Understanding of the interaction of water-based liquids with carbon at the nanoscale is of great importance for exploring the potential of carbon nanotubes in nanofluidic applications. Using hydrothermal synthesis, hydrophilic multiwall carbon nanotubes filled with an aqueous fluid have been produced. These closed tubes were synthesized from an equilibrated C-H-O fluid, in the presence of Ni catalyst, at 730-800 °C under 60-100 MPa pressure. The equilibrium fluid composition, as estimated using Gibbs energy minimization analysis, was 85.2% H_2O, 7.4% CH_4 and 7.4% CO_2. High-resolution analysis of the tube contents shows that the denser fluid is enriched with oxygen, while the lighter phase is oxygen-poor. The presence of oxygen on the outer surface of the tube suggests that tube walls are oxygen (OH) terminated. This may explain the observed hydrophilic behavior of the nanotubes.
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