The method of fluorescence recovery after photobleaching has been used to determine the lateral diffusion coefficient (D) of a fluorescent lipid probe in L-α-dilauroylphosphatidylcholine monolayers. The surface mass density was varied over a wide range from the liquid expanded/gas coexistence phase (LE/G) to the homogeneous liquid phase. The lateral diffusion coefficient reached a maximum value at the “liftoff” point on the surface pressure-area curve (D ≈ 1.2 × 1(10−10 m2/s, A = 1.1−1.3 nm2/molecule, T = 22−24 °C), corresponding to the onset of the uniphasic LE state, and decreased from this maximum as the mass density was varied to both higher and lower values of area per molecule (A). The results were analyzed according to the effective-medium theory and the free area model, respectively. The D values in the LE/G biphasic monolayers were evaluated as a function of the area fraction of the LE phase, whereby it was shown that the gas bubbles in the LE/G can be regarded as semipermeable obstacles for the lipid probes, with a relative permeability of the gas bubbles with respect to the LE phase of 0.2–0.3.
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