For Langmuir monolayers at the air−water interface, an equation of state is theoretically derived which describes the main phase transition between the gaslike and the condensed phases. The theoretical treatment considers the formation of two-dimensional aggregates and describes the nonhorizontal phase transition of the surface pressure−area isotherms and its dependence on the temperature. The equilibrium between the aggregates and monomers was treated using Buttler's equation for the chemical potential of monomers and aggregates within the surface. The results predicted by the theory agree well with the experimental surface pressure−area isotherms over a large temperature range.

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Adsorption layers of aqueous solutions of the amphiphile N-dodecyl-γ-hydroxybutyric acid amide are investigated using Brewster angle microscopy (BAM) and surface pressure (π) measurements. The experimental conditions for the occurrence of a first-order phase transition in adsorption layers (Gibbs monolayers) are studied. This phase transition is indicated by an inflection point in the π-t adsorption kinetics and the evolution of the condensed phase structures whose the morphological features are visualized by BAM. Langmuir monolayers of this amphiphile can be obtained by the usual monolayer technique and are compared with the adsorption layers. The π-A isotherms of the Langmuir monolayers show the plateau region that corresponds to the two-phase coexistence region for the main transition between a fluid phase of low density and a condensed phase. The critical surface pressures for the main phase transitions in the adsorption layers and Langmuir monolayers are correlated. The morphological features of the condensed phase structures of the Langmuir monolayers and adsorption layers agree completely.

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.