A charge-decoration technique for studying the heterogeneity of coexistent monolayer phases by electron microscopy

Phospholipid or fatty acid monolayers at the air-water interface exhibit a series of pressure and temperature-dependent two-dimensional conformational changes^1-3. New techniques^1-3 of studying the microscopic heterogeneity of lipid monolayers have shown that, with increasing pressure, they undergo first a gas-fluid phase transition via a two-dimensional foam ², and subsequently a transition, extending over a finite pressure region of several mN m^-1, from the fluid to a crystalline state. This could be attributed to the coexistence of these states¹ and has been explained in terms of their different electric polarizations - a new picture of the fluid-solid phase transition. We have demonstrated4 that monolayers may be transferred from the air-water interface to solid substrates, not only from completely condensed, but also from more expanded, states such as these fluid-solid coexistence regions of pure and mixed monolayers. This suggests new possibilities of preparing Langmuir-Blodgett films with mosaic-like arrangements of crystalline and amorphous patches several micrometres in diameter ⁵. We report here a new electron microscopy technique which allows us to study the microstructure of such films, without any staining, up to a resolution of 1,000 Å. Different phases may be distinguished by phase contrast, owing to differences in electric charging caused by a corresponding difference in the electric conductivities, and electron diffraction patterns taken from selected areas can give information on the molecular packing of the phases.