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...

A simple model of a hydrophobic polymer in water is studied. The model polymer, a chain of Lennard-Jones particles with a fixed bond length, is designed in such a way that it undergoes a coil-to-globule conformational change near room temperature upon heating in liquid water. At low temperatures (≲270 K), the polymer chain under vacuum takes a globular conformation, whereas in water, it adopts an extended form. At higher temperatures (≳320 K), the polymer has a more compact conformation in water than under vacuum. The same polymer chain in a nonpolar solvent is always extended and shows no sign of a coil-to-globule transformation up to 360 K. The heat-induced collapse of the polymer uniquely observed in water is not attributed to the hydrophobic effect on individual monomers, but it is correlated with the temperature dependence of the potential of mean force between two monomers at contact distance.

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The adsorption performance of dextransulfate-cellulose gel for low-density lipoproteins (LDL) in human plasma was investigated.Considering the mathematical model for the mass transfer of LDL, it was found that mass transfer was limited by pore diffusion within the gel. Maximum adsorption efficiency was obtained by the gel with an effective internal volume fraction in the range from 0.65 to 0.7.