Depolarized Dynamic Light Scattering and Dielectric Spectroscopy: Two Perspectives on Molecular Reorientation in Supercooled Liquids

Abstract

Broadband dielectric spectroscopy (BDS) can be considered the standard and most widespread method to experimentally access molecular reorientation in supercooled liquids, as it covers a range of time constants from sub picoseconds corresponding to the highly fluid liquid to several thousand seconds below the glass transition temperature. In a similar fashion, depolarized dynamic light scattering (DLS) is able to probe molecular reorientation. A comparable range of time scales is covered by combining Tandem Fabry Perot Interferometry (TFPI) and Photon Correlation Spectroscopy (PCS) with recent multispeckle techniques allowing to access even the non-ergodic regime below \(T_g\). Thus, DLS represents an alternative route to cover the full range of glassy dynamics. Moreover, due to the fact that both methods couple to different molecular properties, extra information in particular on the motional mechanism behind a certain dynamic process can be obtained by comparing experimental data from both techniques. In the present work we explore this approach for several examples, including ionic liquids and monohydroxy alcohols, and discuss the implications for different relaxation processes. For instance in the case of supercooled ionic liquids, i.e., molten salts, which are liquid at room temperature, the combination of both techniques allows to unambiguously disentangle the contribution of molecular reorientation from other polarization features that often mask reorientation in the dielectric spectra, and a detailed analysis reveals indications for a crossover in the motional mechanism involved in the \(\alpha \)-relaxation. In monohydroxy alcohols we discuss the appearance of the Johari-Goldstein \(\beta \)-process in both techniques and what the observations imply for the underlying motional mechansim. Furthermore, we consider the Debye relaxation, which is frequently observed in the dielectric spectra of monoalcohols and is usually ascribed to transient supramolecular structures. Here, such a comparison of data reveals molecular details about the conditions under which the supramolecular structures are formed.