Lipophilic Force Driven Dynamics in Langmuir monolayers: In-plane Coalescence and Out-of-plane Diffusion

Abstract: While monolayer area fraction versus time ($A_{n}-t$) curves obtained from surface pressure-area ($\pi-A$) isotherms for desorption-dominated (DD) processes in Langmuir monolayers of fatty acids represent continuous loss, those from Brewster Angle Microscopy (BAM) also show a 2D coalescence. For nucleation-dominated (ND) processes both techniques suggest competing processes, with BAM showing 2D coalescence alongside multilayer formation. $\pi$ enhances both DD and ND with a lower cut-off for ND, while temperature has a lower cut-off for DD but negligible effect on ND. Hydrocarbon chain length has the strongest effect, causing a cross-over from DD to ND dynamics. Imaging Ellipsometry (IE) of horizontally transferred films onto Si(100) shows Stranski-Krastanov (SK) like growth for ND process in arachidic acid monolayer resulting in succesive stages of monolayer, trilayer, multilayer islands, ridges from lateral island-coalescence and shallow wavelike structures from ridge-coalescence on the film surface. These studies show that lipophilic attraction between hydrocarbon chains is the driving force at all stages of long term monolayer dynamics.