The effects of methanol on the trapping of volatile ice components

Abstract

The evaporation of icy mantles, which have been formed on the surface of dust grains, is acknowledged to give rise to the rich chemistry that has been observed in the vicinity of hot cores and corinos. It has long been established that water ice is the dominant species within many astrophysical ices. However, other molecules found within astrophysical ices, particularly methanol, can influence the desorption of volatile species from the ice. Here we present a detailed investigation of the adsorption and desorption of methanol-containing ices, showing the effect that methanol has on the trapping and release of volatiles from model interstellar ices. OCS and CO 2 have been used as probe molecules since they have been suggested to reside in water-rich and methanol-rich environments. Experiments show that methanol fundamentally changes the desorption characteristics of both OCS and CO 2 , leading to the observation of mainly codesorption of both species with bulk water ice for the tertiary ices and causing a lowering of the temperature of the volcano component of the desorption. In contrast, binary ices are dominated by standard volcano desorption. This observation clearly shows that codepositing astrophysically relevant impurities with water ice, such as methanol, can alter the desorption dynamics of volatiles that become trapped in the pores of the amorphous water ice during the sublimation process. Incorporating experimental data into a simple model to simulate these processes on astrophysical timescales shows that the additional methanol component releases larger amounts of OCS from the ice mantle at lower temperatures and earlier times. These results are of interest to astronomers as they can be used to model the star formation process, hence giving information about the evolution of our Universe.

Trapping and desorption of complex organic molecules in water at 20 K

“…Hence, both RAIRS and TPD show that methyl formate does not affect the thermal processing of water ice and in fact it behaves essentially as a small volatile with respect to water. Similar behaviour has also been reported for large non-polar molecules such as CCl 4 , 43 n-hexane, 53 and decane, 27 in addition to polar molecules such as OCS, 44,50 as well as acetone and dimethyl ether (discussed later). This effect arises because the lower desorption temperature of methyl formate, coupled with its weak interaction with the water, does not inhibit the crystallization or desorption of the water ice.…”

“…48,49 Second, the crystallization of ASW may also be modified by interactions of an impurity, e.g., methanol, which lowers the temperature of the onset of crystallization 27 and hence modifies the desorption of trapped molecules. 50 As already shown above, the three C 2 O 2 H 4 isomers have a varying strength of interaction with water, and hence it is expected that they will have differing effects on the desorption and crystallization of the water ice. Figure 8(a) shows RAIR spectra for the OH stretch region following annealing to 140 K for the layered and mixed ices for all three isomers.…”

“…Furthermore, these molecules directly affect the crystallization of water and therefore may change the overall desorption behaviour of multi-component ices. 50 In this case, the carboxylic acid and alcohol functional groups facilitate hydrogen bonding to the water ice. The ability of these species to directly hydrogen bond to water alters the desorption kinetics of surface bound species with monolayers forming on water ice surfaces for both acetic acid and glycolaldehyde.…”

“…Despite this only containing a single carbon atom, it has been included based on its importance in astrochemistry and ubiquity across astrophysical environments. Methanol was previously categorized as water-like by Collings et al, 23 however, based on our previous results 44,50 its classification is extended to complex water-like. This takes into account the observation that methanol directly affects the crystallisation of water.…”

“…Similar effects have been observed for methanol containing water ices that retain OCS within the ice to higher temperatures. 50 As a consequence, these volatiles may undergo further surface processing and chemistry within the ices.…”