Freezing inside nanotubes

Water in well-characterized pores is a system of general interest because it serves as model system for the non-bulk or inhomogeneous water that is ubiquitous in biological and geological systems, as well as in nano-sfructured materials. Often confined or interfacial water is highly relevant to the properties and functions of entire systems, e.g., those in ion channels and clay minerals. X-ray diffraction studies show that water can fill the inner space of open-ended single-walled carbon nanotubes (SWCNTs) under ambient conditions and freezes into crystalline solids. These are often referred to as ice nanotubes . Ice stmctures in confined systems are characterized as stacked n-membered rings or equivalently as a rolled square-net sheet. The formation of the ice nanotubes in CNTs has also been observed by NMR, neutron diffraction, and vibrational spectroscopy studies. 

Among the properties of water in well-defined nanopores, a global picture of the phase behavior is not yet available. We do not accurately know the pore-size dependence of the melting point in the nanometer scale or the conditions for gradual 

The appearances of such diverse ice structures in CNTs reveal the unique ability of water molecules to form stable or quasi-stable arrangements through HBs. Water is like the mythological demon that can take many shapes as and when required. 

It should be clear from the above that because of sustained efforts over many decades, significant progress has now been achieved in the understanding of the freezing of water into ice. However, there stiU remain many unsolved problems in this area. For example, we do not yet have a quantitative theory of the nucleation of ice in supercooled water. The molecular models we use in simulations are perhaps too primitive, as most of them do not include the polarizabihty of water molecules. The polarizability of water is large due to the two lone pairs of electrons on the lone oxygen atom. Perhaps one would need to consider quantum simulations to fully understand the freezing of ice. 

The consequence of the existence of large-scale density fluctuations due to the existence of the two forms of water (HDL and LDL) in the freezing of water is yet to be understood. So is the case for the freezing (or rather the lack of it) in CNTs. 

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