Microscopic origin of the logarithmic relaxation in molecular glass-forming liquids

Logarithmic relaxation is a unique relaxation process exhibited by a few molecular liquids and biomolecules. However, the microscopic origin of logarithmic relaxation is still unclear. To understand the origin of this process, we studied two liquids that exhibit logarithmic relaxation in a dissolved state using quasielastic neutron scattering (QENS) and depolarized dynamic light scattering (DDLS). Although the intermolecular potential of the liquids is drastically different in the dissolved state from the bulk liquids, we observed that the logarithmic relaxation still persists. Our results indicate that the intermolecular potential does not play a role in determining the logarithmic relaxation process. The coupling of rotational and translational relaxation processes could be the origin of the logarithmic relaxation process exhibited by the molecular liquids.

Figure 1

(left) The wide angle diffraction patterns of bulk BPM, BPM+Deuterated acetone mixture and bulk Deuterated acetone. (right) The wide angle diffraction patterns of bulk Salol, Salol+Deuterated acetone mixture and bulk Deuterated acetone.

Figure 2

The dynamic structure factors of dissolved BPM in deuterated acetone obtained on PELICAN, time-of-flight neutron spectrometer at different temperatures.

Figure 3

(a),(b) The self-intermediate scattering functions of BPM and Salol in deuterated acetone obtained by QENS on PELICAN, time-of-flight neutron scattering spectrometer, respectively. (c),(d) The intensity-intensity autocorrelation functions obtained by DLS for dissolved BPM and Salol, respectively. The solid lines are the fits with Eqs. (1) and (3) on the self-intermediate scattering functions and intensity-intensity autocorrelation functions, respectively.

Figure 4

The self-intermediate scattering functions of (a) dissolved BPM and (b) dissolved Salol at 243 K obtained on the PELICAN, time-of-flight neutron scattering spectrometer.

Figure 5

(a) The self-intermediate scattering function of bulk BPM measured at different temperatures on MARS, high-resolution indirect time-of-flight neutron backscattering spectrometer. (b) The self-intermediate scattering function of bulk Salol measured at different temperatures on PELICAN, time-of-flight neutron spectrometer.

Figure 6

The QENS fitting parameters of dissolved BPM and Salol. (a) ${\mathrm{H}}_1\left(Q,T\right)$ of dissolved BPM and (c) dissolved Salol as a function of $Q$ at different temperatures. The temperature dependence of fitting parameter ${\mathrm{B}}_1\left(T\right)$ of (b) dissolved BPM and (d) dissolved Salol. The linear fitting lines are extrapolated consistently to get the MCT critical temperature, $T_c$.

Figure 7

The fitting parameter ${\mathrm{H}}_2\left(Q,T\right)$ of (a) dissolved BPM and (b) dissolved Salol as a function of $Q$ at different temperatures.

Figure 8

The inverse of mean α-relaxation time of dissolved (a) BPM and (b) Salol are plotted as a function of $Q^2$.

Figure 9

The intensity-intensity autocorrelation functions obtained by DDLS for (a) dissolved BPM, (b) bulk BPM, (c) dissolved Salol and (d) bulk Salol, respectively. The solid lines are the fits with Kohlrausch function [Eq. (3)].

Figure 10

The DDLS data of BPM and Salol in dissolved and bulk states. The inverse of the mean α-relaxation time of (a) dissolved BPM, (b) bulk BPM, (c) dissolved Salol, and (d) bulk Salol as a function of square of momentum transfer, $Q^2$.