Universality of the onset of activated transport in Lennard-Jones liquids with tunable coordination: Implications for the effects of pressure and directional bonding on the crossover to activated transport, configurational entropy, and fragility of glassforming liquids

Abstract

We establish, via classical density functional theory, that the crossover to activated transport in liquids takes place when the depth of the metastable minimum in the free energy corresponding to long-lived aperiodic structures reaches a certain near universal value. We show that the particle vibrational displacement is strongly correlated with this depth in a broad range of pressure and temperature, thus providing basis for a Lindemann-like criterion for the onset of activated transport in liquids. The configurational entropy at the crossover temperature T(cr), too, is found to be nearly system-independent, consistent with the random first order transition theory. We show that to reproduce existing data for the pressure dependence of T(cr), the liquid must increase its coordination with pressure. Upon increasing pressure at fixed coordination, the liquid's fragility is predicted to exhibit re-entrant behavior. This prediction is consistent with glycerol data but is in contrast with data in several organic liquids and polymers, whose fragility monotonically decreases with pressure in the so far accessed pressure range. Allowing for increase in coordination with pressure mitigates the disagreement, owing to the resulting decrease in the thermal expansivity. Finally, we rationalize the correlation between the isobaric and isochoric fragilities put forth by Casalini and Roland [Phys. Rev. E 72, 031503 (2005)] and make predictions on the limiting behavior of the fragility at high pressure.