Observation of a two-step mechanism in the formation of a superstructure of cadmium-behenic acid Langmuir monolayer: evidence of an intermediate structure.

By means of grazing incidence x-ray diffraction, the structure of a behenic acid monolayer spread over chloride salt solutions of cadmium is observed to evolve from the tilted L2 phase to the superstructure (corresponding to an organized monolayer of ions in addition to the ordered organic film), through an intermediate phase. The studied salt concentrations are below the so-called "threshold" needed for the formation of this superstructure. This kinetic process involving two first-order phase transitions is confirmed by Brewster angle microscopy experiments and surface pressure-area isotherms measured at different times after monolayer deposition.