Collapse in binary phospholipid monolayers at the air/water interface.

The behavior in the stochastic collapse of lipid monolayers of a 7:3 mixture of DPPC (dipalmitoylphosphatidylcholine) and POPG (palmitoyloleoylphosphatidylglycerol), a model system of pulmonary surfactant assembly, was studied using Langmuir isotherms, fluorescence microscopy, and atomic force microscopy. The monolayers at the air-water interface appeared to be phase separated under compression and retained the continuous liquid-expanded phase network surrounding islands of condensed phase even at a surface pressure approaching 70 mN/m. When the two-dimensional monolayers were compressed beyond the equilibrium surface pressure, they collapsed and assumed a three-dimensional formation. Collapse events involved folding of the monolayer on a micron scale, and each event produced a macroscopic jerk of the layer. The distribution of waiting times between events was estimated to be an exponential function, indicating that the events were independent. Folded regions coexisted with the flat monolayer, remained attached to the interface, and reversibly reincorporated into the monolayer upon expansion.