Modeling the solid-liquid phase transition in saturated triglycerides.
We investigated theoretically two competing published scenarios for the melting transition of the triglyceride trilaurin (TL): those of (1) Corkery et al. [Langmuir 23, 7241 (2007)], in which the average state of each TL molecule in the liquid phase is a discotic "Y" conformer whose three chains are dynamically twisted, with an average angle of approximately 120 degrees between them, and those of (2) Cebula et al. [J. Am. Oil Chem. Soc. 69, 130 (1992)], in which the liquid-state conformation of the TL molecule in the liquid phase is a nematic h*-conformer whose three chains are in a modified "chair" conformation. We developed two competing models for the two scenarios, in which TL molecules are in a nematic compact-chair (or "h") conformation, with extended, possibly all-trans, chains at low-temperatures, and in either a Y conformation or an h* conformation in the liquid state at temperatures higher than the phase-transition temperature, T*=319 K. We defined an h-Y model as a realization of the proposal of Corkery et al. [Langmuir 23, 7241 (2007)], and explored its predictions by mapping it onto an Ising model in a temperature-dependent field, performing a mean-field approximation, and calculating the transition enthalpy DeltaH. We found that the most plausible realization of the h-Y model, as applied to the solid-liquid phase transition in TL, and likely to all saturated triglycerides, gave a value of DeltaH in reasonable agreement with the experiment. We then defined an alternative h-h* model as a realization of the proposal of Cebula et al. [J. Am. Oil Chem. Soc. 69, 130 (1992)], in which the liquid phase exhibits an average symmetry breaking similar to an h conformation, but with twisted chains, to see whether it could describe the TL phase transition. The h-h* model gave a value of DeltaH that was too small by a factor of approximately 3-4. We also predicted the temperature dependence of the 1132 cm(-1) Raman band for both models, and performed measurements of the ratios of three TL Raman bands in the temperature range of -20 degrees C < or = T < or = 90 degrees C. The experimental results were in accord with the predictions of the h-Y model and support the proposal of Corkery et al. [Langmuir 23, 7241 (2007)] that the liquid state is made up of molecules that are each, on average, in a Y conformation. Finally, we carried out computer simulations of minimal-model TLs in the liquid phase, and concluded that although the individual TL molecules are, on average, Y conformers, long-range discotic order is unlikely to exist.
Department of Physics, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada. scorpiocarla@gmail.com
David A
DA
Pink
Charles B
CB
Hanna
Christophe
C
Sandt
Adam J
AJ
MacDonald
Ronald
R
MacEachern
Robert
R
Corkery
Dérick
D
Rousseau
D003198:Computer Simulation; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D044367:Phase Transition; D055585:Physical Phenomena; D013499:Surface Properties; D013696:Temperature; D013816:Thermodynamics; D014280:Triglycerides / Q000737:chemistry*; D014961:X-Ray Diffraction
D014280:Triglycerides; C004565:trilaurin