Guest-free monolayer clathrate and its coexistence with two-dimensional high-density ice

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 · 8 2 -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-I h ∕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-I h and laboratory water.2D high-density ice | 2D low-density ice | 2D monolayer ice clathrate | Ostwald rule of stages | tensile limit of 2D liquid N atural gas is largely reserved in nature in the form of gas clathrates on the world's ocean floors (1-7). In the laboratory, gas clathrates can be produced by bringing a nonpolar gas in direct contact with liquid water under moderate pressure and at low temperature (1,(8)(9)(10)(11)). Since gas clathrates possess an open-cage structure to accommodate small gas molecules, gas clathrates can be characterized by two parameters: (1) the minimum gas pressure needed to stabilize the clathrates, and (2) the degree of occupancy or the fraction of cages occupied by the gas molecules. Although structural and thermodynamic properties of many gas clathrates are well characterized, the kinetic process of 3D clathrate formation is still less understood (1, 12). In an attempt to gain molecular insight into kinetics of clathrate formation, we carried out molecular dynamics (MD) simulations of clathrate formation within a slit nanopore whose width D is on the scale of 0.6 nm (see Materials and Methods). Because the slit nanopore can only accommodate one molecular layer of water, the timescale required for the formation of the quasi-2D gas clathrate is within the reach of MD simulation (typically 10-10 2 ns). Results and DiscussionsWe first considered a binary fluid mixture of water and argon (with 11.1% mole fraction of Ar) confined to the slit nanopore (D ¼ 0.62 nm) whose two opposing walls are smooth and hydrophobic (13-15). The fluid mixture was ...

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

“…Here, the polyamorphism is plausible because the metastable phase is strongly path-dependent; namely, the final structure depends on the initial structure. In stark contrast, no such metastable phase region was found between the ML-LDI and ML-HDI phases for ML water (15).…”

“…simulations. The stability lines of the stable phases and metastability lines of metastable amorphous phases are also estimated from the molecular dynamics (MD) simulation (15,27) and shown schematically in Fig. 3B.…”

“…Evidence of amorphous-to-amorphous transition in quasi-1D pores, however, has not been observed either from experiments or simulations. For water confined to quasi-2D slit pores with smooth walls and a width <6 Å, two distinctive crystalline forms of monolayer (ML) ice have been reported based on computer simulations, namely, the Archimedean 4·8 2 monolayer low-density ice (ML-LDI) and the puckered square monolayer high-density ice (ML-HDI) (14,15). Such ML-LDI has also been found as a stable structure on a hydroxylated silica surface, but it is not free-standing due to strong interaction between water and the hydrophilic surface (16).…”

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…3,[14][15] Theoretical investigations of the structures of the two-dimensional (2D) water confined in between the flat walls have suggested puckered rhombic monolayer ice, planar hexagonal, or amorphous phases depending on the conditions and models employed in the simulations. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Although the structures of confined water have been predicted for a variety of dimensions and materials using molecular dynamics (MD) simulations, the first experimental observation of the 2D water in between the two graphene sheets was obtained very recently using high-resolution transmission electron microscopy measurements (TEM). 30 This observation revealed the formation of a monolayer of planar "square" ice with a high packing density and, depending on the inter-graphene distance, the formation of bi-and trilayer crystallites of water.…”

AA-stacked bilayer square ice between graphene layers

“…The phase diagram of water and its extraordinary properties have been an interesting topic of research in biology, chemistry, and physics for many decades. Depending on the hydrophobic confinement width several two-dimensional ice structures can be formed [1][2][3][4] . Different theoretical methods, e.g.…”