Domain formation in Gibbs monolayers at oil/water interfaces studied by Brewster angle microscopy

Brewster angle microscopy (BAM) was applied to a study of temperature-induc ed phase transitions in Gibbs monolayer films of two long-chain alkanols (o ctadecanol and 1,1,2,2-tetrahydroperfluorododecanol) at the hexane/water in terface. Changes of the interfacial tension along the experimental temperat ure scans were monitored simultaneously with the BAM micrographs by quasiel astic light scattering from the interface. In this way it was possible to l ocate the phase transition temperature T-t in situ from the observed break. in the temperature derivative of the interfacial tension. Domains of the c ondensed phase are seen below T-t, and the morphological features of these domains show remarkable similarities with those observed in Langmuir films at the free surface of water. The coexistence of the condensed phase with a dilute (gaslike or expanded) phase is not limited to a single temperature (as expected for a first-order phase transition) but extends over a tempert ure range of ca. 15 K below the transition temperature. This finding is att ributed to surface-active impurities which will accumulate in the dilute ph ase of the interfacial film.