OPTICAL MEASUREMENTS OF THE PHASE-DIAGRAMS OF LANGMUIR MONOLAYERS OF FATTY-ACID, ESTER, AND ALCOHOL MIXTURES BY BREWSTER-ANGLE MICROSCOPY

Surface pressure-temperature phase diagrams have been determined by Br ewster-angle microscopy for Langmuir monolayers of heneicosanoic acid with the esters methyl and ethyl heneicosanoate and octadecanoic acid with methyl, ethyl, and propyl octadecanoate. The behavior is similar to that found previously in mixtures of an acid and an alcohol. In eac h case with increasing ester concentration the L(2)/L(2)' phase bounda ry moves toward lower pressure and higher temperature while the L(2)/O v boundary moves toward lower pressure and lower temperature. The L(2) ' and Ov phases eventually merge and the boundary with the L(2) phase moves to zero pressure. The phase diagram of eicosyl acetate is simila r to that of the fatty acids. We attribute the variations in the diagr ams to the extent of hydrogen bonding between the head group and the s ubphase. (C) 1997 American Institute of Physics.