Acceleration of amorphous solid water crystallizationby acidic impurities

Abstract

It is well known that impurities usually retard crystallization of solutions. However, amorphous solid water (ASW) with acidic impurities showed an opposite phenomenon; protons from hydrogen chloride (HCl) accelerates the crystallization of ASW. ASW is a solid form of water and has similarities to supercooled liquid water. ASW is metastable, so it transforms into crystalline ice (CI) at temperatures higher than ~135 K. In this study, we investigated the crystallization of ASW thin films affected by dissociated acid molecules. ASW film samples with excess protons from dissociated HCl were prepared by vapor deposition onto a Pt(111) single crystal in an ultrahigh vacuum chamber. After the film preparation, crystallization kinetics in isothermal conditions were measured using OH stretch peak changes by reflectionabsorption infrared spectroscopy (RAIRS). Rapid phase transition observed in HCl-doped ASW films, but not in NaCl-doped films indicated that the excess protons promoted the crystallization of ASW. The apparent activation energy of the ASW crystallization was lowered from 63.4 kJ/mol without HCl to 48.5 kJ/mol with 0.1 ML HCl. This acid-promoted crystallization started near the location where the acid was injected, regardless of the film surface or interior. These results suggest that the excess protons from dissociated HCl accelerated the nucleation step of the crystallization process. A possible acceleration mechanism for this process is discussed based on the configurational entropy of excess protons. This study, crystallization of ASW with acidic impurities, would be helpful to understand the unique phenomenon of protons in water and ice, and the behavior of low-temperature water in interstellar clouds.