Atomic-force and fluorescence microscopy of Langmuir-Blodgett monolayers ofL-α-dimyristoylphosphatidic acid

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Abstract
We have obtained fluorescence and atomic-force-microscopy images of the liquid and solid phases of Langmuir-Blodgett monolayers of L-α-dimyristoylphosphatidic acid, some containing ∼1 mol % of L-α-phosphatidylethanolamine-N-4-nitrobenzo-2-oxa-1,3-diazole. The fluorescence microscopy images are similar to those of monolayers at the air-water interface, but slight differences indicate that changes occur in the monolayers as a result of the deposition process. The atomic-force-microscopy images show the clean glass substrates to be very much rougher than when the monolayers are deposited on them. We propose that a water layer provides a smooth surface on which the Langmuir-Blodgett monolayers rest. We measured a ∼25-Å height difference between the liquid and solid phase region, one explanation being that the alkyl chains of the molecule are standing up in the solid phase and lying down in the liquid phase. This result is consistent with the fluorescence and atomic-force-microscopy images which show an increase in the amount of solid phase domains as a result of the deposition process. Further, the atomic-force-microscopy images of the monolayers show features which have not been described before and which could not be attributed to the presence of the dye. Finally, we showed that if the Langmuir-Blodgett monolayers are not stored in a humid environment, they begin to collapse into multilayers.