Phase transitions in confined water nanofilms

Abstract

Bulk water has three phases: solid, liquid and vapour. In addition to undergoing a phase transition (of the first order) between them, liquid and vapour can deform continuously into each other without crossing a transition line-in other words, there is no intrinsic distinction between the two phases. Hence, the first-order line of the liquid-vapour phase transition should terminate at a critical point. In contrast, the firstorder transition line between solid and liquid is believed to persist indefinitely without terminating at a critical point 1 . In recent years, however, it was reported that inside carbon nanotubes, freezing of water may occur continuously as well as discontinuously through a first-order phase transition 2 . Here we present simulation results for water in a quasi-twodimensional hydrophobic nanopore slit, which are consistent with the idea that water may freeze by means of both firstorder and continuous phase transitions. Our results lead us to hypothesize the existence of a connection point at which first-order and continuous transition lines meet 3,4 .It is widely believed that a solid-liquid phase transition line does not terminate at a critical point, although it has not been possible to prove rigorously the non-existence of the solid-liquid critical point 1 . On the other hand, after the discovery of carbon nanotubes 5 , a solid-liquid phase transition inside these structures has been reported, together with the intriguing possibility of a solid-liquid critical point, presumably because of the constraints in packing inside carbon nanotubes 6 . In recent years, therefore, there has been a great deal of interest in the simulation report that freezing of water inside carbon nanotubes may occur continuously as well as discontinuously through a first-order transition 2,6,7 .As for the solid-liquid phase transition, two-dimensional systems are of particular interest. According to the Mermin-Wagner theorem, continuous symmetry in two dimensions cannot be broken at non-zero temperatures, which means that there is no true long-ranged crystalline order in two-dimensional systems 8 . However, this does not necessarily rule out the presence of a phase transition; instead the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory states that melting in two dimensions may involve two continuous transitions, one from the solid phase to a hexatic phase and the other from the hexatic phase to the liquid phase 9 . Although the KTHNY theory seems rather appealing and general, it is still controversial whether the KTHNY theory provides the only mechanism of the melting in two dimensions 9,10 . Here we present simulation results supporting the possibility that a solid-liquid phase transition of water in quasi-twodimensions may occur through both first-order and continuous phase transitions, depending on the density or pressure.We carry out extensive molecular dynamics simulations of the TIP5P model 11 of N = 256 water molecules confined between two unstructured and smooth hydrophobic plates (see ref. 12

Solid−liquid critical behavior of water in nanopores

“…Then, the variation of W ′ caused by the tube diameter or pressure might be responsible for the confined water undergoing the first-order or continuous solid-liquid phase change. As remarked above, there are other confined systems that exhibit both continuous and discontinuous solid-liquid phase transitions (3,4,6). Phase behaviors of these quasi-1D and quasi-2D systems, too, may be understood by examining a correspondence with the solvable models (34,35) or the 2D versions of such models.…”

“…water | solid−liquid critical point | carbon nanotube | ice | Widom line T he possibility of the solid-liquid critical point has been reported by computer simulation studies of various systems in quasi-one, quasi-two, and three dimensions that exhibit both continuous and discontinuous changes in thermodynamic functions and other order parameters (1)(2)(3)(4)(5)(6)(7). However, the idea that a solid-liquid phase boundary never terminates at the critical point is still commonly accepted as a law of nature, largely because of the famous symmetry argument (8,9) together with the lack of experimental observations.…”

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

“…[20][21][22] These simulations illustrate that at layer thicknesses below 1 nm, water molecules interacting with the outside surface (known as the exohedral surface) of the CNT have preferred orientations, and assume a spatially varying density profile (see Figure 1b) that is independent of CNT outer diameter. [21,22] On the other hand, these simulations show that water molecules interacting with the inside surface (known as the endohedral surface) are strongly influenced by confinement effects, [27][28][29] and assume density profiles that are a strong function of the CNT inner diameter. [20,21,30] While the scaling behavior of the water layer density observed in these simulations should be similar for CNTs with native wall defects (such as those ones used here), differences in interaction potentials mean that the results of these previous studies may not be representative of the current system of aligned CNTs.…”

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.…”