Abstract
Single-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 Twd ≈ 220-230 K and above a critical diameter Dc ≈ 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 > Dc) evaporate and condense into ice Ih outside the SWCNTs at Twd 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 Twd 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 < Dc) 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.
Highlights
Hydrophobicity is believed to play an important role in the underlying mechanism of many natural phenomena, such as protein-folding,1 self-assembly of biological structures,2 and the self-cleaning function of lotus leaves.3 Promising applications of hydrophobicity include the fabrication of innovative biomedical materials,4 self-cleaning materials,5 and nanofluidic devices.6 Hydrophobicity is often characterized macroscopically by the contact angle of a water droplet on a surface
After X-ray diffraction (XRD) evidence for the wet-dry transition (WDT) or the emptying-filling phenomena in water-SWCNTs is shortly reviewed along with a new analysis to reveal effect of surface water adsorbed on the SWCNT bundles, it is clarified that water emerged from the SWCNTs below Twd is crystallized into ice Ih outside SWCNTs
The present study revealed that water inside SWCNTs with diameters D > Dc evaporates and crystallizes into ice Ih outside SWCNTs at Twd upon cooling
Summary
Hydrophobicity is believed to play an important role in the underlying mechanism of many natural phenomena, such as protein-folding, self-assembly of biological structures, and the self-cleaning function of lotus leaves. Promising applications of hydrophobicity include the fabrication of innovative biomedical materials, self-cleaning materials, and nanofluidic devices. Hydrophobicity is often characterized macroscopically by the contact angle of a water droplet on a surface. Water in SWCNTs that are smaller than a critical diameter Dc of around 1.4-1.6 nm freezes into crystal-like structures with one-dimensional periodicity.. Water in SWCNTs that are smaller than a critical diameter Dc of around 1.4-1.6 nm freezes into crystal-like structures with one-dimensional periodicity.26,28,29 These are referred to as ice-nanotubes (ice-NTs) or filled ice-NTs.. We present the results obtained using optical microscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) measurements to elucidate the details of this phenomenon. These results, combined with classical molecular dynamics (MD) simulations, are used to discuss the mechanism for the WDT and its relation to the water structures in cylindrical SWCNT pores
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