Elasticity of Langmuir monolayer detected by dynamic oscillation method

A dynamic oscillation method has been used to measure the dynamic elasticity as a function of mean molecular area for Langmuir monolayers of stearic acid on the subphases of pure water, CdCl₂ and TbCl₃ solution, respectively. The elasticity-area curves nicely revealed the phase transition(s) taking place during the compression. A local sudden fall in elasticity was found for stearic acid monolayer in liquid state on water as well as on CdCl₂ subphase. By contrast, no such discontinuities could be observed in the case of TbCl₃ subphase. It is convinced that the nature of subphase plays an important role in the formation of the monolayer. However, from the suggested molecular structure model, it can be concluded that the elasticity of monolayer is mainly dominated by the interaction between long hydrocarbon chains of stearic acid molecules, but not the static electric interaction between the positive metal ions.