SIMILARITIES IN THE PHASE PROPERTIES OF GIBBS AND LANGMUIR MONOLAYERS

A direct comparison of thermodynamical behavior and condensed-phase st ructures of adsorbed Gibbs monolayers with those of spread Langmuir mo nolayers of pure amphiphilic acid amide compounds at the air/water int erface is presented. Thermodynamical behavior of adsorbed and spread m onolayers of N-(gamma-hydroxypropyl)tridecanoic acid amide (HTRAA) and N-(gamma-hydroxypropyl)tetradecanoic acid amide (HTEAA) have been inv estigated with surface-pressure measurements (pi-A isotherms for HTRAA and HTEAA; pi-t adsorption kinetics for HTRAA). Those measurements we re combined with Brewster angle microscopy (BAM) and synchrotron graci ng 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 characteristi cs are related to a first-order phase transition from a fluidlike to a condensed phase in the monolayers. A simple model enables the compari son of the adsorption kinetics with surface-pressure-area isotherms. S imilar features of condensed phase domains are visualized by BAM for a dsorbed and spread monolayers. Dendritic growth structures are found f or all compounds and both types of monolayers, The molecules are arran ged 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.