In a study just published in Science, Graphene Flagship researchers describe the dielectric properties of water layers only a few molecules thick. Previously, scientists had predicted that such thin layers should exhibit a reduced electric response. Now, this new study shows that atomically thin layers of water near solid surfaces do not respond to electric fields, a finding with very important implications for understanding of many phenomena, including life.
Although water molecules are small and seemingly simple, they have rather complex properties –many of which remain poorly understood. Among them is its ability to dissolve substances much better than any other solvent – the reason water is known as 'the universal solvent.' Water molecules have two poles, two opposite electric charges placed at the ends of its structure. This allows water to easily dissolve salts and sugars, whereas substances like oils are repelled. This property – known as polarizability – plays also an important role in the structuring of the molecules of life, like proteins and nucleic acids.
For decades, scientists have tried to figure out how water behaves on a microscopic scale, in the immediate vicinity of other substances, solid surfaces and macromolecules. The quest has finally succeeded due to collaborative efforts of the groups of Laura Fumagalli and Nobel Prize winner Professor Sir Andre Geim, researchers at the National Graphene Institute of the University of Manchester, one of the partners of the Graphene Flagship project. They combined two recently developed technologies. First, they created nanometric channels and accommodated a few layers of water there. Then, they developed an innovative technique to probe the dielectric constant of water inside such nanochannels.
Fumagalli, lead author of the paper, explains how 'the existence of a low-polarizable water layer near surfaces is central to many scientific disciplines, and its nature has been much debated for almost a century.' To resolve this, scientists needed very specific tools to controllably measure the dielectric constant on a very small scale. 'We have done this now,' she says.
The researchers have found that the electric response of the confined water is not only suppressed but completely absent. In other words, the water inside nanochannels was 'electrically dead' with its dipoles immobilized and unable to screen an external field. This contrasts with bulk water, where molecules easily align along an electric field. The thickness of the dead layer was found to be less than one nanometer, two to three molecules thick. 'Water covers every surface around us. This layer is only a few atoms thick. We don't see it, but it is there and important,' explains Fumagalli.
'This anomaly is not just an academic curiosity but has clear implications for many fields and for life sciences in particular,' explains Geim. 'Electric interactions with water molecules play an important role in shaping biological molecules such as proteins. One can probably claim that interfacial water shapes the life as we know it, both literally and figuratively,' he adds.
Vladimir Falko, leader of the Enabling Science Work Package of Graphene Flagship project, thinks that 'this work represents an important experimental proof of the suppressed polarisability of water molecules on surfaces, which is important for understanding the water dynamics at the nanoscale.'
Professor Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship, and Chair of its Management Panel, added: 'As the Flagship moves along the roadmap planned to take graphene and related materials from the lab to the factory floor, fundamental research is always at the core of what we do. Flagship researchers continue to produce state of the art results keeping Europe at the forefront of basic science. This work, uncovering novel properties of a most common substance like water, thanks to the interaction with graphene, shows yet again how the unique properties of this material can be used to shape the world around us.'
The work was done in collaboration with a group led by Prof Gabriel Gomila at the University of Barcelona (Spain) who carried out computer simulations and a group led by Prof K. Watanabe at the National Institute for Materials Science (Japan) who provided hexagonal boron nitride crystals.
Anomalously low dielectric constant of confined water, by L. Fumagalli, A. Esfandiar, R. Fabregas, S. Hu, P. Ares, A. Janardanan, Q. Yang, B. Radha, T. Taniguchi, K. Watanabe, G. Gomila, K. S. Novoselov, and A. K. Geim. Science 2018, 360 (6395), 1339-1342, DOI: 10.1126/science.aat4191.