Modeling the solid-liquid phase transition in saturated triglycerides

• We investigated theor. two competing published scenarios for the melting transition of the triglyceride trilaurin (TL): those of (1) Corkery et al., in which the av. state of each TL mol. in the liq. phase is a discotic "Y" conformer whose three chains are dynamically twisted, with an av. angle of ∌120° between them, and those of (2) Cebula et al., in which the liq.-state conformation of the TL mol. in the liq. 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 mols. are in a nematic compact-chair (or "h") conformation, with extended, possibly all-trans, chains at low-temps., and in either a Y conformation or an h* conformation in the liq. state at temps. higher than the phase-transition temp., T* = 319 K. We defined an h-Y model as a realization of the proposal of R. Corkery et al. (2007), and explored its predictions by mapping it onto an Ising model in a temp.-dependent field, performing a mean-field approxn., and calcg. the transition enthalpy ΔH. We found that the most plausible realization of the h-Y model, as applied to the solid-liq. phase transition in TL, and likely to all satd. triglycerides, gave a value of ΔH in reasonable agreement with the expt. We then defined an alternative h-h* model as a realization of the proposal of D. J. Cebula et al. (1992), in which the liq. phase exhibits an av. 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 temp. dependence of the 1132 cm-1 Raman band for both models, and performed measurements of the ratios of three TL Raman bands in the temp. range of -20° ≀ T ≀ 90°. The exptl. results were in accord with the predictions of the h-Y model and support the proposal of that the liq. state is made up of mols. that are each, on av., in a Y conformation. Finally, we carried out computer simulations of minimal-model TLs in the liq. phase, and concluded that although the individual TL mols. are, on av., Y conformers, long-range discotic order is unlikely to exist.

Ämnesord

NATURVETENSKAP  -- Kemi -- Annan kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Other Chemistry Topics (hsv//eng)

Nyckelord

modeling solid liq phase transition satd triglyceride

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