Continuous and First-Order Liquid–Solid Phase Transitions in Two-Dimensional Water

Abstract

Understanding the phase behaviors of nanoconfined water is of importance in fundamental physical science and nanofluidic applications. Herein, we perform sub-microsecond to microsecond long molecular-dynamics (MD) simulations to show evidence of continuous and first-order phase transitions of water confined between two smooth walls with width of h = 1.0 nm. At either relatively low lateral pressure (P L ≤ 10 MPa) or relatively high lateral pressure (P L ≥ 400 MPa), the freezing of the confined water undergoes a first-order phase transition and gives rise to bilayer low-density amorphous (BL-LDA) ice and the trilayer puckered high-density ice (TL-pHDI), respectively. Very interestingly, within a moderate range of lateral pressures (100 MPa ≤ P L ≤ 300 MPa), the confined water appears to undergo a continuous phase transition in the isobaric condition to form a new phase, namely, the bilayer and puckered high-density amorphous (BL-pHDA) ice. A similar continuous phase transition behavior has been reported previously in tens of nanoseconds MD simulations of the freezing of BL water into the BL flat rhombic ice within a narrower hydrophobic nanoslit (h = 0.8 nm) and in the isochoric condition at high densities of water (Nat. Phys.20106685). Our simulation results on the pressure-dependent continuous and first-order phase transitions of the confined water extend the previous study in a different way and thereby provide new insights into the novel thermodynamic phase behavior of low-dimensional water in nanoscale confinement.

New insights into the interactions between two‐dimensional ice and two‐dimensional materials

“…Meanwhile, the 2D water-2D ice phase transition can either be discontinuous (first order) or continuous (second order) depending on the confined water density. 46,58 The density of molecules is an uncertain factor in the open system, as opposed to the finite number of molecules in simulation work. Therefore, intercalating vapor or liquid water into 2D layered materials may not necessarily result in the formation of 2D ice because the density of intercalating water molecules is difficult to control.…”

“…Conversely, the unique vdW interaction can also impact the behavior of confined 2D ice. Meanwhile, the 2D water–2D ice phase transition can either be discontinuous (first order) or continuous (second order) depending on the confined water density 46,58 . The density of molecules is an uncertain factor in the open system, as opposed to the finite number of molecules in simulation work.…”

“…34,44,45 It can accumulate as liquid droplets that are sealed between the 2D layers 25 or transform into different phases of 2D ice through continuous or discontinuous transitions (Figure 1a). 16,26,46 On the other hand, atomically thin self-flattened 2D ice can serve as a supportive layer to reduce the instability of 2D layered materials, thereby improving their intrinsic properties. In this context, recent studies have revealed intriguing findings on confined 2D ice, such as its ability to enhance lubricity on the surface of 2D layered materials.…”

Nanoporous ices: an emerging class in the water/ice family

“…Research studies on nanoporous ices have gained increasing attention in the field of water science, 7,8 motived in part by the intensive studies of low-dimensional ices, [9][10][11][12][13][14] heterogeneous ice nucleation on surfaces, [15][16][17][18] as well as gas clathrate hydrates. [19][20][21][22][23][24] The latter belongs to a class of host-guest compounds with the hydrogen-bonded water framework as the host, while the nano-cavities embedded in the frameworks are occupied by gas molecules as the guest.…”