Texture change separate from the transition between two tilted phases in Langmuir monolayers

Palmityl acetate monolayers are studied by grazing-incidence X-ray diffraction (GID). The monolayers are found to undergo a transition from a phase with alkyl chains tilted toward nearest neighbors (NN) to a phase with chains tilted toward next-nearest neighbors (NNN) upon monolayer compression. This transition gives rise to a sudden reorientation of the molecules that is determined by Brewster angle microscopy (BAM). At T > 15 degrees C, palmityl acetate monolayers undergo a first-order transition from a fluid phase to a condensed phase indicated by a plateau region in the pi-A isotherm. The transition pressure increases with increasing temperature. At low temperatures a phase with alkyl chains tilted toward nearest neighbors is formed from the fluid phase. The critical pressure for the transition between the tilted phases changes only slightly with temperature. Consequently, a phase with chains tilted toward next-nearest neighbors is formed directly from the fluid phase at high temperatures. Then no reorientation of molecules is observed by BAM. The textures of the condensed-phase domains formed during the transition from a fluid phase to a condensed phase are temperature-dependent. Some domains are subdivided into six segments of different orientation. In such domains the molecular chains are tilted along the bisector of the segments at T < 22 degrees C, whereas the tilt azimuth is perpendicular to the bisector at T > 24 degrees C. The crossover from one texture to the other is separate from the transition between the tilted phases. At T less than or equal to 26 degrees C, the sudden reorientation of molecules corresponding to the NN-NNN phase transition is still observed.