How many phases and phase transitions do exist in Gibbs adsorption layers at the air–water interface?

Abstract

Four different phases and four different first-order phase transitions have been shown to exist in Gibbs adsorption layers of mixtures containing n-hexadecyl dihydrogen phosphate ( n-HDP) and l-arginine ( l-arg) at a molar ratio of 1:2. These conclusions have been made from surface pressure–time ( π – t ) adsorption isotherms measured with a film balance and from monolayer morphology observed with a Brewster angle microscopy (BAM). The observed four phases are gas (G), liquid expanded (LE), liquid condensed (LC) and LC ′ phases. Three first-order phase transitions are G–LE, LE–LC and LC–LC ′ . However, the thermodynamically allowed G–LC phase transition in a 1.2 × 10 −4 M mixture at 2 °C, which is below the so-called triple point, is kinetically separated into the G–LE and LE–LC phase transitions. The most interesting observation is that the homogeneous LC phase shows a new first-order phase transition named as LC–LC ′ at 2 or 5 °C. The LE and LC phases represent circular and fractal shaped domains, respectively, whereas the LC ′ phase shows very bright, anisotropic and characteristic shaped domains.