Differential adsorption of complex organic molecule isomers on interstellar ice surfaces

We present a combined theoretical and experimental study of the adsorption of two pairs of organic isomers, ( i ) acetic acid AA (CH 3 COOH) and methyl formate MF (HCOOCH 3 ), and ( ii ) ethanol EtOH (CH 3 CH 2 OH) and dimethyl ether DME (CH 3 OCH 3 ), 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.