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Thermotropic Mesomorphism of a Model System for the Plant Epicuticular Wax Layer
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Advisor(s)
Abstract(s)
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°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 (SHEX), which at lower temperatures transforms
to an orthorhombically-packed solid phase (SORT). On heating the SORT 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, SHEX, and SORT 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 (X18) of 0.12 and 0.18 and a
temperature of 32°C. Below 32°C, a binary mixture of solid phases, one an SORT phase and the other an SHEX phase, coexist
up to 25°C, below which both solid phases are SORT 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