Universal Intrinsic Dynamics and Freezing of Water in Small Nanotubes

Abstract

Although it is well known that confinement alters the freezing point of water, the mechanism by which this occurs, particularly in small nanotubes, is still under active research. It was recently claimed [Agrawal et al. Nat. Nanotechnol. 12, 2017, 267] that the freezing point of water in a carbon nanotube of a certain size may be as high as its boiling point under bulk conditions or even higher. A more recent paper [Chiashi et al. ACS Nano. 13, 2019, 1177 reported that the change in the freezing point may not be as drastic as what was claimed by Agrawal et al., and may in fact be close to the bulk freezing point. Thus, aside from the need to resolve the issue, an important question that arises is whether such a behavior is universal and may happen in any type of nanotube. In other words, can the interactions between water molecules and the wall atoms in other types of nanotube suppress this effect, or is this a universal feature occurring in every type of nanotube? In this paper, we address these issues by carrying out extensive molecular dynamics (MD) simulations and studying the dynamics of water in silicon carbide nanotubes. The results indicate that the melting point can be as much as 100 K lower than the bulk value. A comparison between the hydrogen-bond networks formed in carbon and silicon carbon nanotubes indicates that weaker HB networks are the main cause of the depression of the freezing point. Several types of ice, including those with trigonal, square, and pentagonal cross sections, are identified, each of which exhibits a different melting point that depends on the nanotube's diameter. The effect of the partial charges of the atoms of the nanotube's walls on the strength of the hydrogen-bond network was also studied. Larger partial charges lead to a more rapid decay of the hydrogen-bond correlation function, signaling a lower melting point.

Spontaneous Dipole Reorientation in Confined Water and Its Effect on Wetting/Dewetting of Hydrophobic Nanopores

“…Does the tube shape affect the formation of square ice? Computer simulations demonstrate that this ice is formed in cylindrical CNTs, 23 silicon carbide nanotubes, 53 and irregularly shaped cylindrical channels of AFI-type PSZ. 54 These investigations show that ice formation has a general character and that the type of ice depends on the width of the pore and, to a small extent, on the chemistry or pore shape.…”

Hydrogen Bond Dynamics and Phase Transitions of Water inside Carbon Nanotubes

“…It is known that the sharp decrease in the peak of the radial distribution function is a sign of phase transition [ 35 ]. To quantify the possible transition temperature, we computed radial distribution functions of water molecules confined inside the CNT, which are defined as: where N is the total number of water molecules and is the distance between the i th and j th molecules along the x–y axis.…”

Molecular-Level Insights into Unique Behavior of Water Molecules Confined in the Heterojunction between One- and Two-Dimensional Nanochannels

“…Recently, Koḧler et al 35 have performed MD simulations to reveal that water molecules confined in different MoS 2 nanotubes display distinct structural transitions from singlefile to pentagonal ice to bulk-like patterns due to the variation of the HB network as the nanotube diameter. Sahimi and coworkers 41 further verified that weakened HBs should be responsible for the depression of the freezing by comparing the formation of HB networks in CNT and silicon-based nanotube. Besides the structures in 1-D nanoconfinement, the milestone MD work of Hummer et al 31 revealed that single-file water chain in (6,6) CNT has a pulselike diffusion pattern via bursts of their HB network (denoted as the singlefile diffusion) and yield an extraordinary flow speed of around 51 × 10 −14 cm 3 s −1 .…”

“…One-dimensional (1-D) materials, such as carbon nanotube (CNT), − MoS 2 nanotube, − silicon-based nanotube, − and other nanotubes, − have been researched as host materials to explore the nanoconfined behavior of water molecules, including structures, dynamics, and their relationships with the HB properties. For example, earlier MD simulations of Wang et al revealed that single-file water chains can be formed only in narrow CNTs with a diameter of 0.676–0.811 nm and the helicity of CNT rarely affects the structural properties.…”