PNNL

Abstract

Molecular beam techniques are used to create layered nanoscale composite films of amorphous methanol and ethanol at 20 K. The films are then heated and temperature programmed desorption (TPD) and FTIR spectroscopy are used to observe the mixing, desorption, and crystallization behavior from the initially unmixed amorphous layers. We find that after heating above Tg, the layers completely intermix to form a deeply supercooled liquid solution. Modeling of the desorption kinetics shows that the supercooled liquid films behave as ideal solutions. Deviations from ideal solution desorption behavior are observed when the metastable supercooled solution remains for longer times in regions of the phase diagram where crystallization is thermodynamically favorable. In those cases, the finite lifetime of the metastable solutions results in the precipitation of crystalline solids. Finally, in very thick films at temperatures and compositions where a stable liquid should exist, we unexpectedly observe deviations from ideal solution behavior. Visual inspection of the sample indicates that these apparent departures from ideality arise from dewetting of the liquid film from the substrate. We conclude that compositionally tailored nanoscale amorphous films provide a useful means for preparing and examining deeply-supercooled solutions in metastable regions of the phase diagram.