Phase behaviour of binary mixtures involving tristearin, stearyl stearate and stearic acid: thermodynamic study and BAM observation at the air–water interface and AFM analysis of LB films

The binary mixtures involving tristearin (TS), stearyl stearate (SS) and stearic acid (SA) were studied by surface pressure–area (π–A) measurements and by Brewster angle microscopy (BAM), at the air–water interface, and the Langmuir–Blodgett (LB) monolayers, transferred onto mica substrates, were analysed by AFM. The thermodynamic analysis indicated miscibility in the whole composition range for the system SA/TS, and partial miscibility for systems SA/SS and TS/SS. This behaviour was further confirmed by BAM observation and AFM analysis of LB films. The AFM imaging of collapsed monolayers revealed domains with a multilayered structure varying with system and composition. The layers thickness determined by cross section analysis are consistent with estimated molecular lengths and conformations proposed for the molecules, assuming nearly perpendicular or tilted orientations of the hydrocarbon chains to the interface.