Probing Langmuir monolayer self-assembly in condensed and collapsed phases: grazing-incidence X-ray diffraction and X-ray standing wave studies

Cerium-induced effects on the self-assembly of arachidic acid monolayers have been studied. The monolayers were formed on a cerium nitrate solution and studied in condensed and collapsed phases at two different temperatures, T = 21°C and T = 23°C. Grazing-incidence X-ray diffraction and X-ray standing waves were applied to monitor the changes in molecular packing in real time. A new type of molecular organization was found, in which the monolayer maintains its structure despite being overcompressed. Instead of forming crystalline aggregates or anisotropic cracking, the collapsed monolayer at T = 21°C appeared to be corrugated. The diffraction pattern for the monolayer in this new collapse mode is represented by unclosed diffraction rings with maxima near Q z = 0. A theoretical formalism based on the distorted-wave Born approximation has been developed for quantitative analysis of the full 2D maps of diffraction scattering. The collapsed monolayer at T = 21°C is shown to consist of flat crystalline domains, which are inclined from the horizontal position. The crystalline structure of one such domain was identified as a pseudo-herringbone packing mode.