Phase transitions in adsorption layers: comparison with Langmuir monolayers

Abstract

The adsorption layers of aqueous solutions of N-dodecyl-γ-hydroxy-butyric acid amide show a first-order phase transition between a fluid-like low-density phase and a condensed phase. Two-dimensional domains are formed which were visualized by Brewster angle microscopy. The π- A isotherms of the Langmuir monolayers show the wellknown plateau region with the critical surface pressure as the starting point for the first-order phase transition in the Langmuir monolayer. The measurements of π-t adsorption kinetics revealed a conspicuous inflection point at the same critical surface pressure. This inflection point is also related to the start of a first-order phase transition in the Gibbs monolayer. The results of the σ-log c isotherms are interpreted by the Frumkin's equation of state. Based on the calculated interaction parameters, the formation of a condensed phase in Gibbs monolayers is concluded.Thermodynamic behavior and morphological features of the Langmuir and Gibbs monolayers were compared and the similarities in morphology and thermodynamically properties of spread and of adsorbed molecules at the air-water interface demonstrated.Key wordsPhase transitionadsorptionisothermBrewster angle microscopyGibbs monolayerLangmuir monolayer