechanical spectroscopy 36 , triplet solvatation state with mechanical probes 37 as well as in differential scanning calorimetry 38 . Concerning microscopic density fluctuations, neutron scattering experiments on 1-propanol, another monohydroxyl alcohol showing a similar Debye feature in DS spectra, have revealed that at the maximum of S ( q ) the relaxation dynamics is dominated by the α process 39 .…”

Influence of Molecular Architecture on the Dynamics of H-Bonded Supramolecular Structures in Phenyl-Propanols

“…Furthermore, the interpolation of the PCS-α-process reveals a common spectral shape for 1P and its associated phenyl alcohol isomers. We note that prior studies of the monoalcohols 5-methyl-2-hexanol and 2-ethyl-1-butanol also revealed a very similar α-process shape [15], which indicates a common α-process shape in a broader range of monoalcohols.…”