Glass-liquid transition of vapor-deposited hexane studied using TOF-SIMS

Thermodynamic connection between liquid and glass is not obvious for poor glass formers. In this study, microscopic molecular diffusivity and macroscopic fluidity of vapor-deposited thin films of n-hexane were investigated using TOF-SIMS to elucidate the mechanism of the glass-liquid transition. The C 6H 14 film deposited at 15 K is characterized by a porous structure, as inferred from the intermixing with adsorbed C 6D 14 and D 2O molecules, as well as the formation of D 2O nanoclusters on the surface. The hexane molecules are reoriented at temperatures higher than 60-70 K, resulting in smoothing of the surface and densification of the film. Self-diffusion of the hexane molecules commences at 110 K; then, the film dewets the Ni(111) substrate after some aging time. Results indicate that ultraviscous liquid formed at the glass transition temperature of 110 K transforms into fluidized liquid immediately before crystallization. The D 2O molecules adsorbed onto the surface play a role as a surfactant, as evidenced by quenching the film dewetting. The ultraviscous liquid is likely to be a distinct phase, which might explain the absence of calorimetric glass transition for poor glass formers like hexane.