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 °C. 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 °C, 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 °C, 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 °C, 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.