“Water confined in nanoscale one-dimensional channels is of great interest to biology, geology and materials science,” Alexander Kolesnikov of Argonne told nanotechweb.org. “An excellent model for such a system is water in single-walled carbon nanotubes, realized by the unique geometry of nanotubes and the weak interaction between the water molecules and carbon atoms.”
Kolesnikov and colleagues found that the nanotube-water consisted of a “square ice sheet wrapped into a cylinder next to the inner nanotube wall, with a water chain inside”. This configuration produces a hydrogen bond connectivity that is noticeably different from that in bulk water.
“The hydrogen bonds in nanotube-water are weakened and dynamic fluctuations due to bond formation/dissociation are enhanced, particularly along the direction of the water chain,” said Kolesnikov. “This leads to fluid-like behaviour at temperatures far below the freezing point of normal water.”
The researchers say that combining neutron-scattering data, large-scale molecular-dynamics calculations, and transport and thermodynamic properties can validate models of interatomic potentials, formation and dissociation of chemical bonds involving the water molecules. “This approach melds the different techniques toward a quantitative understanding of the fundamental processes and mechanisms for biological objects, which in turn provides guidance for the design and synthesis of new materials with optimal performance,” said Kolesnikov.
Now the plan is to look at water in smaller diameter single-walled carbon nanotubes - closer to the size of some of the narrow channels in transmembrane proteins. More detailed quasielastic-scattering measurements should also provide greater insight into the mechanism of nanotube-water diffusion. The team also intends to measure the thermodynamic properties of nanotube-water, which they expect to show strong temperature anomalies compared with bulk water.
The researchers reported their work in Physical Review Letters.