Structures of surfactant films: a scanning force microscopy study

The alveolar lining layer is thought to consist of a continuous duplex laye r i.e., an aqueous hypophase covered by a thin surfactant film which is a m onolayer with dipalmitoyl-phosphatidylcholine (DPPC) as its most important component. Findings obtained by electron microscopy and results from in vit ro experiments suggest, however, that the structure and hence the structure -function relations of surfactant films are more complex. In order to bette r define their structures films of surfactants were studied by scanning for ce microscopy. Four different surfactants were spread on a Langmuir-Wilhelmy balance, and then transferred onto a solid mica plate by the Langmuir-Blodgett technique , under various states of film compression. Imaging of the films by scannin g force microscopy was performed in the contact (repulsive) made in air. The scanning force micrographs revealed that surfactant films are not homog eneous, but rather undergo phase transitions depending on the surface press ures. Even at comparable surface pressures different surfactants show quite different surface patterns. Differences in surface structure can even be o bserved in films containing surfactant proteins (SP)-B and SP-C. These observations give further evidence that the widely accepted hypothesi s of a regular monolayer of phospholipids governing the surface tension pro bably does not hold true, but that the structure-function relationship of s urface active surfactant films is even more complex than hitherto thought.