Morphology of Vapor-Deposited Acetonitrile Films.

Crystalline acetonitrile has two polymorphs, a high-temperature (HT) phase that is stable between 217 K and its melting point at 229 K and a low-temperature (LT) phase that is stable below 217 K. Solid acetonitrile films can be prepared by vapor deposition in an ultrahigh vacuum chamber. To prevent sublimation of the film, temperatures are often kept below 150 K. While the LT phase is thermodynamically favored at these low temperatures, such preparation usually results in the formation of the metastable HT polymorph. In this work we use reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments to investigate the effects of the deposition temperature and underlying substrate on the morphology of acetonitrile films prepared with molecular beam deposition. We obtained the elusive LT phase when dosing at 120 K on a graphene substrate and on a crystalline decane layer. Dosing acetonitrile on other surfaces produced the HT phase, as did annealing of amorphous films. We used TPD experiments to determine the Gibbs energy difference between the HT and the LT phases. Our ΔG values agree with extrapolation of equilibrium calorimetry data. We also observed that acetonitrile films were amorphous when dosed at temperatures ≤ 60 K and porous for temperatures ≤ 50 K.