Theory of Relaxation Dynamics in Glass-Forming Hydrogen-Bonded Liquids

Abstract

We address the relaxation dynamics in hydrogen-bonded super-cooled liquids near the glass transition, measured via Broad-Band Dielectric Spectroscopy (BDS). We propose a theory based on decomposing the relaxation of the macroscopic dipole moment into contributions from hydrogen bonded clusters of $s$ molecules, with $s_{min}\le s \le s_{max}$. The existence of $s_{max}$ is due to dynamical arrest and its value may depend on the cooling protocol and on the aging time. The existence of $s_{max}$ is translated into a sum-rule on the concentrations of clusters of size $s$. We construct the statistical mechanics of the super-cooled liquid subject to this sum-rule as a constraint, to estimate the temperature-dependent density of clusters of size $s$. With a theoretical estimate of the relaxation time of each cluster we provide predictions for the real and imaginary part of the frequency dependent dielectric response. The predicted spectra and their temperature dependence are in accord with measurements, explaining a host of phenomenological fits like the Vogel-Fulcher fit and the stretched exponential fit. Using glycerol as a particular example we demonstrate quantitative correspondence between theory and experiments. The theory also demonstrates that the $α$ peak and the "excess wing" stem from the same physics in this material. The theory also shows that in other hydrogen -bonded glass formers the "excess wing" can develop into a $β$ peak, depending on the molecular material parameters (predominantly the surface energy of the clusters). We thus argue that $α$ and $β$ peaks can stem from the same physics. Finally we address the BDS in constrained geometries (pores) and explain why recent experiments on glycerol did not show a deviation from bul k spectra.