Structure formation and phase transitions in Gibbs and Langmuir monolayers of amphiphilic acid amides

A comprehensive study of the two-dimensional crystal structure of Gibbs and Langmuir monolayers corroborates recent results that first order phase transition can occur in adsorption layers. The crystal structures and morphological features of the condensed phase of $N$-alkyl-γ-hydroxy-butyric acid amide monolayers with different alkyl chain lengths [dodecyl (DHBAA) and tetradecyl (THBAA)], but the same head group structure at the air-water interface are investigated. Surface pressure measurements ($\pi -A$ isotherms for DHBAA and THBAA; $\pi -t$ adsorption kinetics for DHBAA) are combined with synchrotron x-ray gracing incidence diffraction and Brewster angle microscopy measurements. Twinned domains are formed at lower temperatures $(T<\mathrm{}10\mathrm{}°\mathrm{C})$ and crosslike domains at higher $(T>\mathrm{}10\mathrm{}°\mathrm{C})$ temperatures. The oblique crystal structure found in all monolayers is independent of the process of monolayer formation. Macroscopic textures and structures of domains are correlated to the observed crystal structure. The comparison of Gibbs and Langmuir monolayers shows that their crystal structures, morphological textures, and thermodynamic properties are similar.