Phase transition in adsorption layers at the air–water interface

Abstract

Conclusive evidence is presented for a first order phase transition in adsorption layers. Representative studies were performed with a special tailored amphiphile N-dodecyl-γ-hydroxybutyric acid amide (DHBAA) although main phase transitions can occur in the adsorption layers of numerous amphiphiles. The general conditions necessary for the formation of a two-phase coexistence in adsorption layers are investigated using surface pressure (π) transients, Brewster angle microscopy (BAM) and synchrotron X-ray diffraction at grazing incidence (GIXD). During the adsorption kinetics, appearance and location of the phase transition point depend largely on the concentration of the amphiphile in the aqueous solution and on the temperature. In different temperature regions, various types of morphological textures of the condensed phase are formed. Lattice structure and tilt angle of the alkyl chains in the condensed phase of the adsorption layer are determined using GIXD. The main growth directions of the condensed phase textures are correlated with the lattice structure. The experimental bridging to the Langmuir monolayers supports the conclusions of a first order main transition during the adsorption kinetics. A new theoretical model is presented which describes the adsorption kinetics of the 2D first-order phase transition in an adsorption layer. The theory comprises a kinetic model for a phase transition in Langmuir and Gibbs monolayers, the adsorption from the bulk solution and the dissociation kinetics of bulk micelles. The theoretical data calculated with the new kinetic model are in agreement with the experimental results.