Acid-Promoted Crystallization of Amorphous Solid Water

Abstract

In this study, the effect of externally added hydrogen chloride (HCl) on the crystallization of amorphous solid water (ASW) has been examined. ASW films containing small amounts of HCl, which dissociated into H+ (excess protons) and Cl– ions, and having a thickness of 90–360 monolayers (ML) were prepared on a Pt(111) substrate under an ultrahigh vacuum environment. The location of HCl in the samples was varied by controlling the stacking sequence of H2O and HCl during the film growth. Crystallization kinetics of these samples were examined by conducting temperature-programmed desorption experiments and isothermal kinetic measurements with reflection–absorption infrared spectroscopy for the temperature range of 137–145 K. Crystallization behaviors of pure and NaCl-doped ASW films were also examined for comparison. The results indicated that the excess protons accelerated the crystallization of the ASW films, in contrast to the retardation effect of Na+. In the presence of 0.1 ML HCl, the overall activation energy of ASW crystallization was reduced from 63.4 kJ·mol–1 in the absence of HCl to 48.5 kJ·mol–1, which is close to the activation energy of crystal growth. The crystallization started near the location of HCl injection in the samples, regardless of whether it was the vacuum/ASW interface or the film interior. These observations indicated that the excess protons facilitated the nucleation process and changed the rate-limiting step of ASW crystallization from nucleation to growth. An explanation based on thermodynamics has been proposed, that is, the configurational entropy of the excess protons in ice likely reduces the free-energy barrier of nucleation for the acid-doped ASW samples.