Critical Instability Leads To Labyrinthine Transition In Binary Lipid/Polymer Monolayers

Abstract

Near critical points, the microscopic structures of fluid mixtures become unimportant and systems are characterized by strong susceptibility to small perturbations, large fluctuations and mesoscopic ordering. In biological membranes, these fluctuations in physical properties (composition, density, and curvature) may provide a spatially and temporally defined structural means to organize important functions such as formation of lipid rafts, caveolar invaginations or molecular colocalization. We report an observation of characteristic critical fluctuations in lipid monolayers composed of binary mixtures of 1,2 dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and polyoxyethylene stearate (PegxS of varying PEG chain length) confined to an air/water interface. Using fluorescence and Brewster angle microscopy, compositional changes that occur during the expansion of an initially phase-separated binary monolayer reveal a passage through a critical composition at vanishing lateral surface pressure. The corresponding morphological transformation from an optically homogeneous liquid expanded state to a gas bubble phase via a striped, labyrinthine phase reflects the competition between long-range electrostatic repulsion and line tension forces caused by interactions between DPPC and PegxS.