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 grazing incidence X-ray diffraction and Brewster-angle microscopy studies on mixtures of hexadecanoic acid with methyl n-hexadecanoate. We have determined how the boundary between the L-2 (NN) and Ov (NNN) phases change with composition. Beyond a mole fraction of ester x = 0.70, the boundary moves sharply toward lower temperature with increasing concentration of the ester, and eventually the only tilted mesophase remaining has NN tilt. This is in contrast 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 result of disorder in the chain ends of the amphiphile.