Particle Size Effects on Collapse in Monolayers

We report on the impact of differently sized particles on the collapse of a Langmuir monolayer. We use an SDS–DODAB monolayer because it is known to collapse reversibly under compression and expansion cycles. Particles with diameters of 1 μm, 0.5 μm, 0.1 μm, and 20 nm are deposited on the SDS–DODAB monolayer. We find a critical particle size range of 0.1 to 0.5 μm that produces a transition from reversible to irreversible collapse. The nature of the collapse is determined through optical observations and surface pressure measurements. In addition, although 20 nm particles do not cause irreversible collapse in the monolayer, they significantly decrease the collapse pressure relative to the other systems. Therefore, we observe three distinct collapse behaviors—reversible, irreversible, and reversible at a reduced surface pressure.