Electron microscopy and electron diffraction study of coexisting phases of pure and mixed monolayers transferred onto solid substrates

A recently developed method to transfer continuous lipid monolayers from various condensed as well as expanded (homogeneous or heterogeneous) states on the air/water interface onto solid substrates combined with new electron microscopy (EM) techniques is applied to study the microstructure of arachidic acid and lateral phase separation in dimyristoylphosphatidic acid cholesterol mixed monolayers. Due to a difference in the surface charge formation it is possible to distinguish amorphous and crystalline phases: (1) by a special phase contrast technique based on defocussing; (2) by darkfield-scanning-transmission electronmicroscopy (STEM); or (3) by secondary electron emission electron microscopy. Quantitative information concerning the lateral packing density in the various monolayer phases is obtained by mass density mapping and platinum shadowing. Evidence is provided that, in contrast to phospholipids, the fatty acid monolayer undergoes a (very weak first-order) solid-solid transition at a pressure π_c which is associated with chain tilting as postulated earlier (Albrecht et al., J. Phys. 39, 301 (1978)). At T < 24°C arachidic acid goes over from a (two-dimensional) gas-crystal coexistence region at π = 0 to a continuous crystalline state (I) with a twofold symmetry at 0 ≤ π ≤ π_c and to a second crystal phase (II) with purely hexagonal symmetry at π > π_c. The twofold symmetry is attributed to chain tilting. At T ≥ 25°C the transition to the solid state I occurs via the fluid phase. Mass density mapping shows a reduction in the lateral packing density at the solid-to-fluid transition of 25% as expected. For the gaslike phase an astonishing high packing density amounting to 50% of the solid phase is found.