Influence of temperature on the phase behavior of 1,2-diol derivatives

Monolayers of the racemic 1-palmitoylglycerol (1) and 1-hexadecylglycerol (2) were investigated by grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy (BAM) at temperatures from 5 to 33 degrees C. Above 13 degrees C a liquid-expanded phase was found for 1 and 2, and both compounds show a transition to a condensed phase with a NN tilted centered rectangular structure. This transition pressure increases with increasing temperature. A second transition to a NNN tilted phase was observed using GIXD. Zn the case of 1 this transition pressure has a stronger temperature dependence than for 2. The NN-NNN transition pressures were additionally found to be much higher for 1 than 2. The surface pressure at which upright oriented molecules should be found was extrapolated from GIXD measurements. The transition was visualized for 1 using Brewster angle microscopy. A nonzero lattice distortion at zero tilt angle was obtained for both compounds, indicating an ordering of chain backbones, possibly herringbone packing. Phase diagrams have been constructed for 1 and 2.