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

TL;DR

This study investigates the complex dynamics of Langmuir monolayers, revealing how lipophilic forces influence in-plane coalescence and out-of-plane diffusion, with effects of temperature, pressure, and chain length on monolayer behavior.

Contribution

It provides new insights into the role of lipophilic forces in monolayer dynamics, combining surface pressure measurements, microscopy, and imaging ellipsometry to elucidate growth and coalescence mechanisms.

Findings

Lipophilic attraction drives monolayer coalescence and multilayer formation.

Hydrocarbon chain length determines transition from desorption to nucleation dynamics.

Imaging ellipsometry shows SK-like growth with multilayer island formation.

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.