Observation of continuous and quantized domain size and shape evolution in monolayers at air-water interface

Langmuir monolayers of pentadecanoic acid (C15) and dipalmitoyl phosphatidylcholine (DPPC) are investigated in liquid expanded-liquid condensed (LE-LC) phase co-existence region in order to study evolution of circular and undulating LC domains. The Brewster angle microscopy is analyzed to evaluate LC domain size and shape for various average molecular areas. For C15 monolayer circular domain shape and continuous increase of domain size are observed. In contrast, for DPPC monolayer undulating domain shapes are observed, where the mode of oscillations m is quantized depending on the average molecular area. Surprisingly, also the average domain size is quantized and depends on m. These observations are discussed in accordance with the phase rule modified for the monolayer phase co-existence region and observed levels of domain sizes are compared with theories of m-fold distortion.