Similarities in the Phase Properties of Gibbs and Langmuir Monolayers

A direct comparison of thermodynamical behavior and condensed-phase structures of adsorbed Gibbs monolayers with those of spread Langmuir monolayers of pure amphiphilic acid amide compounds at the air/water interface is presented. Thermodynamical behavior of adsorbed and spread monolayers of N-(γ-hydroxypropyl)tridecanoic acid amide (HTRAA) and N-(γ-hydroxypropyl)tetradecanoic acid amide (HTEAA) have been investigated with surface-pressure measurements (π−A isotherms for HTRAA and HTEAA; π−t adsorption kinetics for HTRAA). Those measurements were combined with Brewster angle microscopy (BAM) and synchrotron gracing incidence X-ray diffraction (GIXD) to study morphological features and crystal structures of the condensed phase. Adsorption kinetics and surface-pressure−area isotherms show a well-pronounced plateau region that starts at a characteristic inflection point. These characteristics are related to a first-order phase transition from a fluidlike to a condensed phase in the monolayers. A simple model enables the comparison of the adsorption kinetics with surface-pressure−area isotherms. Similar features of condensed phase domains are visualized by BAM for adsorbed and spread monolayers. Dendritic growth structures are found for all compounds and both types of monolayers. The molecules are arranged in an oblique lattice structure. The condensed phase structure is independent of the process of monolayer formation, but the density of defects is significantly smaller in the more homogeneously grown Gibbs monolayers