Molecular order and thermodynamics of the solid-liquid transition in triglycerides via Raman spectroscopy

The conformational and thermodynamic behavior of five monoacid saturated triglycerides (TGs) before, during, and above the beta polymorph --> liquid phase transition was studied using Raman spectroscopy. The Raman ratio I[upsilon(s)(CH(2))]/I[upsilon(as)(CH(2))], used to identify intramolecular order about TG hydrocarbon chains, demonstrated that a single conformation, geometry and symmetry existed in liquid-state TGs. The Raman ratio I(1080)/I(1130), used to determine the intermolecular order/disorder about the hydrocarbon chains and relative trans/gauche content, remained constant for TGs in the crystalline state, but steadily increased as a function of temperature in the liquid state. Use of the van't Hoff relation and the spectroscopically-determined trans/gauche content indicated the presence of distinctive pre- and post-transition enthalpies/entropies indicating that the beta --> liquid phase transition is "soft", with possible intermediate conformations. The liquid-state ester carbonyl stretching region, which gave rise to a broad peak between 1780-1700 cm(-1), was decomposed into multiple components. It demonstrated solid-like character 2-3 degrees C above the TG beta-polymorph melting point, above which no further change in spectral character was observed. These results indicate that the solid-liquid transition in TGs is of the "soft" type with non-lamellar conformations likely present in the melt.