Abstract:
Since the beginning of this century, an important idea of density scaling (DS) has been developing successfully. This idea relates macroscopic properties of supercooled liquids to an effective short-range intermolecular Lennard-Jones-type potential and bears hallmarks of universality in cases of different quantities (structural relaxation time, viscosity, segmental relaxation time, Debye-like relaxation time, conductivity relaxation time, dc-conductivity, and diffusivity) and various materials. This includes van der Waals liquids, polymer melts, protic and aprotic ionic liquids, and liquid crystals[1].
According to the basic DS law, the dynamic quantities typically measured in isobaric or/and isothermal conditions can be plotted onto one master curve as a single-variable function f(Gamma), where the scaling variable Gamma=densitygamma/temperature and the scaling exponent, gamma, is a material constant related to the exponent of the repulsive part of the effective intermolecular potential. An additional advantage of the DS law consists in its powerful predictive capabilities, remaining in effect in both the supercooled and normal liquid states [1-3]. Recently, we have shown that the DS based transformation from the temperature-pressure domain to the temperature-density domain enables to reveal some invariant quantities, including a new invariant that is the ratio of dynamic and thermodynamic moduli in both the supercooled and normal liquid states [1,2]. Very recently, we have established that the inflection points observed in isothermal dependences of dynamic quantities, reported first by Herbst et al. in Nature in 1993, can be numerically predicted for different materials from the DS law [1,3]. This surprising finding has resulted in an even more spectacular outcome that is a breakdown of the Arrhenius law regarded, since the end of the 19th century, as a standard rule. This rule is valid for various physicochemical processes, including the thermodynamic evolution of dynamic quantities (such as primary relaxation time, viscosity, and dc-conductivity) in the normal liquid state in isobaric conditions. Combining numerical and analytical arguments based on experimental data measured at ambient and high pressures, we have justified that the standard Arrhenius law, log(X) ~ Eact/temperature, considered for a dynamic quantity X on the assumption of constant apparent activation energy Eact, cannot generally be valid, not only in the supercooled liquid state characterized by super-Arrhenian cooperative molecular dynamics, but also between the boiling and melting points in isobaric conditions, including ambient pressure, at least if the DS law is satisfied [1,3].
References:[1] A. Grzybowski, M. Paluch, Universality of Density Scaling, p. 77-120 in The Scaling of Relaxation Processes, (Eds) F. Kremer, A. Loidl, Springer Nature 2018.
[2] A. Jedrzejowska, A. Grzybowski, M. Paluch, Phys. Chem. Chem. Phys. 19 (2017) 18348-18355.
[3] E. Thoms, A. Grzybowski, S. Pawlus, M. Paluch, J. Phys. Chem. Lett. 9 (2018) 1783-1787.
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