Nanoscale ice formations resembling the double helices of DNA will form when water molecules are frozen inside carbon nanotubes, detailed computer simulations suggest.

Researchers at the University of Nebraska, US, used a supercomputer to run detailed mathematical models of the behaviour of water molecules. In their simulations, they inserted the molecules into carbon nanotubes under high pressure, before cooling them to -23°C.

The scientists were surprised to see the molecules organise themselves into "spiral staircase" arrangements similar to those of a DNA helix. "It was very unexpected," Xiao Cheng Zeng, the computational nanotechnology expert who led the research told New Scientist. "We had expected ice to form into tube structures that have been observed before inside carbon nanotubes."

The simulations involved modelling the behaviour of water molecules packed inside nanotubes measuring between 1.35 and 1.9 nanometres in diameter, under pressures of 10 to 40,000 atmospheres. The combination of such a confined environment and such extreme pressures distorted the hydrogen bonds within each water molecule in ways never seen before, Zeng says.

Spectacular sight

Under most conditions the simulated molecules formed the expected tubular structures. However, those in which water was squeezed into a 1.35-nm-diameter nanotube at around 40,000 atmospheres of pressure saw ice form into a spectacular double walled ice helix (Fig. 1, right).

The inner wall is a four-stranded helix (Fig. 2) and the outer wall consists of four double-stranded helices, each also resembling a DNA double helix (Fig. 3).

Zeng was part of a team that previously discovered the formation of tiny tubes of ice inside carbon nanotubes in 2001, confirming computer simulations performed several years earlier. "I expect these new predictions to also be confirmed," Zeng adds. "Experimental researchers can use infrared spectroscopy or neutron scattering to look for these new structures."

Journal reference: Proceedings of the National Academy of Sciences (DOI:10.1073_pnas.0608401104)