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 ~ 120° 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 ΔH. 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 ΔH 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 ΔH that was too small by a factor of ∼ 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 °C ≤ T ≤ 90 °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.
Pink, David A.; Hanna, Charles B.; Sandt, Christophe; MacDonald, Adam J.; MacEachern, Ronald; Corkery, Robert; and Rousseau, Dérick. (2010). "Modeling the Solid-Liquid Phase Transition in Saturated Triglycerides". Journal of Chemical Physics, 132(5), 054502-1 - 054502-12.