A near-field microscopy study of submicron domain structure in a model lung surfactant monolayer

The submicron domain structure of coexisting liquid condensed (LC) and liquid expanded (LE) phases in monolayers composed of palmitic acid and 20 wt% of a lung surfactant protein B fragment has been investigated. Near-field microscopy was used to simultaneously measure topography and fluorescence images of monolayers that were prepared at a surface pressure of 15 mN/m and a temperature of 22°C. The use of a fluorescently tagged peptide allowed for unambiguous determination of the peptide location in the two-component system. The LC and LE phases in the monolayers are measured on the submicron length scale. A 6–11 Å height difference between the LC and LE phases was evident in the height images. Gradual transitions between the LC and LE domains were observed across a 1.3 μm length scale in the near-field fluorescence images, but were significantly sharper in the simultaneously collected topography images and in the separately measured AFM images. These results may reflect the occurrence of peptide encroachment into the LC domains.