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.
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.