Differential adsorption of complex organic molecule isomers on interstellar ice surfaces

¹ UPMC, Université Paris 6, Laboratoire de Physique Moléculaire pour l’Atmosphère et l’Astrophysique (LPMAA) – CNRS UMR 7092, 75252 Paris, France
² UPMC Université Paris 6, Laboratoire de Chimie Théorique (LCT) – CNRS UMR 7616, 75252 Paris, France
³ Observatoire de Paris et Université de Cergy Pontoise, LAMAp/LERMA – CNRS UMR 8112, 95000 Cergy Pontoise Cedex, France

Abstract

We present a combined theoretical and experimental study of the adsorption of two pairs of organic isomers, (i) acetic acid AA (CH₃COOH) and methyl formate MF (HCOOCH₃), and (ii) ethanol EtOH (CH₃CH₂OH) and dimethyl ether DME (CH₃OCH₃), onto crystalline water ice surfaces at low temperatures. Both approaches show that, for each pair, the most stable isomer from a thermodynamical point of view, i.e. AA and EtOH, is also the one which is the more tightly bound to the water ice surface compared to the less stable isomers (MF and DME). This finding, which can be explained by the ability of AA or EtOH to efficiently interact with the ice surface via hydrogen bondings, may have important consequences in an astrophysical context, since it could explain why the most stable isomer is not the most abundant observed in the interstellar gas phase.