Highly parallel, nanoscale stripe morphology in mixed phospholipid monolayers formed by Langmuir-Blodgett transfer

Atomic force microscopy (AFM) has been used to investigate the morphology of mixed monolayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) deposited onto mica by the Langmuir-Blodgett technique. At compositions near the critical point where phase separation is first observed by AFM, this binary lipid mixture yields a regular nanometer-scale pattern of highly parallel stripes. The area fraction covered by the stripes in the AFM images, their height, and mechanical properties are consistent with phase separation into solidlike DPPC stripe domains and a liquidlike DLPC matrix. The orientation of the stripes has been found to always be perpendicular to the film transfer direction, even when the substrate orientation is varied. This indicates that the film deposition process plays an important role in the generation of the stripe pattern. AFM images of films deposited onto mica at different surface pressures and substrate pulling speeds provide physical insight into the process of stripe formation.