Unified description of the Arrhenian and super-Arrhenian behavior of OTP by the excess internal energy model

As previously reported (Caruthers, Medvedev 2018), the excess internal energy model, 1/U_x, unlike the excess entropy model of Adam and Gibbs, is able to describe the mobility of liquids in the super-cooled state as a function of temperature and pressure. The 1/U_x model prediction of the temperature effect on mobility was validated for 21 molecular glass formers and of the temperature and pressure – for ortho-terphenyl (OTP) – the only material for which there was sufficient data. In the current work MD simulations were performed on model OTP in the high temperature region that includes the onset of the super-Arrhenian behavior. The excess internal energy was obtained at pressures from 0.1 MPa to 5 GPa. The translational and rotation diffusivities were determined at these temperatures and pressures. The MD mobility data (including extremely long time 0.1 MPa simulations from Eastwood et al. 2013) were combined with experimental data, where it was shown that in both the high temperature Arrhenian region and the lower temperature super-Arrhenian region the mobility is a linear function of 1/U_x, albeit with different proportionality constants; the transition between the Arrhenian and super-Arrhenian behaviors is relatively sharp at a critical internal energy U_x,α.