Grazing incidence diffraction and Brewster-angle microscope studies of mixtures of hexadecanoic acid and methyl hexadecanoate: The unexpected appearance of a phase with nearest-neighbor tilt

To examine the relative stability of monolayer phases with nearest-neighbor (NN) and next-nearest-neighbor (NNN) tilt directions, we have performed gr azing incidence X-ray diffraction and Brewster-angle microscopy studies on mixtures of hexadecanoic acid with methyl n-hexadecanoate. We have determin ed how the boundary between the L-2 (NN) and Ov (NNN) phases change with co mposition. Beyond a mole fraction of ester x = 0.70, the boundary moves sha rply toward lower temperature with increasing concentration of the ester, a nd eventually the only tilted mesophase remaining has NN tilt. This is in c ontrast to many other esters that have been studied for which there is only a NNN mesophase. It is argued that the stability of the NN phase is the re sult of disorder in the chain ends of the amphiphile.