Polymorphism and polyamorphism in bilayer water confined to slit nanopore under high pressure

• Main Authors: Bai, Jaeil, Zeng, Xiao Cheng
• Format: Text
• Language: unknown
• Published: DigitalCommons@University of Nebraska - Lincoln 2012
• Subjects: bilayer water and ice; molecular dynamics simulation; bilayer methane hydrate; amorphous-to-amorphous transition; Analytical Chemistry; Chemistry; Materials Chemistry; Physical Chemistry; Methane hydrate
• Online Access: https://digitalcommons.unl.edu/chemzeng/132; https://digitalcommons.unl.edu/context/chemzeng/article/1130/viewcontent/Zeng_PNAS_2012_Polymorphism_and_polyamorphism.pdf

A distinctive physical property of bulk water is its rich solid-state phase behavior, which includes 15 crystalline (ice I–ice XIV) and at least 3 glassy forms ofwater, namely, low-density amorphous, highdensity amorphous, and very-high-density amorphous (VHDA). Nanoscale confinement adds a new physical variable that can result in a wealth of new quasi-2D phases of ice and amorphous ice. Previous computer simulations have revealed that when water is confined between two flat hydrophobic plates about 7–9 Å apart, numerous bilayer (BL) ices (or polymorphs) can arise [e.g., BL-hexagonal ice (BL-ice I)]. Indeed, growth of the BL-ice I through vapor deposition on graphene/Pt(111) substrate has been achieved experimentally. Herein, we report computer simulation evidence of pressure- induced amorphization from BL-ice I to BL-amorphous and then to BL-VHDA2 at 250 K and 3 GPa. In particular, BL-VHDA2 can transform into BL-VHDA1 via decompression from 3 to 1.5 GPa at 250 K. This phenomenon of 2D polyamorphic transition is akin to the pressure- induced amorphization in 3D ice (e.g., from hexagonal ice to HDA and then to VHDA via isobaric annealing). Moreover, when the BL-ice I is compressed instantly to 6 GPa, a new very-high-density BL ice is formed. This new phase of BL ice can be viewed as an array of square ice nanotubes. Insights obtained from pressure-induced amorphization and crystallization of confined water offer a guide with which to seek a thermodynamic path to grow a new form of methane clathrate whose BL ice framework exhibits the Archimedean 4·82 (square-octagon) pattern.