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...

Methanol is used in various applications, such as fuel for transportation vehicles, fuel cells, and in chemical industrial processes. Conventionally, separation of methanol from aqueous solution is by distillation. However, this method consumes a large amount of energy; hence development of a new method is needed. In this work, molecular dynamics simulations are performed to investigate the effect of an electric field on water–methanol separation by carbon nanotubes (CNTs) with diameters of 0.81 to 4.07 nm. Without an electric field, methanol molecules fill the CNTs in preference to water molecules. The preference of methanol to occupy the CNTs over water results in a separation effect. This separation effect is strong for small CNT diameters and significantly decreases with increasing diameter. In contrast, under an electric field, water molecules strongly prefer to occupy the CNTs over methanol molecules, resulting in a separation effect for water. More interestingly, the separation effect for water does not decrease with increasing CNT diameter. Formation of water structures in CNTs induced by an electric field has an important role in the separation of water from methanol.

Carbon nanotubes (CNTs) are promising for water transport through membranes and for use as nano-pumps. The development of CNT-based nanofluidic devices, however, requires a better understanding of the properties of water molecules in CNTs because they can be very different from those in the bulk. Using all-atom molecular dynamics simulations, we investigate the effect of axial electric fields on the structure of water molecules in CNTs having diameters ranging from (7,7) to (10,10). The water dipole moments were aligned parallel to the electric field, which increases the density of water inside the CNTs and forms ordered ice-like structures. The electric field induces the transition from liquid to ice nanotubes in a wide range of CNT diameters. Moreover, we found an increase in the lifetime of hydrogen bonds for water structures in the CNTs. Fast librational motion breaks some hydrogen bonds, but the molecular pairs do not separate and the hydrogen bonds reform. Thus, hydrogen bonds maintain the water structure in the CNTs, and the water molecules move collectively, decreasing the axial diffusion coefficient and permeation rate.

We report ordered structures of water in carbon nanotubes that are different from previously found n-gonal ice nanotubes and may be called filled ice nanotubes. Spontaneous formation of the filled ice nanotubes is observed in molecular dynamics (MD) simulations of water at fixed densities and a fixed temperature. Outer layers of filled ice nanotubes are characterized by a roll-up vector (n, m) while inner files of molecules do not have definite ordered structures. With this notation the filled ice nanotubes are of the (6, 0), (7, 0), (8, 0) and (8, 1) types, the last of which has a helical structure in its outer layer whereas the outer layers of the first three have the same achiral structures as the n-gonal ice nanotubes. Structure analysis is done for their hydrogen-bond networks and average dipole moments.

Bioethanol has been used as an alternative energy source for transportation vehicles to reduce the use of fossil fuels. The separation of water-ethanol solutions from fermentation processes is still an important issue in the production of anhydrous ethanol. Using molecular dynamics simulations, we investigate the effect of axial electric fields on the separation of water-ethanol solutions with carbon nanotubes (CNTs). In the absence of an electric field, CNT-ethanol van der Waals interactions allow ethanol to fill the CNTs in preference to water, i.e., a separation effect for ethanol. However, as the CNT diameter increases, this ethanol separation effect significantly decreases owing to a decrease in the strength of the van der Waals interactions. In contrast, under an electric field, the energy of the electrostatic interactions within the water molecule structure induces water molecules to fill the CNTs in preference to ethanol, i.e., a separation effect for water. More importantly, the electrostatic interactions are dependent on the water molecule structure in the CNT instead of the CNT diameter. As a result, the separation effect observed under an electric field does not diminish over a wide CNT diameter range. Moreover, CNTs and electric fields can be used to separate methanol-ethanol solutions too. Under an electric field, methanol preferentially fills CNTs over ethanol in a wide CNT diameter range.