Role of Cationic Organization on Water Dynamics in Saponite Clays

Water dynamics impact many phenomena, from geosciences to biology, especially in confined environments. In the presence of charged interfaces, there are some ions, the role of which with regard to the water dynamics is unclear. Here, a synthetic saponite clay, which is oriented in a film, is used as a confining medium in the bilayer state. It confines two water layers between negatively charged planes, the charge of which is compensated for by sodium cations. Water dynamics is determined both parallel and perpendicular to the charged clay layers with neutron spin echo. This technique gives access to long enough times and directly provides the intermediate scattering function that is calculated, on the other hand, by molecular dynamics simulations. These latter also enable the study of cation dynamics, which is not experimentally accessible on this time scale. The results point toward a huge role of these cations on the water dynamics, mainly through their local structure and localization between the charged confining planes.