Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules

Abstract

Using molecular dynamics computer simulations, we investigate the dynamics of the rotational degrees of freedom in a supercooled system composed of rigid, diatomic molecules. The interaction between the molecules is given by the sum of interaction-site potentials of the Lennard-Jones type. In agreement with mode-coupling theory (MCT), we find that the relaxation times of the orientational time correlation functions C (s)2 (t) and C 1 (t) show at low temperatures a power-law with the same critical temperature T c , and which is also identical to the critical temperature for the translational degrees of freedom. In contrast to MCT we find, however, that for these correlators the time-temperature superposition principle does not hold well and that also the critical exponent γ depends on the correlator. We also study the temperature dependence of the rotational diffusion constant D r and demonstrate that at high temperatures D r is proportional to the translational diffusion constant D and that when the system starts to become supercooled the former shows an Arrhenius behavior whereas the latter exhibits a power-law dependence. We discuss the origin for the difference in the temperature dependence of D (or the relaxation times of C

Rotational dynamics of a dipolar supercooled liquid

“…These results were also found for a system of nonpolar molecules. 9,10 When fitting D r to a power law, a critical temperature of 58 K is obtained, which is lower than those associated with translational and reorientational relaxation times, and also with translational diffusion coefficients. Then, rotational dynamics about a direction perpendicular to the molecular axis is less hindered than translational dynamics upon cooling, and it is still active when translational dynamics is already frozen.…”

“…Rotational dynamics and its connection with translation has been analyzed in simulations of several molecular glass formers as ortho-terphenyl 5 and water, 6,7 where a decoupling was encountered. 8 For a model system of rigid dumbbells it has been found that jumps are important at temperatures close to the critical one, 9 and that angular jumps are more relevant than translational ones in the deeply supercooled liquid. 10 The analysis of the translational-rotational coupling in that nonpolar system has raised some questions on the adequacy of the diffusive model to describe rotational dynamics in the supercooled state.…”

Dynamic arrest in a liquid of symmetric dumbbells: Reorientational hopping for small molecular elongations

“…A similar phenomenon is expected to be present in the studied system at small ζ, preventing the possibility of an accurate determination of the asymptotic laws close to the MCT critical temperature T B c . In addition, we recall that ideal MCT neglects activated processes which are known to be present in reality around and below T B c [10]. Only when hopping phenomena can be neglected, an MCT study can be feasible very close to T To provide a lower bound to T B c at small ζ, we also fit the data in terms of the well-known phenomeno-…”

Semischematic model for the center-of-mass dynamics in supercooled molecular liquids

“…It has now become apparent that the ideal modecoupling theory (MCT) for structural relaxation [2,8] contains the essential ingredients for the description of the molecular dynamics in supercooled states, down to a cross-over temperature T c , where "hopping" processes become dominant [9,10]. Although the MCT was originally developed for simple (atomic) liquids, it appears to be able to rationalize in a coherent way experimental [3,11,12] and numerical simulation results [6,13,14] also for molecular liquids, i.e. for molecules with anisotropic interaction potentials.…”