MD simulations of charged binary mixtures reveal a generic relation between high- and low-temperature behavior

Abstract

Experimental studies of the glassy slowdown in molecular liquids indicate that the high-temperature activation energy E∞ of glass-forming liquids is directly related to their glass transition temperature Tg. To further investigate such a possible relation between high- and low-temperature dynamics in glass-forming liquids, we analyze the glassy dynamics of binary mixtures using molecular dynamics simulations. We consider a binary mixture of charged Lennard-Jones particles and vary the partial charges of the particles and, thus, the high-temperature activation energy and the glass transition temperature of the system. Based on previous results, we introduce a phenomenological model describing relaxation times over the whole temperature regime from high temperatures to temperatures well inside the supercooled regime. By investigating the dynamics of both particle species on molecular and diffusive length scales along isochoric and isobaric pathways, we find a quadratic charge dependence of both E∞ and Tg, resulting in an approximately constant ratio of both quantities independent of the underlying observable, the thermodynamic ensemble, and the particle species, and this result is robust against the actual definition of Tg. This generic relation between the activation energy and the glass transition temperature indicates that high-temperature dynamics and the glassy slowdown are related phenomena, and the knowledge of E∞ may allow us to approximately predict Tg.

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies

“…We emphasize that the temperature dependence of the α relaxation in bulk liquids is still debated and functional forms other than the VFT relation were proposed for the non-Arrhenius behavior. , In particular, experimental and computational studies − consistently conjectured to decompose the apparent activation energy into a temperature-independent contribution E ∞ , which is given by the activation energy of the regular liquid at high temperatures, and a temperature-dependent contribution E c ( T ), which describes the growing cooperativity of the α relaxation upon supercooling, i.e., Following this approach, it was found for various liquids that − where T a is a reference temperature and μ was termed a generalized fragility. Thus, the non-Arrhenius behavior was traced back to an exponential temperature dependence of E c .…”

“…Next, we discuss possible confinement effects on the α relaxation near T g . To obtain a typical scenario for the temperature dependence in the bulk liquid and thus in the pore center, we use eqs and with typical parameters. − For the structural relaxation at the pore wall, we assume in a first model that eq applies down to the glass transition and use the above values of Δ E / E ∞ (Figure ) for the additional energy barriers. The resulting correlation times of the α relaxation in the center and at the wall decouple very strongly upon cooling (Supporting Information, Figure S10).…”

Melt Properties and String Model Description of Glass Formation in Graft Polymers of Different Side-Chain Lengths

“…This topic has attracted intense interest since a close link between these properties would enable determinations of the low-T properties (such as T g ) from the quantities obtained in the high-T regime. Recently, Vogel and co-workers 108,109 have demonstrated based on simulations of ionic liquids that while both ΔH 0 and T g change as the partial charges of the ions are varied, the ratio, ΔH 0 /k B T g , is nearly constant, indicating a close link between the high-T dynamics and the low-T glassy dynamics. Figure 10 examines the variation of ΔH 0 /k B T g with n s in our model graft polymers, where T g has been determined in our previous work.…”