Compression-induced Phase Transition in Adsorbed Monolayers of Alkylgalactosides at the Air/Water Interface

Compression-induced formation of condensed-phase domains in adsorbed monolayers of alkylgalactosides (AGs) at the air/water interface was observed. When an aqueous solution of AGs was poured into a Langmuir trough, the AG molecules were spontaneously adsorbed from the solution at the air/water interface to form the adsorbed or Gibbs monolayer in an expanded, liquid-like phase at equilibrium. The monolayer was subsequently laterally compressed by the barriers of the trough, while simultaneously observing the system using a Brewster angle microscope (BAM). The surface pressure-film area isotherm upon compression showed a kink at a surface pressure (π_kink) comparable to or several mN・m^-1 higher than the surface pressure at the critical micelle concentration (π_CMC), followed by a plateau region. BAM observations revealed that condensed-phase domains were formed in the homogeneous expanded phase at the plateau. Hence, the plateau corresponds to a first-order phase transition from the expanded phase to the condensed phase. As expected, the compressed adsorbed monolayer was in a metastable state because the surface pressure decreased with time, and the condensed-phase domains disappeared when compression was discontinued. The transient formation of a quasi-stable condensed phase may originate from the combined effect of the lower solubility of AG molecules in water, moderately strong attractive intermolecular interactions between AG molecules at the air/water interface, and high-rate compression.