We study the impact of the addition of particles of a range of sizes on the phase transition behavior of lung surfactant under compression. Charged particles ranging from micro- to nanoscale are deposited on lung surfactant films in a Langmuir trough. Surface area versus surface pressure isotherms and fluorescent microscope observations are utilized to determine changes in the phase transition behavior. We find that the deposition of particles close to 20 nm in diameter significantly impacts the coexistence of the liquid-condensed phase and liquid-expanded phase. This includes morphological changes of the liquid-condensed domains and the elimination of the squeeze-out phase in isotherms. Finally, a drastic increase of the domain fraction of the liquid-condensed phase can be observed for the deposition of 20-nm particles. As the particle size is increased, we observe a return to normal phase behavior. The net result is the observation of a critical particle size that may impact the functionality of the lung surfactant during respiration.

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.