One of the unsolved problems of dynamics in supercooled liquids are the differences in spectral shape of the structural relaxation observed among different methods and substances, and a possible generic line shape has long been debated. We show that the light scattering spectra of very different systems, e.g., hydrogen bonding, van der Waals liquids, and ionic systems, almost perfectly superimpose and show a generic line shape of the structural relaxation, following ∝ ω–1/2 at high frequencies. In dielectric spectra the generic behavior is recovered only for systems with low dipole moment, while in strongly dipolar liquids additional cross-correlation contributions mask the generic structural relaxation.

The relaxation behaviour of monohydroxy alcohols (monoalcohols) in broadband dielectric spectroscopy (BDS) is usually dominated by the Debye process. This process is regarded as a signature of the dynamics of transient supramolecular structures formed by H-bonding. In phenyl propanols the steric hindrance of the phenyl ring is assumed to influence chain formation and thereby to decrease or even suppress the intensity of the Debye process. In the present paper we study this effect in a systematic series of structural isomers of phenyl-1-propanol in comparison with 1-propanol. It turns out that by combining BDS, Photon Correlation Spectroscopy (PCS) and calorimetry the dynamics of supramolecular structures can be uncovered. While light scattering spectra show the same spectral shape of the main relaxation for all investigated monoalcohols, the dielectric spectra differ in the Debye contribution. Thus it becomes possible for the first time to unambiguously disentangle both relaxation modes in the dielectric spectra. It turns out that the Debye relaxation gets weaker the closer the position of phenyl ring is to the hydroxy group, in accordance with the analysis of the Kirkwood-Fröhlich correlation factor. Even in 1-phenyl-1-propanol, which has the phenyl group attached at the closest position to the hydroxy group, we can separate a Debye-contribution in the dielectric spectrum. From this we conclude that hydrogen bonds are not generally suppressed by the increased steric hindrance of the phenyl ring, but rather an equilibrium of ring and chain-like structures is shifted towards ring-like shapes on shifting the phenyl ring closer to the hydroxy group. Moreover, the shape of the α-relaxation as monitored by PCS and BDS remains unaffected by the degree of hydrogen bonding and is the same among the investigated alcohols. arXiv:1909.00646v1 [cond-mat.soft]

We revisit the reorientational dynamics of 1-propanol as a prototype of a monohydroxy alcohol and H-bonding system by dielectric spectroscopy (DS) and depolarized dynamic light scattering (DDLS). In particular, we address the question of whether the Debye relaxation, which is seen as a dominant process in DS, is visible in light scattering and discuss how the Johari-Goldstein (JG) β-process, which is also a prominent feature of the dielectric spectrum, appears in photon correlation spectroscopy. For that purpose we performed depolarized photon correlation experiments with an improved setup and performed additional time domain dielectric experiments which gives us the possibility to compare dielectric and light scattering data in a broad temperature range. It turns out that the improved setup allows to unambiguously identify the JG β-process, which shows almost identical properties in DDLS as in the dielectric spectra, but a Debye relaxation is not present in the DDLS data and can be excluded down to a level of 2.5% of the α-process amplitude.

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.

We suggest a way to disentangle self- from cross-correlations in the dynamic susceptibility of the glass former glycerol by combining dielectric relaxation (BDS) with light scattering (DDLS), field cycling NMR, and shear compliance (SC) data.