Morphology and collapse transitions in binary phospholipid monolayers

We have concurrently studied the microscopic phase behavior, morphology, and surface pressure-area isotherms of Langmuir monolayers of a 7:3 mixture of DPPC (dipalmitoylphosphatidylcholine) and POPG (palmitoyloleoylphosphatidylglycerol) 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 collapse. At temperatures below 28 degreesC, the monolayer is biphasic and collapses by forming large-scale folds, which reliably unfold upon expansion. These 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 vesicular structures that are globular or tubular in shape. Collapse occurs via both folding and vesiculation at temperatures between 28 and 33.5 degreesC, leading to the coexistence of the monolayer with both folds and vesicles. Analogous 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 monolayer, three-dimensional folds, and three-dimensional vesicles. In addition to this "triple point", the monolayer collapse mode is found to be independent of the path taken in the temperature - pressure parameter plane. The transition between the collapse modes thus resembles an equilibrium first-order phase transition.