Dark-field electron microscopy of Langmuir-Blodgett films of fatty acids and their barium salts

Abstract

Monolayers of fatty acids and their barium salts on water were transferred onto supporting films of carbon or silicon dioxide by horizontal attachment, down-stroke or up-stroke modes at various stages of surface-pressure-area (π− A) isotherms. Dark-field electron microscope images and electron diffraction patterns were used to determine the structure and molecular ordering of one-layer Langmuir-Blodgett (LB) films without any presurface treatment such as shadow casting and replication. The observations revealed the polycrystalline powder structure of one-layer LB films of stearic acid, arachidic acid and barium arachidate prepared by the horizontal attachment mode at no compression. With monolayer compression the LB films tended to form a structure consisting of growing island films which included a number of scattered small holes. A dark-field electron micrograph and electron diffraction pattern taken with an electron cryomicroscope led us to conclude that the presence of these scattered small holes in the LB films was due neither to the sublimation of LB film molecules in vacuum nor to electron radiation damage. Similar porous structures were found when the monolayers of barium arachidate with flexible alkyl chains of an intermediate length were transferred by the down-stroke mode onto supporting films of carbon or silicon dioxide. Arachidic acid and its barium salt formed LB films of a rather homogeneous structure with a few large holes by the up-stroke mode. Because the cohesive force increased with the length of alkyl chains, the one-layer LB films of melissic acid (C 30) showed a similar porous structure, irrespective of the mode of transfer of the monolayers.