Characterization of the three major polymorphic forms and liquid state of tristearin by Raman spectroscopy

Raman spectroscopy was used to distinguish the differences in the molecular organization of the alpha, beta' and beta polymorphs, as well as the liquid state, of tristearin with focus placed on the C=O, C-H and C-C Raman-active stretching regions. The ester carbonyl stretching region permitted polymorphic discrimination due to significant differences in the number of modes, their relative frequencies and their full-widths at half-maximum. In the liquid state, the absence of obvious signatures in this region indicated that many local micro-environments likely exist about the ester carbonyl of molten tristearin. The ratio between the symmetrical and asymmetrical C-H stretching modes was linearly correlated with the enthalpy of fusion for each polymorph. The C-C stretching modes, which provided insight into the trans/gauche content, were polymorph independent, but changed significantly upon transition into the liquid state (p < 0.05). Overall, Raman spectroscopy allowed for the quick discrimination of tristearin polymorphs from a conformational and thermodynamic perspective.