Boiling temperature as a scaling parameter for the microscopic relaxation dynamics in molecular liquids

At sufficiently high temperatures, the center-of-mass microscopic diffusion dynamics of liquids is characterized by a single component, often with weak temperature dependence. In this regime, the effective cage made by the neighbor particles cannot be sustained and readily breaks down, enabling long-range diffusion. As the temperature is decreased, the cage relaxation becomes impeded, leading to a higher viscosity with more pronounced temperature dependence. On the microscopic scale, the sustained caging effect leads to a separation between a faster in-cage relaxation component and a slower cage-breaking relaxation component. The evidence for the separate dynamic components, as opposed to a single stretched component, is provided by quasielastic neutron scattering experiments. We use a simple method to evaluate the extent of the dynamic components separation as a function of temperature in a group of related aromatic molecular liquids. We find that, regardless of the glass-forming capabilities or lack thereof, progressively more pronounced separation between the in-cage and cage-breaking dynamic components develops on cooling down as the ratio of T_b/T, where T_b is the boiling temperature, increases. This reflects the microscopic mechanism behind the empirical rule for the glass forming capability based on the ratio of boiling and melting temperatures, T_b/T_m. When a liquid's T_b/T _m happens to be high, the liquid can readily be supercooled below its T_m because the liquid's microscopic relaxation dynamics is already impeded at T_m, as evidenced by a sustained caging effect manifested through the separation of the in-cage and cage-breaking dynamic components. Our findings suggest certain universality in the temperature dependence of the microscopic diffusion dynamics in molecular liquids, regardless of their glass-forming capabilities. Unless the insufficiently low (with respect to T_b) melting temperature, T_m, intervenes and makes crystallization thermodynamically favorable when cage-breaking is still unimpeded and the structural relaxation is fast, the liquid is likely to become supercooled. The propensity to supercooling and eventually forming a glass is thus determined by a purely thermodynamic factor, T_b/T_m.