Hydration of polar and nonpolar molecules at the surface of amorphous solid water

On the basis of time-of-flight secondary ion mass spectrometry, properties of amorphous solid water above the glass transition temperature $\left(136\mathrm{K}\right)$ and the hydration of polar $(\mathrm{HCOOH},{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH})$ and nonpolar $({\mathrm{C}}_6{\mathrm{H}}_{14},{\mathrm{C}}_6{\mathrm{F}}_{14})$ molecules on the ${\mathrm{D}}_2\mathrm{O}$-ice surface have been investigated. No evidence was obtained for the irreversible transition of the amorphous solid water into the crystalline phase: the self-diffusion of water molecules occurs above $140\mathrm{K}$ irrespective of the preparation temperatures of the water-ice film ranging from $15\mathrm{K}$ to $165\mathrm{K}$, whereas the morphology of the film changes drastically at $165\mathrm{K}$ due to the evolution of liquidlike water. It is also demonstrated that the change in conformation of the hydrated $\mathrm{HCOOH}$ molecule, as well as the occurrence of hydrophilic/hydrophobic hydration of the ${\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH}$ molecule, can be analyzed successfully from the temperature evolutions of the secondary-ion intensities. These polar molecules basically stay on the surface and tend to quench the morphological change of the water film due to the reduction of surface tension. The nonpolar ${\mathrm{C}}_6{\mathrm{H}}_{14}$ and ${\mathrm{C}}_6{\mathrm{F}}_{14}$ molecules readily dissolve in the ${\mathrm{D}}_2\mathrm{O}$ layer below $100\mathrm{K}$ and dehydration of the incorporated molecules occurs at $165\mathrm{K}$ concomitantly with the evolution of the liquidlike water. It is thus concluded that the hydrophobic hydration of nonpolar molecules is intimately related to the properties of water films.