Molecular order and thermodynamics of the solid–liquid transition in triglyceridesviaRaman spectroscopy

Abstract

The conformational and thermodynamic behavior of five monoacid saturated triglycerides (TGs) before, during, and above the β polymorph → liquid phase transition was studied using Raman spectroscopy. The Raman ratio I[υ_s(CH₂)]/I[υ_as(CH₂)], 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 I₁₀₈₀/I₁₁₃₀, 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 β → 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 °C above the TG β-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.