Phase diagram of water in carbon nanotubes

Abstract

A phase diagram of water in single-walled carbon nanotubes at atmospheric pressure is proposed, which summarizes ice structures and their melting points as a function of the tube diameter up to 1.7 nm. The investigation is based on extensive molecular dynamics simulations over numerous thermodynamic states on the temperature-diameter plane. Spontaneous freezing of water in the simulations and the analysis of ice structures at 0 K suggest that there exist at least nine ice phases in the cylindrical space, including those reported by x-ray diffraction studies and those unreported by simulation or experiment. Each ice has a structure that maximizes the number of hydrogen bonds under the cylindrical confinement. The results show that the melting curve has many local maxima, each corresponding to the highest melting point for each ice form. The global maximum in the melting curve is located at Ϸ11 Å, where water freezes in a square ice nanotube.ice ͉ nanopore ͉ melting point W ater in well characterized pores is a system of general interest because it serves as model systems for ''nonbulk'' or inhomogeneous water ubiquitous in biological (1) and geological (2, 3) systems as well as in nanostructured materials (4). Studies of such nonbulk water are of fundamental importance because it is believed that confined or interfacial water is highly relevant to properties and functions of the entire systems, e.g., those of ion channels (1) and clay minerals (2). X-ray diffraction studies (5, 6) show that water can fill inner space of open-ended single-walled carbon nanotubes (SWNTs) at ambient conditions and freezes into crystalline solids often referred to as ''ice nanotubes.'' The ice structures are characterized as stacked n-membered rings or equivalently as a rolled square-net sheet (7). The formation of the ice nanotubes in carbon nanotubes has also been observed by NMR (8), neutron diffraction (9), and vibrational spectroscopy (10) studies. A prediction of the spontaneous ice formation in carbon nanotubes was made in a molecular dynamics (MD) simulation study (11). It was shown that the confined water freezes into square, pentagonal, hexagonal, and heptagonal ice nanotubes, and unexpectedly it does so either continuously (unlike any bulk substances, including bulk water) or discontinuously (despite of the fact that it is essentially in one dimension), depending on the diameter of carbon nanotubes or the applied pressure. Recent simulation studies predicted spontaneous formations of octagonal ice nanotubes (10, 12), ice nanotubes with hydrophobic guest molecules (13), single-layer helical ice sheets (14), and multiwalled ice helices and ice nanotubes (15)(16)(17). The versatility of ice we know for bulk water seems to survive in the nano confinement.Of the properties of water in the well defined nanopores, a fundamental yet little known aspect is a global picture of the phase behavior: we do not know pore-size dependence of the melting point in the nanometer scale or conditions for gradual and abrupt freezing. Previous re...

Diameter-dependent ion transport through the interior of isolated single-walled carbon nanotubes

“…The temperature dependence of pore blocking can shed light on the transport mechanism, and determine the influence of structured, solid water as suggested in simulations 27,37 . Reversibly heating the platform (Fig.…”

“…These properties potentially allow for interesting desalination 22 and DNA-sequencing applications 13 , as well as the ability to probe basic fluid structure properties at the smallest of possible scales. For this reason, a large number of molecular simulations have demonstrated interesting diameterdependent phenomena such as ion selectivity 15,22 , rapid proton conduction 18,23 that can be caused by the unique water structures and geometric confinements [24][25][26] , and altered water-ice phase behaviour 27 . However, most studies have focused on theoretical simulations, and their experimental verification is still a subject of debate [28][29][30][31] .…”

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…[1][2][3][4][5][6][7][8][9][10][11] Inside a one-dimensional channel of carbon nanotubes, for example, water molecules could undergo unconventional phase transitions [12][13] and form ice-like structures at room temperatures depending on the channel diameter. Also, a delicate balance between entropy and enthalpy can render these confined water thermodynamically more stable than the bulk water.…”

Critical behaviour of the Ising ferromagnet confined in quasi-cylindrical pores: A Monte Carlo study

“…the inset of Figure 9), the best fit of our results yields ξ (R) ∝ R with a proportionality constant B = 0.43 (2). It is worth mentioning that the value of the proportionality constant cannot be fixed by any simple phenomenological scaling argument and it depends on the boundary conditions used.…”

“…[1][2][3][4] The interplay between the finite-size of the confinement geometry and the surface effects due to the interaction of the physical system with the walls of the container modifies the phase behaviour of the confined system as compared to the bulk. [5][6][7][8][9] In fact, phase transitions are often shifted due to surface effects and rounded due to finite-size effects.…”