Confinement changes water properties
doi:10.1038/nindia.2010.143 Published online 15 October 2010
The behaviour of water has remained largely unexplored at the molecular level.
A joint research team from the Wayne State University, Michigan and Indian Institute of Science Education and Research (IISER), Pune has pried into extremely thin layers of 'ultrapure' water. They have looked at water compressed to single molecular layers confined in nano sized spaces. Their research is expected to shed new light on the viscosity and elastic properties of water molecules.
"This will help better understand water confined in small spaces such as in between molecules in living cells. This will also help study the influence of water structure on the structure of biological macromolecules such as proteins," says lead researcher Peter M. Hoffmann from the Department of Physics and Astronomy, Wayne State University. The research also describes how liquids behave in narrow channels aiding in the design of lab-on-chip devices, he says.
In nanotribology (study of friction at atomic length and time scales) and nanofluidics, the behavior of molecularly thin water films is crucial. Water confined in nano-spaces behaves in a unique manner. Atomic Force Microscope (AFM) and surface force apparatus measurements have earlier suggested that water layers confined between water-loving surfaces exhibit sharp rise in viscosity and elasticity. But, several other studies under similar conditions have shown little change in viscosity of water.
To get a definite picture, the researchers used a small-amplitude AFM technique to perform visco-elastic measurements of molecularly confined ultrapure water layers. The AFM consists of a cantilever with a sharp tip made of silicon dioxide. The AFM tip was made to oscillate and move towards a mica surface. Both the tip and mica surface were immersed in a cell full of water.
The study showed a sharp transition in water behaviour – it behaved liquid-like below 8 angstrom per second (Å/s) speed of the tip and 50% solid-like at 8 Å/s when the water layer was squeezed to a single molecular layer. (An angstrom is 10−10 meter or 0.1 nanometre).
As the speed of the tip was increased, the probability of a single layer behaving solid-like increased to almost 100%. The probability of solid-like behavior increased with approach speed and decreased with film thickness. Dynamic solidification was observed only when the film was less than 1 nm in thickness.
"These findings may explain previous contradictory findings and may have important implications for nanofluidic systems and dynamics of macromolecular motion in cells," Hoffmann says. For nanomechanical devices, the fact that water and other liquids switch their behavior so dramatically upon changes of squeeze rates may allow for the design of 'smart devices' where motions of nanoscale parts need to be controlled, he says.
"This has a lot of implication in nanoscale lubrication," says co-researcher Shivprasad Patil of IISER. "Another interesting situation is the interfacial water in biological systems such as cells where 70% water is confined water," he adds.