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-octadecanol and their binary mixtures between 5 and 70 degrees C, 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 CH(2)-stretching (symmetric and asymmetric) and CH(2)-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 approximately 32 degrees C. Below 32 degrees C, a binary mixture of solid phases, one an S(ORT) phase and the other an S(HEX) phase, coexist up to approximately 25 degrees C, 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.