Diameter-dependent hydrophobicity in carbon nanotubesSingle-wall carbon nanotubes (SWCNTs) are a good model system that provides atomically smooth nanocavities. It has been reported that water-SWCNTs exhibit hydrophobicity depending on the temperature T and the SWCNT diameter D. SWCNTs adsorb water molecules spontaneously in their cylindrical pores around room temperature, whereas they exhibit a hydrophilic-hydrophobic transition or wet-dry transition (WDT) at a critical temperature T wd ≈ 220-230 K and above a critical diameter D c ≈ 1.4-1.6 nm. However, details of the WDT phenomenon and its mechanism remain unknown. Here, we report a systematic experimental study involving X-ray diffraction, optical microscopy, and differential scanning calorimetry. It is found that water molecules inside thick SWCNTs (D > D c ) evaporate and condense into ice Ih outside the SWCNTs at T wd upon cooling, and the ice Ih evaporates and condenses inside the SWCNTs upon heating. On the other hand, residual water trapped inside the SWCNTs below T wd freezes. Molecular dynamics simulations indicate that upon lowering T, the hydrophobicity of thick SWCNTs increases without any structural transition, while the water inside thin SWCNTs (D < D c ) exhibits a structural transition, forming an ordered ice. This ice has a well-developed hydrogen bonding network adapting to the cylindrical pores of the SWCNTs. Thus, the unusual diameter dependence of the WDT is attributed to the adaptability of the structure of water to the pore dimension and shape. Published by AIP Publishing.[http://dx

Water in a nanoconfined geometry has attracted great interest from the viewpoint of not only basic science but also nanofluidic applications. Here, the rotational dynamics of water inside single-walled carbon nanotubes (SWCNTs) with mean diameters larger than ca. 1.4 nm were investigated systematically using 2H nuclear magnetic resonance spectroscopy with high-purity SWCNTs and molecular dynamics calculations. The results were compared with those for hydrophilic pores. It was found that faster water dynamics could be achieved by increasing the hydrophobicity of the pore walls and decreasing the pore diameters. These results suggest a strategy that paves the way for emerging high-performance filtration/separation devices. Upon cooling below 220 K, it was found that water undergoes a transition from fast to slow dynamics states. These results strongly suggest that the observed transition is linked to a liquid-liquid crossover or transition proposed in a two-liquid states scenario for bulk water.

We report the observation of the intrinsic magnetic susceptibility of highly purified SWCNT samples prepared by a combination of acid treatment and density gradient ultracentrifugation (DGU). We observed that the diamagnetic susceptibility of SWCNTs increases linearly with increasing nanotube diameter. We found that the magnetic susceptibility divided by the diameter is a universal function of the scaled temperature. Furthermore, the estimated magnetic susceptibilities of pure semiconducting and pure metallic SWCNT samples suggest that they respond differently to changes in carrier density, which is consistent with theory. These findings provide experimental verification of the theoretically predicted diameter, temperature, and metallicity dependence of the magnetic susceptibility.The magnetism of carbon-based materials (i.e. graphene, graphite, carbon nanotubes, and fullerenes) has recently been the subject of intense research.1 These materials exhibit exotic magnetism such as ferromagnetism above room temperature, which is believed to be induced by defects in the graphitic network. Our understanding of the intrinsic magnetism in these materials without such intentionally introduced defects, however, has also been very limited due to magnetic impurities remaining in samples. In the present work, we investigated single-wall carbon nanotubes (SWCNTs), which are rolled-up tubes of graphene sheets that exhibit unusually anisotropic electrical and magnetic properties. 2-6In the present paper, we focus our attention on the magnetism of SWCNTs, because despite theoretical predictions of novel magnetic features, experimental studies of SWCNT magnetism have been very limited.The magnetism of SWCNTs is expected to be dominated by their orbital magnetic susceptibility, which is 2 orders of magnitude higher than the spin magnetic susceptibility. The magnetic susceptibility is strongly anisotropic: it shows a large diamagnetic response in a magnetic field perpendicular to the tube axis (χ ⊥ ). In contrast, the magnetic susceptibility in a magnetic field parallel to the tube axis (χ ) depends on whether the SWCNT is metallic or semiconducting. Approximately one-third of SWCNTs are metallic and the rest are semiconducting, depending on the chirality. The cylindrical shape of the SWCNT leads to an Aharonov-Bohm effect when a magnetic field is introduced parallel to the tube axis, 4-6 resulting in a paramagnetic (diamagnetic) χ for metallic (semiconducting) SWCNTs. Actually, the predicted large magnetic susceptibility anisotropy ∆χ = χ − χ ⊥ was estimated indirectly by magneto-optical experiments of aligned SWCNT sample in high magnetic fields.7-10 Furthermore, the magnetic susceptibility for SWCNTs depends linearly on the nanotube diameter d, and that there is universal scaling in scaled magnetic susceptibility χ/d as a function of scaled temperature k B T /∆ 0 , where ∆ 0 is the characteristic energy, and corresponds to the bandgap for a semiconducting SWCNT. 6Magnetic susceptibility measurements of SWCNTs, however, are extre...

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We have studied the manipulation and the separation of substances using the horizontal magneto-Archimedes effect. By ingenerating a high magnetic field gradient in the horizontal direction, it is possible to move the target substance to a designated position in the horizontal direction. We attempted to separate several kinds of valuable metals by applying the horizontal magnetic force to the metals falling vertically into a paramagnetic medium. In order to enhance the magnetic force, we set up the ferromagnetic core array in magnetic fields. According to the trajectory analysis for the metal particles and the magnetic separation experiments, we succeeded in dynamically separating the metal mixture dropped to the vertical direction due to the strong horizontal magnetic force in relatively low magnetic fields. It is expected that this method creates a new device for the separation of substances.