赤外分光法により、エチレングリコールが水混和性アストロケミカル氷の結晶化防止剤であることが明らかに

Ethylene glycol (EG), known as an anti-freeze for water on planet Earth is also known to be present in the interstellar medium and both long and/ short period comets. The ubiquitous EG might have an influence on the physico-chemical nature of other molecules, such as water, in a coexistence scenario within the cometary ices and icy mantles of cold dust in the ISM. However the laboratory studies on the influence of EG molecules on water molecules in astrochemical icy conditions are limited to date. Hence we have carried out mid-infrared (MIR) spectroscopic study of the layered and mixture of EG and water in the astrochemical icy conditions in the laboratory starting from 10 K to higher temperatures, until their sublimation. We found that the presence of EG can restrict the amorphous → crystalline phase change of water ice and can elevate the sublimation temperature of water molecules bonded with EG to the sublimation temperature of EG (230 K). This is about ∼ 50 K higher than the usually known water ice sublimation temperature. We attribute these to the hydrogen bonding present between EG and water molecules. Thus the presence of EG in water ice can bring water into the reaction matrix on an icy mantle at higher temperatures than previously known. Therefore hydrogen bonding should be included in the cometary models, especially for the comets where EG and water coexists.