Phase behavior of n-hexadecyl phosphate in Gibbs adsorption layers

Phase behavior of sparingly water soluble n-hexadecyl phosphate in adsorption layers at the air–water interface has been investigated by Brewster angle microscopy. Two distinct phase transitions have been found to occur before attaining the condensed monolayer phase at the global equilibrium. A surface pressure relaxation in the π–t curve which is accompanied by two phase coexistent state is a direct evidence for the first-order phase transition. A second-order phase transition is attributed to a gradual change in the surface morphology from bright isotropic to anisotropic condensed phase. The domains formed during the former transition have a very characteristic worm-like structure at 28 °C although the equilibrium shapes of the domains are circular at lower temperatures.