We report six phases of high-density nano-ice predicted to form within carbon nanotubes (CNTs) at high pressure. High-density nano-ice self-assembled within smaller-diameter CNT (17,0) exhibits a double-walled helical structure where the outer wall consists of four double-stranded helixes, which resemble a DNA double helix, and the inner wall is a quadruple-stranded helix. Four other double-walled nano-ices, self-assembled respectively in two largerdiameter CNTs (20,0 and 22,0), display tubular structure. Within CNT (24,0), the confined water can freeze spontaneously into a triple-walled helical nano-ice where the outer wall is an 18-stranded helix and the middle and inner walls are hextuplestranded helixes.carbon nanotube ͉ high density nano-ice ͉ nano-ice helix

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 of water, 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-VHDA 2 at 250 K and 3 GPa. In particular, BL-VHDA 2 can transform into BL-VHDA 1 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 4·8 2 (square-octagon) pattern.bilayer water and ice | molecular dynamics simulation | bilayer methane hydrate | amorphous-to-amorphous transition T he special character of confined water stems not only from unique properties of a hydrogen-bonding network, such as its ability to expand when cooled below the freezing point or to form rich structures of polymorphs under strong compressions, but from its spatial inhomogeneity, particularly in the nanoscale spaces. Forced to pack into the nanoscale spaces severely constricted by confining surfaces, water molecules in the vicinity of a flat surface tend to arrange themselves in layers parallel to the surface. The resulting oscillations in local density are reflected in properties of the confined water that can differ drastically from those of the bulk water. Not only are intriguing properties of confined water of fundamental interest, but they have implications for diverse practical phenomena at the intersection between chemistry, biological sciences, engineering, and physics: boundary lubrication in nanofluidic and laboratory-on-a-chip devices; frost heaving in soil; synthesis of antifreeze proteins for ice-growth inhibition; rapid cooling of biological suspensions or quenching emulsified water under high pressure; storage of gas hydrates; and hydrogen fuel cells that generate electricity by passing hydrogen ions across a membrane, where water is confined in nanoscale channels. Hence, an improved understanding of the behavi...

Understanding phase behavior of highly confined water, ice, amorphous ice, and clathrate hydrates (or gas hydrates), not only enriches our view of phase transitions and structures of quasi-two-dimensional (Q2D) solids not seen in the bulk phases but also has important implications for diverse phenomena at the intersection between physical chemistry, cell biology, chemical engineering, and nanoscience. Relevant examples include, among others, boundary lubrication in nanofluidic and lab-on-a-chip devices, synthesis of antifreeze proteins for ice-growth inhibition, rapid cooling of biological suspensions or quenching emulsified water under high pressure, and storage of H2 and CO2 in gas hydrates. Classical molecular simulation (MD) is an indispensable tool to explore states and properties of highly confined water and ice. It also has the advantage of precisely monitoring the time and spatial domains in the sub-picosecond and sub-nanometer scales, which are difficult to control in laboratory experiments, and yet allows relatively long simulation at the 10(2) ns time scale that is impractical with ab initio molecular dynamics simulations. In this Account, we present an overview of our MD simulation studies of the structures and phase behaviors of highly confined water, ice, amorphous ice, and clathrate, in slit graphene nanopores. We survey six crystalline phases of monolayer (ML) ice revealed from MD simulations, including one low-density, one mid-density, and four high-density ML ices. We show additional supporting evidence on the structural stabilities of the four high-density ML ices in the vacuum (without the graphene confinement), for the first time, through quantum density-functional theory optimization of their free-standing structures at zero temperature. In addition, we summarize various low-density, high-density, and very-high-density Q2D bilayer (BL) ice and amorphous ice structures revealed from MD simulations. These simulations reinforce the notion that the nanoscale confinement not only can disrupt the hydrogen bonding network in bulk water but also can allow satisfaction of the ice rule for low-density and high-density Q2D crystalline structures. Highly confined water can serve as a generic model system for understanding a variety of Q2D materials science phenomena, for example, liquid-solid, solid-solid, solid-amorphous, and amorphous-amorphous transitions in real time, as well as the Ostwald staging during these transitions. Our simulations also bring new molecular insights into the formation of gas hydrate from a gas and water mixture at low temperature.

Evaluation of the tensile/compression limit of a solid under conditions of tension or compression is often performed to provide mechanical properties that are critical for structure design and assessment. Algara-Siller et al. recently demonstrated that when water is constrained between two sheets of graphene, it becomes a two-dimensional (2D) liquid and then is turned into an intriguing monolayer solid with a square pattern under high lateral pressure [Nature2015519443445]. From a mechanics point of view, this liquid-to-solid transformation characterizes the compression limit (or metastability limit) of the 2D monolayer water. Here, we perform a simulation study of the compression limit of 2D monolayer, bilayer, and trilayer water constrained in graphene nanocapillaries. At 300 K, a myriad of 2D ice polymorphs (both crystalline-like and amorphous) are formed from the liquid water at different widths of the nanocapillaries, ranging from 6.0 to11.6 Å. For monolayer water, the compression limit is typically a few hundred MPa, while for the bilayer and trilayer water, the compression limit is 1.5 GPa or higher, reflecting the ultrahigh van der Waals pressure within the graphene nanocapillaries. The compression-limit (phase) diagram is obtained at the nanocapillary width versus pressure (h–P) plane, based on the comprehensive molecular dynamics simulations at numerous thermodynamic states as well as on the Clapeyron equation. Interestingly, the compression-limit curves exhibit multiple local minima.

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 ...