Time-scale coupling in hydrogen- and van der Waals-bonded liquids.

The coupling behavior of time scales of structural relaxation is investigated on the basis of five different response functions for 1,2,6-hexanetriol, a hydrogen-bonded 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 time-scale indices are temperature-independent, while a decoupling of time scales is observed for squalane in accordance with 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 coupling is 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 \textit{et al.}, J. Chem. Phys. \textbf{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-molecuar interactions apparently play a subordinate role, suggest a rather generic nature of the process of structural relaxation.