DIRECT OBSERVATIONS OF TRANSITIONS BETWEEN CONDENSED LANGMUIR MONOLAYER PHASES BY POLARIZED FLUORESCENCE MICROSCOPY

Polarized fluorescence microscopy of Langmuir monolayers can distinguish domains of different tilt azimuth in condensed phases of fatty acids. The texture within a one-phase region is a mosaic of well-defined domains that coarsen with time. Boundaries often contain sharp kinks, implying a significant anisotropy in the line tension between domains. The contrast between domains disappears at transitions between tilted and untilted phases. We report the first direct observation of the L2''-CS transition and the swiveling transition (L2-L2'), which is marked by the instantaneous and reversible motion of isolated domain boundaries. This transition leaves the domain morphology and the absolute tilt direction largely unchanged, implying that molecules rearrange locally to change the nearest-neighbor direction into the next nearest neighbor. We also confirm the existence of a new phase transition observed by Overbeck et al.1 with Brewster angle microscopy.