Nobody ever said water was simple. Now, scientists in Switzerland have added another complication to an already complex picture of this ubiquitous and essential solvent - they have chilled water to 10 Kelvin without it freezing. Of course, the trick relies on confining the water molecules in nanoscopic lipid channels to preclude ice formation. [Manni, L. S, et al., Nature Nanotechnol. (2019) DOI: 10.1038/s41565-019-0415-0]

Raffaele Mezzenga and Ehud Landau and their colleagues at ETH Zurich and the University of Zurich, Switzerland, have developed an intriguing way to prevent water from forming ice crystals, so that it retains the amorphous nature of the liquid even close to absolute zero. The first step was to come up with designer lipids that could form a "soft" material, a lipidic mesophase. The lipids undergo spontaneous self-assembly and form membranes just as would natural fat molecules. The lipidic mesophase contains a network of interconnected nano-channels, the exact internal structure is dependent on temperature and the amount of water present. Regardless, the channels are far too narrow to allow water molecules to lock together through hydrogen bonds when freezing, no matter how cold the water gets.

At liquid helium temperature, the team could cool a lipidic mesophase made from a modified monoacylglycerol 10 Kelvin without ice crystallization occurring. The water in the channels is not like liquid water, but adopts a "glassy" phase, at least according to the team's simulations.

"The key factor is the ratio of lipids to water," Mezzenga from ETH's Laboratory of Food & Soft Materials explains. The soft biomaterial formed from the lipid membranes and water has a complex structure that minimises the water's contact with the hydrophobic part of the lipids and maximises its interaction with the hydrophilic component. The systems resemble lipid membranes used by certain bacteria that allows them to survive extreme cold.

The new lipidic mesophases will be a useful research tool for studying large biomolecules in an environment that mimics a natural membrane allowing cryogenic electron microscopy to be carried out more easily with ice crystals damaging the membranes and ruining such experiments. In addition, the materials might be useful as a kind of biomimetic antifreeze for a range of applications.

Mezzenga points out that "Our main focus was to give researchers a new tool to facilitate the study of molecular structures at low temperature without ice-interfering crystals, and ultimately to understand how two main components of life, i.e. water and lipids, interact under extreme conditions of temperature and geometrical confinement."

3D model of the cubic motif of the novel lipid mesophase. Credit: ETH Zurich
3D model of the cubic motif of the novel lipid mesophase. Credit: ETH Zurich