Low-Temperature Mixing of Polar Hydrogen Bond-Forming Molecules in Amorphous Solid Water

Abstract

The level of mixing of guest molecules within low-temperature amorphous solid water (ASW) (as a host) is of interest and relevance to model studies of photochemistry that take place in the interstellar medium (ISM). In this study, we explore how the physical properties of guest molecules, methanol and ammonia, which strongly interact with ASW films, affect the level of mixing and distribution of these molecules throughout the ASW film at adsorption temperatures of 35–100 K, while they are deposited on a Ru(0001) substrate under ultra-high vacuum conditions. Both methanol and ammonia have gas-phase dipole moments similar to that of water, and they are both capable of forming hydrogen bonds with water. The level of mixing and dispersion of these molecules in ASW films is explored through the isothermal noninvasive contact potential difference (ΔCPD) measurements and temperature programmed contact potential difference experiments (TP-ΔCPD). Upon adsorption at 35 K, both molecules form hydrogen bonds with the surrounding water molecules but do not move too far from their initial location in the ASW film. This is confirmed by the observation of an “inverse volcano” process, where guest molecules initially placed within the interaction range of the ruthenium substrate migrate to the substrate upon water crystallization instead of desorbing to the vacuum as expected in a typical “volcano” process. The adsorption temperature at which extensive mixing is achieved is correlated to the glass transition and crystallization temperatures of the guest molecules, which in this case is lower for both guest molecules than that of water. Homogeneous mixing is only achieved when the films are heated above the glass transition temperature of the host (water) molecules.