Morphology and collapse transitions in binary phospholipid monolayers

We have concurrently studied the microscopic phase behavior, morphology, an d surface pressure-area isotherms of Langmuir monolayers of a 7:3 mixture o f DPPC (dipalmitoylphosphatidylcholine) and POPG (palmitoyloleoylphosphatid ylglycerol) at various temperatures between 20 and 40 degreesC. The manner in which the monolayer, under compression, explores the third dimension at monolayer collapse correlates with the monolayer morphology prior to collap se. At temperatures below 28 degreesC, the monolayer is biphasic and collap ses by forming large-scale folds, which reliably unfold upon expansion. The se folded structures can be five to several hundred micrometers wide and up to millimeters long. Above 33.5 degreesC, the monolayer is homogeneous and , upon further compression, prefers to collapse through micron-scale vesicu lar structures that are globular or tubular in shape. Collapse occurs via b oth folding and vesiculation at temperatures between 28 and 33.5 degreesC, leading to the coexistence of the monolayer with both folds and vesicles. A nalogous to equilibrium phase transitions, there may exist a temperature in this range, that can be thought of as a "triple point" temperature for the coexistence of the three "phases" corresponding to the two-dimensional mon olayer, three-dimensional folds, and three-dimensional vesicles. In additio n to this "triple point", the monolayer collapse mode is found to be indepe ndent of the path taken in the temperature - pressure parameter plane. The transition between the collapse modes thus resembles an equilibrium first-o rder phase transition.