Researchers long suspected that the dielectric constant of water is lower at interfaces with other materials – what no-one knew was how much. “This is a huge issue,” says University of Manchester condensed matter lecturer and National Graphene Institute researcher Laura Fumagalli. “The value of the dielectric constant at the nanoscale was not clear at all and it has a lot of impact on a lot of phenomena.” These range from the study of proteins and DNA to electrochemistry and batteries. Now Fumagalli has teamed up with 2010 Nobel Laureate for the discovery of graphene Andre Geim, as well as colleagues in the UK, Iran, Spain and Japan, to report experimental evidence that the effect of interfaces on the dielectric constant of water is far greater than previously suspected.
The dielectric constant gives a measure of how well electric dipoles of molecules orient in an electric field. Water is a highly polar substance, so although the molecules can readily reorient in an electric field in the bulk, their alignment at surfaces can be inhibited, potentially diminishing the dielectric constant in interfacial water near surfaces compared with values found in bulk water. Establishing definite values for these effects has flummoxed researchers for decades.
Dielectric measurements get ultrasensitive
Fumagalli has long specialized in investigating the dielectric properties of structures at the nanoscale. In 2012 during her time at Institut de Bioenginyeria de Catalunya and Universitat de Barcelona, Fumagalli and colleagues in Barcelona and Madrid reported on a technique using electrostatic force microscopy with piconewton sensitivity that could identify nanoparticles with identical shape but different chemical composition by ultrasensitive measurements of the dielectric constant.
These experiments did not focus on water, but as Fumagalli points out, “Water is everywhere, even where you don’t want it there is a layer of water from the humidity of the environment.” Yet while her interest was piqued, applying the technique to water proved far from trivial. The success of the latest experiment hinged on Geim’s expertise in 2D materials.
Confinement device yields success
One of the challenges was producing a system to confine water at the nanoscale. Happily one of the many things 2D materials are good at is trapping water, so when Fumagalli joined the National Graphene Institute at Manchester, and spoke to Geim about the problem, a solution proved to be in sight.
“We started with something simpler but the results were not so convincing,” says Fumagalli. “We clearly needed the most advanced devices, and Andre Geim was able to produce them.” She describes how in 2016 and 2017 Geim introduced a new technology that allows the assembly of two-dimensional materials into devices with the smallest possible man-made channels. “Among many other possible applications, these devices allow us to study the transport and properties of water inside such tiny channels.”
The final system comprised slit-like channels fabricated from atomically flat crystals of graphite and hexagonal boron nitride. The researchers could set the heights of the channels to be as low as one nanometre in size so that they only accommodated a few layers of water.
“When you reduce the quantity of water you have, and the water is confined near surfaces so that you have just a few molecular layers there, the molecules are not free to move like in bulk water, and the dielectric constant goes down to two,” says Fumagalli. “This is the minimum value imaginable – so low people had not expected it.”
She highlights that this anomalously low value of the dielectric constant in confined water is in stark contrast to the anomalously high dielectric constant of bulk water, which is around 80. “Water is full of anomalies,” she adds.
When cold warms faster than hot
Result’s impact reaches far
Water, described as the universal solvent because so many other substances are soluble in it, has been dubbed “the solvent of life”. These solvation properties are directly linked to the dielectric constant, which highlights the impact of these results. No small wonder then that the researchers in Manchester remain very interested in water.
“It would be interesting to see if it behaves similarly with other surfaces and how it behaves near bio surfaces,” says Fumagalli. “How water is polarized near biomolecules makes a difference to the forces they experience and has a huge impact on their structuring and functions.”
Geim also emphasized the significance of the results in a press statement: “This anomaly in the dielectric constant of interfacial water is not just an academic curiosity but has clear implications for many fields and for life sciences, in particular. Our results can help to improve the understanding of the role of water in technological processes, and why it is so crucial for life. Electric interactions with water molecules play an important role in shaping biological molecules such as proteins. One can probably claim that interfacial water shapes life as we know it, both literally and figuratively.”
Other substances that may be subject to the same effect include any polar liquids. Studies of those used in batteries for energy storage may have particular relevance for industry.
Full details are reported in Science.
