Universal Intrinsic Dynamics and Freezing of Water in Small Nanotubes

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.