Fragile structured layers on surfaces detected by dynamic atomic force microscopy in aqueous electrolyte solutions

In the present study, the detailed characteristics of the layers of water molecules, ions and hydrated ions adsorbed on surfaces in NaCl solutions were investigated by introducing the dynamic method in the use of atomic force microscopy (AFM). We found the following. (1) There exist two kinds of structured layers on solid surfaces in electrolyte solutions — one is a thin, but firm primary layer in which water molecules, cations and hydrated cations are adsorbed directly on the solid surface, and the other is a thick, but fragile secondary layer outside the primary layer. (2) The thickness of the primary layer varies from 0.35 to 1.0 nm with the concentration of NaCl solution. (3) The secondary structure is detectable by introducing the dynamic method in the use of AFM, and the maximum gap at which the fragile structure starts to interact between surfaces was found to be around 4.0 nm at the maximum sensitivity of the present study.