Compression-Induced Topographic Corrugation of Air/Surfactant/Water Interface: Effect of Nanoparticles Adsorbed beneath the Interface

The compression isotherm of a Langmuir film can provide information on the structural order and density of interfacial molecules. When nanoparticles are introduced to the Langmuir film at an air/liquid interface, the compression isotherm is altered substantially. In this study, we report experimental evidence of topographic corrugation of the air/surfactant/water interface due to compression of nanoparticles adsorbed underneath the surfactant layer. A monolayer of cationic surfactants is suspended in a Langmuir trough with negatively charged nanoparticles in the aqueous subsurface. Changes in structural orders of the interfacial molecules and nanoparticles were characterized using in situ sum-frequency generation (SFG) spectroscopy during compression and associated with the modified compression isotherm in the presence of nanoparticles. This work elucidated changes in the compression isotherm due to the variation of surface geometry, which is required to produce a desired Langmuir film made of nanoparticles.