Department of Chemistry
Date of this Version
3-30-2010
Abstract
Three-dimensional (3D) gas clathrates are ice-like but distinguished from bulk ices by containing polyhedral nano-cages to accommodate small gas molecules. Without space filling by gas molecules, standalone 3D clathrates have not been observed to form in the laboratory, and they appear to be unstable except at negative pressure. Thus far, experimental evidence for guest‐free clathrates has only been found in germanium and silicon, although guest‐free hydrate clathrates have been found, in recent simulations, able to grow from cold stretched water, if first nucleated. Herein, we report simulation evidence of spontaneous formation of monolayer clathrate ice, with or without gas molecules, within hydrophobic nano-slit at low temperatures. The guest-free monolayer clathrate ice is a low-density ice (LDI) whose geometric pattern is identical to Archimedean 4 · 82-truncated square tiling, i.e. a mosaic of tetragons and octagons. At large positive pressure, a second phase of 2D monolayer ice, i.e. the puckered square high-density ice (HDI) can form. The triple point of the LDI/liquid/HDI three-phase coexistence resembles that of the ice-Ih∕water∕ice-III three-phase coexistence. More interestingly, when the LDI is under a strong compression at 200 K, it transforms into the HDI via a liquid intermediate state, the first direct evidence of Ostwald’s rule of stages at 2D. The tensile limit of the 2D LDI and water are close to that of bulk ice-Ih and laboratory water.
Includes "Supporting Information." Five movies (.mpg) are attached (below) as "Related files."
Movie S1
Zeng PNAS 2010 SM2.mpg (6866 kB)
Movie S2
Zeng PNAS 2010 SM3.mpg (10906 kB)
Movie S3
Zeng PNAS 2010 SM4.mpg (9946 kB)
Movie S4
Zeng PNAS 2010 SM5.mpg (11021 kB)
Comments
Published in Proceedings of the National Academy of Sciences USA 107:13 (March 30, 2010), pp. 5718-5722; doi: 10.1073/pnas.0906437107 Copyright © 2010 National Academy of Sciences USA. Used by permission.