Asymmetric Transport Mechanisms of Hydronium and Hydroxide Ions in Amorphous Solid Water: Hydroxide Goes Brownian while Hydronium Hops

Du Hyeong Lee; Cheol Ho Choi; Tae Hoon Choi; Bong June Sung; Heon Kang

doi:10.1021/jz501235y

Asymmetric Transport Mechanisms of Hydronium and Hydroxide Ions in Amorphous Solid Water: Hydroxide Goes Brownian while Hydronium Hops

J Phys Chem Lett. 2014 Aug 7;5(15):2568-72. doi: 10.1021/jz501235y. Epub 2014 Jul 23.

Authors

Du Hyeong Lee¹, Cheol Ho Choi², Tae Hoon Choi³, Bong June Sung⁴, Heon Kang¹

Affiliations

¹ †Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-747, Republic of Korea.
² ‡Department of Chemistry and Green-Nano Materials Research Center, College of Natural Sciences, Kyungpook National University, Taegu 702-701, Republic of Korea.
³ §Department of Chemical Engineering Education, Chungnam National University, Daejeon 305-764, Republic of Korea.
⁴ ∥Department of Chemistry and Research Institute of Basic Science, Sogang University, Seoul 121-742, Republic of Korea.

PMID: 26277944
DOI: 10.1021/jz501235y

Abstract

The diffusion of hydronium (H3O(+)) and hydroxide (OH(-)) ions is one of the most intriguing topics in aqueous chemistry. It is considered that these ions in aqueous solutions move via sequential proton transfer events, known as the Grotthuss mechanisms. Here, we present an experimental study of the diffusion and H/D exchange of hydronium and hydroxide ions in amorphous solid water (ASW) at 140-180 K by using low-energy sputtering (LES) and temperature-programmed desorption (TPD) measurements. The study shows that the two species transport in ASW via fundamentally different molecular mechanisms. Whereas hydronium ions migrate via efficient proton transfer, hydroxide ions move via Brownian molecular diffusion without proton transfer. The molecular hydroxide diffusion in ASW is in stark contrast to the current view of the hydroxide diffusion mechanism in aqueous solution, which involves proton transfer.

Keywords: amorphous solid water; hydronium; hydroxide; ice; platinum; surface analysis.