Desorption and crystallisation of binary 2-propanol and water ices adsorbed on graphite

Abstract

Alcohols, including 2-propanol, are important in a range of industrial applications, and are also found in cold astrophysical environments such as comets and interstellar space, where they are often frozen out on carbonaceous grain surfaces. In these regions, the interaction between alcohols and water ice plays a crucial role in the surface chemistry. We have therefore undertaken a detailed temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) investigation to elucidate the physical chemistry of the adsorption, desorption and crystallisation of 2-propanol and water ices adsorbed on graphite at 26 K. Hydrogen bonding plays a critical role in the physical chemistry of both pure 2-propanol and of binary ices containing 2-propanol and water ice. Monolayer 2-propanol physisorbs strongly on the graphite surface, and with increasing coverage, annealing leads to the desorption of first amorphous, and then crystalline, 2-propanol multilayers. Crystallisation is also evident in RAIR spectra, which show marked changes on annealing of the adsorbed 2-propanol layer. In binary ice systems containing amorphous solid water and 2-propanol, the desorption and crystallisation of the alcohol is modified. The water ice inhibits the 2-propanol crystallisation and gives rise to co-desorption in TPD. In addition, the 2-propanol also strongly influences the behaviour of the water, with even small amounts of the alcohol changing the crystallisation kinetics of water ice, leading to the desorption of water solely in its amorphous form.