Time-scale ordering in hydrogen and van der Waals-bonded liquids

The time scales of structural relaxation are investigated on the basis of five different response functions for 1,2, 6-hexanetriol, a hydrogenbonded
liquid with a minor secondary contribution, and 2,6,10,15,19,23-hexamethyl-tetracosane (squalane), a van der Waals-bonded liquid
with a prominent secondary relaxation process. Time scales of structural relaxation are derived as inverse peak frequencies for each investigated
response function. For 1,2,6-hexanetriol, the ratios of the time scales are temperature-independent, while a decoupling of time scales is
observed for squalane in accordance with the literature. An alternative evaluation approach is made on the squalane data, extracting time scales
from the terminal relaxation mode instead of the peak position, and in this case, temperature-independent time-scale ratios are also found for
squalane, despite its strong secondary relaxation contribution. Interestingly, the very same ordering of response-function-specific time scales
is observed for these two liquids, which is also consistent with the observation made for simple van der Waals-bonded liquids reported previously
[Jakobsen et al., J. Chem. Phys. 136, 081102 (2012)]. This time-scale ordering is based on the following response functions, from fast to
slow dynamics: shear modulus, bulk modulus, dielectric permittivity, longitudinal thermal expansivity coefficient, and longitudinal specific
heat. These findings indicate a general relation between the time scales of different response functions and, as inter-molecular interactions
apparently play a subordinate role, suggest a rather generic nature of the process of structural relaxation.