Assembly of new vic-dihydroxyoctadecanoic acid methyl esters at the air-water interface

Members of a new class of amphiphiles with two vicinal hydroxyl groups as a second polar moiety have been synthesized by dihydroxylation of trans-octa decenoates made of natural fatty acids or synthetic fatty acid esters. The phase behavior is studied by measuring surface pressure - area isotherms (p i-A isotherms) and equilibrium spreading pressures (ESP) as well as Brewste r angle microscopy (BAM) and fluorescence microscopy. Both, pi-A isotherms and the micrographs of all compounds show a two-phase coexistence region, i ndicating a first-order phase transition from an expanded to a condensed ph ase. Depending on the position of the vicinal hydroxyl groups along the hyd rophobic alkyl chain, the film pressure in the plateau region as well as th e onset of the plateau region differ. Additionally, the position of the hyd roxyl groups influences the formation of a stable condensed phase: only the methyl dihydroxyoctadecanoates with no or the largest distance between the two polar groups show an increase in surface pressure at small molecular a reas. A molecular model for the processes during film compression is develo ped, where the two-phase coexistence region is interpreted as the continuou s removal of one polar group from the water surface during compression.