Atomic force microscopy (AFM) was used to investigate the structure, stability, and defects of the hydrophilic surfaces of Langmuir-Blodgett bilayer films of distearoylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylethanolamine (DPPE) in the solid phase, and dilinoleoylphosphatidylethanolamine (DLPE) in the fluid phase. Their relative resilience to external mechanical stress by the scanning tip and by fluid exchange were also investigated. DPPE monolayers showed parallel ridges at the surface with a period of 0.49 nm, corresponding to the rows of aligned headgroups consistent with the known crystallographic structure. DSPC and DLPE monolayers did not show any periodic order. The solid DSPC and DPPE monolayers were stable to continued rastering by the AFM tip; however, the stability of DLPE monolayers depended on the pH of the aqueous environment. Structural defects in the form of monolayer gaps and holes were observed after fluid exchange, but the defects in DLPE monolayer at pH 11 were stable during consecutive scanning. At pH 9 and below, the defects induced by fluid exchange over DLPE monolayers were more extensive and were deformed easily by consecutive scanning of the AFM tip at a force of 10 nN. The pH dependence of resilience was explained by the increasing bending energy or frustration due to the high spontaneous curvature of DLPE monolayers at low pH. The tangential stress exerted by the AFM tip on the deformable monolayers eventually produced a ripple pattern, which could be described as a periodic buckling known as Shallamach waves.
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We thank Frank Grunfeld of NIMA Technology for technical assistance. The work was accomplished when S.W.H. was at UCSB on sabbatical leave. We appreciate the advice and help of Dr. Jorgen Garnaes with the liquid cell AFM and Dr. Deborah Leckband for discussions on lipid phase behavior. This work was supported by grants GM28120 (S.W.H.) and GM 47334 (J.I. and J.A.Z.) from the National Institutes of Health, grant N00014-90-J-1551 (J.A.Z.) from the Office of Naval Research, and grants CTS90-15537 and CTS-9212790 (J.A.Z.) from the National Science Foundation.