Molecular packing changes of octadecylamine monolayers on mica induced by pressure and humidity

The molecular packing structure of octadecylamine molecules forming self-assembled monolayers on mica has been studied by AFM as a function of mechanical pressure and humidity. Upon increasing the load applied by the AFM tip, the thickness of the monolayer was observed to change discretely, which we explain as the result of stepwise tilting of the molecules. Thickness changes were also observed when the films were exposed to high humidity. In this case, we propose that the driving force for such a behavior is the relaxation of electrostatic repulsions between protonated amino groups.