Surface Reflectivity from Absorbed Films of Pulmonary Surfactant at the Air-Liquid Interface

Abstract

Pulmonary surfactant (PS) is the complex mixture of lipids and proteins that forms a thin film on the liquid layer that lines the alveolar air-sacks of the lungs. When compressed by the decreasing alveolar surface area during exhalation, the surfactant films reduce surface tension to exceptionally low levels. This behavior in situ contrasts with the performance of spread monolayers in vitro that contain the complete set of surfactant constituents, which collapse promptly when compressed below the equilibrium spreading pressure. The structural characteristics that provide the basis for this functional difference remain controversial. The studies here used X-ray reflectivity to compare the structure of adsorbed films and spread monolayers of extracted calf surfactant on the air-water interface. Our results show the presence in the adsorbed film of additional double layers of distinct electron densities underneath the interfacial monolayer. These results support previous evidence that adsorbed films of pulmonary surfactant have multilamellar thickness, and that the additional layers may have functional significance.