Amorphous 1-propanol interstellar ice beyond its melting point

The recent discovery of 1-propanol (CH3CH2CH2OH) in the interstellar medium (ISM) is of tremendous interest since fatty alcohols have been proposed as constituents of proto-cell membranes. Motivated by this discovery, we present the laboratory midinfrared (MIR) and vacuum ultra-violet (VUV) absorption spectra of 1-propanol ice under astrochemical conditions, mimicking an icy mantle on cold dust in the ISM. Both MIR and VUV spectra were recorded at ultra-high vacuum (UHV) of ∼ 10-9 mbar and at temperatures ranging from 10 K to sublimation. The morphology of the 1-propanol ice deposited at 10 K was amorphous. By warming the ice to temperatures of 140 K and above, with subsequent recording of IR spectra, we observe complete sublimation of 1-propanol molecules from the substrate around 170 K. No amorphous-to-crystalline phase change was observed upon warming to higher temperatures. Additionally, We observe the IR and VUV signatures of 1-propanol ice on the substrate well beyond its melting point (147 K). To the best of our knowledge, this is the first reported observation of a molecular ice staying well beyond its melting point under such conditions. This result shows that the morphology of icy mantles on ISM cold dust grains is more complex than previously thought. Our atomistic molecular dynamics (MD) simulations capture the experimental trends and shed light on the microscopic origin of this unusual phase behaviour of 1-propanol.