Thermotropic mesomorphism of a model system for the plant epicuticular wax layer

As a model for the epicuticular wax layer of plant cuticular membranes, we have studied the phase behavior of 1-tetradecanol and 1-octaclecanol and their binary mixtures between 5 and 70degreesC, using differential scanning calorimetry and Fourier transform infrared spectroscopy (FTIR). Both pure compounds show two exothermic phase transitions corresponding to a transformation from a liquid phase to a hexagonally packed solid phase (S-HEX), which at lower temperatures transforms to an orthorhombically-packed solid phase (S-ORT). On heating the S-ORT solid a single endothermic transition with a transition enthalpy corresponding to the sum of the exothermic transition enthalpies is obtained. These transitions were also followed using FTIR spectroscopy in the CH2-stretching (symmetric and asymmetric) and CH2-rocking vibration modes. The FTIR spectra of the pure compounds in the liquid, S-HEX, and S-ORT phases were used to simulate experimental spectra in the phase transition regions. The simulations allowed us to estimate the molar fractions of each phase in the transition regions of the pure compounds. A phase diagram for the binary mixture of 1-tetradecanol and 1-octadecanol was obtained using differential scanning calorimetry and FTIR. FTIR studies on binary mixtures prepared from one perdeuterated component and the other nondeuterated permitted studying the thermotropic behavior of each component in the mixture independently. The mixture shows an eutectic behavior with an eutectic point between a molar fraction of octadecanol (X-18) of 0.12 and 0.18 and a temperature of similar to32degreesC. Below 32degreesC, a binary mixture of solid phases, one an S-ORT phase and the other an S-HEX phase, coexist up to similar to25degreesC, below which both solid phases are S-ORT phases. We discuss the possible relevance of this complex phase behavior in a simple binary mixture of two long-chain alkanols in the context of the far more complex phase behavior expected for the plant epicuticular wax layer.