The effects of small alcohols on the crystallisation behaviour of amorphous solid water Jack Fulker and Wendy Brown Department of Chemistry, University of Sussex, Falmer, Brighton, BN1 9QJ, UK Water is one of the most abundant molecules in the interstellar medium and can be found frozen out in icy mantles on the surface of dust grains. [1] Understanding the desorption behaviour of water under astronomical conditions is therefore extremely important in astrochemistry, as the conversion between gas and solid phase water can help to determine the size and age of environments such as nebulae and protoplanetary disks. However, these icy mantles are not single component, but rather a mixture of water and other small molecules, making the desorption kinetics very complex. [2] To investigate these more complex systems, surface science techniques including temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) have been used to investigate the effects of alcohols on the behaviour of multi-component water ices adsorbed on graphite at ~30 K. Water ice grown on a cold surface undergoes an irreversible, thermally induced phase change from amorphous solid water (ASW) to crystalline ice (CI) when heated above 148 K. [3] Due to the porous nature of the ASW ice, small molecules can become trapped within the cavities of the water ice and are then held on the surface above their natural desorption temperature. While trapped, these species can influence the water ice crystallisation behaviour and desorption kinetics. Our investigations show that small alcohols such as methanol promote crystallisation of the water ice by lowering the temperature of crystallisation (T c ) by as much as 20 K, while larger alcohols such as isopropyl alcohol and 1-butanol increase T c to the point of inhibiting the phase change completely. These changes in the water crystallisation behaviour are attributed to the disruption of the hydrogen bond network within the pores of the ASW ice. The crystallisation and desorption behaviours of water ices are important to a number of astronomy groups, particularly when investigating snowlines in star forming regions as well as the ice composition of comets. [4] The phase behaviour of water ices is also important in atmospheric chemistry, as water droplets in the Earth’s mesosphere go through a similar phase change to release trapped molecular species upon thermal processing. [5] References
1. D. A. Williams and E. Herbst, Surf. Sci. , 2002, 500 , 823–837. 2. T. Draine, Annu. Rev. Astron. Astrophys. , 2003, 41 , 241–289.
3. D.J. Burke, F. Puletti, P. M. Woods, S. Viti, B. Slater and W. A. Brown, J. Chem. Phys. , 2015, 143 , 164704. 4. M. N. Fomenkova, S. Chang and L. M. Mukhin, Geochim. Cosmochim. Acta , 1994, 58 , 4503–4512. 5. J. D. Graham and J. T. Roberts, J. Phys. Chem. , 1994, 98 , 5974–5983.
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