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

Abstract

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 foam², 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 diameter⁵. 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.